draft-ietf-mptcp-multiaddressed-02.txt   draft-ietf-mptcp-multiaddressed-03.txt 
Internet Engineering Task Force A. Ford Internet Engineering Task Force A. Ford
Internet-Draft Roke Manor Research Internet-Draft Roke Manor Research
Intended status: Experimental C. Raiciu Intended status: Experimental C. Raiciu
Expires: April 28, 2011 M. Handley Expires: September 15, 2011 M. Handley
University College London University College London
October 25, 2010 O. Bonaventure
Universite catholique de
Louvain
March 14, 2011
TCP Extensions for Multipath Operation with Multiple Addresses TCP Extensions for Multipath Operation with Multiple Addresses
draft-ietf-mptcp-multiaddressed-02 draft-ietf-mptcp-multiaddressed-03
Abstract Abstract
TCP/IP communication is currently restricted to a single path per TCP/IP communication is currently restricted to a single path per
connection, yet multiple paths often exist between peers. The connection, yet multiple paths often exist between peers. The
simultaneous use of these multiple paths for a TCP/IP session would simultaneous use of these multiple paths for a TCP/IP session would
improve resource usage within the network, and thus improve user improve resource usage within the network, and thus improve user
experience through higher throughput and improved resilience to experience through higher throughput and improved resilience to
network failure. network failure.
Multipath TCP provides the ability to simultaneously use multiple Multipath TCP provides the ability to simultaneously use multiple
paths between peers. This document presents a set of extensions to paths between peers. This document presents a set of extensions to
traditional TCP to support multipath operation. The protocol offers traditional TCP to support multipath operation. The protocol offers
the same type of service to applications as TCP - reliable bytestream the same type of service to applications as TCP (i.e. reliable
- and provides the components necessary to establish and use multiple bytestream), and provides the components necessary to establish and
TCP flows across potentially disjoint paths. use multiple TCP flows across potentially disjoint paths.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 28, 2011. This Internet-Draft will expire on September 15, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Design Assumptions . . . . . . . . . . . . . . . . . . . . 4 1.1. Design Assumptions . . . . . . . . . . . . . . . . . . . . 4
1.2. Multipath TCP in the Networking Stack . . . . . . . . . . 5 1.2. Multipath TCP in the Networking Stack . . . . . . . . . . 5
1.3. Operation Summary . . . . . . . . . . . . . . . . . . . . 6 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. Requirements Language . . . . . . . . . . . . . . . . . . 7 1.4. MPTCP Concept . . . . . . . . . . . . . . . . . . . . . . 6
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.5. Requirements Language . . . . . . . . . . . . . . . . . . 7
3. MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 8 2. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Connection Initiation . . . . . . . . . . . . . . . . . . 8 3. MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Starting a New Subflow . . . . . . . . . . . . . . . . . . 11 3.1. Connection Initiation . . . . . . . . . . . . . . . . . . 10
3.3. General MPTCP Operation . . . . . . . . . . . . . . . . . 15 3.2. Starting a New Subflow . . . . . . . . . . . . . . . . . . 14
3.3.1. Data Sequence Numbering . . . . . . . . . . . . . . . 15 3.3. General MPTCP Operation . . . . . . . . . . . . . . . . . 18
3.3.2. Data Acknowledgements . . . . . . . . . . . . . . . . 17 3.3.1. Data Sequence Mapping . . . . . . . . . . . . . . . . 20
3.3.3. Receiver Considerations . . . . . . . . . . . . . . . 18 3.3.2. Data Acknowledgements . . . . . . . . . . . . . . . . 23
3.3.4. Sender Considerations . . . . . . . . . . . . . . . . 19 3.3.3. Closing a Connection . . . . . . . . . . . . . . . . . 24
3.3.5. Congestion Control Considerations . . . . . . . . . . 21 3.3.4. Receiver Considerations . . . . . . . . . . . . . . . 25
3.3.6. Subflow Policy . . . . . . . . . . . . . . . . . . . . 21 3.3.5. Sender Considerations . . . . . . . . . . . . . . . . 26
3.4. Closing a Connection . . . . . . . . . . . . . . . . . . . 22 3.3.6. Reliability and Retransmissions . . . . . . . . . . . 27
3.5. Address Knowledge Exchange (Path Management) . . . . . . . 24 3.3.7. Congestion Control Considerations . . . . . . . . . . 28
3.5.1. Address Advertisement . . . . . . . . . . . . . . . . 25 3.3.8. Subflow Policy . . . . . . . . . . . . . . . . . . . . 28
3.5.2. Remove Address . . . . . . . . . . . . . . . . . . . . 27 3.4. Address Knowledge Exchange (Path Management) . . . . . . . 30
3.6. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.4.1. Address Advertisement . . . . . . . . . . . . . . . . 31
3.7. Error Handling . . . . . . . . . . . . . . . . . . . . . . 31 3.4.2. Remove Address . . . . . . . . . . . . . . . . . . . . 33
3.8. Heuristics . . . . . . . . . . . . . . . . . . . . . . . . 32 3.5. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.8.1. Port Usage . . . . . . . . . . . . . . . . . . . . . . 32 3.6. Error Handling . . . . . . . . . . . . . . . . . . . . . . 37
4. Semantic Issues . . . . . . . . . . . . . . . . . . . . . . . 32 3.7. Heuristics . . . . . . . . . . . . . . . . . . . . . . . . 37
5. Security Considerations . . . . . . . . . . . . . . . . . . . 34 3.7.1. Port Usage . . . . . . . . . . . . . . . . . . . . . . 38
6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 34 3.7.2. Delayed Subflow Start . . . . . . . . . . . . . . . . 38
7. Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3.7.3. Failure Handling . . . . . . . . . . . . . . . . . . . 39
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 38 4. Semantic Issues . . . . . . . . . . . . . . . . . . . . . . . 39
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38 5. Security Considerations . . . . . . . . . . . . . . . . . . . 41
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39 6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 42
10.1. Normative References . . . . . . . . . . . . . . . . . . . 39 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 45
10.2. Informative References . . . . . . . . . . . . . . . . . . 39 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45
Appendix A. Notes on use of TCP Options . . . . . . . . . . . . . 40 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Appendix B. Resync Packet . . . . . . . . . . . . . . . . . . . . 42 9.1. Normative References . . . . . . . . . . . . . . . . . . . 46
Appendix C. Changelog . . . . . . . . . . . . . . . . . . . . . . 42 9.2. Informative References . . . . . . . . . . . . . . . . . . 46
C.1. Changes since draft-ietf-mptcp-multiaddressed-01 . . . . . 42 Appendix A. Notes on use of TCP Options . . . . . . . . . . . . . 48
C.2. Changes since draft-ietf-mptcp-multiaddressed-00 . . . . . 43 Appendix B. Control Blocks . . . . . . . . . . . . . . . . . . . 49
C.3. Changes since draft-ford-mptcp-multiaddressed-03 . . . . . 43 B.1. MPTCP Control Block . . . . . . . . . . . . . . . . . . . 50
C.4. Changes since draft-ford-mptcp-multiaddressed-02 . . . . . 43 B.1.1. Authentication and Metadata . . . . . . . . . . . . . 50
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43 B.1.2. Sending Side . . . . . . . . . . . . . . . . . . . . . 50
B.1.3. Receiving Side . . . . . . . . . . . . . . . . . . . . 51
B.2. TCP Control Blocks . . . . . . . . . . . . . . . . . . . . 51
B.2.1. Sending Side . . . . . . . . . . . . . . . . . . . . . 51
B.2.2. Receiving Side . . . . . . . . . . . . . . . . . . . . 51
Appendix C. Changelog . . . . . . . . . . . . . . . . . . . . . . 52
C.1. Changes since draft-ietf-mptcp-multiaddressed-02 . . . . . 52
C.2. Changes since draft-ietf-mptcp-multiaddressed-01 . . . . . 52
C.3. Changes since draft-ietf-mptcp-multiaddressed-00 . . . . . 52
C.4. Changes since draft-ford-mptcp-multiaddressed-03 . . . . . 52
C.5. Changes since draft-ford-mptcp-multiaddressed-02 . . . . . 53
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 53
1. Introduction 1. Introduction
Multipath TCP (henceforth referred to as MPTCP) is a set of MPTCP is a set of extensions to regular TCP [2] to provide a
extensions to regular TCP [2] to allow a transport connection to Multipath TCP [3] service, which enables a transport connection to
operate across multiple paths simultaneously. This document presents operate across multiple paths simultaneously. This document presents
the protocol changes required to add multipath capability to TCP; the protocol changes required to add multipath capability to TCP;
specifically, those for signalling and setting up multiple paths specifically, those for signalling and setting up multiple paths
("subflows"), managing these subflows, reassembly of data, and ("subflows"), managing these subflows, reassembly of data, and
termination of sessions. This is not the only information required termination of sessions. This is not the only information required
to create a Multipath TCP implementation, however. This document is to create a Multipath TCP implementation, however. This document is
complemented by several others: complemented by three others:
o Architecture [3], which explains the motivations behind Multipath o Architecture [3], which explains the motivations behind Multipath
TCP, contains a discussion of high-level design decisions on which TCP, contains a discussion of high-level design decisions on which
this design is based, and an explanation of a functional this design is based, and an explanation of a functional
separation through which an extensible MPTCP implementation can be separation through which an extensible MPTCP implementation can be
developed. developed.
o Congestion Control [4], presenting a safe congestion control o Congestion Control [4], presenting a safe congestion control
algorithm for coupling the behaviour of the multiple paths in algorithm for coupling the behaviour of the multiple paths in
order to "do no harm" to other network users. order to "do no harm" to other network users.
skipping to change at page 4, line 41 skipping to change at page 4, line 41
1.1. Design Assumptions 1.1. Design Assumptions
In order to limit the potentially huge design space, the authors In order to limit the potentially huge design space, the authors
imposed two key constraints on the multipath TCP design presented in imposed two key constraints on the multipath TCP design presented in
this document: this document:
o It must be backwards-compatible with current, regular TCP, to o It must be backwards-compatible with current, regular TCP, to
increase its chances of deployment increase its chances of deployment
o It can be assumed that one or both endpoints are multihomed and o It can be assumed that one or both hosts are multihomed and
multiaddressed multiaddressed
To simplify the design we assume that the presence of multiple To simplify the design we assume that the presence of multiple
addresses at an endpoint is sufficient to indicate the existence of addresses at a host is sufficient to indicate the existence of
multiple paths. These paths need not be entirely disjoint: they may multiple paths. These paths need not be entirely disjoint: they may
share one or many routers between them. Even in such a situation share one or many routers between them. Even in such a situation
making use of multiple paths is beneficial, improving resource making use of multiple paths is beneficial, improving resource
utilisation and resilience to a subset of node failures. The utilisation and resilience to a subset of node failures. The
congestion control algorithms as discussed in [4] ensure this does congestion control algorithms as discussed in [4] ensure this does
not act detrimentally. not act detrimentally.
There are three aspects to the backwards-compatibility listed above There are three aspects to the backwards-compatibility listed above
(discussed in more detail in [3]): (discussed in more detail in [3]):
skipping to change at page 5, line 26 skipping to change at page 5, line 26
Application Constraints: The protocol must be usable with no change Application Constraints: The protocol must be usable with no change
to existing applications that use the standard TCP API (although to existing applications that use the standard TCP API (although
it is reasonable that not all features would be available to such it is reasonable that not all features would be available to such
legacy applications). Furthermore, the protocol must provide the legacy applications). Furthermore, the protocol must provide the
same service model as regular TCP to the application. same service model as regular TCP to the application.
Fall-back: The protocol should be able to fall back to standard TCP Fall-back: The protocol should be able to fall back to standard TCP
with no interference from the user, to be able to communicate with with no interference from the user, to be able to communicate with
legacy hosts. legacy hosts.
Areas for further study: Further discussion of the design constraints and associated design
decisions are given in the MPTCP Architecture document [3].
o In theory, since this is purely a TCP extension, it should be
possible to use MPTCP with both IPv4 and IPv6 subflows for the
same connection on dual-stack hosts, thus having the additional
possible benefit of aiding transition.
o The design presented should work with network provided multipath,
for instance ECMP routing; subflows could be opened with different
source/destination ports between the same addreses to allow ECMP
to place the subflows on different paths.
1.2. Multipath TCP in the Networking Stack 1.2. Multipath TCP in the Networking Stack
MPTCP operates at the transport layer and aims to be transparent to 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 both higher and lower layers. It is a set of additional features on
top of standard TCP; Figure 1 illustrates this layering. MPTCP is top of standard TCP; Figure 1 illustrates this layering. MPTCP is
designed to be usable by legacy applications with no changes; designed to be usable by legacy applications with no changes;
detailed discussion of its interactions with applications is given in detailed discussion of its interactions with applications is given in
[5]. [5].
skipping to change at page 6, line 17 skipping to change at page 6, line 5
+---------------+ +-------------------------------+ +---------------+ +-------------------------------+
| Application | | MPTCP | | Application | | MPTCP |
+---------------+ + - - - - - - - + - - - - - - - + +---------------+ + - - - - - - - + - - - - - - - +
| TCP | | Subflow (TCP) | Subflow (TCP) | | TCP | | Subflow (TCP) | Subflow (TCP) |
+---------------+ +-------------------------------+ +---------------+ +-------------------------------+
| IP | | IP | IP | | IP | | IP | IP |
+---------------+ +-------------------------------+ +---------------+ +-------------------------------+
Figure 1: Comparison of Standard TCP and MPTCP Protocol Stacks Figure 1: Comparison of Standard TCP and MPTCP Protocol Stacks
Detailed discussion of an architecture for developing a multipath TCP 1.3. Terminology
implementation, especially regarding the functional separation by
which different components should be developed, is given in [3].
1.3. Operation Summary Path: A sequence of links between a sender and a receiver, defined
in this context by a source and destination address pair.
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: A end host operating an MPTCP implementation, and either
initiating or accepting an MPTCP connection.
1.4. MPTCP Concept
This section provides a high-level summary of normal operation of This section provides a high-level summary of normal operation of
MPTCP, and is illustrated by the scenario shown in Figure 2. A MPTCP, and is illustrated by the scenario shown in Figure 2. A
detailed description of operation is given in Section 3. detailed description of operation is given in Section 3.
o To a non-MPTCP-aware application, MPTCP will behave the same as o To a non-MPTCP-aware application, MPTCP will behave the same as
normal TCP. Extended APIs could provide additional control to normal TCP. Extended APIs could provide additional control to
MPTCP-aware applications [5]. An application begins by opening a MPTCP-aware applications [5]. An application begins by opening a
TCP socket in the normal way. MPTCP signaling and operation is TCP socket in the normal way. MPTCP signaling and operation is
handled by the MPTCP implementation. handled by the MPTCP implementation.
skipping to change at page 6, line 46 skipping to change at page 7, line 6
respectively. respectively.
o If extra paths are available, additional TCP sessions (termed o If extra paths are available, additional TCP sessions (termed
"subflows") are created on these paths, and are combined with the "subflows") are created on these paths, and are combined with the
existing session, which continues to appear as a single connection existing session, which continues to appear as a single connection
to the applications at both ends. The creation of the additional to the applications at both ends. The creation of the additional
TCP session is illustrated between Address A2 on Host A and TCP session is illustrated between Address A2 on Host A and
Address B1 on Host B. Address B1 on Host B.
o MPTCP identifies multiple paths by the presence of multiple o MPTCP identifies multiple paths by the presence of multiple
addresses at endpoints. Combinations of these multiple addresses addresses at hosts. Combinations of these multiple addresses
equate to the additional paths. In the example, other potential equate to the additional paths. In the example, other potential
paths that could be set up are A1<->B2 and A2<->B2. Although this 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 additional session is shown as being initiated from A2, it could
equally have been initiated from B1. equally have been initiated from B1.
o The discovery and setup of additional subflows will be achieved o The discovery and setup of additional subflows will be achieved
through a path management method; this document describes a through a path management method; this document describes a
mechanism by which an endpoint can initiate new subflows by using mechanism by which a host can initiate new subflows by using its
its own additional addresses, or by signalling its available own additional addresses, or by signalling its available addresses
addresses to the other endpoint. to the other host.
o MPTCP adds connection-level sequence numbers to allow the o MPTCP adds connection-level sequence numbers to allow the
reassembly of the in-order data stream from multiple subflows reassembly of the in-order data stream from multiple subflows
which may deliver packets out-of-order due to differing network which may deliver packets out-of-order due to differing network
delays. delays.
o Subflows are terminated as regular TCP connections, with a four o Subflows are terminated as regular TCP connections, with a four
way FIN handshake. The MPTCP connection is terminated by a way FIN handshake. The MPTCP connection is terminated by a
connection-level FIN packet, sent together with the FIN on the connection-level FIN.
last subflow of the connection.
Host A Host B Host A Host B
------------------------ ------------------------ ------------------------ ------------------------
Address A1 Address A2 Address B1 Address B2 Address A1 Address A2 Address B1 Address B2
---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------
| | | | | | | |
| (initial connection setup) | | | (initial connection setup) | |
|----------------------------------->| | |----------------------------------->| |
|<-----------------------------------| | |<-----------------------------------| |
| | | | | | | |
| (additional subflow setup) | | (additional subflow setup) |
| |--------------------->| | | |--------------------->| |
| |<---------------------| | | |<---------------------| |
| | | | | | | |
| | | | | | | |
Figure 2: Example MPTCP Usage Scenario Figure 2: Example MPTCP Usage Scenario
1.4. Requirements Language 1.5. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1]. document are to be interpreted as described in RFC 2119 [1].
2. Terminology 2. Operation Overview
Path: A sequence of links between a sender and a receiver, defined This section presents a single description of standard MPTCP
in this context by a source and destination address pair. operation, with reference to the protocol operation. The detailed
protocol specification follows in Section 3.
Subflow: A stream of TCP packets sent over a path, started and To understand the operation of Multipath TCP, let us consider a very
terminated similarly to a regular TCP connection. simple case where a client having two addresses, A1 and A2
establishes an MPTCP connection with a dual homed server having
addresses B1 and B2, as illustrated in Figure 2 in the previous
section. MPTCP offers the same bidirectional bytestream service as
regular TCP.
(MPTCP) Connection: A collection of one or more subflows, over which To open an MPTCP connection, the client sends a SYN segment from one
an application can communicate between two endpoints. There is a of its addresses (say A1) to one of the server's addresses (say B1).
one-to-one mapping between a connection and an application socket. This SYN segment contains the MP_CAPABLE option that indicates that
the client supports MPTCP and contains the client's key for this
MPTCP connection. The server replies with a SYN segment that also
contains the MP_CAPABLE option to confirm that it supports MPTCP.
