RFC 8684 - TCP Extensions for Multipath Operation with Multiple Addresses
RFC 8684
Multipath TCP
March 2020
Ford, et al.
Standards Track
[Page]
Stream:
Internet Engineering Task Force (IETF)
RFC:
8684
Obsoletes:
6824
Category:
Standards Track
Published:
March 2020
ISSN:
2070-1721
Authors:
A. Ford
Pexip
C. Raiciu
U. Politehnica of Bucharest
M. Handley
U. College London
O. Bonaventure
U. catholique de Louvain
C. Paasch
Apple, Inc.
RFC 8684
TCP Extensions for Multipath Operation with Multiple Addresses
Abstract
TCP/IP communication is currently restricted to a single path per connection, yet multiple paths often exist between peers. The simultaneous use of these multiple paths for a TCP/IP session would improve resource usage within the network and thus improve user experience through higher throughput and improved resilience to network failure.
Multipath TCP provides the ability to simultaneously use multiple
paths between peers. This document presents a set of extensions to
traditional TCP to support multipath operation. The protocol offers the
same type of service to applications as TCP (i.e., a reliable bytestream), and it provides the components necessary to establish and use multiple TCP flows across potentially disjoint paths.
This document specifies v1 of Multipath TCP, obsoleting v0 as
specified in RFC 6824, through clarifications and modifications primarily driven by deployment experience.
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 7841.
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) 2020 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.
Table of Contents
1.
Introduction
Multipath TCP (MPTCP) is a set of extensions to regular TCP
RFC0793
to provide a Multipath TCP service
RFC6182
, which enables a transport connection to operate across multiple paths
simultaneously. This document presents the protocol changes required to add
multipath capability to TCP -- specifically, those for signaling and setting
up multiple paths ("subflows"), managing these subflows, reassembly of data,
and termination of sessions. This is not the only information required to create a Multipath TCP implementation, however. This document is complemented by three others:
RFC6182
(MPTCP architecture), which
explains the motivations behind Multipath TCP, contains a discussion
of high-level design decisions on which this design is based, and provides an explanation of a functional separation through which an extensible MPTCP implementation can be developed.
RFC6356
(congestion control), which presents a safe congestion control algorithm for coupling the behavior of the multiple paths in order to "do no harm" to other network users.
RFC6897
(application considerations), which discusses what impact MPTCP will have on applications, what applications will want to do with MPTCP, and as a consequence of these factors, what API extensions an MPTCP implementation should present.
This document obsoletes the v0 specification of
Multipath TCP
RFC6824
. This document specifies MPTCP v1, which is not backward compatible with MPTCP v0. This document additionally defines version negotiation procedures for implementations that support both versions.
1.1.
Design Assumptions
In order to limit the potentially huge design space, the
MPTCP Working Group imposed two key constraints on the Multipath TCP design presented in this document:
It must be backward compatible with current, regular TCP, to increase its chances of deployment.
It can be assumed that one or both hosts are multihomed and multiaddressed.
To simplify the design, we assume that the presence of multiple
addresses at a host is sufficient to indicate the existence of
multiple paths. These paths need not be entirely disjoint: they may
share one or many routers between them. Even in such a situation,
making use of multiple paths is beneficial, improving resource
utilization and resilience to a subset of node failures. The
congestion control algorithm defined in
RFC6356
ensures that the use of multiple paths does not act detrimentally.
Furthermore, there may be some scenarios where different TCP ports on a
single host can provide disjoint paths (such as through certain
Equal-Cost Multipath (ECMP) implementations
RFC2992
), and so the MPTCP design also supports the use of
ports in path identifiers.
There are three aspects to the backward compatibility listed above (discussed in more detail in
RFC6182
):
External Constraints:
The protocol must function through the vast majority of existing
middleboxes such as NATs, firewalls, and proxies, and as such must resemble existing TCP as far as possible on the
wire. Furthermore, the protocol must not assume that the segments it sends on the wire arrive unmodified at the destination:
they may be split or coalesced; TCP options may be removed or duplicated.
Application Constraints:
The protocol must be usable with no change to existing applications that use the common TCP API (although it is reasonable that not all features would be available to such legacy applications). Furthermore, the protocol must provide the same service model as regular TCP to the application.
Fallback:
The protocol should be able to fall back to standard TCP with no interference from the user, to be able to communicate with legacy hosts.
The complementary application considerations document
RFC6897
discusses the necessary features
of an API to provide backward compatibility, as well as API extensions to convey the behavior of MPTCP at a level of control and information equivalent to that available with regular, single-path TCP.
Further discussion of the design constraints and associated design decisions is given in the MPTCP architecture document
RFC6182
and in
howhard
1.2.
Multipath TCP in the Networking Stack
MPTCP operates at the transport layer and aims to be transparent to both higher and lower
layers. It is a set of additional features on top of standard TCP;
Figure 1
illustrates
this layering. MPTCP is designed to be usable by legacy applications with no changes; detailed discussion
of its interactions with applications is given in
RFC6897
+-------------------------------+
| Application |
+---------------+ +-------------------------------+
| Application | | MPTCP |
+---------------+ + - - - - - - - + - - - - - - - +
| TCP | | Subflow (TCP) | Subflow (TCP) |
+---------------+ +-------------------------------+
| IP | | IP | IP |
+---------------+ +-------------------------------+
Figure 1
Comparison of Standard TCP and MPTCP Protocol Stacks
1.3.
Terminology
This document makes use of a number of terms that are either MPTCP specific or have defined meaning in the context of MPTCP, as follows:
Path:
A sequence of links between a sender and a receiver, defined in this context by a 4-tuple of source and destination address/port pairs.
Subflow:
A flow of TCP segments operating over an individual path, which forms part of a larger MPTCP connection. A subflow is started and terminated similarly to a regular TCP connection.
(MPTCP) Connection:
A set of one or more subflows, over which an application can communicate between two hosts. There is a one‑to‑one mapping between a connection and an application socket.
Data-level:
The payload data is nominally transferred over a connection, which in turn is transported over subflows. Thus, the term "data-level" is synonymous with "connection-level", in contrast to "subflow-level", which refers to properties of an individual subflow.
Token:
A locally unique identifier given to a multipath connection by a host. May also be referred to as a "Connection ID".
Host:
An end host operating an MPTCP implementation, and either initiating or accepting an MPTCP connection.
In addition to these terms, note that MPTCP's interpretation of, and effect on, regular single-path TCP semantics are discussed in
Section 4
1.4.
MPTCP Concept
This section provides a high-level summary of normal
operation of MPTCP; this type of scenario is illustrated in
Figure 2
. A detailed description of how
MPTCP operates is given in
Section 3
Host A Host B
------------------------ ------------------------
Address A1 Address A2 Address B1 Address B2
---------- ---------- ---------- ----------
| | | |
| (initial connection setup) | |
|----------------------------------->| |
|<-----------------------------------| |
| | | |
| (additional subflow setup) |
| |--------------------->| |
| |<---------------------| |
| | | |
| | | |
Figure 2
Example MPTCP Usage Scenario
To a non-MPTCP-aware application, MPTCP will behave the same as normal TCP. Extended APIs could provide
additional control to MPTCP-aware applications
RFC6897
An application begins by opening a TCP socket in the normal way.
MPTCP signaling and operation are handled by the MPTCP implementation.
An MPTCP connection begins similarly to a regular TCP connection. This is
illustrated in
Figure 2
, where an MPTCP connection is established between
addresses A1 and B1 on Hosts A and B, respectively.
If extra paths are available, additional TCP sessions (termed MPTCP "subflows")
are created on these paths and are combined with the existing session, which continues
to appear as a single connection to the applications at both ends. The creation of the
additional TCP session is illustrated between Address A2 on Host A and Address B1 on
Host B.
MPTCP identifies multiple paths by the presence of multiple addresses
at hosts. Combinations of these multiple addresses equate to the additional paths.
In the example, other potential paths that could be set up are A1<->B2 and A2<->B2.
Although this additional session is shown as being initiated from A2, it could equally have
been initiated from B1 or B2.
The discovery and setup of additional subflows
will be achieved through a path management method; this document describes a mechanism
by which a host can initiate new subflows by using its own additional addresses or by
signaling its available addresses to the other host.
MPTCP adds connection-level sequence numbers to allow the reassembly of
segments arriving on multiple subflows with differing network delays.
Subflows are terminated as regular TCP connections, with a four‑way FIN
handshake. The MPTCP connection is terminated by a connection-level FIN.
1.5.
Requirements Language
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 BCP 14
RFC2119
RFC8174
when, and only when, they appear in all capitals, as
shown here.
2.
Operation Overview
This section presents a single description of common MPTCP operation, with reference to the protocol operation. This is a high-level overview of the key functions; the full specification follows in
Section 3
. Extensibility and negotiated features are not discussed here. Considerable reference is made to symbolic names of MPTCP options throughout this section -- these are subtypes of the IANA‑assigned MPTCP option (see
Section 7
), and their formats are defined in the detailed protocol specification provided in
Section 3
A Multipath TCP connection provides a bidirectional bytestream between two hosts communicating like normal TCP and thus does not require any change to the applications. However, Multipath TCP enables the hosts to use different paths with different IP addresses to exchange packets belonging to the MPTCP connection. A Multipath TCP connection appears like a normal TCP connection to an application. However, to the network layer, each MPTCP subflow looks like a regular TCP flow whose segments carry a new TCP option type. Multipath TCP manages the creation, removal, and utilization of these subflows to send data. The number of subflows that are managed within a Multipath TCP connection is not fixed, and it can fluctuate during the lifetime of the Multipath TCP connection.
All MPTCP operations are signaled with a TCP option -- a single numerical type for MPTCP, with "subtypes" for each MPTCP message. What follows is a summary of the purpose and rationale of these messages.
2.1.
Initiating an MPTCP Connection
This is the same signaling as for initiating a normal TCP connection, but the SYN, SYN/ACK, and initial ACK (and data) packets also carry the MP_CAPABLE option. This option has a variable length and serves multiple purposes. Firstly, it verifies whether the remote host supports Multipath TCP; secondly, this option allows the hosts to exchange some information to authenticate the establishment of additional subflows. Further details are given in
Section 3.1
Host A Host B
------ ------
MP_CAPABLE ->
[flags]
<- MP_CAPABLE
[B's key, flags]
ACK + MP_CAPABLE (+ data) ->
[A's key, B's key, flags, (data-level details)]
Retransmission of the ACK + MP_CAPABLE can occur if it is not known if it has been received. The following diagrams show all possible exchanges for the initial subflow setup to ensure this reliability.
Host A (with data to send immediately) Host B
------ ------
MP_CAPABLE ->
[flags]
<- MP_CAPABLE
[B's key, flags]
ACK + MP_CAPABLE + data ->
[A's key, B's key, flags, data-level details]
Host A (with data to send later) Host B
------ ------
MP_CAPABLE ->
[flags]
<- MP_CAPABLE
[B's key, flags]
ACK + MP_CAPABLE ->
[A's key, B's key, flags]
ACK + MP_CAPABLE + data ->
[A's key, B's key, flags, data-level details]
Host A Host B (sending first)
------ ------
MP_CAPABLE ->
[flags]
<- MP_CAPABLE
[B's key, flags]
ACK + MP_CAPABLE ->
[A's key, B's key, flags]
<- ACK + DSS + data
[data-level details]
2.2.
Associating a New Subflow with an Existing MPTCP Connection
The exchange of keys in the MP_CAPABLE handshake provides material that can be used to authenticate the endpoints when new subflows will be set up.
Additional subflows begin in the same way as initiating a normal TCP connection, but the SYN, SYN/ACK, and ACK packets also carry the MP_JOIN option.
Host A initiates a new subflow between one of its addresses and one
of Host B's addresses. The token -- generated from the key -- is used
to identify which MPTCP connection it is joining, and the Hash‑based
Message Authentication Code (HMAC) is used for authentication. The HMAC uses the keys exchanged in the MP_CAPABLE handshake and the random numbers (nonces) exchanged in these MP_JOIN options. MP_JOIN also contains flags and an Address ID that can be used to refer to the source address without the sender needing to know if it has been changed by a NAT. Further details are given in
Section 3.2
Host A Host B
------ ------
MP_JOIN ->
[B's token, A's nonce,
A's Address ID, flags]
<- MP_JOIN
[B's HMAC, B's nonce,
B's Address ID, flags]
ACK + MP_JOIN ->
[A's HMAC]
<- ACK
2.3.
Informing the Other Host about Another Potential Address
The set of IP addresses associated to a multihomed host may change during the lifetime of an MPTCP connection. MPTCP supports the addition and removal of addresses on a host both implicitly and explicitly. If Host A has established a subflow starting at address/port pair IP#-A1 and wants to open a second subflow starting at address/port pair IP#-A2, it simply initiates the establishment of the subflow as explained above. The remote host will then be implicitly informed about the new address.
In some circumstances, a host may want to advertise to the remote
host the availability of an address without establishing a new subflow
-- for example, when a NAT prevents setup in one direction. In the example below, Host A informs Host B about its alternative IP address/port pair (IP#-A2). Host B may later send an MP_JOIN to this new address. The ADD_ADDR option contains an HMAC to authenticate the address as having been sent from the originator of the connection. The receiver of this option echoes it back to the client to indicate successful receipt. Further details are given in
Section 3.4.1
Host A Host B
------ ------
ADD_ADDR ->
[Echo-flag=0,
IP#-A2,
IP#-A2's Address ID,
HMAC of IP#-A2]
<- ADD_ADDR
[Echo-flag=1,
IP#-A2,
IP#-A2's Address ID,
HMAC of IP#-A2]
There is a corresponding signal for address removal, making use of
the Address ID that is signaled in the ADD_ADDR handshake.
Further details are given in
Section 3.4.2
Host A Host B
------ ------
REMOVE_ADDR ->
[IP#-A2's Address ID]
2.4.
Data Transfer Using MPTCP
To ensure reliable, in-order delivery of data over subflows that may appear and disappear at any time, MPTCP uses a 64-bit Data Sequence Number (DSN) to number all data sent over the MPTCP connection. Each subflow has its own 32-bit sequence number space, utilizing the regular TCP sequence number header, and an MPTCP option maps the subflow sequence space to the data sequence space. In this way, data can be retransmitted on different subflows (mapped to the same DSN) in the event of failure.
The Data Sequence Signal (DSS) carries the Data Sequence Mapping. The Data Sequence Mapping consists of the subflow sequence number, data sequence number, and length for which this mapping is valid. This option can also carry a connection-level acknowledgment (the "Data ACK") for the received DSN.
With MPTCP, all subflows share the same receive buffer and advertise the same receive window. There are two levels of acknowledgment in MPTCP. Regular TCP acknowledgments are used on each subflow to acknowledge the reception of the segments sent over the subflow independently of their DSN. In addition, there are connection-level acknowledgments for the data sequence space. These acknowledgments track the advancement of the bytestream and slide the receive window.
Further details are given in
Section 3.3
Host A Host B
------ ------
DSS ->
[Data Sequence Mapping]
[Data ACK]
[Checksum]
2.5.
Requesting a Change in a Path's Priority
Hosts can indicate at initial subflow setup whether they wish the subflow to be used as a regular or backup path -- a backup path only being used if there are no regular paths available. During a connection, Host A can request a change in the priority of a subflow through the MP_PRIO signal to Host B. Further details are given in
Section 3.3.8
Host A Host B
------ ------
MP_PRIO ->
2.6.
Closing an MPTCP Connection
When a host wants to close an existing subflow but not the whole connection, it can initiate a regular TCP FIN/ACK exchange.
When Host A wants to inform Host B that it has no more data to send, it signals this "Data FIN" as part of the DSS (see above). It has the same semantics and behavior as a regular TCP FIN, but at the connection level. Once all the data on the MPTCP connection has been successfully received, this message is acknowledged at the connection level with a Data ACK. Further details are given in
Section 3.3.3
Host A Host B
------ ------
DSS ->
[Data FIN]
<- DSS
[Data ACK]
There is an additional method of connection closure, referred to as
"Fast Close", which is analogous to closing a single-path TCP
connection with a RST signal. The MP_FASTCLOSE signal is used to
indicate to the peer that the connection will be abruptly closed and
no data will be accepted anymore. This can be used on an ACK (which
ensures reliability of the signal) or a RST (which does not).
Both examples are shown in the following diagrams. Further details are given in
Section 3.5
Host A Host B
------ ------
ACK + MP_FASTCLOSE ->
[B's key]
[RST on all other subflows] ->
<- [RST on all subflows]
Host A Host B
------ ------
RST + MP_FASTCLOSE ->
[B's key] [on all subflows]
<- [RST on all subflows]
2.7.
Notable Features
It is worth highlighting that MPTCP's signaling has been designed with several key requirements in mind:
To cope with NATs on the path, addresses are referred to by Address IDs, in case the IP packet's source
address gets changed by a NAT. Setting up a new TCP flow is not possible if the receiver of the SYN is behind a NAT;
to allow subflows to be created when either end is behind a NAT, MPTCP uses the ADD_ADDR message.
MPTCP falls back to ordinary TCP if MPTCP operation is not
possible -- for example, if one host is not MPTCP capable or if a middlebox alters the payload. This is discussed in
Section 3.7
To address the threats identified in
RFC6181
, the following steps are taken: keys are sent in
the clear in the MP_CAPABLE messages; MP_JOIN messages are secured
with HMAC-SHA256 (
RFC2104
using
the algorithm in
RFC6234
) using those keys; and standard
TCP validity checks are made on the other messages (ensuring that
sequence numbers are in‑window
RFC5961
).
Residual threats to MPTCP v0 were identified in
RFC7430
, and those affecting the protocol (i.e., modifications to
ADD_ADDR) have been incorporated in this document.
Further discussion of security can be found in
Section 5
3.
MPTCP Operations: An Overview
This section describes the operation of MPTCP. The
subsections below discuss each key part of the protocol operation.
All MPTCP operations are signaled using optional TCP header fields. A single TCP option number ("Kind") has been assigned by IANA for MPTCP (see
Section 7
), and then individual messages will be determined by a "subtype", the values of which are also stored in an IANA registry (and are also listed in
Section 7
). As with all TCP options, the Length field is specified in bytes and includes the 2 bytes of Kind and Length.
