Internet Engineering Task Force                                  A. Ford
Internet-Draft                                       Roke Manor Research
Intended status: Experimental                                  C. Raiciu
Expires: April 29, 2010                                       M. Handley
                                               University College London
                                                        October 26, 2009

     TCP Extensions for Multipath Operation with Multiple Addresses

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   TCP/IP communication is currently restricted to a single path per

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   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Design Assumptions . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Layered Representation . . . . . . . . . . . . . . . . . .  4
     1.3.  Operation Summary  . . . . . . . . . . . . . . . . . . . .  5
     1.4.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . .  6
     1.5.  Requirements Language  . . . . . . . . . . . . . . . . . .  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Semantic Issues  . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Session Initiation . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Starting a New Subflow . . . . . . . . . . . . . . . . . . 10
     4.3.  Address Knowledge Exchange (Path Management) . . . . . . . 11
       4.3.1.  Adding Addresses . . . . . . . . . . . . . . . . . . . 13
       4.3.2.  Remove Address . . . . . . . . . . . . . . . . . . . . 14
     4.4.  General MPTCP Operation  . . . . . . . . . . . . . . . . . 14
       4.4.1.  Receive Window Considerations  . . . . . . . . . . . . 16
       4.4.2.  Congestion Control Considerations  . . . . . . . . . . 17
       4.4.3.  Subflow Policy . . . . . . . . . . . . . . . . . . . . 17
       4.4.4.  Retransmissions  . . . . . . . . . . . . . . . . . . . 18
     4.5.  Closing a Connection . . . . . . . . . . . . . . . . . . . 19
     4.6.  Error Handling . . . . . . . . . . . . . . . . . . . . . . 20
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   6.  Interactions with Middleboxes  . . . . . . . . . . . . . . . . 21
   7.  Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     10.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Notes on use of TCP Options . . . . . . . . . . . . . 23
   Appendix B.  Resync Packet . . . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25

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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:

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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 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
      (potenitally proxied) multipath.

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 in order to
      reassemble the data stream in-order from multiple subflows.
      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.  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
      security that could be included (potentially optionally) within
      this protocol.

   o  Issues around simulataneous 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 4.4.3).  The ECN signal is currently proposed but other
      alternatives, including per subflow receive windows or path

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      property options, 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
      make implementation of IDS much easier?

   o  Best way of ensuring data/subflow sequence numbering mapping
      through middleboxes (Section 4.4)?

   o  Is there any benefit to a data-level acknowlegement?

1.5.  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.

   Token:  A 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.

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   Sequence Number:  The (in-header) TCP sequence number is subflow-
      specific.  To allow the receiver to reorder application data, an
      additional data-level sequence space is used.  In this 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.

   Receive Window:  The receive window exists at the connection level,
      rather than at the subflow level, as it tries to regulate the
      sending rate of the sender to a slower receiver.  With multipath
      TCP, each subflow MUST report the same global receive window,
      describing the per connection receive buffer.

   FIN:  The FIN only applies to a subflow, not to a connection.  For a
      connection-level FIN, use the DATA FIN option.

   ACK:  The ACK acknowledges the subflow sequence number only, and the
      mapping to the data sequence number is handled out-of-band.

   RST:  The RST only applies to a subflow.  There is no connection-
      level RST, since it would be impossible to distinguish the two,
      i.e. if there is no state about a subflow, the host cannot know to
      what connection the subflow is related.  A connection is
      considered reset if every subflow sends a RST in response.

   Address List:  The address management is handled per-connection to
      permit the application of per-connection local policy.

   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 9, and are defined throughout the following

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4.1.  Session Initiation

   Session Initiation begins with a SYN, SYN/ACK exchange on a single
   path.  Each of these packets will additionally feature the Multipath
   Capable TCP option (Figure 3), which declares the sender's locally
   unique 32-bit token for this connection, and contains a version

   The "Multipath Capable" option declares an endpoint to be capable of
   operating Multipath TCP (or rather, more accurately, a desire to
   operate Multipath TCP on this particular connection).  As well as
   this declaration, this field presents a token, which 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.

