Coupled Congestion Control for Multipath Transport Protocols
RFC 6356

Received changes through RFC Editor sync (changed abstract to 'Often endpoints are connected by multiple paths, but communications are usually restricted to a single path per connection. Resource usage within the network would be more efficient were it possible for these multiple paths to be used concurrently. Multipath TCP is a proposal to achieve multipath transport in TCP.

New congestion control algorithms are needed for multipath transport protocols such as Multipath TCP, as single path algorithms have a series of issues in the multipath context. One of the prominent problems is that running existing algorithms such as standard TCP independently on each path would give the multipath flow more than its fair share at a bottleneck link traversed by more than one of its subflows. Further, it is desirable that a source with multiple paths available will transfer more traffic using the least congested of the paths, achieving a property called "resource pooling" where a bundle of links effectively behaves like one shared link with bigger capacity. This would increase the overall efficiency of the network and also its robustness to failure.

This document presents a congestion control algorithm that couples the congestion control algorithms running on different subflows by linking their increase functions, and dynamically controls the overall aggressiveness of the multipath flow. The result is a practical algorithm that is fair to TCP at bottlenecks while moving traffic away from congested links. This document defines an Experimental Protocol for the Internet community.')

State changed to Approved-announcement to be sent from Approved-announcement to be sent::Point Raised - writeup needed.<br>the authors have submitted a revision that incorporates COMMENTs received from the IESG, and an RFC Editor note on the equations has been added.

[Ballot comment]
I know that the ASCII text that we use makes it very difficult to publish maths, but I found that the method of presenting the math chosen by the authors, particularly the math embedded in the paragraphs, and the absence of equation numbers made it extremely difficult to understand what they were saying.

I have the three worst examples below, but I would request that the authors work on the rest of the math to find a clearer way to present it.

=========

This is worse in my browser than in the text itself, but even so it is difficult to understand the equation

The formula to compute alpha is:

                                      cwnd_i
                                max --------
                                  i        2
                                      rtt_i
            alpha = tot_cwnd * ----------------
                              /      cwnd_i \ 2
                              | sum ---------|
                              \  i  rtt_i  /

Please can a clearer specification of the equation be provided

=====

Hence, it is possible to compute alpha only once per drop according
  to the formula above, by replacing rtt_i with rtt_avg_i.

There were lots of formulii above

======

The following is just about readable.

For each ack received on subflow i, increase cwnd_i by min (
      (alpha*bytes_acked*mss_i/tot_cwnd)/alpha_scale ,
      bytes_acked*mss_i/cwnd_i )

[Ballot comment]
I agree with Sean's comment.

I didn't have time to check the references but do they cover the case
where someone manipulates one link (e.g. dropping packets) to cause
more traffic on another link they can monitor or want to DoS?

[Ballot comment]
For the Security Considerations, I think maybe you meant to say something like:

This coupled congestion control algorithm defined in this draft adds no new security considerations to those found in [I-D.ford-mptcp-multiaddressed] and [RFC6181].  Detailed security analysis for the Multipath TCP protocol itself is included in [I-D.ford-mptcp-multiaddressed] and [RFC6181].

Instead of None followed by where the security considerations are found.

[Ballot comment]
I don't understand why this document is being put forward for Experimental instead of standards track. The document writeup says there is strong consensus in the WG for this document, there is a good discussion of how this mechanism will not cause congestion problems, and there are already some implementations. It doesn't seem research or something that isn't appropriate for attempted deployment.

[Ballot discuss]
I know that the ASCII text that we use makes it very difficult to publish maths, but I found that the method of presenting the math chosen by the authors, particularly the math embedded in the paragraphs, and the absence of equation numbers made it extremely difficult to understand what they were saying.

I have the three worst examples below, but I would request that the authors work on the rest of the math to find a clearer way to present it.

=========

This is worse in my browser than in the text itself, but even so it is difficult to understand the equation

The formula to compute alpha is:

                                      cwnd_i
                                max --------
                                  i        2
                                      rtt_i
            alpha = tot_cwnd * ----------------
                              /      cwnd_i \ 2
                              | sum ---------|
                              \  i  rtt_i  /

Please can a clearer specification of the equation be provided

=====

Hence, it is possible to compute alpha only once per drop according
  to the formula above, by replacing rtt_i with rtt_avg_i.

There were lots of formulii above

======

The following is just about readable.

For each ack received on subflow i, increase cwnd_i by min (
      (alpha*bytes_acked*mss_i/tot_cwnd)/alpha_scale ,
      bytes_acked*mss_i/cwnd_i )

[Ballot comment]
the authors should consider the comments sent by David Black in his TSVDIR review; a minor revision may be necessary in order to answer the main question raised.

State changed to In Last Call from Last Call Requested.

The following Last Call Announcement was sent out:

From: The IESG <iesg-secretary@ietf.org>
To: IETF-Announce <ietf-announce@ietf.org>
CC: <multipathtcp@ietf.org>
Reply-To: ietf@ietf.org
Subject: Last Call: <draft-ietf-mptcp-congestion-05.txt> (Coupled Congestion Control for Multipath Transport Protocols) to Experimental RFC


The IESG has received a request from the Multipath TCP WG (mptcp) to
consider the following document:
- 'Coupled Congestion Control for Multipath Transport Protocols'
  <draft-ietf-mptcp-congestion-05.txt> as an Experimental RFC

The IESG plans to make a decision in the next few weeks, and solicits
final comments on this action. Please send substantive comments to the
ietf@ietf.org mailing lists by 2011-07-05. Exceptionally, comments may be
sent to iesg@ietf.org instead. In either case, please retain the
beginning of the Subject line to allow automated sorting.

