draft-ietf-mptcp-multiaddressed-07.txt   draft-ietf-mptcp-multiaddressed-08.txt 
Internet Engineering Task Force A. Ford Internet Engineering Task Force A. Ford
Internet-Draft Cisco Internet-Draft Cisco
Intended status: Experimental C. Raiciu Intended status: Experimental C. Raiciu
Expires: September 27, 2012 University Politehnica of Expires: November 26, 2012 University Politehnica of
Bucharest Bucharest
M. Handley M. Handley
University College London University College London
O. Bonaventure O. Bonaventure
Universite catholique de Universite catholique de
Louvain Louvain
March 26, 2012 May 25, 2012
TCP Extensions for Multipath Operation with Multiple Addresses TCP Extensions for Multipath Operation with Multiple Addresses
draft-ietf-mptcp-multiaddressed-07 draft-ietf-mptcp-multiaddressed-08
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.
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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 September 27, 2012. This Internet-Draft will expire on November 26, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
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. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. MPTCP Concept . . . . . . . . . . . . . . . . . . . . . . 6 1.4. MPTCP Concept . . . . . . . . . . . . . . . . . . . . . . 6
1.5. Requirements Language . . . . . . . . . . . . . . . . . . 7 1.5. Requirements Language . . . . . . . . . . . . . . . . . . 8
2. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 8 2. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Initiating an MPTCP connection . . . . . . . . . . . . . . 8 2.1. Initiating an MPTCP connection . . . . . . . . . . . . . . 8
2.2. Associating a new subflow with an existing MPTCP 2.2. Associating a new subflow with an existing MPTCP
connection . . . . . . . . . . . . . . . . . . . . . . . . 9 connection . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3. Informing the other Host about another potential 2.3. Informing the other Host about another potential
address . . . . . . . . . . . . . . . . . . . . . . . . . 9 address . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4. Data transfer using MPTCP . . . . . . . . . . . . . . . . 10 2.4. Data transfer using MPTCP . . . . . . . . . . . . . . . . 10
2.5. Requesting a change in a path's priority . . . . . . . . . 11 2.5. Requesting a change in a path's priority . . . . . . . . . 11
2.6. Closing an MPTCP connection . . . . . . . . . . . . . . . 11 2.6. Closing an MPTCP connection . . . . . . . . . . . . . . . 11
2.7. Notable features . . . . . . . . . . . . . . . . . . . . . 11 2.7. Notable features . . . . . . . . . . . . . . . . . . . . . 11
3. MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 12 3. MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1. Connection Initiation . . . . . . . . . . . . . . . . . . 13 3.1. Connection Initiation . . . . . . . . . . . . . . . . . . 13
3.2. Starting a New Subflow . . . . . . . . . . . . . . . . . . 16 3.2. Starting a New Subflow . . . . . . . . . . . . . . . . . . 17
3.3. General MPTCP Operation . . . . . . . . . . . . . . . . . 21 3.3. General MPTCP Operation . . . . . . . . . . . . . . . . . 21
3.3.1. Data Sequence Mapping . . . . . . . . . . . . . . . . 22 3.3.1. Data Sequence Mapping . . . . . . . . . . . . . . . . 23
3.3.2. Data Acknowledgements . . . . . . . . . . . . . . . . 25 3.3.2. Data Acknowledgements . . . . . . . . . . . . . . . . 26
3.3.3. Closing a Connection . . . . . . . . . . . . . . . . . 27 3.3.3. Closing a Connection . . . . . . . . . . . . . . . . . 27
3.3.4. Receiver Considerations . . . . . . . . . . . . . . . 28 3.3.4. Receiver Considerations . . . . . . . . . . . . . . . 28
3.3.5. Sender Considerations . . . . . . . . . . . . . . . . 29 3.3.5. Sender Considerations . . . . . . . . . . . . . . . . 29
3.3.6. Reliability and Retransmissions . . . . . . . . . . . 30 3.3.6. Reliability and Retransmissions . . . . . . . . . . . 30
3.3.7. Congestion Control Considerations . . . . . . . . . . 31 3.3.7. Congestion Control Considerations . . . . . . . . . . 31
3.3.8. Subflow Policy . . . . . . . . . . . . . . . . . . . . 31 3.3.8. Subflow Policy . . . . . . . . . . . . . . . . . . . . 32
3.4. Address Knowledge Exchange (Path Management) . . . . . . . 33 3.4. Address Knowledge Exchange (Path Management) . . . . . . . 33
3.4.1. Address Advertisement . . . . . . . . . . . . . . . . 34 3.4.1. Address Advertisement . . . . . . . . . . . . . . . . 34
3.4.2. Remove Address . . . . . . . . . . . . . . . . . . . . 36 3.4.2. Remove Address . . . . . . . . . . . . . . . . . . . . 37
3.5. Fast Close . . . . . . . . . . . . . . . . . . . . . . . . 37 3.5. Fast Close . . . . . . . . . . . . . . . . . . . . . . . . 38
3.6. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.6. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.7. Error Handling . . . . . . . . . . . . . . . . . . . . . . 42 3.7. Error Handling . . . . . . . . . . . . . . . . . . . . . . 42
3.8. Heuristics . . . . . . . . . . . . . . . . . . . . . . . . 42 3.8. Heuristics . . . . . . . . . . . . . . . . . . . . . . . . 43
3.8.1. Port Usage . . . . . . . . . . . . . . . . . . . . . . 42 3.8.1. Port Usage . . . . . . . . . . . . . . . . . . . . . . 43
3.8.2. Delayed Subflow Start . . . . . . . . . . . . . . . . 43 3.8.2. Delayed Subflow Start . . . . . . . . . . . . . . . . 43
3.8.3. Failure Handling . . . . . . . . . . . . . . . . . . . 44 3.8.3. Failure Handling . . . . . . . . . . . . . . . . . . . 44
4. Semantic Issues . . . . . . . . . . . . . . . . . . . . . . . 44 4. Semantic Issues . . . . . . . . . . . . . . . . . . . . . . . 45
5. Security Considerations . . . . . . . . . . . . . . . . . . . 46 5. Security Considerations . . . . . . . . . . . . . . . . . . . 46
6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 47 6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 47
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 50 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 51
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 50 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 51 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53
9.1. Normative References . . . . . . . . . . . . . . . . . . . 51 9.1. Normative References . . . . . . . . . . . . . . . . . . . 53
9.2. Informative References . . . . . . . . . . . . . . . . . . 52 9.2. Informative References . . . . . . . . . . . . . . . . . . 53
Appendix A. Notes on use of TCP Options . . . . . . . . . . . . . 53 Appendix A. Notes on use of TCP Options . . . . . . . . . . . . . 54
Appendix B. Control Blocks . . . . . . . . . . . . . . . . . . . 54 Appendix B. Control Blocks . . . . . . . . . . . . . . . . . . . 56
B.1. MPTCP Control Block . . . . . . . . . . . . . . . . . . . 55 B.1. MPTCP Control Block . . . . . . . . . . . . . . . . . . . 56
B.1.1. Authentication and Metadata . . . . . . . . . . . . . 55 B.1.1. Authentication and Metadata . . . . . . . . . . . . . 56
B.1.2. Sending Side . . . . . . . . . . . . . . . . . . . . . 55 B.1.2. Sending Side . . . . . . . . . . . . . . . . . . . . . 57
B.1.3. Receiving Side . . . . . . . . . . . . . . . . . . . . 56 B.1.3. Receiving Side . . . . . . . . . . . . . . . . . . . . 57
B.2. TCP Control Blocks . . . . . . . . . . . . . . . . . . . . 56 B.2. TCP Control Blocks . . . . . . . . . . . . . . . . . . . . 57
B.2.1. Sending Side . . . . . . . . . . . . . . . . . . . . . 56 B.2.1. Sending Side . . . . . . . . . . . . . . . . . . . . . 57
B.2.2. Receiving Side . . . . . . . . . . . . . . . . . . . . 56 B.2.2. Receiving Side . . . . . . . . . . . . . . . . . . . . 58
Appendix C. Finite State Machine . . . . . . . . . . . . . . . . 57 Appendix C. Finite State Machine . . . . . . . . . . . . . . . . 58
Appendix D. Changelog . . . . . . . . . . . . . . . . . . . . . . 57 Appendix D. Changelog . . . . . . . . . . . . . . . . . . . . . . 59
D.1. Changes since draft-ietf-mptcp-multiaddressed-05 . . . . . 57 D.1. Changes since draft-ietf-mptcp-multiaddressed-05 . . . . . 59
D.2. Changes since draft-ietf-mptcp-multiaddressed-04 . . . . . 58 D.2. Changes since draft-ietf-mptcp-multiaddressed-04 . . . . . 59
D.3. Changes since draft-ietf-mptcp-multiaddressed-03 . . . . . 58 D.3. Changes since draft-ietf-mptcp-multiaddressed-03 . . . . . 60
D.4. Changes since draft-ietf-mptcp-multiaddressed-02 . . . . . 58 D.4. Changes since draft-ietf-mptcp-multiaddressed-02 . . . . . 60
D.5. Changes since draft-ietf-mptcp-multiaddressed-01 . . . . . 58 D.5. Changes since draft-ietf-mptcp-multiaddressed-01 . . . . . 60
D.6. Changes since draft-ietf-mptcp-multiaddressed-00 . . . . . 58 D.6. Changes since draft-ietf-mptcp-multiaddressed-00 . . . . . 60
D.7. Changes since draft-ford-mptcp-multiaddressed-03 . . . . . 59 D.7. Changes since draft-ford-mptcp-multiaddressed-03 . . . . . 60
D.8. Changes since draft-ford-mptcp-multiaddressed-02 . . . . . 59 D.8. Changes since draft-ford-mptcp-multiaddressed-02 . . . . . 61
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 59 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 61
1. Introduction 1. Introduction
MPTCP is a set of extensions to regular TCP [1] to provide a MPTCP is a set of extensions to regular TCP [1] to provide a
Multipath TCP [2] service, which enables a transport connection to Multipath TCP [2] 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 signaling and setting up multiple paths specifically, those for signaling 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
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o It can be assumed that one or both hosts 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 a host 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 [5] ensure this does congestion control algorithms defined in [5] ensure this does not act
not act detrimentally. 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 [2]): (discussed in more detail in [2]):
External Constraints: The protocol must function through the vast External Constraints: The protocol must function through the vast
majority of existing middleboxes such as NATs, firewalls and majority of existing middleboxes such as NATs, firewalls and
proxies, and as such must resemble existing TCP as far as possible proxies, and as such must resemble existing TCP as far as possible
on the wire. Furthermore, the protocol must not assume the on the wire. Furthermore, the protocol must not assume the
segments it sends on the wire arrive unmodified at the segments it sends on the wire arrive unmodified at the
destination: they may be split or coalesced; TCP options may be destination: they may be split or coalesced; TCP options may be
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+---------------+ + - - - - - - - + - - - - - - - + +---------------+ + - - - - - - - + - - - - - - - +
| 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
1.3. Terminology 1.3. Terminology
This document introduces a number of MPTCP-specific terms, defined
below:
Path: A sequence of links between a sender and a receiver, defined Path: A sequence of links between a sender and a receiver, defined
in this context by a source and destination address pair. in this context by a source and destination address pair.
Subflow: A flow of TCP segments operating over an individual path, Subflow: A flow of TCP segments operating over an individual path,
which forms part of a larger MPTCP connection. A subflow is which forms part of a larger MPTCP connection. A subflow is
started and terminated similarly to a regular TCP connection. started and terminated similarly to a regular TCP connection.
(MPTCP) Connection: A set of one or more subflows, over which an (MPTCP) Connection: A set of one or more subflows, over which an
application can communicate between two hosts. There is a one-to- application can communicate between two hosts. There is a one-to-
one mapping between a connection and an application socket. one mapping between a connection and an application socket.
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term "data-level" is synonymous with "connection level", in term "data-level" is synonymous with "connection level", in
contrast to "subflow-level" which refers to properties of an contrast to "subflow-level" which refers to properties of an
individual subflow. individual subflow.
Token: A locally unique identifier given to a multipath connection Token: A locally unique identifier given to a multipath connection
by a host. May also be referred to as a "Connection ID". by a host. May also be referred to as a "Connection ID".
Host: A end host operating an MPTCP implementation, and either Host: A end host operating an MPTCP implementation, and either
initiating or accepting an MPTCP connection. initiating or accepting an MPTCP connection.
MPTCP's interpretation of, and effect on, regular TCP semantics are
discussed in Section 4.
1.4. MPTCP Concept 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. Changes in
semantics from regular, single-path TCP are discussed in Section 4.
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 [6]. An application begins by opening a MPTCP-aware applications [6]. 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.
o An MPTCP connection begins similarly to a regular TCP connection. o An MPTCP connection begins similarly to a regular TCP connection.
This is illustrated in Figure 2 where a TCP connection is This is illustrated in Figure 2 where an MPTCP connection is
established between addresses A1 and B1 on Hosts A and B established between addresses A1 and B1 on Hosts A and B
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 MPTCP "subflows") are created on these paths, and are combined
existing session, which continues to appear as a single connection with the existing session, which continues to appear as a single
to the applications at both ends. The creation of the additional connection to the applications at both ends. The creation of the
TCP session is illustrated between Address A2 on Host A and additional TCP session is illustrated between Address A2 on Host A
Address B1 on Host B. and 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 hosts. 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
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1.5. 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 [3]. document are to be interpreted as described in RFC 2119 [3].
2. Operation Overview 2. Operation Overview
This section presents a single description of standard MPTCP This section presents a single description of standard MPTCP
operation, with reference to the protocol operation. Considerable operation, with reference to the protocol operation. This is a high-
reference is made to symbolic names of MPTCP options throughout this level overview of the key functions; the full specification follows
section - these are subtypes of the IANA-assigned MPTCP option (see in Section 3. Considerable reference is made to symbolic names of
Section 8), and their formats are defined in the detailed protocol MPTCP options throughout this section - these are subtypes of the
specification which follows in Section 3. IANA-assigned MPTCP option (see Section 8), and their formats are
defined in the detailed protocol specification which follows in
Section 3.
