draft-ietf-mptcp-multiaddressed-03.txt   draft-ietf-mptcp-multiaddressed-04.txt 
Internet Engineering Task Force A. Ford Internet Engineering Task Force A. Ford
Internet-Draft Roke Manor Research Internet-Draft Roke Manor Research
Intended status: Experimental C. Raiciu Intended status: Experimental C. Raiciu
Expires: September 15, 2011 M. Handley Expires: January 12, 2012 M. Handley
University College London University College London
O. Bonaventure O. Bonaventure
Universite catholique de Universite catholique de
Louvain Louvain
March 14, 2011 July 11, 2011
TCP Extensions for Multipath Operation with Multiple Addresses TCP Extensions for Multipath Operation with Multiple Addresses
draft-ietf-mptcp-multiaddressed-03 draft-ietf-mptcp-multiaddressed-04
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.
skipping to change at page 1, line 47 skipping to change at page 1, line 47
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 15, 2011. This Internet-Draft will expire on January 12, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 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
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Design Assumptions . . . . . . . . . . . . . . . . . . . . 4 1.1. Design Assumptions . . . . . . . . . . . . . . . . . . . . 4
1.2. Multipath TCP in the Networking Stack . . . . . . . . . . 5 1.2. Multipath TCP in the Networking Stack . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. MPTCP Concept . . . . . . . . . . . . . . . . . . . . . . 6 1.4. MPTCP Concept . . . . . . . . . . . . . . . . . . . . . . 6
1.5. Requirements Language . . . . . . . . . . . . . . . . . . 7 1.5. Requirements Language . . . . . . . . . . . . . . . . . . 7
2. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 8 2. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 8
3. MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1. Initiating an MPTCP connection . . . . . . . . . . . . . . 8
3.1. Connection Initiation . . . . . . . . . . . . . . . . . . 10 2.2. Associating a new subflow with an existing MPTCP
3.2. Starting a New Subflow . . . . . . . . . . . . . . . . . . 14 connection . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. General MPTCP Operation . . . . . . . . . . . . . . . . . 18 2.3. Informing the other Host about another potential
3.3.1. Data Sequence Mapping . . . . . . . . . . . . . . . . 20 address . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3.2. Data Acknowledgements . . . . . . . . . . . . . . . . 23 2.4. Data transfer using MPTCP . . . . . . . . . . . . . . . . 10
3.3.3. Closing a Connection . . . . . . . . . . . . . . . . . 24 2.5. Requesting a change in a path's priority . . . . . . . . . 11
3.3.4. Receiver Considerations . . . . . . . . . . . . . . . 25 2.6. Closing an MPTCP connection . . . . . . . . . . . . . . . 11
3.3.5. Sender Considerations . . . . . . . . . . . . . . . . 26 2.7. Notable features . . . . . . . . . . . . . . . . . . . . . 11
3.3.6. Reliability and Retransmissions . . . . . . . . . . . 27 3. MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3.7. Congestion Control Considerations . . . . . . . . . . 28 3.1. Connection Initiation . . . . . . . . . . . . . . . . . . 13
3.3.8. Subflow Policy . . . . . . . . . . . . . . . . . . . . 28 3.2. Starting a New Subflow . . . . . . . . . . . . . . . . . . 16
3.4. Address Knowledge Exchange (Path Management) . . . . . . . 30 3.3. General MPTCP Operation . . . . . . . . . . . . . . . . . 21
3.4.1. Address Advertisement . . . . . . . . . . . . . . . . 31 3.3.1. Data Sequence Mapping . . . . . . . . . . . . . . . . 22
3.4.2. Remove Address . . . . . . . . . . . . . . . . . . . . 33 3.3.2. Data Acknowledgements . . . . . . . . . . . . . . . . 25
3.5. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.3.3. Closing a Connection . . . . . . . . . . . . . . . . . 27
3.6. Error Handling . . . . . . . . . . . . . . . . . . . . . . 37 3.3.4. Receiver Considerations . . . . . . . . . . . . . . . 28
3.7. Heuristics . . . . . . . . . . . . . . . . . . . . . . . . 37 3.3.5. Sender Considerations . . . . . . . . . . . . . . . . 29
3.7.1. Port Usage . . . . . . . . . . . . . . . . . . . . . . 38 3.3.6. Reliability and Retransmissions . . . . . . . . . . . 30
3.7.2. Delayed Subflow Start . . . . . . . . . . . . . . . . 38 3.3.7. Congestion Control Considerations . . . . . . . . . . 31
3.7.3. Failure Handling . . . . . . . . . . . . . . . . . . . 39 3.3.8. Subflow Policy . . . . . . . . . . . . . . . . . . . . 31
4. Semantic Issues . . . . . . . . . . . . . . . . . . . . . . . 39 3.4. Address Knowledge Exchange (Path Management) . . . . . . . 33
5. Security Considerations . . . . . . . . . . . . . . . . . . . 41 3.4.1. Address Advertisement . . . . . . . . . . . . . . . . 34
6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 42 3.4.2. Remove Address . . . . . . . . . . . . . . . . . . . . 36
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 45 3.5. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 37
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45 3.6. Error Handling . . . . . . . . . . . . . . . . . . . . . . 40
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.7. Heuristics . . . . . . . . . . . . . . . . . . . . . . . . 41
9.1. Normative References . . . . . . . . . . . . . . . . . . . 46 3.7.1. Port Usage . . . . . . . . . . . . . . . . . . . . . . 41
9.2. Informative References . . . . . . . . . . . . . . . . . . 46 3.7.2. Delayed Subflow Start . . . . . . . . . . . . . . . . 41
Appendix A. Notes on use of TCP Options . . . . . . . . . . . . . 48 3.7.3. Failure Handling . . . . . . . . . . . . . . . . . . . 42
Appendix B. Control Blocks . . . . . . . . . . . . . . . . . . . 49 4. Semantic Issues . . . . . . . . . . . . . . . . . . . . . . . 43
B.1. MPTCP Control Block . . . . . . . . . . . . . . . . . . . 50 5. Security Considerations . . . . . . . . . . . . . . . . . . . 44
B.1.1. Authentication and Metadata . . . . . . . . . . . . . 50 6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 45
B.1.2. Sending Side . . . . . . . . . . . . . . . . . . . . . 50 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 48
B.1.3. Receiving Side . . . . . . . . . . . . . . . . . . . . 51 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
B.2. TCP Control Blocks . . . . . . . . . . . . . . . . . . . . 51 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
B.2.1. Sending Side . . . . . . . . . . . . . . . . . . . . . 51 9.1. Normative References . . . . . . . . . . . . . . . . . . . 50
B.2.2. Receiving Side . . . . . . . . . . . . . . . . . . . . 51 9.2. Informative References . . . . . . . . . . . . . . . . . . 50
Appendix C. Changelog . . . . . . . . . . . . . . . . . . . . . . 52 Appendix A. Notes on use of TCP Options . . . . . . . . . . . . . 51
C.1. Changes since draft-ietf-mptcp-multiaddressed-02 . . . . . 52 Appendix B. Control Blocks . . . . . . . . . . . . . . . . . . . 53
C.2. Changes since draft-ietf-mptcp-multiaddressed-01 . . . . . 52 B.1. MPTCP Control Block . . . . . . . . . . . . . . . . . . . 53
C.3. Changes since draft-ietf-mptcp-multiaddressed-00 . . . . . 52 B.1.1. Authentication and Metadata . . . . . . . . . . . . . 53
C.4. Changes since draft-ford-mptcp-multiaddressed-03 . . . . . 52 B.1.2. Sending Side . . . . . . . . . . . . . . . . . . . . . 54
C.5. Changes since draft-ford-mptcp-multiaddressed-02 . . . . . 53 B.1.3. Receiving Side . . . . . . . . . . . . . . . . . . . . 54
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 53 B.2. TCP Control Blocks . . . . . . . . . . . . . . . . . . . . 54
B.2.1. Sending Side . . . . . . . . . . . . . . . . . . . . . 55
B.2.2. Receiving Side . . . . . . . . . . . . . . . . . . . . 55
Appendix C. Finite State Machine . . . . . . . . . . . . . . . . 55
Appendix D. Changelog . . . . . . . . . . . . . . . . . . . . . . 56
D.1. Changes since draft-ietf-mptcp-multiaddressed-03 . . . . . 56
D.2. Changes since draft-ietf-mptcp-multiaddressed-02 . . . . . 56
D.3. Changes since draft-ietf-mptcp-multiaddressed-01 . . . . . 57
D.4. Changes since draft-ietf-mptcp-multiaddressed-00 . . . . . 57
D.5. Changes since draft-ford-mptcp-multiaddressed-03 . . . . . 57
D.6. Changes since draft-ford-mptcp-multiaddressed-02 . . . . . 58
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 58
1. Introduction 1. Introduction
MPTCP is a set of extensions to regular TCP [2] to provide a MPTCP is a set of extensions to regular TCP [2] to provide a
Multipath TCP [3] service, which enables a transport connection to Multipath TCP [3] service, which enables a transport connection to
operate across multiple paths simultaneously. This document presents operate across multiple paths simultaneously. This document presents
the protocol changes required to add multipath capability to TCP; the protocol changes required to add multipath capability to TCP;
specifically, those for signalling and setting up multiple paths specifically, those for signalling and setting up multiple paths
("subflows"), managing these subflows, reassembly of data, and ("subflows"), managing these subflows, reassembly of data, and
termination of sessions. This is not the only information required termination of sessions. This is not the only information required
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not act detrimentally. not act detrimentally.
There are three aspects to the backwards-compatibility listed above There are three aspects to the backwards-compatibility listed above
(discussed in more detail in [3]): (discussed in more detail in [3]):
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; 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 standard TCP API (although
it is reasonable that not all features would be available to such it is reasonable that not all features would be available to such
legacy applications). Furthermore, the protocol must provide the legacy applications). Furthermore, the protocol must provide the
same service model as regular TCP to the application. same service model as regular TCP to the application.
Fall-back: The protocol should be able to fall back to standard TCP Fall-back: The protocol should be able to fall back to standard TCP
with no interference from the user, to be able to communicate with with no interference from the user, to be able to communicate with
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1.5. Requirements Language 1.5. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1]. document are to be interpreted as described in RFC 2119 [1].
2. Operation Overview 2. Operation Overview
This section presents a single description of standard MPTCP This section presents a single description of standard MPTCP
operation, with reference to the protocol operation. The detailed operation, with reference to the protocol operation. Considerable
protocol specification follows in Section 3. reference is made to symbolic names of MPTCP options throughout this
section - these are subtypes of the IANA-assigned MPTCP option (see
Section 8), and their formats are defined in the detailed protocol
specification which follows in Section 3.
To understand the operation of Multipath TCP, let us consider a very A Multipath TCP connection provides a bidirectionnal bytestream
simple case where a client having two addresses, A1 and A2 between two hosts communicating hosts like normal TCP and thus does
establishes an MPTCP connection with a dual homed server having not require any change to the applications. However, Multipath TCP
addresses B1 and B2, as illustrated in Figure 2 in the previous enables the hosts to use different paths with different IP addresses
section. MPTCP offers the same bidirectional bytestream service as to exchange packets belonging to the MPTCP connection. A Multipath
regular TCP. TCP connection appears like a normal TCP connection to an
application. However, to the network layer it appears as a set of
coordinated TCP subflows. These TCP subflows are coordinated by
Multipath TCP. Multipath TCP manages the creation, removal and
utilization of these subflows to send data. The number of
coordinated TCP subflows that are managed within a Multipath TCP
connection is not fixed and it can fluctuate during the lifetime of
the Multipath TCP connection.
To open an MPTCP connection, the client sends a SYN segment from one All MPTCP operations are signaled with a TCP option - a single
of its addresses (say A1) to one of the server's addresses (say B1). numerical type for MPTCP, with "sub-types" for each MPTCP message.
This SYN segment contains the MP_CAPABLE option that indicates that What follows is a summary of the purpose and rationale of these
the client supports MPTCP and contains the client's key for this messages.
MPTCP connection. The server replies with a SYN segment that also
contains the MP_CAPABLE option to confirm that it supports MPTCP.
The MP_CAPABLE option returned by the server includes the server's
key. The client are server keys are used for different purposes by
MPTCP. First, each host derives a 32 bits token that uniquely
identifies the MPTCP connection on this host. Second, the keys are
used to authenticate the utilisation of other addresses. Additional
details about the utilisation of the MP_CAPABLE option may be found
in Section 3.1.
To enable the client and the server to use their multiple addresses 2.1. Initiating an MPTCP connection
to support the same MPTCP connection, MPTCP allows the client and the
server to open additional subflows. These subflows are TCP This is the same signalling as for initiating a normal TCP
connections that are linked to the MPTCP connection and can be used connection, but the SYN, SYN/ACK and ACK packets also carry the
to send and receive data. The client can open an additional subflow MP_CAPABLE option. This is variable-length and serves multiple
by sending a SYN segment from another address (e.g. A2) with the purposes. Firstly, it verifies whether the remote host supports
MP_JOIN option to the server. The MP_JOIN option contains the Multipath TCP; and secondly, the second and third instances of this
server's token that uniquely identifies the MPTCP connection to which option allow the hosts to exchange some information that is used to
the subflow must be associated and a random number. To accept the authenticating the establishment of additional subflows. Further
subflow, the server replies by sending a SYN+ACK segment with the details are given in Section 3.1.
