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Versions: 00 01 02 03 draft-ietf-mptcp-multiaddressed

Internet Engineering Task Force                                  A. Ford
Internet-Draft                                       Roke Manor Research
Intended status: Experimental                                  C. Raiciu
Expires: September 9, 2010                                    M. Handley
                                               University College London
                                                           March 8, 2010

     TCP Extensions for Multipath Operation with Multiple Addresses


   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 - 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 to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on September 9, 2010.

Copyright Notice

   Copyright (c) 2010 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
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   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 BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Design Assumptions . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Layered Representation . . . . . . . . . . . . . . . . . .  5
     1.3.  Operation Summary  . . . . . . . . . . . . . . . . . . . .  6
     1.4.  Requirements Language  . . . . . . . . . . . . . . . . . .  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Semantic Issues  . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Connection Initiation  . . . . . . . . . . . . . . . . . .  9
     4.2.  Starting a New Subflow . . . . . . . . . . . . . . . . . . 11
     4.3.  Address Knowledge Exchange (Path Management) . . . . . . . 13
       4.3.1.  Address Advertisement  . . . . . . . . . . . . . . . . 14
       4.3.2.  Remove Address . . . . . . . . . . . . . . . . . . . . 16
     4.4.  General MPTCP Operation  . . . . . . . . . . . . . . . . . 16
       4.4.1.  Data Sequence Numbering  . . . . . . . . . . . . . . . 17
       4.4.2.  Data Acknowledgements  . . . . . . . . . . . . . . . . 19
       4.4.3.  Receiver Considerations  . . . . . . . . . . . . . . . 19
       4.4.4.  Sender Considerations  . . . . . . . . . . . . . . . . 20
       4.4.5.  Congestion Control Considerations  . . . . . . . . . . 21
       4.4.6.  Subflow Policy . . . . . . . . . . . . . . . . . . . . 21
     4.5.  Closing a Connection . . . . . . . . . . . . . . . . . . . 22
     4.6.  Error Handling . . . . . . . . . . . . . . . . . . . . . . 24
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   6.  Interactions with Middleboxes  . . . . . . . . . . . . . . . . 25
   7.  Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 28
   8.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 28
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 30
     11.2. Informative References . . . . . . . . . . . . . . . . . . 30
   Appendix A.  Notes on use of TCP Options . . . . . . . . . . . . . 31
   Appendix B.  Signaling Control Information in the Payload  . . . . 32
   Appendix C.  Resync Packet . . . . . . . . . . . . . . . . . . . . 32
   Appendix D.  Changelog . . . . . . . . . . . . . . . . . . . . . . 34
     D.1.  Changes since draft-ford-mptcp-multiaddressed-02 . . . . . 34
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34

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1.  Introduction

   Multipath TCP (henceforth referred to as MPTCP) is set of extensions
   to regular TCP [2] to allow a transport connection to operate across
   multiple paths simultaneously.  This document presents the protocol
   changes required by Multipath 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 several others:

   o  Architecture [3], which explains the motivations behind Multipath
      TCP and 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 endpoints are multihomed and

   To simplify the design we assume that the presence of multiple
   addresses at an endpoint 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.

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

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   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; 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).

   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.

   Areas for further study:

   o  In theory, since this is purely a TCP extension, it should be
      possible to use MPTCP with both IPv4 and IPv6 subflows for the
      same connection on dual-stack hosts, thus having the additional
      possible benefit of aiding transition.

   o  Some features of the design presented here could be extended to
      work with non-multi-addressed hosts by using other packet metadata
      (such as ports or flow label), packet marking, or partial
      multipath (such as by using a proxy).

1.2.  Layered Representation

   MPTCP operates at the transport layer, and its existence aims to be
   transparent to both higher and lower layers.  It is a set of
   additional features on top of standard TCP, and as such MPTCP is
   designed to be usable by legacy applications with no changes.  A
   possible implementation would be for such a feature to be a system-
   wide setting: "Use multipath TCP by default?  Y/N".  Multipath-aware
   applications would be able to use an extended sockets API to have
   further influence on the behaviour of MPTCP.  Figure 1 illustrates
   this layering.

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

      Figure 1: Comparison of Standard TCP and MPTCP Protocol Stacks

   Detailed discussion of an architecture for developing a multipath TCP
   implementation, especially regarding the functional separation by
   which different components should be developed, is given in [3].

1.3.  Operation Summary

   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 4.

   o  To a non-MPTCP-aware application, MPTCP will be indistinguishable
      from normal TCP.  All MPTCP operation is handled by the MPTCP
      implementation, although extended APIs could provide additional
      control and influence [5].  An application begins by opening a TCP
      socket in the normal way.

   o  An MPTCP connection begins as a single TCP session.  This is
      illustrated in Figure 2 as being between Addresses A1 and B1 on
      Hosts A and B respectively.

   o  If extra paths are available, additional TCP sessions 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.

   o  MPTCP identifies multiple paths by the presence of multiple
      addresses at endpoints.  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 TCP sessions (termed
      'subflows') will be achieved through a path management method.
      This document describes a mechanism by which an endpoint can

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      initiate new subflows by using its additional addresses, or by
      signalling its available addresses to the other endpoint.

   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.  Connections are terminated by connection-level FIN
      packets as well as those relating to the individual subflows.

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

                  Figure 2: Example MPTCP Usage Scenario

1.4.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [1].

2.  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 stream of TCP packets sent over a path.  A subflow is a
      component part of a connection between two endpoints.

   Connection:  A collection of one or more subflows, over which an
      application can communicate between two endpoints.  There is a
      one-to-one mapping between a connection and a socket.

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   Data-level:  The payload data is nominally transfered 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 an endpoint.  May also be referred to as a "Connection ID".

