RFC 6120 - Extensible Messaging and Presence Protocol (XMPP): Core
Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 6120 Cisco
Obsoletes:
3920
March 2011
Category: Standards Track
ISSN: 2070-1721
Extensible Messaging and Presence Protocol (XMPP): Core
Abstract
The Extensible Messaging and Presence Protocol (XMPP) is an
application profile of the Extensible Markup Language (XML) that
enables the near-real-time exchange of structured yet extensible data
between any two or more network entities. This document defines
XMPP's core protocol methods: setup and teardown of XML streams,
channel encryption, authentication, error handling, and communication
primitives for messaging, network availability ("presence"), and
request-response interactions. This document obsoletes
RFC 3920
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in
Section 2 of RFC 5741
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to
BCP 78
and the IETF Trust's Legal
Provisions Relating to IETF Documents
) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Saint-Andre Standards Track [Page 1]
RFC 6120
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Table of Contents
. Introduction . . . . . . . . . . . . . . . . . . . . . . . .
1.1
. Overview . . . . . . . . . . . . . . . . . . . . . . . .
1.2
. History . . . . . . . . . . . . . . . . . . . . . . . .
1.3
. Functional Summary . . . . . . . . . . . . . . . . . . .
1.4
. Terminology . . . . . . . . . . . . . . . . . . . . . .
11
. Architecture . . . . . . . . . . . . . . . . . . . . . . . .
13
2.1
. Global Addresses . . . . . . . . . . . . . . . . . . . .
13
2.2
. Presence . . . . . . . . . . . . . . . . . . . . . . . .
14
2.3
. Persistent Streams . . . . . . . . . . . . . . . . . . .
14
2.4
. Structured Data . . . . . . . . . . . . . . . . . . . .
14
2.5
. Distributed Network of Clients and Servers . . . . . . .
14
. TCP Binding . . . . . . . . . . . . . . . . . . . . . . . . .
16
3.1
. Scope . . . . . . . . . . . . . . . . . . . . . . . . .
16
3.2
. Resolution of Fully Qualified Domain Names . . . . . . .
17
3.2.1
. Preferred Process: SRV Lookup . . . . . . . . . . .
17
3.2.2
. Fallback Processes . . . . . . . . . . . . . . . . .
18
3.2.3
. When Not to Use SRV . . . . . . . . . . . . . . . .
18
3.2.4
. Use of SRV Records with Add-On Services . . . . . .
19
3.3
. Reconnection . . . . . . . . . . . . . . . . . . . . . .
19
3.4
. Reliability . . . . . . . . . . . . . . . . . . . . . .
20
. XML Streams . . . . . . . . . . . . . . . . . . . . . . . . .
20
4.1
. Stream Fundamentals . . . . . . . . . . . . . . . . . .
20
4.2
. Opening a Stream . . . . . . . . . . . . . . . . . . . .
23
4.3
. Stream Negotiation . . . . . . . . . . . . . . . . . . .
24
4.3.1
. Basic Concepts . . . . . . . . . . . . . . . . . . .
24
4.3.2
. Stream Features Format . . . . . . . . . . . . . . .
25
4.3.3
. Restarts . . . . . . . . . . . . . . . . . . . . . .
27
4.3.4
. Resending Features . . . . . . . . . . . . . . . . .
27
4.3.5
. Completion of Stream Negotiation . . . . . . . . . .
27
4.3.6
. Determination of Addresses . . . . . . . . . . . . .
28
4.3.7
. Flow Chart . . . . . . . . . . . . . . . . . . . . .
29
4.4
. Closing a Stream . . . . . . . . . . . . . . . . . . . .
31
4.5
. Directionality . . . . . . . . . . . . . . . . . . . . .
32
4.6
. Handling of Silent Peers . . . . . . . . . . . . . . . .
33
4.6.1
. Dead Connection . . . . . . . . . . . . . . . . . .
34
4.6.2
. Broken Stream . . . . . . . . . . . . . . . . . . .
34
4.6.3
. Idle Peer . . . . . . . . . . . . . . . . . . . . .
34
4.6.4
. Use of Checking Methods . . . . . . . . . . . . . .
35
4.7
. Stream Attributes . . . . . . . . . . . . . . . . . . .
35
4.7.1
. from . . . . . . . . . . . . . . . . . . . . . . . .
35
4.7.2
. to . . . . . . . . . . . . . . . . . . . . . . . . .
37
4.7.3
. id . . . . . . . . . . . . . . . . . . . . . . . . .
38
4.7.4
. xml:lang . . . . . . . . . . . . . . . . . . . . . .
39
4.7.5
. version . . . . . . . . . . . . . . . . . . . . . .
41
4.7.6
. Summary of Stream Attributes . . . . . . . . . . . .
43
4.8
. XML Namespaces . . . . . . . . . . . . . . . . . . . . .
43
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4.8.1
. Stream Namespace . . . . . . . . . . . . . . . . . .
43
4.8.2
. Content Namespace . . . . . . . . . . . . . . . . .
43
4.8.3
. XMPP Content Namespaces . . . . . . . . . . . . . .
44
4.8.4
. Other Namespaces . . . . . . . . . . . . . . . . . .
46
4.8.5
. Namespace Declarations and Prefixes . . . . . . . .
47
4.9
. Stream Errors . . . . . . . . . . . . . . . . . . . . .
48
4.9.1
. Rules . . . . . . . . . . . . . . . . . . . . . . .
48
4.9.1.1
. Stream Errors Are Unrecoverable . . . . . . . . .
48
4.9.1.2
. Stream Errors Can Occur During Setup . . . . . .
49
4.9.1.3. Stream Errors When the Host Is Unspecified or
Unknown . . . . . . . . . . . . . . . . . . . . .
50
4.9.1.4
. Where Stream Errors Are Sent . . . . . . . . . .
50
4.9.2
. Syntax . . . . . . . . . . . . . . . . . . . . . . .
51
4.9.3
. Defined Stream Error Conditions . . . . . . . . . .
52
4.9.3.1
. bad-format . . . . . . . . . . . . . . . . . . .
52
4.9.3.2
. bad-namespace-prefix . . . . . . . . . . . . . .
52
4.9.3.3
. conflict . . . . . . . . . . . . . . . . . . . .
53
4.9.3.4
. connection-timeout . . . . . . . . . . . . . . .
54
4.9.3.5
. host-gone . . . . . . . . . . . . . . . . . . . .
54
4.9.3.6
. host-unknown . . . . . . . . . . . . . . . . . .
55
4.9.3.7
. improper-addressing . . . . . . . . . . . . . . .
56
4.9.3.8
. internal-server-error . . . . . . . . . . . . . .
56
4.9.3.9
. invalid-from . . . . . . . . . . . . . . . . . .
56
4.9.3.10
. invalid-namespace . . . . . . . . . . . . . . . .
57
4.9.3.11
. invalid-xml . . . . . . . . . . . . . . . . . . .
57
4.9.3.12
. not-authorized . . . . . . . . . . . . . . . . .
58
4.9.3.13
. not-well-formed . . . . . . . . . . . . . . . . .
59
4.9.3.14
. policy-violation . . . . . . . . . . . . . . . .
59
4.9.3.15
. remote-connection-failed . . . . . . . . . . . .
60
4.9.3.16
. reset . . . . . . . . . . . . . . . . . . . . . .
60
4.9.3.17
. resource-constraint . . . . . . . . . . . . . . .
61
4.9.3.18
. restricted-xml . . . . . . . . . . . . . . . . .
61
4.9.3.19
. see-other-host . . . . . . . . . . . . . . . . .
62
4.9.3.20
. system-shutdown . . . . . . . . . . . . . . . . .
64
4.9.3.21
. undefined-condition . . . . . . . . . . . . . . .
64
4.9.3.22
. unsupported-encoding . . . . . . . . . . . . . .
64
4.9.3.23
. unsupported-feature . . . . . . . . . . . . . . .
65
4.9.3.24
. unsupported-stanza-type . . . . . . . . . . . . .
65
4.9.3.25
. unsupported-version . . . . . . . . . . . . . . .
66
4.9.4
. Application-Specific Conditions . . . . . . . . . .
67
4.10
. Simplified Stream Examples . . . . . . . . . . . . . . .
68
. STARTTLS Negotiation . . . . . . . . . . . . . . . . . . . .
69
5.1
. Fundamentals . . . . . . . . . . . . . . . . . . . . . .
69
5.2
. Support . . . . . . . . . . . . . . . . . . . . . . . .
70
5.3
. Stream Negotiation Rules . . . . . . . . . . . . . . . .
70
5.3.1
. Mandatory-to-Negotiate . . . . . . . . . . . . . . .
70
5.3.2
. Restart . . . . . . . . . . . . . . . . . . . . . .
70
5.3.3
. Data Formatting . . . . . . . . . . . . . . . . . .
70
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5.3.4
. Order of TLS and SASL Negotiations . . . . . . . . .
71
5.3.5
. TLS Renegotiation . . . . . . . . . . . . . . . . .
71
5.3.6
. TLS Extensions . . . . . . . . . . . . . . . . . . .
72
5.4
. Process . . . . . . . . . . . . . . . . . . . . . . . .
72
5.4.1
. Exchange of Stream Headers and Stream Features . . .
72
5.4.2
. Initiation of STARTTLS Negotiation . . . . . . . . .
73
5.4.2.1
. STARTTLS Command . . . . . . . . . . . . . . . .
73
5.4.2.2
. Failure Case . . . . . . . . . . . . . . . . . .
73
5.4.2.3
. Proceed Case . . . . . . . . . . . . . . . . . .
74
5.4.3
. TLS Negotiation . . . . . . . . . . . . . . . . . .
74
5.4.3.1
. Rules . . . . . . . . . . . . . . . . . . . . . .
74
5.4.3.2
. TLS Failure . . . . . . . . . . . . . . . . . . .
75
5.4.3.3
. TLS Success . . . . . . . . . . . . . . . . . . .
76
. SASL Negotiation . . . . . . . . . . . . . . . . . . . . . .
77
6.1
. Fundamentals . . . . . . . . . . . . . . . . . . . . . .
77
6.2
. Support . . . . . . . . . . . . . . . . . . . . . . . .
77
6.3
. Stream Negotiation Rules . . . . . . . . . . . . . . . .
77
6.3.1
. Mandatory-to-Negotiate . . . . . . . . . . . . . . .
77
6.3.2
. Restart . . . . . . . . . . . . . . . . . . . . . .
78
6.3.3
. Mechanism Preferences . . . . . . . . . . . . . . .
78
6.3.4
. Mechanism Offers . . . . . . . . . . . . . . . . . .
78
6.3.5
. Data Formatting . . . . . . . . . . . . . . . . . .
79
6.3.6
. Security Layers . . . . . . . . . . . . . . . . . .
80
6.3.7
. Simple User Name . . . . . . . . . . . . . . . . . .
80
6.3.8
. Authorization Identity . . . . . . . . . . . . . . .
80
6.3.9
. Realms . . . . . . . . . . . . . . . . . . . . . . .
81
6.3.10
. Round Trips . . . . . . . . . . . . . . . . . . . .
81
6.4
. Process . . . . . . . . . . . . . . . . . . . . . . . .
82
6.4.1
. Exchange of Stream Headers and Stream Features . . .
82
6.4.2
. Initiation . . . . . . . . . . . . . . . . . . . . .
83
6.4.3
. Challenge-Response Sequence . . . . . . . . . . . .
84
6.4.4
. Abort . . . . . . . . . . . . . . . . . . . . . . .
84
6.4.5
. SASL Failure . . . . . . . . . . . . . . . . . . . .
85
6.4.6
. SASL Success . . . . . . . . . . . . . . . . . . . .
86
6.5
. SASL Errors . . . . . . . . . . . . . . . . . . . . . .
87
6.5.1
. aborted . . . . . . . . . . . . . . . . . . . . . .
88
6.5.2
. account-disabled . . . . . . . . . . . . . . . . . .
88
6.5.3
. credentials-expired . . . . . . . . . . . . . . . .
88
6.5.4
. encryption-required . . . . . . . . . . . . . . . .
89
6.5.5
. incorrect-encoding . . . . . . . . . . . . . . . . .
89
6.5.6
. invalid-authzid . . . . . . . . . . . . . . . . . .
89
6.5.7
. invalid-mechanism . . . . . . . . . . . . . . . . .
90
6.5.8
. malformed-request . . . . . . . . . . . . . . . . .
90
6.5.9
. mechanism-too-weak . . . . . . . . . . . . . . . . .
90
6.5.10
. not-authorized . . . . . . . . . . . . . . . . . . .
91
6.5.11
. temporary-auth-failure . . . . . . . . . . . . . . .
91
6.6
. SASL Definition . . . . . . . . . . . . . . . . . . . .
91
. Resource Binding . . . . . . . . . . . . . . . . . . . . . .
92
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7.1
. Fundamentals . . . . . . . . . . . . . . . . . . . . . .
92
7.2
. Support . . . . . . . . . . . . . . . . . . . . . . . .
93
7.3
. Stream Negotiation Rules . . . . . . . . . . . . . . . .
93
7.3.1
. Mandatory-to-Negotiate . . . . . . . . . . . . . . .
93
7.3.2
. Restart . . . . . . . . . . . . . . . . . . . . . .
93
7.4
. Advertising Support . . . . . . . . . . . . . . . . . .
93
7.5
. Generation of Resource Identifiers . . . . . . . . . . .
94
7.6
. Server-Generated Resource Identifier . . . . . . . . . .
94
7.6.1
. Success Case . . . . . . . . . . . . . . . . . . . .
94
7.6.2
. Error Cases . . . . . . . . . . . . . . . . . . . .
95
7.6.2.1
. Resource Constraint . . . . . . . . . . . . . . .
95
7.6.2.2
. Not Allowed . . . . . . . . . . . . . . . . . . .
96
7.7
. Client-Submitted Resource Identifier . . . . . . . . . .
96
7.7.1
. Success Case . . . . . . . . . . . . . . . . . . . .
96
7.7.2
. Error Cases . . . . . . . . . . . . . . . . . . . .
97
7.7.2.1
. Bad Request . . . . . . . . . . . . . . . . . . .
97
7.7.2.2
. Conflict . . . . . . . . . . . . . . . . . . . .
97
7.7.3
. Retries . . . . . . . . . . . . . . . . . . . . . .
99
. XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . .
99
8.1
. Common Attributes . . . . . . . . . . . . . . . . . . .
100
8.1.1
. to . . . . . . . . . . . . . . . . . . . . . . . . .
100
8.1.1.1
. Client-to-Server Streams . . . . . . . . . . . .
100
8.1.1.2
. Server-to-Server Streams . . . . . . . . . . . .
101
8.1.2
. from . . . . . . . . . . . . . . . . . . . . . . . .
101
8.1.2.1
. Client-to-Server Streams . . . . . . . . . . . .
101
8.1.2.2
. Server-to-Server Streams . . . . . . . . . . . .
102
8.1.3
. id . . . . . . . . . . . . . . . . . . . . . . . . .
103
8.1.4
. type . . . . . . . . . . . . . . . . . . . . . . . .
103
8.1.5
. xml:lang . . . . . . . . . . . . . . . . . . . . . .
103
8.2
. Basic Semantics . . . . . . . . . . . . . . . . . . . .
105
8.2.1
. Message Semantics . . . . . . . . . . . . . . . . .
105
8.2.2
. Presence Semantics . . . . . . . . . . . . . . . . .
105
8.2.3
. IQ Semantics . . . . . . . . . . . . . . . . . . . .
105
8.3
. Stanza Errors . . . . . . . . . . . . . . . . . . . . .
107
8.3.1
. Rules . . . . . . . . . . . . . . . . . . . . . . .
108
8.3.2
. Syntax . . . . . . . . . . . . . . . . . . . . . . .
109
8.3.3
. Defined Conditions . . . . . . . . . . . . . . . . .
110
8.3.3.1
. bad-request . . . . . . . . . . . . . . . . . . .
110
8.3.3.2
. conflict . . . . . . . . . . . . . . . . . . . .
111
8.3.3.3
. feature-not-implemented . . . . . . . . . . . . .
111
8.3.3.4
. forbidden . . . . . . . . . . . . . . . . . . . .
112
8.3.3.5
. gone . . . . . . . . . . . . . . . . . . . . . .
113
8.3.3.6
. internal-server-error . . . . . . . . . . . . . .
113
8.3.3.7
. item-not-found . . . . . . . . . . . . . . . . .
114
8.3.3.8
. jid-malformed . . . . . . . . . . . . . . . . . .
114
8.3.3.9
. not-acceptable . . . . . . . . . . . . . . . . .
115
8.3.3.10
. not-allowed . . . . . . . . . . . . . . . . . . .
116
8.3.3.11
. not-authorized . . . . . . . . . . . . . . . . .
116
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8.3.3.12
. policy-violation . . . . . . . . . . . . . . . .
117
8.3.3.13
. recipient-unavailable . . . . . . . . . . . . . .
117
8.3.3.14
. redirect . . . . . . . . . . . . . . . . . . . .
118
8.3.3.15
. registration-required . . . . . . . . . . . . . .
119
8.3.3.16
. remote-server-not-found . . . . . . . . . . . . .
119
8.3.3.17
. remote-server-timeout . . . . . . . . . . . . . .
120
8.3.3.18
. resource-constraint . . . . . . . . . . . . . . .
121
8.3.3.19
. service-unavailable . . . . . . . . . . . . . . .
121
8.3.3.20
. subscription-required . . . . . . . . . . . . . .
122
8.3.3.21
. undefined-condition . . . . . . . . . . . . . . .
123
8.3.3.22
. unexpected-request . . . . . . . . . . . . . . .
123
8.3.4
. Application-Specific Conditions . . . . . . . . . .
124
8.4
. Extended Content . . . . . . . . . . . . . . . . . . . .
125
. Detailed Examples . . . . . . . . . . . . . . . . . . . . . .
128
9.1
. Client-to-Server Examples . . . . . . . . . . . . . . .
128
9.1.1
. TLS . . . . . . . . . . . . . . . . . . . . . . . .
128
9.1.2
. SASL . . . . . . . . . . . . . . . . . . . . . . . .
130
9.1.3
. Resource Binding . . . . . . . . . . . . . . . . . .
132
9.1.4
. Stanza Exchange . . . . . . . . . . . . . . . . . .
133
9.1.5
. Close . . . . . . . . . . . . . . . . . . . . . . .
134
9.2
. Server-to-Server Examples . . . . . . . . . . . . . . .
134
9.2.1
. TLS . . . . . . . . . . . . . . . . . . . . . . . .
134
9.2.2
. SASL . . . . . . . . . . . . . . . . . . . . . . . .
136
9.2.3
. Stanza Exchange . . . . . . . . . . . . . . . . . .
137
9.2.4
. Close . . . . . . . . . . . . . . . . . . . . . . .
137
10
. Server Rules for Processing XML Stanzas . . . . . . . . . . .
138
10.1
. In-Order Processing . . . . . . . . . . . . . . . . . .
138
10.2
. General Considerations . . . . . . . . . . . . . . . . .
140
10.3
. No 'to' Address . . . . . . . . . . . . . . . . . . . .
141
10.3.1
. Message . . . . . . . . . . . . . . . . . . . . . .
141
10.3.2
. Presence . . . . . . . . . . . . . . . . . . . . . .
141
10.3.3
. IQ . . . . . . . . . . . . . . . . . . . . . . . . .
141
10.4
. Remote Domain . . . . . . . . . . . . . . . . . . . . .
142
10.4.1
. Existing Stream . . . . . . . . . . . . . . . . . .
142
10.4.2
. No Existing Stream . . . . . . . . . . . . . . . . .
142
10.4.3
. Error Handling . . . . . . . . . . . . . . . . . . .
143
10.5
. Local Domain . . . . . . . . . . . . . . . . . . . . . .
143
10.5.1
. domainpart . . . . . . . . . . . . . . . . . . . . .
143
10.5.2
. domainpart/resourcepart . . . . . . . . . . . . . .
143
10.5.3
. localpart@domainpart . . . . . . . . . . . . . . . .
143
10.5.3.1
. No Such User . . . . . . . . . . . . . . . . . .
144
10.5.3.2
. User Exists . . . . . . . . . . . . . . . . . . .
144
10.5.4
. localpart@domainpart/resourcepart . . . . . . . . .
144
11
. XML Usage . . . . . . . . . . . . . . . . . . . . . . . . . .
145
11.1
. XML Restrictions . . . . . . . . . . . . . . . . . . . .
145
11.2
. XML Namespace Names and Prefixes . . . . . . . . . . . .
146
11.3
. Well-Formedness . . . . . . . . . . . . . . . . . . . .
146
11.4
. Validation . . . . . . . . . . . . . . . . . . . . . . .
147
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11.5
. Inclusion of XML Declaration . . . . . . . . . . . . . .
147
11.6
. Character Encoding . . . . . . . . . . . . . . . . . . .
147
11.7
. Whitespace . . . . . . . . . . . . . . . . . . . . . . .