Throughout this document, when reference is made to an MPTCP option by symbolic name, such as "MP_CAPABLE", this refers to a TCP option with the single MPTCP option type, and with the subtype value of the symbolic name as defined in
Section 7
. This subtype is a 4-bit field -- the first 4 bits of the option payload, as shown in
Figure 3
. The MPTCP messages are defined in the following sections.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-----------------------+
| Kind | Length |Subtype| |
+---------------+---------------+-------+ |
| Subtype-specific data |
| (variable length) |
+---------------------------------------------------------------+
Figure 3
MPTCP Option Format
Those MPTCP options associated with subflow initiation are used on
packets with the SYN flag set. Additionally, there is one MPTCP option
for signaling metadata to ensure that segmented data can be recombined for delivery to the application.
The remaining options, however, are signals that do not need to be on
a specific packet, such as those for signaling additional
addresses. While an implementation may desire to send MPTCP options as
soon as possible, it may not be possible to combine all desired options
(both those for MPTCP and for regular TCP, such as SACK (selective
acknowledgment)
RFC2018
) on a single
packet. Therefore, an implementation may choose to send duplicate ACKs
containing the additional signaling information. This changes the
semantics of a duplicate ACK; these are usually only sent as a signal of
a lost segment
RFC5681
in regular
TCP. Therefore, an MPTCP implementation receiving a duplicate ACK that
contains an MPTCP option
MUST NOT
treat it as a signal of
congestion. Additionally, an MPTCP implementation
SHOULD NOT
send more than two duplicate ACKs in a row for the purposes
of sending MPTCP options alone, in order to ensure that no middleboxes misinterpret this as a sign of congestion.
Furthermore, standard TCP validity checks (such as ensuring that the
sequence number and acknowledgment number are within the window)
MUST
be undertaken before processing any MPTCP signals, as described in
RFC5961
, and initial subflow sequence numbers
SHOULD
be generated according to the recommendations in
RFC6528
3.1.
Connection Initiation
Connection initiation begins with a SYN, SYN/ACK, ACK exchange
on a single path. Each packet
contains the Multipath Capable (MP_CAPABLE) MPTCP option
Figure 4
). This option declares its
sender capable of performing Multipath TCP and wishes to do
so on this particular connection.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-------+---------------+
| Kind | Length |Subtype|Version|A|B|C|D|E|F|G|H|
+---------------+---------------+-------+-------+---------------+
| Option Sender's Key (64 bits) |
| (if option Length > 4) |
| |
+---------------------------------------------------------------+
| Option Receiver's Key (64 bits) |
| (if option Length > 12) |
| |
+-------------------------------+-------------------------------+
| Data-Level Length (16 bits) | Checksum (16 bits, optional) |
+-------------------------------+-------------------------------+
Figure 4
Multipath Capable (MP_CAPABLE) Option
The MP_CAPABLE exchange in this specification (v1) is different than
that specified in v0. If a host supports multiple versions
of MPTCP, the sender of the MP_CAPABLE option
SHOULD
signal the
highest version number it supports. In return, in its MP_CAPABLE option,
the receiver will signal the version number it wishes to use, which
MUST
be equal to or lower than the version number indicated in the initial
MP_CAPABLE.
There is a caveat, though, with respect to this version negotiation with
old listeners that only support v0. A listener that supports v0 expects that
the MP_CAPABLE option in the SYN segment will include the initiator's
key. If, however,
the initiator already upgraded to v1, it won't include the key in the
SYN segment. Thus, the listener will ignore the MP_CAPABLE of this SYN segment
and reply with a SYN/ACK that does not include an MP_CAPABLE. The initiator
MAY
choose to immediately fall back to TCP or
MAY
choose to attempt a connection
using MPTCP v0 (if the initiator supports v0), in order to discover whether the
listener supports the earlier version of MPTCP. In general, an MPTCP v0 connection
will likely be preferred over a TCP connection; however, in a particular deployment scenario,
it may be known that the listener is unlikely to support MPTCP v0 and so the
initiator may prefer not to attempt a v0 connection. An initiator
MAY
cache
information for a peer about what version of MPTCP it supports, if any, and use
this information for future connection attempts.
The MP_CAPABLE option is of variable length, with different fields
included, depending on which packet the option is used on. The full
MP_CAPABLE option is shown in
Figure 4
The MP_CAPABLE option is carried on the SYN, SYN/ACK, and ACK packets that start the first subflow of an MPTCP connection, as well as the first packet that carries data, if the initiator wishes to send first. The data carried by each option is as follows, where A = initiator and B = listener.
SYN (A->B): only the first 4 octets (Length = 4).
SYN/ACK (B->A): B's key for this connection (Length = 12).
ACK (no data) (A->B): A's key followed by B's key (Length = 20).
ACK (with first data) (A->B): A's key followed by B's key followed by Data-Level Length, and optional Checksum (Length = 22 or 24).
The contents of the option are determined by the SYN and ACK flags of the packet, along with the option's Length field. In
Figure 4
, "Sender" and "Receiver" refer to the sender or receiver of the TCP packet (which can be either host).
The initial SYN, containing just the MP_CAPABLE header, is used
to define the version of MPTCP being requested and also to exchange
flags to negotiate connection features, as described later.
This option is used to declare the 64-bit keys that the end hosts
have generated for this MPTCP connection. These keys are used to
authenticate the addition of future subflows to this connection. This
is the only time the key will be sent in the clear on the wire (unless "Fast Close" (
Section 3.5
) is used); all future subflows will identify the connection using a 32-bit "token". This token is a cryptographic hash of this key. The algorithm for this process is dependent on the authentication algorithm selected; the method of selection is defined later in this section.
Upon reception of the initial SYN segment, a stateful server generates a random key and replies with a SYN/ACK. The key's method of generation is implementation specific. The key
MUST
be hard to guess, and it
MUST
be unique for the sending host across all its current MPTCP connections. Recommendations for generating random numbers for use in keys are given in
RFC4086
. Connections will be indexed at each host by the token (a one-way hash of the key). Therefore, an implementation will require a mapping from each token to the corresponding connection, and in turn to the keys for the connection.
There is a risk that two different keys will hash to the same
token. The risk of hash collisions is usually small, unless the host
is handling many tens of thousands of connections. Therefore, an
implementation
SHOULD
check its list of connection
tokens to ensure that there is no collision before sending its key,
and if there is, then it should generate a new key. This would,
however, be costly for a server with thousands of connections. The
subflow handshake mechanism (
Section 3.2
) will ensure that new subflows only join the
correct connection, however, through the cryptographic handshake, as
well as checking the connection tokens in both directions, and
ensuring that sequence numbers are in-window. So, in the worst case, if there was a token collision, the new subflow would not succeed, but the MPTCP connection would continue to provide a regular TCP service.
Since key generation is implementation specific, there is no
requirement that they simply be random numbers. An implementation is
free to exchange cryptographic material out of band and generate these
keys from this material, in order to provide additional mechanisms by which to verify the identity of the communicating entities. For example, an implementation could choose to link its MPTCP keys to those used in higher-layer TLS or SSH connections.
If the server behaves in a
stateless manner, it has to generate its own key in a verifiable
fashion. This verifiable way of generating the key can be done by
using a hash of the 4-tuple, sequence number, and a local secret
(similar to what is done for the TCP sequence number
RFC4987
).
It will thus be able to verify whether it is indeed the originator of
the key echoed back in the subsequent MP_CAPABLE option.
As for a stateful server, the tokens
SHOULD
be checked for uniqueness; however,
if uniqueness is not met and there is no way to generate an alternative verifiable
key, then the connection
MUST
fall back to using regular TCP by not sending an
MP_CAPABLE in the SYN/ACK.
The ACK carries both A's key and B's key. This is the first time that A's key is seen on the wire, although it is expected that A will have generated a key locally before the initial SYN. The echoing of B's key allows B to operate statelessly, as described above. Therefore, A's key must be delivered reliably to B, and in order to do this, the transmission of this packet must be made reliable.
If B has data to send first, then the reliable delivery of the
ACK + MP_CAPABLE is ensured by the receipt of this data with an
MPTCP Data Sequence Signal (DSS) option (
Section 3.3
) containing a DATA_ACK for the MP_CAPABLE (which is
the first octet of the data sequence space). If, however, A wishes to send data first, it has
two options to ensure the reliable delivery of the ACK + MP_CAPABLE. If
it immediately has data to send, then the first ACK (with data) would
also contain an MP_CAPABLE option with additional data parameters (the
Data-Level Length and optional Checksum as shown in
Figure 4
). If A does not immediately
have data to send, it
MUST
include the MP_CAPABLE on
the first ACK, but without the additional data parameters. When A does
have data to send, it must repeat the sending of the MP_CAPABLE option
from the first ACK, with additional data parameters. This MP_CAPABLE
option is used in place of the DSS and simply specifies (1) the Data-Level
Length of the payload and (2) the checksum (if the use of checksums is
negotiated). This is the minimal data required to establish an MPTCP
connection -- it allows validation of the payload, and given that it is the
first data, the Initial Data Sequence Number (IDSN) is also known (as
it is generated from the key, as described below). Conveying the keys
on the first data packet allows the TCP reliability mechanisms to
ensure that the packet is successfully delivered. The receiver will acknowledge this data at the connection level with a Data ACK, as if a DSS option has been received.
There could be situations where both A and B attempt to transmit
initial data at the same time. For example, if A did not initially
have data to send but then needed to transmit data before it had
received anything from B, it would use an MP_CAPABLE option with data
parameters (since it would not know if the MP_CAPABLE on the ACK was
received). In such a situation, B may also have transmitted data with
a DSS option, but it had not yet been received at A. Therefore, B has
received data with an MP_CAPABLE mapping after it has sent data with a
DSS option. To ensure that these situations can be handled, it follows that the data parameters in an MP_CAPABLE are semantically equivalent to those in a DSS option and can be used interchangeably. Similar situations could occur when the MP_CAPABLE with data is lost and retransmitted. Furthermore, in the case of TCP segmentation offloading, the MP_CAPABLE with data parameters may be duplicated across multiple packets, and implementations must also be able to cope with duplicate MP_CAPABLE mappings as well as duplicate DSS mappings.
Additionally, the MP_CAPABLE exchange allows the safe passage of
MPTCP options on SYN packets to be determined. If any of these options
are dropped, MPTCP will gracefully fall back to regular single-path
TCP, as documented in
Section 3.7
If at any point in the handshake either party thinks the MPTCP
negotiation is compromised -- for example, by a middlebox corrupting
the TCP options or by unexpected ACK numbers being present -- the host
MUST
stop using MPTCP and no longer include MPTCP options in future TCP packets. The other host will then also fall back to regular TCP using the fallback mechanism. Note that new subflows
MUST NOT
be established (using the process documented in
Section 3.2
) until a DSS option has been successfully received across the path (as documented in
Section 3.3
).
Like all MPTCP options, the MP_CAPABLE option starts with the Kind
and Length to specify the TCP option's kind and length. This
information is followed by the MP_CAPABLE option. The first 4 bits of
the first octet in the MP_CAPABLE option (
Figure 4
) define the MPTCP Option Subtype (see
Section 7
; for MP_CAPABLE, this value is
0x0), and the remaining 4 bits of this octet specify the MPTCP
version in use (for this specification, this value is 1).
The second octet is reserved for flags, allocated as follows:
A:
The leftmost bit, labeled "A",
SHOULD
be set to 1 to indicate "Checksum required", unless the system administrator has decided that checksums are not required (for example, if the environment is controlled and no middleboxes exist that might adjust the payload).
B:
The second bit, labeled "B", is an extensibility flag. It
MUST
be set to 0 for current implementations. This
flag will be used for an extensibility mechanism in a future specification, and the impact of this flag will be defined at a later date. It is expected, but not mandated, that this flag would be used as part of an alternative security mechanism that does not require a full version upgrade of the protocol but does require redefining some elements of the handshake. If receiving a message with the "B" flag set to 1 and this is not understood, then the MP_CAPABLE in this SYN
MUST
be silently ignored, which triggers a fallback to regular TCP; the sender is expected to retry with a format compatible with this legacy specification. Note that the length of the MP_CAPABLE option, and the meanings of bits "D" through "H", may be altered by setting B=1.
C:
The third bit, labeled "C", is set to 1 to indicate that the
sender of this option will not accept additional MPTCP subflows to
the source address and port, and therefore the receiver
MUST NOT
try to open any additional subflows toward this address
and port. This improves efficiency in situations where the
sender knows a restriction is in place -- for example, if the sender is behind a strict NAT or operating behind a legacy Layer 4 load balancer.
D through H:
The remaining bits, labeled "D" through "H", are used for
crypto algorithm negotiation. In this specification, only the
rightmost bit, labeled "H", is assigned. Bit "H" indicates the use
of HMAC-SHA256 (as defined in
Section 3.2
). An implementation that only supports this
method
MUST
set bit "H" to 1 and bits "D"
through "G" to 0.
A crypto algorithm
MUST
be specified. If flag bits "D" through "H" are all 0, the MP_CAPABLE option
MUST
be treated as invalid and ignored (that is, it must be treated as a regular TCP handshake).
The selection of the authentication algorithm also impacts the algorithm used to generate the token and the IDSN. In this specification, with only the SHA-256 algorithm (bit "H") specified and selected, the token
MUST
be a truncated (most significant 32 bits) SHA-256 hash
RFC6234
of the key. A different, 64-bit truncation (the least significant 64 bits) of the SHA-256 hash of the key
MUST
be used as the IDSN. Note that the key
MUST
be hashed in network byte order. Also note that the "least significant" bits
MUST
be the rightmost bits of the SHA-256 digest, as per
RFC6234
. Future specifications of the use of the crypto bits may choose to specify different algorithms for token and IDSN generation.
Both the crypto and checksum bits negotiate capabilities in similar
ways. For the "Checksum required" bit (labeled "A"), if either host
requires the use of checksums, checksums
MUST
be
used. In other words, the only way for checksums not to be used is if
both hosts in their SYNs set A=0. This decision is confirmed by the
setting of the "A" bit in the third packet (the ACK) of the
handshake. For example, if the initiator sets A=0 in the SYN but the
responder sets A=1 in the SYN/ACK, checksums
MUST
be
used in both directions, and the initiator will set A=1 in the
ACK. The decision regarding whether to use checksums will be stored by an implementation in a per-connection binary state variable. If A=1 is received by a host that does not want to use checksums, it
MUST
fall back to regular TCP by ignoring the MP_CAPABLE option as if it was invalid.
For crypto negotiation, the responder has the choice. The initiator
creates a proposal setting a bit for each algorithm it supports to 1
(in this version of the specification, there is only one proposal, so
bit "H" will always be set to 1). The responder responds with only 1 bit set -- this is the chosen algorithm. The rationale for this behavior is that the responder will typically be a server with potentially many thousands of connections, so it may wish to choose an algorithm with minimal computational complexity, depending on the load. If a responder does not support (or does not want to support) any of the initiator's proposals, it
MUST
respond without an MP_CAPABLE option, thus forcing a fallback to regular TCP.
The MP_CAPABLE option is only used in the first subflow of a
connection, in order to identify the connection; all subsequent
subflows will use the MP_JOIN option (see
Section 3.2
) to join the existing connection.
If a SYN contains an MP_CAPABLE option but the
SYN/ACK does not, it is assumed that the sender of the SYN/ACK is not
multipath capable; thus, the MPTCP session
MUST
operate as
a regular, single-path TCP session. If a SYN does not contain an
MP_CAPABLE option, the SYN/ACK
MUST NOT
contain one
in response. If the third packet (the ACK) does not contain
the MP_CAPABLE option, then the session
MUST
fall back to
operating as a regular, single-path TCP session. This is done to maintain
compatibility with middleboxes on the path that drop some
or all TCP options. Note that an implementation
MAY
choose
to attempt sending MPTCP options more than one time before
making this decision to operate as regular TCP (see
Section 3.9
).
If the SYN packets are unacknowledged, it is up to local
policy to decide how to respond. It is expected that a sender
will eventually fall back to single-path TCP (i.e., without the
MP_CAPABLE option) in order to work around middleboxes that
may drop packets with unknown options; however, the number of
multipath-capable attempts that are made first will be up to
local policy.
It is possible that MPTCP and non-MPTCP SYNs could get reordered
in the network. Therefore, the final state is inferred from the
presence or absence of the MP_CAPABLE option in the third packet
of the TCP handshake. If this option is not present, the
connection
SHOULD
fall back to regular TCP, as documented in
Section 3.7
The IDSN on an MPTCP connection
is generated from the key. The algorithm for IDSN generation is
also determined from the negotiated authentication algorithm.
In this specification, with only the SHA-256 algorithm specified and
selected, the IDSN of a host
MUST
be the least significant 64 bits of the
SHA-256 hash of its key, i.e., IDSN-A = Hash(Key-A) and IDSN-B = Hash(Key-B).
This deterministic generation of the IDSN allows a receiver to ensure
that there are no gaps in sequence space at the start of the connection.
The SYN with MP_CAPABLE occupies the first octet of data sequence space,
although this does not need to be acknowledged at the connection level
until the first data is sent (see
Section 3.3
).
3.2.
Starting a New Subflow
Once an MPTCP connection has begun with the MP_CAPABLE
exchange, further subflows can be added to the connection.
Hosts have knowledge of their own address(es) and can
become aware of the other host's addresses through
signaling exchanges as described in
Section 3.4
. Using this knowledge, a host
can initiate a new subflow over a currently unused pair of
addresses. It is permissible for either host in a connection
to initiate the creation of a new subflow, but it is expected
that this will normally be the original connection initiator
(see
Section 3.9
for heuristics).
A new subflow is started as a normal TCP SYN/ACK
exchange. The Join Connection (MP_JOIN) MPTCP option
is used to identify the connection to be joined by the new subflow.
It uses keying material that was exchanged in the initial MP_CAPABLE
handshake (
Section 3.1
), and that handshake also
negotiates the crypto algorithm in use for the MP_JOIN handshake.
This section specifies the behavior of MP_JOIN using the HMAC-SHA256
algorithm. An MP_JOIN option is present in the SYN, SYN/ACK,
and ACK of the three-way handshake, although in each case with a
different format.
In the first MP_JOIN on the SYN packet, illustrated in
Figure 5
, the initiator sends a token, random
number, and Address ID.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-----+-+---------------+
| Kind | Length = 12 |Subtype|(rsv)|B| Address ID |
+---------------+---------------+-------+-----+-+---------------+
| Receiver's Token (32 bits) |
+---------------------------------------------------------------+
| Sender's Random Number (32 bits) |
+---------------------------------------------------------------+
Figure 5
Join Connection (MP_JOIN) Option (for Initial SYN)
The token is used to identify the MPTCP connection and is a
cryptographic hash of the receiver's key, as exchanged
in the initial MP_CAPABLE handshake (
Section 3.1
).
