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Versions: (draft-ford-mptcp-multiaddressed) 00 01 02 03 04 05 06 07 08 09 RFC 6824

Internet Engineering Task Force                                  A. Ford
Internet-Draft                                       Roke Manor Research
Intended status: Experimental                                  C. Raiciu
Expires: January 12, 2012                                     M. Handley
                                               University College London
                                                          O. Bonaventure
                                                Universite catholique de
                                                                 Louvain
                                                           July 11, 2011


     TCP Extensions for Multipath Operation with Multiple Addresses
                   draft-ietf-mptcp-multiaddressed-04

Abstract

   TCP/IP communication is currently restricted to a single path per
   connection, yet multiple paths often exist between peers.  The
   simultaneous use of these multiple paths for a TCP/IP session would
   improve resource usage within the network, and thus improve user
   experience through higher throughput and improved resilience to
   network failure.

   Multipath TCP provides the ability to simultaneously use multiple
   paths between peers.  This document presents a set of extensions to
   traditional TCP to support multipath operation.  The protocol offers
   the same type of service to applications as TCP (i.e. reliable
   bytestream), and provides the components necessary to establish and
   use multiple TCP flows across potentially disjoint paths.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 12, 2012.

Copyright Notice



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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Design Assumptions . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Multipath TCP in the Networking Stack  . . . . . . . . . .  5
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
     1.4.  MPTCP Concept  . . . . . . . . . . . . . . . . . . . . . .  6
     1.5.  Requirements Language  . . . . . . . . . . . . . . . . . .  7
   2.  Operation Overview . . . . . . . . . . . . . . . . . . . . . .  8
     2.1.  Initiating an MPTCP connection . . . . . . . . . . . . . .  8
     2.2.  Associating a new subflow with an existing MPTCP
           connection . . . . . . . . . . . . . . . . . . . . . . . .  9
     2.3.  Informing the other Host about another potential
           address  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     2.4.  Data transfer using MPTCP  . . . . . . . . . . . . . . . . 10
     2.5.  Requesting a change in a path's priority . . . . . . . . . 11
     2.6.  Closing an MPTCP connection  . . . . . . . . . . . . . . . 11
     2.7.  Notable features . . . . . . . . . . . . . . . . . . . . . 11
   3.  MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.1.  Connection Initiation  . . . . . . . . . . . . . . . . . . 13
     3.2.  Starting a New Subflow . . . . . . . . . . . . . . . . . . 16
     3.3.  General MPTCP Operation  . . . . . . . . . . . . . . . . . 21
       3.3.1.  Data Sequence Mapping  . . . . . . . . . . . . . . . . 22
       3.3.2.  Data Acknowledgements  . . . . . . . . . . . . . . . . 25
       3.3.3.  Closing a Connection . . . . . . . . . . . . . . . . . 27
       3.3.4.  Receiver Considerations  . . . . . . . . . . . . . . . 28
       3.3.5.  Sender Considerations  . . . . . . . . . . . . . . . . 29
       3.3.6.  Reliability and Retransmissions  . . . . . . . . . . . 30
       3.3.7.  Congestion Control Considerations  . . . . . . . . . . 31
       3.3.8.  Subflow Policy . . . . . . . . . . . . . . . . . . . . 31
     3.4.  Address Knowledge Exchange (Path Management) . . . . . . . 33
       3.4.1.  Address Advertisement  . . . . . . . . . . . . . . . . 34
       3.4.2.  Remove Address . . . . . . . . . . . . . . . . . . . . 36
     3.5.  Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 37



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     3.6.  Error Handling . . . . . . . . . . . . . . . . . . . . . . 40
     3.7.  Heuristics . . . . . . . . . . . . . . . . . . . . . . . . 41
       3.7.1.  Port Usage . . . . . . . . . . . . . . . . . . . . . . 41
       3.7.2.  Delayed Subflow Start  . . . . . . . . . . . . . . . . 41
       3.7.3.  Failure Handling . . . . . . . . . . . . . . . . . . . 42
   4.  Semantic Issues  . . . . . . . . . . . . . . . . . . . . . . . 43
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 44
   6.  Interactions with Middleboxes  . . . . . . . . . . . . . . . . 45
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 48
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 49
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 50
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 50
   Appendix A.  Notes on use of TCP Options . . . . . . . . . . . . . 51
   Appendix B.  Control Blocks  . . . . . . . . . . . . . . . . . . . 53
     B.1.  MPTCP Control Block  . . . . . . . . . . . . . . . . . . . 53
       B.1.1.  Authentication and Metadata  . . . . . . . . . . . . . 53
       B.1.2.  Sending Side . . . . . . . . . . . . . . . . . . . . . 54
       B.1.3.  Receiving Side . . . . . . . . . . . . . . . . . . . . 54
     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




















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1.  Introduction

   MPTCP is a set of extensions to regular TCP [2] to provide a
   Multipath TCP [3] service, which enables a transport connection to
   operate across multiple paths simultaneously.  This document presents
   the protocol changes required to add multipath capability to TCP;
   specifically, those for signalling and setting up multiple paths
   ("subflows"), managing these subflows, reassembly of data, and
   termination of sessions.  This is not the only information required
   to create a Multipath TCP implementation, however.  This document is
   complemented by three others:

   o  Architecture [3], which explains the motivations behind Multipath
      TCP, contains a discussion of high-level design decisions on which
      this design is based, and an explanation of a functional
      separation through which an extensible MPTCP implementation can be
      developed.

   o  Congestion Control [4], presenting a safe congestion control
      algorithm for coupling the behaviour of the multiple paths in
      order to "do no harm" to other network users.

   o  Application Considerations [5], discussing what impact MPTCP will
      have on applications, what applications will want to do with
      MPTCP, and as a consequence of these factors, what API extensions
      an MPTCP implementation should present.

1.1.  Design Assumptions

   In order to limit the potentially huge design space, the authors
   imposed two key constraints on the multipath TCP design presented in
   this document:

   o  It must be backwards-compatible with current, regular TCP, to
      increase its chances of deployment

   o  It can be assumed that one or both hosts are multihomed and
      multiaddressed

   To simplify the design we assume that the presence of multiple
   addresses at a host is sufficient to indicate the existence of
   multiple paths.  These paths need not be entirely disjoint: they may
   share one or many routers between them.  Even in such a situation
   making use of multiple paths is beneficial, improving resource
   utilisation and resilience to a subset of node failures.  The
   congestion control algorithms as discussed in [4] ensure this does
   not act detrimentally.




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   There are three aspects to the backwards-compatibility listed above
   (discussed in more detail in [3]):

   External Constraints:  The protocol must function through the vast
      majority of existing middleboxes such as NATs, firewalls and
      proxies, and as such must resemble existing TCP as far as possible
      on the wire.  Furthermore, the protocol must not assume the
      segments it sends on the wire arrive unmodified at the
      destination: they may be split or coalesced; TCP options may be
      removed or duplicated.

   Application Constraints:  The protocol must be usable with no change
      to existing applications that use the standard TCP API (although
      it is reasonable that not all features would be available to such
      legacy applications).  Furthermore, the protocol must provide the
      same service model as regular TCP to the application.

   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
      legacy hosts.

   Further discussion of the design constraints and associated design
   decisions are given in the MPTCP Architecture document [3].

1.2.  Multipath TCP in the Networking Stack

   MPTCP operates at the transport layer and aims to be transparent to
   both higher and lower layers.  It is a set of additional features on
   top of standard TCP; Figure 1 illustrates this layering.  MPTCP is
   designed to be usable by legacy applications with no changes;
   detailed discussion of its interactions with applications is given in
   [5].

                                   +-------------------------------+
                                   |           Application         |
      +---------------+            +-------------------------------+
      |  Application  |            |             MPTCP             |
      +---------------+            + - - - - - - - + - - - - - - - +
      |      TCP      |            | Subflow (TCP) | Subflow (TCP) |
      +---------------+            +-------------------------------+
      |      IP       |            |       IP      |      IP       |
      +---------------+            +-------------------------------+

      Figure 1: Comparison of Standard TCP and MPTCP Protocol Stacks







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1.3.  Terminology

   Path:  A sequence of links between a sender and a receiver, defined
      in this context by a source and destination address pair.

   Subflow:  A flow of TCP segments operating over an individual path,
      which forms part of a larger MPTCP connection.  A subflow is
      started and terminated similarly to a regular TCP connection.

   (MPTCP) Connection:  A set of one or more subflows, over which an
      application can communicate between two hosts.  There is a one-to-
      one mapping between a connection and an application socket.

   Data-level:  The payload data is nominally transferred over a
      connection, which in turn is transported over subflows.  Thus the
      term "data-level" is synonymous with "connection level", in
      contrast to "subflow-level" which refers to properties of an
      individual subflow.

   Token:  A locally unique identifier given to a multipath connection
      by a host.  May also be referred to as a "Connection ID".

   Host:  A end host operating an MPTCP implementation, and either
      initiating or accepting an MPTCP connection.

1.4.  MPTCP Concept

   This section provides a high-level summary of normal operation of
   MPTCP, and is illustrated by the scenario shown in Figure 2.  A
   detailed description of operation is given in Section 3.

   o  To a non-MPTCP-aware application, MPTCP will behave the same as
      normal TCP.  Extended APIs could provide additional control to
      MPTCP-aware applications [5].  An application begins by opening a
      TCP socket in the normal way.  MPTCP signaling and operation is
      handled by the MPTCP implementation.

   o  An MPTCP connection begins similarly to a regular TCP connection.
      This is illustrated in Figure 2 where a TCP connection is
      established between addresses A1 and B1 on Hosts A and B
      respectively.

   o  If extra paths are available, additional TCP sessions (termed
      "subflows") are created on these paths, and are combined with the
      existing session, which continues to appear as a single connection
      to the applications at both ends.  The creation of the additional
      TCP session is illustrated between Address A2 on Host A and
      Address B1 on Host B.



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   o  MPTCP identifies multiple paths by the presence of multiple
      addresses at hosts.  Combinations of these multiple addresses
      equate to the additional paths.  In the example, other potential
      paths that could be set up are A1<->B2 and A2<->B2.  Although this
      additional session is shown as being initiated from A2, it could
      equally have been initiated from B1.

   o  The discovery and setup of additional subflows will be achieved
      through a path management method; this document describes a
      mechanism by which a host can initiate new subflows by using its
      own additional addresses, or by signalling its available addresses
      to the other host.

   o  MPTCP adds connection-level sequence numbers to allow the
      reassembly of the in-order data stream from multiple subflows
      which may deliver packets out-of-order due to differing network
      delays.

   o  Subflows are terminated as regular TCP connections, with a four
      way FIN handshake.  The MPTCP connection is terminated by a
      connection-level FIN.


               Host A                               Host B
      ------------------------             ------------------------
      Address A1    Address A2             Address B1    Address B2
      ----------    ----------             ----------    ----------
          |             |                      |             |
          |     (initial connection setup)     |             |
          |----------------------------------->|             |
          |<-----------------------------------|             |
          |             |                      |             |
          |            (additional subflow setup)            |
          |             |--------------------->|             |
          |             |<---------------------|             |
          |             |                      |             |
          |             |                      |             |

                  Figure 2: Example MPTCP Usage Scenario

1.5.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].






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2.  Operation Overview

   This section presents a single description of standard MPTCP
   operation, with reference to the protocol operation.  Considerable
   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.

   A Multipath TCP connection provides a bidirectionnal bytestream
   between two hosts communicating hosts like normal TCP and thus does
   not require any change to the applications.  However, Multipath TCP
   enables the hosts to use different paths with different IP addresses
   to exchange packets belonging to the MPTCP connection.  A Multipath
   TCP connection appears like a normal TCP connection to an
   application.  However, to the network layer 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.

   All MPTCP operations are signaled with a TCP option - a single
   numerical type for MPTCP, with "sub-types" for each MPTCP message.
   What follows is a summary of the purpose and rationale of these
   messages.

2.1.  Initiating an MPTCP connection

   This is the same signalling as for initiating a normal TCP
   connection, but the SYN, SYN/ACK and ACK packets also carry the
   MP_CAPABLE option.  This is variable-length and serves multiple
   purposes.  Firstly, it verifies whether the remote host supports
   Multipath TCP; and secondly, the second and third instances of this
   option allow the hosts to exchange some information that is used to
   authenticating the establishment of additional subflows.  Further
   details are given in Section 3.1.

      Host-A                                  Host-B
      ------                                  ------
      MP_CAPABLE            ->

                            <-                MP_CAPABLE
                                              [B's key, flags]
      ACK MP_CAPABLE        ->
      [A's key, flags]




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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.