148
11.8
. XML Versions . . . . . . . . . . . . . . . . . . . . . .
148
12
. Internationalization Considerations . . . . . . . . . . . . .
148
13
. Security Considerations . . . . . . . . . . . . . . . . . . .
148
13.1
. Fundamentals . . . . . . . . . . . . . . . . . . . . . .
148
13.2
. Threat Model . . . . . . . . . . . . . . . . . . . . . .
149
13.3
. Order of Layers . . . . . . . . . . . . . . . . . . . .
150
13.4
. Confidentiality and Integrity . . . . . . . . . . . . .
150
13.5
. Peer Entity Authentication . . . . . . . . . . . . . . .
151
13.6
. Strong Security . . . . . . . . . . . . . . . . . . . .
151
13.7
. Certificates . . . . . . . . . . . . . . . . . . . . . .
152
13.7.1
. Certificate Generation . . . . . . . . . . . . . . .
152
13.7.1.1
. General Considerations . . . . . . . . . . . . .
152
13.7.1.2
. Server Certificates . . . . . . . . . . . . . . .
153
13.7.1.3
. Client Certificates . . . . . . . . . . . . . . .
156
13.7.1.4
. XmppAddr Identifier Type . . . . . . . . . . . .
156
13.7.2
. Certificate Validation . . . . . . . . . . . . . . .
157
13.7.2.1
. Server Certificates . . . . . . . . . . . . . . .
158
13.7.2.2
. Client Certificates . . . . . . . . . . . . . . .
158
13.7.2.3. Checking of Certificates in Long-Lived Streams . 160
13.7.2.4
. Use of Certificates in XMPP Extensions . . . . .
160
13.8
. Mandatory-to-Implement TLS and SASL Technologies . . . .
160
13.8.1
. For Authentication Only . . . . . . . . . . . . . .
161
13.8.2
. For Confidentiality Only . . . . . . . . . . . . . .
161
13.8.3. For Confidentiality and Authentication with
Passwords . . . . . . . . . . . . . . . . . . . . .
162
13.8.4. For Confidentiality and Authentication without
Passwords . . . . . . . . . . . . . . . . . . . . .
163
13.9
. Technology Reuse . . . . . . . . . . . . . . . . . . . .
163
13.9.1
. Use of Base 64 in SASL . . . . . . . . . . . . . . .
163
13.9.2
. Use of DNS . . . . . . . . . . . . . . . . . . . . .
163
13.9.3
. Use of Hash Functions . . . . . . . . . . . . . . .
164
13.9.4
. Use of SASL . . . . . . . . . . . . . . . . . . . .
164
13.9.5
. Use of TLS . . . . . . . . . . . . . . . . . . . . .
165
13.9.6
. Use of UTF-8 . . . . . . . . . . . . . . . . . . . .
165
13.9.7
. Use of XML . . . . . . . . . . . . . . . . . . . . .
166
13.10
. Information Leaks . . . . . . . . . . . . . . . . . . .
166
13.10.1
. IP Addresses . . . . . . . . . . . . . . . . . . . .
166
13.10.2
. Presence Information . . . . . . . . . . . . . . . .
166
13.11
. Directory Harvesting . . . . . . . . . . . . . . . . . .
166
13.12
. Denial of Service . . . . . . . . . . . . . . . . . . .
167
13.13
. Firewalls . . . . . . . . . . . . . . . . . . . . . . .
169
13.14
. Interdomain Federation . . . . . . . . . . . . . . . . .
169
13.15
. Non-Repudiation . . . . . . . . . . . . . . . . . . . .
169
14
. IANA Considerations . . . . . . . . . . . . . . . . . . . . .
170
14.1
. XML Namespace Name for TLS Data . . . . . . . . . . . .
170
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14.2
. XML Namespace Name for SASL Data . . . . . . . . . . . .
170
14.3
. XML Namespace Name for Stream Errors . . . . . . . . . .
170
14.4
. XML Namespace Name for Resource Binding . . . . . . . .
171
14.5
. XML Namespace Name for Stanza Errors . . . . . . . . . .
171
14.6
. GSSAPI Service Name . . . . . . . . . . . . . . . . . .
171
14.7
. Port Numbers and Service Names . . . . . . . . . . . . .
171
15
. Conformance Requirements . . . . . . . . . . . . . . . . . .
172
16
. References . . . . . . . . . . . . . . . . . . . . . . . . .
181
16.1
. Normative References . . . . . . . . . . . . . . . . . .
181
16.2
. Informative References . . . . . . . . . . . . . . . . .
184
Appendix A
. XML Schemas . . . . . . . . . . . . . . . . . . . .
190
A.1
. Stream Namespace . . . . . . . . . . . . . . . . . . . .
190
A.2
. Stream Error Namespace . . . . . . . . . . . . . . . . .
192
A.3
. STARTTLS Namespace . . . . . . . . . . . . . . . . . . .
193
A.4
. SASL Namespace . . . . . . . . . . . . . . . . . . . . .
194
A.5
. Client Namespace . . . . . . . . . . . . . . . . . . . .
196
A.6
. Server Namespace . . . . . . . . . . . . . . . . . . . .
201
A.7
. Resource Binding Namespace . . . . . . . . . . . . . . .
206
A.8
. Stanza Error Namespace . . . . . . . . . . . . . . . . .
206
Appendix B
. Contact Addresses . . . . . . . . . . . . . . . . .
208
Appendix C
. Account Provisioning . . . . . . . . . . . . . . . .
208
Appendix D
. Differences from
RFC 3920
. . . . . . . . . . . . .
208
Appendix E
. Acknowledgements . . . . . . . . . . . . . . . . . .
210
. Introduction
1.1
. Overview
The Extensible Messaging and Presence Protocol (XMPP) is an
application profile of the Extensible Markup Language [
XML
] that
enables the near-real-time exchange of structured yet extensible data
between any two or more network entities. This document defines
XMPP's core protocol methods: setup and teardown of XML streams,
channel encryption, authentication, error handling, and communication
primitives for messaging, network availability ("presence"), and
request-response interactions.
1.2
. History
The basic syntax and semantics of XMPP were developed originally
within the Jabber open-source community, mainly in 1999. In late
2002, the XMPP Working Group was chartered with developing an
adaptation of the base Jabber protocol that would be suitable as an
IETF instant messaging (IM) and presence technology in accordance
with [
IMP-REQS
]. In October 2004, [
RFC3920
] and [
RFC3921
] were
published, representing the most complete definition of XMPP at that
time.
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Since 2004 the Internet community has gained extensive implementation
and deployment experience with XMPP, including formal
interoperability testing carried out under the auspices of the XMPP
Standards Foundation (XSF). This document incorporates comprehensive
feedback from software developers and XMPP service providers,
including a number of backward-compatible modifications summarized
under
Appendix D
. As a result, this document reflects the rough
consensus of the Internet community regarding the core features of
XMPP 1.0, thus obsoleting
RFC 3920
1.3
. Functional Summary
This non-normative section provides a developer-friendly, functional
summary of XMPP; refer to the sections that follow for a normative
definition of XMPP.
The purpose of XMPP is to enable the exchange of relatively small
pieces of structured data (called "XML stanzas") over a network
between any two (or more) entities. XMPP is typically implemented
using a distributed client-server architecture, wherein a client
needs to connect to a server in order to gain access to the network
and thus be allowed to exchange XML stanzas with other entities
(which can be associated with other servers). The process whereby a
client connects to a server, exchanges XML stanzas, and ends the
connection is:
1. Determine the IP address and port at which to connect, typically
based on resolution of a fully qualified domain name
Section 3.2
2. Open a Transmission Control Protocol [
TCP
] connection
3. Open an XML stream over TCP (
Section 4.2
4. Preferably negotiate Transport Layer Security [
TLS
] for channel
encryption (
Section 5
5. Authenticate using a Simple Authentication and Security Layer
SASL
] mechanism (
Section 6
6. Bind a resource to the stream (
Section 7
7. Exchange an unbounded number of XML stanzas with other entities
on the network (
Section 8
8. Close the XML stream (
Section 4.4
9. Close the TCP connection
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Within XMPP, one server can optionally connect to another server to
enable inter-domain or inter-server communication. For this to
happen, the two servers need to negotiate a connection between
themselves and then exchange XML stanzas; the process for doing so
is:
1. Determine the IP address and port at which to connect, typically
based on resolution of a fully qualified domain name
Section 3.2
2. Open a TCP connection
3. Open an XML stream (
Section 4.2
4. Preferably negotiate TLS for channel encryption (
Section 5
5. Authenticate using a Simple Authentication and Security Layer
SASL
] mechanism (
Section 6
) *
6. Exchange an unbounded number of XML stanzas both directly for the
servers and indirectly on behalf of entities associated with each
server, such as connected clients (
Section 8
7. Close the XML stream (
Section 4.4
8. Close the TCP connection
* Interoperability Note: At the time of writing, most deployed
servers still use the Server Dialback protocol [
XEP-0220
] to
provide weak identity verification instead of using SASL with PKIX
certificates to provide strong authentication, especially in cases
where SASL negotiation would not result in strong authentication
anyway (e.g., because TLS negotiation was not mandated by the peer
server, or because the PKIX certificate presented by the peer
server during TLS negotiation is self-signed and has not been
previously accepted); for details, see [
XEP-0220
]. The solutions
specified in this document offer a significantly stronger level of
security (see also
Section 13.6
).
This document specifies how clients connect to servers and specifies
the basic semantics of XML stanzas. However, this document does not
define the "payloads" of the XML stanzas that might be exchanged once
a connection is successfully established; instead, those payloads are
defined by various XMPP extensions. For example, [
XMPP-IM
] defines
extensions for basic instant messaging and presence functionality.
In addition, various specifications produced in the XSF's XEP series
XEP-0001
] define extensions for a wide range of applications.
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1.4
. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC
2119
KEYWORDS
].
Certain security-related terms are to be understood in the sense
defined in [
SEC-TERMS
]; such terms include, but are not limited to,
"assurance", "attack", "authentication", "authorization",
"certificate", "certification authority", "certification path",
"confidentiality", "credential", "downgrade", "encryption", "hash
value", "identity", "integrity", "signature", "self-signed
certificate", "sign", "spoof", "tamper", "trust", "trust anchor",
"validate", and "verify".
Certain terms related to certificates, domains, and application
service identity are to be understood in the sense defined in
TLS-CERTS
]; these include, but are not limited to, "PKIX
certificate", "source domain", "derived domain", and the identifier
types "CN-ID", "DNS-ID", and "SRV-ID".
Other security-related terms are to be understood in the sense
defined in the referenced specifications (for example, "denial of
service" as described in [
DOS
] or "end entity certificate" as
described in [
PKIX
]).
The term "whitespace" is used to refer to any character or characters
matching the "S" production from [
XML
], i.e., one or more instances
of the SP, HTAB, CR, or LF rules defined in [
ABNF
].
The terms "localpart", "domainpart", and "resourcepart" are defined
in [
XMPP-ADDR
].
The term "bare JID" refers to an XMPP address of the form
The term "full JID" refers to an XMPP address of the form
client or device associated with an account) or of the form
associated with a server).
The term "XML stream" (also "stream") is defined under
Section 4.1
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The term "XML stanza" (also "stanza") is defined under
Section 4.1
There are three kinds of stanzas: message, presence, and IQ (short
for "Info/Query"). These communication primitives are defined under
Sections
8.2.1
8.2.2
, and
8.2.3
, respectively.
The term "originating entity" refers to the entity that first
generates a stanza that is sent over an XMPP network (e.g., a
connected client, an add-on service, or a server). The term
"generated stanza" refers to the stanza so generated.
The term "input stream" designates an XML stream over which a server
receives data from a connected client or remote server, and the term
"output stream" designates an XML stream over which a server sends
data to a connected client or remote server. The following terms
designate some of the actions that a server can perform when
processing data received over an input stream:
route: pass the data to a remote server for direct processing by
the remote server or eventual delivery to a client associated
with the remote server
deliver: pass the data to a connected client
ignore: discard the data without acting upon it or returning an
error to the sender
When the term "ignore" is used with regard to client processing of
data it receives, the phrase "without acting upon it" explicitly
includes not presenting the data to a human user.
Following the "XML Notation" used in [
IRI
] to represent characters
that cannot be rendered in ASCII-only documents, some examples in
this document use the form "&#x...." as a notational device to
represent [
UNICODE
] characters (e.g., the string "ř" stands
for the Unicode character LATIN SMALL LETTER R WITH CARON); this form
is definitely not to be sent over the wire in XMPP systems.
Consistent with the convention used in [
URI
] to represent Uniform
Resource Identifiers, XMPP addresses in running text are enclosed
between '<' and '>' (although natively they are not URIs).
In examples, lines have been wrapped for improved readability,
"[...]" means elision, and the following prepended strings are used
(these prepended strings are not to be sent over the wire):
o C: = a client
o E: = any XMPP entity
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o I: = an initiating entity
o P: = a peer server
o R: = a receiving entity
o S: = a server
o S1: = server1
o S2: = server2
Readers need to be aware that the examples are not exhaustive and
that, in examples for some protocol flows, the alternate steps shown
would not necessarily be triggered by the exact data sent in the
previous step; in all cases the protocol definitions specified in
this document or in normatively referenced documents rule over any
examples provided here. All examples are fictional and the
information exchanged (e.g., usernames and passwords) does not
represent any existing users or servers.
. Architecture
XMPP provides a technology for the asynchronous, end-to-end exchange
of structured data by means of direct, persistent XML streams among a
distributed network of globally addressable, presence-aware clients
and servers. Because this architectural style involves ubiquitous
knowledge of network availability and a conceptually unlimited number
of concurrent information transactions in the context of a given
client-to-server or server-to-server session, we label it
"Availability for Concurrent Transactions" (ACT) to distinguish it
from the "Representational State Transfer" [
REST
] architectural style
familiar from the World Wide Web. Although the architecture of XMPP
is similar in important ways to that of email (see [
EMAIL-ARCH
]), it
introduces several modifications to facilitate communication in close
to real time. The salient features of this ACTive architectural
style are as follows.
2.1
. Global Addresses
As with email, XMPP uses globally unique addresses (based on the
Domain Name System) in order to route and deliver messages over the
network. All XMPP entities are addressable on the network, most
particularly clients and servers but also various additional services
that can be accessed by clients and servers. In general, server
addresses are of the form
accounts hosted at a server are of the form
(e.g.,
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particular connected device or resource that is currently authorized
for interaction on behalf of an account is of the form
historical reasons, XMPP addresses are often called Jabber IDs or
JIDs. Because the formal specification of the XMPP address format
depends on internationalization technologies that are in flux at the
time of writing, the format is defined in [
XMPP-ADDR
] instead of this
document. The terms "localpart", "domainpart", and "resourcepart"
are defined more formally in [
XMPP-ADDR
].
2.2
. Presence
XMPP includes the ability for an entity to advertise its network
availability or "presence" to other entities. In XMPP, this
availability for communication is signaled end-to-end by means of a
dedicated communication primitive: the
knowledge of network availability is not strictly necessary for the
exchange of XMPP messages, it facilitates real-time interaction
because the originator of a message can know before initiating
communication that the intended recipient is online and available.
End-to-end presence is defined in [
XMPP-IM
].
2.3
. Persistent Streams
Availability for communication is also built into each point-to-point
"hop" through the use of persistent XML streams over long-lived TCP
connections. These "always-on" client-to-server and server-to-server
streams enable each party to push data to the other party at any time
for immediate routing or delivery. XML streams are defined under
Section 4
2.4
. Structured Data
The basic protocol data unit in XMPP is not an XML stream (which
simply provides the transport for point-to-point communication) but
an XML "stanza", which is essentially a fragment of XML that is sent
over a stream. The root element of a stanza includes routing
attributes (such as "from" and "to" addresses), and the child
elements of the stanza contain a payload for delivery to the intended
recipient. XML stanzas are defined under
Section 8
2.5
. Distributed Network of Clients and Servers
In practice, XMPP consists of a network of clients and servers that
inter-communicate (however, communication between any two given
deployed servers is strictly discretionary and a matter of local
service policy). Thus, for example, the user
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associated with the server
messages, presence, and other structured data with the user
pattern is familiar from messaging protocols that make use of global
addresses, such as the email network (see [
SMTP
] and [
EMAIL-ARCH
]).
As a result, end-to-end communication in XMPP is logically peer-to-
peer but physically client-to-server-to-server-to-client, as
illustrated in the following diagram.
example.net <--------------> im.example.com
^ ^
| |
v v
romeo@example.net juliet@im.example.com
Figure 1: Distributed Client-Server Architecture
Informational Note: Architectures that employ XML streams
Section 4
) and XML stanzas (
Section 8
) but that establish peer-
to-peer connections directly between clients using technologies
based on [
LINKLOCAL
] have been deployed, but such architectures
are not defined in this specification and are best described as
"XMPP-like"; for details, see [
XEP-0174
]. In addition, XML
streams can be established end-to-end over any reliable transport,
including extensions to XMPP itself; however, such methods are out
of scope for this specification.
The following paragraphs describe the responsibilities of clients and
servers on the network.
A client is an entity that establishes an XML stream with a server by
authenticating using the credentials of a registered account (via
SASL negotiation (
Section 6
)) and that then completes resource
binding (
Section 7
) in order to enable delivery of XML stanzas
between the server and the client over the negotiated stream. The
client then uses XMPP to communicate with its server, other clients,
and any other entities on the network, where the server is
responsible for delivering stanzas to other connected clients at the
same server or routing them to remote servers. Multiple clients can
connect simultaneously to a server on behalf of the same registered
account, where each client is differentiated by the resourcepart of
an XMPP address (e.g.,
XMPP-ADDR
] and
Section 7
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A server is an entity whose primary responsibilities are to:
o Manage XML streams (
Section 4
) with connected clients and deliver
XML stanzas (
Section 8
) to those clients over the negotiated
streams; this includes responsibility for ensuring that a client
authenticates with the server before being granted access to the
XMPP network.
o Subject to local service policies on server-to-server
communication, manage XML streams (
Section 4
) with remote servers
and route XML stanzas (
Section 8
) to those servers over the
negotiated streams.
Depending on the application, the secondary responsibilities of an
XMPP server can include:
o Storing data that is used by clients (e.g., contact lists for
users of XMPP-based instant messaging and presence applications as
defined in [
XMPP-IM
]); in this case, the relevant XML stanza is
handled directly by the server itself on behalf of the client and
is not routed to a remote server or delivered to a connected
client.
o Hosting add-on services that also use XMPP as the basis for
communication but that provide additional functionality beyond
that defined in this document or in [
XMPP-IM
]; examples include
multi-user conferencing services as specified in [
XEP-0045
] and
publish-subscribe services as specified in [
XEP-0060
].
. TCP Binding
3.1
. Scope
As XMPP is defined in this specification, an initiating entity
(client or server) MUST open a Transmission Control Protocol [
TCP
connection to the receiving entity (server) before it negotiates XML
streams with the receiving entity. The parties then maintain that
TCP connection for as long as the XML streams are in use. The rules
specified in the following sections apply to the TCP binding.
Informational Note: There is no necessary coupling of XML streams
to TCP, and other transports are possible. For example, two
entities could connect to each other by means of [
HTTP
] as
specified in [
XEP-0124
] and [
XEP-0206
]. However, this
specification defines only a binding of XMPP to TCP.
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3.2
. Resolution of Fully Qualified Domain Names
Because XML streams are sent over TCP, the initiating entity needs to
determine the IPv4 or IPv6 address (and port) of the receiving entity
before it can attempt to open an XML stream. Typically this is done
by resolving the receiving entity's fully qualified domain name or
FQDN (see [
DNS-CONCEPTS
]).
3.2.1
. Preferred Process: SRV Lookup
The preferred process for FQDN resolution is to use [
DNS-SRV
] records
as follows:
1. The initiating entity constructs a DNS SRV query whose inputs
are:
* a Service of "xmpp-client" (for client-to-server connections)
or "xmpp-server" (for server-to-server connections)
* a Proto of "tcp"
* a Name corresponding to the "origin domain" [
TLS-CERTS
] of the
XMPP service to which the initiating entity wishes to connect
(e.g., "example.net" or "im.example.com")
2. The result is a query such as "_xmpp-client._tcp.example.net." or
"_xmpp-server._tcp.im.example.com.".
3. If a response is received, it will contain one or more
combinations of a port and FDQN, each of which is weighted and
prioritized as described in [
DNS-SRV
]. (However, if the result
of the SRV lookup is a single resource record with a Target of
".", i.e., the root domain, then the initiating entity MUST abort
SRV processing at this point because according to [
DNS-SRV
] such
a Target "means that the service is decidedly not available at
this domain".)
4. The initiating entity chooses at least one of the returned FQDNs
to resolve (following the rules in [
DNS-SRV
]), which it does by
performing DNS "A" or "AAAA" lookups on the FDQN; this will
result in an IPv4 or IPv6 address.
5. The initiating entity uses the IP address(es) from the
successfully resolved FDQN (with the corresponding port number
returned by the SRV lookup) as the connection address for the
receiving entity.