In this specification, the tokens presented in this
option are generated by the SHA-256 algorithm
RFC6234
, truncated to the most significant 32 bits. The token
included in the MP_JOIN option is the token that the receiver
of the packet uses to identify this connection; i.e., Host A
will send Token-B (which is generated from Key-B). Note that the
hash generation algorithm can be overridden by the choice of
cryptographic handshake algorithm, as defined in
Section 3.1
The MP_JOIN SYN sends not only the token (which is static for a
connection) but also random numbers (nonces) that are used to prevent
replay attacks on the authentication method. Recommendations for the
generation of random numbers for this purpose are given in
RFC4086
The MP_JOIN option includes an "Address ID". This is an identifier
generated by the sender of the option, used to identify the source address
of this packet, even if the IP header has been changed in transit by a middlebox.
The numeric value of this field is generated by the sender and must map uniquely
to a source IP address for the sending host.
The Address ID allows address removal (
Section 3.4.2
without needing to know what the source address at the
receiver is, thus allowing address removal through NATs.
The Address ID also allows correlation between new subflow setup attempts
and address signaling (
Section 3.4.1
),
to prevent setting up duplicate subflows on the same path, if an MP_JOIN
and ADD_ADDR are sent at the same time.
The Address IDs of the subflow used in the initial SYN
exchange of the first subflow in the connection are implicit
and have the value zero. A host
MUST
store the mappings between
Address IDs and addresses both for itself and the remote host.
An implementation will also need to know which local and remote
Address IDs are associated with which established subflows, for
when addresses are removed from a local or remote host.
The MP_JOIN option on packets with the SYN flag set also includes
4 bits of flags, 3 of which are currently reserved and
MUST
be set to 0 by the sender. The final bit, labeled
"B", indicates whether the sender of this option (1) wishes this
subflow to be used as a backup path (B=1) in the event of failure of
other paths or (2) wants the subflow to be used as part of the
connection immediately. By setting B=1, the sender of the option is
requesting that the other host only send data on this subflow if there
are no available subflows where B=0. Subflow policy is discussed in more
detail in
Section 3.3.8
When receiving a SYN with an MP_JOIN option that contains
a valid token for an existing MPTCP connection, the recipient
SHOULD
respond with a SYN/ACK also containing an MP_JOIN
option containing a random number and a truncated (leftmost 64 bits) HMAC. This
version of the option is shown in
Figure 6
. If the token is unknown or the host wants to refuse subflow
establishment (for example, due to a limit on the number of
subflows it will permit), the receiver will send back a reset
(RST) signal, analogous to an unknown port in TCP, containing an
MP_TCPRST option (
Section 3.6
) with an "MPTCP
specific error" reason code. Although calculating an HMAC
requires cryptographic operations, it is believed that the
32-bit token in the MP_JOIN SYN gives sufficient protection against blind state
exhaustion attacks; therefore, there is no need to provide
mechanisms to allow a responder to operate statelessly at the
MP_JOIN stage.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-----+-+---------------+
| Kind | Length = 16 |Subtype|(rsv)|B| Address ID |
+---------------+---------------+-------+-----+-+---------------+
| |
| Sender's Truncated HMAC (64 bits) |
| |
+---------------------------------------------------------------+
| Sender's Random Number (32 bits) |
+---------------------------------------------------------------+
Figure 6
Join Connection (MP_JOIN) Option (for Responding SYN/ACK)
An HMAC is sent by both hosts -- by the initiator (Host A)
in the third packet (the ACK) and by the responder (Host B) in
the second packet (the SYN/ACK). Doing the HMAC exchange at this
stage allows both hosts to have first exchanged random data (in the
first two SYN packets) that is used as the "message". This
specification defines that HMAC as defined in
RFC2104
is used, along with the SHA-256 hash algorithm
RFC6234
and that the output is truncated to the leftmost 160 bits (20 octets).
Due to option space limitations, the HMAC included in
the SYN/ACK is truncated to the leftmost 64 bits, but this is
acceptable, since random numbers are used; thus, an attacker
only has one chance to correctly guess the HMAC that matches the random
number previously sent by the peer (if the HMAC is
incorrect, the TCP connection is closed, so a new MP_JOIN negotiation
with a new random number is required).
The initiator's authentication information is sent in its
first ACK (the third packet of the handshake), as shown in
Figure 7
. This data needs to be sent reliably,
since it is the only time this HMAC is sent;
therefore, receipt of this packet
MUST
trigger a regular TCP ACK
in response, and the packet
MUST
be retransmitted if this
ACK is not received. In other words, sending the ACK/MP_JOIN
packet places the subflow in the PRE_ESTABLISHED state, and it
moves to the ESTABLISHED state only on receipt of an ACK from
the receiver. It is not permissible to send data while in the
PRE_ESTABLISHED state. The reserved bits in this option
MUST
be set
to 0 by the sender.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-----------------------+
| Kind | Length = 24 |Subtype| (reserved) |
+---------------+---------------+-------+-----------------------+
| |
| |
| Sender's Truncated HMAC (160 bits) |
| |
| |
+---------------------------------------------------------------+
Figure 7
Join Connection (MP_JOIN) Option (for Initiator's First ACK)
The key for the HMAC algorithm, in the case of the message
transmitted by Host A, will be Key-A followed by Key-B; and in the
case of Host B, Key-B followed by Key-A. These are the keys that were
exchanged in the original MP_CAPABLE handshake. The "message" for the
HMAC algorithm in each case is the concatenations of random numbers for
each host (denoted by R): for Host A, R-A followed by R-B; and for
Host B, R-B followed by R-A.
These various MPTCP options fit together to enable authenticated subflow setup as illustrated in
Figure 8
Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| | SYN + MP_CAPABLE |
|--------------------------------------------->|
|<---------------------------------------------|
| SYN/ACK + MP_CAPABLE(Key-B) |
| | |
| ACK + MP_CAPABLE(Key-A, Key-B) |
|--------------------------------------------->|
| | |
| | SYN + MP_JOIN(Token-B, R-A) |
| |------------------------------->|
| |<-------------------------------|
| | SYN/ACK + MP_JOIN(HMAC-B, R-B) |
| | |
| | ACK + MP_JOIN(HMAC-A) |
| |------------------------------->|
| |<-------------------------------|
| | ACK |
HMAC-A = HMAC(Key=(Key-A + Key-B), Msg=(R-A + R-B))
HMAC-B = HMAC(Key=(Key-B + Key-A), Msg=(R-B + R-A))
Figure 8
Example Use of MPTCP Authentication
If the token received at Host B is unknown or local policy
prohibits the acceptance of the new subflow, the recipient
MUST
respond with a TCP RST for the subflow. If appropriate, an MP_TCPRST
option with an "Administratively prohibited" reason code
Section 3.6
) should be included.
If the token is accepted at Host B but the HMAC returned to
Host A does not match the one expected, Host A
MUST
close the
subflow with a TCP RST. In this and all subsequent cases of sending
a RST as described in this section, the sender
SHOULD
send an MP_TCPRST option
Section 3.6
) on this RST packet with the reason
code for an "MPTCP-specific error".
If Host B does not receive the expected HMAC or the MP_JOIN
option is missing from the ACK, it
MUST
close the subflow with a
TCP RST.
If the HMACs are verified as correct, then both hosts have
verified each other as being the same peers as those that existed at
the start of the connection, and they have agreed of which
connection this subflow will become a part.
If the SYN/ACK as received at Host A does not have an MP_JOIN
option, Host A
MUST
close the subflow with a TCP RST.
This covers all cases of the loss of an MP_JOIN. In more detail,
if an MP_JOIN is stripped from the SYN on the path from A to
B and Host B does not have a listener on the relevant
port, it will respond with a RST in the normal way. If in
response to a SYN with an MP_JOIN option a SYN/ACK is
received without the MP_JOIN option (because it was either
stripped on the return path, or stripped on the
outgoing path leading to Host B responding as if
it was a new regular TCP session), then the subflow is
unusable and Host A
MUST
close it with a RST.
Note that additional subflows can be created
between any pair of ports (but see
Section 3.9
for
heuristics); no explicit application-level accept calls or
bind calls are required to open additional subflows. To
associate a new subflow with an existing connection, the token
supplied in the subflow's SYN exchange is used for
demultiplexing. This then binds the 5-tuple of the TCP
subflow to the local token of the connection. One consequence is
that it is possible to allow any port pairs to be used for a
connection.
Demultiplexing subflow SYNs
MUST
be done using the token;
this is unlike traditional TCP, where the destination port is
used for demultiplexing SYN packets. Once a subflow is set up,
demultiplexing packets is done using the 5-tuple, as in
traditional TCP. The 5-tuples will be mapped to the local
connection identifier (token). Note that Host A will know its
local token for the subflow even though it is not sent on the
wire -- only the responder's token is sent.
3.3.
MPTCP Operation and Data Transfer
This section discusses the operation of MPTCP for data transfer. At a high level, an MPTCP implementation will take one input data stream from an application and split it into one or more subflows, with sufficient control information to allow it to be reassembled and delivered reliably and in order to the recipient application. The following subsections define this behavior in detail.
The Data Sequence Mapping and the Data ACK are signaled in the DSS option (
Figure 9
). Either or both can be signaled in one DSS, depending on the flags set. The Data Sequence Mapping defines how the sequence space on the subflow maps to the connection level, and the Data ACK acknowledges receipt of data at the connection level. These functions are described in more detail in the following two subsections.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+----------------------+
| Kind | Length |Subtype| (reserved) |F|m|M|a|A|
+---------------+---------------+-------+----------------------+
| Data ACK (4 or 8 octets, depending on flags) |
+--------------------------------------------------------------+
| Data Sequence Number (4 or 8 octets, depending on flags) |
+--------------------------------------------------------------+
| Subflow Sequence Number (4 octets) |
+-------------------------------+------------------------------+
| Data-Level Length (2 octets) | Checksum (2 octets) |
+-------------------------------+------------------------------+
Figure 9
Data Sequence Signal (DSS) Option
The flags, when set, define the contents of this option, as follows:
A = Data ACK present
a = Data ACK is 8 octets (if not set, Data ACK is 4 octets)
M = Data Sequence Number (DSN), Subflow Sequence Number (SSN), Data-Level Length, and Checksum (if negotiated) present
m = Data Sequence Number is 8 octets (if not set, DSN is 4 octets)
The flags "a" and "m" only have meaning if the corresponding "A" or "M" flags are set; otherwise, they will be ignored. The maximum length of this option, with all flags set, is 28 octets.
The "F" flag indicates "Data FIN". If present, this means that this
mapping covers the final data from the sender. This is the
connection-level equivalent of the FIN flag in single-path TCP. A connection is not closed unless there has been a Data FIN exchange, an MP_FASTCLOSE (
Section 3.5
) message, or an implementation-specific connection-level send timeout. The purpose of the Data FIN and the interactions between this flag, the subflow-level FIN flag, and the Data Sequence Mapping are described in
Section 3.3.3
The remaining reserved bits
MUST
be set to 0 by an implementation of this specification.
Note that the checksum is only present in this option if the use of
MPTCP checksumming has been negotiated at the MP_CAPABLE handshake
(see
Section 3.1
). The presence of the
checksum can be inferred from the length of the option. If a checksum
is present but its use had not been negotiated in the MP_CAPABLE
handshake, the receiver
MUST
close the subflow with a
RST, as it is not behaving as negotiated. If a checksum is not present when its use has been negotiated, the receiver
MUST
close the subflow with a RST, as it is considered broken. In both cases, this RST
SHOULD
be accompanied by an MP_TCPRST option (
Section 3.6
) with the reason code for an "MPTCP-specific error".
3.3.1.
Data Sequence Mapping
The data stream as a whole can be reassembled through the use of the Data Sequence Mapping components of the DSS option (
Figure 9
), which define the
mapping from the subflow sequence number to the data sequence number. This is
used by the receiver to ensure in-order delivery to the application
layer. Meanwhile, the subflow-level sequence numbers (i.e., the
regular sequence numbers in the TCP header) are only relevant to the subflow. It is expected (but not mandated) that SACK
RFC2018
will be used at the subflow level to improve efficiency.
The Data Sequence Mapping specifies a mapping from the subflow
sequence space to the data sequence space. This is expressed in terms of starting sequence numbers for the subflow and the data level, and a length of bytes for which this mapping is valid.
This explicit mapping for a range of data, rather than per‑packet signaling, was chosen to assist with compatibility with
situations where TCP/IP segmentation or coalescing is undertaken
separately from the stack that is generating the data flow (e.g.,
through the use of TCP segmentation offloading on network interface
cards, or by middleboxes such as Performance Enhancing Proxies
(PEPs)
RFC3135
). It
also allows a single mapping to cover many packets; this may be useful in bulk‑transfer situations.
A mapping is fixed, in that the subflow sequence number is bound to the data sequence number after the mapping has been processed. A sender
MUST NOT
change this mapping
after it has been declared; however, the same data sequence number can be
mapped to by different subflows for retransmission purposes (see
Section 3.3.6
). This would also
permit the same data to be sent simultaneously on multiple subflows
for resilience or efficiency purposes, especially in the case of
lossy links. Although the detailed specification of such operation
is outside the scope of this document, an implementation
SHOULD
treat the first data that is received at a
subflow for the data sequence space as the data that should be delivered to the application, and any subsequent data for that sequence space
SHOULD
be ignored.
The data sequence number is specified as an absolute value,
whereas the subflow sequence numbering is relative (the SYN at the
start of the subflow has a relative subflow sequence number of
0). This is done to allow middleboxes to change the Initial Sequence
Number (ISN) of a subflow, such as firewalls that undertake ISN randomization.
The Data Sequence Mapping also contains a checksum of the data
that this mapping covers, if the use of checksums has been negotiated at
the MP_CAPABLE exchange. Checksums are used to detect if the payload
has been adjusted in any way by a non-MPTCP-aware middlebox. If this
checksum fails, it will trigger a failure of the subflow, or a
fallback to regular TCP, as documented in
Section 3.7
, since MPTCP can no longer
reliably know the subflow sequence space at the receiver to build
Data Sequence Mappings. Without checksumming enabled, corrupt data
may be delivered to the application if a middlebox alters segment
boundaries, alters content, or does not deliver all segments covered
by a Data Sequence Mapping. It is therefore
RECOMMENDED
that checksumming be used, unless it is known
that the network path contains no such devices.
The checksum algorithm used is the standard TCP checksum
RFC0793
, operating over the data covered by this mapping, along with a pseudo‑header as shown in
Figure 10
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+--------------------------------------------------------------+
| |
| Data Sequence Number (8 octets) |
| |
+--------------------------------------------------------------+
| Subflow Sequence Number (4 octets) |
+-------------------------------+------------------------------+
| Data-Level Length (2 octets) | Zeros (2 octets) |
+-------------------------------+------------------------------+
Figure 10
Pseudo-Header for DSS Checksum
Note that the data sequence number used in the pseudo-header is always the 64-bit value, irrespective of what length is used in the DSS option itself. The standard TCP checksum algorithm has been chosen, since it will be calculated anyway for the TCP subflow, and if calculated first over the data before adding the pseudo-headers, it only needs to be calculated once. Furthermore, since the TCP checksum is additive, the checksum for a DSN_MAP can be constructed by simply adding together the checksums for the data of each constituent TCP segment and adding the checksum for the DSS pseudo‑header.
Note that checksumming relies on the TCP subflow containing contiguous data; therefore, a TCP subflow
MUST NOT
use the Urgent Pointer to interrupt an existing mapping. Further note, however, that if Urgent data is received on a subflow, it
SHOULD
be mapped to the data sequence space and delivered to the application, analogous to Urgent data in regular TCP.
To avoid possible deadlock scenarios, subflow-level
processing should be undertaken separately from processing at the
connection level. Therefore, even if a mapping does not exist
from the subflow space to the data‑level space, the data
SHOULD
still be ACKed at the subflow (if it is in-window).
This data cannot, however, be acknowledged at the data level
Section 3.3.2
) because its data sequence
numbers are unknown. Implementations
MAY
hold onto such
unmapped data for a short while, in the expectation that a
mapping will arrive shortly. Such unmapped data cannot be
counted as being within the connection-level receive window because this is
relative to the data sequence numbers, so if the receiver runs
out of memory to hold this data, it will have to be discarded.
If a mapping for that subflow-level sequence space does not
arrive within a receive window of data, that subflow
SHOULD
be
treated as broken, closed with a RST, and any unmapped data
silently discarded.
Data sequence numbers are always 64-bit quantities and
MUST
be maintained as such in implementations. If a
connection is progressing at a slow rate, so protection
against wrapped sequence numbers is not required,
then an implementation
MAY
include just the lower 32
bits of the data sequence number in the Data Sequence Mapping and/or
Data ACK as an optimization, and an implementation can make this choice
independently for each packet. An implementation
MUST
be able to receive
and process both 64-bit and 32-bit sequence number values, but it is not
required that an implementation be able to send both.
An implementation
MUST
send the full 64-bit data sequence number
if it is transmitting at a sufficiently high rate that the 32-bit value
could wrap within the Maximum Segment Lifetime
(MSL)
RFC7323
. The lengths of the DSNs used in these
values (which may be different) are declared with flags in the
DSS option. Implementations
MUST
accept a 32-bit DSN and implicitly
promote it to a 64-bit quantity by incrementing the upper 32
bits of the sequence number each time the lower 32
bits wrap. A sanity check
MUST
be implemented to ensure that
a wrap occurs at an expected time (e.g., the sequence number jumps
from a very high number to a very low number) and is not triggered
by out‑of-order packets.
As with the standard TCP sequence number, the data sequence
number should not start at zero, but at a random value to make
blind session hijacking harder. This specification requires
setting the IDSN of each host to the
least significant 64 bits of the SHA-256 hash of the host's key, as
described in
Section 3.1
. This is also required in
order for the receiver to know what the expected IDSN is and thus
determine if any initial connection-level packets are missing; this
is particularly relevant if two subflows start transmitting simultaneously.
The mapping provided by a Data Sequence Mapping MUST apply to
some or all of the subflow sequence space in the TCP segment that
carries the option. It does not need to be included in
every MPTCP packet, as long as the subflow sequence space in
that packet is covered by a mapping known at the receiver. This
can be used to reduce overhead in cases where the mapping is
known in advance. One such case is when there is a single
subflow between the hosts, and another is when segments of
data are scheduled in larger-than-packet-sized chunks.