A Multipath TCP connection provides a bidirectionnal bytestream A Multipath TCP connection provides a bidirectionnal bytestream
between two hosts communicating like normal TCP and thus does not between two hosts communicating like normal TCP and thus does not
require any change to the applications. However, Multipath TCP require any change to the applications. However, Multipath TCP
enables the hosts to use different paths with different IP addresses enables the hosts to use different paths with different IP addresses
to exchange packets belonging to the MPTCP connection. A Multipath to exchange packets belonging to the MPTCP connection. A Multipath
TCP connection appears like a normal TCP connection to an TCP connection appears like a normal TCP connection to an
application. However, to the network layer each MPTCP subflows looks application. However, to the network layer each MPTCP subflows looks
like a regular TCP flow whose segments carry a new TCP option type. like a regular TCP flow whose segments carry a new TCP option type.
Multipath TCP manages the creation, removal and utilization of these Multipath TCP manages the creation, removal and utilization of these
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What follows is a summary of the purpose and rationale of these What follows is a summary of the purpose and rationale of these
messages. messages.
2.1. Initiating an MPTCP connection 2.1. Initiating an MPTCP connection
This is the same signaling as for initiating a normal TCP connection, This is the same signaling as for initiating a normal TCP connection,
but the SYN, SYN/ACK and ACK packets also carry the MP_CAPABLE but the SYN, SYN/ACK and ACK packets also carry the MP_CAPABLE
option. This is variable-length and serves multiple purposes. option. This is variable-length and serves multiple purposes.
Firstly, it verifies whether the remote host supports Multipath TCP; Firstly, it verifies whether the remote host supports Multipath TCP;
and secondly, this option allows the hosts to exchange some and secondly, this option allows the hosts to exchange some
information that is used to authenticate the establishment of information to authenticate the establishment of additional subflows.
additional subflows. Further details are given in Section 3.1. Further details are given in Section 3.1.
Host-A Host-B Host-A Host-B
------ ------ ------ ------
MP_CAPABLE -> MP_CAPABLE ->
[A's key, flags] [A's key, flags]
<- MP_CAPABLE <- MP_CAPABLE
[B's key, flags] [B's key, flags]
ACK MP_CAPABLE -> ACK + MP_CAPABLE ->
[A's key, B's key, flags] [A's key, B's key, flags]
2.2. Associating a new subflow with an existing MPTCP connection 2.2. Associating a new subflow with an existing MPTCP connection
The exchange of keys in the MP_CAPABLE handshake provides material The exchange of keys in the MP_CAPABLE handshake provides material
that can be used to authenticate the endpoints when new subflows will that can be used to authenticate the endpoints when new subflows will
be setup. Additional subflows begin in the same way as initiating a be setup. Additional subflows begin in the same way as initiating a
normal TCP connection, but the SYN, SYN/ACK and ACK packets also normal TCP connection, but the SYN, SYN/ACK and ACK packets also
carry the MP_JOIN option. carry the MP_JOIN option.
Host-A initiates a new subflow between one of its addresses and one Host-A initiates a new subflow between one of its addresses and one
of Host-B's addresses. The token - generated from the key - is used of Host-B's addresses. The token - generated from the key - is used
to identify which MPTCP connection it is joining, and the MAC is used to identify which MPTCP connection it is joining, and the MAC is used
for authentication. MP_JOIN also contains flags and an Address ID for authentication. The MAC uses the keys exchanged in the
MP_CAPABLE handshake, and the random numbers (nonces) exchanged in
these MP_JOIN options. MP_JOIN also contains flags and an Address ID
that can be used to refer to the source address without the sender that can be used to refer to the source address without the sender
needing to know if it has been changed by a NAT. Further details in needing to know if it has been changed by a NAT. Further details in
Section 3.2. Section 3.2.
Host-A Host-B Host-A Host-B
------ ------ ------ ------
MP_JOIN -> MP_JOIN ->
[B's token, A's nonce, [B's token, A's nonce,
A's Address ID, flags] A's Address ID, flags]
<- MP_JOIN <- MP_JOIN
[B's MAC, B's nonce, [B's MAC, B's nonce,
B's Address ID, flags] B's Address ID, flags]
ACK MP_JOIN -> ACK + MP_JOIN ->
[A's MAC] [A's MAC]
<- ACK
2.3. Informing the other Host about another potential address 2.3. Informing the other Host about another potential address
The set of IP addresses associated to a multihomed host may change The set of IP addresses associated to a multihomed host may change
during the lifetime of an MPTCP connection. MPTCP supports the during the lifetime of an MPTCP connection. MPTCP supports the
addition and removal of addresses on a host both implicitly and addition and removal of addresses on a host both implicitly and
explicitly. If Host-A has established a subflow starting at address explicitly. If Host-A has established a subflow starting at address
IP#-A1 and wants to open a second subflow starting at address IP#-A2, IP#-A1 and wants to open a second subflow starting at address IP#-A2,
it simply initiates the establishment of the subflow as explained it simply initiates the establishment of the subflow as explained
above. The remote host will then be implictly informed about the new above. The remote host will then be implictly informed about the new
address. address.
skipping to change at page 10, line 30 skipping to change at page 10, line 41
2.4. Data transfer using MPTCP 2.4. Data transfer using MPTCP
To ensure reliable, in-order delivery of data over subflows that may To ensure reliable, in-order delivery of data over subflows that may
appear and disappear at any time, MPTCP uses a 64-bit Data Sequence appear and disappear at any time, MPTCP uses a 64-bit Data Sequence
Number (DSN) to number all data sent over the MPTCP connection. Each Number (DSN) to number all data sent over the MPTCP connection. Each
subflow has its own 32 bits sequence number space and an MPTCP option subflow has its own 32 bits sequence number space and an MPTCP option
maps the subflow sequence space to the data sequence space. In this maps the subflow sequence space to the data sequence space. In this
way, data can be retransmitted on different subflows (mapped to the way, data can be retransmitted on different subflows (mapped to the
same DSN) in the event of failure. same DSN) in the event of failure.
The "Data Sequence Signal" option which carries this "Data Sequence The "Data Sequence Signal" carries the "Data Sequence Mapping". The
Mapping", which consists of the subflow sequence number, data Data Sequence Mapping consists of the subflow sequence number, data
sequence number, and length for which this mapping is valid. This sequence number, and length for which this mapping is valid. This
option can also carry a connection-level acknowledgement (the "Data option can also carry a connection-level acknowledgement (the "Data
ACK") for the received DSN. ACK") for the received DSN.
With MPTCP, all subflows share the same receive buffer and advertise With MPTCP, all subflows share the same receive buffer and advertise
the same receive window. There are two levels of acknowledgement in the same receive window. There are two levels of acknowledgement in
MPTCP. Regular TCP acknowledgements are used on each subflow to MPTCP. Regular TCP acknowledgements are used on each subflow to
acknowledge the reception of the segments sent over the subflow acknowledge the reception of the segments sent over the subflow
independently of their DSN. In addition, there are connection-level independently of their DSN. In addition, there are connection-level
acknowledgements for the data sequence space. These acknowledgements acknowledgements for the data sequence space. These acknowledgements
skipping to change at page 11, line 25 skipping to change at page 11, line 37
------ ------ ------ ------
MP_PRIO -> MP_PRIO ->
2.6. Closing an MPTCP connection 2.6. Closing an MPTCP connection
When Host-A wants to inform Host-B that it has no more data to send, When Host-A wants to inform Host-B that it has no more data to send,
it signals this "Data FIN" as part of the Data Sequence Signal (see it signals this "Data FIN" as part of the Data Sequence Signal (see
above). It has the same semantics and behaviour as a regular TCP above). It has the same semantics and behaviour as a regular TCP
FIN, but at the connection level. Once all the data on the MPTCP FIN, but at the connection level. Once all the data on the MPTCP
connection has been successfully received, then this message is connection has been successfully received, then this message is
acknowledged at the connection level with a DATA ACK. Further acknowledged at the connection level with a DATA_ACK. Further
details in Section 3.3.3. details in Section 3.3.3.
Host-A Host-B Host-A Host-B
------ ------ ------ ------
DATA_SEQUENCE_SIGNAL -> DATA_SEQUENCE_SIGNAL ->
[Data FIN] [Data FIN]
<- (MPTCP DATA ACK) <- (MPTCP DATA_ACK)
2.7. Notable features 2.7. Notable features
It is worth highlighting that MPTCP's signaling has been designed It is worth highlighting that MPTCP's signaling has been designed
with several key requirements in mind: with several key requirements in mind:
o To cope with NATs on the path, addresses are referred to by o To cope with NATs on the path, addresses are referred to by
Address IDs, in case the IP packet's source address gets changed Address IDs, in case the IP packet's source address gets changed
by a NAT. Setting up a new TCP flow is not possible if the by a NAT. Setting up a new TCP flow is not possible if the
passive opener is behind a NAT; to allow subflows to be created passive opener is behind a NAT; to allow subflows to be created
when either end is behind a NAT, MPTCP uses the MP-ADD-ADDR when either end is behind a NAT, MPTCP uses the ADD_ADDR message.
message.
o MPTCP falls back to ordinary TCP if MPTCP operation is not o MPTCP falls back to ordinary TCP if MPTCP operation is not
possible. For example if one host is not MPTCP capable, or if a possible. For example if one host is not MPTCP capable, or if a
middlebox alters the payload. middlebox alters the payload.
o To meet the threats identified in [7], the following steps are o To meet the threats identified in [7], the following steps are
taken: keys are sent in the clear in the MP_CAPABLE messages; taken: keys are sent in the clear in the MP_CAPABLE messages;
MP_JOIN messages are secured with HMAC-SHA1 using those keys; and MP_JOIN messages are secured with HMAC-SHA1 using those keys; and
standard TCP validity checks are made on the other messages ( standard TCP validity checks are made on the other messages (
ensuring sequence numbers are in-window). ensuring sequence numbers are in-window).
skipping to change at page 13, line 23 skipping to change at page 13, line 35
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 4). 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 is used to declare the sender's 64 bit key, which is used This option is used to declare the sender's 64 bit key, which is used
to authenticate the addition of future subflows. This is the only to authenticate the addition of future subflows to this MPTCP
time the key will be sent in clear on the wire; all future subflows connection. This is the only time the key will be sent in clear on
will identify the connection using a 32 bit "token". This token is a the wire (unless "fast close", Section 3.5, is used); all future
cryptographic hash of this key. The token will be a truncated (most subflows will identify the connection using a 32 bit "token". This
significant 32 bits) SHA-1 hash [4]. A different, 64 bit truncation token is a cryptographic hash of this key. The token will be a
(the least significant 64 bits) of the hash of the key will be used truncated (most significant 32 bits) SHA-1 hash [4]. A different, 64
as the Initial Data Sequence Number. bit truncation (the least significant 64 bits) of the hash of the key
will be used as the Initial Data Sequence Number.
This key is generated by its 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 MUST be its method of generation is implementation-specific. The key MUST be
hard to guess, and it MUST be unique for the sending host at any one hard to guess, and it MUST be unique for the sending host at any one
time. Recommendations for generating random keys are given in [11]. time. Recommendations for generating random keys are given in [11].
Connections will be indexed at each host by the token (the truncated Connections will be indexed at each host by the token (the truncated
SHA-1 hash of the key). Therefore, an implementation will require a SHA-1 hash of the key). Therefore, an implementation will require a
mapping from each token to the corresponding connection, and in turn mapping from each token to the corresponding connection, and in turn
to the keys for the connection. to the keys for the connection.
There is a very small risk that two different keys will hash to the There is a very small risk that two different keys will hash to the
same token. An implementation SHOULD check its list of connection same token. An implementation SHOULD check its list of connection
tokens to ensure there is not a collision before sending its key in 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 the SYN/ACK. This would, however, be costly for a server with
thousands of connections. The subflow handshake mechanism thousands of connections. The subflow handshake mechanism
(Section 3.2) will ensure that new subflows only join the correct (Section 3.2) will ensure that new subflows only join the correct
connection, however, so in the worst case if there was a token connection, however, by checking tokens in both directions, and
collision, the second connection cannot support multiple subflows, ensuring sequence numbers are in-window, so in the worst case if
but will otherwise provide a regular TCP service. there was a token collision, the new subflow would be closed, but the
MPTCP connection would continue to 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.
skipping to change at page 14, line 39 skipping to change at page 15, line 5
Section 3.6. Note that new subflows MUST NOT be established (using Section 3.6. Note that new subflows MUST NOT be established (using
the process documented in Section 3.2) until a DSS option has been the process documented in Section 3.2) until a DSS option has been
successfully received across the path (as documented in Section 3.3). successfully received across the path (as documented in Section 3.3).
The first four bits of the first octet in the MP_CAPABLE option The first four bits of the first octet in the MP_CAPABLE option
(Figure 4) define the MPTCP option subtype (see Section 8; for (Figure 4) define the MPTCP option subtype (see Section 8; for
MP_CAPABLE, this is 0), and the remaining four bits of this octet MP_CAPABLE, this is 0), and the remaining four bits of this octet
specifies the MPTCP version in use (for this specification, this is specifies the MPTCP version in use (for this specification, this is
0). 0).