MP_JOIN option that contains a random number chosen by the server and
a HMAC computed over the client and server's random numbers with the Host-A Host-B
client and server keys. This HMAC authenticates the server to the ------ ------
client. Upon reception of this SYN+ACK segment, the client replies MP_CAPABLE ->
with an ACK segment that contains an MP_JOIN option that includes
another HMAC that authenticates the client to the server. Additional <- MP_CAPABLE
details about the utilisation of the MP_JOIN option may be found in [B's key, flags]
ACK MP_CAPABLE ->
[A's key, flags]
2.2. Associating a new subflow with an existing MPTCP connection
The exchange of keys in the MP_CAPABLE handshake provides material
that can be used to authenticate the endpoints in a handshake setting
up a new subflow. Additional subflows begin in the same way as
initiating a normal TCP connection, but the SYN, SYN/ACK and ACK
packets also carry the MP_JOIN option.
Host-A initiates a new subflow between one of its addresses and one
of Host-B's addresses. The token - generated from the key - is used
to identify which MPTCP connection it is joining, and the MAC is used
for authentication. MP_JOIN also contains flags and an Address ID
that can be used to refer to the source address without the sender
needing to know if it has been changed by a NAT. Further details in
Section 3.2. Section 3.2.
The server may also establish one or more subflows with the client by Host-A Host-B
sending SYN segments with the MP_JOIN option that has been briefly ------ ------
described above. Furthermore, a host my also inform the other host MP_JOIN ->
of the IP addresses that it owns. MPTCP uses two options for this [B's token, A's nonce,
purpose. The ADD_ADDR option allows a host to indicates that it owns A's Address ID, flags]
another address. For example, in the above scenario, the server <- MP_JOIN
could use the ADD_ADDR option to indicate that it also owns address [B's MAC, B's nonce,
B2. If a host becomes unable to use a previously advertised address, B's Address ID, flags]
it uses the REMOVE_ADDR option to indicate the address that it lost ACK MP_JOIN ->
to its peer. Additional details about the utilisation of the [A's MAC]
ADD_ADDR and REMOVE_ADDR options may be found in Section 3.4.
The data produced by the client and the server can be sent over any 2.3. Informing the other Host about another potential address
of the subflows that compose an MPTCP connection, and if a subflow
fails, data may need to be retransmitted over another subflow. For The set of IP addresses associated to a multihomed host may change
this, MPTCP relies on two principles. First, each subflow is during the lifetime of an MPTCP connection. MPTCP supports the
equivalent to a normal TCP connection with its own 32-bits sequence addition and removal of addresses on a host both implicitly and
numbering space. This enables MPTCP to traverse complex middle-boxes explicitly. If Host-A has established a subflow starting at address
like transparent proxies or traffic normalizers. Second, MPTCP IP#-A1 and wants to open a second subflow starting at address IP#-A2,
maintains a 64-bits data sequence numbering space. The DSS MPTCP it simply initiates the establishment of the subflow as explained
option is used to send the data sequence numbers and data sequence above. The remote host will then be implictly informed about the new
acknowledgements. When a host sends a TCP segment over one subflow, address.
it indicates inside the segment, by using the DSS option, the mapping
between the 64-bits data sequence number and the 32-bits sequence In some circumstances, a host may want to advertise to the remote
number used by the subflow. Thanks to this mapping, the receiving host the availability of an address without establishing a new
host can reorder the data received, possibly out-of-sequence over the subflow, for example when a NAT prevents setup in one direction. In
different subflows. In MPTCP, a received segment is acknowledged at the example below, Host-A informs Host-B about its alternative IP
two different levels. First, the TCP cumulative or selective address (IP#-A2). Host-B may later send an MP_JOIN to this new
acknowledgements are used to acknowledge the reception of the data on address. Due to the presence of middleboxes that may translate IP
each subflow. Second, the acknowledgements field in the DSS option addresses, this option uses an address identifier to unambiguously
is returned by the receiving host to provide cumulative identify an address on a host. Further details in Section 3.4.1.
acknowledgements at the data sequence level. When a segment is lost,
the receiver detects the gap in the received 32-bits sequence number Host-A Host-B
and traditional TCP retransmission mechanisms are triggered to ------ ------
recover from the loss. When a subflow fails, MPTCP detects the ADD_ADDR ->
failure and retransmits the unacknowledged data over another subflow [IP#-A2,
that is still active. The DSS option also includes an optional IP#-A2's Address ID]
checksum that covers data at the MPTCP connection level to enable a
receiver to detect whether an middlebox has inserted, deleted or There is a corresponding signal for address removal, making use of
modified data on-the-fly. The transmission of data by MPTCP is the Address ID that is signalled in the add address handshake.
discussed in details in Section 3.3. Further details in Section 3.4.2.
Host-A Host-B
------ ------
REMOVE_ADDR ->
[IP#-A2's Address ID]
2.4. Data transfer using MPTCP
To ensure reliable, in-order delivery of data over subflows that may
appear and disappear at anytime, MPTCP uses a 64-bit Data Sequence
Number (DSN) to number all data sent over the MPTCP connection. Each
subflow has its own 32 bits sequence number space and a MPTCP option
allows to map the subflow sequence space to the data sequence space.
In this way, data can be retransmitted on different subflows (mapped
to the same DSN) in the event of failure.
The "Data Sequence Signal" option which carries this mapping can also
carry a connection-level acknowledgement (the "Data ACK") for the
received DSN.
With MPTCP, all subflows share the same receive buffer and advertise
the same receive window. There are two levels of acknowledgement in
MPTCP. Regular TCP acknowledgements are used on each subflow to
acknowledge the reception of the segments sent over the subflow
independently of their DSN. In addition, there are connection-level
acknowledgements for the data sequence space. These acknowledgements
track the advancement of the bytestream and slide the receiving
window.
Further details are in Section 3.3.
Host-A Host-B
------ ------
DATA_SEQUENCE_SIGNAL ->
[Data Sequence Mapping]
[Data ACK]
[Checksum]
2.5. Requesting a change in a path's priority
Hosts can indicate at initial subflow setup whether they wish the
subflow to be used as a regular or backup path - a backup path being
only used if there are no regular paths available. During a
connection, Host-A can request a change in the priority of a subflow
through the MP_PRIO signal to Host-B. Further details in
Section 3.3.8.
Host-A Host-B
------ ------
MP_PRIO ->
2.6. Closing an MPTCP connection
When Host-A wants to inform Host-B that it has no more data to send,
it signals this "Data FIN" as partof the Data Sequence Signal (see
above). It has the same semantics and behaviour as a regular TCP
FIN, but at the connection level. Once all the data on the MPTCP
connection has been successfully received, then this message is
acknowledged at the connection level with a DATA ACK. Further
details in Section 3.3.3.
Host-A Host-B
------ ------
DATA_SEQUENCE_SIGNAL ->
[Data FIN]
<- (MPTCP DATA ACK)
2.7. Notable features
MPTCP's signalling has been designed with several key requirements in
mind that are worth highlighting:
o To cope with NATs on the path, addresses are referred to by
Address IDs, in case the IP packetAs source address gets changed
by a NAT.An MP_JOIN may be blocked by a NAT in one direction but
not the other; hence the MP-ADD-ADDR message improves the chances
of being able to establish multiple paths. Data ACKs explicitly
acknowledge data at the MPTCP connection level. At the subflow
level, the sequence numbers (for data exchange) are identical to
TCPAs. A special Data Sequence Mapping indicates to fallback to
regular TCP for the remainder of the connection. All MPTCP's
signalling is done using TCP options.
o To fall back to ordinary TCP if MPTCP is not possible. For
example if one host is not MPTCP capable, or if a middlebox does
something strange to a MPTCP message (perhaps it alters the
content).
o To meet the threats identified [6], the following steps are taken:
keys are sent in the clear in the MP_CAPABLE messages; MP_JOIN
messages are secured with HMAC-SHA1 using those keys; and standard
TCP validity checks are made on the other messages (ie ensuring
sequence numbers are correct).
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 will be assigned by IANA (see Section 8), A single TCP option number ("Kind") will be assigned by IANA for
and then individual messages will be determined by a "sub-type", the MPTCP (see Section 8), and then individual messages will be
values of which will also be stored in an IANA registry (and are also determined by a "sub-type", the values of which will also be stored
listed in Section 8). This sub-type is a four-bit field - the first in an IANA registry (and are also listed in Section 8).
four bits of the option payload, as shown in Figure 3. The MPTCP
messages are defined in the following sections. Throughout this document, when reference is made to an MPTCP option
by symbolic name, such as "MP_CAPABLE", this refers to a TCP option
with the single MPTCP option type, and with the sub-type value of the
symbolic name as defined in Section 8. This sub-type is a four-bit
field - the first four bits of the option payload, as shown in
Figure 3. The MPTCP messages are defined in the following sections.
1 2 3 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-----------------------+ +---------------+---------------+-------+-----------------------+
| Kind | Length |Subtype| | | Kind | Length |Subtype| |
+---------------+---------------+-------+ | +---------------+---------------+-------+ |
| Subtype-specific data | | Subtype-specific data |
| (variable length) | | (variable length) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
skipping to change at page 10, line 30 skipping to change at page 13, line 4
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 signalling metadata to ensure segmented data can one MPTCP option for signalling 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 signalling additional addresses. a specific packet, such as those for signalling 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 [6]) on a single those for MPTCP and for regular TCP, such as SACK [7]) 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 signalling information. This changes ACKs containing the additional signalling 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 [7] in regular TCP. Therefore, an MPTCP signal of a lost segment [8] 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 signalling purposes, so as to ensure no middleboxes a row for signalling purposes, so as to ensure no middleboxes
misinterpret this as a sign of congestion. 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 [8]. undertaken before processing any MPTCP signals, as described in [9].
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 contains a 64-bit key that is used to authenticate the This option is used to declare the sender's 64 bit key, which is used
addition of future subflows. This is the only time the key will be to authenticate the addition of future subflows. This is the only
sent in clear on the wire; all future subflows will identify the time the key will be sent in clear on the wire; all future subflows
connection using a 32-bit "token". This token is a cryptographic will identify the connection using a 32 bit "token". This token is a
hash of this key. This will be a truncated (most significant 32 cryptographic hash of this key. The token will be a truncated (most
bits) SHA-1 hash [9]. A different, 64-bit truncation (the least significant 32 bits) SHA-1 hash [10]. A different, 64 bit truncation
significant 64 bits) of the hash of the key will be used as the (the least significant 64 bits) of the hash of the key will be used
Initial Data Sequence Number. as the Initial Data Sequence Number.
This key is generated by its sender and has local meaning only, and This key is generated by its sender and has local meaning only, and
its method of generation is implementation-specific. The key MUST be its method of generation is implementation-specific. The key MUST be
hard to guess, and it MUST be unique for the sending host at any one hard to guess, and it MUST be unique for the sending host at any one
time. Recommendations for generating random keys are given in [10]. time. Recommendations for generating random keys are given in [11].
Connections will be indexed at each host by the token (the truncated Connections will be indexed at each host by the token (the truncated
SHA-1 hash of the key). Therefore, an implementation will require a SHA-1 hash of the key). Therefore, an implementation will require a
mapping from each token to the corresponding connection, and in turn mapping from each token to the corresponding connection, and in turn
to the keys for the connection. to the keys for the connection.
There is a very small risk that two different keys will hash to the There is a very small risk that two different keys will hash to the
same token. An implementation SHOULD check its list of connection same token. An implementation SHOULD check its list of connection
tokens to ensure there is not a collision before sending its key in tokens to ensure there is not a collision before sending its key in
the SYN/ACK. This would, however, be costly for a server with the SYN/ACK. This would, however, be costly for a server with
thousands of connections. The subflow handshake mechanism thousands of connections. The subflow handshake mechanism
(Section 3.2) will ensure that new subflows only join the correct (Section 3.2) will ensure that new subflows only join the correct
connection, however, so in the worst case if there was a token connection, however, so in the worst case if there was a token
collision, it just means that the second connection cannot support collision, the second connection cannot support multiple subflows,
multiple subflows, but will otherwise provide a regular TCP service. but will otherwise provide a regular TCP service.
The MP_CAPABLE option is carried on the SYN, SYN/ACK, and ACK packets The MP_CAPABLE option is carried on the SYN, SYN/ACK, and ACK packets
that start the first subflow of an MPTCP connection. The data that start the first subflow of an MPTCP connection. The data
carried by each packet is as follows, where A = initiator and B = carried by each packet is as follows, where A = initiator and B =
listener. listener.
o SYN (A->B): A's Key. o SYN (A->B): no key, just capability signalling.
o SYN/ACK (B->A): B's Key. o SYN/ACK (B->A): B's Key.
o ACK (A->B): Both A's Key and B's Key. o ACK (A->B): A's Key followed by B's Key.