   Endpoint:  A host operating an MPTCP implementation, and either
      initiating or terminating a MPTCP connection.

3.  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.  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 the subflow
      sequence number only, not the data-level sequence space.  Although
      data acknowledgments could be inferred from the subflow ACK, an
      explicit connection-level DATA_ACK is used to ensure end-to-end
      reliability in the presense of certain types of middlebox.

   Receive Window:  The receive window in the TCP header indicates the
      amount of free buffer space for this connection (as opposed to for
      this subflow) that is available at the receiver.  This is a change
      to the semantics of the field.  With regular TCP the window is
      relative to the acknowledgment number in the TCP header.  This is
      not meaningful for multipath TCP.  Instead with multipath TCP the
      receive window is relative to the DATA_ACK field, indicating the
      amount of buffer space available at the data-level.  This permits
      the receive window to serve its original purpose and provide flow-
      control of the data sent by the TCP sending application.

   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.

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   RST:  The RST flag in the TCP header applies only to the subflow it
      is sent on, not to the whole connection.  A connection is
      considered reset if a RST is received on every subflow.

   Address List:  Address management 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 has
      no implication whatsoever for other connections between the same
      pair of hosts.

   5-tuple:  The 5-tuple (protocol, local address, local port, remote
      address, remote port) presented 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.
      These API issues are discussed in more detail in [5].

4.  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.
   These TCP Options will have option numbers allocated by IANA, as
   listed in Section 10, and are defined throughout the following

4.1.  Connection Initiation

   Connection Initiation begins with a SYN, SYN/ACK exchange on a single
   path.  Each of these packets will additionally feature the MP_CAPABLE
   TCP option (Figure 3) This option declares its sender is capable of
   performing multipath TCP and wishes to do so on this particular
   connection).  As well as this declaration, this field presents a
   locally-unique token identifying this connection.  This is used when
   adding additional subflows to this connection.

   This token is generated by the sender and has local meaning only,
   hence it MUST be unique for the sender.  The token MUST be difficult
   for an attacker to guess, and thus it is recommended it SHOULD be
   generated randomly.  (However, see further discussions about security
   in Section 5, including the possibility of 64-bit tokens.)

   This option is only present in packets with the SYN flag set.  It is
   only used in the first TCP session of a connection, in order to
   identify the connection; all following connections will use path
   management options (see Section 4.2) to join the existing connection.

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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=MP_CAP   |   Length=11   |       Sender Token            :
      : Sender Token (cont 4 octets)  | Initial Data Sequence Number  :
      :        Initial Data Sequence Number  (cont - 6 bytes)         |

                    Figure 3: Multipath 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
   the MPTCP session will operate as 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 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.  Once the active opener has
   sent a SYN without the MP_CAPABLE option, it MUST fall back to
   regular TCP behavior, even if it subsequently receives a SYN/ACK that
   contains an MP_CAPABLE option.  This might happen if the MP_CAPABLE
   SYN and subsequent non-MP-capable SYN are reordered.  This is to
   ensure that the two endpoints end up in an interoperable state, no
   matter what order the SYNs arrive at the passive opener.  This final
   state is inferred from the presence or absence of the DATA_ACK option
   in the third packet of the TCP handshake.

   The MPC option includes the most significant 6 bytes of the 8-byte
   initial Data Sequence Number option (discussed in Section 4.4).  The
   least significant two bytes should be zeroed.  This is also used as
   an implicit mapping of the SYN to the data sequence space (and this
   initial SYN counts as one octet in this space, as for a regular SYN
   in single-path TCP).  This will be used to ensure both ends agree on
   whether the connection is multipath or standard TCP, regardless of
   middlebox behaviour.  This could also have some (minor) security
   benefits, discussed in Section 5.  To preserve option space, only the
   most significant six bytes are sent in the SYN, as there is no
   significant security benefit from randomizing the values of the lower
   two bytes given that these fall within typical receive windows sizes.

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4.2.  Starting a New Subflow

   Endpoints have knowledge of their own address(es), and can become
   aware of the other endpoint's addresses through signalling exchanges
   as described in Section 4.3.  Using this knowledge, an endpoint can
   initiate a new subflow over a currently unused pair of addresses.
   Either endpoint that is part of a connection can initiate the
   creation of a new subflow.

   A new subflow is started as a normal TCP SYN/ACK exchange.  The
   "Join" TCP option (Figure 4) is used to identify of which connection
   the new subflow should become a part.  The token used is the locally
   unique token of the destination for the subflow, as defined by the
   MP_CAPABLE option received in the first SYN/ACK exchange.

                           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=OPT_JOIN |  Length = 7   |Receiver Token (4 octets total):
      :  Receiver Token (continued)   |   Address ID   |

                     Figure 4: Join Connection option

   In response to a SYN with the "Join" option, if the token is valid
   for an existing MPTCP connection, the recipient MUST respond with a
   SYN/ACK also containing a "Join" option, with the initiator's token.
   This serves two purposes: firstly, to ensure both endpoints agree on
   the connection being referred to (this is particularly relevant when
   both addresses being used are new to the connection); and secondly,
   to ensure there are no middleboxes in the path that will drop MPTCP
   options on the return path.  This behaviour is illustred in Figure 5.

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               Host A                               Host B
      ------------------------             ------------------------
      Address A1    Address A2             Address B1    Address B2
      ----------    ----------             ----------    ----------
          |             |                      |             |
          |       SYN + OPT_MPC(Token A)       |             |
          |----------------------------------->|             |
          |<-----------------------------------|             |
          |     SYN/ACK + OPT_MPC(Token B)     |             |
          |             |                      |             |
          |             |       SYN + OPT_JOIN(Token B)      |
          |             |----------------------------------->|
          |             |<-----------------------------------|
          |             |     SYN/ACK + OPT_JOIN(Token A)    |
          |             |                      |             |

                   Figure 5: Example use of MPTCP Tokens

   If the token is unknown, the recipient MUST respond with a TCP RST in
   the same way as when an unknown TCP port is used.