An "infinite" mapping can be used to fall back to regular TCP by
mapping the subflow-level data to the connection-level data
for the remainder of the connection (see
Section 3.7
). This is achieved by setting
the Data-Level Length field of the DSS option to the reserved value of 0. The
checksum, in such a case, will also be set to 0.
3.3.2.
Data Acknowledgments
To provide full end-to-end resilience, MPTCP provides a
connection-level acknowledgment, to act as a cumulative ACK for
the connection as a whole. This is done via the "Data ACK" field of
the DSS option (
Figure 9
). The Data ACK
is analogous to the behavior
of the standard TCP cumulative ACK -- indicating
how much data has been successfully received (with no
holes). This can be compared to the subflow-level ACK, which
acts in a fashion analogous to TCP SACK, given that there may still be
holes in the data stream at the connection level.
The Data ACK specifies the next data sequence number
it expects to receive.
The Data ACK, as for the DSN, can be sent as the full 64-bit
value or as the lower 32 bits. If data is received with a 64-bit DSN,
it
MUST
be acknowledged with a 64-bit Data ACK. If the DSN received
is 32 bits, an implementation can choose whether to send a 32-bit or
64-bit Data ACK, and an implementation
MUST
accept either in this situation.
The Data ACK proves that the data, and all required MPTCP
signaling, have been received and accepted by the remote end.
One key use of the Data ACK signal is that it is used to indicate
the left edge of the advertised receive window. As explained in
Section 3.3.4
, the receive window is shared by all
subflows and is relative to the Data ACK. Because of this, an
implementation
MUST NOT
use the RCV.WND field of a TCP segment
at the connection level if it does not also carry a DSS option with
a Data ACK field. Furthermore,
separating the connection-level acknowledgments from the
subflow level allows processing to be done separately, and
a receiver has the freedom to drop segments after acknowledgment
at the subflow level -- for example, due to memory constraints
when many segments arrive out of order.
An MPTCP sender
MUST NOT
free data from the send buffer until
it has been acknowledged by both a Data ACK received on any subflow
and at the subflow level by all subflows on which the data was sent.
The former condition ensures liveness of the
connection, and the latter condition ensures liveness and
self-consistence of a subflow when data needs to be
retransmitted.
Note, however, that if some data needs to be retransmitted multiple
times over a subflow, there is a risk of blocking the send
window. In this case, the MPTCP sender can decide to terminate the
subflow that is behaving badly by sending a RST, using an appropriate
MP_TCPRST (
Section 3.6
) error code.
The Data ACK
MAY
be included in all segments; however, optimizations
SHOULD
be considered in more advanced implementations, where the
Data ACK is present in segments
only when the Data ACK value advances, and this behavior
MUST
be treated as valid. This behavior ensures that the send buffer
is freed, while reducing overhead when the data transfer is
unidirectional.
3.3.3.
Closing a Connection
In regular TCP, a FIN announces to the receiver that the sender has no more data to send.
In order to allow subflows to operate independently and to keep the appearance of TCP over the wire,
a FIN in MPTCP only affects the subflow on which it is sent. This
allows nodes to exercise considerable freedom over which paths are in use at any one time.
The semantics of a FIN remain as for regular TCP; i.e., it is not until both sides have ACKed
each other's FINs that the subflow is fully closed.
When an application calls close() on a socket, this indicates that it has no more
data to send; for regular TCP, this would result in a FIN on the connection. For MPTCP, an
equivalent mechanism is needed; this is referred to as the DATA_FIN.
A DATA_FIN is an indication that the sender has no more data to send, and
as such it can be used to verify that all data has been successfully received. A DATA_FIN,
as with the FIN on a regular TCP connection, is a unidirectional signal.
The DATA_FIN is signaled by setting the "F" flag in the DSS
option (
Figure 9
to 1. A DATA_FIN occupies 1 octet (the final octet) of the
connection-level sequence space. Note that the
DATA_FIN is included in the Data-Level Length but not at the subflow
level: for example, a segment with a DSN value of 80 and a
Data-Level Length of 11, with DATA_FIN set, would map 10 octets from
the subflow into data sequence space 80-89, and the DATA_FIN would
be DSN 90; therefore, this segment, including DATA_FIN, would be
acknowledged with a DATA_ACK of 91.
Note that when the DATA_FIN is not attached to a TCP segment containing data, the DSS
MUST
have a subflow sequence number of 0, a Data-Level Length of 1, and the data sequence number that corresponds with the DATA_FIN itself. The checksum in this case will only cover the pseudo-header.
A DATA_FIN has the same semantics and behavior as a regular TCP FIN, but at the connection level. Notably, it is only DATA_ACKed once all data has been successfully received at the connection level. Note, therefore, that a DATA_FIN is decoupled from a subflow FIN. It is only permissible to combine these signals on one subflow if there is no data outstanding on other subflows. Otherwise, it may be necessary to retransmit data on different subflows. Essentially, a host
MUST NOT
close all functioning subflows unless it is safe to do so, i.e., until all outstanding data has been DATA_ACKed or until the segment with the DATA_FIN flag set is the only outstanding segment.
Once a DATA_FIN has been acknowledged, all remaining subflows
MUST
be closed with standard FIN exchanges. Both
hosts
SHOULD
send FINs on all subflows, as a courtesy,
to allow middleboxes to clean up state even if an individual subflow
has failed. Reducing the timeouts (MSL) on subflows at end hosts after receiving a
DATA_FIN is also encouraged. In particular, any subflows where there is still
outstanding data queued (which has been retransmitted on other
subflows in order to get the DATA_FIN acknowledged)
MAY
be closed with a RST with an MP_TCPRST (
Section 3.6
) error code for "too much outstanding data".
A connection is considered closed once both hosts' DATA_FINs have been acknowledged by DATA_ACKs.
As specified above, a standard TCP FIN on an individual subflow
only shuts down the subflow on which it was sent. If all subflows
have been closed with a FIN exchange but no DATA_FIN has been
received and acknowledged, the MPTCP connection is treated as closed
only after a timeout. This implies that an implementation will have
TIME_WAIT states at both the subflow level and the connection level (see
Appendix D
). This permits "break-before-make" scenarios where connectivity is lost on all subflows before a new one can be re‑established.
3.3.4.
Receiver Considerations
Regular TCP advertises a receive window in each packet, telling the sender how much data the receiver
is willing to accept past the cumulative ACK. The receive window is used to implement flow control, throttling
down fast senders when receivers cannot keep up.
MPTCP also uses a unique receive window, shared between the subflows. The idea is to allow any
subflow to send data as long as the receiver is willing to accept it. The
alternative -- maintaining per-subflow
receive windows -- could end up stalling some subflows while others would not use up their window.
The receive window is relative to the DATA_ACK. As in TCP, a receiver
MUST NOT
shrink the right edge of the receive window (i.e., DATA_ACK + receive window). The receiver will
use the data sequence number to tell if a packet should be accepted at the connection level.
When deciding to accept packets at the subflow level, regular TCP checks
the sequence number in the packet against the allowed receive window.
With MPTCP, such a check is done using only the connection-level window. A sanity
check
SHOULD
be performed at the subflow level to ensure that the subflow and mapped sequence
numbers meet the following test: SSN - SUBFLOW_ACK <= DSN - DATA_ACK, where SSN is the subflow sequence number of the received packet and SUBFLOW_ACK is the RCV.NXT (next expected sequence number) of the subflow (with the equivalent connection-level definitions for DSN and DATA_ACK).
In regular TCP, once a segment is deemed in-window, it is put in either
the in-order receive queue or the out-of-order queue.
In Multipath TCP, the same thing happens, but at the connection level: a segment
is placed in the connection-level in-order or out-of-order queue if
it is in-window at both the connection level and the subflow level.
The stack still has to remember, for each subflow, which segments were
received successfully so that it can ACK them at the subflow level appropriately.
Typically, this will be implemented by keeping per-subflow out-of-order
queues (containing only message headers -- not the payloads) and remembering
the value of the cumulative ACK.
It is important for implementers to understand how large
a receive buffer is appropriate. The lower bound for full
network utilization is the maximum bandwidth-delay product
of any one of the paths. However, this might be insufficient
when a packet is lost on a slower subflow and needs to be
retransmitted (see
Section 3.3.6
). A tight
upper bound would be the maximum round-trip time (RTT) of any path multiplied
by the total bandwidth available across all paths. This
permits all subflows to continue at full speed while a
packet is fast-retransmitted on the maximum RTT path. Even
this might be insufficient to maintain full performance in
the event of a retransmit timeout on the maximum RTT path.
Determining the relationship between
retransmission strategies and receive buffer sizing is left for future study.
3.3.5.
Sender Considerations
The sender remembers receive window advertisements from the
receiver. It should only update its local receive window values when
the largest sequence number allowed (i.e., DATA_ACK + receive
window) increases on the receipt of a DATA_ACK. This is important
for allowing the use of paths with different RTTs and thus different feedback loops.
MPTCP uses a single receive window across all subflows, and if
the receive window was guaranteed to be unchanged end to end, a host could always read the most recent receive window value. However, some classes of middleboxes may alter the TCP-level receive window. Typically, these will
shrink the offered window, although for short periods of time it may be possible for the window to be larger (however,
note that this would not continue for long periods, since ultimately the middlebox must keep up with
delivering data to the receiver). Therefore, if receive window sizes differ on multiple subflows,
when sending data MPTCP
SHOULD
take the largest of the most recent window sizes as the one to use in calculations.
This rule is implicit in the requirement not to reduce the right edge of the window.
The sender
MUST
also remember the receive windows advertised by each subflow.
The allowed window for subflow i is (ack_i, ack_i + rcv_wnd_i), where ack_i is the
subflow-level cumulative ACK of subflow i. This ensures that data will not be sent to a middlebox
unless there is enough buffering for the data.
Putting the two rules together, we get the following: a sender is allowed to send
data segments with data-level sequence numbers between (DATA_ACK, DATA_ACK + receive_window).
Each of these segments will be mapped onto subflows, as long as subflow sequence numbers
are in the allowed windows for those subflows. Note that subflow sequence numbers do not
generally affect flow control if the same receive window is advertised across all subflows.
They will perform flow control for those subflows with a smaller advertised receive window.
The send buffer
MUST
, at a minimum, be as big as the receive buffer, to enable the sender to reach maximum throughput.
3.3.6.
Reliability and Retransmissions
The Data Sequence Mapping allows senders to resend data with the
same data sequence number on a different subflow. When doing this, a
host
MUST
still retransmit the original data on the
original subflow, in order to preserve the subflow's integrity
(middleboxes could replay old data and/or could reject holes in
subflows), and a receiver will ignore these retransmissions. While
this is clearly suboptimal, for compatibility reasons this is
sensible behavior. Optimizations could be negotiated in future
versions of this protocol. Note also that this property would also permit a sender to always send the same data, with the same data sequence number, on multiple subflows, if desired for reliability reasons.
This protocol specification does not mandate any mechanisms for handling retransmissions, and much will be dependent upon local policy
(as discussed in
Section 3.3.8
). One can imagine aggressive connection-level retransmission policies where every packet lost at the subflow level is retransmitted on
a different subflow (hence wasting bandwidth but possibly reducing application-to-application delays) or conservative retransmission policies where connection-level retransmissions
are only used after a few subflow-level retransmission timeouts occur.
It is envisaged that a standard connection-level retransmission mechanism
would be implemented around a connection-level data queue: all segments that haven't
been DATA_ACKed are stored. A timer is set when
the head of the connection level is ACKed at the subflow level but is not DATA_ACKed at the data level. This timer will guard against retransmission failures
by middleboxes that proactively ACK data.
The sender
MUST
keep data in its send buffer as
long as the data has not been acknowledged both (1) at the
connection level and (2) on all subflows on which it
has been sent. In this way, the sender can always retransmit the data if needed, on the same subflow or on a different one. A special case is when a subflow fails: the sender
will typically resend the data on other working subflows after a timeout and will keep trying to retransmit the data
on the failed subflow too. The sender will declare the subflow failed after a predefined upper bound on retransmissions is reached (which
MAY
be lower than the usual TCP limits of the MSL) or on the receipt of an ICMP error, and only then delete the outstanding data segments.
If multiple retransmissions that indicate that a
subflow is performing badly are triggered, this
MAY
lead to a host resetting the subflow with a RST. However, additional research is required to understand the heuristics of how and when to reset underperforming subflows. For example, a highly asymmetric path may be misdiagnosed as underperforming. A RST for this purpose
SHOULD
be accompanied by an "Unacceptable performance" MP_TCPRST option (
Section 3.6
).
3.3.7.
Congestion Control Considerations
Different subflows in an MPTCP connection have different congestion windows.
To achieve fairness at bottlenecks and resource pooling, it is necessary to couple the
congestion windows in use on each subflow, in order to push most traffic to uncongested links.
One algorithm for achieving this is presented in
RFC6356
the algorithm does not achieve perfect resource pooling but is "safe" in that it is readily
deployable in the current Internet. By this we mean that it does not take up more capacity
on any one path than if it was a single path flow using only that route, so this ensures
fair coexistence with single-path TCP at shared bottlenecks.
It is foreseeable that different congestion controllers will be
implemented for MPTCP, each aiming to achieve different properties
in the resource pooling / fairness / stability design space, as well as those for achieving different properties in quality of service, reliability, and resilience.
Regardless of the algorithm used,
the design of MPTCP aims to provide the congestion control
implementations with sufficient information
to make the right decisions; this information includes, for each subflow, which packets were lost and when.
3.3.8.
Subflow Policy
Within a local MPTCP implementation, a host may use any local policy it wishes to decide how to share the traffic to be sent over the available paths.
In the typical use case, where the goal is to maximize throughput, all available paths will be used simultaneously for data transfer, using coupled congestion control as described in
RFC6356
. It is expected, however, that other use cases will appear.
For instance, one possibility is an "all-or-nothing" approach, i.e., have a second path ready for use in the event of
failure of the first path, but alternatives could include entirely saturating one path before using an additional
path (the "overflow" case). Such choices would be most likely based on the monetary cost of links but may also be
based on properties such as the delay or jitter of links, where stability (of delay or bandwidth) is more important than throughput. Application
requirements such as these are discussed in detail in
RFC6897
The ability to make effective choices at the sender requires full knowledge of the path "cost", which
is unlikely to be the case. It would be desirable for a receiver to be able to signal their own preferences for paths,
since they will often be the multihomed party and may have to pay for metered incoming bandwidth.
To enable this behavior, the MP_JOIN option (see
Section 3.2
) contains the "B" bit,
which allows a host to indicate to its peer that this path should be
treated as a backup path to use only in the event of failure of
other working subflows (i.e., a subflow where the receiver has
indicated that B=1
SHOULD NOT
be used to send data unless there are no usable subflows where B=0).
In the event that the available set of paths changes, a host may
wish to signal a change in priority of subflows to the peer (e.g., a
subflow that was previously set as a backup should now take priority
over all remaining subflows). Therefore, the MP_PRIO option, shown
in
Figure 11
, can be used to
change the "B" flag of the subflow on which it is sent.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-----+-+
| Kind | Length |Subtype|(rsv)|B|
+---------------+---------------+-------+-----+-+
Figure 11
Change Subflow Priority (MP_PRIO) Option
Another use of the MP_PRIO option is to set the "B" flag on a
subflow to cleanly "retire" its use before closing it and removing it
with REMOVE_ADDR (
Section 3.4.2
) -- for example, to support make-before-break session continuity, where new subflows are added before the previously used subflows are closed.
It should be noted that the backup flag is a request from a data receiver to a data sender only, and the data sender
SHOULD
adhere to these requests. A host cannot assume that the data sender will do so, however, since local policies -- or technical difficulties -- may override MP_PRIO requests. Note also that this signal applies to a single direction, and so the sender of this option could choose to continue using the subflow to send data even if it has signaled B=1 to the other host.
3.4.
Address Knowledge Exchange (Path Management)
We use the term "path management" to refer to the exchange of information about additional paths between hosts, which in this design is managed by multiple addresses at hosts. For more details regarding the architectural thinking behind this design, see the MPTCP architecture document
RFC6182
This design makes use of two methods of sharing such
information, and both can be used on a connection.
The first is the direct
setup of new subflows (described in
Section 3.2
), where the initiator has an
additional address. The second method (described in the
following subsections) signals addresses explicitly to the
other host to allow it to initiate new subflows. The
two mechanisms are complementary: the first is implicit and
simple, while the second (explicit) is more complex but is more
robust. Together, these mechanisms allow addresses to change in
flight (and thus support operation through NATs, since the
source address need not be known); they also allow the
signaling of previously unknown addresses and of addresses
belonging to other address families (e.g., both IPv4 and IPv6).
Here is an example of typical operation of the protocol:
An MPTCP connection is initially set up between address/port A1 of Host A
and address/port B1 of Host B. If Host A is multihomed and
multiaddressed, it can start an additional subflow from
its address A2 to B1, by sending a SYN with an MP_JOIN
option from A2 to B1, using B's previously declared
token for this connection. Alternatively, if B is
multihomed, it can try to set up a new subflow from B2 to
A1, using A's previously declared token. In either
case, the SYN will be sent to the port already in use
for the original subflow on the receiving host.
Simultaneously (or after a timeout), an ADD_ADDR option
Section 3.4.1
) is sent on an existing subflow, informing
the receiver of the sender's alternative address(es). The recipient can use
this information to open a new subflow to the sender's additional address(es).
In our example, A will send the ADD_ADDR option informing B of address/port A2.
The mix of using the SYN‑based option and the ADD_ADDR option, including
timeouts, is implementation specific and can be tailored to agree with local policy.
If subflow A2-B1 is successfully set up, Host B can use the Address ID in
the MP_JOIN option to correlate this source address with the ADD_ADDR option that will also arrive on
an existing subflow; now B knows not to open A2-B1, ignoring the ADD_ADDR.
Otherwise, if B has not received the A2-B1 MP_JOIN SYN but received the ADD_ADDR,
it can try to initiate a new subflow from one or more of its addresses to address
A2. This permits new sessions to be opened if one host is behind a NAT.
Other ways of using the two signaling mechanisms are possible; for instance,
signaling addresses in other address families can only be done explicitly
using the Add Address (ADD_ADDR) option.
3.4.1.
Address Advertisement
The ADD_ADDR MPTCP option announces additional addresses (and, optionally, ports) on which a
host can be reached (
Figure 12
).
