Internet Engineering Task Force                                  A. Ford
Internet-Draft                                       Roke Manor Research
Intended status: Experimental                                  C. Raiciu
Expires: April 28, September 15, 2011                                   M. Handley
                                               University College London
                                                        October 25, 2010
                                                          O. Bonaventure
                                                Universite catholique de
                                                                 Louvain
                                                          March 14, 2011

     TCP Extensions for Multipath Operation with Multiple Addresses
                   draft-ietf-mptcp-multiaddressed-02
                   draft-ietf-mptcp-multiaddressed-03

Abstract

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

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

Status of this Memo

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   This Internet-Draft will expire on April 28, September 15, 2011.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Design Assumptions . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Multipath TCP in the Networking Stack  . . . . . . . . . .  5
     1.3.  Operation Summary  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
     1.4.  MPTCP Concept  . . . . . . . . . . . . . . . . . . . . . .  6
     1.5.  Requirements Language  . . . . . . . . . . . . . . . . . .  7
   2.  Terminology  . . .  Operation Overview . . . . . . . . . . . . . . . . . . . . . .  7  8
   3.  MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . .  8  9
     3.1.  Connection Initiation  . . . . . . . . . . . . . . . . . .  8 10
     3.2.  Starting a New Subflow . . . . . . . . . . . . . . . . . . 11 14
     3.3.  General MPTCP Operation  . . . . . . . . . . . . . . . . . 15 18
       3.3.1.  Data Sequence Numbering Mapping  . . . . . . . . . . . . . . . 15 . 20
       3.3.2.  Data Acknowledgements  . . . . . . . . . . . . . . . . 17 23
       3.3.3.  Receiver Considerations  Closing a Connection . . . . . . . . . . . . . . . 18 . . 24
       3.3.4.  Sender  Receiver Considerations  . . . . . . . . . . . . . . . . 19 25
       3.3.5.  Congestion Control  Sender Considerations  . . . . . . . . . . 21 . . . . . . 26
       3.3.6.  Subflow Policy  Reliability and Retransmissions  . . . . . . . . . . . 27
       3.3.7.  Congestion Control Considerations  . . . . . . . . . 21
     3.4.  Closing a Connection . 28
       3.3.8.  Subflow Policy . . . . . . . . . . . . . . . . . . 22
     3.5. . . 28
     3.4.  Address Knowledge Exchange (Path Management) . . . . . . . 24
       3.5.1. 30
       3.4.1.  Address Advertisement  . . . . . . . . . . . . . . . . 25
       3.5.2. 31
       3.4.2.  Remove Address . . . . . . . . . . . . . . . . . . . . 27
     3.6. 33
     3.5.  Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 28
     3.7. 34
     3.6.  Error Handling . . . . . . . . . . . . . . . . . . . . . . 31
     3.8. 37
     3.7.  Heuristics . . . . . . . . . . . . . . . . . . . . . . . . 32
       3.8.1. 37
       3.7.1.  Port Usage . . . . . . . . . . . . . . . . . . . . . . 32
   4.  Semantic Issues 38
       3.7.2.  Delayed Subflow Start  . . . . . . . . . . . . . . . . 38
       3.7.3.  Failure Handling . . . . . . . 32
   5.  Security Considerations . . . . . . . . . . . . 39
   4.  Semantic Issues  . . . . . . . 34
   6.  Interactions with Middleboxes . . . . . . . . . . . . . . . . 34
   7.  Interfaces 39
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 41
   6.  Interactions with Middleboxes  . . . . . . . . . . 38
   8. . . . . . . 42
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 38
   9. 45
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
   10. 45
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     10.1. 46
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 39
     10.2. 46
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 39 46
   Appendix A.  Notes on use of TCP Options . . . . . . . . . . . . . 40 48
   Appendix B.  Resync Packet  Control Blocks  . . . . . . . . . . . . . . . . . . . 49
     B.1.  MPTCP Control Block  . 42
   Appendix C.  Changelog . . . . . . . . . . . . . . . . . . 50
       B.1.1.  Authentication and Metadata  . . . . 42
     C.1.  Changes since draft-ietf-mptcp-multiaddressed-01 . . . . . 42
     C.2.  Changes since draft-ietf-mptcp-multiaddressed-00 . . . . 50
       B.1.2.  Sending Side . 43
     C.3.  Changes since draft-ford-mptcp-multiaddressed-03 . . . . . 43
     C.4.  Changes since draft-ford-mptcp-multiaddressed-02 . . . . . 43
   Authors' Addresses . . . . . . . . . . 50
       B.1.3.  Receiving Side . . . . . . . . . . . . . . 43

1.  Introduction

   Multipath TCP (henceforth referred to as MPTCP) is a set of
   extensions to regular . . . . . . 51
     B.2.  TCP [2] to allow a transport connection to
   operate across multiple paths simultaneously.  This document presents
   the protocol changes required to add multipath capability to TCP;
   specifically, those for signalling and setting Control Blocks . . . . . . . . . . . . . . . . . . . . 51
       B.2.1.  Sending Side . . . . . . . . . . . . . . . . . . . . . 51
       B.2.2.  Receiving Side . . . . . . . . . . . . . . . . . . . . 51
   Appendix C.  Changelog . . . . . . . . . . . . . . . . . . . . . . 52
     C.1.  Changes since draft-ietf-mptcp-multiaddressed-02 . . . . . 52
     C.2.  Changes since draft-ietf-mptcp-multiaddressed-01 . . . . . 52
     C.3.  Changes since draft-ietf-mptcp-multiaddressed-00 . . . . . 52
     C.4.  Changes since draft-ford-mptcp-multiaddressed-03 . . . . . 52
     C.5.  Changes since draft-ford-mptcp-multiaddressed-02 . . . . . 53
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 53

1.  Introduction

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

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

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

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

1.1.  Design Assumptions

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

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

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

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

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

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

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

   Fall-back:  The protocol should be able to fall back to standard TCP
      with no interference from the user, to be able to communicate with
      legacy hosts.

   Areas for further study:

   o  In theory, since this is purely a

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

1.2.  Multipath TCP extension, it should be
      possible to use in the Networking Stack

   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  The design presented should work with network provided multipath,
      for instance ECMP routing; subflows could be opened with different
      source/destination ports between the same addreses to allow ECMP
      to place the subflows on different paths.

1.2.  Multipath TCP in the Networking Stack

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

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

      Figure 1: Comparison of Standard TCP and MPTCP Protocol Stacks

   Detailed discussion

1.3.  Terminology

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

   Subflow:  A flow of TCP segments operating over an architecture for developing individual path,
      which forms part of a multipath larger MPTCP connection.  A subflow is
      started and terminated similarly to a regular TCP
   implementation, especially regarding the functional separation by connection.

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

   Data-level:  The payload data is nominally transferred over a
      connection, which in [3].

1.3.  Operation Summary turn is transported over subflows.  Thus the
      term "data-level" is synonymous with "connection level", in
      contrast to "subflow-level" which refers to properties of an
      individual subflow.

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

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

1.4.  MPTCP Concept

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

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

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

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

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

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

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

   o  Subflows are terminated as regular TCP connections, with a four
      way FIN handshake.  The MPTCP connection is terminated by a
      connection-level FIN packet, sent together with the FIN on the
      last subflow of the connection. FIN.

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

                  Figure 2: Example MPTCP Usage Scenario

1.4.

1.5.  Requirements Language

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

2.  Terminology

   Path:  A sequence of links between a sender and a receiver, defined
      in this context by  Operation Overview

   This section presents a source and destination address pair.

   Subflow:  A stream single description of TCP packets sent over a path, started and
      terminated similarly standard MPTCP
   operation, with reference to a regular TCP connection.

   (MPTCP) 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 an application socket.

   Data-level: the protocol operation.  The payload data is nominally transfered over a
      connection, which detailed
   protocol specification follows in turn is transported over subflows.  Thus Section 3.

   To understand the
      term "data-level" is synonymous with "connection level", in
      contrast to "subflow-level" which refers to properties operation of an
      individual subflow.

   Token:  A locally unique identifier given to Multipath TCP, let us consider a multipath connection
      by an endpoint.  May also be referred to as very
   simple case where a "Connection ID".

   Endpoint:  A host operating client having two addresses, A1 and A2
   establishes an MPTCP implementation, connection with a dual homed server having
   addresses B1 and either
      initiating or accepting an B2, as illustrated in Figure 2 in the previous
   section.  MPTCP connection.

3. offers the same bidirectional bytestream service as
   regular TCP.

   To open an MPTCP Protocol

   This section describes connection, the operation client sends a SYN segment from one
   of the MPTCP protocol, and is
   subdivided into sections for each key part its addresses (say A1) to one of the protocol operation.

   All MPTCP operations are signalled using optional TCP header fields.
   These TCP Options will have server's addresses (say B1).
   This SYN segment contains the MP_CAPABLE option numbers allocated by IANA, as
   listed in Section 9, that indicates that
   the client supports MPTCP and are defined throughout contains the following
   subsections.

3.1.  Connection Initiation

   Connection Initiation begins client's key for this
   MPTCP connection.  The server replies with a SYN, SYN/ACK, ACK exchange on a
   single path.  Each packet SYN segment that also
   contains the Multipath Capable (MP_CAPABLE)
   TCP option (Figure 3).  This MP_CAPABLE option declares its sender is capable of
   performing multipath TCP and wishes to do so on this particular
   connection.

   This confirm that it supports MPTCP.
   The MP_CAPABLE option contains returned by the server includes the server's
   key.  The client are server keys are used for different purposes by
   MPTCP.  First, each host derives a 64-bit key 32 bits token that is uniquely
   identifies the MPTCP connection on this host.  Second, the keys are
   used to authenticate the
   addition utilisation of future subflows.  This is other addresses.  Additional
   details about the only time utilisation of the key will MP_CAPABLE option may be
   sent found
   in clear on Section 3.1.

   To enable the wire; all future subflows will identify client and the
   connection using a 32-bit "token".  This token is a cryptographically
   secure hash of this key.  This will be a truncated (most significant
   32 bits) SHA-1 hash [6].  A different, 64-bit truncation (the least
   significant 64 bits) of server to use their multiple addresses
   to support the hash of same MPTCP connection, MPTCP allows the key will be used as client and the
   Initial Data Sequence Number.

   This key is generated by
   server to open additional subflows.  These subflows are TCP
   connections that are linked to the sender and has local meaning only, MPTCP connection and
   its method of generation is implementation-specific.  The key SHOULD can be hard used
   to guess, send and it MUST be unique for the receive data.  The client can open an additional subflow
   by sending host at any
   one time.  Connections will be indexed at each host by the token (the
   truncated SHA-1 hash of the key), but an implementation will require a mapping SYN segment from another address (e.g.  A2) with the token
   MP_JOIN option to the key for each connection. server.  The MP_CAPABLE MP_JOIN option is carried on contains the SYN, SYN/ACK, and ACK packets
   server's token that start uniquely identifies the first subflow of an MPTCP connection.  The data
   carried by each packet is as follows, where A = initiator and B =
   listener.

   o  SYN (A->B): A's Key.

   o  SYN/ACK (B->A): B's Key.

   o  ACK (A->B): Both A's Key connection to which
   the subflow must be associated and B's Key.

   The contents of a random number.  To accept the
   subflow, the server replies by sending a SYN+ACK segment with the
   MP_JOIN option is determined that contains a random number chosen by the SYN server and ACK flags of
   the packet, verified by
   a HMAC computed over the option's length field.  For client and server's random numbers with the diagram
   shown in Figure 3, "sender"
   client and "receiver" refer server keys.  This HMAC authenticates the server to the sender or
   receiver
   client.  Upon reception of this SYN+ACK segment, the TCP packet.

   The keys are echoed in the client replies
   with an ACK in order to allow segment that contains an MP_JOIN option that includes
   another HMAC that authenticates the listener client to act
   statelessly until the TCP connection reaches server.  Additional
   details about the ESTABLISHED state.
   If utilisation of the listener acts in this way, however, it MUST generate its key MP_JOIN option may be found in a verifiable fashion, allowing it to verify that it generated
   Section 3.2.

   The server may also establish one or more subflows with the
   key when it is echoed in client by
   sending SYN segments with the ACK.  If this ACK does not carry data,
   it MUST still be ACKed by the receiver in order for the sender to
   ensure the ACK with MP_JOIN option that has been received.

   The first octet briefly
   described above.  Furthermore, a host my also inform the other host
   of this option specifies the IP addresses that it owns.  MPTCP version in use
   (for this specification, uses two options for this is 0).
   purpose.  The second octet is reserved
   for flags, and currently MUST be set ADD_ADDR option allows a host to all zeros.  The meaning indicates that it owns
   another address.  For example, in the above scenario, the server
   could use the ADD_ADDR option to indicate that it also owns address
   B2.  If a host becomes unable to use a previously advertised address,
   it uses the REMOVE_ADDR option to indicate the address that it lost
   to its peer.  Additional details about the utilisation of
   such flags will the
   ADD_ADDR and REMOVE_ADDR options may be determined found in future revisions Section 3.4.