Abstract


  Often endpoints are connected by multiple paths, but communications
  are usually restricted to a single path per connection.  Resource
  usage within the network would be more efficient were it possible for
  these multiple paths to be used concurrently.  Multipath TCP is a
  proposal to achieve multipath transport in TCP.

  New congestion control algorithms are needed for multipath transport
  protocols such as Multipath TCP, as single path algorithms have a
  series of issues in the multipath context.  One of the prominent
  problems is that running existing algorithms such as standard TCP
  independently on each path would give the multipath flow more than
  its fair share at a bottleneck link traversed by more than one of its
  subflows.  Further, it is desirable that a source with multiple paths
  available will transfer more traffic using the least congested of the
  paths, hence achieving resource pooling.  This would increase the
  overall efficiency of the network and also its robustness to failure.

  This document presents a congestion control algorithm which couples
  the congestion control algorithms running on different subflows by
  linking their increase functions, and dynamically controls the
  overall aggressiveness of the multipath flow.  The result is a
  practical algorithm that is fair to TCP at bottlenecks while moving
  traffic away from congested links.




The file can be obtained via
http://datatracker.ietf.org/doc/draft-ietf-mptcp-congestion/

IESG discussion can be tracked via
http://datatracker.ietf.org/doc/draft-ietf-mptcp-congestion/


No IPR declarations have been submitted directly on this I-D.

  (1.a) Who is the Document Shepherd for this document? Has the
        Document Shepherd personally reviewed this version of the
        document and, in particular, does he or she believe this
        version is ready for forwarding to the IESG for publication?

Yoshifumi Nishida is the document shepherd. I have personally reviewed the document
and believe it is ready for publication.

  (1.b) Has the document had adequate review both from key WG members
        and from key non-WG members? Does the Document Shepherd have
        any concerns about the depth or breadth of the reviews that
        have been performed? 

The document has been reviewed by both key WG members and Key non-WG members.
There is no concern about the reviews.

  (1.c) Does the Document Shepherd have concerns that the document
        needs more review from a particular or broader perspective,
        e.g., security, operational complexity, someone familiar with
        AAA, internationalization or XML?

None.

  (1.d) Does the Document Shepherd have any specific concerns or
        issues with this document that the Responsible Area Director
        and/or the IESG should be aware of? For example, perhaps he
        or she is uncomfortable with certain parts of the document, or
        has concerns whether there really is a need for it. In any
        event, if the WG has discussed those issues and has indicated
        that it still wishes to advance the document, detail those
        concerns here. Has an IPR disclosure related to this document
        been filed? If so, please include a reference to the
        disclosure and summarize the WG discussion and conclusion on
        this issue.

None.

  (1.e) How solid is the WG consensus behind this document? Does it
        represent the strong concurrence of a few individuals, with
        others being silent, or does the WG as a whole understand and
        agree with it? 

There is solid consensus on this document in the WG.

  (1.f) Has anyone threatened an appeal or otherwise indicated extreme
        discontent? If so, please summarise the areas of conflict in
        separate email messages to the Responsible Area Director. (It
        should be in a separate email because this questionnaire is
        entered into the ID Tracker.)

None.

  (1.g) Has the Document Shepherd personally verified that the
        document satisfies all ID nits? (See the Internet-Drafts Checklist
        and http://tools.ietf.org/tools/idnits/). Boilerplate checks are
        not enough; this check needs to be thorough. Has the document
        met all formal review criteria it needs to, such as the MIB
        Doctor, media type and URI type reviews?

This document is verified with idnits 2.12.12.

  (1.h) Has the document split its references into normative and
        informative? Are there normative references to documents that
        are not ready for advancement or are otherwise in an unclear
        state? If such normative references exist, what is the
        strategy for their completion? Are there normative references
        that are downward references, as described in [RFC3967]? If
        so, list these downward references to support the Area
        Director in the Last Call procedure for them [RFC3967].

The document splits the references. There is no downward reference
in the normative reference.

  (1.i) Has the Document Shepherd verified that the document IANA
        consideration section exists and is consistent with the body
        of the document? If the document specifies protocol
        extensions, are reservations requested in appropriate IANA
        registries? Are the IANA registries clearly identified? If
        the document creates a new registry, does it define the
        proposed initial contents of the registry and an allocation
        procedure for future registrations? Does it suggest a
        reasonable name for the new registry? See [RFC5226]. If the
        document describes an Expert Review process has Shepherd
        conferred with the Responsible Area Director so that the IESG
        can appoint the needed Expert during the IESG Evaluation?

IANA consideration section exists in the document, but no action is required.

  (1.j) Has the Document Shepherd verified that sections of the
        document that are written in a formal language, such as XML
        code, BNF rules, MIB definitions, etc., validate correctly in
        an automated checker?

The document contains no formal language.

  (1.k) The IESG approval announcement includes a Document
        Announcement Write-Up. Please provide such a Document
        Announcement Write-Up? Recent examples can be found in the
        "Action" announcements for approved documents. The approval
        announcement contains the following sections:

    Technical Summary

This document presents a congestion control algorithm which couples
TCP standard congestion control algorithms running on different subflows.
This coupled congestion control algorithm allows a flow that uses
multiple paths to utilize bandwidth efficiently while keeping the
fairness to other flows at the bottleneck.
Protocols such as Multipath TCP and SCTP which support multiple
end-points for a single flow benefit from this algorithm.

    Working Group Summary

This document has been discussed in the WG meetings and the mailing list.
The WG reached consensus on this document.

    Document Quality

This document was reviewed by various people and has been through
WGLC successfully. No substantial issues were raised during the process.
We have linux implementation for this draft and has been actively maintained.
We also notice that some companies show some interests in implementing this.

    Personnel

Yoshifumi Nishida is the Document Shepherd for this document.
The Responsible Area Director is Wesley Eddy.