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DCCP Extensions for Multipath Operation with Multiple Addresses
draft-ietf-tsvwg-multipath-dccp-16

Document Type Active Internet-Draft (tsvwg WG)
Authors Markus Amend , Anna Brunstrom , Andreas Kassler , Veselin Rakocevic , Stephen Johnson
Last updated 2024-05-13 (Latest revision 2024-05-04)
Replaces draft-amend-tsvwg-multipath-dccp
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Submit " DCCP Extensions for Multipath Operation with Multiple Addresses" as a Proposed Standard RFC
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draft-ietf-tsvwg-multipath-dccp-16
Transport Area Working Group                               M. Amend, Ed.
Internet-Draft                                                        DT
Intended status: Standards Track                            A. Brunstrom
Expires: 5 November 2024                                      A. Kassler
                                                     Karlstad University
                                                            V. Rakocevic
                                              City, University of London
                                                              S. Johnson
                                                                      BT
                                                              4 May 2024

    DCCP Extensions for Multipath Operation with Multiple Addresses
                   draft-ietf-tsvwg-multipath-dccp-16

Abstract

   DCCP communications as defined in RFC 4340 are restricted to a single
   path per connection, yet multiple paths often exist between peers.
   The simultaneous use of available multiple paths for a DCCP session
   could improve resource usage within the network and, thus, improve
   user experience through higher throughput and improved resilience to
   network failures.  Use cases for Multipath DCCP (MP-DCCP) are mobile
   devices (e.g., handsets, vehicles) and residential home gateways
   simultaneously connected to distinct networks as, e.g., a cellular
   and a Wireless Local Area (WLAN) network or a cellular and a fixed
   access network.  Compared to existing multipath protocols such as
   MPTCP, MP-DCCP offers special support for latency-sensitive services
   with different requirements for reliability and in-order delivery.

   This document specifies a set of extensions to DCCP to support
   multipath operations.  The protocol offers the same type of service
   to applications as DCCP and provides the components necessary to
   establish and use multiple DCCP flows across different paths
   simultaneously.

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/.

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

   This Internet-Draft will expire on 5 November 2024.

Copyright Notice

   Copyright (c) 2024 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Multipath DCCP in the Networking Stack  . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Operation Overview  . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  MP-DCCP Concept . . . . . . . . . . . . . . . . . . . . .   6
   3.  MP-DCCP Protocol  . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Multipath Capable Feature . . . . . . . . . . . . . . . .   9
     3.2.  Multipath Option  . . . . . . . . . . . . . . . . . . . .  11
       3.2.1.  MP_CONFIRM  . . . . . . . . . . . . . . . . . . . . .  12
       3.2.2.  MP_JOIN . . . . . . . . . . . . . . . . . . . . . . .  15
       3.2.3.  MP_FAST_CLOSE . . . . . . . . . . . . . . . . . . . .  16
       3.2.4.  MP_KEY  . . . . . . . . . . . . . . . . . . . . . . .  17
       3.2.5.  MP_SEQ  . . . . . . . . . . . . . . . . . . . . . . .  19
       3.2.6.  MP_HMAC . . . . . . . . . . . . . . . . . . . . . . .  19
       3.2.7.  MP_RTT  . . . . . . . . . . . . . . . . . . . . . . .  21
       3.2.8.  MP_ADDADDR  . . . . . . . . . . . . . . . . . . . . .  23
       3.2.9.  MP_REMOVEADDR . . . . . . . . . . . . . . . . . . . .  25
       3.2.10. MP_PRIO . . . . . . . . . . . . . . . . . . . . . . .  27
       3.2.11. MP_CLOSE  . . . . . . . . . . . . . . . . . . . . . .  28
       3.2.12. Experimental Multipath option MP_EXP for private
               use . . . . . . . . . . . . . . . . . . . . . . . . .  30
     3.3.  MP-DCCP Handshaking Procedure . . . . . . . . . . . . . .  30
     3.4.  Address knowledge exchange  . . . . . . . . . . . . . . .  32
       3.4.1.  Advertising a new path (MP_ADDADDR) . . . . . . . . .  32

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       3.4.2.  Removing a path (MP_REMOVEADDR) . . . . . . . . . . .  33
     3.5.  Closing an MP-DCCP connection . . . . . . . . . . . . . .  34
     3.6.  Fallback  . . . . . . . . . . . . . . . . . . . . . . . .  35
     3.7.  State Diagram . . . . . . . . . . . . . . . . . . . . . .  36
     3.8.  Congestion Control Considerations . . . . . . . . . . . .  38
     3.9.  Maximum Packet Size Considerations  . . . . . . . . . . .  38
     3.10. Maximum number of Subflows  . . . . . . . . . . . . . . .  38
     3.11. Path usage strategies . . . . . . . . . . . . . . . . . .  38
       3.11.1.  Path mobility  . . . . . . . . . . . . . . . . . . .  39
       3.11.2.  Concurrent path usage  . . . . . . . . . . . . . . .  39
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  40
   5.  Interactions with Middleboxes . . . . . . . . . . . . . . . .  42
   6.  Implementation  . . . . . . . . . . . . . . . . . . . . . . .  42
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  42
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  42
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  45
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  45
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  46
   Appendix A.  Differences from Multipath TCP . . . . . . . . . . .  50
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  52

1.  Introduction

   Datagram Congestion Control Protocol (DCCP) [RFC4340] is a transport
   protocol that provides bidirectional unicast connections of
   congestion-controlled unreliable datagrams.  DCCP communications are
   restricted to one single path.  This document specifies a set of
   protocol changes that add multipath support to DCCP; specifically,
   support for signaling and setting up multiple paths (a.k.a,
   "subflows"), managing these subflows, reordering of data, and
   termination of sessions.

   Multipath DCCP (MP-DCCP) enables a DCCP connection to simultaneously
   establish a flow across multiple paths.  This can be beneficial to
   applications that transfer large amounts of data, by utilizing the
   capacity/connectivity offered by multiple paths.  In addition, the
   multipath extensions enable to tradeoff timeliness and reliability,
   which is important for low-latency applications that do not require
   guaranteed delivery services, such as Audio/Video streaming.

   In addition to the integration into DCCP services, implementers or
   future specification could choose MP-DCCP for other use cases like
   3GPP 5G multi-access solutions (e.g., Access Traffic Steering,
   Switching, and Splitting (ATSSS) specified in [TS23.501]) or hybrid
   access networks [I-D.lhwxz-hybrid-access-network-architecture][I-D.mu
   ley-network-based-bonding-hybrid-access], in which MP-DCCP can be
   used for load balancing, seamless session handover and bandwidth
   aggregation using a framework such as

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   [I-D.amend-tsvwg-multipath-framework-mpdccp].  More details on
   potential use cases for MP-DCCP are provided in [multipath-dccp.org],
   [IETF115.Slides], and [MP-DCCP.Paper].  All these use cases profit
   from an Open Source Linux reference implementation provided under
   [multipath-dccp.org].

   The encapsulation of non-DCCP traffic (e.g., UDP or IP) in MP-DCCP to
   enable the above-mentioned use cases is not considered in this
   specification; such matters are further detailed in
   [I-D.amend-tsvwg-multipath-framework-mpdccp].  Also out of scope is
   the encapsulation of DCCP traffic in UDP to pass middleboxes that do
   not support DCCP.  A possible method is defined in [RFC6773] or is
   considered in [I-D.amend-tsvwg-dccp-udp-header-conversion] to achieve
   the same with less overhead.

   Similar to MP-DCCP, MP-QUIC is designed to enable the simultaneous
   usage of multiple paths for a single connection
   [I-D.ietf-quic-multipath].  MP-QUIC is based on QUIC in a similar way
   as MP-DCCP is based on DCCP.  MP-QUIC inherits the properties of QUIC
   with its various facets of encryption, multi-streaming and the STREAM
   and DATAGRAM transport characteristic.  This makes a practical
   multipath implementation very complex.  In contrast, MP-DCCP is based
   exclusively on the lean concept of DCCP.  For traffic that is already
   encrypted, MP-DCCP is the more efficient choice as it does not apply
   its own encryption mechanisms.  Also, the procedures defined by MP-
   DCCP, which allow subsequent reordering of traffic, improve
   performance, as shown in [MP-DCCP.Paper], and are not available in
   MP-QUIC.

1.1.  Multipath DCCP in the Networking Stack

   MP-DCCP provides a set of features to DCCP; Figure 1 illustrates this
   layering.  MP-DCCP operates at the transport layer and can be used as
   a transport for both higher and lower layers.  The protocol is
   designed to be used by applications in the same way as DCCP with no
   changes to the application itself.

                                +-------------------------------+
                                |           Application         |
   +---------------+            +-------------------------------+
   |  Application  |            |            MP-DCCP            |
   +---------------+            + - - - - - - - + - - - - - - - +
   |      DCCP     |            |Subflow (DCCP) |Subflow (DCCP) |
   +---------------+            +-------------------------------+
   |      IP       |            |       IP      |      IP       |
   +---------------+            +-------------------------------+

     Figure 1: Comparison of standard DCCP and MP-DCCP protocol stacks

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1.2.  Terminology

   This document uses terms that are either specific for multipath
   transport or are defined in the context of MP-DCCP, similar to
   [RFC8684], 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.

   (MP-DCCP) Connection: A set of one or more subflows, over which an
   application can communicate between two hosts.  The MP-DCCP
   connection is exposed as single DCCP socket to the application.

   Connection Identifier (CI): A locally unique identifier given to a
   multipath connection by a host.

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

   Subflow: A subflow refers to a DCCP flow transmitted using a specific
   path (4-tuple of source and destination address/port pairs) that
   forms one of the multipath flows used by a single connection.

   In addition to these terms, within the framework of MP-DCCP, the
   interpretation of, and effect on, regular single-path DCCP semantics
   is discussed in Section 3.

1.3.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Operation Overview

   DCCP transmits congestion-controlled unreliable datagrams over a
   single path.
   Various congestion control mechanisms have been specified to optimize
   DCCP performance for specific traffic types in terms of profiles
   denoted by a Congestion Control IDentifier (CCID).  However, DCCP
   does not provide built-in support for managing multiple subflows
   within one DCCP connection.

   The extension of DCCP for Multipath-DCCP (MP-DCCP) is described in
   detail in Section 3.

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   At a high level of the MP-DCCP operation, the data stream from a DCCP
   application is split by MP-DCCP operation into one or more subflows
   which can be transmitted via different - also physically isolated -
   paths.  The corresponding control information allows the receiver to
   optionally re-assemble and deliver the received data in the
   originally transmitted order to the recipient application.  This may
   be necessary because DCCP does not guarantee in-order delivery.  The
   details of the transmission scheduling mechanism and optional
   reordering mechanism are up to the sender and receiver, respectively,
   and are outside the scope of this document.

   A Multipath DCCP connection provides a bidirectional connection of
   datagrams between two hosts exchanging data using DCCP.  It does not
   require any change to the applications.  Multipath DCCP enables the
   hosts to use multiple paths with different IP addresses to transport
   the packets of an MP-DCCP connection.  MP-DCCP manages the request,
   set-up, authentication, prioritization, modification, and removal of
   the DCCP subflows on different paths as well as the exchange of
   performance parameters.
   The number of DCCP subflows can vary during the lifetime of a
   Multipath DCCP connection.  The details of the path management
   decisions for when to add or remove subflows are outside the scope of
   this document.

   The Multipath Capability for MP-DCCP is negotiated with a new DCCP
   feature, as specified in Section 3.1.  Once negotiated, all
   subsequent MP-DCCP operations for that connection are signalled with
   a variable length multipath-related option, as described in
   Section 3.  All MP-DCCP operations are signaled by Multipath options
   described in Section 3.2.  Options that require confirmation from the
   remote peer are retransmitted by the sender until confirmed or until
   confirmation is no longer considered relevant.

   The following sections define MP-DCCP behavior in detail.

2.1.  MP-DCCP Concept

   Figure 2 provides a general overview of the MP-DCCP working mode,
   whose main characteristics are summarized in this section.

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              Host A                               Host B
   ------------------------             ------------------------
   Address A1    Address A2             Address B1    Address B2
   ----------    ----------             ----------    ----------
     |             |                      |             |
     |         (DCCP subflow setup)       |             |
     |----------------------------------->|             |
     |<-----------------------------------|             |
     |             |                      |             |
     |             |  (DCCP subflow setup)|             |
     |             |--------------------->|             |
     |             |<---------------------|             |
     | merge individual DCCP subflows to one MP-DCCP connection
     |             |                      |             |

                  Figure 2: Example MP-DCCP usage scenario

   *  An MP-DCCP connection begins with a 4-way handshake, between two
      hosts.  In Figure 2, an MP-DCCP connection is established between
      addresses A1 and B1 on Hosts A and B, respectively.  In the
      handshake, a Multipath Capable feature is used to negotiate
      multipath support for the connection.  Host specific keys are also
      exchanged between Host A and Host B during the handshake.  The
      details of the MP-DCCP handshaking procedure is described in
      Section 3.3.  MP-DCCP does not require both peers to have more
      than one address.