   The data produced by the client and the server can be sent over any
   of MPTCP, however
   some possible uses the subflows that compose an MPTCP connection, and if a subflow
   fails, data may need to be retransmitted over another subflow.  For
   this, MPTCP relies on two principles.  First, each subflow is
   equivalent to enable or disable certain a normal TCP connection with its own 32-bits sequence
   numbering space.  This enables MPTCP
   features, and to provide traverse complex middle-boxes
   like transparent proxies or traffic normalizers.  Second, MPTCP
   maintains a mechanism for crypto agility. 64-bits data sequence numbering space.  The MP_CAPABLE DSS MPTCP
   option is only used in to send the first subflow of data sequence numbers and data sequence
   acknowledgements.  When a
   connection, in order to identify host sends a TCP segment over one subflow,
   it indicates inside the connection; all following
   subflows will use segment, by using the "Join" option (see Section 3.2) DSS option, the mapping
   between the 64-bits data sequence number and the 32-bits sequence
   number used by the subflow.  Thanks to join this mapping, the
   existing connection. receiving
   host can reorder the data received, possibly out-of-sequence over the
   different subflows.  In MPTCP, a received segment is acknowledged at
   two different levels.  First, the TCP cumulative or selective
   acknowledgements are used to acknowledge the reception of the data on
   each subflow.  Second, the acknowledgements field in the DSS option
   is returned by the receiving host to provide cumulative
   acknowledgements at the data sequence level.  When a segment is lost,
   the receiver detects the gap in the received 32-bits sequence number
   and traditional TCP retransmission mechanisms are triggered to
   recover from the loss.  When a subflow fails, MPTCP detects the
   failure and retransmits the unacknowledged data over another subflow
   that is still active.  The DSS option also includes an optional
   checksum that covers data at the MPTCP connection level to enable a
   receiver to detect whether an middlebox has inserted, deleted or
   modified data on-the-fly.  The transmission of data by MPTCP is
   discussed in details in Section 3.3.

3.  MPTCP Protocol

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

   All MPTCP operations are signalled using optional TCP header fields.
   A single TCP option number will be assigned by IANA (see Section 8),
   and then individual messages will be determined by a "sub-type", the
   values of which will also be stored in an IANA registry (and are also
   listed in Section 8).  This sub-type is a four-bit field - the first
   four bits of the option payload, as shown in Figure 3.  The MPTCP
   messages are defined in the following sections.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+---------------+---------------+
      |Kind=MP_CAPABLE|    Length
      +---------------+---------------+-------+-----------------------+
      |    Version     Kind      |   (reserved)    Length     |Subtype|                       |
      +---------------+---------------+---------------+---------------+
      |                           Sender Key                          |
      |                            (64 bits)                          |
      |                                                               |
      +---------------------------------------------------------------+
      |                      Receiver Key (64 bits)
      +---------------+---------------+-------+                       |
      |                         (if Length==20)                     Subtype-specific data                     |
      |                       (variable length)                       |
      +---------------------------------------------------------------+

                       Figure 3: Multipath Capable (MP_CAPABLE) option

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

   Those 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 options associated with subflow initiation must be
   included on packets with the SYN packets are unacknowledged, it flag set.  Additionally, there is up to local policy to
   decide how
   one MPTCP option for signalling metadata to respond.  It is expected that a sender will eventually
   fall back ensure segmented data can
   be recombined for delivery to single-path TCP (i.e. without the MP_CAPABLE Option) in
   order to work around middleboxes that may drop packets with unknown
   options; application.

   The remaining options, however, the number of multipath-capable attempts that are
   made first will be up signals that do not need to local policy.  Once the active opener has
   sent be on
   a SYN without the MP_CAPABLE option, specific packet, such as those for signalling additional addresses.
   Whilst an implementation may desire to send MPTCP options as soon as
   possible, it MUST fall back may not be possible to combine all desired options (both
   those for MPTCP and for regular TCP behavior, even if it subsequently receives TCP, such as SACK [6]) on a SYN/ACK that
   contains single
   packet.  Therefore, an MP_CAPABLE option.  This might happen if implementation may choose to send duplicate
   ACKs containing the MP_CAPABLE
   SYN and subsequent non-MP-capable SYN are reordered. additional signalling information.  This is to
   ensure that changes
   the two endpoints end up semantics of a duplicate ACK, these are usually only sent as a
   signal of a lost segment [7] in regular TCP.  Therefore, an interoperable state, MPTCP
   implementation receiving a duplicate ACK which contains an MPTCP
   option MUST NOT treat it as a signal of congestion.  Additionally, an
   MPTCP implementation SHOULD NOT send more than two duplicate ACKs in
   a row for signalling purposes, so as to ensure no
   matter what order the SYNs arrive at the passive opener.  This final
   state is inferred from the presence or absence middleboxes
   misinterpret this as a sign of congestion.

   Furthermore, standard TCP validity checks (such as ensuring the MP_CAPABLE
   option
   Sequence Number and Acknowledgement Number are within window) MUST be
   undertaken before processing any MPTCP signals, as described in the third [8].

3.1.  Connection Initiation

   Connection Initiation begins with a SYN, SYN/ACK, ACK exchange on a
   single path.  Each packet of contains the Multipath Capable (MP_CAPABLE)
   TCP handshake.  If this option (Figure 4).  This option declares its sender is
   not present, the connection should fall back capable of
   performing multipath TCP and wishes to regular TCP, as
   documented in Section 3.6.

   The initial Data Sequence Number (IDSN) is generated as do so on this particular
   connection.

   This option contains a hash from 64-bit key that is used to authenticate the Key, in
   addition of future subflows.  This is the same way as only time the token, i.e.  IDSN-A = Hash(Key-A) and
   IDSN-B = Hash(Key-B).  The Hash mechanism here provides key will be
   sent in clear on the least
   significant 64 bits of wire; all future subflows will identify the SHA-1
   connection using a 32-bit "token".  This token is a cryptographic
   hash of the this key.  The SYN with
   MP_CAPABLE occupies the first octet of Data Sequence Space.

3.2.  Starting a New Subflow

   Once a MPTCP connection has begun with the MP_CAPABLE exchange,
   further subflows can  This will be added to the connection.  Endpoints have
   knowledge a truncated (most significant 32
   bits) SHA-1 hash [9].  A different, 64-bit truncation (the least
   significant 64 bits) of their own address(es), and can become aware the hash of the other
   endpoint's addresses through signalling exchanges key will be used as described in
   Section 3.5.  Using this knowledge, an endpoint can initiate a new
   subflow over a currently unused pair of addresses.  The protocol
   permits either endpoint of a connection to initiate the creation
   Initial Data Sequence Number.

   This key is generated by its sender and has local meaning only, and
   its method of a
   new subflow (but see Section 3.8 for heuristics).

   A new subflow generation is started as a normal TCP SYN/ACK exchange. implementation-specific.  The Join
   Connection (MP_JOIN) TCP option (Figure 4) is used to identify the
   connection to key MUST be joined by the new subflow.  The tokens used
   hard to
   identify the MPTCP connection are cryptographically secure hashes of guess, and it MUST be unique for the sending host at any one
   time.  Recommendations for generating random keys exchanged in the initial MP_CAPABLE handshake.  The tokens
   presented in this option are generated given in [10].
   Connections will be indexed at each host by the SHA-1 [6] algorithm, token (the truncated to
   SHA-1 hash of the most significant 32 bits.  The key).  Therefore, an implementation will require a
   mapping from each token included to the corresponding connection, and in turn
   to the
   MP_JOIN option is keys for the token connection.

   There is a very small risk that two different keys will hash to the receiver of the packet uses
   same token.  An implementation SHOULD check its list of connection
   tokens to
   identify this connection, i.e.  Host A will send Token-B (which ensure there is
   generated from Key-B), and vice versa.

   The MP_JOIN SYN/SYN-ACK handshake not only exchanges a collision before sending its key in
   the tokens
   (which are static SYN/ACK.  This would, however, be costly for a connection) but also Random Numbers (nonces) server with
   thousands of connections.  The subflow handshake mechanism
   (Section 3.2) will ensure that are used to prevent replay attacks on new subflows only join the authentication method.
   Whilst these data are transferred correct
   connection, however, so in the SYN exchange, worst case if there was a token
   collision, it just means that the actual
   cryptographic authentication second connection cannot support
   multiple subflows, but will otherwise provide a regular TCP service.

   The MP_CAPABLE option is undertaken in carried on the SYN, SYN/ACK, and ACK packets
   that start the first two payload
   segments subflow of the an MPTCP connection.  Once the peers have successfully
   authenticated themselves,  The data
   carried by each packet is as follows, where A = initiator and B =
   listener.

   o  SYN (A->B): A's Key.

   o  SYN/ACK (B->A): B's Key.

   o  ACK (A->B): Both A's Key and B's Key.

   The contents of the subflow option is handed over to determined by the scheduler
   to be used for data (the presense SYN and ACK flags of a DSN_MAP option Section 3.3
   indicates this).

   The MP_JOIN option also contains an "Address ID"
   the packet, verified by the option's length field.  For the diagram
   shown in Figure 4, "sender" and "receiver" refer to identify the
   source address sender or
   receiver of this the TCP packet if it has changed in transit; (which can be either host).  If the
   behaviour of this ID SYN
   flag is explained later in this section.

                           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_JOIN  |  Length = 8   |   Address ID   | (reserved) |B|
      +---------------+---------------+----------------+--------------+
      |                    Receiver Token (32 bits)                   |
      +---------------------------------------------------------------+
      |                 Sender Random Number (32 bits)                |
      +---------------------------------------------------------------+

       Figure 4: Join Connection (MP_JOIN) option (only valid on SYN
                                 packets)

   On the third and fourth packets of the handshake, the following data set, a single key is sent included; if only an ACK flag is set,
   both keys are present.

   The keys are echoed in the TCP payload:

                           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_AUTH  |    Length     |          (reserved)           |
      +---------------+---------------+-------------------------------+
      |                                                               |
      |                                                               |
      |                  HMAC (256 bits for SHA-256)                  |
      |                                                               |
      |                                                               |
      +---------------------------------------------------------------+

                       Figure 5: Authentication Data

   For consistancy, this follows ACK in order to allow the listener (host
   B) to act statelessly until the same format as a TCP Option,
   although connection reaches the
   ESTABLISHED state.  If the listener acts in this way, however, it is sent
   MUST generate its key in a verifiable fashion, allowing it to verify
   that it generated the TCP payload.  The HMAC algorithm key when it is as
   defined echoed in [6], using the SHA-256 hash algorithm (thus generating a
   256-bit / 32 octet HMAC), however ACK.

   Furthermore, in the future some of the reserved
   bits could be used order to enable alternative algorithms.

   The key for the HMAC algorithm, in the case ensure reliable delivery of the message
   transmitted by Host A, ACK
   containing the MP_CAPABLE option, a server MUST respond with an ACK
   segment on receipt of this, which may contain data, or will be Key-A followed by Key-B, and in a pure
   ACK if it does not have any data to send immediately.  If the
   case
   initiator does not receive this ACK within the RTO, it MUST re-send
   the ACK containing MP_CAPABLE.  In effect, an MPTCP connection is in
   a "PRE_ESTABLISHED" state while awaiting this ACK, and only upon
   receipt of Host B, Key-B followed by Key-A. the ACK will it move to the ESTABLISHED state.

   The message first four bits of the first octet in each case the MP_CAPABLE option
   (Figure 4) define the MPTCP option subtype (see Section 8; for
   MP_CAPABLE, this is 0), and the concatenations remaining four bits of Random Number for each host (denoted by R): this octet
   specifies the MPTCP version in use (for this specification, this is
   0).

   The second octet is reserved for
   Host A, R-A followed by R-B; flags.  The leftmost bit - labeled C
   - indicates "Checksum required", and for Host B, R-B followed by R-A.

   When receiving a SYN with a MP_JOIN option that contains a valid
   token for an existing MPTCP connection, the recipient SHOULD respond
   with a SYN/ACK also containing an MP_JOIN option containing the
   initiator's token.  This will then lead on be set to 1 unless
   specifically overridden (for example, if the authentication HMAC
   exchange described above.  This behaviour system administrator has
   decided that checksums are not required - see Section 3.3 for more
   discussion).  The remaining bits are used for crypto algorithm
   negotiation.  Currently only the rightmost bit - labeled S - is illustrated
   assigned, and indicates the use of HMAC-SHA1 (as defined in Figure 6.

              Host A                                     Host B
     ------------------------                   ------------------------
     Address A1    Address A2                   Address B1    Address B2
     ----------    ----------                   ----------    ----------
         |             |                            |             |
         |         SYN + MP_CAPABLE(Key-A)          |             |
         |----------------------------------------->|             |
         |<-----------------------------------------|             |
         |       SYN/ACK + MP_CAPABLE(Key-B)        |             |
         |             |                            |             |
         |      ACK + MP_CAPABLE(Key-A, Key-B)      |             |
         |----------------------------------------->|             |
         |             |                            |             |
         |             |        SYN + MP_JOIN(Token-B, R-A)       |
         |             |----------------------------------------->|
         |             |<-----------------------------------------|
         |             |      SYN/ACK + MP_JOIN(Token-A, R-B)     |
         |             |                            |             |
         |             |  HMAC(Key=(Key-A+Key-B), Msg=(R-A+R-B))  |
         |             |----------------------------------------->|
         |             |<-----------------------------------------|
         |             |  HMAC(Key=(Key-B+Key-A), Msg=(R-B+R-A))  |
         |             |                            |             |

               Figure 6: Example use of MPTCP Authentication
   Section 3.2).  An implementation that only supports this method MUST
   set this bit to 1 and all other currently reserved bits to zero.  If the token received at Host B is unknown or local policy prohibits
   the acceptance
   none of these flags are set, the new subflow, the recipient MP_CAPABLE option MUST respond with be treated as
   invalid and ignored (i.e. it must be treated as a regular TCP RST.