The contents of the option is determined by the SYN and ACK flags of The contents of the option is determined by the SYN and ACK flags of
the packet, verified by the option's length field. For the diagram the packet, verified by the option's length field. For the diagram
shown in Figure 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).
flag is set, a single key is included; if only an ACK flag is set,
both keys are present.
The keys are echoed in the ACK in order to allow the listener (host B's Key is echoed in the ACK in order to allow the listener (host B)
B) to act statelessly until the TCP connection reaches the to act statelessly until the TCP connection reaches the ESTABLISHED
ESTABLISHED state. If the listener acts in this way, however, it state. If the listener acts in this way, however, it MUST generate
MUST generate its key in a verifiable fashion, allowing it to verify its key in a verifiable fashion, allowing it to verify that it
that it generated the key when it is echoed in the ACK. generated the key when it is echoed in the ACK.
Furthermore, in order to ensure reliable delivery of the ACK Furthermore, in order to ensure reliable delivery of the ACK
containing the MP_CAPABLE option, a server MUST respond with an ACK containing the MP_CAPABLE option, a server MUST respond with an ACK
segment on receipt of this, which may contain data, or will be a pure segment on receipt of this, which may contain data, or will be a pure
ACK if it does not have any data to send immediately. If the ACK if it does not have any data to send immediately. If the
initiator does not receive this ACK within the RTO, it MUST re-send initiator does not receive this ACK within the RTO, it MUST re-send
the ACK containing MP_CAPABLE. In effect, an MPTCP connection is in the ACK containing MP_CAPABLE. In effect, an MPTCP connection is in
a "PRE_ESTABLISHED" state while awaiting this ACK, and only upon a "PRE_ESTABLISHED" state while awaiting this ACK, and only upon
receipt of the ACK will it move to the ESTABLISHED state. receipt of the ACK will it move to the ESTABLISHED state.
skipping to change at page 12, line 49 skipping to change at page 15, line 22
hosts in their SYNs set C=0. The decision whether to use checksums hosts in their SYNs set C=0. The decision whether to use checksums
will be stored by an implementation in a per-connection binary state will be stored by an implementation in a per-connection binary state
variable. variable.
For crypto negotiation, the responder has the choice. The initiator For crypto negotiation, the responder has the choice. The initiator
creates a proposal setting a bit for each algorithm it supports to 1 creates a proposal setting a bit for each algorithm it supports to 1
(in this version of the specification, there is only one proposal, so (in this version of the specification, there is only one proposal, so
S will be always set to 1). The responder responds with only one bit S will be always set to 1). The responder responds with only one bit
set - this is the chosen algorithm. The rationale for this behaviour set - this is the chosen algorithm. The rationale for this behaviour
is that the responder will typically be a server with potentially is that the responder will typically be a server with potentially
many thousands of connections, so may wish to choose an algorithm many thousands of connections, so it may wish to choose an algorithm
with minimal computational complexity, depending on load. If a with minimal computational complexity, depending on the load. If a
responder does not support (or does not want to support) any of the responder does not support (or does not want to support) any of the
initiator's proposals, it can respond without an MP_CAPABLE option, initiator's proposals, it can respond without an MP_CAPABLE option,
thus forcing a fall-back to regular TCP. thus forcing a fall-back to regular TCP.
The MP_CAPABLE option is only used in the first subflow of a The MP_CAPABLE option is only used in the first subflow of a
connection, in order to identify the connection; all following connection, in order to identify the connection; all following
subflows will use the "Join" option (see Section 3.2) to join the subflows will use the "Join" option (see Section 3.2) to join the
existing connection. existing connection.
1 2 3 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-------+-+-----------+-+ +---------------+---------------+-------+-------+-+-----------+-+
| Kind | Length |Subtype|Version|C| (reservd) |S| | Kind | Length |Subtype|Version|C| (reservd) |S|
+---------------+---------------+-------+-------+-+-----------+-+ +---------------+---------------+-------+-------+-+-----------+-+
| Sender's Key | | Option Sender's Key (64 bits) |
| (64 bits) | | (if option Length == 12 or 20) |
| | | |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Receiver's Key (64 bits) | | Option Receiver's Key (64 bits) |
| (if Length==20) | | (if option Length == 20) |
| | | |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
Figure 4: Multipath Capable (MP_CAPABLE) option Figure 4: Multipath Capable (MP_CAPABLE) option
If a SYN contains an MP_CAPABLE option but the SYN/ACK does not, it If a SYN contains an MP_CAPABLE option but the SYN/ACK does not, it
is assumed that the passive opener is not multipath capable and thus is assumed that the passive opener is not multipath capable and thus
the MPTCP session MUST operate as regular, single-path TCP. If a SYN the MPTCP session MUST operate as a regular, single-path TCP. If a
does not contain a MP_CAPABLE option, the SYN/ACK MUST NOT contain SYN does not contain a MP_CAPABLE option, the SYN/ACK MUST NOT
one in response. If the third packet (the ACK) does not contain the contain one in response. If the third packet (the ACK) does not
MP_CAPABLE option, then the session MUST fall back to operating as contain the MP_CAPABLE option, then the session MUST fall back to
regular, single-path TCP. This is to maintain compatibility with operating as a regular, single-path TCP. This is to maintain
middleboxes on the path that drop some or all TCP options. compatibility with middleboxes on the path that drop some or all TCP
options.
If the SYN packets are unacknowledged, it is up to local policy to If the SYN packets are unacknowledged, it is up to local policy to
decide how to respond. It is expected that a sender will eventually decide how to respond. It is expected that a sender will eventually
fall back to single-path TCP (i.e. without the MP_CAPABLE Option) in fall back to single-path TCP (i.e. without the MP_CAPABLE Option) in
order to work around middleboxes that may drop packets with unknown order to work around middleboxes that may drop packets with unknown
options; however, the number of multipath-capable attempts that are options; however, the number of multipath-capable attempts that are
made first will be up to local policy. 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 3.5. TCP, as documented in Section 3.5.
The initial Data Sequence Number (IDSN) is generated as a hash from The initial Data Sequence Number (IDSN) is generated as a hash from
the Key, in the same way as the token, i.e. IDSN-A = Hash(Key-A) and the Key, in the same way as the token, i.e. IDSN-A = Hash(Key-A) and
IDSN-B = Hash(Key-B). The Hash mechanism here provides the least IDSN-B = Hash(Key-B). The Hash mechanism here provides the least
significant 64 bits of the SHA-1 hash of the key. The SYN with significant 64 bits of the SHA-1 hash of the key. The SYN with
MP_CAPABLE occupies the first octet of Data Sequence Space. MP_CAPABLE occupies the first octet of Data Sequence Space, 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 signalling exchanges as described in host's addresses through signalling 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 14, line 38 skipping to change at page 17, line 14
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). The tokens presented in this
option are generated by the SHA-1 [9] algorithm, truncated to the option are generated by the SHA-1 [10] algorithm, truncated to the
most significant 32 bits. The token included in the MP_JOIN option most significant 32 bits. The token included in the MP_JOIN option
is the token that the receiver of the packet uses to identify this is the token that the receiver of the packet uses to identify this
connection, i.e. Host A will send Token-B (which is generated from connection, i.e. Host A will send Token-B (which is generated from
Key-B). Key-B).
The MP_JOIN SYN not only sends the token (which is static for a The MP_JOIN SYN not only sends the token (which is static for a
connection) but also Random Numbers (nonces) that are used to prevent connection) but also Random Numbers (nonces) that are used to prevent
replay attacks on the authentication method. replay attacks on the authentication method.
The MP_JOIN option includes an "Address ID". This is an identifier The MP_JOIN option includes an "Address ID". This is an identifier
skipping to change at page 15, line 18 skipping to change at page 17, line 42
setup attempts and address signalling (Section 3.4.1), to prevent setup attempts and address signalling (Section 3.4.1), to prevent
setting up duplicate subflows on the same path. setting up duplicate subflows on the same path.
The Address IDs of the subflow used in the initial SYN exchange of The Address IDs of the subflow used in the initial SYN exchange of
the first subflow in the connection are implicit, and have the value the first subflow in the connection are implicit, and have the value
zero. A host MUST store the Address IDs associated with all zero. A host MUST store the Address IDs associated with all
established subflows. established subflows.
The MP_JOIN option on SYNs also includes 4 bits of flags, 3 of which The MP_JOIN option on SYNs also includes 4 bits of flags, 3 of which
are currently reserved and MUST be set to zero by the sender. The are currently reserved and MUST be set to zero by the sender. The
final bit, labelled 'B', indicates whether the initiator wishes this final bit, labelled 'B', indicates whether the sender of this option
subflow to be used purely as a backup path (B=1) in the event of wishes this subflow to be used as a backup path (B=1) in the event of
failure of other paths, or whether it wants it to be used as part of failure of other paths, or whether it wants it to be used as part of
the connection immediately. Subflow policy is discussed in more the connection immediately. By setting B=1, the sender of the option
detail in Section 3.3.8. is requesting the other host to only send data on this subflow if
there are no available subflows where B=0. Subflow policy is
discussed in more detail in Section 3.3.8.
1 2 3 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-----+-+---------------+ +---------------+---------------+-------+-----+-+---------------+
| Kind | Length = 12 |Subtype| |B| Address ID | | Kind | Length = 12 |Subtype| |B| Address ID |
+---------------+---------------+-------+-----+-+---------------+ +---------------+---------------+-------+-----+-+---------------+
| Receiver's Token (32 bits) | | Receiver's Token (32 bits) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Sender's Random Number (32 bits) | | Sender's Random Number (32 bits) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
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 a 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) MAC. This version of the number and a truncated (leftmost 64 bits) Message Authentication Code
option is shown in Figure 6. If the token is unknown, or the host (MAC). This version of the option is shown in Figure 6. If the
wants to refuse subflow establishment (for example, due to a limit on token is unknown, or the host wants to refuse subflow establishment
the number of subflows it will permit), the receiver will send back (for example, due to a limit on the number of subflows it will
an RST, analogous to an unknown port in TCP. Although cryptographic permit), the receiver will send back an RST, analogous to an unknown
calculations are required in the SYN/ACK, it is felt that the 32-bit port in TCP. Although cryptographic calculations are required in the
token gives sufficient protection against blind state exhaustion SYN/ACK, it is felt that the 32 bit token gives sufficient protection
attacks and therefore there is no need to provide mechanisms to allow against blind state exhaustion attacks and therefore there is no need
a responder to operate statelessly at the MP_JOIN stage. to provide mechanisms to allow a responder to operate statelessly at
the MP_JOIN stage.
An MAC is sent by both hosts - by the initiator (Host A) in the third An MAC is sent by both hosts - by the initiator (Host A) in the third
packet (the ACK) and by the responder (Host B) in the second packet packet (the ACK) and by the responder (Host B) in the second packet
(the SYN/ACK). This is to allow both hosts to have exchanged random (the SYN/ACK). This is to allow both hosts to have exchanged random
data to be used as the message before generating the MAC. In both data to be used as the message before generating the MAC. In both
cases, the MAC algorithm is HMAC as defined in [11], using the SHA-1 cases, the MAC algorithm is HMAC as defined in [12], using the SHA-1
hash algorithm [9] (thus generating a 160-bit / 20 octet HMAC). Due hash algorithm [10] (thus generating a 160-bit / 20 octet HMAC). Due
to option space limitations, the MAC included in the SYN/ACK is to option space limitations, the MAC included in the SYN/ACK is
truncated to the leftmost 64 bits, but this is acceptable since while truncated to the leftmost 64 bits, but this is acceptable since while
in an attacker-initiated attack, the attacker can retry many times; in an attacker-initiated attack, the attacker can retry many times;
if the attacker is the responder, he only has one chance to get the if the attacker is the responder, he only has one chance to get the
MAC correct. MAC correct.
The initiator's authentication information is sent in its first ACK, The initiator's authentication information is sent in its first ACK,
and is shown in Figure 7. The same reliability algorithm for this and is shown in Figure 7. The same reliability algorithm for this
packet as for the MP_CAPABLE ACK is applied: receipt of this packet packet as for the MP_CAPABLE ACK is applied: receipt of this packet
MUST trigger an ACK in response, and the packet MUST be retransmitted MUST trigger an ACK in response, and the packet MUST be retransmitted
skipping to change at page 17, line 26 skipping to change at page 20, line 10
Figure 7: Join Connection (MP_JOIN) option (for third ACK) Figure 7: Join Connection (MP_JOIN) option (for third ACK)
These various TCP options fit together to enable authenticated These various TCP options fit together to enable authenticated
subflow setup as illustrated in Figure 8. subflow setup as illustrated in Figure 8.