The MP_CAPABLE option returned by the server includes the server's
key. The client are server keys are used for different purposes by
MPTCP. First, each host derives a 32 bits token that uniquely
identifies the MPTCP connection on this host. Second, the keys are
used to authenticate the utilisation of other addresses. Additional
details about the utilisation of the MP_CAPABLE option may be found
in Section 3.1.
Data-level: The payload data is nominally transfered over a To enable the client and the server to use their multiple addresses
connection, which in turn is transported over subflows. Thus the to support the same MPTCP connection, MPTCP allows the client and the
term "data-level" is synonymous with "connection level", in server to open additional subflows. These subflows are TCP
contrast to "subflow-level" which refers to properties of an connections that are linked to the MPTCP connection and can be used
individual subflow. to send and receive data. The client can open an additional subflow
by sending a SYN segment from another address (e.g. A2) with the
MP_JOIN option to the server. The MP_JOIN option contains the
server's token that uniquely identifies the MPTCP connection to which
the subflow must be associated and a random number. To accept the
subflow, the server replies by sending a SYN+ACK segment with the
MP_JOIN option that contains a random number chosen by the server and
a HMAC computed over the client and server's random numbers with the
client and server keys. This HMAC authenticates the server to the
client. Upon reception of this SYN+ACK segment, the client replies
with an ACK segment that contains an MP_JOIN option that includes
another HMAC that authenticates the client to the server. Additional
details about the utilisation of the MP_JOIN option may be found in
Section 3.2.
Token: A locally unique identifier given to a multipath connection The server may also establish one or more subflows with the client by
by an endpoint. May also be referred to as a "Connection ID". sending SYN segments with the MP_JOIN option that has been briefly
described above. Furthermore, a host my also inform the other host
of the IP addresses that it owns. MPTCP uses two options for this
purpose. The ADD_ADDR option allows a host to indicates that it owns
another address. For example, in the above scenario, the server
could use the ADD_ADDR option to indicate that it also owns address
B2. If a host becomes unable to use a previously advertised address,
it uses the REMOVE_ADDR option to indicate the address that it lost
to its peer. Additional details about the utilisation of the
ADD_ADDR and REMOVE_ADDR options may be found in Section 3.4.
Endpoint: A host operating an MPTCP implementation, and either The data produced by the client and the server can be sent over any
initiating or accepting an MPTCP connection. of the subflows that compose an MPTCP connection, and if a subflow
fails, data may need to be retransmitted over another subflow. For
this, MPTCP relies on two principles. First, each subflow is
equivalent to a normal TCP connection with its own 32-bits sequence
numbering space. This enables MPTCP to traverse complex middle-boxes
like transparent proxies or traffic normalizers. Second, MPTCP
maintains a 64-bits data sequence numbering space. The DSS MPTCP
option is used to send the data sequence numbers and data sequence
acknowledgements. When a host sends a TCP segment over one subflow,
it indicates inside the segment, by using the DSS option, the mapping
between the 64-bits data sequence number and the 32-bits sequence
number used by the subflow. Thanks to this mapping, the receiving
host can reorder the data received, possibly out-of-sequence over the
different subflows. In MPTCP, a received segment is acknowledged at
two different levels. First, the TCP cumulative or selective
acknowledgements are used to acknowledge the reception of the data on
each subflow. Second, the acknowledgements field in the DSS option
is returned by the receiving host to provide cumulative
acknowledgements at the data sequence level. When a segment is lost,
the receiver detects the gap in the received 32-bits sequence number
and traditional TCP retransmission mechanisms are triggered to
recover from the loss. When a subflow fails, MPTCP detects the
failure and retransmits the unacknowledged data over another subflow
that is still active. The DSS option also includes an optional
checksum that covers data at the MPTCP connection level to enable a
receiver to detect whether an middlebox has inserted, deleted or
modified data on-the-fly. The transmission of data by MPTCP is
discussed in details in Section 3.3.
3. MPTCP Protocol 3. MPTCP Protocol
This section describes the operation of the MPTCP protocol, and is This section describes the operation of the MPTCP protocol, and is
subdivided into sections for each key part of the protocol operation. subdivided into sections for each key part of the protocol operation.
All MPTCP operations are signalled using optional TCP header fields. All MPTCP operations are signalled using optional TCP header fields.
These TCP Options will have option numbers allocated by IANA, as A single TCP option number will be assigned by IANA (see Section 8),
listed in Section 9, and are defined throughout the following and then individual messages will be determined by a "sub-type", the
subsections. values of which will also be stored in an IANA registry (and are also
listed in Section 8). This sub-type is a four-bit field - the first
four 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 must be
included on packets with the SYN flag set. Additionally, there is
one MPTCP option for signalling metadata to ensure 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 signalling additional addresses.
Whilst 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 [6]) on a single
packet. Therefore, an implementation may choose to send duplicate
ACKs containing the additional signalling information. This changes
the semantics of a duplicate ACK, these are usually only sent as a
signal of a lost segment [7] in regular TCP. Therefore, an MPTCP
implementation receiving a duplicate ACK which 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 signalling purposes, so as to ensure no middleboxes
misinterpret this as a sign of congestion.
Furthermore, standard TCP validity checks (such as ensuring the
Sequence Number and Acknowledgement Number are within window) MUST be
undertaken before processing any MPTCP signals, as described in [8].
3.1. Connection Initiation 3.1. Connection Initiation
Connection Initiation begins with a SYN, SYN/ACK, ACK exchange on a Connection Initiation begins with a SYN, SYN/ACK, ACK exchange on a
single path. Each packet contains the Multipath Capable (MP_CAPABLE) single path. Each packet contains the Multipath Capable (MP_CAPABLE)
TCP option (Figure 3). This option declares its sender is capable of TCP option (Figure 4). This option declares its sender is capable of
performing multipath TCP and wishes to do so on this particular performing multipath TCP and wishes to do so on this particular
connection. connection.
This option contains a 64-bit key that is used to authenticate the This option contains a 64-bit key that is used to authenticate the
addition of future subflows. This is the only time the key will be addition of future subflows. This is the only time the key will be
sent in clear on the wire; all future subflows will identify the sent in clear on the wire; all future subflows will identify the
connection using a 32-bit "token". This token is a cryptographically connection using a 32-bit "token". This token is a cryptographic
secure hash of this key. This will be a truncated (most significant hash of this key. This will be a truncated (most significant 32
32 bits) SHA-1 hash [6]. A different, 64-bit truncation (the least bits) SHA-1 hash [9]. A different, 64-bit truncation (the least
significant 64 bits) of the hash of the key will be used as the significant 64 bits) of the hash of the key will be used as the
Initial Data Sequence Number. Initial Data Sequence Number.
This key is generated by the sender and has local meaning only, and This key is generated by its sender and has local meaning only, and
its method of generation is implementation-specific. The key SHOULD its method of generation is implementation-specific. The key MUST be
be hard to guess, and it MUST be unique for the sending host at any hard to guess, and it MUST be unique for the sending host at any one
one time. Connections will be indexed at each host by the token (the time. Recommendations for generating random keys are given in [10].
truncated SHA-1 hash of the key), but an implementation will require Connections will be indexed at each host by the token (the truncated
a mapping from the token to the key for each connection. SHA-1 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 very small risk that two different keys will hash to the
same token. An implementation SHOULD check its list of connection
tokens to ensure there is not a collision before sending its key in
the SYN/ACK. 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, so in the worst case if there was a token
collision, it just means that the second connection cannot support
multiple subflows, but will otherwise provide a regular TCP service.
The MP_CAPABLE option is carried on the SYN, SYN/ACK, and ACK packets The MP_CAPABLE option is carried on the SYN, SYN/ACK, and ACK packets
that start the first subflow of an MPTCP connection. The data that start the first subflow of an MPTCP connection. The data
carried by each packet is as follows, where A = initiator and B = carried by each packet is as follows, where A = initiator and B =
listener. listener.
o SYN (A->B): A's Key. o SYN (A->B): A's Key.
o SYN/ACK (B->A): B's Key. o SYN/ACK (B->A): B's Key.
o ACK (A->B): Both A's Key and B's Key. o ACK (A->B): Both A's Key and B's Key.
The contents of the option is determined by the SYN and ACK flags of The contents of the option is determined by the SYN and ACK flags of
the packet, verified by the option's length field. For the diagram the packet, verified by the option's length field. For the diagram
shown in Figure 3, "sender" and "receiver" refer to the sender or shown in Figure 4, "sender" and "receiver" refer to the sender or
receiver of the TCP packet. receiver of the TCP packet (which can be either host). If the SYN
flag is set, a single key is included; if only an ACK flag is set,
both keys are present.
The keys are echoed in the ACK in order to allow the listener to act The keys are echoed in the ACK in order to allow the listener (host
statelessly until the TCP connection reaches the ESTABLISHED state. B) to act statelessly until the TCP connection reaches the
If the listener acts in this way, however, it MUST generate its key ESTABLISHED state. If the listener acts in this way, however, it
in a verifiable fashion, allowing it to verify that it generated the MUST generate its key in a verifiable fashion, allowing it to verify
key when it is echoed in the ACK. If this ACK does not carry data, that it generated the key when it is echoed in the ACK.
it MUST still be ACKed by the receiver in order for the sender to
ensure the ACK with MP_JOIN option has been received.
The first octet of this option specifies the MPTCP version in use Furthermore, in order to ensure reliable delivery of the ACK
(for this specification, this is 0). The second octet is reserved containing the MP_CAPABLE option, a server MUST respond with an ACK
for flags, and currently MUST be set to all zeros. The meaning of segment on receipt of this, which may contain data, or will be a pure
such flags will be determined in future revisions of MPTCP, however ACK if it does not have any data to send immediately. If the
some possible uses may be to enable or disable certain MPTCP initiator does not receive this ACK within the RTO, it MUST re-send
features, and to provide a mechanism for crypto agility. the ACK containing MP_CAPABLE. In effect, an MPTCP connection is in
a "PRE_ESTABLISHED" state while awaiting this ACK, and only upon
receipt of the ACK will it move to the ESTABLISHED state.
The first four bits of the first octet in the MP_CAPABLE option
(Figure 4) define the MPTCP option subtype (see Section 8; for
MP_CAPABLE, this is 0), and the remaining four bits of this octet
specifies the MPTCP version in use (for this specification, this is
0).
The second octet is reserved for flags. The leftmost bit - labeled C
- indicates "Checksum required", and SHOULD be set to 1 unless
specifically overridden (for example, if the system administrator has
decided that checksums are not required - see Section 3.3 for more
discussion). The remaining bits are used for crypto algorithm
negotiation. Currently only the rightmost bit - labeled S - is
assigned, and indicates the use of HMAC-SHA1 (as defined in
Section 3.2). An implementation that only supports this method MUST
set this bit to 1 and all other currently reserved bits to zero. If
none of these flags are set, the MP_CAPABLE option MUST be treated as
invalid and ignored (i.e. it must be treated as a regular TCP
handshake).
These bits negotiate capabilities in similar ways. For the 'C' bit,
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 C=0. The decision whether to use checksums
will be stored by an implementation in a per-connection binary state
variable.
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
S will be always set to 1). The responder responds with only one bit
set - this is the chosen algorithm. The rationale for this behaviour
is that the responder will typically be a server with potentially
many thousands of connections, so may wish to choose an algorithm
with minimal computational complexity, depending on load. If a
responder does not support (or does not want to support) any of the
initiator's proposals, it can respond without an MP_CAPABLE option,
thus forcing a fall-back to regular TCP.
The MP_CAPABLE option is only used in the first subflow of a The MP_CAPABLE option is only used in the first subflow of a
connection, in order to identify the connection; all following connection, in order to identify the connection; all following
subflows will use the "Join" option (see Section 3.2) to join the subflows will use the "Join" option (see Section 3.2) to join the
existing connection. existing connection.
1 2 3 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 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=MP_CAPABLE| Length | Version | (reserved) | | Kind | Length |Subtype|Version|C| (reservd) |S|
+---------------+---------------+---------------+---------------+ +---------------+---------------+-------+-------+-+-----------+-+
| Sender Key | | Sender's Key |
| (64 bits) | | (64 bits) |
| | | |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Receiver Key (64 bits) | | Receiver's Key (64 bits) |
| (if Length==20) | | (if Length==20) |
| | | |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
Figure 3: Multipath Capable (MP_CAPABLE) option Figure 4: Multipath Capable (MP_CAPABLE) option
If a SYN contains an MP_CAPABLE option but the SYN/ACK does not, it If a SYN contains an MP_CAPABLE option but the SYN/ACK does not, it
is assumed that the passive opener is not multipath capable and thus is assumed that the passive opener is not multipath capable and thus
the MPTCP session will operate as regular, single-path TCP. If a SYN the MPTCP session MUST operate as regular, single-path TCP. If a SYN
does not contain a MP_CAPABLE option, the SYN/ACK MUST NOT contain does not contain a MP_CAPABLE option, the SYN/ACK MUST NOT contain
one in response. 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
regular, single-path TCP. This is to maintain compatibility with
middleboxes on the path that drop some or all TCP options.
If the SYN packets are unacknowledged, it is up to local policy to 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 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 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 order to work around middleboxes that may drop packets with unknown
options; however, the number of multipath-capable attempts that are options; however, the number of multipath-capable attempts that are
made first will be up to local policy. Once the active opener has made first will be up to local policy. It is possible that MPTCP and
sent a SYN without the MP_CAPABLE option, it MUST fall back to non-MPTCP SYNs could get re-ordered in the network. Therefore, the
regular TCP behavior, even if it subsequently receives a SYN/ACK that final state is inferred from the presence or absence of the
contains an MP_CAPABLE option. This might happen if the MP_CAPABLE MP_CAPABLE option in the third packet of the TCP handshake. If this
SYN and subsequent non-MP-capable SYN are reordered. This is to option is not present, the connection should fall back to regular
ensure that the two endpoints end up in an interoperable state, no TCP, as documented in Section 3.5.
matter what order the SYNs arrive at the passive opener. This 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 3.6.
The initial Data Sequence Number (IDSN) is generated as a hash from The initial Data Sequence Number (IDSN) is generated as a hash from
the Key, in the same way as the token, i.e. IDSN-A = Hash(Key-A) and the Key, in the same way as the token, i.e. IDSN-A = Hash(Key-A) and
IDSN-B = Hash(Key-B). The Hash mechanism here provides the least IDSN-B = Hash(Key-B). The Hash mechanism here provides the least
significant 64 bits of the SHA-1 hash of the key. The SYN with significant 64 bits of the SHA-1 hash of the key. The SYN with
MP_CAPABLE occupies the first octet of Data Sequence Space. MP_CAPABLE occupies the first octet of Data Sequence Space.
3.2. Starting a New Subflow 3.2. Starting a New Subflow
Once a MPTCP connection has begun with the MP_CAPABLE exchange, Once an MPTCP connection has begun with the MP_CAPABLE exchange,
further subflows can be added to the connection. Endpoints have further subflows can be added to the connection. Hosts have
knowledge of their own address(es), and can become aware of the other knowledge of their own address(es), and can become aware of the other
endpoint's addresses through signalling exchanges as described in host's addresses through signalling exchanges as described in
Section 3.5. Using this knowledge, an endpoint can initiate a new Section 3.4. Using this knowledge, a host can initiate a new subflow
subflow over a currently unused pair of addresses. The protocol over a currently unused pair of addresses. It is permitted for
permits either endpoint of a connection to initiate the creation of a either host in a connection to initiate the creation of a new
new subflow (but see Section 3.8 for heuristics). subflow, but it is expected that this will normally be the original
connection initiator (see Section 3.7 for heuristics).
A new subflow is started as a normal TCP SYN/ACK exchange. The Join A new subflow is started as a normal TCP SYN/ACK exchange. The Join
Connection (MP_JOIN) TCP option (Figure 4) is used to identify the Connection (MP_JOIN) TCP option is used to identify the connection to
connection to be joined by the new subflow. The tokens used to be joined by the new subflow. It uses keying material that was
identify the MPTCP connection are cryptographically secure hashes of exchanged in the initial MP_CAPABLE handshake (Section 3.1), and that
the keys exchanged in the initial MP_CAPABLE handshake. The tokens handshake also negotiates the crypto algorithm in use for the MP_JOIN
presented in this option are generated by the SHA-1 [6] algorithm, handshake.
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), and vice versa.
The MP_JOIN SYN/SYN-ACK handshake not only exchanges the tokens This section specifies the behaviour of MP_JOIN using the HMAC-SHA1
(which are static for a connection) but also Random Numbers (nonces) algorithm. An MP_JOIN option is present in the SYN, SYN/ACK and ACK
that are used to prevent replay attacks on the authentication method. of the three-way handshake, although in each case with a different
Whilst these data are transferred in the SYN exchange, the actual format.
cryptographic authentication is undertaken in the first two payload
segments of the connection. Once the peers have successfully
authenticated themselves, the subflow is handed over to the scheduler
to be used for data (the presense of a DSN_MAP option Section 3.3
indicates this).
The MP_JOIN option also contains an "Address ID" to identify the In the first MP_JOIN on the SYN packet, illustrated in Figure 5, the
source address of this packet if it has changed in transit; the initiator sends a token, random number, and address ID.
behaviour of this ID is explained later in this section.
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). The tokens presented in this
option are generated by the SHA-1 [9] algorithm, 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).
The MP_JOIN SYN not only sends the token (which is static for a
connection) but also Random Numbers (nonces) that are used to prevent
replay attacks on the authentication method.
The MP_JOIN option includes an "Address ID". This is an identifier
that only has significance within a single connection, where it
identifies the source address of this packet, even if the address
itself has been changed in transit by a middlebox. This allows
address removal without needing to know what the source address at
the receiver is, thus this allows address removal through NATs. The
sender can signal this to the receiver via the REMOVE_ADDR option
(Section 3.4.2). It also allows correlation between new subflow
setup attempts and address signalling (Section 3.4.1), to prevent
setting up duplicate subflows on the same path.
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 Address IDs associated with all
established subflows.
The MP_JOIN option on SYNs also includes 4 bits of flags, 3 of which
are currently reserved and MUST be set to zero by the sender. The
final bit, labelled 'B', indicates whether the initiator wishes this
subflow to be used purely as a backup path (B=1) in the event of
failure of other paths, or whether it wants it to be used as part of
the connection immediately. Subflow policy is discussed in more
detail in Section 3.3.8.
1 2 3 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 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=MP_JOIN | Length = 8 | Address ID | (reserved) |B| | Kind | Length = 12 |Subtype| |B| Address ID |
+---------------+---------------+----------------+--------------+ +---------------+---------------+-------+-----+-+---------------+
| Receiver Token (32 bits) | | Receiver's Token (32 bits) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Sender Random Number (32 bits) | | Sender's Random Number (32 bits) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
Figure 4: Join Connection (MP_JOIN) option (only valid on SYN Figure 5: Join Connection (MP_JOIN) option (for initial SYN)
packets)
On the third and fourth packets of the handshake, the following data When receiving a SYN with a MP_JOIN option that contains a valid
is sent in the TCP payload: 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) MAC. 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
an RST, analogous to an unknown port in TCP. Although cryptographic
calculations are required in the SYN/ACK, it is felt that the 32-bit
token gives sufficient protection against blind state exhaustion
attacks and therefore there is no need to provide mechanisms to allow
a responder to operate statelessly at the MP_JOIN stage.
