draft-ietf-mptcp-multiaddressed-09.txt   draft-ietf-mptcp-multiaddressed-10.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: December 8, 2012 University Politehnica of Expires: April 6, 2013 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
June 6, 2012 October 3, 2012
TCP Extensions for Multipath Operation with Multiple Addresses TCP Extensions for Multipath Operation with Multiple Addresses
draft-ietf-mptcp-multiaddressed-09 draft-ietf-mptcp-multiaddressed-10
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 December 8, 2012. This Internet-Draft will expire on April 6, 2013.
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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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 . . . . . . . . . . . . . . . . . . 17 3.2. Starting a New Subflow . . . . . . . . . . . . . . . . . . 17
3.3. General MPTCP Operation . . . . . . . . . . . . . . . . . 21 3.3. General MPTCP Operation . . . . . . . . . . . . . . . . . 22
3.3.1. Data Sequence Mapping . . . . . . . . . . . . . . . . 23 3.3.1. Data Sequence Mapping . . . . . . . . . . . . . . . . 24
3.3.2. Data Acknowledgements . . . . . . . . . . . . . . . . 26 3.3.2. Data Acknowledgments . . . . . . . . . . . . . . . . . 27
3.3.3. Closing a Connection . . . . . . . . . . . . . . . . . 27 3.3.3. Closing a Connection . . . . . . . . . . . . . . . . . 28
3.3.4. Receiver Considerations . . . . . . . . . . . . . . . 28 3.3.4. Receiver Considerations . . . . . . . . . . . . . . . 29
3.3.5. Sender Considerations . . . . . . . . . . . . . . . . 29 3.3.5. Sender Considerations . . . . . . . . . . . . . . . . 30
3.3.6. Reliability and Retransmissions . . . . . . . . . . . 30 3.3.6. Reliability and Retransmissions . . . . . . . . . . . 31
3.3.7. Congestion Control Considerations . . . . . . . . . . 31 3.3.7. Congestion Control Considerations . . . . . . . . . . 32
3.3.8. Subflow Policy . . . . . . . . . . . . . . . . . . . . 32 3.3.8. Subflow Policy . . . . . . . . . . . . . . . . . . . . 33
3.4. Address Knowledge Exchange (Path Management) . . . . . . . 33 3.4. Address Knowledge Exchange (Path Management) . . . . . . . 34
3.4.1. Address Advertisement . . . . . . . . . . . . . . . . 34 3.4.1. Address Advertisement . . . . . . . . . . . . . . . . 35
3.4.2. Remove Address . . . . . . . . . . . . . . . . . . . . 37 3.4.2. Remove Address . . . . . . . . . . . . . . . . . . . . 38
3.5. Fast Close . . . . . . . . . . . . . . . . . . . . . . . . 38 3.5. Fast Close . . . . . . . . . . . . . . . . . . . . . . . . 39
3.6. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.6. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.7. Error Handling . . . . . . . . . . . . . . . . . . . . . . 42 3.7. Error Handling . . . . . . . . . . . . . . . . . . . . . . 43
3.8. Heuristics . . . . . . . . . . . . . . . . . . . . . . . . 43 3.8. Heuristics . . . . . . . . . . . . . . . . . . . . . . . . 44
3.8.1. Port Usage . . . . . . . . . . . . . . . . . . . . . . 43 3.8.1. Port Usage . . . . . . . . . . . . . . . . . . . . . . 44
3.8.2. Delayed Subflow Start . . . . . . . . . . . . . . . . 43 3.8.2. Delayed Subflow Start . . . . . . . . . . . . . . . . 44
3.8.3. Failure Handling . . . . . . . . . . . . . . . . . . . 44 3.8.3. Failure Handling . . . . . . . . . . . . . . . . . . . 45
4. Semantic Issues . . . . . . . . . . . . . . . . . . . . . . . 45 4. Semantic Issues . . . . . . . . . . . . . . . . . . . . . . . 46
5. Security Considerations . . . . . . . . . . . . . . . . . . . 46 5. Security Considerations . . . . . . . . . . . . . . . . . . . 47
6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 47 6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 49
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 51 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 53
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 55
9.1. Normative References . . . . . . . . . . . . . . . . . . . 53 9.1. Normative References . . . . . . . . . . . . . . . . . . . 55
9.2. Informative References . . . . . . . . . . . . . . . . . . 53 9.2. Informative References . . . . . . . . . . . . . . . . . . 55
Appendix A. Notes on use of TCP Options . . . . . . . . . . . . . 54 Appendix A. Notes on use of TCP Options . . . . . . . . . . . . . 57
Appendix B. Control Blocks . . . . . . . . . . . . . . . . . . . 56 Appendix B. Control Blocks . . . . . . . . . . . . . . . . . . . 58
B.1. MPTCP Control Block . . . . . . . . . . . . . . . . . . . 56 B.1. MPTCP Control Block . . . . . . . . . . . . . . . . . . . 59
B.1.1. Authentication and Metadata . . . . . . . . . . . . . 56 B.1.1. Authentication and Metadata . . . . . . . . . . . . . 59
B.1.2. Sending Side . . . . . . . . . . . . . . . . . . . . . 57 B.1.2. Sending Side . . . . . . . . . . . . . . . . . . . . . 59
B.1.3. Receiving Side . . . . . . . . . . . . . . . . . . . . 57 B.1.3. Receiving Side . . . . . . . . . . . . . . . . . . . . 60
B.2. TCP Control Blocks . . . . . . . . . . . . . . . . . . . . 57 B.2. TCP Control Blocks . . . . . . . . . . . . . . . . . . . . 60
B.2.1. Sending Side . . . . . . . . . . . . . . . . . . . . . 57 B.2.1. Sending Side . . . . . . . . . . . . . . . . . . . . . 60
B.2.2. Receiving Side . . . . . . . . . . . . . . . . . . . . 58 B.2.2. Receiving Side . . . . . . . . . . . . . . . . . . . . 60
Appendix C. Finite State Machine . . . . . . . . . . . . . . . . 58 Appendix C. Finite State Machine . . . . . . . . . . . . . . . . 61
Appendix D. Changelog . . . . . . . . . . . . . . . . . . . . . . 59 Appendix D. Changelog . . . . . . . . . . . . . . . . . . . . . . 61
D.1. Changes since draft-ietf-mptcp-multiaddressed-05 . . . . . 59 D.1. Changes since draft-ietf-mptcp-multiaddressed-05 . . . . . 62
D.2. Changes since draft-ietf-mptcp-multiaddressed-04 . . . . . 59 D.2. Changes since draft-ietf-mptcp-multiaddressed-04 . . . . . 62
D.3. Changes since draft-ietf-mptcp-multiaddressed-03 . . . . . 60 D.3. Changes since draft-ietf-mptcp-multiaddressed-03 . . . . . 62
D.4. Changes since draft-ietf-mptcp-multiaddressed-02 . . . . . 60 D.4. Changes since draft-ietf-mptcp-multiaddressed-02 . . . . . 62
D.5. Changes since draft-ietf-mptcp-multiaddressed-01 . . . . . 60 D.5. Changes since draft-ietf-mptcp-multiaddressed-01 . . . . . 62
D.6. Changes since draft-ietf-mptcp-multiaddressed-00 . . . . . 60 D.6. Changes since draft-ietf-mptcp-multiaddressed-00 . . . . . 63
D.7. Changes since draft-ford-mptcp-multiaddressed-03 . . . . . 60 D.7. Changes since draft-ford-mptcp-multiaddressed-03 . . . . . 63
D.8. Changes since draft-ford-mptcp-multiaddressed-02 . . . . . 61 D.8. Changes since draft-ford-mptcp-multiaddressed-02 . . . . . 63
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 61 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 63
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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algorithm for coupling the behaviour of the multiple paths in algorithm for coupling the behaviour of the multiple paths in
order to "do no harm" to other network users. order to "do no harm" to other network users.
o Application Considerations [6], discussing what impact MPTCP will o Application Considerations [6], discussing what impact MPTCP will
have on applications, what applications will want to do with have on applications, what applications will want to do with
MPTCP, and as a consequence of these factors, what API extensions MPTCP, and as a consequence of these factors, what API extensions
an MPTCP implementation should present. an MPTCP implementation should present.