This option can be used at any time during a connection, depending on when the
sender wishes to enable multiple paths and/or when paths become available. As with all MPTCP
signals, the receiver
MUST
undertake standard TCP validity
checks, e.g., per
RFC5961
, before
acting upon it.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-------+---------------+
| Kind | Length |Subtype|(rsv)|E| Address ID |
+---------------+---------------+-------+-------+---------------+
| Address (IPv4: 4 octets / IPv6: 16 octets) |
+-------------------------------+-------------------------------+
| Port (2 octets, optional) | |
+-------------------------------+ |
| Truncated HMAC (8 octets, if E=0) |
| +-------------------------------+
| |
+-------------------------------+
Figure 12
Add Address (ADD_ADDR) Option
Every address has an Address ID that can be used for uniquely identifying the address within a connection for address removal. The Address ID is also
used to identify MP_JOIN options (see
Section 3.2
) relating to
the same address, even when address translators are in use. The Address ID
MUST
uniquely
identify the address for the sender of the option (within the scope of the connection); the mechanism for
allocating such IDs is implementation specific.
All Address IDs learned via either MP_JOIN or ADD_ADDR
SHOULD
be stored by the receiver in a data structure
that gathers all the Address-ID-to-address mappings for a connection
(identified by a token pair). In this way, there is
a stored mapping between the Address ID, observed source address, and token pair for
future processing of control information for a connection. Note that an implementation
MAY
discard incoming address advertisements at will -- for example, to avoid updating
mapping state or because advertised addresses are of no use to it (for
example, IPv6 addresses when it has IPv4 only). Therefore, a host
MUST
treat address
advertisements as soft state, and it
MAY
choose to refresh advertisements periodically.
Note also that an implementation
MAY
choose to cache these address advertisements even
if they are not currently relevant but may be relevant in the future, such as IPv4
addresses when IPv6 connectivity is available but IPv4 is awaiting DHCP.
This option is shown in
Figure 12
. The illustration is sized for
IPv4 addresses. For IPv6, the length of the address will be 16 octets (instead of 4).
The 2 octets that specify the TCP port number to use are optional, and their presence
can be inferred from the length of the option. Although it is expected that the majority of
use cases will use the same port pairs as those used for the initial subflow (e.g., port
80 remains port 80 on all subflows, as does the ephemeral port at the client), there
may be cases (such as port-based load balancing) where the explicit specification of
a different port is required. If no port is specified, MPTCP
SHOULD
attempt to
connect to the specified address on the same port as the port that is already in use by the subflow
on which the ADD_ADDR signal was sent; this is discussed in more detail in
Section 3.9
The Truncated HMAC parameter present in this option is the rightmost 64 bits of an HMAC, negotiated and
calculated in the same way as for MP_JOIN as described in
Section 3.2
. For this
specification of MPTCP, as there is only one hash algorithm option specified, this will be HMAC as
defined in
RFC2104
, using the SHA-256 hash algorithm
RFC6234
In the same way as for MP_JOIN, the key for the HMAC
algorithm, in the case of the message transmitted by Host A, will be Key-A followed by Key-B, and in
the case of Host B, Key-B followed by Key-A. These are the keys that were exchanged in the original
MP_CAPABLE handshake. The message for the HMAC is the Address ID, IP address, and port that precede
the HMAC in the ADD_ADDR option. If the port is not present in the ADD_ADDR option, the HMAC message
will nevertheless include 2 octets of value zero. The rationale for the HMAC is to
prevent unauthorized entities from injecting ADD_ADDR signals in an attempt to hijack a connection.
Note that, additionally, the presence of this HMAC prevents the
address from being changed in flight unless
the key is known by an intermediary. If a host receives an ADD_ADDR option for which it cannot
validate the HMAC, it
SHOULD
silently ignore the option.
A set of four flags is present after the subtype and before the Address ID. Only the rightmost
bit -- labeled "E" -- is assigned in this specification. The other
bits are currently unassigned; they
MUST
be set to 0 by a sender and
MUST
be ignored by the receiver.
The "E" flag exists to provide reliability for this option. Because this option will often be sent
on pure ACKs, there is no guarantee of reliability. Therefore, a receiver receiving a fresh ADD_ADDR
option (where E=0) will send the same option back to the sender, but not including the HMAC and
with E=1, to indicate receipt. According to local policy, the lack of
this type of "echo" can indicate to the initial ADD_ADDR sender that the
ADD_ADDR needs to be retransmitted.
Due to the proliferation of NATs, it is reasonably likely that
one host may attempt to advertise private addresses
RFC1918
. It is not desirable to prohibit
this behavior, since there may be cases where both hosts have additional
interfaces on the same private network, and a host
MAY
advertise such addresses. The MP_JOIN handshake
to create a new subflow (
Section 3.2
provides mechanisms to minimize security risks. The MP_JOIN message
contains a 32-bit token that uniquely identifies the connection to
the receiving host. If the token is unknown, the host will respond
with a RST. In the unlikely event that the token is valid at the
receiving host, subflow setup will continue, but the HMAC exchange
must occur for authentication. The HMAC exchange
will fail and will provide
sufficient protection against two unconnected hosts accidentally
setting up a new subflow upon the signal of a private address.
Further security considerations around the issue of ADD_ADDR messages that accidentally misdirect, or maliciously direct, new MP_JOIN attempts are discussed in
Section 5
A host that receives an ADD_ADDR but finds that a connection set up to that IP address and port number is unsuccessful
SHOULD NOT
perform further connection attempts to this address/port combination for this connection. A sender that wants to trigger a new incoming connection attempt on a previously advertised address/port combination can therefore refresh ADD_ADDR information by sending the option again.
A host can therefore send an ADD_ADDR message with an
already-assigned Address ID, but the address
MUST
be
the same as the address previously assigned to this Address ID. A
new ADD_ADDR may have the same port number or a different port number. If the port number is different, the receiving host
SHOULD
try to set up a new subflow to this new address/port combination.
A host wishing to replace an existing Address ID
MUST
first remove the existing one (
Section 3.4.2
).
During normal MPTCP operation, it is unlikely that there will be sufficient TCP option space for ADD_ADDR to be included along with those for data sequence numbering (
Section 3.3.1
). Therefore, it is expected that an MPTCP implementation will send the ADD_ADDR option on separate ACKs. As discussed earlier, however, an MPTCP implementation
MUST NOT
treat duplicate ACKs with any MPTCP option, with the exception of the DSS option, as indications of congestion
RFC5681
, and an MPTCP implementation
SHOULD NOT
send more than two duplicate ACKs in a row for signaling purposes.
3.4.2.
Remove Address
If, during the lifetime of an MPTCP connection, a previously
announced address becomes invalid (e.g., if the interface
disappears or an IPv6 address is no longer preferred), the affected
host
SHOULD
announce this situation so that the peer can remove
subflows related to this address. Even if an address is not in use
by an MPTCP connection, if it has been previously announced, an
implementation
SHOULD
announce its removal. A host
MAY
also choose to announce that a valid IP address
should not be used any longer -- for example, for make‑before-break session continuity.
This is achieved through the Remove Address (REMOVE_ADDR) option
Figure 13
), which will remove a
previously added address (or list of addresses) from a connection
and terminate any subflows currently using that address.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-------+---------------+
| Kind |Length = 3 + n |Subtype|(resvd)| Address ID | ...
+---------------+---------------+-------+-------+---------------+
(followed by n-1 Address IDs, if required)
Figure 13
Remove Address (REMOVE_ADDR) Option
For security purposes, if a host receives a REMOVE_ADDR option,
it must ensure that the affected path or paths are no longer in use
before it instigates closure. The receipt of REMOVE_ADDR
SHOULD
first trigger the sending of a TCP keepalive
RFC1122
on the path, and if a
response is received, the path
SHOULD NOT
be
removed. If the path is found to still be alive, the receiving host
SHOULD
no longer use the specified address for future
connections, but it is the responsibility of the host that sent the
REMOVE_ADDR to shut down the subflow. Before the address is removed,
the requesting host
MAY
also use MP_PRIO (
Section 3.3.8
) to request that a path no longer be used. Typical TCP validity tests on the subflow (e.g., ensuring
that sequence and ACK numbers are correct)
MUST
also be undertaken. An implementation can use indications of these test failures as part of intrusion detection or error logging.
The sending and receipt (if no keepalive response was received)
of this message
SHOULD
trigger the sending of RSTs by
both hosts on the affected subflow(s) (if possible), as a courtesy,
to allow the cleanup of middlebox state before cleaning up any local state.
Address removal is undertaken according to the Address ID, so as to
permit the use of NATs and other middleboxes that rewrite source
addresses. If an Address ID is not known, the receiver will
silently ignore the request.
A subflow that is still functioning
MUST
be closed with a FIN exchange as in regular TCP, rather than using this option. For more information, see
Section 3.3.3
3.5.
Fast Close
Regular TCP has the means of sending a RST signal to abruptly
close a connection. With MPTCP, a regular RST only has the scope of
the subflow; it
will only close the applicable subflow and will not affect the remaining
subflows. MPTCP's connection will stay alive at the data level, in order
to permit break-before-make handover between subflows. It is therefore
necessary to provide an MPTCP-level "reset" to allow the abrupt closure
of the whole MPTCP connection; this is done via the MP_FASTCLOSE option.
MP_FASTCLOSE is used to indicate to the peer that the connection will be
abruptly closed and no data will be accepted anymore. The reasons for
triggering an MP_FASTCLOSE are implementation specific. Regular TCP does
not allow the sending of a RST while the connection is in a synchronized
state
RFC0793
. Nevertheless, implementations allow
the sending of a RST in this state if, for example, the operating
system is running out of resources. In these cases, MPTCP should send
the MP_FASTCLOSE. This option is illustrated in
Figure 14
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-----------------------+
| Kind | Length |Subtype| (reserved) |
+---------------+---------------+-------+-----------------------+
| Option Receiver's Key |
| (64 bits) |
| |
+---------------------------------------------------------------+
Figure 14
Fast Close (MP_FASTCLOSE) Option
If Host A wants to force the closure of an MPTCP connection, it can
do so via two
options:
Option A (ACK): Host A sends an ACK containing the MP_FASTCLOSE
option on one subflow, containing the key of Host B as declared in
the initial connection handshake. On all the other subflows, Host A
sends a regular TCP RST to close these subflows and tears them down.
Host A now enters FASTCLOSE_WAIT state.
Option R (RST): Host A sends a RST containing the MP_FASTCLOSE
option on all subflows, containing the key of Host B as declared in
the initial connection handshake. Host A can tear down the subflows and
the connection immediately.
If Host A decides to force the closure by using Option A and sending
an ACK with the MP_FASTCLOSE option, the connection shall proceed as follows:
Upon receipt of an ACK with MP_FASTCLOSE by Host B, containing the valid key, Host B answers
on the same subflow with a TCP RST and tears down all subflows
also through sending TCP RST signals. Host B can
now close the whole MPTCP connection (it transitions directly to CLOSED state).
As soon as Host A has received the TCP RST on the remaining subflow, it
can close this subflow and tear down the whole connection (transition from
FASTCLOSE_WAIT state to CLOSED state). If Host A receives an MP_FASTCLOSE instead
of a TCP RST, both hosts attempted fast closure simultaneously. Host A should
reply with a TCP RST and tear down the connection.
If Host A does not receive a TCP RST in reply to its MP_FASTCLOSE after one
retransmission timeout (RTO) (the RTO of the subflow where the MP_FASTCLOSE has been sent), it
SHOULD
retransmit the MP_FASTCLOSE. To keep this connection from being
retained for a long time, the number of retransmissions
SHOULD
be
limited;
this limit is implementation specific. A
RECOMMENDED
number is 3. If no TCP RST
is received in response, Host A
SHOULD
send a TCP RST with the MP_FASTCLOSE option
itself when it releases state in order to clear any remaining state at middleboxes.
If, however, Host A decides to force the closure by using Option R and
sending a RST with the MP_FASTCLOSE option, Host B will act as follows:
upon receipt of a RST with MP_FASTCLOSE, containing the valid key,
Host B tears down all subflows by sending a TCP RST. Host B can now close the whole MPTCP
connection (it transitions directly to CLOSED state).
3.6.
Subflow Reset
An implementation of MPTCP may also need to send a regular TCP RST to force
the closure of a subflow. A host sends a TCP RST in order to close a subflow
or reject an attempt to open a subflow (MP_JOIN). In order to let the
receiving host know why a subflow is being closed or rejected, the TCP RST packet
MAY
include the MP_TCPRST option (
Figure 15
). The host
MAY
use this information to
decide, for example, whether it tries to re-establish the subflow
immediately, later, or never.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-----------------------+
| Kind | Length |Subtype|U|V|W|T| Reason |
+---------------+---------------+-------+-----------------------+
Figure 15
TCP RST Reason (MP_TCPRST) Option
The MP_TCPRST option contains a reason code that allows the
sender of the option to provide more information about the reason for
the termination of the subflow. Using 12 bits of option space, the
first 4 bits are reserved for flags (only one of which is currently
defined), and the remaining octet is used to express a reason code for
this subflow termination, from which a receiver
MAY
infer information
about the usability of this path.
The "T" flag is used by the sender to indicate whether the error
condition that is reported is Transient ("T" bit set to 1) or Permanent
("T" bit set to 0). If the error condition is considered to be
Transient by the sender of the RST segment, the recipient of this
segment
MAY
try to re-establish a subflow for this connection over the
failed path. The time at which a receiver may try to
re‑establish this subflow
is implementation specific but
SHOULD
take into account the properties
of the failure as defined by the provided reason code. If the error condition
is considered to be Permanent, the receiver of the RST segment
SHOULD NOT
try
to re‑establish a subflow for this connection over this path. The "U", "V",
and "W" flags are not defined by this specification and are reserved for
future use. An implementation of this specification
MUST
set these flags
to 0, and a receiver
MUST
ignore them.
"Reason" is an 8-bit field that indicates the reason code for
the termination of the subflow. The following codes are defined in
this document:
Unspecified error (code 0x00). This is the default error;
it implies that the
subflow is no longer available. The presence of this option shows
that the RST was generated by an MPTCP-aware device.
MPTCP-specific error (code 0x01). An error has been detected in the
processing of MPTCP options. This is the usual reason code to return
in the cases where a RST is being sent to close a subflow because
of an invalid response.
Lack of resources (code 0x02). This code indicates that the
sending host does not have enough resources to support the
terminated subflow.
Administratively prohibited (code 0x03). This code indicates that
the requested subflow is prohibited by the policies of the sending
host.
Too much outstanding data (code 0x04). This code indicates that
there is an excessive amount of data that needs to be transmitted
over the terminated subflow while having already been acknowledged
over one or more other subflows. This may occur if a path has been
unavailable for a short period and it is more efficient to reset and
start again than it is to retransmit the queued data.
Unacceptable performance (code 0x05). This code indicates that
the performance of this subflow was too low compared to the other
subflows of this Multipath TCP connection.
Middlebox interference (code 0x06). Middlebox interference has
been detected over this subflow, making MPTCP signaling invalid. For
example, this may be sent if the checksum does not validate.
3.7.
Fallback
Sometimes, middleboxes will exist on a path that could prevent the
operation of MPTCP. MPTCP has been designed to cope with many
middlebox modifications (see
Section 6
), but there are still some cases where a subflow
could fail to operate within the MPTCP requirements. Notably, these cases are the following: the loss of MPTCP options on a path and the modification of payload data. If such an event occurs, it is necessary to "fall back" to the previous, safe operation. This may be either falling back to regular TCP or removing a problematic subflow.
At the start of an MPTCP connection (i.e., the first subflow), it is important to ensure that the path is fully MPTCP capable and the necessary MPTCP options can reach each host. The handshake as described in
Section 3.1
SHOULD
fall back to regular TCP if either of the SYN messages does not have the MPTCP options: this is the same, and desired, behavior in the case where a host is not MPTCP capable or the path does not support the MPTCP options. When attempting to join an existing MPTCP connection (
Section 3.2
), if a path is not MPTCP capable and the MPTCP options do not get through on the SYNs, the subflow will be closed according to the MP_JOIN logic.
There is, however, another corner case that should be addressed:
the case where MPTCP options get through on the SYN but not on regular
packets. If the subflow is the first subflow and thus all data in
flight is contiguous, this situation can be resolved by using the following rules:
A sender
MUST
include a DSS option with Data Sequence Mapping in every segment until one of the sent segments has been acknowledged with a DSS option containing a Data ACK. Upon reception of the acknowledgment, the sender has the confirmation that the DSS option passes in both directions and may choose to send fewer DSS options than once per segment.
If, however, an ACK is received for data (not just for the SYN)
without a DSS option containing a Data ACK, the sender determines that the path is not MPTCP capable. In the case of this occurring on an additional subflow (i.e., one started with MP_JOIN), the host
MUST
close the subflow with a RST, which
SHOULD
contain an MP_TCPRST option (
Section 3.6
) with a "Middlebox interference" reason code.
In the case of such an ACK being received on the first subflow
(i.e., that started with MP_CAPABLE), before any additional subflows
are added, the implementation
MUST
drop out of MPTCP
mode and fall back to regular TCP. The sender will send one final Data Sequence Mapping, with the Data-Level Length value of 0 indicating an infinite mapping (to inform the other end in case the path drops options in one direction only), and then revert to sending data on the single subflow without any MPTCP options.
If a subflow breaks during operation, e.g., if it is rerouted and
MPTCP options are no longer permitted, then once this is detected (by
the subflow-level receive buffer filling up, since there is no mapping
available in order to DATA_ACK this data), the subflow
SHOULD
be treated as broken and closed with a RST,
since no data can be delivered to the application layer and no
fallback signal can be reliably sent. This RST
SHOULD
include the MP_TCPRST option (
Section 3.6
) with a "Middlebox interference" reason code.
These rules should cover all cases where such a failure could
happen -- whether it's on the forward or reverse path and whether the server or the client first sends data.
So far, this section has discussed the loss of MPTCP options,
either initially or during the course of the connection. As described
in
Section 3.3
, each portion of
data for which there is a mapping is protected by a checksum, if
checksums have been negotiated. This mechanism is used to detect if
middleboxes have made any adjustments to the payload (added, removed,
or changed data). A checksum will fail if the data has been changed in
any way. The use of a checksum will also detect whether the length of data on the subflow is
increased or decreased, and this means the Data Sequence Mapping is no
longer valid. The sender no longer knows what subflow-level sequence
number the receiver is genuinely operating at (the middlebox will be
faking ACKs in return), and it cannot signal any further
mappings. Furthermore, in addition to the possibility of payload
modifications that are valid at the application layer, it is possible that such modifications could be triggered across MPTCP segment boundaries, corrupting the data. Therefore, all data from the start of the segment that failed the checksum onward is not trustworthy.