   It should be noted that additional subflows can exist between any
   pair of ports; no explicit accept calls or bind calls are required to
   open additional subflows.  To associate a new subflow to an existing
   connection, the token supplied in the subflow's SYN exchange is used
   for demultiplexing.  This means that port numbers on subflow SYN
   exchanges are not important, and a receiver of a SYN SHOULD allow any
   values to be used, as long as the 5-tuple is unique for each host.
   However the sender of a SYN containing a JOIN option SHOULD send the
   SYN to the port used by the remote party for the first subflow in the
   connection.  The local port for such SYNs MAY be chosen locally,
   either dynamically, or by the application if an API allows the
   application to do so.  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.

   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 JOIN option includes an "Address ID".  This is an identifier,
   locally unique to the sender of this option, and with only per-
   connection relevance, which identifies the source address of this
   packet.  The key purpose of this identifier is, if an address becomes
   unexpectedly unavailable on the sender, it can signal this to the
   receiver via a remove address option (Section 4.3.2) without needing
   to know what the source address actually is (thus allowing the use of

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   NATs).  It also allows correlation between new connection attempts
   and address signalling (Section 4.3.1), to prevent duplicate subflow

   The Address IDs of the subflow used in the initial SYN exchange of
   the first subflow in the connection are implicit, and have the value

   The Address ID must be stored by the receiver in a data structure
   that gathers all the Address ID to address mappings for a connection
   identified by a token pair.  In this way there is a stored mapping
   between Address ID, observed source address and token pair for future
   processing of control information for a connection.

   This option can only be present when the SYN flag is set.

4.3.  Address Knowledge Exchange (Path Management)

   We use the term "path management" to refer to the exchange of
   information about additional paths between endpoints, which in this
   design is managed by multiple addresses at endpoints.  For more
   detail of the architectural thinking behind this design, see the
   separate 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 4.2, where the initiator has an
   additional address.  The second method is described in the following
   subsections, whereby addresses are signalled explicitly to the other
   endpoint, to allow it to initiate new connections.  This approach, of
   two complementary mechanisms, has been chosen to allow addresses to
   change in flight, and thus support operation through NATs, whilst
   also allowing the signalling of previously unknown addresses, such as
   those belonging to other address families (e.g.  IPv4 and IPv6).

   Here is an example of typical operation of the protocol:

   o  An endpoint that is multihomed starts an additional TCP session to
      an address/port pair that is already in use on the other endpoint,
      using a token to identify the flow (Section 4.2).  (A multihomed
      destination may open a new subflow from its new address to an
      existing subflow's source address and port, or a multihomed source
      may open a new subflow from its new address to an existing
      subflow's destination and port).

   o  More concretely, say a connection is intiated from host "A" on
      (address, port) combination A1 to destination (address, port) B1
      on host "B".  If host A is multihomed, it starts an additional

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      connection from new (address, port) A2 to B1, using B's previously
      declared token.  Alternatively, if B is multhomed, it will try to
      set up a new TCP connection from B2 to A1, using A's previously
      declared token.

   o  Simultaneously (or after a timeout), an "Add Address" option
      (Section 4.3.1) 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.  Using the previous notation, this would be an
      Add Address packet sent from A1 to B1, informing B of address A2.

   o  The mix of using the SYN-based option and the Add Address option,
      including timeouts, is implementation-specific and can be tailored
      to agree with local policy.

   o  If host B successfully receives the first SYN, starting a new
      subflow, it can use the Address ID in the Join option to correlate
      this with the Add Address option that will also arrive on an
      existing subflow.  Assuming the endpoint has already responded to
      the SYN with a SYN/ACK, it will know to ignore the Add Address
      option.  Otherwise, if it has not received such a SYN, it will try
      to initiate a new subflow from one or more of its addresses to
      address A2 (triggered by the Add Address option).  This is
      intended to permit new sessions to be opened if one endpoint is
      behind a NAT.  A slight security improvement can be gained if a
      host ensures there is a correlated Add Address option before
      responding to the SYN.

   Other scenarios are valid, however, such as those where entirely new
   addresses are signalled, e.g. to allow an IPv6 and an IPv4 path to be
   used simultaneously.

4.3.1.  Address Advertisement

   The Add Address TCP Option announces additional addresses on which an
   endpoint can be reached (Figure 6), which allows several (ID,
   address) pairs to be announced to the other endpoint.  Multiple
   addresses can be added 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.

   Every address has an ID which can be used for address removal, and
   therefore endpoints must cache the mapping between ID and address.
   This is also used to identify Join Connection options (Section 4.2)
   relating to the same address, even when address translators are in

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   use.  The ID must be unique to the sender and connection, per
   address, but its mechanism for allocating such IDs is implementation-

   This option is shown for IPv4.  For IPv6, the IPVer field will read
   6, and the length of the address will be 16 octets not 4, and thus
   the length of the option will be 2 + (18 * number_of_entries).  If
   there is sufficient TCP option space, multiple addresses can be
   included, with an ID following on immediately from the previous
   address, and their existance can be inferred through the option
   length and version fields.