1.1. Design Assumptions 1.1. Design Assumptions
In order to limit the potentially huge design space, the authors In order to limit the potentially huge design space, the working
imposed two key constraints on the multipath TCP design presented in group imposed two key constraints on the multipath TCP design
this document: presented in this document:
o It must be backwards-compatible with current, regular TCP, to o It must be backwards-compatible with current, regular TCP, to
increase its chances of deployment increase its chances of deployment
o It can be assumed that one or both 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
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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
removed or duplicated. removed or duplicated.
Application Constraints: The protocol must be usable with no change Application Constraints: The protocol must be usable with no change
to existing applications that use the standard TCP API (although to existing applications that use the common TCP API (although it
it is reasonable that not all features would be available to such is reasonable that not all features would be available to such
legacy applications). Furthermore, the protocol must provide the legacy applications). Furthermore, the protocol must provide the
same service model as regular TCP to the application. same service model as regular TCP to the application.
Fall-back: The protocol should be able to fall back to standard TCP Fall-back: The protocol should be able to fall back to standard TCP
with no interference from the user, to be able to communicate with with no interference from the user, to be able to communicate with
legacy hosts. legacy hosts.
Further discussion of the design constraints and associated design Further discussion of the design constraints and associated design
decisions are given in the MPTCP Architecture document [2]. decisions are given in the MPTCP Architecture document [2].
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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 This document makes use of a number of terms which are either MPTCP-
below: specific, or have defined meaning in the context of MPTCP, as
follows:
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-
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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 HMAC is
for authentication. The MAC uses the keys exchanged in the used for authentication. The HMAC uses the keys exchanged in the
MP_CAPABLE handshake, and the random numbers (nonces) exchanged in MP_CAPABLE handshake, and the random numbers (nonces) exchanged in
these MP_JOIN options. MP_JOIN also contains flags and an Address ID 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 HMAC, B's nonce,
B's Address ID, flags] B's Address ID, flags]
ACK + MP_JOIN -> ACK + MP_JOIN ->
[A's MAC] [A's HMAC]
<- ACK <- 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 implicitly informed about the
address. new address.
In some circumstances, a host may want to advertise to the remote In some circumstances, a host may want to advertise to the remote
host the availability of an address without establishing a new host the availability of an address without establishing a new
subflow, for example when a NAT prevents setup in one direction. In subflow, for example when a NAT prevents setup in one direction. In
the example below, Host-A informs Host-B about its alternative IP the example below, Host-A informs Host-B about its alternative IP
address (IP#-A2). Host-B may later send an MP_JOIN to this new address (IP#-A2). Host-B may later send an MP_JOIN to this new
address. Due to the presence of middleboxes that may translate IP address. Due to the presence of middleboxes that may translate IP
addresses, this option uses an address identifier to unambiguously addresses, this option uses an address identifier to unambiguously
identify an address on a host. Further details in Section 3.4.1. identify an address on a host. Further details in Section 3.4.1.
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same DSN) in the event of failure. same DSN) in the event of failure.
The "Data Sequence Signal" carries the "Data Sequence Mapping". The The "Data Sequence Signal" carries the "Data Sequence Mapping". The
Data Sequence Mapping 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 acknowledgments 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 acknowledgments for the data sequence space. These acknowledgments
track the advancement of the bytestream and slide the receiving track the advancement of the bytestream and slide the receiving
window. window.
Further details are in Section 3.3. Further details are in Section 3.3.
Host-A Host-B Host-A Host-B
------ ------ ------ ------
DATA_SEQUENCE_SIGNAL -> DATA_SEQUENCE_SIGNAL ->
[Data Sequence Mapping] [Data Sequence Mapping]
[Data ACK] [Data ACK]
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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 ADD_ADDR message. when either end is behind a NAT, MPTCP uses the ADD_ADDR 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 ([8], [4]) using those
standard TCP validity checks are made on the other messages ( keys; and standard TCP validity checks are made on the other
ensuring sequence numbers are in-window). messages (ensuring sequence numbers are in-window).
3. MPTCP Protocol 3. MPTCP Protocol
This section describes the operation of the MPTCP protocol, and is This section describes the operation of the MPTCP protocol, and is
subdivided into sections for each key part of the protocol operation. subdivided into sections for each key part of the protocol operation.
All MPTCP operations are signalled using optional TCP header fields. All MPTCP operations are signalled using optional TCP header fields.
A single TCP option number ("Kind") will be assigned by IANA for A single TCP option number ("Kind") will be assigned by IANA for
MPTCP (see Section 8), and then individual messages will be MPTCP (see Section 8), and then individual messages will be
determined by a "sub-type", the values of which will also be stored determined by a "sub-type", the values of which will also be stored
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Those MPTCP options associated with subflow initiation must be Those MPTCP options associated with subflow initiation must be
included on packets with the SYN flag set. Additionally, there is included on packets with the SYN flag set. Additionally, there is
one MPTCP option for signaling metadata to ensure segmented data can one MPTCP option for signaling metadata to ensure segmented data can
be recombined for delivery to the application. be recombined for delivery to the application.
The remaining options, however, are signals that do not need to be on The remaining options, however, are signals that do not need to be on
a specific packet, such as those for signaling additional addresses. a specific packet, such as those for signaling additional addresses.
Whilst an implementation may desire to send MPTCP options as soon as Whilst an implementation may desire to send MPTCP options as soon as
possible, it may not be possible to combine all desired options (both possible, it may not be possible to combine all desired options (both
those for MPTCP and for regular TCP, such as SACK [8]) on a single those for MPTCP and for regular TCP, such as SACK [9]) on a single
packet. Therefore, an implementation may choose to send duplicate packet. Therefore, an implementation may choose to send duplicate
ACKs containing the additional signaling information. This changes ACKs containing the additional signaling information. This changes
the semantics of a duplicate ACK, these are usually only sent as a the semantics of a duplicate ACK, these are usually only sent as a
signal of a lost segment [9] in regular TCP. Therefore, an MPTCP signal of a lost segment [10] in regular TCP. Therefore, an MPTCP
implementation receiving a duplicate ACK which contains an MPTCP implementation receiving a duplicate ACK which contains an MPTCP
option MUST NOT treat it as a signal of congestion. Additionally, an option MUST NOT treat it as a signal of congestion. Additionally, an
MPTCP implementation SHOULD NOT send more than two duplicate ACKs in MPTCP implementation SHOULD NOT send more than two duplicate ACKs in
a row for signaling purposes, so as to ensure no middleboxes a row for the purposes of sending MPTCP options alone, in order to
misinterpret this as a sign of congestion. ensure no middleboxes misinterpret this as a sign of congestion.
Furthermore, standard TCP validity checks (such as ensuring the Furthermore, standard TCP validity checks (such as ensuring the
Sequence Number and Acknowledgement Number are within window) MUST be Sequence Number and Acknowledgement Number are within window) MUST be
undertaken before processing any MPTCP signals, as described in [10]. undertaken before processing any MPTCP signals, as described in [11].
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 This option is used to declare the sender's 64 bit key, which is
uniquely linked to this MPTCP connection. This key is used to uniquely linked to this MPTCP connection. This key is used to
authenticate the addition of future subflows to this connection. authenticate the addition of future subflows to this connection.
This is the only time the key will be sent in clear on the wire This is the only time the key will be sent in clear on the wire
(unless "fast close", Section 3.5, is used); all future subflows will (unless "fast close", Section 3.5, is used); all future subflows will
identify the connection using a 32 bit "token". This token is a identify the connection using a 32 bit "token". This token is a
cryptographic hash of this key. The token will be a truncated (most cryptographic hash of this key. The algorithm for this process is
significant 32 bits) SHA-1 hash [4]. A different, 64 bit truncation dependent on the authentication algorithm selected; the method of
(the least significant 64 bits) of the hash of the key will be used selection is defined later in this section.
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 its method of generation is
its method of generation is implementation-specific. The key MUST be implementation-specific. The key MUST be hard to guess, and it MUST
hard to guess, and it MUST be unique for the sending host at any one be unique for the sending host at any one time. Recommendations for
time. Recommendations for generating random keys are given in [11]. generating random numers for use in keys are given in [12].