Note that if checksum usage has not been negotiated, this fallback mechanism cannot be used unless there is some higher-layer or lower‑layer signal to inform the MPTCP implementation that the payload has been tampered with.
When multiple subflows are in use, the data in flight on a subflow
will likely involve data that is not contiguously part of the
connection-level stream, since segments will be spread across the
multiple subflows. Due to the problems identified above, it is not
possible to determine what adjustments have been done to the data (notably,
any changes to the subflow sequence numbering). Therefore, it is not
possible to recover the subflow, and the affected subflow must be
immediately closed with a RST that includes an MP_FAIL option (
Figure 16
), which defines the data sequence number at the start of the segment (defined by the Data Sequence Mapping) that had the checksum failure. Note that the MP_FAIL option requires the use of the full 64-bit sequence number, even if 32-bit sequence numbers are normally in use in the DSS signals on the path.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+----------------------+
| Kind | Length=12 |Subtype| (reserved) |
+---------------+---------------+-------+----------------------+
| |
| Data Sequence Number (8 octets) |
| |
+--------------------------------------------------------------+
Figure 16
Fallback (MP_FAIL) Option
The receiver of this option
MUST
discard all data following the data sequence number specified.
Failed data
MUST NOT
be DATA_ACKed and so will be retransmitted on other subflows (
Section 3.3.6
).
A special case is when there is a single subflow and it fails with a checksum error. If it is known that all unacknowledged data in
flight is contiguous (which will usually be the case with a single
subflow), an infinite mapping can be applied to the subflow without
the need to close it first, essentially turning off all further
MPTCP signaling.
In this case, if a receiver identifies a checksum failure
when there is only one path, it will send back an MP_FAIL option on the subflow-level ACK, referring to the data-level sequence number of the start of the
segment on which the checksum error was detected. The sender will receive
this information and, if all unacknowledged data in flight is contiguous, will signal an infinite mapping.
This infinite mapping will be a DSS option (
Section 3.3
on the first new packet, containing a Data Sequence Mapping that acts retroactively, referring to the start of the subflow sequence
number of the most recent segment that was known to be delivered intact (i.e., was successfully DATA_ACKed). From that point onward, data can be altered
by a middlebox without affecting MPTCP, as the data stream is equivalent to a regular, legacy TCP session.
While in theory paths may only be damaged in one direction -- and the MP_FAIL
signal affects only one direction of traffic --
for simplicity of implementation, the receiver of an MP_FAIL
MUST
also respond with an MP_FAIL in the reverse direction and entirely revert to a regular TCP session.
In the rare case that the data is not contiguous (which could happen when there is only one subflow but it is retransmitting data from a subflow
that has recently been uncleanly closed), the receiver
MUST
close the subflow with a RST with MP_FAIL. The receiver
MUST
discard all data that follows the
data sequence number specified. The sender
MAY
attempt to
create a new subflow belonging to the same connection and, if it chooses to do
so,
SHOULD
immediately place
the single subflow in single-path mode by setting an infinite Data Sequence Mapping. This mapping will begin from the data-level sequence number
that was declared in the MP_FAIL.
After a sender signals an infinite mapping, it
MUST
only use subflow ACKs to clear its send buffer.
This is because Data ACKs may become misaligned with the subflow ACKs when middleboxes insert or delete data.
The receiver
SHOULD
stop generating Data ACKs after it receives
an infinite mapping.
When a connection has fallen back with an infinite mapping, only one subflow can send data; otherwise, the receiver would not know how to reorder the data. In practice, this means that all MPTCP subflows will have to be terminated except one. Once MPTCP falls back to regular TCP, it
MUST NOT
revert to MPTCP later in the connection.
It should be emphasized that MPTCP is not attempting to prevent the use of middleboxes that want to adjust the payload. An MPTCP-aware middlebox could provide such functionality by also rewriting checksums.
3.8.
Error Handling
In addition to the fallback mechanism described above, the standard classes of TCP errors may need to be handled in an MPTCP‑specific way. Note that changing semantics -- such as the relevance of a RST -- are covered in
Section 4
. Where possible, we do not want to deviate from regular TCP behavior.
The following list covers possible errors and the appropriate MPTCP behavior:
Unknown token in MP_JOIN (or HMAC failure in MP_JOIN ACK, or missing MP_JOIN in SYN/ACK response): send RST (analogous to TCP's behavior on an unknown port)
DSN out of window (during normal operation): drop the data; do not send Data ACKs
Remove request for unknown Address ID: silently ignore
3.9.
Heuristics
There are a number of heuristics that are needed for
performance or deployment but that are not required for
protocol correctness. In this section, we detail such
heuristics. Note that discussions of buffering and certain
sender and receiver window behaviors are presented in Sections
3.3.4
and
3.3.5
and retransmission is discussed in
Section 3.3.6
3.9.1.
Port Usage
Under typical operation, an MPTCP implementation
SHOULD
use
the same ports as the ports that are already in use. In other words, the
destination port of a SYN containing an MP_JOIN option
SHOULD
be the same as the remote port of the first subflow in the
connection. The local port for such SYNs
SHOULD
also be the
same as the port for the first subflow (and as such, an
implementation
SHOULD
reserve ephemeral ports across all
local IP addresses), although there may be cases where this
is infeasible. This strategy is intended to maximize the
probability of the SYN being permitted by a firewall or NAT
at the recipient and to avoid confusing any network-monitoring software.
There may also be cases, however, where a host wishes to
signal that a specific port should be used; this facility
is provided in the ADD_ADDR option as documented in
Section 3.4.1
. It is therefore feasible
to allow multiple subflows between the same two addresses
but using different port pairs, and
such a facility could be used to allow load balancing within
the network based on 5-tuples (e.g., some ECMP implementations
RFC2992
).
3.9.2.
Delayed Subflow Start and Subflow Symmetry
Many TCP connections are short-lived and consist only of a few
segments, and so the overhead
of using MPTCP outweighs any benefits. A heuristic is required,
therefore, to decide when to start using additional subflows in
an MPTCP connection. Experimental deployments have shown that
MPTCP can be applied in a range of scenarios, so an implementation
will likely need to take into account such factors as the type of
traffic being sent and the duration of the session; this information
MAY
be signaled by the application layer.
However, for standard TCP traffic, a suggested general-purpose
heuristic that an implementation
MAY
choose to employ is as follows.
If a host has data buffered for its peer (which implies that the
application has received a request for data), the host opens one
subflow for each initial window's worth of data that is buffered.
Consideration should also be given to limiting the rate of adding
new subflows, as well as limiting the total number of subflows open
for a particular connection. A host may choose to vary these values
based on its load or knowledge of traffic and path characteristics.
Note that this heuristic alone is probably insufficient. Traffic
for many common applications, such as downloads, is highly asymmetric, and
the host that is multihomed may well be the client that will never fill
its buffers and thus never use MPTCP according to this heuristic. Advanced APIs that allow an
application to signal its traffic requirements would aid in these decisions.
An additional time-based heuristic could be applied, opening additional
subflows after a given period of time has passed. This would alleviate the
above issue and also provide resilience for low‑bandwidth but long-lived
applications.
Another issue is that both communicating hosts may simultaneously try to
set up a subflow between the same pair of addresses. This leads to an
inefficient use of resources.
If the same ports are used on all subflows, as recommended above,
then standard TCP simultaneous-open logic should take care of this situation
and only one subflow will be established between the address pairs. However,
this relies on the same ports being used at both end hosts. If a host does
not support TCP simultaneous open, it is
RECOMMENDED
that some element
of randomization be applied to the time to wait before opening new subflows,
so that only one subflow is created between a given address pair. If, however,
hosts signal additional ports to use (for example, for leveraging ECMP on-path),
this heuristic is not appropriate.
This section has shown some of the factors that an implementer
should consider when developing MPTCP heuristics, but it is not intended to be
prescriptive.
3.9.3.
Failure Handling
Requirements for MPTCP's handling of unexpected signals are
given in
Section 3.8
. There are other failure cases,
however, where hosts can choose appropriate behavior.
For example,
Section 3.1
suggests that a host
SHOULD
fall back to trying regular TCP SYNs after one or more failures of MPTCP
SYNs for a connection. A host may keep a system-wide cache of such
information, so that it can back off from using MPTCP, firstly for that
particular destination host and, eventually, on a whole interface, if
MPTCP connections continue to fail. The duration of such a cache would
be implementation specific.
Another failure could occur when the MP_JOIN handshake fails.
Section 3.8
specifies that an incorrect handshake
MUST
lead to the subflow being closed with a RST. A host operating an active
intrusion-detection system may choose to start blocking MP_JOIN packets
from the source host if multiple failed MP_JOIN attempts are seen. From
the connection initiator's point of view, if an MP_JOIN fails, it
SHOULD NOT
attempt to connect to the same IP address and port during the lifetime
of the connection, unless the other host refreshes the information with
another ADD_ADDR option. Note that the ADD_ADDR option is informational
only and does not guarantee that the other host will attempt a connection.
In addition, an implementation may learn, over a number of connections,
that certain interfaces or destination addresses consistently fail and
may default to not trying to use MPTCP for such interfaces or
addresses. The behavior of subflows that perform particularly badly
or subflows that regularly fail during use could also
be learned, so that an implementation can temporarily choose not to use
these paths.
4.
Semantic Issues
In order to support multipath operation, the semantics of some TCP
components have changed. To help clarify, this section lists these
semantic changes as a point of reference.
Sequence number:
The (in-header) TCP sequence
number is specific to the subflow. To allow the receiver to
reorder application data, an additional data-level
sequence space is used. In this data‑level sequence space, the initial SYN and
the final DATA_FIN occupy 1 octet of sequence space. This is done to
ensure that these
signals are acknowledged at the connection level. There is an explicit
mapping of data sequence space to subflow sequence space,
which is signaled through TCP options in data
packets.
ACK:
The ACK field in the TCP header
acknowledges only the subflow sequence number -- not the
data-level sequence space. Implementations
SHOULD NOT
attempt to infer a data-level acknowledgment from the
subflow ACKs.
This separates subflow-level and connection-level processing
at an end host.
Duplicate ACK:
A duplicate ACK that includes any MPTCP signaling
(with the exception of the DSS option)
MUST NOT
be treated as a signal of congestion.
To limit the chances of non-MPTCP-aware entities mistakenly interpreting duplicate
ACKs as a signal of congestion, MPTCP
SHOULD NOT
send more than two duplicate ACKs
containing (non-DSS) MPTCP signals in a row.
Receive Window:
The receive window in the TCP
header indicates the amount of free buffer space for the
whole data-level connection (as opposed to the amount of space for this
subflow) that is available at the receiver. The
semantics are the same as for regular TCP, but to maintain these
semantics the receive window must be interpreted at the
sender as relative to the sequence number given in the
DATA_ACK rather than the subflow ACK in the TCP header.
In this way, the original role of flow control is preserved.
Note that some middleboxes may change the receive window,
and so a host
SHOULD
use the maximum value of those recently
seen on the constituent subflows for the connection-level
receive window and also needs to maintain a subflow-level
window for subflow-level processing.
FIN:
The FIN flag in the TCP header applies
only to the subflow it is sent on -- not to the whole
connection. For connection-level FIN semantics, the
DATA_FIN option is used.
RST:
The RST flag in the TCP header applies
only to the subflow it is sent on -- not to the whole
connection. The MP_FASTCLOSE option provides the Fast Close
functionality of a RST at the MPTCP connection level.
Address List:
Address list management (i.e.,
knowledge of the local and remote hosts' lists of
available IP addresses) is handled
on a per-connection basis (as opposed to per subflow, per
host, or per pair of communicating hosts). This permits
the application of per-connection local policy. Adding an
address to one connection (either explicitly through an
ADD_ADDR message or implicitly through an MP_JOIN) has no implications
for other connections between the same pair of hosts.
5-tuple:
The 5-tuple (protocol, local
address, local port, remote address, remote port)
presented by kernel APIs to the application layer in a
non-multipath-aware application is that of the first
subflow, even if the subflow has since been closed and
removed from the connection. This decision, and other
related API issues, are discussed in more detail in
RFC6897
5.
Security Considerations
As identified in
RFC6181
, the
addition of multipath capability to TCP will bring with it a number of
new classes of threats. In order to prevent these threats,
RFC6182
presents a set of requirements for a security
solution for MPTCP. The fundamental goal is for the security of MPTCP to
be "no worse" than regular TCP today. The key security requirements
are as follows:
Provide a mechanism to confirm that the parties in a subflow
handshake are the same as the parties in the original connection setup.
Provide verification that the peer can receive traffic at a new address before using it as part of a connection.
Provide replay protection, i.e., ensure that a request to add/remove a subflow is "fresh".
In order to achieve these goals, MPTCP includes a hash-based handshake
algorithm, as documented in Sections
3.1
and
3.2
The security of the MPTCP connection hangs on the use of keys that
are shared once at the start of the first subflow and are never sent
again over the network (unless used in the Fast Close mechanism (
Section 3.5
)). To ease demultiplexing
while not giving away any cryptographic material, future subflows use a
truncated cryptographic hash of this key as the connection
identification "token". The keys are concatenated and used as keys for
creating Hash-based Message Authentication Codes (HMACs) used on subflow
setup, in order to verify that the parties in the handshake are the same
as the parties in the original connection setup. It also provides verification that
the peer can receive traffic at this new address. Replay attacks would
still be possible when only keys are used; therefore, the handshakes use
single-use random numbers (nonces) at both ends -- this ensures that the HMAC will never be the same on two handshakes. Guidance on generating random numbers suitable for use as keys is given in
RFC4086
and discussed in
Section 3.1
. The nonces are valid for the lifetime of the TCP connection attempt. HMAC is also used to secure the ADD_ADDR option, due to the threats identified in
RFC7430
The use of crypto capability bits in the initial connection handshake
to negotiate the use of a particular algorithm allows the deployment of additional crypto mechanisms in the future. This negotiation would nevertheless be susceptible to a bid-down attack by an on-path active attacker who could modify the crypto capability bits in the response from the receiver to use a less secure crypto mechanism. The security mechanism presented in this document should therefore protect against all forms of flooding and hijacking attacks discussed in
RFC6181
The version negotiation specified in
Section 3.1
, if differing MPTCP versions shared a common
negotiation format, would allow an on-path attacker to apply a
theoretical bid-down attack. Since the v1 and v0 protocols have a
different handshake, such an attack would require that the client
re-establish the connection using v0 and that the server support v0.
Note that an on-path attacker would have access to the raw data, negating any other TCP-level security mechanisms. As also noted in
Appendix E
, this document specifies the removal of the AddrID field
RFC6824
in the MP_PRIO option (
Section 3.3.8
).
This change eliminates the possibility of a theoretical attack where
a subflow could be placed in "backup" mode by an attacker.
During normal operation, regular TCP protection mechanisms (such as
ensuring that sequence numbers are in-window) will provide the same
level of protection against attacks on individual TCP subflows as the
level of protection that exists for regular TCP today. Implementations will introduce additional buffers compared to regular TCP, to reassemble data at the connection level. The application of window sizing will minimize the risk of denial-of-service attacks consuming resources.
As discussed in
Section 3.4.1
, a host may advertise its private addresses, but these might point to different hosts in the receiver's network. The MP_JOIN handshake (
Section 3.2
) will ensure that this does not succeed in setting up a subflow to the incorrect host. However, it could still create unwanted TCP handshake traffic. This feature of MPTCP could be a target for denial-of-service exploits, with malicious participants in MPTCP connections encouraging the recipient to target other hosts in the network. Therefore, implementations should consider heuristics (
Section 3.9
) at both the sender and receiver to reduce the impact of this.
To further protect against malicious ADD_ADDR messages sent by an off-path attacker, the ADD_ADDR includes an HMAC using the keys negotiated during the handshake. This effectively prevents an attacker from diverting an MPTCP connection through an off-path ADD_ADDR injection into the stream.
A small security risk could theoretically exist with key reuse, but in order to accomplish a replay attack, both the sender and receiver keys, and the sender and receiver random numbers, in the MP_JOIN handshake (
Section 3.2
) would have to match.
While this specification defines a "medium" security solution,
meeting the criteria specified at the start of this section and in the
threat analysis document
RFC6181
, since attacks
only ever get worse, it is likely that a future version of MPTCP would
need to be able to support stronger security.
There are several ways the security of MPTCP could potentially be improved; some of these would be compatible with MPTCP as defined in this document, while others may not be. For now, the best approach is to gain experience with the current approach, establish what might work, and check that the threat analysis is still accurate.
Possible ways of improving MPTCP security could include:
defining a new MPTCP cryptographic algorithm, as negotiated in
MP_CAPABLE. If an implementation was being deployed in a controlled
environment where additional assumptions could be made, such as the
ability for the servers to store state during the TCP handshake, then
it may be possible to use a stronger cryptographic algorithm than
would otherwise be possible.
defining how to secure data transfer with MPTCP, while not changing the signaling part of the protocol.
defining security that requires more option space, perhaps in
conjunction with a "long options" proposal for extending the TCP
option space (such as those surveyed in
TCPLO
), or perhaps
building on the current approach with a second stage of
security based on MPTCP options.
revisiting the working group's decision to exclusively use TCP
options for MPTCP signaling and instead looking at the
possibility of using TCP payloads as well.
MPTCP has been designed with several methods available to indicate a new security mechanism, including:
available flags in MP_CAPABLE (
Figure 4
).
available subtypes in the MPTCP option (
Figure 3
).
the Version field in MP_CAPABLE (
Figure 4
).
6.
Interactions with Middleboxes
Multipath TCP was designed to be deployable in the present world. Its design takes into account "reasonable"
existing middlebox behavior. In this section, we outline a few representative middlebox-related failure scenarios and
show how Multipath TCP handles them. Next, we list the design decisions
Multipath TCP has made to accommodate the different
middleboxes.
A primary concern is our use of a new TCP option. Middleboxes should forward packets
with unknown options unchanged, yet there are some that don't. We expect these
middleboxes to strip options and pass the data,
drop packets with new options, copy the same option into multiple segments (e.g., when doing segmentation), or drop
options during segment coalescing.