   NB: by having a IPVer field, we get four free reserved bits.  These
   could be used in later versions of this protocol for expressing
   sender policy, e.g. one bit for "use now" or similar, to
   differentiate between subflows for backup purposes and those for

                           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=OPT_ADDR |     Length    |  Address ID   | IPVer |(resvd)|
      |                   Address (IPv4 - 4 octets)                   |
          ( ... further ID/Version/Address fields as required ... )

                  Figure 6: Add Address option (for IPv4)

   Ideally, we'd like to ensure the Add Address (and Remove Address)
   option is sent reliably and in order to the other end.  This is to
   ensure that we don't close the connection when remove/add addresses
   are processed in reverse order, and to ensure that all possible paths
   are used.  We note, however, that losing reliability and ordering it
   will not break the multipath connections; they will just reduce the
   opportunity to open multipath paths and to survive different patterns
   of path failures.

   Subflow level ACKs do not cover options, so if we want explicit
   guarantees we need to build in other mechanisms.  Solutions include
   echoing the options and sending one option per RTT, or adding a
   sequence number to the option which is explicitly acked in another
   option.  However, we feel these mechanisms' added complexity is not
   worth the benefits they bring.  There are two basic failure modes for
   options: a) every new option gets stripped or b) some options get
   stripped, randomly.  The second option looks more like a middlebox
   implementation error, so we believe it is not worth optimizing for.
   In the first case, resending the option on a different subflow is the

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   thing to do.  To achieve similar reliability without explicit ACKs,
   we propose sending all Add/Remove Address options on all existing
   subflows.  If ordering is needed, we should only send one add/remove
   option per RTT (modulo lost packets at subflow level).

   If an address index is in use, the Add Address option SHOULD be
   silently ignored.

4.3.2.  Remove Address

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

   This is achieved through the Remove Address option (Figure 7), which
   will remove a previously-added address (or list of addresses) from a
   connection and terminate any subflows currently using that address.

   The sending and receipt of this message should trigger the sending of
   FINs by both endpoints on the affected subflow(s) (if possible), as a
   courtesy to cleaning up middlebox state, but endpoints may clean up
   their internal state without a long timeout.

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

   The standard way to close a subflow (so long as it is still
   functioning) is to use a FIN exchange as in regular TCP - for more
   information, see Section 4.5.

                           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=OPT_REMADR|  Length = 2+n |  Address ID   | ...

                      Figure 7: Remove Address option

4.4.  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.

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4.4.1.  Data Sequence Numbering

   The data stream as a whole can be reassembled through the use of the
   Data Sequence Mapping (Figure 8) option, which defines the mapping
   from the data sequence number to the subflow sequence number.  This
   is used by the receiver to ensure in-order delivery to the
   application layer.  Meanwhile, the subflow-level sequence numbers
   (i.e. the regular sequence numbers in the TCP header) have subflow-
   only relevance.  It is expected (but not mandated) that SACK [6] is
   used at the subflow level to improve efficiency.

                           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=OPT_DSN  |    Length     |     Data Sequence Number ... :
      : ... ( (length-8) octets )     | Data-level Length (2 octets) |
      |            Subflow Sequence Number (4 octets)                |

                  Figure 8: Data Sequence Mapping option

   This option specifies a full mapping from data sequence number to
   subflow sequence number, informing the receiver that there is a one-
   to-one correspondence between the two sequence spaces for the
   specified length.  The purpose of the explicit mapping is to assist
   with compatibility with situations where TCP/IP segmentation or
   coalescing is undertaken separately from the stack that is generating
   the data flow (e.g. through the use of TCP segmentation offloading on
   network interface cards, or by middleboxes such as performance
   enhancing proxies).

   The data sequence number specified in this option is absolute,
   whereas the subflow sequence numbering is relative (the SYN at the
   start of the subflow has subflow sequence number 1).  This is to
   permit middleboxes that may wish to alter sequence numbering, since
   the data stream itself will not be affected.

   TBD: if we used absolute sequence numbers that would make receiver
   code a bit simpler, and would make it more difficult to inject data
   as the attacker needs to guess both Data Sequence Number and Subflow
   Sequence Number.  How many middleboxes are there that change the
   sequence numbers, and should we optimize for them?

   A mapping is unique, in that the subflow sequence number is bound to
   the data sequence number after the mapping has been processed.  It is
   not possible to change this mapping afterwards; however, the same

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   data sequence number can be mapped on different subflows for
   retransmission purposes (see Section 4.4.4).

   A receiver MUST NOT accept data for which it does not have a mapping
   to the data sequence space.  To do this, the receiver will not
   acknowledge the unmapped data at subflow level.  It is better to have
   a subflow fail than to accept data in the wrong order.  However, if
   there was a lost packet in the subflow, the receiver SHOULD wait for
   this to be retransmitted before closing the subflow, since the lost
   packet may contain the necessary mapping information.

   NOTE: if the subflow did ACK data for which it did not have a
   mapping, it would be possible to use the DATA_ACK to detect when the
   mapping was lost.  This will likely not increase reliability, as the
   subflow will likely drop all unknown options.  In addition, the
   receiver is now storing potentially useless data: what happens if the
   mapping never arrives?  Should the receiver have a timer to delete
   this data?

   Data sequence numbers are always 64-bit quantities, and should be
   maintained as such in implementations.  If a connection is
   progressing at a slow rate, so that protection against wrapped
   sequence numbers is not required, and security requirements against
   blind insertion attacks are not stringent, then it is permissible to
   include just the lower 32 bits of the sequence number in the OPT_DSN
   option as an optimization.  Implementations MUST accept this and
   implicitly promote it to a 64-bit quantity.  In all other cases, the
   full 64 bits should be included.  Security implications are discussed
   in Section 5.