Connections will be indexed at each host by the token (the truncated Connections will be indexed at each host by the token (a one-way hash
SHA-1 hash of the key). Therefore, an implementation will require a of the key). Therefore, an implementation will require a mapping
mapping from each token to the corresponding connection, and in turn from each token to the corresponding connection, and in turn to the
to the keys for the connection. 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, by checking tokens in both directions, and connection, however, by checking tokens in both directions, and
ensuring sequence numbers are in-window, so in the worst case if ensuring sequence numbers are in-window, so in the worst case if
there was a token collision, the new subflow would be closed, but the there was a token collision, the new subflow would be closed, but the
skipping to change at page 14, line 38 skipping to change at page 14, line 37
The contents of the option is determined by the SYN and ACK flags of The contents of the option is determined by the SYN and ACK flags of
the packet, verified by the option's length field. For the diagram the packet, verified by the option's length field. For the diagram
shown in Figure 4, "sender" and "receiver" refer to the sender or shown in Figure 4, "sender" and "receiver" refer to the sender or
receiver of the TCP packet (which can be either host). If the SYN receiver of the TCP packet (which can be either host). If the SYN
flag is set, a single key is included; if only an ACK flag is set, flag is set, a single key is included; if only an ACK flag is set,
both keys are present. both keys are present.
B's Key is echoed in the ACK in order to allow the listener (host B) B's Key is echoed in the ACK in order to allow the listener (host B)
to act statelessly until the TCP connection reaches the ESTABLISHED to act statelessly until the TCP connection reaches the ESTABLISHED
state. If the listener acts in this way, however, it MUST generate state. If the listener acts in this way, however, it MUST generate
its key in a verifiable fashion, allowing it to verify that it its key in a way that would allow it to verify that it generated the
generated the key when it is echoed in the ACK. key when it is echoed in the ACK.
This exchange allows the safe passage of MPTCP options on SYN packets This exchange allows the safe passage of MPTCP options on SYN packets
to be determined. If any of these options are dropped, MPTCP SHOULD to be determined. If any of these options are dropped, MPTCP will
gracefully fall back to regular single-path TCP, as documented in gracefully fall back to regular single-path TCP, as documented in
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).
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-------+---------------+
| Kind | Length |Subtype|Version|A|B|C|D|E|F|G|H|
+---------------+---------------+-------+-------+---------------+
| Option Sender's Key (64 bits) |
| |
| |
+---------------------------------------------------------------+
| Option Receiver's Key (64 bits) |
| (if option Length == 20) |
| |
+---------------------------------------------------------------+
Figure 4: Multipath Capable (MP_CAPABLE) option
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 - labelled The second octet is reserved for flags, allocated as follows:
C - SHOULD be set to 1 to indicate "Checksum required", unless the
system administrator has decided that checksums are not required (for
example, if the environment is controlled and no middleboxes exist
that may adjust the payload). The remaining bits are used for crypto
algorithm negotiation. Currently only the rightmost bit - labeled S
- is assigned, and indicates the use of HMAC-SHA1 (as defined in
Section 3.2). An implementation that only supports this method MUST
set this bit to 1 and all other currently reserved bits to zero. If
none of these flags are set, the MP_CAPABLE option MUST be treated as
invalid and ignored (i.e. it must be treated as a regular TCP
handshake).
These bits negotiate capabilities in similar ways. For the 'C' bit, A: The leftmost bit, labelled "A", SHOULD be set to 1 to indicate
if either host requires the use of checksums, checksums MUST be used. "Checksum Required", unless the system administrator has decided
In other words, the only way for checksums not to be used is if both that checksums are not required (for example, if the environment
hosts in their SYNs set C=0. This decision is confirmed by the is controlled and no middleboxes exist that might adjust the
setting of the 'C' bit in the third packet (the ACK) of the payload).
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 in both B: The second bit, labelled "B", is an extensibility flag, and MUST
directions, and the initiator will set C=1 in the ACK. The decision be set to 0 for current implementations. This will be used for an
whether to use checksums will be stored by an implementation in a extensibility mechanism in a future specification, and the impact
per-connection binary state variable. of this flag will be defined at a later date. If receiving a
message with the "B" flag set to 1, and this is not understood,
then this SYN MUST be silently ignored; the sender is expected to
retry with a format compatible with this legacy specification.
Note that the length of the MP_CAPABLE option, and the meanings of
bits "C" through "H", may be altered by setting B=1.
C through H: The remaining bits, labelled "C" through "H", are used
for crypto algorithm negotiation. Currently only the rightmost
bit, labelled "H", is assigned. Bit "H" indicates the use of
HMAC-SHA1 (as defined in Section 3.2). An implementation that
only supports this method MUST set bit "H" to 1, and bits "C"
through "G" to 0.
A crypto algorithm MUST be specified. If flag bits C through H are
all 0, the MP_CAPABLE option MUST be treated as invalid and ignored
(that is, it must be treated as a regular TCP handshake).
The selection of the authentication algorithm also impacts the
algorithm used to generate the token. In this specification, with
only the SHA-1 algorithm (bit "H") specified and selected, the token
MUST be a truncated (most significant 32 bits) SHA-1 hash [4]. A
different, 64 bit truncation (the least significant 64 bits) of the
hash of the key is RECOMMENDED to be used as the Initial Data
Sequence Number. Future specifications of the use of the crypto bits
may choose to specify different token generation algorithms.
Both the crypto and checksum bits negotiate capabilities in similar
ways. For the Checksum Required bit (labelled "A"), if either host
requires the use of checksums, checksums MUST be used. In other
words, the only way for checksums not to be used is if both hosts in
their SYNs set A=0. This decision is confirmed by the setting of the
"A" bit in the third packet (the ACK) of the handshake. For example,
if the initiator sets A=0 in the SYN, but the responder sets A=1 in
the SYN/ACK, checksums MUST be used in both directions, and the
initiator will set A=1 in the ACK. The decision whether to use
checksums will be stored by an implementation in a per-connection
binary state variable.
For crypto negotiation, the responder has the choice. The initiator 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 bit "H" will be always set to 1). The responder responds with only
set - this is the chosen algorithm. The rationale for this behaviour one bit set - this is the chosen algorithm. The rationale for this
is that the responder will typically be a server with potentially behaviour is that the responder will typically be a server with
many thousands of connections, so it may wish to choose an algorithm potentially many thousands of connections, so it may wish to choose
with minimal computational complexity, depending on the load. If a an algorithm with minimal computational complexity, depending on the
responder does not support (or does not want to support) any of the load. If a responder does not support (or does not want to support)
initiator's proposals, it can respond without an MP_CAPABLE option, any of the initiator's proposals, it can respond without an
thus forcing a fall-back to regular TCP. MP_CAPABLE option, thus forcing a fall-back to regular TCP.
The MP_CAPABLE option is only used in the first subflow of a The MP_CAPABLE option is only used in the first subflow of a
connection, in order to identify the connection; all following connection, in order to identify the connection; all following
subflows will use the "Join" option (see Section 3.2) to join the subflows will use the "Join" option (see Section 3.2) to join the
existing connection. existing connection.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-------+-+-----------+-+
| Kind | Length |Subtype|Version|C| (reservd) |S|
+---------------+---------------+-------+-------+-+-----------+-+
| Option Sender's Key (64 bits) |
| |
| |
+---------------------------------------------------------------+
| Option Receiver's Key (64 bits) |
| (if option Length == 20) |
| |
+---------------------------------------------------------------+
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. Note that an implementation MAY choose to attempt sending options. Note that an implementation MAY choose to attempt sending
MPTCP options more than one time before making this decision to MPTCP options more than one time before making this decision to
skipping to change at page 16, line 45 skipping to change at page 17, line 22
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) on a MPTCP connection MUST be
the Key, i.e. IDSN-A = Hash(Key-A) and IDSN-B = Hash(Key-B). The hard to guess. It is RECOMMENDED that the IDSN is generated as a
Hash mechanism here provides the least significant 64 bits of the hash from the Key, i.e. IDSN-A = Hash(Key-A) and IDSN-B =
SHA-1 hash of the key. The SYN with MP_CAPABLE occupies the first Hash(Key-B). A suggested algorithm for this version of the
octet of Data Sequence Space, although this does not need to be specification is to use the least significant 64 bits of the SHA-1
acknowledged at the connection level until the first data is sent hash of the key, however a receiver MUST NOT make any assumptions
(see Section 3.3). about the IDSN generation algorithm. The SYN with MP_CAPABLE
occupies the first octet of Data Sequence Space, although this does
not need to be acknowledged at the connection level until the first
data is sent (see Section 3.3).