   *  When additional paths and corresponding addresses/ports are
      available, additional DCCP subflows can be created on these paths
      and attached to the existing MP-DCCP connection.  An MP_JOIN
      option is used to connect a new DCCP subflow to an existing MP-
      DCCP connection.  It contains a Connection Identifier during the
      setup of the initial subflow and is exchanged in the 4-way
      handshake for the subflow together with the Multipath Capable
      feature.  The example in Figure 2 illustrates creation of an
      additional DCCP subflow between Address A2 on Host A and Address
      B1 on Host B.  The two subflows continues to provide a single
      connection to the applications at both endpoints.

   *  MP-DCCP identifies multiple paths by the presence of multiple
      addresses/ports at hosts.  Combinations of these multiple
      addresses/ports indicate the additional paths.  In the example,
      other potential paths that could be set up are A1<->B2 and
      A2<->B2.  Although the additional subflow in the example is shown
      as being initiated from A2, an additional subflow could
      alternatively have been initiated from B1 or B2.

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   *  The discovery and setup of additional subflows is achieved through
      a path management method including the logic and details of the
      procedures for adding/removing subflows; this document describes
      measures to allow a host to initiate new subflows and signal
      available addresses between peers.  The definition of a path
      management method is, however, out of scope of this document and
      subject to a corresponding policy and the specifics of the
      implementation.  If a MP-DCCP peer host limits the maximum number
      of paths that can be maintained (e.g., similar to what is
      discussed in Section 3.4 of [RFC8041], the creation of new
      subflows from that peer host needs to be avoided and incoming
      subflow requests terminated.

   *  Through the use of multipath options, MP-DCCP adds connection-
      level sequence numbers and exchange of Round-Trip Time (RTT)
      information to enable optional reordering features.  As a hint for
      scheduling decisions, a multipath option that allows a peer to
      indicate its priorities for what path to use is also defined.

   *  Subflows are terminated in the same way as regular DCCP
      connections, as described in ([RFC4340], Section 8.3).  MP-DCCP
      connections are closed by including an MP_CLOSE option in subflow
      DCCP-CloseReq or DCCP-Close messages.  An MP-DCCP connection may
      also be reset through the use of an MP_FAST_CLOSE option.  Key
      data from the initial handshake is included in the MP_CLOSE and
      MP_FAST_CLOSE to protect from unauthorized shutdown of MP-DCCP
      connections.

3.  MP-DCCP Protocol

   The DCCP protocol feature list (Section 6.4 of [RFC4340]) is updated
   by adding a new Multipath feature with Feature number 10, as shown in
   Table 1.

    +========+===================+============+===============+=======+
    | Number | Meaning           | Rec'n Rule | Initial Value | Req'd |
    +========+===================+============+===============+=======+
    |   10   | Multipath Capable |     SP     |       0       |   N   |
    +--------+-------------------+------------+---------------+-------+

                         Table 1: Multipath feature

   Rec'n Rule:  The reconciliation rule used for the feature.  SP
      indicates the server-priority.

   Initial Value:  The initial value for the feature.  Every feature has
      a known initial value.

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   Req'd:  This column is "Y" if and only if every DCCP implementation
      MUST understand the feature.  If it is "N", then the feature
      behaves like an extension, and it is safe to respond to Change
      options for the feature with empty Confirm options.

   This specification adds a DCCP protocol option as defined in
   ([RFC4340], Section 5.8) providing a new Multipath related variable-
   length option with option type 46, as shown in Table 2.

             +======+===============+===========+============+
             | Type | Option Length |  Meaning  | DCCP-Data? |
             +======+===============+===========+============+
             |  46  |    variable   | Multipath |     Y      |
             +------+---------------+-----------+------------+

                       Table 2: Multipath option set

      Note to the RFC Editor: Please update the Number and Type with the
      value assigned by IANA.

3.1.  Multipath Capable Feature

   A DCCP endpoint negotiates the Multipath Capable Feature to determine
   whether multipath extensions can be enabled for a DCCP connection.

   The Multipath Capable feature (MP_CAPABLE) has feature number 10 and
   follows the structure for features given in [RFC4340] Section 6.
   Beside the negotiation of the feature itself, also one or several
   values can be exchanged.  The value field specified here for the
   Multipath Capable feature has a length of one-byte and can be
   repeated several times within the DCCP option for feature negotiation
   if required for example to announce support of different versions of
   the protocol.  For that, the leftmost four bits in Figure 3 specify
   the compatible version of the MP-DCCP implementation and MUST be set
   to 0 following this specification.  The four bits following the
   Version field are unassigned in version 0 and MUST be set to zero by
   the sender and MUST be ignored by the receiver.

       0  1  2  3   4  5  6  7
      +-----------+------------+
      |  Version  | Unassigned |
      +-----------+------------+

       Figure 3: Format of the Multipath Capable feature value field

   The setting of the MP_CAPABLE feature MUST follow the server-priority
   reconciliation rule described in ([RFC4340], Section 6.3.1).  This
   allows multiple versions to be specified in order of priority.

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   The negotiation MUST be a part of the initial handshake procedure
   described in Section 3.3.  No subsequent re-negotiation of the
   MP_CAPABLE feature is allowed for the same MP-DCCP connection.

   Clients MUST include a Change R option during the initial handshake
   request to supply a list of supported MP-DCCP protocol versions,
   ordered by preference.

   Servers MUST include a Confirm L option in the subsequent response to
   agree on an MP-DCCP version to be used from the Client list, followed
   by its own supported version(s), ordered by preference.  Any subflow
   added to an existing MP-DCCP connection MUST use the version
   negotiated for the first subflow.

   If no agreement is found, the Server MUST reply with an empty Confirm
   L option with feature number 10 and no values.

   An example of successful version negotiation is shown hereafter and
   follows the negotiation example shown in [RFC4340] Section 6.5.  For
   better understanding, this example uses the unspecified MP-DCCP
   versions 1 and 2 in addition to the MP-DCCP version 0 specified in
   this document:

         Client                                             Server
         ------                                             ------
         DCCP-Req + Change R(MP_CAPABLE, 1 0)
                        ----------------------------------->

                         DCCP-Resp + Confirm L(MP_CAPABLE, 1, 2 1 0)
               <-----------------------------------

                    * agreement on version = 1 *

     Figure 4: Example of MP-DCCP support negotiation using MP_CAPABLE

   1.  The Client indicates support for both MP-DCCP versions 1 and 0,
       with a preference for version 1.

   2.  Server agrees on using MP-DCCP version 1 indicated by the first
       value, and supplies its own preference list with the following
       values.

   3.  MP-DCCP is then enabled between the Client and Server with
       version 1.

   Unlike the example in Figure 4, this document only allows the
   negotiation of MP-DCCP version 0, which means that client and server
   must support it.

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   If the version negotiation fails or the MP_CAPABLE feature is not
   present in the DCCP-Request or DCCP-Response packets of the initial
   handshake procedure, the MP-DCCP connection SHOULD fallback to
   regular DCCP or MUST close the connection.  Further details are
   specified in Section 3.6

3.2.  Multipath Option

   MP-DCCP uses one single option to signal various multipath-related
   operations.  The format of this multipath option is shown in
   Figure 5.

               1          2          3
    01234567 89012345 67890123 45678901 23456789
   +--------+--------+--------+--------+--------+
   |00101110| Length | MP_OPT | Value(s) ...
   +--------+--------+--------+--------+--------+
    Type=46

                     Figure 5: Multipath option format

   The fields used by the multipath option are described in Table 3.
   MP_OPT refers to an Multipath option.

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    +======+========+================+================================+
    | Type | Option | MP_OPT         | Meaning                        |
    |      | Length |                |                                |
    +======+========+================+================================+
    | 46   | var    | 0 =MP_CONFIRM  | Confirm reception and          |
    |      |        |                | processing of an MP_OPT option |
    +------+--------+----------------+--------------------------------+
    | 46   | 12     | 1 =MP_JOIN     | Join path to an existing MP-   |
    |      |        |                | DCCP connection                |
    +------+--------+----------------+--------------------------------+
    | 46   | var    | 2              | Close an MP-DCCP connection    |
    |      |        | =MP_FAST_CLOSE | unconditionally                |
    +------+--------+----------------+--------------------------------+
    | 46   | var    | 3 =MP_KEY      | Exchange key material for      |
    |      |        |                | MP_HMAC                        |
    +------+--------+----------------+--------------------------------+
    | 46   | 9      | 4 =MP_SEQ      | Multipath Sequence Number      |
    +------+--------+----------------+--------------------------------+
    | 46   | 23     | 5 =MP_HMAC     | HMA Code for authentication    |
    +------+--------+----------------+--------------------------------+
    | 46   | 12     | 6 =MP_RTT      | Transmit RTT values            |
    +------+--------+----------------+--------------------------------+
    | 46   | var    | 7 =MP_ADDADDR  | Advertise additional Address   |
    +------+--------+----------------+--------------------------------+
    | 46   | 4      | 8              | Remove Address                 |
    |      |        | =MP_REMOVEADDR |                                |
    +------+--------+----------------+--------------------------------+
    | 46   | 4      | 9 =MP_PRIO     | Change subflow Priority        |
    +------+--------+----------------+--------------------------------+
    | 46   | var    | 10 =MP_CLOSE   | Close an MP-DCCP subflow       |
    +------+--------+----------------+--------------------------------+
    | 46   | var    | 11 =MP_EXP     | Experimental for private use   |
    +------+--------+----------------+--------------------------------+
    | 46   | TBD    | >11            | Reserved for future MP         |
    |      |        |                | options.                       |
    +------+--------+----------------+--------------------------------+

                        Table 3: MP_OPT option types

   Future MP options could be defined in a later version or extension to
   this specification.

   These operations are largely inspired by the signals defined in
   [RFC8684].

3.2.1.  MP_CONFIRM

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                 1          2          3           4          5
      01234567 89012345 67890123 45678901 23456789 01234567 89012345
     +--------+--------+--------+--------+--------+--------+--------+
     |00101110|  var   |00000000| List of confirmations ...
     +--------+--------+--------+--------+--------+--------+--------+
      Type=46   Length  MP_OPT=0

                 Figure 6: Format of the MP_CONFIRM option

   Some multipath options require confirmation from the remote peer (see
   Table 4).  Such options will be retransmitted by the sender until an
   MP_CONFIRM is received or confirmation of options is identified
   outdated.  The further processing of the multipath options in the
   receiving host is not the subject of MP_CONFIRM.

   Multipath options could arrive out-of-order, therefore multipath
   options defined in Table 4 MUST be sent in a DCCP datagram with
   MP_SEQ Section 3.2.5.  This allows a receiver to identify whether
   multipath options are associated with obsolete datasets (information
   carried in the option header) that would otherwise conflict with
   newer datasets.  In the case of MP_ADDADDR or MP_REMOVEADDR the same
   dataset is identified based on AddressID, whereas the same dataset
   for MP_PRIO is identified by the subflow in use.  An outdated
   multipath option is detected at the receiver if a previous multipath
   option referring to the same dataset contained a higher sequence
   number in the MP_SEQ.  An MP_CONFIRM MAY be generated for multipath
   options that are identified as outdated.

   Similarly an MP_CONFIRM could arrive out of order.  The associated
   MP_SEQ received MUST be echoed to ensure that the most recent
   multipath option is confirmed.  This protects from inconsistencies
   that could occur, e.g. if three MP_PRIO options are sent one after
   the other on one path in order to first set the path priority to 0,
   then to 1 and finally to 0 again.  Without an associated MP_SEQ, a
   loss of the third MP_PRIO option and a loss of the MP_CONFIRM of the
   second update and the third update would cause the sender to
   incorrectly interpret that the priority value was set to 0 without
   recognizing that the receiver has applied priority value 1.

   The length of the MP_CONFIRM option and the path over which the
   option is sent depend on the confirmed multipath options and the
   received MP_SEQ, which are both copied verbatim and appended as a
   list of confirmations.  The list is structured by first listing the
   received MP_SEQ followed by the related multipath option or options
   to confirm.  The same rules apply when multipath options with
   different MP_SEQs are confirmed at once.  This could happen if a
   datagram with MP_PRIO and a first MP_SEQ_1 and another datagram with
   MP_ADDADDR and a second MP_SEQ_2 are received in short succession.