   If
   handshake).

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

   For crypto negotiation, the one expected, Host A MUST close responder has the subflow with choice.  The initiator
   creates a TCP
   RST.

   If either host receives an incorrect HMAC (i.e. it does not match
   what the host believes it should be), proposal setting a bit for each algorithm it MUST close supports to 1
   (in this version of the subflow specification, there is only one proposal, so
   S will be always set to 1).  The responder responds with only one bit
   set - this is the chosen algorithm.  The rationale for this behaviour
   is that the responder will typically be a server with potentially
   many thousands of connections, so may wish to choose an algorithm
   with minimal computational complexity, depending on load.  If a
   responder does not support (or does not want to support) any of the
   initiator's proposals, it can respond without an MP_CAPABLE option,
   thus forcing a fall-back to regular TCP.

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

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

              Figure 4: Multipath Capable (MP_CAPABLE) option

   If a SYN contains an MP_CAPABLE option but the SYN/ACK does not, it
   is assumed that the passive opener is not multipath capable and thus
   the MPTCP session MUST 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 third packet (the ACK) does not contain the
   MP_CAPABLE option, then the session MUST fall back to operating as
   regular, single-path TCP.  This is to maintain compatibility with
   middleboxes on the path that drop some or all TCP options.

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

   The initial Data Sequence Number (IDSN) is generated as a hash from
   the Key, in the same way as the token, i.e.  IDSN-A = Hash(Key-A) and
   IDSN-B = Hash(Key-B).  The Hash mechanism here provides the least
   significant 64 bits of the SHA-1 hash of the key.  The SYN with
   MP_CAPABLE occupies the first octet of Data Sequence Space.

3.2.  Starting a New Subflow

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

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

   This section specifies the behaviour of MP_JOIN using the HMAC-SHA1
   algorithm.  An MP_JOIN option is present in the SYN, SYN/ACK and ACK
   of the three-way handshake, although in each case with a different
   format.

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

   The token is used to identify the MPTCP connection and is a
   cryptographic hash of the receiver's key, as exchanged in the initial
   MP_CAPABLE handshake (Section 3.1).  The tokens presented in this
   option are generated by the SHA-1 [9] algorithm, truncated to the
   most significant 32 bits.  The token included in the MP_JOIN option
   is the token that the receiver of the packet uses to identify this
   connection, i.e.  Host A will send Token-B (which is generated from
   Key-B).

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

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

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

   The MP_JOIN option on SYNs also includes 4 bits of flags, 3 of which
   are currently reserved and MUST be set to zero by the sender.  The
   final bit, labelled 'B', indicates whether the initiator wishes this
   subflow to be used purely as a backup path (B=1) in the event of
   failure of other paths, or whether it wants it to be used as part of
   the connection immediately.  Subflow policy is discussed in more
   detail in Section 3.3.8.

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

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

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

   An MAC is sent by both hosts - by the initiator (Host A) in the third
   packet (the ACK) and by the responder (Host B) in the second packet
   (the SYN/ACK).  This is to allow both hosts to have exchanged random
   data to be used as the message before generating the MAC.  In both
   cases, the MAC algorithm is HMAC as defined in [11], using the SHA-1
   hash algorithm [9] (thus generating a 160-bit / 20 octet HMAC).  Due
   to option space limitations, the MAC included in the SYN/ACK is
   truncated to the leftmost 64 bits, but this is acceptable since while
   in an attacker-initiated attack, the attacker can retry many times;
   if the attacker is the responder, he only has one chance to get the
   MAC correct.

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

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

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

    Figure 6: Join Connection (MP_JOIN) option (for responding SYN/ACK)
                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-----------------------+
      |     Kind      |  Length = 24  |Subtype|      (reserved)       |
      +---------------+---------------+-------+-----------------------+
      |                                                               |
      |                                                               |
      |                 Sender's MAC (160 bits SHA-1)                 |
      |                                                               |
      |                                                               |
      +---------------------------------------------------------------+

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

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

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

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

               Figure 8: Example use of MPTCP Authentication

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

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

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

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

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

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

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

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

3.3.  General MPTCP Operation

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

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

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

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

                Figure 9: Data Sequence Signal (DSS) option

   The echoing of the token serves two purposes: it ensures both
   endpoints agree on the connection being referred to (this is
   particularly relevant flags when both addresses being used are new to set define the
   connection); and it ensures there are no middleboxes on contents of this new path
   that will drop MPTCP options on the return path.

   If the SYN/ACK as received at Host A does not have an MP_JOIN option,
   Host as follows:

   o  A MUST close the subflow with = Data ACK present

   o  a RST.

   If MP_JOIN = Data ACK is stripped from the SYN on the path from A to B, 8 octets (if not set, Data ACK is 4 octets)

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

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

   The flags 'a' and 'm' only have a passive opener on meaning if the relevant port, it corresponding 'A' or
   'M' flags are set, otherwise they will
   respond be ignored.  The maximum
   length of this option, with an RST in the normal way. all flags set, is 28 octets.

   The 'F' flag indicates "DATA FIN".  If in response to a SYN with
   an MP_JOIN option, a SYN/ACK present, this means that this
   mapping covers the final data from the sender.  This is received without the MP_JOIN option
   (either since it was stripped on
   connection-level equivalent to the return path, or it was stripped
   on FIN flag in single-path TCP.  The
   purpose of the outgoing path but DATA FIN, along with the passive opener on Host B responded as if
   it were a new regular TCP session), then interactions between this
   flag, the subflow is unusable subflow-level FIN flag, and
   Host A the data sequence mapping are
   described in Section 3.3.3.  The remaining reserved bits MUST close it with a RST.

   It should be noted that additional subflows can be created between
   any pair of ports (but see Section 3.8 for heuristics); no explicit
   application-level accept calls or bind calls are required set
   to open
   additional subflows.  To associate a new subflow with zero by an existing
   connection, implementation of this specification.

   Note that the token supplied Checksum is only present in this option if the subflow's SYN exchange is used
   for demultiplexing.  This then binds use of
   MPTCP checksumming has been negotiated at the 5-tuple MP_CAPABLE handshake
   (see Section 3.1).  The presence of the TCP subflow
   to checksum can be inferred from
   the local token length of the connection.  A consequence is that it is
   possible to allow any port pairs to be used for option.

3.3.1.  Data Sequence Mapping

   The data stream as a connection.

   Deumultiplexing subflow SYNs MUST whole can be done using reassembled through the token; this is
   unlike traditional TCP, where use of the
   Data Sequence Mapping components of the DSS option (Figure 9), which
   define the mapping from the destination port is used for
   demultiplexing SYN packets.  Once a subflow sequence number to the data
   sequence number.  This is setup, demultiplexing
   packets is done using used by the five-tuple, as in traditional TCP.  The
   five-tuples will be mapped receiver to ensure in-order
   delivery to the local connection ID.

   The MP_JOIN option includes an "Address ID".  This application layer.  Meanwhile, the subflow-level
   sequence numbers (i.e. the regular sequence numbers in the TCP
   header) have subflow-only relevance.  It is an identifier expected (but not
   mandated) that only has significance within SACK [6] is used at the subflow level to improve
   efficiency.

   The Data Sequence Mapping specifies a single connection, where it
   identifies full mapping from subflow
   sequence space to data sequence space, for the source address specified length
   (number of this packet. bytes of data) starting at the specified Subflow and Data
   Sequence Numbers.  The key purpose of
   this identifier the explicit mapping is to allow address removal without needing to know
   what assist
   with compatibility with situations where TCP/IP segmentation or
   coalescing is undertaken separately from the source address at stack that is generating
   the receiver is, thus allowing data flow (e.g. through the use of
   NATs.  The sender can signal this to the receiver via the REMOVE_ADDR
   option (Section 3.5.2). TCP segmentation offloading on
   network interface cards, or by middleboxes such as performance
   enhancing proxies).  It also allows correlation between new
   subflow setup attempts and address signalling (Section 3.5.1), a single mapping to
   prevent setting up duplicate subflows on the same path.

   The Address IDs of the subflow used cover many
   packets, which may be useful in the initial SYN exchange of
   the first subflow bulk transfer situations.

   A mapping is unique, in that the connection are implicit, and have the value
   zero.

   The Address ID must be stored by subflow sequence number is bound to
   the receiver in a data structure
   that gathers all sequence number after the Address ID mapping has been processed.  It is
   not possible to address mappings for a connection
   identified by a token pair.  In change this way there is a stored mapping
   between Address ID, observed source address and token pair for future
   processing of control information afterwards; however, the same
   data sequence number can be mapped to different subflows for a connection.

   The MP_JOIN option
   retransmission purposes (see Section 3.3.6).  It would also includes 8 bits of flags, 7 of which are
   currently reserved.  The final bit, labelled 'B', indicates whether permit
   the initiator wishes this subflow same data to be used purely as a backup path
   (B=1) in sent simultaneously on multiple subflows for
   resilience purposes, although the event detailed specification of failure such
   operation is outside the scope of other paths, or whether it wants it
   to be used this document.

   The data sequence number is specified as part of an absolute value, whereas
   the connection immediately.  Subflow policy subflow sequence numbering is
   discussed in more detail in Section 3.3.6.

3.3.  General MPTCP Operation

   This section discusses operation relative (the SYN at the start 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
   the subflow has relative subflow sequence number 0).  This is to
   allow it to be reassembled and
   delivered reliably and in-order middleboxes to change the recipient application.  The
   following subsections define this behaviour in detail.

3.3.1.  Data Initial Sequence Numbering Number of a subflow,
   such as firewalls that undertake ISN randomization.

   The data stream as sequence mapping also contains a whole can be reassembled through the use checksum of the
   Data Sequence Mapping (DSN_MAP, Figure 7) option, which defines the data that
   this mapping from covers.  This is used to detect if the data sequence number payload has been
   adjusted in any way by a non-MPTCP-aware middlebox.  If this checksum
   fails, it will trigger a failure of the subflow, or a fallback to
   regular TCP, as documented in Section 3.5, since MPTCP can no longer
   reliably know the subflow sequence number.
   This is used by space at the receiver to ensure in-order delivery to the
   application layer.  Meanwhile, the subflow-level sequence numbers
   (i.e. the regular build
   data sequence numbers in mappings.

   The checksum algorithm used is the standard TCP header) have subflow-
   only relevance.  It is expected (but not mandated) that SACK [7] is
   used at checksum [2],
   operating over the subflow level to improve efficiency. data covered by this mapping, along with a pseudo-
   header as shown in Figure 10.

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+------------------------------+
     +--------------------------------------------------------------+
     | Kind=DSN_MAP                                                              |    Length
     |                Data Sequence Number ... :
      +---------------+---------------+------------------------------+
      : ... ( (length-10) octets )    | Data-level Length (2 (8 octets)               |
      +-------------------------------+------------------------------+
     |                                                              |
     +--------------------------------------------------------------+
     |              Subflow Sequence Number (4 octets)              |
     +-------------------------------+------------------------------+
     |      Checksum  Data-level Length (2 octets) |
      +-------------------------------+        Zeros (2 octets)      |
     +-------------------------------+------------------------------+

                 Figure 7: Data Sequence Mapping (DSN_MAP) 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 10: Pseudo-Header for the
   specified length (number of bytes of data).  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 DSS Checksum

   Note that is generating the data flow (e.g. through the use of
   TCP segmentation offloading on network interface cards, or by
   middleboxes such as performance enhancing proxies).  It also allows a
   single mapping to cover many packets, which may be useful in bulk
   transfer situations.

   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 relative subflow sequence number 1).  This
   is allow middleboxes to change the Initial Sequence Number of a
   subflow, since Data Sequence Number used in the data stream itself will not be affected (some
   firewalls do ISN randomization).

   The final two octets of this option contain a checksum of pseudo-header is
   always the data
   that this mapping covers.  This 64-bit value, irrespective of what length is used to detect if the payload has
   been adjusted in any way by a non-MPTCP-aware middlebox.  If this
   checksum fails, it will trigger a failure of the subflow, or a
   fallback to regular TCP, as documented in Section 3.6.
   DSS option itself.  The checksum
   algorithm used is the standard TCP checksum [2], operating only over
   the data covered by this DSN_MAP (i.e. there is no pseudo-header).
   This algorithm has been
   chosen since it will be calculated anyway for the TCP subflow, and if
   calculated first over the data before adding the pseudo-header, pseudo-headers, it
   only needs to be calculated once.  Furthermore, since the TCP
   checksum is additive, the checksum for a DSN_MAP can be constructed
   by simply adding together the checksums for the data of each
   constituent TCP segment.  This segment, and adding the checksum for the DSS pseudo-
   header.

