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Versions: (draft-bonaventure-mptcp-exp-option) 00 01 02 03 04 05 06 07 08 09 10 11 12

Internet Engineering Task Force                                  A. Ford
Internet-Draft                                                     Pexip
Obsoletes: 6824 (if approved)                                  C. Raiciu
Intended status: Standards Track            U. Politechnica of Bucharest
Expires: April 6, 2019                                        M. Handley
                                                       U. College London
                                                          O. Bonaventure
                                                U. catholique de Louvain
                                                               C. Paasch
                                                             Apple, Inc.
                                                         October 3, 2018


     TCP Extensions for Multipath Operation with Multiple Addresses
                     draft-ietf-mptcp-rfc6824bis-12

Abstract

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

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

   This document specifies v1 of Multipath TCP, obsoleting v0 as
   specified in RFC6824 [RFC6824] through clarifications and
   modifications primarily driven by deployment experience.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 6, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Design Assumptions  . . . . . . . . . . . . . . . . . . .   4
     1.2.  Multipath TCP in the Networking Stack . . . . . . . . . .   5
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
     1.4.  MPTCP Concept . . . . . . . . . . . . . . . . . . . . . .   7
     1.5.  Requirements Language . . . . . . . . . . . . . . . . . .   8
   2.  Operation Overview  . . . . . . . . . . . . . . . . . . . . .   8
     2.1.  Initiating an MPTCP Connection  . . . . . . . . . . . . .   9
     2.2.  Associating a New Subflow with an Existing MPTCP
           Connection  . . . . . . . . . . . . . . . . . . . . . . .  10
     2.3.  Informing the Other Host about Another Potential Address   11
     2.4.  Data Transfer Using MPTCP . . . . . . . . . . . . . . . .  12
     2.5.  Requesting a Change in a Path's Priority  . . . . . . . .  13
     2.6.  Closing an MPTCP Connection . . . . . . . . . . . . . . .  13
     2.7.  Notable Features  . . . . . . . . . . . . . . . . . . . .  14
   3.  MPTCP Protocol  . . . . . . . . . . . . . . . . . . . . . . .  15
     3.1.  Connection Initiation . . . . . . . . . . . . . . . . . .  16
     3.2.  Starting a New Subflow  . . . . . . . . . . . . . . . . .  23
     3.3.  General MPTCP Operation . . . . . . . . . . . . . . . . .  28
       3.3.1.  Data Sequence Mapping . . . . . . . . . . . . . . . .  30
       3.3.2.  Data Acknowledgments  . . . . . . . . . . . . . . . .  33
       3.3.3.  Closing a Connection  . . . . . . . . . . . . . . . .  34
       3.3.4.  Receiver Considerations . . . . . . . . . . . . . . .  36
       3.3.5.  Sender Considerations . . . . . . . . . . . . . . . .  37
       3.3.6.  Reliability and Retransmissions . . . . . . . . . . .  38
       3.3.7.  Congestion Control Considerations . . . . . . . . . .  39



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       3.3.8.  Subflow Policy  . . . . . . . . . . . . . . . . . . .  39
     3.4.  Address Knowledge Exchange (Path Management)  . . . . . .  41
       3.4.1.  Address Advertisement . . . . . . . . . . . . . . . .  42
       3.4.2.  Remove Address  . . . . . . . . . . . . . . . . . . .  45
     3.5.  Fast Close  . . . . . . . . . . . . . . . . . . . . . . .  46
     3.6.  Subflow Reset . . . . . . . . . . . . . . . . . . . . . .  48
     3.7.  Fallback  . . . . . . . . . . . . . . . . . . . . . . . .  50
     3.8.  Error Handling  . . . . . . . . . . . . . . . . . . . . .  53
     3.9.  Heuristics  . . . . . . . . . . . . . . . . . . . . . . .  54
       3.9.1.  Port Usage  . . . . . . . . . . . . . . . . . . . . .  54
       3.9.2.  Delayed Subflow Start and Subflow Symmetry  . . . . .  54
       3.9.3.  Failure Handling  . . . . . . . . . . . . . . . . . .  55
   4.  Semantic Issues . . . . . . . . . . . . . . . . . . . . . . .  56
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  57
   6.  Interactions with Middleboxes . . . . . . . . . . . . . . . .  60
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  63
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  64
     8.1.  MPTCP Option Subtypes . . . . . . . . . . . . . . . . . .  64
     8.2.  MPTCP Handshake Algorithms  . . . . . . . . . . . . . . .  65
     8.3.  MP_TCPRST Reason Codes  . . . . . . . . . . . . . . . . .  66
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  67
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  67
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  67
   Appendix A.  Notes on Use of TCP Options  . . . . . . . . . . . .  71
   Appendix B.  TCP Fast Open and MPTCP  . . . . . . . . . . . . . .  72
     B.1.  TFO cookie request with MPTCP . . . . . . . . . . . . . .  73
     B.2.  Data sequence mapping under TFO . . . . . . . . . . . . .  73
     B.3.  Connection establishment examples . . . . . . . . . . . .  74
   Appendix C.  Control Blocks . . . . . . . . . . . . . . . . . . .  76
     C.1.  MPTCP Control Block . . . . . . . . . . . . . . . . . . .  76
       C.1.1.  Authentication and Metadata . . . . . . . . . . . . .  76
       C.1.2.  Sending Side  . . . . . . . . . . . . . . . . . . . .  77
       C.1.3.  Receiving Side  . . . . . . . . . . . . . . . . . . .  77
     C.2.  TCP Control Blocks  . . . . . . . . . . . . . . . . . . .  77
       C.2.1.  Sending Side  . . . . . . . . . . . . . . . . . . . .  78
       C.2.2.  Receiving Side  . . . . . . . . . . . . . . . . . . .  78
   Appendix D.  Finite State Machine . . . . . . . . . . . . . . . .  78
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  79

1.  Introduction

   Multipath TCP (MPTCP) is a set of extensions to regular TCP [RFC0793]
   to provide a Multipath TCP [RFC6182] 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 signaling and setting up
   multiple paths ("subflows"), managing these subflows, reassembly of
   data, and termination of sessions.  This is not the only information



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   required to create a Multipath TCP implementation, however.  This
   document is complemented by three others:

   o  Architecture [RFC6182], 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 [RFC6356] presents a safe congestion control
      algorithm for coupling the behavior of the multiple paths in order
      to "do no harm" to other network users.

   o  Application considerations [RFC6897] discusses 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.

   This document is an update to, and obsoletes, the v0 specification of
   Multipath TCP [RFC6824].  This document specifies MPTCP v1, which is
   not backward compatible with MPTCP v0.  This document additionally
   defines version negotiation procedures for implementations that
   support both versions.

1.1.  Design Assumptions

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

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

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

   To simplify the design, we assume that the presence of multiple
   addresses at a host is sufficient to indicate the existence of
   multiple paths.  These paths need not be entirely disjoint: they may
   share one or many routers between them.  Even in such a situation,
   making use of multiple paths is beneficial, improving resource
   utilization and resilience to a subset of node failures.  The
   congestion control algorithms defined in [RFC6356] ensure this does
   not act detrimentally.  Furthermore, there may be some scenarios
   where different TCP ports on a single host can provide disjoint paths
   (such as through certain Equal-Cost Multipath (ECMP) implementations
   [RFC2992]), and so the MPTCP design also supports the use of ports in
   path identifiers.



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   There are three aspects to the backwards-compatibility listed above
   (discussed in more detail in [RFC6182]):

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

   Application Constraints:  The protocol must be usable with no change
      to existing applications that use the common 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.

   Fallback:  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.

   The complementary application considerations document [RFC6897]
   discusses the necessary features of an API to provide backwards-
   compatibility, as well as API extensions to convey the behavior of
   MPTCP at a level of control and information equivalent to that
   available with regular, single-path TCP.

   Further discussion of the design constraints and associated design
   decisions are given in the MPTCP Architecture document [RFC6182] and
   in [howhard].

1.2.  Multipath TCP in the Networking Stack

   MPTCP operates at the transport layer and aims to be transparent to
   both higher and lower layers.  It is a set of additional features on
   top of standard TCP; Figure 1 illustrates this layering.  MPTCP is
   designed to be usable by legacy applications with no changes;
   detailed discussion of its interactions with applications is given in
   [RFC6897].












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

      Figure 1: Comparison of Standard TCP and MPTCP Protocol Stacks

1.3.  Terminology

   This document makes use of a number of terms that are either MPTCP-
   specific or have defined meaning in the context of MPTCP, as follows:

   Path:  A sequence of links between a sender and a receiver, defined
      in this context by a 4-tuple of source and destination address/
      port pairs.

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

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

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

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

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

   In addition to these terms, note that MPTCP's interpretation of, and
   effect on, regular single-path TCP semantics are discussed in
   Section 4.







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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 [RFC6897].  An application begins by
      opening a TCP socket in the normal way.  MPTCP signaling and
      operation are handled by the MPTCP implementation.

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

   o  If extra paths are available, additional TCP sessions (termed
      MPTCP "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 hosts.  Combinations of these multiple addresses
      equate to the additional paths.  In the example, other potential
      paths that could be set up are A1<->B2 and A2<->B2.  Although this
      additional session is shown as being initiated from A2, it could
      equally have been initiated from B1.

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

   o  MPTCP adds connection-level sequence numbers to allow the
      reassembly of segments arriving on multiple subflows with
      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.







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

                  Figure 2: Example MPTCP Usage Scenario

1.5.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 RFC 2119 [RFC2119] RFC 8174 [RFC8174] when, and only when,
   they appear in all capitals, as shown here.

2.  Operation Overview

   This section presents a single description of common MPTCP operation,
   with reference to the protocol operation.  This is a high-level
   overview of the key functions; the full specification follows in
   Section 3.  Extensibility and negotiated features are not discussed
   here.  Considerable reference is made to symbolic names of MPTCP
   options throughout this section -- these are subtypes of the IANA-
   assigned MPTCP option (see Section 8), and their formats are defined
   in the detailed protocol specification that follows in Section 3.

   A Multipath TCP connection provides a bidirectional bytestream
   between two hosts communicating like normal TCP and, thus, does not
   require any change to the applications.  However, Multipath TCP
   enables the hosts to use different paths with different IP addresses
   to exchange packets belonging to the MPTCP connection.  A Multipath
   TCP connection appears like a normal TCP connection to an
   application.  However, to the network layer, each MPTCP subflow looks
   like a regular TCP flow whose segments carry a new TCP option type.
   Multipath TCP manages the creation, removal, and utilization of these
   subflows to send data.  The number of subflows that are managed
   within a Multipath TCP connection is not fixed and it can fluctuate
   during the lifetime of the Multipath TCP connection.



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

2.1.  Initiating an MPTCP Connection

   This is the same signaling as for initiating a normal TCP connection,
   but the SYN, SYN/ACK, and initial ACK (and data) packets also carry
   the MP_CAPABLE option.  This option has a variable length and serves
   multiple purposes.  Firstly, it verifies whether the remote host
   supports Multipath TCP; secondly, this option allows the hosts to
   exchange some information to authenticate the establishment of
   additional subflows.  Further details are given in Section 3.1.

      Host A                                  Host B
      ------                                  ------
      MP_CAPABLE                ->
      [flags]
                                <-            MP_CAPABLE
                                              [B's key, flags]
      ACK + MP_CAPABLE (+ data) ->
      [A's key, B's key, flags, (data-level details)]

   Retransmission of the ACK + MP_CAPABLE can occur if it is not known
   if it has been received.  The following diagrams show all possible
   exchanges for the initial subflow setup to ensure this reliability.
























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      Host A (with data to send immediately)  Host B
      ------                                  ------
      MP_CAPABLE                ->
      [flags]
                                <-            MP_CAPABLE
                                              [B's key, flags]
      ACK + MP_CAPABLE + data   ->
      [A's key, B's key, flags, data-level details]


      Host A (with data to send later)        Host B
      ------                                  ------
      MP_CAPABLE                ->
      [flags]
                                <-            MP_CAPABLE
                                              [B's key, flags]
      ACK + MP_CAPABLE          ->
      [A's key, B's key, flags]

      ACK + MP_CAPABLE + data   ->
      [A's key, B's key, flags, data-level details]


      Host A                                  Host B (sending first)
      ------                                  ------
      MP_CAPABLE                ->
      [flags]
                                <-            MP_CAPABLE
                                              [B's key, flags]
      ACK + MP_CAPABLE          ->
      [A's key, B's key, flags]

                                <-            ACK + DSS + data
                                              [data-level details]

2.2.  Associating a New Subflow with an Existing MPTCP Connection

   The exchange of keys in the MP_CAPABLE handshake provides material
   that can be used to authenticate the endpoints when new subflows will
   be set up.  Additional subflows begin in the same way as initiating a
   normal TCP connection, but the SYN, SYN/ACK, and ACK packets also
   carry the MP_JOIN option.

   Host A initiates a new subflow between one of its addresses and one
   of Host B's addresses.  The token -- generated from the key -- is
   used to identify which MPTCP connection it is joining, and the HMAC
   is used for authentication.  The Hash-based Message Authentication
   Code (HMAC) uses the keys exchanged in the MP_CAPABLE handshake, and



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   the random numbers (nonces) exchanged in these MP_JOIN options.
   MP_JOIN also contains flags and an Address ID that can be used to
   refer to the source address without the sender needing to know if it
   has been changed by a NAT.  Further details are in Section 3.2.

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

                            <-                ACK

2.3.  Informing the Other Host about Another Potential Address

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

   In some circumstances, a host may want to advertise to the remote
   host the availability of an address without establishing a new
   subflow, for example, when a NAT prevents setup in one direction.  In
   the example below, Host A informs Host B about its alternative IP
   address/port pair (IP#-A2).  Host B may later send an MP_JOIN to this
   new address.  The ADD_ADDR option contains a HMAC to authenticate the
   address as having been sent from the originator of the connection.
   The receiver of this option echoes it back to the client to indicate
   successful reception.  Further details are in Section 3.4.1.













