Probing MPTCP Subflows
draft-boucadair-mptcp-probe-subflow-00

Abstract

This document specifies an extension to Multipath TCP (MPTCP) that is meant to assess whether a path used to establish a given subflow is MPTCP-friendly, i.e., intermediate nodes involved in that path do not alter nor strip MPTCP options, which would prevent the establishment of MPTCP communications along that path. A new flag bit, called Probe Flag (P-flag) is defined for this purpose.

This document updates RFC6824.

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].

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 http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on January 7, 2016.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. The Problem
• 3. Probe Flag (P-flag)
• 4. Security Considerations
• 5. IANA Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Authors' Addresses

1. Introduction

This document specifies an extension to Multipath TCP (MPTCP, [RFC6824]) that is meant to assess whether a path used to establish a given subflow is MPTCP-friendly. That is, intermediate nodes involved in that path do not alter nor strip MPTCP options, which would prevent the establishment of MPTCP communications along that path.

The problem is summarized briefly in Section 2 while the probe flag is defined in Section 3.

The solution proposed in this document allows to anticipate failures due to the presence of MPTCP-unfriendly devices in the communication paths.

2. The Problem

MPTCP supports a backup mode that relies on a dedicated flag, called backup flag carried in the MP_JOIN option: when set, this flag informs the remote peer that this is a backup subflow. Several problems may be arise such as. For example:

• A peer decides to use a backup subflow but MPTCP cannot be used for that subflow because an intermediate function removes DSS options, for example. This failure will have a negative impact on the quality of experience.
• Several subflows can be candidate to be used as backup but the participating nodes do not know in advance whether associated forwarding paths are MPTCP-friendly, i.e., they can actually support MPTCP subflows. The participating nodes need some "hints" to decide which subflows are to be used as regular ones and those as backup. This lack of information may also affect the perceived quality of experience.

3. Probe Flag (P-flag)

As a solution to the problem described in Section 2, a meaning is associated with one of the reserved bits in MP_JOIN. This new flag bit is called: Probe Flag (P-flag). This flag bit is used to explicitly inform the remote peer that a probing procedure is associated with the corresponding subflow.

Figure 1 and Figure 2 show the required update to the MP_JOIN option format in SYN and SYN/ACK.

OLD: 

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

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

      where "rr" are reserved bits for future assignment as
      additional flag bits. r bits MUST each be sent as zero and MUST be
      ignored on receipt.   

OLD:
                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-----+-+---------------+
      |     Kind      |  Length = 16  |Subtype|     |B|   Address ID  |
      +---------------+---------------+-------+-----+-+---------------+
      |                                                               |
      |                Sender's Truncated HMAC (64 bits)              |
      |                                                               |
      +---------------------------------------------------------------+
      |                Sender's Random Number (32 bits)               |
      +---------------------------------------------------------------+

NEW:
                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-+-+-+-+---------------+
      |     Kind      |  Length = 16  |Subtype|r|r|P|B|   Address ID  |
      +---------------+---------------+-------+-+-+-+-+---------------+
      |                                                               |
      |                Sender's Truncated HMAC (64 bits)              |
      |                                                               |
      +---------------------------------------------------------------+
      |                Sender's Random Number (32 bits)               |
      +---------------------------------------------------------------+

      where "rr" are reserved bits for future assignment as
      additional flag bits. r bits MUST each be sent as zero and MUST be
      ignored on receipt.