   Note that checksumming relies on the TCP subflow containing
   contiguous data, however, and thus therefore a TCP subflow MUST NOT use the Urgent
   Pointer (i.e. the URG flag MUST be zero).

   A mapping is unique, in to interrupt an existing mapping.  Further note, however,
   that the subflow sequence number if Urgent data is bound received on a subflow, it SHOULD be mapped to
   the data sequence number after the mapping has been processed.  It is
   not possible space and delivered to change this mapping afterwards (although the length
   of a mapping can extend); however, the same application analogous to
   Urgent data sequence number can
   be mapped on different subflows for retransmission purposes (see
   Section 3.3.4). in regular TCP.

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

   Data sequence numbers are always 64-bit quantities, and MUST be
   maintained as such in implementations.  If a connection is
   progressing at a slow rate, so protection against wrapped sequence
   numbers is not required, and if security requirements against blind
   insertion attacks are not stringent, then it is permissible to include just the
   lower 32 bits of the data sequence number in the DSN_MAP
   option Data Sequence
   Mapping and/or Data ACK as an optimization.  An implementation MUST
   send the full 64 bit Data Sequence Number if it is transmitting at a
   sufficiently high rate that it could wrap within the MSL [12].  The
   lengths of the DSNs used in these values (which may be different) are
   declared with flags in the DSS option.  Implementations MUST accept this a
   32-bit DSN and implicitly promote it to a 64-bit quantity by
   incrementing the upper 32 bits of sequence number each time the lower
   32 bits wrap.  By
   defauly, the full 64 bit DSN_MAP should  A sanity check MUST be sent.  Security
   implications are discussed in Section 5. implemented to ensure that a
   wrap occurs at an expected time (e.g. the sequence number jumps from
   a very high number to a very low number) and is not triggered by out-
   of-order packets.

   As with the standard TCP sequence number, the data sequence number
   should not start at zero, but at a random value to make blind session
   hijacking harder.  This is done by setting the initial data sequence
   number (IDSN) of each host to the least significant 64 bits of the
   SHA-1 hash of the host's key (as declared in the MP_CAPABLE option in
   the initial connection SYN, which itself occupies the first octet of
   data sequence space).  This handshake is key, as described in more detail in Section 3.1.

   The DSN_MAP option

   A Data Sequence Mapping does not need to be included in every MPTCP
   packet, as long as the subflow sequence space in that packet is
   covered by a mapping known at the receiver.  This can be used to
   reduce overhead in cases where the mapping is known in advance; one
   such case is when there is a single subflow between the endpoints, hosts,
   another is when segments of data are scheduled in larger than packet-
   sized chunks.  An "infinite" mapping can be used to fallback to
   regular TCP by mapping the subflow-level data to the connection-level
   data for the remainder of the connection (see Section 3.6). 3.5).  This is
   achieved by setting the data-level length field to the reserved value
   of 0.  The checksum, in such a case, will also be set to zero.

3.3.2.  Data Acknowledgements

   To provide full end-to-end resilience, MPTCP provides a connection-
   level acknowledgement, the DATA_ACK, illustrated in Figure 8, to act as a cumulative ACK for the connection
   as a whole.  This is the "Data ACK" field of the DSS option
   (Figure 9).  The Data ACK is analogous to the behaviour of the
   standard TCP cumulative ACK in TCP SACK - indicating how much data
   has been successfully received (with no holes).  The Data ACK
   specifies the next Data Sequence Number it expects to receive.

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

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

   Another reason for including the Data ACK is that it indicates the receiver.  This sort
   left edge of bad
   interaction might be expecially prevalent when the receiver is
   mobile.  The DATA_ACK ensures advertised receive window.  As explained in
   Section 3.3.4, the data has been delieverd receive window is shared by all subflows and is
   relative to the
   receiver. Data ACK.  Because of this, an implementation MUST
   NOT use the RCV.WND field of a TCP segment at connection-level if it
   does not also carry a DSS option with a Data ACK field.

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

   The DATA_ACK option Data ACK MAY be included in all segments, analogous to a
   standard TCP ACK.  However, however optimisations
   SHOULD be considered in more advanced implementations, where the DATA_ACK option Data
   ACK is present in segments (data or pure ACKs) only when the DATA_ACK Data ACK value advances, and
   this behaviour MUST be treated as valid.  This behaviour ensures the
   sender buffer is freed, while reducing overhead when the data
   transfer is unidirectional.

                           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=DATA_ACK |    Length     |     Data Sequence Number ... :
      +---------------+---------------+------------------------------+
      : ... ( (length-2) octets )     |
      +-------------------------------+

           Figure 8: Connection-level Acknowledgement (DATA_ACK)

3.3.3.  Receiver Considerations

   Regular  Closing a Connection

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

   When an application calls close() on a socket, this indicates that it
   has no more data to accept past send, and for regular TCP this would result in a
   FIN on the
   cumulative ack.  The receive window connection.  For MPTCP, an equivalent mechanism is used needed,
   and this is referred to implement flow
   control, throttling down fast senders when receivers cannot keep up.

   MPTCP also uses a unique receive window, shared between as the subflows.
   The idea DATA FIN.

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

   The DATA FIN is signalled by setting the 'F' flag in the Data
   Sequence Signal option (Figure 9) to accept it; 1.  A DATA FIN occupies one
   octet (the final octet) of the connection-level sequence space.  Note
   that the DATA FIN is included in the Data-level Length, but not at
   the alternative, maintaining per subflow
   receive windows, could end-up stalling some subflows while others level: for example, a segment with DSN 80, and length 11,
   with DATA FIN set, would not use up their window.

   The receive window map 10 octets from the subflow into data
   sequnce space 80-89, the DATA FIN is DSN 90, and therefore this
   segment including DATA FIN would be acknowledged with a DATA ACK of
   91.

   Note that when the DATA FIN is relative not attached to the DATA_ACK.  As in TCP, a
   receiver MUST NOT shrink TCP segment
   containing data, the right edge Data Sequence Mapping MUST have Subflow Sequence
   Number of the receive window (e.g.
   DATA_ACK + receive window).  The receiver will use 0, a Length of 1, and 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 that
   corresponds with the
   sequence number DATA FIN itself.  The checksum in the packet and checks it against the allowed
   receive window.  With multipath, such a check is done using this case will
   only cover the
   connection level window. pseudo-header.

   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?  An
   implementation might wait until they arrive in order at subflow
   level, DATA FIN has the semantics and only then do behaviour as a regular TCP FIN, but
   at the connection level processing.  However, if
   many segments of data are restransmitted on more than one subflow,
   then because some data level.  Notably, it is duplicated then the sum total of
   unacknowledged data on only DATA ACKed once all subflows might exceed the receive window
   that was advertised, which indicates buffering available for
   data
   sequence space.  This such a strategy is probably undesirable.

   An alternative implementation might process segments has been successfully received at the connection level segments level.  Note
   therefore that have not yet been acked at a DATA FIN is decoupled from a subflow
   level; the FIN.  It is
   only requirement for this permissable to combine these signals on one subflow if there is
   no data oustanding on other subflows.  Otherwise, it may be necessary
   to have retransmit data on different subflows.  Essentially, a valid host MUST
   NOT FIN all subflows unless it is safe to do so, i.e. until all data sequence
   mapping for
   has been DATA ACKed, or that the segment.  This removes such duplicate data from segment with the
   receive buffer, so avoids running out of buffer space.  Such
   implementations SHOULD keep track of which subflow sequence numbers
   have already FIN flag set is the
   only outstanding segment.

   Once a DATA FIN has been accepted in this way, so they can acknowledged, all remaining subflows MUST be ACKed
   appropriately when the hole in
   closed with standard FIN exchanges.  Both hosts SHOULD send FINs, as
   a courtesy to allow middleboxes to clean up state even if the subflow sequence space in
   subsequently filled.  An implementation that does store such metadata
   would still progress (the rules for freeing data at the sender ensure
   this), but unnecessary retransmissions will result.
   has failed.  It is important for implementers also encouraged to understand how large a receiver
   buffer is appropriate.  The lower bound for full network utilization
   is reduce the maximum bandwidth-delay product of timeouts (Maximum
   Segment Life) on subflows at end hosts.  In particular, any of the paths.  However
   this might be insufficient when a packet subflows
   where there is lost still outstanding data queued (which has been
   retransmitted on a slower subflow
   and needs other subflows in order to get the DATA FIN
   acknowledged) MAY be retransmitted (see Section 3.3.4). closed with an RST.

   A tight upper
   bound would be the maximum RTT of any path multiplied connection is considered closed once both hosts' DATA FINs have
   been acknowledged by the total
   bandwidth available across all paths.  This permits all subflows to
   continue at full speed while DATA ACKs.

   Note that a packet is fast-retransmitted host may also send a FIN on the
   maximum RTT path.  Even an individual subflow to shut
   it down, but this might be insufficient impact is limited to maintain full
   performance in the event of subflow in question.  If
   all subflows have been closed with a retransmit timeout on FIN exchange, but no DATA FIN
   has been received and acknowledged, the maximum RTT
   path.  It MPTCP connection is for future study to determine treated
   as closed only after a timeout.  This implies that an implementation
   will have TIME_WAIT states at both the relationship between
   retransmission strategies subflow and receive buffer sizing. connection levels.

3.3.4.  Sender  Receiver Considerations

   The sender remembers receiver window advertisements from the
   receiver.  It should only update its local

   Regular TCP advertises a receive window values when in each packet, telling the largest sequence number allowed (i.e.  DATA_ACK + receive window)
   increases.  This
   sender how much data the receiver is important willing to allow using paths with different
   RTTs, and thus different feedback loops.

   Some classes of middleboxes may alter accept past the TCP-level
   cumulative ack.  The receive window.
   Typically these will shrink the offered window, although for short
   periods of time it may be possible for the window is used to be larger
   (however note that this would not continue for long periods since
   ultimately the middlebox must keep up with delivering data to the
   receiver).  Therefore, if receive window sizes differ on multiple
   subflows, implement flow
   control, throttling down fast senders when sending data receivers cannot keep up.

   MPTCP SHOULD take the largest of the most
   recent window sizes as also uses a unique receive window, shared between the one subflows.
   The idea is to use in calculations. (this rule allow any subflow to send data as long as the receiver
   is
   implicit in willing to accept it; the requirement 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 move back the DATA_ACK.  As in TCP, a
   receiver MUST NOT shrink the right edge of the
   window).

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

   Putting the two rules together, we get use the following: a sender is
   allowed Data Sequence
   Number to send data segments with data-level sequence numbers
   between (DATA_ACK, DATA_ACK + receive_window).  Each of these
   segments will tell if a packet should be mapped onto subflows, as long as accepted at connection level.

   When deciding to accept packets at subflow level, normal TCP uses the
   sequence
   numbers are number in the packet and checks it against the allowed windows for those subflows.  Note
   receive window.  With multipath, such a check is done using only the
   connection level window.  A sanity check SHOULD be performed at
   subflow level to ensure that the subflow and mapped sequence numbers do not generally affect flow control if
   meet the
   same receive window is advertised across all subflows.  They will
   perform flow control for those subflows with following test: SSN - SUBFLOW_ACK <= DSN - DATA_ACK.

   In regular TCP, once a smaller advertised
   receive window.

   The data sequence mapping allows senders to re-send data with segment is deemed in-window, it is either put
   in the in-order receive queue or in the out-of-order queue.  In
   multipath TCP, the same data sequence number on happens but at connection-level: a different subflow.  When doing this,
   an endpoint must still retransmit the original data on segment is
   placed in the original connection level in-order or out-of-order queue if it
   is in-window at both connection and subflow level.  The stack still
   has to remember, for each subflow, in which segments were received
   succesfully so that it can ACK them at subflow level appropriately.
   Typically, this will be implemented by keeping per subflow out-of-
   order to preserve queues (containing only message headers, not the subflow integrity (middleboxes
   could replay old data, and/or could reject holes in subflows), payloads) and
   remembering the value of the cumulative ACK.

   It is important for implementers to understand how large a receiver will ignore these retransmissions.  While this
   buffer is clearly
   suboptimal, appropriate.  The lower bound for compatibility reasons this full network utilization
   is the best behaviour.
   Optimisations could be negotiated in future versions maximum bandwidth-delay product of this
   protocol.

   This protocol specification does not mandate any mechanisms for
   handling retransmissions, and much will of the paths.  However
   this might be dependent upon local
   policy (as discussed in Section 3.3.6).  One can imagine aggressive
   connection level retransmissions policies where every insufficient when a packet lost at
   subflow level is retransmitted lost on a different slower subflow (hence wasting
   and needs to be retransmitted (see Section 3.3.6).  A tight upper
   bound would be the maximum RTT of any path multiplied by the total
   bandwidth but possibly reducing application-to-application delays),
   or conservative retransmission policies where connection-level
   retransmits are only used after available across all paths.  This permits all subflows to
   continue at full speed while a packet is fast-retransmitted on the
   maximum RTT path.  Even this might be insufficient to maintain full
   performance in the event of a few subflow level retransmit timeout on the maximum RTT
   path.  It is for future study to determine the relationship between
   retransmission
   timeouts occur. strategies and receive buffer sizing.