MPTCP uses a single new TCP option called "Kind", and all message types are defined by "subtype" values (see
Section 7
). This should reduce the chances of only some types of MPTCP options being passed; instead, the key differing characteristics are different paths and the presence of the SYN flag.
MPTCP SYN packets on the first subflow of a connection contain the MP_CAPABLE option (
Section 3.1
). If this is dropped, MPTCP
SHOULD
fall back to regular TCP. If packets with the MP_JOIN option (
Section 3.2
) are dropped, the paths will simply not be used.
If a middlebox strips options but otherwise passes the packets
unchanged, MPTCP will behave safely. If an MP_CAPABLE option is dropped
on either the outgoing path or the return path, the initiating host can
fall back to regular TCP, as illustrated in
Figure 17
and discussed in
Section 3.1
Host A Host B
| Middlebox M |
| | |
| SYN (MP_CAPABLE) | SYN |
|-------------------|---------------->|
| SYN/ACK |
|<------------------------------------|
a) MP_CAPABLE option stripped on outgoing path
Host A Host B
| SYN (MP_CAPABLE) |
|-------------------------------------->|
| Middlebox M |
| | |
| SYN/ACK |SYN/ACK (MP_CAPABLE)|
|<-----------------|--------------------|
b) MP_CAPABLE option stripped on return path
Figure 17
Connection Setup with Middleboxes That Strip Options from Packets
Subflow SYNs contain the MP_JOIN option. If this option is stripped on the outgoing path,
the SYN will appear to be a regular SYN to Host B. Depending on whether there is a listening socket on
the target port, Host B will reply with either a SYN/ACK or a RST (subflow connection fails). When Host A
receives the SYN/ACK, it sends a RST because the SYN/ACK does not contain the MP_JOIN option and its token.
Either way, the subflow setup fails but otherwise does not affect the MPTCP connection as a whole.
We now examine data flow with MPTCP, assuming that the flow is
correctly set up, which implies that the options in the SYN
packets were allowed through by the relevant middleboxes. If options are allowed through and there is no resegmentation or
coalescing to TCP segments, Multipath TCP flows can proceed without problems.
The case when options get stripped on data packets is discussed
in
Section 3.7
If only some MPTCP options are stripped, behavior is not deterministic. If some Data Sequence Mappings are lost, the connection can continue so long as mappings exist for the subflow-level data (e.g., if multiple maps have been sent that reinforce each other). If some subflow-level space is left unmapped, however, the subflow is treated as broken and is closed, using the process described in
Section 3.7
. MPTCP should survive with a loss of some Data ACKs, but performance will degrade as the fraction of stripped options increases.
We do not expect such cases to appear in practice, though: most
middleboxes will either strip all options or let them all through.
We end this section with a list of middlebox classes, their behavior, and the elements in the MPTCP design
that allow operation through such middleboxes. Issues surrounding dropping packets with options
or stripping options were discussed above and are not included here:
NATs (Network Address (and port) Translators)
RFC3022
change the source address (and
often the source port) of packets. This means that a host will not know its
public-facing address for signaling in MPTCP. Therefore, MPTCP permits implicit address addition via the MP_JOIN option,
and the handshake mechanism ensures that connection attempts to private addresses
RFC1918
, since they are authenticated, will only set up subflows to the correct hosts.
Explicit address removal is undertaken by an Address ID to allow no knowledge of the source address.
Performance Enhancing Proxies (PEPs)
RFC3135
might proactively ACK data to increase performance. MPTCP, however, relies on accurate congestion control signals from the end host, and non‑MPTCP-aware PEPs will not be able to provide such signals. MPTCP will, therefore, fall back to single-path TCP or close the problematic subflow (see
Section 3.7
).
Traffic normalizers
norm
may not
allow holes in sequence numbers, and they may cache packets and retransmit the same data.
MPTCP looks like standard TCP on the wire and will not retransmit different data on the same subflow sequence number. In the event of a retransmission, the same data will be retransmitted on the original TCP subflow even if it is additionally retransmitted at the connection level on a different subflow.
Firewalls
RFC2979
might perform
Initial Sequence Number (ISN) randomization on TCP connections. MPTCP uses relative
sequence numbers in Data Sequence Mappings to cope with this. Like NATs, firewalls will not permit many incoming connections, so
MPTCP supports address signaling (ADD_ADDR) so that a multiaddressed host can invite its peer behind the firewall/NAT to connect
out to its additional interface.
Intrusion Detection Systems / Intrusion Prevention Systems (IDSs/IPSs) observe packet streams for patterns and content that could threaten a network. MPTCP may require the
instrumentation of additional paths, and an MPTCP-aware IDS or IPS would need to read MPTCP tokens to correlate data from multiple subflows to maintain comparable visibility into all of the traffic between devices. Without such changes, an IDS would get an incomplete view of the traffic, increasing the risk of missing traffic of interest (false negatives) and increasing the chances of erroneously identifying a subflow as a risk due to only seeing partial data (false positives).
Application-level middleboxes such as content-aware firewalls may
alter the payload within a subflow -- for example, rewriting URIs in
HTTP traffic. MPTCP will detect such changes using the checksum
and close the affected subflow(s), if there are other subflows that can be used. If all subflows are affected, MPTCP
will fall back to TCP, allowing such middleboxes to change the payload. MPTCP-aware middleboxes should be able to adjust the payload and MPTCP metadata in order not to break the connection.
In addition, all classes of middleboxes may affect TCP traffic in the following ways:
TCP options may be removed, or packets with unknown options dropped, by many classes of middleboxes. It is intended
that the initial SYN exchange, with a TCP option, will be sufficient to identify the path's capabilities. If such a packet does
not get through, MPTCP will end up falling back to regular TCP.
Segmentation/coalescing (e.g., TCP segmentation offloading) might copy options between packets and might
strip some options. MPTCP's Data Sequence Mapping includes the relative subflow sequence number instead of using the sequence
number in the segment. In this way, the mapping is independent of the packets that carry it.
The receive window may be shrunk by some middleboxes at the
subflow level. MPTCP will use the maximum window at the data level but will also obey
subflow-specific windows.
7.
IANA Considerations
This document obsoletes
RFC6824
. As such, IANA has updated
several registries to point to this document. In addition, this document
creates one new registry. These topics are described in the following subsections.
7.1.
TCP Option Kind Numbers
IANA has
updated the "TCP Option Kind Numbers" registry to point to this document
for Multipath TCP, as shown in
Table 1
Table 1
TCP Option Kind Numbers
Kind
Length
Meaning
Reference
30
Multipath TCP (MPTCP)
RFC 8684
7.2.
MPTCP Option Subtypes
The 4-bit MPTCP subtype in the "MPTCP Option Subtypes"
subregistry under the "Transmission Control Protocol (TCP) Parameters"
registry was defined in
RFC6824
. Since
RFC6824
is an
Experimental RFC and not a Standards Track RFC, and since no further
entries have occurred beyond those pointing to
RFC6824
, IANA has
replaced the existing registry with the contents of
Table 2
and with the following
explanatory note.
Note: This registry specifies the MPTCP Option Subtypes for MPTCP v1, which obsoletes the Experimental MPTCP v0. For the MPTCP v0 subtypes, please refer to
RFC6824
Table 2
MPTCP Option Subtypes
Value
Symbol
Name
Reference
0x0
MP_CAPABLE
Multipath Capable
RFC 8684,
Section 3.1
0x1
MP_JOIN
Join Connection
RFC 8684,
Section 3.2
0x2
DSS
Data Sequence Signal (Data ACK and Data Sequence Mapping)
RFC 8684,
Section 3.3
0x3
ADD_ADDR
Add Address
RFC 8684,
Section 3.4.1
0x4
REMOVE_ADDR
Remove Address
RFC 8684,
Section 3.4.2
0x5
MP_PRIO
Change Subflow Priority
RFC 8684,
Section 3.3.8
0x6
MP_FAIL
Fallback
RFC 8684,
Section 3.7
0x7
MP_FASTCLOSE
Fast Close
RFC 8684,
Section 3.5
0x8
MP_TCPRST
Subflow Reset
RFC 8684,
Section 3.6
0xf
MP_EXPERIMENTAL
Reserved for Private Use
Values 0x9 through 0xe are currently unassigned. Option 0xf is reserved for use by private experiments. Its use may be formalized in a future specification. Future assignments in this registry are to be defined by Standards Action as defined by
RFC8126
. Assignments consist of the MPTCP subtype's symbolic name, its associated value, and a reference to its specification.
7.3.
MPTCP Handshake Algorithms
The "MPTCP Handshake Algorithms" subregistry under the
"Transmission Control Protocol (TCP) Parameters" registry was defined
in
RFC6824
. Since
RFC6824
is an Experimental RFC and not
a Standards Track RFC, and since no further entries have occurred
beyond those pointing to
RFC6824
, IANA has replaced
the existing registry with the contents of
Table 3
and with the following explanatory note.
Note: This registry specifies the MPTCP Handshake Algorithms for MPTCP v1, which obsoletes the Experimental MPTCP v0. For the MPTCP v0 subtypes, please refer to
RFC6824
Table 3
MPTCP Handshake Algorithms
Flag Bit
Meaning
Reference
Checksum required
RFC 8684,
Section 3.1
Extensibility
RFC 8684,
Section 3.1
Do not attempt to establish new subflows to the source address.
RFC 8684,
Section 3.1
D-G
Unassigned
HMAC-SHA256
RFC 8684,
Section 3.2
Note that the meanings of bits "D" through "H" can be dependent upon bit "B",
depending on how the Extensibility parameter is defined in future specifications; see
Section 3.1
for more information.
Future assignments in this registry are also
to be defined by Standards Action as defined by
RFC8126
Assignments consist of the value of the flags, a symbolic name for the algorithm,
and a reference to its specification.
7.4.
MP_TCPRST Reason Codes
IANA has created a further subregistry, "MPTCP MP_TCPRST
Reason Codes" under the "Transmission Control Protocol (TCP)
Parameters" registry, based on the reason code in the MP_TCPRST (
Section 3.6
) message. Initial values for this registry are given in
Table 4
; future assignments are to be defined by Specification Required as defined by
RFC8126
. Assignments consist of the value of the code, a short description of its meaning, and a reference to its specification. The maximum value is 0xff.
Table 4
MPTCP MP_TCPRST Reason Codes
Code
Meaning
Reference
0x00
Unspecified error
RFC 8684,
Section 3.6
0x01
MPTCP-specific error
RFC 8684,
Section 3.6
0x02
Lack of resources
RFC 8684,
Section 3.6
0x03
Administratively prohibited
RFC 8684,
Section 3.6
0x04
Too much outstanding data
RFC 8684,
Section 3.6
0x05
Unacceptable performance
RFC 8684,
Section 3.6
0x06
Middlebox interference
RFC 8684,
Section 3.6
As guidance to the designated expert
RFC8126
, assignments should not normally be refused unless
codepoint space is becoming scarce, provided that there is a clear
distinction from other, already-existing codes and also provided that there is sufficient guidance for implementers both sending and receiving these codes.
8.
References
8.1.
Normative References
[RFC0793]
Postel, J.
"Transmission Control Protocol"
STD 7
RFC 793
DOI 10.17487/RFC0793
September 1981
[RFC2104]
Krawczyk, H.
, Bellare, M.
, and R. Canetti
"HMAC: Keyed-Hashing for Message Authentication"
RFC 2104
DOI 10.17487/RFC2104
February 1997
[RFC2119]
Bradner, S.
"Key words for use in RFCs to Indicate Requirement Levels"
BCP 14
RFC 2119
DOI 10.17487/RFC2119
March 1997
[RFC5961]
Ramaiah, A.
, Stewart, R.
, and M. Dalal
"Improving TCP's Robustness to Blind In-Window Attacks"
RFC 5961
DOI 10.17487/RFC5961
August 2010
[RFC6234]
Eastlake 3rd, D.
and T. Hansen
"US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)"
RFC 6234
DOI 10.17487/RFC6234
May 2011
[RFC8174]
Leiba, B.
"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"
BCP 14
RFC 8174
DOI 10.17487/RFC8174
May 2017
8.2.
Informative References
[deployments]
Bonaventure, O.
and S. Seo
"Multipath TCP Deployments"
IETF Journal 2016
November 2016
[howhard]
Raiciu, C.
, Paasch, C.
, Barre, S.
, Ford, A.
, Honda, M.
, Duchene, F.
, Bonaventure, O.
, and M. Handley
"How Hard Can It Be? Designing and Implementing a Deployable Multipath TCP"
Usenix Symposium on Networked Systems Design and Implementation 2012
April 2012
[norm]
Handley, M.
, Paxson, V.
, and C. Kreibich
"Network Intrusion Detection: Evasion, Traffic Normalization, and End-to-End Protocol Semantics"
Usenix Security Symposium 2001
August 2001
[RFC1122]
Braden, R., Ed.
"Requirements for Internet Hosts - Communication Layers"
STD 3
RFC 1122
DOI 10.17487/RFC1122
October 1989
[RFC1918]
Rekhter, Y.
, Moskowitz, B.
, Karrenberg, D.
, de Groot, G. J.
, and E. Lear
"Address Allocation for Private Internets"
BCP 5
RFC 1918
DOI 10.17487/RFC1918
February 1996
[RFC2018]
Mathis, M.
, Mahdavi, J.
, Floyd, S.
, and A. Romanow
"TCP Selective Acknowledgment Options"
RFC 2018
DOI 10.17487/RFC2018
October 1996
[RFC2979]
Freed, N.
"Behavior of and Requirements for Internet Firewalls"
RFC 2979
DOI 10.17487/RFC2979
October 2000
[RFC2992]
Hopps, C.
"Analysis of an Equal-Cost Multi-Path Algorithm"
RFC 2992
DOI 10.17487/RFC2992
November 2000
[RFC3022]
Srisuresh, P.
and K. Egevang
"Traditional IP Network Address Translator (Traditional NAT)"
RFC 3022
DOI 10.17487/RFC3022
January 2001
[RFC3135]
Border, J.
, Kojo, M.
, Griner, J.
, Montenegro, G.
, and Z. Shelby
"Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations"
RFC 3135
DOI 10.17487/RFC3135
June 2001
[RFC4086]
Eastlake 3rd, D.
, Schiller, J.
, and S. Crocker
"Randomness Requirements for Security"
BCP 106
RFC 4086
DOI 10.17487/RFC4086
June 2005
[RFC4987]
Eddy, W.
"TCP SYN Flooding Attacks and Common Mitigations"
RFC 4987
DOI 10.17487/RFC4987
August 2007
[RFC5681]
Allman, M.
, Paxson, V.
, and E. Blanton
"TCP Congestion Control"
RFC 5681
DOI 10.17487/RFC5681
September 2009
[RFC6181]
Bagnulo, M.
"Threat Analysis for TCP Extensions for Multipath Operation with Multiple Addresses"
RFC 6181
DOI 10.17487/RFC6181
March 2011
[RFC6182]
Ford, A.
, Raiciu, C.
, Handley, M.
, Barre, S.
, and J. Iyengar
"Architectural Guidelines for Multipath TCP Development"
RFC 6182
DOI 10.17487/RFC6182
March 2011
[RFC6356]
Raiciu, C.
, Handley, M.
, and D. Wischik
"Coupled Congestion Control for Multipath Transport Protocols"
RFC 6356
DOI 10.17487/RFC6356
October 2011
[RFC6528]
Gont, F.
and S. Bellovin
"Defending against Sequence Number Attacks"
RFC 6528
DOI 10.17487/RFC6528
February 2012
[RFC6824]
Ford, A.
, Raiciu, C.
, Handley, M.
, and O. Bonaventure
"TCP Extensions for Multipath Operation with Multiple Addresses"
RFC 6824
DOI 10.17487/RFC6824
January 2013
[RFC6897]
Scharf, M.
and A. Ford
"Multipath TCP (MPTCP) Application Interface Considerations"
RFC 6897
DOI 10.17487/RFC6897
March 2013
[RFC7323]
Borman, D.
, Braden, B.
, Jacobson, V.
, and R. Scheffenegger, Ed.
"TCP Extensions for High Performance"
RFC 7323
DOI 10.17487/RFC7323
September 2014
[RFC7413]
Cheng, Y.
, Chu, J.
, Radhakrishnan, S.
, and A. Jain
"TCP Fast Open"
RFC 7413
DOI 10.17487/RFC7413
December 2014
[RFC7430]
Bagnulo, M.
, Paasch, C.
, Gont, F.
, Bonaventure, O.
, and C. Raiciu
"Analysis of Residual Threats and Possible Fixes for Multipath TCP (MPTCP)"
RFC 7430
DOI 10.17487/RFC7430
July 2015
[RFC8041]
Bonaventure, O.
, Paasch, C.
, and G. Detal
"Use Cases and Operational Experience with Multipath TCP"
RFC 8041
DOI 10.17487/RFC8041
January 2017
[RFC8126]
Cotton, M.
, Leiba, B.
, and T. Narten
"Guidelines for Writing an IANA Considerations Section in RFCs"
BCP 26
RFC 8126
DOI 10.17487/RFC8126
June 2017
[TCPLO]
Ramaiah, A.
"TCP option space extension"
Work in Progress
Internet-Draft, draft-ananth-tcpm-tcpoptext-00
26 March 2012
Appendix A.
Notes on Use of TCP Options
The TCP option space is limited due to the length of the Data Offset field in the TCP header (4 bits), which defines the TCP header length in 32-bit words. With the standard TCP header being 20 bytes, this leaves a maximum of 40 bytes for options, and many of these may already be used by options such as timestamp and SACK.
We performed a brief study on the commonly used TCP options in SYN,
data, and pure ACK packets and found that there is enough room
to fit all the options discussed in this document.
SYN packets typically include the following options: Maximum Segment Size (MSS) (4 bytes),
window scale (3 bytes), SACK permitted (2 bytes), and timestamp
(10 bytes). The sum of these options is 19 bytes. Some operating
systems appear to pad each option up to a word boundary, thus using 24
bytes (a brief survey suggests that Windows XP and Mac OS X do this, whereas Linux does not).
Optimistically, therefore, we have 21 bytes available, or 16 if options have to be
word-aligned. In either case, however, the SYN versions of
MP_CAPABLE (12 bytes) and MP_JOIN (12 or 16 bytes) will fit in this remaining space.