      Host-A                                  Host-B
      ------                                  ------
      MP_JOIN               ->
      [B's token, A's nonce,
       A's Address ID, flags]
                            <-                MP_JOIN
                                              [B's MAC, B's nonce,
                                               B's Address ID, flags]
      ACK MP_JOIN           ->
      [A's MAC]

2.3.  Informing the other Host about another potential address

   The set of IP addresses associated to a multihomed host may change
   during the lifetime of an MPTCP connection.  MPTCP supports the
   addition and removal of addresses on a host both implicitly and
   explicitly.  If Host-A has established a subflow starting at address
   IP#-A1 and wants to open a second subflow starting at address IP#-A2,
   it simply initiates the establishment of the subflow as explained
   above.  The remote host will then be implictly informed about the new
   address.

   In some circumstances, a host may want to advertise to the remote
   host the availability of an address without establishing a new
   subflow, for example when a NAT prevents setup in one direction.  In
   the example below, Host-A informs Host-B about its alternative IP
   address (IP#-A2).  Host-B may later send an MP_JOIN to this new
   address.  Due to the presence of middleboxes that may translate IP
   addresses, this option uses an address identifier to unambiguously
   identify an address on a host.  Further details in Section 3.4.1.





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      Host-A                                 Host-B
      ------                                 ------
      ADD_ADDR                  ->
      [IP#-A2,
       IP#-A2's Address ID]

   There is a corresponding signal for address removal, making use of
   the Address ID that is signalled in the add address handshake.
   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]





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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



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      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

   This section describes the operation of the MPTCP protocol, and is
   subdivided into sections for each key part of the protocol operation.

   All MPTCP operations are signalled using optional TCP header fields.
   A single TCP option number ("Kind") will be assigned by IANA for
   MPTCP (see Section 8), and then individual messages will be
   determined by a "sub-type", the values of which will also be stored
   in an IANA registry (and are also listed in Section 8).

   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
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-----------------------+
      |     Kind      |    Length     |Subtype|                       |
      +---------------+---------------+-------+                       |
      |                     Subtype-specific data                     |
      |                       (variable length)                       |
      +---------------------------------------------------------------+

                       Figure 3: MPTCP option format

   Those MPTCP options associated with subflow initiation must be
   included on packets with the SYN flag set.  Additionally, there is
   one MPTCP option for signalling metadata to ensure segmented data can
   be recombined for delivery to the application.

   The remaining options, however, are signals that do not need to be on
   a specific packet, such as those for signalling additional addresses.
   Whilst an implementation may desire to send MPTCP options as soon as
   possible, it may not be possible to combine all desired options (both



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   those for MPTCP and for regular TCP, such as SACK [7]) on a single
   packet.  Therefore, an implementation may choose to send duplicate
   ACKs containing the additional signalling information.  This changes
   the semantics of a duplicate ACK, these are usually only sent as a
   signal of a lost segment [8] in regular TCP.  Therefore, an MPTCP
   implementation receiving a duplicate ACK which contains an MPTCP
   option MUST NOT treat it as a signal of congestion.  Additionally, an
   MPTCP implementation SHOULD NOT send more than two duplicate ACKs in
   a row for signalling purposes, so as to ensure no middleboxes
   misinterpret this as a sign of congestion.

   Furthermore, standard TCP validity checks (such as ensuring the
   Sequence Number and Acknowledgement Number are within window) MUST be
   undertaken before processing any MPTCP signals, as described in [9].

3.1.  Connection Initiation

   Connection Initiation begins with a SYN, SYN/ACK, ACK exchange on a
   single path.  Each packet contains the Multipath Capable (MP_CAPABLE)
   TCP option (Figure 4).  This option declares its sender is capable of
   performing multipath TCP and wishes to do so on this particular
   connection.

   This option is used to declare the sender's 64 bit key, which is used
   to authenticate the addition of future subflows.  This is the only
   time the key will be sent in clear on the wire; all future subflows
   will identify the connection using a 32 bit "token".  This token is a
   cryptographic hash of this key.  The token will be a truncated (most
   significant 32 bits) SHA-1 hash [10].  A different, 64 bit truncation
   (the least significant 64 bits) of the hash of the key will be used
   as the Initial Data Sequence Number.

   This key is generated by its sender and has local meaning only, and
   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
   time.  Recommendations for generating random keys are given in [11].
   Connections will be indexed at each host by the token (the truncated
   SHA-1 hash of the key).  Therefore, an implementation will require a
   mapping from each token to the corresponding connection, and in turn
   to the keys for the connection.

   There is a very small risk that two different keys will hash to the
   same token.  An implementation SHOULD check its list of connection
   tokens to ensure there is not a collision before sending its key in
   the SYN/ACK.  This would, however, be costly for a server with
   thousands of connections.  The subflow handshake mechanism
   (Section 3.2) will ensure that new subflows only join the correct
   connection, however, so in the worst case if there was a token



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   collision, the second connection cannot support multiple subflows,
   but will otherwise provide a regular TCP service.

   The MP_CAPABLE option is carried on the SYN, SYN/ACK, and ACK packets
   that start the first subflow of an MPTCP connection.  The data
   carried by each packet is as follows, where A = initiator and B =
   listener.

   o  SYN (A->B): no key, just capability signalling.

   o  SYN/ACK (B->A): 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 packet, verified by the option's length field.  For the diagram
   shown in Figure 4, "sender" and "receiver" refer to the sender or
   receiver of the TCP packet (which can be either host).

   B's Key is echoed in the ACK in order to allow the listener (host B)
   to act statelessly until the TCP connection reaches the ESTABLISHED
   state.  If the listener acts in this way, however, it MUST generate
   its key in a verifiable fashion, allowing it to verify that it
   generated the key when it is echoed in the ACK.

   Furthermore, in order to ensure reliable delivery of the 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
   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
   the ACK containing MP_CAPABLE.  In effect, an MPTCP connection is in
   a "PRE_ESTABLISHED" state while awaiting this ACK, and only upon
   receipt of the ACK will it move to the ESTABLISHED state.

   The first four bits of the first octet in the MP_CAPABLE option
   (Figure 4) define the MPTCP option subtype (see Section 8; for
   MP_CAPABLE, this is 0), and the remaining four bits of this octet
   specifies the MPTCP version in use (for this specification, this is
   0).

   The second octet is reserved for flags.  The leftmost bit - labeled C
   - indicates "Checksum required", and SHOULD be set to 1 unless
   specifically overridden (for example, if the system administrator has
   decided that checksums are not required - see Section 3.3 for more
   discussion).  The remaining bits are used for crypto algorithm
   negotiation.  Currently only the rightmost bit - labeled S - is
   assigned, and indicates the use of HMAC-SHA1 (as defined in
   Section 3.2).  An implementation that only supports this method MUST



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   set this bit to 1 and all other currently reserved bits to zero.  If
   none of these flags are set, the MP_CAPABLE option MUST be treated as
   invalid and ignored (i.e. it must be treated as a regular TCP
   handshake).

   These bits negotiate capabilities in similar ways.  For the 'C' bit,
   if either host requires the use of checksums, checksums MUST be used.
   In other words, the only way for checksums not to be used is if both
   hosts in their SYNs set C=0.  The decision whether to use checksums
   will be stored by an implementation in a per-connection binary state
   variable.

   For crypto negotiation, the responder has the choice.  The initiator
   creates a proposal setting a bit for each algorithm it supports to 1
   (in this version of the specification, there is only one proposal, so
   S will be always set to 1).  The responder responds with only one bit
   set - this is the chosen algorithm.  The rationale for this behaviour
   is that the responder will typically be a server with potentially
   many thousands of connections, so it may wish to choose an algorithm
   with minimal computational complexity, depending on the load.  If a
   responder does not support (or does not want to support) any of the
   initiator's proposals, it can respond without an MP_CAPABLE option,
   thus forcing a fall-back to regular TCP.

   The MP_CAPABLE option is only used in the first subflow of a
   connection, in order to identify the connection; all following
   subflows will use the "Join" option (see Section 3.2) to join the
   existing connection.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-------+-+-----------+-+
      |     Kind      |    Length     |Subtype|Version|C| (reservd) |S|
      +---------------+---------------+-------+-------+-+-----------+-+
      |                   Option Sender's Key (64 bits)               |
      |                  (if option Length == 12 or 20)               |
      |                                                               |
      +---------------------------------------------------------------+
      |                  Option Receiver's Key (64 bits)              |
      |                     (if option Length == 20)                  |
      |                                                               |
      +---------------------------------------------------------------+


              Figure 4: Multipath Capable (MP_CAPABLE) option

   If a SYN contains an MP_CAPABLE option but the SYN/ACK does not, it
   is assumed that the passive opener is not multipath capable and thus



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   the MPTCP session MUST operate as a regular, single-path TCP.  If a
   SYN does not contain a MP_CAPABLE option, the SYN/ACK MUST NOT
   contain one in response.  If the third packet (the ACK) does not
   contain the MP_CAPABLE option, then the session MUST fall back to
   operating as a regular, single-path TCP.  This is to maintain
   compatibility with middleboxes on the path that drop some or all TCP
   options.

   If the SYN packets are unacknowledged, it is up to local policy to
   decide how to respond.  It is expected that a sender will eventually
   fall back to single-path TCP (i.e. without the MP_CAPABLE Option) in
   order to work around middleboxes that may drop packets with unknown
   options; however, the number of multipath-capable attempts that are
   made first will be up to local policy.  It is possible that MPTCP and
   non-MPTCP SYNs could get re-ordered in the network.  Therefore, the
   final state is inferred from the presence or absence of the
   MP_CAPABLE option in the third packet of the TCP handshake.  If this
   option is not present, the connection should fall back to regular
   TCP, as documented in Section 3.5.

   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
   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
   MP_CAPABLE occupies the first octet of Data Sequence Space, although
   this does not need to be acknowledged at the connection level until
   the first data is sent (see Section 3.3).

3.2.  Starting a New Subflow

   Once an MPTCP connection has begun with the MP_CAPABLE exchange,
   further subflows can be added to the connection.  Hosts have
   knowledge of their own address(es), and can become aware of the other
   host's addresses through signalling exchanges as described in
   Section 3.4.  Using this knowledge, a host can initiate a new subflow
   over a currently unused pair of addresses.  It is permitted for
   either host in a connection to initiate the creation of a new
   subflow, but it is expected that this will normally be the original
   connection initiator (see Section 3.7 for heuristics).

   A new subflow is started as a normal TCP SYN/ACK exchange.  The Join
   Connection (MP_JOIN) TCP option is used to identify the connection to
   be joined by the new subflow.  It uses keying material that was
   exchanged in the initial MP_CAPABLE handshake (Section 3.1), and that
   handshake also negotiates the crypto algorithm in use for the MP_JOIN
   handshake.

   This section specifies the behaviour of MP_JOIN using the HMAC-SHA1



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   algorithm.  An MP_JOIN option is present in the SYN, SYN/ACK and ACK
   of the three-way handshake, although in each case with a different
   format.

   In the first MP_JOIN on the SYN packet, illustrated in Figure 5, the
   initiator sends a token, random number, and address ID.

   The token is used to identify the MPTCP connection and is a
   cryptographic hash of the receiver's key, as exchanged in the initial
   MP_CAPABLE handshake (Section 3.1).  The tokens presented in this
   option are generated by the SHA-1 [10] algorithm, truncated to the
   most significant 32 bits.  The token included in the MP_JOIN option
   is the token that the receiver of the packet uses to identify this
   connection, i.e.  Host A will send Token-B (which is generated from
   Key-B).

   The MP_JOIN SYN not only sends the token (which is static for a
   connection) but also Random Numbers (nonces) that are used to prevent
   replay attacks on the authentication method.

   The MP_JOIN option includes an "Address ID".  This is an identifier
   that only has significance within a single connection, where it
   identifies the source address of this packet, even if the address
   itself has been changed in transit by a middlebox.  This allows
   address removal without needing to know what the source address at
   the receiver is, thus this allows address removal through NATs.  The
   sender can signal this to the receiver via the REMOVE_ADDR option
   (Section 3.4.2).  It also allows correlation between new subflow
   setup attempts and address signalling (Section 3.4.1), to prevent
   setting up duplicate subflows on the same path.

   The Address IDs of the subflow used in the initial SYN exchange of
   the first subflow in the connection are implicit, and have the value
   zero.  A host MUST store the Address IDs associated with all
   established subflows.

   The MP_JOIN option on SYNs also includes 4 bits of flags, 3 of which
   are currently reserved and MUST be set to zero by the sender.  The
   final bit, labelled 'B', indicates whether the sender of this option
   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
   the connection immediately.  By setting B=1, the sender of the option
   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.