The second octet is reserved for flags. The leftmost bit - labeled C The second octet is reserved for flags. The leftmost bit - labelled
- indicates "Checksum required", and SHOULD be set to 1 unless C - SHOULD be set to 1 to indicate "Checksum required", unless the
specifically overridden (for example, if the system administrator has system administrator has decided that checksums are not required (for
decided that checksums are not required - see Section 3.3 for more example, if the environment is controlled and no middleboxes exist
discussion). The remaining bits are used for crypto algorithm that may adjust the payload). The remaining bits are used for crypto
negotiation. Currently only the rightmost bit - labeled S - is algorithm negotiation. Currently only the rightmost bit - labeled S
assigned, and indicates the use of HMAC-SHA1 (as defined in - is assigned, and indicates the use of HMAC-SHA1 (as defined in
Section 3.2). An implementation that only supports this method MUST 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 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 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 invalid and ignored (i.e. it must be treated as a regular TCP
handshake). handshake).
These bits negotiate capabilities in similar ways. For the 'C' bit, These bits negotiate capabilities in similar ways. For the 'C' bit,
if either host requires the use of checksums, checksums MUST be used. 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 In other words, the only way for checksums not to be used is if both
hosts in their SYNs set C=0. This decision is confirmed by the hosts in their SYNs set C=0. This decision is confirmed by the
setting of the 'C' bit in the third packet (the ACK) of the setting of the 'C' bit in the third packet (the ACK) of the
handshake. For example, if the initiator sets C=0 in the SYN, but handshake. For example, if the initiator sets C=0 in the SYN, but
the responder sets C=1 in the SYN/ACK, checksums must be used and the the responder sets C=1 in the SYN/ACK, checksums MUST be used in both
initiator will set C=1 in the ACK. The decision whether to use directions, and the initiator will set C=1 in the ACK. The decision
checksums will be stored by an implementation in a per-connection whether to use checksums will be stored by an implementation in a
binary state variable. per-connection binary state variable.
For crypto negotiation, the responder has the choice. The initiator For crypto negotiation, the responder has the choice. The initiator
creates a proposal setting a bit for each algorithm it supports to 1 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 (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 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 set - this is the chosen algorithm. The rationale for this behaviour
is that the responder will typically be a server with potentially is that the responder will typically be a server with potentially
many thousands of connections, so it may wish to choose an algorithm many thousands of connections, so it may wish to choose an algorithm
with minimal computational complexity, depending on the load. If a with minimal computational complexity, depending on the load. If a
responder does not support (or does not want to support) any of the responder does not support (or does not want to support) any of the
skipping to change at page 16, line 9 skipping to change at page 16, line 29
Figure 4: 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 MUST operate as a regular, single-path TCP. If a the MPTCP session MUST operate as a regular, single-path TCP. If a
SYN does not contain a MP_CAPABLE option, the SYN/ACK MUST NOT SYN does not contain a MP_CAPABLE option, the SYN/ACK MUST NOT
contain one in response. If the third packet (the ACK) does not contain one in response. If the third packet (the ACK) does not
contain the MP_CAPABLE option, then the session MUST fall back to contain the MP_CAPABLE option, then the session MUST fall back to
operating as a regular, single-path TCP. This is to maintain operating as a regular, single-path TCP. This is to maintain
compatibility with middleboxes on the path that drop some or all TCP compatibility with middleboxes on the path that drop some or all TCP
options. options. Note that an implementation MAY choose to attempt sending
MPTCP options more than one time before making this decision to
operate as regular TCP (see Section 3.8).
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. It is possible that MPTCP and made first will be up to local policy. It is possible that MPTCP and
non-MPTCP SYNs could get re-ordered in the network. Therefore, the non-MPTCP SYNs could get re-ordered in the network. Therefore, the
final state is inferred from the presence or absence of the final state is inferred from the presence or absence of the
MP_CAPABLE option in the third packet of the TCP handshake. If this MP_CAPABLE option in the third packet of the TCP handshake. If this
option is not present, the connection should fall back to regular option is not present, the connection SHOULD fall back to regular
TCP, as documented in Section 3.6. 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, i.e. IDSN-A = Hash(Key-A) and IDSN-B = Hash(Key-B). The
IDSN-B = Hash(Key-B). The Hash mechanism here provides the least Hash mechanism here provides the least significant 64 bits of the
significant 64 bits of the SHA-1 hash of the key. The SYN with SHA-1 hash of the key. The SYN with MP_CAPABLE occupies the first
MP_CAPABLE occupies the first octet of Data Sequence Space, although octet of Data Sequence Space, although this does not need to be
this does not need to be acknowledged at the connection level until acknowledged at the connection level until the first data is sent
the first data is sent (see Section 3.3). (see Section 3.3).
3.2. Starting a New Subflow 3.2. Starting a New Subflow
Once an 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. Hosts 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
host's addresses through signaling exchanges as described in host's addresses through signaling exchanges as described in
Section 3.4. Using this knowledge, a host can initiate a new subflow Section 3.4. Using this knowledge, a host can initiate a new subflow
over a currently unused pair of addresses. It is permitted for over a currently unused pair of addresses. It is permitted for
either host in a connection to initiate the creation of a new either host in a connection to initiate the creation of a new
skipping to change at page 17, line 25 skipping to change at page 17, line 47
is the token that the receiver of the packet uses to identify this 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 connection, i.e. Host A will send Token-B (which is generated from
Key-B). Key-B).
The MP_JOIN SYN not only sends the token (which is static for a 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 connection) but also Random Numbers (nonces) that are used to prevent
replay attacks on the authentication method. replay attacks on the authentication method.
The MP_JOIN option includes an "Address ID". This is an identifier The MP_JOIN option includes an "Address ID". This is an identifier
that only has significance within a single connection, where it that only has significance within a single connection, where it
identifies the source address of this packet, even if the address identifies the source address of this packet, even if the IP header
itself has been changed in transit by a middlebox. This allows has been changed in transit by a middlebox. The Address ID allows
address removal without needing to know what the source address at address removal (Section 3.4.2) without needing to know what the
the receiver is, thus this allows address removal through NATs. The source address at the receiver is, thus allowing address removal
sender can signal this to the receiver via the REMOVE_ADDR option through NATs. The Address ID also allows correlation between new
(Section 3.4.2). It also allows correlation between new subflow subflow setup attempts and address signaling (Section 3.4.1), to
setup attempts and address signaling (Section 3.4.1), to prevent prevent setting up duplicate subflows on the same path.
setting up duplicate subflows on the same path.
The Address IDs of the subflow used in the initial SYN exchange of 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 the first subflow in the connection are implicit, and have the value
zero. A host MUST store the Address IDs associated with all zero. A host MUST store the mappings between Address IDs and
established subflows. addresses both for itself and the remote host. An implementation
will also need to know which local and remote Address IDs are
associated with which established subflows, for when addresses are
removed from a local or remote host.
The MP_JOIN option on SYNs also includes 4 bits of flags, 3 of which The MP_JOIN option on packets with the SYN flag set also includes 4
are currently reserved and MUST be set to zero by the sender. The bits of flags, 3 of which are currently reserved and MUST be set to
final bit, labelled 'B', indicates whether the sender of this option zero by the sender. The final bit, labelled 'B', indicates whether
wishes this subflow to be used as a backup path (B=1) in the event of the sender of this option wishes this subflow to be used as a backup
failure of other paths, or whether it wants it to be used as part of path (B=1) in the event of failure of other paths, or whether it
the connection immediately. By setting B=1, the sender of the option wants it to be used as part of the connection immediately. By
is requesting the other host to only send data on this subflow if setting B=1, the sender of the option is requesting the other host to
there are no available subflows where B=0. Subflow policy is only send data on this subflow if there are no available subflows
discussed in more detail in Section 3.3.8. where B=0. 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 | Length = 12 |Subtype| |B| Address ID | | Kind | Length = 12 |Subtype| |B| Address ID |
+---------------+---------------+-------+-----+-+---------------+ +---------------+---------------+-------+-----+-+---------------+
| Receiver's Token (32 bits) | | Receiver's Token (32 bits) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Sender's Random Number (32 bits) | | Sender's Random Number (32 bits) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
skipping to change at page 18, line 25 skipping to change at page 18, line 45
Figure 5: Join Connection (MP_JOIN) option (for initial SYN) Figure 5: Join Connection (MP_JOIN) option (for initial SYN)
When receiving a SYN with an MP_JOIN option that contains a valid When receiving a SYN with an MP_JOIN option that contains a valid
token for an existing MPTCP connection, the recipient SHOULD respond token for an existing MPTCP connection, the recipient SHOULD respond
with a SYN/ACK also containing an MP_JOIN option containing a random with a SYN/ACK also containing an MP_JOIN option containing a random
number and a truncated (leftmost 64 bits) Message Authentication Code number and a truncated (leftmost 64 bits) Message Authentication Code
(MAC). This version of the option is shown in Figure 6. If the (MAC). This version of the option is shown in Figure 6. If the
token is unknown, or the host wants to refuse subflow establishment token is unknown, or the host wants to refuse subflow establishment
(for example, due to a limit on the number of subflows it will (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 permit), the receiver will send back an RST, analogous to an unknown
port in TCP. Although cryptographic calculations are required in the port in TCP. Although calculating a MAC requires cryptographic
SYN/ACK, it is felt that the 32 bit token gives sufficient protection operations, it is believed that the 32 bit token in the MP_JOIN SYN
against blind state exhaustion attacks and therefore there is no need gives sufficient protection against blind state exhaustion attacks
to provide mechanisms to allow a responder to operate statelessly at and therefore there is no need to provide mechanisms to allow a
the MP_JOIN stage. responder to operate statelessly at the MP_JOIN stage.
An MAC is sent by both hosts - by the initiator (Host A) in the third 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 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 (the SYN/ACK). Doing the MAC exchange at this stage allow both hosts
data to be used as the message before generating the MAC. In both to have first exchanged random data (in the first two SYN packets)
cases, the MAC algorithm is HMAC as defined in [12], using the SHA-1 that is used as the "message". Both MACs are generated according to
hash algorithm [4] (thus generating a 160-bit / 20 octet HMAC). Due HMAC as defined in [12], using the SHA-1 hash algorithm [4] (thus
to option space limitations, the MAC included in the SYN/ACK is generating a 160-bit / 20 octet HMAC). Due to option space
truncated to the leftmost 64 bits, but this is acceptable since while limitations, the MAC included in the SYN/ACK is truncated to the
in an attacker-initiated attack, the attacker can retry many times; leftmost 64 bits, but this is acceptable since an attacker only has
if the attacker is the responder, he only has one chance to get the once chance to guess the MAC correctly.
MAC correct.
The initiator's authentication information is sent in its first ACK The initiator's authentication information is sent in its first ACK
(the third packet of the handshake), and this is shown in Figure 7. (the third packet of the handshake), as shown in Figure 7. This data
This data needs to be sent reliably, and therefore receipt of this needs to be sent reliably, since it is the only time this MAC is sent
packet MUST trigger an ACK in response, and the packet MUST be and therefore receipt of this packet MUST trigger a regular TCP ACK
retransmitted if this ACK is not received. In other words, sending in response, and the packet MUST be retransmitted if this ACK is not
the ACK/MP_JOIN packet places the subflow in the PRE_ESTABLISHED received. In other words, sending the ACK/MP_JOIN packet places the
state, and it moves to the ESTABLISHED state only on receipt of an subflow in the PRE_ESTABLISHED state, and it moves to the ESTABLISHED
ACK from the receiver. It is not permitted to send data while in the state only on receipt of an ACK from the receiver. It is not
PRE_ESTABLISHED state. The reserved bits in this option MUST be set permitted to send data while in the PRE_ESTABLISHED state. The
to zero by the sender. 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 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 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 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 in the original MP_CAPABLE handshake. The "message" for the MAC
the concatenations of Random Number for each host (denoted by R): for algorithm in each case is the concatenations of Random Number for
Host A, R-A followed by R-B; and for Host B, R-B followed by R-A. 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 | Length = 16 |Subtype| |B| Address ID | | Kind | Length = 16 |Subtype| |B| Address ID |
+---------------+---------------+-------+-----+-+---------------+ +---------------+---------------+-------+-----+-+---------------+
| | | |
| Sender's Truncated MAC (64 bits) | | Sender's Truncated MAC (64 bits) |
| | | |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
skipping to change at page 20, line 10 skipping to change at page 20, line 26
Figure 7: Join Connection (MP_JOIN) option (for third ACK) Figure 7: Join Connection (MP_JOIN) option (for third ACK)
These various TCP options fit together to enable authenticated These various TCP options fit together to enable authenticated
subflow setup as illustrated in Figure 8. subflow setup as illustrated in Figure 8.
Host A Host B Host A Host B
------------------------ ---------- ------------------------ ----------
Address A1 Address A2 Address B1 Address A1 Address A2 Address B1
---------- ---------- ---------- ---------- ---------- ----------
| | | | | |
| | SYN + MP_CAPABLE | | SYN + MP_CAPABLE(Key-A) |
|--------------------------------------------->| |--------------------------------------------->|
|<---------------------------------------------| |<---------------------------------------------|
| SYN/ACK + MP_CAPABLE(Key-B) | | SYN/ACK + MP_CAPABLE(Key-B) |
| | | | | |
| ACK + MP_CAPABLE(Key-A, Key-B) | | ACK + MP_CAPABLE(Key-A, Key-B) |
|--------------------------------------------->| |--------------------------------------------->|
| | | | | |
| | SYN + MP_JOIN(Token-B, R-A) | | | SYN + MP_JOIN(Token-B, R-A) |
| |------------------------------->| | |------------------------------->|
| |<-------------------------------| | |<-------------------------------|
| | SYN/ACK + MP_JOIN(MAC-B, R-B) | | | SYN/ACK + MP_JOIN(MAC-B, R-B) |
| | | | | |
| | ACK + MP_JOIN(MAC-A) | | | ACK + MP_JOIN(MAC-A) |
| |------------------------------->| | |------------------------------->|
| | | | |<-------------------------------|
| | ACK |
MAC-A = MAC(Key=(Key-A+Key-B), Msg=(R-A+R-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)) MAC-B = MAC(Key=(Key-B+Key-A), Msg=(R-B+R-A))
Figure 8: 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 for the subflow. TCP RST for the subflow.
skipping to change at page 21, line 39 skipping to change at page 22, line 8
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.