3.2. Starting a New Subflow 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 34 skipping to change at page 18, line 15
This section specifies the behaviour of MP_JOIN using the HMAC-SHA1 This section specifies the behaviour of MP_JOIN using the HMAC-SHA1
algorithm. An MP_JOIN option is present in the SYN, SYN/ACK and ACK algorithm. An MP_JOIN option is present in the SYN, SYN/ACK and ACK
of the three-way handshake, although in each case with a different of the three-way handshake, although in each case with a different
format. format.
In the first MP_JOIN on the SYN packet, illustrated in Figure 5, the In the first MP_JOIN on the SYN packet, illustrated in Figure 5, the
initiator sends a token, random number, and address ID. initiator sends a token, random number, and address ID.
The token is used to identify the MPTCP connection and is a The token is used to identify the MPTCP connection and is a
cryptographic hash of the receiver's key, as exchanged in the initial cryptographic hash of the receiver's key, as exchanged in the initial
MP_CAPABLE handshake (Section 3.1). The tokens presented in this MP_CAPABLE handshake (Section 3.1). In this specification, the
option are generated by the SHA-1 [4] algorithm, truncated to the tokens presented in this option are generated by the SHA-1 [4]
most significant 32 bits. The token included in the MP_JOIN option algorithm, truncated to the most significant 32 bits. The token
is the token that the receiver of the packet uses to identify this included in the MP_JOIN option is the token that the receiver of the
connection, i.e. Host A will send Token-B (which is generated from packet uses to identify this connection, i.e. Host A will send
Key-B). Token-B (which is generated from Key-B). Note that the hash
generation algorithm can be overridden by the choice of cryptographic
handshake algorithm, as defined in Section 3.1.
The MP_JOIN SYN 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 IP header identifies the source address of this packet, even if the IP header
has been changed in transit by a middlebox. The Address ID allows has been changed in transit by a middlebox. The Address ID allows
address removal (Section 3.4.2) without needing to know what the address removal (Section 3.4.2) without needing to know what the
source address at the receiver is, thus allowing address removal source address at the receiver is, thus allowing address removal
through NATs. The Address ID also allows correlation between new through NATs. The Address ID also allows correlation between new
subflow setup attempts and address signaling (Section 3.4.1), to subflow setup attempts and address signaling (Section 3.4.1), to
prevent setting up duplicate subflows on the same path. prevent setting up duplicate subflows on the same path, if a MP_JOIN
and ADD_ADDR are sent at the same time.
The Address IDs of the subflow used in the initial SYN exchange of The 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 mappings between Address IDs and zero. A host MUST store the mappings between Address IDs and
addresses both for itself and the remote host. An implementation addresses both for itself and the remote host. An implementation
will also need to know which local and remote Address IDs are will also need to know which local and remote Address IDs are
associated with which established subflows, for when addresses are associated with which established subflows, for when addresses are
removed from a local or remote host. removed from a local or remote host.
The MP_JOIN option on packets with the SYN flag set also includes 4 The MP_JOIN option on packets with the SYN flag set also includes 4
skipping to change at page 18, line 40 skipping to change at page 19, line 24
| Receiver's Token (32 bits) | | Receiver's Token (32 bits) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Sender's Random Number (32 bits) | | Sender's Random Number (32 bits) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
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) Hash-based Message
(MAC). This version of the option is shown in Figure 6. If the Authentication Code (HMAC). This version of the option is shown in
token is unknown, or the host wants to refuse subflow establishment Figure 6. If the token is unknown, or the host wants to refuse
(for example, due to a limit on the number of subflows it will subflow establishment (for example, due to a limit on the number of
permit), the receiver will send back an RST, analogous to an unknown subflows it will permit), the receiver will send back an RST,
port in TCP. Although calculating a MAC requires cryptographic analogous to an unknown port in TCP. Although calculating an HMAC
operations, it is believed that the 32 bit token in the MP_JOIN SYN requires cryptographic operations, it is believed that the 32 bit
gives sufficient protection against blind state exhaustion attacks token in the MP_JOIN SYN gives sufficient protection against blind
and therefore there is no need to provide mechanisms to allow a state exhaustion attacks and therefore there is no need to provide
responder to operate statelessly at the MP_JOIN stage. mechanisms to allow a responder to operate statelessly at the MP_JOIN
stage.
An MAC is sent by both hosts - by the initiator (Host A) in the third An HMAC is sent by both hosts - by the initiator (Host A) in the
packet (the ACK) and by the responder (Host B) in the second packet third packet (the ACK) and by the responder (Host B) in the second
(the SYN/ACK). Doing the MAC exchange at this stage allow both hosts packet (the SYN/ACK). Doing the HMAC exchange at this stage allows
to have first exchanged random data (in the first two SYN packets) both hosts to have first exchanged random data (in the first two SYN
that is used as the "message". Both MACs are generated according to packets) that is used as the "message". This specification defines
HMAC as defined in [12], using the SHA-1 hash algorithm [4] (thus that HMAC as defined in [8] is used, along with the SHA-1 hash
generating a 160-bit / 20 octet HMAC). Due to option space algorithm [4] (thus generating a 160-bit / 20 octet HMAC). Due to
limitations, the MAC included in the SYN/ACK is truncated to the option space limitations, the HMAC included in the SYN/ACK is
leftmost 64 bits, but this is acceptable since an attacker only has truncated to the leftmost 64 bits, but this is acceptable since
once chance to guess the MAC correctly. random numbers are used, and thus an attacker only has one chance to
guess the HMAC correctly (if the HMAC is incorrect, the TCP
connection is closed, so a new MP_JOIN negotiation with a new random
number is required).
The initiator's authentication information is sent in its first ACK The initiator's authentication information is sent in its first ACK
(the third packet of the handshake), as shown in Figure 7. This data (the third packet of the handshake), as shown in Figure 7. This data
needs to be sent reliably, since it is the only time this MAC is sent needs to be sent reliably, since it is the only time this HMAC is
and therefore receipt of this packet MUST trigger a regular TCP ACK sent and therefore receipt of this packet MUST trigger a regular TCP
in response, and the packet MUST be retransmitted if this ACK is not ACK in response, and the packet MUST be retransmitted if this ACK is
received. In other words, sending the ACK/MP_JOIN packet places the not received. In other words, sending the ACK/MP_JOIN packet places
subflow in the PRE_ESTABLISHED state, and it moves to the ESTABLISHED the subflow in the PRE_ESTABLISHED state, and it moves to the
state only on receipt of an ACK from the receiver. It is not ESTABLISHED state only on receipt of an ACK from the receiver. It is
permitted to send data while in the PRE_ESTABLISHED state. The not permitted to send data while in the PRE_ESTABLISHED state. The
reserved bits in this option MUST be set 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 HMAC algorithm, in the case of the message
by Host A, will be Key-A followed by Key-B, and in the case of Host transmitted by Host A, will be Key-A followed by Key-B, and in the
B, Key-B followed by Key-A. These are the keys that were exchanged case of Host B, Key-B followed by Key-A. These are the keys that
in the original MP_CAPABLE handshake. The "message" for the MAC were exchanged in the original MP_CAPABLE handshake. The "message"
algorithm in each case is the concatenations of Random Number for for the HMAC algorithm in each case is the concatenations of Random
each host (denoted by R): for Host A, R-A followed by R-B; and for Number for each host (denoted by R): for Host A, R-A followed by R-B;
Host B, R-B followed by R-A. 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 HMAC (64 bits) |
| | | |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Sender's Random Number (32 bits) | | Sender's Random Number (32 bits) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
Figure 6: Join Connection (MP_JOIN) option (for responding SYN/ACK) Figure 6: Join Connection (MP_JOIN) option (for responding SYN/ACK)
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 = 24 |Subtype| (reserved) | | Kind | Length = 24 |Subtype| (reserved) |
+---------------+---------------+-------+-----------------------+ +---------------+---------------+-------+-----------------------+
| | | |
| | | |
| Sender's MAC (160 bits SHA-1) | | Sender's HMAC (160 bits) |
| | | |
| | | |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
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
skipping to change at page 20, line 37 skipping to change at page 21, line 21
|--------------------------------------------->| |--------------------------------------------->|
|<---------------------------------------------| |<---------------------------------------------|
| 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(HMAC-B, R-B) |
| | | | | |
| | ACK + MP_JOIN(MAC-A) | | | ACK + MP_JOIN(HMAC-A) |
| |------------------------------->| | |------------------------------->|
| |<-------------------------------| | |<-------------------------------|
| | ACK | | | ACK |
MAC-A = MAC(Key=(Key-A+Key-B), Msg=(R-A+R-B)) HMAC-A = HMAC(Key=(Key-A+Key-B), Msg=(R-A+R-B))
MAC-B = MAC(Key=(Key-B+Key-A), Msg=(R-B+R-A)) HMAC-B = HMAC(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.