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      Host A                                 Host B
      ------                                 ------
      ADD_ADDR                  ->
      [Echo-flag=0,
       IP#-A2,
       IP#-A2's Address ID,
       HMAC of IP#-A2]

                                <-          ADD_ADDR
                                            [Echo-flag=1,
                                             IP#-A2,
                                             IP#-A2's Address ID,
                                             HMAC of IP#-A2]

   There is a corresponding signal for address removal, making use of
   the Address ID that is signaled in the add address handshake.
   Further details in Section 3.4.2.

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

2.4.  Data Transfer Using MPTCP

   To ensure reliable, in-order delivery of data over subflows that may
   appear and disappear at any time, MPTCP uses a 64-bit data sequence
   number (DSN) to number all data sent over the MPTCP connection.  Each
   subflow has its own 32-bit sequence number space, utilising the
   regular TCP sequence number header, and an MPTCP option maps the
   subflow sequence space to the data sequence space.  In this way, data
   can be retransmitted on different subflows (mapped to the same DSN)
   in the event of failure.

   The Data Sequence Signal (DSS) carries the Data Sequence Mapping.
   The Data Sequence Mapping consists of the subflow sequence number,
   data sequence number, and length for which this mapping is valid.
   This option can also carry a connection-level acknowledgment (the
   "Data ACK") for the received DSN.

   With MPTCP, all subflows share the same receive buffer and advertise
   the same receive window.  There are two levels of acknowledgment in
   MPTCP.  Regular TCP acknowledgments are used on each subflow to
   acknowledge the reception of the segments sent over the subflow
   independently of their DSN.  In addition, there are connection-level
   acknowledgments for the data sequence space.  These acknowledgments
   track the advancement of the bytestream and slide the receiving
   window.



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   Further details are in Section 3.3.

      Host A                                 Host B
      ------                                 ------
      DSS                       ->
      [Data Sequence Mapping]
      [Data ACK]
      [Checksum]

2.5.  Requesting a Change in a Path's Priority

   Hosts can indicate at initial subflow setup whether they wish the
   subflow to be used as a regular or backup path -- a backup path only
   being used if there are no regular paths available.  During a
   connection, Host A can request a change in the priority of a subflow
   through the MP_PRIO signal to Host B.  Further details are in
   Section 3.3.8.

      Host A                                 Host B
      ------                                 ------
      MP_PRIO                   ->

2.6.  Closing an MPTCP Connection

   When a host wants to close an existing subflow, but not the whole
   connection, it can initiate a regular TCP FIN/ACK exchange.

   When Host A wants to inform Host B that it has no more data to send,
   it signals this "Data FIN" as part of the Data Sequence Signal (see
   above).  It has the same semantics and behavior as a regular TCP FIN,
   but at the connection level.  Once all the data on the MPTCP
   connection has been successfully received, then this message is
   acknowledged at the connection level with a DATA_ACK.  Further
   details are in Section 3.3.3.

      Host A                                 Host B
      ------                                 ------
      DATA_SEQUENCE_SIGNAL      ->
      [Data FIN]
                                <-           (MPTCP DATA_ACK)

   There is an additional method of connection closure, referred to as
   "Fast Close", which is analogous to closing a single-path TCP
   connection with a RST signal.  The MP_FASTCLOSE signal is used to
   indicate to the peer that the connection will be abruptly closed and
   no data will be accepted anymore.  This can be used on an ACK
   (ensuring reliability of the signal), or a RST (which is not).  Both




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   examples are shown in the following diagrams.  Further details are in
   Section 3.5.

      Host A                                 Host B
      ------                                 ------
      ACK + MP_FASTCLOSE          ->
      [B's key]

      [RST on all other subflows] ->

                                  <-         [RST on all subflows]


      Host A                                 Host B
      ------                                 ------
      RST + MP_FASTCLOSE          ->
      [B's key] [on all subflows]

                                  <-         [RST on all subflows]

2.7.  Notable Features

   It is worth highlighting that MPTCP's signaling has been designed
   with several key requirements in mind:

   o  To cope with NATs on the path, addresses are referred to by
      Address IDs, in case the IP packet's source address gets changed
      by a NAT.  Setting up a new TCP flow is not possible if the
      receiver of the SYN is behind a NAT; to allow subflows to be
      created when either end is behind a NAT, MPTCP uses the ADD_ADDR
      message.

   o  MPTCP falls back to ordinary TCP if MPTCP operation is not
      possible, for example, if one host is not MPTCP capable or if a
      middlebox alters the payload.  This is discussed in Section 3.7.

   o  To address the threats identified in [RFC6181], the following
      steps are taken: keys are sent in the clear in the MP_CAPABLE
      messages; MP_JOIN messages are secured with HMAC-SHA256
      ([RFC2104], [SHS]) using those keys; and standard TCP validity
      checks are made on the other messages (ensuring sequence numbers
      are in-window [RFC5961]).  Residual threats to MPTCP v0 [RFC6824]
      were identified in [RFC7430], and those affecting the protocol
      (i.e. modification to ADD_ADDR) have been incorporated in this
      document.  Further discussion of security can be found in
      Section 5.





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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 signaled using optional TCP header fields.
   A single TCP option number ("Kind") has been assigned by IANA for
   MPTCP (see Section 8), and then individual messages will be
   determined by a "subtype", the values of which are also stored in an
   IANA registry (and are also listed in Section 8).  As with all TCP
   options, the Length field is specified in bytes, and includes the 2
   bytes of Kind and Length.

   Throughout this document, when reference is made to an MPTCP option
   by symbolic name, such as "MP_CAPABLE", this refers to a TCP option
   with the single MPTCP option type, and with the subtype value of the
   symbolic name as defined in Section 8.  This subtype is a 4-bit field
   -- the first 4 bits of the option payload, as shown in Figure 3.  The
   MPTCP messages are defined in the following sections.

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

                       Figure 3: MPTCP Option Format

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

   The remaining options, however, are signals that do not need to be on
   a specific packet, such as those for signaling additional addresses.
   Whilst an implementation may desire to send MPTCP options as soon as
   possible, it may not be possible to combine all desired options (both
   those for MPTCP and for regular TCP, such as SACK (selective
   acknowledgment) [RFC2018]) on a single packet.  Therefore, an
   implementation may choose to send duplicate ACKs containing the
   additional signaling information.  This changes the semantics of a
   duplicate ACK; these are usually only sent as a signal of a lost
   segment [RFC5681] in regular TCP.  Therefore, an MPTCP implementation
   receiving a duplicate ACK that contains an MPTCP option MUST NOT
   treat it as a signal of congestion.  Additionally, an MPTCP



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   implementation SHOULD NOT send more than two duplicate ACKs in a row
   for the purposes of sending MPTCP options alone, in order to ensure
   no middleboxes misinterpret this as a sign of congestion.

   Furthermore, standard TCP validity checks (such as ensuring the
   sequence number and acknowledgment number are within window) MUST be
   undertaken before processing any MPTCP signals, as described in
   [RFC5961], and initial subflow sequence numbers SHOULD be generated
   according to the recommendations in [RFC6528].

3.1.  Connection Initiation

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

   The MP_CAPABLE exchange in this specification (v1) is different to
   that specified in v0 [RFC6824].  If a host supports multiple versions
   of MPTCP, the sender of the MP_CAPABLE option SHOULD signal the
   highest version number it supports.  In return, in its MP_CAPABLE
   option, the receiver will signal the version number it wishes to use,
   which MUST be equal to or lower than the version number indicated in
   the initial MP_CAPABLE.  There is a caveat though with respect to
   this version negotiation with old listeners that only support v0.  A
   listener that supports v0 expects that the MP_CAPABLE option in the
   SYN-segment includes the initiator's key.  If the initiator however
   already upgraded to v1, it won't include the key in the SYN-segment.
   Thus, the listener will ignore the MP_CAPABLE of this SYN-segment and
   reply with a SYN/ACK that does not include an MP_CAPABLE, thus
   leading to a fallback to regular TCP.  An initiator MAY cache this
   information about a peer and for future connections, MAY choose to
   attempt using MPTCP v0, if supported, before recording the host as
   not supporting MPTCP.

   The MP_CAPABLE option is variable-length, with different fields
   included depending on which packet the option is used on.  The full
   MP_CAPABLE option is shown in Figure 4.












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                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-------+---------------+
      |     Kind      |    Length     |Subtype|Version|A|B|C|D|E|F|G|H|
      +---------------+---------------+-------+-------+---------------+
      |                   Option Sender's Key (64 bits)               |
      |                      (if option Length > 4)                   |
      |                                                               |
      +---------------------------------------------------------------+
      |                  Option Receiver's Key (64 bits)              |
      |                      (if option Length > 12)                  |
      |                                                               |
      +-------------------------------+-------------------------------+
      |  Data-Level Length (16 bits)  |  Checksum (16 bits, optional) |
      +-------------------------------+-------------------------------+

              Figure 4: Multipath Capable (MP_CAPABLE) Option

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

   o  SYN (A->B): only the first four octets (Length = 4).

   o  SYN/ACK (B->A): B's Key for this connection (Length = 12).

   o  ACK (no data) (A->B): A's Key followed by B's Key (Length = 20).

   o  ACK (with first data) (A->B): A's Key followed by B's Key followed
      by Data-Level Length, and optional Checksum (Length = 22 or 24).

   The contents of the option is determined by the SYN and ACK flags of
   the packet, along with the option's length field.  For the diagram
   shown in Figure 4, "sender" and "receiver" refer to the sender or
   receiver of the TCP packet (which can be either host).

   The initial SYN, containing just the MP_CAPABLE header, is used to
   define the version of MPTCP being requested, as well as exchanging
   flags to negotiate connection features, described later.

   This option is used to declare the 64-bit keys that the end hosts
   have generated for this MPTCP connection.  These keys are used to
   authenticate the addition of future subflows to this connection.
   This is the only time the key will be sent in clear on the wire
   (unless "fast close", Section 3.5, is used); all future subflows will
   identify the connection using a 32-bit "token".  This token is a



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   cryptographic hash of this key.  The algorithm for this process is
   dependent on the authentication algorithm selected; the method of
   selection is defined later in this section.

   Upon reception of the initial SYN-segment, a stateful server
   generates a random key and replies with a SYN/ACK.  The key's method
   of generation is implementation specific.  The key MUST be hard to
   guess, and it MUST be unique for the sending host across all its
   current MPTCP connections.  Recommendations for generating random
   numbers for use in keys are given in [RFC4086].  Connections will be
   indexed at each host by the token (a one-way hash of the key).
   Therefore, an implementation will require a mapping from each token
   to the corresponding connection, and in turn to the keys for the
   connection.

   There is a risk that two different keys will hash to the same token.
   The risk of hash collisions is usually small, unless the host is
   handling many tens of thousands of connections.  Therefore, an
   implementation SHOULD check its list of connection tokens to ensure
   there is no collision before sending its key, and if there is, then
   it should generate a new key.  This would, however, be costly for a
   server with thousands of connections.  The subflow handshake
   mechanism (Section 3.2) will ensure that new subflows only join the
   correct connection, however, through the cryptographic handshake, as
   well as checking the connection tokens in both directions, and
   ensuring sequence numbers are in-window.  So in the worst case if
   there was a token collision, the new subflow would not succeed, but
   the MPTCP connection would continue to provide a regular TCP service.

   Since key generation is implementation-specific, there is no
   requirement that they be simply random numbers.  An implementation is
   free to exchange cryptographic material out-of-band and generate
   these keys from this, in order to provide additional mechanisms by
   which to verify the identity of the communicating entities.  For
   example, an implementation could choose to link its MPTCP keys to
   those used in higher-layer TLS or SSH connections.

   If the server behaves in a stateless manner, it has to generate its
   own key in a verifiable fashion.  This verifiable way of generating
   the key can be done by using a hash of the 4-tuple, sequence number
   and a local secret (similar to what is done for the TCP-sequence
   number [RFC4987]).  It will thus be able to verify whether it is
   indeed the originator of the key echoed back in the later MP_CAPABLE
   option.  As for a stateful server, the tokens SHOULD be checked for
   uniqueness, however if uniqueness is not met, and there is no way to
   generate an alternative verifiable key, then the connection MUST fall
   back to using regular TCP by not sending a MP_CAPABLE in the SYN/ACK.




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   The ACK carries both A's key and B's key.  This is the first time
   that A's key is seen on the wire, although it is expected that A will
   have generated a key locally before the initial SYN.  The echoing of
   B's key allows B to operate statelessly, as described above.
   Therefore, A's key must be delivered reliably to B, and in order to
   do this, the transmission of this packet must be made reliable.

   If B has data to send first, then the reliable delivery of the
   ACK+MP_CAPABLE can be inferred by the receipt of this data with a
   MPTCP Data Sequence Signal (DSS) option (Section 3.3).  If, however,
   A wishes to send data first, it has two options to ensure the
   reliable delivery of the ACK+MP_CAPABLE.  If it immediately has data
   to send, then the third ACK (with data) would also contain an
   MP_CAPABLE option with additional data parameters (the Data-Level
   Length and optional Checksum as shown in Figure 4).  If A does not
   immediately have data to send, it MUST include the MP_CAPABLE on the
   third ACK, but without the additional data parameters.  When A does
   have data to send, it must repeat the sending of the MP_CAPABLE
   option from the third ACK, with additional data parameters.  This
   MP_CAPABLE option is in place of the DSS, and simply specifies the
   data-level length of the payload, and the checksum (if the use of
   checksums is negotiated).  This is the minimal data required to
   establish a MPTCP connection - it allows validation of the payload,
   and given it is the first data, the Initial Data Sequence Number
   (IDSN) is also known (as it is generated from the key, as described
   below).  Conveying the keys on the first data packet allows the TCP
   reliability mechanisms to ensure the packet is successfully
   delivered.  The receiver will acknowledge this data at the connection
   level with a Data ACK, as if a DSS option has been received.