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6. If the initiating entity fails to connect using that IP address
but the "A" or "AAAA" lookups returned more than one IP address,
then the initiating entity uses the next resolved IP address for
that FDQN as the connection address.
7. If the initiating entity fails to connect using all resolved IP
addresses for a given FDQN, then it repeats the process of
resolution and connection for the next FQDN returned by the SRV
lookup based on the priority and weight as defined in [
DNS-SRV
].
8. If the initiating entity receives a response to its SRV query but
it is not able to establish an XMPP connection using the data
received in the response, it SHOULD NOT attempt the fallback
process described in the next section (this helps to prevent a
state mismatch between inbound and outbound connections).
9. If the initiating entity does not receive a response to its SRV
query, it SHOULD attempt the fallback process described in the
next section.
3.2.2
. Fallback Processes
The fallback process SHOULD be a normal "A" or "AAAA" address record
resolution to determine the IPv4 or IPv6 address of the origin
domain, where the port used is the "xmpp-client" port of 5222 for
client-to-server connections or the "xmpp-server" port of 5269 for
server-to-server connections (these are the default ports as
registered with the IANA as described under
Section 14.7
).
If connections via TCP are unsuccessful, the initiating entity might
attempt to find and use alternative connection methods such as the
HTTP binding (see [
XEP-0124
] and [
XEP-0206
]), which might be
discovered using [
DNS-TXT
] records as described in [
XEP-0156
].
3.2.3
. When Not to Use SRV
If the initiating entity has been explicitly configured to associate
a particular FQDN (and potentially port) with the origin domain of
the receiving entity (say, to "hardcode" an association from an
origin domain of example.net to a configured FQDN of
apps.example.com), the initiating entity is encouraged to use the
configured name instead of performing the preferred SRV resolution
process on the origin domain.
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3.2.4
. Use of SRV Records with Add-On Services
Many XMPP servers are implemented in such a way that they can host
add-on services (beyond those defined in this specification and
XMPP-IM
]) at DNS domain names that typically are "subdomains" of the
main XMPP service (e.g., conference.example.net for a [
XEP-0045
service associated with the example.net XMPP service) or "subdomains"
of the first-level domain of the underlying service (e.g.,
muc.example.com for a [
XEP-0045
] service associated with the
im.example.com XMPP service). If an entity associated with a remote
XMPP server wishes to communicate with such an add-on service, it
would generate an appropriate XML stanza and the remote server would
attempt to resolve the add-on service's DNS domain name via an SRV
lookup on resource records such as "_xmpp-
server._tcp.conference.example.net." or "_xmpp-
server._tcp.muc.example.com.". Therefore, if the administrators of
an XMPP service wish to enable entities associated with remote
servers to access such add-on services, they need to advertise the
appropriate "_xmpp-server" SRV records in addition to the "_xmpp-
server" record for their main XMPP service. In case SRV records are
not available, the fallback methods described under
Section 3.2.2
can
be used to resolve the DNS domain names of add-on services.
3.3
. Reconnection
It can happen that an XMPP server goes offline unexpectedly while
servicing TCP connections from connected clients and remote servers.
Because the number of such connections can be quite large, the
reconnection algorithm employed by entities that seek to reconnect
can have a significant impact on software performance and network
congestion. If an entity chooses to reconnect, it:
o SHOULD set the number of seconds that expire before reconnecting
to an unpredictable number between 0 and 60 (this helps to ensure
that not all entities attempt to reconnect at exactly the same
number of seconds after being disconnected).
o SHOULD back off increasingly on the time between subsequent
reconnection attempts (e.g., in accordance with "truncated binary
exponential backoff" as described in [
ETHERNET
]) if the first
reconnection attempt does not succeed.
It is RECOMMENDED to make use of TLS session resumption [
TLS-RESUME
when reconnecting. A future version of this document, or a separate
specification, might provide more detailed guidelines regarding
methods for speeding the reconnection process.
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3.4
. Reliability
The use of long-lived TCP connections in XMPP implies that the
sending of XML stanzas over XML streams can be unreliable, since the
parties to a long-lived TCP connection might not discover a
connectivity disruption in a timely manner. At the XMPP application
layer, long connectivity disruptions can result in undelivered
stanzas. Although the core XMPP technology defined in this
specification does not contain features to overcome this lack of
reliability, there exist XMPP extensions for doing so (e.g.,
XEP-0198
]).
. XML Streams
4.1
. Stream Fundamentals
Two fundamental concepts make possible the rapid, asynchronous
exchange of relatively small payloads of structured information
between XMPP entities: XML streams and XML stanzas. These terms are
defined as follows.
Definition of XML Stream: An XML stream is a container for the
exchange of XML elements between any two entities over a network.
The start of an XML stream is denoted unambiguously by an opening
"stream header" (i.e., an XML
attributes and namespace declarations), while the end of the XML
stream is denoted unambiguously by a closing XML
During the life of the stream, the entity that initiated it can
send an unbounded number of XML elements over the stream, either
elements used to negotiate the stream (e.g., to complete TLS
negotiation (
Section 5
) or SASL negotiation (
Section 6
)) or XML
stanzas. The "initial stream" is negotiated from the initiating
entity (typically a client or server) to the receiving entity
(typically a server), and can be seen as corresponding to the
initiating entity's "connection to" or "session with" the
receiving entity. The initial stream enables unidirectional
communication from the initiating entity to the receiving entity;
in order to enable exchange of stanzas from the receiving entity
to the initiating entity, the receiving entity MUST negotiate a
stream in the opposite direction (the "response stream").
Definition of XML Stanza: An XML stanza is the basic unit of meaning
in XMPP. A stanza is a first-level element (at depth=1 of the
stream) whose element name is "message", "presence", or "iq" and
whose qualifying namespace is 'jabber:client' or 'jabber:server'.
By contrast, a first-level element qualified by any other
namespace is not an XML stanza (stream errors, stream features,
TLS-related elements, SASL-related elements, etc.), nor is a
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'jabber:client' or 'jabber:server' namespace but that occurs at a
depth other than one (e.g., a
an extension element (
Section 8.4
) for reporting purposes), nor is
a
namespace other than 'jabber:client' or 'jabber:server'. An XML
stanza typically contains one or more child elements (with
accompanying attributes, elements, and XML character data) as
necessary in order to convey the desired information, which MAY be
qualified by any XML namespace (see [
XML-NAMES
] as well as
Section 8.4
in this specification).
There are three kinds of stanzas: message, presence, and IQ (short
for "Info/Query"). These stanza types provide three different
communication primitives: a "push" mechanism for generalized
messaging, a specialized "publish-subscribe" mechanism for
broadcasting information about network availability, and a "request-
response" mechanism for more structured exchanges of data (similar to
HTTP
]). Further explanations are provided under
Section 8.2.1
Section 8.2.2, and
Section 8.2.3
, respectively.
Consider the example of a client's connection to a server. The
client initiates an XML stream by sending a stream header to the
server, preferably preceded by an XML declaration specifying the XML
version and the character encoding supported (see
Section 11.5
and
Section 11.6
). Subject to local policies and service provisioning,
the server then replies with a second XML stream back to the client,
again preferably preceded by an XML declaration. Once the client has
completed SASL negotiation (
Section 6
) and resource binding
Section 7
), the client can send an unbounded number of XML stanzas
over the stream. When the client desires to close the stream, it
simply sends a closing tag to the server as further
described under
Section 4.4
In essence, then, one XML stream functions as an envelope for the XML
stanzas sent during a session and another XML stream functions as an
envelope for the XML stanzas received during a session. We can
represent this in a simplistic fashion as follows.
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+--------------------+--------------------+
| INITIAL STREAM | RESPONSE STREAM |
+--------------------+--------------------+
|
|--------------------|--------------------|
| |
|--------------------|--------------------|
|
|
|
|--------------------|--------------------|
|
|
| | |
|--------------------|--------------------|
|
|
|
|--------------------|--------------------|
| |
| |
| |
|--------------------|--------------------|
| [ ... ] | |
|--------------------|--------------------|
| | [ ... ] |
|--------------------|--------------------|
|
|--------------------|--------------------|
| |
+--------------------+--------------------+
Figure 2: A Simplistic View of Two Streams
Those who are accustomed to thinking of XML in a document-centric
manner might find the following analogies useful:
o The two XML streams are like two "documents" (matching the
"document" production from [
XML
]) that are built up through the
accumulation of XML stanzas.
o The root
"document" (as described in Section 4.8 of [
XML
]).
o The XML stanzas sent over the streams are like "fragments" of the
"documents" (as described in [
XML-FRAG
]).
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However, these descriptions are merely analogies, because XMPP does
not deal in documents and fragments but in streams and stanzas.
The remainder of this section defines the following aspects of XML
streams (along with related topics):
o How to open a stream (
Section 4.2
o The stream negotiation process (
Section 4.3
o How to close a stream (
Section 4.4
o The directionality of XML streams (
Section 4.5
o How to handle peers that are silent (
Section 4.6
o The XML attributes of a stream (
Section 4.7
o The XML namespaces of a stream (
Section 4.8
o Error handling related to XML streams (
Section 4.9
4.2
. Opening a Stream
After connecting to the appropriate IP address and port of the
receiving entity, the initiating entity opens a stream by sending a
stream header (the "initial stream header") to the receiving entity.
I:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The receiving entity then replies by sending a stream header of its
own (the "response stream header") to the initiating entity.
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R:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The entities can then proceed with the remainder of the stream
negotiation process.
4.3
. Stream Negotiation
4.3.1
. Basic Concepts
Because the receiving entity for a stream acts as a gatekeeper to the
domains it services, it imposes certain conditions for connecting as
a client or as a peer server. At a minimum, the initiating entity
needs to authenticate with the receiving entity before it is allowed
to send stanzas to the receiving entity (for client-to-server streams
this means using SASL as described under
Section 6
). However, the
receiving entity can consider conditions other than authentication to
be mandatory-to-negotiate, such as encryption using TLS as described
under
Section 5
. The receiving entity informs the initiating entity
about such conditions by communicating "stream features": the set of
particular protocol interactions that the initiating entity needs to
complete before the receiving entity will accept XML stanzas from the
initiating entity, as well as any protocol interactions that are
voluntary-to-negotiate but that might improve the handling of an XML
stream (e.g., establishment of application-layer compression as
described in [
XEP-0138
]).
The existence of conditions for connecting implies that streams need
to be negotiated. The order of layers (TCP, then TLS, then SASL,
then XMPP as described under
Section 13.3
) implies that stream
negotiation is a multi-stage process. Further structure is imposed
by two factors: (1) a given stream feature might be offered only to
certain entities or only after certain other features have been
negotiated (e.g., resource binding is offered only after SASL
authentication), and (2) stream features can be either mandatory-to-
negotiate or voluntary-to-negotiate. Finally, for security reasons
the parties to a stream need to discard knowledge that they gained
during the negotiation process after successfully completing the
protocol interactions defined for certain features (e.g., TLS in all
cases and SASL in the case when a security layer might be
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established, as defined in the specification for the relevant SASL
mechanism). This is done by flushing the old stream context and
exchanging new stream headers over the existing TCP connection.
4.3.2
. Stream Features Format
If the initiating entity includes in the initial stream header the
'version' attribute set to a value of at least "1.0" (see
Section 4.7.5
), after sending the response stream header the
receiving entity MUST send a
prefixed by the stream namespace prefix as described under
Section 4.8.5
) to the initiating entity in order to announce any
conditions for continuation of the stream negotiation process. Each
condition takes the form of a child element of the
element, qualified by a namespace that is different from the stream
namespace and the content namespace. The
contain one child, contain multiple children, or be empty.
Implementation Note: The order of child elements contained in any
given
If a particular stream feature is or can be mandatory-to-negotiate,
the definition of that feature needs to do one of the following:
1. Declare that the feature is always mandatory-to-negotiate (e.g.,
this is true of resource binding for XMPP clients); or
2. Specify a way for the receiving entity to flag the feature as
mandatory-to-negotiate for this interaction (e.g., for STARTTLS,
this is done by including an empty
advertisement for that stream feature, but that is not a generic
format for all stream features); it is RECOMMENDED that stream
feature definitions for new mandatory-to-negotiate features do so
by including an empty
STARTTLS.
Informational Note: Because there is no generic format for
indicating that a feature is mandatory-to-negotiate, it is
possible that a feature that is not understood by the initiating
entity might be considered mandatory-to-negotiate by the receiving
entity, resulting in failure of the stream negotiation process.
Although such an outcome would be undesirable, the working group
deemed it rare enough that a generic format was not needed.
For security reasons, certain stream features necessitate the
initiating entity to send a new initial stream header upon successful
negotiation of the feature (e.g., TLS in all cases and SASL in the
case when a security layer might be established). If this is true of
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a given stream feature, the definition of that feature needs to
specify that a stream restart is expected after negotiation of the
feature.
A
negotiate feature indicates that the stream negotiation is not
complete and that the initiating entity MUST negotiate further
features.
R:
A
negotiate feature. This means that the initiating entity can choose
among the mandatory-to-negotiate features at this stage of the stream
negotiation process. As an example, perhaps a future technology will
perform roughly the same function as TLS, so the receiving entity
might advertise support for both TLS and the future technology at the
same stage of the stream negotiation process. However, this applies
only at a given stage of the stream negotiation process and does not
apply to features that are mandatory-to-negotiate at different stages
(e.g., the receiving entity would not advertise both STARTTLS and
SASL as mandatory-to-negotiate, or both SASL and resource binding as
mandatory-to-negotiate, because TLS would need to be negotiated
before SASL and because SASL would need to be negotiated before
resource binding).
A
voluntary-to-negotiate features indicates that the negotiation is not
complete but that the initiating entity MAY complete the voluntary-
to-negotiate feature(s) before it attempts to negotiate the
mandatory-to-negotiate feature(s).
R:
A
features indicates that the stream negotiation is complete and that
the initiating entity is cleared to send XML stanzas, but that the
initiating entity MAY negotiate further features if desired.
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R:
An empty
complete and that the initiating entity is cleared to send XML
stanzas.
R:
4.3.3
. Restarts
On successful negotiation of a feature that necessitates a stream
restart, both parties MUST consider the previous stream to be
replaced but MUST NOT send a closing tag and MUST NOT
terminate the underlying TCP connection; instead, the parties MUST
reuse the existing connection, which might be in a new state (e.g.,
encrypted as a result of TLS negotiation). The initiating entity
then MUST send a new initial stream header, which SHOULD be preceded
by an XML declaration as described under
Section 11.5
. When the
receiving entity receives the new initial stream header, it MUST
generate a new stream ID (instead of reusing the old stream ID)
before sending a new response stream header (which SHOULD be preceded
by an XML declaration as described under
Section 11.5
).
4.3.4
. Resending Features
The receiving entity MUST send an updated list of stream features to
the initiating entity after a stream restart. The list of updated
features MAY be empty if there are no further features to be
advertised or MAY include any combination of features.
4.3.5
. Completion of Stream Negotiation
The receiving entity indicates completion of the stream negotiation
process by sending to the initiating entity either an empty
voluntary-to-negotiate features. After doing so, the receiving
entity MAY send an empty
of such voluntary-to-negotiate features) but MUST NOT send additional
stream features to the initiating entity (if the receiving entity has
new features to offer, preferably limited to mandatory-to-negotiate
or security-critical features, it can simply close the stream with a
Section 4.9.3.16
) and then advertise the new
features when the initiating entity reconnects, preferably closing
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existing streams in a staggered way so that not all of the initiating
entities reconnect at once). Once stream negotiation is complete,
the initiating entity is cleared to send XML stanzas over the stream
for as long as the stream is maintained by both parties.
Informational Note: Resource binding as specified under
Section 7
is an historical exception to the foregoing rule, since it is
mandatory-to-negotiate for clients but uses XML stanzas for
negotiation purposes.
The initiating entity MUST NOT attempt to send XML stanzas
Section 8
) to entities other than itself (i.e., the client's
connected resource or any other authenticated resource of the
client's account) or the server to which it is connected until stream
negotiation has been completed. Even if the initiating entity does
attempt to do so, the receiving entity MUST NOT accept such stanzas
and MUST close the stream with a
Section 4.9.3.12
). This rule applies to XML stanzas only (i.e.,
namespace) and not to XML elements used for stream negotiation (e.g.,
elements used to complete TLS negotiation (
Section 5
) or SASL
negotiation (
Section 6
)).
4.3.6
. Determination of Addresses
After the parties to an XML stream have completed the appropriate
aspects of stream negotiation, the receiving entity for a stream MUST
determine the initiating entity's JID.
For client-to-server communication, both SASL negotiation (
Section 6
and resource binding (
Section 7
) MUST be completed before the server
can determine the client's address. The client's bare JID
(
defined by [
SASL
]), either (1) as directly communicated by the client
during SASL negotiation (
Section 6
) or (2) as derived by the server
from the authentication identity if no authorization identity was
specified during SASL negotiation. The resourcepart of the full JID
(
by the client and server during resource binding (
Section 7
). A
client MUST NOT attempt to guess at its JID but instead MUST consider
its JID to be whatever the server returns to it during resource
binding. The server MUST ensure that the resulting JID (including
localpart, domainpart, resourcepart, and separator characters)
conforms to the canonical format for XMPP addresses defined in
XMPP-ADDR
]; to meet this restriction, the server MAY replace the JID
sent by the client with the canonicalized JID as determined by the
server and communicate that JID to the client during resource
binding.
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For server-to-server communication, the initiating server's bare JID
(
SASL
]), either (1) as directly communicated by the initiating server
during SASL negotiation (
Section 6
) or (2) as derived by the
receiving server from the authentication identity if no authorization
identity was specified during SASL negotiation. In the absence of
SASL negotiation, the receiving server MAY consider the authorization
identity to be an identity negotiated within the relevant
verification protocol (e.g., the 'from' attribute of the
element in Server Dialback [
XEP-0220
]).
Security Warning: Because it is possible for a third party to
tamper with information that is sent over the stream before a
security layer such as TLS is successfully negotiated, it is
advisable for the receiving server to treat any such unprotected
information with caution; this applies especially to the 'from'
and 'to' addresses on the first initial stream header sent by the
initiating entity.
4.3.7
. Flow Chart
We summarize the foregoing rules in the following non-normative flow
chart for the stream negotiation process, presented from the
perspective of the initiating entity.
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+---------------------+
| open TCP connection |
+---------------------+
+---------------+
| send initial |<-------------------------+
| stream header | ^
+---------------+ |
| |
v |
+------------------+ |
| receive response | |
| stream header | |
+------------------+ |
| |
v |
+----------------+ |
| receive stream | |
+------------------>| features | |
^ {OPTIONAL} +----------------+ |
| | |
| v |
| +<-----------------+ |
| | |
| {empty?} ----> {all voluntary?} ----> {some mandatory?} |
| | no | no | |
| | yes | yes | yes |
| | v v |
| | +---------------+ +----------------+ |
| | | MAY negotiate | | MUST negotiate | |
| | | any or none | | one feature | |
| | +---------------+ +----------------+ |
| v | | |
| +---------+ v | |
| | DONE |<----- {negotiate?} | |
| +---------+ no | | |
| yes | | |
| v v |
| +--------->+<---------+ |
| | |
| v |
+<-------------------------- {restart mandatory?} ------------>+
no yes
Figure 3: Stream Negotiation Flow Chart
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4.4
. Closing a Stream
An XML stream from one entity to another can be closed at any time,
either because a specific stream error (
Section 4.9
) has occurred or
in the absence of an error (e.g., when a client simply ends its
session).
A stream is closed by sending a closing tag.
E:
If the parties are using either two streams over a single TCP
connection or two streams over two TCP connections, the entity that
sends the closing stream tag MUST behave as follows:
1. Wait for the other party to also close its outbound stream before
terminating the underlying TCP connection(s); this gives the
other party an opportunity to finish transmitting any outbound
data to the closing entity before the termination of the TCP
connection(s).
2. Refrain from sending any further data over its outbound stream to
the other entity, but continue to process data received from the
other entity (and, if necessary, process such data).
3. Consider both streams to be void if the other party does not send
its closing stream tag within a reasonable amount of time (where
the definition of "reasonable" is a matter of implementation or
deployment).
4. After receiving a reciprocal closing stream tag from the other
party or waiting a reasonable amount of time with no response,
terminate the underlying TCP connection(s).
Security Warning: In accordance with Section 7.2.1 of [
TLS
], to
help prevent a truncation attack the party that is closing the
stream MUST send a TLS close_notify alert and MUST receive a
responding close_notify alert from the other party before
terminating the underlying TCP connection(s).
If the parties are using multiple streams over multiple TCP
connections, there is no defined pairing of streams and therefore the
behavior is a matter for implementation.
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4.5
. Directionality
An XML stream is always unidirectional, by which is meant that XML
stanzas can be sent in only one direction over the stream (either
from the initiating entity to the receiving entity or from the
receiving entity to the initiating entity).