3.3.5.  Sender Considerations

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

   MPTCP uses a standard connection-level retransmission
   mechanism would single receive window across all subflows, and if the
   receive window was guaranteed to be implemented around unchanged end-to-end, a connection-level data queue:
   all segments that haven't been DATA_ACKed are stored.  A timer (based
   on host
   could always read the subflow timer values) is set when most recent receive window value.  However,
   some classes of middleboxes may alter the head TCP-level receive window.
   Typically these will shrink the offered window, although for short
   periods of time it may be possible for the connection-
   level is ACKed at subflow level but its corresponding data is window to be larger
   (however note that this would not
   acked at data level.

   The sender MUST keep data in its send buffer as continue for long as periods since
   ultimately the middlebox must keep up with delivering data has
   not been acked at connection level and to the
   receiver).  Therefore, if receive window sizes differ on all subflows it has been
   sent on.  In this way, multiple
   subflows, when sending data MPTCP SHOULD take the sender can always retransmit largest of the data if
   needed, on most
   recent window sizes as the same subflow or on a different one.  A special case one to use in calculations.  This rule is
   when a subflow fails: the sender will typically resend
   implicit in the data on
   other working subflows, and will keep trying requirement not to retransmit reduce the data
   on right edge of the failed subflow too.
   window.

   The sender will declare also remembers the receive windows advertised by each
   subflow.  The allowed window for subflow
   failed after a predefined upper bound on retransmissions i is (ack_i, ack_i +
   rcv_wnd_i), where ack_i is reached,
   and only then delete the outstanding subflow-level cumulative ack of
   subflow i.  This ensures data segments.

   A sender will maintain connection level timers for unacknowledged
   segments.  These timers will not be based on the subflow timers, and will
   guard against pro-active acking by middleboxes.

   The send buffer must be, at sent to a middlebox unless
   there is enough buffering for the minimum, as big as data.

   Putting the receive
   buffer, to enable two rules together, we get the following: a sender to reach maximum throughput.

3.3.5.  Congestion Control Considerations

   Different subflows in an MPTCP connection have different congestion
   windows.  To achieve fairness at bottlenecks and resource pooling, it is necessary
   allowed to couple send data segments with data-level sequence numbers
   between (DATA_ACK, DATA_ACK + receive_window).  Each of these
   segments will be mapped onto subflows, as long as subflow sequence
   numbers are in the congestion the allowed 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 those subflows.  Note that it is readily
   deployable in
   subflow sequence numbers do not generally affect flow control if the current Internet.

   It
   same receive window is foreseeable that different congestion controllers advertised across all subflows.  They will be
   implemented
   perform flow control 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, those subflows with a smaller advertised
   receive window.

   The send buffer must be, at the design of minimum, as big as the MPTCP protocol
   aims receive
   buffer, to provide enable the congestion control implementations sufficient
   information sender to take the right decisions; this information includes,
   for each subflow, which packets where lost and when. reach maximum throughput.

3.3.6.  Subflow Policy

   Within  Reliability and Retransmissions

   The data sequence mapping allows senders to re-send data with the
   same data sequence number on a local MPTCP implementation, different subflow.  When doing this, a
   host may use any local policy
   it wishes to decide how to share the traffic to be sent over the
   available paths.

   In must still retransmit the typical use case, where original data on the goal is original subflow,
   in order to maximise throughput,
   all available paths will be used simultaneously for data transfer,
   using coupled congestion control as described preserve the subflow integrity (middleboxes could replay
   old data, and/or could reject holes in [4].  It is
   expected, however, that other use cases will appear.

   For instance, subflows), and a possibility receiver will
   ignore these retransmissions.  While this is an 'all-or-nothing' approach, i.e.
   have a second path ready clearly suboptimal, for use in the event of failure of
   compatibility reasons this is the first
   path, but alternatives best behaviour.  Optimisations
   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 negotiated in future versions of links, but may
   also this protocol.

   This protocol specification does not mandate any mechanisms for
   handling retransmissions, and much will 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 dependent upon local
   policy (as discussed in detail in [5].

   The ability to make effective choices Section 3.3.8).  One can imagine aggressive
   connection level retransmissions policies where every packet lost at the sender requires full
   knowledge of the path "cost", which
   subflow level is unlikely to be the case. retransmitted on a different subflow (hence wasting
   bandwidth but possibly reducing application-to-application delays),
   or conservative retransmission policies where connection-level
   retransmits are only used after a few subflow level retransmission
   timeouts occur.

   It is envisaged that a standard connection-level retransmission
   mechanism would be desirable for implemented around a receiver to be able to signal their own
   preferences for paths, since they will often be connection-level data queue:
   all segments that haven't been DATA_ACKed are stored.  A timer is set
   when the multihomed party,
   and may have to pay for metered incoming bandwidth.

   Whilst fine-grained control may be head of the most powerful solution, connection-level is ACKed at subflow level but
   its corresponding data is not ACKed at data level.  This timer will
   guard against failures in re-transmission by middleboxes that
   would require some mechanism such pro-
   active ACK data.

   The sender MUST keep data in its send buffer as long as overloading the ECN signal [8],
   which is undesirable, data has
   not been acknowledged at both connection level and on all subflows it is felt that there would not be
   sufficient benefit to justify an entirely new signal.  Therefore
   has been sent on.  In this way, the
   MP_JOIN Section 3.2 and ADD_ADDR Section 3.5 options contain sender can always retransmit the 'B'
   bit, which allows
   data if needed, on the same subflow or on a host to indicate to its peer that this path
   should be treated as different one.  A special
   case is when a backup path to use only in subflow fails: the event of
   failure of sender will typically resend the
   data on other working subflows (i.e. after a subflow where the receiver
   has indicated B=1 SHOULD NOT be used timeout, and will keep trying
   to send retransmit the data unless there are no
   usable subflows where B=0).

   In on the event that failed subflow too.  The sender will
   declare the available set of paths changes, a host may wish
   to signal subflow failed after a change in priority of subflows to the peer.  Therefore,
   the MP_PRIO option, shown in Figure 9, can predefined upper bound on
   retransmissions is reached (which MAY be used to change lower than the 'B'
   flag usual TCP
   limits of the subflow Maximum Segment Life), or on which it is sent.

                           1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------------+-+
      | Kind=MP_PRIO  |   Length=3    |  (reserved) |B|
      +---------------+---------------+-------------+-+

                         Figure 9: MP_PRIO option

   It should be noted that the backup flag is receipt of an ICMP
   error, and only then delete the outstanding data segments.

   Multiple retransmissions are triggers that will indicate that a request from
   subflow performs badly and could lead to a host resetting the
   receiver subflow
   with an RST.  However, additional research is required to understand
   the sender only, heuristics of how and the sender SHOULD adhere when to these
   requests.  The reciever, however, reset underperforming subflows.
   For example, subflows that perform highly asymmetrically may continue using the subflow to
   send data even if be mis-
   diagnosed as underperforming.

3.3.7.  Congestion Control Considerations

   Different subflows in an MPTCP connection have different congestion
   windows.  To achieve fairness at bottlenecks and resource pooling, it has signalled B=1
   is necessary to couple the other host.

3.4.  Closing a Connection

   In regular TCP a FIN announces 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 receiver algorithm does not
   achieve perfect resource pooling but is "safe" in that it is readily
   deployable in the sender has no current Internet.  By this, we mean that it does
   not take up more data capacity on any one path than if it was a single
   path flow using only that route, so this ensures fair coexistence
   with single-path TCP at shared bottlenecks.

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

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

3.3.8.  Subflow Policy

   Within a local MPTCP implementation, a host may use any local policy
   it wishes to keep decide how to share the appearance of TCP traffic to be sent over the wire, a FIN
   in MPTCP only affects
   available paths.

   In the subflow on which it typical use case, where the goal is sent.  This allows
   nodes to exercise considerable freedom over which maximise throughput,
   all available paths are in use at
   any one time.  The semantics of a FIN remain as will be used simultaneously for regular TCP, i.e.
   it data transfer,
   using coupled congestion control as described in [4].  It is not until both sides have ACKed each other's FINs
   expected, however, that the
   subflow other use cases will appear.

   For instance, a possibility is fully closed.

   When an application calls close() on 'all-or-nothing' approach, i.e.
   have a socket, this indicates that it
   has no more data to send, and second path ready for regular TCP this would result use in a
   FIN on the connection.  For MPTCP, event of failure of the first
   path, but alternatives could include entirely saturating one path
   before using an equivalent mechanism is needed,
   and this is additional path (the 'overflow' case).  Such choices
   would be most likely based on the DATA_FIN.  This option, shown in Figure 10, is
   attached to a regular FIN option monetary cost of links, but may
   also be based on a subflow.

   A DATA_FIN is an indication that properties such as the sender has no delay or jitter of links,
   where stability (of delay or bandwidth) is more data to send,
   and as such can be used important than
   throughput.  Application requirements such as a rapid indication of these are discussed in
   detail in [5].

   The ability to make effective choices at the end sender requires full
   knowledge 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 path "cost", which is included in unlikely to be the option, and will case.  It
   would be
   ACKed at data level desirable for a receiver to ensure reliable delivery.

   The DATA_FIN is an optimisation be able to rapidly indicate signal their own
   preferences for paths, since they will often be the end of a data
   stream multihomed party,
   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 to pay for metered incoming bandwidth.

   Whilst fine-grained control 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.

   The interactions between a DATA_FIN and subflow properties are most powerful solution, that
   would require some mechanism such as
   follows:

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

   o  When DATA_FIN ECN signal [13],
   which is sent, undesirable, and it should be sent on all active subflows.

   o  There is a one-to-one mapping between felt that there would not be
   sufficient benefit to justify an entirely new signal.  Therefore the DATA_FIN and
   MP_JOIN option (see Section 3.2) contains the
      subflow's FIN flag (and its associated sequence space and thus 'B' bit, which allows a
   host to indicate to its
      acknowlegement).

   o  The data sequence number included peer that this path should be treated as a
   backup path to use only in the DATA_FIN is event of failure of other working
   subflows (i.e. a subflow where the receiver has indicated B=1 SHOULD
   NOT be used to
      verify that all send data has been successfully received.

   It should be noted unless there are no usable subflows where
   B=0).

   In the event that an endpoint may also send the available set of paths changes, a FIN on an
   individual subflow host may wish
   to shut it down, but this impact is limited signal a change in priority of subflows to the
   subflow peer.  Therefore,
   the MP_PRIO option, shown in question.  If all subflows have been closed with a FIN,
   that is equivalent Figure 11, can be used to having closed change the connection with a DATA_FIN.

   The full eight-byte data sequence number 'B'
   flag of the subflow on which it is always included in a
   DATA_FIN. sent.

                           1                   2
       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=DATA_FIN     Kind      |   Length=10   |   Data Sequence Number (8B)  :
      +---------------+---------------+------------------------------+
      :                  Data Sequence Number (contd.)               :
      +-------------------------------+------------------------------+
      :  Data Sequence Number (contd.)|
      +-------------------------------+     Length    |Subtype|     |B|
      +---------------+---------------+-------+-----+-+

                         Figure 10: DATA_FIN 11: MP_PRIO option

3.5.

   It should be noted that the backup flag is a request from the
   receiver to the sender only, and the sender SHOULD adhere to these
   requests.  The receiver, however, may continue using the subflow to
   send data even if it has signalled B=1 to the other host.

3.4.  Address Knowledge Exchange (Path Management)

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

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

   Here is an example of typical operation of the protocol:

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

   o  Simultaneously (or after a timeout), an ADD_ADDR option
      (Section 3.5.1) 3.4.1) is sent on an existing subflow, informing the
      receiver of the sender's alternative address(es).  The recipient
      can use this information to open a new subflow to the sender's
      additional address.  In our example, A will send ADD_ADDR option
      informing B of address address/port A2.  The mix of using the SYN-based
      option and the ADD_ADDR option, including timeouts, is implementation-
      specific
      implementation-specific and can be tailored to agree with local
      policy.

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

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

3.5.1.

3.4.1.  Address Advertisement

   The Add Address (ADD_ADDR) TCP Option announces additional addresses
   (and optionally, ports) on which an endpoint a host can be reached (Figure 11).  It can be used to
   announce several (ID, address) pairs to be announced to the other
   endpoint. 12).
   Multiple addresses instances of this TCP option can be added in a single
   message if there is sufficient TCP option space, otherwise multiple
   TCP messages containing this option will be sent.  This option can be
   used at any time during a connection, depending on when the sender
   wishes to enable multiple paths and/or when paths become available.

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

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

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

   The 'P' bit is used to indicate the 4).

   The presence of an additional the final two
   octets octets, specifying the TCP port number
   to use. use, are optional and can be inferred from the length of the
   option.  Although it is expected that the majority of use cases will
   use the same port pairs as used for the initial subflow (e.g. port 80
   remains port 80 on all
   subflows, subflows), as does the ephemeral port at the
   client, there may be cases (such as port-based load balancing) where
   the explicit specification of a different port is required.  If the P bit no
   port is not specified, MPTCP MUST SHOULD attempt to connect to the specified
   address on same port as is already in use by the signalling subflow.