Note that due to the use of a 64-bit data-level sequence space, it is
feasible that MPTCP will not require the timestamp option for
protection against wrapped sequence numbers (per the Protection
Against Wrapped Sequences (PAWS) mechanism, as described in
RFC7323
), since the data-level sequence space has far less
chance of wrapping. Confirmation of the validity of this optimization is
left for further study.
TCP data packets typically carry timestamp options in every packet,
taking 10 bytes (or 12, with padding). That leaves 30 bytes (or 28, if
word-aligned). The DSS option varies in length, depending on (1) whether the
Data Sequence Mapping, DATA_ACK, or both are included, (2) whether the
sequence numbers in use are 4 or 8 octets, and (3) whether the
checksum is present. The maximum size of the DSS option is 28 bytes, so even that will fit in the available space. But unless a connection is both bidirectional and high-bandwidth, it is unlikely that all that option space will be required on each DSS option.
Within the DSS option, it is not necessary to include the Data Sequence Mapping and DATA_ACK in each packet, and in many cases it may be possible to alternate their presence (so long as the mapping covers the data being sent in the subsequent packet). It would also be possible to alternate between 4-byte and 8-byte sequence numbers in each option.
On subflow and connection setup, an MPTCP option is also set on the third packet (an ACK). These are 20 bytes (for MP_CAPABLE) and 24 bytes (for MP_JOIN), both of which will fit in the available option space.
Pure ACKs in TCP typically contain only timestamps (10 bytes). Here, Multipath TCP typically
needs to encode only the DATA_ACK (maximum of 12 bytes). Occasionally, ACKs will contain SACK information. Depending
on the number of lost packets, SACK may utilize the entire option space. If a DATA_ACK had to be
included, then it is probably necessary to reduce the number of SACK blocks to accommodate the
DATA_ACK. However, the presence of the DATA_ACK is unlikely to be necessary in a case where SACK is
in use, since until at least some of the SACK blocks have been retransmitted, the cumulative
data-level ACK will not be moving forward (or if it does, due to retransmissions on another path,
then that path can also be used to transmit the new DATA_ACK).
The ADD_ADDR option can be between 16 and 30 bytes, depending on
(1) whether IPv4 or IPv6 is used and (2) whether or not the port number is
present. It is unlikely that such signaling would fit in a data packet
(although if there is space, it is fine to include it). It is
recommended that duplicate ACKs not be used with any other payload or options, in
order to transmit these rare signals. Note that this is the reason for
mandating that duplicate ACKs with MPTCP options not be taken as a signal of congestion.
Appendix B.
TCP Fast Open and MPTCP
TCP Fast Open (TFO) is an experimental TCP extension, described in
RFC7413
, which has been introduced to
allow the sending of data
one RTT earlier than with regular TCP. This is
considered a valuable gain, as very short connections are very common,
especially for HTTP request/response schemes. It achieves this by sending
the SYN segment together with the application's data and allowing the listener to reply
immediately with data after the SYN/ACK.
RFC7413
secures
this mechanism by using a new TCP option that includes a cookie that
is negotiated in a preceding connection.
When using TFO in conjunction with MPTCP, there are two key
points to take into account, as detailed below.
B.1.
TFO Cookie Request with MPTCP
When a TFO initiator first connects to a listener, it cannot immediately
include data in the SYN for security reasons
RFC7413
Instead, it requests a cookie that will be used in subsequent
connections. This is done with the TCP cookie request/response options,
of 2 bytes and 6-18 bytes, respectively (depending on the chosen cookie length).
TFO and MPTCP can be combined, provided that the total length of all the
options does not exceed the maximum 40 bytes possible in TCP:
In the SYN: MPTCP uses a 4-byte MP_CAPABLE option. The sum
of the MPTCP and TFO options is 6 bytes. With typical TCP options using up
to 19 bytes in the SYN (24 bytes if options are padded at a word boundary),
there is enough space to combine the MP_CAPABLE with the TFO cookie request.
In the SYN + ACK: MPTCP uses a 12-byte MP_CAPABLE option, but
now the TFO option can be as long as 18 bytes. Since the maximum option length
may be exceeded, it is up to the listener to avoid this problem by using a
shorter cookie.
As an example, if we consider that 19 bytes are used for classical
TCP options, the maximum possible cookie length would be
7 bytes. Note that, for the SYN packet, the same limitation applies to subsequent
connections (because the initiator then echoes
the cookie back to the listener). Finally, if the security impact of reducing
the cookie size is not deemed acceptable, the listener can reduce the
amount of space used by other TCP options by omitting the TCP timestamps (as
outlined in
Appendix A
).
B.2.
Data Sequence Mapping under TFO
In the TCP establishment phase, MPTCP uses a key exchange that is
used to generate the Initial Data Sequence Numbers (IDSNs). In particular,
the SYN with MP_CAPABLE occupies the first octet of data sequence
space. With TFO, one way to handle the data sent together with the SYN
would be to consider an implicit DSS mapping that covers that SYN segment
(since there is not enough space in the SYN to include a DSS option).
The problem with that approach is that if a middlebox modifies the TFO
data, this will not be noticed by MPTCP because of the absence of a
DSS checksum. For example, a TCP‑aware (but not MPTCP-aware) middlebox could
insert bytes at the beginning of the stream and adapt the TCP checksum
and sequence numbers accordingly. With an implicit mapping, this information would
give to the initiator and listener a different view of the DSS
mapping; there would be no
way to detect this inconsistency, because the DSS checksum is not present.
To solve this issue, the TFO data must not be considered part of the
data sequence number space: the SYN with MP_CAPABLE still occupies
the first octet of data sequence space, but then the first non-TFO
data byte occupies the second octet. This guarantees that, if the
use of the DSS checksum is negotiated, all data in the data sequence
number space is checksummed. We also note that this does not entail
a loss of functionality, because TFO data is always only sent on the
initial subflow, before any attempt to create additional subflows.
B.3.
Connection Establishment Examples
A few examples of possible "TFO + MPTCP"
establishment scenarios are shown below.
Before an initiator can send data together with the SYN, it must request
a cookie from the listener, as shown in
Figure 18
. (Note: The sequence number
and length are annotated in
Figure 18
as
Seq(Length) (e.g., "S. 0(0)") and used as such in the subsequent figures
(e.g., "S 0(20)" in
Figure 19
).) This is done by simply combining the TFO and MPTCP options.
initiator listener
| |
| S Seq=0(Length=0)
| --------------------------------------------------------> |
| |
| S. 0(0) ack 1
| <-------------------------------------------------------- |
| |
| . 0(0) ack 1
| --------------------------------------------------------> |
| |
Figure 18
Cookie Request
Once this is done, the received cookie can be used for TFO, as shown
in
Figure 19
. In this example, the initiator first
sends 20 bytes in the SYN. The listener immediately replies with 100 bytes
following the SYN-ACK, to which the initiator replies with 20 more bytes.
Note that the last segment in the figure
has a TCP sequence number of 21, while the DSS subflow sequence
number is 1 (because the TFO data is not part of the data sequence
number space, as explained in
Appendix B.2
initiator listener
| |
| S 0(20)
| --------------------------------------------------------> |
| |
| S. 0(0) ack 21
| <-------------------------------------------------------- |
| |
| . 1(100) ack 21
| <-------------------------------------------------------- |
| |
| . 21(0) ack 1
| --------------------------------------------------------> |
| |
| . 21(20) ack 101
| --------------------------------------------------------> |
| |
Figure 19
The Listener Supports TFO
In
Figure 20
, the listener does not support TFO. The initiator detects
that no state is created in the listener (as no data is ACKed) and now
sends the MP_CAPABLE in the third packet, in order for the listener to
build its MPTCP context at the end of the establishment. Now, the
TFO data, when retransmitted, becomes part of the Data Sequence Mapping
because it is effectively sent (in fact re‑sent) after the
establishment.
initiator listener
| |
| S 0(20)
| --------------------------------------------------------> |
| |
| S. 0(0) ack 1
| <-------------------------------------------------------- |
| |
| . 1(0) ack 1
| --------------------------------------------------------> |
| |
| . 1(20) ack 1
| --------------------------------------------------------> |
| |
| . 0(0) ack 21
| <-------------------------------------------------------- |
| |
Figure 20
The Listener Does Not Support TFO
It is also possible that the listener acknowledges only part of the TFO
data, as illustrated in
Figure 21
. The
initiator will simply retransmit the missing data together with a
DSS mapping.
initiator listener
| |
| S 0(1000)
| --------------------------------------------------------> |
| |
| S. 0(0) ack 501
| <-------------------------------------------------------- |
| |
| . 501(0) ack 1
| --------------------------------------------------------> |
| |
| . 501(500) ack 1
| --------------------------------------------------------> |
| |
Figure 21
Partial Data Acknowledgment
Appendix C.
Control Blocks
Conceptually, an MPTCP connection can be represented as an MPTCP protocol control
block (PCB) that contains several variables that track the progress and the
state of the MPTCP connection and a set of linked TCP control blocks
that correspond to the subflows that have been established.
RFC 793
RFC0793
specifies several state variables. Whenever possible, we reuse
the same terminology as RFC 793 to describe the state variables that are
maintained by MPTCP.
C.1.
MPTCP Control Block
The MPTCP control block contains the following variables per connection.
C.1.1.
Authentication and Metadata
Local.Token (32 bits):
This is the token chosen by the local host on
this MPTCP connection. The token must be unique among all established
MPTCP connections and is generated from the local key.
Local.Key (64 bits):
This is the key sent by the local host on this
MPTCP connection.
Remote.Token (32 bits):
This is the token chosen by the remote host on
this MPTCP connection, generated from the remote key.
Remote.Key (64 bits):
This is the key chosen by the remote host on
this MPTCP connection.
MPTCP.Checksum (flag):
This flag is set to true if at least one of the
hosts has set the "A" bit in the MP_CAPABLE options exchanged during
connection establishment; otherwise,
it is set to false. If this flag is set, the checksum must be computed in
all DSS options.
C.1.2.
Sending Side
SND.UNA (64 bits):
This is the data sequence number of the next byte to be
acknowledged, at the MPTCP connection level. This variable is updated
upon reception of a DSS option containing a DATA_ACK.
SND.NXT (64 bits):
This is the data sequence number of the next byte to be
sent. SND.NXT is used to determine the value of the DSN in the DSS option.
SND.WND (32 bits):
This is the send window. 32 bits if the features in RFC
7323 are used; 16 bits otherwise. MPTCP maintains the send window at the MPTCP connection level, and the same
window is shared by all subflows. All subflows use the MPTCP connection-level
SND.WND to compute the SEQ.WND value that is sent in each
transmitted segment.
C.1.3.
Receiving Side
RCV.NXT (64 bits):
This is the data sequence number of the next byte that
is expected on the MPTCP connection. This state variable is modified
upon reception of in-order data. The value of RCV.NXT is used to specify
the DATA_ACK that is sent in the DSS option on all subflows.
RCV.WND (32 bits):
This is the connection-level receive window, which is the
maximum of the RCV.WND on all the subflows. 32 bits if the features in RFC 7323 are used; 16 bits otherwise.
C.2.
TCP Control Blocks
The MPTCP control block also contains a list of the TCP control blocks
that are associated with the MPTCP connection.
Note that the TCP control block on the TCP subflows does not contain the
RCV.WND and SND.WND state variables, as these are maintained at the MPTCP
connection level and not at the subflow level.
Inside each TCP control block, the following state variables are defined.
C.2.1.
Sending Side
SND.UNA (32 bits):
This is the sequence number of the next byte to be
acknowledged on the subflow. This variable is updated upon reception of
each TCP acknowledgment on the subflow.
SND.NXT (32 bits):
This is the sequence number of the next byte to be
sent on the subflow. SND.NXT is used to set the value of SEG.SEQ upon
transmission of the next segment.
C.2.2.
Receiving Side
RCV.NXT (32 bits):
This is the sequence number of the next byte that
is expected on the subflow. This state variable is modified upon
reception of in-order segments. The value of RCV.NXT is copied to the
SEG.ACK field of the next segments transmitted on the subflow.
RCV.WND (32 bits):
This is the subflow-level receive window that is updated with
the window field from the segments received on this subflow. 32
bits if the features in RFC 7323 are used; 16 bits otherwise.
Appendix D.
Finite State Machine
The diagram in
Figure 22
shows the
Finite State Machine for connection-level closure. This illustrates how
the DATA_FIN connection-level signal (indicated in the diagram as the
DFIN flag on a DATA_ACK) (1) interacts with subflow-level FINs and (2) permits break-before-make handover between subflows.
+---------+
| M_ESTAB |
+---------+
M_CLOSE | | rcv DATA_FIN
------- | | -------
+---------+ snd DATA_FIN / \ snd DATA_ACK[DFIN] +-------+
| M_FIN |<----------------- ------------------->|M_CLOSE|
| WAIT-1 |--------------------------- | WAIT |
+---------+ rcv DATA_FIN \ +-------+
| rcv DATA_ACK[DFIN] ------- | M_CLOSE |
| -------------- snd DATA_ACK | ------- |
| CLOSE all subflows | snd DATA_FIN |
V V V
+-----------+ +-----------+ +----------+
|M_FINWAIT-2| | M_CLOSING | |M_LAST-ACK|
+-----------+ +-----------+ +----------+
| rcv DATA_ACK[DFIN] | rcv DATA_ACK[DFIN] |
| rcv DATA_FIN -------------- | -------------- |
| ------- CLOSE all subflows | CLOSE all subflows |
| snd DATA_ACK[DFIN] V delete MPTCP PCB V
\ +-----------+ +--------+
------------------------>|M_TIME WAIT|---------------->|M_CLOSED|
+-----------+ +--------+
All subflows in CLOSED
------------
delete MPTCP PCB
Figure 22
Finite State Machine for Connection Closure
Appendix E.
Changes from RFC 6824
This appendix lists the key technical changes between
RFC6824
which specifies MPTCP v0; and this document, which obsoletes
RFC6824
and specifies MPTCP v1. Note that this specification is not backward compatible with
RFC6824
This document incorporates lessons learned from the various implementations, deployments, and experiments gathered in the documents "Use Cases and Operational Experience with Multipath TCP"
RFC8041
and the IETF Journal article "Multipath TCP Deployments"
deployments
Connection initiation, through the exchange of the MP_CAPABLE
MPTCP option, is different from
RFC6824
. The SYN no longer
includes the initiator's key, to allow the MP_CAPABLE option on the SYN to be shorter in length and to avoid duplicating the sending of keying material.
This also ensures reliable delivery of the key on the MP_CAPABLE
option by allowing its transmission to be combined with data and thus
using TCP's built-in reliability mechanism. If the initiator does not
immediately have data to send, the MP_CAPABLE option with the keys
will be repeated on the first data packet. If the other end is the first to send, then the presence of the DSS option implicitly confirms the receipt of the MP_CAPABLE.
In the Flags field of MP_CAPABLE, "C" is now assigned to mean that
the sender of this option will not accept additional MPTCP subflows to
the source address and port. This improves efficiency -- for example,
in cases where the sender is behind a strict NAT.
In the Flags field of MP_CAPABLE, "H" now indicates the use of HMAC-SHA256 (rather than HMAC-SHA1).
Connection initiation also defines the procedure for version negotiation, for implementations that support both v0
RFC6824
and v1 (this document).
The HMAC-SHA256 (rather than HMAC-SHA1) algorithm is used, as it provides better security. It is used to generate the token in the MP_JOIN and ADD_ADDR messages and to set the IDSN.
A new subflow-level option exists to signal reasons for sending a
RST on a subflow (MP_TCPRST (
Section 3.6
)); this can help an implementation decide whether to attempt later reconnection.
The MP_PRIO option (
Section 3.3.8
),
which is used to signal a change of priority for a subflow, no longer
includes the AddrID field. Its purpose was to allow the changed
priority to be applied on a subflow other than the one it was sent
on. However, it was determined that this could be used by a
man-in-the-middle to divert all traffic onto its own path, and MP_PRIO
does not include a token or other type of security mechanism.
The ADD_ADDR option (
Section 3.4.1
), which is used to inform the other host about another potential address, is different in several ways. It now includes an HMAC of the added address, for enhanced security. In addition, reliability for the ADD_ADDR option has been added: the IPVer field is replaced with a flag field, and one flag is assigned ("E") that is used as an "echo" so a host can indicate that it has received the option.
This document describes an additional way of performing a Fast
Close -- by sending an MP_FASTCLOSE option on a RST on all subflows. This allows the host to tear down the subflows and the connection immediately.
IANA has reserved the MPTCP option subtype of value 0xf for
Private Use (
Section 7.2
). This document doesn't define how to use that value.
This document adds a new appendix (
Appendix B
), which discusses the usage of both MPTCP options
and TFO options on the same packet.
Acknowledgments
The authors gratefully acknowledge significant input into this
document from
Sebastien Barre
and
Andrew McDonald
The authors also wish to acknowledge reviews and contributions from
Iljitsch van Beijnum
Lars Eggert
Marcelo Bagnulo
Robert Hancock
Pasi Sarolahti
Toby Moncaster
Philip Eardley
Sergio Lembo
Lawrence Conroy
Yoshifumi Nishida
Bob Briscoe
Stein Gjessing
Andrew McGregor
Georg Hampel
Anumita Biswas
Wes Eddy
Alexey Melnikov
Francis Dupont
Adrian Farrel
Barry Leiba
Robert Sparks
Sean Turner
Stephen Farrell
Martin Stiemerling
Gregory Detal
Fabien Duchene
Xavier de Foy
Rahul Jadhav
Klemens Schragel
Mirja Kühlewind
Sheng Jiang
Alissa Cooper
Ines Robles
Roman Danyliw
Adam Roach
Eric Vyncke
, and
Ben Kaduk
Authors' Addresses
Alan Ford
Pexip
Email:
alan.ford@gmail.com
Costin Raiciu
University Politehnica of Bucharest
Splaiul Independentei 313
Bucharest
Romania
Email:
costin.raiciu@cs.pub.ro
Mark Handley
University College London
Gower Street
London
WC1E 6BT
United Kingdom
Email:
m.handley@cs.ucl.ac.uk
Olivier Bonaventure
Université catholique de Louvain
Pl. Ste Barbe, 2
1348
Louvain-la-Neuve
Belgium
Email:
olivier.bonaventure@uclouvain.be
Christoph Paasch
Apple, Inc.
Cupertino
CA
United States of America
Email:
cpaasch@apple.com
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- Proposed Standard
March 2020
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Mark J. Handley
Olivier Bonaventure
Christoph Paasch
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