Depending on the type of session that has been negotiated and the
nature of the entities involved, the entities might use:
o Two streams over a single TCP connection, where the security
context negotiated for the first stream is applied to the second
stream. This is typical for client-to-server sessions, and a
server MUST allow a client to use the same TCP connection for both
streams.
o Two streams over two TCP connections, where each stream is
separately secured. In this approach, one TCP connection is used
for the stream in which stanzas are sent from the initiating
entity to the receiving entity, and the other TCP connection is
used for the stream in which stanzas are sent from the receiving
entity to the initiating entity. This is typical for server-to-
server sessions.
o Multiple streams over two or more TCP connections, where each
stream is separately secured. This approach is sometimes used for
server-to-server communication between two large XMPP service
providers; however, this can make it difficult to maintain
coherence of data received over multiple streams in situations
described under
Section 10.1
, which is why a server MAY close the
stream with a
Section 4.9.3.3
) if a
remote server attempts to negotiate more than one stream (as
described under
Section 4.9.3.3
).
This concept of directionality applies only to stanzas and explicitly
does not apply to first-level children of the stream root that are
used to bootstrap or manage the stream (e.g., first-level elements
used for TLS negotiation, SASL negotiation, Server Dialback
XEP-0220
], and Stream Management [
XEP-0198
]).
The foregoing considerations imply that while completing STARTTLS
negotiation (
Section 5
) and SASL negotiation (
Section 6
) two servers
would use one TCP connection, but after the stream negotiation
process is done that original TCP connection would be used only for
the initiating server to send XML stanzas to the receiving server.
In order for the receiving server to send XML stanzas to the
initiating server, the receiving server would need to reverse the
roles and negotiate an XML stream from the receiving server to the
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initiating server over a separate TCP connection. This separate TCP
connection is then secured using a new round of TLS and/or SASL
negotiation.
Implementation Note: For historical reasons, a server-to-server
session always uses two TCP connections. While that approach
remains the standard behavior described in this document,
extensions such as [
XEP-0288
] enable servers to negotiate the use
of a single TCP connection for bidirectional stanza exchange.
Informational Note: Although XMPP developers sometimes apply the
terms "unidirectional" and "bidirectional" to the underlying TCP
connection (e.g., calling the TCP connection for a client-to-
server session "bidirectional" and the TCP connection for a
server-to-server session "unidirectional"), strictly speaking a
stream is always unidirectional (because the initiating entity and
receiving entity always have a minimum of two streams, one in each
direction) and a TCP connection is always bidirectional (because
TCP traffic can be sent in both directions). Directionality
applies to the application-layer traffic sent over the TCP
connection, not to the transport-layer traffic sent over the TCP
connection itself.
4.6
. Handling of Silent Peers
When an entity that is a party to a stream has not received any XMPP
traffic from its stream peer for some period of time, the peer might
appear to be silent. There are several reasons why this might
happen:
1. The underlying TCP connection is dead.
2. The XML stream is broken despite the fact that the underlying TCP
connection is alive.
3. The peer is idle and simply has not sent any XMPP traffic over
its XML stream to the entity.
These three conditions are best handled separately, as described in
the following sections.
Implementation Note: For the purpose of handling silent peers, we
treat a two unidirectional TCP connections as conceptually
equivalent to a single bidirectional TCP connection (see
Section 4.5
); however, implementers need to be aware that, in the
case of two unidirectional TCP connections, responses to traffic
at the XMPP application layer will come back from the peer on the
second TCP connection. In addition, the use of multiple streams
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in each direction (which is a somewhat frequent deployment choice
for server-to-server connectivity among large XMPP service
providers) further complicates application-level checking of XMPP
streams and their underlying TCP connections, because there is no
necessary correlation between any given initial stream and any
given response stream.
4.6.1
. Dead Connection
If the underlying TCP connection is dead, stream-level checks (e.g.,
XEP-0199
] and [
XEP-0198
]) are ineffective. Therefore, it is
unnecessary to close the stream with or without an error, and it is
appropriate instead to simply terminate the TCP connection.
One common method for checking the TCP connection is to send a space
character (U+0020) between XML stanzas, which is allowed for XML
streams as described under
Section 11.7
; the sending of such a space
character is properly called a "whitespace keepalive" (the term
"whitespace ping" is often used, despite the fact that it is not a
ping since no "pong" is possible). However, this is not allowed
during TLS negotiation or SASL negotiation, as described under
Section 5.3.3
and
Section 6.3.5
4.6.2
. Broken Stream
Even if the underlying TCP connection is alive, the peer might never
respond to XMPP traffic that the entity sends, whether normal stanzas
or specialized stream-checking traffic such as the application-level
pings defined in [
XEP-0199
] or the more comprehensive Stream
Management protocol defined in [
XEP-0198
]. In this case, it is
appropriate for the entity to close a broken stream with a
Section 4.9.3.4
).
4.6.3
. Idle Peer
Even if the underlying TCP connection is alive and the stream is not
broken, the peer might have sent no stanzas for a certain period of
time. In this case, the peer itself MAY close the stream (as
described under
Section 4.4
) rather than leaving an unused stream
open. If the idle peer does not close the stream, the other party
MAY either close the stream using the handshake described under
Section 4.4
or close the stream with a stream error (e.g.,
Section 4.9.3.17
) if the entity has reached a limit on
the number of open TCP connections or
Section 4.9.3.14
) if the connection has exceeded a local timeout
policy). However, consistent with the order of layers (specified
under
Section 13.3
), the other party is advised to verify that the
underlying TCP connection is alive and the stream is unbroken (as
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described above) before concluding that the peer is idle.
Furthermore, it is preferable to be liberal in accepting idle peers,
since experience has shown that doing so improves the reliability of
communication over XMPP networks and that it is typically more
efficient to maintain a stream between two servers than to
aggressively time out such a stream.
4.6.4
. Use of Checking Methods
Implementers are advised to support whichever stream-checking and
connection-checking methods they deem appropriate, but to carefully
weigh the network impact of such methods against the benefits of
discovering broken streams and dead TCP connections in a timely
manner. The length of time between the use of any particular check
is very much a matter of local service policy and depends strongly on
the network environment and usage scenarios of a given deployment and
connection type. At the time of writing, it is RECOMMENDED that any
such check be performed not more than once every 5 minutes and that,
ideally, such checks will be initiated by clients rather than
servers. Those who implement XMPP software and deploy XMPP services
are encouraged to seek additional advice regarding appropriate timing
of stream-checking and connection-checking methods, particularly when
power-constrained devices are being used (e.g., in mobile
environments).
4.7
. Stream Attributes
The attributes of the root
following sections.
Security Warning: Until and unless the confidentiality and
integrity of the stream are protected via TLS as described under
Section 5
or an equivalent security layer (such as the SASL GSSAPI
mechanism), the attributes provided in a stream header could be
tampered with by an attacker.
Implementation Note: The attributes of the root
are not prepended by a namespace prefix because, as explained in
XML-NAMES
], "[d]efault namespace declarations do not apply
directly to attribute names; the interpretation of unprefixed
attributes is determined by the element on which they appear."
4.7.1
. from
The 'from' attribute specifies an XMPP identity of the entity sending
the stream element.
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For initial stream headers in client-to-server communication, the
'from' attribute is the XMPP identity of the principal controlling
the client, i.e., a JID of the form
client might not know the XMPP identity, e.g., because the XMPP
identity is assigned at a level other than the XMPP application layer
(as in the Generic Security Service Application Program Interface
GSS-API
]) or is derived by the server from information provided by
the client (as in some deployments of end-user certificates with the
SASL EXTERNAL mechanism). Furthermore, if the client considers the
XMPP identity to be private information then it is advised not to
include a 'from' attribute before the confidentiality and integrity
of the stream are protected via TLS or an equivalent security layer.
However, if the client knows the XMPP identity then it SHOULD include
the 'from' attribute after the confidentiality and integrity of the
stream are protected via TLS or an equivalent security layer.
I:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For initial stream headers in server-to-server communication, the
'from' attribute is one of the configured FQDNs of the server, i.e.,
a JID of the form
more than one XMPP identity, e.g., in the case of a server that
provides virtual hosting, so it will need to choose an identity that
is associated with this output stream (e.g., based on the 'to'
attribute of the stanza that triggered the stream negotiation
attempt). Because a server is a "public entity" on the XMPP network,
it MUST include the 'from' attribute after the confidentiality and
integrity of the stream are protected via TLS or an equivalent
security layer.
I:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
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For response stream headers in both client-to-server and server-to-
server communication, the receiving entity MUST include the 'from'
attribute and MUST set its value to one of the receiving entity's
FQDNs (which MAY be an FQDN other than that specified in the 'to'
attribute of the initial stream header, as described under
Section 4.9.1.3
and
Section 4.9.3.6
).
R:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
Whether or not the 'from' attribute is included, each entity MUST
verify the identity of the other entity before exchanging XML stanzas
with it, as described under
Section 13.5
Interoperability Note: It is possible that implementations based
on [
RFC3920
] will not include the 'from' address on any stream
headers (even ones whose confidentiality and integrity are
protected); an entity SHOULD be liberal in accepting such stream
headers.
4.7.2
. to
For initial stream headers in both client-to-server and server-to-
server communication, the initiating entity MUST include the 'to'
attribute and MUST set its value to a domainpart that the initiating
entity knows or expects the receiving entity to service. (The same
information can be provided in other ways, such as a Server Name
Indication during TLS negotiation as described in [
TLS-EXT
].)
I:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For response stream headers in client-to-server communication, if the
client included a 'from' attribute in the initial stream header then
the server MUST include a 'to' attribute in the response stream
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header and MUST set its value to the bare JID specified in the 'from'
attribute of the initial stream header. If the client did not
include a 'from' attribute in the initial stream header then the
server MUST NOT include a 'to' attribute in the response stream
header.
R:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For response stream headers in server-to-server communication, the
receiving entity MUST include a 'to' attribute in the response stream
header and MUST set its value to the domainpart specified in the
'from' attribute of the initial stream header.
R:
id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
to='example.net'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
Whether or not the 'to' attribute is included, each entity MUST
verify the identity of the other entity before exchanging XML stanzas
with it, as described under
Section 13.5
Interoperability Note: It is possible that implementations based
on [
RFC3920
] will not include the 'to' address on stream headers;
an entity SHOULD be liberal in accepting such stream headers.
4.7.3
. id
The 'id' attribute specifies a unique identifier for the stream,
called a "stream ID". The stream ID MUST be generated by the
receiving entity when it sends a response stream header and MUST BE
unique within the receiving application (normally a server).
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Security Warning: The stream ID MUST be both unpredictable and
non-repeating because it can be security-critical when reused by
an authentication mechanisms, as is the case for Server Dialback
XEP-0220
] and the "XMPP 0.9" authentication mechanism used before
RFC 3920
defined the use of SASL in XMPP; for recommendations
regarding randomness for security purposes, see [
RANDOM
].
For initial stream headers, the initiating entity MUST NOT include
the 'id' attribute; however, if the 'id' attribute is included, the
receiving entity MUST ignore it.
For response stream headers, the receiving entity MUST include the
'id' attribute.
R:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
Interoperability Note: In
RFC 3920
, the text regarding inclusion
of the 'id' attribute was ambiguous, leading some implementations
to leave the attribute off the response stream header.
4.7.4
. xml:lang
The 'xml:lang' attribute specifies an entity's preferred or default
language for any human-readable XML character data to be sent over
the stream (an XML stanza can also possess an 'xml:lang' attribute,
as discussed under
Section 8.1.5
). The syntax of this attribute is
defined in Section 2.12 of [
XML
]; in particular, the value of the
'xml:lang' attribute MUST conform to the NMTOKEN datatype (as defined
in Section 2.3 of [
XML
]) and MUST conform to the language identifier
format defined in [
LANGTAGS
].
For initial stream headers, the initiating entity SHOULD include the
'xml:lang' attribute.
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I:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For response stream headers, the receiving entity MUST include the
'xml:lang' attribute. The following rules apply:
o If the initiating entity included an 'xml:lang' attribute in its
initial stream header and the receiving entity supports that
language in the human-readable XML character data that it
generates and sends to the initiating entity (e.g., in the
element for stream and stanza errors), the value of the 'xml:lang'
attribute MUST be the identifier for the initiating entity's
preferred language (e.g., "de-CH").
o If the receiving entity supports a language that matches the
initiating entity's preferred language according to the "lookup
scheme" specified in Section 3.4 of [
LANGMATCH
] (e.g., "de"
instead of "de-CH"), then the value of the 'xml:lang' attribute
SHOULD be the identifier for the matching language.
o If the receiving entity does not support the initiating entity's
preferred language or a matching language according to the lookup
scheme (or if the initiating entity did not include the 'xml:lang'
attribute in its initial stream header), then the value of the
'xml:lang' attribute MUST be the identifier for the default
language of the receiving entity (e.g., "en").
R:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
If the initiating entity included the 'xml:lang' attribute in its
initial stream header, the receiving entity SHOULD remember that
value as the default xml:lang for all stanzas sent by the initiating
entity over the current stream. As described under
Section 8.1.5
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the initiating entity MAY include the 'xml:lang' attribute in any XML
stanzas it sends over the stream. If the initiating entity does not
include the 'xml:lang' attribute in any such stanza, the receiving
entity SHOULD add the 'xml:lang' attribute to the stanza when routing
it to a remote server or delivering it to a connected client, where
the value of the attribute MUST be the identifier for the language
preferred by the initiating entity (even if the receiving entity does
not support that language for human-readable XML character data it
generates and sends to the initiating entity, such as in stream or
stanza errors). If the initiating entity includes the 'xml:lang'
attribute in any such stanza, the receiving entity MUST NOT modify or
delete it when routing it to a remote server or delivering it to a
connected client.
4.7.5
. version
The inclusion of the version attribute set to a value of at least
"1.0" signals support for the stream-related protocols defined in
this specification, including TLS negotiation (
Section 5
), SASL
negotiation (
Section 6
), stream features (
Section 4.3.2
), and stream
errors (
Section 4.9
).
The version of XMPP specified in this specification is "1.0"; in
particular, XMPP 1.0 encapsulates the stream-related protocols as
well as the basic semantics of the three defined XML stanza types
(
8.2.1, 8.2.2, and 8.2.3, respectively).
The numbering scheme for XMPP versions is "
major and minor numbers MUST be treated as separate integers and each
number MAY be incremented higher than a single digit. Thus, "XMPP
2.4" would be a lower version than "XMPP 2.13", which in turn would
be lower than "XMPP 12.3". Leading zeros (e.g., "XMPP 6.01") MUST be
ignored by recipients and MUST NOT be sent.
The major version number will be incremented only if the stream and
stanza formats or obligatory actions have changed so dramatically
that an older version entity would not be able to interoperate with a
newer version entity if it simply ignored the elements and attributes
it did not understand and took the actions defined in the older
specification.
The minor version number will be incremented only if significant new
capabilities have been added to the core protocol (e.g., a newly
defined value of the 'type' attribute for message, presence, or IQ
stanzas). The minor version number MUST be ignored by an entity with
a smaller minor version number, but MAY be used for informational
purposes by the entity with the larger minor version number (e.g.,
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the entity with the larger minor version number would simply note
that its correspondent would not be able to understand that value of
the 'type' attribute and therefore would not send it).
The following rules apply to the generation and handling of the
'version' attribute within stream headers:
1. The initiating entity MUST set the value of the 'version'
attribute in the initial stream header to the highest version
number it supports (e.g., if the highest version number it
supports is that defined in this specification, it MUST set the
value to "1.0").
2. The receiving entity MUST set the value of the 'version'
attribute in the response stream header to either the value
supplied by the initiating entity or the highest version number
supported by the receiving entity, whichever is lower. The
receiving entity MUST perform a numeric comparison on the major
and minor version numbers, not a string match on
"
3. If the version number included in the response stream header is
at least one major version lower than the version number included
in the initial stream header and newer version entities cannot
interoperate with older version entities as described, the
initiating entity SHOULD close the stream with an
Section 4.9.3.25
).
4. If either entity receives a stream header with no 'version'
attribute, the entity MUST consider the version supported by the
other entity to be "0.9" and SHOULD NOT include a 'version'
attribute in the response stream header.
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4.7.6
. Summary of Stream Attributes
The following table summarizes the attributes of the root
element.
+----------+--------------------------+-------------------------+
| | initiating to receiving | receiving to initiating |
+----------+--------------------------+-------------------------+
| to | JID of receiver | JID of initiator |
| from | JID of initiator | JID of receiver |
| id | ignored | stream identifier |
| xml:lang | default language | default language |
| version | XMPP 1.0+ supported | XMPP 1.0+ supported |
+----------+--------------------------+-------------------------+
Figure 4: Stream Attributes
4.8
. XML Namespaces
Readers are referred to the specification of XML namespaces
XML-NAMES
] for a full understanding of the concepts used in this
section, especially the concept of a "default namespace" as provided
in
Section 3
and
Section 6.2
of that specification.
4.8.1
. Stream Namespace
The root
namespace 'http://etherx.jabber.org/streams' (the "stream
namespace"). If this rule is violated, the entity that receives the
offending stream header MUST close the stream with a stream error,
which SHOULD be
Section 4.9.3.10
), although
some existing implementations send
Section 4.9.3.1
instead.
4.8.2
. Content Namespace
An entity MAY declare a "content namespace" as the default namespace
for data sent over the stream (i.e., data other than elements
qualified by the stream namespace). If so, (1) the content namespace
MUST be other than the stream namespace, and (2) the content
namespace MUST be the same for the initial stream and the response
stream so that both streams are qualified consistently. The content
namespace applies to all first-level child elements sent over the
stream unless explicitly qualified by another namespace (i.e., the
content namespace is the default namespace).
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Alternatively (i.e., instead of declaring the content namespace as
the default namespace), an entity MAY explicitly qualify the
namespace for each first-level child element of the stream, using so-
called "prefix-free canonicalization". These two styles are shown in
the following examples.
When a content namespace is declared as the default namespace, in
rough outline a stream will look something like the following.
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
foo
When a content namespace is not declared as the default namespace and
so-called "prefix-free canonicalization" is used instead, in rough
outline a stream will look something like the following.
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='http://etherx.jabber.org/streams'>
foo
Traditionally, most XMPP implementations have used the content-
namespace-as-default-namespace style rather than the prefix-free
canonicalization style for stream headers; however, both styles are
acceptable since they are semantically equivalent.
4.8.3
. XMPP Content Namespaces
XMPP as defined in this specification uses two content namespaces:
'jabber:client' and 'jabber:server'. These namespaces are nearly
identical but are used in different contexts (client-to-server
communication for 'jabber:client' and server-to-server communication
for 'jabber:server'). The only difference between the two is that
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the 'to' and 'from' attributes are OPTIONAL on stanzas sent over XML
streams qualified by the 'jabber:client' namespace, whereas they are
REQUIRED on stanzas sent over XML streams qualified by the 'jabber:
server' namespace. Support for these content namespaces implies
support for the common attributes (
Section 8.1
) and basic semantics
Section 8.2
) of all three core stanza types (message, presence, and
IQ).
An implementation MAY support content namespaces other than 'jabber:
client' or 'jabber:server'. However, because such namespaces would
define applications other than XMPP, they are to be defined in
separate specifications.
An implementation MAY refuse to support any other content namespaces
as default namespaces. If an entity receives a first-level child
element qualified by a content namespace it does not support, it MUST
close the stream with an
Section 4.9.3.10
).
Client implementations MUST support the 'jabber:client' content
namespace as a default namespace. The 'jabber:server' content
namespace is out of scope for an XMPP client, and a client MUST NOT
send stanzas qualified by the 'jabber:server' namespace.
Server implementations MUST support as default content namespaces
both the 'jabber:client' namespace (when the stream is used for
communication between a client and a server) and the 'jabber:server'
namespace (when the stream is used for communication between two
servers). When communicating with a connected client, a server MUST
NOT send stanzas qualified by the 'jabber:server' namespace; when
communicating with a peer server, a server MUST NOT send stanzas
qualified by the 'jabber:client' namespace.
Implementation Note: Because a client sends stanzas over a stream
whose content namespace is 'jabber:client', if a server routes to
a peer server a stanza it has received from a connected client
then it needs to "re-scope" the stanza so that its content
namespace is 'jabber:server'. Similarly, if a server delivers to
a connected client a stanza it has received from a peer server
then it needs to "re-scope" the stanza so that its content
namespace is 'jabber:client'. This rule applies to XML stanzas as
defined under
Section 4.1
(i.e., a first-level
'jabber:server' namespace), and by namespace inheritance to all
child elements of a stanza. However, the rule does not apply to
elements qualified by namespaces other than 'jabber:client' and
'jabber:server' nor to any children of such elements (e.g., a
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Section 8.4
) for reporting purposes). Although it is not
forbidden for an entity to generate stanzas in which an extension
element contains a child element qualified by the 'jabber:client'
or 'jabber:server' namespace, existing implementations handle such
stanzas inconsistently; therefore, implementers are advised to
weigh the likely lack of interoperability against the possible
utility of such stanzas. Finally, servers are advised to apply
stanza re-scoping to other stream connection methods and
alternative XMPP connection methods, such as those specified in
XEP-0124
], [
XEP-0206
], [
XEP-0114
], and [
XEP-0225
].