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                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-----+-+---------------+
      |     Kind      |  Length = 12  |Subtype|     |B|   Address ID  |
      +---------------+---------------+-------+-----+-+---------------+
      |                   Receiver's Token (32 bits)                  |
      +---------------------------------------------------------------+
      |                Sender's Random Number (32 bits)               |
      +---------------------------------------------------------------+

       Figure 5: Join Connection (MP_JOIN) option (for initial SYN)

   When receiving a SYN with an MP_JOIN option that contains a valid
   token for an existing MPTCP connection, the recipient SHOULD respond
   with a SYN/ACK also containing an MP_JOIN option containing a random
   number and a truncated (leftmost 64 bits) Message Authentication Code
   (MAC).  This version of the option is shown in Figure 6.  If the
   token is unknown, or the host wants to refuse subflow establishment
   (for example, due to a limit on the number of subflows it will
   permit), the receiver will send back an RST, analogous to an unknown
   port in TCP.  Although cryptographic calculations are required in the
   SYN/ACK, it is felt that the 32 bit token gives sufficient protection
   against blind state exhaustion attacks and therefore there is no need
   to provide mechanisms to allow a responder to operate statelessly at
   the MP_JOIN stage.

   An MAC is sent by both hosts - by the initiator (Host A) in the third
   packet (the ACK) and by the responder (Host B) in the second packet
   (the SYN/ACK).  This is to allow both hosts to have exchanged random
   data to be used as the message before generating the MAC.  In both
   cases, the MAC algorithm is HMAC as defined in [12], using the SHA-1
   hash algorithm [10] (thus generating a 160-bit / 20 octet HMAC).  Due
   to option space limitations, the MAC included in the SYN/ACK is
   truncated to the leftmost 64 bits, but this is acceptable since while
   in an attacker-initiated attack, the attacker can retry many times;
   if the attacker is the responder, he only has one chance to get the
   MAC correct.

   The initiator's authentication information is sent in its first ACK,
   and is shown in Figure 7.  The same reliability algorithm for this
   packet as for the MP_CAPABLE ACK is applied: receipt of this packet
   MUST trigger an ACK in response, and the packet MUST be retransmitted
   if this ACK is not received.  In other words, sending the ACK/MP_JOIN
   packet places the subflow in the PRE_ESTABLISHED state, and it moves
   to the ESTABLISHED state only on receipt of an ACK from the receiver.
   The reserved bits in this option MUST be set to zero by the sender.

   The key for the MAC algorithm, in the case of the message transmitted



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   by Host A, will be Key-A followed by Key-B, and in the case of Host
   B, Key-B followed by Key-A.  These are the keys that were exchanged
   in the original MP_CAPABLE handshake.  The message in each case is
   the concatenations of Random Number for each host (denoted by R): for
   Host A, R-A followed by R-B; and for Host B, R-B followed by R-A.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-----+-+---------------+
      |     Kind      |  Length = 16  |Subtype|     |B|   Address ID  |
      +---------------+---------------+-------+-----+-+---------------+
      |                                                               |
      |                Sender's Truncated MAC (64 bits)               |
      |                                                               |
      +---------------------------------------------------------------+
      |                Sender's Random Number (32 bits)               |
      +---------------------------------------------------------------+

    Figure 6: Join Connection (MP_JOIN) option (for responding SYN/ACK)


                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-----------------------+
      |     Kind      |  Length = 24  |Subtype|      (reserved)       |
      +---------------+---------------+-------+-----------------------+
      |                                                               |
      |                                                               |
      |                 Sender's MAC (160 bits SHA-1)                 |
      |                                                               |
      |                                                               |
      +---------------------------------------------------------------+

        Figure 7: Join Connection (MP_JOIN) option (for third ACK)

   These various TCP options fit together to enable authenticated
   subflow setup as illustrated in Figure 8.














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              Host A                                  Host B
     ------------------------                       ----------
     Address A1    Address A2                       Address B1
     ----------    ----------                       ----------
         |             |                                |
         |             | SYN + MP_CAPABLE               |
         |--------------------------------------------->|
         |<---------------------------------------------|
         |          SYN/ACK + MP_CAPABLE(Key-B)         |
         |             |                                |
         |        ACK + MP_CAPABLE(Key-A, Key-B)        |
         |--------------------------------------------->|
         |             |                                |
         |             |   SYN + MP_JOIN(Token-B, R-A)  |
         |             |------------------------------->|
         |             |<-------------------------------|
         |             |  SYN/ACK + MP_JOIN(MAC-B, R-B) |
         |             |                                |
         |             |      ACK + MP_JOIN(MAC-A)      |
         |             |------------------------------->|
         |             |                                |

   MAC-A = MAC(Key=(Key-A+Key-B), Msg=(R-A+R-B))
   MAC-B = MAC(Key=(Key-B+Key-A), Msg=(R-B+R-A))

               Figure 8: Example use of MPTCP Authentication

   If the token received at Host B is unknown or local policy prohibits
   the acceptance of the new subflow, the recipient MUST respond with a
   TCP RST for the subflow.

   If the token is accepted at Host B, but the MAC returned to Host A
   does not match the one expected, Host A MUST close the subflow with a
   TCP RST.

   If Host B does not receive the expected MAC, or the MP_JOIN option is
   missing from the ACK, it MUST close the subflow with a TCP RST.

   If the MACs are verified as correct, then both hosts have
   authenticated each other as being the same peers as existed at the
   start of the connection, and they have agreed of which connection
   this subflow will become a part.

   If the SYN/ACK as received at Host A does not have an MP_JOIN option,
   Host A MUST close the subflow with a RST.

   This covers all cases of the loss of an MP_JOIN.  In more detail, if
   MP_JOIN is stripped from the SYN on the path from A to B, and Host B



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   does not have a passive opener on the relevant port, it will respond
   with an RST in the normal way.  If in response to a SYN with an
   MP_JOIN option, a SYN/ACK is received without the MP_JOIN option
   (either since it was stripped on the return path, or it was stripped
   on the outgoing path but the passive opener on Host B responded as if
   it were a new regular TCP session), then the subflow is unusable and
   Host A MUST close it with a RST.

   Note that additional subflows can be created between any pair of
   ports (but see Section 3.7 for heuristics); no explicit application-
   level accept calls or bind calls are required to open additional
   subflows.  To associate a new subflow with an existing connection,
   the token supplied in the subflow's SYN exchange is used for
   demultiplexing.  This then binds the 5-tuple of the TCP subflow to
   the local token of the connection.  A consequence is that it is
   possible to allow any port pairs to be used for a connection.

   Deumultiplexing subflow SYNs MUST be done using the token; this is
   unlike traditional TCP, where the destination port is used for
   demultiplexing SYN packets.  Once a subflow is setup, demultiplexing
   packets is done using the five-tuple, as in traditional TCP.  The
   five-tuples will be mapped to the local connection identifier
   (token).  Note that Host A will know its local token for the subflow
   even though it is not sent on the wire - only the responder's token
   is sent.

3.3.  General MPTCP Operation

   This section discusses operation of MPTCP for data transfer.  At a
   high level, an MPTCP implementation will take one input data stream
   from an application, and split it into one or more subflows, with
   sufficient control information to allow it to be reassembled and
   delivered reliably and in-order to the recipient application.  The
   following subsections define this behaviour in detail.

   During normal MPTCP operation, the Data Sequence Signal (DSS) TCP
   option (shown in Figure 9) is used to signal the data required to
   enable multipath transport.  This data comprises: the Data Sequence
   Mapping, which defines how the sequence space on the subflow maps to
   the connection level; and the Data ACK, for acknowledging receipt of
   data at the connection level.  These functions are described in more
   detail in the following two subsections.

   Either or both of the Data Sequence Mapping or the Data ACK can be
   signalled in the DSS option, dependent on the flags set.






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                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +---------------+---------------+-------+----------------------+
     |     Kind      |    Length     |Subtype| (reserved) |F|m|M|a|A|
     +---------------+---------------+-------+----------------------+
     |           Data ACK (4 or 8 octets, depending on flags)       |
     +--------------------------------------------------------------+
     |   Data Sequence Number (4 or 8 octets, depending on flags)   |
     +--------------------------------------------------------------+
     |              Subflow Sequence Number (4 octets)              |
     +-------------------------------+------------------------------+
     |  Data-level Length (2 octets) |      Checksum (2 octets)     |
     +-------------------------------+------------------------------+

                Figure 9: Data Sequence Signal (DSS) option

   The flags when set define the contents of this option, as follows:

   o  A = Data ACK present

   o  a = Data ACK is 8 octets (if not set, Data ACK is 4 octets)

   o  M = Data Sequence Number, Subflow Sequence Number, Data-level
      Length, and Checksum present

   o  m = Data Sequence Number is 8 octets (if not set, DSN is 4 octets)

   The flags 'a' and 'm' only have meaning if the corresponding 'A' or
   'M' flags are set, otherwise they will be ignored.  The maximum
   length of this option, with all flags set, is 28 octets.

   The 'F' flag indicates "DATA FIN".  If present, this means that this
   mapping covers the final data from the sender.  This is the
   connection-level equivalent to the FIN flag in single-path TCP.  The
   purpose of the DATA FIN, along with the interactions between this
   flag, the subflow-level FIN flag, and the data sequence mapping are
   described in Section 3.3.3.  The remaining reserved bits MUST be set
   to zero by an implementation of this specification.

   Note that the Checksum is only present in this option if the use of
   MPTCP checksumming has been negotiated at the MP_CAPABLE handshake
   (see Section 3.1).  The presence of the checksum can be inferred from
   the length of the option.

3.3.1.  Data Sequence Mapping

   The data stream as a whole can be reassembled through the use of the
   Data Sequence Mapping components of the DSS option (Figure 9), which



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   define the mapping from the subflow sequence number to the data
   sequence number.  This is used by the receiver to ensure in-order
   delivery to the application layer.  Meanwhile, the subflow-level
   sequence numbers (i.e. the regular sequence numbers in the TCP
   header) have subflow-only relevance.  It is expected (but not
   mandated) that SACK [7] is used at the subflow level to improve
   efficiency.

   The Data Sequence Mapping specifies a mapping from subflow sequence
   space to data sequence space.  This is expressed in terms of starting
   sequence numbers for the subflow and the data level, and a length of
   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
   coalescing is undertaken separately from the stack that is generating
   the data flow (e.g. through the use of TCP segmentation offloading on
   network interface cards, or by middleboxes such as performance
   enhancing proxies).  It also allows a single mapping to cover many
   packets, which may be useful in bulk transfer situations.

   A mapping is fixed, in that the subflow sequence number is bound to
   the data sequence number after the mapping has been processed.  A
   sender MUST NOT change this mapping after it has been declared;
   however, the same data sequence number can be mapped to by different
   subflows for retransmission purposes (see Section 3.3.6).  This would
   also permit the same data to be sent simultaneously on multiple
   subflows for resilience or efficiency purposes, especially in the
   case of lossy links.  Although the detailed specification of such
   operation is outside the scope of this document, an implementation
   SHOULD treat the first data that is received at a subflow for the
   data sequence space as that which should be delivered to the
   application.

   The data sequence number is specified as an absolute value, whereas
   the subflow sequence numbering is relative (the SYN at the start of
   the subflow has relative subflow sequence number 0).  This is to
   allow middleboxes to change the Initial Sequence Number of a subflow,
   such as firewalls that undertake ISN randomization.

   The data sequence mapping also contains a checksum of the data that
   this mapping covers.  This is used to detect if the payload has been
   adjusted in any way by a non-MPTCP-aware middlebox.  If this checksum
   fails, it will trigger a failure of the subflow, or a fallback to
   regular TCP, as documented in Section 3.5, since MPTCP can no longer
   reliably know the subflow sequence space at the receiver to build
   data sequence mappings.

   The checksum algorithm used is the standard TCP checksum [2],



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   operating over the data covered by this mapping, along with a pseudo-
   header as shown in Figure 10.

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +--------------------------------------------------------------+
     |                                                              |
     |                Data Sequence Number (8 octets)               |
     |                                                              |
     +--------------------------------------------------------------+
     |              Subflow Sequence Number (4 octets)              |
     +-------------------------------+------------------------------+
     |  Data-level Length (2 octets) |        Zeros (2 octets)      |
     +-------------------------------+------------------------------+

                 Figure 10: Pseudo-Header for DSS Checksum

   Note that the Data Sequence Number used in the pseudo-header is
   always the 64 bit value, irrespective of what length is used in the
   DSS option itself.  The standard TCP checksum algorithm has been
   chosen since it will be calculated anyway for the TCP subflow, and if
   calculated first over the data before adding the pseudo-headers, it
   only needs to be calculated once.  Furthermore, since the TCP
   checksum is additive, the checksum for a DSN_MAP can be constructed
   by simply adding together the checksums for the data of each
   constituent TCP segment, and adding the checksum for the DSS pseudo-
   header.