An MAC 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). This is to allow both hosts to have exchanged random
data to be used as the message before generating the MAC. In both
cases, the MAC algorithm is HMAC as defined in [11], using the SHA-1
hash algorithm [9] (thus generating a 160-bit / 20 octet HMAC). Due
to option space limitations, the MAC included in the SYN/ACK is
truncated to the leftmost 64 bits, but this is acceptable since while
in an attacker-initiated attack, the attacker can retry many times;
if the attacker is the responder, he only has one chance to get the
MAC correct.
The initiator's authentication information is sent in its first ACK,
and is shown in Figure 7. The same reliability algorithm for this
packet as for the MP_CAPABLE ACK is applied: receipt of this packet
MUST trigger an 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.
The reserved bits in this option MUST be set to zero by the sender.
The key for the MAC 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 in each case is
the concatenations of Random Number 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.
1 2 3 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 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=MP_AUTH | Length | (reserved) | | Kind | Length = 16 |Subtype| |B| Address ID |
+---------------+---------------+-------------------------------+ +---------------+---------------+-------+-----+-+---------------+
| | | |
| Sender's Truncated MAC (64 bits) |
| | | |
| HMAC (256 bits for SHA-256) | +---------------------------------------------------------------+
| Sender's Random Number (32 bits) |
+---------------------------------------------------------------+
Figure 6: Join Connection (MP_JOIN) option (for responding SYN/ACK)
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 MAC (160 bits SHA-1) |
| | | |
| | | |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
Figure 5: Authentication Data Figure 7: Join Connection (MP_JOIN) option (for third ACK)
For consistancy, this follows the same format as a TCP Option,
although it is sent in the TCP payload. The HMAC algorithm is as
defined in [6], using the SHA-256 hash algorithm (thus generating a
256-bit / 32 octet HMAC), however in the future some of the reserved
bits could be used to enable alternative algorithms.
The key for the HMAC algorithm, in the case of the message These various TCP options fit together to enable authenticated
transmitted by Host A, will be Key-A followed by Key-B, and in the subflow setup as illustrated in Figure 8.
case of Host B, Key-B followed by Key-A. The message in each case is
the concatenations of Random Number 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.
When receiving a SYN with a MP_JOIN option that contains a valid Host A Host B
token for an existing MPTCP connection, the recipient SHOULD respond ------------------------ ----------
with a SYN/ACK also containing an MP_JOIN option containing the Address A1 Address A2 Address B1
initiator's token. This will then lead on to the authentication HMAC ---------- ---------- ----------
exchange described above. This behaviour is illustrated in Figure 6. | | |
| SYN + MP_CAPABLE(Key-A) |
|--------------------------------------------->|
|<---------------------------------------------|
| SYN/ACK + MP_CAPABLE(Key-B) |
| | |
| ACK + MP_CAPABLE(Key-A, Key-B) |
|--------------------------------------------->|
| | |
| | SYN + MP_JOIN(Token-B, R-A) |
| |------------------------------->|
| |<-------------------------------|
| | SYN/ACK + MP_JOIN(MAC-B, R-B) |
| | |
| | ACK + MP_JOIN(MAC-A) |
| |------------------------------->|
| | |
Host A Host B MAC-A = MAC(Key=(Key-A+Key-B), Msg=(R-A+R-B))
------------------------ ------------------------ MAC-B = MAC(Key=(Key-B+Key-A), Msg=(R-B+R-A))
Address A1 Address A2 Address B1 Address B2
---------- ---------- ---------- ----------
| | | |
| SYN + MP_CAPABLE(Key-A) | |
|----------------------------------------->| |
|<-----------------------------------------| |
| SYN/ACK + MP_CAPABLE(Key-B) | |
| | | |
| ACK + MP_CAPABLE(Key-A, Key-B) | |
|----------------------------------------->| |
| | | |
| | SYN + MP_JOIN(Token-B, R-A) |
| |----------------------------------------->|
| |<-----------------------------------------|
| | SYN/ACK + MP_JOIN(Token-A, R-B) |
| | | |
| | HMAC(Key=(Key-A+Key-B), Msg=(R-A+R-B)) |
| |----------------------------------------->|
| |<-----------------------------------------|
| | HMAC(Key=(Key-B+Key-A), Msg=(R-B+R-A)) |
| | | |
Figure 6: Example use of MPTCP Authentication Figure 8: Example use of MPTCP Authentication
If the token received at Host B is unknown or local policy prohibits 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 the acceptance of the new subflow, the recipient MUST respond with a
TCP RST. TCP RST for the subflow.
If the token is accepted at Host B, but the token returned to Host A If the token is accepted at Host B, but the MAC returned to Host A
is not the one expected, Host A MUST close the subflow with a TCP does not match the one expected, Host A MUST close the subflow with a
RST. TCP RST.
If either host receives an incorrect HMAC (i.e. it does not match If Host B does not receive the expected MAC, or the MP_JOIN option is
what the host believes it should be), it MUST close the subflow with missing from the ACK, it MUST close the subflow with a TCP RST.
a TCP RST.
The echoing of the token serves two purposes: it ensures both If the MACs are verified as correct, then both hosts have
endpoints agree on the connection being referred to (this is authenticated each other as being the same peers as existed at the
particularly relevant when both addresses being used are new to the start of the connection, and they have agreed of which connection
connection); and it ensures there are no middleboxes on this new path this subflow will become a part.
that will drop MPTCP options on the return path.
If the SYN/ACK as received at Host A does not have an MP_JOIN option, If the SYN/ACK as received at Host A does not have an MP_JOIN option,
Host A MUST close the subflow with a RST. Host A MUST close the subflow with a RST.
If MP_JOIN is stripped from the SYN on the path from A to B, and Host This covers all cases of the loss of an MP_JOIN. In more detail, if
B does not have a passive opener on the relevant port, it will MP_JOIN is stripped from the SYN on the path from A to B, and Host B
respond with an RST in the normal way. If in response to a SYN with does not have a passive opener on the relevant port, it will respond
an MP_JOIN option, a SYN/ACK is received without the MP_JOIN option with an 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
(either since it was stripped on the return path, or it was stripped (either since it was stripped on the return path, or it was stripped
on the outgoing path but the passive opener on Host B responded as if on the outgoing path but the passive opener on Host B responded as if
it were a new regular TCP session), then the subflow is unusable and it were a new regular TCP session), then the subflow is unusable and
Host A MUST close it with a RST. Host A MUST close it with a RST.
It should be noted that additional subflows can be created between Note that additional subflows can be created between any pair of
any pair of ports (but see Section 3.8 for heuristics); no explicit ports (but see Section 3.7 for heuristics); no explicit application-
application-level accept calls or bind calls are required to open level accept calls or bind calls are required to open additional
additional subflows. To associate a new subflow with an existing subflows. To associate a new subflow with an existing connection,
connection, the token supplied in the subflow's SYN exchange is used the token supplied in the subflow's SYN exchange is used for
for demultiplexing. This then binds the 5-tuple of the TCP subflow demultiplexing. This then binds the 5-tuple of the TCP subflow to
to the local token of the connection. A consequence is that it is the local token of the connection. A consequence is that it is
possible to allow any port pairs to be used for a connection. possible to allow any port pairs to be used for a connection.
Deumultiplexing subflow SYNs MUST be done using the token; this is Deumultiplexing subflow SYNs MUST be done using the token; this is
unlike traditional TCP, where the destination port is used for unlike traditional TCP, where the destination port is used for
demultiplexing SYN packets. Once a subflow is setup, demultiplexing demultiplexing SYN packets. Once a subflow is setup, demultiplexing
packets is done using the five-tuple, as in traditional TCP. The packets is done using the five-tuple, as in traditional TCP. The
five-tuples will be mapped to the local connection ID. five-tuples will be mapped to the local connection identifier
(token). Note that Host A will know its local token for the subflow
The MP_JOIN option includes an "Address ID". This is an identifier even though it is not sent on the wire - only the responder's token
that only has significance within a single connection, where it is sent.
identifies the source address of this packet. The key purpose of
this identifier is to allow address removal without needing to know
what the source address at the receiver is, thus allowing the use of
NATs. The sender can signal this to the receiver via the REMOVE_ADDR
option (Section 3.5.2). It also allows correlation between new
subflow setup attempts and address signalling (Section 3.5.1), to
prevent setting up duplicate subflows on the same path.
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.
The Address ID must 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 Address ID, observed source address and token pair for future
processing of control information for a connection.
The MP_JOIN option also includes 8 bits of flags, 7 of which are
currently reserved. The final bit, labelled 'B', indicates whether
the initiator wishes this subflow to be used purely as a backup path
(B=1) in the event of failure of other paths, or whether it wants it
to be used as part of the connection immediately. Subflow policy is
discussed in more detail in Section 3.3.6.
3.3. General MPTCP Operation 3.3. General MPTCP Operation
This section discusses operation of MPTCP for data transfer. At a This section discusses operation of MPTCP for data transfer. At a
high level, an MPTCP implementation will take one input data stream high level, an MPTCP implementation will take one input data stream
from an application, and split it into one or more subflows, with from an application, and split it into one or more subflows, with
sufficient control information to allow it to be reassembled and sufficient control information to allow it to be reassembled and
delivered reliably and in-order to the recipient application. The delivered reliably and in-order to the recipient application. The
following subsections define this behaviour in detail. following subsections define this behaviour in detail.
3.3.1. Data Sequence Numbering During normal MPTCP operation, the Data Sequence Signal (DSS) TCP
option (shown in Figure 9) is used to signal the data required to
enable multipath transport. This data comprises: the Data Sequence
Mapping (DSM), which defines how the sequence space on the subflow
maps to the connection level; and the Data ACK, for acknowledging
receipt of data at the connection level. These functions are
described in more detail in the following two subsections.
The data stream as a whole can be reassembled through the use of the Either or both of the Data Sequence Mapping or the Data ACK can be
Data Sequence Mapping (DSN_MAP, Figure 7) option, which defines the signalled in the DSS option, dependent on the flags set.
mapping from the data sequence number to the subflow 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) have subflow-
only relevance. It is expected (but not mandated) that SACK [7] is
used at the subflow level to improve efficiency.
1 2 3 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 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=DSN_MAP | Length | Data Sequence Number ... : | Kind | Length |Subtype| (reserved) |F|m|M|a|A|
+---------------+---------------+------------------------------+ +---------------+---------------+-------+----------------------+
: ... ( (length-10) octets ) | Data-level Length (2 octets) | | Data ACK (4 or 8 octets, depending on flags) |
+-------------------------------+------------------------------+ +--------------------------------------------------------------+
| Subflow Sequence Number (4 octets) | | Data Sequence Number (4 or 8 octets, depending on flags) |
+-------------------------------+------------------------------+ +--------------------------------------------------------------+
| Checksum (2 octets) | | Subflow Sequence Number (4 octets) |
+-------------------------------+ +-------------------------------+------------------------------+
| Data-level Length (2 octets) | Checksum (2 octets) |
+-------------------------------+------------------------------+
Figure 7: Data Sequence Mapping (DSN_MAP) option Figure 9: Data Sequence Signal (DSS) option
This option specifies a full mapping from data sequence number to The flags when set define the contents of this option, as follows:
subflow sequence number, informing the receiver that there is a one-
to-one correspondence between the two sequence spaces for the
specified length (number of bytes of data). The purpose of the
explicit mapping is 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). It also allows a
single mapping to cover many packets, which may be useful in bulk
transfer situations.
The data sequence number specified in this option is absolute, o A = Data ACK present
whereas the subflow sequence numbering is relative (the SYN at the
start of the subflow has relative subflow sequence number 1). This
is allow middleboxes to change the Initial Sequence Number of a
subflow, since the data stream itself will not be affected (some
firewalls do ISN randomization).
The final two octets of this option contain a checksum of the data o a = Data ACK is 8 octets (if not set, Data ACK is 4 octets)
that this mapping covers. This is used to detect if the payload has
been adjusted in any way by a non-MPTCP-aware middlebox. If this o M = Data Sequence Number, Subflow Sequence Number, Data-level
checksum fails, it will trigger a failure of the subflow, or a Length, and Checksum present
fallback to regular TCP, as documented in Section 3.6. The checksum
algorithm used is the standard TCP checksum [2], operating only over o m = Data Sequence Number is 8 octets (if not set, DSN is 4 octets)
the data covered by this DSN_MAP (i.e. there is no pseudo-header).
This algorithm has been chosen since it will be calculated anyway for The flags 'a' and 'm' only have meaning if the corresponding 'A' or
the TCP subflow, and if calculated first over the data before adding 'M' flags are set, otherwise they will be ignored. The maximum
the pseudo-header, it only needs to be calculated once. Furthermore, length of this option, with all flags set, is 28 octets.
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 The 'F' flag indicates "DATA FIN". If present, this means that this
each constituent TCP segment. This relies on the TCP subflow mapping covers the final data from the sender. This is the
containing contiguous data, however, and thus a TCP subflow MUST NOT connection-level equivalent to the FIN flag in single-path TCP. The
use the Urgent Pointer (i.e. the URG flag MUST be zero). purpose of the DATA FIN, along with 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 zero 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.
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) have subflow-only relevance. It is expected (but not
mandated) that SACK [6] is used at the subflow level to improve
efficiency.
The Data Sequence Mapping specifies a full mapping from subflow
sequence space to data sequence space, for the specified length
(number of bytes of data) starting at the specified Subflow and Data
Sequence Numbers. The purpose of the explicit mapping is 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). It also allows a single mapping to cover many
packets, which may be useful in bulk transfer situations.
A mapping is unique, in that the subflow sequence number is bound to A mapping is unique, in that the subflow sequence number is bound to
the data sequence number after the mapping has been processed. It is the data sequence number after the mapping has been processed. It is
not possible to change this mapping afterwards (although the length not possible to change this mapping afterwards; however, the same
of a mapping can extend); however, the same data sequence number can data sequence number can be mapped to different subflows for
be mapped on different subflows for retransmission purposes (see retransmission purposes (see Section 3.3.6). It would also permit
Section 3.3.4). the same data to be sent simultaneously on multiple subflows for
resilience purposes, although the detailed specification of such
operation is outside the scope of this document.
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 relative subflow sequence number 0). This is to
allow middleboxes to change the Initial Sequence Number 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. This is 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 3.5, since MPTCP can no longer
reliably know the subflow sequence space at the receiver to build
data sequence mappings.
The checksum algorithm used is the standard TCP checksum [2],
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, and 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 To avoid possible deadlock scenarios, subflow-level processing should
be undertaken separately from that at connection-level. Therefore, be undertaken separately from that at connection-level. Therefore,
even if a mapping does not exist from the subflow space to the data- 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. This level space, the data SHOULD still be ACKed at the subflow (if it is
data cannot, however, be acknowledged at the data level in-window). This data cannot, however, be acknowledged at the data
(Section 3.3.2) because its data sequence numbers are unknown. level (Section 3.3.2) because its data sequence numbers are unknown.
Implementations MAY hold onto such unmapped data for a short while in Implementations MAY hold onto such unmapped data for a short while in
the expectation than a mapping will arrive shortly. Such unmapped the expectation that a mapping will arrive shortly. Such unmapped
data cannot be counted as being within the receive window because data cannot be counted as being within the connection-level receive
this is relative to the data sequence numbers, so if the receiver window because this is relative to the data sequence numbers, so if
runs out of memory to hold this data, it will have to be discarded. the receiver runs out of memory to hold this data, it will have to be
If a mapping for that subflow-level sequence space does not arrive discarded. If a mapping for that subflow-level sequence space does
within a receive window of data, that subflow should be treated as not arrive within a receive window of data, that subflow SHOULD be
broken, closed with an RST, and an unmapped data silently discarded. treated as broken, closed with an RST, and an unmapped data silently
discarded.
Data sequence numbers are always 64-bit quantities, and MUST be Data sequence numbers are always 64-bit quantities, and MUST be
maintained as such in implementations. If a connection is maintained as such in implementations. If a connection is
progressing at a slow rate, so protection against wrapped sequence progressing at a slow rate, so protection against wrapped sequence
numbers is not required, and if security requirements against blind numbers is not required, then it is permissible to include just the
insertion attacks are not stringent, then it is permissible to lower 32 bits of the data sequence number in the Data Sequence
include just the lower 32 bits of the sequence number in the DSN_MAP Mapping and/or Data ACK as an optimization. An implementation MUST
option as an optimization. Implementations MUST accept this and send the full 64 bit Data Sequence Number if it is transmitting at a
implicitly promote it to a 64-bit quantity by incrementing the upper sufficiently high rate that it could wrap within the MSL [12]. The
32 bits of sequence number each time the lower 32 bits wrap. By lengths of the DSNs used in these values (which may be different) are
defauly, the full 64 bit DSN_MAP should be sent. Security declared with flags in the DSS option. Implementations MUST accept a
implications are discussed in Section 5. 32-bit DSN and implicitly promote it to a 64-bit quantity by
incrementing the upper 32 bits of 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 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 should not start at zero, but at a random value to make blind session
hijacking harder. This is done by setting the initial data sequence hijacking harder. This is done by setting the initial data sequence
number (IDSN) of each host to the least significant 64 bits of the number (IDSN) of each host to the least significant 64 bits of the
SHA-1 hash of the host's key (as declared in the MP_CAPABLE option in SHA-1 hash of the host's key, as described in Section 3.1.
the initial connection SYN, which itself occupies the first octet of
data sequence space). This handshake is described in more detail in
Section 3.1.
The DSN_MAP option does not need to be included in every MPTCP A Data Sequence Mapping does not need to be included in every MPTCP
packet, as long as the subflow sequence space in that packet is 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 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 reduce overhead in cases where the mapping is known in advance; one
such case is when there is a single subflow between the endpoints, such case is when there is a single subflow between the hosts,
another is when segments of data are scheduled in larger than packet- another is when segments of data are scheduled in larger than packet-
sized chunks. An "infinite" mapping can be used to fallback to sized chunks. An "infinite" mapping can be used to fallback to
regular TCP by mapping the subflow-level data to the connection-level regular TCP by mapping the subflow-level data to the connection-level
data for the remainder of the connection (see Section 3.6). This is data for the remainder of the connection (see Section 3.5). This is
achieved by setting the data-level length field to the reserved value achieved by setting the data-level length field to the reserved value
of 0. of 0. The checksum, in such a case, will also be set to zero.