   There could be situations where both A and B attempt to transmit
   initial data at the same time.  For example, if A did not initially
   have data to send, but then needed to transmit data before it had
   received anything from B, it would use a MP_CAPABLE option with data
   parameters (since it would not know if the MP_CAPABLE on the ACK was
   received).  In such a situation, B may also have transmitted data
   with a DSS option, but it had not yet been received at A.  Therefore,
   B has received data with a MP_CAPABLE mapping after it has sent data
   with a DSS option.  To ensure these situations can be handled, it
   follows that the data parameters in a MP_CAPABLE are semantically
   equivalent to those in a DSS option and can be used interchangeably.
   Similar situations could occur when the MP_CAPABLE with data is lost
   and retransmitted.  Furthermore, in the case of TCP Segmentation
   Offloading, the MP_CAPABLE with data parameters may be duplicated
   across multiple packets, and implementations must also be able to
   cope with duplicate MP_CAPABLE mappings as well as duplicate DSS
   mappings.




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   Additionally, the MP_CAPABLE exchange allows the safe passage of
   MPTCP options on SYN packets to be determined.  If any of these
   options are dropped, MPTCP will gracefully fall back to regular
   single-path TCP, as documented in Section 3.7.  If at any point in
   the handshake either party thinks the MPTCP negotiation is
   compromised, for example by a middlebox corrupting the TCP options,
   or unexpected ACK numbers being present, the host MUST stop using
   MPTCP and no longer include MPTCP options in future TCP packets.  The
   other host will then also fall back to regular TCP using the fall
   back mechanism.  Note that new subflows MUST NOT be established
   (using the process documented in Section 3.2) until a Data Sequence
   Signal (DSS) option has been successfully received across the path
   (as documented in Section 3.3).

   Like all MPTCP options, the MP_CAPABLE option starts with the Kind
   and Length to specify the TCP-option kind and its length.  Followed
   by that is the MP_CAPABLE option.  The first 4 bits of the first
   octet in the MP_CAPABLE option (Figure 4) define the MPTCP option
   subtype (see Section 8; for MP_CAPABLE, this is 0x0), and the
   remaining 4 bits of this octet specify the MPTCP version in use (for
   this specification, this is 1).

   The second octet is reserved for flags, allocated as follows:

   A: The leftmost bit, labeled "A", SHOULD be set to 1 to indicate
      "Checksum Required", unless the system administrator has decided
      that checksums are not required (for example, if the environment
      is controlled and no middleboxes exist that might adjust the
      payload).

   B: The second bit, labeled "B", is an extensibility flag, and MUST be
      set to 0 for current implementations.  This will be used for an
      extensibility mechanism in a future specification, and the impact
      of this flag will be defined at a later date.  If receiving a
      message with the 'B' flag set to 1, and this is not understood,
      then the MP_CAPABLE in this SYN MUST be silently ignored, which
      triggers a fallback to regular TCP; the sender is expected to
      retry with a format compatible with this legacy specification.
      Note that the length of the MP_CAPABLE option, and the meanings of
      bits "C" through "H", may be altered by setting B=1.

   C: The third bit, labeled "C", is set to "1" to indicate that the
      sender of this option will not accept additional MPTCP subflows to
      the source address and port, and therefore the receiver MUST NOT
      try to open any additional subflows towards this address and port.
      This is an efficiency improvement for situations where the sender
      knows a restriction is in place, for example if the sender is




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      behind a strict NAT, or operating behind a legacy Layer 4 load
      balancer.

   D through H:  The remaining bits, labeled "D" through "H", are used
      for crypto algorithm negotiation.  Currently only the rightmost
      bit, labeled "H", is assigned.  Bit "H" indicates the use of HMAC-
      SHA256 (as defined in Section 3.2).  An implementation that only
      supports this method MUST set bit "H" to 1, and bits "D" through
      "G" to 0.

   A crypto algorithm MUST be specified.  If flag bits D through H are
   all 0, the MP_CAPABLE option MUST be treated as invalid and ignored
   (that is, it must be treated as a regular TCP handshake).

   The selection of the authentication algorithm also impacts the
   algorithm used to generate the token and the Initial Data Sequence
   Number (IDSN).  In this specification, with only the SHA-256
   algorithm (bit "H") specified and selected, the token MUST be a
   truncated (most significant 32 bits) SHA-256 hash ([SHS], [RFC6234])
   of the key.  A different, 64-bit truncation (the least significant 64
   bits) of the SHA-256 hash of the key MUST be used as the IDSN.  Note
   that the key MUST be hashed in network byte order.  Also note that
   the "least significant" bits MUST be the rightmost bits of the
   SHA-256 digest, as per [SHS].  Future specifications of the use of
   the crypto bits may choose to specify different algorithms for token
   and IDSN generation.

   Both the crypto and checksum bits negotiate capabilities in similar
   ways.  For the Checksum Required bit (labeled "A"), if either host
   requires the use of checksums, checksums MUST be used.  In other
   words, the only way for checksums not to be used is if both hosts in
   their SYNs set A=0.  This decision is confirmed by the setting of the
   "A" bit in the third packet (the ACK) of the handshake.  For example,
   if the initiator sets A=0 in the SYN, but the responder sets A=1 in
   the SYN/ACK, checksums MUST be used in both directions, and the
   initiator will set A=1 in the ACK.  The decision whether to use
   checksums will be stored by an implementation in a per-connection
   binary state variable.  If A=1 is received by a host that does not
   want to use checksums, it MUST fall back to regular TCP by ignoring
   the MP_CAPABLE option as if it was invalid.

   For crypto negotiation, the responder has the choice.  The initiator
   creates a proposal setting a bit for each algorithm it supports to 1
   (in this version of the specification, there is only one proposal, so
   bit "H" will be always set to 1).  The responder responds with only 1
   bit set -- this is the chosen algorithm.  The rationale for this
   behavior is that the responder will typically be a server with
   potentially many thousands of connections, so it may wish to choose



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   an algorithm with minimal computational complexity, depending on the
   load.  If a responder does not support (or does not want to support)
   any of the initiator's proposals, it can respond without an
   MP_CAPABLE option, thus forcing a fallback 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.

   If a SYN contains an MP_CAPABLE option but the SYN/ACK does not, it
   is assumed that sender of the SYN/ACK is not multipath capable; thus,
   the MPTCP session MUST operate as a regular, single-path TCP.  If a
   SYN does not contain a MP_CAPABLE option, the SYN/ACK MUST NOT
   contain one in response.  If the third packet (the ACK) does not
   contain the MP_CAPABLE option, then the session MUST fall back to
   operating as a regular, single-path TCP.  This is to maintain
   compatibility with middleboxes on the path that drop some or all TCP
   options.  Note that an implementation MAY choose to attempt sending
   MPTCP options more than one time before making this decision to
   operate as regular TCP (see Section 3.9).

   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 reordered 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 3.7.

   The initial data sequence number on an MPTCP connection is generated
   from the key.  The algorithm for IDSN generation is also determined
   from the negotiated authentication algorithm.  In this specification,
   with only the SHA-256 algorithm specified and selected, the IDSN of a
   host MUST be the least significant 64 bits of the SHA-256 hash of its
   key, i.e., IDSN-A = Hash(Key-A) and IDSN-B = Hash(Key-B).  This
   deterministic generation of the IDSN allows a receiver to ensure that
   there are no gaps in sequence space at the start of the connection.
   The SYN with MP_CAPABLE occupies the first octet of data sequence
   space, although this does not need to be acknowledged at the
   connection level until the first data is sent (see Section 3.3).






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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 signaling 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 3.9 for heuristics).

   A new subflow is started as a normal TCP SYN/ACK exchange.  The Join
   Connection (MP_JOIN) MPTCP 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 behavior of MP_JOIN using the HMAC-SHA256
   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).  In this specification, the
   tokens presented in this option are generated by the SHA-256 ([SHS],
   [RFC6234]) 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).  Note that the hash
   generation algorithm can be overridden by the choice of cryptographic
   handshake algorithm, as defined in Section 3.1.

   The MP_JOIN SYN sends not only the token (which is static for a
   connection) but also random numbers (nonces) that are used to prevent
   replay attacks on the authentication method.  Recommendations for the
   generation of random numbers for this purpose are given in [RFC4086].

   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 IP header
   has been changed in transit by a middlebox.  The Address ID allows
   address removal (Section 3.4.2) without needing to know what the



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   source address at the receiver is, thus allowing address removal
   through NATs.  The Address ID also allows correlation between new
   subflow setup attempts and address signaling (Section 3.4.1), to
   prevent setting up duplicate subflows on the same path, if an MP_JOIN
   and ADD_ADDR are sent at the same time.

   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 mappings between Address IDs and
   addresses both for itself and the remote host.  An implementation
   will also need to know which local and remote Address IDs are
   associated with which established subflows, for when addresses are
   removed from a local or remote host.

   The MP_JOIN option on packets with the SYN flag set also includes 4
   bits of flags, 3 of which are currently reserved and MUST be set to
   zero by the sender.  The final bit, labeled "B", indicates whether
   the sender of this option wishes this subflow to be used as a backup
   path (B=1) in the event of failure of other paths, or whether it
   wants it to be used as part of the connection immediately.  By
   setting B=1, the sender of the option is requesting the other host to
   only send data on this subflow if there are no available subflows
   where B=0.  Subflow policy is discussed in more detail in
   Section 3.3.8.

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

       Figure 5: Join Connection (MP_JOIN) Option (for Initial SYN)

   When receiving a SYN with an MP_JOIN option that contains a valid
   token for an existing MPTCP connection, the recipient SHOULD respond
   with a SYN/ACK also containing an MP_JOIN option containing a random
   number and a truncated (leftmost 64 bits) Hash-based Message
   Authentication Code (HMAC).  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 a reset (RST)
   signal, analogous to an unknown port in TCP, containing a MP_TCPRST
   option (Section 3.6) with a "MPTCP specific error" reason code.
   Although calculating an HMAC requires cryptographic operations, it is



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   believed that the 32-bit token in the MP_JOIN SYN gives sufficient
   protection against blind state exhaustion attacks; therefore, there
   is no need to provide mechanisms to allow a responder to operate
   statelessly at the MP_JOIN stage.

   An HMAC 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).  Doing the HMAC exchange at this stage allows
   both hosts to have first exchanged random data (in the first two SYN
   packets) that is used as the "message".  This specification defines
   that HMAC as defined in [RFC2104] is used, along with the SHA-256
   hash algorithm [SHS] (potentially implemented as in [RFC6234]), thus
   generating a 160-bit / 20-octet HMAC.  Due to option space
   limitations, the HMAC included in the SYN/ACK is truncated to the
   leftmost 64 bits, but this is acceptable since random numbers are
   used; thus, an attacker only has one chance to guess the HMAC
   correctly (if the HMAC is incorrect, the TCP connection is closed, so
   a new MP_JOIN negotiation with a new random number is required).

   The initiator's authentication information is sent in its first ACK
   (the third packet of the handshake), as shown in Figure 7.  This data
   needs to be sent reliably, since it is the only time this HMAC is
   sent; therefore, receipt of this packet MUST trigger a regular TCP
   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.  It is
   not permitted to send data while in the PRE_ESTABLISHED state.  The
   reserved bits in this option MUST be set to zero by the sender.

   The key for the HMAC 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"
   for the HMAC algorithm 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.














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                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-----+-+---------------+
      |     Kind      |  Length = 16  |Subtype|     |B|   Address ID  |
      +---------------+---------------+-------+-----+-+---------------+
      |                                                               |
      |                Sender's Truncated HMAC (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 HMAC (160 bits)                    |
      |                                                               |
      |                                                               |
      +---------------------------------------------------------------+

        Figure 7: Join Connection (MP_JOIN) Option (for Third ACK)

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





















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

   HMAC-A = HMAC(Key=(Key-A+Key-B), Msg=(R-A+R-B))
   HMAC-B = HMAC(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 appropriate, a MP_TCPRST option with a
   "Administratively prohibited" reason code (Section 3.6) should be
   included.

   If the token is accepted at Host B, but the HMAC returned to Host A
   does not match the one expected, Host A MUST close the subflow with a
   TCP RST.  In this, and all following cases of sending a RST in this
   section, the sender SHOULD send a MP_TCPRST option (Section 3.6) on
   this RST packet with the reason code for a "MPTCP specific error".

   If Host B does not receive the expected HMAC, or the MP_JOIN option
   is missing from the ACK, it MUST close the subflow with a TCP RST
   with a MP_TCPRST (Section 3.6) option with the reason code for "MPTCP
   specific error".

   If the HMACs are verified as correct, then both hosts have
   authenticated each other as being the same peers as existed at the




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   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 TCP RST with a MP_TCPRST
   (Section 3.6) option with the reason code for "MPTCP specific error".

   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 listener on the relevant port, it will respond with a
   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 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 3.9 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.

   Demultiplexing 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 set up, demultiplexing
   packets is done using the 5-tuple, as in traditional TCP.  The
   5-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 behavior in detail.

   The data sequence mapping and the Data ACK are signaled in the Data
   Sequence Signal (DSS) option (Figure 9).  Either or both can be
   signaled in one DSS, depending on the flags set.  The data sequence
   mapping defines how the sequence space on the subflow maps to the



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   connection level, and the Data ACK acknowledges receipt of data at
   the connection level.  These functions are described in more detail
   in the following two subsections.