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   In this case, the structure described above is concatenated resulting
   in MP_SEQ_2 + MP_ADDADDR + MP_SEQ_1 + MP_PRIO.  The order of the
   confirmed multipath options in the list of confirmations MUST reflect
   the incoming order at the host who sends the MP_CONFIRM, with the
   most recent suboption received listed first.  This could allow the
   host receiving the MP_CONFIRM to verify that the options were applied
   in the correct order and to take countermeasures if they were not,
   e.g., if an MP_REMOVEADDR overtakes an MP_ADDADDR that refers to the
   same dataset.

   +======+===============+==================+=========================+
   | Type | Option Length | MP_OPT           | MP_CONFIRM Sending path |
   +======+===============+==================+=========================+
   | 46   | var           | 7 =MP_ADDADDR    | Any available           |
   +------+---------------+------------------+-------------------------+
   | 46   | 4             | 8                | Any available           |
   |      |               | =MP_REMOVEADDR   |                         |
   +------+---------------+------------------+-------------------------+
   | 46   | 4             | 9 =MP_PRIO       | Any available           |
   +------+---------------+------------------+-------------------------+

             Table 4: Multipath options requiring confirmation

   An example to illustrate the MP-DCCP confirm procedure for the
   MP_PRIO option is shown in Figure 7.  The host A sends a DCCP-Request
   on path A2-B2 with an MP_PRIO option with value 1 and associated
   sequence number of 1.  Host B replies on the same path in this
   instance (any path can be used) with a DCCP-Response containing the
   MP_CONFIRM option and a list containing the original sequence number
   (1) together with the associated option (MP_PRIO).

             Host A                                     Host B
   ------------------------                     ------------------------
   Address A1    Address A2                     Address B1    Address B2
   ----------    ----------                     ----------    ----------
        |             |                                   |       |
        |             | DCCP-Request(seqno 1) + MP_PRIO(1)|       |
        |             |------------------------------------------>|
        |             |                                   |       |
        |             | DCCP-Response +                   |       |
        |             |<---- MP_CONFIRM(seqno 1, MP_PRIO) --------|
        |             |                                   |       |

                Figure 7: Example MP-DCCP CONFIRM procedure

   A second example to illustrate the same MP-DCCP confirm procedure but
   where an out of date option is also delivered is shown in (Figure 8.
   Here, the first DCCP-Data is sent from Host A to Host B with option

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   MP_PRIO set to 4.  Host A subsequently sends the second DCCP-Data
   with option MP_PRIO set to 1.  In this case, the delivery of the
   first MP_PRIO is delayed in the network between Host A and Host B and
   arrives after the second MP_PRIO.  Host B ignores this second MP_PRIO
   as the associated sequence number is earlier than the first.  Host B
   sends a DCCP-Ack confirming receipt of the MP_PRIO(1) with sequence
   number 2.

             Host A                                     Host B
   ------------------------                     ------------------------
   Address A1    Address A2                     Address B1    Address B2
   ----------    ----------                     ----------    ----------
        |             |                                   |       |
        |             | DCCP-Data(seqno 1) +  MP_PRIO(4)  |       |
        |             |------------                       |       |
        |             |            \                      |       |
        |             | DCCP-Data(seqno 2) +  MP_PRIO(1)  |       |
        |             |--------------\--------------------------->|
        |             |               \                   |       |
        |             |                -------------------------->|
        |             |                                   |       |
        |             | DCCP-Ack +                        |       |
        |             |<---- MP_CONFIRM(seqno 2, MP_PRIO) --------|
        |             |                                   |       |

    Figure 8: Example MP-DCCP CONFIRM procedure with outdated suboption

3.2.2.  MP_JOIN

                 1          2          3
      01234567 89012345 67890123 45678901
     +--------+--------+--------+--------+
     |00101110|00001100|00000001| Addr ID|
     +--------+--------+--------+--------+
     | Connection Identifier             |
     +--------+--------+--------+--------+
     | Nonce                             |
     +--------+--------+--------+--------+
      Type=46  Length=12 MP_OPT=1

                 Figure 9: Format of the MP_JOIN suboption

   The MP_JOIN option is used to add a new subflow to an existing MP-
   DCCP connection and REQUIRES a successful establishment of the first
   subflow using MP_KEY.  The Connection Identifier (CI) is the one from
   the peer host, which was previously exchanged with the MP_KEY option.
   MP_HMAC MUST be set when using MP_JOIN within a DCCP-Response packet
   (See Section 3.2.6 for details).

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   The MP_JOIN option includes an "Addr ID" (Address ID) generated by
   the sender of the option, used to identify the source address of this
   packet, even if the IP header was changed in transit by a middlebox.
   The value of this field is generated by the sender and MUST map
   uniquely to a source IP address for the sending host.  The Address ID
   allows address removal (Section 3.2.9) without needing to know what
   the 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 3.2.8), to
   prevent setting up duplicate subflows on the same path, if an MP_JOIN
   and MP_ADDADDR are sent at the same time.

   The Address IDs of the subflow used in the initial DCCP Request/
   Response 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.  An
   Address ID always MUST be unique over the lifetime of a subflow and
   can only be re-assigned if sender and receiver no longer have them in
   use.

   The Nonce is a 32-bit random value locally generated for every
   MP_JOIN option.  Together with the CI, the Nonce value builds the
   basis to calculate the HMAC used in the handshaking process as
   described in Section 3.3.

   If the CI cannot be verified by the receiving host during a handshake
   negotiation, the new subflow MUST be closed, as specified in
   Section 3.6.

3.2.3.  MP_FAST_CLOSE

   DCCP can send a Close or Reset signal to abruptly close a connection.
   Using MP-DCCP, a regular Close or Reset only has the scope of the
   subflow over which a signal was received.  As such, it will only
   close the subflow and does not affect other remaining subflows or the
   MP-DCCP connection (unless it is the last subflow).  This permits
   break-before-make handover between subflows.

   In order to provide an MP-DCCP-level "reset" and thus allow the
   abrupt closure of the MP-DCCP connection, the MP_FAST_CLOSE suboption
   can be used.

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                 1          2          3
      01234567 89012345 67890123 45678901 23456789
     +--------+--------+--------+--------+--------+
     |00101110|  var   |00000010| Key Data ...
     +--------+--------+--------+--------+--------+
      Type=46   Length  MP_OPT=2

              Figure 10: Format of the MP_FAST_CLOSE suboption

   When host A wants to abruptly close an MP-DCCP connection with host
   B, it will send out the MP_FAST_CLOSE.  The MP_FAST_CLOSE suboption
   MUST be sent from host A on all subflows using a DCCP-Reset packet
   with Reset Code 13.  The requirement to send the MP_FAST_CLOSE on all
   subflows increases the probability that host B will receive the
   MP_FAST_CLOSE to take the same action.  To protect from unauthorized
   shutdown of a MP-DCCP connection, the selected Key Data of the peer
   host during the handshaking procedure is carried by the MP_FAST_CLOSE
   option.

   After sending the MP_FAST_CLOSE on all subflows, host A will tear
   down all subflows and the multipath DCCP connection immediately
   terminates.

   Upon reception of the first MP_FAST_CLOSE with successfully validated
   Key Data, host B will send a DCCP Reset packet response on all
   subflows to host A with Reset Code 13 to clean potential middlebox
   states.  Host B will then tear down all subflows and terminate the
   MP-DCCP connection.

3.2.4.  MP_KEY

                          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
     +---------------+---------------+---------------+---------------+
     |0 0 1 0 1 1 1 0|      var      |0 0 0 0 0 0 1 1|     resvd     |
     +---------------+---------------+---------------+---------------+
     |                     Connection Identifier                     |
     +---------------+---------------+---------------+---------------+
     |  Key Type (1) |  Key Data (1) |  Key Type (2) |  Key Data (2) |
     +---------------+---------------+---------------+---------------+
     |  Key Type (3) | ...
     +---------------+---------------+
         Type=46          Length         MP_OPT=3

                 Figure 11: Format of the MP_KEY suboption

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   The MP_KEY suboption is used to exchange a Connection Identifier (CI)
   and key material between hosts for a given connection.  The CI is a
   unique number that is configured per host during the initial exchange
   of a connection with MP_KEY and is necessary to connect other DCCP
   subflows to an MP-DCCP connection with MP_JOIN (Section 3.2.2).  Its
   size of 32-bits also defines the maximum number of simultaneous MP-
   DCCP connections in a host to 2^32.  According to the Key related
   elements of the MP_KEY suboption, the Length varies between 17 and 73
   Bytes for a single-key message, and up to 115 Bytes when all
   specified Key Types 0-2 are provided.  The Key Type field specifies
   the type of the following key data.  The set of key types are shown
   in Table 5.

    +========================+====================+===================+
    | Key Type               | Key Length (Bytes) | Meaning           |
    +========================+====================+===================+
    | 0 =Plain Text          | 8                  | Plain Text Key    |
    +------------------------+--------------------+-------------------+
    | 1 =ECDHE-C25519-SHA256 | 32                 | ECDHE with SHA256 |
    |                        |                    | and Curve25519    |
    +------------------------+--------------------+-------------------+
    | 2 =ECDHE-C25519-SHA512 | 64                 | ECDHE with SHA512 |
    |                        |                    | and Curve25519    |
    +------------------------+--------------------+-------------------+
    | 3-255                  |                    | Unassigned        |
    +------------------------+--------------------+-------------------+

                         Table 5: MP_KEY key types

   Plain Text
      Key Material is exchanged in plain text between hosts, and the key
      parts (key-a, key-b) are used by each host to generate the derived
      key (d-key) by concatenating the two parts with the local key in
      front (e.g. hostA d-key(A)=(key-a+key-b), hostB d-key(B)=(key-
      b+key-a)).

   ECDHE-SHA256-C25519
      Public Key Material is exchanged via ECDHE key exchange with
      SHA256 and Curve 25519 to generate the derived key (d-key) from
      the shared secret.  The full potential of ECDHE use is realized
      when it is combined with peer authentication technologies to
      protect against men-in-the-middle attacks.  This can be achieved,
      for example, with separate use and verification of certificates
      issued by a certificate authority.

   ECDHE-SHA512-C25519

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      Public Key Material is exchanged via ECDHE key exchange with
      SHA512 and Curve 25519 to generate the derived key (d-key) from
      the shared secret.

   Multiple keys are only permitted in the DCCP-Request message of the
   handshake procedure for the first subflow.  This allows the hosts to
   agree on a single key type to be used, as described in Section 3.3

   It is possible that not all hosts will have all key types.  If the
   key type cannot be agreed in the handshake procedure, the MP-DCCP
   connection MUST fall back to not using MP-DCCP, as indicated in
   Section 3.6.

3.2.5.  MP_SEQ

                 1          2          3           4          5
      01234567 89012345 67890123 45678901 23456789 01234567 89012345
     +--------+--------+--------+--------+--------+--------+--------+
     |00101110|00001001|00000100| Multipath Sequence Number
     +--------+--------+--------+--------+--------+--------+--------+
                       |
     +--------+--------+
      Type=46  Length=9 MP_OPT=4

                 Figure 12: Format of the MP_SEQ suboption

   The MP_SEQ suboption is used for end-to-end 48-bit datagram-based
   sequence numbers of an MP-DCCP connection.  The initial data sequence
   number (IDSN) SHOULD be set randomly [RFC4086].

   The MP_SEQ number space is independent from the path individual
   sequence number space and MUST be sent with all DCCP-Data and DCCP-
   DataACK packets.

   When the sequence number space is exhausted, the sequence number MUST
   be wrapped.  [RFC7323] provides guidance on selecting an
   appropriately sized sequence number space according to the maximum
   segment lifetime of TCP. 64 bits is the recommended size for TCP to
   avoid the sequence number space going through within the segment
   lifetime.  For DCCP, the Maximum Segment Lifetime is the same as that
   of TCP as specified in Section 3.4 of [RFC4340].  Compared to TCP,
   the sequence number for DCCP is incremented per packet rather than
   per byte transmitted.  For this reason, the 48 bits chosen in MP_SEQ
   are considered sufficiently large.

3.2.6.  MP_HMAC

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                 1          2          3           4
      01234567 89012345 67890123 45678901 23456789 01234567
     +--------+--------+--------+--------+--------+--------+
     |00101110|00010111|00000101| HMAC-SHA256 (20 bytes) ...
     +--------+--------+--------+--------+--------+--------+
      Type=46  Length=23 MP_OPT=5

                 Figure 13: Format of the MP_HMAC suboption

   The MP_HMAC suboption is used to provide authentication for the
   MP_ADDADDR, and MP_REMOVEADDR suboptions.  In addition, it provides
   authentication for subflows joining an existing MP_DCCP connection,
   as described in the second and third step of the handshake of a
   subsequent subflow in Section 3.3.  For this specification of MP-
   DCCP, the HMAC code is generated according to [RFC2104] in
   combination with the SHA256 hash algorithm described in [RFC6234],
   with the output truncated to the leftmost 160 bits (20 bytes).