   Note that checksumming relies on the TCP subflow containing
   contiguous data, and therefore a TCP subflow MUST NOT use the Urgent
   Pointer to interrupt an existing mapping.  Further note, however,
   that if Urgent data is received on a subflow, it SHOULD be mapped to
   the data sequence space and delivered to the application analogous to
   Urgent data in regular TCP.

   To avoid possible deadlock scenarios, subflow-level processing should
   be undertaken separately from that at connection-level.  Therefore,
   even if a mapping does not exist from the subflow space to the data-
   level space, the data SHOULD still be ACKed at the subflow (if it is
   in-window).  This data cannot, however, be acknowledged at the data
   level (Section 3.3.2) because its data sequence numbers are unknown.
   Implementations MAY hold onto such unmapped data for a short while in
   the expectation that a mapping will arrive shortly.  Such unmapped
   data cannot be counted as being within the connection-level receive
   window because this is relative to the data sequence numbers, so if
   the receiver runs out of memory to hold this data, it will have to be
   discarded.  If a mapping for that subflow-level sequence space does
   not arrive within a receive window of data, that subflow SHOULD be



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   treated as broken, closed with an RST, and any unmapped data silently
   discarded.

   Data sequence numbers are always 64 bit quantities, and MUST be
   maintained as such in implementations.  If a connection is
   progressing at a slow rate, so protection against wrapped sequence
   numbers is not required, then it is permissible to include just the
   lower 32 bits of the data sequence number in the Data Sequence
   Mapping and/or Data ACK as an optimization, and an implementation can
   make this choice independently for each packet.

   An implementation MUST 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
   declared with flags in the DSS option.  Implementations MUST accept a
   32 bit DSN and implicitly promote it to a 64 bit quantity by
   incrementing the upper 32 bits of sequence number each time the lower
   32 bits wrap.  A sanity check MUST be implemented to ensure that a
   wrap occurs at an expected time (e.g. the sequence number jumps from
   a very high number to a very low number) and is not triggered by out-
   of-order packets.

   As with the standard TCP sequence number, the data sequence number
   should not start at zero, but at a random value to make blind session
   hijacking harder.  This is done by setting the initial data sequence
   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.

   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
   covered by a mapping known at the receiver.  This can be used to
   reduce overhead in cases where the mapping is known in advance; one
   such case is when there is a single subflow between the hosts,
   another is when segments of data are scheduled in larger than packet-
   sized chunks.

   An "infinite" mapping can be used to fallback to 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 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

   To provide full end-to-end resilience, MPTCP provides a 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



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   (Figure 9).  The Data ACK is analogous to the behaviour of the
   standard TCP cumulative ACK in TCP SACK - indicating how much data
   has been successfully received (with no holes).  The Data ACK
   specifies the next Data Sequence Number it expects to receive.

   The Data ACK, as for the DSN, can be sent as the full 64 bit value,
   or as the lower 32 bits.  If data is received with a 64 bit DSN, it
   MUST be acknowledged with a 64 bit Data ACK.  If the DSN received is
   32 bits, it is valid for the implementation to choose whether to send
   a 32 bit or 64 bit Data ACK.

   The rationale for the inclusion of the Data ACK includes the
   existence of certain middleboxes that pro-actively ACK packets, and
   thus might cause deadlock conditions if data were acked at the
   subflow level but then fails to reach the receiver.  This sort of bad
   interaction might be especially prevalent when the receiver is
   mobile.  The Data ACK ensures the data has been delivered to the
   receiver.  Furthermore, separating the connection-level
   acknowledgements from the subflow-level allows processing to be done
   separately, and a receiver has the freedom to drop segments after
   acknowledgement at the subflow level, for example due to memory
   constraints when many segments arrive out-of-order.

   Another reason for including the Data ACK is that it indicates the
   left edge of the advertised receive window.  As explained in
   Section 3.3.4, the receive window is shared by all subflows and is
   relative to the Data ACK.  Because of this, an implementation MUST
   NOT use the RCV.WND field of a TCP segment at connection-level if it
   does not also carry a DSS option with a Data ACK field.

   An MPTCP sender MUST only free data from the send buffer when it has
   been acknowledged by both a Data ACK received on any subflow and at
   the subflow level by any subflows the data was sent on.  The former
   condition ensures liveness of the connection and the latter condition
   ensures liveness and self-consistence of a subflow when data needs to
   be restransmited.  Note, however, that if some data needs to be
   retransmitted multiple times over a subflow, there is a risk of
   blocking the sending window.  In this case, the MPTCP sender can
   decide to cancel the subflow that is behaving badly by sending a RST.

   The Data ACK MAY be included in all segments, however optimisations
   SHOULD be considered in more advanced implementations, where the Data
   ACK is present in segments only when the Data ACK value advances, and
   this behaviour MUST be treated as valid.  This behaviour ensures the
   sender buffer is freed, while reducing overhead when the data
   transfer is unidirectional.





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3.3.3.  Closing a Connection

   In regular TCP a FIN announces the receiver that the sender has no
   more data to send.  In order to allow subflows to operate
   independently and to keep the appearance of TCP over the wire, a FIN
   in MPTCP only affects the subflow on which it is sent.  This allows
   nodes to exercise considerable freedom over which paths are in use at
   any one time.  The semantics of a FIN remain as for regular TCP, i.e.
   it is not until both sides have ACKed each other's FINs that the
   subflow is fully closed.

   When an application calls close() on a socket, this indicates that it
   has no more data to send, and for regular TCP this would result in a
   FIN on the connection.  For MPTCP, an equivalent mechanism is needed,
   and this is referred to as the DATA FIN.

   A DATA FIN is an indication that the sender has no more data to send,
   and as such can be used to verify that all data has been successfully
   received.  A DATA_FIN, as with the FIN on a regular TCP connection,
   is a unidirectional signal.

   The DATA FIN is signalled by setting the 'F' flag in the Data
   Sequence Signal option (Figure 9) to 1.  A DATA FIN occupies one
   octet (the final octet) of the connection-level sequence space.  Note
   that the DATA FIN is included in the Data-level Length, but not at
   the subflow level: for example, a segment with DSN 80, and length 11,
   with DATA FIN set, would map 10 octets from the subflow into data
   sequnce space 80-89, the DATA FIN is DSN 90, and therefore this
   segment including DATA FIN would be acknowledged with a DATA ACK of
   91.

   Note that when the DATA FIN is not attached to a TCP segment
   containing data, the Data Sequence Mapping MUST have Subflow Sequence
   Number of 0, a Length of 1, and the Data Sequence Number that
   corresponds with the DATA FIN itself.  The checksum in this case will
   only cover the pseudo-header.

   A DATA FIN has the semantics and behaviour as a regular TCP FIN, but
   at the connection level.  Notably, it is only DATA ACKed once all
   data has been successfully received at the connection level.  Note
   therefore that a DATA FIN is decoupled from a subflow FIN.  It is
   only permissable to combine these signals on one subflow if there is
   no data oustanding on other subflows.  Otherwise, it may be necessary
   to retransmit data on different subflows.  Essentially, a host MUST
   NOT FIN all functioning subflows unless it is safe to do so, i.e.
   until all outstanding data has been DATA ACKed, or that the segment
   with the FIN flag set is the only outstanding segment.




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   Once a DATA FIN has been acknowledged, all remaining subflows MUST be
   closed with standard FIN exchanges.  Both hosts SHOULD send FINs, as
   a courtesy to allow middleboxes to clean up state even if the subflow
   has failed.  It is also encouraged to reduce the timeouts (Maximum
   Segment Life) on subflows at end hosts.  In particular, any subflows
   where there is still outstanding data queued (which has been
   retransmitted on other subflows in order to get the DATA FIN
   acknowledged) MAY be closed with an RST.

   A connection is considered closed once both hosts' DATA FINs have
   been acknowledged by DATA ACKs.

   Note that a host may also send a FIN on an individual subflow to shut
   it down, but this impact is limited to the subflow in question.  If
   all subflows have been closed with a FIN exchange, but no DATA FIN
   has been received and acknowledged, the MPTCP connection is treated
   as closed only after a timeout.  This implies that an implementation
   will have TIME_WAIT states at both the subflow and connection levels
   (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

   Regular TCP advertises a receive window in each packet, telling the
   sender how much data the receiver is willing to accept past the
   cumulative ack.  The receive window is used to implement flow
   control, throttling down fast senders when receivers cannot keep up.

   MPTCP also uses a unique receive window, shared between the subflows.
   The idea is to allow any subflow to send data as long as the receiver
   is willing to accept it; the alternative, maintaining per subflow
   receive windows, could end-up stalling some subflows while others
   would not use up their window.

   The receive window is relative to the DATA_ACK.  As in TCP, a
   receiver MUST NOT shrink the right edge of the receive window (i.e.
   DATA_ACK + receive window).  The receiver will use the Data Sequence
   Number to tell if a packet should be accepted at connection level.

   When deciding to accept packets at subflow level, normal TCP uses the
   sequence number in the packet and checks it against the allowed
   receive window.  With multipath, such a check is done using only the
   connection level window.  A sanity check SHOULD be performed at
   subflow level to ensure that the subflow and mapped sequence numbers
   meet the following test: SSN - SUBFLOW_ACK <= DSN - DATA_ACK, where
   SSN is the subflow sequence number of the received packet and
   SUBFLOW_ACK is the rcv_next of the subflow (with the equivalent



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   connection-level definitions for DSN and DATA_ACK).

   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
   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
   is in-window at both connection and subflow level.  The stack still
   has to remember, for each subflow, which segments were received
   succesfully so that it can ACK them at subflow level appropriately.
   Typically, this will be implemented by keeping per subflow out-of-
   order queues (containing only message headers, not the payloads) and
   remembering the value of the cumulative ACK.

   It is important for implementers to understand how large a receiver
   buffer is appropriate.  The lower bound for full network utilization
   is the maximum bandwidth-delay product of any of the paths.  However
   this might be insufficient when a packet is lost on a slower subflow
   and needs to be retransmitted (see Section 3.3.6).  A tight upper
   bound would be the maximum RTT of any path multiplied by the total
   bandwidth available across all paths.  This permits all subflows to
   continue at full speed while a packet is fast-retransmitted on the
   maximum RTT path.  Even this might be insufficient to maintain full
   performance in the event of a retransmit timeout on the maximum RTT
   path.  It is for future study to determine the relationship between
   retransmission strategies and receive buffer sizing.

3.3.5.  Sender Considerations

   The sender remembers receiver window advertisements from the
   receiver.  It should only update its local receive window values when
   the largest sequence number allowed (i.e.  DATA_ACK + receive window)
   increases.  This is important to allow using paths with different
   RTTs, and thus different feedback loops.

   MPTCP uses a single receive window across all subflows, and if the
   receive window was guaranteed to be unchanged end-to-end, a host
   could always read the most recent receive window value.  However,
   some classes of middleboxes may alter the TCP-level receive window.
   Typically these will shrink the offered window, although for short
   periods of time it may be possible for the window to be larger
   (however note that this would not continue for long periods since
   ultimately the middlebox must keep up with delivering data to the
   receiver).  Therefore, if receive window sizes differ on multiple
   subflows, when sending data MPTCP SHOULD take the largest of the most
   recent window sizes as the one to use in calculations.  This rule is
   implicit in the requirement not to reduce the right edge of the
   window.




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   The sender also remembers the receive windows advertised by each
   subflow.  The allowed window for subflow i is (ack_i, ack_i +
   rcv_wnd_i), where ack_i is the subflow-level cumulative ack of
   subflow i.  This ensures data will not be sent to a middlebox unless
   there is enough buffering for the data.

   Putting the two rules together, we get the following: a sender is
   allowed to send data segments with data-level sequence numbers
   between (DATA_ACK, DATA_ACK + receive_window).  Each of these
   segments will be mapped onto subflows, as long as subflow sequence
   numbers are in the the allowed windows for those subflows.  Note that
   subflow sequence numbers do not generally affect flow control if the
   same receive window is advertised across all subflows.  They will
   perform flow control for those subflows with a smaller advertised
   receive window.

   The send buffer must be, at the minimum, as big as the receive
   buffer, to enable the sender to reach maximum throughput.

3.3.6.  Reliability and Retransmissions

   The data sequence mapping allows senders to re-send data with the
   same data sequence number on a different subflow.  When doing this, a
   host must still retransmit the original data on the original subflow,
   in order to preserve the subflow integrity (middleboxes could replay
   old data, and/or could reject holes in subflows), and a receiver will
   ignore these retransmissions.  While this is clearly suboptimal, for
   compatibility reasons this is the best behaviour.  Optimisations
   could be negotiated in future versions of this protocol.