During normal MPTCP operation, the Data Sequence Signal (DSS) TCP The Data Sequence Mapping and the Data ACK are signalled in the Data
option (shown in Figure 9) is used to signal the data required to Sequence Signal (DSS) option. Either or both can be signalled in one
enable multipath transport. This data comprises: the Data Sequence DSS, dependent on the flags set. The Data Sequence Mapping defines
Mapping, which defines how the sequence space on the subflow maps to how the sequence space on the subflow maps to the connection level,
the connection level; and the Data ACK, for acknowledging receipt of and the Data ACK acknowledges receipt of data at the connection
data at the connection level. These functions are described in more level. These functions are described in more detail in the following
detail in the following two subsections. two subsections.
Either or both the Data Sequence Mapping and the Data ACK can be Either or both the Data Sequence Mapping and the Data ACK can be
signalled in the DSS option, dependent on the flags set. signalled in the DSS option, dependent on the flags set.
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 | Length |Subtype| (reserved) |F|m|M|a|A| | Kind | Length |Subtype| (reserved) |F|m|M|a|A|
+---------------+---------------+-------+----------------------+ +---------------+---------------+-------+----------------------+
| Data ACK (4 or 8 octets, depending on flags) | | Data ACK (4 or 8 octets, depending on flags) |
skipping to change at page 22, line 36 skipping to change at page 22, line 50
o M = Data Sequence Number, Subflow Sequence Number, Data-level o M = Data Sequence Number, Subflow Sequence Number, Data-level
Length, and Checksum present Length, and Checksum present
o m = Data Sequence Number is 8 octets (if not set, DSN is 4 octets) o m = Data Sequence Number is 8 octets (if not set, DSN is 4 octets)
The flags 'a' and 'm' only have meaning if the corresponding 'A' or The flags 'a' and 'm' only have meaning if the corresponding 'A' or
'M' flags are set, otherwise they will be ignored. The maximum 'M' flags are set, otherwise they will be ignored. The maximum
length of this option, with all flags set, is 28 octets. length of this option, with all flags set, is 28 octets.
The 'F' flag indicates "DATA FIN". If present, this means that this The 'F' flag indicates "DATA_FIN". If present, this means that this
mapping covers the final data from the sender. This is the mapping covers the final data from the sender. This is the
connection-level equivalent to the FIN flag in single-path TCP. The connection-level equivalent to the FIN flag in single-path TCP. The
purpose of the DATA FIN, along with the interactions between this purpose of the DATA_FIN, along with the interactions between this
flag, the subflow-level FIN flag, and the data sequence mapping are 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 described in Section 3.3.3. The remaining reserved bits MUST be set
to zero by an implementation of this specification. to zero by an implementation of this specification.
Note that the Checksum is only present in this option if the use of Note that the Checksum is only present in this option if the use of
MPTCP checksumming has been negotiated at the MP_CAPABLE handshake MPTCP checksumming has been negotiated at the MP_CAPABLE handshake
(see Section 3.1). The presence of the checksum can be inferred from (see Section 3.1). The presence of the checksum can be inferred from
the length of the option. the length of the option. If a checksum is present, but its use had
not been negotiated in the MP_CAPABLE handshake, it SHOULD be
ignored. If a checksum is not present when its use has been
negotiated, the receiver SHOULD close the subflow with a RST as it is
considered broken.
3.3.1. Data Sequence Mapping 3.3.1. Data Sequence Mapping
The data stream as a whole can be reassembled through the use of the 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 Data Sequence Mapping components of the DSS option (Figure 9), which
define the mapping from the subflow sequence number to the data define the mapping from the subflow sequence number to the data
sequence number. This is used by the receiver to ensure in-order sequence number. This is used by the receiver to ensure in-order
delivery to the application layer. Meanwhile, the subflow-level delivery to the application layer. Meanwhile, the subflow-level
sequence numbers (i.e. the regular sequence numbers in the TCP sequence numbers (i.e. the regular sequence numbers in the TCP
header) have subflow-only relevance. It is expected (but not header) have subflow-only relevance. It is expected (but not
skipping to change at page 23, line 44 skipping to change at page 24, line 15
data sequence space as that which should be delivered to the data sequence space as that which should be delivered to the
application. application.
The data sequence number is specified as an absolute value, whereas 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 sequence numbering is relative (the SYN at the start of
the subflow has relative subflow sequence number 0). This is to the subflow has relative subflow sequence number 0). This is to
allow middleboxes to change the Initial Sequence Number of a subflow, allow middleboxes to change the Initial Sequence Number of a subflow,
such as firewalls that undertake ISN randomization. such as firewalls that undertake ISN randomization.
The data sequence mapping also contains a checksum of the data that 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 this mapping covers, if use of checksums has been neogiated at the
adjusted in any way by a non-MPTCP-aware middlebox. If this checksum MP_CAPABLE exchange. Checksums are used to detect if the payload has
fails, it will trigger a failure of the subflow, or a fallback to been adjusted in any way by a non-MPTCP-aware middlebox. If this
regular TCP, as documented in Section 3.6, since MPTCP can no longer checksum fails, it will trigger a failure of the subflow, or a
reliably know the subflow sequence space at the receiver to build fallback to regular TCP, as documented in Section 3.6, since MPTCP
data sequence mappings. 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 [1], The checksum algorithm used is the standard TCP checksum [1],
operating over the data covered by this mapping, along with a pseudo- operating over the data covered by this mapping, along with a pseudo-
header as shown in Figure 10. header as shown in Figure 10.
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
+--------------------------------------------------------------+ +--------------------------------------------------------------+
| | | |
| Data Sequence Number (8 octets) | | Data Sequence Number (8 octets) |
skipping to change at page 25, line 44 skipping to change at page 26, line 16
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 hosts, 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. sized chunks.
An "infinite" mapping can be used to fallback to regular TCP by An "infinite" mapping can be used to fallback to regular TCP by
mapping the subflow-level data to the connection-level data for the mapping the subflow-level data to the connection-level data for the
remainder of the connection (see Section 3.6). This is achieved by remainder of the connection (see Section 3.6). This is achieved by
setting the data-level length field to the reserved value of 0. The setting the Data-level Length field of the DSS option to the reserved
checksum, in such a case, will also be set to zero. value 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, to act as a cumulative ACK for the connection level acknowledgement, to act as a cumulative ACK for the connection
as a whole. This is the "Data ACK" field of the DSS option as a whole. This is the "Data ACK" field of the DSS option
(Figure 9). The Data ACK is analogous to the behaviour of the (Figure 9). The Data ACK is analogous to the behaviour of the
standard TCP cumulative ACK - indicating how much data has been standard TCP cumulative ACK - indicating how much data has been
successfully received (with no holes). This is in comparison to the successfully received (with no holes). This is in comparison to the
subflow-level ACK, which acts analogous to TCP SACK, given that there subflow-level ACK, which acts analogous to TCP SACK, given that there
skipping to change at page 26, line 31 skipping to change at page 26, line 51
advertised receive window. As explained in Section 3.3.4, 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 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 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 field of a TCP segment at connection-level if it does not also carry
a DSS option with a Data ACK field. Furthermore, separating the a DSS option with a Data ACK field. Furthermore, separating the
connection-level acknowledgements from the subflow-level allows connection-level acknowledgements from the subflow-level allows
processing to be done separately, and a receiver has the freedom to processing to be done separately, and a receiver has the freedom to
drop segments after acknowledgement at the subflow level, for example drop segments after acknowledgement at the subflow level, for example
due to memory constraints when many segments arrive out-of-order. due to memory constraints when many segments arrive out-of-order.
An MPTCP sender MUST only free data from the send buffer when it has An MPTCP sender MUST NOT free data from the send buffer until it has
been acknowledged by both a Data ACK received on any subflow and at 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 all 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. Note, however, that if some data needs to be be restransmited. Note, however, that if some data needs to be
retransmitted multiple times over a subflow, there is a risk of retransmitted multiple times over a subflow, there is a risk of
blocking the sending window. In this case, the MPTCP sender can blocking the sending window. In this case, the MPTCP sender can
decide to terminate the subflow that is behaving badly by sending a decide to terminate the subflow that is behaving badly by sending a
RST. RST.
The Data ACK MAY be included in all segments, however optimisations The Data ACK MAY be included in all segments, however optimisations
SHOULD be considered in more advanced implementations, where the Data SHOULD be considered in more advanced implementations, where the Data
skipping to change at page 27, line 19 skipping to change at page 27, line 34
independently and to keep the appearance of TCP over the wire, a FIN 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 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 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. 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 it is not until both sides have ACKed each other's FINs that the
subflow is fully closed. subflow is fully closed.
When an application calls close() on a socket, this indicates that it 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 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, FIN on the connection. For MPTCP, an equivalent mechanism is needed,
and this is referred to as the DATA FIN. and this is referred to as the DATA_FIN.
A DATA FIN is an indication that the sender has no more data to send, 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 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, received. A DATA_FIN, as with the FIN on a regular TCP connection,
is a unidirectional signal. is a unidirectional signal.
The DATA FIN is signalled by setting the 'F' flag in the Data 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 Sequence Signal option (Figure 9) to 1. A DATA_FIN occupies one
octet (the final octet) of the connection-level sequence space. Note 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 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, the subflow level: for example, a segment with DSN 80, and Data-Level
with DATA FIN set, would map 10 octets from the subflow into data Length 11, with DATA_FIN set, would map 10 octets from the subflow
sequnce space 80-89, the DATA FIN is DSN 90, and therefore this into data sequnce space 80-89, the DATA_FIN is DSN 90, and therefore
segment including DATA FIN would be acknowledged with a DATA ACK of this segment including DATA_FIN would be acknowledged with a DATA_ACK
91. of 91.
Note that when the DATA FIN is not attached to a TCP segment Note that when the DATA_FIN is not attached to a TCP segment
containing data, the Data Sequence Mapping MUST have Subflow Sequence containing data, the Data Sequence Signal MUST have Subflow Sequence
Number of 0, a Length of 1, and the Data Sequence Number that Number of 0, a Data-Level Length of 1, and the Data Sequence Number
corresponds with the DATA FIN itself. The checksum in this case will that corresponds with the DATA_FIN itself. The checksum in this case
only cover the pseudo-header. will only cover the pseudo-header.
A DATA FIN has the semantics and behaviour as a regular TCP FIN, but 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 at the connection level. Notably, it is only DATA_ACKed once all
data has been successfully received at the connection level. Note data has been successfully received at the connection level. Note
therefore that a DATA FIN is decoupled from a subflow FIN. It is 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 only permissable to combine these signals on one subflow if there is
no data oustanding on other subflows. Otherwise, it may be necessary no data oustanding on other subflows. Otherwise, it may be necessary
to retransmit data on different subflows. Essentially, a host MUST to retransmit data on different subflows. Essentially, a host MUST
NOT FIN all functioning subflows unless it is safe to do so, i.e. NOT close all functioning subflows unless it is safe to do so, i.e.
until all outstanding data has been DATA ACKed, or that the segment until all outstanding data has been DATA_ACKed, or that the segment
with the FIN flag set is the only outstanding segment. with the DATA_FIN flag set is the only outstanding segment.
Once a DATA FIN has been acknowledged, all remaining subflows MUST be Once a DATA_FIN has been acknowledged, all remaining subflows MUST be
closed with standard FIN exchanges. Both hosts SHOULD send FINs, as closed with standard FIN exchanges. Both hosts SHOULD send FINs on
a courtesy to allow middleboxes to clean up state even if the subflow all subflows, as a courtesy to allow middleboxes to clean up state
has failed. It is also encouraged to reduce the timeouts (Maximum even if an individual subflow has failed. It is also encouraged to
Segment Life) on subflows at end hosts. In particular, any subflows reduce the timeouts (Maximum Segment Life) on subflows at end hosts.
where there is still outstanding data queued (which has been In particular, any subflows where there is still outstanding data
retransmitted on other subflows in order to get the DATA FIN queued (which has been retransmitted on other subflows in order to
acknowledged) MAY be closed with an RST. get the DATA_FIN acknowledged) MAY be closed with an RST.
A connection is considered closed once both hosts' DATA FINs have A connection is considered closed once both hosts' DATA_FINs have
been acknowledged by DATA ACKs. been acknowledged by DATA_ACKs.
Note that a host may also send a FIN on an individual subflow to shut As specified above, a standard TCP FIN on an individual subflow only
it down, but this impact is limited to the subflow in question. If shuts down the subflow on which it was sent. If all subflows have
all subflows have been closed with a FIN exchange, but no DATA FIN been closed with a FIN exchange, but no DATA_FIN has been received
has been received and acknowledged, the MPTCP connection is treated and acknowledged, the MPTCP connection is treated as closed only
as closed only after a timeout. This implies that an implementation after a timeout. This implies that an implementation will have
will have TIME_WAIT states at both the subflow and connection levels TIME_WAIT states at both the subflow and connection levels (see
(see Appendix C). This permits "break-before-make" scenarios where Appendix C). This permits "break-before-make" scenarios where
connectivity is lost on all subflows before a new one can be re- connectivity is lost on all subflows before a new one can be re-
established. established.
3.3.4. Receiver Considerations 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.
skipping to change at page 28, line 45 skipping to change at page 29, line 12
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 (i.e. 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, regular TCP checks
sequence number in the packet and checks it against the allowed the sequence number in the packet against the allowed receive window.
receive window. With multipath, such a check is done using only the With multipath, such a check is done using only the connection level
connection level window. A sanity check SHOULD be performed at window. A sanity check SHOULD be performed at subflow level to
subflow level to ensure that the subflow and mapped sequence numbers ensure that the subflow and mapped sequence numbers meet the
meet the following test: SSN - SUBFLOW_ACK <= DSN - DATA_ACK, where following test: SSN - SUBFLOW_ACK <= DSN - DATA_ACK, where SSN is the
SSN is the subflow sequence number of the received packet and subflow sequence number of the received packet and SUBFLOW_ACK is the
SUBFLOW_ACK is the rcv_next of the subflow (with the equivalent RCV.NXT (next expected sequence number) of the subflow (with the
connection-level definitions for DSN and DATA_ACK). equivalent connection-level definitions for DSN and DATA_ACK).