   The "Key" used for the HMAC computation is the derived key (d-key)
   described in Section 3.2.4, while the HMAC "Message" for MP_JOIN,
   MP_ADDADDR and MP_REMOVEADDR is a concatenation of:

   *  for MP_JOIN: The nonces of the MP_JOIN messages for which
      authentication shall be performed.  Depending on whether Host A or
      Host B performs the HMAC-SHA256 calculation, it is carried out as
      follows: MP_HMAC(A) = HMAC-SHA256(Key=d-key(A), Msg=RA+RB)
      MP_HMAC(B) = HMAC-SHA256(Key=d-key(B), Msg=RB+RA) An usage example
      is shown in Figure 21.

   *  for MP_ADDADDR: The Address ID with associated IP address and if
      defined port, otherwise two octets of value 0.  IP address and
      port MUST be used in network byte order (NBO).  Depending on
      whether Host A or Host B performs the HMAC-SHA256 calculation, it
      is carried out as follows: MP_HMAC(A) = HMAC-SHA256(Key=d-key(A),
      Msg=Address ID+NBO(IP)+NBO(Port)) MP_HMAC(B) = HMAC-
      SHA256(Key=d-key(B), Msg=Address ID+NBO(IP)+NBO(Port))

   *  for MP_REMOVEADDR: Solely the Address ID.  Depending on whether
      Host A or Host B performs the HMAC-SHA256 calculation, it is
      carried out as follows: MP_HMAC(A) = HMAC-SHA256(Key=d-key(A),
      Msg=Address ID) MP_HMAC(B) = HMAC-SHA256(Key=d-key(B), Msg=Address
      ID)

   MP_JOIN, MP_ADDADDR and MP_REMOVEADDR can co-exist or be used
   multiple times within a single DCCP packet.  All these multipath
   options require an individual MP_HMAC option.  This ensures that the
   MP_HMAC is correctly associated.  Otherwise, the receiver cannot
   validate multiple MP_JOIN, MP_ADDADDR or MP_REMOVEADDR.  Therefore, a

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   MP_HMAC MUST directly follow its associated multipath option.  In the
   likely case of sending a MP_JOIN together with a MP_ADDADDR, this
   results in concatenating MP_JOIN + MP_HMAC_1 + MP_ADDADDR +
   MP_HMAC_2, whereas the first MP_HMAC_1 is associated with the MP_JOIN
   and the second MP_HMAC_2 is associated with the MP_ADDADDR suboption.

   On the receiver side, the HMAC validation of the suboptions MUST be
   carried out according to the sending sequence in which the associated
   MP_HMAC follows a suboption.  If the suboption cannot be validated by
   a receiving host because the HMAC validation fails, the subsequent
   handling depends on which suboption was being verified.  If the
   suboption to be authenticated was either MP_ADDADDR or MP_REMOVEADDR,
   the receiving host MUST silently ignore it (see Section 3.2.8 and
   Section 3.2.9).  If the suboption to be authenticated was MP_JOIN,
   the subflow MUST be closed (see Section 3.6).  In the event that an
   MP_HMAC cannot be associated with a suboption, unless it is an
   MP_HMAC sent in DCCP-Ack in response to a DCCP-Response packet
   containing an MP_JOIN option, this MP_HMAC MUST be ignored.

3.2.7.  MP_RTT

                 1          2          3           4          5
      01234567 89012345 67890123 45678901 23456789 01234567 89012345
     +--------+--------+--------+--------+--------+--------+--------+
     |00101110|00001100|00000110|RTT Type| RTT
     +--------+--------+--------+--------+--------+--------+--------+
              | Age                               |
     +--------+--------+--------+--------+--------+
      Type=46  Length=12 MP_OPT=6

                 Figure 14: Format of the MP_RTT suboption

   The MP_RTT suboption is used to transmit RTT values and age
   (represented in milliseconds) that belong to the path over which this
   information is transmitted.  This information is useful for the
   receiving host to calculate the RTT difference between the subflows
   and to estimate whether missing data has been lost.

   The RTT and Age information is a 32-bit integer.  This covers a
   period of approximately 1193 hours.

   The Field RTT type indicates the type of RTT estimation, according to
   the following description:

   Raw RTT (=0)
      Raw RTT value of the last Datagram Round-Trip

   Min RTT (=1)

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      Min RTT value over a given period

   Max RTT (=2)
      Max RTT value over a given period

   Smooth RTT (=3)
      Averaged RTT value over a given period

   Each CCID specifies the algorithms and period applied for their
   corresponding RTT estimations.The availability of the above described
   types, to be used in the MP_RTT option, depends on the CCID
   implementation in place.

   Age
      The Age parameter defines the time difference between now -
      creation of the MP_RTT option - and the conducted RTT measurement
      in milliseconds.  If no previous measurement exists, e.g., when
      initialized, the value is 0.

   An example of a flow showing the exchange of path individual RTT
   information is provided in Figure 15.  RTT1 refers to the first path
   and RTT2 to the second path.  The RTT values could be extracted from
   the sender's Congestion Control procedure and are conveyed to the
   receiving host using the MP_RTT suboption.  With the reception of
   RTT1 and RTT2, the receiver is able to calculate the path_delta which
   corresponds to the absolute difference of both values.  In the case
   that the path individual RTTs are symmetric in the down- and uplink
   directions and there is no jitter, packets with missing sequence
   number MP_SEQ, e.g., in a reordering process, can be assumed lost
   after path_delta/2.

      MP-DCCP                   MP-DCCP
      Sender                    Receiver
      +--------+  MP_RTT(RTT1)  +-------------+
      |   RTT1 |----------------|             |
      |        |                | path_delta= |
      |        |  MP_RTT(RTT2)  | |RTT1-RTT2| |
      |   RTT2 |----------------|             |
      +--------+                +-------------+

           Figure 15: Exemplary flow of MP_RTT exchange and usage

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3.2.8.  MP_ADDADDR

   The MP_ADDADDR suboption announces additional addresses (and,
   optionally, port numbers) by which a host can be reached.  This can
   be sent at any time during an existing MP-DCCP connection, when the
   sender wishes to enable multiple paths and/or when additional paths
   become available.  Multiple instances of this suboption within a
   packet can simultaneously advertise new addresses.

   The Length is variable depending on the address family (IPv4 or IPv6)
   and whether a port number is used.  This field is in range between 8
   and 22 bytes.

   The final 2 octets, optionally specify the DCCP port number to use,
   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 could be cases (such as port-based load balancing)
   where the explicit specification of a different port is required.  If
   no port is specified, the receiving host MUST assume that any attempt
   to connect to the specified address uses the port already used by the
   subflow on which the MP_ADDADDR signal was sent.

   Along with the MP_ADDADDR option an MP_HMAC option MUST be sent for
   authentication.  The truncated HMAC parameter present in this MP_HMAC
   option is the leftmost 20 bytes of an HMAC, negotiated and calculated
   as described in Section 3.2.6.  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 and
   selected in the original MP_KEY handshake.  The message for the HMAC
   is the Address ID, IP address, and port number that precede the HMAC
   in the MP_ADDADDR option.  If the port number is not present in the
   MP_ADDADDR option, the HMAC message will include 2 octets of value
   zero.  The rationale for the HMAC is to prevent unauthorized entities
   from injecting MP_ADDADDR signals in an attempt to hijack a
   connection.  Note that, additionally, the presence of this HMAC
   prevents the address from being changed in flight unless the key is
   known by an intermediary.  If a host receives an MP_ADDADDR option
   for which it cannot validate the HMAC, it SHOULD silently ignore the
   option.

   The presence of an MP_SEQ Section 3.2.5 MUST be ensured in a DCCP
   datagram in which MP_ADDADDR is sent, as described in Section 3.2.1.

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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
     +---------------+---------------+-------+-------+---------------+
     |0 0 1 0 1 1 1 0|      var      |0 0 0 0 0 1 1 1|  Address ID   |
     +---------------+---------------+-------+-------+---------------+
     |          Address (IPv4 - 4 bytes / IPv6 - 16 bytes)           |
     +-------------------------------+-------------------------------+
     |   Port (2 bytes, optional)    | + MP_HMAC option
     +-------------------------------+
          Type=46         Length         MP_OPT=7

               Figure 16: Format of the MP_ADDADDR suboption

   Each address has an Address ID that could be used for uniquely
   identifying the address within a connection for address removal.
   Each host maintains a list of unique Address IDs and it manages these
   as it wishes.  The Address ID is also used to identify MP_JOIN
   options (see Section 3.2.2) 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); the mechanism for allocating such IDs is
   implementation specific.

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

   A host MAY advertise private addresses, e.g., because there is a NAT
   on the path.  It is desirable to allow this, since there could be
   cases where both hosts have additional interfaces on the same private
   network.

   The MP_JOIN handshake to create a new subflow (Section 3.2.2)
   provides mechanisms to minimize security risks.  The MP_JOIN message
   contains a 32-bit CI that uniquely identifies a connection to the
   receiving host.  If the CI is unknown, the host MUST send a DCCP-
   Reset.

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   Further security considerations around the issue of MP_ADDADDR
   messages that accidentally misdirect, or maliciously direct, new
   MP_JOIN attempts are discussed in Section 4.  If a sending host of an
   MP_ADDADDR knows that no incoming subflows can be established at a
   particular address, an MP_ADDADDR SHOULD NOT announce that address
   unless the sending host has new knowledge about the possibility to do
   so.  This information can be obtained from local firewall or routing
   settings, knowledge about availability of external NAT or firewall,
   or from connectivity checks performed by the host/application.

   The reception of an MP_ADDADDR message is acknowledged using
   MP_CONFIRM (Section 3.2.1).  This ensures reliable exchange of
   address information.

   A host MAY send an MP_ADDADDR message with an already assigned
   Address ID, but the Address MUST be the same as previously assigned
   to this Address ID, and the Port MUST be different from one already
   in use for this Address ID.  If these conditions are not met, the
   receiver SHOULD silently ignore the MP_ADDADDR.  A host wishing to
   replace an existing Address ID MUST first remove the existing one
   (Section 3.2.9).

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

3.2.9.  MP_REMOVEADDR

   If, during the lifetime of an MP-DCCP connection, a previously
   announced address becomes invalid (e.g., if an interface disappears),
   the affected host SHOULD announce this.  The peer can remove a
   previously added address with an Address ID from a connection using
   the Remove Address (MP_REMOVEADDR) suboption.  This will terminate
   any subflows currently using that address.

   Along with the MP_REMOVEADDR suboption a MP_HMAC option MUST be sent
   for authentication.  The truncated HMAC parameter present in this
   MP_HMAC option is the leftmost 20 bytes of an HMAC, negotiated and
   calculated as described in Section 3.2.6.  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 and selected in the original MP_KEY handshake.  The
   message for the HMAC is the Address ID.

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   The rationale for using a HMAC is to prevent unauthorized entities
   from injecting MP_REMOVEADDR signals in an attempt to hijack a
   connection.  Note that, additionally, the presence of this HMAC
   prevents the address from being modified in flight unless the key is
   known by an intermediary.  If a host receives an MP_REMOVEADDR option
   for which it cannot validate the HMAC, it SHOULD silently ignore the
   option.

   A receiver MUST include a MP_SEQ Section 3.2.5 in a DCCP datagram
   that sends an MP_REMOVEADDR.  Further details are given in
   Section 3.2.1.

   The reception of an MP_REMOVEADDR message is acknowledged using
   MP_CONFIRM (Section 3.2.1).  This ensures reliable exchange of
   address information.  To avoid inconsistent states, the sender
   releases the address ID only after MP_REMOVEADDR has been confirmed.

   The sending and receiving of this message SHOULD trigger the closing
   procedure described in [RFC4340] between the client and the server,
   respectively on the affected subflow(s) (if possible).  This helps
   remove middlebox state, before removing any local state.

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

                        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
   +---------------+---------------+---------------+---------------+
   |0 0 1 0 1 1 1 0|0 0 0 0 0 1 0 0|0 0 0 0 1 0 0 0|   Address ID  |
   +---------------+---------------+---------------+---------------+
        Type=46        Length=4         MP_OPT=8

   -> followed by MP_HMAC option

              Figure 17: Format of the MP_REMOVEADDR suboption

   A subflow that is still functioning MUST be closed with a DCCP-Close
   exchange as in regular DCCP, rather than using this option.  For more
   information, see Section 3.5.