If the token is accepted at Host B, but the MAC returned to Host A If the token is accepted at Host B, but the HMAC returned to Host A
does not match the one expected, Host A MUST close the subflow with a does not match the one expected, Host A MUST close the subflow with a
TCP RST. TCP RST.
If Host B does not receive the expected MAC, or the MP_JOIN option is If Host B does not receive the expected HMAC, or the MP_JOIN option
missing from the ACK, it MUST close the subflow with a TCP RST. is missing from the ACK, it MUST close the subflow with a TCP RST.
If the MACs are verified as correct, then both hosts have If the HMACs are verified as correct, then both hosts have
authenticated each other as being the same peers as existed at the authenticated each other as being the same peers as existed at the
start of the connection, and they have agreed of which connection start of the connection, and they have agreed of which connection
this subflow will become a part. this subflow will become a part.
If the SYN/ACK as received at Host A does not have an MP_JOIN option, If the SYN/ACK as received at Host A does not have an MP_JOIN option,
Host A MUST close the subflow with a RST. Host A MUST close the subflow with a RST.
This covers all cases of the loss of an MP_JOIN. In more detail, if This covers all cases of the loss of an MP_JOIN. In more detail, if
MP_JOIN is stripped from the SYN on the path from A to B, and Host B MP_JOIN is stripped from the SYN on the path from A to B, and Host B
does not have a passive opener on the relevant port, it will respond does not have a passive opener on the relevant port, it will respond
skipping to change at page 21, line 39 skipping to change at page 22, line 22
Note that additional subflows can be created between any pair of Note that additional subflows can be created between any pair of
ports (but see Section 3.8 for heuristics); no explicit application- ports (but see Section 3.8 for heuristics); no explicit application-
level accept calls or bind calls are required to open additional level accept calls or bind calls are required to open additional
subflows. To associate a new subflow with an existing connection, subflows. To associate a new subflow with an existing connection,
the token supplied in the subflow's SYN exchange is used for the token supplied in the subflow's SYN exchange is used for
demultiplexing. This then binds the 5-tuple of the TCP subflow to demultiplexing. This then binds the 5-tuple of the TCP subflow to
the local token of the connection. A consequence is that it is the local token of the connection. A consequence is that it is
possible to allow any port pairs to be used for a connection. possible to allow any port pairs to be used for a connection.
Deumultiplexing subflow SYNs MUST be done using the token; this is Demultiplexing subflow SYNs MUST be done using the token; this is
unlike traditional TCP, where the destination port is used for unlike traditional TCP, where the destination port is used for
demultiplexing SYN packets. Once a subflow is setup, demultiplexing demultiplexing SYN packets. Once a subflow is setup, demultiplexing
packets is done using the five-tuple, as in traditional TCP. The packets is done using the five-tuple, as in traditional TCP. The
five-tuples will be mapped to the local connection identifier five-tuples will be mapped to the local connection identifier
(token). Note that Host A will know its local token for the subflow (token). Note that Host A will know its local token for the subflow
even though it is not sent on the wire - only the responder's token even though it is not sent on the wire - only the responder's token
is sent. is sent.
3.3. General MPTCP Operation 3.3. General MPTCP Operation
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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. A
purpose of the DATA_FIN, along with the interactions between this connection is not closed unless there has been a DATA_FIN exchange,
flag, the subflow-level FIN flag, and the data sequence mapping are or a timeout. The purpose of the DATA_FIN, along with the
described in Section 3.3.3. The remaining reserved bits MUST be set interactions between this flag, the subflow-level FIN flag, and the
to zero by an implementation of this specification. data sequence mapping are described in Section 3.3.3. The remaining
reserved bits MUST be set to zero by an implementation of this
specification.
Note that the Checksum is only present in this option if the use of 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. If a checksum is present, but its use had 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 not been negotiated in the MP_CAPABLE handshake, the checksum field
ignored. If a checksum is not present when its use has been MUST 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 negotiated, the receiver SHOULD close the subflow with a RST as it is
considered broken. 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
mandated) that SACK [8] is used at the subflow level to improve mandated) that SACK [9] is used at the subflow level to improve
efficiency. efficiency.
The Data Sequence Mapping specifies a mapping from subflow sequence The Data Sequence Mapping specifies a mapping from subflow sequence
space to data sequence space. This is expressed in terms of starting space to data sequence space. This is expressed in terms of starting
sequence numbers for the subflow and the data level, and a length of sequence numbers for the subflow and the data level, and a length of
bytes for which this mapping is valid. This explicit mapping for a bytes for which this mapping is valid. This explicit mapping for a
range of data was chosen rather than per-packet signaling to assist range of data was chosen rather than per-packet signaling to assist
with compatibility with situations where TCP/IP segmentation or with compatibility with situations where TCP/IP segmentation or
coalescing is undertaken separately from the stack that is generating coalescing is undertaken separately from the stack that is generating
the data flow (e.g. through the use of TCP segmentation offloading on the data flow (e.g. through the use of TCP segmentation offloading on
skipping to change at page 24, line 6 skipping to change at page 24, line 41
the data sequence number after the mapping has been processed. A the data sequence number after the mapping has been processed. A
sender MUST NOT change this mapping after it has been declared; sender MUST NOT change this mapping after it has been declared;
however, the same data sequence number can be mapped to by different however, the same data sequence number can be mapped to by different
subflows for retransmission purposes (see Section 3.3.6). This would subflows for retransmission purposes (see Section 3.3.6). This would
also permit the same data to be sent simultaneously on multiple also permit the same data to be sent simultaneously on multiple
subflows for resilience or efficiency purposes, especially in the subflows for resilience or efficiency purposes, especially in the
case of lossy links. Although the detailed specification of such case of lossy links. Although the detailed specification of such
operation is outside the scope of this document, an implementation operation is outside the scope of this document, an implementation
SHOULD treat the first data that is received at a subflow for the SHOULD treat the first data that is received at a subflow for the
data sequence space as that which should be delivered to the data sequence space as that which should be delivered to the
application. application, and any later data for that sequence space ignored.
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, if use of checksums has been neogiated at the this mapping covers, if use of checksums has been negotiated at the
MP_CAPABLE exchange. Checksums are used to detect if the payload has MP_CAPABLE exchange. Checksums are used to detect if the payload has
been adjusted in any way by a non-MPTCP-aware middlebox. If this been adjusted in any way by a non-MPTCP-aware middlebox. If this
checksum fails, it will trigger a failure of the subflow, or a checksum fails, it will trigger a failure of the subflow, or a
fallback to regular TCP, as documented in Section 3.6, since MPTCP fallback to regular TCP, as documented in Section 3.6, since MPTCP
can no longer reliably know the subflow sequence space at the can no longer reliably know the subflow sequence space at the
receiver to build data sequence mappings. 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.
skipping to change at page 25, line 50 skipping to change at page 26, line 37
declared with flags in the DSS option. Implementations MUST accept a declared with flags in the DSS option. Implementations MUST accept a
32 bit DSN and implicitly promote it to a 64 bit quantity by 32 bit DSN and implicitly promote it to a 64 bit quantity by
incrementing the upper 32 bits of sequence number each time the lower incrementing the upper 32 bits of sequence number each time the lower
32 bits wrap. A sanity check MUST be implemented to ensure that a 32 bits wrap. A sanity check MUST be implemented to ensure that a
wrap occurs at an expected time (e.g. the sequence number jumps from wrap occurs at an expected time (e.g. the sequence number jumps from
a very high number to a very low number) and is not triggered by out- a very high number to a very low number) and is not triggered by out-
of-order packets. of-order packets.
As with the standard TCP sequence number, the data sequence number As with the standard TCP sequence number, the data sequence number
should not start at zero, but at a random value to make blind session should not start at zero, but at a random value to make blind session
hijacking harder. This is done by setting the initial data sequence hijacking harder. This specification suggests setting the initial
number (IDSN) of each host to the least significant 64 bits of the data sequence number (IDSN) of each host to the least significant 64
SHA-1 hash of the host's key, as described in Section 3.1. bits of the SHA-1 hash of the host's key, as described in
Section 3.1.