In regular TCP, once a segment is deemed in-window, it is either put In regular TCP, once a segment is deemed in-window, it is either put
in the in-order receive queue or in the out-of-order queue. In in the in-order receive queue or in the out-of-order queue. In
multipath TCP, the same happens but at connection-level: a segment is multipath TCP, the same happens but at connection-level: a segment is
placed in the connection level in-order or out-of-order queue if it placed in the connection level in-order or out-of-order queue if it
is in-window at both connection and subflow level. The stack still is in-window at both connection and subflow level. The stack still
has to remember, for each subflow, which segments were received has to remember, for each subflow, which segments were received
succesfully so that it can ACK them at subflow level appropriately. succesfully so that it can ACK them at subflow level appropriately.
Typically, this will be implemented by keeping per subflow out-of- Typically, this will be implemented by keeping per subflow out-of-
order queues (containing only message headers, not the payloads) and order queues (containing only message headers, not the payloads) and
remembering the value of the cumulative ACK. remembering the value of the cumulative ACK.
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 one of the paths.
this might be insufficient when a packet is lost on a slower subflow However this might be insufficient when a packet is lost on a slower
and needs to be retransmitted (see Section 3.3.6). A tight upper subflow and needs to be retransmitted (see Section 3.3.6). A tight
bound would be the maximum RTT of any path multiplied by the total upper bound would be the maximum RTT of any path multiplied by the
bandwidth available across all paths. This permits all subflows to total bandwidth available across all paths. This permits all
continue at full speed while a packet is fast-retransmitted on the subflows to continue at full speed while a packet is fast-
maximum RTT path. Even this might be insufficient to maintain full retransmitted on the maximum RTT path. Even this might be
performance in the event of a retransmit timeout on the maximum RTT insufficient to maintain full performance in the event of a
path. It is for future study to determine the relationship between retransmit timeout on the maximum RTT path. It is for future study
retransmission strategies and receive buffer sizing. to determine the relationship between retransmission strategies and
receive buffer sizing.
3.3.5. 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.
MPTCP uses a single receive window across all subflows, and if the MPTCP uses a single receive window across all subflows, and if the
skipping to change at page 30, line 5 skipping to change at page 30, line 21
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 reduce 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 MUST also remember 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 The send buffer MUST, at a minimum, be as big as the receive buffer,
buffer, to enable the sender to reach maximum throughput. to enable the sender to reach maximum throughput.
3.3.6. Reliability and Retransmissions 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, a same data sequence number on a different subflow. When doing this, a
host must still retransmit the original data on the original subflow, host MUST still retransmit the original data on the original subflow,
in order to preserve the subflow integrity (middleboxes could replay in order to preserve the subflow integrity (middleboxes could replay
old data, and/or could reject holes in subflows), and a receiver will old data, and/or could reject holes in subflows), and a receiver will
ignore these retransmissions. While this is clearly suboptimal, for ignore these retransmissions. While this is clearly suboptimal, for
compatibility reasons this is the best behaviour. Optimisations compatibility reasons this is sensible behaviour. Optimisations
could be negotiated in future versions of this protocol. could be negotiated in future versions of this 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 3.3.8). 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 is set all segments that haven't been DATA_ACKed are stored. A timer is set
when the head of the connection-level is ACKed at subflow level but when the head of the connection-level is ACKed at subflow level but
its corresponding data is not ACKed at data level. This timer will its corresponding data is not ACKed at data level. This timer will
guard against failures in re-transmission by middleboxes that pro- guard against failures in re-transmission by middleboxes that pro-
active ACK data. actively 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 acknowledged at both connection level and on all subflows it not been acknowledged at both connection level and on all subflows it
has been sent on. In this way, the sender can always retransmit the has been sent on. In this way, the sender can always retransmit the
data if needed, on the same subflow or on a different one. A special data if needed, on the same subflow or on a different one. A special
case is when a subflow fails: the sender will typically resend the case is when a subflow fails: the sender will typically resend the
data on other working subflows after a timeout, and will keep trying data on other working subflows after a timeout, and will keep trying
to retransmit the data on the failed subflow too. The sender will to retransmit the data on the failed subflow too. The sender will
declare the subflow failed after a predefined upper bound on declare the subflow failed after a predefined upper bound on
retransmissions is reached (which MAY be lower than the usual TCP retransmissions is reached (which MAY be lower than the usual TCP
limits of the Maximum Segment Life), or on the receipt of an ICMP limits of the Maximum Segment Life), or on the receipt of an ICMP
error, and only then delete the outstanding data segments. error, and only then delete the outstanding data segments.
Multiple retransmissions are triggers that will indicate that a Multiple retransmissions are triggers that will indicate that a
subflow performs badly and could lead to a host resetting the subflow subflow performs badly and could lead to a host resetting the subflow
with an RST. However, additional research is required to understand with an RST. However, additional research is required to understand
the heuristics of how and when to reset underperforming subflows. the heuristics of how and when to reset underperforming subflows.
For example, subflows that perform highly asymmetrically may be mis- For example, a highly asymmetric path may be mis-diagnosed as
diagnosed as underperforming. underperforming.
3.3.7. 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 [5]; the algorithm does not for achieving this is presented in [5]; 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. By this, we mean that it does deployable in the current Internet. By this, we mean that it does
skipping to change at page 33, line 7 skipping to change at page 33, line 24
+---------------+---------------+-------+-----+-+--------------+ +---------------+---------------+-------+-----+-+--------------+
| Kind | Length |Subtype| |B| AddrID (opt) | | Kind | Length |Subtype| |B| AddrID (opt) |
+---------------+---------------+-------+-----+-+--------------+ +---------------+---------------+-------+-----+-+--------------+
Figure 11: MP_PRIO option Figure 11: MP_PRIO option
It should be noted that the backup flag is a request from a data It should be noted that the backup flag is a request from a data
receiver to a data sender only, and the data sender SHOULD adhere to receiver to a data sender only, and the data sender SHOULD adhere to
these requests. A host cannot assume that the data sender will do these requests. A host cannot assume that the data sender will do
so, however, since local policies - or technical difficulties - may so, however, since local policies - or technical difficulties - may
override MP_PRIO requests. The signal applies to a single direction: override MP_PRIO requests. Note also that this signal applies to a
the sender of this option, however, may continue using the subflow to single direction, and so the sender of this option could choose to
send data even if it has signalled B=1 to the other host. continue using the subflow to send data even if it has signalled B=1
to the other host.
This option can also be applied to other subflows than the one on This option can also be applied to other subflows than the one on
which it is sent, by setting the optional Address ID field. This which it is sent, by setting the optional Address ID field. This
applies the given setting of B to all subflows that use the address applies the given setting of B to all subflows in this connection
identified by the given Address ID. The presence of this field is that use the address identified by the given Address ID. The
determined by the option length; if Length==4 then it is present, if presence of this field is determined by the option length; if
Length==3 then it applies to the current subflow only. The use case Length==4 then it is present, if Length==3 then it applies to the
of this is that a host can signal to its peer that an address is current subflow only. The use case of this is that a host can signal
temporarily unavailable (for example, if it has radio coverage to its peer that an address is temporarily unavailable (for example,
issues) and the peer should therefore drop to backup state on all if it has radio coverage issues) and the peer should therefore drop
subflows using that Address ID. to backup state on all subflows using that Address ID.
3.4. Address Knowledge Exchange (Path Management) 3.4. 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 hosts, which in this information about additional paths between hosts, which in this
design is managed by multiple addresses at hosts. For more detail of design is managed by multiple addresses at hosts. For more detail of
the architectural thinking behind this design, see the separate the architectural thinking behind this design, see the separate
architecture document [2]. architecture document [2].
This design makes use of two methods of sharing such information, This design makes use of two methods of sharing such information, and
used simultaneously. The first is the direct setup of new subflows, both can be used on a connection. The first is the direct setup of
already described in Section 3.2, where the initiator has an new subflows, already described in Section 3.2, where the initiator
additional address. The second method, described in the following has an additional address. The second method, described in the
subsections, signals addresses explicitly to the other host to allow following subsections, signals addresses explicitly to the other host
it to initiate new subflows. The two mechanisms are complementary: to allow it to initiate new subflows. The two mechanisms are
the first is implicit and simple, while the explicit is more complex complementary: the first is implicit and simple, while the explicit
but is more robust. Together, the mechanisms allow addresses to is more complex but is more robust. Together, the mechanisms allow
change in flight (and thus support operation through NATs, since the addresses to change in flight (and thus support operation through
source address need not be known), and also allow the signaling of NATs, since the source address need not be known), and also allow the
previously unknown addresses, and of addresses belonging to other signaling of previously unknown addresses, and of addresses belonging
address families (e.g. both IPv4 and IPv6). to other 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 An MPTCP connection is initially set up between address/port A1 of o An MPTCP connection is initially set up between address/port A1 of
host A and address/port B1 of host B. If host A is multihomed and host A and address/port B1 of host B. If host A is multihomed and
multi-addressed, it can start an additional subflow from its multi-addressed, it can start an additional subflow from its
address A2 to B1, by sending a SYN with a Join option from A2 to address A2 to B1, by sending a SYN with a Join option from A2 to
B1, using B's previously declared token for this connection. B1, using B's previously declared token for this connection.
Alternatively, if B is multihomed, it can try to set up a new Alternatively, if B is multihomed, it can try to set up a new
subflow from B2 to A1, using A's previously declared token. In subflow from B2 to A1, using A's previously declared token. In
skipping to change at page 34, line 39 skipping to change at page 35, line 8
3.4.1. 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
(and optionally, ports) on which a host can be reached (Figure 12). (and optionally, ports) on which a host can be reached (Figure 12).
Multiple instances of this TCP option can be added in a single Multiple instances of this TCP option can be added in a single
message if there is sufficient TCP option space, otherwise multiple message if there is sufficient TCP option space, otherwise multiple
TCP messages containing this option will be sent. This option can be TCP messages containing this option will be sent. This option can be
used at any time during a connection, depending on when the sender used at any time during a connection, depending on when the sender
wishes to enable multiple paths and/or when paths become available. wishes to enable multiple paths and/or when paths become available.
As with all MPTCP signals, the receiver MUST understake standard TCP As with all MPTCP signals, the receiver MUST undertake standard TCP
validity checks before acting upon it. validity checks before acting upon it.
Every address has an ID which can be used for uniquely identifying Every address has an Address ID which can be used for uniquely
the address within a connection, for address removal. This is also identifying the address within a connection, for address removal.
used to identify MP_JOIN options (see Section 3.2) relating to the This is also used to identify MP_JOIN options (see Section 3.2)
same address, even when address translators are in use. The ID MUST relating to the same address, even when address translators are in
uniquely identify the address to the sender (within the scope of the use. The Address ID MUST uniquely identify the address to the sender
connection), but the mechanism for allocating such IDs is (within the scope of the connection), but the mechanism for
implementation-specific. allocating such IDs is implementation-specific.
All address IDs learnt via either MP_JOIN or ADD_ADDR SHOULD be All address IDs learnt via either MP_JOIN or ADD_ADDR SHOULD be
stored by the receiver in a data structure that gathers all the stored by the receiver in a data structure that gathers all the
Address ID to address mappings for a connection (identified by a Address ID to address mappings for a connection (identified by a
token pair). In this way there is a stored mapping between Address token pair). In this way there is a stored mapping between Address
ID, observed source address and token pair for future processing of ID, observed source address and token pair for future processing of
control information for a connection. Note that an implementation control information for a connection. Note that an implementation
MAY discard incoming address advertisements at will, for example for MAY discard incoming address advertisements at will, for example for
avoiding the required mapping state, or because advertised addresses avoiding the required mapping state, or because advertised addresses
are of no use to it (for example, IPv6 addresses when it has IPv4 are of no use to it (for example, IPv6 addresses when it has IPv4
skipping to change at page 35, line 22 skipping to change at page 35, line 39
state, and MAY choose to refresh advertisements periodically. state, and MAY choose to refresh advertisements periodically.
This option is shown in Figure 12. The illustration is sized for This option is shown in Figure 12. The illustration is sized for
IPv4 addresses (IPVer = 4). For IPv6, the IPVer field will read 6, IPv4 addresses (IPVer = 4). For IPv6, the IPVer field will read 6,
and the length of the address will be 16 octets (instead of 4). and the length of the address will be 16 octets (instead of 4).