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3.2.10.  MP_PRIO

   The path priority SHOULD be considered as hints for the packet
   scheduler when making decisions which path to use for payload
   traffic.  When a single specific path from the set of available paths
   is treated with higher priority compared to the others when making
   scheduling decisions for payload traffic, a host can signal such
   change in priority to the peer.  This could be used when there are
   different costs for using different paths (e.g., WiFi is free while
   cellular has limit on volume, 5G has higher energy consumption).  The
   priority of a path could also change, for example, when a mobile host
   runs out of battery, the usage of only a single path may be the
   preferred choice of the user.

   The MP_PRIO suboption, shown below, can be used to set a priority
   flag for the subflow over which the suboption is received.

                           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
      +---------------+---------------+---------------+--------------+
      |0 0 1 0 1 1 1 0|0 0 0 0 0 1 0 0|0 0 0 0 1 0 0 1|(resvd)| prio |
      +---------------+---------------+---------------+--------------+
          Type=46         Length=4        MP_OPT=9

                 Figure 18: Format of the MP_PRIO suboption

   The following values are available for the Prio field:

   *  0: Do not use.  The path is not available.

   *  1: Standby: do not use this path for traffic scheduling, if
      another path (secondary or primary) is available.  The path will
      only be used if other secondary or primary paths are not
      established.

   *  2: Secondary: do not use this path for traffic scheduling, if the
      other paths are good enough.  The path will be used occasionally
      for increasing temporarily the available capacity, e.g. when
      primary paths are congested or are not available.  This is the
      recommended setting for paths that have costs or data caps as
      these paths will be used less frequently then primary paths.

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   *  3 - 15: Primary: The path can be used for packet scheduling
      decisions.  The priority number indicates the relative priority of
      one path over the other for primary paths.  Higher numbers
      indicate higher priority.  The peer should consider sending
      traffic first over higher priority paths.  This is the recommended
      setting for paths that do not have a cost or data caps associated
      with them as these paths will be frequently used.

   Example use cases include:

   1) Setting Wi-Fi path to Primary and Cellular paths to Secondary.  In
   this case Wi-Fi will be used and Cellular will be used only if the
   Wi-Fi path is congested or not available.  Such setting results in
   using the Cellular path only temporally, if more capacity is needed
   than the WiFi path can provide, indicating a clear priority of the
   Wi-Fi path over the Cellular due to, e.g., cost reasons. 2) Setting
   Wi-Fi path to Primary and Cellular to Standby.  In this case Wi-Fi
   will be used and Cellular will be used only if the Wi-Fi path is not
   available.  3) Setting Wi-Fi path to Primary and Cellular path to
   Primary.  In this case, both paths can be used when making packet
   scheduling decisions.

   If not specified, the default behavior is to always use a path for
   packet scheduling decisions (MP_PRIO=3), when the path has been
   established and added to an existing MP-DCCP connection.  At least
   one path ought to have a MP_PRIO value greater or equal to one for it
   to be allowed to send on the connection.  It is RECOMMENDED to update
   at least one path to a non-zero MP_PRIO value when an MP-DCCP
   connection enters a state where all paths remain with an MP_PRIO
   value of zero.  This helps an MP-DCCP connection to schedule when the
   multipath scheduler strictly respects MP_PRIO value 0.  MP_PRIO is
   assumed to be exchanged reliably using the MP_CONFIRM mechanisms (see
   Table 4).

   The relative ratio of the primary path values 3-15 depends on the
   path usage strategy, which is described in more detail in
   Section 3.11.  In the case of path mobility (Section 3.11.1), only
   one path can be used at a time and MUST be the appropriate one that
   has the highest available priority value including also the prio
   numbers 1 and 2.  In the other case of concurrent path usage
   (Section 3.11.2), the definition is up to the multipath scheduler
   logic.

   A MP_SEQ (Section 3.2.5) MUST be present in a DCCP datagram in which
   MP_PRIO is sent.  Further details are given in Section 3.2.1.

3.2.11.  MP_CLOSE

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                 1          2          3
      01234567 89012345 67890123 45678901 23456789
     +--------+--------+--------+--------+--------+
     |00101110|  var   |00001010| Key Data ...
     +--------+--------+--------+--------+--------+
      Type=46   Length  MP_OPT=10

                Figure 19: Format of the MP_CLOSE suboption

   An MP-DCCP connection can be gracefully closed by sending and
   MP_CLOSE to the peer host.  On all subflows, the regular termination
   procedure as described in [RFC4340] MUST be initiated using MP_CLOSE
   in the initial packet (either a DCCP-CloseReq or a DCCP-Close).  When
   a DCCP-CloseReq is used, the following DCCP-Close MUST also carry the
   MP_CLOSE to avoid keeping a state in the sender of the DCCP-CloseReq.
   At the initiator of the DCCP-CloseReq, all sockets including the MP-
   DCCP connection socket, transition to CLOSEREQ state.  To protect
   from unauthorized shutdown of a multi-path connection, the selected
   Key Data of the peer host during the handshaking procedure MUST be
   included in by the MP_CLOSE option and must be validated by the peer
   host.  Note, the Key Data is different between MP_CLOSE option
   carried by DCCP-CloseReq or DCCP-Close.

   On reception of the first DCCP-CloseReq carrying a MP_CLOSE with
   valid Key Data, or due to a local decision, all subflows transition
   to the CLOSING state before transmitting a DCCP-Close carrying
   MP_CLOSE.  The MP-DCCP connection socket on the host sending the
   DCCP-Close reflects the state of the initial subflow during handshake
   with MP_KEY option.  If the initial subflow no longer exists, the
   state moves immediately to CLOSED.

   Upon reception of the first DCCP-Close carrying a MP_CLOSE with valid
   Key Data at the peer host, all subflows, as well as the MP-DCCP
   connection socket, move to the CLOSED state.  After this, a DCCP-
   Reset with Reset Code 1 MUST be sent on any subflow in response to a
   received DCCP-Close containing a valid MP_CLOSE option.

   When the MP-DCCP connection socket is in CLOSEREQ or CLOSE state, new
   subflow requests using MP_JOIN MUST be ignored.

   Contrary to a MP_FAST_CLOSE (Section 3.2.3), no single-sided abrupt
   termination is applied.

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3.2.12.  Experimental Multipath option MP_EXP for private use

   This section reserves a Multipath option to define and specify any
   experimental additional feature for improving and optimization of the
   MP-DCCP protocol.  This option could be applicable to specific
   environments or scenarios according to potential new requirements and
   is meant for private use only.  MP_OPT feature number 11 is specified
   with an exemplary description as below:

                        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
   +---------------+---------------+---------------+---------------+
   |0 0 1 0 1 1 1 0|      var      |0 0 0 0 1 0 1 1|   Data TBD    |
   +---------------+---------------+---------------+---------------+
   |   ...
   +---------------------------------------------------------------+
        Type=46         Length         MP_OPT=11

                 Figure 20: Format of the MP_EXP suboption

   The Data field can carry any data according to the foreseen use by
   the experimenters with a maximum length of 252 Bytes.

3.3.  MP-DCCP Handshaking Procedure

   An example to illustrate the MP-DCCP handshake procedure is shown in
   Figure 21.

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             Host A                                         Host B
   ------------------------                              ----------
   Address A1    Address A2                              Address B1
   ----------    ----------                              ----------
        |             |                                       |
        |           DCCP-Request + Change R (MP_CAPABLE,...)  |
        |---- MP_KEY(CI-A + Key-A(1), Key-A(2),...) --------->|
        |<------------------- MP_KEY(CI-B + Key-B) -----------|
        |       DCCP-Response +  Confirm L (MP_CAPABLE, ...)  |
        |             |                                       |
        |   DCCP-Ack  |                                       |
        |---------------------------------------------------->|
        |<----------------------------------------------------|
        |   DCCP-Ack  |                                       |
        |             |                                       |
        |             |DCCP-Request + Change R(MP_CAPABLE,...)|
        |             |--- MP_JOIN(CI-B,RA) ----------------->|
        |             |<------MP_JOIN(CI-A,RB) + MP_HMAC(B)---|
        |             |DCCP-Response+Confirm L(MP_CAPABLE,...)|
        |             |                                       |
        |             |DCCP-Ack                               |
        |             |-------- MP_HMAC(A) ------------------>|
        |             |<--------------------------------------|
        |             |DCCP-ACK                               |

                    Figure 21: Example MP-DCCP handshake

   The basic initial handshake for the first subflow is as follows:

   *  Host A sends a DCCP-Request with the MP-Capable feature Change
      request and the MP_KEY option with a Host-specific CI-A and a
      Key-A for each of the supported key types as described in
      Section 3.2.4.  CI-A is a unique identifier during the lifetime of
      a MP-DCCP connection.

   *  Host B sends a DCCP-Response with Confirm feature for MP-Capable
      and the MP_Key option with a unique Host-specific CI-B and a
      single Host-specific Key-B.  The type of the key is chosen from
      the list of supported types from the previous request.

   *  Host A sends a DCCP-Ack to confirm the proper key exchange.

   *  Host B sends a DCCP-Ack to complete the handshake and set both
      connection ends to the OPEN state.

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   It should be noted that DCCP is protected against corruption of DCCP
   header data (section 9 of [RFC4340]), so no additional mechanisms
   beyond the general confirmation are required to ensure that the
   header data has been properly received.

   Host A waits for the final DCCP-Ack from host B before starting any
   establishment of additional subflow connections.

   The handshake for subsequent subflows based on a successful initial
   handshake is as follows:

   *  Host A sends a DCCP-Request with the MP-Capable feature Change
      request and the MP_JOIN option with Host B's CI-B, obtained during
      the initial handshake.  Additionally, an own random nonce RA is
      transmitted with the MP_JOIN.

   *  Host B computes the HMAC of the DCCP-Request and sends a DCCP-
      Response with Confirm feature option for MP-Capable and the
      MP_JOIN option with the CI-A and a random nonce RB together with
      the computed MP_HMAC.  The HMAC is calculated by taking the
      leftmost 20 bytes from the SHA256 hash of a HMAC code created by
      using the nonce received with MP_JOIN(A) and the local nonce RB as
      message and the derived key described in Section 3.2.4 as key:

      MP_HMAC(B) = HMAC-SHA256(Key=d-key(B), Msg=RB+RA)

   *  Host A sends a DCCP-Ack with the HMAC computed for the DCCP-
      Response.  The HMAC is calculated by taking the leftmost 20 bytes
      from the SHA256 hash of a HMAC code created by using the local
      nonce RA and the nonce received with MP_JOIN(B) as message and the
      derived key described in Section 3.2.4 as key:

      MP_HMAC(A) = HMAC-SHA256(Key=d-key(A), Msg=RA+RB)

   *  Host B sends a DCCP-Ack to confirm the HMAC and to conclude the
      handshaking.

3.4.  Address knowledge exchange

3.4.1.  Advertising a new path (MP_ADDADDR)

   When a host (Host A) wants to advertise the availability of a new
   path, it should use the MP_ADDADDR option (Section 3.2.8) as shown in
   the example in Figure 22.  The MP_ADDADDR option passed in the DCCP-
   Data contains the following parameters:

   *  an identifier (id 2) for the new IP address which is used as a
      reference in subsequent control exchanges.

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   *  the IP address of the new path (A2_IP)

   *  A pair of octets specifying the port number associated with this
      IP address.  The value of 00 here indicates that the port number
      is the same as that used for the initial subflow address A1_IP

   The following options MUST be included in a packet carrying
   MP_ADDADDR:

   *  the leftmost 20 bytes of the HMAC(A) generated during the initial
      handshaking procedure described in Section 3.3 and Section 3.2.6

   *  the MP_SEQ option with the sequence number (seqno 12) for this
      message according to Section 3.2.5.

   Host B acknowledges receipt of the MP_ADDADDR message with a DCCP-Ack
   containing the MP_CONFIRM option.  The parameters supplied in this
   response are as follows:

   *  an MP_CONFIRM containing the MP_SEQ number (seqno 12) of the
      packet carrying the option that we are confirming together with
      the MP_ADDADDR option

   *  the leftmost 20 bytes of the HMAC(B) generated during the initial
      handshaking procedure Section 3.3

             Host A                                         Host B
   ------------------------                              -----------
   Address A1    Address A2                               Address B1
   ----------    ----------                              -----------
        |             |                                       |
        |   DCCP-Data +  MP_ADDADDR(id 2, A2_IP, 00) +        |
        |------- MP_HMAC(A) + MP_SEQ(seqno 12) -------------->|
        |             |                                       |
        |   DCCP-Ack + MP_HMAC(B) +                           |
        |<----- MP_CONFIRM(seqno 12, MP_ADDADDR) -------------|

                Figure 22: Example MP-DCCP ADDADDR procedure

3.4.2.  Removing a path (MP_REMOVEADDR)

   When a host (Host A) wants to indicate that a path is no longer
   available, it should use the MP_REMOVEADDR option (Section 3.2.9) as
   shown in the example in Figure 23.  The MP_REMOVEADDR option passed
   in the DCCP-Data contains the following parameters:

   *  an identifier (id 2) for the IP address to remove (A2_IP) and
      which was specified in a previous MP_ADDADDR message.

