



TCP Maintenance and Minor Extensions                           M. Baerts
Internet-Draft                                 UCLouvain & NGI Zero Core
Intended status: Standards Track                             7 July 2025
Expires: 8 January 2026


                 Multipath TCP with longer DSS mappings
                     draft-baerts-tcpm-mptcpdss-00

Abstract

   This document proposes an extension to improve Multipath TCP based on
   operational experience by allowing Multipath TCP to use DSS mappings
   that are longer than 64 KBytes.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://ipnetworkinglab.github.io/draft-mptcp-dss/draft-baerts-tcpm-
   mptcpdss.html.  Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-baerts-tcpm-mptcpdss/.

   Discussion of this document takes place on the TCP Maintenance and
   Minor Extensions Working Group mailing list (mailto:tcpm@ietf.org),
   which is archived at https://mailarchive.ietf.org/arch/browse/tcpm/.
   Subscribe at https://www.ietf.org/mailman/listinfo/tcpm/.

   Source for this draft and an issue tracker can be found at
   https://github.com/IPNetworkingLab/draft-mptcp-dss.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 8 January 2026.



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Copyright Notice

   Copyright (c) 2025 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   2
   3.  Extending the DSS option  . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   From a performance viewpoint, TCP stacks are optimised to leverage
   large segments and use TCP Segment Offload / Generic Receive Offload
   (TSO/GRO).  The DSS option defined in Multipath TCP allows to map a
   series of bytes from the bytestream on a specific subflow.
   Unfortunately, the length of this mapping is encoded in a 16-bit
   field.  Since each Multipath TCP segment must include a DSS mapping
   before being sent to the network interface, this restricts the size
   of the segments that Multipath TCP can use.  In particular in IPv6,
   it is impossible for Multipath TCP to leverage IPv6 jumbograms
   [RFC2675] in contrast to regular TCP.  This document proposes a
   modification of the DSS option to support longer mappings.

2.  Conventions and Definitions

   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.



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3.  Extending the DSS option

   The extension proposed in this document is pretty simple.  Given that
   the DSS Checksum is rarely used in practice, we propose to reuse the
   space reserved for this checksum in the DSS option to support 32-bit
   data-level mappings.  This enables Multipath TCP servers to send
   segments that are longer than 64 KBytes.  This extension is
   negotiated using the TBD flag in the MP_CAPABLE option during the
   handshake.

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

     Figure 1: Negotiation of the DSS option with longer mappings

   The DSS option defined in [RFC8684] reserves 16 bits for the
   Checksum.  However, operational experience indicates that this
   checksum is almost never used by Multipath TCP deployments.  It was
   designed to detect middlebox interference caused notably by
   Application Level Gateways that modify TCP payloads [RFC8041].  Given
   the widespread adoption of TLS, such ALGs are rarely used by
   applications using Multipath TCP [MPTCP-longitudinal].

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

                    Figure 2: The DSS option in RFC8684







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   This document proposes to use a 32-bit Data-Level Length to support
   large TCP segments.  The new DSS option is shown in Figure 3.  The
   other fields of this option and the procedures defined in [RFC8684]
   are unchanged.

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

                        Figure 3: The new DSS option

   [RFC8684] defines the MP_CAPABLE option as shown in Figure 4.  This
   option contains several flags, A-H.  Flags A, B, C, and H are
   specified in [RFC8684].  This document uses Flag TBD to indicate in a
   SYN that the initiator of a connection requests the utilization of
   32-bit Data-Level Length.  If this Flag is set in a SYN, Flag A must
   also obviously be set to 0 to indicate that the Checksum is not
   required on this connection.  If both Flags A and TBD are set in a
   SYN, the receiver MUST not continue the MPTCP connection, and SHOULD
   fallback to TCP.  A server that receives a SYN with the TBD Flag set
   can reply with:

   *  a SYN+ACK with the TBD Flag set to 1 to confirm that it accepts to
      use 32-bit Data-Level Length

   *  a SYN+ACK with the TBD Flag set to 0 to indicate that it prefers
      to use 16-bit Data-Level Length

   Even when the TBD Flag is set to 1, the MP_CAPABLE options continue
   to use a 16-bit Data-Level Length like before, to allow fallback if
   the receiver doesn't support a 32-bit Data-Level Length.










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

                     Figure 4: The MP_CAPABLE option

4.  Security Considerations

   This document does not change the security considerations defined in
   [RFC8684].

5.  IANA Considerations

   This document requests the IANA to reserve flag TBD of the MP_CAPABLE
   option as defined in this document.  It also proposes to change the
   format of the DSS option.  This document suggests using the D flag of
   the MP_CAPABLE option.

6.  References

6.1.  Normative References

   [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>.

   [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>.

   [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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6.2.  Informative References

   [MPTCP-longitudinal]
              Shreedhar, T., Zeynali, D., Gasser, O., Mohan, N., and J.
              Ott, "A Longitudinal View at the Adoption of Multipath
              TCP", arXiv, DOI 10.48550/ARXIV.2205.12138, 2022,
              <https://doi.org/10.48550/ARXIV.2205.12138>.

   [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, DOI 10.17487/RFC2675, August 1999,
              <https://www.rfc-editor.org/rfc/rfc2675>.

   [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>.

Acknowledgments

   This project is funded through NGI Zero Core, a fund established by
   NLnet with financial support from the European Commission's Next
   Generation Internet program.

Author's Address

   Matthieu Baerts
   UCLouvain & NGI Zero Core
   Email: matttbe@kernel.org























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