3.3.2. Data Acknowledgements 3.3.2. Data Acknowledgements
To provide full end-to-end resilience, MPTCP provides a connection- To provide full end-to-end resilience, MPTCP provides a connection-
level acknowledgement, the DATA_ACK, illustrated in Figure 8, to act level acknowledgement, to act as a cumulative ACK for the connection
as a cumulative ACK for the connection as a whole. This is analogous as a whole. This is the "Data ACK" field of the DSS option
to the behaviour of the standard TCP cumulative ACK in TCP SACK - (Figure 9). The Data ACK is analogous to the behaviour of the
indicating how much data has been successfully received (with no standard TCP cumulative ACK in TCP SACK - indicating how much data
holes). has been successfully received (with no holes). The Data ACK
specifies the next Data Sequence Number it expects to receive.
The rationale for the inclusion of the DATA_ACK includes the 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, it is valid for the implementation to choose whether to send
a 32 bit or 64 bit Data ACK.
The rationale for the inclusion of the Data ACK includes the
existence of certain middleboxes that pro-actively ACK packets, and existence of certain middleboxes that pro-actively ACK packets, and
thus might cause deadlock conditions if data were acked at the thus might cause deadlock conditions if data were acked at the
subflow level but then fails to reach the receiver. This sort of bad subflow level but then fails to reach the receiver. This sort of bad
interaction might be expecially prevalent when the receiver is interaction might be especially prevalent when the receiver is
mobile. The DATA_ACK ensures the data has been delieverd to the mobile. The Data ACK ensures the data has been delivered to the
receiver. receiver. Furthermore, separating the connection-level
acknowledgements from the subflow-level allows processing to be done
separately, and a receiver has the freedom to drop segments after
acknowledgement at the subflow level, for example due to memory
constraints when many segments arrive out-of-order.
Another reason for including the Data ACK is that it indicates 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 connection-level if it
does not also carry a DSS option with a Data ACK field.
An MPTCP sender MUST only free data from the send buffer when it has An MPTCP sender MUST only free data from the send buffer when it has
been acknowledged by both a DATA_ACK received on any subflow and at been acknowledged by both a Data ACK received on any subflow and at
the subflow level by any subflows the data was sent on. The former the subflow level by any subflows the data was sent on. The former
condition ensures liveness of the connection and the latter condition condition ensures liveness of the connection and the latter condition
ensures liveness and self-consistence of a subflow when data needs to ensures liveness and self-consistence of a subflow when data needs to
be restransmited. be restransmited. Note, however, that if some data needs to be
retransmitted multiple times over a subflow, there is a risk of
blocking the sending window. In this case, the MPTCP sender can
decide to cancel the subflow that is behaving badly by sending a RST.
The DATA_ACK option MAY be included in all segments, analogous to a The Data ACK MAY be included in all segments, however optimisations
standard TCP ACK. However, optimisations SHOULD be considered in SHOULD be considered in more advanced implementations, where the Data
more advanced implementations, where the DATA_ACK option is present ACK is present in segments only when the Data ACK value advances, and
in segments (data or pure ACKs) only when the DATA_ACK advances, and
this behaviour MUST be treated as valid. This behaviour ensures the this behaviour MUST be treated as valid. This behaviour ensures the
sender buffer is freed, while reducing overhead when the data sender buffer is freed, while reducing overhead when the data
transfer is unidirectional. transfer is unidirectional.
1 2 3 3.3.3. Closing a Connection
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=DATA_ACK | Length | Data Sequence Number ... :
+---------------+---------------+------------------------------+
: ... ( (length-2) octets ) |
+-------------------------------+
Figure 8: Connection-level Acknowledgement (DATA_ACK) In regular TCP a FIN announces 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.
3.3.3. Receiver Considerations When an application calls close() on a socket, this indicates that it
has no more data to send, and for regular TCP this would result in a
FIN on the connection. For MPTCP, an equivalent mechanism is needed,
and 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 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 signalled by setting the 'F' flag in the Data
Sequence Signal option (Figure 9) to 1. A DATA FIN occupies one
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 DSN 80, and length 11,
with DATA FIN set, would map 10 octets from the subflow into data
sequnce space 80-89, the DATA FIN is DSN 90, and 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 Data Sequence Mapping MUST have Subflow Sequence
Number of 0, a 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 semantics and behaviour 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 permissable to combine these signals on one subflow if there is
no data oustanding on other subflows. Otherwise, it may be necessary
to retransmit data on different subflows. Essentially, a host MUST
NOT FIN all subflows unless it is safe to do so, i.e. until all data
has been DATA ACKed, or that the segment with the 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, as
a courtesy to allow middleboxes to clean up state even if the subflow
has failed. It is also encouraged to reduce the timeouts (Maximum
Segment Life) on subflows at end hosts. 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 an RST.
A connection is considered closed once both hosts' DATA FINs have
been acknowledged by DATA ACKs.
Note that a host may also send a FIN on an individual subflow to shut
it down, but this impact is limited to the subflow in question. 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 and connection levels.
3.3.4. Receiver Considerations
Regular TCP advertises a receive window in each packet, telling the Regular TCP advertises a receive window in each packet, telling the
sender how much data the receiver is willing to accept past the sender how much data the receiver is willing to accept past the
cumulative ack. The receive window is used to implement flow cumulative ack. The receive window is used to implement flow
control, throttling down fast senders when receivers cannot keep up. control, throttling down fast senders when receivers cannot keep up.
MPTCP also uses a unique receive window, shared between the subflows. 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 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 is willing to accept it; the alternative, maintaining per subflow
receive windows, could end-up stalling some subflows while others receive windows, could end-up stalling some subflows while others
would not use up their window. would not use up their window.
The receive window is relative to the DATA_ACK. As in TCP, a The receive window is relative to the DATA_ACK. As in TCP, a
receiver MUST NOT shrink the right edge of the receive window (e.g. receiver MUST NOT shrink the right edge of the receive window (i.e.
DATA_ACK + receive window). The receiver will use the Data Sequence DATA_ACK + receive window). The receiver will use the Data Sequence
Number to tell if a packet should be accepted at connection level. Number to tell if a packet should be accepted at connection level.
When deciding to accept packets at subflow level, normal TCP uses the When deciding to accept packets at subflow level, normal TCP uses the
sequence number in the packet and checks it against the allowed sequence number in the packet and checks it against the allowed
receive window. With multipath, such a check is done using only the receive window. With multipath, such a check is done using only the
connection level window. A sanity check could be performed at connection level window. A sanity check SHOULD be performed at
subflow level to ensure that: SSN - SUBFLOW_ACK <= DSN - DATA_ACK. subflow level to ensure that the subflow and mapped sequence numbers
meet the following test: SSN - SUBFLOW_ACK <= DSN - DATA_ACK.
When should segments be processed at connection level? An
implementation might wait until they arrive in order at subflow
level, and only then do connection level processing. However, if
many segments of data are restransmitted on more than one subflow,
then because some data is duplicated then the sum total of
unacknowledged data on all subflows might exceed the receive window
that was advertised, which indicates buffering available for data
sequence space. This such a strategy is probably undesirable.
An alternative implementation might process segments at the In regular TCP, once a segment is deemed in-window, it is either put
connection level segments that have not yet been acked at subflow in the in-order receive queue or in the out-of-order queue. In
level; the only requirement for this is to have a valid data sequence multipath TCP, the same happens but at connection-level: a segment is
mapping for the segment. This removes such duplicate data from the placed in the connection level in-order or out-of-order queue if it
receive buffer, so avoids running out of buffer space. Such is in-window at both connection and subflow level. The stack still
implementations SHOULD keep track of which subflow sequence numbers has to remember, for each subflow, which segments were received
have already been accepted in this way, so they can be ACKed succesfully so that it can ACK them at subflow level appropriately.
appropriately when the hole in the subflow sequence space in Typically, this will be implemented by keeping per subflow out-of-
subsequently filled. An implementation that does store such metadata order queues (containing only message headers, not the payloads) and
would still progress (the rules for freeing data at the sender ensure remembering the value of the cumulative ACK.
this), but unnecessary retransmissions will result.
It is important for implementers to understand how large a receiver It is important for implementers to understand how large a receiver
buffer is appropriate. The lower bound for full network utilization buffer is appropriate. The lower bound for full network utilization
is the maximum bandwidth-delay product of any of the paths. However is the maximum bandwidth-delay product of any of the paths. However
this might be insufficient when a packet is lost on a slower subflow this might be insufficient when a packet is lost on a slower subflow
and needs to be retransmitted (see Section 3.3.4). A tight upper and needs to be retransmitted (see Section 3.3.6). A tight upper
bound would be the maximum RTT of any path multiplied by the total bound would be the maximum RTT of any path multiplied by the total
bandwidth available across all paths. This permits all subflows to bandwidth available across all paths. This permits all subflows to
continue at full speed while a packet is fast-retransmitted on the continue at full speed while a packet is fast-retransmitted on the
maximum RTT path. Even this might be insufficient to maintain full maximum RTT path. Even this might be insufficient to maintain full
performance in the event of a retransmit timeout on the maximum RTT performance in the event of a retransmit timeout on the maximum RTT
path. It is for future study to determine the relationship between path. It is for future study to determine the relationship between
retransmission strategies and receive buffer sizing. retransmission strategies and receive buffer sizing.
3.3.4. Sender Considerations 3.3.5. Sender Considerations
The sender remembers receiver window advertisements from the The sender remembers receiver window advertisements from the
receiver. It should only update its local receive window values when receiver. It should only update its local receive window values when
the largest sequence number allowed (i.e. DATA_ACK + receive window) the largest sequence number allowed (i.e. DATA_ACK + receive window)
increases. This is important to allow using paths with different increases. This is important to allow using paths with different
RTTs, and thus different feedback loops. RTTs, and thus different feedback loops.
Some classes of middleboxes may alter the TCP-level receive window. 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 Typically these will shrink the offered window, although for short
periods of time it may be possible for the window to be larger periods of time it may be possible for the window to be larger
(however note that this would not continue for long periods since (however note that this would not continue for long periods since
ultimately the middlebox must keep up with delivering data to the ultimately the middlebox must keep up with delivering data to the
receiver). Therefore, if receive window sizes differ on multiple receiver). Therefore, if receive window sizes differ on multiple
subflows, when sending data MPTCP SHOULD take the largest of the most 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 recent window sizes as the one to use in calculations. This rule is
implicit in the requirement not to move back the right edge of the implicit in the requirement not to reduce the right edge of the
window). window.
The sender also remembers the receive windows advertised by each The sender also remembers the receive windows advertised by each
subflow. The allowed window for subflow i is (ack_i, ack_i + 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 rcv_wnd_i), where ack_i is the subflow-level cumulative ack of
subflow i. This ensures data will not be sent to a middlebox unless subflow i. This ensures data will not be sent to a middlebox unless
there is enough buffering for the data. there is enough buffering for the data.
Putting the two rules together, we get the following: a sender is Putting the two rules together, we get the following: a sender is
allowed to send data segments with data-level sequence numbers allowed to send data segments with data-level sequence numbers
between (DATA_ACK, DATA_ACK + receive_window). Each of these between (DATA_ACK, DATA_ACK + receive_window). Each of these
segments will be mapped onto subflows, as long as subflow sequence segments will be mapped onto subflows, as long as subflow sequence
numbers are in the the allowed windows for those subflows. Note that numbers are in the the allowed windows for those subflows. Note that
subflow sequence numbers do not generally affect flow control if the subflow sequence numbers do not generally affect flow control if the
same receive window is advertised across all subflows. They will same receive window is advertised across all subflows. They will
perform flow control for those subflows with a smaller advertised perform flow control for those subflows with a smaller advertised
receive window. receive window.
The send buffer must be, at the minimum, 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 re-send data with the The data sequence mapping allows senders to re-send data with the
same data sequence number on a different subflow. When doing this, same data sequence number on a different subflow. When doing this, a
an endpoint must still retransmit the original data on the original host must still retransmit the original data on the original subflow,
subflow, in order to preserve the subflow integrity (middleboxes in order to preserve the subflow integrity (middleboxes could replay
could replay old data, and/or could reject holes in subflows), and a old data, and/or could reject holes in subflows), and a receiver will
receiver will ignore these retransmissions. While this is clearly ignore these retransmissions. While this is clearly suboptimal, for
suboptimal, for compatibility reasons this is the best behaviour. compatibility reasons this is the best behaviour. Optimisations
Optimisations could be negotiated in future versions of this could be negotiated in future versions of this protocol.
protocol.
This protocol specification does not mandate any mechanisms for This protocol specification does not mandate any mechanisms for
handling retransmissions, and much will be dependent upon local handling retransmissions, and much will be dependent upon local
policy (as discussed in Section 3.3.6). One can imagine aggressive policy (as discussed in Section 3.3.8). One can imagine aggressive
connection level retransmissions policies where every packet lost at connection level retransmissions policies where every packet lost at
subflow level is retransmitted on a different subflow (hence wasting subflow level is retransmitted on a different subflow (hence wasting
bandwidth but possibly reducing application-to-application delays), bandwidth but possibly reducing application-to-application delays),
or conservative retransmission policies where connection-level or conservative retransmission policies where connection-level
retransmits are only used after a few subflow level retransmission retransmits are only used after a few subflow level retransmission
timeouts occur. timeouts occur.
It is envisaged that a standard connection-level retransmission It is envisaged that a standard connection-level retransmission
mechanism would be implemented around a connection-level data queue: mechanism would be implemented around a connection-level data queue:
all segments that haven't been DATA_ACKed are stored. A timer (based all segments that haven't been DATA_ACKed are stored. A timer is set
on the subflow timer values) is set when the head of the connection- when the head of the connection-level is ACKed at subflow level but
level is ACKed at subflow level but its corresponding data is not its corresponding data is not ACKed at data level. This timer will
acked at data level. guard against failures in re-transmission by middleboxes that pro-
active ACK data.
The sender MUST keep data in its send buffer as long as the data has The sender MUST keep data in its send buffer as long as the data has
not been acked at connection level and on all subflows it has been not been acknowledged at both connection level and on all subflows it
sent on. In this way, the sender can always retransmit the data if has been sent on. In this way, the sender can always retransmit the
needed, on the same subflow or on a different one. A special case is data if needed, on the same subflow or on a different one. A special
when a subflow fails: the sender will typically resend the data on case is when a subflow fails: the sender will typically resend the
other working subflows, and will keep trying to retransmit the data data on other working subflows after a timeout, and will keep trying
on the failed subflow too. The sender will declare the subflow to retransmit the data on the failed subflow too. The sender will
failed after a predefined upper bound on retransmissions is reached, declare the subflow failed after a predefined upper bound on
and only then delete the outstanding data segments. retransmissions is reached (which MAY be lower than the usual TCP
limits of the Maximum Segment Life), or on the receipt of an ICMP
A sender will maintain connection level timers for unacknowledged error, and only then delete the outstanding data segments.
segments. These timers will be based on the subflow timers, and will
guard against pro-active acking by middleboxes.
The send buffer must be, at the minimum, as big as the receive Multiple retransmissions are triggers that will indicate that a
buffer, to enable the sender to reach maximum throughput. subflow performs badly and could lead to a host resetting the subflow
with an RST. However, additional research is required to understand
the heuristics of how and when to reset underperforming subflows.
For example, subflows that perform highly asymmetrically may be mis-
diagnosed as underperforming.
3.3.5. Congestion Control Considerations 3.3.7. Congestion Control Considerations
Different subflows in an MPTCP connection have different congestion Different subflows in an MPTCP connection have different congestion
windows. To achieve fairness at bottlenecks and resource pooling, it windows. To achieve fairness at bottlenecks and resource pooling, it
is necessary to couple the congestion windows in use on each subflow, is necessary to couple the congestion windows in use on each subflow,
in order to push most traffic to uncongested links. One algorithm in order to push most traffic to uncongested links. One algorithm
for achieving this is presented in [4]; the algorithm does not for achieving this is presented in [4]; the algorithm does not
achieve perfect resource pooling but is "safe" in that it is readily achieve perfect resource pooling but is "safe" in that it is readily
deployable in the current Internet. 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 It is foreseeable that different congestion controllers will be
implemented for MPTCP, each aiming to achieve different properties in implemented for MPTCP, each aiming to achieve different properties in
the resource pooling/fairness/stability design space. Much research the resource pooling/fairness/stability design space, as well as
is expected in this area in the near future. those for achieving different properties in quality of service,
reliability and resilience.
Regardless of the algorithm used, the design of the MPTCP protocol Regardless of the algorithm used, the design of the MPTCP protocol
aims to provide the congestion control implementations sufficient aims to provide the congestion control implementations sufficient
information to take the right decisions; this information includes, information to take the right decisions; this information includes,
for each subflow, which packets where lost and when. for each subflow, which packets were lost and when.
3.3.6. Subflow Policy 3.3.8. Subflow Policy
Within a local MPTCP implementation, a host may use any local 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 it wishes to decide how to share the traffic to be sent over the
available paths. available paths.
In the typical use case, where the goal is to maximise throughput, In the typical use case, where the goal is to maximise throughput,
all available paths will be used simultaneously for data transfer, all available paths will be used simultaneously for data transfer,
using coupled congestion control as described in [4]. It is using coupled congestion control as described in [4]. It is
expected, however, that other use cases will appear. expected, however, that other use cases will appear.
For instance, a possibility is an 'all-or-nothing' approach, i.e. For instance, a possibility is an 'all-or-nothing' approach, i.e.
have a second path ready for use in the event of failure of the first have a second path ready for use in the event of failure of the first
path, but alternatives could include entirely saturating one path path, but alternatives could include entirely saturating one path
before using an additional path (the 'overflow' case). Such choices before using an additional path (the 'overflow' case). Such choices
would be most likely based on the monetary cost of links, but may 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, also be based on properties such as the delay or jitter of links,
where stability is more important than throughput. Application where stability (of delay or bandwidth) is more important than
requirements such as these are discussed in detail in [5]. throughput. Application requirements such as these are discussed in
detail in [5].
The ability to make effective choices at the sender requires full The ability to make effective choices at the sender requires full
knowledge of the path "cost", which is unlikely to be the case. It 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 would be desirable for a receiver to be able to signal their own
preferences for paths, since they will often be the multihomed party, preferences for paths, since they will often be the multihomed party,
and may have to pay for metered incoming bandwidth. and may have to pay for metered incoming bandwidth.
Whilst fine-grained control may be the most powerful solution, that Whilst fine-grained control may be the most powerful solution, that
would require some mechanism such as overloading the ECN signal [8], would require some mechanism such as overloading the ECN signal [13],
which is undesirable, and it is felt that there would not be which is undesirable, and it is felt that there would not be
sufficient benefit to justify an entirely new signal. Therefore the sufficient benefit to justify an entirely new signal. Therefore the
MP_JOIN Section 3.2 and ADD_ADDR Section 3.5 options contain the 'B' MP_JOIN option (see Section 3.2) contains the 'B' bit, which allows a
bit, which allows a host to indicate to its peer that this path host to indicate to its peer that this path should be treated as a
should be treated as a backup path to use only in the event of backup path to use only in the event of failure of other working
failure of other working subflows (i.e. a subflow where the receiver subflows (i.e. a subflow where the receiver has indicated B=1 SHOULD
has indicated B=1 SHOULD NOT be used to send data unless there are no NOT be used to send data unless there are no usable subflows where
usable subflows where B=0). B=0).