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   The following options MUST be included in a packet carrying
   MP_REMOVEADDR:

   *  the leftmost 20 bytes of the HMAC(A) generated during the initial
      handshaking procedure described in Section 3.3 and Section 3.2.6

   *  the MP_SEQ option with the sequence number (seqno 33) for this
      message according to Section 3.2.5.

   Host B acknowledges receipt of the MP_REMOVEADDR message with a DCCP-
   Ack containing the MP_CONFIRM option.  The parameters supplied in
   this response are as follows:

   *  an MP_CONFIRM containing the MP_SEQ number (seqno 33) of the
      packet carrying the option that we are confirming, together with
      the MP_REMOVEADDR option

   *  the leftmost 20 bytes of the HMAC(B) generated during the initial
      handshaking procedure Section 3.3

             Host A                                         Host B
   ------------------------                              -----------
   Address A1    Address A2                               Address B1
   ----------    ----------                              -----------
        |             |                                       |
        |   DCCP-Data +  MP_REMOVEADDR(id 2) +                |
        |------- MP_HMAC(A) + MP_SEQ(seqno 33) -------------->|
        |             |                                       |
        |   DCCP-Ack + MP_HMAC(B) +                           |
        |<----- MP_CONFIRM(seqno 33, MP_REMOVEADDR) ----------|

              Figure 23: Example MP-DCCP REMOVEADDR procedure

3.5.  Closing an MP-DCCP connection

   When a host wants to close an existing subflow but not the whole MP-
   DCCP connection, it MUST initiate the regular DCCP connection
   termination procedure as described in Section 5.6 of [RFC4340], i.e.,
   it sends a DCCP-Close/DCCP-Reset on the subflow.  This may be
   preceded by a DCCP-CloseReq.  In the event of an irregular
   termination of a subflow, e.g., during subflow establishment, it MUST
   use an appropriate DCCP reset code as specified in IANA
   [DCCP.Parameter] for DCCP operations.  This could be, for example,
   sending reset code 5 (Option Error) when an MP-DCCP option provides
   invalid data or reset code 9 (Too Busy) when the maximum number of
   maintainable paths is reached.  Note that receiving a reset code 9
   for secondary subflows SHOULD NOT impact already existing active
   subflows.  If necessary, these subflows are terminated in a

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   subsequent step using the procedures described in this section.

   A host terminates an MP-DCCP connection using the DCCP connection
   termination specified in section 5.5 of [RFC4340] on each subflow
   with the first packet on each subflow carrying MP_CLOSE (see
   Section 3.2.11).

     Host A                                   Host B
     ------                                   ------
                                      <-      Optional DCCP-CloseReq +
                                              MP_CLOSE [A's key]
                                              [on all subflows]
     DCCP-Close + MP_CLOSE            ->
     [B's key] [on all subflows]
                                      <-      DCCP-Reset
                                              [on all subflows]

   Additionally, an MP-DCCP connection may be closed abruptly using the
   "Fast Close" procedure described in Section 3.2.3, where a DCCP-Reset
   is sent on all subflows, each carrying the MP_FAST_CLOSE option.

     Host A                                   Host B
     ------                                   ------
     DCCP-Reset + MP_FAST_CLOSE       ->
     [B's key] [on all subflows]
                                      <-      DCCP-Reset
                                              [on all subflows]

3.6.  Fallback

   When a subflow fails to operate following MP-DCCP intended behavior,
   it is necessary to proceed with a fallback.  This may be either
   falling back to regular DCCP [RFC4340] or removing a problematic
   subflow.  The main reasons for subflow failing include: no MP support
   at peer host, failure to negotiate protocol version, loss of
   Multipath options, faulty/non-supported MP-DCCP options or
   modification of payload data.

   At the start of an MP-DCCP connection, the handshake ensures exchange
   of MP-DCCP feature and options and thus ensures that the path is
   fully MP-DCCP capable.  If during the handshake procedure it appears
   that DCCP-Request or DCCP-Response messages do not carry the
   MP_CAPABLE feature, the MP-DCCP connection will not be established
   and the handshake SHOULD fallback to regular DCCP (if this is not
   possible it MUST be closed).

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   A connection SHOULD fallback to regular DCCP if the endpoints fail to
   agree on a protocol version to use during the Multipath Capable
   feature negotiation.  This is described in Section 3.1.  The protocol
   version negotiation distinguishes between negotiation for the initial
   connection establishment, and addition of subsequent subflows.  If
   protocol version negotiation is not successful during the initial
   connection establishment, MP-DCCP connection will fallback to regular
   DCCP.

   The fallback procedure to regular DCCP MUST be also applied if the
   MP_KEY Section 3.2.4 Key Type cannot be negotiated.

   If a subflow attempts to join an existing MP-DCCP connection, but MP-
   DCCP options or MP_CAPABLE feature are not present or are faulty in
   the handshake procedure, that subflow MUST be closed.  This is
   especially the case if a different MP_CAPABLE version than the
   originally negotiated version is used.  Reception of a non-verifiable
   MP_HMAC (Section 3.2.6) or an invalid CI used in MP_JOIN
   (Section 3.2.2) during flow establishment MUST cause the subflow to
   be closed.

   The subflow closing procedure MUST be also applied if a final ACK
   carrying MP_KEY with wrong Key-A/Key-B is received or MP_KEY option
   is malformed.

   Another relevant case is when payload data is modified by
   middleboxes.  DCCP uses checksum to protect the data, as described in
   section 9 of [RFC4340].  A checksum will fail if the data has been
   changed in any way.  All data from the start of the segment that
   failed the checksum onwards cannot be considered trustworthy.  DCCP
   defines that if the checksum fails, the receiving endpoint MUST drop
   the application data and report that data as dropped due to
   corruption using a Data Dropped option (Drop Code 3, Corrupt).  If
   data is dropped due to corruption for an MP-DCCP connection, the
   affected subflow MAY be closed.

3.7.  State Diagram

   The MP-DCCP per subflow state transitions to a large extent follow
   the state transitions defined for DCCP in [RFC4340], with some
   modifications due to the MP-DCCP four-way handshake and fast close
   procedures.  The state diagram below illustrates the most common
   state transitions.  The diagram is illustrative.  For example, there
   are arcs (not shown) from several additional states to TIMEWAIT,
   contingent on the receipt of a valid DCCP-Reset.

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   The states transitioned when moving from the CLOSED to OPEN state
   during the four-way handshake remain the same as for DCCP, but it is
   no longer possible to transmit application data while in the REQUEST
   state.  The fast close procedure can be triggered by either the
   client or the server and results in the transmission of a Reset
   packet.  The fast close procedure moves the state of the client and
   server directly to TIMEWAIT and CLOSED, respectively.

     +----------------------------+    +------------------------------+
     |                            v    v                              |
     |                         +----------+                           |
     |           +-------------+  CLOSED  +-------------+             |
     |           | passive     +----------+   active    |             |
     |           |  open                       open     |             |
     |           |                          snd Request |             |
     |           v                                      v             |
     |     +-----------+                           +----------+       |
     |     |  LISTEN   |                           | REQUEST  |       |
     |     +-----+-----+                           +----+-----+       |
     |           | rcv Request             rcv Response |             |
     |           | snd Response              snd Ack    |             |
     |           v                                      v             |
     |     +-----------+                           +----------+       |
     |     |  RESPOND  |                           | PARTOPEN |       |
     |     +-----+-----+                           +----+-----+       |
     |           | rcv Ack             rcv Ack/DataAck  |             |
     |           | snd Ack                              |             |
     |           |             +-----------+            |             |
     |           +------------>|   OPEN    |<-----------+             |
     |                         +--+-+-+-+--+                          |
     |        server active close | | | |   active close              |
     |            snd CloseReq    | | | | or rcv CloseReq             |
     |                            | | | |    snd Close                |
     |                            | | | |                             |
     |     +-----------+          | | | |            +----------+     |
     |     | CLOSEREQ  |<---------+ | | +----------->| CLOSING  |     |
     |     +-----+-----+            | |              +----+-----+     |
     |           | rcv Close        | |         rcv Reset |           |
     |           | snd Reset        | |                   |           |
     |           |                  | | active FastClose  |           |
     |<----------+        rcv Close | | or rcv FastClose  v           |
     |   or server active FastClose | | snd Reset    +----+-----+     |
     |      or server rcv FastClose | +------------->| TIMEWAIT |     |
     |                    snd Reset |                +----+-----+     |
     +------------------------------+                     |           |
                                                          +-----------+
                                                      2MSL timer expires

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      Figure 24: Most common state transitions of a MP-DCCP subflow

3.8.  Congestion Control Considerations

   Senders MUST manage per-path congestion status, and avoid to sending
   more data on a given path than congestion control for each path
   allows.

3.9.  Maximum Packet Size Considerations

   A DCCP implementation maintains the maximum packet size (MPS) during
   operation of a DCCP session.  This procedure is specified for single-
   path DCCP in [RFC4340], Section 14.  Without any restrictions, this
   is adopted for MP-DCCP operations, in particular the PMTU measurement
   and the Sender Behaviour.  The DCCP application interface SHOULD
   allow the application to discover the current MPS.  This reflects the
   current supported largest size for the data stream that can be used
   across the set of all active MP-DCCP subflows.

3.10.  Maximum number of Subflows

   In theory, an infinite number of subflows can be created within an
   MP-DCCP connection, as there is no element in the protocol that
   represents a restriction.  In practical scenarios, however, there
   will be resource limitations on the host or use cases that do not
   benefit from additional subflows.

   It is RECOMMENDED to limit the number of subflows in implementations
   and to reject incoming subflow requests with a DCCP-Reset using the
   Reset Code "too busy" as specified in [RFC4340] if this limit is
   exceeded.

3.11.  Path usage strategies

   MP-DCCP can be configured to realize one of several strategies for
   path usage, via selecting one DCCP subflow of the multiple DCCP
   subflows within a MP-DCCP connection for data transmission.  This can
   be a dynamic process further facilitated by the means of DCCP and MP-
   DCCP defined options such as path preference using MP-PRIO, adding or
   removing DCCP subflows using MP_REMOVEADDR, MP_ADDADDR or DCCP-Close/
   DCCP-Reset and also path metrics such as packet-loss-rate, CWND or
   RTT provided by the Congestion Control Algorithm.  Selecting an
   appropriate method can allow MP-DCCP to realize different path
   utilization strategies that make MP-DCCP suitable for end-to-end
   implementation over the Internet or in controlled environments such
   as Hybrid Access or 5G ATSSS.

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3.11.1.  Path mobility

   The path mobility strategy provides the use of a single path with a
   seamless handover function to continue the connection when the
   currently used path is deemed unsuitable for service delivery.  Some
   of the DCCP subflows of a MP-DCCP connection might become inactive
   due to either the occurrence of certain error conditions (e.g., DCCP
   timeout, packet loss threshold, RTT threshold, closed/removed) or
   adjustments from the MP-DCCP user.  When there is outbound data to
   send and the primary path becomes inactive (e.g., due to failures) or
   de-prioritized, the MP-DCCP endpoint SHOULD try to send the data
   through an alternate path with a different source or destination
   address (depending on the point of failure), if one exists.  This
   process SHOULD respect the path priority configured by MP_PRIO or if
   not available pick the most divergent source-destination pair from
   the original used source-destination pair.

      Note: Rules for picking the most appropriate source-destination
      pair are an implementation decision and are not specified within
      this document.  Path mobility is supported in the current Linux
      reference implementation [multipath-dccp.org].

3.11.2.  Concurrent path usage

   Different to a path mobility strategy, the selection between MP-DCCP
   subflows is a per-packet decision that is a part of the multipath
   scheduling process.  This method would allow multiple subflows to be
   simultaneously used to aggregate the path resources to obtain higher
   connection throughput.

   In this scenario, the selection of congestion control, per-packet
   scheduling and potential re-ordering method determines a concurrent
   path utilization strategy and result in a particular transport
   characteristic.  A concurrent path usage method uses a scheduling
   design that could seek to maximize reliability, throughput,
   minimizing latency, etc.