   The 'B' bit is used to indicate that subflow,
   and this specified address (and
   port, if applicable) should be treated as a backup subflow to use
   only is discussed in the event of failure of other working subflows.  A receiver
   of this option SHOULD set up a TCP subflow to the specified address
   and port, but SHOULD NOT send data on it until the other paths have
   failed. more detail in Section 3.7.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+---------------+-------+---+-+-+
      +---------------+---------------+-------+-------+---------------+
      | Kind=ADD_ADDR     Kind      |     Length    |Subtype| IPVer |  Address ID   | IPVer |   |B|P|
      +---------------+---------------+---------------+-------+---+-+-+
      +---------------+---------------+-------+-------+---------------+
      |          Address (IPv4 - 4 octets / IPv6 - 16 octets)         |
      +-------------------------------+-------------------------------+
      +-------------------------------+---------------+---------------+
      |   Port (2 octets if P=1) octets, optional)   | ...
      +-------------------------------+
       ( ... further ID/Version/Address/Port fields as required ... )

         Figure 11: 12: Add Address (ADD_ADDR) option (shown for IPv4)

   Due to the proliferation of NATs, it is reasonably likely that one
   endpoint
   host may attempt to advertise private addresses [9]. [14].  We do not wish
   to blanket prohibit this, since there may be cases where both
   endpoints hosts
   have additional interfaces on the same private network.  We must
   ensure, however, that such advertisements do not cause harm.  The
   standard mechanism to create a new subflow (Section 3.2) contains a
   32-bit token that uniquely identifies the connection to the receiving endpoint .
   host.  If the token is unknown, the endpoint host will return with a RST.  If  In
   the unlikely event that the token is known, subflow setup will
   continue, but the sender's token will be sent back.  In order for a
   new subflow to be setup, both tokens must match what each endpoint
   expects.  This will be further followed by the HMAC MAC exchange must occur for authentication.  This
   will fail, and will provide sufficient protection against two
   unconnected endpoints hosts accidentally setting up a new subflow upon the
   signal of a private address.

   Ideally, we'd like to ensure the ADD_ADDR (and REMOVE_ADDR) option is and REMOVE_ADDR options are
   sent reliably reliably, and in order order, to the other end.  This is to ensure
   that we don't close do not unnecessarily cause an outage in the connection when
   remove/add addresses are processed in reverse order, and also to
   ensure that all possible paths are used.  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

   Therefore, implementing reliability signals for these TCP options is
   not worth optimizing for. necessary.  In order to minimise the first case, resending impact of the option on a different subflow loss of these
   options, however, it is the
   thing to do.  To achieve similar reliability without explicit ACKs,
   we propose sending all ADD_ADDR/REMOVE_ADDR RECOMMENDED that a sender should send these
   options on all existing available subflows.  If ordering is needed, we should these options need to be
   received in-order, an implementation SHOULD only send one ADD_ADDR/
   REMOVE_ADDR option per RTT (modulo lost packets at subflow level). RTT, to minimise the risk of misordering.

   When receiving an ADD_ADDR message with an address Address ID already in use
   for that a live subflow within the connection, the receiver SHOULD
   silently ignore the ADD_ADDR.  If the Address ID is not in use on a
   live subflow, but is stored by the receiver, a new ADD_ADDR SHOULD
   take precedence and replace the stored address.

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

   During normal MPTCP operation, it is unlikely that there will be
   sufficient TCP option space for ADD_ADDR to be included along with
   those for data sequence numbering (Section 3.3.1).  Therefore, it is
   expected that an MPTCP implementation will send the ADD_ADDR option
   on separate (either duplicate, or normal but lacking any payload) ACKs.  As discussed earlier, however, an MPTCP
   implementation MUST NOT treat duplicate ACKs with all TCP Options, the ADD_ADDR option does not have reliable
   delivery.  Therefore, a sender should MPTCP options as
   indications of congestion [7], and an MPTCP implementation SHOULD NOT
   send a more than two duplicate ACK with this
   option on all available subflows.

3.5.2. ACKs in a row for signalling purposes.

3.4.2.  Remove Address

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

   This is achieved through the Remove Address (REMOVE_ADDR) option
   (Figure 12), 13), which will remove a previously-added address (or list of
   addresses) from a connection and terminate any subflows currently
   using that address.

   For security purposes, if a host receives a REMOVE_ADDR option, it
   must ensure the affected path(s) are no longer in use before it
   instigates closure.  The receipt of REMOVE_ADDR should SHOULD first trigger
   the sending of a TCP Keepalive [10] [15] on the path, and if a response is
   received the path is not removed.  Typical TCP validity tests on the
   subflow (e.g. ensuring sequence and ack numbers are correct) MUST
   also be undertaken.

   The sending and receipt (if no keepalive response was received) of
   this message SHOULD trigger the sending of RSTs by both endpoints hosts on the
   affected subflow(s) (if possible), as a courtesy to cleaning up
   middlebox state, but endpoints may clean before cleaning up their internal state
   without a long timeout. any local state.

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

   The standard way to close a

   A subflow (so long as it is still
   functioning) is to use that is still functioning MUST be closed with a FIN
   exchange as in regular TCP - for more information, see Section 3.4. 3.3.3.

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

              Figure 12: 13: Remove Address (REMOVE_ADDR) option

3.6.

3.5.  Fallback

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

   There is, however, another corner case which should be addressed.
   That is one of MPTCP options getting through on the SYN, but not on
   regular packets.  This can be resolved if the subflow is the first
   subflow, and thus all data in flight is contiguous.  This resolution
   mechanism is as follows:

   o  The first window's worth of data MUST be DATA_ACKed on every
      packet

   o  If contiguous, using the first data packet does not have
   following rules.

   A sender MUST include a DSS option with Data Sequence Mapping
      option, drop out in
   every segment until one of MPTCP mode back to regular TCP (and thus send the sent segments has been acknowledged
   with a regular, subflow-level ACK, without DSS option containing a DATA_ACK)

   o  If Data ACK.  Upon reception of the
   acknowledgement, the sender has the confirmation that the DSS option
   passes in both directions and may choose to send fewer DSS options
   than once per segment.

   If, however, an ACK is received for data (not just for the SYN)
   without a DATA_ACK within Data ACK in a DSS option, the sender determines the path is
   not MPTCP capable.  In the case of this occurring on an additional
   subflow (i.e. one started with MP_JOIN), the host MUST close the
   subflow with an RST.  In the case of the first window, subflow (i.e. that
   started with MP_CAPABLE), it MUST drop out of a MPTCP mode back to
   regular TCP (and thus stop TCP.  The sender will send one final Data Sequence Mapping,
   with the length value of 0 indicating an infinite mapping (in case
   the path drops options in one direction only), and then revert to
   sending data on the single subflow without any MPTCP options.

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

   These rules should cover all cases where such a failure could happen:
   whether it's on the forward or reverse path, and whether the server
   or the client first sends data.  If lost options on data packets
   occur on any other subflow apart from the start of the initial subflow, it
   should be treated as a standard path failure.  The data would not be
   DATA_ACKed (since there is no mapping for the data), and the subflow
   can be closed with an RST.  (Note that these rules do not apply if an
   infinite mapping is included from the start - in which case, each end
   will send DSS options declaring the infinite mapping.)

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

   As described in Section 3.3, each portion of data for which there is
   a mapping is protected by a checksum.  This mechanism is used to
   detect if middleboxes have made any adjustments to the payload
   (added, removed, or changed data).  A checksum will fail if the data
   has been changed in any way.  This will also detect if the length of
   data on the subflow is increased or decreased, and this means the
   Data Sequence Mapping is no longer valid.  The sender no longer knows
   what subflow-level sequence number the receiver is genuinely
   operating at (the middlebox will be faking ACKs in return), and
   cannot signal any further mappings.  Furthermore, in addition to the
   possibility of payload modifications that are valid at the
   application layer, there is the possibility that false-positives
   could be hit across MPTCP segment boundaries, corrupting the data.
   Therefore, all data from the start of the segment that failed the
   checksum onwards is not trustworthy.

   When multiple subflows are in use, the data in-flight on a subflow
   will likely involve data that is not contiguously part of the
   connection-level stream, since segments will be spread across the
   multiple subflows.  Due to the problems identified above, it is not
   possible to determine what the adjustment has done to the data
   (notably, any changes to the subflow sequence numbering).  Therefore,
   it is not possible to recover the subflow, and the affected subflow
   must be immediately closed with an RST, featuring a "checksum failed"
   option, MP_FAIL option
   (Figure 14), which defines the Data Sequence Number at the start of
   the segment (defined by the Data Sequence Mapping) which had the
   checksum
   failure (see Figure 13). failure.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+---------------+--------------+
      +---------------+---------------+-------+----------------------+
      | Kind=MP_FAIL     Kind      |   Length=10   Length=12   |Subtype|      (reserved)      |
      +---------------+---------------+-------+----------------------+
      |                 Data Sequence Number (8B)  :
      +---------------+---------------+------------------------------+
      :                  Data Sequence Number (contd.) (8 octets)              :
      +-------------------------------+------------------------------+
      +--------------------------------------------------------------+
      :                Data Sequence Number (contd.)|
      +-------------------------------+ (continued)              |
      +--------------------------------------------------------------+

                   Figure 13: 14: Fallback (MP_FAIL) option

   TBD:

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

   A special case is when there is a single subflow and it fails with a
   checksum error.  Here, MPTCP should be able to recover and continue
   sending data.  There are two possible mechanisms to support this.
   The first and simplest is to establish a new subflow as part of the
   same MPTCP connection, and then close the original one with a RST.
   Since it is known that the path may be compromised, it is not
   desirable to use MPTCP's segmentation on this path any longer.  The
   new subflow will begin and will signal an infinite mapping (indicated
   by length=0 in the Data Sequence Mapping option, Section 3.3) from
   the data sequence number of the segment that failed the checksum.
   This connection will then continue to appear as a regular TCP
   session, and a middlebox may change the payload without causing
   unintentional harm.

   An optimisation is possible, however.  If it is known that all
   unacknowledged data in flight is contiguous, an infinite mapping
   could be applied to the subflow without the need to close it first,
   and essentially turn off all further MPTCP signalling.  In this case, is
   if a receiver identifies a checksum failure when there any point in signalling Checksum Failed,
   or could we just RST is only one
   path, it will send back an MP_FAIL option on the subflow? subflow-level ACK.
   The signal would allow the sender
   to know there will receive this, and if all unacknowledged data in
   flight is something wrong with contiguous, will signal an infinite mapping (if the path and data is
   not try contiguous, the sender MUST send an RST).  This infinite mapping
   will be a DSS option (Section 3.3) on the first new packet,
   containing a Data Sequence Mapping that acts retroactively, referring
   to re-
   establish the start of the subflow (if sequence number of the last segment that
   was otherwise the policy).

   Failed data will not known to be DATA_ACKed and so will delivered intact.  From that point onwards data can
   be re-transmitted on
   other subflows (Section 3.3.4).

   A special case is when there altered by a middlebox without affecting MPTCP, as the data stream
   is equivalent to a single subflow and it fails with regular, legacy TCP session.

   After a
   checksum error.  Here, MPTCP should be able sender signals an infinite mapping it MUST only use subflow
   ACKs to recover and continue
   sending clear its send buffer.  This is because Data ACKs may become
   misaligned with the subflow ACKs when middleboxes insert or delete
   data.  There are two possible mechanisms to support this.  The first and simplest receive SHOULD stop generating Data ACKs after it receives
   an infinite mapping.

   When a connection is in fallback mode, only one subflow can send data
   at a time.  Otherwise, the receiver would not know how to nevertheless close reorder the subflow with a
   RST,
   data.  However, subflows can be opened and immediately establish closed as necessary, as
   long as a new single one as part of the same MPTCP
   connection.  Since it is known that the path may active at any point.

   It should be compromised, it
   is emphasised that we are not desirable attempting to use MPTCP's segmentation on this path any longer.
   The new subflow will begin and will signal an infinite mapping
   (indicated by length=0 in the Data Sequence Mapping option,
   Section 3.3) from prevent the data sequence number use
   of the segment middleboxes that failed
   the checksum.  This connection will then continue want to appear as a
   regular TCP session, and a middlebox may change adjust the payload without
   causing unintentional harm. payload.  An optimisation is possible, however.  If it is known that all
   unacknowledged data in flight is contiguous, an infinite mapping MPTCP-aware
   middlebox to provide such functionality could be applied designed that would
   re-write checksums if needed, and additionally would be able to parse
   the subflow without data sequence mappings, and thus not hit false positives though
   not knowing where data boundaries lie.

3.6.  Error Handling

   In addition to the fallback mechanism as described above, the
   standard classes of TCP errors may need to close it first,
   and essentially turn off all further MPTCP signalling.  In this case,
   if a receiver identifies a checksum failure when there is only one
   path, it will send back be handled in an MP_FAIL option on MPTCP-
   specific way.  Note that changing semantics - such as the subflow-level ACK. relevance
   of an RST - are covered in Section 4.  Where possible, we do not want
   to deviate from regular TCP behaviour.