In the event that the available set of paths changes, a host may wish 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. Therefore, to signal a change in priority of subflows to the peer. Therefore,
the MP_PRIO option, shown in Figure 9, can be used to change the 'B' the MP_PRIO option, shown in Figure 11, can be used to change the 'B'
flag of the subflow on which it is sent. flag of the subflow on which it is sent.
1 1 2
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 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+---------------+---------------+-------------+-+ +---------------+---------------+-------+-----+-+
| Kind=MP_PRIO | Length=3 | (reserved) |B| | Kind | Length |Subtype| |B|
+---------------+---------------+-------------+-+ +---------------+---------------+-------+-----+-+
Figure 9: MP_PRIO option Figure 11: MP_PRIO option
It should be noted that the backup flag is a request from the It should be noted that the backup flag is a request from the
receiver to the sender only, and the sender SHOULD adhere to these receiver to the sender only, and the sender SHOULD adhere to these
requests. The reciever, however, may continue using the subflow to requests. The receiver, however, may continue using the subflow to
send data even if it has signalled B=1 to the other host. send data even if it has signalled B=1 to the other host.
3.4. Closing a Connection 3.4. Address Knowledge Exchange (Path Management)
In regular TCP a FIN announces 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, and for regular TCP this would result in a
FIN on the connection. For MPTCP, an equivalent mechanism is needed,
and this is the DATA_FIN. This option, shown in Figure 10, is
attached to a regular FIN option on a subflow.
A DATA_FIN is an indication that the sender has no more data to send,
and as such can be used as a rapid indication of the end of data from
a sender. A DATA_FIN, as with the FIN on a regular TCP connection,
is a unidirectional signal.
A DATA_FIN occupies one octet (the final octet) of Data Sequence
Number space. This number is included in the option, and will be
ACKed at data level to ensure reliable delivery.
The DATA_FIN is an optimisation to rapidly indicate the end of a data
stream and clean up state associated with a MPTCP connection,
especially when some subflows may have failed. Specifically, when a
DATA_FIN has been received, IF all data has been successfully
received, timeouts on all subflows MAY be reduced. Similarly, when
sending a DATA_FIN, once all data (including the DATA_FIN, since it
occupies one octet of data sequence space) has been acknowledged,
FINs must be sent on every subflow. This applies to both endpoints,
and is required in order to clean up state in middleboxes.
The interactions between a DATA_FIN and subflow properties are as
follows:
o A DATA_FIN MUST only be sent on a packet which also has the FIN
flag set.
o When DATA_FIN is sent, it should be sent on all active subflows.
o There is a one-to-one mapping between the DATA_FIN and the
subflow's FIN flag (and its associated sequence space and thus its
acknowlegement).
o The data sequence number included in the DATA_FIN is used to
verify that all data has been successfully received.
It should be noted that an endpoint may also send a FIN on an
individual subflow to shut it down, but this impact is limited to the
subflow in question. If all subflows have been closed with a FIN,
that is equivalent to having closed the connection with a DATA_FIN.
The full eight-byte data sequence number is always included in a
DATA_FIN.
1
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=DATA_FIN | Length=10 | Data Sequence Number (8B) :
+---------------+---------------+------------------------------+
: Data Sequence Number (contd.) :
+-------------------------------+------------------------------+
: Data Sequence Number (contd.)|
+-------------------------------+
Figure 10: DATA_FIN option
3.5. Address Knowledge Exchange (Path Management)
We use the term "path management" to refer to the exchange of We use the term "path management" to refer to the exchange of
information about additional paths between endpoints, which in this information about additional paths between hosts, which in this
design is managed by multiple addresses at endpoints. For more design is managed by multiple addresses at hosts. For more detail of
detail of the architectural thinking behind this design, see the the architectural thinking behind this design, see the separate
separate architecture document [3]. architecture document [3].
This design makes use of two methods of sharing such information, This design makes use of two methods of sharing such information,
used simultaneously. The first is the direct setup of new subflows, used simultaneously. The first is the direct setup of new subflows,
already described in Section 3.2, where the initiator has an already described in Section 3.2, where the initiator has an
additional address. The second method, described in the following additional address. The second method, described in the following
subsections, signals addresses explicitly to the other endpoint to subsections, signals addresses explicitly to the other host to allow
allow it to initiate new subflows. The two mechanisms are it to initiate new subflows. The two mechanisms are complementary:
complementary: the first is implicit and simple, while the explicit the first is implicit and simple, while the explicit is more complex
is more complex but is more robust. Together, the mechanisms allow but is more robust. Together, the mechanisms allow addresses to
addresses to change in flight (and thus support operation through change in flight (and thus support operation through NATs, since the
NATs, since the source address need not be known), and also allow the source address need not be known), and also allow the signalling of
signalling of previously unknown addresses, and of addresses previously unknown addresses, and of addresses belonging to other
belonging to other address families (e.g. IPv4 and IPv6). address families (e.g. both IPv4 and IPv6).
Here is an example of typical operation of the protocol: Here is an example of typical operation of the protocol:
o A1 of host A and address/port B1 of host B. If host A is o An MPTCP connection is initially set up between address/port A1 of
multihomed and multi-addressed, it can start an additional subflow host A and address/port B1 of host B. If host A is multihomed and
from its address A2 to B1, by sending a SYN with a Join option multi-addressed, it can start an additional subflow from its
from A2 to B1, using B's previously declared token for this address A2 to B1, by sending a SYN with a Join option from A2 to
connection. Alternatively, if B is multhomed, it can try to set B1, using B's previously declared token for this connection.
up a new subflow from B2 to A1, using A's previously declared Alternatively, if B is multihomed, it can try to set up a new
token. In either case, the SYN will be sent to the port already subflow from B2 to A1, using A's previously declared token. In
in use for the original subflow on the receiving host. either case, the SYN will be sent to the port already in use for
the original subflow on the receiving host.
o Simultaneously (or after a timeout), an ADD_ADDR option o Simultaneously (or after a timeout), an ADD_ADDR option
(Section 3.5.1) is sent on an existing subflow, informing the (Section 3.4.1) is sent on an existing subflow, informing the
receiver of the sender's alternative address(es). The recipient receiver of the sender's alternative address(es). The recipient
can use this information to open a new subflow to the sender's can use this information to open a new subflow to the sender's
additional address. In our example, A will send ADD_ADDR option additional address. In our example, A will send ADD_ADDR option
informing B of address A2. The mix of using the SYN-based option informing B of address/port A2. The mix of using the SYN-based
and the ADD_ADDR option, including timeouts, is implementation- option and the ADD_ADDR option, including timeouts, is
specific and can be tailored to agree with local policy. implementation-specific and can be tailored to agree with local
policy.
o If subflow A2-B1 is succesfully setup, host B1 can use the Address o If subflow A2-B1 is succesfully setup, host B can use the Address
ID in the Join option to correlate this with the ADD_ADDR option ID in the Join option to correlate this with the ADD_ADDR option
that will also arrive on an existing subflow; now B knows not to 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 open A2-B1, ignoring the ADD_ADDR. Otherwise, if B has not
received the A2-B1 SYN join but received the ADD_ADDR, it will try received the A2-B1 MP_JOIN SYN but received the ADD_ADDR, it can
to initiate a new subflow from one or more of its addresses to 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 address A2. This permits new sessions to be opened if one host is
endpoint is behind a NAT. A slight security improvement can be behind a NAT.
gained if a host ensures there is a correlated ADD_ADDR option
before responding to the SYN.
Other ways of using the two signaling mechanisms are possible; for Other ways of using the two signaling mechanisms are possible; for
instance, signaling addresses in other address families can only be instance, signaling addresses in other address families can only be
done explicitly using the Add Address option. done explicitly using the Add Address option.
3.5.1. Address Advertisement 3.4.1. Address Advertisement
The Add Address (ADD_ADDR) TCP Option announces additional addresses The Add Address (ADD_ADDR) TCP Option announces additional addresses
on which an endpoint can be reached (Figure 11). It can be used to (and optionally, ports) on which a host can be reached (Figure 12).
announce several (ID, address) pairs to be announced to the other Multiple instances of this TCP option can be added in a single
endpoint. Multiple addresses can be added in a single message if message if there is sufficient TCP option space, otherwise multiple
there is sufficient TCP option space, otherwise multiple TCP messages TCP messages containing this option will be sent. This option can be
containing this option will be sent. This option can be used at any used at any time during a connection, depending on when the sender
time during a connection, depending on when the sender wishes to wishes to enable multiple paths and/or when paths become available.
enable multiple paths and/or when paths become available.
Every address has an ID which can be used for address removal, and Every address has an ID which can be used for uniquely identifying
therefore endpoints must cache the mapping between ID and address. the address within a connection, for address removal. This is also
This is also used to identify Join Connection options (Section 3.2) used to identify MP_JOIN options (see Section 3.2) relating to the
relating to the same address, even when address translators are in same address, even when address translators are in use. The ID MUST
use. The ID must uniquely identify the address to the sender (within uniquely identify the address to the sender (within the scope of the
the connection), but its mechanism for allocating such IDs is connection), but the mechanism for allocating such IDs is
implementation-specific. implementation-specific.
This option is shown for IPv4. For IPv6, the IPVer field will read All address IDs learnt via either MP_JOIN or ADD_ADDR SHOULD be
6, and the length of the address will be 16 octets (instead of 4), stored by the receiver in a data structure that gathers all the
and the length of the option will be 2 + (18 * number_of_entries). Address ID to address mappings for a connection (identified by a
If there is sufficient TCP option space, multiple addresses can be token pair). In this way there is a stored mapping between Address
included, with an ID following on immediately from the previous ID, observed source address and token pair for future processing of
address. The number of addresses can be deduced from the option control information for a connection. Note that an implementation
length and version fields. MAY discard incoming address advertisements at will, for example for
avoiding the required 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 MAY choose to refresh advertisements periodically.
The 'P' bit is used to indicate the presence of an additional two This option is shown in Figure 12. The illustration is sized for
octets specifying the port number to use. Although it is expected IPv4 addresses (IPVer = 4). For IPv6, the IPVer field will read 6,
that the majority of use cases will use the same port pairs as used and the length of the address will be 16 octets (instead of 4).
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 the P bit is not
specified, MPTCP MUST attempt to connect to the specified address on
same port as is already in use by the signalling subflow.
The 'B' bit is used to indicate that this specified address (and The presence of the final two octets, specifying the TCP port number
port, if applicable) should be treated as a backup subflow to use to use, are optional and can be inferred from the length of the
only in the event of failure of other working subflows. A receiver option. Although it is expected that the majority of use cases will
of this option SHOULD set up a TCP subflow to the specified address use the same port pairs as used for the initial subflow (e.g. port 80
and port, but SHOULD NOT send data on it until the other paths have remains port 80 on all subflows), as does the ephemeral port at the
failed. 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 same port as is already in use by the signalling subflow,
and this is discussed in more detail in Section 3.7.
1 2 3 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 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=ADD_ADDR | Length | Address ID | IPVer | |B|P| | Kind | Length |Subtype| IPVer | Address ID |
+---------------+---------------+---------------+-------+---+-+-+ +---------------+---------------+-------+-------+---------------+
| Address (IPv4 - 4 octets / IPv6 - 16 octets) | | Address (IPv4 - 4 octets / IPv6 - 16 octets) |
+-------------------------------+-------------------------------+ +-------------------------------+---------------+---------------+
| Port (2 octets if P=1) | ... | Port (2 octets, optional) |
+-------------------------------+ +-------------------------------+
( ... further ID/Version/Address/Port fields as required ... )
Figure 11: Add Address (ADD_ADDR) option (shown for IPv4) Figure 12: Add Address (ADD_ADDR) option (shown for IPv4)
Due to the proliferation of NATs, it is reasonably likely that one Due to the proliferation of NATs, it is reasonably likely that one
endpoint may attempt to advertise private addresses [9]. We do not host may attempt to advertise private addresses [14]. We do not wish
wish to blanket prohibit this, since there may be cases where both to blanket prohibit this, since there may be cases where both hosts
endpoints have additional interfaces on the same private network. We have additional interfaces on the same private network. We must
must ensure, however, that such advertisements do not cause harm. ensure, however, that such advertisements do not cause harm. The
The standard mechanism to create a new subflow (Section 3.2) contains standard mechanism to create a new subflow (Section 3.2) contains a
a 32-bit token that uniquely identifies the connection to the 32-bit token that uniquely identifies the connection to the receiving
receiving endpoint . If the token is unknown, the endpoint will host. If the token is unknown, the host will return with a RST. In
return with a RST. If the token is known, subflow setup will the unlikely event that the token is known, subflow setup will
continue, but the sender's token will be sent back. In order for a continue, but the MAC exchange must occur for authentication. This
new subflow to be setup, both tokens must match what each endpoint will fail, and will provide sufficient protection against two
expects. This will be further followed by the HMAC exchange for unconnected hosts accidentally setting up a new subflow upon the
authentication. This will provide sufficient protection against two
unconnected endpoints accidentally setting up a new subflow upon the
signal of a private address. signal of a private address.
Ideally, we'd like to ensure the ADD_ADDR (and REMOVE_ADDR) option is Ideally, we'd like to ensure the ADD_ADDR and REMOVE_ADDR options are
sent reliably and in order to the other end. This is to ensure that sent reliably, and in order, to the other end. This is to ensure
we don't close the connection when remove/add addresses are processed that we do not unnecessarily cause an outage in the connection when
in reverse order, and to ensure that all possible paths are used. We remove/add addresses are processed in reverse order, and also to
note, however, that losing reliability and ordering it will not break ensure that all possible paths are used. We note, however, that
the multipath connections; they will just reduce the opportunity to losing reliability and ordering it will not break the multipath
open multipath paths and to survive different patterns of path connections; they will just reduce the opportunity to open multipath
failures. paths and to survive different patterns of path failures.
Subflow level ACKs do not cover options, so if we want explicit Therefore, implementing reliability signals for these TCP options is
guarantees we need to build in other mechanisms. Solutions include not necessary. In order to minimise the impact of the loss of these
echoing the options and sending one option per RTT, or adding a options, however, it is RECOMMENDED that a sender should send these
sequence number to the option which is explicitly acked in another options on all available subflows. If these options need to be
option. However, we feel these mechanisms' added complexity is not received in-order, an implementation SHOULD only send one ADD_ADDR/
worth the benefits they bring. There are two basic failure modes for REMOVE_ADDR option per RTT, to minimise the risk of misordering.
options: a) every new option gets stripped or b) some options get
stripped, randomly. The second option looks more like a middlebox
implementation error, so we believe it is not worth optimizing for.
In the first case, resending the option on a different subflow is the
thing to do. To achieve similar reliability without explicit ACKs,
we propose sending all ADD_ADDR/REMOVE_ADDR options on all existing
subflows. If ordering is needed, we should only send one ADD_ADDR/
REMOVE_ADDR option per RTT (modulo lost packets at subflow level).
When receiving an ADD_ADDR message with an address ID already in use When receiving an ADD_ADDR message with an Address ID already in use
for that connection, the receiver SHOULD silently ignore the for a live subflow within the connection, the receiver SHOULD
ADD_ADDR. silently ignore the ADD_ADDR. If the Address ID is not in use on a
live subflow, but is stored by the receiver, a new ADD_ADDR SHOULD
take precedence and replace the stored address.
A host that receives an ADD_ADDR but finds a connection setup to that
address is unsuccessful SHOULD NOT perform further connection
attempts to this address for this connection. A sender that wants to
trigger a new incoming connection attempt on a previously advertised
address can therefore refresh ADD_ADDR information by sending the
option again.
During normal MPTCP operation, it is unlikely that there will be During normal MPTCP operation, it is unlikely that there will be
sufficient TCP option space for ADD_ADDR to be included along with sufficient TCP option space for ADD_ADDR to be included along with
those for data sequence numbering (Section 3.3.1). Therefore, it is those for data sequence numbering (Section 3.3.1). Therefore, it is
expected that an MPTCP implementation will send the ADD_ADDR option expected that an MPTCP implementation will send the ADD_ADDR option
on separate (either duplicate, or normal but lacking any payload) on separate ACKs. As discussed earlier, however, an MPTCP
ACKs. implementation MUST NOT treat duplicate ACKs with MPTCP options as
indications of congestion [7], and an MPTCP implementation SHOULD NOT
As with all TCP Options, the ADD_ADDR option does not have reliable send more than two duplicate ACKs in a row for signalling purposes.
delivery. Therefore, a sender should send a duplicate ACK with this
option on all available subflows.
3.5.2. Remove Address 3.4.2. Remove Address
If, during the lifetime of a MPTCP connection, a previously-announced If, during the lifetime of an MPTCP connection, a previously-
address becomes invalid (e.g. if the interface disappears), the announced address becomes invalid (e.g. if the interface disappears),
affected endpoint should announce this so that the other endpoint can the affected host SHOULD announce this so that the peer can remove
remove subflows related to this address. subflows related to this address.
This is achieved through the Remove Address (REMOVE_ADDR) option This is achieved through the Remove Address (REMOVE_ADDR) option
(Figure 12), which will remove a previously-added address (or list of (Figure 13), which will remove a previously-added address (or list of
addresses) from a connection and terminate any subflows currently addresses) from a connection and terminate any subflows currently
using that address. using that address.
For security purposes, if a host receives a REMOVE_ADDR option, it For security purposes, if a host receives a REMOVE_ADDR option, it
must ensure the affected path(s) are no longer in use before it must ensure the affected path(s) are no longer in use before it
instigates closure. The receipt of REMOVE_ADDR should first trigger instigates closure. The receipt of REMOVE_ADDR SHOULD first trigger
the sending of a TCP Keepalive [10] on the path, and if a response is the sending of a TCP Keepalive [15] on the path, and if a response is
received the path is not removed. Typical TCP validity tests on the received the path is not removed. Typical TCP validity tests on the
subflow (e.g. ensuring sequence and ack numbers are correct) MUST subflow (e.g. ensuring sequence and ack numbers are correct) MUST
also be undertaken. also be undertaken.
The sending and receipt (if no keepalive response was received) of The sending and receipt (if no keepalive response was received) of
this message SHOULD trigger the sending of RSTs by both endpoints on this message SHOULD trigger the sending of RSTs by both hosts on the
the affected subflow(s) (if possible), as a courtesy to cleaning up affected subflow(s) (if possible), as a courtesy to cleaning up
middlebox state, but endpoints may clean up their internal state middlebox state, before cleaning up any local state.
without a long timeout.