Host A Host B Host A Host B
------------------------ ---------- ------------------------ ----------
Address A1 Address A2 Address B1 Address A1 Address A2 Address B1
---------- ---------- ---------- ---------- ---------- ----------
| | | | | |
| SYN + MP_CAPABLE(Key-A) | | | SYN + MP_CAPABLE |
|--------------------------------------------->| |--------------------------------------------->|
|<---------------------------------------------| |<---------------------------------------------|
| 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) |
| |------------------------------->| | |------------------------------->|
| |<-------------------------------| | |<-------------------------------|
skipping to change at page 19, line 11 skipping to change at page 21, line 42
This section discusses operation of MPTCP for data transfer. At a This section discusses operation of MPTCP for data transfer. At a
high level, an MPTCP implementation will take one input data stream high level, an MPTCP implementation will take one input data stream
from an application, and split it into one or more subflows, with from an application, and split it into one or more subflows, with
sufficient control information to allow it to be reassembled and sufficient control information to allow it to be reassembled and
delivered reliably and in-order to the recipient application. The delivered reliably and in-order to the recipient application. The
following subsections define this behaviour in detail. following subsections define this behaviour in detail.
During normal MPTCP operation, the Data Sequence Signal (DSS) TCP During normal MPTCP operation, the Data Sequence Signal (DSS) TCP
option (shown in Figure 9) is used to signal the data required to option (shown in Figure 9) is used to signal the data required to
enable multipath transport. This data comprises: the Data Sequence enable multipath transport. This data comprises: the Data Sequence
Mapping (DSM), which defines how the sequence space on the subflow Mapping, which defines how the sequence space on the subflow maps to
maps to the connection level; and the Data ACK, for acknowledging the connection level; and the Data ACK, for acknowledging receipt of
receipt of data at the connection level. These functions are data at the connection level. These functions are described in more
described in more detail in the following two subsections. detail in the following two subsections.
Either or both of the Data Sequence Mapping or the Data ACK can be Either or both of the Data Sequence Mapping or the Data ACK can be
signalled in the DSS option, dependent on the flags set. signalled in the DSS option, dependent on the flags set.
1 2 3 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+----------------------+ +---------------+---------------+-------+----------------------+
| Kind | Length |Subtype| (reserved) |F|m|M|a|A| | Kind | Length |Subtype| (reserved) |F|m|M|a|A|
+---------------+---------------+-------+----------------------+ +---------------+---------------+-------+----------------------+
| Data ACK (4 or 8 octets, depending on flags) | | Data ACK (4 or 8 octets, depending on flags) |
skipping to change at page 20, line 24 skipping to change at page 23, line 9
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 [6] is used at the subflow level to improve mandated) that SACK [7] is used at the subflow level to improve
efficiency. efficiency.
The Data Sequence Mapping specifies a full mapping from subflow The Data Sequence Mapping specifies a mapping from subflow sequence
sequence space to data sequence space, for the specified length space to data sequence space. This is expressed in terms of starting
(number of bytes of data) starting at the specified Subflow and Data sequence numbers for the subflow and the data level, and a length of
Sequence Numbers. The purpose of the explicit mapping is to assist bytes for which this mapping is valid. This explicit mapping for a
range of data was chosen rather than per-packet signalling 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
network interface cards, or by middleboxes such as performance network interface cards, or by middleboxes such as performance
enhancing proxies). It also allows a single mapping to cover many enhancing proxies). It also allows a single mapping to cover many
packets, which may be useful in bulk transfer situations. packets, which may be useful in bulk transfer situations.
A mapping is unique, in that the subflow sequence number is bound to A mapping is fixed, in that the subflow sequence number is bound to
the data sequence number after the mapping has been processed. It is the data sequence number after the mapping has been processed. A
not possible to change this mapping afterwards; however, the same sender MUST NOT change this mapping after it has been declared;
data sequence number can be mapped to different subflows for however, the same data sequence number can be mapped to by different
retransmission purposes (see Section 3.3.6). It would also permit subflows for retransmission purposes (see Section 3.3.6). This would
the same data to be sent simultaneously on multiple subflows for also permit the same data to be sent simultaneously on multiple
resilience purposes, although the detailed specification of such subflows for resilience or efficiency purposes, especially in the
operation is outside the scope of this document. case of lossy links. Although the detailed specification of such
operation is outside the scope of this document, an implementation
SHOULD treat the first data that is received at a subflow for the
data sequence space as that which should be delivered to the
application.
The data sequence number is specified as an absolute value, whereas The data sequence number is specified as an absolute value, whereas
the subflow sequence numbering is relative (the SYN at the start of the subflow sequence numbering is relative (the SYN at the start of
the subflow has relative subflow sequence number 0). This is to the subflow has relative subflow sequence number 0). This is to
allow middleboxes to change the Initial Sequence Number of a subflow, allow middleboxes to change the Initial Sequence Number of a subflow,
such as firewalls that undertake ISN randomization. such as firewalls that undertake ISN randomization.
The data sequence mapping also contains a checksum of the data that The data sequence mapping also contains a checksum of the data that
this mapping covers. This is used to detect if the payload has been this mapping covers. This is used to detect if the payload has been
adjusted in any way by a non-MPTCP-aware middlebox. If this checksum adjusted in any way by a non-MPTCP-aware middlebox. If this checksum
skipping to change at page 21, line 32 skipping to change at page 24, line 22
| | | |
+--------------------------------------------------------------+ +--------------------------------------------------------------+
| Subflow Sequence Number (4 octets) | | Subflow Sequence Number (4 octets) |
+-------------------------------+------------------------------+ +-------------------------------+------------------------------+
| Data-level Length (2 octets) | Zeros (2 octets) | | Data-level Length (2 octets) | Zeros (2 octets) |
+-------------------------------+------------------------------+ +-------------------------------+------------------------------+
Figure 10: Pseudo-Header for DSS Checksum Figure 10: Pseudo-Header for DSS Checksum
Note that the Data Sequence Number used in the pseudo-header is Note that the Data Sequence Number used in the pseudo-header is
always the 64-bit value, irrespective of what length is used in the always the 64 bit value, irrespective of what length is used in the
DSS option itself. The standard TCP checksum algorithm has been DSS option itself. The standard TCP checksum algorithm has been
chosen since it will be calculated anyway for the TCP subflow, and if chosen since it will be calculated anyway for the TCP subflow, and if
calculated first over the data before adding the pseudo-headers, it calculated first over the data before adding the pseudo-headers, it
only needs to be calculated once. Furthermore, since the TCP only needs to be calculated once. Furthermore, since the TCP
checksum is additive, the checksum for a DSN_MAP can be constructed checksum is additive, the checksum for a DSN_MAP can be constructed
by simply adding together the checksums for the data of each by simply adding together the checksums for the data of each
constituent TCP segment, and adding the checksum for the DSS pseudo- constituent TCP segment, and adding the checksum for the DSS pseudo-
header. header.
Note that checksumming relies on the TCP subflow containing Note that checksumming relies on the TCP subflow containing
skipping to change at page 22, line 13 skipping to change at page 25, line 4
level space, the data SHOULD still be ACKed at the subflow (if it is level space, the data SHOULD still be ACKed at the subflow (if it is
in-window). This data cannot, however, be acknowledged at the data in-window). This data cannot, however, be acknowledged at the data
level (Section 3.3.2) because its data sequence numbers are unknown. level (Section 3.3.2) because its data sequence numbers are unknown.
Implementations MAY hold onto such unmapped data for a short while in Implementations MAY hold onto such unmapped data for a short while in
the expectation that a mapping will arrive shortly. Such unmapped the expectation that a mapping will arrive shortly. Such unmapped
data cannot be counted as being within the connection-level receive data cannot be counted as being within the connection-level receive
window because this is relative to the data sequence numbers, so if window because this is relative to the data sequence numbers, so if
the receiver runs out of memory to hold this data, it will have to be the receiver runs out of memory to hold this data, it will have to be
discarded. If a mapping for that subflow-level sequence space does discarded. If a mapping for that subflow-level sequence space does
not arrive within a receive window of data, that subflow SHOULD be not arrive within a receive window of data, that subflow SHOULD be
treated as broken, closed with an RST, and an unmapped data silently treated as broken, closed with an RST, and any unmapped data silently
discarded. discarded.
Data sequence numbers are always 64-bit quantities, and MUST be Data sequence numbers are always 64 bit quantities, and MUST be
maintained as such in implementations. If a connection is maintained as such in implementations. If a connection is
progressing at a slow rate, so protection against wrapped sequence progressing at a slow rate, so protection against wrapped sequence
numbers is not required, then it is permissible to include just the numbers is not required, then it is permissible to include just the
lower 32 bits of the data sequence number in the Data Sequence lower 32 bits of the data sequence number in the Data Sequence
Mapping and/or Data ACK as an optimization. An implementation MUST Mapping and/or Data ACK as an optimization, and an implementation can
send the full 64 bit Data Sequence Number if it is transmitting at a make this choice independently for each packet.
sufficiently high rate that it could wrap within the MSL [12]. The
An implementation MUST send the full 64 bit Data Sequence Number if
it is transmitting at a sufficiently high rate that the 32 bit value
could wrap within the Maximum Segment Lifetime (MSL) [13]. The
lengths of the DSNs used in these values (which may be different) are lengths of the DSNs used in these values (which may be different) are
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 is done by setting the initial data sequence
number (IDSN) of each host to the least significant 64 bits of the number (IDSN) of each host to the least significant 64 bits of the
SHA-1 hash of the host's key, as described in Section 3.1. 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. An "infinite" mapping can be used to fallback to sized chunks.
regular TCP by mapping the subflow-level data to the connection-level
data for the remainder of the connection (see Section 3.5). This is An "infinite" mapping can be used to fallback to regular TCP by
achieved by setting the data-level length field to the reserved value mapping the subflow-level data to the connection-level data for the
of 0. The checksum, in such a case, will also be set to zero. remainder of the connection (see Section 3.5). This is achieved by
setting the data-level length field to the reserved value of 0. The
checksum, in such a case, will also be set to zero.
3.3.2. Data Acknowledgements 3.3.2. Data Acknowledgements
To provide full end-to-end resilience, MPTCP provides a connection- To provide full end-to-end resilience, MPTCP provides a connection-
level acknowledgement, to act as a cumulative ACK for the connection level acknowledgement, to act as a cumulative ACK for the connection
as a whole. This is the "Data ACK" field of the DSS option as a whole. This is the "Data ACK" field of the DSS option
(Figure 9). The Data ACK is analogous to the behaviour of the (Figure 9). The Data ACK is analogous to the behaviour of the
standard TCP cumulative ACK in TCP SACK - indicating how much data standard TCP cumulative ACK in TCP SACK - indicating how much data
has been successfully received (with no holes). The Data ACK has been successfully received (with no holes). The Data ACK
specifies the next Data Sequence Number it expects to receive. specifies the next Data Sequence Number it expects to receive.
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corresponds with the DATA FIN itself. The checksum in this case will corresponds with the DATA FIN itself. The checksum in this case will
only cover the pseudo-header. only cover the pseudo-header.
A DATA FIN has the semantics and behaviour as a regular TCP FIN, but A DATA FIN has the semantics and behaviour as a regular TCP FIN, but
at the connection level. Notably, it is only DATA ACKed once all at the connection level. Notably, it is only DATA ACKed once all
data has been successfully received at the connection level. Note data has been successfully received at the connection level. Note
therefore that a DATA FIN is decoupled from a subflow FIN. It is therefore that a DATA FIN is decoupled from a subflow FIN. It is
only permissable to combine these signals on one subflow if there is only permissable to combine these signals on one subflow if there is
no data oustanding on other subflows. Otherwise, it may be necessary no data oustanding on other subflows. Otherwise, it may be necessary
to retransmit data on different subflows. Essentially, a host MUST to retransmit data on different subflows. Essentially, a host MUST
NOT FIN all subflows unless it is safe to do so, i.e. until all data NOT FIN all functioning subflows unless it is safe to do so, i.e.
has been DATA ACKed, or that the segment with the FIN flag set is the until all outstanding data has been DATA ACKed, or that the segment
only outstanding segment. with the FIN flag set is the only outstanding segment.
Once a DATA FIN has been acknowledged, all remaining subflows MUST be Once a DATA FIN has been acknowledged, all remaining subflows MUST be
closed with standard FIN exchanges. Both hosts SHOULD send FINs, as closed with standard FIN exchanges. Both hosts SHOULD send FINs, as
a courtesy to allow middleboxes to clean up state even if the subflow a courtesy to allow middleboxes to clean up state even if the subflow
has failed. It is also encouraged to reduce the timeouts (Maximum has failed. It is also encouraged to reduce the timeouts (Maximum
Segment Life) on subflows at end hosts. In particular, any subflows Segment Life) on subflows at end hosts. In particular, any subflows
where there is still outstanding data queued (which has been where there is still outstanding data queued (which has been
retransmitted on other subflows in order to get the DATA FIN retransmitted on other subflows in order to get the DATA FIN
acknowledged) MAY be closed with an RST. acknowledged) MAY be closed with an RST.