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

                Figure 9: Data Sequence Signal (DSS) Option

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

   o  A = Data ACK present

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

   o  M = Data Sequence Number (DSN), Subflow Sequence Number (SSN),
      Data-Level Length, and Checksum (if negotiated) present

   o  m = Data sequence number is 8 octets (if not set, DSN is 4 octets)

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

   The 'F' flag indicates "DATA_FIN".  If present, this means that this
   mapping covers the final data from the sender.  This is the
   connection-level equivalent to the FIN flag in single-path TCP.  A
   connection is not closed unless there has been a DATA_FIN exchange,
   or an implementation-specific, connection-level timeout.  The purpose
   of the DATA_FIN and the interactions between this flag, the subflow-
   level FIN flag, and the data sequence mapping are described in
   Section 3.3.3.  The remaining reserved bits MUST be set to zero by an
   implementation of this specification.

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



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   the length of the option.  If a checksum is present, but its use had
   not been negotiated in the MP_CAPABLE handshake, the checksum field
   MUST be ignored.  If a checksum is not present when its use has been
   negotiated, the receiver MUST close the subflow with a RST as it is
   considered broken.  This RST SHOULD be accompanied with a MP_TCPRST
   option (Section 3.6) with the reason code for a "MPTCP specific
   error".

3.3.1.  Data Sequence Mapping

   The data stream as a whole can be reassembled through the use of the
   data sequence mapping components of the DSS option (Figure 9), which
   define the mapping from the subflow sequence number to the data
   sequence number.  This is used by the receiver to ensure in-order
   delivery to the application layer.  Meanwhile, the subflow-level
   sequence numbers (i.e., the regular sequence numbers in the TCP
   header) have subflow-only relevance.  It is expected (but not
   mandated) that SACK [RFC2018] is used at the subflow level to improve
   efficiency.

   The data sequence mapping specifies a mapping from subflow sequence
   space to data sequence space.  This is expressed in terms of starting
   sequence numbers for the subflow and the data level, and a length of
   bytes for which this mapping is valid.  This explicit mapping for a
   range of data was chosen rather than per-packet signaling to assist
   with compatibility with situations where TCP/IP segmentation or
   coalescing is undertaken separately from the stack that is generating
   the data flow (e.g., through the use of TCP segmentation offloading
   on network interface cards, or by middleboxes such as performance
   enhancing proxies).  It also allows a single mapping to cover many
   packets, which may be useful in bulk transfer situations.

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

   The data sequence number is specified as an absolute value, whereas
   the subflow sequence numbering is relative (the SYN at the start of



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   the subflow has relative subflow sequence number 0).  This is to
   allow middleboxes to change the initial sequence number of a subflow,
   such as firewalls that undertake Initial Sequence Number (ISN)
   randomization.

   The data sequence mapping also contains a checksum of the data that
   this mapping covers, if use of checksums has been negotiated at the
   MP_CAPABLE exchange.  Checksums are 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 3.7, since MPTCP
   can no longer reliably know the subflow sequence space at the
   receiver to build data sequence mappings.

   The checksum algorithm used is the standard TCP checksum [RFC0793],
   operating over the data covered by this mapping, along with a pseudo-
   header as shown in Figure 10.

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

                 Figure 10: Pseudo-Header for DSS Checksum

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

   Note that checksumming relies on the TCP subflow containing
   contiguous data; therefore, a TCP subflow MUST NOT use the Urgent
   Pointer to interrupt an existing mapping.  Further note, however,
   that if Urgent data is received on a subflow, it SHOULD be mapped to




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   the data sequence space and delivered to the application analogous to
   Urgent data in regular TCP.

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

   Data sequence numbers are always 64-bit quantities, and MUST be
   maintained as such in implementations.  If a connection is
   progressing at a slow rate, so protection against wrapped sequence
   numbers is not required, then an implementation MAY include just the
   lower 32 bits of the data sequence number in the data sequence
   mapping and/or Data ACK as an optimization, and an implementation can
   make this choice independently for each packet.  An implementation
   MUST be able to receive and process both 64-bit or 32-bit sequence
   number values, but it is not required that an implementation is able
   to send both.

   An implementation MUST send the full 64-bit data sequence number if
   it is transmitting at a sufficiently high rate that the 32-bit value
   could wrap within the Maximum Segment Lifetime (MSL) [RFC1323].  The
   lengths of the DSNs used in these values (which may be different) are
   declared with flags in the DSS option.  Implementations MUST accept a
   32-bit DSN and implicitly promote it to a 64-bit quantity by
   incrementing the upper 32 bits of sequence number each time the lower
   32 bits wrap.  A sanity check MUST be implemented to ensure that a
   wrap occurs at an expected time (e.g., the sequence number jumps from
   a very high number to a very low number) and is not triggered by out-
   of-order packets.

   As with the standard TCP sequence number, the data sequence number
   should not start at zero, but at a random value to make blind session
   hijacking harder.  This specification requires setting the initial
   data sequence number (IDSN) of each host to the least significant 64
   bits of the SHA-256 hash of the host's key, as described in
   Section 3.1.  This is required also in order for the receiver to know



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   what the expected IDSN is, and thus determine if any initial
   connection-level packets are missing; this is particularly relevant
   if two subflows start transmitting simultaneously.

   A data sequence mapping does not need to be included in every MPTCP
   packet, as long as the subflow sequence space in that packet is
   covered by a mapping known at the receiver.  This can be used to
   reduce overhead in cases where the mapping is known in advance; one
   such case is when there is a single subflow between the hosts,
   another is when segments of data are scheduled in larger than packet-
   sized chunks.

   An "infinite" mapping can be used to fall back to regular TCP by
   mapping the subflow-level data to the connection-level data for the
   remainder of the connection (see Section 3.7).  This is achieved by
   setting the Data-Level Length field of the DSS option to the reserved
   value of 0.  The checksum, in such a case, will also be set to zero.

3.3.2.  Data Acknowledgments

   To provide full end-to-end resilience, MPTCP provides a connection-
   level acknowledgment, 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 behavior of the
   standard TCP cumulative ACK -- indicating how much data has been
   successfully received (with no holes).  This is in comparison to the
   subflow-level ACK, which acts analogous to TCP SACK, given that there
   may still be holes in the data stream at the connection level.  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 Data ACK proves that the data, and all required MPTCP signaling,
   has been received and accepted by the remote end.  One key use of the
   Data ACK signal is that it is used to indicate the left edge of the
   advertised receive window.  As explained in Section 3.3.4, the
   receive window is shared by all subflows and is relative to the Data
   ACK.  Because of this, an implementation MUST NOT use the RCV.WND
   field of a TCP segment at the connection level if it does not also
   carry a DSS option with a Data ACK field.  Furthermore, separating
   the connection-level acknowledgments from the subflow level allows
   processing to be done separately, and a receiver has the freedom to




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   drop segments after acknowledgment at the subflow level, for example,
   due to memory constraints when many segments arrive out of order.

   An MPTCP sender MUST NOT free data from the send buffer until it has
   been acknowledged by both a Data ACK received on any subflow and at
   the subflow level by all subflows on which the data was sent.  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 retransmitted.  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 terminate the subflow that is behaving badly by sending
   a RST, using an appropriate MP_TCPRST (Section 3.6) error code.

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

3.3.3.  Closing a Connection

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

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

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

   The DATA_FIN is signaled by setting the 'F' flag in the Data Sequence
   Signal option (Figure 9) to 1.  A DATA_FIN occupies 1 octet (the
   final octet) of the connection-level sequence space.  Note that the
   DATA_FIN is included in the Data-Level Length, but not at the subflow
   level: for example, a segment with DSN 80, and Data-Level Length 11,
   with DATA_FIN set, would map 10 octets from the subflow into data



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   sequence space 80-89, the DATA_FIN is DSN 90; therefore, this segment
   including DATA_FIN would be acknowledged with a DATA_ACK of 91.

   Note that when the DATA_FIN is not attached to a TCP segment
   containing data, the Data Sequence Signal MUST have a subflow
   sequence number of 0, a Data-Level Length of 1, and the data sequence
   number that corresponds with the DATA_FIN itself.  The checksum in
   this case will only cover the pseudo-header.

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

   Once a DATA_FIN has been acknowledged, all remaining subflows MUST be
   closed with standard FIN exchanges.  Both hosts SHOULD send FINs on
   all subflows, as a courtesy to allow middleboxes to clean up state
   even if an individual subflow has failed.  It is also encouraged to
   reduce the timeouts (Maximum Segment Lifetime) on subflows at end
   hosts after receiving a DATA_FIN.  In particular, any subflows where
   there is still outstanding data queued (which has been retransmitted
   on other subflows in order to get the DATA_FIN acknowledged) MAY be
   closed with a RST with MP_TCPRST (Section 3.6) error code for "too
   much outstanding data".

   A connection is considered closed once both hosts' DATA_FINs have
   been acknowledged by DATA_ACKs.

   As specified above, a standard TCP FIN on an individual subflow only
   shuts down the subflow on which it was sent.  If all subflows have
   been closed with a FIN exchange, but no DATA_FIN has been received
   and acknowledged, the MPTCP connection is treated as closed only
   after a timeout.  This implies that an implementation will have
   TIME_WAIT states at both the subflow and connection levels (see
   Appendix D).  This permits "break-before-make" scenarios where
   connectivity is lost on all subflows before a new one can be re-
   established.







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3.3.4.  Receiver Considerations

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

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

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

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

   In regular TCP, once a segment is deemed in-window, it is put either
   in the in-order receive queue or in the out-of-order queue.  In
   Multipath TCP, the same happens but at the connection level: a
   segment is placed in the connection level in-order or out-of-order
   queue if it is in-window at both connection and subflow levels.  The
   stack still has to remember, for each subflow, which segments were
   received successfully so that it can ACK them at subflow level
   appropriately.  Typically, this will be implemented by keeping per
   subflow out-of-order queues (containing only message headers, not the
   payloads) and remembering the value of the cumulative ACK.

   It is important for implementers to understand how large a receiver
   buffer is appropriate.  The lower bound for full network utilization
   is the maximum bandwidth-delay product of any one of the paths.
   However, this might be insufficient when a packet is lost on a slower
   subflow and needs to be retransmitted (see Section 3.3.6).  A tight
   upper bound would be the maximum round-trip time (RTT) of any path
   multiplied by the total bandwidth available across all paths.  This
   permits all subflows to continue at full speed while a packet is



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   fast-retransmitted on the maximum RTT path.  Even this might be
   insufficient to maintain full performance in the event of a
   retransmit timeout on the maximum RTT path.  It is for future study
   to determine the relationship between retransmission strategies and
   receive buffer sizing.

3.3.5.  Sender Considerations

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

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

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

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

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





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3.3.6.  Reliability and Retransmissions

   The data sequence mapping allows senders to resend data with the same
   data sequence number on a different subflow.  When doing this, a host
   MUST still retransmit the original data on the original subflow, in
   order to preserve the subflow integrity (middleboxes could replay old
   data, and/or could reject holes in subflows), and a receiver will
   ignore these retransmissions.  While this is clearly suboptimal, for
   compatibility reasons this is sensible behavior.  Optimizations could
   be negotiated in future versions of this protocol.  Note also that
   this property would also permit a sender to always send the same
   data, with the same data sequence number, on multiple subflows, if it
   so desired for reliability reasons.

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

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

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

   Multiple retransmissions are triggers that will indicate that a
   subflow performs badly and could lead to a host resetting the subflow
   with a RST.  However, additional research is required to understand
   the heuristics of how and when to reset underperforming subflows.



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   For example, a highly asymmetric path may be misdiagnosed as
   underperforming.  A RST for this purpose SHOULD be accompanied with
   an "Unacceptable performance" MP_TCPRST option (Section 3.6).

3.3.7.  Congestion Control Considerations

   Different subflows in an MPTCP connection have different congestion
   windows.  To achieve fairness at bottlenecks and resource pooling, it
   is necessary to couple the congestion windows in use on each subflow,
   in order to push most traffic to uncongested links.  One algorithm
   for achieving this is presented in [RFC6356]; the algorithm does not
   achieve perfect resource pooling but is "safe" in that it is readily
   deployable in the current Internet.  By this, we mean that it does
   not take up more capacity on any one path than if it was a single
   path flow using only that route, so this ensures fair coexistence
   with single-path TCP at shared bottlenecks.

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

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

3.3.8.  Subflow Policy

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

   In the typical use case, where the goal is to maximize throughput,
   all available paths will be used simultaneously for data transfer,
   using coupled congestion control as described in [RFC6356].  It is
   expected, however, that other use cases will appear.

   For instance, a possibility is an 'all-or-nothing' approach, i.e.,
   have a second path ready for use in the event of failure of the first
   path, but alternatives could include entirely saturating one path
   before using an additional path (the 'overflow' case).  Such choices
   would be most likely based on the monetary cost of links, but may
   also be based on properties such as the delay or jitter of links,
   where stability (of delay or bandwidth) is more important than
   throughput.  Application requirements such as these are discussed in
   detail in [RFC6897].



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

   Whilst fine-grained control may be the most powerful solution, that
   would require some mechanism such as overloading the Explicit
   Congestion Notification (ECN) signal [RFC3168], which is undesirable,
   and it is felt that there would not be sufficient benefit to justify
   an entirely new signal.  Therefore, the MP_JOIN option (see
   Section 3.2) contains the 'B' bit, which allows a host to indicate to
   its peer that this path should be treated as a backup path to use
   only in the event of failure of other working subflows (i.e., a
   subflow where the receiver has indicated B=1 SHOULD NOT be used to
   send data unless there are no usable subflows where B=0).

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

   Another use of the MP_PRIO option is to set the 'B' flag on a subflow
   to cleanly retire its use before closing it and removing it with
   REMOVE_ADDR Section 3.4.2, for example to support make-before-break
   session continuity.