A Data Sequence Mapping does not need to be included in every MPTCP A Data Sequence Mapping does not need to be included in every MPTCP
packet, as long as the subflow sequence space in that packet is packet, as long as the subflow sequence space in that packet is
covered by a mapping known at the receiver. This can be used to covered by a mapping known at the receiver. This can be used to
reduce overhead in cases where the mapping is known in advance; one reduce overhead in cases where the mapping is known in advance; one
such case is when there is a single subflow between the 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 of the DSS option to the reserved setting the Data-level Length field of the DSS option to the reserved
value of 0. The checksum, in such a case, will also be set to 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 Acknowledgments
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
may still be holes in the data stream at the connection level. The may still be holes in the data stream at the connection level. The
Data ACK specifies the next Data Sequence Number it expects to Data ACK specifies the next Data Sequence Number it expects to
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a 32 bit or 64 bit Data ACK. a 32 bit or 64 bit Data ACK.
The Data ACK proves that the data, and all required MPTCP signaling, The Data ACK proves that the data, and all required MPTCP signaling,
has been received and accepted by the remote end. One key use of the has been received and accepted by the remote end. One key use of the
Data ACK signal is that it is used to indicate the left edge of the Data ACK signal is that it is used to indicate the left edge of the
advertised receive window. As explained in Section 3.3.4, the 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 acknowledgments 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 NOT free data from the send buffer until 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 all 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 retransmitted. 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
ACK is present in segments only when the Data ACK value advances, and ACK is present in segments only when the Data ACK value advances, and
this behaviour MUST be treated as valid. This behaviour ensures the this behaviour MUST be treated as valid. This behaviour ensures the
sender buffer is freed, while reducing overhead when the data sender buffer is freed, while reducing overhead when the data
skipping to change at page 28, line 13 skipping to change at page 28, line 50
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 Signal MUST have Subflow Sequence containing data, the Data Sequence Signal MUST have Subflow Sequence
Number of 0, a Data-Level Length of 1, and the Data Sequence Number Number of 0, a Data-Level Length of 1, and the Data Sequence Number
that corresponds with the DATA_FIN itself. The checksum in this case that corresponds with the DATA_FIN itself. The checksum in this case
will 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 permissible to combine these signals on one subflow if there is
no data oustanding on other subflows. Otherwise, it may be necessary no data outstanding on other subflows. Otherwise, it may be
to retransmit data on different subflows. Essentially, a host MUST necessary to retransmit data on different subflows. Essentially, a
NOT close all functioning subflows unless it is safe to do so, i.e. host MUST NOT close all functioning subflows unless it is safe to do
until all outstanding data has been DATA_ACKed, or that the segment so, i.e. until all outstanding data has been DATA_ACKed, or that the
with the DATA_FIN flag set is the only outstanding segment. 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 on closed with standard FIN exchanges. Both hosts SHOULD send FINs on
all subflows, as a courtesy to allow middleboxes to clean up state all subflows, as a courtesy to allow middleboxes to clean up state
even if an individual subflow has failed. It is also encouraged to even if an individual subflow has failed. It is also encouraged to
reduce the timeouts (Maximum Segment Life) on subflows at end hosts. reduce the timeouts (Maximum Segment Life) on subflows at end hosts.
In particular, any subflows where there is still outstanding data In particular, any subflows where there is still outstanding data
queued (which has been retransmitted on other subflows in order to queued (which has been retransmitted on other subflows in order to
get the DATA_FIN acknowledged) MAY be closed with an RST. get the DATA_FIN acknowledged) MAY be closed with an RST.
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subflow sequence number of the received packet and SUBFLOW_ACK is the subflow sequence number of the received packet and SUBFLOW_ACK is the
RCV.NXT (next expected sequence number) of the subflow (with the RCV.NXT (next expected sequence number) of the subflow (with the
equivalent 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. successfully 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 one of the paths. is the maximum bandwidth-delay product of any one of the paths.
However this might be insufficient when a packet is lost on a slower However this might be insufficient when a packet is lost on a slower
subflow and needs to be retransmitted (see Section 3.3.6). A tight subflow and needs to be retransmitted (see Section 3.3.6). A tight
upper bound would be the maximum RTT of any path multiplied by the upper bound would be the maximum RTT of any path multiplied by the
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insufficient to maintain full performance in the event of a insufficient to maintain full performance in the event of a
retransmit timeout on the maximum RTT path. It is for future study retransmit timeout on the maximum RTT path. It is for future study
to determine the relationship between retransmission strategies and to determine the relationship between retransmission strategies and
receive buffer sizing. 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, on the receipt of a DATA_ACK. This is important to allow
RTTs, and thus different feedback loops. using paths with different 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
receive window was guaranteed to be unchanged end-to-end, a host receive window was guaranteed to be unchanged end-to-end, a host
could always read the most recent receive window value. However, could always read the most recent receive window value. However,
some classes of middleboxes may alter the TCP-level receive window. some classes of middleboxes may alter the TCP-level receive window.
Typically these will shrink the offered window, although for short Typically these will shrink the offered window, although for short
periods of time it may be possible for the window to be larger periods of time it may be possible for the window to be larger
(however note that this would not continue for long periods since (however note that this would not continue for long periods since
ultimately the middlebox must keep up with delivering data to the ultimately the middlebox must keep up with delivering data to the
receiver). Therefore, if receive window sizes differ on multiple receiver). Therefore, if receive window sizes differ on multiple
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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 HMAC 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 ensure that this address in order, to the other end. This would ensure that this address
management does not unnecessarily cause an outage in the connection management does not unnecessarily cause an outage in the connection
when remove/add addresses are processed in reverse order, and also to when remove/add addresses are processed in reverse order, and also to
ensure that all possible paths are used. Note, however, that losing ensure that all possible paths are used. Note, however, that losing
reliability and ordering will not break the multipath connections, it reliability and ordering will not break the multipath connections, it
will just reduce the opportunity to open multipath paths and to will just reduce the opportunity to open multipath 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 A host can send an ADD_ADDR message with an already assigned Address
for a live subflow within the connection, the receiver SHOULD ID, but the Address MUST be the same as previously assigned to this
silently ignore the ADD_ADDR. A host wishing to replace an existing Address ID, and the Port MUST be different to one already in use for
Address ID MUST first remove the existing one (Section 3.4.2). this Address ID. If these conditions are not met, the receiver
SHOULD silently ignore the ADD_ADDR. A host wishing to replace an
existing Address ID MUST first remove the existing one
(Section 3.4.2).
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,
with the exception of the DSS option, as indications of congestion with the exception of the DSS option, as indications of congestion
[9], and an MPTCP implementation SHOULD NOT send more than two [10], and an MPTCP implementation SHOULD NOT send more than two
duplicate ACKs in a row 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 SHOULD NOT be removed. Typical TCP validity tests received the path SHOULD NOT be removed. Typical TCP validity tests
on the subflow (e.g. ensuring sequence and ack numbers are correct) on the subflow (e.g. ensuring sequence and ack numbers are correct)
MUST also be undertaken. MUST also be undertaken. An implementation can use indications of
these test failures as part of intrusion detection or error logging.
The sending and receipt (if no keepalive response was received) of 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, rather than using this option. For more exchange as in regular TCP, rather than using this option. For more
information, see Section 3.3.3. 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|(resvd)| Address ID | ...
+---------------+---------------+-------+-------+---------------+ +---------------+---------------+-------+-------+---------------+
(followed by n-1 Address IDs, if required)
Figure 13: Remove Address (REMOVE_ADDR) option Figure 13: Remove Address (REMOVE_ADDR) option
3.5. Fast Close 3.5. Fast Close
Regular TCP has the means of sending a reset signal (RST) to abruptly Regular TCP has the means of sending a reset signal (RST) to abruptly
close a connection. With MPTCP, the RST only has the scope of the close a connection. With MPTCP, the RST only has the scope of the
subflow and will only close the concerned subflow but not affect the subflow and will only close the concerned subflow but not affect the
remaining subflows. MPTCP's connection will stay alive at the data- remaining subflows. MPTCP's connection will stay alive at the data-
level, in order to permit break-before-make handover between level, in order to permit break-before-make handover between
skipping to change at page 39, line 25 skipping to change at page 40, line 24
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. A RECOMMENDED number is 3.
