Internet-Draft Subtype-Cap Exchange February 2021
Kang & Liang Expires 10 August 2021 [Page]
TCP Maintenance and Minor Extensions
Intended Status:
J. Kang, Ed.
Q. Liang

Subtype Capability Exchange During MPTCP Handshake


Multipath TCP provides the ability to simultaneously use multiple paths between peers. MPTCP protocol defines seven subtypes in MPTCP v0 [RFC6824] and ten subtypes in MPTCP v1 [RFC8684] to differentiate message types and implement some additional functions during a session.

This draft proposes an enhancement to support Subtype Capability Exchange during MPTCP connection establishment in order to improve elastic scalability of MPTCP protocol. It includes: 1) requirements for which this kind of capability exchange during handshake is important for a MPTCP session; 2) a typical flow for Subtype Capability Exchange between peers; 3) a feasible solution on protocol design is suggested.

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

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This Internet-Draft will expire on 10 August 2021.

Table of Contents

1. Introduction

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Background

Table 1 lists all subtypes that have been specified in current MPTCP versions. Besides version negotiation, MPTCP peers can not interact with each other on the granularity of subtype capability. This feature may cause inflexible protocol extension. For example, if a new message type A is added in future extension, a higher version should be released to import it and a new subtype may need to be allocated. Another case is that if a sender does not know the subtypes supported by a receiver in a MPTCP session, as a result, invalid data packets may been sent from the sender during data transmission and the receiver will discard it which causes system overhead on receiver side.

Table 1: Overview MPTCP Subtypes
Value Symbol Name MPTCPv0 MPTCPv1
0x0 MP_CAPABLE Multipath Capable Supported Supported
0x1 MP_JOIN Join Connection Supported Supported
0x2 DSS Data Sequence Signal (Data ACK and Data Sequence Mapping) Supported Supported
0x3 ADD_ADDR Add Address Supported Supported
0x4 REMOVE_ADDR Remove Address Supported Supported
0x5 MP_PRIO Change Subflow Priority Supported Supported
0x6 MP_FAIL Fallback Supported Supported
0x7 MP_FASTCLOSE Fast Close Supported Supported
0x8 MP_TCPRST Subflow Reset / Supported
0xf MP_EXPERIMENTAL Reserved for Private Use / Supported

This document suggests a new function of Subtype Capability Exchange during MPTCP handshake in the scenario that MPTCP peers in a session support same MPTCP protocol version but with different subtype sets.

2. One Typical Flow

Figure 1 illustrates a typical flow for this Subtype Capability Exchange during MPTCP connection setup. The field of Subtype Capability is used to indicate whether these subtypes are supported by the sender, for example, Host A Subtype Capabilities indicates the status of the subtypes on Host A and Host B Subtype Capabilities indicates that on Host B. Through the transmission of this information between both parties, a sender can determine whether a message can be properly processed by its receiver and only send the message that can be supported by the receiver during data transmission.

            Host A                                  Host B
     ------------------------                       ----------
     Address A1    Address A2                       Address B1
     ----------    ----------                       ----------
         |             |                                |
         |        SYN + Host A Subtype Capability       |
         |                                        Determine and
         |                                    Cache the capabilities
         |                                          of Host A
         |      SYN/ACK + Host B Subtype Capability     |
Determine and Cache the                                 |
 capabilities of Host B                                 |
         |                         ACK                  |
         |             |                                |
         |             |        SYN + MP_JOIN           |
         |             |------------------------------->|
         |             |<-------------------------------|
         |             |      SYN/ACK + MP_JOIN         |
         |             |                                |
         |             |        ACK + MP_JOIN           |
         |             |------------------------------->|
         |             |<-------------------------------|
         |             |             ACK                |
         |             |                                |
         |   Data Transmission(with subtype messages)   |
         |             |                                |
         |             | Data Transmission(with subtype |
         |             |<------------------------------>|
         |             |             messages)          |
         |             |                                |
Figure 1: MPTCP Subtype Capability Exchange

In practice, another possible implementation is as follows: after receiving the subtype capability information sent by Host A, Host B determines the common subtype sets supported by both parties, and returns this common subtype sets in the reponse. Host A caches this common subtype sets locally. In data transmission phase, Host A sends the specified subtype messages to Host B that are included in the common subtype sets. As an alternative solution, its protocol design on MPTCP will be considered and updated in later versions.

3. Protocol Implementation

This document describes one solution on the modifications to MPTCP protocol to support this mechanism.In this solution, MP_CAPABLE option is used and extended to add bits to carry subtype capabilities information. There should be other possible solutions that can be defined in subsequent discussions.

3.1. Carrying Subtype Capabilities in MP_CAPABLE Option

In Figure 2, a 32-bit "OptionSupported" is added to MP_CAPABLE option to indicate whether the subtypes are supported by the sender.

                       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)               |
  |                                                               |
  |                    OptionSupported (32 bits)                  |
  |  Data-Level Length (16 bits)  |  Checksum (16 bits, optional) |
Figure 2: OptionSupported Format

For MPTCP v1, ten subtypes has been defined and applied in practice. So the first 10-bits in OptionSupported field is used for indicating whether these subtypes is supported by sender. The order is listed below:



2: DSS








11~31: Reserved for Future Use

Two values, that is 0 and 1, can be set to these bits in OptionSupported field. The value of 0 indicates that the sender does not support this subtype. The value of 1 indicates that the sender supports this subtype.

4. Security Considerations

To be added.

5. IANA Considerations

To be added.

6. References

6.1. Normative References

Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, DOI 10.17487/RFC6824, , <>.
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, , <>.

6.2. Informative References

Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, DOI 10.17487/RFC2629, , <>.

Authors' Addresses

Jiao Kang (editor)
D2-03,Huawei Industrial Base
Qiandeng Liang
No. 207, Jiufeng 3rd Road, East Lake High-tech Development Zone