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   Concurrent path usage over the Internet can have implications.  When
   a Multipath DCCP connection uses two or more paths, there is no
   guarantee that these paths are fully disjoint.  When two (or more)
   subflows share the same bottleneck, using a standard congestion
   control scheme could result in an unfair distribution of the capacity
   with the multipath connection using more capacity than competing
   single path connections.
   Multipath TCP uses the coupled congestion control Linked Increases
   Algorithm (LIA) specified in the experimental specification [RFC6356]
   to solve this problem.  This scheme could also be specified for
   Multipath DCCP.  The same applies to other coupled congestion control
   schemes that have been proposed for Multipath TCP such as
   Opportunistic Linked Increases Algorithm [OLIA].

   The specification of scheduling for concurrent multipath and related
   the congestion control algorithms and re-ordering methods for use in
   the general Internet are outside the scope of this document.  If, and
   when, the IETF specifies a method for concurrent usage of multiple
   paths for the general Internet, the framework specified in this
   document could be used to provide an IETF recommended method for MP-
   DCCP.

4.  Security Considerations

   Similar to DCCP, MP-DCCP does not provide cryptographic security
   guarantees inherently.  Thus, if applications need cryptographic
   security (integrity, authentication, confidentiality, access control,
   and anti-replay protection) the use of IPsec, DTLS over DCCP
   [RFC5238] or other end-to-end security is recommended; Secure Real-
   time Transport Protocol (SRTP) [RFC3711] is one candidate protocol
   for authentication.  Together with Encryption of Header Extensions in
   SRTP, as provided by [RFC6904], also integrity would be provided.

   DCCP [RFC4340] provides protection against hijacking and limits the
   potential impact of some denial-of-service attacks, but DCCP provides
   no inherent protection against an on-path attacker snooping on data
   packets.  Regarding the security of MP-DCCP no additional risks
   should be introduced compared to regular DCCP.  Thereof derived are
   the following key security requirements to be fulfilled by MP-DCCP:

   *  Provide a mechanism to confirm that parties involved in a subflow
      handshake are identical to those in the original connection setup.

   *  Provide verification that the new address to be included in a MP
      connection is valid for a peer to receive traffic at before using
      it.

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

   To achieve these goals, MP-DCCP includes a hash-based handshake
   algorithm documented in Sections Section 3.2.4, Section 3.2.6 and
   Section 3.3.  The security of the MP-DCCP connection depends on the
   use of keys that are shared once at the start of the first subflow
   and are never sent again over the network.  To ease demultiplexing
   while not revealing cryptographic material, subsequent subflows use
   the initially exchanged CI information.  The keys exchanged once at
   the beginning 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 of subsequent
   subflows 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 that the HMAC will never be the
   same on two handshakes.  Guidance on generating random numbers
   suitable for use as keys is given in [RFC4086].  During normal
   operation, regular DCCP protection mechanisms (such as header
   checksum to protect DCCP headers against corruption) is designed to
   provide the same level of protection against attacks on individual
   DCCP subflows as exists for regular DCCP.

   As discussed in Section 3.2.8, a host may advertise its private
   addresses, but these might point to different hosts in the receiver's
   network.  The MP_JOIN handshake (Section 3.2.2) is designed to ensure
   that this does not set up a subflow to the incorrect host.  However,
   it could still create unwanted DCCP handshake traffic.  This feature
   of MP-DCCP could be a target for denial-of-service exploits, with
   malicious participants in MP-DCCP connections encouraging the
   recipient to target other hosts in the network.  Therefore,
   implementations should consider heuristics at both the sender and
   receiver to reduce the impact of this.

   As described in Section 3.9, a Maximum Packet Size (MPS) is
   maintained for a MP-DCCP connection.  If MP-DCCP exposes a minimum
   MPS across all paths, any change to one path impacts the sender for
   all paths.  To mitigate attacks that seek to force a low MPS, MP-DCCP
   could detect an attempt to reduce the MPS less than a minimum MPS,
   and then stop using these paths.

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5.  Interactions with Middleboxes

   Issues from interaction with on-path middleboxes such as NATs,
   firewalls, proxies, intrusion detection systems (IDSs), and others
   have to be considered for all extensions to standard protocols since
   otherwise unexpected reactions of middleboxes may hinder its
   deployment.  DCCP already provides means to mitigate the potential
   impact of middleboxes, also in comparison to TCP (see [RFC4043],
   Section 16).  When both hosts are located behind a NAT or firewall
   entity, specific measures have to be applied such as the [RFC5596]
   specified simultaneous-open technique that update the (traditionally
   asymmetric) connection-establishment procedures for DCCP.  Further
   standardized technologies addressing middleboxes operating as NATs
   are provided in [RFC5597].

   [RFC6773] specifies UDP Encapsulation for NAT Traversal of DCCP
   sessions, similar to other UDP encapsulations such as for SCTP
   [RFC6951].  Future specifications by the IETF could specify other
   methods for DCCP encapsulation.

   The security impact of MP-DCCP aware middleboxes is discussed in
   Section 4

6.  Implementation

   The approach described above has been implemented in open source
   across different testbeds and a new scheduling algorithm has been
   extensively tested.  Also demonstrations of a laboratory setup have
   been executed and have been published at [multipath-dccp.org].

7.  Acknowledgments

   [RFC8684] defines Multipath TCP and provided important inputs for
   this specification.

   The authors gratefully acknowledge significant input into this
   document from Dirk von Hugo, Nathalie Romo Moreno, Omar Nassef,
   Mohamed Boucadair, Simone Ferlin, Olivier Bonaventure, Gorry
   Fairhurst and Behcet Sarikaya.

8.  IANA Considerations

   This section provides guidance to the Internet Assigned Numbers
   Authority (IANA) regarding registration of values related to the MP
   extension of the DCCP protocol in accordance with [RFC8126].  This
   document defines one new value which is requested to be allocated in
   the IANA DCCP Feature Numbers registry and three new registries to be
   allocated in the DCCP registry group.

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   This document requests IANA to assign a new DCCP feature parameter
   for negotiating the support of multipath capability for DCCP sessions
   between hosts as described in Section 3.  The following entry in
   Table 6 should be added to the Feature Numbers registry in the DCCP
   registry group according to [RFC4340], Section 19.4. under the "DCCP
   Protocol" heading.

           +==============+===================+================+
           |    Value     |    Feature Name   | Specification  |
           +==============+===================+================+
           | 10 suggested | Multipath Capable | [ThisDocument] |
           +--------------+-------------------+----------------+

             Table 6: Addition to DCCP Feature Numbers registry

      Note to RFC Editor: Please replace [ThisDocument] with a reference
      to the final RFC

   Sect.  Section 3.1 specifies the new 1-Byte entry above includes a
   4-bit part to specify the version of the used MP-DCCP implementation.
   This document requests IANA to create a new 'MP-DCCP Versions'
   registry within the DCCP registry group to track the MP-DCCP version.
   The initial content of this registry is as follows:

             +============+================+================+
             |  Version   |     Value      | Specification  |
             +============+================+================+
             |     0      | 0000 suggested | [ThisDocument] |
             +------------+----------------+----------------+
             | Unassigned |  0001 - 1111   |                |
             +------------+----------------+----------------+

                    Table 7: MP-DCCP Versions Registry

      Note to RFC Editor: Please replace [ThisDocument] with a reference
      to the final RFC

   Future MP-DCCP versions 1 to 15 are assigned from this registry using
   the Specification Required policy (Section 4.6 of [RFC8126]).

   This document requests IANA to assign value 46 in the DCCP "Option
   Types" registry to "Multipath Options", as described in Section 3.2.

   IANA is requested to create a new 'Multipath Options' registry within
   the DCCP registry group.  The following entries in Table 8 should be
   added to the new 'Multipath Options' registry.  The registry in
   Table 8 has an upper boundary of 255 in the numeric value field.

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      +===========+===============+=====================+===========+
      | Multipath |      Name     |     Description     | Reference |
      |   Option  |               |                     |           |
      +===========+===============+=====================+===========+
      |  MP_OPT=0 |   MP_CONFIRM  |  Confirm reception/ |  Section  |
      |           |               |   processing of an  |   3.2.1   |
      |           |               |    MP_OPT option    |           |
      +-----------+---------------+---------------------+-----------+
      |  MP_OPT=1 |    MP_JOIN    |   Join subflow to   |  Section  |
      |           |               |   existing MP-DCCP  |   3.2.2   |
      |           |               |      connection     |           |
      +-----------+---------------+---------------------+-----------+
      |  MP_OPT=2 | MP_FAST_CLOSE |    Close MP-DCCP    |  Section  |
      |           |               |      connection     |   3.2.3   |
      +-----------+---------------+---------------------+-----------+
      |  MP_OPT=3 |     MP_KEY    |     Exchange key    |  Section  |
      |           |               |     material for    |   3.2.4   |
      |           |               |       MP_HMAC       |           |
      +-----------+---------------+---------------------+-----------+
      |  MP_OPT=4 |     MP_SEQ    |  Multipath sequence |  Section  |
      |           |               |        number       |   3.2.5   |
      +-----------+---------------+---------------------+-----------+
      |  MP_OPT=5 |    MP_HMAC    |  Hash-based message |  Section  |
      |           |               |  auth. code for MP- |   3.2.6   |
      |           |               |         DCCP        |           |
      +-----------+---------------+---------------------+-----------+
      |  MP_OPT=6 |     MP_RTT    | Transmit RTT values |  Section  |
      |           |               |   and calculation   |   3.2.7   |
      |           |               |      parameters     |           |
      +-----------+---------------+---------------------+-----------+
      |  MP_OPT=7 |   MP_ADDADDR  |      Advertise      |  Section  |
      |           |               |      additional     |   3.2.8   |
      |           |               | address(es)/port(s) |           |
      +-----------+---------------+---------------------+-----------+
      |  MP_OPT=8 | MP_REMOVEADDR | Remove address(es)/ |  Section  |
      |           |               |       port(s)       |   3.2.9   |
      +-----------+---------------+---------------------+-----------+
      |  MP_OPT=9 |    MP_PRIO    |    Change subflow   |  Section  |
      |           |               |       priority      |   3.2.10  |
      +-----------+---------------+---------------------+-----------+
      | MP_OPT=10 |    MP_CLOSE   |    Close MP-DCCP    |  Section  |
      |           |               |       subflow       |   3.2.11  |
      +-----------+---------------+---------------------+-----------+
      | MP_OPT=11 |     MP_EXP    |     Experimental    |  Section  |
      |           |               |    suboption for    |   3.2.12  |
      |           |               |     private use     |           |
      +-----------+---------------+---------------------+-----------+
      | MP_OPT>11 |   Unassigned  | Reserved for future |           |

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      |           |               |  Multipath Options  |           |
      +-----------+---------------+---------------------+-----------+

                    Table 8: Multipath Options registry

   Future Multipath options with MP_OPT>11 are assigned from this
   registry using the Specification Required policy (Section 4.6 of
   [RFC8126]).

   In addition IANA is requested to assign a new DCCP Reset Code value
   13 suggested in the DCCP Reset Codes Registry, with the short
   description "Abrupt MP termination".  Use of this reset code is
   defined in section Section 3.2.3.

   In addition IANA is requested to assign for this version of the MP-
   DCCP protocol a new 'Multipath Key Type' registry containing three
   different suboptions to the MP_KEY option to identify the MP_KEY Key
   types in terms of 8-bit values as specified in Section 3.2.4
   according to the entries in Table 9 below.  Values in range 3-255
   (decimal) inclusive remain unassigned in this here specified version
   0 of the protocol and are assigned via Specification Required
   [RFC8126] in potential future versions of the MP-DCCP protocol.

    +=======+=====================+===================+===============+
    |  Type |         Name        |      Meaning      | Reference     |
    +=======+=====================+===================+===============+
    |   0   |      Plain Text     |   Plain text key  | Section 3.2.4 |
    +-------+---------------------+-------------------+---------------+
    |   1   | ECDHE-C25519-SHA256 | ECDHE with SHA256 | Section 3.2.4 |
    |       |                     |   and Curve25519  |               |
    +-------+---------------------+-------------------+---------------+
    |   2   | ECDHE-C25519-SHA512 | ECDHE with SHA512 | Section 3.2.4 |
    |       |                     |   and Curve25519  |               |
    +-------+---------------------+-------------------+---------------+
    | 3-255 |      Unassigned     |    Reserved for   | Section 3.2.4 |
    |       |                     |     future use    |               |
    +-------+---------------------+-------------------+---------------+

       Table 9: Multipath Key Type registry with the MP_KEY Key Types
                  for key data exchange on different paths

9.  References

9.1.  Normative References

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   [DCCP.Parameter]
              "IANA Datagram Congestion Control Protocol (DCCP)
              Parameters", n.d., <https://www.iana.org/assignments/dccp-
              parameters/dccp-parameters.xhtml>.

   [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/rfc/rfc2119>.