4.8.4
. Other Namespaces
Either party to a stream MAY send data qualified by namespaces other
than the content namespace and the stream namespace. For example,
this is how data related to TLS negotiation and SASL negotiation are
exchanged, as well as XMPP extensions such as Stream Management
XEP-0198
] and Server Dialback [
XEP-0220
].
Interoperability Note: For historical reasons, some server
implementations expect a declaration of the 'jabber:server:
dialback' namespace on server-to-server streams, as explained in
XEP-0220
].
However, an XMPP server MUST NOT route or deliver data received over
an input stream if that data is (a) qualified by another namespace
and (b) addressed to an entity other than the server, unless the
other party to the output stream over which the server would send the
data has explicitly negotiated or advertised support for receiving
arbitrary data from the server. This rule is included because XMPP
is designed for the exchange of XML stanzas (not arbitrary XML data),
and because allowing an entity to send arbitrary data to other
entities could significantly increase the potential for exchanging
malicious information. As an example of this rule, the server
hosting the example.net domain would not route the following first-
level XML element from
to='juliet@example.com'>
This rule also applies to first-level elements that look like stanzas
but that are improperly namespaced and therefore really are not
stanzas at all (see also
Section 4.8.5
), for example:
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to='juliet@example.com'>
hi
Upon receiving arbitrary first-level XML elements over an input
stream, a server MUST either ignore the data or close the stream with
a stream error, which SHOULD be
Section 4.9.3.24
).
4.8.5
. Namespace Declarations and Prefixes
Because the content namespace is other than the stream namespace, if
a content namespace is declared as the default namespace then the
following statements are true:
1. The stream header needs to contain a namespace declaration for
both the content namespace and the stream namespace.
2. The stream namespace declaration needs to include a namespace
prefix for the stream namespace.
Interoperability Note: For historical reasons, an implementation
MAY accept only the prefix 'stream' for the stream namespace
(resulting in prefixed names such as
RFC3920
] for the purpose of backward compatibility.
Implementations are advised that using a prefix other than
'stream' for the stream namespace might result in interoperability
problems. If an entity receives a stream header with a stream
namespace prefix it does not accept, it MUST close the stream with
a stream error, which SHOULD be
Section 4.9.3.2
), although some existing implementations send
Section 4.9.3.1
) instead.
An implementation MUST NOT generate namespace prefixes for elements
qualified by the content namespace (i.e., the default namespace for
data sent over the stream) if the content namespace is 'jabber:
client' or 'jabber:server'. For example, the following is illegal:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
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An XMPP entity SHOULD NOT accept data that violates this rule (in
particular, an XMPP server MUST NOT route or deliver such data to
another entity without first correcting the error); instead it SHOULD
either ignore the data or close the stream with a stream error, which
SHOULD be
Section 4.9.3.2
).
Namespaces declared in a stream header MUST apply only to that stream
(e.g., the 'jabber:server:dialback' namespace used in Server Dialback
XEP-0220
]). In particular, because XML stanzas intended for routing
or delivery over streams with other entities will lose the namespace
context declared in the header of the stream in which those stanzas
originated, namespaces for extended content within such stanzas MUST
NOT be declared in that stream header (see also
Section 8.4
). If
either party to a stream declares such namespaces, the other party to
the stream SHOULD close the stream with an
stream error (
Section 4.9.3.10
). In any case, an entity MUST ensure
that such namespaces are properly declared (according to this
section) when routing or delivering stanzas from an input stream to
an output stream.
4.9
. Stream Errors
The root stream element MAY contain an
qualified by the stream namespace. The error child SHALL be sent by
a compliant entity if it perceives that a stream-level error has
occurred.
4.9.1
. Rules
The following rules apply to stream-level errors.
4.9.1.1
. Stream Errors Are Unrecoverable
Stream-level errors are unrecoverable. Therefore, if an error occurs
at the level of the stream, the entity that detects the error MUST
send an
the error condition and then immediately close the stream as
described under
Section 4.4
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C:
S:
The entity that receives the stream error then SHALL close the stream
as explained under
Section 4.4
C:
4.9.1.2
. Stream Errors Can Occur During Setup
If the error is triggered by the initial stream header, the receiving
entity MUST still send the opening
element as a child of the stream element, and send the closing
C:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://wrong.namespace.example.org/'>
S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
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4.9.1.3
. Stream Errors When the Host Is Unspecified or Unknown
If the initiating entity provides no 'to' attribute or provides an
unknown host in the 'to' attribute and the error occurs during stream
setup, the value of the 'from' attribute returned by the receiving
entity in the stream header sent before closing the stream MUST be
either an authoritative FQDN for the receiving entity or the empty
string.
C:
to='unknown.host.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
4.9.1.4
. Where Stream Errors Are Sent
When two TCP connections are used between the initiating entity and
the receiving entity (one in each direction) rather than using a
single bidirectional connection, the following rules apply:
o Stream-level errors related to the initial stream are returned by
the receiving entity on the response stream via the same TCP
connection.
o Stanza errors triggered by outbound stanzas sent from the
initiating entity over the initial stream via the same TCP
connection are returned by the receiving entity on the response
stream via the other ("return") TCP connection, since they are
inbound stanzas from the perspective of the initiating entity.
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4.9.2
. Syntax
The syntax for stream errors is as follows, where XML data shown
within the square brackets '[' and ']' is OPTIONAL.
[
OPTIONAL descriptive text
[OPTIONAL application-specific condition element]
The "defined-condition" MUST correspond to one of the stream error
conditions defined under
Section 4.9.3
. However, because additional
error conditions might be defined in the future, if an entity
receives a stream error condition that it does not understand then it
MUST treat the unknown condition as equivalent to
Section 4.9.3.21
). If the designers of an XMPP protocol
extension or the developers of an XMPP implementation need to
communicate a stream error condition that is not defined in this
specification, they can do so by defining an application-specific
error condition element qualified by an application-specific
namespace.
The
o MUST contain a child element corresponding to one of the defined
stream error conditions (
Section 4.9.3
); this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace.
o MAY contain a
that describes the error in more detail; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace
and SHOULD possess an 'xml:lang' attribute specifying the natural
language of the XML character data.
o MAY contain a child element for an application-specific error
condition; this element MUST be qualified by an application-
defined namespace, and its structure is defined by that namespace
(see
Section 4.9.4
).
The
to provide descriptive or diagnostic information that supplements the
meaning of a defined condition or application-specific condition. It
MUST NOT be interpreted programmatically by an application. It MUST
NOT be used as the error message presented to a human user, but MAY
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be shown in addition to the error message associated with the defined
condition element (and, optionally, the application-specific
condition element).
4.9.3
. Defined Stream Error Conditions
The following stream-level error conditions are defined.
4.9.3.1
. bad-format
The entity has sent XML that cannot be processed.
(In the following example, the client sends an XMPP message that is
not well-formed XML, which alternatively might trigger a
Section 4.9.3.13
).)
C:
No closing tag!
S:
This error can be used instead of the more specific XML-related
errors, such as
the more specific errors are RECOMMENDED.
4.9.3.2
. bad-namespace-prefix
The entity has sent a namespace prefix that is unsupported, or has
sent no namespace prefix on an element that needs such a prefix (see
Section 11.2
).
(In the following example, the client specifies a namespace prefix of
"foobar" for the XML stream namespace.)
C:
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:foobar='http://etherx.jabber.org/streams'>
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S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
4.9.3.3
. conflict
The server either (1) is closing the existing stream for this entity
because a new stream has been initiated that conflicts with the
existing stream, or (2) is refusing a new stream for this entity
because allowing the new stream would conflict with an existing
stream (e.g., because the server allows only a certain number of
connections from the same IP address or allows only one server-to-
server stream for a given domain pair as a way of helping to ensure
in-order processing as described under
Section 10.1
).
C:
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
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If a client receives a
Section 4.9.3.3
),
during the resource binding aspect of its reconnection attempt it
MUST NOT blindly request the resourcepart it used during the former
session but instead MUST choose a different resourcepart; details are
provided under
Section 7
4.9.3.4
. connection-timeout
One party is closing the stream because it has reason to believe that
the other party has permanently lost the ability to communicate over
the stream. The lack of ability to communicate can be discovered
using various methods, such as whitespace keepalives as specified
under
Section 4.4
, XMPP-level pings as defined in [
XEP-0199
], and
XMPP Stream Management as defined in [
XEP-0198
].
P:
Interoperability Note:
RFC 3920
specified that the
Section 4.9.3.4
) is to be used if the peer
has not generated any traffic over the stream for some period of
time. That behavior is no longer recommended; instead, the error
SHOULD be used only if the connected client or peer server has not
responded to data sent over the stream.
4.9.3.5
. host-gone
The value of the 'to' attribute provided in the initial stream header
corresponds to an FQDN that is no longer serviced by the receiving
entity.
(In the following example, the peer specifies a 'to' address of
"foo.im.example.com" when connecting to the "im.example.com" server,
but the server no longer hosts a service at that address.)
P:
to='foo.im.example.com'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
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S:
id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
to='example.net'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
4.9.3.6
. host-unknown
The value of the 'to' attribute provided in the initial stream header
does not correspond to an FQDN that is serviced by the receiving
entity.
(In the following example, the peer specifies a 'to' address of
"example.org" when connecting to the "im.example.com" server, but the
server knows nothing of that address.)
P:
to='example.org'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
S:
id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
to='example.net'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
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4.9.3.7
. improper-addressing
A stanza sent between two servers lacks a 'to' or 'from' attribute,
the 'from' or 'to' attribute has no value, or the value violates the
rules for XMPP addresses [
XMPP-ADDR
].
(In the following example, the peer sends a stanza without a 'to'
address over a server-to-server stream.)
P:
Wherefore art thou?
S:
4.9.3.8
. internal-server-error
The server has experienced a misconfiguration or other internal error
that prevents it from servicing the stream.
S:
4.9.3.9
. invalid-from
The data provided in a 'from' attribute does not match an authorized
JID or validated domain as negotiated (1) between two servers using
SASL or Server Dialback, or (2) between a client and a server via
SASL authentication and resource binding.
(In the following example, a peer that has authenticated only as
"example.net" attempts to send a stanza from an address at
"example.org".)
P:
Neither, fair saint, if either thee dislike.
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S:
4.9.3.10
. invalid-namespace
The stream namespace name is something other than
"http://etherx.jabber.org/streams" (see
Section 11.2
) or the content
namespace declared as the default namespace is not supported (e.g.,
something other than "jabber:client" or "jabber:server").
(In the following example, the client specifies a namespace of
'http://wrong.namespace.example.org/' for the stream.)
C:
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://wrong.namespace.example.org/'>
S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
4.9.3.11
. invalid-xml
The entity has sent invalid XML over the stream to a server that
performs validation (see
Section 11.4
).
(In the following example, the peer attempts to send an IQ stanza of
type "subscribe", but the XML schema defines no such value for the
'type' attribute.)
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P:
to='im.example.com'
type='subscribe'>
S:
4.9.3.12
. not-authorized
The entity has attempted to send XML stanzas or other outbound data
before the stream has been authenticated, or otherwise is not
authorized to perform an action related to stream negotiation; the
receiving entity MUST NOT process the offending data before sending
the stream error.
(In the following example, the client attempts to send XML stanzas
before authenticating with the server.)
C:
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
C:
Wherefore art thou?
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S:
4.9.3.13
. not-well-formed
The initiating entity has sent XML that violates the well-formedness
rules of [
XML
] or [
XML-NAMES
].
(In the following example, the client sends an XMPP message that is
not namespace-well-formed.)
C:
S:
Interoperability Note: In
RFC 3920
, the name of this error
condition was "xml-not-well-formed" instead of "not-well-formed".
The name was changed because the element name
XML
] that
"names beginning with a match to (('X'|'x')('M'|'m')('L'|'l')) are
reserved for standardization in this or future versions of this
specification".
4.9.3.14
. policy-violation
The entity has violated some local service policy (e.g., a stanza
exceeds a configured size limit); the server MAY choose to specify
the policy in the
condition element.
(In the following example, the client sends an XMPP message that is
too large according to the server's local service policy.)
C:
[ ... the-emacs-manual ... ]
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S:
S:
4.9.3.15
. remote-connection-failed
The server is unable to properly connect to a remote entity that is
needed for authentication or authorization (e.g., in certain
scenarios related to Server Dialback [
XEP-0220
]); this condition is
not to be used when the cause of the error is within the
administrative domain of the XMPP service provider, in which case the
C:
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
4.9.3.16
. reset
The server is closing the stream because it has new (typically
security-critical) features to offer, because the keys or
certificates used to establish a secure context for the stream have
expired or have been revoked during the life of the stream
Section 13.7.2.3
), because the TLS sequence number has wrapped
Section 5.3.5
), etc. The reset applies to the stream and to any
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security context established for that stream (e.g., via TLS and
SASL), which means that encryption and authentication need to be
negotiated again for the new stream (e.g., TLS session resumption
cannot be used).
S:
4.9.3.17
. resource-constraint
The server lacks the system resources necessary to service the
stream.
C:
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
4.9.3.18
. restricted-xml
The entity has attempted to send restricted XML features such as a
comment, processing instruction, DTD subset, or XML entity reference
(see
Section 11.1
).
(In the following example, the client sends an XMPP message
containing an XML comment.)
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C:
This message has no subject.
S:
4.9.3.19
. see-other-host
The server will not provide service to the initiating entity but is
redirecting traffic to another host under the administrative control
of the same service provider. The XML character data of the
alternate FQDN or IP address at which to connect, which MUST be a
valid domainpart or a domainpart plus port number (separated by the
':' character in the form "domainpart:port"). If the domainpart is
the same as the source domain, derived domain, or resolved IPv4 or
IPv6 address to which the initiating entity originally connected
(differing only by the port number), then the initiating entity
SHOULD simply attempt to reconnect at that address. (The format of
an IPv6 address MUST follow [
IPv6-ADDR
], which includes the enclosing
the IPv6 address in square brackets '[' and ']' as originally defined
by [
URI
].) Otherwise, the initiating entity MUST resolve the FQDN
specified in the
Section 3.2
C:
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
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S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
[2001:41D0:1:A49b::1]:9222
When negotiating a stream with the host to which it has been
redirected, the initiating entity MUST apply the same policies it
would have applied to the original connection attempt (e.g., a policy
requiring TLS), MUST specify the same 'to' address on the initial
stream header, and MUST verify the identity of the new host using the
same reference identifier(s) it would have used for the original
connection attempt (in accordance with [
TLS-CERTS
]). Even if the
receiving entity returns a
confidentiality and integrity of the stream have been established
(thus introducing the possibility of a denial-of-service attack), the
fact that the initiating entity needs to verify the identity of the
XMPP service based on the same reference identifiers implies that the
initiating entity will not connect to a malicious entity. To reduce
the possibility of a denial-of-service attack, (a) the receiving
entity SHOULD NOT close the stream with a
error until after the confidentiality and integrity of the stream
have been protected via TLS or an equivalent security layer (such as
the SASL GSSAPI mechanism), and (b) the receiving entity MAY have a
policy of following redirects only if it has authenticated the
receiving entity. In addition, the initiating entity SHOULD abort
the connection attempt after a certain number of successive redirects
(e.g., at least 2 but no more than 5).
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4.9.3.20
. system-shutdown
The server is being shut down and all active streams are being
closed.
S:
4.9.3.21
. undefined-condition
The error condition is not one of those defined by the other
conditions in this list; this error condition SHOULD NOT be used
except in conjunction with an application-specific condition.
S:
4.9.3.22
. unsupported-encoding
The initiating entity has encoded the stream in an encoding that is
not supported by the server (see
Section 11.6
) or has otherwise
improperly encoded the stream (e.g., by violating the rules of the
UTF-8
] encoding).
(In the following example, the client attempts to encode data using
UTF-16 instead of UTF-8.)
C:
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
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S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
4.9.3.23
. unsupported-feature
The receiving entity has advertised a mandatory-to-negotiate stream
feature that the initiating entity does not support, and has offered
no other mandatory-to-negotiate feature alongside the unsupported
feature.
(In the following example, the receiving entity requires negotiation
of an example feature, but the initiating entity does not support the
feature.)
R:
I:
4.9.3.24
. unsupported-stanza-type
The initiating entity has sent a first-level child of the stream that
is not supported by the server, either because the receiving entity
does not understand the namespace or because the receiving entity
does not understand the element name for the applicable namespace
(which might be the content namespace declared as the default
namespace).
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(In the following example, the client attempts to send a first-level
child element of
namespace, but the schema for that namespace defines no such
element.)
C:
To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them?
href='http://denmark.example/2003/12/13/atom03'/>
S:
4.9.3.25
. unsupported-version
The 'version' attribute provided by the initiating entity in the
stream header specifies a version of XMPP that is not supported by
the server.
C:
to='im.example.com'
version='11.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
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S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
4.9.4
. Application-Specific Conditions
As noted, an application MAY provide application-specific stream
error information by including a properly namespaced child in the
error element. The application-specific element SHOULD supplement or
further qualify a defined element. Thus, the
contain two or three child elements.
C:
My keyboard layout is:
QWERTYUIOP{}|
ASDFGHJKL:"
ZXCVBNM<>?
S:
Some special application diagnostic information!
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4.10
. Simplified Stream Examples
This section contains two highly simplified examples of a stream-
based connection between a client and a server; these examples are
included for the purpose of illustrating the concepts introduced thus
far, but the reader needs to be aware that these examples elide many
details (see
Section 9
for more complete examples).
A basic connection:
C:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
[ ... stream negotiation ... ]
C:
xml:lang='en'>
Art thou not Romeo, and a Montague?
S:
xml:lang='en'>
Neither, fair saint, if either thee dislike.
C:
S:
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A connection gone bad:
C:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S:
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
[ ... stream negotiation ... ]
C:
xml:lang='en'>
No closing tag!
S:
More detailed examples are provided under
Section 9
. STARTTLS Negotiation
5.1
. Fundamentals
XMPP includes a method for securing the stream from tampering and
eavesdropping. This channel encryption method makes use of the
Transport Layer Security [
TLS
] protocol, specifically a "STARTTLS"
extension that is modeled after similar extensions for the [
IMAP
],
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POP3
], and [
ACAP
] protocols as described in [
USINGTLS
]. The XML
namespace name for the STARTTLS extension is
'urn:ietf:params:xml:ns:xmpp-tls'.
5.2
. Support
Support for STARTTLS is REQUIRED in XMPP client and server
implementations. An administrator of a given deployment MAY specify
that TLS is mandatory-to-negotiate for client-to-server
communication, server-to-server communication, or both. An
initiating entity SHOULD use TLS to secure its stream with the
receiving entity before proceeding with SASL authentication.
5.3
. Stream Negotiation Rules
5.3.1
. Mandatory-to-Negotiate
If the receiving entity advertises only the STARTTLS feature or if
the receiving entity includes the
explained under
Section 5.4.1
, the parties MUST consider TLS as
mandatory-to-negotiate. If TLS is mandatory-to-negotiate, the
receiving entity SHOULD NOT advertise support for any stream feature
except STARTTLS during the initial stage of the stream negotiation
process, because further stream features might depend on prior
negotiation of TLS given the order of layers in XMPP (e.g., the
particular SASL mechanisms offered by the receiving entity will
likely depend on whether TLS has been negotiated).
5.3.2
. Restart
After TLS negotiation, the parties MUST restart the stream.
5.3.3
. Data Formatting
During STARTTLS negotiation, the entities MUST NOT send any
whitespace as separators between XML elements (i.e., from the last
character of the first-level
'urn:ietf:params:xml:ns:xmpp-tls' namespace as sent by the initiating
entity, until the last character of the first-level
element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace
as sent by the receiving entity). This prohibition helps to ensure
proper security layer byte precision. Any such whitespace shown in
the STARTTLS examples provided in this document is included only for
the sake of readability.
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5.3.4
. Order of TLS and SASL Negotiations
If the initiating entity chooses to use TLS, STARTTLS negotiation
MUST be completed before proceeding to SASL negotiation (
Section 6
);
this order of negotiation is necessary to help safeguard
authentication information sent during SASL negotiation, as well as
to make it possible to base the use of the SASL EXTERNAL mechanism on
a certificate (or other credentials) provided during prior TLS
negotiation.
5.3.5
. TLS Renegotiation
The TLS protocol allows either party in a TLS-protected channel to
initiate a new handshake that establishes new cryptographic
parameters (see [
TLS-NEG
]). The cases most commonly mentioned are:
1. Refreshing encryption keys.
2. Wrapping the TLS sequence number as explained in Section 6.1 of
TLS
].
3. Protecting client credentials by completing server authentication
first and then completing client authentication over the
protected channel.