The presence of the final two octets, specifying the TCP port number The presence of the final two octets, specifying the TCP port number
to use, are optional and can be inferred from the length of the to use, are optional and can be inferred from the length of the
option. Although it is expected that the majority of use cases will option. Although it is expected that the majority of use cases will
use the same port pairs as used for the initial subflow (e.g. port 80 use the same port pairs as used for the initial subflow (e.g. port 80
remains port 80 on all subflows), as does the ephemeral port at the 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 client), there may be cases (such as port-based load balancing) where
the explicit specification of a different port is required. If no the explicit specification of a different port is required. If no
port is specified, MPTCP SHOULD attempt to connect to the specified port is specified, MPTCP SHOULD attempt to connect to the specified
address on the same port as is already in use by the signaling address on the same port as is already in use by the subflow on which
subflow, and this is discussed in more detail in Section 3.8. the ADD_ADDR signal was sent; this is discussed in more detail in
Section 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 | Length |Subtype| IPVer | Address ID | | Kind | Length |Subtype| IPVer | Address ID |
+---------------+---------------+-------+-------+---------------+ +---------------+---------------+-------+-------+---------------+
| Address (IPv4 - 4 octets / IPv6 - 16 octets) | | Address (IPv4 - 4 octets / IPv6 - 16 octets) |
+-------------------------------+---------------+---------------+ +-------------------------------+-------------------------------+
| Port (2 octets, optional) | | Port (2 octets, optional) |
+-------------------------------+ +-------------------------------+
Figure 12: Add Address (ADD_ADDR) option (shown for IPv4) Figure 12: Add Address (ADD_ADDR) option
Due to the proliferation of NATs, it is reasonably likely that one Due to the proliferation of NATs, it is reasonably likely that one
host may attempt to advertise private addresses [15]. It is not host may attempt to advertise private addresses [15]. It is not
desirable to prohibit this, since there may be cases where both hosts desirable to prohibit this, since there may be cases where both hosts
have additional interfaces on the same private network, and a host have additional interfaces on the same private network, and a host
MAY want to advertise such addresses. Such advertisements must not, MAY want to advertise such addresses. Such advertisements must not,
however, cause harm or security vulnerabilities. The standard however, cause harm or security vulnerabilities. The standard
mechanism to create a new subflow (Section 3.2) contains a 32 bit mechanism to create a new subflow (Section 3.2) contains a 32 bit
token that uniquely identifies the connection to the receiving host. token that uniquely identifies the connection to the receiving host.
If the token is unknown, the host will return with a RST. In the If the token is unknown, the host will return with a RST. In the
unlikely event that the token is known, subflow setup will continue, unlikely event that the token is known, subflow setup will continue,
but the MAC exchange must occur for authentication. This will fail, but the MAC exchange must occur for authentication. This will fail,
and will provide sufficient protection against two unconnected hosts and will provide sufficient protection against two unconnected hosts
accidentally setting up a new subflow upon the signal of a private accidentally setting up a new subflow upon the signal of a private
address. address.
Ideally, ADD_ADDR and REMOVE_ADDR options would be sent reliably, and Ideally, ADD_ADDR and REMOVE_ADDR options would be sent reliably, and
in order, to the other end. This would be to ensure that this in order, to the other end. This would ensure that this address
address management does not unnecessarily cause an outage in the management does not unnecessarily cause an outage in the connection
connection when remove/add addresses are processed in reverse order, when remove/add addresses are processed in reverse order, and also to
and also to ensure that all possible paths are used. Note, however, ensure that all possible paths are used. Note, however, that losing
that losing reliability and ordering will not break the multipath reliability and ordering will not break the multipath connections, it
connections, it will just reduce the opportunity to open multipath will just reduce the opportunity to open multipath paths and to
paths and to survive different patterns of path failures. survive different patterns of path failures.
Therefore, implementing reliability signals for these TCP options is Therefore, implementing reliability signals for these TCP options is
not necessary. In order to minimise the impact of the loss of these not necessary. In order to minimise the impact of the loss of these
options, however, it is RECOMMENDED that a sender should send these options, however, it is RECOMMENDED that a sender should send these
options on all available subflows. If these options need to be options on all available subflows. If these options need to be
received in-order, an implementation SHOULD only send one ADD_ADDR/ received in-order, an implementation SHOULD only send one ADD_ADDR/
REMOVE_ADDR option per RTT, to minimise the risk of misordering. REMOVE_ADDR option per RTT, to minimise the risk of misordering.
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 a live subflow within the connection, the receiver SHOULD for a live subflow within the connection, the receiver SHOULD
silently ignore the ADD_ADDR. If the Address ID is not in use on a silently ignore the ADD_ADDR. A host wishing to replace an existing
live subflow, but is stored by the receiver, a new ADD_ADDR SHOULD Address ID MUST first remove the existing one (Section 3.4.2).
take precedence and replace the stored address.
A host that receives an ADD_ADDR but finds a connection setup to that A host that receives an ADD_ADDR but finds a connection setup to that
IP address and port number is unsuccessful SHOULD NOT perform further IP address and port number is unsuccessful SHOULD NOT perform further
connection attempts to this address/port combination for this connection attempts to this address/port combination for this
connection. A sender that wants to trigger a new incoming connection connection. A sender that wants to trigger a new incoming connection
attempt on a previously advertised address/port combination can attempt on a previously advertised address/port combination can
therefore refresh ADD_ADDR information by sending the option again. 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 ACKs. As discussed earlier, however, an MPTCP on separate ACKs. As discussed earlier, however, an MPTCP
implementation MUST NOT treat duplicate ACKs with any MPTCP option implementation MUST NOT treat duplicate ACKs with any MPTCP option,
apart from DSS as indications of congestion [9], and an MPTCP with the exception of the DSS option, as indications of congestion
implementation SHOULD NOT send more than two duplicate ACKs in a row [9], and an MPTCP implementation SHOULD NOT send more than two
for signaling purposes. duplicate ACKs in a row for signaling purposes.
3.4.2. Remove Address 3.4.2. Remove Address
If, during the lifetime of an MPTCP connection, a previously- If, during the lifetime of an MPTCP connection, a previously-
announced address becomes invalid (e.g. if the interface disappears), announced address becomes invalid (e.g. if the interface disappears),
the affected host SHOULD announce this so that the peer can remove the affected host SHOULD announce this so that the peer can 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 13), 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 [16] on the path, and if a response is the sending of a TCP Keepalive [16] on the path, and if a response is
received the path is not removed. Typical TCP validity tests on the received the path SHOULD NOT be removed. Typical TCP validity tests
subflow (e.g. ensuring sequence and ack numbers are correct) MUST on the subflow (e.g. ensuring sequence and ack numbers are correct)
also be undertaken. MUST 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 hosts on the this message SHOULD trigger the sending of RSTs by both hosts on 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, before cleaning up any local state. middlebox state, before cleaning up any local state.
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 that rewrite source addresses. If there is no and other middleboxes that rewrite source addresses. If there is no
address at the requested ID, the receiver will silently ignore the address at the requested ID, the receiver will silently ignore the
request. request.
A subflow that is still functioning MUST be closed with a FIN A subflow that is still functioning MUST be closed with a FIN
exchange as in regular TCP - for more information, see Section 3.3.3. exchange as in regular TCP, rather than using this option. For more
information, see Section 3.3.3.
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 | Length = 3+n |Subtype| | Address ID | ... | Kind | Length = 3+n |Subtype| | Address ID | ...
+---------------+---------------+-------+-------+---------------+ +---------------+---------------+-------+-------+---------------+
Figure 13: Remove Address (REMOVE_ADDR) option Figure 13: Remove Address (REMOVE_ADDR) option
3.5. Fast Close 3.5. Fast Close
skipping to change at page 38, line 48 skipping to change at page 39, line 23
o As soon as Host A has received the TCP RST on the remaining o As soon as Host A has received the TCP RST on the remaining
subflow, it can close this subflow and tear down the whole subflow, it can close this subflow and tear down the whole
connection (transition from FASTCLOSE_WAIT to CLOSED states). If connection (transition from FASTCLOSE_WAIT to CLOSED states). If
Host A receives an MP_FASTCLOSE instead of a TCP RST, both hosts Host A receives an MP_FASTCLOSE instead of a TCP RST, both hosts
attempted fast closure simultaneously. Hose A should reply with a attempted fast closure simultaneously. Hose A should reply with a
TCP RST and tear down the connection. TCP RST and tear down the connection.
o If host A does not receive a TCP RST in reply to its MP_FASTCLOSE o If host A does not receive a TCP RST in reply to its MP_FASTCLOSE
after one RTO (the RTO of the subflow where the MPTCP_RST has been after one RTO (the RTO of the subflow where the MPTCP_RST has been
sent), it SHOULD retransmit the MP_FASTCLOSE. The number of sent), it SHOULD retransmit the MP_FASTCLOSE. The number of
retransmissions should be limited to avoid this connection from retransmissions SHOULD be limited to avoid this connection from
being retained for a long time, but this limit is implementation- being retained for a long time, but this limit is implementation-
specific. specific.
3.6. Fallback 3.6. Fallback
Sometimes, middleboxes will exist on a path that could prevent the
operation of MPTCP. MPTCP has been designed in order to cope with
many middlebox modifications (see Section 6), but there are still
some cases where a subflow could fail to operate within the MPTCP
requirements. These cases are notably: the loss of TCP options on a
path; and the modification of payload data. If such an event occurs,
it is necessary to "fall back" to the previous, safe operation. This
may either be falling back to regular TCP, or removing a problematic
subflow.
At the start of an 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 host. 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 SHOULD 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 a host is not MPTCP capable, or desired, behaviour in the case where a host is not MPTCP capable, or
the path does not support the MPTCP options. When attempting to join the path does not support the MPTCP options. When attempting to join
an existing MPTCP connection (Section 3.2), if a path is not MPTCP an existing MPTCP connection (Section 3.2), if a path is not MPTCP
capable, the TCP options will not get through on the SYNs and the capable and the TCP options do not get through on the SYNs, the
subflow will be closed. subflow will be closed according to the MP_JOIN logic.
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, using the subflow, and thus all data in flight is contiguous, using the
following rules. following rules.
A sender MUST include a DSS option with Data Sequence Mapping in A sender MUST include a DSS option with Data Sequence Mapping in
every segment until one of the sent segments has been acknowledged every segment until one of the sent segments has been acknowledged
with a DSS option containing a Data ACK. Upon reception of the with a DSS option containing a Data ACK. Upon reception of the
skipping to change at page 39, line 38 skipping to change at page 40, line 22
passes in both directions and may choose to send fewer DSS options passes in both directions and may choose to send fewer DSS options
than once per segment. than once per segment.
If, however, an ACK is received for data (not just for the SYN) If, however, an ACK is received for data (not just for the SYN)
without a DSS option containing a Data ACK, the sender determines the without a DSS option containing a Data ACK, the sender determines the
path is not MPTCP capable. In the case of this occurring on an path is not MPTCP capable. In the case of this occurring on an
additional subflow (i.e. one started with MP_JOIN), the host MUST 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 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 an MPTCP (i.e. that started with MP_CAPABLE), it MUST drop out of an MPTCP
mode back to regular TCP. The sender will send one final Data mode back to regular TCP. The sender will send one final Data
Sequence Mapping, with the length value of 0 indicating an infinite Sequence Mapping, with the Data-Level Length value of 0 indicating an
mapping (in case the path drops options in one direction only), and infinite mapping (in case the path drops options in one direction
then revert to sending data on the single subflow without any MPTCP only), and then revert to sending data on the single subflow without
options. any MPTCP options.
Note that this rule essentially prohibits the sending of data on the Note that this rule essentially prohibits the sending of data on the
third packet of an MP_CAPABLE or MP_JOIN handshake, since both that third packet of an MP_CAPABLE or MP_JOIN handshake, since both that
option and a DSS cannot fit in TCP option space. If the initiator is 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 to send first, another segment must be sent that contains the data
and DSS. Note also that an additional subflow cannot be used until and DSS. Note also that an additional subflow cannot be used until
the initial path has been verified as MPTCP-capable. 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 the initial subflow, it occur on any other subflow apart from the the initial subflow, it
should be treated as a standard path failure. The data would not be should be treated as a standard path failure. The data would not be
DATA_ACKed (since there is no mapping for the data), and the subflow DATA_ACKed (since there is no mapping for the data), and the subflow
can be closed with an RST. (Note that these rules do not apply if an can be closed with an RST.
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, for when
generally, fallback to regular TCP can become necessary at any point the lack of MPTCP support is detected before any data is acknowledged
during a connection if a non-MPTCP-aware middlebox changes the data at the connection level on a subflow. More generally, fallback
stream. (either closing a subflow, or to regular TCP) can become necessary at
any point during a connection if a non-MPTCP-aware middlebox changes
the data 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
skipping to change at page 40, line 42 skipping to change at page 41, line 27
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 an MP_FAIL option must be immediately closed with an RST, featuring an MP_FAIL option
(Figure 15), which defines the Data Sequence Number at the start of (Figure 15), which defines the Data Sequence Number at the start of
the segment (defined by the Data Sequence Mapping) which had the the segment (defined by the Data Sequence Mapping) which had the
checksum failure. checksum failure. Note that the MP_FAIL option requires the use of
the full 64-bit sequence number, even if 32-bit sequence numbers are
normally in use in the DSS signals on the path.
1 2 3 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 | Length=12 |Subtype| (reserved) | | Kind | Length=12 |Subtype| (reserved) |
+---------------+---------------+-------+----------------------+ +---------------+---------------+-------+----------------------+
| | | |
| Data Sequence Number (8 octets) | | Data Sequence Number (8 octets) |
| | | |
+--------------------------------------------------------------+ +--------------------------------------------------------------+
Figure 15: Fallback (MP_FAIL) option Figure 15: Fallback (MP_FAIL) option
The receiver MUST discard all data following the data sequence number The receiver MUST discard all data following the data sequence number
specified. Failed data will not be DATA_ACKed and so will be re- specified. Failed data MUST NOT be DATA_ACKed and so will be re-
transmitted on other subflows (Section 3.3.6). transmitted on 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. If it is known that all unacknowledged data in checksum error. If it is known that all unacknowledged data in
flight is contiguous (which will usually be the case with a single flight is contiguous (which will usually be the case with a single
subflow), an infinite mapping can be applied to the subflow without subflow), an infinite mapping can be applied to the subflow without
the need to close it first, and essentially turn off all further the need to close it first, and essentially turn off all further
MPTCP signaling. In this case, if a receiver identifies a checksum MPTCP signaling. In this case, if a receiver identifies a checksum
failure when there is only one path, it will send back an MP_FAIL failure when there is only one path, it will send back an MP_FAIL
option on the subflow-level ACK, refering to the data-level sequence option on the subflow-level ACK, refering to the data-level sequence
skipping to change at page 41, line 46 skipping to change at page 42, line 25
onwards data can be altered by a middlebox without affecting MPTCP, onwards data can be altered by a middlebox without affecting MPTCP,
as the data stream is equivalent to a regular, legacy TCP session. as the data stream is equivalent to a regular, legacy TCP session.