Address removal is undertaken by ID, so as to permit the use of NATs Address removal is undertaken by ID, so as to permit the use of NATs
and other middleboxes. If there is no address at the requested ID, and other middleboxes that rewrite source addresses. If there is no
the receiver will silently ignore the request. address at the requested ID, the receiver will silently ignore the
request.
The standard way to close a subflow (so long as it is still A subflow that is still functioning MUST be closed with a FIN
functioning) is to use a FIN exchange as in regular TCP - for more exchange as in regular TCP - for more information, see Section 3.3.3.
information, see Section 3.4.
1 2 3 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 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=REMOVEADDR| Length = 2+n | Address ID | ... | Kind | Length = 3+n |Subtype| | Address ID | ...
+---------------+---------------+---------------+ +---------------+---------------+-------+-------+---------------+
Figure 12: Remove Address (REMOVE_ADDR) option Figure 13: Remove Address (REMOVE_ADDR) option
3.6. Fallback 3.5. Fallback
At the start of a MPTCP connection (i.e. the first subflow), it is 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 important to ensure that the path is fully MPTCP-capable and the
necessary TCP options can reach each endpoint. The handshake as necessary TCP options can reach each host. The handshake as
described in Section 3.1 will fall back to regular TCP if either of described in Section 3.1 will fall back to regular TCP if either of
the SYN messages do not have the MPTCP options: this is the same, and the SYN messages do not have the MPTCP options: this is the same, and
desired, behaviour in the case where an endpoint is not MPTCP desired, behaviour in the case where a host is not MPTCP capable, or
capable, or the path does not support he MPTCP options. When the path does not support the MPTCP options. When attempting to join
attempting to join an existing MPTCP connection (Section 3.2), if a an existing MPTCP connection (Section 3.2), if a path is not MPTCP
path is not MPTCP capable, the TCP options will not get through on capable, the TCP options will not get through on the SYNs and the
the SYNs and the subflow will be closed. subflow will be closed.
There is, however, another corner case which should be addressed. There is, however, another corner case which should be addressed.
That is one of MPTCP options getting through on the SYN, but not on That is one of MPTCP options getting through on the SYN, but not on
regular packets. This can be resolved if the subflow is the first regular packets. This can be resolved if the subflow is the first
subflow, and thus all data in flight is contiguous. This resolution subflow, and thus all data in flight is contiguous, using the
mechanism is as follows: following rules.
o The first window's worth of data MUST be DATA_ACKed on every A sender MUST include a DSS option with Data Sequence Mapping in
packet every segment until one of the sent segments has been acknowledged
with a DSS option containing a Data ACK. Upon reception of the
acknowledgement, 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.
o If the first data packet does not have a Data Sequence Mapping If, however, an ACK is received for data (not just for the SYN)
option, drop out of MPTCP mode back to regular TCP (and thus send without a Data ACK in a DSS option, the sender determines the path is
a regular, subflow-level ACK, without a DATA_ACK) 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 an RST. In the case of the first subflow (i.e. that
started with MP_CAPABLE), it MUST drop out of a MPTCP mode back to
regular TCP. The sender will send one final Data Sequence Mapping,
with the length value of 0 indicating an infinite mapping (in case
the path drops options in one direction only), and then revert to
sending data on the single subflow without any MPTCP options.
o If an ACK is received without a DATA_ACK within the first window, Note that this rule essentially prohibits the sending of data on the
drop out of MPTCP mode back to regular TCP (and thus stop sending third packet of a MP_CAPABLE or MP_JOIN handshake, since both that
data with a Data Sequence Mapping) option and a DSS cannot fit in TCP option space. If the initiator is
to send first, another segment must be sent that contains the data
and DSS. Note also that an additional subflow cannot be used until
the initial path has been verified as MPTCP-capable.
These rules should cover all cases where such a failure could happen: 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 whether it's on the forward or reverse path, and whether the server
or the client first sends data. If lost options on data packets or the client first sends data. If lost options on data packets
occur on any other subflow apart from the start of the initial occur on any other subflow apart from the the initial subflow, it
subflow, it should be treated as a standard path failure. The data should be treated as a standard path failure. The data would not be
would not be DATA_ACKed (since there is no mapping for the data), and DATA_ACKed (since there is no mapping for the data), and the subflow
the subflow can be closed with an RST. can be closed with an RST. (Note that these rules do not apply if an
infinite mapping is included from the start - in which case, each end
will send DSS options declaring the infinite mapping.)
The case described above is a specialised case of fallback. More The case described above is a specialised case of fallback. More
generally, fallback to regular TCP can become necessary at any point generally, fallback to regular TCP can become necessary at any point
during a connection if a non-MPTCP-aware middlebox changes the data during a connection if a non-MPTCP-aware middlebox changes the data
stream. stream.
As described in Section 3.3, each portion of data for which there is As described in Section 3.3, each portion of data for which there is
a mapping is protected by a checksum. This mechanism is used to a mapping is protected by a checksum. This mechanism is used to
detect if middleboxes have made any adjustments to the payload detect if middleboxes have made any adjustments to the payload
(added, removed, or changed data). A checksum will fail if the data (added, removed, or changed data). A checksum will fail if the data
has been changed in any way. This will also detect if the length of has been changed in any way. This will also detect if the length of
data on the subflow is increased or decreased, and this means the data on the subflow is increased or decreased, and this means the
Data Sequence Mapping is no longer valid. The sender no longer knows Data Sequence Mapping is no longer valid. The sender no longer knows
what subflow-level sequence number the receiver is genuinely what subflow-level sequence number the receiver is genuinely
operating at (the middlebox will be faking ACKs in return), and operating at (the middlebox will be faking ACKs in return), and
cannot signal any further mappings. Furthermore, in addition to the cannot signal any further mappings. Furthermore, in addition to the
possibility of payload modifications that are valid at the possibility of payload modifications that are valid at the
application layer, there is the possibility that false-positives application layer, there is the possibility that false-positives
could be hit across segment boundaries, corrupting the data. could be hit across MPTCP segment boundaries, corrupting the data.
Therefore, all data from the start of the segment that failed the Therefore, all data from the start of the segment that failed the
checksum onwards is not trustworthy. checksum onwards is not trustworthy.
When multiple subflows are in use, the data in-flight on a subflow When multiple subflows are in use, the data in-flight on a subflow
will likely involve data that is not contiguously part of the will likely involve data that is not contiguously part of the
connection-level stream, since segments will be spread across the connection-level stream, since segments will be spread across the
multiple subflows. Due to the problems identified above, it is not multiple subflows. Due to the problems identified above, it is not
possible to determine what the adjustment has done to the data possible to determine what the adjustment has done to the data
(notably, any changes to the subflow sequence numbering). Therefore, (notably, any changes to the subflow sequence numbering). Therefore,
it is not possible to recover the subflow, and the affected subflow it is not possible to recover the subflow, and the affected subflow
must be immediately closed with an RST, featuring a "checksum failed" must be immediately closed with an RST, featuring a MP_FAIL option
option, which defines the Data Sequence Number at the start of the (Figure 14), which defines the Data Sequence Number at the start of
segment (defined by the Data Sequence Mapping) which had the checksum the segment (defined by the Data Sequence Mapping) which had the
failure (see Figure 13). checksum failure.
1 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 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=MP_FAIL | Length=10 | Data Sequence Number (8B) : | Kind | Length=12 |Subtype| (reserved) |
+---------------+---------------+------------------------------+ +---------------+---------------+-------+----------------------+
: Data Sequence Number (contd.) : | Data Sequence Number (8 octets) :
+-------------------------------+------------------------------+ +--------------------------------------------------------------+
: Data Sequence Number (contd.)| : Data Sequence Number (continued) |
+-------------------------------+ +--------------------------------------------------------------+
Figure 13: Fallback (MP_FAIL) option
TBD: In this case, is there any point in signalling Checksum Failed, Figure 14: Fallback (MP_FAIL) option
or could we just RST the subflow? The signal would allow the sender
to know there is something wrong with the path and not try to re-
establish the subflow (if that was otherwise the policy).
Failed data will not be DATA_ACKed and so will be re-transmitted on Failed data will not be DATA_ACKed and so will be re-transmitted on
other subflows (Section 3.3.4). other subflows (Section 3.3.6).
A special case is when there is a single subflow and it fails with a A special case is when there is a single subflow and it fails with a
checksum error. Here, MPTCP should be able to recover and continue checksum error. Here, MPTCP should be able to recover and continue
sending data. There are two possible mechanisms to support this. sending data. There are two possible mechanisms to support this.
The first and simplest is to nevertheless close the subflow with a The first and simplest is to establish a new subflow as part of the
RST, and immediately establish a new one as part of the same MPTCP same MPTCP connection, and then close the original one with a RST.
connection. Since it is known that the path may be compromised, it Since it is known that the path may be compromised, it is not
is not desirable to use MPTCP's segmentation on this path any longer. desirable to use MPTCP's segmentation on this path any longer. The
The new subflow will begin and will signal an infinite mapping new subflow will begin and will signal an infinite mapping (indicated
(indicated by length=0 in the Data Sequence Mapping option, by length=0 in the Data Sequence Mapping option, Section 3.3) from
Section 3.3) from the data sequence number of the segment that failed the data sequence number of the segment that failed the checksum.
the checksum. This connection will then continue to appear as a This connection will then continue to appear as a regular TCP
regular TCP session, and a middlebox may change the payload without session, and a middlebox may change the payload without causing
causing unintentional harm. unintentional harm.
An optimisation is possible, however. If it is known that all An optimisation is possible, however. If it is known that all
unacknowledged data in flight is contiguous, an infinite mapping unacknowledged data in flight is contiguous, an infinite mapping
could be applied to the subflow without the need to close it first, could be applied to the subflow without the need to close it first,
and essentially turn off all further MPTCP signalling. In this case, and essentially turn off all further MPTCP signalling. In this case,
if a receiver identifies a checksum failure when there is only one 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. path, it will send back an MP_FAIL option on the subflow-level ACK.
The sender will receive this, and if all unacknowledged data in The sender will receive this, and if all unacknowledged data in
flight is contiguous, will signal an infinite mapping (if the data is flight is contiguous, will signal an infinite mapping (if the data is
not contiguous, the sender MUST send an RST). This infinite mapping not contiguous, the sender MUST send an RST). This infinite mapping
will be a Data Sequence Mapping option on the first new packet, but will be a DSS option (Section 3.3) on the first new packet,
it acts retroactively, referring to the start of the subflow sequence containing a Data Sequence Mapping that acts retroactively, referring
number of the last segment that was known to be delivered intact. to the start of the subflow sequence number of the last segment that
From that point onwards data can be altered by a middlebox without was known to be delivered intact. From that point onwards data can
affecting MPTCP, as the data stream is equivalent to a regular, be altered by a middlebox without affecting MPTCP, as the data stream
legacy TCP session. is equivalent to a regular, legacy TCP session.
After a sender signals an infinite mapping it MUST only use subflow 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 ACKs to clear its send buffer. This is because Data ACKs may become
misaligned with the subflow ACKs when middleboxes insert or delete misaligned with the subflow ACKs when middleboxes insert or delete
data. The receive SHOULD stop generating Data ACKs after it receives data. The receive SHOULD stop generating Data ACKs after it receives
an infinite mapping. an infinite mapping.
When a connection is in fallback mode, only one subflow can send data When a connection is in fallback mode, only one subflow can send data
at a time. Otherwise, the receiver would not know how to reorder the at a time. Otherwise, the receiver would not know how to reorder the
data. However, subflows can be opened and closed as necessary, as data. However, subflows can be opened and closed as necessary, as
long as a single one is active at any point. long as a single one is active at any point.
It should be emphasised that we are not attempting to prevent the use It should be emphasised that we are not attempting to prevent the use
of middleboxes that want to adjust the payload. An MPTCP-aware of middleboxes that want to adjust the payload. An MPTCP-aware
middlebox to provide such functionality could be designed that would middlebox to provide such functionality could be designed that would
re-write checksums if needed, and additionally would be able to parse re-write checksums if needed, and additionally would be able to parse
the data sequence mappings, and thus not hit false positives though the data sequence mappings, and thus not hit false positives though
not knowing where data boundaries lie. not knowing where data boundaries lie.
3.7. Error Handling 3.6. Error Handling
In addition to the fallback mechanism as described above, the In addition to the fallback mechanism as described above, the
standard classes of TCP errors may need to be handled in an MPTCP- standard classes of TCP errors may need to be handled in an MPTCP-
specific way. Note that changing semantics - such as the relevance specific way. Note that changing semantics - such as the relevance
of an RST - has already been covered in Section 4. Where possible, of an RST - are covered in Section 4. Where possible, we do not want
we do not want to deviate from regular TCP behaviour. to deviate from regular TCP behaviour.
The following list covers possible errors and the appropriate MPTCP The following list covers possible errors and the appropriate MPTCP
behaviour: behaviour:
o Unknown token in MP_JOIN (or token mismatch in MP_JOIN ACK, or o Unknown token in MP_JOIN (or MAC failure in MP_JOIN ACK, or
missing MP_JOIN in SYN/ACK response): send RST (analogous to TCP's missing MP_JOIN in SYN/ACK response): send RST (analogous to TCP's
behaviour on an unknown port) behaviour on an unknown port)
o DSN out of Window (during normal operation): just ignore, however o DSN out of Window (during normal operation): drop the data, do not
if at the beginning of a new subflow we might want to RST it as a send Data ACKs.
security mechanism
o Remove request for unknown address ID: silently ignore o Remove request for unknown address ID: silently ignore
o DATA_ACK for data not yet sent: abort connection by RST on every 3.7. Heuristics
subflow.
3.8. Heuristics
There are a number of heuristics that are needed for performance or There are a number of heuristics that are needed for performance or
deployment but which are not required for protocol correctness. In deployment but which are not required for protocol correctness. In
this section we detail such heuristics this section we detail such heuristics. Note that discussion of
buffering and certain sender and receiver window behaviours are
presented in Section 3.3.4 and Section 3.3.5, as well as
retransmission in Section 3.3.6.
3.8.1. Port Usage 3.7.1. Port Usage
Under typical operation an MPTCP implementation SHOULD use the same Under typical operation an MPTCP implementation SHOULD use the same
ports as already in use. In other words, the destination port of a ports as already in use. In other words, the destination port of a
SYN containing a MP_JOIN option SHOULD be the same as the remote port SYN containing a MP_JOIN option SHOULD be the same as the remote port
of the first subflow in the connection. The local port for such SYNs of the first subflow in the connection. The local port for such SYNs
SHOULD also be the same as for the first subflow (and as such, an SHOULD also be the same as for the first subflow (and as such, an
implementation SHOULD reserve ephemeral ports across all local IP implementation SHOULD reserve ephemeral ports across all local IP
addresses), although there may be cases where this is infeasible. addresses), although there may be cases where this is infeasible.
This strategy is intended to maximize the probability of the SYN This strategy is intended to maximize the probability of the SYN
being permitted by a firewall or NAT at the recipient and to avoid being permitted by a firewall or NAT at the recipient and to avoid
confusing any network monitoring software. confusing any network monitoring software.
There may also be cases, however, where the passive opener wishes to There may also be cases, however, where the passive opener wishes to
signal to the other endpoint that a specific port should be used, and signal to the other host that a specific port should be used, and
this facility is provided in the Add Address option as documented in this facility is provided in the Add Address option as documented in
Section 3.5.1. It is therefore feasible to allow multiple subflows Section 3.4.1. It is therefore feasible to allow multiple subflows
between the same two addresses but using different port pairs, and between the same two addresses but using different port pairs, and
such a facility could be such a facility could be used to allow load such a facility could be used to allow load balancing within the
balancing within the network based on 5-tuples (e.g. ECMP). network based on 5-tuples (e.g. some ECMP implementations).
3.7.2. Delayed Subflow Start
Many TCP connections are short-lived and consist only of a few
segments, and so the overheads of using MPTCP outweigh any benefits.
A heuristic is required, therefore, to decide when to start using
additional subflows in an MPTCP connection. We expect that
experience gathered from deployments will provide further guidance on
this, and will be affected by particular application characteristics
(which are likely to change over time). However, a suggested
general-purpose heuristic that an implementation MAY choose to employ
is as follows. Results from experimental deployments are needed in
order to verify the correctness of this proposal.
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 which will never
fill its buffers, and thus never use MPTCP. 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.
This section has shown some of the considerations than an implementer
should give when developing MPTCP heuristics, but is not intended to
be prescriptive.
3.7.3. Failure Handling
Requirements for MPTCP's handling of unexpected signals have been
given in Section 3.6. There are other failure cases, however, where
a hosts can choose appropriate behaviour.
For example, Section 3.1 suggests that a host should fall back to
trying regular TCP SYNs after several failures of MPTCP SYNs. 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
failing.
Another failure could occur when the MP_JOIN handshake fails.
Section 3.6 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 during
the lifetime of the connection, unless the other host refreshes the
information with a REMOVE_ADDR and then an ADD_ADDR for the same
address.
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 these. Behaviour
could also be learnt for particularly badly performing subflows or
subflows that regularly fail during use, in order to temporarily
choose not to use these paths.
4. Semantic Issues 4. Semantic Issues
In order to support multipath operation, the semantics of some TCP In order to support multipath operation, the semantics of some TCP
components have changed. To aid clarity, this section collects these components have changed. To aid clarity, this section collects these
semantic changes as a reference. semantic changes as a reference.
Sequence Number: The (in-header) TCP sequence number is specific to Sequence Number: The (in-header) TCP sequence number is specific to
the subflow. To allow the receiver to reorder application data, the subflow. To allow the receiver to reorder application data,
an additional data-level sequence space is used. In this data- an additional data-level sequence space is used. In this data-
level sequence space, the initial SYN and the final DATA_FIN level sequence space, the initial SYN and the final DATA_FIN
occupy one octet of sequence space. There is an explicit mapping occupy one octet of sequence space. There is an explicit mapping
of data sequence space to subflow sequence space, which is of data sequence space to subflow sequence space, which is
signalled through TCP options in data packets. signalled through TCP options in data packets.
ACK: The ACK field in the TCP header acknowledges only the subflow ACK: The ACK field in the TCP header acknowledges only the subflow
sequence number, not the data-level sequence space. sequence number, not the data-level sequence space.
Implementations SHOULD NOT attempt to infer a data-level Implementations SHOULD NOT attempt to infer a data-level
acknowledgement from the subflow ACKs. Instead an explicit data- acknowledgement from the subflow ACKs. Instead an explicit data-
level DATA_ACK is used. This avoids possible deadlock scenarios level ACK is used. This avoids possible deadlock scenarios when a
when a non-TCP-aware middlebox pro-actively ACKs at the subflow non-TCP-aware middlebox pro-actively ACKs at the subflow level,
level. and separates subflow- and connection-level processing at an end
host.