Because it is relatively inexpensive to establish streams in XMPP,
for the first two cases it is preferable to use an XMPP stream reset
(as described under
Section 4.9.3.16
) instead of performing TLS
renegotiation.
The third case has improved security characteristics when the TLS
client (which might be an XMPP server) presents credentials to the
TLS server. If communicating such credentials to an unauthenticated
TLS server might leak private information, it can be appropriate to
complete TLS negotiation for the purpose of authenticating the TLS
server to the TLS client and then attempt TLS renegotiation for the
purpose of authenticating the TLS client to the TLS server. However,
this case is extremely rare because the credentials presented by an
XMPP server or XMPP client acting as a TLS client are almost always
public (i.e., a PKIX certificate), and therefore providing those
credentials before authenticating the XMPP server acting as a TLS
server would not in general leak private information.
As a result, implementers are encouraged to carefully weigh the costs
and benefits of TLS renegotiation before supporting it in their
software, and XMPP entities that act as TLS clients are discouraged
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from attempting TLS renegotiation unless the certificate (or other
credential information) sent during TLS negotiation is known to be
private.
Support for TLS renegotiation is strictly OPTIONAL. However,
implementations that support TLS renegotiation MUST implement and use
the TLS Renegotiation Extension [
TLS-NEG
].
If an entity that does not support TLS renegotiation detects a
renegotiation attempt, then it MUST immediately close the underlying
TCP connection without returning a stream error (since the violation
has occurred at the TLS layer, not the XMPP layer, as described under
Section 13.3
).
If an entity that supports TLS renegotiation detects a TLS
renegotiation attempt that does not use the TLS Renegotiation
Extension [
TLS-NEG
], then it MUST immediately close the underlying
TCP connection without returning a stream error (since the violation
has occurred at the TLS layer, not the XMPP layer as described under
Section 13.3
).
5.3.6
. TLS Extensions
Either party to a stream MAY include any TLS extension during the TLS
negotiation itself. This is a matter for the TLS layer, not the XMPP
layer.
5.4
. Process
5.4.1
. Exchange of Stream Headers and Stream Features
The initiating entity resolves the FQDN of the receiving entity as
specified under
Section 3
, opens a TCP connection to the advertised
port at the resolved IP address, and sends an initial stream header
to the receiving entity.
I:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The receiving entity MUST send a response stream header to the
initiating entity over the TCP connection opened by the initiating
entity.
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R:
id='t7AMCin9zjMNwQKDnplntZPIDEI='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The receiving entity then MUST send stream features to the initiating
entity. If the receiving entity supports TLS, the stream features
MUST include an advertisement for support of STARTTLS negotiation,
i.e., a
'urn:ietf:params:xml:ns:xmpp-tls' namespace.
If the receiving entity considers STARTTLS negotiation to be
mandatory-to-negotiate, the
R:
5.4.2
. Initiation of STARTTLS Negotiation
5.4.2.1
. STARTTLS Command
In order to begin the STARTTLS negotiation, the initiating entity
issues the STARTTLS command (i.e., a
the 'urn:ietf:params:xml:ns:xmpp-tls' namespace) to instruct the
receiving entity that it wishes to begin a STARTTLS negotiation to
secure the stream.
I:
The receiving entity MUST reply with either a
(proceed case) or a
the 'urn:ietf:params:xml:ns:xmpp-tls' namespace.
5.4.2.2
. Failure Case
If the failure case occurs, the receiving entity MUST return a
namespace, close the XML stream, and terminate the underlying TCP
connection.
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R:
R:
Causes for the failure case include but are not limited to:
1. The initiating entity has sent a malformed STARTTLS command.
2. The receiving entity did not offer the STARTTLS feature in its
stream features.
3. The receiving entity cannot complete STARTTLS negotiation because
of an internal error.
Informational Note: STARTTLS failure is not triggered by TLS
errors such as bad_certificate or handshake_failure, which are
generated and handled during the TLS negotiation itself as
described in [
TLS
].
If the failure case occurs, the initiating entity MAY attempt to
reconnect as explained under
Section 3.3
5.4.2.3
. Proceed Case
If the proceed case occurs, the receiving entity MUST return a
namespace.
R:
The receiving entity MUST consider the TLS negotiation to have begun
immediately after sending the closing '>' character of the
element to the initiating entity. The initiating entity MUST
consider the TLS negotiation to have begun immediately after
receiving the closing '>' character of the
the receiving entity.
The entities now proceed to TLS negotiation as explained in the next
section.
5.4.3
. TLS Negotiation
5.4.3.1
. Rules
In order to complete TLS negotiation over the TCP connection, the
entities MUST follow the process defined in [
TLS
].
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The following rules apply:
1. The entities MUST NOT send any further XML data until the TLS
negotiation is complete.
2. When using any of the mandatory-to-implement (MTI) ciphersuites
specified under
Section 13.8
, the receiving entity MUST present a
certificate.
3. So that mutual certificate authentication will be possible, the
receiving entity SHOULD send a certificate request to the
initiating entity, and the initiating entity SHOULD send a
certificate to the receiving entity (but for privacy reasons
might opt not to send a certificate until after the receiving
entity has authenticated to the initiating entity).
4. The receiving entity SHOULD choose which certificate to present
based on the domainpart contained in the 'to' attribute of the
initial stream header (in essence, this domainpart is
functionally equivalent to the Server Name Indication defined for
TLS in [
TLS-EXT
]).
5. To determine if the TLS negotiation will succeed, the initiating
entity MUST attempt to validate the receiving entity's
certificate in accordance with the certificate validation
procedures specified under
Section 13.7.2
6. If the initiating entity presents a certificate, the receiving
entity too MUST attempt to validate the initiating entity's
certificate in accordance with the certificate validation
procedures specified under
Section 13.7.2
7. Following successful TLS negotiation, all further data
transmitted by either party MUST be protected with the negotiated
algorithms, keys, and secrets (i.e., encrypted, integrity-
protected, or both depending on the ciphersuite used).
Security Warning: See
Section 13.8
regarding ciphersuites that
MUST be supported for TLS; naturally, other ciphersuites MAY be
supported as well.
5.4.3.2
. TLS Failure
If the TLS negotiation results in failure, the receiving entity MUST
terminate the TCP connection.
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The receiving entity MUST NOT send a closing tag before
terminating the TCP connection (since the failure has occurred at the
TLS layer, not the XMPP layer as described under
Section 13.3
).
The initiating entity MAY attempt to reconnect as explained under
Section 3.3
, with or without attempting TLS negotiation (in
accordance with local service policy, user-configured preferences,
etc.).
5.4.3.3
. TLS Success
If the TLS negotiation is successful, then the entities MUST proceed
as follows.
1. The initiating entity MUST discard any information transmitted in
layers above TCP that it obtained from the receiving entity in an
insecure manner before TLS took effect (e.g., the receiving
entity's 'from' address or the stream ID and stream features
received from the receiving entity).
2. The receiving entity MUST discard any information transmitted in
layers above TCP that it obtained from the initiating entity in
an insecure manner before TLS took effect (e.g., the initiating
entity's 'from' address).
3. The initiating entity MUST send a new initial stream header to
the receiving entity over the encrypted connection (as specified
under
Section 4.3.3
, the initiating entity MUST NOT send a
closing tag before sending the new initial stream
header, since the receiving entity and initiating entity MUST
consider the original stream to be replaced upon success of the
TLS negotiation).
I:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
4. The receiving entity MUST respond with a new response stream
header over the encrypted connection (for which it MUST generate
a new stream ID instead of reusing the old stream ID).
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R:
id='vgKi/bkYME8OAj4rlXMkpucAqe4='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
5. The receiving entity also MUST send stream features to the
initiating entity, which MUST NOT include the STARTTLS feature
but which SHOULD include the SASL stream feature as described
under
Section 6
(see especially
Section 6.4.1
regarding the few
reasons why the SASL stream feature would not be offered here).
R:
. SASL Negotiation
6.1
. Fundamentals
XMPP includes a method for authenticating a stream by means of an
XMPP-specific profile of the Simple Authentication and Security Layer
protocol (see [
SASL
]). SASL provides a generalized method for adding
authentication support to connection-based protocols, and XMPP uses
an XML namespace profile of SASL that conforms to the profiling
requirements of [
SASL
]. The XML namespace name for the SASL
extension is 'urn:ietf:params:xml:ns:xmpp-sasl'.
6.2
. Support
Support for SASL negotiation is REQUIRED in XMPP client and server
implementations.
6.3
. Stream Negotiation Rules
6.3.1
. Mandatory-to-Negotiate
The parties to a stream MUST consider SASL as mandatory-to-negotiate.
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6.3.2
. Restart
After SASL negotiation, the parties MUST restart the stream.
6.3.3
. Mechanism Preferences
Any entity that will act as a SASL client or a SASL server MUST
maintain an ordered list of its preferred SASL mechanisms according
to the client or server, where the list is ordered according to local
policy or user configuration (which SHOULD be in order of perceived
strength to enable the strongest authentication possible). The
initiating entity MUST maintain its own preference order independent
of the preference order of the receiving entity. A client MUST try
SASL mechanisms in its preference order. For example, if the server
offers the ordered list "PLAIN SCRAM-SHA-1 GSSAPI" or "SCRAM-SHA-1
GSSAPI PLAIN" but the client's ordered list is "GSSAPI SCRAM-SHA-1",
the client MUST try GSSAPI first and then SCRAM-SHA-1 but MUST NOT
try PLAIN (since PLAIN is not on its list).
6.3.4
. Mechanism Offers
If the receiving entity considers TLS negotiation (
Section 5
) to be
mandatory-to-negotiate before it will accept authentication with a
particular SASL mechanism, it MUST NOT advertise that mechanism in
its list of available SASL mechanisms before TLS negotiation has been
completed.
The receiving entity SHOULD offer the SASL EXTERNAL mechanism if both
of the following conditions hold:
1. During TLS negotiation the initiating entity presented a
certificate that is acceptable to the receiving entity for
purposes of strong identity verification in accordance with local
service policies (e.g., because said certificate is unexpired, is
unrevoked, and is anchored to a root trusted by the receiving
entity).
2. The receiving entity expects that the initiating entity will be
able to authenticate and authorize as the identity provided in
the certificate; in the case of a server-to-server stream, the
receiving entity might have such an expectation because a DNS
domain name presented in the initiating entity's certificate
matches the domain referenced in the 'from' attribute of the
initial stream header, where the matching rules of [
TLS-CERTS
apply; in the case of a client-to-server stream, the receiving
entity might have such an expectation because the bare JID
presented in the initiating entity's certificate matches a user
account that is registered with the server or because other
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information contained in the initiating entity's certificate
matches that of an entity that has permission to use the server
for access to an XMPP network.
However, the receiving entity MAY offer the SASL EXTERNAL mechanism
under other circumstances, as well.
When the receiving entity offers the SASL EXTERNAL mechanism, the
receiving entity SHOULD list the EXTERNAL mechanism first among its
offered SASL mechanisms and the initiating entity SHOULD attempt SASL
negotiation using the EXTERNAL mechanism first (this preference will
tend to increase the likelihood that the parties can negotiate mutual
certificate authentication).
Section 13.8
specifies SASL mechanisms that MUST be supported;
naturally, other SASL mechanisms MAY be supported as well.
Informational Note: Best practices for the use of SASL in the
context of XMPP are described in [
XEP-0175
] for the ANONYMOUS
mechanism and in [
XEP-0178
] for the EXTERNAL mechanism.
6.3.5
. Data Formatting
The following data formatting rules apply to the SASL negotiation:
1. During SASL negotiation, the entities MUST NOT send any
whitespace as separators between XML elements (i.e., from the
last character of the first-level
the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace as sent by the
initiating entity, until the last character of the first-level
'urn:ietf:params:xml:ns:xmpp-sasl' namespace as sent by the
receiving entity). This prohibition helps to ensure proper
security layer byte precision. Any such whitespace shown in the
SASL examples provided in this document is included only for the
sake of readability.
2. Any XML character data contained within the XML elements MUST be
encoded using base 64, where the encoding adheres to the
definition in Section 4 of [
BASE64
] and where the padding bits
are set to zero.
3. As formally specified in the XML schema for the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace under
Appendix A.4
the receiving entity MAY include one or more application-specific
child elements inside the
information that might be needed by the initiating entity in
order to complete successful SASL negotiation using one or more
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of the offered mechanisms; however, the syntax and semantics of
all such elements are out of scope for this specification (see
XEP-0233
] for one example).
6.3.6
. Security Layers
Upon successful SASL negotiation that involves negotiation of a
security layer, both the initiating entity and the receiving entity
MUST discard any application-layer state (i.e, state from the XMPP
layer, excluding state from the TLS negotiation or SASL negotiation).
6.3.7
. Simple User Name
Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify
that the authentication identity used in the context of such
mechanisms is a "simple user name" (see Section 2 of [
SASL
] as well
as [
SASLPREP
]). The exact form of the simple user name in any
particular mechanism or deployment thereof is a local matter, and a
simple user name does not necessarily map to an application
identifier such as a JID or JID component (e.g., a localpart).
However, in the absence of local information provided by the server,
an XMPP client SHOULD assume that the authentication identity for
such a SASL mechanism is a simple user name equal to the localpart of
the user's JID.
6.3.8
. Authorization Identity
An authorization identity is an OPTIONAL identity included by the
initiating entity to specify an identity to act as (see Section 2 of
SASL
]). In client-to-server streams, it would most likely be used
by an administrator to perform some management task on behalf of
another user, whereas in server-to-server streams it would most
likely be used to specify a particular add-on service at an XMPP
service (e.g., a multi-user chat server at conference.example.com
that is hosted by the example.com XMPP service). If the initiating
entity wishes to act on behalf of another entity and the selected
SASL mechanism supports transmission of an authorization identity,
the initiating entity MUST provide an authorization identity during
SASL negotiation. If the initiating entity does not wish to act on
behalf of another entity, it MUST NOT provide an authorization
identity.
In the case of client-to-server communication, the value of an
authorization identity MUST be a bare JID (
rather than a full JID (
In the case of server-to-server communication, the value of an
authorization identity MUST be a domainpart only (
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If the initiating entity provides an authorization identity during
SASL negotiation, the receiving entity is responsible for verifying
that the initiating entity is in fact allowed to assume the specified
authorization identity; if not, the receiving entity MUST return an
Section 6.5.6
6.3.9
. Realms
The receiving entity MAY include a realm when negotiating certain
SASL mechanisms (e.g., both the GSSAPI and DIGEST-MD5 mechanisms
allow the authentication exchange to include a realm, though in
different ways, whereas the EXTERNAL, SCRAM, and PLAIN mechanisms do
not). If the receiving entity does not communicate a realm, the
initiating entity MUST NOT assume that any realm exists. The realm
MUST be used only for the purpose of authentication; in particular,
an initiating entity MUST NOT attempt to derive an XMPP domainpart
from the realm information provided by the receiving entity.
6.3.10
. Round Trips
SASL
] specifies that a using protocol such as XMPP can define two
methods by which the protocol can save round trips where allowed for
the SASL mechanism:
1. When the SASL client (the XMPP "initiating entity") requests an
authentication exchange, it can include "initial response" data
with its request if appropriate for the SASL mechanism in use.
In XMPP, this is done by including the initial response as the
XML character data of the
2. At the end of the authentication exchange, the SASL server (the
XMPP "receiving entity") can include "additional data with
success" if appropriate for the SASL mechanism in use. In XMPP,
this is done by including the additional data as the XML
character data of the
For the sake of protocol efficiency, it is REQUIRED for clients and
servers to support these methods and RECOMMENDED to use them;
however, clients and servers MUST support the less efficient modes as
well.
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6.4
. Process
The process for SASL negotiation is as follows.
6.4.1
. Exchange of Stream Headers and Stream Features
If SASL negotiation follows successful STARTTLS negotiation
Section 5
), then the SASL negotiation occurs over the protected
stream that has already been negotiated. If not, the initiating
entity resolves the FQDN of the receiving entity as specified under
Section 3
, opens a TCP connection to the advertised port at the
resolved IP address, and sends an initial stream header to the
receiving entity. In either case, the receiving entity will receive
an initial stream from the initiating entity.
I:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
When the receiving entity processes an initial stream header from the
initiating entity, it MUST send a response stream header to the
initiating entity (for which it MUST generate a unique stream ID. If
TLS negotiation has already succeeded, then this stream ID MUST be
different from the stream ID sent before TLS negotiation succeeded).
R:
id='vgKi/bkYME8OAj4rlXMkpucAqe4='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The receiving entity also MUST send stream features to the initiating
entity. The stream features SHOULD include an advertisement for
support of SASL negotiation, i.e., a
by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace. Typically there
are only three cases in which support for SASL negotiation would not
be advertised here:
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o TLS negotiation needs to happen before SASL can be offered (i.e.,
TLS is required and the receiving entity is responding to the very
first initial stream header it has received for this connection
attempt).
o SASL negotiation is impossible for a server-to-server connection
(i.e., the initiating server has not provided a certificate that
would enable strong authentication and therefore the receiving
server is falling back to weak identity verification using the
Server Dialback protocol [
XEP-0220
]).
o SASL has already been negotiated (i.e., the receiving entity is
responding to an initial stream header sent as a stream restart
after successful SASL negotiation).
The
for each authentication mechanism the receiving entity offers to the
initiating entity. As noted, the order of
the XML indicates the preference order of the SASL mechanisms
according to the receiving entity (which is not necessarily the
preference order according to the initiating entity).
R:
6.4.2
. Initiation
In order to begin the SASL negotiation, the initiating entity sends
an
'urn:ietf:params:xml:ns:xmpp-sasl' namespace and includes an
appropriate value for the 'mechanism' attribute, thus starting the
handshake for that particular authentication mechanism. This element
MAY contain XML character data (in SASL terminology, the "initial
response") if the mechanism supports or requires it. If the
initiating entity needs to send a zero-length initial response, it
MUST transmit the response as a single equals sign character ("="),
which indicates that the response is present but contains no data.
I:
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If the initiating entity subsequently sends another
and the ongoing authentication handshake has not yet completed, the
receiving entity MUST discard the ongoing handshake and MUST process
a new handshake for the subsequently requested SASL mechanism.
6.4.3
. Challenge-Response Sequence
If necessary, the receiving entity challenges the initiating entity
by sending a
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (which MUST be generated in accordance
with the definition of the SASL mechanism chosen by the initiating
entity).
The initiating entity responds to the challenge by sending a
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (which MUST be generated in accordance
with the definition of the SASL mechanism chosen by the initiating
entity).
If necessary, the receiving entity sends more challenges and the
initiating entity sends more responses.
This series of challenge/response pairs continues until one of three
things happens:
o The initiating entity aborts the handshake for this authentication
mechanism.
o The receiving entity reports failure of the handshake.
o The receiving entity reports success of the handshake.
These scenarios are described in the following sections.
6.4.4
. Abort
The initiating entity aborts the handshake for this authentication
mechanism by sending an
'urn:ietf:params:xml:ns:xmpp-sasl' namespace.
I:
Upon receiving an
a
'urn:ietf:params:xml:ns:xmpp-sasl' namespace and containing an
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R:
6.4.5
. SASL Failure
The receiving entity reports failure of the handshake for this
authentication mechanism by sending a
the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace (the particular
cause of failure MUST be communicated in an appropriate child element
of the
Section 6.5
).
R:
Where appropriate for the chosen SASL mechanism, the receiving entity
SHOULD allow a configurable but reasonable number of retries (at
least 2 and no more than 5); this enables the initiating entity
(e.g., an end-user client) to tolerate incorrectly provided
credentials (e.g., a mistyped password) without being forced to
reconnect (which it would if the receiving entity immediately
returned a SASL failure and closed the stream).
If the initiating entity attempts a reasonable number of retries with
the same SASL mechanism and all attempts fail, it MAY fall back to
the next mechanism in its ordered list by sending a new
request to the receiving entity, thus starting a new handshake for
that authentication mechanism. If all handshakes fail and there are
no remaining mechanisms in the initiating entity's list of supported
and acceptable mechanisms, the initiating entity SHOULD simply close
the stream as described under
Section 4.4
(instead of waiting for the
stream to time out).
If the initiating entity exceeds the number of retries, the receiving
entity MUST close the stream with a stream error, which SHOULD be
Section 4.9.3.14
), although some existing
implementations send
Section 4.9.3.12
) instead.
Implementation Note: For server-to-server streams, if the
receiving entity cannot offer the SASL EXTERNAL mechanism or any
other SASL mechanism based on the security context established
during TLS negotiation, the receiving entity MAY attempt to
complete weak identity verification using the Server Dialback
protocol [
XEP-0220
]; however, if according to local service
policies weak identity verification is insufficient then the
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receiving entity SHOULD instead close the stream with a
Section 4.9.3.14
) instead of waiting for
the stream to time out.