   As with the standard TCP sequence number, the data sequence number
   should not start at zero, but at a random value to make session
   hijacking harder.  This is done by including a Data Sequence Mapping
   option along with the MP_CAPABLE option in the initial SYN (which
   occupies one octet of data sequence space; see Section 4.1).  In this
   case, to save option space, neither the data-level length nor the
   subflow sequence number fields are present in this option, so the
   Length field will be the length of the Data Sequence Number, plus two

   The 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 a 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 endpoints, another
   is when segments of data are scheduled in larger than packet-sized

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4.4.2.  Data Acknowledgements

   In a perfect world, it would be possible to make do with only
   subflow-level acknowledgements, with the sender keeping track of
   these acknowledgements to derive what data has been successfully
   received.  If there are ever cases where the subflow data is dropped
   after it has been acked (which may occur if a proxy middlebox fails,
   or if a buffer fills on a host), the connection will break entirely
   since the sender will assume the data has been received when it

   Therefore, MPTCP provides a connection-level acknowledgement (the
   DATA_ACK) to act as a cumulative ACK for the connection as a whole.
   This is analogous to the behaviour of the standard TCP cumulative ACK
   in SACK - indicating how much data has been successfully received
   (with no holes).  This option, illustrated in Figure 9, is expected
   to be included in every packet by an MPTCP host.

                           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=OPT_DACK |    Length     |     Data Sequence Number ... :
      : ... ( (length-8) octets )     |

           Figure 9: Connection-level Acknowledgement (DATA_ACK)

4.4.3.  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 (e.g.
   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

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   receive window.  With multipath, such a check is done using only the
   connection level window.  A sanity check could be performed at
   subflow level to ensure that: SSN-SUBFLOW_ACK <= DSN - DATA_ACK.

   When should segments be processed at connection level?  The default
   is to wait until they arrive in order at subflow level, and only then
   do connection level processing.  However, one can optimize for speed
   by processing at connection level segments that have not yet been
   acked at subflow level; the only requirement for this optimization is
   to have a valid data sequence mapping for the segment.  Note that the
   segment can be dropped at subflow level afterwards (e.g. because it
   is out of order and there is more pressure); the DATA_ACK ensure the
   connection can make progress without having to wait for the subflow

   An issue will arise regarding how large a receive buffer to
   implement.  The lower bound would be the maximum bandwidth/delay
   product of all paths, however this could easily fill when a packet is
   lost on a slower subflow and needs to be retransmitted (see
   Section 4.4.4).  The upper bound would be the maximum RTT multiplied
   by the maximum total bandwidth available.  This will cover most
   eventualities, but could easily become very large.  It is FFS what
   the best approach is.

4.4.4.  Sender Considerations

   The sender should only consider receive window advertisements where
   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.

   The data sequence mapping allows senders to re-send data with the
   same data sequence number on a different subflow.  When doing this,
   an endpoint 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

   This protocol specification does not mandate any mechanisms for
   handling retransmissions, and much will be dependent upon local
   policy (as discussed in Section 4.4.6).  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

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   retransmits are only used after a few subflow level retransmission
   timeouts occur.

   Whichever the retransmission strategy, the sender MUST keep data in
   its send buffer as long as the data has not been acked at connection
   level and on all subflows it has been 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, 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, and only then delete the
   outstanding data segments.

   A sender will maintain connection level timers for unacknowledged
   segments.  These timers will be based on the subflow timers, and will
   guard against pro-active acking by middleboxes.

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

4.4.5.  Congestion Control Considerations

   Different subflows in an MPTCP connection have different congestion
   windows.  To achieve 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

   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.  Much research
   is expected in this area in the near future.

   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 where lost and when.

4.4.6.  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.

   In the typical use case, where the goal is to maximise throughput,

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   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 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.
   There is no mechanism in MPTCP for a receiver to signal their own
   particular preferences for paths, but this is a necessary feature
   since receivers will often be the multihomed party, and may have to
   pay for metered incoming bandwidth.  Instead of incorporating complex
   signalling, it is proposed to use existing TCP features to signal
   priority implicitly.  If a receiver wishes to keep a path active as a
   backup but wishes to prevent data being sent on that path, it could
   stop sending ACKs for any data it receives on that path.  The sender
   would interpret this as severe congestion or a broken path and stop
   using it.  We do not advocate this method, however, since this will
   result in unnecessary retransmissions.

   Therefore, a proposal is to use ECN [7] to to provide fake congestion
   signals on paths that a receiver wishes to stop being used for data.
   This has the benefit of causing the sender to back off without the
   need to retransmit data unnecessarily, as in the case of a lost ACK.
   This should be sufficient to allow a receiver to express their
   policy, although does not permit a rapid increase in throughput when
   switching to such a path.

   TBD: This is clearly an overload of the ECN signal, and as such other
   solutions, such as explicitly signalling path operation preferences
   (such as in the reserved bits of certain TCP options, or through
   entirely new options) may be a preferred solution.

4.5.  Closing a Connection

   Under single path TCP, a FIN signifies that the sender has no more
   data to send.  In order to allow subflows to operate independently,
   however, and with as little change from regular TCP as possible, 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

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   TCP, i.e. it is not until both sides have ACKed each other's FINs
   that the subflow is fully closed.

   When an application calls close() on a socket, this indicates that it
   has no more data to send, and for regular TCP this would result in a
   FIN on the connection.  For MPTCP, an equivalent mechanism is needed,
   and this is the DATA_FIN.  This option, shown in Figure 10, is
   attached to a regular FIN option on a subflow.

   A DATA_FIN is an indication that the sender has no more data to send,
   and as such can be used as a rapid indication of the end of data from
   a sender.  A DATA_FIN, as with the FIN on a regular TCP connection,
   is a unidirectional signal.

   A DATA_FIN occupies one octet (the final octet) of Data Sequence
   Number space.  This number is included in the option, and will be
   ACKed at data level to ensure reliable delivery.