                           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_MPC  |  Length = 7   |(resvd)|Version|  Sender Token :
      : Sender Token (continued - 4 octets total)     |

                    Figure 3: Multipath Capable option

   The version field represents the version of MPTCP in use.  The
   version provided in this specification is 0.  The reserved bits may
   be used for connection-specific flags in later versions, or may be
   used to indicate an authentication method.

   If a SYN contains a "multipath capable" option but the SYN/ACK does
   not, it is assumed that the recipient is not multipath capable and
   thus the MPTCP session will operate as regular, single-path TCP.  If
   a SYN does not contain a "multipath capable" option, the SYN/ACK MUST
   NOT contain one in response.

   If these packets are unacknowledged, it is up to local policy to
   decide how to respond.  It is expected that a sender will eventually

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   fall back to single-path TCP (i.e. without the Multipath 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.  In the case
   of out-of-order packets, i.e. if a multipath-capable SYN/ACK is
   received in response to a multipath-capable SYN, after a standard SYN
   has been sent, then once again it is up to the initiator to choose
   how to behave.  For example, it could respond to new connections
   using the previously declared token, or it could simply drop any new
   multipath options within the flow.

   If an endpoint is known to be multiaddressed (e.g. through multiple
   addresses returned in a DNS lookup), alternative destination
   addresses SHOULD be tried first, before falling back to regular TCP.

   In addition to this option, a Data Sequence Number option (discussed
   in Section 4.4) is included to provide an initial data-level sequence
   number (and this initial SYN counts as one octet in this space, as
   for a regular SYN in single-path TCP).  This could also have some
   (minor) security benefits, discussed in Section 5.

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 will
   initiate a new subflow over a currently unused pair of addresses.

   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
   Multipath Capable option received in the first SYN/ACK exchange.

   It should be noted that, in theory, 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 any values can be used,
   as long as the 5-tuple is unique for each host.  In practice, it is
   envisaged that most new subflows will connect to a port that is
   already in use as the source or destination port of an existing
   subflow, in order to have a greater chance of getting through
   firewalls and other middleboxes, and to support traffic engineering
   of the flows.

   Deumultiplexing subflow SYNs MUST be done using the token; this is

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   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.  This serves two purposes.  Firstly, 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
   NATs).  Secondly, it allows correlation between new connection
   attempts and address signalling (Section 4.3.1), to prevent duplicate
   subflow initiation.

   TBD: Instead of an Address ID, 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 address?

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

                           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

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

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   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  To expand upon this, 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
      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.

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4.3.1.  Adding Addresses

   The Add Address TCP Option announces additional addresses on which an
   endpoint can be reached (Figure 5), 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
   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 5: Add Address option (for IPv4)

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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 6), 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 6: 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.  The
   data stream as a whole can be reassembled through the use of the Data
   Sequence Mapping (Figure 7) 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

   The only acknowledgements are those at the subflow-level, so the
   sender must be able to map these acknowledgements to the data
   sequence numbers that were contained in the relevant packets.  The

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   sender thus knows, if subflow data goes unackowledged, which part of
   the original data stream this equates to, and thus what data must be
   retransmitted.  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 7: 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).

   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
   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.

   Although it is expected that initial implementations will use 32-bit
   data sequence numbers (i.e. 4 octets, so a length field of 12),

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   setting the length field to 16 and including a 64-bit sequence number
   (eight octets) MUST be considered valid and processed appropriately.
   This may have also have useful security implications, 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 Number
   option along with the Multipath 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

   The MPTCP data and subflow level sequence numbering could be seen to
   be analogous to that used in SACK, however there are subtle
   differences.  The key similarity is that it is possible to have
   temporary "holes" in the received data sequence space - later data
   may have arrived earlier (most likely on a different subflow), but
   does not need to be retransmitted.  The "holes" are later filled in.
   The key difference, however, is that while SACK can rely on the
   regular TCP cumulative acknowledgements to indicate how much data has
   been successfully received (with no holes), there is no similar
   method in MPTCP.  Instead, the sender must keep track of the
   acknowledgements to derive what data has been successfully received.
   This leads to some oddities especially with session termination (see
   Section 4.5).