Duplicate ACK: A duplicate ACK that includes MPTCP signalling MUST
NOT be treated as a signal of congestion. To avoid any non-MPTCP-
aware entities also mistakenly seeing duplicate ACKs in such
cases, MPTCP SHOULD NOT send more than two duplicate ACKs
containing MPTCP signals in a row.
Receive Window: The receive window in the TCP header indicates the Receive Window: The receive window in the TCP header indicates the
amount of free buffer space for the whole data-level connection amount of free buffer space for the whole data-level connection
(as opposed to for this subflow) that is available at the (as opposed to for this subflow) that is available at the
receiver. This is the same semantics as regular TCP, but to receiver. This is the same semantics as regular TCP, but to
maintain these semantics the receive window must be interpreted at maintain these semantics the receive window must be interpreted at
the sender as relative to the sequence number given in the the sender as relative to the sequence number given in the
DATA_ACK rather than the subflow ACK in the TCP header. In this DATA_ACK rather than the subflow ACK in the TCP header. In this
way the original flow control role is preserved. way the original flow control role is preserved. Note that some
middleboxes may change the receive window, and so a host must use
the maximum value of those recently seen on the constituent
subflows for the connection-level receive window, and also need to
maintain a subflow-level window for subflow-level processing.
FIN: The FIN flag in the TCP header applies only to the subflow it 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 is sent on, not to the whole connection. For connection-level FIN
semantics, the DATA_FIN option is used. semantics, the DATA_FIN option is used.
RST: The RST flag in the TCP header applies only to the subflow it RST: The RST flag in the TCP header applies only to the subflow it
is sent on, not to the whole connection. A connection is is sent on, not to the whole connection. A connection is
considered reset if a RST is received on every subflow. considered reset if a RST is received on every subflow.
Address List: Address list management (i.e. knowledge of the local Address List: Address list management (i.e. knowledge of the local
skipping to change at page 34, line 7 skipping to change at page 41, line 23
5-tuple: The 5-tuple (protocol, local address, local port, remote 5-tuple: The 5-tuple (protocol, local address, local port, remote
address, remote port) presented by kernel APIs to the application address, remote port) presented by kernel APIs to the application
layer in a non-multipath-aware application is that of the first layer in a non-multipath-aware application is that of the first
subflow, even if the subflow has since been closed and removed subflow, even if the subflow has since been closed and removed
from the connection. This decision, and other related API issues, from the connection. This decision, and other related API issues,
are discussed in more detail in [5]. are discussed in more detail in [5].
5. Security Considerations 5. Security Considerations
As identified in [11], the addition of multipath capability to TCP As identified in [16], the addition of multipath capability to TCP
will bring with it a number of new classes of threat. In order to will bring with it a number of new classes of threat. In order to
prevent these, [3] presents a set of requirements for a security prevent these, [3] presents a set of requirements for a security
solution for MPTCP. The fundamental goal is for the security of solution for MPTCP. The fundamental goal is for the security of
MPTCP to be "no worse" than regular TCP today, and the key security MPTCP to be "no worse" than regular TCP today, and the key security
requirements are: requirements are:
o Provide a mechanism to confirm that the parties in a subflow o Provide a mechanism to confirm that the parties in a subflow
handshake are the same as in the original connection setup. handshake are the same as in the original connection setup.
o Provide verification that the peer can receive traffic at a new o Provide verification that the peer can receive traffic at a new
skipping to change at page 34, line 30 skipping to change at page 41, line 46
o Provide replay protection, i.e. ensure that a request to add/ o Provide replay protection, i.e. ensure that a request to add/
remove a subflow is 'fresh'. remove a subflow is 'fresh'.
In order to achieve these goals, MPTCP includes a hash-based In order to achieve these goals, MPTCP includes a hash-based
handshake algorithm documented in Section 3.1 and Section 3.2. handshake algorithm documented in Section 3.1 and Section 3.2.
The security of the MPTCP connection hangs on the use of keys that The security of the MPTCP connection hangs on the use of keys that
are shared once at the start of the first subflow, and never again in are shared once at the start of the first subflow, and never again in
the clear. To ease demultiplexing whilst not giving away any the clear. To ease demultiplexing whilst not giving away any
cryptographic material, future subflows use a truncated SHA-1 hash of cryptographic material, future subflows use a truncated SHA-1 hash of
this key as the connection identification "token". The keys are used this key as the connection identification "token". The keys are
as keys in a HMAC, and this should verify that the parties in the combined and used as keys in a MAC, and this should verify that the
handshake are the same as in the original connection setup. It also parties in the handshake are the same as in the original connection
provides verification that the peer can receive traffic at this new setup. It also provides verification that the peer can receive
address. Replay attacks would still be possible in this scenario, traffic at this new address. Replay attacks would still be possible
and therefore the handshakes use single-use random numbers (nonces) when only keys are used, and therefore the handshakes use single-use
at both ends - this ensures the HMAC will never be the same on two random numbers (nonces) at both ends - this ensures the MAC will
handshakes. The security mechanism presented in this draft should never be the same on two handshakes. The use of crypto capability
therefore protect against all forms of flooding and hijacking attacks bits in the initial connection handshake to negotiate use of a
suggested in [11]. particular algorithm will allow the deployment of additional crypto
mechanisms in the future. Note that this would be susceptible to
bid-down attacks only if the attacker was on-path (and thus would be
able to modify the data anyway). The security mechanism presented in
this draft should therefore protect against all forms of flooding and
hijacking attacks suggested in [16].
6. Interactions with Middleboxes 6. Interactions with Middleboxes
Multipath TCP was designed to be deployable in the present world. Multipath TCP was designed to be deployable in the present world.
Its design takes into account "reasonable" existing middlebox Its design takes into account "reasonable" existing middlebox
behaviour. In this section we outline a few representative behaviour. In this section we outline a few representative
middlebox-related failure scenarios and show how multipath TCP middlebox-related failure scenarios and show how multipath TCP
handles them. Next, we list the design decisions multipath has made handles them. Next, we list the design decisions multipath has made
to accomodate the different middleboxes. to accomodate the different middleboxes.
A primary concern is our use of new TCP options. Most middleboxes A primary concern is our use of a new TCP option. Most middleboxes
should just forward packets with new options unchanged, yet there are should just forward packets with new options unchanged, yet there are
some that don't. These we expect will either strip options and pass some that don't. These we expect will either strip options and pass
the data, drop packets with new options, copy the same option into the data, drop packets with new options, copy the same option into
multiple segments (e.g. when doing segmentation) or drop options multiple segments (e.g. when doing segmentation) or drop options
during segment coalescing. during segment coalescing.
MPTCP SYN packets contain the MP_CAPABLE option to indicate the use MPTCP uses a single new TCP option "Kind", and all message types are
of MPTCP. When the middlebox drops the packet containing the defined by "subtype" values (see Section 8). This should reduce the
MP_CAPABLE option either on the outgoing or the return path, the chances of only some types of MPTCP options being passed, and instead
connection will fail. Host A SHOULD fall back to TCP in such cases the key differing characteristics are different paths, and the
(studies suggest that few middleboxes drop packets with unknown presence of the SYN flag.
options). The same applies for subflow setup.
The second case is when the middleboxes strip options. Let's first MPTCP SYN packets on the first subflow of a connection contain the
discuss behaviour for initial connection SYNs (see Figure 14). If MP_CAPABLE option (Section 3.1). If this is dropped, MPTCP SHOULD
the option is stripped from the packet on the outgoing path, the fall back to regular TCP. If packets with the MP_JOIN option
connection falls back to regular TCP. If the option is stripped on (Section 3.2) are dropped, the paths will simply not be used.
the return path, host B will wait for a DATA_ACK of its connection
SYN, retransmitting the SYN/ACK until it declares the connection If a middlebox strips options but otherwise passes the packets
failed. Host A thinks it is talking to a regular host, and may send unchanged, MPTCP will behave safely. If an MP_CAPABLE option is
data segments, but these will not be acked by host B as they do not dropped on either the outgoing or the return path, the initiating
have the proper mapping. Hence the connection fails. Host A SHOULD host can fall back to regular TCP, as illustred in Figure 15 and
fall back to regular TCP after the connection times out. discussed in Section 3.1.
Subflow SYNs contain the MP_JOIN option. If this option is stripped 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 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 B. Depending on whether there is a listening socket on the target
port, host B will reply either with SYN/ACK or RST (subflow port, host B will reply either with SYN/ACK or RST (subflow
connection fails). When host A receives the SYN/ACK it sends a RST 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 because the SYN/ACK does not contain the MP_JOIN option and its
token. Either way, the connection fails. token. Either way, the subflow setup fails, but otherwise does not
affect the MPTCP connection as a whole.
Host A Host B Host A Host B
| Middlebox M | | Middlebox M |
| | | | | |
| SYN(MP_CAPABLE) | SYN | | SYN(MP_CAPABLE) | SYN |
|-------------------|---------------->| |-------------------|---------------->|
| SYN/ACK | | SYN/ACK |
|<------------------------------------| |<------------------------------------|
a) MP_CAPABLE option stripped on outgoing path a) MP_CAPABLE option stripped on outgoing path
Host A Host B Host A Host B
| SYN(MP_CAPABLE) | | SYN(MP_CAPABLE) |
|------------------------------------>| |------------------------------------>|
| Middlebox M | | Middlebox M |
| | | | | |
| SYN/ACK |SYN/ACK(MP_CAPABLE)| | SYN/ACK |SYN/ACK(MP_CAPABLE)|
|<----------------|-------------------| |<----------------|-------------------|
b) MP_CAPABLE option stripped on return path b) MP_CAPABLE option stripped on return path
Figure 14: Connection Setup with Middleboxes that Strip Options from Figure 15: Connection Setup with Middleboxes that Strip Options from
Packets Packets
We now examine data flow with MPTCP, assuming the flow is correctly We now examine data flow with MPTCP, assuming the flow is correctly
setup which implies the options in the SYN packets were allowed setup, which implies the options in the SYN packets were allowed
through by the relevant middleboxes. If options are allowed through through by the relevant middleboxes. If options are allowed through
and there is no resegmentation or coalescing to TCP segments, and there is no resegmentation or coalescing to TCP segments,
multipath TCP flows can proceed without problems. multipath TCP flows can proceed without problems.
The case when options get stripped on data packets has been discussed The case when options get stripped on data packets has been discussed
in the Fallback section. We can further analyze what happens when a in the Fallback section. If a fraction of options are stripped,
fraction of options is stripped. The multipath subflow should behaviour is not deterministic. If some Data Sequence Mappings are
survive losing a fraction of DATA_ACKs and data sequence mappings, 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, as discussed
in Section 3.3. MPTCP should survive with a loss of some Data ACKs,
but performance will degrade as the fraction of stripped options but performance will degrade as the fraction of stripped options
increases. We do not expect such cases to appear in practice, increases. We do not expect such cases to appear in practice,
though: most middleboxes will either strip all options or let them though: most middleboxes will either strip all options or let them
all through. all through.
We end this section with a list of middlebox classes, their behaviour We end this section with a list of middlebox classes, their behaviour
and the elements in the MPTCP design that allow operation through and the elements in the MPTCP design that allow operation through
such middleboxes. Issues surrounding dropping packets with options such middleboxes. Issues surrounding dropping packets with options
or stripping options were discussed above, and are not included here: or stripping options were discussed above, and are not included here:
o NAT [12]: changes the source address and port of packets. This o NAT [17]: Network Address (and Port) Translators change the source
means that a host will not know its public-facing address for address (and often source port) of packets. This means that a
signalling in MPTCP. Therefore, MPTCP permits implicit address host will not know its public-facing address for signalling in
addition via the MP_JOIN option, and has heuristics to ensure that MPTCP. Therefore, MPTCP permits implicit address addition via the
connection attempts to private addresses [9] do not cause MP_JOIN option, and the handshake mechanism ensures that
problems. Address removal is undertaken by an ID number to allow connection attempts to private addresses [14] do not cause
no knowledge of the source address. problems. Explicit address removal is undertaken by an ID number
to allow no knowledge of the source address.
o Performance Enhancing Proxies (PEPs) [13]: might pro-actively ACK o Performance Enhancing Proxies (PEPs) [18]: might pro-actively ACK
data to increase performance. Problems will occur if a PEP ACKs data to increase performance. Problems will occur if a PEP ACKs
data and then fails before sending it on to the receiver, of it data and then fails before sending it on to the receiver, of if
the receiver is mobile and moves away before proactively ACKed the receiver is mobile and moves away before proactively ACKed
data is forwarded on. If subflow ACKs were used to control send data is forwarded on. If subflow ACKs were used to control send
buffering, the data could be lost and never be retransmitted, thus buffering, the data could be lost and never be retransmitted, thus
causing the subflow to permanently stall. MPTCP therefore uses causing the subflow to permanently stall. MPTCP therefore uses
the DATA_ACK to make progress when one of its subflows fails in the DATA_ACK to make progress when one of its subflows fails in
this way. This is why MPTCP does not use subflow ACKs to infer this way. This is why MPTCP does not use subflow ACKs to infer
connection level ACKs. connection level ACKs.
o Traffic Normalizers [14]: do not allow holes in sequence numbers, o Traffic Normalizers [19]: may not allow holes in sequence numbers,
cache packets and retransmit the same data. MPTCP looks like and may cache packets and retransmit the same data. MPTCP looks
standard TCP on the wire, and will not retransmit different data like standard TCP on the wire, and will not retransmit different
on the same subflow sequence number. data on the same subflow sequence number.
o Firewalls [15]: might perform sequence number randomization on TCP o Firewalls [20]: might perform initial sequence number
connections. MPTCP uses relative sequence numbers in data randomization on TCP connections. MPTCP uses relative sequence
sequence mapping to cope with this. Like NATs, firewalls will not numbers in data sequence mapping to cope with this. Like NATs,
permit many incoming connections, so MPTCP supports address firewalls will not permit many incoming connections, so MPTCP
signalling (ADD_ADDR) so that a multi-addressed endpoint can supports address signalling (ADD_ADDR) so that a multi-addressed
invite its peer behind the firewall/NAT to connect out to its host can invite its peer behind the firewall/NAT to connect out to
additional interface. its additional interface.
o Intrusion Detection Systems: look out for traffic patterns and o Intrusion Detection Systems: look out for traffic patterns and
content that could threaten a network. Multipath will mean that content that could threaten a network. Multipath will mean that
such data is potentially spread, so it is more difficult for an such data is potentially spread, so it is more difficult for an
IDS to analyse the whole traffic, and potentially increasint the IDS to analyse the whole traffic, and potentially increases the
risk of false positives. However, for an MPTCP-aware IDS, risk of false positives. However, for an MPTCP-aware IDS, tokens
connection IDs can be easily read by such systems to correlate can be read by such systems to correlate multiple subflows and re-
multiple subflows and re-assemble for analysis. assemble for analysis.
o Application level NATs: may alter the payload within a subflow.
Multipath TCP will detect these using the checksum and close the
affected subflow(s), if there are other subflows that can be used.
If all subflows are affected multipath will fallback to TCP,
allowing middleboxes to change the payload.
o Middleboxes that alter the receive window: MPTCP will use the o Application level middleboxes: such as content-aware firewalls may
maximum window at data-level, but will also obey subflow specific alter the payload within a subflow, such as re-writing URIs in
windows. HTTP traffic. MPTCP will detect these using the checksum and
close the affected subflow(s), if there are other subflows that
can be used. If all subflows are affected multipath will fallback
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 In addition, all classes of middleboxes may affect TCP traffic in the
following ways: following ways:
o TCP Options: may be removed, or packets with unknown options o TCP Options: may be removed, or packets with unknown options
dropped, by many classes of middleboxes. It is intended that the dropped, by many classes of middleboxes. It is intended that the
initial SYN exchange, with a TCP Option, will be sufficient to initial SYN exchange, with a TCP Option, will be sufficient to
identify the path capabilities. If such a packet does not get identify the path capabilities. If such a packet does not get
through, MPTCP will end up falling back to regular TCP. through, MPTCP will end up falling back to regular TCP.
o Segmentation/Coalescing (e.g. tcp segmentation offloading, etc): o Segmentation/Coalescing (e.g. TCP segmentation offloading): might
might copy options between packets and might strip some options. copy options between packets and might strip some options.
MPTCP's data sequence mapping includes the subflow sequence number MPTCP's data sequence mapping includes the relative subflow
instead of using the sequence number in the segment. In this way, sequence number instead of using the sequence number in the
the mapping is independent of the packets that carry it. segment. In this way, the mapping is independent of the packets
that carry it.
7. Interfaces
TBD
Interface with applications, interface with TCP, interface with lower
layers...
Discussion of interaction with applications (both in terms of how o The Receive Window: may be shrunk by some middleboxes at the
MPTCP will affect an application's assumptions of the transport subflow level. MPTCP will use the maximum window at data-level,
layer, and what API extensions an application may wish to use with but will also obey subflow specific windows.
MPTCP) are discussed in [5].
8. Acknowledgements 7. Acknowledgements
The authors are supported by Trilogy The authors are supported by Trilogy
(http://www.trilogy-project.org), a research project (ICT-216372) (http://www.trilogy-project.org), a research project (ICT-216372)
partially funded by the European Community under its Seventh partially funded by the European Community under its Seventh
Framework Program. The views expressed here are those of the Framework Program. The views expressed here are those of the
author(s) only. The European Commission is not liable for any use author(s) only. The European Commission is not liable for any use
that may be made of the information in this document. that may be made of the information in this document.
The authors gratefully acknowledge significant input into this The authors gratefully acknowledge significant input into this
document from Olivier Bonaventure and Andrew McDonald. document from Sebastien Barre, Christoph Paasch and Andrew McDonald.
The authors also wish to acknowledge reviews and contributions from The authors also wish to acknowledge reviews and contributions from
Iljitsch van Beijnum, Lars Eggert, Marcelo Bagnulo, Robert Hancock, Iljitsch van Beijnum, Lars Eggert, Marcelo Bagnulo, Robert Hancock,
Pasi Sarolahti, Toby Moncaster, Philip Eardley, Sergio Lembo, and Pasi Sarolahti, Toby Moncaster, Philip Eardley, Sergio Lembo,
Lawrence Conroy. Lawrence Conroy, Yoshifumi Nishida and Bob Briscoe.
9. IANA Considerations 8. IANA Considerations
This document will make a request to IANA to allocate new values for This document will make a request to IANA to allocate a new TCP
TCP Option identifiers, as follows: option value for MPTCP. This value will be the value of the "Kind"
field seen in all MPTCP options in this document.
