Multipath TCP with variable-length DSS mappings

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Table of Contents

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 and the Subflow Sequence Number always uses 4 bytes. 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. Conversely, short connections waste option space by always using a 4-byte Subflow Sequence Number. This document proposes a modification of the DSS option to use variable-length encoding for both the Subflow Sequence Number and the Data-Level Length, allowing shorter encodings when possible and longer mappings when needed.¶

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.¶

3. Extending the DSS option

The extension proposed in this document modifies the DSS option to use variable-length encoding for the Subflow Sequence Number (SSN) and the Data-Level Length (DLL). Given that the DSS Checksum is rarely used in practice, we propose to remove it and use flag bits in the reserved field to indicate the length of the SSN and DLL fields. 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)]

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)     |
     +-------------------------------+------------------------------+

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +---------------+---------------+-------+---+----+----+-+-+-+-+-+
     |     Kind      |    Length     |Subtype|rsv| ss | dd |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 (2, 3, or 4 octets, see ss)      |
     +--------------------------------------------------------------+
     |       Data-Level Length (2, 3, or 4 octets, see dd)          |
     +--------------------------------------------------------------+

                          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) |
     +-------------------------------+-------------------------------+

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 by introducing variable-length encoding for the Subflow Sequence Number and Data-Level Length fields using 4 bits (ss, dd) from the reserved field of the DSS option. This document suggests using the D flag of the MP_CAPABLE option.¶

6. References

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.¶

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