4.4.1.  Receive Window 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

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   would not use up their window.

   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.

4.4.2.  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.

4.4.3.  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,
   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,

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   where stability is more important than throughput.  Application
   requirements such as this 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 is
   brutal, naive, and 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.

4.4.4.  Retransmissions

   This protocol specification does not mandate any mechanisms for
   handling retransmissions in the event of path failures, and much will
   be dependent upon local policy (as discussed in Section 4.4.3).  The
   data sequence number, as given in a TCP option, is used to reassemble
   the incoming streams before presentation to the application layers,
   so a sender is free 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 we feel this is the best behaviour.
   Optimisations could be negotiated in future versions of this

   Of course, retransmissions on alternative subflows will only occur if

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   this is what local policy suggests.  Indeed, it may be equally valid
   to retransmit on the same subflow if alternative paths have
   considerably worse quality of service, or are only kept for backup
   purposes.  Additionally, it may be possible for some implementations
   to signal from lower layers if there are problems with the paths, and
   so more appropriate responses could occur.

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 will only affect 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 the DATA FIN.  This option, shown in Figure 8, 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.

   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  A DATA FIN occupies one octet (the final octet) of Data Sequence
      Number space.  Therefore, even if there is no user data, a Data
      Sequence Number option MUST be added to a packet containing the

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      DATA FIN option.  This allows the receiver to easily determine the
      last data sequence number that should have been received.

   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  As such, the acknowledgement of a FIN and DATA FIN DOES NOT
      indicate that all data has been successfully received.  Because
      the data level ack is inferred from subflow acks, an endpoint must
      use subflow acks to discover when all data up to and including the
      DATA FIN has been received.

   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,
   that is equivalent to having closed the connection with a DATA FIN.

       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      | Kind=OPT_DFIN |   Length = 2  |

                         Figure 8: 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

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   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.

   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 behard 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.

   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


   How we get around NATs, firewalls.  Problems with TCP proxies.  How
   to make an MPTCP-aware middlebox, ...

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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.  Acknowledgements

   The authors are supported by Trilogy
   (, 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 and
   Andrew McDonald.

9.  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_MPC  |   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 Number |  Section 4.4  | (tbc) |
       |  OPT_DFIN  |       DATA FIN       |  Section 4.5  | (tbc) |

                      Table 1: TCP Options for MPTCP

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10.  References

10.1.  Normative References

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

10.2.  Informative References

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

   [3]  Ford, A., Raiciu, C., Barre, S., Iyengar, J., and B. Ford,
        "Architectural Guidelines for Multipath TCP Development",
        draft-ford-mptcp-architecture-00 (work in progress),
        October 2009.

   [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.

   [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-bagnulo-mptcp-threat-00 (work in progress),
        October 2009.

   [9]  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.

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   As such, when doing address list manipulation, not all data may fit.
   This can be mitigated in one of two ways:

   o  Using an option to extend the option space, such as that proposed
      in [9], which proposes an option providing a 16-bit header length
      field.  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.

   o  Alternatively, since at least one IP address option field should
      be able to fit per packet, address list manipulation can be
      undertaken with one address per packet.  One method could be to
      wait for data to send, and then append one new address per packet.
      This would seem reasonable if the TCP session begins rapidly, but
      if it is required that the multipath session is ready before the
      first data is to be sent, address list manipulation would be
      required on empty data (signalling only) packets.  Issues may
      arise regarding acknowledged delivery of signalling versus data -
      this is discussed in Section 3 below.

Appendix B.  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
   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

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   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.

                           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 9: Resync option

Authors' Addresses

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

   Phone: +44 1794 833 465

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   Costin Raiciu
   University College London
   Gower Street
   London  WC1E 6BT


   Mark Handley
   University College London
   Gower Street
   London  WC1E 6BT


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