   This protocol specification does not mandate any mechanisms for
   handling retransmissions, and much will be dependent upon local
   policy (as discussed in Section 3.3.8).  One can imagine aggressive
   connection level retransmissions policies where every packet lost at
   subflow level is retransmitted on a different subflow (hence wasting
   bandwidth but possibly reducing application-to-application delays),
   or conservative retransmission policies where connection-level
   retransmits are only used after a few subflow level retransmission
   timeouts occur.

   It is envisaged that a standard connection-level retransmission
   mechanism would be implemented around a connection-level data queue:
   all segments that haven't been DATA_ACKed are stored.  A timer is set
   when the head of the connection-level is ACKed at subflow level but
   its corresponding data is not ACKed at data level.  This timer will
   guard against failures in re-transmission by middleboxes that pro-
   active ACK data.




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   The sender MUST keep data in its send buffer as long as the data has
   not been acknowledged at both connection level and on all subflows it
   has been sent on.  In this way, the sender can always retransmit the
   data if needed, on the same subflow or on a different one.  A special
   case is when a subflow fails: the sender will typically resend the
   data on other working subflows after a timeout, and will keep trying
   to retransmit the data on the failed subflow too.  The sender will
   declare the subflow failed after a predefined upper bound on
   retransmissions is reached (which MAY be lower than the usual TCP
   limits of the Maximum Segment Life), or on the receipt of an ICMP
   error, and only then delete the outstanding data segments.

   Multiple retransmissions are triggers that will indicate that a
   subflow performs badly and could lead to a host resetting the subflow
   with an RST.  However, additional research is required to understand
   the heuristics of how and when to reset underperforming subflows.
   For example, subflows that perform highly asymmetrically may be mis-
   diagnosed as underperforming.

3.3.7.  Congestion Control Considerations

   Different subflows in an MPTCP connection have different congestion
   windows.  To achieve fairness at bottlenecks and resource pooling, it
   is necessary to couple the congestion windows in use on each subflow,
   in order to push most traffic to uncongested links.  One algorithm
   for achieving this is presented in [4]; the algorithm does not
   achieve perfect resource pooling but is "safe" in that it is readily
   deployable in the current Internet.  By this, we mean that it does
   not take up more capacity on any one path than if it was a single
   path flow using only that route, so this ensures fair coexistence
   with single-path TCP at shared bottlenecks.

   It is foreseeable that different congestion controllers will be
   implemented for MPTCP, each aiming to achieve different properties in
   the resource pooling/fairness/stability design space, as well as
   those for achieving different properties in quality of service,
   reliability and resilience.

   Regardless of the algorithm used, the design of the MPTCP protocol
   aims to provide the congestion control implementations sufficient
   information to take the right decisions; this information includes,
   for each subflow, which packets were lost and when.

3.3.8.  Subflow Policy

   Within a local MPTCP implementation, a host may use any local policy
   it wishes to decide how to share the traffic to be sent over the
   available paths.



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   In the typical use case, where the goal is to maximise throughput,
   all available paths will be used simultaneously for data transfer,
   using coupled congestion control as described in [4].  It is
   expected, however, that other use cases will appear.

   For instance, a possibility is an 'all-or-nothing' approach, i.e.
   have a second path ready for use in the event of failure of the first
   path, but alternatives could include entirely saturating one path
   before using an additional path (the 'overflow' case).  Such choices
   would be most likely based on the monetary cost of links, but may
   also be based on properties such as the delay or jitter of links,
   where stability (of delay or bandwidth) is more important than
   throughput.  Application requirements such as these are discussed in
   detail in [5].

   The ability to make effective choices at the sender requires full
   knowledge of the path "cost", which is unlikely to be the case.  It
   would be desirable for a receiver to be able to signal their own
   preferences for paths, since they will often be the multihomed party,
   and may have to pay for metered incoming bandwidth.

   Whilst fine-grained control may be the most powerful solution, that
   would require some mechanism such as overloading the ECN signal [14],
   which is undesirable, and it is felt that there would not be
   sufficient benefit to justify an entirely new signal.  Therefore the
   MP_JOIN option (see Section 3.2) contains the 'B' bit, which allows a
   host to indicate to its peer that this path should be treated as a
   backup path to use only in the event of failure of other working
   subflows (i.e. a subflow where the receiver has indicated B=1 SHOULD
   NOT be used to send data unless there are no usable subflows where
   B=0).

   In the event that the available set of paths changes, a host may wish
   to signal a change in priority of subflows to the peer (e.g. a
   subflow that was previously set as backup should now take priority
   over all remaining subflows).  Therefore, the MP_PRIO option, shown
   in Figure 11, can be used to change the 'B' flag of the subflow on
   which it is sent.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-----+-+--------------+
      |     Kind      |     Length    |Subtype|     |B| AddrID (opt) |
      +---------------+---------------+-------+-----+-+--------------+

                         Figure 11: MP_PRIO option

   It should be noted that the backup flag is a request from a data



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   receiver to a data sender only, and the data sender SHOULD adhere to
   these requests.  A host cannot assume that the data sender will do
   so, however, since local policies - or technical difficulties - may
   override MP_PRIO requests.  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.

   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)

   We use the term "path management" to refer to the exchange of
   information about additional paths between hosts, which in this
   design is managed by multiple addresses at hosts.  For more detail of
   the architectural thinking behind this design, see the separate
   architecture document [3].

   This design makes use of two methods of sharing such information,
   used simultaneously.  The first is the direct setup of new subflows,
   already described in Section 3.2, where the initiator has an
   additional address.  The second method, described in the following
   subsections, signals addresses explicitly to the other host to allow
   it to initiate new subflows.  The two mechanisms are complementary:
   the first is implicit and simple, while the explicit is more complex
   but is more robust.  Together, the mechanisms allow addresses to
   change in flight (and thus support operation through NATs, since the
   source address need not be known), and also allow the signalling of
   previously unknown addresses, and of addresses belonging to other
   address families (e.g. both IPv4 and IPv6).

   Here is an example of typical operation of the protocol:

   o  An MPTCP connection is initially set up between address/port A1 of
      host A and address/port B1 of host B. If host A is multihomed and
      multi-addressed, it can start an additional subflow from its
      address A2 to B1, by sending a SYN with a Join option from A2 to
      B1, using B's previously declared token for this connection.
      Alternatively, if B is multihomed, it can try to set up a new
      subflow from B2 to A1, using A's previously declared token.  In



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      either case, the SYN will be sent to the port already in use for
      the original subflow on the receiving host.

   o  Simultaneously (or after a timeout), an ADD_ADDR option
      (Section 3.4.1) is sent on an existing subflow, informing the
      receiver of the sender's alternative address(es).  The recipient
      can use this information to open a new subflow to the sender's
      additional address.  In our example, A will send ADD_ADDR option
      informing B of address/port A2.  The mix of using the SYN-based
      option and the ADD_ADDR option, including timeouts, is
      implementation-specific and can be tailored to agree with local
      policy.

   o  If subflow A2-B1 is succesfully setup, host B can use the Address
      ID in the Join option to correlate this with the ADD_ADDR option
      that will also arrive on an existing subflow; now B knows not to
      open A2-B1, ignoring the ADD_ADDR.  Otherwise, if B has not
      received the A2-B1 MP_JOIN SYN but received the ADD_ADDR, it can
      try to initiate a new subflow from one or more of its addresses to
      address A2.  This permits new sessions to be opened if one host is
      behind a NAT.

   Other ways of using the two signaling mechanisms are possible; for
   instance, signaling addresses in other address families can only be
   done explicitly using the Add Address option.

3.4.1.  Address Advertisement

   The Add Address (ADD_ADDR) TCP Option announces additional addresses
   (and optionally, ports) on which a host can be reached (Figure 12).
   Multiple instances of this TCP option can be added in a single
   message if there is sufficient TCP option space, otherwise multiple
   TCP messages containing this option will be sent.  This option can be
   used at any time during a connection, depending on when the sender
   wishes to enable multiple paths and/or when paths become available.
   As with all MPTCP signals, the receiver MUST understake standard TCP
   validity checks before acting upon it.

   Every address has an ID which can be used for uniquely identifying
   the address within a connection, for address removal.  This is also
   used to identify MP_JOIN options (see Section 3.2) relating to the
   same address, even when address translators are in use.  The ID MUST
   uniquely identify the address to the sender (within the scope of the
   connection), but the mechanism for allocating such IDs is
   implementation-specific.

   All address IDs learnt via either MP_JOIN or ADD_ADDR SHOULD be
   stored by the receiver in a data structure that gathers all the



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   Address ID to address mappings for a connection (identified by a
   token pair).  In this way there is a stored mapping between Address
   ID, observed source address and token pair for future processing of
   control information for a connection.  Note that an implementation
   MAY discard incoming address advertisements at will, for example for
   avoiding the required mapping state, or because advertised addresses
   are of no use to it (for example, IPv6 addresses when it has IPv4
   only).  Therefore, a host MUST treat address advertisements as soft
   state, and MAY choose to refresh advertisements periodically.

   This option is shown in Figure 12.  The illustration is sized for
   IPv4 addresses (IPVer = 4).  For IPv6, the IPVer field will read 6,
   and the length of the address will be 16 octets (instead of 4).

   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
   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
   remains port 80 on all subflows), as does the ephemeral port at the
   client, there may be cases (such as port-based load balancing) where
   the explicit specification of a different port is required.  If no
   port is specified, MPTCP SHOULD attempt to connect to the specified
   address on the same port as is already in use by the signalling
   subflow, and this is discussed in more detail in Section 3.7.

                           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| IPVer |  Address ID   |
      +---------------+---------------+-------+-------+---------------+
      |          Address (IPv4 - 4 octets / IPv6 - 16 octets)         |
      +-------------------------------+---------------+---------------+
      |   Port (2 octets, optional)   |
      +-------------------------------+

         Figure 12: Add Address (ADD_ADDR) option (shown for IPv4)

   Due to the proliferation of NATs, it is reasonably likely that one
   host may attempt to advertise private addresses [15].  It is not
   desirable to prohibit this, since there may be cases where both hosts
   have additional interfaces on the same private network, and a host
   MAY want to advertise such addresses.  Such advertisements must not,
   however, cause harm or security vulnerabilities.  The standard
   mechanism to create a new subflow (Section 3.2) contains a 32 bit
   token that uniquely identifies the connection to the receiving host.
   If the token is unknown, the host will return with a RST.  In the
   unlikely event that the token is known, subflow setup will continue,
   but the MAC exchange must occur for authentication.  This will fail,



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   and will provide sufficient protection against two unconnected hosts
   accidentally setting up a new subflow upon the signal of a private
   address.

   Ideally, ADD_ADDR and REMOVE_ADDR options would be sent reliably, and
   in order, to the other end.  This would be to ensure that this
   address management does not unnecessarily cause an outage in the
   connection when remove/add addresses are processed in reverse order,
   and also to ensure that all possible paths are used.  Note, however,
   that losing reliability and ordering will not break the multipath
   connections, it will just reduce the opportunity to open multipath
   paths and to survive different patterns of path failures.

   Therefore, implementing reliability signals for these TCP options is
   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 on all available subflows.  If these options need to be
   received in-order, an implementation SHOULD only send one ADD_ADDR/
   REMOVE_ADDR option per RTT, to minimise the risk of misordering.

   When receiving an ADD_ADDR message with an Address ID already in use
   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
   live subflow, but is stored by the receiver, a new ADD_ADDR SHOULD
   take precedence and replace the stored address.

   A host that receives an ADD_ADDR but finds a connection setup to that
   IP address and port number is unsuccessful SHOULD NOT perform further
   connection attempts to this address/port combination for this
   connection.  A sender that wants to trigger a new incoming connection
   attempt on a previously advertised address/port combination can
   therefore refresh ADD_ADDR information by sending the option again.

   During normal MPTCP operation, it is unlikely that there will be
   sufficient TCP option space for ADD_ADDR to be included along with
   those for data sequence numbering (Section 3.3.1).  Therefore, it is
   expected that an MPTCP implementation will send the ADD_ADDR option
   on separate ACKs.  As discussed earlier, however, an MPTCP
   implementation MUST NOT treat duplicate ACKs with MPTCP options as
   indications of congestion [8], and an MPTCP implementation SHOULD NOT
   send more than two duplicate ACKs in a row for signalling purposes.

3.4.2.  Remove Address

   If, during the lifetime of an MPTCP connection, a previously-
   announced address becomes invalid (e.g. if the interface disappears),
   the affected host SHOULD announce this so that the peer can remove
   subflows related to this address.



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   This is achieved through the Remove Address (REMOVE_ADDR) option
   (Figure 13), which will remove a previously-added address (or list of
   addresses) from a connection and terminate any subflows currently
   using that address.