6.4.6
. SASL Success
Before considering the SASL handshake to be a success, if the
initiating entity provided a 'from' attribute on an initial stream
header whose confidentiality and integrity were protected via TLS or
an equivalent security layer (such as the SASL GSSAPI mechanism) then
the receiving entity SHOULD correlate the authentication identity
resulting from the SASL negotiation with that 'from' address; if the
two identities do not match then the receiving entity SHOULD
terminate the connection attempt (however, the receiving entity might
have legitimate reasons not to terminate the connection attempt, for
example, because it has overridden a connecting client's address to
correct the JID format or assign a JID based on information presented
in an end-user certificate).
The receiving entity reports success of the handshake by sending a
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (in SASL terminology, "additional data
with success") if the chosen SASL mechanism supports or requires it.
If the receiving entity needs to send additional data of zero length,
it MUST transmit the data as a single equals sign character ("=").
R:
Informational Note: For client-to-server streams, the
authorization identity communicated during SASL negotiation is
used to determine the canonical address for the initiating client
according to the receiving server, as described under
Section 4.3.6
Upon receiving the
initiate a new stream over the existing TCP connection by sending a
new initial stream header to the receiving entity (as specified under
Section 4.3.3
, the initiating entity MUST NOT send a closing
tag before sending the new initial stream header, since the
receiving entity and initiating entity MUST consider the original
stream to be replaced upon success of the SASL negotiation).
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I:
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
Upon receiving the new initial stream header from the initiating
entity, the receiving entity MUST respond by sending a new response
stream header to the initiating entity (for which it MUST generate a
new stream ID instead of reusing the old stream ID).
R:
id='gPybzaOzBmaADgxKXu9UClbprp0='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The receiving entity MUST also send stream features, containing any
further available features or containing no features (via an empty
R:
6.5
. SASL Errors
The syntax of SASL errors is as follows, where the XML data shown
within the square brackets '[' and ']' is OPTIONAL.
[
OPTIONAL descriptive text
The "defined-condition" MUST be one of the SASL-related error
conditions defined in the following sections. However, because
additional error conditions might be defined in the future, if an
entity receives a SASL error condition that it does not understand
then it MUST treat the unknown condition as a generic authentication
failure, i.e., as equivalent to
Section 6.5.10
).
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Inclusion of the
provide application-specific information about the error condition,
which information MAY be displayed to a human but only as a
supplement to the defined condition.
Because XMPP itself defines an application profile of SASL and there
is no expectation that more specialized XMPP applications will be
built on top of SASL, the SASL error format does not provide
extensibility for application-specific error conditions as is done
for XML streams (
Section 4.9.4
) and XML stanzas (
Section 8.3.4
).
6.5.1
. aborted
The receiving entity acknowledges that the authentication handshake
has been aborted by the initiating entity; sent in reply to the
I:
R:
6.5.2
. account-disabled
The account of the initiating entity has been temporarily disabled;
sent in reply to an
data) or a
I:
R:
6.5.3
. credentials-expired
The authentication failed because the initiating entity provided
credentials that have expired; sent in reply to a
or an
I:
[ ... ]
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R:
6.5.4
. encryption-required
The mechanism requested by the initiating entity cannot be used
unless the confidentiality and integrity of the underlying stream are
protected (typically via TLS); sent in reply to an
(with or without initial response data).
I:
R:
6.5.5
. incorrect-encoding
The data provided by the initiating entity could not be processed
because the base 64 encoding is incorrect (e.g., because the encoding
does not adhere to the definition in Section 4 of [
BASE64
]); sent in
reply to a
response data.
I:
R:
6.5.6
. invalid-authzid
The authzid provided by the initiating entity is invalid, either
because it is incorrectly formatted or because the initiating entity
does not have permissions to authorize that ID; sent in reply to a
I:
[ ... ]
R:
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6.5.7
. invalid-mechanism
The initiating entity did not specify a mechanism, or requested a
mechanism that is not supported by the receiving entity; sent in
reply to an
I:
R:
6.5.8
. malformed-request
The request is malformed (e.g., the
response data but the mechanism does not allow that, or the data sent
violates the syntax for the specified SASL mechanism); sent in reply
to an
(In the following example, the XML character data of the
element contains more than 255 UTF-8-encoded Unicode characters and
therefore violates the "token" production for the SASL ANONYMOUS
mechanism as specified in [
ANONYMOUS
].)
I:
R:
6.5.9
. mechanism-too-weak
The mechanism requested by the initiating entity is weaker than
server policy permits for that initiating entity; sent in reply to an
I:
R:
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6.5.10
. not-authorized
The authentication failed because the initiating entity did not
provide proper credentials, or because some generic authentication
failure has occurred but the receiving entity does not wish to
disclose specific information about the cause of the failure; sent in
reply to a
response data.
I:
[ ... ]
R:
Security Warning: This error condition includes but is not limited
to the case of incorrect credentials or a nonexistent username.
In order to discourage directory harvest attacks, no
differentiation is made between incorrect credentials and a
nonexistent username.
6.5.11
. temporary-auth-failure
The authentication failed because of a temporary error condition
within the receiving entity, and it is advisable for the initiating
entity to try again later; sent in reply to an
I:
[ ... ]
R:
6.6
. SASL Definition
The profiling requirements of [
SASL
] require that the following
information be supplied by the definition of a using protocol.
service name: "xmpp"
initiation sequence: After the initiating entity provides an opening
XML stream header and the receiving entity replies in kind, the
receiving entity provides a list of acceptable authentication
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methods. The initiating entity chooses one method from the list
and sends it to the receiving entity as the value of the
'mechanism' attribute possessed by an
including an initial response to avoid a round trip.
exchange sequence: Challenges and responses are carried through the
exchange of
initiating entity and
to receiving entity. The receiving entity reports failure by
sending a
element; the initiating entity aborts the exchange by sending an
consider the original XML stream to be closed and new stream
headers are sent by both entities.
security layer negotiation: The security layer takes effect
immediately after sending the closing '>' character of the
receiving the closing '>' character of the
the initiating entity. The order of layers is first [
TCP
], then
TLS
], then [
SASL
], then XMPP.
use of the authorization identity: The authorization identity can be
used in XMPP to denote the non-default
client; an empty string is equivalent to an absent authorization
identity.
. Resource Binding
7.1
. Fundamentals
After a client authenticates with a server, it MUST bind a specific
resource to the stream so that the server can properly address the
client. That is, there MUST be an XMPP resource associated with the
bare JID (
for use over that stream is a full JID of the form
ensures that the server can deliver XML stanzas to and receive XML
stanzas from the client in relation to entities other than the server
itself or the client's account, as explained under
Section 10
Informational Note: The client could exchange stanzas with the
server itself or the client's account before binding a resource
since the full JID is needed only for addressing outside the
context of the stream negotiated between the client and the
server, but this is not commonly done.
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After a client has bound a resource to the stream, it is referred to
as a "connected resource". A server SHOULD allow an entity to
maintain multiple connected resources simultaneously, where each
connected resource is associated with a distinct XML stream and is
differentiated from the other connected resources by a distinct
resourcepart.
Security Warning: A server SHOULD enable the administrator of an
XMPP service to limit the number of connected resources in order
to prevent certain denial-of-service attacks as described under
Section 13.12
If, before completing the resource binding step, the client attempts
to send an XML stanza to an entity other than the server itself or
the client's account, the server MUST NOT process the stanza and MUST
close the stream with a
Section 4.9.3.12
).
The XML namespace name for the resource binding extension is
'urn:ietf:params:xml:ns:xmpp-bind'.
7.2
. Support
Support for resource binding is REQUIRED in XMPP client and server
implementations.
7.3
. Stream Negotiation Rules
7.3.1
. Mandatory-to-Negotiate
The parties to a stream MUST consider resource binding as mandatory-
to-negotiate.
7.3.2
. Restart
After resource binding, the parties MUST NOT restart the stream.
7.4
. Advertising Support
Upon sending a new response stream header to the client after
successful SASL negotiation, the server MUST include a
element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace
in the stream features it presents to the client.
The server MUST NOT include the resource binding stream feature until
after the client has authenticated, typically by means of successful
SASL negotiation.
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S:
id='gPybzaOzBmaADgxKXu9UClbprp0='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S:
Upon being informed that resource binding is mandatory-to-negotiate,
the client MUST bind a resource to the stream as described in the
following sections.
7.5
. Generation of Resource Identifiers
A resourcepart MUST at a minimum be unique among the connected
resources for that
policy is the responsibility of the server.
Security Warning: A resourcepart can be security-critical. For
example, if a malicious entity can guess a client's resourcepart
then it might be able to determine if the client (and therefore
the controlling principal) is online or offline, thus resulting in
a presence leak as described under
Section 13.10.2
. To prevent
that possibility, a client can either (1) generate a random
resourcepart on its own or (2) ask the server to generate a
resourcepart on its behalf. One method for ensuring that the
resourcepart is random is to generate a Universally Unique
Identifier (UUID) as specified in [
UUID
].
7.6
. Server-Generated Resource Identifier
A server MUST be able to generate an XMPP resourcepart on behalf of a
client. The resourcepart generated by the server MUST be random (see
RANDOM
]).
7.6.1
. Success Case
A client requests a server-generated resourcepart by sending an IQ
stanza of type "set" (see
Section 8.2.3
) containing an empty
element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind'
namespace.
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C:
Once the server has generated an XMPP resourcepart for the client, it
MUST return an IQ stanza of type "result" to the client, which MUST
include a
connected resource as determined by the server.
S:
juliet@im.example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb
7.6.2
. Error Cases
When a client asks the server to generate a resourcepart during
resource binding, the following stanza error conditions are defined:
o The account has reached a limit on the number of simultaneous
connected resources allowed.
o The client is otherwise not allowed to bind a resource to the
stream.
Naturally, it is possible that error conditions not specified here
might occur, as described under
Section 8.3
7.6.2.1
. Resource Constraint
If the account has reached a limit on the number of simultaneous
connected resources allowed, the server MUST return a
Section 8.3.3.18
).
S:
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7.6.2.2
. Not Allowed
If the client is otherwise not allowed to bind a resource to the
stream, the server MUST return a
Section 8.3.3.10
).
S:
7.7
. Client-Submitted Resource Identifier
Instead of asking the server to generate a resourcepart on its
behalf, a client MAY attempt to submit a resourcepart that it has
generated or that the controlling user has provided.
7.7.1
. Success Case
A client asks its server to accept a client-submitted resourcepart by
sending an IQ stanza of type "set" containing a
a child
data.
C:
The server SHOULD accept the client-submitted resourcepart. It does
so by returning an IQ stanza of type "result" to the client,
including a
connected resource and contains without modification the client-
submitted text.
S:
Alternatively, in accordance with local service policies the server
MAY refuse the client-submitted resourcepart and override it with a
resourcepart that the server generates.
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S:
juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
7.7.2
. Error Cases
When a client attempts to submit its own XMPP resourcepart during
resource binding, the following stanza error conditions are defined
in addition to those described under
Section 7.6.2
o The provided resourcepart cannot be processed by the server.
o The provided resourcepart is already in use.
Naturally, it is possible that error conditions not specified here
might occur, as described under
Section 8.3
7.7.2.1
. Bad Request
If the provided resourcepart cannot be processed by the server (e.g.,
because it is of zero length or because it otherwise violates the
rules for resourceparts specified in [
XMPP-ADDR
]), the server can
return a
Section 8.3.3.1
) but SHOULD
instead process the resourcepart so that it is in conformance.
S:
7.7.2.2
. Conflict
If there is a currently connected client whose session has the
resourcepart being requested by the newly connecting client, the
server MUST do one of the following (which of these the server does
is a matter for implementation or local service policy, although
suggestions are provided below).
1. Override the resourcepart provided by the newly connecting client
with a server-generated resourcepart. This behavior is
encouraged, because it simplifies the resource binding process
for client implementations.
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2. Disallow the resource binding attempt of the newly connecting
client and maintain the session of the currently connected
client. This behavior is neither encouraged nor discouraged,
despite the fact that it was implicitly encouraged in
RFC 3920
however, note that handling of the
supported among existing client implementations, which often
treat it as an authentication error and have been observed to
discard cached credentials when receiving it.
3. Terminate the session of the currently connected client and allow
the resource binding attempt of the newly connecting client.
Although this was the traditional behavior of early XMPP server
implementations, it is now discouraged because it can lead to a
never-ending cycle of two clients effectively disconnecting each
other; however, note that this behavior can be appropriate in
some deployment scenarios or if the server knows that the
currently connected client has a dead connection or broken stream
as described under
Section 4.6
If the server follows behavior #1, it returns an
"result" to the newly connecting client, where the
the
full JID of the client, including the resourcepart that was generated
by the server.
S:
juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
If the server follows behavior #2, it sends a
error (
Section 8.3.3.2
) in response to the resource binding attempt
of the newly connecting client but maintains the XML stream so that
the newly connecting client has an opportunity to negotiate a non-
conflicting resourcepart (i.e., the newly connecting client needs to
choose a different resourcepart before making another attempt to bind
a resource).
S:
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If the server follows behavior #3, it returns a
error (
Section 4.9.3.3
) to the currently connected client (as
described under
Section 4.9.3.3
) and returns an IQ stanza of type
"result" (indicating success) in response to the resource binding
attempt of the newly connecting client.
S:
juliet@im.example.com/balcony
7.7.3
. Retries
If an error occurs when a client submits a resourcepart, the server
SHOULD allow a configurable but reasonable number of retries (at
least 5 and no more than 10); this enables the client to tolerate
incorrectly provided resourceparts (e.g., bad data formats or
duplicate text strings) without being forced to reconnect.
After the client has reached the retry limit, the server MUST close
the stream with a
Section 4.9.3.14
).
. XML Stanzas
After a client and a server (or two servers) have completed stream
negotiation, either party can send XML stanzas. Three kinds of XML
stanza are defined for the 'jabber:client' and 'jabber:server'
namespaces:
are five common attributes for these stanza types. These common
attributes, as well as the basic semantics of the three stanza types,
are defined in this specification; more detailed information
regarding the syntax of XML stanzas for instant messaging and
presence applications is provided in [
XMPP-IM
], and for other
applications in the relevant XMPP extension specifications.
Support for the XML stanza syntax and semantics defined in this
specification is REQUIRED in XMPP client and server implementations.
Security Warning: A server MUST NOT process a partial stanza and
MUST NOT attach meaning to the transmission timing of any part of
a stanza (before receipt of the closing tag).
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8.1
. Common Attributes
The following five attributes are common to message, presence, and IQ
stanzas.
8.1.1
. to
The 'to' attribute specifies the JID of the intended recipient for
the stanza.
Art thou not Romeo, and a Montague?
For information about server processing of inbound and outbound XML
stanzas based on the 'to' address, refer to
Section 10
8.1.1.1
. Client-to-Server Streams
The following rules apply to inclusion of the 'to' attribute in
stanzas sent from a connected client to its server over an XML stream
qualified by the 'jabber:client' namespace.
1. A stanza with a specific intended recipient (e.g., a conversation
partner, a remote service, the server itself, even another
resource associated with the user's bare JID) MUST possess a 'to'
attribute whose value is an XMPP address.
2. A stanza sent from a client to a server for direct processing by
the server (e.g., roster processing as described in [
XMPP-IM
] or
presence sent to the server for broadcasting to other entities)
MUST NOT possess a 'to' attribute.
The following rules apply to inclusion of the 'to' attribute in
stanzas sent from a server to a connected client over an XML stream
qualified by the 'jabber:client' namespace.
1. If the server has received the stanza from another connected
client or from a peer server, the server MUST NOT modify the 'to'
address before delivering the stanza to the client.
2. If the server has itself generated the stanza (e.g., a response
to an IQ stanza of type "get" or "set", even if the stanza did
not include a 'to' address), the stanza MAY include a 'to'
address, which MUST be the full JID of the client; however, if
the stanza does not include a 'to' address then the client MUST
treat it as if the 'to' address were included with a value of the
client's full JID.
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Implementation Note: It is the server's responsibility to deliver
only stanzas that are addressed to the client's full JID or the
user's bare JID; thus, there is no need for the client to check
the 'to' address of incoming stanzas. However, if the client does
check the 'to' address then it is suggested to check at most the
bare JID portion (not the full JID), since the 'to' address might
be the user's bare JID, the client's current full JID, or even a
full JID with a different resourcepart (e.g., in the case of so-
called "offline messages" as described in [
XEP-0160
]).
8.1.1.2
. Server-to-Server Streams
The following rules apply to inclusion of the 'to' attribute in the
context of XML streams qualified by the 'jabber:server' namespace
(i.e., server-to-server streams).
1. A stanza MUST possess a 'to' attribute whose value is an XMPP
address; if a server receives a stanza that does not meet this
restriction, it MUST close the stream with an
Section 4.9.3.7
).
2. The domainpart of the JID contained in the stanza's 'to'
attribute MUST match the FQDN of the receiving server (or any
validated domain thereof) as communicated via SASL negotiation
(see
Section 6
), Server Dialback (see [
XEP-0220
]), or similar
means; if a server receives a stanza that does not meet this
restriction, it MUST close the stream with a
stream error (
Section 4.9.3.6
) or a
Section 4.9.3.5
).
8.1.2
. from
The 'from' attribute specifies the JID of the sender.
Art thou not Romeo, and a Montague?
8.1.2.1
. Client-to-Server Streams
The following rules apply to the 'from' attribute in the context of
XML streams qualified by the 'jabber:client' namespace (i.e., client-
to-server streams).
1. When a server receives an XML stanza from a connected client, the
server MUST add a 'from' attribute to the stanza or override the
'from' attribute specified by the client, where the value of the
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'from' attribute MUST be the full JID
(
the connected resource that generated the stanza (see
Section 4.3.6
), or the bare JID (
case of subscription-related presence stanzas (see [
XMPP-IM
]).
2. When the server generates a stanza on its own behalf for delivery
to the client from the server itself, the stanza MUST include a
'from' attribute whose value is the bare JID (i.e.,
of the server as agreed upon during stream negotiation (e.g.,
based on the 'to' attribute of the initial stream header).
3. When the server generates a stanza from the server for delivery
to the client on behalf of the account of the connected client
(e.g., in the context of data storage services provided by the
server on behalf of the client), the stanza MUST either (a) not
include a 'from' attribute or (b) include a 'from' attribute
whose value is the account's bare JID (
4. A server MUST NOT send to the client a stanza without a 'from'
attribute if the stanza was not generated by the server on its
own behalf (e.g., if it was generated by another client or a peer
server and the server is merely delivering it to the client on
behalf of some other entity); therefore, when a client receives a
stanza that does not include a 'from' attribute, it MUST assume
that the stanza is from the user's account on the server.
8.1.2.2
. Server-to-Server Streams
The following rules apply to the 'from' attribute in the context of
XML streams qualified by the 'jabber:server' namespace (i.e., server-
to-server streams).
1. A stanza MUST possess a 'from' attribute whose value is an XMPP
address; if a server receives a stanza that does not meet this
restriction, it MUST close the stream with an
Section 4.9.3.7
).
2. The domainpart of the JID contained in the stanza's 'from'
attribute MUST match the FQDN of the sending server (or any
validated domain thereof) as communicated via SASL negotiation
(see
Section 6
), Server Dialback (see [
XEP-0220
]), or similar
means; if a server receives a stanza that does not meet this
restriction, it MUST close the stream with an
stream error (
Section 4.9.3.9
).
Enforcement of these rules helps to prevent certain denial-of-service
attacks as described under
Section 13.12
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8.1.3
. id
The 'id' attribute is used by the originating entity to track any
response or error stanza that it might receive in relation to the
generated stanza from another entity (such as an intermediate server
or the intended recipient).
It is up to the originating entity whether the value of the 'id'
attribute is unique only within its current stream or unique
globally.
For
originating entity to include an 'id' attribute; for
it is REQUIRED.
If the generated stanza includes an 'id' attribute then it is
REQUIRED for the response or error stanza to also include an 'id'
attribute, where the value of the 'id' attribute MUST match that of
the generated stanza.
The semantics of IQ stanzas impose additional restrictions as
described under
Section 8.2.3
8.1.4
. type
The 'type' attribute specifies the purpose or context of the message,
presence, or IQ stanza. The particular allowable values for the
'type' attribute vary depending on whether the stanza is a message,
presence, or IQ stanza. The defined values for message and presence
stanzas are specific to instant messaging and presence applications
and therefore are defined in [
XMPP-IM
], whereas the values for IQ
stanzas specify the part of the semantics for all structured request-
response exchanges (no matter what the payload) and therefore are
specified under
Section 8.2.3
. The only 'type' value common to all
three kinds of stanzas is "error" as described under
Section 8.3
8.1.5
. xml:lang
A stanza SHOULD possess an 'xml:lang' attribute (as defined in
Section 2.12 of [
XML
]) if the stanza contains XML character data that
is intended to be presented to a human user (as explained in
CHARSETS
], "internationalization is for humans"). The value of the
'xml:lang' attribute specifies the default language of any such
human-readable XML character data.
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The value of the 'xml:lang' attribute MAY be overridden by the 'xml:
lang' attribute of a specific child element.