   The DATA_FIN is an optimisation to rapidly indicate the end of a data
   stream and clean up state associated with a MPTCP connection,
   especially when some subflows may have failed.  Specifically, when a
   DATA_FIN has been received, IF all data has been successfully
   received, timeouts on all subflows MAY be reduced.  Similarly, when
   sending a DATA_FIN, once all data (including the DATA_FIN, since it
   occupies one octet of data sequence space) has been acknowledged,
   FINs must be sent on every subflow.  This applies to both endpoints,
   and is required in order to clean up state in middleboxes.

   There are complex interactions, however, between a DATA_FIN and
   subflow properties:

   o  A DATA_FIN MUST only be sent on a packet which also has the FIN
      flag set.

   o  When DATA_FIN is sent, it should be sent on all subflows.

   o  There is a one-to-one mapping between the DATA_FIN and the
      subflow's FIN flag (and its associated sequence space and thus its
      acknowlegement).  In other words, when a subflow's FIN flag has
      been acknowledged, the associated DATA_FIN is also acknowledged.

   o  The DATA_ACK (Section 4.4.2), which will be included with a
      DATA_FIN, is used to verify that all data has been successfully

   It should be noted that an endpoint may also send a FIN on an
   individual subflow to shut it down, but this impact is limited to the
   subflow in question.  If all subflows have been closed with a FIN,

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   that is equivalent to having closed the connection with a DATA_FIN.

   The full eight-byte data sequence number is always included in a

       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=OPT_DFIN |    Length=10  | Data Sequence Number (8B)    :
      :                  Data Sequence Number (contd.)               :
      :  Data Sequence Number (contd.)|

                        Figure 10: DATA_FIN option

4.6.  Error Handling


   Unknown token in MPTCP SYN should equate to an unknown port, e.g. a
   TCP reset?  We should make this as silent and tolerant as possible.
   Where possible, we should keep this close to the semantics of TCP.
   However, some MPTCP-specific issues such as where a data sequence
   number is missing from a subflow, will definitely need MPTCP-specific
   errors handling in those cases.

5.  Security Considerations


   (Token generation, handshake mechanisms, new subflow authentication,

   A generic threat analysis for the addition of multipath capabilities
   to TCP is presented in [8].  The protocol presented here has been
   designed to minimise or eliminate these identified threats.  (A
   future version of this document will explicitly address the presented

   The development of a TCP extension such as this will bring with it
   many additional security concerns.  We have set out here to produce a
   solution that is "no worse" than current TCP, with the possibility
   that more secure extensions could be proposed later.

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   The primary area of concern will be around the handshake to start new
   subflows which join existing connections.  The proposal set out in
   Section 4.1 and Section 4.2 is for the initiator of the new subflow
   to include the token of the other endpoint in the handshake.  The
   purpose of this is to indicate that the sender of this token was the
   same entity that received this token at the initial handshake.

   One area of concern is that the token could be simply brute-forced.
   The token must be hard to guess, and as such could be randomly
   generated.  This may still not be strong enough, however, and so the
   use of 64 bits for the token would alleviate this somewhat.

   The two tokens don't need to be the same length.  Token B could be 64
   bits and token A 32 bits.  If JOIN always contains Token B, this
   would provide adequate security while saving scarce space in the
   initial SYN, where it is most at a premium.

   Use of these tokens only provide an indication that the token is the
   same as at the initial handshake, and does not say anything about the
   current sender of the token.  Therefore, another approach would be to
   bring a new measure of freshness in to the handshake, so instead of
   using the initial token a sender could request a new token from the
   receiver to use in the next handshake.  Hash chains could also be
   used for this purpose.

   Yet another alternative would be for all SYN packets to include a
   data sequence number.  This could either be used as a passive
   identifier to indicate an awareness of the current data sequence
   number (although a reasonable window would have to be allowed for
   delays).  Or, the SYN could form part of the data sequence space -
   but this would cause issues in the event of lost SYNs (if a new
   subflow is never established), thus causing unnecessary delays for

6.  Interactions with Middleboxes

   Multipath TCP will be deployed in a network that no longer provides
   just basic datagram delivery.  A miriad of middleboxes are deployed
   to optimize various perceived problems with the Internet protocols:
   NATs primarily address space shortage [9], Performance Enhancing
   Proxies (PEPs) optimize TCP for different link characteristics [10],
   firewalls [11] and intrusion detection systems try to block malicious
   content from reaching a host, and traffic normalizers [12] ensure a
   consistent view of the traffic stream to IDSes and hosts.

   All these middleboxes optimize current applications at the expense of
   future applications.  In effect, future applications must mimic

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   existing ones if they want to be deployed.  Further, the precise
   behaviour of all these middleboxes is not clearly specified, and
   implementation errors make matters worse, raising the bar for the
   deployment of new technologies.

   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 new TCP options.  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 SYN packets contain the MPC option to indicate the use of
   MPTCP.  When the middlebox drops the packet containing the MPC option
   either on the outgoing or the return path, the connection will fail.
   Host A SHOULD fall back to TCP in such cases (studies suggest that
   few middleboxes drop packets with unknown options).  The same applies
   for subflow setup.

   The second case is when the middleboxes strip options.  Let's first
   discuss behaviour for initial connection SYNs (see Figure 11).  If
   the option is stripped from the packet on the outgoing path, the
   connection falls back to regular TCP.  If the option is stripped on
   the return path, host B will wait for a DATA_ACK of its connection
   SYN, retransmitting the SYN/ACK until it declares the connection
   failed.  Host A thinks it is talking to a regular host, and may send
   data segments, but these will not be acked by host B as they do not
   have the proper mapping.  Hence the connection fails.  Host A SHOULD
   fall back to regular TCP after the connection times out.