   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
   instigates closure.  The receipt of REMOVE_ADDR SHOULD first trigger
   the sending of a TCP Keepalive [16] on the path, and if a response is
   received the path is not removed.  Typical TCP validity tests on the
   subflow (e.g. ensuring sequence and ack numbers are correct) MUST
   also be undertaken.

   The sending and receipt (if no keepalive response was received) of
   this message SHOULD trigger the sending of RSTs by both hosts on the
   affected subflow(s) (if possible), as a courtesy to cleaning up
   middlebox state, before cleaning up any local state.

   Address removal is undertaken by ID, so as to permit the use of NATs
   and other middleboxes that rewrite source addresses.  If there is no
   address at the requested ID, the receiver will silently ignore the
   request.

   A subflow that is still functioning MUST be closed with a FIN
   exchange as in regular TCP - for more information, see Section 3.3.3.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------+-------+---------------+
   |     Kind      |  Length = 3+n |Subtype|       |   Address ID  | ...
   +---------------+---------------+-------+-------+---------------+

              Figure 13: Remove Address (REMOVE_ADDR) option

3.5.  Fallback

   At the start of an MPTCP connection (i.e. the first subflow), it is
   important to ensure that the path is fully MPTCP-capable and the
   necessary TCP options can reach each host.  The handshake as
   described in Section 3.1 will fall back to regular TCP if either of
   the SYN messages do not have the MPTCP options: this is the same, and
   desired, behaviour in the case where a host is not MPTCP capable, or
   the path does not support the MPTCP options.  When attempting to join
   an existing MPTCP connection (Section 3.2), if a path is not MPTCP
   capable, the TCP options will not get through on the SYNs and the
   subflow will be closed.

   There is, however, another corner case which should be addressed.



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   That is one of MPTCP options getting through on the SYN, but not on
   regular packets.  This can be resolved if the subflow is the first
   subflow, and thus all data in flight is contiguous, using the
   following rules.

   A sender MUST include a DSS option with Data Sequence Mapping in
   every segment until one of the sent segments has been acknowledged
   with a DSS option containing a Data ACK.  Upon reception of the
   acknowledgement, the sender has the confirmation that the DSS option
   passes in both directions and may choose to send fewer DSS options
   than once per segment.

   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
   not MPTCP capable.  In the case of this occurring on an additional
   subflow (i.e. one started with MP_JOIN), the host MUST close the
   subflow with an RST.  In the case of the first subflow (i.e. that
   started with MP_CAPABLE), it MUST drop out of an MPTCP mode back to
   regular TCP.  The sender will send one final Data Sequence Mapping,
   with the length value of 0 indicating an infinite mapping (in case
   the path drops options in one direction only), and then revert to
   sending data on the single subflow without any MPTCP options.

   Note that this rule essentially prohibits the sending of data on the
   third packet of an MP_CAPABLE or MP_JOIN handshake, since both that
   option and a DSS cannot fit in TCP option space.  If the initiator is
   to send first, another segment must be sent that contains the data
   and DSS.  Note also that an additional subflow cannot be used until
   the initial path has been verified as MPTCP-capable.

   These rules should cover all cases where such a failure could happen:
   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
   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
   DATA_ACKed (since there is no mapping for the data), and the subflow
   can be closed with an RST.  (Note that these rules do not apply if an
   infinite mapping is included from the start - in which case, each end
   will send DSS options declaring the infinite mapping.)

   The case described above is a specialised case of fallback.  More
   generally, fallback to regular TCP can become necessary at any point
   during a connection if a non-MPTCP-aware middlebox changes the data
   stream.

   As described in Section 3.3, each portion of data for which there is
   a mapping is protected by a checksum.  This mechanism is used to
   detect if middleboxes have made any adjustments to the payload



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   (added, removed, or changed data).  A checksum will fail if the data
   has been changed in any way.  This will also detect if the length of
   data on the subflow is increased or decreased, and this means the
   Data Sequence Mapping is no longer valid.  The sender no longer knows
   what subflow-level sequence number the receiver is genuinely
   operating at (the middlebox will be faking ACKs in return), and
   cannot signal any further mappings.  Furthermore, in addition to the
   possibility of payload modifications that are valid at the
   application layer, there is the possibility that false-positives
   could be hit across MPTCP segment boundaries, corrupting the data.
   Therefore, all data from the start of the segment that failed the
   checksum onwards is not trustworthy.

   When multiple subflows are in use, the data in-flight on a subflow
   will likely involve data that is not contiguously part of the
   connection-level stream, since segments will be spread across the
   multiple subflows.  Due to the problems identified above, it is not
   possible to determine what the adjustment has done to the data
   (notably, any changes to the subflow sequence numbering).  Therefore,
   it is not possible to recover the subflow, and the affected subflow
   must be immediately closed with an RST, featuring an MP_FAIL option
   (Figure 14), which defines the Data Sequence Number at the start of
   the segment (defined by the Data Sequence Mapping) which had the
   checksum failure.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+----------------------+
      |     Kind      |   Length=12   |Subtype|      (reserved)      |
      +---------------+---------------+-------+----------------------+
      |                 Data Sequence Number (8 octets)              :
      +--------------------------------------------------------------+
      :                Data Sequence Number (continued)              |
      +--------------------------------------------------------------+


                   Figure 14: Fallback (MP_FAIL) option

   Failed data will not be DATA_ACKed and so will be re-transmitted on
   other subflows (Section 3.3.6).

   A special case is when there is a single subflow and it fails with a
   checksum error.  Here, MPTCP should be able to recover and continue
   sending data.  There are two possible mechanisms to support this.
   The first and simplest is to establish a new subflow as part of the
   same MPTCP connection, and then close the original one with a RST.
   Since it is known that the path may be compromised, it is not
   desirable to use MPTCP's segmentation on this path any longer.  The



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   new subflow will begin and will signal an infinite mapping (indicated
   by length=0 in the Data Sequence Mapping option, Section 3.3) from
   the data sequence number of the segment that failed the checksum.
   This connection will then continue to appear as a regular TCP
   session, and a middlebox may change the payload without causing
   unintentional harm.

   An optimisation is possible, however.  If it is known that all
   unacknowledged data in flight is contiguous, an infinite mapping
   could be applied to the subflow without the need to close it first,
   and essentially turn off all further MPTCP signalling.  In this case,
   if a receiver identifies a checksum failure when there is only one
   path, it will send back an MP_FAIL option on the subflow-level ACK.
   The sender will receive this, and if all unacknowledged data in
   flight is contiguous, will signal an infinite mapping (if the data is
   not contiguous, the sender MUST send an RST).  This infinite mapping
   will be a DSS option (Section 3.3) on the first new packet,
   containing a Data Sequence Mapping that acts retroactively, referring
   to the start of the subflow sequence number of the last segment that
   was known to be delivered intact.  From that point onwards data can
   be altered by a middlebox without affecting MPTCP, as the data stream
   is equivalent to a regular, legacy TCP session.

   After a sender signals an infinite mapping it MUST only use subflow
   ACKs to clear its send buffer.  This is because Data ACKs may become
   misaligned with the subflow ACKs when middleboxes insert or delete
   data.  The receive SHOULD stop generating Data ACKs after it receives
   an infinite mapping.

   When a connection is in fallback mode, only one subflow can send data
   at a time.  Otherwise, the receiver would not know how to reorder the
   data.  However, subflows can be opened and closed as necessary, as
   long as a single one is active at any point.

   It should be emphasised that we are not attempting to prevent the use
   of middleboxes that want to adjust the payload.  An MPTCP-aware
   middlebox to provide such functionality could be designed that would
   re-write checksums if needed, and additionally would be able to parse
   the data sequence mappings, and thus not hit false positives though
   not knowing where data boundaries lie.

3.6.  Error Handling

   In addition to the fallback mechanism as described above, the
   standard classes of TCP errors may need to be handled in an MPTCP-
   specific way.  Note that changing semantics - such as the relevance
   of an RST - are covered in Section 4.  Where possible, we do not want
   to deviate from regular TCP behaviour.



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   The following list covers possible errors and the appropriate MPTCP
   behaviour:

   o  Unknown token in MP_JOIN (or MAC failure in MP_JOIN ACK, or
      missing MP_JOIN in SYN/ACK response): send RST (analogous to TCP's
      behaviour on an unknown port)

   o  DSN out of Window (during normal operation): drop the data, do not
      send Data ACKs.

   o  Remove request for unknown address ID: silently ignore

3.7.  Heuristics

   There are a number of heuristics that are needed for performance or
   deployment but which are not required for protocol correctness.  In
   this section we detail such heuristics.  Note that discussion of
   buffering and certain sender and receiver window behaviours are
   presented in Section 3.3.4 and Section 3.3.5, as well as
   retransmission in Section 3.3.6.

3.7.1.  Port Usage

   Under typical operation an MPTCP implementation SHOULD use the same
   ports as already in use.  In other words, the destination port of a
   SYN containing an MP_JOIN option SHOULD be the same as the remote
   port of the first subflow in the connection.  The local port for such
   SYNs SHOULD also be the same as for the first subflow (and as such,
   an implementation SHOULD reserve ephemeral ports across all local IP
   addresses), although there may be cases where this is infeasible.
   This strategy is intended to maximize the probability of the SYN
   being permitted by a firewall or NAT at the recipient and to avoid
   confusing any network monitoring software.

   There may also be cases, however, where the passive opener wishes to
   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
   Section 3.4.1.  It is therefore feasible to allow multiple subflows
   between the same two addresses but using different port pairs, and
   such a facility could be used to allow load balancing within the
   network based on 5-tuples (e.g. some ECMP implementations).

3.7.2.  Delayed Subflow Start

   Many TCP connections are short-lived and consist only of a few
   segments, and so the overheads of using MPTCP outweigh any benefits.
   A heuristic is required, therefore, to decide when to start using
   additional subflows in an MPTCP connection.  We expect that



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   experience gathered from deployments will provide further guidance on
   this, and will be affected by particular application characteristics
   (which are likely to change over time).  However, a suggested
   general-purpose heuristic that an implementation MAY choose to employ
   is as follows.  Results from experimental deployments are needed in
   order to verify the correctness of this proposal.

   If a host has data buffered for its peer (which implies that the
   application has received a request for data), the host opens one
   subflow for each initial window's worth of data that is buffered.

   Consideration should also be given to limiting the rate of adding new
   subflows, as well as limiting the total number of subflows open for a
   particular connection.  A host may choose to vary these values based
   on its load or knowledge of traffic and path characteristics.

   Note that this heuristic alone is probably insufficient.  Traffic for
   many common applications, such as downloads, is highly asymmetric and
   the host that is multihomed may well be the client which will never
   fill its buffers, and thus never use MPTCP.  Advanced APIs that allow
   an application to signal its traffic requirements would aid in these
   decisions.

   An additional time-based heuristic could be applied, opening
   additional subflows after a given period of time has passed.  This
   would alleviate the above issue, and also provide resilience for low-
   bandwidth but long-lived applications.

   This section has shown some of the considerations that an implementer
   should give when developing MPTCP heuristics, but is not intended to
   be prescriptive.

3.7.3.  Failure Handling

   Requirements for MPTCP's handling of unexpected signals have been
   given in Section 3.6.  There are other failure cases, however, where
   a hosts can choose appropriate behaviour.

   For example, Section 3.1 suggests that a host should fall back to
   trying regular TCP SYNs after several failures of MPTCP SYNs.  A host
   may keep a system-wide cache of such information, so that it can back
   off from using MPTCP, firstly for that particular destination host,
   and eventually on a whole interface, if MPTCP connections continue
   failing.

   Another failure could occur when the MP_JOIN handshake fails.
   Section 3.6 specifies that an incorrect handshake MUST lead to the
   subflow being closed with a RST.  A host operating an active



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   intrusion detection system may choose to start blocking MP_JOIN
   packets from the source host if multiple failed MP_JOIN attempts are
   seen.  From the connection initiator's point of view, if an MP_JOIN
   fails, it SHOULD NOT attempt to connect to the same IP address and
   port during the lifetime of the connection, unless the other host
   refreshes the information with another ADD_ADDR option.  Note that
   the ADD_ADDR option is informational only, and does not guarantee the
   other host will attempt a connection.

   In addition, an implementation may learn over a number of connections
   that certain interfaces or destination addresses consistently fail
   and may default to not trying to use MPTCP for these.  Behaviour
   could also be learnt for particularly badly performing subflows or
   subflows that regularly fail during use, in order to temporarily
   choose not to use these paths.


4.  Semantic Issues

   In order to support multipath operation, the semantics of some TCP
   components have changed.  To aid clarity, this section collects these
   semantic changes as a reference.

   Sequence Number:  The (in-header) TCP sequence number is specific to
      the subflow.  To allow the receiver to reorder application data,
      an additional data-level sequence space is used.  In this data-
      level sequence space, the initial SYN and the final DATA_FIN
      occupy one octet of sequence space.  There is an explicit mapping
      of data sequence space to subflow sequence space, which is
      signalled through TCP options in data packets.