A connection is considered closed once both hosts' DATA FINs have A connection is considered closed once both hosts' DATA FINs have
been acknowledged by DATA ACKs. been acknowledged by DATA ACKs.
Note that a host may also send a FIN on an individual subflow to shut Note that a host may also send a FIN on an individual subflow to shut
it down, but this impact is limited to the subflow in question. If it down, but this impact is limited to the subflow in question. If
all subflows have been closed with a FIN exchange, but no DATA FIN all subflows have been closed with a FIN exchange, but no DATA FIN
has been received and acknowledged, the MPTCP connection is treated has been received and acknowledged, the MPTCP connection is treated
as closed only after a timeout. This implies that an implementation as closed only after a timeout. This implies that an implementation
will have TIME_WAIT states at both the subflow and connection levels. will have TIME_WAIT states at both the subflow and connection levels
(see Appendix C). This permits "break-before-make" scenarios where
connectivity is lost on all subflows before a new one can be re-
established.
3.3.4. Receiver Considerations 3.3.4. Receiver Considerations
Regular TCP advertises a receive window in each packet, telling the Regular TCP advertises a receive window in each packet, telling the
sender how much data the receiver is willing to accept past the sender how much data the receiver is willing to accept past the
cumulative ack. The receive window is used to implement flow cumulative ack. The receive window is used to implement flow
control, throttling down fast senders when receivers cannot keep up. control, throttling down fast senders when receivers cannot keep up.
MPTCP also uses a unique receive window, shared between the subflows. MPTCP also uses a unique receive window, shared between the subflows.
The idea is to allow any subflow to send data as long as the receiver The idea is to allow any subflow to send data as long as the receiver
skipping to change at page 25, line 50 skipping to change at page 28, line 50
The receive window is relative to the DATA_ACK. As in TCP, a The receive window is relative to the DATA_ACK. As in TCP, a
receiver MUST NOT shrink the right edge of the receive window (i.e. receiver MUST NOT shrink the right edge of the receive window (i.e.
DATA_ACK + receive window). The receiver will use the Data Sequence DATA_ACK + receive window). The receiver will use the Data Sequence
Number to tell if a packet should be accepted at connection level. Number to tell if a packet should be accepted at connection level.
When deciding to accept packets at subflow level, normal TCP uses the When deciding to accept packets at subflow level, normal TCP uses the
sequence number in the packet and checks it against the allowed sequence number in the packet and checks it against the allowed
receive window. With multipath, such a check is done using only the receive window. With multipath, such a check is done using only the
connection level window. A sanity check SHOULD be performed at connection level window. A sanity check SHOULD be performed at
subflow level to ensure that the subflow and mapped sequence numbers subflow level to ensure that the subflow and mapped sequence numbers
meet the following test: SSN - SUBFLOW_ACK <= DSN - DATA_ACK. meet the following test: SSN - SUBFLOW_ACK <= DSN - DATA_ACK, where
SSN is the subflow sequence number of the received packet and
SUBFLOW_ACK is the rcv_next of the subflow (with the equivalent
connection-level definitions for DSN and DATA_ACK).
In regular TCP, once a segment is deemed in-window, it is either put In regular TCP, once a segment is deemed in-window, it is either put
in the in-order receive queue or in the out-of-order queue. In in the in-order receive queue or in the out-of-order queue. In
multipath TCP, the same happens but at connection-level: a segment is multipath TCP, the same happens but at connection-level: a segment is
placed in the connection level in-order or out-of-order queue if it placed in the connection level in-order or out-of-order queue if it
is in-window at both connection and subflow level. The stack still is in-window at both connection and subflow level. The stack still
has to remember, for each subflow, which segments were received has to remember, for each subflow, which segments were received
succesfully so that it can ACK them at subflow level appropriately. succesfully so that it can ACK them at subflow level appropriately.
Typically, this will be implemented by keeping per subflow out-of- Typically, this will be implemented by keeping per subflow out-of-
order queues (containing only message headers, not the payloads) and order queues (containing only message headers, not the payloads) and
skipping to change at page 29, line 25 skipping to change at page 32, line 27
throughput. Application requirements such as these are discussed in throughput. Application requirements such as these are discussed in
detail in [5]. detail in [5].
The ability to make effective choices at the sender requires full The ability to make effective choices at the sender requires full
knowledge of the path "cost", which is unlikely to be the case. It knowledge of the path "cost", which is unlikely to be the case. It
would be desirable for a receiver to be able to signal their own would be desirable for a receiver to be able to signal their own
preferences for paths, since they will often be the multihomed party, preferences for paths, since they will often be the multihomed party,
and may have to pay for metered incoming bandwidth. and may have to pay for metered incoming bandwidth.
Whilst fine-grained control may be the most powerful solution, that Whilst fine-grained control may be the most powerful solution, that
would require some mechanism such as overloading the ECN signal [13], would require some mechanism such as overloading the ECN signal [14],
which is undesirable, and it is felt that there would not be which is undesirable, and it is felt that there would not be
sufficient benefit to justify an entirely new signal. Therefore the sufficient benefit to justify an entirely new signal. Therefore the
MP_JOIN option (see Section 3.2) contains the 'B' bit, which allows a MP_JOIN option (see Section 3.2) contains the 'B' bit, which allows a
host to indicate to its peer that this path should be treated as a host to indicate to its peer that this path should be treated as a
backup path to use only in the event of failure of other working backup path to use only in the event of failure of other working
subflows (i.e. a subflow where the receiver has indicated B=1 SHOULD subflows (i.e. a subflow where the receiver has indicated B=1 SHOULD
NOT be used to send data unless there are no usable subflows where NOT be used to send data unless there are no usable subflows where
B=0). B=0).
In the event that the available set of paths changes, a host may wish In the event that the available set of paths changes, a host may wish
to signal a change in priority of subflows to the peer. Therefore, to signal a change in priority of subflows to the peer (e.g. a
the MP_PRIO option, shown in Figure 11, can be used to change the 'B' subflow that was previously set as backup should now take priority
flag of the subflow on which it is sent. over all remaining subflows). Therefore, the MP_PRIO option, shown
in Figure 11, can be used to change the 'B' flag of the subflow on
which it is sent.
1 2 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 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| |B| | Kind | Length |Subtype| |B| AddrID (opt) |
+---------------+---------------+-------+-----+-+ +---------------+---------------+-------+-----+-+--------------+
Figure 11: MP_PRIO option Figure 11: MP_PRIO option
It should be noted that the backup flag is a request from the It should be noted that the backup flag is a request from a data
receiver to the sender only, and the sender SHOULD adhere to these receiver to a data sender only, and the data sender SHOULD adhere to
requests. The receiver, however, may continue using the subflow to these requests. A host cannot assume that the data sender will do
so, however, since local policies - or technical difficulties - may
override MP_PRIO requests. The signal applies to a single direction:
the sender of this option, however, may continue using the subflow to
send data even if it has signalled B=1 to the other host. send data even if it has signalled B=1 to the other host.
This option can also be applied to other subflows than the one on
which it is sent, by setting the optional Address ID field. This
applies the given setting of B to all subflows that use the address
identified by the given Address ID. The presence of this field is
determined by the option length; if Length==4 then it is present, if
Length==3 then it applies to the current subflow only. The use case
of this is that a host can signal to its peer that an address is
temporarily unavailable (for example, if it has radio coverage
issues) and the peer should therefore drop to backup state on all
subflows using that Address ID.
3.4. Address Knowledge Exchange (Path Management) 3.4. Address Knowledge Exchange (Path Management)
We use the term "path management" to refer to the exchange of We use the term "path management" to refer to the exchange of
information about additional paths between hosts, which in this information about additional paths between hosts, which in this
design is managed by multiple addresses at hosts. For more detail of design is managed by multiple addresses at hosts. For more detail of
the architectural thinking behind this design, see the separate the architectural thinking behind this design, see the separate
architecture document [3]. architecture document [3].
This design makes use of two methods of sharing such information, This design makes use of two methods of sharing such information,
used simultaneously. The first is the direct setup of new subflows, used simultaneously. The first is the direct setup of new subflows,
skipping to change at page 31, line 21 skipping to change at page 34, line 39
3.4.1. Address Advertisement 3.4.1. Address Advertisement
The Add Address (ADD_ADDR) TCP Option announces additional addresses The Add Address (ADD_ADDR) TCP Option announces additional addresses
(and optionally, ports) on which a host can be reached (Figure 12). (and optionally, ports) on which a host can be reached (Figure 12).
Multiple instances of this TCP option can be added in a single Multiple instances of this TCP option can be added in a single
message if there is sufficient TCP option space, otherwise multiple message if there is sufficient TCP option space, otherwise multiple
TCP messages containing this option will be sent. This option can be TCP messages containing this option will be sent. This option can be
used at any time during a connection, depending on when the sender used at any time during a connection, depending on when the sender
wishes to enable multiple paths and/or when paths become available. wishes to enable multiple paths and/or when paths become available.
As with all MPTCP signals, the receiver MUST understake standard TCP
validity checks before acting upon it.
Every address has an ID which can be used for uniquely identifying Every address has an ID which can be used for uniquely identifying
the address within a connection, for address removal. This is also the address within a connection, for address removal. This is also
used to identify MP_JOIN options (see Section 3.2) relating to the used to identify MP_JOIN options (see Section 3.2) relating to the
same address, even when address translators are in use. The ID MUST same address, even when address translators are in use. The ID MUST
uniquely identify the address to the sender (within the scope of the uniquely identify the address to the sender (within the scope of the
connection), but the mechanism for allocating such IDs is connection), but the mechanism for allocating such IDs is
implementation-specific. implementation-specific.
All address IDs learnt via either MP_JOIN or ADD_ADDR SHOULD be All address IDs learnt via either MP_JOIN or ADD_ADDR SHOULD be
skipping to change at page 32, line 6 skipping to change at page 35, line 26
and the length of the address will be 16 octets (instead of 4). and the length of the address will be 16 octets (instead of 4).
The presence of the final two octets, specifying the TCP port number The presence of the final two octets, specifying the TCP port number
to use, are optional and can be inferred from the length of the to use, are optional and can be inferred from the length of the
option. Although it is expected that the majority of use cases will option. Although it is expected that the majority of use cases will
use the same port pairs as used for the initial subflow (e.g. port 80 use the same port pairs as used for the initial subflow (e.g. port 80
remains port 80 on all subflows), as does the ephemeral port at the remains port 80 on all subflows), as does the ephemeral port at the
client, there may be cases (such as port-based load balancing) where client, there may be cases (such as port-based load balancing) where
the explicit specification of a different port is required. If no the explicit specification of a different port is required. If no
port is specified, MPTCP SHOULD attempt to connect to the specified port is specified, MPTCP SHOULD attempt to connect to the specified
address on same port as is already in use by the signalling subflow, address on the same port as is already in use by the signalling
and this is discussed in more detail in Section 3.7. subflow, and this is discussed in more detail in Section 3.7.
1 2 3 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-------+---------------+ +---------------+---------------+-------+-------+---------------+
| Kind | Length |Subtype| IPVer | Address ID | | Kind | Length |Subtype| IPVer | Address ID |
+---------------+---------------+-------+-------+---------------+ +---------------+---------------+-------+-------+---------------+
| Address (IPv4 - 4 octets / IPv6 - 16 octets) | | Address (IPv4 - 4 octets / IPv6 - 16 octets) |
+-------------------------------+---------------+---------------+ +-------------------------------+---------------+---------------+
| Port (2 octets, optional) | | Port (2 octets, optional) |
+-------------------------------+ +-------------------------------+
Figure 12: Add Address (ADD_ADDR) option (shown for IPv4) Figure 12: Add Address (ADD_ADDR) option (shown for IPv4)
Due to the proliferation of NATs, it is reasonably likely that one Due to the proliferation of NATs, it is reasonably likely that one
host may attempt to advertise private addresses [14]. We do not wish host may attempt to advertise private addresses [15]. It is not
to blanket 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. We must have additional interfaces on the same private network, and a host
ensure, however, that such advertisements do not cause harm. The MAY want to advertise such addresses. Such advertisements must not,
standard mechanism to create a new subflow (Section 3.2) contains a however, cause harm or security vulnerabilities. The standard
32-bit token that uniquely identifies the connection to the receiving mechanism to create a new subflow (Section 3.2) contains a 32 bit
host. If the token is unknown, the host will return with a RST. In token that uniquely identifies the connection to the receiving host.
the unlikely event that the token is known, subflow setup will If the token is unknown, the host will return with a RST. In the
continue, but the MAC exchange must occur for authentication. This unlikely event that the token is known, subflow setup will continue,
will fail, and will provide sufficient protection against two but the MAC exchange must occur for authentication. This will fail,
unconnected hosts accidentally setting up a new subflow upon the and will provide sufficient protection against two unconnected hosts
signal of a private address. accidentally setting up a new subflow upon the signal of a private
address.