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

            Figure 11: Change Subflow Priority (MP_PRIO) Option

   It should be noted that the backup flag is a request from a data
   receiver to a data sender only, and the data sender SHOULD adhere to
   these requests.  A host cannot assume that the data sender will do
   so, however, since local policies -- or technical difficulties -- may
   override MP_PRIO requests.  Note also that this signal applies to a
   single direction, and so the sender of this option could choose to
   continue using the subflow to send data even if it has signaled B=1
   to the other host.






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3.4.  Address Knowledge Exchange (Path Management)

   We use the term "path management" to refer to the exchange of
   information about additional paths between hosts, which in this
   design is managed by multiple addresses at hosts.  For more detail of
   the architectural thinking behind this design, see the MPTCP
   Architecture document [RFC6182].

   This design makes use of two methods of sharing such information, and
   both can be used on a connection.  The first is the direct setup of
   new subflows, already described in Section 3.2, where the initiator
   has an additional address.  The second method, described in the
   following subsections, signals addresses explicitly to the other host
   to allow it to initiate new subflows.  The two mechanisms are
   complementary: the first is implicit and simple, while the explicit
   is more complex but is more robust.  Together, the mechanisms allow
   addresses to change in flight (and thus support operation through
   NATs, since the source address need not be known), and also allow the
   signaling 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
      multiaddressed, it can start an additional subflow from its
      address A2 to B1, by sending a SYN with a Join option from A2 to
      B1, using B's previously declared token for this connection.
      Alternatively, if B is multihomed, it can try to set up a new
      subflow from B2 to A1, using A's previously declared token.  In
      either case, the SYN will be sent to the port already in use for
      the original subflow on the receiving host.

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

   o  If subflow A2-B1 is successfully set up, Host B can use the
      Address ID in the Join option to correlate this with the ADD_ADDR
      option that will also arrive on an existing subflow; now B knows
      not to open A2-B1, ignoring the ADD_ADDR.  Otherwise, if B has not
      received the A2-B1 MP_JOIN SYN but received the ADD_ADDR, it can



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      try to initiate a new subflow from one or more of its addresses to
      address A2.  This permits new sessions to be opened if one host is
      behind a NAT.

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

3.4.1.  Address Advertisement

   The Add Address (ADD_ADDR) MPTCP option announces additional
   addresses (and optionally, ports) on which a host can be reached
   (Figure 12).  This option can be used at any time during a
   connection, depending on when the sender wishes to enable multiple
   paths and/or when paths become available.  As with all MPTCP signals,
   the receiver MUST undertake standard TCP validity checks, e.g.
   [RFC5961], before acting upon it.

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

   All address IDs learned 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 updating 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 it MAY choose to refresh advertisements periodically.

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

   The 2 octets that specify the TCP port number to use are optional and
   their presence can be inferred from the length of the option.
   Although it is expected that the majority of use cases will use the
   same port pairs as used for the initial subflow (e.g., port 80
   remains port 80 on all subflows, as does the ephemeral port at the
   client), there may be cases (such as port-based load balancing) where



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   the explicit specification of a different port is required.  If no
   port is specified, MPTCP SHOULD attempt to connect to the specified
   address on the same port as is already in use by the subflow on which
   the ADD_ADDR signal was sent; this is discussed in more detail in
   Section 3.9.

   The Truncated HMAC present in this Option is the rightmost 64 bits of
   an HMAC, negotiated and calculated in the same way as for MP_JOIN as
   described in Section 3.2.  For this specification of MPTCP, as there
   is only one hash algorithm option specified, this will be HMAC as
   defined in [RFC2104], using the SHA-256 hash algorithm [SHS],
   implemented as in [RFC6234].  In the same way as for MP_JOIN, the key
   for the HMAC 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 for the HMAC is the
   Address ID, IP Address, and Port which precede the HMAC in the
   ADD_ADDR option.  If the port is not present in the ADD_ADDR option,
   the HMAC message will nevertheless include two octets of value zero.
   The rationale for the HMAC is to prevent unauthorized entities from
   injecting ADD_ADDR signals in an attempt to hijack a connection.
   Note that additionally the presence of this HMAC prevents the address
   being changed in flight unless the key is known by an intermediary.
   If a host receives an ADD_ADDR option for which it cannot validate
   the HMAC, it SHOULD silently ignore the option.

   A set of four flags are present after the subtype and before the
   Address ID.  Only the rightmost bit - labelled 'E' - is assigned
   today.  The other bits are currently unassigned and MUST be set to
   zero by a sender and MUST be ignored by the receiver.

   The 'E' flag exists to provide reliability for this option.  Because
   this option will often be sent on pure ACKs, there is no guarantee of
   reliability.  Therefore, a receiver receiving a fresh ADD_ADDR option
   (where E=0), will send the same option back to the sender, but not
   including the HMAC, and with E=1.  The lack of this echo can be used
   by the initial ADD_ADDR sender to retransmit the ADD_ADDR according
   to local policy.













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                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-------+---------------+
      |     Kind      |     Length    |Subtype|(rsv)|E|  Address ID   |
      +---------------+---------------+-------+-------+---------------+
      |          Address (IPv4 - 4 octets / IPv6 - 16 octets)         |
      +-------------------------------+-------------------------------+
      |   Port (2 octets, optional)   |                               |
      +-------------------------------+                               |
      |        Truncated HMAC (8 octets, if length > 10 octets)       |
      |                               +-------------------------------+
      |                               |
      +-------------------------------+

                 Figure 12: Add Address (ADD_ADDR) Option

   Due to the proliferation of NATs, it is reasonably likely that one
   host may attempt to advertise private addresses [RFC1918].  It is not
   desirable to prohibit this, since there may be cases where both hosts
   have additional interfaces on the same private network, and a host
   MAY want to advertise such addresses.  The MP_JOIN handshake to
   create a new subflow (Section 3.2) provides mechanisms to minimize
   security risks.  The MP_JOIN message contains a 32-bit token that
   uniquely identifies the connection to the receiving host.  If the
   token is unknown, the host will return with a RST.  In the unlikely
   event that the token is valid at the receiving host, subflow setup
   will continue, but the HMAC exchange must occur for authentication.
   This will fail, and will provide sufficient protection against two
   unconnected hosts accidentally setting up a new subflow upon the
   signal of a private address.  Further security considerations around
   the issue of ADD_ADDR messages that accidentally misdirect, or
   maliciously direct, new MP_JOIN attempts are discussed in Section 5.

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

   Therefore, implementing reliability signals for these MPTCP options
   is not necessary.  In order to minimize the impact of the loss of
   these options, however, it is RECOMMENDED that a sender should send
   these options on all available subflows.  If these options need to be
   received in order, an implementation SHOULD only send one ADD_ADDR/
   REMOVE_ADDR option per RTT, to minimize the risk of misordering.



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

   A host can therefore send an ADD_ADDR message with an already
   assigned Address ID, but the Address MUST be the same as previously
   assigned to this Address ID.  A new ADD_ADDR may have the same, or
   different, port number.  If the port number is different, the
   receiving host SHOULD try to set up a new subflow to this new
   address/port combination.

   A host wishing to replace an existing Address ID MUST first remove
   the existing one (Section 3.4.2).

   During normal MPTCP operation, it is unlikely that there will be
   sufficient TCP option space for ADD_ADDR to be included along with
   those for data sequence numbering (Section 3.3.1).  Therefore, it is
   expected that an MPTCP implementation will send the ADD_ADDR option
   on separate ACKs.  As discussed earlier, however, an MPTCP
   implementation MUST NOT treat duplicate ACKs with any MPTCP option,
   with the exception of the DSS option, as indications of congestion
   [RFC5681], and an MPTCP implementation SHOULD NOT send more than two
   duplicate ACKs in a row for signaling purposes.

3.4.2.  Remove Address

   If, during the lifetime of an MPTCP connection, a previously
   announced address becomes invalid (e.g., if the interface
   disappears), the affected host SHOULD announce this so that the peer
   can remove subflows related to this address.  A host MAY also choose
   to announce that a valid IP address should not be used any longer,
   for example for make-before-break session continuity.

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

   For security purposes, if a host receives a REMOVE_ADDR option, it
   must ensure the affected path(s) are no longer in use before it
   instigates closure.  The receipt of REMOVE_ADDR SHOULD first trigger
   the sending of a TCP keepalive [RFC1122] on the path, and if a
   response is received the path SHOULD NOT be removed.  If the path is
   found to still be alive, the receiving host SHOULD no longer use the
   specified address for future connections, but it is the



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   responsibility of the host which sent the REMOVE_ADDR to shut down
   the subflow.  The requesting host MAY also use MP_PRIO
   (Section 3.3.8) to request a path is no longer used, before removal.
   Typical TCP validity tests on the subflow (e.g., ensuring sequence
   and ACK numbers are correct) MUST also be undertaken.  An
   implementation can use indications of these test failures as part of
   intrusion detection or error logging.

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

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

   A subflow that is still functioning MUST be closed with a FIN
   exchange as in regular TCP, rather than using this option.  For more
   information, see Section 3.3.3.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------+-------+---------------+
   |     Kind      |  Length = 3+n |Subtype|(resvd)|   Address ID  | ...
   +---------------+---------------+-------+-------+---------------+
                              (followed by n-1 Address IDs, if required)

              Figure 13: Remove Address (REMOVE_ADDR) Option

3.5.  Fast Close

   Regular TCP has the means of sending a reset (RST) signal to abruptly
   close a connection.  With MPTCP, a regular RST only has the scope of
   the subflow and will only close the concerned subflow but not affect
   the remaining subflows.  MPTCP's connection will stay alive at the
   data level, in order to permit break-before-make handover between
   subflows.  It is therefore necessary to provide an MPTCP-level
   "reset" to allow the abrupt closure of the whole MPTCP connection,
   and this is the MP_FASTCLOSE option.

   MP_FASTCLOSE is used to indicate to the peer that the connection will
   be abruptly closed and no data will be accepted anymore.  The reasons
   for triggering an MP_FASTCLOSE are implementation specific.  Regular
   TCP does not allow sending a RST while the connection is in a
   synchronized state [RFC0793].  Nevertheless, implementations allow
   the sending of a RST in this state, if, for example, the operating



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   system is running out of resources.  In these cases, MPTCP should
   send the MP_FASTCLOSE.  This option is illustrated in Figure 14.

                            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)       |
       +---------------+---------------+-------+-----------------------+
       |                      Option Receiver's Key                    |
       |                            (64 bits)                          |
       |                                                               |
       +---------------------------------------------------------------+

                Figure 14: Fast Close (MP_FASTCLOSE) Option

   If Host A wants to force the closure of an MPTCP connection, it has
   two different options:

   o  Option A (ACK) : Host A sends an ACK containing the MP_FASTCLOSE
      option on one subflow, containing the key of Host B as declared in
      the initial connection handshake.  On all the other subflows, Host
      A sends a regular TCP RST to close these subflows, and tears them
      down.  Host A now enters FASTCLOSE_WAIT state.

   o  Option R (RST) : Host A sends a RST containing the MP_FASTCLOSE
      option on all subflows, containing the key of Host B as declared
      in the initial connection handshake.  Host A can tear the subflows
      and the connection down immediately.

   If host A decides to force the closure by using Option A and sending
   an ACK with the MP_FASTCLOSE option, the connection shall proceed as
   follows:

   o  Upon receipt of an ACK with MP_FASTCLOSE by Host B, containing the
      valid key, Host B answers on the same subflow with a TCP RST and
      tears down all subflows also through sending TCP RST signals.
      Host B can now close the whole MPTCP connection (it transitions
      directly to CLOSED state).

   o  As soon as Host A has received the TCP RST on the remaining
      subflow, it can close this subflow and tear down the whole
      connection (transition from FASTCLOSE_WAIT to CLOSED states).  If
      Host A receives an MP_FASTCLOSE instead of a TCP RST, both hosts
      attempted fast closure simultaneously.  Host A should reply with a
      TCP RST and tear down the connection.

   o  If Host A does not receive a TCP RST in reply to its MP_FASTCLOSE
      after one retransmission timeout (RTO) (the RTO of the subflow



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      where the MP_FASTCLOSE has been sent), it SHOULD retransmit the
      MP_FASTCLOSE.  The number of retransmissions SHOULD be limited to
      avoid this connection from being retained for a long time, but
      this limit is implementation specific.  A RECOMMENDED number is 3.
      If no TCP RST is received in response, Host A SHOULD send a TCP
      RST with the MP_FASTCLOSE option itself when it releases state in
      order to clear any remaining state at middleboxes.

   If however host A decides to force the closure by using Option R and
   sending a RST with the MP_FASTCLOSE option, Host B will act as
   follows: Upon receipt of a RST with MP_FASTCLOSE, containing the
   valid key, Host B tears down all subflows by sending a TCP RST.  Host
   B can now close the whole MPTCP connection (it transitions directly
   to CLOSED state).

3.6.  Subflow Reset

   An implementation of MPTCP may also need to send a regular TCP RST to
   force the closure of a subflow.  A host sends a TCP RST in order to
   close a subflow or reject an attempt to open a subflow (MP_JOIN).  In
   order to inform the receiving host why a subflow is being closed or
   rejected, the TCP RST packet MAY include the MP_TCPRST Option.  The
   host MAY use this information to decide, for example, whether it
   tries to re-establish the subflow immediately, later, or never.

                            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|U|V|W|T|    Reason     |
       +---------------+---------------+-------+-----------------------+

               Figure 15: TCP RST Reason (MP_TCPRST) Option

   The MP_TCPRST option contains a reason code that allows the sender of
   the option to provide more information about the reason for the
   termination of the subflow.  Using 12 bits of option space, the first
   four bits are reserved for flags (only one of which is currently
   defined), and the remaining octet is used to express a reason code
   for this subflow termination, from which a receiver MAY infer
   information about the usability of this path.