   ACK:  The ACK field in the TCP header acknowledges only the subflow
      sequence number, not the data-level sequence space.
      Implementations SHOULD NOT attempt to infer a data-level
      acknowledgement from the subflow ACKs.  Instead an explicit data-
      level ACK is used.  This avoids possible deadlock scenarios when a
      non-TCP-aware middlebox pro-actively ACKs at the subflow level,
      and separates subflow- and connection-level processing at an end
      host.

   Duplicate ACK:  A duplicate ACK that includes MPTCP signalling MUST
      NOT be treated as a signal of congestion.  To avoid any non-MPTCP-
      aware entities also mistakenly seeing duplicate ACKs in such
      cases, MPTCP SHOULD NOT send more than two duplicate ACKs
      containing MPTCP signals in a row.






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   Receive Window:  The receive window in the TCP header indicates the
      amount of free buffer space for the whole data-level connection
      (as opposed to for this subflow) that is available at the
      receiver.  This is the same semantics as regular TCP, but to
      maintain these semantics the receive window must be interpreted at
      the sender as relative to the sequence number given in the
      DATA_ACK rather than the subflow ACK in the TCP header.  In this
      way the original flow control role is preserved.  Note that some
      middleboxes may change the receive window, and so a host must use
      the maximum value of those recently seen on the constituent
      subflows for the connection-level receive window, and also need to
      maintain a subflow-level window for subflow-level processing.

   FIN:  The FIN flag in the TCP header applies only to the subflow it
      is sent on, not to the whole connection.  For connection-level FIN
      semantics, the DATA_FIN option is used.

   RST:  The RST flag in the TCP header applies only to the subflow it
      is sent on, not to the whole connection.

   Address List:  Address list management (i.e. knowledge of the local
      and remote hosts' lists of available IP addresses) is handled on a
      per-connection basis (as opposed to per-subflow, per host, or per
      pair of communicating hosts).  This permits the application of
      per-connection local policy.  Adding an address to one connection
      (either explicitly through an Add Address message, or implicitly
      through a Join) has no implication for other connections between
      the same pair of hosts.

   5-tuple:  The 5-tuple (protocol, local address, local port, remote
      address, remote port) presented by kernel APIs to the application
      layer in a non-multipath-aware application is that of the first
      subflow, even if the subflow has since been closed and removed
      from the connection.  This decision, and other related API issues,
      are discussed in more detail in [5].


5.  Security Considerations

   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
   prevent these, [3] presents a set of requirements for a security
   solution for MPTCP.  The fundamental goal is for the security of
   MPTCP to be "no worse" than regular TCP today, and the key security
   requirements are:

   o  Provide a mechanism to confirm that the parties in a subflow
      handshake are the same as in the original connection setup.



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   o  Provide verification that the peer can receive traffic at a new
      address before using it as part of a connection.

   o  Provide replay protection, i.e. ensure that a request to add/
      remove a subflow is 'fresh'.

   In order to achieve these goals, MPTCP includes a hash-based
   handshake algorithm documented in Section 3.1 and Section 3.2.

   The security of the MPTCP connection hangs on the use of keys that
   are shared once at the start of the first subflow, and never again in
   the clear.  To ease demultiplexing whilst not giving away any
   cryptographic material, future subflows use a truncated SHA-1 hash of
   this key as the connection identification "token".  The keys are
   combined and used as keys in a MAC, and this should verify that the
   parties in the handshake are the same as in the original connection
   setup.  It also provides verification that the peer can receive
   traffic at this new address.  Replay attacks would still be possible
   when only keys are used, and therefore the handshakes use single-use
   random numbers (nonces) at both ends - this ensures the MAC will
   never be the same on two handshakes.  The use of crypto capability
   bits in the initial connection handshake to negotiate use of a
   particular algorithm will allow the deployment of additional crypto
   mechanisms in the future.  Note that this would be susceptible to
   bid-down attacks only if the attacker was on-path (and thus would be
   able to modify the data anyway).  The security mechanism presented in
   this draft should therefore protect against all forms of flooding and
   hijacking attacks suggested in [6].


6.  Interactions with Middleboxes

   Multipath TCP was designed to be deployable in the present world.
   Its design takes into account "reasonable" existing middlebox
   behaviour.  In this section we outline a few representative
   middlebox-related failure scenarios and show how multipath TCP
   handles them.  Next, we list the design decisions multipath has made
   to accomodate the different middleboxes.

   A primary concern is our use of a new TCP option.  Most middleboxes
   should just forward packets with new options unchanged, yet there are
   some that don't.  These we expect will either strip options and pass
   the data, drop packets with new options, copy the same option into
   multiple segments (e.g. when doing segmentation) or drop options
   during segment coalescing.

   MPTCP uses a single new TCP option "Kind", and all message types are
   defined by "subtype" values (see Section 8).  This should reduce the



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   chances of only some types of MPTCP options being passed, and instead
   the key differing characteristics are different paths, and the
   presence of the SYN flag.

   MPTCP SYN packets on the first subflow of a connection contain the
   MP_CAPABLE option (Section 3.1).  If this is dropped, MPTCP SHOULD
   fall back to regular TCP.  If packets with the MP_JOIN option
   (Section 3.2) are dropped, the paths will simply not be used.

   If a middlebox strips options but otherwise passes the packets
   unchanged, MPTCP will behave safely.  If an MP_CAPABLE option is
   dropped on either the outgoing or the return path, the initiating
   host can fall back to regular TCP, as illustred in Figure 15 and
   discussed in Section 3.1.

   Subflow SYNs contain the MP_JOIN option.  If this option is stripped
   on the outgoing path the SYN will appear to be a regular SYN to host
   B. Depending on whether there is a listening socket on the target
   port, host B will reply either with SYN/ACK or RST (subflow
   connection fails).  When host A receives the SYN/ACK it sends a RST
   because the SYN/ACK does not contain the MP_JOIN option and its
   token.  Either way, the subflow setup fails, but otherwise does not
   affect the MPTCP connection as a whole.

        Host A                             Host B
         |              Middlebox M            |
         |                   |                 |
         |  SYN(MP_CAPABLE)  |        SYN      |
         |-------------------|---------------->|
         |                SYN/ACK              |
         |<------------------------------------|
     a) MP_CAPABLE option stripped on outgoing path

       Host A                               Host B
         |            SYN(MP_CAPABLE)          |
         |------------------------------------>|
         |             Middlebox M             |
         |                 |                   |
         |    SYN/ACK      |SYN/ACK(MP_CAPABLE)|
         |<----------------|-------------------|
     b) MP_CAPABLE option stripped on return path

   Figure 15: Connection Setup with Middleboxes that Strip Options from
                                  Packets

   We now examine data flow with MPTCP, assuming the flow is correctly
   setup, which implies the options in the SYN packets were allowed
   through by the relevant middleboxes.  If options are allowed through



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   and there is no resegmentation or coalescing to TCP segments,
   multipath TCP flows can proceed without problems.

   The case when options get stripped on data packets has been discussed
   in the Fallback section.  If a fraction of options are stripped,
   behaviour is not deterministic.  If some Data Sequence Mappings are
   lost, the connection can continue so long as mappings exist for the
   subflow-level data (e.g. if multiple maps have been sent that
   reinforce each other).  If some subflow-level space is left unmapped,
   however, the subflow is treated as broken and is closed, as discussed
   in Section 3.3.  MPTCP should survive with a loss of some Data ACKs,
   but performance will degrade as the fraction of stripped options
   increases.  We do not expect such cases to appear in practice,
   though: most middleboxes will either strip all options or let them
   all through.

   We end this section with a list of middlebox classes, their behaviour
   and the elements in the MPTCP design that allow operation through
   such middleboxes.  Issues surrounding dropping packets with options
   or stripping options were discussed above, and are not included here:

   o  NATs [17] (Network Address (and Port) Translators) change the
      source address (and often source port) of packets.  This means
      that a host will not know its public-facing address for signalling
      in MPTCP.  Therefore, MPTCP permits implicit address addition via
      the MP_JOIN option, and the handshake mechanism ensures that
      connection attempts to private addresses [15] do not cause
      problems.  Explicit address removal is undertaken by an ID number
      to allow no knowledge of the source address.

   o  Performance Enhancing Proxies (PEPs) [18] might pro-actively ACK
      data to increase performance.  Problems will occur if a PEP ACKs
      data and then fails before sending it on to the receiver, or if
      the receiver is mobile and moves away before proactively ACKed
      data is forwarded on.  If subflow ACKs were used to control send
      buffering, the data could be lost and never be retransmitted, thus
      causing the subflow to permanently stall.  MPTCP therefore uses
      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
      connection level ACKs.

   o  Traffic Normalizers [19] may not allow holes in sequence numbers,
      and may cache packets and retransmit the same data.  MPTCP looks
      like standard TCP on the wire, and will not retransmit different
      data on the same subflow sequence number.

   o  Firewalls [20] might perform initial sequence number randomization
      on TCP connections.  MPTCP uses relative sequence numbers in data



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      sequence mapping to cope with this.  Like NATs, firewalls will not
      permit many incoming connections, so MPTCP supports address
      signalling (ADD_ADDR) so that a multi-addressed host can invite
      its peer behind the firewall/NAT to connect out to its additional
      interface.

   o  Intrusion Detection Systems look out for traffic patterns and
      content that could threaten a network.  Multipath will mean that
      such data is potentially spread, so it is more difficult for an
      IDS to analyse the whole traffic, and potentially increases the
      risk of false positives.  However, for an MPTCP-aware IDS, tokens
      can be read by such systems to correlate multiple subflows and re-
      assemble for analysis.

   o  Application level middleboxes such as content-aware firewalls may
      alter the payload within a subflow, such as re-writing URIs in
      HTTP traffic.  MPTCP will detect these using the checksum and
      close the affected subflow(s), if there are other subflows that
      can be used.  If all subflows are affected multipath will fallback
      to TCP, allowing such middleboxes to change the payload.  MPTCP-
      aware middleboxes should be able to adjust the payload and MPTCP
      metadata in order not to break the connection.

   In addition, all classes of middleboxes may affect TCP traffic in the
   following ways:

   o  TCP Options may be removed, or packets with unknown options
      dropped, by many classes of middleboxes.  It is intended that the
      initial SYN exchange, with a TCP Option, will be sufficient to
      identify the path capabilities.  If such a packet does not get
      through, MPTCP will end up falling back to regular TCP.

   o  Segmentation/Coalescing (e.g.  TCP segmentation offloading) might
      copy options between packets and might strip some options.
      MPTCP's data sequence mapping includes the relative subflow
      sequence number instead of using the sequence number in the
      segment.  In this way, the mapping is independent of the packets
      that carry it.

   o  The Receive Window may be shrunk by some middleboxes at the
      subflow level.  MPTCP will use the maximum window at data-level,
      but will also obey subflow specific windows.


7.  Acknowledgements

   The authors are supported by Trilogy
   (http://www.trilogy-project.org), a research project (ICT-216372)



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   partially funded by the European Community under its Seventh
   Framework Program.  The views expressed here are those of the
   author(s) only.  The European Commission is not liable for any use
   that may be made of the information in this document.

   The authors gratefully acknowledge significant input into this
   document from Sebastien Barre, Christoph Paasch, and Andrew McDonald.

   The authors also wish to acknowledge reviews and contributions from
   Iljitsch van Beijnum, Lars Eggert, Marcelo Bagnulo, Robert Hancock,
   Pasi Sarolahti, Toby Moncaster, Philip Eardley, Sergio Lembo,
   Lawrence Conroy, Yoshifumi Nishida, Bob Briscoe, Stein Gjessing,
   Andrew McGregor, and Georg Hampel.


8.  IANA Considerations

   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"
   field seen in all MPTCP options in this document.

   This document will also request IANA operates a registry for MPTCP
   option subtype values.  The values as defined by this specification
   are as follows:

   +-------------+-----------------------------+---------------+-------+
   |    Symbol   |             Name            |      Ref      | Value |
   +-------------+-----------------------------+---------------+-------+
   |  MP_CAPABLE |      Multipath Capable      |  Section 3.1  |  0x0  |
   |   MP_JOIN   |       Join Connection       |  Section 3.2  |  0x1  |
   |     DSS     |  Data Sequence Signal (Data |  Section 3.3  |  0x2  |
   |             |    ACK and Data Sequence    |               |       |
   |             |           Mapping)          |               |       |
   |   ADD_ADDR  |         Add Address         | Section 3.4.1 |  0x3  |
   | REMOVE_ADDR |        Remove Address       | Section 3.4.2 |  0x4  |
   |   MP_PRIO   |   Change Subflow Priority   | Section 3.3.8 |  0x5  |
   |   MP_FAIL   |           Fallback          |  Section 3.5  |  0x6  |
   +-------------+-----------------------------+---------------+-------+

                      Table 1: MPTCP Option Subtypes

   This document also requests that IANA keeps a registry of
   cryptographic handshake algorithms based on the flags in MP_CAPABLE
   (Section 3.1).  This document specifies only one algorithm:







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            +-------+-----------+----------------------------+
            | Flags | Algorithm |          Document          |
            +-------+-----------+----------------------------+
            |  0x1  | HMAC-SHA1 | This document, Section 3.2 |
            +-------+-----------+----------------------------+

                    Table 2: MPTCP Handshake Algorithms


9.  References

9.1.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

9.2.  Informative References

   [2]   Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
         September 1981.