Ideally, we'd like to ensure the ADD_ADDR and REMOVE_ADDR options are Ideally, ADD_ADDR and REMOVE_ADDR options would be sent reliably, and
sent reliably, and in order, to the other end. This is to ensure in order, to the other end. This would be to ensure that this
that we do not unnecessarily cause an outage in the connection when address management does not unnecessarily cause an outage in the
remove/add addresses are processed in reverse order, and also to connection when remove/add addresses are processed in reverse order,
ensure that all possible paths are used. We note, however, that and also to ensure that all possible paths are used. Note, however,
losing reliability and ordering it will not break the multipath that losing reliability and ordering will not break the multipath
connections; they will just reduce the opportunity to open multipath connections, it will just reduce the opportunity to open multipath
paths and to survive different patterns of path failures. paths and to survive different patterns of path failures.
Therefore, implementing reliability signals for these TCP options is Therefore, implementing reliability signals for these TCP options is
not necessary. In order to minimise the impact of the loss of these not necessary. In order to minimise the impact of the loss of these
options, however, it is RECOMMENDED that a sender should send these options, however, it is RECOMMENDED that a sender should send these
options on all available subflows. If these options need to be options on all available subflows. If these options need to be
received in-order, an implementation SHOULD only send one ADD_ADDR/ received in-order, an implementation SHOULD only send one ADD_ADDR/
REMOVE_ADDR option per RTT, to minimise the risk of misordering. REMOVE_ADDR option per RTT, to minimise the risk of misordering.
When receiving an ADD_ADDR message with an Address ID already in use When receiving an ADD_ADDR message with an Address ID already in use
for a live subflow within the connection, the receiver SHOULD for a live subflow within the connection, the receiver SHOULD
silently ignore the ADD_ADDR. If the Address ID is not in use on a silently ignore the ADD_ADDR. If the Address ID is not in use on a
live subflow, but is stored by the receiver, a new ADD_ADDR SHOULD live subflow, but is stored by the receiver, a new ADD_ADDR SHOULD
take precedence and replace the stored address. take precedence and replace the stored address.
A host that receives an ADD_ADDR but finds a connection setup to that A host that receives an ADD_ADDR but finds a connection setup to that
address is unsuccessful SHOULD NOT perform further connection IP address and port number is unsuccessful SHOULD NOT perform further
attempts to this address for this connection. A sender that wants to connection attempts to this address/port combination for this
trigger a new incoming connection attempt on a previously advertised connection. A sender that wants to trigger a new incoming connection
address can therefore refresh ADD_ADDR information by sending the attempt on a previously advertised address/port combination can
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 MPTCP options as implementation MUST NOT treat duplicate ACKs with MPTCP options as
indications of congestion [7], and an MPTCP implementation SHOULD NOT indications of congestion [8], and an MPTCP implementation SHOULD NOT
send more than two duplicate ACKs in a row for signalling purposes. send more than two duplicate ACKs in a row for signalling 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 [15] on the path, and if a response is the sending of a TCP Keepalive [16] on the path, and if a response is
received the path is not removed. Typical TCP validity tests on the received the path is not removed. Typical TCP validity tests on the
subflow (e.g. ensuring sequence and ack numbers are correct) MUST subflow (e.g. ensuring sequence and ack numbers are correct) MUST
also be undertaken. also be undertaken.
The sending and receipt (if no keepalive response was received) of The sending and receipt (if no keepalive response was received) of
this message SHOULD trigger the sending of RSTs by both 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
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with a DSS option containing a Data ACK. Upon reception of the with a DSS option containing a Data ACK. Upon reception of the
acknowledgement, the sender has the confirmation that the DSS option acknowledgement, the sender has the confirmation that the DSS option
passes in both directions and may choose to send fewer DSS options passes in both directions and may choose to send fewer DSS options
than once per segment. than once per segment.
If, however, an ACK is received for data (not just for the SYN) If, however, an ACK is received for data (not just for the SYN)
without a Data ACK in a DSS option, the sender determines the path is without a Data ACK in a DSS option, the sender determines the path is
not MPTCP capable. In the case of this occurring on an additional not MPTCP capable. In the case of this occurring on an additional
subflow (i.e. one started with MP_JOIN), the host MUST close the subflow (i.e. one started with MP_JOIN), the host MUST close the
subflow with an RST. In the case of the first subflow (i.e. that subflow with an RST. In the case of the first subflow (i.e. that
started with MP_CAPABLE), it MUST drop out of a MPTCP mode back to started with MP_CAPABLE), it MUST drop out of an MPTCP mode back to
regular TCP. The sender will send one final Data Sequence Mapping, regular TCP. The sender will send one final Data Sequence Mapping,
with the length value of 0 indicating an infinite mapping (in case with the length value of 0 indicating an infinite mapping (in case
the path drops options in one direction only), and then revert to the path drops options in one direction only), and then revert to
sending data on the single subflow without any MPTCP options. sending data on the single subflow without any MPTCP options.
Note that this rule essentially prohibits the sending of data on the Note that this rule essentially prohibits the sending of data on the
third packet of a MP_CAPABLE or MP_JOIN handshake, since both that third packet of an MP_CAPABLE or MP_JOIN handshake, since both that
option and a DSS cannot fit in TCP option space. If the initiator is option and a DSS cannot fit in TCP option space. If the initiator is
to send first, another segment must be sent that contains the data to send first, another segment must be sent that contains the data
and DSS. Note also that an additional subflow cannot be used until and DSS. Note also that an additional subflow cannot be used until
the initial path has been verified as MPTCP-capable. the initial path has been verified as MPTCP-capable.
These rules should cover all cases where such a failure could happen: These rules should cover all cases where such a failure could happen:
whether it's on the forward or reverse path, and whether the server whether it's on the forward or reverse path, and whether the server
or the client first sends data. If lost options on data packets or the client first sends data. If lost options on data packets
occur on any other subflow apart from the the initial subflow, it occur on any other subflow apart from the the initial subflow, it
should be treated as a standard path failure. The data would not be should be treated as a standard path failure. The data would not be
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Therefore, all data from the start of the segment that failed the Therefore, all data from the start of the segment that failed the
checksum onwards is not trustworthy. checksum onwards is not trustworthy.
When multiple subflows are in use, the data in-flight on a subflow When multiple subflows are in use, the data in-flight on a subflow
will likely involve data that is not contiguously part of the will likely involve data that is not contiguously part of the
connection-level stream, since segments will be spread across the connection-level stream, since segments will be spread across the
multiple subflows. Due to the problems identified above, it is not multiple subflows. Due to the problems identified above, it is not
possible to determine what the adjustment has done to the data possible to determine what the adjustment has done to the data
(notably, any changes to the subflow sequence numbering). Therefore, (notably, any changes to the subflow sequence numbering). Therefore,
it is not possible to recover the subflow, and the affected subflow it is not possible to recover the subflow, and the affected subflow
must be immediately closed with an RST, featuring a MP_FAIL option must be immediately closed with an RST, featuring an MP_FAIL option
(Figure 14), which defines the Data Sequence Number at the start of (Figure 14), which defines the Data Sequence Number at the start of
the segment (defined by the Data Sequence Mapping) which had the the segment (defined by the Data Sequence Mapping) which had the
checksum failure. checksum failure.
1 2 3 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+----------------------+ +---------------+---------------+-------+----------------------+
| Kind | Length=12 |Subtype| (reserved) | | Kind | Length=12 |Subtype| (reserved) |
+---------------+---------------+-------+----------------------+ +---------------+---------------+-------+----------------------+
| Data Sequence Number (8 octets) : | Data Sequence Number (8 octets) :
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deployment but which are not required for protocol correctness. In deployment but which are not required for protocol correctness. In
this section we detail such heuristics. Note that discussion of this section we detail such heuristics. Note that discussion of
buffering and certain sender and receiver window behaviours are buffering and certain sender and receiver window behaviours are
presented in Section 3.3.4 and Section 3.3.5, as well as presented in Section 3.3.4 and Section 3.3.5, as well as
retransmission in Section 3.3.6. retransmission in Section 3.3.6.
3.7.1. Port Usage 3.7.1. Port Usage
Under typical operation an MPTCP implementation SHOULD use the same Under typical operation an MPTCP implementation SHOULD use the same
ports as already in use. In other words, the destination port of a ports as already in use. In other words, the destination port of a
SYN containing a MP_JOIN option SHOULD be the same as the remote port SYN containing an MP_JOIN option SHOULD be the same as the remote
of the first subflow in the connection. The local port for such SYNs port of the first subflow in the connection. The local port for such
SHOULD also be the same as for the first subflow (and as such, an SYNs SHOULD also be the same as for the first subflow (and as such,
implementation SHOULD reserve ephemeral ports across all local IP an implementation SHOULD reserve ephemeral ports across all local IP
addresses), although there may be cases where this is infeasible. addresses), although there may be cases where this is infeasible.
This strategy is intended to maximize the probability of the SYN This strategy is intended to maximize the probability of the SYN
being permitted by a firewall or NAT at the recipient and to avoid being permitted by a firewall or NAT at the recipient and to avoid
confusing any network monitoring software. confusing any network monitoring software.
There may also be cases, however, where the passive opener wishes to There may also be cases, however, where the passive opener wishes to
signal to the other host that a specific port should be used, and signal to the other host that a specific port should be used, and
this facility is provided in the Add Address option as documented in this facility is provided in the Add Address option as documented in
Section 3.4.1. It is therefore feasible to allow multiple subflows Section 3.4.1. It is therefore feasible to allow multiple subflows
between the same two addresses but using different port pairs, and between the same two addresses but using different port pairs, and
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the host that is multihomed may well be the client which will never the host that is multihomed may well be the client which will never
fill its buffers, and thus never use MPTCP. Advanced APIs that allow fill its buffers, and thus never use MPTCP. Advanced APIs that allow
an application to signal its traffic requirements would aid in these an application to signal its traffic requirements would aid in these
decisions. decisions.
An additional time-based heuristic could be applied, opening An additional time-based heuristic could be applied, opening
additional subflows after a given period of time has passed. This additional subflows after a given period of time has passed. This
would alleviate the above issue, and also provide resilience for low- would alleviate the above issue, and also provide resilience for low-
bandwidth but long-lived applications. bandwidth but long-lived applications.
This section has shown some of the considerations than an implementer This section has shown some of the considerations that an implementer
should give when developing MPTCP heuristics, but is not intended to should give when developing MPTCP heuristics, but is not intended to
be prescriptive. be prescriptive.
3.7.3. Failure Handling 3.7.3. Failure Handling
Requirements for MPTCP's handling of unexpected signals have been Requirements for MPTCP's handling of unexpected signals have been
given in Section 3.6. There are other failure cases, however, where given in Section 3.6. There are other failure cases, however, where
a hosts can choose appropriate behaviour. a hosts can choose appropriate behaviour.
For example, Section 3.1 suggests that a host should fall back to For example, Section 3.1 suggests that a host should fall back to
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off from using MPTCP, firstly for that particular destination host, off from using MPTCP, firstly for that particular destination host,
and eventually on a whole interface, if MPTCP connections continue and eventually on a whole interface, if MPTCP connections continue
failing. failing.
Another failure could occur when the MP_JOIN handshake fails. Another failure could occur when the MP_JOIN handshake fails.
Section 3.6 specifies that an incorrect handshake MUST lead to the Section 3.6 specifies that an incorrect handshake MUST lead to the
subflow being closed with a RST. A host operating an active subflow being closed with a RST. A host operating an active
intrusion detection system may choose to start blocking MP_JOIN intrusion detection system may choose to start blocking MP_JOIN
packets from the source host if multiple failed MP_JOIN attempts are packets from the source host if multiple failed MP_JOIN attempts are
seen. From the connection initiator's point of view, if an MP_JOIN seen. From the connection initiator's point of view, if an MP_JOIN
fails, it SHOULD NOT attempt to connect to the same IP address during fails, it SHOULD NOT attempt to connect to the same IP address and
the lifetime of the connection, unless the other host refreshes the port during the lifetime of the connection, unless the other host
information with a REMOVE_ADDR and then an ADD_ADDR for the same refreshes the information with another ADD_ADDR option. Note that
address. the ADD_ADDR option is informational only, and does not guarantee the
other host will attempt a connection.
In addition, an implementation may learn over a number of connections In addition, an implementation may learn over a number of connections
that certain interfaces or destination addresses consistently fail that certain interfaces or destination addresses consistently fail
and may default to not trying to use MPTCP for these. Behaviour and may default to not trying to use MPTCP for these. Behaviour
could also be learnt for particularly badly performing subflows or could also be learnt for particularly badly performing subflows or
subflows that regularly fail during use, in order to temporarily subflows that regularly fail during use, in order to temporarily
choose not to use these paths. choose not to use these paths.