   The "T" flag is used by the sender to indicate whether the error
   condition that is reported is Transient (T bit set to 1) or Permanent
   (T bit set to 0).  If the error condition is considered to be
   Transient by the sender of the RST segment, the recipient of this
   segment MAY try to reestablish a subflow for this connection over the
   failed path.  The time at which a receiver may try to re-establish
   this is implementation-specific, but SHOULD take into account the



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   properties of the failure defined by the following reason code.  If
   the error condition is considered to be permanent, the receiver of
   the RST segment SHOULD NOT try to reestablish a subflow for this
   connection over this path.  The "U", "V" and "W" flags are not
   defined by this specification and are reserved for future use.  An
   implementation of this specification MUST set these flags to 0, and a
   receiver MUST ignore them.

   The "Reason" code is an 8-bit field that indicates the reason for the
   termination of the subflow.  The following codes are defined in this
   document:

   o  Unspecified error (code 0x0).  This is the default error implying
      the subflow is no longer available.  The presence of this option
      shows that the RST was generated by a MPTCP-aware device.

   o  MPTCP specific error (code 0x01).  An error has been detected in
      the processing of MPTCP options.  This is the usual reason code to
      return in the cases where a RST is being sent to close a subflow
      for reasons of an invalid response.

   o  Lack of resources (code 0x02).  This code indicates that the
      sending host does not have enough resources to support the
      terminated subflow.

   o  Administratively prohibited (code 0x03).  This code indicates that
      the requested subflow is prohibited by the policies of the sending
      host.

   o  Too much outstanding data (code 0x04).  This code indicates that
      there is an excessive amount of data that need to be transmitted
      over the terminated subflow while having already been acknowledged
      over one or more other subflows.  This may occur if a path has
      been unavailable for a short period and it is more efficient to
      reset and start again than it is to retransmit the queued data.

   o  Unacceptable performance (code 0x05).  This code indicates that
      the performance of this subflow was too low compared to the other
      subflows of this Multipath TCP connection.

   o  Middlebox interference (code 0x06).  Middlebox interference has
      been detected over this subflow making MPTCP signaling invalid.
      For example, this may be sent if the checksum does not validate.








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3.7.  Fallback

   Sometimes, middleboxes will exist on a path that could prevent the
   operation of MPTCP.  MPTCP has been designed in order to cope with
   many middlebox modifications (see Section 6), but there are still
   some cases where a subflow could fail to operate within the MPTCP
   requirements.  These cases are notably the following: the loss of
   MPTCP options on a path, and the modification of payload data.  If
   such an event occurs, it is necessary to "fall back" to the previous,
   safe operation.  This may be either falling back to regular TCP or
   removing a problematic subflow.

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

   There is, however, another corner case that 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, using the
   following rules.

   A sender MUST include a DSS option with data sequence mapping in
   every segment until one of the sent segments has been acknowledged
   with a DSS option containing a Data ACK.  Upon reception of the
   acknowledgment, 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 DSS option containing a Data ACK, 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 a RST, which SHOULD contain a MP_TCPRST option
   (Section 3.6) with a "Middlebox interferance" reason code.

   In the case of such an ACK being received on the first subflow (i.e.,
   that started with MP_CAPABLE), before any additional subflows are
   added, the implementation MUST drop out of an MPTCP mode, back to
   regular TCP.  The sender will send one final data sequence mapping,
   with the Data-Level Length value of 0 indicating an infinite mapping



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   (to inform the other end in case the path drops options in one
   direction only), and then revert to sending data on the single
   subflow without any MPTCP options.

   If a subflow breaks during operation, e.g. if it is re-routed and
   MPTCP options are no longer permitted, then once this is detected (by
   the subflow-level receive buffer filling up), the subflow SHOULD be
   treated as broken and closed with a RST, since no data can be
   delivered to the application layer, and no fallback signal can be
   reliably sent.  This RST SHOULD include the MP_TCPRST option
   (Section 3.6) with a "Middlebox interferance" reason code.

   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 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 a RST, containing a MP_TCPRST option (Section 3.6) with a
   "Middlebox interferance" reason code.

   So far this section has discussed the lost of MPTCP options, either
   initially, or during the course of the connection.  As described in
   Section 3.3, each portion of data for which there is a mapping is
   protected by a checksum, if checksums have been negotiated.  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 it 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 such modifications
   could be triggered across MPTCP segment boundaries, corrupting the
   data.  Therefore, all data from the start of the segment that failed
   the checksum onwards is not trustworthy.

   Note that if checksum usage has not been negotiated, this fallback
   mechanism cannot be used unless there is some higher or lower layer
   signal to inform the MPTCP implementation that the payload has been
   tampered with.

   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



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   possible to determine what 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 a RST, featuring an MP_FAIL option
   (Figure 16), which defines the data sequence number at the start of
   the segment (defined by the data sequence mapping) that had the
   checksum failure.  Note that the MP_FAIL option requires the use of
   the full 64-bit sequence number, even if 32-bit sequence numbers are
   normally in use in the DSS signals on the path.

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


                   Figure 16: Fallback (MP_FAIL) Option

   The receiver of this option MUST discard all data following the data
   sequence number specified.  Failed data MUST NOT be DATA_ACKed and so
   will be retransmitted on other subflows (Section 3.3.6).

   A special case is when there is a single subflow and it fails with a
   checksum error.  If it is known that all unacknowledged data in
   flight is contiguous (which will usually be the case with a single
   subflow), an infinite mapping can be applied to the subflow without
   the need to close it first, and essentially turn off all further
   MPTCP signaling.  In this case, if a receiver identifies a checksum
   failure when there is only one path, it will send back an MP_FAIL
   option on the subflow-level ACK, referring to the data-level sequence
   number of the start of the segment on which the checksum error was
   detected.  The sender will receive this, and if all unacknowledged
   data in flight is contiguous, will signal an infinite mapping.  This
   infinite mapping will be a DSS option (Section 3.3) on the first new
   packet, containing a data sequence mapping that acts retroactively,
   referring to the start of the subflow sequence number of the most
   recent segment that was known to be delivered intact (i.e. was
   successfully DATA_ACKed).  From that point onwards, data can be
   altered by a middlebox without affecting MPTCP, as the data stream is
   equivalent to a regular, legacy TCP session.  Whilst in theory paths
   may only be damaged in one direction, and the MP_FAIL signal affects
   only one direction of traffic, for implementation simplicity, the




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   receiver of an MP_FAIL MUST also respond with an MP_FAIL in the
   reverse direction and entirely revert to a regular TCP session.

   In the rare case that the data is not contiguous (which could happen
   when there is only one subflow but it is retransmitting data from a
   subflow that has recently been uncleanly closed), the receiver MUST
   close the subflow with a RST with MP_FAIL.  The receiver MUST discard
   all data that follows the data sequence number specified.  The sender
   MAY attempt to create a new subflow belonging to the same connection,
   and, if it chooses to do so, SHOULD place the single subflow
   immediately in single-path mode by setting an infinite data sequence
   mapping.  This mapping will begin from the data-level sequence number
   that was declared in the MP_FAIL.

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

   When a connection has fallen back with an infinite mapping, only one
   subflow can send data; otherwise, the receiver would not know how to
   reorder the data.  In practice, this means that all MPTCP subflows
   will have to be terminated except one.  Once MPTCP falls back to
   regular TCP, it MUST NOT revert to MPTCP later in the connection.

   It should be emphasized that MPTCP is not attempting to prevent the
   use of middleboxes that want to adjust the payload.  An MPTCP-aware
   middlebox could provide such functionality by also rewriting
   checksums.

3.8.  Error Handling

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

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

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

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



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   o  Remove request for unknown address ID: silently ignore

3.9.  Heuristics

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

3.9.1.  Port Usage

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

   There may also be cases, however, where a host wishes to signal that
   a specific port should be used, and this facility is provided in the
   ADD_ADDR option as documented in Section 3.4.1.  It is therefore
   feasible to allow multiple subflows between the same two addresses
   but using different port pairs, and such a facility could be used to
   allow load balancing within the network based on 5-tuples (e.g., some
   ECMP implementations [RFC2992]).

3.9.2.  Delayed Subflow Start and Subflow Symmetry

   Many TCP connections are short-lived and consist only of a few
   segments, and so the overheads of using MPTCP outweigh any benefits.
   A heuristic is required, therefore, to decide when to start using
   additional subflows in an MPTCP connection.  We expect that
   experience gathered from deployments will provide further guidance on
   this, and will be affected by particular application characteristics
   (which are likely to change over time).  However, a suggested
   general-purpose heuristic that an implementation MAY choose to employ
   is as follows.  Results from experimental deployments are needed in
   order to verify the correctness of this proposal.

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



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

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

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

   Another issue is that both communicating hosts may simultaneously try
   to set up a subflow between the same pair of addresses.  This leads
   to an inefficient use of resources.

   If the the same ports are used on all subflows, as recommended above,
   then standard TCP simultaneous open logic should take care of this
   situation and only one subflow will be established between the
   address pairs.  However, this relies on the same ports being used at
   both end hosts.  If a host does not support TCP simultaneous open, it
   is RECOMMENDED that some element of randomization is applied to the
   time to wait before opening new subflows, so that only one subflow is
   created between a given address pair.  If, however, hosts signal
   additional ports to use (for example, for leveraging ECMP on-path),
   this heuristic is not appropriate.

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

3.9.3.  Failure Handling

   Requirements for MPTCP's handling of unexpected signals have been
   given in Section 3.8.  There are other failure cases, however, where
   a hosts can choose appropriate behavior.

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



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   Another failure could occur when the MP_JOIN handshake fails.
   Section 3.8 specifies that an incorrect handshake MUST lead to the
   subflow being closed with a RST.  A host operating an active
   intrusion detection system may choose to start blocking MP_JOIN
   packets from the source host if multiple failed MP_JOIN attempts are
   seen.  From the connection initiator's point of view, if an MP_JOIN
   fails, it SHOULD NOT attempt to connect to the same IP address and
   port during the lifetime of the connection, unless the other host
   refreshes the information with another ADD_ADDR option.  Note that
   the ADD_ADDR option is informational only, and does not guarantee the
   other host will attempt a connection.

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

4.  Semantic Issues

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

   Sequence number:  The (in-header) TCP sequence number is specific to
      the subflow.  To allow the receiver to reorder application data,
      an additional data-level sequence space is used.  In this data-
      level sequence space, the initial SYN and the final DATA_FIN
      occupy 1 octet of sequence space.  This is to ensure these signals
      are acknowledged at the connection level.  There is an explicit
      mapping of data sequence space to subflow sequence space, which is
      signaled 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
      acknowledgment from the subflow ACKs.  This separates subflow- and
      connection-level processing at an end host.

   Duplicate ACK:  A duplicate ACK that includes any MPTCP signaling
      (with the exception of the DSS option) MUST NOT be treated as a
      signal of congestion.  To limit the chances of non-MPTCP-aware
      entities mistakenly interpreting duplicate ACKs as a signal of
      congestion, MPTCP SHOULD NOT send more than two duplicate ACKs
      containing (non-DSS) MPTCP signals in a row.





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

   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 [RFC6897].

5.  Security Considerations

   As identified in [RFC6181], the addition of multipath capability to
   TCP will bring with it a number of new classes of threat.  In order
   to prevent these, [RFC6182] 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:





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   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 Sections 3.1 and 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 are never sent
   again over the network (unless used in the fast close mechanism,
   Section 3.5).  To ease demultiplexing while not giving away any
   cryptographic material, future subflows use a truncated cryptographic
   hash of this key as the connection identification "token".  The keys
   are concatenated and used as keys for creating Hash-based Message
   Authentication Codes (HMACs) used on subflow setup, in order to
   verify that the parties in the handshake are the same as in the
   original connection setup.  It also provides verification that the
   peer can receive traffic at this new address.  Replay attacks would
   still be possible when only keys are used; therefore, the handshakes
   use single-use random numbers (nonces) at both ends -- this ensures
   the HMAC will never be the same on two handshakes.  Guidance on
   generating random numbers suitable for use as keys is given in
   [RFC4086] and discussed in Section 3.1.  HMAC is also used to secure
   the ADD_ADDR option, due to the threats identified in [RFC7430].

   The use of crypto capability bits in the initial connection handshake
   to negotiate use of a particular algorithm allows 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 document should therefore protect against
   all forms of flooding and hijacking attacks discussed in [RFC6181].

   The version negotiation specified in Section 3.1, if differing MPTCP
   versions shared a common negotiation format, would allow an on-path
   attacker to apply a theoretical bid-down attack.  However, since the
   v1 and v0 protocols have a different handshake, this is not an attack
   that can be applied here.  Furthermore, an on-path attacker would
   have access to the raw data, negating any other TCP-level security
   mechanisms.  Also a change from [RFC6824] has removed the subflow
   identifier from the MP_PRIO option (Section 3.3.8), to remove the
   theoretical attack where a subflow could be placed in "backup" mode
   by an attacker.



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   During normal operation, regular TCP protection mechanisms (such as
   ensuring sequence numbers are in-window) will provide the same level
   of protection against attacks on individual TCP subflows as exists
   for regular TCP today.  Implementations will introduce additional
   buffers compared to regular TCP, to reassemble data at the connection
   level.  The application of window sizing will minimize the risk of
   denial-of-service attacks consuming resources.

   As discussed in Section 3.4.1, a host may advertise its private
   addresses, but these might point to different hosts in the receiver's
   network.  The MP_JOIN handshake (Section 3.2) will ensure that this
   does not succeed in setting up a subflow to the incorrect host.
   However, it could still create unwanted TCP handshake traffic.  This
   feature of MPTCP could be a target for denial-of-service exploits,
   with malicious participants in MPTCP connections encouraging the
   recipient to target other hosts in the network.  Therefore,
   implementations should consider heuristics (Section 3.9) at both the
   sender and receiver to reduce the impact of this.