3.6. Fallback 3.6. Fallback
Sometimes, middleboxes will exist on a path that could prevent the Sometimes, middleboxes will exist on a path that could prevent the
operation of MPTCP. MPTCP has been designed in order to cope with operation of MPTCP. MPTCP has been designed in order to cope with
many middlebox modifications (see Section 6), but there are still many middlebox modifications (see Section 6), but there are still
some cases where a subflow could fail to operate within the MPTCP some cases where a subflow could fail to operate within the MPTCP
requirements. These cases are notably: the loss of TCP options on a requirements. These cases are notably: the loss of TCP options on a
path; and the modification of payload data. If such an event occurs, path; and the modification of payload data. If such an event occurs,
it is necessary to "fall back" to the previous, safe operation. This it is necessary to "fall back" to the previous, safe operation. This
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In addition to the fallback mechanism as described above, the In addition to the fallback mechanism as described above, the
standard classes of TCP errors may need to be handled in an MPTCP- standard classes of TCP errors may need to be handled in an MPTCP-
specific way. Note that changing semantics - such as the relevance specific way. Note that changing semantics - such as the relevance
of an RST - are covered in Section 4. Where possible, we do not want of an RST - are covered in Section 4. Where possible, we do not want
to deviate from regular TCP behaviour. to deviate from regular TCP behaviour.
The following list covers possible errors and the appropriate MPTCP The following list covers possible errors and the appropriate MPTCP
behaviour: behaviour:
o Unknown token in MP_JOIN (or MAC failure in MP_JOIN ACK, or o Unknown token in MP_JOIN (or HMAC failure in MP_JOIN ACK, or
missing MP_JOIN in SYN/ACK response): send RST (analogous to TCP's missing MP_JOIN in SYN/ACK response): send RST (analogous to TCP's
behaviour on an unknown port) behaviour on an unknown port)
o DSN out of Window (during normal operation): drop the data, do not o DSN out of Window (during normal operation): drop the data, do not
send Data ACKs. send Data ACKs.
o Remove request for unknown address ID: silently ignore o Remove request for unknown address ID: silently ignore
3.8. Heuristics 3.8. Heuristics
skipping to change at page 47, line 19 skipping to change at page 48, line 16
address before using it as part of a connection. address before using it as part of a connection.
o Provide replay protection, i.e. ensure that a request to add/ o Provide replay protection, i.e. ensure that a request to add/
remove a subflow is 'fresh'. remove a subflow is 'fresh'.
In order to achieve these goals, MPTCP includes a hash-based In order to achieve these goals, MPTCP includes a hash-based
handshake algorithm documented in Section 3.1 and Section 3.2. handshake algorithm documented in Section 3.1 and Section 3.2.
The security of the MPTCP connection hangs on the use of keys that The security of the MPTCP connection hangs on the use of keys that
are shared once at the start of the first subflow, and 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 (unless used in the fast close mechanism,
away any cryptographic material, future subflows use a truncated Section 3.5). To ease demultiplexing whilst not giving away any
SHA-1 hash of this key as the connection identification "token". The cryptographic material, future subflows use a truncated cryptographic
keys are concatenated and used as keys for creating Message hash of this key as the connection identification "token". The keys
Authentication Codes (MAC) used on subflow setup, in order to verify are concatenated and used as keys for creating Hash-based Message
Authentication Codes (HMAC) used on subflow setup, in order to verify
that the parties in the handshake are the same as in the original that the parties in the handshake are the same as in the original
connection setup. It also provides verification that the peer can connection setup. It also provides verification that the peer can
receive traffic at this new address. Replay attacks would still be receive traffic at this new address. Replay attacks would still be
possible when only keys are used, and therefore the handshakes use possible when only keys are used, and therefore the handshakes use
single-use random numbers (nonces) at both ends - this ensures the single-use random numbers (nonces) at both ends - this ensures the
MAC will never be the same on two handshakes. HMAC will never be the same on two handshakes. Guidance on
generating random numbers suitable for use as keys is given in [12]
and discussed in Section 3.1.
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 allows the deployment of to negotiate use of a particular algorithm allows the deployment of
additional crypto mechanisms in the future. Note that this would be additional crypto mechanisms in the future. Note that this would be
susceptible to bid-down attacks only if the attacker was on-path (and susceptible to bid-down attacks only if the attacker was on-path (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].
During normal operation, regular TCP protection mechanisms (such as
ensuring sequence numbers are in-window) will provide the same level
of protection against attacks on indivudal TCP subflows as exists for
regular TCP today. Implementations will introduce additional buffers
compared to regular TCP, to reassemble data at the connection level.
The application of window sizing will minimize the risk of denial-of-
service attacks consuming resources.
A small security risk could theoretically exist with key reuse, but
in order to accomplish a replay attack, both the sender and receiver
keys, and the sender and receiver random numbers, in the MP_JOIN
handshake (Section 3.2 would have to match.
Whilst this specification defines a "medium" security solution,
meeting the criteria specified at the start of this section and the
threat analyis ([7]), since attacks only ever get worse, it is likely
that a future standards-track version of MPTCP would need to be able
to support stronger security. There are several ways the security of
MPTCP could potentially be improved; some of these would be
compatible with MPTCP as defined in this document, whilst others may
not be. For now, the best approach is to get experience with the
current approach, establish what might work and check that the threat
analysis is still accurate.
Possible ways of improving MPTCP security could include:
o defining a new MPCTP cryptographic algorithm, as negotiated in
MP_CAPABLE. A sub-case could be to include an additional
deployment assumption, such as stateful servers, in order to allow
a more powerful algorithm to be used.
o defining how to secure data transfer with MPTCP, whilst not
changing the signalling part of the protocol.
o defining security that requires more option space, perhaps in
conjunction with a "long options" proposal for extending the TCP
options space (such as those surveyed in [17]), or perhaps
building on the current approach with a second stage of MPTCP-
option-based security.
o re-visiting the working group's decision to exclusively use TCP
options for MPTCP signalling, and instead look at also making use
of the TCP payloads.
MPTCP has been designed with several methods available to indicate a
new security mechanism, including:
o available flags in MP_CAPABLE (Figure 4);
o available subtypes in the MPTCP Option Figure 3);
o the version field in MP_CAPABLE (Figure 4);
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 accommodate the different middleboxes. to accommodate the different middleboxes.
A primary concern is our use of a new TCP option. Middleboxes should A primary concern is our use of a new TCP option. Middleboxes should
skipping to change at page 48, line 21 skipping to change at page 50, line 26
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
MP_CAPABLE option (Section 3.1). If this is dropped, MPTCP SHOULD MP_CAPABLE option (Section 3.1). If this is dropped, MPTCP SHOULD
fall back to regular TCP. If packets with the MP_JOIN option fall back to regular TCP. If packets with the MP_JOIN option
(Section 3.2) are dropped, the paths will simply not be used. (Section 3.2) are dropped, the paths will simply not be used.
If a middlebox strips options but otherwise passes the packets If a middlebox strips options but otherwise passes the packets
unchanged, MPTCP will behave safely. If an MP_CAPABLE option is unchanged, MPTCP will behave safely. If an MP_CAPABLE option is
dropped on either the outgoing or the return path, the initiating dropped on either the outgoing or the return path, the initiating
host can fall back to regular TCP, as illustred in Figure 16 and host can fall back to regular TCP, as illustrated in Figure 16 and
discussed in Section 3.1. discussed in Section 3.1.