4. Semantic Issues 4. Semantic Issues
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middleboxes may change the receive window, and so a host must use middleboxes may change the receive window, and so a host must use
the maximum value of those recently seen on the constituent the maximum value of those recently seen on the constituent
subflows for the connection-level receive window, and also need to subflows for the connection-level receive window, and also need to
maintain a subflow-level window for subflow-level processing. maintain a subflow-level window for subflow-level processing.
FIN: The FIN flag in the TCP header applies only to the subflow it FIN: The FIN flag in the TCP header applies only to the subflow it
is sent on, not to the whole connection. For connection-level FIN is sent on, not to the whole connection. For connection-level FIN
semantics, the DATA_FIN option is used. semantics, the DATA_FIN option is used.
RST: The RST flag in the TCP header applies only to the subflow it RST: The RST flag in the TCP header applies only to the subflow it
is sent on, not to the whole connection. A connection is is sent on, not to the whole connection.
considered reset if a RST is received on every subflow.
Address List: Address list management (i.e. knowledge of the local Address List: Address list management (i.e. knowledge of the local
and remote hosts' lists of available IP addresses) is handled on a and remote hosts' lists of available IP addresses) is handled on a
per-connection basis (as opposed to per-subflow, per host, or per per-connection basis (as opposed to per-subflow, per host, or per
pair of communicating hosts). This permits the application of pair of communicating hosts). This permits the application of
per-connection local policy. Adding an address to one connection per-connection local policy. Adding an address to one connection
(either explicitly through an Add Address message, or implicitly (either explicitly through an Add Address message, or implicitly
through a Join) has no implication for other connections between through a Join) has no implication for other connections between
the same pair of hosts. the same pair of hosts.
5-tuple: The 5-tuple (protocol, local address, local port, remote 5-tuple: The 5-tuple (protocol, local address, local port, remote
address, remote port) presented by kernel APIs to the application address, remote port) presented by kernel APIs to the application
layer in a non-multipath-aware application is that of the first layer in a non-multipath-aware application is that of the first
subflow, even if the subflow has since been closed and removed subflow, even if the subflow has since been closed and removed
from the connection. This decision, and other related API issues, from the connection. This decision, and other related API issues,
are discussed in more detail in [5]. are discussed in more detail in [5].
5. Security Considerations 5. Security Considerations
As identified in [16], the addition of multipath capability to TCP As identified in [6], the addition of multipath capability to TCP
will bring with it a number of new classes of threat. In order to will bring with it a number of new classes of threat. In order to
prevent these, [3] presents a set of requirements for a security prevent these, [3] presents a set of requirements for a security
solution for MPTCP. The fundamental goal is for the security of solution for MPTCP. The fundamental goal is for the security of
MPTCP to be "no worse" than regular TCP today, and the key security MPTCP to be "no worse" than regular TCP today, and the key security
requirements are: requirements are:
o Provide a mechanism to confirm that the parties in a subflow o Provide a mechanism to confirm that the parties in a subflow
handshake are the same as in the original connection setup. handshake are the same as in the original connection setup.
o Provide verification that the peer can receive traffic at a new o Provide verification that the peer can receive traffic at a new
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traffic at this new address. Replay attacks would still be possible traffic at this new address. Replay attacks would still be possible
when only keys are used, and therefore the handshakes use single-use when only keys are used, and therefore the handshakes use single-use
random numbers (nonces) at both ends - this ensures the MAC will random numbers (nonces) at both ends - this ensures the MAC will
never be the same on two handshakes. The use of crypto capability never be the same on two handshakes. The use of crypto capability
bits in the initial connection handshake to negotiate use of a bits in the initial connection handshake to negotiate use of a
particular algorithm will allow the deployment of additional crypto particular algorithm will allow the deployment of additional crypto
mechanisms in the future. Note that this would be susceptible to mechanisms in the future. Note that this would be susceptible to
bid-down attacks only if the attacker was on-path (and thus would be bid-down attacks only if the attacker was on-path (and thus would be
able to modify the data anyway). The security mechanism presented in able to modify the data anyway). The security mechanism presented in
this draft should therefore protect against all forms of flooding and this draft should therefore protect against all forms of flooding and
hijacking attacks suggested in [16]. hijacking attacks suggested in [6].
6. Interactions with Middleboxes 6. Interactions with Middleboxes
Multipath TCP was designed to be deployable in the present world. Multipath TCP was designed to be deployable in the present world.
Its design takes into account "reasonable" existing middlebox Its design takes into account "reasonable" existing middlebox
behaviour. In this section we outline a few representative behaviour. In this section we outline a few representative
middlebox-related failure scenarios and show how multipath TCP middlebox-related failure scenarios and show how multipath TCP
handles them. Next, we list the design decisions multipath has made handles them. Next, we list the design decisions multipath has made
to accomodate the different middleboxes. to accomodate the different middleboxes.
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but performance will degrade as the fraction of stripped options but performance will degrade as the fraction of stripped options
increases. We do not expect such cases to appear in practice, increases. We do not expect such cases to appear in practice,
though: most middleboxes will either strip all options or let them though: most middleboxes will either strip all options or let them
all through. all through.
We end this section with a list of middlebox classes, their behaviour We end this section with a list of middlebox classes, their behaviour
and the elements in the MPTCP design that allow operation through and the elements in the MPTCP design that allow operation through
such middleboxes. Issues surrounding dropping packets with options such middleboxes. Issues surrounding dropping packets with options
or stripping options were discussed above, and are not included here: or stripping options were discussed above, and are not included here:
o NAT [17]: Network Address (and Port) Translators change the source o NATs [17] (Network Address (and Port) Translators) change the
address (and often source port) of packets. This means that a source address (and often source port) of packets. This means
host will not know its public-facing address for signalling in that a host will not know its public-facing address for signalling
MPTCP. Therefore, MPTCP permits implicit address addition via the in MPTCP. Therefore, MPTCP permits implicit address addition via
MP_JOIN option, and the handshake mechanism ensures that the MP_JOIN option, and the handshake mechanism ensures that
connection attempts to private addresses [14] do not cause connection attempts to private addresses [15] do not cause
problems. Explicit address removal is undertaken by an ID number problems. Explicit address removal is undertaken by an ID number
to allow no knowledge of the source address. to allow no knowledge of the source address.
o Performance Enhancing Proxies (PEPs) [18]: might pro-actively ACK o Performance Enhancing Proxies (PEPs) [18] might pro-actively ACK
data to increase performance. Problems will occur if a PEP ACKs data to increase performance. Problems will occur if a PEP ACKs
data and then fails before sending it on to the receiver, of if data and then fails before sending it on to the receiver, or if
the receiver is mobile and moves away before proactively ACKed the receiver is mobile and moves away before proactively ACKed
data is forwarded on. If subflow ACKs were used to control send data is forwarded on. If subflow ACKs were used to control send
buffering, the data could be lost and never be retransmitted, thus buffering, the data could be lost and never be retransmitted, thus
causing the subflow to permanently stall. MPTCP therefore uses causing the subflow to permanently stall. MPTCP therefore uses
the DATA_ACK to make progress when one of its subflows fails in the DATA_ACK to make progress when one of its subflows fails in
this way. This is why MPTCP does not use subflow ACKs to infer this way. This is why MPTCP does not use subflow ACKs to infer
connection level ACKs. connection level ACKs.
o Traffic Normalizers [19]: may not allow holes in sequence numbers, o Traffic Normalizers [19] may not allow holes in sequence numbers,
and may cache packets and retransmit the same data. MPTCP looks and may cache packets and retransmit the same data. MPTCP looks
like standard TCP on the wire, and will not retransmit different like standard TCP on the wire, and will not retransmit different
data on the same subflow sequence number. data on the same subflow sequence number.
o Firewalls [20]: might perform initial sequence number o Firewalls [20] might perform initial sequence number randomization
randomization on TCP connections. MPTCP uses relative sequence on TCP connections. MPTCP uses relative sequence numbers in data
numbers in data sequence mapping to cope with this. Like NATs, sequence mapping to cope with this. Like NATs, firewalls will not
firewalls will not permit many incoming connections, so MPTCP permit many incoming connections, so MPTCP supports address
supports address signalling (ADD_ADDR) so that a multi-addressed signalling (ADD_ADDR) so that a multi-addressed host can invite
host can invite its peer behind the firewall/NAT to connect out to its peer behind the firewall/NAT to connect out to its additional
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
IDS to analyse the whole traffic, and potentially increases the IDS to analyse the whole traffic, and potentially increases the
risk of false positives. However, for an MPTCP-aware IDS, tokens risk of false positives. However, for an MPTCP-aware IDS, tokens
can be read by such systems to correlate multiple subflows and re- can be read by such systems to correlate multiple subflows and re-
assemble for analysis. assemble for analysis.
o Application level middleboxes: such as content-aware firewalls may o Application level middleboxes such as content-aware firewalls may
alter the payload within a subflow, such as re-writing URIs in alter the payload within a subflow, such as re-writing URIs in
HTTP traffic. MPTCP will detect these using the checksum and HTTP traffic. MPTCP will detect these using the checksum and
close the affected subflow(s), if there are other subflows that close the affected subflow(s), if there are other subflows that
can be used. If all subflows are affected multipath will fallback can be used. If all subflows are affected multipath will fallback
to TCP, allowing such middleboxes to change the payload. MPTCP- to TCP, allowing such middleboxes to change the payload. MPTCP-
aware middleboxes should be able to adjust the payload and MPTCP aware middleboxes should be able to adjust the payload and MPTCP
metadata in order not to break the connection. metadata in order not to break the connection.
In addition, all classes of middleboxes may affect TCP traffic in the In addition, all classes of middleboxes may affect TCP traffic in the
following ways: following ways:
o TCP Options: may be removed, or packets with unknown options o TCP Options may be removed, or packets with unknown options
dropped, by many classes of middleboxes. It is intended that the dropped, by many classes of middleboxes. It is intended that the
initial SYN exchange, with a TCP Option, will be sufficient to initial SYN exchange, with a TCP Option, will be sufficient to
identify the path capabilities. If such a packet does not get identify the path capabilities. If such a packet does not get
through, MPTCP will end up falling back to regular TCP. through, MPTCP will end up falling back to regular TCP.
o Segmentation/Coalescing (e.g. TCP segmentation offloading): might o Segmentation/Coalescing (e.g. TCP segmentation offloading) might
copy options between packets and might strip some options. copy options between packets and might strip some options.
MPTCP's data sequence mapping includes the 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. Acknowledgements
The authors are supported by Trilogy The authors are supported by Trilogy
(http://www.trilogy-project.org), a research project (ICT-216372) (http://www.trilogy-project.org), a research project (ICT-216372)
partially funded by the European Community under its Seventh partially funded by the European Community under its Seventh
Framework Program. The views expressed here are those of the Framework Program. The views expressed here are those of the
author(s) only. The European Commission is not liable for any use author(s) only. The European Commission is not liable for any use
that may be made of the information in this document. that may be made of the information in this document.
The authors gratefully acknowledge significant input into this The authors gratefully acknowledge significant input into this
document from 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 and Bob Briscoe. Lawrence Conroy, Yoshifumi Nishida, Bob Briscoe, Stein Gjessing,
Andrew McGregor, and Georg Hampel.
8. IANA Considerations 8. IANA Considerations
This document will make a request to IANA to allocate a new TCP This document will make a request to IANA to allocate a new TCP
option value for MPTCP. This value will be the value of the "Kind" option value for MPTCP. This value will be the value of the "Kind"
field seen in all MPTCP options in this document. field seen in all MPTCP options in this document.
This document will also request IANA operates a registry for MPTCP This document will also request IANA operates a registry for MPTCP
option subtype values. The values as defined by this specification option subtype values. The values as defined by this specification
are as follows: are as follows:
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[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References 9.2. Informative References
[2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, [2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981. September 1981.
[3] Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar, [3] Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar,
"Architectural Guidelines for Multipath TCP Development", "Architectural Guidelines for Multipath TCP Development",
draft-ietf-mptcp-architecture-05 (work in progress), RFC 6182, March 2011.
January 2011.
[4] Raiciu, C., Handley, M., and D. Wischik, "Coupled Congestion [4] Raiciu, C., Handley, M., and D. Wischik, "Coupled Congestion
Control for Multipath Transport Protocols", Control for Multipath Transport Protocols",
draft-ietf-mptcp-congestion-01 (work in progress), draft-ietf-mptcp-congestion-05 (work in progress), June 2011.
January 2011.
[5] Scharf, M. and A. Ford, "MPTCP Application Interface [5] Scharf, M. and A. Ford, "MPTCP Application Interface
Considerations", draft-ietf-mptcp-api-00 (work in progress), Considerations", draft-ietf-mptcp-api-02 (work in progress),
November 2010. June 2011.
[6] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP [6] Bagnulo, M., "Threat Analysis for TCP Extensions for Multipath
Operation with Multiple Addresses", RFC 6181, March 2011.
[7] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996. Selective Acknowledgment Options", RFC 2018, October 1996.