   The sender will receive this, following list covers possible errors and if all unacknowledged data the appropriate MPTCP
   behaviour:

   o  Unknown token in
   flight is contiguous, will signal MP_JOIN (or MAC failure in MP_JOIN ACK, or
      missing MP_JOIN in SYN/ACK response): send RST (analogous to TCP's
      behaviour on an infinite mapping (if unknown port)

   o  DSN out of Window (during normal operation): drop the data is data, do not contiguous, the sender MUST
      send an RST).  This infinite mapping
   will be a Data Sequence Mapping option on the first new packet, but
   it acts retroactively, referring to the start of the subflow sequence ACKs.

   o  Remove request for unknown address ID: silently ignore

3.7.  Heuristics

   There are a number of the last segment heuristics that was known to be delivered intact.
   From are needed for performance or
   deployment but which are not required for protocol correctness.  In
   this section we detail such heuristics.  Note that point onwards data can be altered by a middlebox without
   affecting MPTCP, discussion of
   buffering and certain sender and receiver window behaviours are
   presented in Section 3.3.4 and Section 3.3.5, as well as
   retransmission in Section 3.3.6.

3.7.1.  Port Usage

   Under typical operation an MPTCP implementation SHOULD use the same
   ports as already in use.  In other words, the data stream is equivalent to destination port of a regular,
   legacy TCP session.

   After
   SYN containing a sender signals an infinite mapping it MUST only use subflow
   ACKs to clear its send buffer.  This is because Data ACKs may become
   misaligned with MP_JOIN option SHOULD be the same as the remote port
   of the first subflow ACKs when middleboxes insert or delete
   data. in the connection.  The receive local port for such SYNs
   SHOULD stop generating Data ACKs after it receives also be the same as for the first subflow (and as such, an infinite mapping.

   When a connection
   implementation SHOULD reserve ephemeral ports across all local IP
   addresses), although there may be cases where this is in fallback mode, only one subflow can send data
   at a time.  Otherwise, the receiver would not know how infeasible.
   This strategy is intended to reorder maximize the
   data.  However, subflows can be opened and closed as necessary, as
   long as probability of the SYN
   being permitted by a single one is active firewall or NAT at the recipient and to avoid
   confusing any point.

   It should network monitoring software.

   There may also be emphasised that we are not attempting to prevent cases, however, where the use
   of middleboxes that want passive opener wishes to adjust the payload.  An MPTCP-aware
   middlebox
   signal to provide such functionality could be designed the other host that would
   re-write checksums if needed, and additionally would a specific port should be able used, and
   this facility is provided in the Add Address option as documented in
   Section 3.4.1.  It is therefore feasible to parse allow multiple subflows
   between the data sequence mappings, same two addresses but using different port pairs, and thus not hit false positives though
   not knowing where data boundaries lie.

3.7.  Error Handling

   In addition
   such a facility could be used to allow load balancing within the fallback mechanism as described above,
   network based on 5-tuples (e.g. some ECMP implementations).

3.7.2.  Delayed Subflow Start

   Many TCP connections are short-lived and consist only of a few
   segments, and so the
   standard classes overheads of TCP errors may need using MPTCP outweigh any benefits.
   A heuristic is required, therefore, to be handled decide when to start using
   additional subflows in an MPTCP-
   specific way.  Note MPTCP connection.  We expect that
   experience gathered from deployments will provide further guidance on
   this, and will be affected by particular application characteristics
   (which are likely to change over time).  However, a suggested
   general-purpose heuristic that changing semantics - such as the relevance
   of an RST - has already been covered in Section 4.  Where possible,
   we do not want implementation MAY choose to deviate employ
   is as follows.  Results from regular TCP behaviour.

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

   o  Unknown token in MP_JOIN (or token mismatch in MP_JOIN ACK, or
      missing MP_JOIN experimental deployments are needed in SYN/ACK response): send RST (analogous
   order to TCP's
      behaviour on an unknown port)

   o  DSN out of Window (during normal operation): just ignore, however
      if at verify the beginning correctness of this proposal.

   If a new subflow we might want to RST it as host has data buffered for its peer (which implies that the
   application has received a
      security mechanism

   o  Remove request for unknown address ID: silently ignore

   o  DATA_ACK data), the host opens one
   subflow for each initial window's worth of data not yet sent: abort connection by RST that is buffered.

   Consideration should also be given to limiting the rate of adding new
   subflows, as well as limiting the total number of subflows open for a
   particular connection.  A host may choose to vary these values based
   on every
      subflow.

3.8.  Heuristics

   There are its load or knowledge of traffic and path characteristics.

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

   An additional time-based heuristic could be applied, opening
   additional subflows after a number given period of heuristics that are needed time has passed.  This
   would alleviate the above issue, and also provide resilience for performance or
   deployment low-
   bandwidth but which are not required for protocol correctness.  In
   this long-lived applications.

   This section we detail such heuristics

3.8.1.  Port Usage

   Under typical operation has shown some of the considerations than an implementer
   should give when developing MPTCP implementation SHOULD use the same
   ports as already heuristics, but is not intended to
   be prescriptive.

3.7.3.  Failure Handling

   Requirements for MPTCP's handling of unexpected signals have been
   given in use.  In Section 3.6.  There are other words, the destination port of failure cases, however, where
   a
   SYN containing hosts can choose appropriate behaviour.

   For example, Section 3.1 suggests that a MP_JOIN option SHOULD be the same as the remote port host should fall back to
   trying regular TCP SYNs after several failures of MPTCP SYNs.  A host
   may keep a system-wide cache of the first subflow in the connection.  The local port for such SYNs
   SHOULD also be the same as information, so that it can back
   off from using MPTCP, firstly for that particular destination host,
   and eventually on a whole interface, if MPTCP connections continue
   failing.

   Another failure could occur when the MP_JOIN handshake fails.
   Section 3.6 specifies that an incorrect handshake MUST lead to the first
   subflow (and as such, being closed with a RST.  A host operating an
   implementation SHOULD reserve ephemeral ports across all local IP
   addresses), although there active
   intrusion detection system may be cases where this is infeasible.
   This strategy is intended choose to maximize start blocking MP_JOIN
   packets from the probability source host if multiple failed MP_JOIN attempts are
   seen.  From the connection initiator's point of view, if an MP_JOIN
   fails, it SHOULD NOT attempt to connect to the SYN
   being permitted by a firewall or NAT at same IP address during
   the recipient and to avoid
   confusing any network monitoring software.

   There may also be cases, however, where lifetime of the passive opener wishes to
   signal to connection, unless the other endpoint that host refreshes the
   information with a specific port should be used, REMOVE_ADDR and
   this facility is provided in the Add Address option as documented in
   Section 3.5.1.  It is therefore feasible to allow multiple subflows
   between then an ADD_ADDR for the same two
   address.

   In addition, an implementation may learn over a number of connections
   that certain interfaces or destination addresses but using different port pairs, consistently fail
   and
   such a facility could be such a facility may default to not trying to use MPTCP for these.  Behaviour
   could also be used learnt for particularly badly performing subflows or
   subflows that regularly fail during use, in order to allow load
   balancing within the network based on 5-tuples (e.g.  ECMP). temporarily
   choose not to use these paths.

4.  Semantic Issues

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

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

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

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

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

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

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

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

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

5.  Security Considerations

   As identified in [11], [16], the addition of multipath capability to TCP
   will bring with it a number of new classes of threat.  In order to
   prevent these, [3] presents a set of requirements for a security
   solution for MPTCP.  The fundamental goal is for the security of
   MPTCP to be "no worse" than regular TCP today, and the key security
   requirements are:

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

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

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

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

   The security of the MPTCP connection hangs on the use of keys that
   are shared once at the start of the first subflow, and never again in
   the clear.  To ease demultiplexing whilst not giving away any
   cryptographic material, future subflows use a truncated SHA-1 hash of
   this key as the connection identification "token".  The keys are
   combined and used as keys in a HMAC, MAC, and this should verify that the
   parties in the handshake are the same as in the original connection
   setup.  It also provides verification that the peer can receive
   traffic at this new address.  Replay attacks would still be possible in this scenario,
   when only keys are used, and therefore the handshakes use single-use
   random numbers (nonces) at both ends - this ensures the HMAC MAC will
   never be the same on two handshakes.  The use of crypto capability
   bits in the initial connection handshake to negotiate use of a
   particular algorithm will allow the deployment of additional crypto
   mechanisms in the future.  Note that this would be susceptible to
   bid-down attacks only if the attacker was on-path (and thus would be
   able to modify the data anyway).  The security mechanism presented in
   this draft should therefore protect against all forms of flooding and
   hijacking attacks suggested in [11]. [16].

6.  Interactions with Middleboxes

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

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

   MPTCP SYN packets contain the MP_CAPABLE uses a single new TCP option to indicate "Kind", and all message types are
   defined by "subtype" values (see Section 8).  This should reduce the use
   chances of MPTCP.  When the middlebox drops the packet containing the
   MP_CAPABLE option either on only some types of MPTCP options being passed, and instead
   the outgoing or key differing characteristics are different paths, and the return path,
   presence of the
   connection will fail.  Host A SHOULD fall back to TCP in such cases
   (studies suggest that few middleboxes drop SYN flag.

   MPTCP SYN packets with unknown
   options).  The same applies for subflow setup.

   The second case is when on the middleboxes strip options.  Let's first
   discuss behaviour for initial subflow of a connection SYNs (see Figure 14).  If contain the
   MP_CAPABLE option (Section 3.1).  If this is stripped from the packet on the outgoing path, the
   connection falls dropped, MPTCP SHOULD
   fall back to regular TCP.  If packets with the MP_JOIN 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
   (Section 3.2) are dropped, the connection
   failed.  Host A thinks it is talking to a regular host, and may send
   data segments, but these paths will simply not be acked by host B as they do not
   have used.

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

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

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

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

   Figure 14: 15: Connection Setup with Middleboxes that Strip Options from
                                  Packets

   We now examine data flow with MPTCP, assuming the flow is correctly
   setup
   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.

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

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

   o  NAT [12]: changes [17]: Network Address (and Port) Translators change the source
      address and port (and often source port) of packets.  This means that a
      host will not know its public-facing address for signalling in
      MPTCP.  Therefore, MPTCP permits implicit address addition via the
      MP_JOIN option, and has heuristics to ensure the handshake mechanism ensures that
      connection attempts to private addresses [9] [14] do not cause
      problems.  Address  Explicit address removal is undertaken by an ID number
      to allow no knowledge of the source address.

   o  Performance Enhancing Proxies (PEPs) [13]: [18]: might pro-actively ACK
      data to increase performance.  Problems will occur if a PEP ACKs
      data and then fails before sending it on to the receiver, of it if
      the receiver is mobile and moves away before proactively ACKed
      data is forwarded on.  If subflow ACKs were used to control send
      buffering, the data could be lost and never be retransmitted, thus
      causing the subflow to permanently stall.  MPTCP therefore uses
      the DATA_ACK to make progress when one of its subflows fails in
      this way.  This is why MPTCP does not use subflow ACKs to infer
      connection level ACKs.

   o  Traffic Normalizers [14]: do [19]: may not allow holes in sequence numbers,
      and may cache packets and retransmit the same data.  MPTCP looks
      like standard TCP on the wire, and will not retransmit different
      data on the same subflow sequence number.

   o  Firewalls [15]: [20]: might perform initial sequence number
      randomization on TCP connections.  MPTCP uses relative sequence
      numbers in data sequence mapping to cope with this.  Like NATs,
      firewalls will not permit many incoming connections, so MPTCP
      supports address signalling (ADD_ADDR) so that a multi-addressed endpoint
      host can invite its peer behind the firewall/NAT to connect out to
      its additional interface.

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

   o  Application level NATs: middleboxes: such as content-aware firewalls may
      alter the payload within a subflow.
      Multipath TCP subflow, such as re-writing URIs in
      HTTP traffic.  MPTCP will detect these using the checksum and
      close the affected subflow(s), if there are other subflows that
      can be used.  If all subflows are affected multipath will fallback
      to TCP, allowing such middleboxes to change the payload.

   o  Middleboxes that alter  MPTCP-
      aware middleboxes should be able to adjust the receive window: payload and MPTCP will use
      metadata in order not to break the
      maximum window at data-level, but will also obey subflow specific
      windows. connection.

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

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

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

7.  Interfaces

   TBD

   Interface with applications, interface with TCP, interface with lower
   layers...

   Discussion of interaction with applications (both in terms of how

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

7.  Acknowledgements

   The authors are supported by Trilogy
   (http://www.trilogy-project.org), a research project (ICT-216372)
   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 Olivier Bonaventure Sebastien Barre, Christoph Paasch and Andrew McDonald.

   The authors also wish to acknowledge reviews and contributions from
   Iljitsch van Beijnum, Lars Eggert, Marcelo Bagnulo, Robert Hancock,
   Pasi Sarolahti, Toby Moncaster, Philip Eardley, Sergio Lembo, and
   Lawrence Conroy.