This document will also request IANA operates a registry for MPTCP
option subtype values. The values as defined by this specification
are as follows:
+-------------+-----------------------------+---------------+-------+ +-------------+-----------------------------+---------------+-------+
| Symbol | Name | Ref | Value | | Symbol | Name | Ref | Value |
+-------------+-----------------------------+---------------+-------+ +-------------+-----------------------------+---------------+-------+
| MP_CAPABLE | Multipath Capable | Section 3.1 | (tbc) | | MP_CAPABLE | Multipath Capable | Section 3.1 | 0x0 |
| MP_JOIN | Join Connection | Section 3.2 | (tbc) | | MP_JOIN | Join Connection | Section 3.2 | 0x1 |
| ADD_ADDR | Add Address | Section 3.5.1 | (tbc) | | DSS | Data Sequence Signal (Data | Section 3.3 | 0x2 |
| REMOVE_ADDR | Remove Address | Section 3.5.2 | (tbc) | | | ACK and Data Sequence | | |
| DSN_MAP | Data Sequence Number | Section 3.3 | (tbc) | | | Mapping) | | |
| | Mapping | | | | ADD_ADDR | Add Address | Section 3.4.1 | 0x3 |
| DATA_ACK | Data-level Acknowledgment | Section 3.3 | (tbc) | | REMOVE_ADDR | Remove Address | Section 3.4.2 | 0x4 |
| DATA_FIN | Data-level FIN | Section 3.4 | (tbc) | | MP_PRIO | Change Subflow Priority | Section 3.3.8 | 0x5 |
| MP_PRIO | Change Subflow Priority | Section 3.3.6 | (tbc) | | MP_FAIL | Fallback | Section 3.5 | 0x6 |
| MP_FAIL | Fallback | Section 3.6 | (tbc) |
+-------------+-----------------------------+---------------+-------+ +-------------+-----------------------------+---------------+-------+
Table 1: TCP Options for MPTCP Table 1: MPTCP Option Subtypes
10. References This document also requests that IANA keeps a registry of
cryptographic handshake algorithms based on the flags in MP_CAPABLE
(Section 3.1). This document specifies only one algorithm:
10.1. Normative References +-------+-----------+----------------------------+
| Flags | Algorithm | Document |
+-------+-----------+----------------------------+
| 0x1 | HMAC-SHA1 | This document, Section 3.2 |
+-------+-----------+----------------------------+
Table 2: MPTCP Handshake Algorithms
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References 9.2. Informative References
[2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, [2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981. September 1981.
[3] Ford, A., Raiciu, C., Handley, M., and J. Iyengar, [3] Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar,
"Architectural Guidelines for Multipath TCP Development", "Architectural Guidelines for Multipath TCP Development",
draft-ietf-mptcp-architecture-02 (work in progress), draft-ietf-mptcp-architecture-05 (work in progress),
October 2010. January 2011.
[4] Raiciu, C., Handley, M., and D. Wischik, "Coupled Multipath- [4] Raiciu, C., Handley, M., and D. Wischik, "Coupled Congestion
Aware Congestion Control", draft-ietf-mptcp-congestion-00 (work Control for Multipath Transport Protocols",
in progress), July 2010. draft-ietf-mptcp-congestion-01 (work in progress),
January 2011.
[5] Scharf, M. and A. Ford, "MPTCP Application Interface [5] Scharf, M. and A. Ford, "MPTCP Application Interface
Considerations", draft-scharf-mptcp-api-02 (work in progress), Considerations", draft-ietf-mptcp-api-00 (work in progress),
July 2010. November 2010.
[6] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and [6] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[7] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[8] Gont, F., "Security Assessment of the Transmission Control
Protocol (TCP)", draft-ietf-tcpm-tcp-security-02 (work in
progress), January 2011.
[9] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and
HMAC-SHA)", RFC 4634, July 2006. HMAC-SHA)", RFC 4634, July 2006.
[7] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP [10] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Selective Acknowledgment Options", RFC 2018, October 1996. Requirements for Security", BCP 106, RFC 4086, June 2005.
[8] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of [11] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[12] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.
[13] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168, Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001. September 2001.
[9] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. [14] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E.
Lear, "Address Allocation for Private Internets", BCP 5, Lear, "Address Allocation for Private Internets", BCP 5,
RFC 1918, February 1996. RFC 1918, February 1996.
[10] Braden, R., "Requirements for Internet Hosts - Communication [15] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989. Layers", STD 3, RFC 1122, October 1989.
[11] Bagnulo, M., "Threat Analysis for Multi-addressed/Multi-path [16] Bagnulo, M., "Threat Analysis for TCP Extensions for Multi-path
TCP", draft-ietf-mptcp-threat-03 (work in progress), Operation with Multiple Addresses", draft-ietf-mptcp-threat-08
October 2010. (work in progress), January 2011.
[12] Srisuresh, P. and K. Egevang, "Traditional IP Network Address [17] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001. Translator (Traditional NAT)", RFC 3022, January 2001.
[13] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z. [18] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to Mitigate Shelby, "Performance Enhancing Proxies Intended to Mitigate
Link-Related Degradations", RFC 3135, June 2001. Link-Related Degradations", RFC 3135, June 2001.
[14] Handley, M., Paxson, V., and C. Kreibich, "Network Intrusion [19] Handley, M., Paxson, V., and C. Kreibich, "Network Intrusion
Detection: Evasion, Traffic Normalization, and End-to-End Detection: Evasion, Traffic Normalization, and End-to-End
Protocol Semantics", Usenix Security 2001, 2001, <http:// Protocol Semantics", Usenix Security 2001, 2001, <http://
www.usenix.org/events/sec01/full_papers/handley/handley.pdf>. www.usenix.org/events/sec01/full_papers/handley/handley.pdf>.
[15] Freed, N., "Behavior of and Requirements for Internet [20] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC 2979, October 2000. Firewalls", RFC 2979, October 2000.
Appendix A. Notes on use of TCP Options Appendix A. Notes on use of TCP Options
The TCP option space is limited due to the length of the Data Offset 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 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 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 leaves a maximum of 40 bytes for options, and many of these may
already be used by options such as timestamp and SACK. already be used by options such as timestamp and SACK.
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We have performed a brief study on the commonly used TCP options in We have performed a brief study on the commonly used TCP options in
SYN, data, and pure ACK packets, and found that there is enough room SYN, data, and pure ACK packets, and found that there is enough room
to fit all the options we propose using in this draft. to fit all the options we propose using in this draft.
SYN packets typically include MSS (4 bytes), window scale (3 bytes), SYN packets typically include MSS (4 bytes), window scale (3 bytes),
SACK permitted (2 bytes) and timestamp (10 bytes) options. Together SACK permitted (2 bytes) and timestamp (10 bytes) options. Together
these sum to 19 bytes. Some operating systems appear to pad each these sum to 19 bytes. Some operating systems appear to pad each
option up to a word boundary, thus using 24 bytes (a brief survey option up to a word boundary, thus using 24 bytes (a brief survey
suggests Windows XP and Mac OS X do this, whereas Linux does not). suggests Windows XP and Mac OS X do this, whereas Linux does not).
Optimistically, therefore, we have 21 bytes spare, or 16 if it has to Optimistically, therefore, we have 21 bytes spare, or 16 if it has to
be word-aligned. In either case, however, the Multipath Capable (12 be word-aligned. In either case, however, the SYN versions of
bytes) and Join (12 bytes) options will fit in this remaining space. Multipath Capable (12 bytes) and Join (12 or 16 bytes) options will
fit in this remaining space.
TCP data packets typically carry timestamp options in every packet, TCP data packets typically carry timestamp options in every packet,
taking 10 bytes (or 12 with padding). That leaves 30 bytes (or 28, taking 10 bytes (or 12 with padding). That leaves 30 bytes (or 28,
if word-aligned), which are enough to encode the data sequence if word-aligned). The Data Sequence Signal (DSS) option varies in
mapping (14 or 18 bytes, depending on the length of the sequence length depending on whether the Data Sequence Mapping and DATA ACK
number in use) and the DATA_ACK if the flow is bidirectional (6 or 10 are included, and whether the sequence numbers in use are 4 or 8
bytes). Such options will just fit in the available option space, octets. The maximum size of the DSS option is 28 bytes, so even that
although 8 byte data-level sequence numbers in both will only fit if will fit in the available space. But unless a connection is both bi-
word-alignment is not required. If this proves to be a problem, it directional and high-bandwidth, it is unlikely that all that option
is not necessary to include the Data Sequence Mapping and DATA_ACK in space will be required on each DSS option.
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 It is not necessary to include the Data Sequence Mapping and DATA ACK
following packet). Other options include: wrapping the DATA_ACK into in each packet, and in many cases it may be possible to alternate
the Data Sequence Mapping option; alternating between 4 and 8 byte their presence (so long as the mapping covers the data being sent in
sequence numbers in each option; and sending the DATA_ACK on a the following packet). Other options include: alternating between 4
duplicate subflow-level ACK. and 8 byte sequence numbers in each option; and sending the DATA_ACK
on a duplicate subflow-level ACK (although note that this must not be
taken as a signal of congestion).
On subflow and connection setup, an MPTCP option is also set on the
third packet (an ACK). These are 20 bytes (for Multipath Capable)
and 24 bytes (for Join) - both of which will fit in the available
option space.
Pure ACKs in TCP typically contain only timestamps (10B). Here, Pure ACKs in TCP typically contain only timestamps (10B). Here,
multipath TCP typically needs to encode the DATA_ACK (max 10B). multipath TCP typically needs to encode only the DATA ACK (maximum of
Occasionally ACKs will contain SACK information. Depending on the 12 octets). Occasionally ACKs will contain SACK information.
number of lost packets, SACK may utilize the entire option space. If Depending on the number of lost packets, SACK may utilize the entire
a DATA_ACK had to be included, then it is probably necessary to option space. If a DATA ACK had to be included, then it is probably
reduce the number of SACK blocks by one to accomodate the DATA_ACK. necessary to reduce the number of SACK blocks to accomodate the DATA
However, the presence of the DATA_ACK is unlikely to be necessary in ACK. However, the presence of the DATA ACK is unlikely to be
a case where SACK is in use, however, since until at least some of necessary in a case where SACK is in use, since until at least some
the SACK blocks have been retransmitted, the cumulative data-level of the SACK blocks have been retransmitted, the cumulative data-level
ACK will not be moving forward (or if it does, due to retransmissions ACK will not be moving forward (or if it does, due to retransmissions
on antoher path, then that path can also be used to transmit the new on another path, then that path can also be used to transmit the new
DATA_ACK). DATA ACK).
The ADD_ADDR option can be between 8 and 22 bytes, depending on The ADD_ADDR option can be between 8 and 22 bytes, depending on
whether IPv4 or IPv6 is used, and whether the Port number is present whether IPv4 or IPv6 is used, and whether the port number is present
or not. It is unlikely that such signalling would fit in a data or not. It is unlikely that such signalling would fit in a data
packet (although if there is space, it is fine to include it). It is packet (although if there is space, it is fine to include it). It is
recommended to use duplicate ACKs with no other payload or options in recommended to use duplicate ACKs with no other payload or options in
order to transmit these rare signals. order to transmit these rare signals. Note this is the reason for
mandating that duplicate ACKs with MPTCP options are not taken as a
signal of congestion.
Finally, there are issues with options reliability. As options can Finally, there are issues with reliable delivery of options. As
also be sent on pure ACKs, these are not reliably sent. This is not options can also be sent on pure ACKs, these are not reliably sent.
an issue for DATA_ACK due to their cumulative nature, but may be an This is not an issue for DATA_ACK due to their cumulative nature, but
issue for ADD_ADDR/REMOVE_ADDR options. Here we favour redundant may be an issue for ADD_ADDR/REMOVE_ADDR options. Here, it is
transmissions at the sender (whether on multiple paths, or on the recommended to send these options redundantly (whether on multiple
same path on a number of ACKs). The cases where options are stripped paths, or on the same path on a number of ACKs - but interspersed
by middleboxes are discussed in Section 6. with data in order to avoid interpretation as congestion). The cases
where options are stripped by middleboxes are discussed in Section 6.
Appendix B. Resync Packet Appendix B. Control Blocks
In earlier versions of this draft, we proposed the use of a "re-sync" Conceptually, an MPTCP connection can be represented as an MPTCP
option that would be used in certain circumstances when a sender control block that contains several variables that track the progress
needs to instruct the receiver to skip over certain subflow sequence and the state of the MPTCP connection and a set of linked TCP control
numbers (i.e. to treat the specified sequence space as having been blocks that correspond to the subflows that have been established.
received and acknowledged).
The typical use of this option will be when packets are retransmitted RFC793 [2] specifies several state variables. Whenever possible, we
on different subflows, after failing to be acknowledged on the reuse the same terminology as RFC793 to describe the state variables
original subflow. In such a case, it becomes necessary to move that are maintained by MPTCP.
forward the original subflow's sequence numbering so as not to later
transmit different data with a previously used sequence number (i.e.
when more data comes to be transmitted on the original subflow, it
would be different data, and so must not be sent with previously-used
(but unacknowledged) sequence numbering).
The rationale for needing to do this is two-fold: firstly, when ACKs B.1. MPTCP Control Block
are received they are for the subflow only, and the sender infers
from this the data that was sent - if the same sequence space could
be occupied by different data, the sender won't know whether the
intended data was received. Secondly, certain classes of middleboxes
may cache data and not send the new data on a previously-seen
sequence number.
This option was dropped, however, since some middleboxes may get The MPTCP control block contains the following variable per-
confused when they meet a hole in the sequence space, and do not connection.
understand the resync option. It is therefore felt that the same
data must continue to be retransmitted on a subflow even if it is B.1.1. Authentication and Metadata
already received after being retransmitted on another. There should
not be a significant performance hit from this since the amount of Local.Token (32 bits): This is the token chosen by the local host on
data involved and needing to be retransmitted multiple times will be this MPTCP connection. The token MUST be unique among all
relatively small. established MPTCP connections, 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 C bit the MP_CAPABLE options exchanged
during connection establishment, and is set to false otherwise.
If this flag is set, the checksum must be computed in all DSS
options.
B.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 with RFC1323, 16 bits without): This is the sending
window. MPTCP maintains the sending 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 which is sent in each transmitted segment.
B.1.3. Receiving Side
RCV.NXT (64 bits): This is the Data Sequence Number of the next byte
which 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 which is sent in the DSS option on
all subflows.
RCV.WND (32bits with RFC1323, 16 bits otherwise): This is the
connection-level receive window, which is the maximum of the
RCV.WND on all the subflows.
B.2. TCP Control Blocks
The MPTCP control block also contains a list of the TCP control
blocks that are associated to 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:
B.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 acknowledgement 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.
B.2.2. Receiving Side
RCV.NXT (32 bits): This is the sequence number of the next byte
which 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 with RFC1323, 16 bits otherwise): This is the
subflow-level receive window which is updated with the window
field from the segments received on this subflow.
Appendix C. Changelog Appendix C. Changelog
This section maintains logs of significant changes made to this This section maintains logs of significant changes made to this
document between versions. document between versions.
C.1. Changes since draft-ietf-mptcp-multiaddressed-01 C.1. Changes since draft-ietf-mptcp-multiaddressed-02
o Changed to using a single TCP option with a sub-type field.
o Merged Data Sequence Number, DATA ACK, and DATA FIN.
o Changed DATA FIN behaviour (separated from subflow FIN).
o Added crypto agility and checksum negotiation.
o Redefined MP_JOIN handshake to use only three TCP options.
o Added pseudo-header to checksum.
o Many clarifications and re-structuring.
o Added more discussion on heuristics.
C.2. Changes since draft-ietf-mptcp-multiaddressed-01
o Added proposal for hash-based security mechanism. o Added proposal for hash-based security mechanism.
o Added receiver subflow policy control (backup path flags and o Added receiver subflow policy control (backup path flags and
MP_PRIO option). MP_PRIO option).
o Changed DSN_MAP checksum to use the TCP checksum algorithm. o Changed DSN_MAP checksum to use the TCP checksum algorithm.
C.2. Changes since draft-ietf-mptcp-multiaddressed-00 C.3. Changes since draft-ietf-mptcp-multiaddressed-00
o Various clarifications and minor re-structuring in response to o Various clarifications and minor re-structuring in response to
comments. comments.
C.3. Changes since draft-ford-mptcp-multiaddressed-03 C.4. Changes since draft-ford-mptcp-multiaddressed-03
o Clarified handshake mechanism, especially with regard to error o Clarified handshake mechanism, especially with regard to error
cases (Section 3.2). cases (Section 3.2).
o Added optional port to ADD_ADDR and clarified situation with o Added optional port to ADD_ADDR and clarified situation with
private addresses (Section 3.5.1). private addresses (Section 3.4.1).
o Added path liveness check to REMOVE_ADDR (Section 3.5.2). o Added path liveness check to REMOVE_ADDR (Section 3.4.2).
o Added chunk checksumming to DSN_MAP (Section 3.3.1) to detect o Added chunk checksumming to DSN_MAP (Section 3.3.1) to detect
payload-altering middleboxes, and defined fallback mechanism payload-altering middleboxes, and defined fallback mechanism
(Section 3.6). (Section 3.5).
o Major clarifications to receive window discussion (Section 3.3.4). o Major clarifications to receive window discussion (Section 3.3.5).
o Various textual clarifications, especially in examples. o Various textual clarifications, especially in examples.
C.4. Changes since draft-ford-mptcp-multiaddressed-02 C.5. Changes since draft-ford-mptcp-multiaddressed-02
o Remove Version and Address ID in MP_CAPABLE in Section 3.1, and o Remove Version and Address ID in MP_CAPABLE in Section 3.1, and
make ISN be 6 bytes. make ISN be 6 bytes.
o Data sequence numbers are now always 8 bytes. But in some cases o Data sequence numbers are now always 8 bytes. But in some cases
where it is unambiguous it is permissible to only send the lower 4 where it is unambiguous it is permissible to only send the lower 4
bytes if space is at a premium. bytes if space is at a premium.
o Clarified behaviour of MP_JOIN in Section 3.2. o Clarified behaviour of MP_JOIN in Section 3.2.
skipping to change at page 44, line 23 skipping to change at page 54, line 4
Phone: +44 1794 833 465 Phone: +44 1794 833 465
Email: alan.ford@roke.co.uk Email: alan.ford@roke.co.uk
Costin Raiciu Costin Raiciu
University College London University College London
Gower Street Gower Street
London WC1E 6BT London WC1E 6BT
UK UK
Email: c.raiciu@cs.ucl.ac.uk Email: c.raiciu@cs.ucl.ac.uk
Mark Handley Mark Handley
University College London University College London
Gower Street Gower Street
London WC1E 6BT London WC1E 6BT
UK UK
Email: m.handley@cs.ucl.ac.uk Email: m.handley@cs.ucl.ac.uk
Olivier Bonaventure
Universite catholique de Louvain
Pl. Ste Barbe, 2
Louvain-la-Neuve 1348
Belgium
Email: olivier.bonaventure@uclouvain.be
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