   [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/rfc/rfc4086>.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,
              <https://www.rfc-editor.org/rfc/rfc4340>.

   [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/rfc/rfc6234>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [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/rfc/rfc8174>.

9.2.  Informative References

   [I-D.amend-iccrg-multipath-reordering]
              Amend, M. and D. Von Hugo, "Multipath sequence
              maintenance", Work in Progress, Internet-Draft, draft-
              amend-iccrg-multipath-reordering-03, 25 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-amend-iccrg-
              multipath-reordering-03>.

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   [I-D.amend-tsvwg-dccp-udp-header-conversion]
              Amend, M., Brunstrom, A., Kassler, A., and V. Rakocevic,
              "Lossless and overhead free DCCP - UDP header conversion
              (U-DCCP)", Work in Progress, Internet-Draft, draft-amend-
              tsvwg-dccp-udp-header-conversion-01, 8 July 2019,
              <https://datatracker.ietf.org/doc/html/draft-amend-tsvwg-
              dccp-udp-header-conversion-01>.

   [I-D.amend-tsvwg-multipath-framework-mpdccp]
              Amend, M., Bogenfeld, E., Brunstrom, A., Kassler, A., and
              V. Rakocevic, "A multipath framework for UDP traffic over
              heterogeneous access networks", Work in Progress,
              Internet-Draft, draft-amend-tsvwg-multipath-framework-
              mpdccp-01, 8 July 2019,
              <https://datatracker.ietf.org/doc/html/draft-amend-tsvwg-
              multipath-framework-mpdccp-01>.

   [I-D.ietf-quic-multipath]
              Liu, Y., Ma, Y., De Coninck, Q., Bonaventure, O., Huitema,
              C., and M. Kuehlewind, "Multipath Extension for QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-
              multipath-07, 10 April 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              multipath-07>.

   [I-D.lhwxz-hybrid-access-network-architecture]
              Leymann, N., Heidemann, C., Cullen, M., Xue, L., and M.
              Zhang, "Hybrid Access Network Architecture", Work in
              Progress, Internet-Draft, draft-lhwxz-hybrid-access-
              network-architecture-02, 13 January 2015,
              <https://datatracker.ietf.org/doc/html/draft-lhwxz-hybrid-
              access-network-architecture-02>.

   [I-D.muley-network-based-bonding-hybrid-access]
              Muley, P., Henderickx, W., Geng, L., Liu, H., Cardullo,
              L., Newton, J., Seo, S., Draznin, S., and B. Patil,
              "Network based Bonding solution for Hybrid Access", Work
              in Progress, Internet-Draft, draft-muley-network-based-
              bonding-hybrid-access-03, 22 October 2018,
              <https://datatracker.ietf.org/doc/html/draft-muley-
              network-based-bonding-hybrid-access-03>.

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   [IETF115.Slides]
              Amend, M., "MP-DCCP for enabling transfer of UDP/IP
              traffic over multiple data paths in multi-connectivity
              networks", IETF105 , n.d.,
              <https://datatracker.ietf.org/meeting/105/materials/
              slides-105-tsvwg-sessa-62-dccp-extensions-for-multipath-
              operation-00>.

   [MP-DCCP.Paper]
              Amend, M., Bogenfeld, E., Cvjetkovic, M., Rakocevic, V.,
              Pieska, M., Kassler, A., and A. Brunstrom, "A Framework
              for Multiaccess Support for Unreliable Internet Traffic
              using Multipath DCCP", DOI 10.1109/LCN44214.2019.8990746,
              October 2019,
              <https://doi.org/10.1109/LCN44214.2019.8990746>.

   [multipath-dccp.org]
              "Multipath extension for DCCP", n.d.,
              <https://multipath-dccp.org/>.

   [OLIA]     Khalili, R., Gast, N., Popovic, M., Upadhyay, U., and J.
              Le Boudec, "MPTCP is not pareto-optimal: performance
              issues and a possible solution", Proceedings of the 8th
              international conference on Emerging networking
              experiments and technologies, ACM , 2012.

   [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/rfc/rfc2104>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/rfc/rfc3711>.

   [RFC4043]  Pinkas, D. and T. Gindin, "Internet X.509 Public Key
              Infrastructure Permanent Identifier", RFC 4043,
              DOI 10.17487/RFC4043, May 2005,
              <https://www.rfc-editor.org/rfc/rfc4043>.

   [RFC5238]  Phelan, T., "Datagram Transport Layer Security (DTLS) over
              the Datagram Congestion Control Protocol (DCCP)",
              RFC 5238, DOI 10.17487/RFC5238, May 2008,
              <https://www.rfc-editor.org/rfc/rfc5238>.

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   [RFC5596]  Fairhurst, G., "Datagram Congestion Control Protocol
              (DCCP) Simultaneous-Open Technique to Facilitate NAT/
              Middlebox Traversal", RFC 5596, DOI 10.17487/RFC5596,
              September 2009, <https://www.rfc-editor.org/rfc/rfc5596>.

   [RFC5597]  Denis-Courmont, R., "Network Address Translation (NAT)
              Behavioral Requirements for the Datagram Congestion
              Control Protocol", BCP 150, RFC 5597,
              DOI 10.17487/RFC5597, September 2009,
              <https://www.rfc-editor.org/rfc/rfc5597>.

   [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/rfc/rfc6356>.

   [RFC6773]  Phelan, T., Fairhurst, G., and C. Perkins, "DCCP-UDP: A
              Datagram Congestion Control Protocol UDP Encapsulation for
              NAT Traversal", RFC 6773, DOI 10.17487/RFC6773, November
              2012, <https://www.rfc-editor.org/rfc/rfc6773>.

   [RFC6904]  Lennox, J., "Encryption of Header Extensions in the Secure
              Real-time Transport Protocol (SRTP)", RFC 6904,
              DOI 10.17487/RFC6904, April 2013,
              <https://www.rfc-editor.org/rfc/rfc6904>.

   [RFC6951]  Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
              Control Transmission Protocol (SCTP) Packets for End-Host
              to End-Host Communication", RFC 6951,
              DOI 10.17487/RFC6951, May 2013,
              <https://www.rfc-editor.org/rfc/rfc6951>.

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,
              <https://www.rfc-editor.org/rfc/rfc7323>.

   [RFC8041]  Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and
              Operational Experience with Multipath TCP", RFC 8041,
              DOI 10.17487/RFC8041, January 2017,
              <https://www.rfc-editor.org/rfc/rfc8041>.

   [RFC8684]  Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
              Paasch, "TCP Extensions for Multipath Operation with
              Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
              2020, <https://www.rfc-editor.org/rfc/rfc8684>.

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   [RFC9293]  Eddy, W., Ed., "Transmission Control Protocol (TCP)",
              STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9293>.

   [TS23.501] 3GPP, "System architecture for the 5G System; Stage 2;
              Release 16", December 2020,
              <https://www.3gpp.org/ftp//Specs/
              archive/23_series/23.501/23501-g70.zip>.

Appendix A.  Differences from Multipath TCP

   This appendix is Informative.

   Multipath DCCP is similar to Multipath TCP [RFC8684], in that it
   extends the related basic DCCP transport protocol [RFC4340] with
   multipath capabilities in the same way as Multipath TCP extends TCP
   [RFC9293].  However, because of the differences between the
   underlying TCP and DCCP protocols, the transport characteristics of
   MPTCP and MP-DCCP are different.

   Table 10 compares the protocol characteristics of TCP and DCCP, which
   are by nature inherited by their respective multipath extensions.  A
   major difference lies in the delivery of payload, which is for TCP an
   exact copy of the generated byte-stream.  DCCP behaves in a different
   way and does not guarantee to deliver any payload nor the order of
   delivery.  Since this is mainly affecting the receiving endpoint of a
   TCP or DCCP communication, many similarities on the sender side can
   be identified.  Both transport protocols share the 3-way initiation
   of a communication and both employ congestion control to adapt the
   sending rate to the path characteristics.

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    +=======================+=================+======================+
    |        Feature        |       TCP       |         DCCP         |
    +=======================+=================+======================+
    |      Full-Duplex      |       yes       |         yes          |
    +-----------------------+-----------------+----------------------+
    |  Connection-Oriented  |       yes       |         yes          |
    +-----------------------+-----------------+----------------------+
    |  Header option space  |     40 bytes    | < 1008 bytes or PMTU |
    +-----------------------+-----------------+----------------------+
    |     Data transfer     |     reliable    |      unreliable      |
    +-----------------------+-----------------+----------------------+
    |  Packet-loss handling | re-transmission |     report only      |
    +-----------------------+-----------------+----------------------+
    | Ordered data delivery |       yes       |          no          |
    +-----------------------+-----------------+----------------------+
    |    Sequence numbers   |   one per byte  |     one per PDU      |
    +-----------------------+-----------------+----------------------+
    |      Flow control     |       yes       |          no          |
    +-----------------------+-----------------+----------------------+
    |   Congestion control  |       yes       |         yes          |
    +-----------------------+-----------------+----------------------+
    |      ECN support      |       yes       |         yes          |
    +-----------------------+-----------------+----------------------+
    |     Selective ACK     |       yes       |      depends on      |
    |                       |                 |  congestion control  |
    +-----------------------+-----------------+----------------------+
    |      Fix message      |        no       |         yes          |
    |       boundaries      |                 |                      |
    +-----------------------+-----------------+----------------------+
    |   Path MTU discovery  |       yes       |         yes          |
    +-----------------------+-----------------+----------------------+
    |     Fragmentation     |       yes       |          no          |
    +-----------------------+-----------------+----------------------+
    |  SYN flood protection |       yes       |          no          |
    +-----------------------+-----------------+----------------------+
    | Half-open connections |       yes       |          no          |
    +-----------------------+-----------------+----------------------+

                Table 10: TCP and DCCP protocol comparison

   Consequently, the multipath features, shown in Table 11, are the
   same, supporting volatile paths having varying capacity and latency,
   session handover and path aggregation capabilities.  All of them
   profit by the existence of congestion control.

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          +==================+=======================+==========+
          |     Feature      |         MPTCP         | MP-DCCP  |
          +==================+=======================+==========+
          |  Volatile paths  |          yes          |   yes    |
          +------------------+-----------------------+----------+
          | Session handover |          yes          |   yes    |
          +------------------+-----------------------+----------+
          | Path aggregation |          yes          |   yes    |
          +------------------+-----------------------+----------+
          | Data reordering  |          yes          | optional |
          +------------------+-----------------------+----------+
          |  Expandability   | limited by TCP header | flexible |
          +------------------+-----------------------+----------+

              Table 11: MPTCP and MP-DCCP protocol comparison

   Therefore, the sender logic is not much different between MP-DCCP and
   MPTCP.

   The receiver side for MP-DCCP has to deal with the unreliable
   delivery provided by DCCP.  The multipath sequence numbers included
   in MP-DCCP (see Section 3.2.5) facilitates adding optional mechanisms
   for data stream packet reordering at the receiver.  Information from
   the MP_RTT multipath option (Section 3.2.7), DCCP path sequencing and
   the DCCP Timestamp Option provide further means for advanced
   reordering approaches, e.g., as proposed in
   [I-D.amend-iccrg-multipath-reordering].  Such mechanisms do, however,
   not affect interoperability and are not part of the MP-DCCP protocol.
   Many applications that use unreliable transport protocols can also
   inherently process out-of-sequence data (e.g., through adaptive audio
   and video buffers), and so additional reordering support might not be
   necessary.  The addition of optional reordering mechanisms are likely
   to be needed when the different DCCP subflows are routed across paths
   with different latencies.  In theory, applications using DCCP are
   aware that packet reordering could occur, because DCCP does not
   provide mechanisms to restore the original packet order.

   In contrast to TCP, the receiver processing for MPTCP adopted a rigid
   "just wait" approach, because TCP guarantees reliable in-order
   delivery.

Authors' Addresses

   Markus Amend (editor)
   Deutsche Telekom
   Deutsche-Telekom-Allee 9
   64295 Darmstadt
   Germany

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   Email: Markus.Amend@telekom.de

   Anna Brunstrom
   Karlstad University
   Universitetsgatan 2
   SE-651 88 Karlstad
   Sweden
   Email: anna.brunstrom@kau.se

   Andreas Kassler
   Karlstad University
   Universitetsgatan 2
   SE-651 88 Karlstad
   Sweden
   Email: andreas.kassler@kau.se

   Veselin Rakocevic
   City, University of London
   Northampton Square
   London
   United Kingdom
   Email: veselin.rakocevic.1@city.ac.uk

   Stephen Johnson
   BT
   Adastral Park
   Martlesham Heath
   IP5 3RE
   United Kingdom
   Email: stephen.h.johnson@bt.com

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