In the rare case that the data is not contiguous (which could happen In the rare case that the data is not contiguous (which could happen
when there is only one subflow but it is retransmitting data from a when there is only one subflow but it is retransmitting data from a
subflow that has recently been uncleanly closed), the receiver MUST subflow that has recently been uncleanly closed), the receiver MUST
close the subflow with an RST with MP_FAIL. The receiver MUST close the subflow with an RST with MP_FAIL. The receiver MUST
discard all data that follows the data sequence number specified. discard all data that follows the data sequence number specified.
The sender MAY attempt to create a new subflow belonging to the same The sender MAY attempt to create a new subflow belonging to the same
connection, and if it chooses to do so, SHOULD place the single connection, and if it chooses to do so, SHOULD place the single
subflow immediately in fallback mode by setting an infinite data subflow immediately in single-path mode by setting an infinite data
sequence mapping. This mapping will begin from the data-level sequence mapping. This mapping will begin from the data-level
sequence number that was declared in the MP_FAIL. sequence number that was declared in the MP_FAIL.
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 has fallen back, only one subflow can send data,
at a time. Otherwise, the receiver would not know how to reorder the otherwise the receiver would not know how to reorder the data. In
data; in practice this means that all MPTCP subflows will have to be practice, this means that all MPTCP subflows will have to be
terminated except one. Once MPTCP falls back to regular TCP, it MUST terminated except one. Once MPTCP falls back to regular TCP, it MUST
NOT revert to MPTCP later in the connection. NOT revert to MPTCP later in the connection.
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 could provide such functionality by also rewriting middlebox could provide such functionality by also rewriting
checksums. checksums.
3.7. Error Handling 3.7. Error Handling
skipping to change at page 44, line 17 skipping to change at page 44, line 45
This section has shown some of the considerations that an implementer This section has shown some of the considerations that an implementer
should give when developing MPTCP heuristics, but is not intended to should give when developing MPTCP heuristics, but is not intended to
be prescriptive. be prescriptive.
3.8.3. Failure Handling 3.8.3. Failure Handling
Requirements for MPTCP's handling of unexpected signals have been Requirements for MPTCP's handling of unexpected signals have been
given in Section 3.7. There are other failure cases, however, where given in Section 3.7. There are other failure cases, however, where
a hosts can choose appropriate behaviour. a hosts can choose appropriate behaviour.
For example, Section 3.1 suggests that a host should fall back to 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 trying regular TCP SYNs after one or more failures of MPTCP SYNs for
may keep a system-wide cache of such information, so that it can back a connection. A host may keep a system-wide cache of such
off from using MPTCP, firstly for that particular destination host, information, so that it can back off from using MPTCP, firstly for
and eventually on a whole interface, if MPTCP connections continue that particular destination host, and eventually on a whole
failing. interface, if MPTCP connections continue failing.
Another failure could occur when the MP_JOIN handshake fails. Another failure could occur when the MP_JOIN handshake fails.
Section 3.7 specifies that an incorrect handshake MUST lead to the Section 3.7 specifies that an incorrect handshake MUST lead to the
subflow being closed with a RST. A host operating an active subflow being closed with a RST. A host operating an active
intrusion detection system may choose to start blocking MP_JOIN intrusion detection system may choose to start blocking MP_JOIN
packets from the source host if multiple failed MP_JOIN attempts are 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 seen. From the connection initiator's point of view, if an MP_JOIN
fails, it SHOULD NOT attempt to connect to the same IP address and fails, it SHOULD NOT attempt to connect to the same IP address and
port during the lifetime of the connection, unless the other host port during the lifetime of the connection, unless the other host
refreshes the information with another ADD_ADDR option. Note that refreshes the information with another ADD_ADDR option. Note that
the ADD_ADDR option is informational only, and does not guarantee the the ADD_ADDR option is informational only, and does not guarantee the
other host will attempt a connection. other host will attempt a connection.
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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. This separates subflow- acknowledgement from the subflow ACKs. This separates subflow-
and connection-level processing at an end host. and connection-level processing at an end host.
Duplicate ACK: A duplicate ACK that includes any MPTCP signaling Duplicate ACK: A duplicate ACK that includes any MPTCP signaling
(with the exception of the DSS option) MUST NOT be treated as a (with the exception of the DSS option) MUST NOT be treated as a
signal of congestion. To avoid any non-MPTCP-aware entities also signal of congestion. To limit the chances of non-MPTCP-aware
mistakenly seeing duplicate ACKs in such cases, MPTCP SHOULD NOT entities mistakenly interpreting duplicate ACKs as a signal of
send more than two duplicate ACKs containing MPTCP signals in a congestion, MPTCP SHOULD NOT send more than two duplicate ACKs
row. containing (non-DSS) 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. Note that some way the original flow control role is preserved. Note that some
middleboxes may change the receive window, and so a host must use middleboxes may change the receive window, and so a host SHOULD
the maximum value of those recently seen on the constituent use the maximum value of those recently seen on the constituent
subflows for the connection-level receive window, and also need to subflows for the connection-level receive window, and also needs
maintain a subflow-level window for subflow-level processing. 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. The MP_FASTCLOSE option is sent on, not to the whole connection. The MP_FASTCLOSE option
provides the fast-close functionality of a RST at the MPTCP provides the fast-close functionality of a RST at the MPTCP
connection level. connection level.
skipping to change at page 46, line 42 skipping to change at page 47, line 23
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 are never sent are shared once at the start of the first subflow, and are never sent
again over the network. To ease demultiplexing whilst not giving again over the network. To ease demultiplexing whilst not giving
away any cryptographic material, future subflows use a truncated away any cryptographic material, future subflows use a truncated
SHA-1 hash of this key as the connection identification "token". The SHA-1 hash of this key as the connection identification "token". The
keys are concatenated and used as keys for creating Message keys are concatenated and used as keys for creating Message
Authentication Codes (MAC) used on subflow setup that verify that the Authentication Codes (MAC) used on subflow setup, in order to verify
parties in the handshake are the same as in the original connection that the parties in the handshake are the same as in the original
setup. It also provides verification that the peer can receive connection setup. It also provides verification that the peer can
traffic at this new address. Replay attacks would still be possible receive traffic at this new address. Replay attacks would still be
when only keys are used, and therefore the handshakes use single-use possible when only keys are used, and therefore the handshakes use
random numbers (nonces) at both ends - this ensures the MAC will single-use random numbers (nonces) at both ends - this ensures the
never be the same on two handshakes. MAC will never be the same on two handshakes.
The use of crypto capability bits in the initial connection handshake The use of crypto capability bits in the initial connection handshake
to negotiate use of a particular algorithm will allow the deployment to negotiate use of a particular algorithm allows the deployment of
of additional crypto mechanisms in the future. Note that this would additional crypto mechanisms in the future. Note that this would be
be susceptible to bid-down attacks only if the attacker was on-path susceptible to bid-down attacks only if the attacker was on-path (and
(and thus would be able to modify the data anyway). The security thus would be able to modify the data anyway). The security
mechanism presented in this draft should therefore protect against mechanism presented in this draft should therefore protect against
all forms of flooding and hijacking attacks discussed in [7]. all forms of flooding and hijacking attacks discussed in [7].
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 accommodate the different middleboxes.
A primary concern is our use of a new TCP option. Most middleboxes A primary concern is our use of a new TCP option. Middleboxes should
should just forward packets with new options unchanged, yet there are forward packets with unknown options unchanged, yet there are some
some that don't. These we expect will either strip options and pass that don't. These we expect will either strip options and pass the
the data, drop packets with new options, copy the same option into 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 uses a single new TCP option "Kind", and all message types are MPTCP uses a single new TCP option "Kind", and all message types are
defined by "subtype" values (see Section 8). This should reduce the defined by "subtype" values (see Section 8). This should reduce the
chances of only some types of MPTCP options being passed, and instead chances of only some types of MPTCP options being passed, and instead
the key differing characteristics are different paths, and the the key differing characteristics are different paths, and the
presence of the SYN flag. presence of the SYN flag.
MPTCP SYN packets on the first subflow of a connection contain the MPTCP SYN packets on the first subflow of a connection contain the
skipping to change at page 48, line 39 skipping to change at page 49, line 18
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. If a fraction of options are stripped, in the Fallback section. If a fraction of options are stripped,
behaviour is not deterministic. If some Data Sequence Mappings are behaviour is not deterministic. If some Data Sequence Mappings are
lost, the connection can continue so long as mappings exist for the lost, the connection can continue so long as mappings exist for the
subflow-level data (e.g. if multiple maps have been sent that subflow-level data (e.g. if multiple maps have been sent that
reinforce each other). If some subflow-level space is left unmapped, reinforce each other). If some subflow-level space is left unmapped,
however, the subflow is treated as broken and is closed, as discussed however, the subflow is treated as broken and is closed, through the
in Section 3.3. MPTCP should survive with a loss of some Data ACKs, process described in Section 3.6. MPTCP should survive with a loss
but performance will degrade as the fraction of stripped options of some Data ACKs, but performance will degrade as the fraction of
increases. We do not expect such cases to appear in practice, stripped options increases. We do not expect such cases to appear in
though: most middleboxes will either strip all options or let them practice, though: most middleboxes will either strip all options or
all through. let them 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 NATs [17] (Network Address (and Port) Translators) change the o NATs [17] (Network Address (and Port) Translators) change the
source address (and often source port) of packets. This means source address (and often source port) of packets. This means
that a host will not know its public-facing address for signaling that a host will not know its public-facing address for signaling
in MPTCP. Therefore, MPTCP permits implicit address addition via in MPTCP. Therefore, MPTCP permits implicit address addition via
the MP_JOIN option, and the handshake mechanism ensures that the MP_JOIN option, and the handshake mechanism ensures that
connection attempts to private addresses [15] do not cause connection attempts to private addresses [15] do not cause
problems. Explicit address removal is undertaken by an ID number problems. Explicit address removal is undertaken by an Address ID
to allow no knowledge of the source address. to allow no knowledge of the source address.
o Performance Enhancing Proxies (PEPs) [18] might pro-actively ACK o Performance Enhancing Proxies (PEPs) [18] might pro-actively ACK
data to increase performance. MPTCP, however, relies on accurate data to increase performance. MPTCP, however, relies on accurate
congestion control signals from the end host, and non-MPTCP-aware congestion control signals from the end host, and non-MPTCP-aware
PEPs will not be able to provide such signals. MPTCP will PEPs will not be able to provide such signals. MPTCP will
therefore fall back to single-path TCP (see Section 3.6). therefore fall back to single-path TCP, or close the problematic
subflow (see Section 3.6).
o Traffic Normalizers [19] may not allow holes in sequence numbers, o Traffic Normalizers [19] may not allow holes in sequence numbers,
and may cache packets and retransmit the same data. MPTCP looks and may cache packets and retransmit the same data. MPTCP looks
like standard TCP on the wire, and will not retransmit different like standard TCP on the wire, and will not retransmit different
data on the same subflow sequence number. data on the same subflow sequence number. In the event of a
retransmission, he same data will be retransmitted on the original
TCP subflow even if it is additionally retransmitted at the
connection-level on a different subflow.
o Firewalls [20] might perform initial sequence number randomization o Firewalls [20] might perform initial sequence number randomization
on TCP connections. MPTCP uses relative sequence numbers in data on TCP connections. MPTCP uses relative sequence numbers in data
sequence mapping to cope with this. Like NATs, firewalls will not sequence mapping to cope with this. Like NATs, firewalls will not
permit many incoming connections, so MPTCP supports address permit many incoming connections, so MPTCP supports address
signaling (ADD_ADDR) so that a multi-addressed host can invite its signaling (ADD_ADDR) so that a multi-addressed host can invite its
peer behind the firewall/NAT to connect out to its additional peer behind the firewall/NAT to connect out to its additional
interface. interface.
o Intrusion Detection Systems look out for traffic patterns and o Intrusion Detection Systems look out for traffic patterns and
skipping to change at page 50, line 44 skipping to change at page 51, line 27
document from Sebastien Barre, Christoph Paasch, 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, Pasi Sarolahti, Toby Moncaster, Philip Eardley, Sergio Lembo,
Lawrence Conroy, Yoshifumi Nishida, Bob Briscoe, Stein Gjessing, Lawrence Conroy, Yoshifumi Nishida, Bob Briscoe, Stein Gjessing,
Andrew McGregor, Georg Hampel, and Anumita Biswas. Andrew McGregor, Georg Hampel, and Anumita Biswas.
8. IANA Considerations 8. IANA Considerations
This document will make a request to IANA to allocate a new TCP This document defines a new TCP option for MPTCP, assigned a value of
option value for MPTCP. This value will be the value of the "Kind" 30 (decimal) from the TCP Option space. This value is the value of
field seen in all MPTCP options in this document. "Kind" as seen in all MPTCP options in this document. This value is
defined as:
This document will also request IANA operates a registry for MPTCP +------+--------+---------------+-----------------+
option subtype values. The values as defined by this specification | Kind | Length | Meaning | Reference |
are as follows: +------+--------+---------------+-----------------+
| 30 | N | Multipath TCP | (This document) |
+------+--------+---------------+-----------------+
Table 1: TCP Option Kind Numbers
This document also defines a four-bit subtype field, for which IANA
is to create and maintain a new sub-registry entitled "MPTCP option
subtype values" under the MPTCP option. Initial values for the MPTCP
option subtype registry are given below; future assignments are to be
defined by RFCs (RFC Requirement as defined by [21]) Assignments
consist of the MPTCP subtype's symbolic name and its associated
value, as per the following table.