Subflow SYNs contain the MP_JOIN option. If this option is stripped Subflow SYNs contain the MP_JOIN option. If this option is stripped
on the outgoing path the SYN will appear to be a regular SYN to host on the outgoing path the SYN will appear to be a regular SYN to host
B. Depending on whether there is a listening socket on the target B. Depending on whether there is a listening socket on the target
port, host B will reply either with SYN/ACK or RST (subflow port, host B will reply either with SYN/ACK or RST (subflow
connection fails). When host A receives the SYN/ACK it sends a RST connection fails). When host A receives the SYN/ACK it sends a RST
because the SYN/ACK does not contain the MP_JOIN option and its because the SYN/ACK does not contain the MP_JOIN option and its
token. Either way, the subflow setup fails, but otherwise does not token. Either way, the subflow setup fails, but otherwise does not
affect the MPTCP connection as a whole. affect the MPTCP connection as a whole.
skipping to change at page 49, line 30 skipping to change at page 51, line 50
of some Data ACKs, but performance will degrade as the fraction of of some Data ACKs, but performance will degrade as the fraction of
stripped options increases. We do not expect such cases to appear in stripped options increases. We do not expect such cases to appear in
practice, though: most middleboxes will either strip all options or practice, though: most middleboxes will either strip all options or
let them 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 [18] (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 Address ID 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) [19] 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, or close the problematic therefore fall back to single-path TCP, or close the problematic
subflow (see Section 3.6). subflow (see Section 3.6).
o Traffic Normalizers [19] may not allow holes in sequence numbers, o Traffic Normalizers [20] 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. In the event of a data on the same subflow sequence number. In the event of a
retransmission, he same data will be retransmitted on the original retransmission, the same data will be retransmitted on the
TCP subflow even if it is additionally retransmitted at the original TCP subflow even if it is additionally retransmitted at
connection-level on a different subflow. the connection-level on a different subflow.
o Firewalls [20] might perform initial sequence number randomization o Firewalls [21] 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
content that could threaten a network. Multipath will mean that content that could threaten a network. Multipath will mean that
such data is potentially spread, so it is more difficult for an such data is potentially spread, so it is more difficult for an
skipping to change at page 51, line 5 skipping to change at page 53, line 22
copy options between packets and might strip some options. copy options between packets and might strip some options.
MPTCP's data sequence mapping includes the relative subflow MPTCP's data sequence mapping includes the relative subflow
sequence number instead of using the sequence number in the sequence number instead of using the sequence number in the
segment. In this way, the mapping is independent of the packets segment. In this way, the mapping is independent of the packets
that carry it. that carry it.
o The Receive Window may be shrunk by some middleboxes at the o The Receive Window may be shrunk by some middleboxes at the
subflow level. MPTCP will use the maximum window at data-level, subflow level. MPTCP will use the maximum window at data-level,
but will also obey subflow specific windows. but will also obey subflow specific windows.
7. Acknowledgements 7. Acknowledgments
The authors were originally supported by Trilogy The authors were originally supported by Trilogy
(http://www.trilogy-project.org), a research project (ICT-216372) (http://www.trilogy-project.org), a research project (ICT-216372)
partially funded by the European Community under its Seventh partially funded by the European Community under its Seventh
Framework Program. The views expressed here are those of the Framework Program.
author(s) only. The European Commission is not liable for any use
that may be made of the information in this document.
Alan Ford was originally supported by Roke Manor Research. Alan Ford was originally supported by Roke Manor Research.
The authors gratefully acknowledge significant input into this The authors gratefully acknowledge significant input into this
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,
skipping to change at page 51, line 42 skipping to change at page 54, line 15
+------+--------+---------------+-----------------+ +------+--------+---------------+-----------------+
| Kind | Length | Meaning | Reference | | Kind | Length | Meaning | Reference |
+------+--------+---------------+-----------------+ +------+--------+---------------+-----------------+
| 30 | N | Multipath TCP | (This document) | | 30 | N | Multipath TCP | (This document) |
+------+--------+---------------+-----------------+ +------+--------+---------------+-----------------+
Table 1: TCP Option Kind Numbers Table 1: TCP Option Kind Numbers
This document also defines a four-bit subtype field, for which IANA This document also defines a four-bit subtype field, for which IANA
is to create and maintain a new sub-registry entitled "MPTCP option is to create and maintain a new sub-registry entitled "MPTCP option
subtype values" under the MPTCP option. Initial values for the MPTCP subtype values" under the TCP Parameters registry. Initial values
option subtype registry are given below; future assignments are to be for the MPTCP option subtype registry are given below; future
defined by RFCs (RFC Requirement as defined by [21]) Assignments assignments are to be defined by Standards Action as defined by [22].
consist of the MPTCP subtype's symbolic name and its associated Assignments consist of the MPTCP subtype's symbolic name and its
value, as per the following table. associated value, as per the following table.
+--------------+----------------------------+---------------+-------+ +--------------+----------------------------+---------------+-------+
| Symbol | Name | Reference | 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 2: MPTCP Option Subtypes Table 2: MPTCP Option Subtypes
The value 0xf is reserved for Private Use. The value 0xf is reserved for Private Use within controlled testbeds.
This document also requests that IANA keeps a registry of "MPTCP This document also requests that IANA creates another sub-registry,
cryptographic handshake algorithms" based on the flags in MP_CAPABLE "MPTCP handshake algorithms" under the TCP Paramers registry, based
(Section 3.1). This document specifies only one algorithm: on the flags in MP_CAPABLE (Section 3.1). The flags consist of eight
bits, labelled "A" through "H", and this document assigns the bits as
follows, where "(available)" means that the bit is available for
future assignment:
+-------+----------------+----------------------------+ +----------+-------------------+----------------------------+
| Flags | Algorithm Name | Reference | | Flag Bit | Meaning | Reference |
+-------+----------------+----------------------------+ +----------+-------------------+----------------------------+
| 0x1 | HMAC-SHA1 | This document, Section 3.2 | | A | Checksum required | This document, Section 3.1 |
+-------+----------------+----------------------------+ | B | Extensibility | This document, Section 3.1 |
| C | (available) | |
| D | (available) | |
| E | (available) | |
| F | (available) | |
| G | (available) | |
| H | HMAC-SHA1 | This document, Section 3.2 |
+----------+-------------------+----------------------------+
Table 3: MPTCP Handshake Algorithms Table 3: MPTCP Handshake Algorithms
Future assignments in this registry are also to be defined by RFCs Note that the meanings of bits C through H can be dependent upon bit
(RFC Requirement as defined by [21]) Assignments consist of the value B, depending on how Extensibility is defined in future
of the flags, a symbolic name for the algorithm, and a reference to specifications; see Section 3.1 for more information.
its specification.
Note that the length of this field is not fixed; it is a definition Future assignments in this registry are also to be defined by
of the meaning of each bit in this field (i.e. 0x2, 0x4, 0x8, 0x10, Standards Action as defined by [22]. Assignments consist of the
etc). Future specifications may define additional flags using the value of the flags, a symbolic name for the algorithm, and a
leftmost bits of this field, and therefore the number of bits reference to its specification.
available for cryptographic negotiation may change.
9. References 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
skipping to change at page 53, line 32 skipping to change at page 56, line 12
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-05 (work in progress), Considerations", draft-ietf-mptcp-api-05 (work in progress),
April 2012. 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] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[9] 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 E. Blanton, "TCP Congestion [10] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009. Control", RFC 5681, September 2009.
[10] Gont, F., "Survey of Security Hardening Methods for [11] Gont, F., "Survey of Security Hardening Methods for
Transmission Control Protocol (TCP) Implementations", Transmission Control Protocol (TCP) Implementations",
draft-ietf-tcpm-tcp-security-03 (work in progress), March 2012. draft-ietf-tcpm-tcp-security-03 (work in progress), March 2012.
[11] Eastlake, D., Schiller, J., and S. Crocker, "Randomness [12] 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
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.
[14] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of [14] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168, Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001. September 2001.
[15] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. [15] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E.
Lear, "Address Allocation for Private Internets", BCP 5, Lear, "Address Allocation for Private Internets", BCP 5,
RFC 1918, February 1996. RFC 1918, February 1996.
[16] Braden, R., "Requirements for Internet Hosts - Communication [16] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989. Layers", STD 3, RFC 1122, October 1989.
[17] Srisuresh, P. and K. Egevang, "Traditional IP Network Address [17] Ramaiah, A., "TCP option space extension",
draft-ananth-tcpm-tcpoptext-00 (work in progress), March 2012.
[18] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001. Translator (Traditional NAT)", RFC 3022, January 2001.
[18] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z. [19] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to Mitigate Shelby, "Performance Enhancing Proxies Intended to Mitigate
Link-Related Degradations", RFC 3135, June 2001. Link-Related Degradations", RFC 3135, June 2001.
[19] Handley, M., Paxson, V., and C. Kreibich, "Network Intrusion [20] 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 [21] 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 [22] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 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.
skipping to change at page 55, line 34 skipping to change at page 58, line 16
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 (10 bytes). 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 bytes). 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 necessary to reduce the number of SACK blocks to accommodate the
DATA_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
skipping to change at page 59, line 24 skipping to change at page 61, line 32
| 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 delete MPTCP PCB 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
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