   Subflow SYNs contain the OPT_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 OPT_JOIN option and its
   token.  Either way, the connection fails.

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        Host A                             Host B
         |              Middlebox M            |
         |                   |                 |
         |     SYN(OPT_MPC)  |        SYN      |
         |                SYN/ACK              |
     a) OPT_MPC option stripped on outgoing path

       Host A                               Host B
         |               SYN(OPT_MPC)          |
         |             Middlebox M             |
         |                 |                   |
         |    SYN/ACK      |  SYN/ACK(OPT_MPC) |
     b) OPT_MPC option stripped on return path

   Figure 11: Connection Setup with Middleboxes that Strip Options from

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

   If options are stripped in either direction by middleboxes (this is
   unlikely, as the SYN options did get through), the particular subflow
   will timeout repeatedly while waiting for a DATA_ACK or subflow-level
   ACK, and will be closed.  If the subflow is the initial one, host A
   SHOULD fall back to regular TCP.

   We can further analyze what happens when a fraction of options is
   stripped.  The multipath subflow should survive losing a fraction of
   DATA_ACKs and data sequence mappings, 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  NAT: will prevent flow/subflow setup when the server does not have
      a public address.  MPTCP assumes the server has at least one
      public address (or the client uses standard NAT traversal to reach

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      it) that is used to setup the connection.  If uses Add Address
      messages to signal the existence of other addresses.

   o  Performance Enhancing Proxies: might pro-actively ACK data and
      then fail.  MPTCP 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: do not allow holes in sequence numbers, 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  Segmentation/Coalescing (e.g. tcp segmentation offloading, etc):
      might copy options between packets and might strip some options.
      MPTCP's data sequence mapping includes the 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  Firewalls: might perform sequence number randomization on outgoing
      connections.  MPTCP uses relative sequence numbers in data
      sequence mapping to cope with this.

7.  Interfaces


   Interface with applications, interface with TCP, interface with lower

   Discussion of interaction with applications (both in terms of how
   MPTCP will affect an application's assumptions of the transport
   layer, and what API extensions an application may wish to use with
   MPTCP) are discussed in [5].

8.  Open Issues

   This specification is a work-in-progress, and as such there are many
   issues that are still to be resolved.  This section lists many of the
   key open issues within this specification; these are discussed in
   more detail in the appropriate sections throughout this document.

   o  Best handshake mechanisms (Section 4.1).  This document contains a
      proposed scheme by which connections and subflows can be set up.
      It is felt that, although this is "no worse than regular TCP",
      there could be opportunities for significant improvements in

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      security that could be included (potentially optionally) within
      this protocol.

   o  Issues around simultaneous opens, where both ends attempt to
      create a new subflow simultaneously, need to be investigated and
      behaviour specified.

   o  Appropriate mechanisms for controlling policy/priority of subflow
      usage (specifically regarding controlling incoming traffic,
      Section 4.4.6).  The ECN signal is currently proposed but other
      alternatives, including per subflow receive windows or options
      indicating path properties, could be employed instead.

   o  How much control do we want over subflows from other subflows
      (e.g. closing when interface has failed)?  Do we want to
      differentiate between subflows and addresses (Section 4.2)?

   o  Do we want a connection identifier in every packet?  E.g. would it
      make the implementation of an IDS easier?

   o  Should we do signaling in the TCP payload, rather than options as
      proposed in this draft?  We discuss this alternative in the

   o  Should we explicitly support SYN cookies?  With the current
      design, MPTCP would be downgraded to basic TCP if SYN cookies were
      used.  Is it worth designing the protocol to allow stateless
      server handshake?

   o  Instead of an Address ID in JOIN, are there any cases where a
      Subflow ID (i.e. unique to the subflow) would be useful instead?
      For example, two addresses which become NATted to the same

9.  Acknowledgements

   The authors are supported by Trilogy
   (http://www.trilogy-project.org), a research project (ICT-216372)
   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 many members of the Trilogy project, notably Iljitsch
   van Beijnum, Lars Eggert, Marcelo Bagnulo Braun, Robert Hancock, Pasi
   Sarolahti, Olivier Bonaventure, Toby Moncaster, Philip Eardley,

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   Andrew McDonald and Sergio Lembo.

10.  IANA Considerations

   This document will make a request to IANA to allocate new values for
   TCP Option identifiers, as follows:

      |   Symbol   |          Name         |      Ref      | Value |
      |  OPT_MPCAP |   Multipath Capable   |  Section 4.1  | (tbc) |
      |  OPT_ADDR  |      Add Address      | Section 4.3.1 | (tbc) |
      | OPT_REMADR |     Remove Address    | Section 4.3.2 | (tbc) |
      |  OPT_JOIN  |    Join Connection    |  Section 4.2  | (tbc) |
      |   OPT_DSN  | Data Sequence Mapping |  Section 4.4  | (tbc) |
      |  OPT_DACK  |        DATA_ACK       |  Section 4.4  | (tbc) |
      |  OPT_DFIN  |        DATA_FIN       |  Section 4.5  | (tbc) |

                      Table 1: TCP Options for MPTCP

11.  References

11.1.  Normative References

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

11.2.  Informative References

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

   [3]   Ford, A., Raiciu, C., Barre, S., and J. Iyengar, "Architectural
         Guidelines for Multipath TCP Development",
         draft-ietf-mptcp-architecture-00 (work in progress),
         March 2010.

   [4]   Raiciu, C., Handley, M., and D. Wischik, "Coupled Multipath-
         Aware Congestion Control", draft-raiciu-mptcp-congestion-00
         (work in progress), October 2009.

   [5]   Scharf, M. and A. Ford, "MPTCP Application Interface
         Considerations", draft-scharf-mptcp-api-00 (work in progress),
         October 2009.