   [3]   Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar,
         "Architectural Guidelines for Multipath TCP Development",
         RFC 6182, March 2011.

   [4]   Raiciu, C., Handley, M., and D. Wischik, "Coupled Congestion
         Control for Multipath Transport Protocols",
         draft-ietf-mptcp-congestion-05 (work in progress), June 2011.

   [5]   Scharf, M. and A. Ford, "MPTCP Application Interface
         Considerations", draft-ietf-mptcp-api-02 (work in progress),
         June 2011.

   [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.

   [8]   Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
         Control", RFC 2581, April 1999.

   [9]   Gont, F., "Security Assessment of the Transmission Control
         Protocol (TCP)", draft-ietf-tcpm-tcp-security-02 (work in
         progress), January 2011.

   [10]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and
         HMAC-SHA)", RFC 4634, July 2006.



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   [11]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
         Requirements for Security", BCP 106, RFC 4086, June 2005.

   [12]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.

   [13]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions for
         High Performance", RFC 1323, May 1992.

   [14]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of
         Explicit Congestion Notification (ECN) to IP", RFC 3168,
         September 2001.

   [15]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E.
         Lear, "Address Allocation for Private Internets", BCP 5,
         RFC 1918, February 1996.

   [16]  Braden, R., "Requirements for Internet Hosts - Communication
         Layers", STD 3, RFC 1122, October 1989.

   [17]  Srisuresh, P. and K. Egevang, "Traditional IP Network Address
         Translator (Traditional NAT)", RFC 3022, January 2001.

   [18]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
         Shelby, "Performance Enhancing Proxies Intended to Mitigate
         Link-Related Degradations", RFC 3135, June 2001.

   [19]  Handley, M., Paxson, V., and C. Kreibich, "Network Intrusion
         Detection: Evasion, Traffic Normalization, and End-to-End
         Protocol Semantics", Usenix Security 2001, 2001, <http://
         www.usenix.org/events/sec01/full_papers/handley/handley.pdf>.

   [20]  Freed, N., "Behavior of and Requirements for Internet
         Firewalls", RFC 2979, October 2000.


Appendix A.  Notes on use of TCP Options

   The TCP option space is limited due to the length of the Data Offset
   field in the TCP header (4 bits), which defines the TCP header length
   in 32 bit words.  With the standard TCP header being 20 bytes, this
   leaves a maximum of 40 bytes for options, and many of these may
   already be used by options such as timestamp and SACK.

   We 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
   to fit all the options we propose using in this draft.




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   SYN packets typically include MSS (4 bytes), window scale (3 bytes),
   SACK permitted (2 bytes) and timestamp (10 bytes) options.  Together
   these sum to 19 bytes.  Some operating systems appear to pad each
   option up to a word boundary, thus using 24 bytes (a brief survey
   suggests Windows XP and Mac OS X do this, whereas Linux does not).
   Optimistically, therefore, we have 21 bytes spare, or 16 if it has to
   be word-aligned.  In either case, however, the SYN versions of
   Multipath Capable (12 bytes) and Join (12 or 16 bytes) options will
   fit in this remaining space.

   TCP data packets typically carry timestamp options in every packet,
   taking 10 bytes (or 12 with padding).  That leaves 30 bytes (or 28,
   if word-aligned).  The Data Sequence Signal (DSS) option varies in
   length depending on whether the Data Sequence Mapping and DATA ACK
   are included, and whether the sequence numbers in use are 4 or 8
   octets.  The maximum size of the DSS option is 28 bytes, so even that
   will fit in the available space.  But unless a connection is both bi-
   directional and high-bandwidth, it is unlikely that all that option
   space will be required on each DSS option.

   It is not necessary to include the Data Sequence Mapping and DATA ACK
   in each packet, and in many cases it may be possible to alternate
   their presence (so long as the mapping covers the data being sent in
   the following packet).  Other options include: alternating between 4
   and 8 byte sequence numbers in each option; and sending the DATA_ACK
   on a duplicate subflow-level ACK (although note that this must not be
   taken as a signal of congestion).

   On subflow and connection setup, an MPTCP option is also set on the
   third packet (an ACK).  These are 20 bytes (for Multipath Capable)
   and 24 bytes (for Join), both of which will fit in the available
   option space.

   Pure ACKs in TCP typically contain only timestamps (10B).  Here,
   multipath TCP typically needs to encode only the DATA ACK (maximum of
   12 octets).  Occasionally ACKs will contain SACK information.
   Depending on the number of lost packets, SACK may utilize the entire
   option space.  If a DATA ACK had to be included, then it is probably
   necessary to reduce the number of SACK blocks to accomodate the DATA
   ACK.  However, the presence of the DATA ACK is unlikely to be
   necessary in a case where SACK is in use, since until at least some
   of the SACK blocks have been retransmitted, the cumulative data-level
   ACK will not be moving forward (or if it does, due to retransmissions
   on another path, then that path can also be used to transmit the new
   DATA ACK).

   The ADD_ADDR option can be between 8 and 22 bytes, depending on
   whether IPv4 or IPv6 is used, and whether the port number is present



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   or not.  It is unlikely that such signalling would fit in a data
   packet (although if there is space, it is fine to include it).  It is
   recommended to use duplicate ACKs with no other payload or options in
   order to transmit these rare signals.  Note this is the reason for
   mandating that duplicate ACKs with MPTCP options are not taken as a
   signal of congestion.

   Finally, there are issues with reliable delivery of options.  As
   options can also be sent on pure ACKs, these are not reliably sent.
   This is not an issue for DATA_ACK due to their cumulative nature, but
   may be an issue for ADD_ADDR/REMOVE_ADDR options.  Here, it is
   recommended to send these options redundantly (whether on multiple
   paths, or on the same path on a number of ACKs - but interspersed
   with data in order to avoid interpretation as congestion).  The cases
   where options are stripped by middleboxes are discussed in Section 6.


Appendix B.  Control Blocks

   Conceptually, an MPTCP connection can be represented as an MPTCP
   control block that contains several variables that track the progress
   and the state of the MPTCP connection and a set of linked TCP control
   blocks that correspond to the subflows that have been established.

   RFC793 [2] specifies several state variables.  Whenever possible, we
   reuse the same terminology as RFC793 to describe the state variables
   that are maintained by MPTCP.

B.1.  MPTCP Control Block

   The MPTCP control block contains the following variable per-
   connection.

B.1.1.  Authentication and Metadata

   Local.Token (32 bits):  This is the token chosen by the local host on
      this MPTCP connection.  The token MUST be unique among all
      established MPTCP connections, generated from the local key.

   Local.Key (64 bits):  This is the key sent by the local host on this
      MPTCP connection.

   Remote.Token (32 bits):  This is the token chosen by the remote host
      on this MPTCP connection, generated from the remote key.







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   Remote.Key (64 bits):  This is the key chosen by the remote host on
      this MPTCP connection

   MPTCP.Checksum (flag):  This flag is set to true if at least one of
      the hosts has set the C bit in the MP_CAPABLE options exchanged
      during connection establishment, and is set to false otherwise.
      If this flag is set, the checksum must be computed in all DSS
      options.

B.1.2.  Sending Side

   SND.UNA (64 bits):  This is the Data Sequence Number of the next byte
      to be acknowledged, at the MPTCP connection level.  This variable
      is updated upon reception of a DSS option containing a DATA_ACK.

   SND.NXT (64 bits):  This is the Data Sequence Number of the next byte
      to be sent.  SND.NXT is used to determine the value of the DSN in
      the DSS option.

   SND.WND (32 bits with RFC1323, 16 bits without):  This is the sending
      window.  MPTCP maintains the sending window at the MPTCP
      connection level and the same window is shared by all subflows.
      All subflows use the MPTCP connection level SND.WND to compute the
      SEQ.WND value which is sent in each transmitted segment.

B.1.3.  Receiving Side

   RCV.NXT (64 bits):  This is the Data Sequence Number of the next byte
      which is expected on the MPTCP connection.  This state variable is
      modified upon reception of in-order data.  The value of RCV.NXT is
      used to specify the DATA_ACK which is sent in the DSS option on
      all subflows.

   RCV.WND (32bits with RFC1323, 16 bits otherwise):  This is the
      connection-level receive window, which is the maximum of the
      RCV.WND on all the subflows.

B.2.  TCP Control Blocks

   The MPTCP control block also contains a list of the TCP control
   blocks that are associated to the MPTCP connection.

   Note that the TCP control block on the TCP subflows does not contain
   the RCV.WND and SND.WND state variables as these are maintained at
   the MPTCP connection level and not at the subflow level.

   Inside each TCP control block, the following state variables are
   defined:



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B.2.1.  Sending Side

   SND.UNA (32 bits):  This is the sequence number of the next byte to
      be acknowledged on the subflow.  This variable is updated upon
      reception of each TCP acknowledgement on the subflow.

   SND.NXT (32 bits):  This is the sequence number of the next byte to
      be sent on the subflow.  SND.NXT is used to set the value of
      SEG.SEQ upon transmission of the next segment.

B.2.2.  Receiving Side

   RCV.NXT (32 bits):  This is the sequence number of the next byte
      which is expected on the subflow.  This state variable is modified
      upon reception of in-order segments.  The value of RCV.NXT is
      copied to the SEG.ACK field of the next segments transmitted on
      the subflow.

   RCV.WND (32 bits with RFC1323, 16 bits otherwise):  This is the
      subflow-level receive window which is updated with the window
      field from the segments received on this subflow.


Appendix C.  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.






















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                               +---------+
                               | 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
   document between versions.

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.





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   o  Merged Data Sequence Number, DATA ACK, and DATA FIN.

   o  Changed DATA FIN behaviour (separated from subflow FIN).

   o  Added crypto agility and checksum negotiation.

   o  Redefined MP_JOIN handshake to use only three TCP options.

   o  Added pseudo-header to checksum.

   o  Many clarifications and re-structuring.

   o  Added more discussion on heuristics.

D.3.  Changes since draft-ietf-mptcp-multiaddressed-01

   o  Added proposal for hash-based security mechanism.

   o  Added receiver subflow policy control (backup path flags and
      MP_PRIO option).

   o  Changed DSN_MAP checksum to use the TCP checksum algorithm.

D.4.  Changes since draft-ietf-mptcp-multiaddressed-00

   o  Various clarifications and minor re-structuring in response to
      comments.

D.5.  Changes since draft-ford-mptcp-multiaddressed-03

   o  Clarified handshake mechanism, especially with regard to error
      cases (Section 3.2).

   o  Added optional port to ADD_ADDR and clarified situation with
      private addresses (Section 3.4.1).

   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
      payload-altering middleboxes, and defined fallback mechanism
      (Section 3.5).

   o  Major clarifications to receive window discussion (Section 3.3.5).

   o  Various textual clarifications, especially in examples.






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D.6.  Changes since draft-ford-mptcp-multiaddressed-02

   o  Remove Version and Address ID in MP_CAPABLE in Section 3.1, and
      make ISN be 6 bytes.

   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
      bytes if space is at a premium.

   o  Clarified behaviour of MP_JOIN in Section 3.2.

   o  Added DATA_ACK to Section 3.3.

   o  Clarified fallback to non-multipath once a non-MP-capable SYN is
      sent.


Authors' Addresses

   Alan Ford
   Roke Manor Research
   Old Salisbury Lane
   Romsey, Hampshire  SO51 0ZN
   UK

   Phone: +44 1794 833 465
   Email: alan.ford@roke.co.uk


   Costin Raiciu
   University College London
   Gower Street
   London  WC1E 6BT
   UK

   Email: c.raiciu@cs.ucl.ac.uk


   Mark Handley
   University College London
   Gower Street
   London  WC1E 6BT
   UK

   Email: m.handley@cs.ucl.ac.uk






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   Olivier Bonaventure
   Universite catholique de Louvain
   Pl. Ste Barbe, 2
   Louvain-la-Neuve  1348
   Belgium

   Email: olivier.bonaventure@uclouvain.be












































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