+--------------+----------------------------+---------------+-------+ +--------------+----------------------------+---------------+-------+
| Symbol | Name | Ref | Value | | Symbol | Name | Reference | Value |
+--------------+----------------------------+---------------+-------+ +--------------+----------------------------+---------------+-------+
| MP_CAPABLE | Multipath Capable | Section 3.1 | 0x0 | | MP_CAPABLE | Multipath Capable | Section 3.1 | 0x0 |
| MP_JOIN | Join Connection | Section 3.2 | 0x1 | | MP_JOIN | Join Connection | Section 3.2 | 0x1 |
| DSS | Data Sequence Signal (Data | Section 3.3 | 0x2 | | DSS | Data Sequence Signal (Data | Section 3.3 | 0x2 |
| | ACK and Data Sequence | | | | | ACK and Data Sequence | | |
| | Mapping) | | | | | Mapping) | | |
| ADD_ADDR | Add Address | Section 3.4.1 | 0x3 | | ADD_ADDR | Add Address | Section 3.4.1 | 0x3 |
| REMOVE_ADDR | Remove Address | Section 3.4.2 | 0x4 | | REMOVE_ADDR | Remove Address | Section 3.4.2 | 0x4 |
| MP_PRIO | Change Subflow Priority | Section 3.3.8 | 0x5 | | MP_PRIO | Change Subflow Priority | Section 3.3.8 | 0x5 |
| MP_FAIL | Fallback | Section 3.6 | 0x6 | | MP_FAIL | Fallback | Section 3.6 | 0x6 |
| MP_FASTCLOSE | Fast Close | Section 3.5 | 0x7 | | MP_FASTCLOSE | Fast Close | Section 3.5 | 0x7 |
+--------------+----------------------------+---------------+-------+ +--------------+----------------------------+---------------+-------+
Table 1: MPTCP Option Subtypes Table 2: MPTCP Option Subtypes
This document also requests that IANA keeps a registry of The value 0xf is reserved for Private Use.
cryptographic handshake algorithms based on the flags in MP_CAPABLE
This document also requests that IANA keeps a registry of "MPTCP
cryptographic handshake algorithms" based on the flags in MP_CAPABLE
(Section 3.1). This document specifies only one algorithm: (Section 3.1). This document specifies only one algorithm:
+-------+-----------+----------------------------+ +-------+----------------+----------------------------+
| Flags | Algorithm | Document | | Flags | Algorithm Name | Reference |
+-------+-----------+----------------------------+ +-------+----------------+----------------------------+
| 0x1 | HMAC-SHA1 | This document, Section 3.2 | | 0x1 | HMAC-SHA1 | This document, Section 3.2 |
+-------+-----------+----------------------------+ +-------+----------------+----------------------------+
Table 2: MPTCP Handshake Algorithms Table 3: MPTCP Handshake Algorithms
9. References Future assignments in this registry are also to be defined by RFCs
(RFC Requirement as defined by [21]) Assignments consist of the value
of the flags, a symbolic name for the algorithm, and a reference to
its specification.
Note that the length of this field is not fixed; it is a definition
of the meaning of each bit in this field (i.e. 0x2, 0x4, 0x8, 0x10,
etc). Future specifications may encroach on the non-cryptographic
flags at the other end of this field so the number of flags available
for cryptographic algorithm use may change.
9. References
9.1. Normative References 9.1. Normative References
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, [1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981. September 1981.
[2] Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar, [2] Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar,
"Architectural Guidelines for Multipath TCP Development", "Architectural Guidelines for Multipath TCP Development",
RFC 6182, March 2011. RFC 6182, March 2011.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement [3] 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.
[4] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and [4] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and
HMAC-SHA)", RFC 4634, July 2006. SHA-based HMAC and HKDF)", RFC 6234, May 2011.
9.2. Informative References 9.2. Informative References
[5] Raiciu, C., Handley, M., and D. Wischik, "Coupled Congestion [5] Raiciu, C., Handley, M., and D. Wischik, "Coupled Congestion
Control for Multipath Transport Protocols", RFC 6356, Control for Multipath Transport Protocols", RFC 6356,
October 2011. October 2011.
[6] Scharf, M. and A. Ford, "MPTCP Application Interface [6] Scharf, M. and A. Ford, "MPTCP Application Interface
Considerations", draft-ietf-mptcp-api-03 (work in progress), Considerations", draft-ietf-mptcp-api-05 (work in progress),
November 2011. April 2012.
[7] Bagnulo, M., "Threat Analysis for TCP Extensions for Multipath [7] Bagnulo, M., "Threat Analysis for TCP Extensions for Multipath
Operation with Multiple Addresses", RFC 6181, March 2011. Operation with Multiple Addresses", RFC 6181, March 2011.
[8] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP [8] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996. Selective Acknowledgment Options", RFC 2018, October 1996.
[9] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion [9] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 2581, April 1999. Control", RFC 5681, September 2009.
[10] Gont, F., "Security Assessment of the Transmission Control [10] Gont, F., "Survey of Security Hardening Methods for
Protocol (TCP)", draft-ietf-tcpm-tcp-security-02 (work in Transmission Control Protocol (TCP) Implementations",
progress), January 2011. draft-ietf-tcpm-tcp-security-03 (work in progress), March 2012.
[11] Eastlake, D., Schiller, J., and S. Crocker, "Randomness [11] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005. Requirements for Security", BCP 106, RFC 4086, June 2005.
[12] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing [12] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997. for Message Authentication", RFC 2104, February 1997.
[13] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions for [13] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992. High Performance", RFC 1323, May 1992.
skipping to change at page 53, line 14 skipping to change at page 54, line 29
Link-Related Degradations", RFC 3135, June 2001. Link-Related Degradations", RFC 3135, June 2001.
[19] 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>.
[20] 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.
[21] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
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.
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
skipping to change at page 53, line 39 skipping to change at page 55, line 10
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 SYN versions of be word-aligned. In either case, however, the SYN versions of
Multipath Capable (12 bytes) and Join (12 or 16 bytes) options will Multipath Capable (12 bytes) and Join (12 or 16 bytes) options will
fit in this remaining space. 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). The Data Sequence Signal (DSS) option varies in if word-aligned). The Data Sequence Signal (DSS) option varies in
length depending on whether the Data Sequence Mapping and DATA ACK length depending on whether the Data Sequence Mapping and DATA_ACK
are included, and whether the sequence numbers in use are 4 or 8 are included, and whether the sequence numbers in use are 4 or 8
octets. The maximum size of the DSS option is 28 bytes, so even that octets. The maximum size of the DSS option is 28 bytes, so even that
will fit in the available space. But unless a connection is both bi- will fit in the available space. But unless a connection is both bi-
directional and high-bandwidth, it is unlikely that all that option directional and high-bandwidth, it is unlikely that all that option
space will be required on each DSS option. space will be required on each DSS option.
Within the DSS option, it is not necessary to include the Data Within the DSS option, it is not necessary to include the Data
Sequence Mapping and DATA ACK in each packet, and in many cases it Sequence Mapping and DATA_ACK in each packet, and in many cases it
may be possible to alternate their presence (so long as the mapping may be possible to alternate their presence (so long as the mapping
covers the data being sent in the following packet). It would also covers the data being sent in the following packet). It would also
be possible to alternate between 4 and 8 byte sequence numbers in be possible to alternate between 4 and 8 byte sequence numbers in
each option. each option.
On subflow and connection setup, an MPTCP option is also set on the On subflow and connection setup, an MPTCP option is also set on the
third packet (an ACK). These are 20 bytes (for Multipath Capable) third packet (an ACK). These are 20 bytes (for Multipath Capable)
and 24 bytes (for Join), both of which will fit in the available and 24 bytes (for Join), both of which will fit in the available
option space. option space.
Pure ACKs in TCP typically contain only timestamps (10B). Here, Pure ACKs in TCP typically contain only timestamps (10 bytes). Here,
multipath TCP typically needs to encode only the DATA ACK (maximum of multipath TCP typically needs to encode only the DATA_ACK (maximum of
12 octets). Occasionally ACKs will contain SACK information. 12 bytes). Occasionally ACKs will contain SACK information.
Depending on the number of lost packets, SACK may utilize the entire Depending on the number of lost packets, SACK may utilize the entire
option space. If a DATA ACK had to be included, then it is probably option space. If a DATA_ACK had to be included, then it is probably
necessary to reduce the number of SACK blocks to accomodate the DATA necessary to reduce the number of SACK blocks to accomodate the
ACK. However, the presence of the DATA ACK is unlikely to be DATA_ACK. However, the presence of the DATA_ACK is unlikely to be
necessary in a case where SACK is in use, since until at least some necessary in a case where SACK is in use, since until at least some
of the SACK blocks have been retransmitted, the cumulative data-level 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 another 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 signaling would fit in a data or not. It is unlikely that such signaling would fit in a data
packet (although if there is space, it is fine to include it). It is 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. Note this is the reason for order to transmit these rare signals. Note this is the reason for
mandating that duplicate ACKs with MPTCP options are not taken as a mandating that duplicate ACKs with MPTCP options are not taken as a
signal of congestion. signal of congestion.
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the subflow. the subflow.
RCV.WND (32 bits with RFC1323, 16 bits otherwise): This is the RCV.WND (32 bits with RFC1323, 16 bits otherwise): This is the
subflow-level receive window which is updated with the window subflow-level receive window which is updated with the window
field from the segments received on this subflow. field from the segments received on this subflow.
Appendix C. Finite State Machine Appendix C. Finite State Machine
The diagram in Figure 17 shows the Finite State Machine for The diagram in Figure 17 shows the Finite State Machine for
connection-level closure. This illustrates how the DATA_FIN connection-level closure. This illustrates how the DATA_FIN
connection-level signal interacts with subflow-level FINs, and connection-level signal (indicated as the DFIN flag on a DATA_ACK)
permits "break-before-make" handover between subflows. interacts with subflow-level FINs, and permits "break-before-make"
handover between subflows.
+---------+ +---------+
| M_ESTAB | | M_ESTAB |
+---------+ +---------+
M_CLOSE | | rcv DATA_FIN M_CLOSE | | rcv DATA_FIN
------- | | ------- ------- | | -------
+---------+ snd DATA_FIN / \ snd DATA_ACK +---------+ +---------+ snd DATA_FIN / \ snd DATA_ACK[DFIN] +---------+
| M_FIN |<----------------- ------------------>| M_CLOSE | | M_FIN |<----------------- ------------------->| M_CLOSE |
| WAIT-1 |--------------------------- | WAIT | | WAIT-1 |--------------------------- | WAIT |
+---------+ rcv DATA_FIN \ +---------+ +---------+ rcv DATA_FIN \ +---------+
| rcv DATA_ACK[DFIN] ------- | M_CLOSE | | rcv DATA_ACK[DFIN] ------- | M_CLOSE |
| -------------- snd DATA_ACK | ------- | | -------------- snd DATA_ACK | ------- |
| CLOSE all subflows | snd DATA_FIN | | CLOSE all subflows | snd DATA_FIN |
V V V V V V
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+ +-----------+
|M_FINWAIT-2| | M_CLOSING | | M_LAST-ACK| |M_FINWAIT-2| | M_CLOSING | | M_LAST-ACK|
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+ +-----------+
| rcv DATA_ACK[DFIN] | rcv DATA_ACK[DFIN] | | rcv DATA_ACK[DFIN] | rcv DATA_ACK[DFIN] |
| rcv DATA_FIN -------------- | -------------- | | rcv DATA_FIN -------------- | -------------- |
| ------- CLOSE all subflows | CLOSE all subflows | | ------- CLOSE all subflows | CLOSE all subflows |
| snd DATA_ACK[DFIN] V V | snd DATA_ACK[DFIN] V V
\ +-----------+ +---------+ \ +-----------+ +---------+
------------------------>|M_TIME WAIT|---------------->| M_CLOSED| ------------------------>|M_TIME WAIT|----------------->| M_CLOSED|
+-----------+ +---------+ +-----------+ +---------+
All subflows in CLOSED All subflows in CLOSED
------------ ------------
delete MPTCP PCB delete MPTCP PCB
Figure 17: Finite State Machine for Connection Closure Figure 17: Finite State Machine for Connection Closure
Appendix D. Changelog Appendix D. Changelog
(To be removed by the RFC Editor)
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.
D.1. Changes since draft-ietf-mptcp-multiaddressed-05 D.1. Changes since draft-ietf-mptcp-multiaddressed-05
o Added MP_FASTCLOSE mechanism. o Added MP_FASTCLOSE mechanism.
D.2. Changes since draft-ietf-mptcp-multiaddressed-04 D.2. Changes since draft-ietf-mptcp-multiaddressed-04
o Reverted change to MP_CAPABLE from last revision. o Reverted change to MP_CAPABLE from last revision.
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o Removed Key from MP_CAPABLE on SYN (it is in the ACK). o Removed Key from MP_CAPABLE on SYN (it is in the ACK).
o Added optional Address ID to MP_PRIO. o Added optional Address ID to MP_PRIO.
o Responded to review comments. o Responded to review comments.
D.4. Changes since draft-ietf-mptcp-multiaddressed-02 D.4. Changes since draft-ietf-mptcp-multiaddressed-02
o Changed to using a single TCP option with a sub-type field. o Changed to using a single TCP option with a sub-type field.
o Merged Data Sequence Number, DATA ACK, and DATA FIN. o Merged Data Sequence Number, DATA_ACK, and DATA_FIN.
o Changed DATA FIN behaviour (separated from subflow FIN). o Changed DATA_FIN behaviour (separated from subflow FIN).
o Added crypto agility and checksum negotiation. o Added crypto agility and checksum negotiation.
o Redefined MP_JOIN handshake to use only three TCP options. o Redefined MP_JOIN handshake to use only three TCP options.
o Added pseudo-header to checksum. o Added pseudo-header to checksum.
o Many clarifications and re-structuring. o Many clarifications and re-structuring.
o Added more discussion on heuristics. o Added more discussion on heuristics.
 End of changes. 132 change blocks. 
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