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   [6]   Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
         Selective Acknowledgment Options", RFC 2018, October 1996.

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

   [8]   Bagnulo, M., "Threat Analysis for Multi-addressed/Multi-path
         TCP", draft-ietf-mptcp-threat-00 (work in progress),
         February 2010.

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

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

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

   [12]  Handley, M., Paxson, V., and C. Kreibich, "Network Intrusion
         Detection: Evasion, Traffic Normalization, and End-to-End
         Protocol Semantics", Usenix Security 2001, 2001, <http://

   [13]  Eddy, W. and A. Langley, "Extending the Space Available for TCP
         Options", draft-eddy-tcp-loo-04 (work in progress), July 2008.

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
   both SYN, data packets and pure ACK packets, and found that there is
   enough room to fit all the options we propose using in this draft.

   SYN packets typically include MSS, window scale, sack permitted and
   timestamp options.  Together these sum to 19B. The multipath capable
   (MPC) option requires a max of 16B, and the Join option requires 8B,
   so they both fit the existing space.

   TCP data packets typically carry timestamp options in every packet,

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   taking 10B. That leaves 30B which are enough to encode the data
   sequence mapping (max 16B) and the DATA_ACK if the flow is
   bidirectional (max 10B).

   Pure ACKs in TCP typically contain only timestamps (10B).  Here,
   multipath TCP typically needs to encode the DATA_ACK (max 10B).
   Ocasionally acks will contain SACK information.  Depending on the
   number of lost packets, SACK may utilize the entire option space.  We
   propose reducing the number of SACK blocks by one to accomodate the

   Encoding Add/Remove address options uses at most 10B (for IPv6
   addresses).  These will fit in data packets if the DATA_ACK is not
   present.  Otherwise, the endpoint can insert pure ACKs that contain
   the add address option.  Finally, if SACK information is included in
   the data packets, one further block can be removed to accomodate the
   add address option.

   All the new options fit in the space available yet there is little
   room for adding new options.  We note that if 8B data sequence
   numbers are used, PAWS is no longer needed.  Hence the use for
   timestamps is limited to providing RTT measurements for retransmitted
   packets.  As loss rates are typically low, we believe we can just
   stop using timestamps, claiming 10B of options space on all packets.

   Alternatively, we could use a TCP option to increase the option
   space, such as that proposed in [13].  The proposal extends the 4 bit
   header to 16 bits.  Such an option could only be used between nodes
   that support it, however, and so long options could not be used until
   a handshake is complete.

   Finally, there are issues with options reliability.  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/remove address options.  Here we favour redundant
   transmissions at the sender (whether on multiple paths, or on the
   same path on a number of ACKs).  The cases where options are stripped
   by middleboxes are discussed in Section 6.

Appendix B.  Signaling Control Information in the Payload

Appendix C.  Resync Packet

   In earlier versions of this draft, we proposed the use of a "re-sync"
   option that would be used in certain circumstances when a sender
   needs to instruct the receiver to skip over certain subflow sequence

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   numbers (i.e. to treat the specified sequence space as having been
   received and acknowledged).

   The typical use of this option will be when packets are retransmitted
   on different subflows, after failing to be acknowledged on the
   original subflow.  In such a case, it becomes necessary to move
   forward the original subflow's sequence numbering so as not to later
   transmit different data with a previously used sequence number (i.e.
   when more data comes to be transmitted on the original subflow, it
   would be different data, and so must not be sent with previously-used
   (but unacknowledged) sequence numbering).

   The rationale for needing to do this is two-fold: firstly, when ACKs
   are received they are for the subflow only, and the sender infers
   from this the data that was sent - if the same sequence space could
   be occupied by different data, the sender won't know whether the
   intended data was received.  Secondly, certain classes of middleboxes
   may cache data and not send the new data on a previously-seen
   sequence number.

   This option was dropped, however, since some middleboxes may get
   confused when they meet a hole in the sequence space, and do not
   understand the resync option.  It is therefore felt that the same
   data must continue to be retransmitted on a subflow even if it is
   already received after being retransmitted on another.  There should
   not be a significant performance hit from this since the amount of
   data involved and needing to be retransmitted multiple times will be
   relatively small.

   Therefore, it is necessary to 're-sync' the expected sequence
   numbering at the receiving end of a subflow, using the following TCP
   option.  This packet declares a sequence number space (inclusive)
   which the receiving node should skip over, i.e. if the receiver's
   next expected sequence number was previously within the range
   start_seq_num to end_seq_num, move it forward to end_seq_num + 1.

   This option will be used on the first new packet on the subflow that
   needs its sequence numbering re-synchronised.  It will be continue to
   be included on every packet sent on this subflow until a packet
   containing this option has been acknowledged (i.e. if subflow
   acknowledgements exist for packets beyond the end sequence number).
   If the end sequence number is earlier than the current expected
   sequence number (i.e. if a resync packet has already been received),
   this option should be ignored.

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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=OPT_RESYNC|  Length = 10  |     Start Sequence Number    :
      :          (4 octets)           |      End Sequence Number     :
      :          (4 octets)           |

                         Figure 12: Resync option

Appendix D.  Changelog

   This section maintains logs of significant changes made to this
   document between versions.

D.1.  Changes since draft-ford-mptcp-multiaddressed-02

   o  Remote Version and Address ID in MP_CAPABLE in Section 4.1, 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 OPT_JOIN in Section 4.2.

   o  Added DATA_ACK to Section 4.4.

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

Authors' Addresses

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

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

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   Costin Raiciu
   University College London
   Gower Street
   London  WC1E 6BT

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

   Mark Handley
   University College London
   Gower Street
   London  WC1E 6BT

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

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