9. Conroy, Yoshifumi Nishida and Bob Briscoe.

8.  IANA Considerations

   This document will make a request to IANA to allocate a new values for TCP Option identifiers,
   option value for MPTCP.  This value will be the value of the "Kind"
   field seen in all MPTCP options in this document.

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

   +-------------+-----------------------------+---------------+-------+
   |    Symbol   |             Name            |      Ref      | Value |
   +-------------+-----------------------------+---------------+-------+
   |  MP_CAPABLE |      Multipath Capable      |  Section 3.1  | (tbc)  0x0  |
   |   MP_JOIN   |       Join Connection       |  Section 3.2  | (tbc) |
   |   ADD_ADDR  |         Add Address         | Section 3.5.1 | (tbc)  0x1  |
   | REMOVE_ADDR     DSS     |        Remove Address  Data Sequence Signal (Data |  Section 3.5.2 3.3  | (tbc)  0x2  |
   |   DSN_MAP             |    ACK and Data Sequence Number    |  Section 3.3               | (tbc)       |
   |             |           Mapping           Mapping)          |               |       |
   |   DATA_ACK   ADD_ADDR  |  Data-level Acknowledgment         Add Address         | Section 3.3 3.4.1 | (tbc)  0x3  |
   |   DATA_FIN REMOVE_ADDR |        Data-level FIN        Remove Address       | Section 3.4 3.4.2 | (tbc)  0x4  |
   |   MP_PRIO   |   Change Subflow Priority   | Section 3.3.6 3.3.8 | (tbc)  0x5  |
   |   MP_FAIL   |           Fallback          |  Section 3.6 3.5  | (tbc)  0x6  |
   +-------------+-----------------------------+---------------+-------+

                      Table 1: TCP Options for MPTCP

10. Option Subtypes

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

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

                    Table 2: MPTCP Handshake Algorithms

9.  References

10.1.

9.1.  Normative References

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

10.2.

9.2.  Informative References

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

   [3]   Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar,
         "Architectural Guidelines for Multipath TCP Development",
         draft-ietf-mptcp-architecture-02
         draft-ietf-mptcp-architecture-05 (work in progress),
         October 2010.
         January 2011.

   [4]   Raiciu, C., Handley, M., and D. Wischik, "Coupled Multipath-
         Aware Congestion Control", draft-ietf-mptcp-congestion-00
         Control for Multipath Transport Protocols",
         draft-ietf-mptcp-congestion-01 (work in progress), July 2010.
         January 2011.

   [5]   Scharf, M. and A. Ford, "MPTCP Application Interface
         Considerations", draft-scharf-mptcp-api-02 draft-ietf-mptcp-api-00 (work in progress),
         July
         November 2010.

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

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

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

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

   [7]   Mathis, M., Mahdavi,

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

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

   [12]  Jacobson, V., Braden, B., and D. Borman, "TCP
         Selective Acknowledgment Options", Extensions for
         High Performance", RFC 2018, October 1996.

   [8] 1323, May 1992.

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

   [9]

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

   [10]

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

   [11]

   [16]  Bagnulo, M., "Threat Analysis for Multi-addressed/Multi-path
         TCP", draft-ietf-mptcp-threat-03 TCP Extensions for Multi-path
         Operation with Multiple Addresses", draft-ietf-mptcp-threat-08
         (work in progress),
         October 2010.

   [12] January 2011.

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

   [13]

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

   [14]

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

   [15]

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

Appendix A.  Notes on use of TCP Options

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

   We have performed a brief study on the commonly used TCP options in
   SYN, data, and pure ACK packets, and found that there is enough room
   to fit all the options we propose using in this draft.

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

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

   It is not necessary to include the Data Sequence Mapping and DATA ACK
   in each packet, and in many cases it may
   already be used by options such possible to alternate
   their presence (so long as timestamp and SACK.

   We have performed a brief study on the commonly used TCP mapping covers the data being sent in
   the following packet).  Other options include: alternating between 4
   and 8 byte sequence numbers in
   SYN, data, each option; and pure sending the DATA_ACK
   on a duplicate subflow-level ACK packets, and found (although note that there this must not be
   taken as a signal of congestion).

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

   Pure ACKs in this draft.

   SYN packets TCP typically include MSS (4 bytes), window scale (3 bytes),
   SACK permitted (2 bytes) and timestamp (10 bytes) options.  Together
   these sum to 19 bytes.  Some operating systems appear contain only timestamps (10B).  Here,
   multipath TCP typically needs to pad each encode only the DATA ACK (maximum of
   12 octets).  Occasionally ACKs will contain SACK information.
   Depending on the number of lost packets, SACK may utilize the entire
   option up to space.  If a word boundary, thus using 24 bytes (a brief survey
   suggests Windows XP and Mac OS X do this, whereas Linux does not).

   Optimistically, therefore, we have 21 bytes spare, or 16 if DATA ACK had to be included, then it has is probably
   necessary to reduce the number of SACK blocks to
   be word-aligned.  In either case, however, accomodate the Multipath Capable (12
   bytes) and Join (12 bytes) options will fit DATA
   ACK.  However, the presence of the DATA ACK is unlikely to be
   necessary in this remaining space.

   TCP data packets typically carry timestamp options a case where SACK is in every packet,
   taking 10 bytes (or 12 with padding).  That leaves 30 bytes use, since until at least some
   of the SACK blocks have been retransmitted, the cumulative data-level
   ACK will not be moving forward (or 28, if word-aligned), which are enough it does, due to encode retransmissions
   on another path, then that path can also be used to transmit the data sequence
   mapping (14 or 18 new
   DATA ACK).

   The ADD_ADDR option can be between 8 and 22 bytes, depending on
   whether IPv4 or IPv6 is used, and whether the length of the sequence port number is present
   or not.  It is unlikely that such signalling would fit in use) and the DATA_ACK a data
   packet (although if the flow there is bidirectional (6 space, it is fine to include it).  It is
   recommended to use duplicate ACKs with no other payload or 10
   bytes).  Such options will just fit in
   order to transmit these rare signals.  Note this is the available option space,
   although 8 byte data-level sequence numbers in both will only fit if
   word-alignment reason for
   mandating that duplicate ACKs with MPTCP options are not taken as a
   signal of congestion.

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

Appendix B.  Control Blocks

   Conceptually, an MPTCP connection can be possible to alternate their
   presence (so long represented as an MPTCP
   control block that contains several variables that track the mapping covers progress
   and the data being sent in state of the
   following packet).  Other options include: wrapping MPTCP connection and a set of linked TCP control
   blocks that correspond to the DATA_ACK into subflows that have been established.

   RFC793 [2] specifies several state variables.  Whenever possible, we
   reuse the Data Sequence Mapping option; alternating between 4 and 8 byte
   sequence numbers in each option; same terminology as RFC793 to describe the state variables
   that are maintained by MPTCP.

B.1.  MPTCP Control Block

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

B.1.1.  Authentication and sending Metadata

   Local.Token (32 bits):  This is the DATA_ACK token chosen by the local host on a
   duplicate subflow-level ACK.

   Pure ACKs in TCP typically contain only timestamps (10B).  Here,
   multipath TCP typically needs to encode
      this MPTCP connection.  The token MUST be unique among all
      established MPTCP connections, generated from the DATA_ACK (max 10B).
   Occasionally ACKs will contain SACK information.  Depending local key.

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

   Remote.Token (32 bits):  This is the
   number of lost packets, SACK may utilize token chosen by the entire option space.  If
   a DATA_ACK had to be included, then it remote host
      on this MPTCP connection, generated from the remote key.

   Remote.Key (64 bits):  This is probably necessary to
   reduce the number of SACK blocks key chosen by one the remote host on
      this MPTCP connection

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

B.1.2.  Sending Side

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

   SND.NXT (64 bits):  This is the SACK blocks have been retransmitted, Data Sequence Number of the cumulative data-level
   ACK will not be moving forward (or if it does, due next byte
      to retransmissions
   on antoher path, then that path can also be sent.  SND.NXT is used to transmit determine the new
   DATA_ACK).

   The ADD_ADDR option can be between 8 and 22 bytes, depending on
   whether IPv4 or IPv6 is used, and whether value of the Port number is present
   or not.  It is unlikely that such signalling would fit DSN in a data
   packet (although if there is space, it
      the DSS option.

   SND.WND (32 bits with RFC1323, 16 bits without):  This is fine to include it).  It the sending
      window.  MPTCP maintains the sending window at the MPTCP
      connection level and the same window is
   recommended to shared by all subflows.
      All subflows use duplicate ACKs with no other payload or options in
   order the MPTCP connection level SND.WND to transmit these rare signals.

   Finally, there are issues with options reliability.  As options can
   also be compute the
      SEQ.WND value which is sent on pure ACKs, these are not reliably sent. in each transmitted segment.

B.1.3.  Receiving Side

   RCV.NXT (64 bits):  This is not
   an issue for DATA_ACK due to their cumulative nature, but may be an
   issue for ADD_ADDR/REMOVE_ADDR options.  Here we favour redundant
   transmissions at the sender (whether on multiple paths, or on Data Sequence Number of the
   same path next byte
      which is expected on a number the MPTCP connection.  This state variable is
      modified upon reception of ACKs). in-order data.  The cases where options are stripped
   by middleboxes are discussed in Section 6.

Appendix B.  Resync Packet

   In earlier versions of this draft, we proposed the use value of a "re-sync"
   option that would be RCV.NXT is
      used in certain circumstances when a sender
   needs to instruct specify the receiver to skip over certain subflow sequence
   numbers (i.e. to treat DATA_ACK which is sent in the specified sequence space as having been
   received and acknowledged).

   The typical use of this DSS option will be when packets are retransmitted on different subflows, after failing to be acknowledged
      all subflows.

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

B.2.  TCP Control Blocks

   The MPTCP control block also contains a case, it becomes necessary to move
   forward list of 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 TCP control
   blocks that are associated to be transmitted the MPTCP connection.

   Note that the TCP control block on the original subflow, it
   would be different data, and so must TCP subflows does not be sent with previously-used
   (but unacknowledged) sequence numbering).

   The rationale for needing to do this is two-fold: firstly, when ACKs contain
   the RCV.WND and SND.WND state variables as these are received they maintained at
   the MPTCP connection level and not at the subflow level.

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

B.2.1.  Sending Side

   SND.UNA (32 bits):  This is the subflow only, and sequence number of the sender infers
   from this next byte to
      be acknowledged on the data that was sent - if subflow.  This variable is updated upon
      reception of each TCP acknowledgement on the subflow.

   SND.NXT (32 bits):  This is the same sequence space could number of the next byte to
      be occupied by different data, sent on the sender won't know whether subflow.  SND.NXT is used to set the
   intended data was received.  Secondly, certain classes value of
      SEG.SEQ upon transmission of middleboxes
   may cache data and not send the new data on a previously-seen
   sequence number. next segment.

B.2.2.  Receiving Side

   RCV.NXT (32 bits):  This option was dropped, however, since some middleboxes may get
   confused when they meet a hole in is the sequence space, and do not
   understand number of the resync option.  It next byte
      which is therefore felt that expected on the same
   data must continue subflow.  This state variable is modified
      upon reception of in-order segments.  The value of RCV.NXT is
      copied to be retransmitted the SEG.ACK field of the next segments transmitted on a subflow even if it
      the subflow.

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

Appendix C.  Changelog

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

C.1.  Changes since draft-ietf-mptcp-multiaddressed-02

   o  Changed to using a single TCP option with a sub-type field.

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

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

   o  Added crypto agility and checksum negotiation.

   o  Redefined MP_JOIN handshake to use only three TCP options.

   o  Added pseudo-header to checksum.

   o  Many clarifications and re-structuring.

   o  Added more discussion on heuristics.

C.2.  Changes since draft-ietf-mptcp-multiaddressed-01

   o  Added proposal for hash-based security mechanism.

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

   o  Changed DSN_MAP checksum to use the TCP checksum algorithm.

C.2.

C.3.  Changes since draft-ietf-mptcp-multiaddressed-00

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

C.3.

C.4.  Changes since draft-ford-mptcp-multiaddressed-03

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

   o  Added optional port to ADD_ADDR and clarified situation with
      private addresses (Section 3.5.1). 3.4.1).

   o  Added path liveness check to REMOVE_ADDR (Section 3.5.2). 3.4.2).

   o  Added chunk checksumming to DSN_MAP (Section 3.3.1) to detect
      payload-altering middleboxes, and defined fallback mechanism
      (Section 3.6). 3.5).

   o  Major clarifications to receive window discussion (Section 3.3.4). 3.3.5).

   o  Various textual clarifications, especially in examples.

C.4.

C.5.  Changes since draft-ford-mptcp-multiaddressed-02

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

   o  Data sequence numbers are now always 8 bytes.  But in some cases
      where it is unambiguous it is permissible to only send the lower 4
      bytes if space is at a premium.

   o  Clarified behaviour of MP_JOIN in Section 3.2.

   o  Added DATA_ACK to Section 3.3.

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

Authors' Addresses

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

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

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

   Email: c.raiciu@cs.ucl.ac.uk
   Mark Handley
   University College London
   Gower Street
   London  WC1E 6BT
   UK

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

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

   Email: olivier.bonaventure@uclouvain.be