If an outbound stanza generated by a client does not possess an 'xml:
lang' attribute, the client's server SHOULD add an 'xml:lang'
attribute whose value is that specified for the client's output
stream as defined under
Section 4.7.4
C:
S:
xml:lang='en'>
If an inbound stanza received by a client or server does not possess
an 'xml:lang' attribute, an implementation MUST assume that the
default language is that specified for the entity's input stream as
defined under
Section 4.7.4
The value of the 'xml:lang' attribute MUST conform to the NMTOKEN
datatype (as defined in Section 2.3 of [
XML
]) and MUST conform to the
format defined in [
LANGTAGS
].
A server MUST NOT modify or delete 'xml:lang' attributes on stanzas
it receives from other entities.
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8.2
. Basic Semantics
8.2.1
. Message Semantics
The
information to another entity, similar to the communications that
occur in a system such as email. All message stanzas will possess a
'to' attribute that specifies the intended recipient of the message
(see
Section 8.1.1
and
Section 10.3
), unless the message is being
sent to the bare JID of a connected client's account. Upon receiving
a message stanza with a 'to' address, a server SHOULD attempt to
route or deliver it to the intended recipient (see
Section 10
for
general routing and delivery rules related to XML stanzas).
8.2.2
. Presence Semantics
The
subscribe" mechanism, whereby multiple entities receive information
(in this case, network availability information) about an entity to
which they have subscribed. In general, a publishing client SHOULD
send a presence stanza with no 'to' attribute, in which case the
server to which the client is connected will broadcast that stanza to
all subscribed entities. However, a publishing client MAY also send
a presence stanza with a 'to' attribute, in which case the server
will route or deliver that stanza to the intended recipient.
Although the
it can also be used by servers, add-on services, and any other kind
of XMPP entity. See
Section 10
for general routing and delivery
rules related to XML stanzas, and [
XMPP-IM
] for rules specific to
presence applications.
8.2.3
. IQ Semantics
Info/Query, or IQ, is a "request-response" mechanism, similar in some
ways to the Hypertext Transfer Protocol [
HTTP
]. The semantics of IQ
enable an entity to make a request of, and receive a response from,
another entity. The data content of the request and response is
defined by the schema or other structural definition associated with
the XML namespace that qualifies the direct child element of the IQ
element (see
Section 8.4
), and the interaction is tracked by the
requesting entity through use of the 'id' attribute. Thus, IQ
interactions follow a common pattern of structured data exchange such
as get/result or set/result (although an error can be returned in
reply to a request if appropriate):
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Requesting Responding
Entity Entity
---------- ----------
| |
|
| [ ... payload ... ] |
|
| -------------------------> |
| |
|
| [ ... payload ... ] |
|
| <------------------------- |
| |
|
| [ ... payload ... ] |
|
| -------------------------> |
| |
|
| [ ... condition ... ] |
|
| <------------------------- |
| |
Figure 5: Semantics of IQ Stanzas
To enforce these semantics, the following rules apply:
1. The 'id' attribute is REQUIRED for IQ stanzas.
2. The 'type' attribute is REQUIRED for IQ stanzas. The value MUST
be one of the following; if not, the recipient or an intermediate
router MUST return a
Section 8.3.3.1
).
* get -- The stanza requests information, inquires about what
data is needed in order to complete further operations, etc.
* set -- The stanza provides data that is needed for an
operation to be completed, sets new values, replaces existing
values, etc.
* result -- The stanza is a response to a successful get or set
request.
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* error -- The stanza reports an error that has occurred
regarding processing or delivery of a get or set request (see
Section 8.3
).
3. An entity that receives an IQ request of type "get" or "set" MUST
reply with an IQ response of type "result" or "error". The
response MUST preserve the 'id' attribute of the request (or be
empty if the generated stanza did not include an 'id' attribute).
4. An entity that receives a stanza of type "result" or "error" MUST
NOT respond to the stanza by sending a further IQ response of
type "result" or "error"; however, the requesting entity MAY send
another request (e.g., an IQ of type "set" to provide obligatory
information discovered through a get/result pair).
5. An IQ stanza of type "get" or "set" MUST contain exactly one
child element, which specifies the semantics of the particular
request.
6. An IQ stanza of type "result" MUST include zero or one child
elements.
7. An IQ stanza of type "error" MAY include the child element
contained in the associated "get" or "set" and MUST include an
Section 8.3
8.3
. Stanza Errors
Stanza-related errors are handled in a manner similar to stream
errors (
Section 4.9
). Unlike stream errors, stanza errors are
recoverable; therefore, they do not result in termination of the XML
stream and underlying TCP connection. Instead, the entity that
discovers the error condition returns an error stanza, which is a
stanza that:
o is of the same kind (message, presence, or IQ) as the generated
stanza that triggered the error
o has a 'type' attribute set to a value of "error"
o typically swaps the 'from' and 'to' addresses of the generated
stanza
o mirrors the 'id' attribute (if any) of the generated stanza that
triggered the error
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o contains an
condition and therefore provides a hint regarding actions that the
sender might be able to take in an effort to remedy the error
(however, it is not always possible to remedy the error)
8.3.1
. Rules
The following rules apply to stanza errors:
1. The receiving or processing entity that detects an error
condition in relation to a stanza SHOULD return an error stanza
(and MUST do so for IQ stanzas).
2. The error stanza SHOULD simply swap the 'from' and 'to' addresses
from the generated stanza, unless doing so would (1) result in an
information leak (see under
Section 13.10
) or other breach of
security, or (2) force the sender of the error stanza to include
a malformed JID in the 'from' or 'to' address of the error
stanza.
3. If the generated stanza was
included an 'id' attribute then it is REQUIRED for the error
stanza to also include an 'id' attribute. If the generated
stanza was
attribute. In all cases, the value of the 'id' attribute MUST
match that of the generated stanza (or be empty if the generated
stanza did not include an 'id' attribute).
4. An error stanza MUST contain an
5. The entity that returns an error stanza MAY pass along its JID to
the sender of the generated stanza (e.g., for diagnostic or
tracking purposes) through the addition of a 'by' attribute to
the
6. The entity that returns an error stanza MAY include the original
XML sent so that the sender can inspect and, if necessary,
correct the XML before attempting to resend (however, this is a
courtesy only and the originating entity MUST NOT depend on
receiving the original payload). Naturally, the entity MUST NOT
include the original data if it not well-formed XML, violates the
XML restrictions of XMPP (see under
Section 11.1
), or is
otherwise harmful (e.g., exceeds a size limit).
7. An
has a value other than "error" (or if there is no 'type'
attribute).
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8. An entity that receives an error stanza MUST NOT respond to the
stanza with a further error stanza; this helps to prevent
looping.
8.3.2
. Syntax
The syntax for stanza-related errors is as follows, where XML data
shown within the square brackets '[' and ']' is OPTIONAL, 'intended-
recipient' is the JID of the entity to which the original stanza was
addressed, 'sender' is the JID of the originating entity, and 'error-
generator' is the entity that detects the fact that an error has
occurred and thus returns an error stanza.
[OPTIONAL to include sender XML here]
[
OPTIONAL descriptive text
[OPTIONAL application-specific condition element]
The "stanza-kind" MUST be one of message, presence, or iq.
The "error-type" MUST be one of the following:
o auth -- retry after providing credentials
o cancel -- do not retry (the error cannot be remedied)
o continue -- proceed (the condition was only a warning)
o modify -- retry after changing the data sent
o wait -- retry after waiting (the error is temporary)
The "defined-condition" MUST correspond to one of the stanza error
conditions defined under
Section 8.3.3
. However, because additional
error conditions might be defined in the future, if an entity
receives a stanza error condition that it does not understand then it
MUST treat the unknown condition as equivalent to
Section 8.3.3.21
). If the designers of an XMPP protocol
extension or the developers of an XMPP implementation need to
communicate a stanza error condition that is not defined in this
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specification, they can do so by defining an application-specific
error condition element qualified by an application-specific
namespace.
The
o MUST contain a defined condition element.
o MAY contain a
that describes the error in more detail; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace
and SHOULD possess an 'xml:lang' attribute specifying the natural
language of the XML character data.
o MAY contain a child element for an application-specific error
condition; this element MUST be qualified by an application-
specific namespace that defines the syntax and semantics of the
element.
The
to provide descriptive or diagnostic information that supplements the
meaning of a defined condition or application-specific condition. It
MUST NOT be interpreted programmatically by an application. It
SHOULD NOT be used as the error message presented to a human user,
but MAY be shown in addition to the error message associated with the
defined condition element (and, optionally, the application-specific
condition element).
Interoperability Note: The syntax defined in [
RFC3920
] included a
legacy 'code' attribute, whose semantics have been replaced by the
defined condition elements; information about mapping defined
condition elements to values of the legacy 'code' attribute can be
found in [
XEP-0086
].
8.3.3
. Defined Conditions
The following conditions are defined for use in stanza errors.
The error-type value that is RECOMMENDED for each defined condition
is the usual expected type; however, in some circumstances a
different type might be more appropriate.
8.3.3.1
. bad-request
The sender has sent a stanza containing XML that does not conform to
the appropriate schema or that cannot be processed (e.g., an IQ
stanza that includes an unrecognized value of the 'type' attribute,
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or an element that is qualified by a recognized namespace but that
violates the defined syntax for the element); the associated error
type SHOULD be "modify".
C:
to='im.example.com'
type='subscribe'>
S:
to='juliet@im.example.com/balcony'
type='error'>
8.3.3.2
. conflict
Access cannot be granted because an existing resource exists with the
same name or address; the associated error type SHOULD be "cancel".
C:
S:
8.3.3.3
. feature-not-implemented
The feature represented in the XML stanza is not implemented by the
intended recipient or an intermediate server and therefore the stanza
cannot be processed (e.g., the entity understands the namespace but
does not recognize the element name); the associated error type
SHOULD be "cancel" or "modify".
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C:
to='pubsub.example.com'
type='get'>
E:
to='juliet@im.example.com/balcony'
type='error'>
feature='retrieve-subscriptions'/>
8.3.3.4
. forbidden
The requesting entity does not possess the necessary permissions to
perform an action that only certain authorized roles or individuals
are allowed to complete (i.e., it typically relates to authorization
rather than authentication); the associated error type SHOULD be
"auth".
C:
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
E:
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
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8.3.3.5
. gone
The recipient or server can no longer be contacted at this address,
typically on a permanent basis (as opposed to the
condition, which is used for temporary addressing failures); the
associated error type SHOULD be "cancel" and the error stanza SHOULD
include a new address (if available) as the XML character data of the
URI
] or
Internationalized Resource Identifier [
IRI
] at which the entity can
be contacted, typically an XMPP IRI as specified in [
XMPP-URI
]).
C:
id='sj2b371v'
to='romeo@example.net'
type='chat'>
Thy lips are warm.
S:
id='sj2b371v'
to='juliet@im.example.com/churchyard'
type='error'>
xmpp:romeo@afterlife.example.net
8.3.3.6
. internal-server-error
The server has experienced a misconfiguration or other internal error
that prevents it from processing the stanza; the associated error
type SHOULD be "cancel".
C:
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
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E:
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
8.3.3.7
. item-not-found
The addressed JID or item requested cannot be found; the associated
error type SHOULD be "cancel".
C:
to='nosuchroom@conference.example.org/foo'/>
S:
to='userfoo@example.com/bar'
type='error'>
Security Warning: An application MUST NOT return this error if
doing so would provide information about the intended recipient's
network availability to an entity that is not authorized to know
such information (for a more detailed discussion of presence
authorization, refer to the discussion of presence subscriptions
in [
XMPP-IM
]); instead it MUST return a
stanza error (
Section 8.3.3.19
).
8.3.3.8
. jid-malformed
The sending entity has provided (e.g., during resource binding) or
communicated (e.g., in the 'to' address of a stanza) an XMPP address
or aspect thereof that violates the rules defined in [
XMPP-ADDR
]; the
associated error type SHOULD be "modify".
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C:
id='y2bs71v4'
to='ch@r@cters@muc.example.com/JulieC'>
E:
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
Implementation Note: Enforcement of the format for XMPP localparts
is primarily the responsibility of the service at which the
associated account or entity is located (e.g., the example.com
service is responsible for returning
related to all JIDs of the form
enforcement of the format for XMPP domainparts is primarily the
responsibility of the service that seeks to route a stanza to the
service identified by that domainpart (e.g., the example.org
service is responsible for returning
related to stanzas that users of that service have to tried send
to JIDs of the form
that detects a malformed JID MAY return this error.
8.3.3.9
. not-acceptable
The recipient or server understands the request but cannot process it
because the request does not meet criteria defined by the recipient
or server (e.g., a request to subscribe to information that does not
simultaneously include configuration parameters needed by the
recipient); the associated error type SHOULD be "modify".
C:
[ ... the-emacs-manual ... ]
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S:
8.3.3.10
. not-allowed
The recipient or server does not allow any entity to perform the
action (e.g., sending to entities at a blacklisted domain); the
associated error type SHOULD be "cancel".
C:
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
E:
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
8.3.3.11
. not-authorized
The sender needs to provide credentials before being allowed to
perform the action, or has provided improper credentials (the name
"not-authorized", which was borrowed from the "401 Unauthorized"
error of [
HTTP
], might lead the reader to think that this condition
relates to authorization, but instead it is typically used in
relation to authentication); the associated error type SHOULD be
"auth".
C:
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
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E:
id='y2bs71v4'
to='juliet@im.example.com/balcony'>
8.3.3.12
. policy-violation
The entity has violated some local service policy (e.g., a message
contains words that are prohibited by the service) and the server MAY
choose to specify the policy in the
application-specific condition element; the associated error type
SHOULD be "modify" or "wait" depending on the policy being violated.
(In the following example, the client sends an XMPP message
containing words that are forbidden according to the server's local
service policy.)
C:
id='vq71f4nb'>
%#&@^!!!
S:
to='romeo@example.net/foo'>
8.3.3.13
. recipient-unavailable
The intended recipient is temporarily unavailable, undergoing
maintenance, etc.; the associated error type SHOULD be "wait".
C:
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
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E:
id='y2bs71v4'
to='juliet@im.example.com/balcony'>
Security Warning: An application MUST NOT return this error if
doing so would provide information about the intended recipient's
network availability to an entity that is not authorized to know
such information (for a more detailed discussion of presence
authorization, refer to the discussion of presence subscriptions
in [
XMPP-IM
]); instead it MUST return a
stanza error (
Section 8.3.3.19
).
8.3.3.14
. redirect
The recipient or server is redirecting requests for this information
to another entity, typically in a temporary fashion (as opposed to
the
failures); the associated error type SHOULD be "modify" and the error
stanza SHOULD contain the alternate address in the XML character data
of the
sender can communicate, typically an XMPP IRI as specified in
XMPP-URI
]).
C:
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
E:
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
xmpp:characters@conference.example.org
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Security Warning: An application receiving a stanza-level redirect
SHOULD warn a human user of the redirection attempt and request
approval before proceeding to communicate with the entity whose
address is contained in the XML character data of the
element, because that entity might have a different identity or
might enforce different security policies. The end-to-end
authentication or signing of XMPP stanzas could help to mitigate
this risk, since it would enable the sender to determine if the
entity to which it has been redirected has the same identity as
the entity it originally attempted to contact. An application MAY
have a policy of following redirects only if it has authenticated
the receiving entity. In addition, an application SHOULD abort
the communication attempt after a certain number of successive
redirects (e.g., at least 2 but no more than 5).
8.3.3.15
. registration-required
The requesting entity is not authorized to access the requested
service because prior registration is necessary (examples of prior
registration include members-only rooms in XMPP multi-user chat
XEP-0045
] and gateways to non-XMPP instant messaging services, which
traditionally required registration in order to use the gateway
XEP-0100
]); the associated error type SHOULD be "auth".
C:
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
E:
id='y2bs71v4'
to='juliet@im.example.com/balcony'>
8.3.3.16
. remote-server-not-found
A remote server or service specified as part or all of the JID of the
intended recipient does not exist or cannot be resolved (e.g., there
is no _xmpp-server._tcp DNS SRV record, the A or AAAA fallback
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resolution fails, or A/AAAA lookups succeed but there is no response
on the IANA-registered port 5269); the associated error type SHOULD
be "cancel".
C:
id='ud7n1f4h'
to='bar@example.org'
type='chat'>
yt?
E:
id='ud7n1f4h'
to='romeo@example.net/home'
type='error'>
8.3.3.17
. remote-server-timeout
A remote server or service specified as part or all of the JID of the
intended recipient (or needed to fulfill a request) was resolved but
communications could not be established within a reasonable amount of
time (e.g., an XML stream cannot be established at the resolved IP
address and port, or an XML stream can be established but stream
negotiation fails because of problems with TLS, SASL, Server
Dialback, etc.); the associated error type SHOULD be "wait" (unless
the error is of a more permanent nature, e.g., the remote server is
found but it cannot be authenticated or it violates security
policies).
C:
id='ud7n1f4h'
to='bar@example.org'
type='chat'>
yt?
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E:
id='ud7n1f4h'
to='romeo@example.net/home'
type='error'>
8.3.3.18
. resource-constraint
The server or recipient is busy or lacks the system resources
necessary to service the request; the associated error type SHOULD be
"wait".
C:
to='pubsub.example.com'
type='get'>
E:
to='romeo@example.net/foo'
type='error'>
8.3.3.19
. service-unavailable
The server or recipient does not currently provide the requested
service; the associated error type SHOULD be "cancel".
C:
Hello?
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S:
Security Warning: An application MUST return a
Section 8.3.3.19
) instead of
Section 8.3.3.7
) or
Section 8.3.3.13
) if sending one of the latter errors would
provide information about the intended recipient's network
availability to an entity that is not authorized to know such
information (for a more detailed discussion of presence
authorization, refer to [
XMPP-IM
]).
8.3.3.20
. subscription-required
The requesting entity is not authorized to access the requested
service because a prior subscription is necessary (examples of prior
subscription include authorization to receive presence information as
defined in [
XMPP-IM
] and opt-in data feeds for XMPP publish-subscribe
as defined in [
XEP-0060
]); the associated error type SHOULD be
"auth".
C:
id='pa73b4n7'
to='playwright@shakespeare.example.com'
type='chat'>
help, I forgot my lines!
E:
id='pa73b4n7'
to='romeo@example.net/orchard'
type='error'>
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8.3.3.21
. undefined-condition
The error condition is not one of those defined by the other
conditions in this list; any error type can be associated with this
condition, and it SHOULD NOT be used except in conjunction with an
application-specific condition.
C:
id='richard2-4.1.247'
to='kingrichard@royalty.england.example'>
My lord, dispatch; read o'er these articles.
value='stored'/>
S:
to='northumberland@example.net/field'
type='error'>
status='error'
to='northumberland@example.net/field'>
value='stored'/>
value='stored'/>
8.3.3.22
. unexpected-request
The recipient or server understood the request but was not expecting
it at this time (e.g., the request was out of order); the associated
error type SHOULD be "wait" or "modify".
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C:
to='pubsub.example.com'
type='set'>
jid='romeo@example.net'/>
E:
to='romeo@example.net/foo'
type='error'>
8.3.4
. Application-Specific Conditions
As noted, an application MAY provide application-specific stanza
error information by including a properly namespaced child within the
error element. Typically, the application-specific element
supplements or further qualifies a defined element. Thus, the
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[ ... application-specific information ... ]
An entity that receives an application-specific error condition it
does not understand MUST ignore that condition but appropriately
process the rest of the error stanza.
8.4
. Extended Content
Although the message, presence, and IQ stanzas provide basic
semantics for messaging, availability, and request-response
interactions, XMPP uses XML namespaces (see [
XML-NAMES
]) to extend
the basic stanza syntax for the purpose of providing additional
functionality.
A message or presence stanza MAY contain one or more optional child
elements specifying content that extends the meaning of the message
(e.g., an XHTML-formatted version of the message body as described in
XEP-0071
]), and an IQ stanza of type "get" or "set" MUST contain one
such child element. Such a child element MAY have any name and MUST
possess a namespace declaration (other than "jabber:client", "jabber:
server", or "http://etherx.jabber.org/streams") that defines the data
contained within the child element. Such a child element is called
an "extension element". An extension element can be included either
at the direct child level of the stanza or in any mix of levels.
Similarly, "extension attributes" are allowed. That is: a stanza
itself (i.e., an
by the "jabber:client" or "jabber:server" content namespace) or any
child element of such a stanza (whether an extension element or a
child element qualified by the content namespace) MAY also include
one or more attributes qualified by XML namespaces other than the
content namespace or the reserved
"http://www.w3.org/XML/1998/namespace" namespace (including the so-
called "empty namespace" if the attribute is not prefixed as
described under [
XML-NAMES
]).
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Interoperability Note: For the sake of backward compatibility and
maximum interoperability, an entity that generates a stanza SHOULD
NOT include such attributes in the stanza itself or in child
elements of the stanza that are qualified by the content
namespaces "jabber:client" or "jabber:server" (e.g., the