   To further protect against malicious ADD_ADDR messages sent by an
   off-path attacker, the ADD_ADDR includes an HMAC using the keys
   negotiated during the handshake.  This effectively prevents an
   attacker from diverting an MPTCP connection through an off-path
   ADD_ADDR injection into the stream.

   A small security risk could theoretically exist with key reuse, but
   in order to accomplish a replay attack, both the sender and receiver
   keys, and the sender and receiver random numbers, in the MP_JOIN
   handshake (Section 3.2) would have to match.

   Whilst this specification defines a "medium" security solution,
   meeting the criteria specified at the start of this section and the
   threat analysis ([RFC6181]), since attacks only ever get worse, it is
   likely that a future Standards Track version of MPTCP would need to
   be able to support stronger security.  There are several ways the
   security of MPTCP could potentially be improved; some of these would
   be compatible with MPTCP as defined in this document, whilst others
   may not be.  For now, the best approach is to get experience with the
   current approach, establish what might work, and check that the
   threat analysis is still accurate.

   Possible ways of improving MPTCP security could include:

   o  defining a new MPCTP cryptographic algorithm, as negotiated in
      MP_CAPABLE.  A sub-case could be to include an additional
      deployment assumption, such as stateful servers, in order to allow
      a more powerful algorithm to be used.




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   o  defining how to secure data transfer with MPTCP, whilst not
      changing the signaling part of the protocol.

   o  defining security that requires more option space, perhaps in
      conjunction with a "long options" proposal for extending the TCP
      options space (such as those surveyed in [TCPLO]), or perhaps
      building on the current approach with a second stage of MPTCP-
      option-based security.

   o  revisiting the working group's decision to exclusively use TCP
      options for MPTCP signaling, and instead look at also making use
      of the TCP payloads.

   MPTCP has been designed with several methods available to indicate a
   new security mechanism, including:

   o  available flags in MP_CAPABLE (Figure 4);

   o  available subtypes in the MPTCP option (Figure 3);

   o  the version field in MP_CAPABLE (Figure 4);

6.  Interactions with Middleboxes

   Multipath TCP was designed to be deployable in the present world.
   Its design takes into account "reasonable" existing middlebox
   behavior.  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 accommodate the different middleboxes.

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

   MPTCP uses a single new TCP option "Kind", and all message types are
   defined by "subtype" values (see Section 8).  This should reduce the
   chances of only some types of MPTCP options being passed, and instead
   the key differing characteristics are different paths, and the
   presence of the SYN flag.

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



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   If a middlebox strips options but otherwise passes the packets
   unchanged, MPTCP will behave safely.  If an MP_CAPABLE option is
   dropped on either the outgoing or the return path, the initiating
   host can fall back to regular TCP, as illustrated in Figure 17 and
   discussed in Section 3.1.

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

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

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

   Figure 17: Connection Setup with Middleboxes that Strip Options from
                                  Packets

   We now examine data flow with MPTCP, assuming the flow is correctly
   set up, 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.  If only some MPTCP options are stripped,
   behavior is not deterministic.  If some data sequence mappings are
   lost, the connection can continue so long as mappings exist for the
   subflow-level data (e.g., if multiple maps have been sent that
   reinforce each other).  If some subflow-level space is left unmapped,



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   however, the subflow is treated as broken and is closed, through the
   process described in Section 3.7.  MPTCP should survive with a loss
   of some Data ACKs, but performance will degrade as the fraction of
   stripped options increases.  We do not expect such cases to appear in
   practice, though: most middleboxes will either strip all options or
   let them all through.

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

   o  NATs [RFC3022] (Network Address (and Port) Translators) change the
      source address (and often source port) of packets.  This means
      that a host will not know its public-facing address for signaling
      in MPTCP.  Therefore, MPTCP permits implicit address addition via
      the MP_JOIN option, and the handshake mechanism ensures that
      connection attempts to private addresses [RFC1918], since they are
      authenticated, will only set up subflows to the correct hosts.
      Explicit address removal is undertaken by an Address ID to allow
      no knowledge of the source address.

   o  Performance Enhancing Proxies (PEPs) [RFC3135] might proactively
      ACK data to increase performance.  MPTCP, however, relies on
      accurate congestion control signals from the end host, and non-
      MPTCP-aware PEPs will not be able to provide such signals.  MPTCP
      will, therefore, fall back to single-path TCP, or close the
      problematic subflow (see Section 3.7).

   o  Traffic Normalizers [norm] 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.  In the event
      of a retransmission, the same data will be retransmitted on the
      original TCP subflow even if it is additionally retransmitted at
      the connection level on a different subflow.

   o  Firewalls [RFC2979] 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 signaling (ADD_ADDR) so that a multiaddressed
      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



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      IDS to analyze the whole traffic, and potentially increases the
      risk of false positives.  However, a MPTCP-aware IDS can read
      tokens to correlate multiple subflows and reassemble them for
      analysis.

   o  Application-level middleboxes such as content-aware firewalls may
      alter the payload within a subflow, such as rewriting 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 fall back to TCP,
      allowing such middleboxes to change the payload.  MPTCP-aware
      middleboxes should be able to adjust the payload and MPTCP
      metadata in order not to break the connection.

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

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

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

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

7.  Acknowledgments

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

   The authors also wish to acknowledge reviews and contributions from
   Iljitsch van Beijnum, Lars Eggert, Marcelo Bagnulo, Robert Hancock,
   Pasi Sarolahti, Toby Moncaster, Philip Eardley, Sergio Lembo,
   Lawrence Conroy, Yoshifumi Nishida, Bob Briscoe, Stein Gjessing,
   Andrew McGregor, Georg Hampel, Anumita Biswas, Wes Eddy, Alexey
   Melnikov, Francis Dupont, Adrian Farrel, Barry Leiba, Robert Sparks,
   Sean Turner, Stephen Farrell, Martin Stiemerling, Gregory Detal,
   Fabien Duchene, Xavier de Foy, and Rahul Jadhav.




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8.  IANA Considerations

   This document obsoletes [RFC6824] and as such IANA is requested to
   update the TCP option space registry to point to this document for
   Multipath TCP, as follows:

         +------+--------+-----------------------+---------------+
         | Kind | Length |        Meaning        |   Reference   |
         +------+--------+-----------------------+---------------+
         |  30  |   N    | Multipath TCP (MPTCP) | This document |
         +------+--------+-----------------------+---------------+

                     Table 1: TCP Option Kind Numbers

8.1.  MPTCP Option Subtypes

   The 4-bit MPTCP subtype sub-registry ("MPTCP Option Subtypes" under
   the "Transmission Control Protocol (TCP) Parameters" registry) was
   defined in [RFC6824].  This document defines one additional subtype
   (ADD_ADDR) and updates the references to this document for all sub-
   types except ADD_ADDR, which is deprecated.  The updates are listed
   in the following table.





























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   +-------+-----------------+-------------------------+---------------+
   | Value |      Symbol     |           Name          |   Reference   |
   +-------+-----------------+-------------------------+---------------+
   |  0x0  |    MP_CAPABLE   |    Multipath Capable    |      This     |
   |       |                 |                         |   document,   |
   |       |                 |                         |  Section 3.1  |
   |  0x1  |     MP_JOIN     |     Join Connection     |      This     |
   |       |                 |                         |   document,   |
   |       |                 |                         |  Section 3.2  |
   |  0x2  |       DSS       |   Data Sequence Signal  |      This     |
   |       |                 |    (Data ACK and data   |   document,   |
   |       |                 |    sequence mapping)    |  Section 3.3  |
   |  0x3  |     ADD_ADDR    |       Add Address       |      This     |
   |       |                 |                         |   document,   |
   |       |                 |                         | Section 3.4.1 |
   |  0x4  |   REMOVE_ADDR   |      Remove Address     |      This     |
   |       |                 |                         |   document,   |
   |       |                 |                         | Section 3.4.2 |
   |  0x5  |     MP_PRIO     | Change Subflow Priority |      This     |
   |       |                 |                         |   document,   |
   |       |                 |                         | Section 3.3.8 |
   |  0x6  |     MP_FAIL     |         Fallback        |      This     |
   |       |                 |                         |   document,   |
   |       |                 |                         |  Section 3.7  |
   |  0x7  |   MP_FASTCLOSE  |        Fast Close       |      This     |
   |       |                 |                         |   document,   |
   |       |                 |                         |  Section 3.5  |
   |  0x8  |    MP_TCPRST    |      Subflow Reset      |      This     |
   |       |                 |                         |   document,   |
   |       |                 |                         |  Section 3.6  |
   |  0xf  | MP_EXPERIMENTAL |   Reserved for private  |               |
   |       |                 |       experiments       |               |
   +-------+-----------------+-------------------------+---------------+

                      Table 2: MPTCP Option Subtypes

   Values 0x9 through 0xe are currently unassigned.  Option 0xf is
   reserved for use by private experiments.  Its use may be formalized
   in a future specification.

8.2.  MPTCP Handshake Algorithms

   IANA has created another sub-registry, "MPTCP Handshake Algorithms"
   under the "Transmission Control Protocol (TCP) Parameters" registry,
   based on the flags in MP_CAPABLE (Section 3.1).  IANA is requested to
   update the references of this table to this document, as follows:





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   +-------+----------------------------------------+------------------+
   |  Flag |                Meaning                 |    Reference     |
   |  Bit  |                                        |                  |
   +-------+----------------------------------------+------------------+
   |   A   |           Checksum required            |  This document,  |
   |       |                                        |   Section 3.1    |
   |   B   |             Extensibility              |  This document,  |
   |       |                                        |   Section 3.1    |
   |   C   |    Do not attempt to establish new     |  This document,  |
   |       |    subflows to the source address.     |   Section 3.1    |
   |  D-G  |               Unassigned               |                  |
   |   H   |              HMAC-SHA256               |  This document,  |
   |       |                                        |   Section 3.2    |
   +-------+----------------------------------------+------------------+

                    Table 3: MPTCP Handshake Algorithms

   Note that the meanings of bits D through H can be dependent upon bit
   B, depending on how Extensibility is defined in future
   specifications; see Section 3.1 for more information.

   Future assignments in this registry are also to be defined by
   Standards Action as defined by [RFC5226].  Assignments consist of the
   value of the flags, a symbolic name for the algorithm, and a
   reference to its specification.

8.3.  MP_TCPRST Reason Codes

   IANA is requested to create a further sub-registry, "MP_TCPRST Reason
   Codes" under the "Transmission Control Protocol (TCP) Parameters"
   registry, based on the reason code in MP_TCPRST (Section 3.6):

    +------+-----------------------------+----------------------------+
    | Code |           Meaning           |         Reference          |
    +------+-----------------------------+----------------------------+
    | 0x00 |    Unspecified TCP error    | This document, Section 3.6 |
    | 0x01 |     MPTCP specific error    | This document, Section 3.6 |
    | 0x02 |      Lack of resources      | This document, Section 3.6 |
    | 0x03 | Administratively prohibited | This document, Section 3.6 |
    | 0x04 |  Too much outstanding data  | This document, Section 3.6 |
    | 0x05 |   Unacceptable performance  | This document, Section 3.6 |
    | 0x06 |    Middlebox interference   | This document, Section 3.6 |
    +------+-----------------------------+----------------------------+

                   Table 4: MPTCP MP_TCPRST Reason Codes






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9.  References

9.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6182]  Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
              Iyengar, "Architectural Guidelines for Multipath TCP
              Development", RFC 6182, DOI 10.17487/RFC6182, March 2011,
              <https://www.rfc-editor.org/info/rfc6182>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [SHS]      National Institute of Science and Technology, "Secure Hash
              Standard", Federal Information Processing Standard
              (FIPS) 180-4, August 2015,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

9.2.  Informative References

   [howhard]  Raiciu, C., Paasch, C., Barre, S., Ford, A., Honda, M.,
              Duchene, F., Bonaventure, O., and M. Handley, "How Hard
              Can It Be? Designing and Implementing a Deployable
              Multipath TCP", Usenix Symposium on Networked Systems
              Design and Implementation 2012, 2012,
              <https://www.usenix.org/conference/nsdi12/how-hard-can-it-
              be-designing-and-implementing-deployable-multipath-tcp>.

   [norm]     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>.







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   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1323]  Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, DOI 10.17487/RFC1323, May
              1992, <https://www.rfc-editor.org/info/rfc1323>.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
              <https://www.rfc-editor.org/info/rfc1918>.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,
              <https://www.rfc-editor.org/info/rfc2018>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC2979]  Freed, N., "Behavior of and Requirements for Internet
              Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000,
              <https://www.rfc-editor.org/info/rfc2979>.

   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path
              Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,
              <https://www.rfc-editor.org/info/rfc2992>.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              DOI 10.17487/RFC3022, January 2001,
              <https://www.rfc-editor.org/info/rfc3022>.

   [RFC3135]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
              Shelby, "Performance Enhancing Proxies Intended to
              Mitigate Link-Related Degradations", RFC 3135,
              DOI 10.17487/RFC3135, June 2001,
              <https://www.rfc-editor.org/info/rfc3135>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.




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   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
              <https://www.rfc-editor.org/info/rfc4987>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks", RFC 5961,
              DOI 10.17487/RFC5961, August 2010,
              <https://www.rfc-editor.org/info/rfc5961>.

   [RFC6181]  Bagnulo, M., "Threat Analysis for TCP Extensions for
              Multipath Operation with Multiple Addresses", RFC 6181,
              DOI 10.17487/RFC6181, March 2011,
              <https://www.rfc-editor.org/info/rfc6181>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC6356]  Raiciu, C., Handley, M., and D. Wischik, "Coupled
              Congestion Control for Multipath Transport Protocols",
              RFC 6356, DOI 10.17487/RFC6356, October 2011,
              <https://www.rfc-editor.org/info/rfc6356>.