[7] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion [8] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999. Control", RFC 2581, April 1999.
[8] Gont, F., "Security Assessment of the Transmission Control [9] Gont, F., "Security Assessment of the Transmission Control
Protocol (TCP)", draft-ietf-tcpm-tcp-security-02 (work in Protocol (TCP)", draft-ietf-tcpm-tcp-security-02 (work in
progress), January 2011. progress), January 2011.
[9] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and [10] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and
HMAC-SHA)", RFC 4634, July 2006. HMAC-SHA)", RFC 4634, July 2006.
[10] Eastlake, D., Schiller, J., and S. Crocker, "Randomness [11] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005. Requirements for Security", BCP 106, RFC 4086, June 2005.
[11] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing [12] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997. for Message Authentication", RFC 2104, February 1997.
[12] 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.
[13] 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.
[14] 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.
[15] 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.
[16] Bagnulo, M., "Threat Analysis for TCP Extensions for Multi-path
Operation with Multiple Addresses", draft-ietf-mptcp-threat-08
(work in progress), January 2011.
[17] Srisuresh, P. and K. Egevang, "Traditional IP Network Address [17] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001. Translator (Traditional NAT)", RFC 3022, January 2001.
[18] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z. [18] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to Mitigate Shelby, "Performance Enhancing Proxies Intended to Mitigate
Link-Related Degradations", RFC 3135, June 2001. Link-Related Degradations", RFC 3135, June 2001.
[19] Handley, M., Paxson, V., and C. Kreibich, "Network Intrusion [19] Handley, M., Paxson, V., and C. Kreibich, "Network Intrusion
Detection: Evasion, Traffic Normalization, and End-to-End Detection: Evasion, Traffic Normalization, and End-to-End
Protocol Semantics", Usenix Security 2001, 2001, <http:// Protocol Semantics", Usenix Security 2001, 2001, <http://
www.usenix.org/events/sec01/full_papers/handley/handley.pdf>. www.usenix.org/events/sec01/full_papers/handley/handley.pdf>.
[20] Freed, N., "Behavior of and Requirements for Internet [20] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC 2979, October 2000. Firewalls", RFC 2979, October 2000.
Appendix A. Notes on use of TCP Options Appendix A. Notes on use of TCP Options
The TCP option space is limited due to the length of the Data Offset The TCP option space is limited due to the length of the Data Offset
field in the TCP header (4 bits), which defines the TCP header length field in the TCP header (4 bits), which defines the TCP header length
in 32-bit words. With the standard TCP header being 20 bytes, this in 32 bit words. With the standard TCP header being 20 bytes, this
leaves a maximum of 40 bytes for options, and many of these may leaves a maximum of 40 bytes for options, and many of these may
already be used by options such as timestamp and SACK. already be used by options such as timestamp and SACK.
We have performed a brief study on the commonly used TCP options in We have performed a brief study on the commonly used TCP options in
SYN, data, and pure ACK packets, and found that there is enough room SYN, data, and pure ACK packets, and found that there is enough room
to fit all the options we propose using in this draft. to fit all the options we propose using in this draft.
SYN packets typically include MSS (4 bytes), window scale (3 bytes), SYN packets typically include MSS (4 bytes), window scale (3 bytes),
SACK permitted (2 bytes) and timestamp (10 bytes) options. Together SACK permitted (2 bytes) and timestamp (10 bytes) options. Together
these sum to 19 bytes. Some operating systems appear to pad each these sum to 19 bytes. Some operating systems appear to pad each
skipping to change at page 49, line 15 skipping to change at page 52, line 35
It is not necessary to include the Data Sequence Mapping and DATA ACK It is not necessary to include the Data Sequence Mapping and DATA ACK
in each packet, and in many cases it may be possible to alternate in each packet, and in many cases it may be possible to alternate
their presence (so long as the mapping covers the data being sent in their presence (so long as the mapping covers the data being sent in
the following packet). Other options include: alternating between 4 the following packet). Other options include: alternating between 4
and 8 byte sequence numbers in each option; and sending the DATA_ACK and 8 byte sequence numbers in each option; and sending the DATA_ACK
on a duplicate subflow-level ACK (although note that this must not be on a duplicate subflow-level ACK (although note that this must not be
taken as a signal of congestion). taken as a signal of congestion).
On subflow and connection setup, an MPTCP option is also set on the On subflow and connection setup, an MPTCP option is also set on the
third packet (an ACK). These are 20 bytes (for Multipath Capable) third packet (an ACK). These are 20 bytes (for Multipath Capable)
and 24 bytes (for Join) - both of which will fit in the available and 24 bytes (for Join), both of which will fit in the available
option space. option space.
Pure ACKs in TCP typically contain only timestamps (10B). Here, Pure ACKs in TCP typically contain only timestamps (10B). Here,
multipath TCP typically needs to encode only the DATA ACK (maximum of multipath TCP typically needs to encode only the DATA ACK (maximum of
12 octets). Occasionally ACKs will contain SACK information. 12 octets). Occasionally ACKs will contain SACK information.
Depending on the number of lost packets, SACK may utilize the entire Depending on the number of lost packets, SACK may utilize the entire
option space. If a DATA ACK had to be included, then it is probably option space. If a DATA ACK had to be included, then it is probably
necessary to reduce the number of SACK blocks to accomodate the DATA necessary to reduce the number of SACK blocks to accomodate the DATA
ACK. However, the presence of the DATA ACK is unlikely to be 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
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Local.Key (64 bits): This is the key sent by the local host on this Local.Key (64 bits): This is the key sent by the local host on this
MPTCP connection. MPTCP connection.
Remote.Token (32 bits): This is the token chosen by the remote host Remote.Token (32 bits): This is the token chosen by the remote host
on this MPTCP connection, generated from the remote key. on this MPTCP connection, generated from the remote key.
Remote.Key (64 bits): This is the key chosen by the remote host on Remote.Key (64 bits): This is the key chosen by the remote host on
this MPTCP connection this MPTCP connection
MPTCP.Checksum (flag): This flag is set to true if at least one of MPTCP.Checksum (flag): This flag is set to true if at least one of
the hosts has set the C bit the MP_CAPABLE options exchanged the hosts has set the C bit in the MP_CAPABLE options exchanged
during connection establishment, and is set to false otherwise. during connection establishment, and is set to false otherwise.
If this flag is set, the checksum must be computed in all DSS If this flag is set, the checksum must be computed in all DSS
options. options.
B.1.2. Sending Side B.1.2. Sending Side
SND.UNA (64 bits): This is the Data Sequence Number of the next byte SND.UNA (64 bits): This is the Data Sequence Number of the next byte
to be acknowledged, at the MPTCP connection level. This variable to be acknowledged, at the MPTCP connection level. This variable
is updated upon reception of a DSS option containing a DATA_ACK. is updated upon reception of a DSS option containing a DATA_ACK.
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RCV.NXT (32 bits): This is the sequence number of the next byte RCV.NXT (32 bits): This is the sequence number of the next byte
which is expected on the subflow. This state variable is modified which is expected on the subflow. This state variable is modified
upon reception of in-order segments. The value of RCV.NXT is upon reception of in-order segments. The value of RCV.NXT is
copied to the SEG.ACK field of the next segments transmitted on copied to the SEG.ACK field of the next segments transmitted on
the subflow. the subflow.
RCV.WND (32 bits with RFC1323, 16 bits otherwise): This is the RCV.WND (32 bits with RFC1323, 16 bits otherwise): This is the
subflow-level receive window which is updated with the window subflow-level receive window which is updated with the window
field from the segments received on this subflow. field from the segments received on this subflow.
Appendix C. Changelog Appendix C. Finite State Machine
The diagram in Figure 16 shows the Finite State Machine for
connection-level closure. This illustrates how the DATA_FIN
connection-level signal interacts with subflow-level FINs, and
permits "break-before-make" handover between subflows.
+---------+
| M_ESTAB |
+---------+
M_CLOSE | | rcv DATA_FIN
------- | | -------
+---------+ snd DATA_FIN / \ snd DATA_ACK +---------+
| M_FIN |<----------------- ------------------>| M_CLOSE |
| WAIT-1 |--------------------------- | WAIT |
+---------+ rcv DATA_FIN \ +---------+
| rcv DATA_ACK[DFIN] ------- | M_CLOSE |
| -------------- snd DATA_ACK | ------- |
| CLOSE all subflows | snd DATA_FIN |
V V V
+-----------+ +-----------+ +-----------+
|M_FINWAIT-2| | M_CLOSING | | M_LAST-ACK|
+-----------+ +-----------+ +-----------+
| rcv DATA_ACK[DFIN] | rcv DATA_ACK[DFIN] |
| rcv DATA_FIN -------------- | -------------- |
| ------- CLOSE all subflows | CLOSE all subflows |
| snd DATA_ACK[DFIN] V V
\ +-----------+ +---------+
------------------------>|M_TIME WAIT|---------------->| M_CLOSED|
+-----------+ +---------+
All subflows in CLOSED
------------
delete MPTCP PCB
Figure 16: Finite State Machine for Connection Closure
Appendix D. Changelog
This section maintains logs of significant changes made to this This section maintains logs of significant changes made to this
document between versions. document between versions.
C.1. Changes since draft-ietf-mptcp-multiaddressed-02 D.1. Changes since draft-ietf-mptcp-multiaddressed-03
o Removed Key from MP_CAPABLE on SYN (it is in the ACK).
o Added optional Address ID to MP_PRIO.
o Responded to review comments.
D.2. Changes since draft-ietf-mptcp-multiaddressed-02
o Changed to using a single TCP option with a sub-type field. o Changed to using a single TCP option with a sub-type field.
o Merged Data Sequence Number, DATA ACK, and DATA FIN. o Merged Data Sequence Number, DATA ACK, and DATA FIN.
o Changed DATA FIN behaviour (separated from subflow FIN). o Changed DATA FIN behaviour (separated from subflow FIN).
o Added crypto agility and checksum negotiation. o Added crypto agility and checksum negotiation.
o Redefined MP_JOIN handshake to use only three TCP options. o Redefined MP_JOIN handshake to use only three TCP options.
o Added pseudo-header to checksum. o Added pseudo-header to checksum.
o Many clarifications and re-structuring. o Many clarifications and re-structuring.
o Added more discussion on heuristics. o Added more discussion on heuristics.
C.2. Changes since draft-ietf-mptcp-multiaddressed-01 D.3. Changes since draft-ietf-mptcp-multiaddressed-01
o Added proposal for hash-based security mechanism. o Added proposal for hash-based security mechanism.
o Added receiver subflow policy control (backup path flags and o Added receiver subflow policy control (backup path flags and
MP_PRIO option). MP_PRIO option).
o Changed DSN_MAP checksum to use the TCP checksum algorithm. o Changed DSN_MAP checksum to use the TCP checksum algorithm.
C.3. Changes since draft-ietf-mptcp-multiaddressed-00 D.4. Changes since draft-ietf-mptcp-multiaddressed-00
o Various clarifications and minor re-structuring in response to o Various clarifications and minor re-structuring in response to
comments. comments.
C.4. Changes since draft-ford-mptcp-multiaddressed-03 D.5. Changes since draft-ford-mptcp-multiaddressed-03
o Clarified handshake mechanism, especially with regard to error o Clarified handshake mechanism, especially with regard to error
cases (Section 3.2). cases (Section 3.2).
o Added optional port to ADD_ADDR and clarified situation with o Added optional port to ADD_ADDR and clarified situation with
private addresses (Section 3.4.1). private addresses (Section 3.4.1).
o Added path liveness check to REMOVE_ADDR (Section 3.4.2). o Added path liveness check to REMOVE_ADDR (Section 3.4.2).
o Added chunk checksumming to DSN_MAP (Section 3.3.1) to detect o Added chunk checksumming to DSN_MAP (Section 3.3.1) to detect
payload-altering middleboxes, and defined fallback mechanism payload-altering middleboxes, and defined fallback mechanism
(Section 3.5). (Section 3.5).
o Major clarifications to receive window discussion (Section 3.3.5). o Major clarifications to receive window discussion (Section 3.3.5).
o Various textual clarifications, especially in examples. o Various textual clarifications, especially in examples.
C.5. Changes since draft-ford-mptcp-multiaddressed-02 D.6. Changes since draft-ford-mptcp-multiaddressed-02
o Remove Version and Address ID in MP_CAPABLE in Section 3.1, and o Remove Version and Address ID in MP_CAPABLE in Section 3.1, and
make ISN be 6 bytes. make ISN be 6 bytes.
o Data sequence numbers are now always 8 bytes. But in some cases o Data sequence numbers are now always 8 bytes. But in some cases
where it is unambiguous it is permissible to only send the lower 4 where it is unambiguous it is permissible to only send the lower 4
bytes if space is at a premium. bytes if space is at a premium.
o Clarified behaviour of MP_JOIN in Section 3.2. o Clarified behaviour of MP_JOIN in Section 3.2.
 End of changes. 104 change blocks. 
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