   [RFC6528]  Gont, F. and S. Bellovin, "Defending against Sequence
              Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
              2012, <https://www.rfc-editor.org/info/rfc6528>.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
              <https://www.rfc-editor.org/info/rfc6824>.





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   [RFC6897]  Scharf, M. and A. Ford, "Multipath TCP (MPTCP) Application
              Interface Considerations", RFC 6897, DOI 10.17487/RFC6897,
              March 2013, <https://www.rfc-editor.org/info/rfc6897>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

   [RFC7430]  Bagnulo, M., Paasch, C., Gont, F., Bonaventure, O., and C.
              Raiciu, "Analysis of Residual Threats and Possible Fixes
              for Multipath TCP (MPTCP)", RFC 7430,
              DOI 10.17487/RFC7430, July 2015,
              <https://www.rfc-editor.org/info/rfc7430>.

   [TCPLO]    Ramaiah, A., "TCP option space extension", Work
              in Progress, March 2012.



































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Appendix A.  Notes on Use of TCP Options

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

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

   SYN packets typically include Maximum Segment Size (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.

   Note that due to the use of a 64-bit data-level sequence space, it is
   feasible that MPTCP will not require the timestamp option for
   protection against wrapped sequence numbers (PAWS [RFC1323]), since
   the data-level sequence space has far less chance of wrapping.
   Confirmation of the validity of this optimisation is for further
   study.

   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
   bidirectional and high-bandwidth, it is unlikely that all that option
   space will be required on each DSS option.

   Within the DSS option, it is not necessary to include the data
   sequence mapping and DATA_ACK in each packet, and in many cases it
   may be possible to alternate their presence (so long as the mapping
   covers the data being sent in the following packet).  It would also
   be possible to alternate between 4- and 8-byte sequence numbers in
   each option.

   On subflow and connection setup, an MPTCP option is also set on the
   third packet (an ACK).  These are 20 bytes (for Multipath Capable)



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   and 24 bytes (for Join), both of which will fit in the available
   option space.

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

   The ADD_ADDR option can be between 16 and 30 bytes, depending on
   whether IPv4 or IPv6 is used, and whether or not the port number is
   present.  It is unlikely that such signaling would fit in a data
   packet (although if there is space, it is fine to include it).  It is
   recommended to use duplicate ACKs with no other payload or options in
   order to transmit these rare signals.  Note this is the reason for
   mandating that duplicate ACKs with MPTCP options are not taken as a
   signal of congestion.

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

Appendix B.  TCP Fast Open and MPTCP

   TCP Fast Open (TFO) is an experimental TCP extension, described in
   [RFC7413], which has been introduced to allow sending data one RTT
   earlier than with regular TCP.  This is considered a valuable gain as
   very short connections are very common, especially for HTTP request/
   response schemes.  It achieves this by sending the SYN-segment
   together with the application's data and allowing the listener to
   reply immediately with data after the SYN/ACK.  [RFC7413] secures
   this mechanism, by using a new TCP option that includes a cookie
   which is negotiated in a preceding connection.

   When using TCP Fast Open in conjunction with MPTCP, there are two key
   points to take into account, detailed hereafter.



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B.1.  TFO cookie request with MPTCP

   When a TFO initiator first connects to a listener, it cannot
   immediately include data in the SYN for security reasons [RFC7413].
   Instead, it requests a cookie that will be used in subsequent
   connections.  This is done with the TCP cookie request/response
   options, of respectively 2 bytes and 6-18 bytes (depending on the
   chosen cookie length).

   TFO and MPTCP can be combined provided that the total length of all
   the options does not exceed the maximum 40 bytes possible in TCP:

   o  In the SYN: MPTCP uses a 4-bytes long MP_CAPABLE option.  The
      MPTCP and TFO options sum up to 6 bytes.  With typical TCP-options
      using up to 19 bytes in the SYN (24 bytes if options are padded at
      a word boundary), there is enough space to combine the MP_CAPABLE
      with the TFO Cookie Request.

   o  In the SYN+ACK: MPTCP uses a 12-bytes long MP_CAPABLE option, but
      now TFO can be as long as 18 bytes.  Since the maximum option
      length may be exceeded, it is up to the listener to solve this by
      using a shorter cookie.  As an example, if we consider that 19
      bytes are used for classical TCP options, the maximum possible
      cookie length would be of 7 bytes.  Note that the same limitation
      applies to subsequent connections, for the SYN packet (because the
      initiator then echoes back the cookie to the listener).  Finally,
      if the security impact of reducing the cookie size is not deemed
      acceptable, the listener can reduce the amount of other TCP-
      options by omitting the TCP timestamps (as outlined in
      Appendix A).

B.2.  Data sequence mapping under TFO

   MPTCP uses, in the TCP establishment phase, a key exchange that is
   used to generate the Initial Data Sequence Numbers (IDSNs).  In
   particular, the SYN with MP_CAPABLE occupies the first octet of the
   data sequence space.  With TFO, one way to handle the data sent
   together with the SYN would be to consider an implicit DSS mapping
   that covers that SYN segment (since there is not enough space in the
   SYN to include a DSS option).  The problem with that approach is that
   if a middlebox modifies the TFO data, this will not be noticed by
   MPTCP because of the absence of a DSS-checksum.  For example, a TCP
   (but not MPTCP)-aware middlebox could insert bytes at the beginning
   of the stream and adapt the TCP checksum and sequence numbers
   accordingly.  With an implicit mapping, this would give to initiator
   and listener a different view on the DSS-mapping, with no way to
   detect this inconsistency as the DSS checksum is not present.




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   To solve this, the TFO data must not be considered part of the Data
   Sequence Number space: the SYN with MP_CAPABLE still occupies the
   first octet of data sequence space, but then the first non-TFO data
   byte occupies the second octet.  This guarantees that, if the use of
   DSS-checksum is negotiated, all data in the data sequence number
   space is checksummed.  We also note that this does not entail a loss
   of functionality, because TFO-data is always only sent on the initial
   subflow before any attempt to create additional subflows.

B.3.  Connection establishment examples

   The following shows a few examples of possible TFO+MPTCP
   establishment scenarios.

   Before an initiator can send data together with the SYN, it must
   request a cookie to the listener, as shown in Figure Figure 18.  This
   is done by simply combining the TFO and MPTCP options.

initiator                                                       listener
    |                                                              |
    |    S Seq=0(Length=0) <MP_CAPABLE>, <TFO cookie request>      |
    | -----------------------------------------------------------> |
    |                                                              |
    |    S. 0(0) ack 1 <MP_CAPABLE>, <TFO cookie>                  |
    | <----------------------------------------------------------- |
    |                                                              |
    |    .  0(0) ack 1 <MP_CAPABLE>                                |
    | -----------------------------------------------------------> |
    |                                                              |

   Figure 18: Cookie request - sequence number and length are annotated
             as Seq(Length) and used hereafter in the figures.

   Once this is done, the received cookie can be used for TFO, as shown
   in Figure Figure 19.  In this example, the initiator first sends 20
   bytes in the SYN.  The listener immediately replies with 100 bytes
   following the SYN-ACK upon which the initiator replies with 20 more
   bytes.  Note that the last segment in the figure has a TCP sequence
   number of 21, while the DSS subflow sequence number is 1 (because the
   TFO data is not part of the data sequence number space, as explained
   in Section Appendix B.2.










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initiator                                                       listener
    |                                                              |
    |    S  0(20) <MP_CAPABLE>, <TFO cookie>                       |
    | -----------------------------------------------------------> |
    |                                                              |
    |    S. 0(0) ack 21 <MP_CAPABLE>                               |
    | <----------------------------------------------------------- |
    |                                                              |
    |    .  1(100) ack 21 <DSS ack=1 seq=1 ssn=1 dlen=100>         |
    | <----------------------------------------------------------- |
    |                                                              |
    |    .  21(0) ack 1 <MP_CAPABLE>                               |
    | -----------------------------------------------------------> |
    |                                                              |
    |    .  21(20) ack 101 <DSS ack=101 seq=1 ssn=1 dlen=20>       |
    | -----------------------------------------------------------> |
    |                                                              |

                   Figure 19: The listener supports TFO

   In Figure Figure 20, the listener does not support TFO.  The
   initiator detects that no state is created in the listener (as no
   data is acked), and now sends the MP_CAPABLE in the third ack, in
   order for the listener to build its MPTCP context at then end of the
   establishment.  Now, the tfo data, retransmitted, becomes part of the
   data sequence mapping because it is effectively sent (in fact re-
   sent) after the establishment.

initiator                                                       listener
    |                                                              |
    |    S  0(20) <MP_CAPABLE>, <TFO cookie>                       |
    | -----------------------------------------------------------> |
    |                                                              |
    |    S. 0(0) ack 1 <MP_CAPABLE>                                |
    | <----------------------------------------------------------- |
    |                                                              |
    |    .  1(0) ack 1 <MP_CAPABLE>                                |
    | -----------------------------------------------------------> |
    |                                                              |
    |    .  1(20) ack 1 <DSS ack=1 seq=1 ssn=1 dlen=20>            |
    | -----------------------------------------------------------> |
    |                                                              |
    |    .  0(0) ack 21 <DSS ack=21 seq=1 ssn=1 dlen=0>            |
    | <----------------------------------------------------------- |
    |                                                              |

               Figure 20: The listener does not support TFO




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   It is also possible that the listener acknowledges only part of the
   TFO data, as illustrated in Figure Figure 21.  The initiator will
   simply retransmit the missing data together with a DSS-mapping.

initiator                                                       listener
    |                                                              |
    |  S  0(1000) <MP_CAPABLE>, <TFO cookie>                       |
    | -----------------------------------------------------------> |
    |                                                              |
    |  S. 0(0) ack 501 <MP_CAPABLE>                                |
    | <----------------------------------------------------------- |
    |                                                              |
    |    .  501(0) ack 1 <MP_CAPABLE>                              |
    | -----------------------------------------------------------> |
    |                                                              |
    |   .  501(500) ack 1 <DSS ack=1 seq=1 ssn=1 dlen=500>         |
    | -----------------------------------------------------------> |
    |                                                              |

                  Figure 21: Partial data acknowledgement

Appendix C.  Control Blocks

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

   RFC 793 [RFC0793] specifies several state variables.  Whenever
   possible, we reuse the same terminology as RFC 793 to describe the
   state variables that are maintained by MPTCP.

C.1.  MPTCP Control Block

   The MPTCP control block contains the following variable per
   connection.

C.1.1.  Authentication and Metadata

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

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





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

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

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

C.1.2.  Sending Side

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

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

   SND.WND (32 bits with RFC 1323, 16 bits otherwise):  This is the
      sending window.  MPTCP maintains the sending window at the MPTCP
      connection level and the same window is shared by all subflows.
      All subflows use the MPTCP connection level SND.WND to compute the
      SEQ.WND value that is sent in each transmitted segment.

C.1.3.  Receiving Side

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

   RCV.WND (32 bits with RFC 1323, 16 bits otherwise):  This is the
      connection-level receive window, which is the maximum of the
      RCV.WND on all the subflows.

C.2.  TCP Control Blocks

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

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



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

C.2.1.  Sending Side

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

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

C.2.2.  Receiving Side

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

   RCV.WND (32 bits with RFC 1323, 16 bits otherwise):  This is the
      subflow-level receive window that is updated with the window field
      from the segments received on this subflow.

Appendix D.  Finite State Machine

   The diagram in Figure 22 shows the Finite State Machine for
   connection-level closure.  This illustrates how the DATA_FIN
   connection-level signal (indicated as the DFIN flag on a DATA_ACK)
   interacts with subflow-level FINs, and permits "break-before-make"
   handover between subflows.




















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                              +---------+
                              | M_ESTAB |
                              +---------+
                     M_CLOSE    |     |    rcv DATA_FIN
                      -------   |     |    -------
 +---------+       snd DATA_FIN /       \ snd DATA_ACK[DFIN] +---------+
 |  M_FIN  |<-----------------           ------------------->| M_CLOSE |
 | WAIT-1  |---------------------------                      |   WAIT  |
 +---------+               rcv DATA_FIN \                    +---------+
   | rcv DATA_ACK[DFIN]         ------- |                   M_CLOSE |
   | --------------        snd DATA_ACK |                   ------- |
   | CLOSE all subflows                 |              snd DATA_FIN |
   V                                    V                           V
 +-----------+              +-----------+                  +-----------+
 |M_FINWAIT-2|              | M_CLOSING |                  | M_LAST-ACK|
 +-----------+              +-----------+                  +-----------+
   |              rcv DATA_ACK[DFIN] |           rcv DATA_ACK[DFIN] |
   | rcv DATA_FIN     -------------- |               -------------- |
   |  -------     CLOSE all subflows |           CLOSE all subflows |
   | snd DATA_ACK[DFIN]              V            delete MPTCP PCB  V
   \                          +-----------+                  +---------+
     ------------------------>|M_TIME WAIT|----------------->| M_CLOSED|
                              +-----------+                  +---------+
                                         All subflows in CLOSED
                                             ------------
                                         delete MPTCP PCB

          Figure 22: Finite State Machine for Connection Closure

Authors' Addresses

   Alan Ford
   Pexip

   EMail: alan.ford@gmail.com


   Costin Raiciu
   University Politehnica of Bucharest
   Splaiul Independentei 313
   Bucharest
   Romania

   EMail: costin.raiciu@cs.pub.ro







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   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


   Christoph Paasch
   Apple, Inc.
   Cupertino
   US

   EMail: cpaasch@apple.com



























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