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Versions: (draft-floyd-tsvwg-cc-alt) 00 01 02 03 04 RFC 5033

Internet Engineering Task Force                                 S. Floyd
Internet-Draft                                                 M. Allman
Intended status: Best Current Practice                       ICIR / ICSI
Expires: November 2007                                          May 2007


              Specifying New Congestion Control Algorithms
                    draft-ietf-tsvwg-cc-alt-03.txt

Status of this Memo

    By submitting this Internet-Draft, each author represents that any
    applicable patent or other IPR claims of which he or she is aware
    have been or will be disclosed, and any of which he or she becomes
    aware will be disclosed, in accordance with Section 6 of BCP 79.

    Internet-Drafts are working documents of the Internet Engineering
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    Internet-Drafts.

    Internet-Drafts are draft documents valid for a maximum of six
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    reference material or to cite them other than as "work in progress."

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Copyright Notice

    Copyright (C) The IETF Trust (2007).

Abstract

    The IETF's standard congestion control schemes have been widely
    shown to be inadequate for various environments (e.g., high-speed
    networks).  Recent research has yielded many alternate congestion
    control schemes that significantly differ from the IETF's congestion
    control principles.  Using these new congestion control schemes in
    the global Internet has possible ramifications to both the traffic
    using the new congestion control and to traffic using the currently
    standardized congestion control.  Therefore, the IETF must proceed
    with caution when dealing with alternate congestion control
    proposals.  The goal of this document is to provide guidance for
    considering alternate congestion control algorithms within the IETF.

    TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

    Changes from draft-ietf-tsvwg-cc-alt-01.txt:

    * Removed references from abstract.

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    * Added a note that we are focused on documents produced within the
      IETF (i.e., these are not guidelines that the IRTF or the RFC
      Editor would necessarily have to follow).

    * Added a list of 'difficult environments' the IETF has thought
      about in the past (even while admitting that an exhaustive list of
      'difficult environments' is impossible to produce).

    * Removed section 5 (conclusions).  Some felt that it was redundant
      and not needed.

    * Made a few of the references normative.

    * Various small wording tweaks.

    Changes from draft-ietf-tsvwg-cc-alt-01.txt:

    * Very minor wording tweaks gathered during WGLC.

    Changes from draft-ietf-tsvwg-cc-alt-00.txt:

    * Added text to the introduction to clarify the relationship of this
      document and RFC 2914.  In addition, added a requirement (0) in
      section 3 that says new congestion control schemes that
      significantly diverge from the principles in RFC 2914 must explain
      this divergence.

    Changes from draft-floyd-tsvwg-cc-alt-00.txt:

    * Changed the name to draft-ietf-tsvwg-cc-alt-00.txt.

    * Added a sentence about robustness with various
      queueing algorithms in the routers, especially both RED
      and DropTail.  Suggestion from Jitendra Padhye.

    * Added a sentence about robustness with the routers,
      middleboxes, and such deployed in the current Internet.
      Concern taken from a talk by Henry Sanders.

    * Add a section about minimum requirements necessary for
      approval for deployment in the global Internet.
      Suggestion by Jitendra Padhye.

    * Added more examples to guideline 3 about difficult environments,
      and added that TCP performance in difficult environments is
      still an active research topic.  Suggestion from Doug Leith.

    * Added citations to examples of discussions of these issues
      in Experimental RFCs 3649 and 4782.

    * Added examples of high speed TCP proposals.  Suggestion
      from Bob Braden.


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    * Changed the fairness bullets to better reflect that new congestion
      controllers are expected to assess the impact to standard
      congestion controlled flows---without commenting on how that
      assessment should be done.  From discussions with bob Briscoe.

    * Made numerous editing changes suggested by Gorry Fairhurst.

    Changes from draft-floyd-cc-alt-00.txt:

    * Changed the name to draft-floyd-tsvwg-cc-alt-00.txt.

    * Added a bullet about incremental deployment.  Feedback from
      Colin Perkins

    * Clarified the fairness section;  this section is not saying
      that strict TCP-friendliness is a requirement.

    * Clarified that as an alternative to Full Backoff, a flow
      could stop sending when the packet drop rate is above a
      certain threshold.

    * Clarified that the Full Backoff bullet does not require
      that different flows with different round-trip times
      use the same criteria about when they should back off
      to one packet per round-trip time or less.

    * Added a paragraph about Informational RFCs.

    * Added a bullet about response to transient events, including
      routing events or moving from a private to a shared network.

    END OF NOTES TO BE DELETED.


1.  Introduction

    This document provides guidelines for the IETF to use when
    evaluating suggested congestion control algorithms that
    significantly differ from the general congestion control principles
    outlined in [RFC2914].  The guidance is intended to be useful to
    authors proposing alternate congestion control and for the IETF
    community when evaluating whether a proposal is appropriate for
    publication in the RFC series.

    The guidelines in this document are intended to be consistent with
    the congestion control principles from [RFC2914] of preventing
    congestion collapse, considering fairness, and optimizing the flow's
    own performance in terms of throughput, delay, and loss.  [RFC2914]
    also discusses the goal of avoiding a congestion control `arms race'
    among competing transport protocols.

    This document does not give hard-and-fast requirements for an
    appropriate congestion control scheme.  Rather, the document
    provides a set of criteria that should be considered and weighed by

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    the IETF in the context of each proposal.  The high-order criteria
    for any new proposal is that a serious scientific study of the pros
    and cons of the proposal needs to have been done such that the IETF
    has a well rounded set of information to consider.

    After initial studies, we encourage authors to write a specification
    of their proposals for publication in the RFC series to allow others
    to concretely understand and investigate the wealth of proposals in
    this space.

2.  Document Status

    Following the lead of HighSpeed TCP [RFC3649], alternate congestion
    control algorithms are expected to be published as "Experimental"
    RFCs until such time that the community better understands the
    solution space.  Traditionally, the meaning of "Experimental" status
    has varied in its use and interpretation.  As part of this document
    we define two classes of congestion control proposals that can be
    published with the "Experimental" status.  The first class includes
    algorithms that are judged to be safe to deploy for best-effort
    traffic in the global Internet and further investigated in that
    environment.  The second class includes algorithms that, while
    promising, are not deemed safe enough for widespread deployment as
    best-effort traffic on the Internet, but are being specified to
    facilitate investigations in simulation, testbeds, or controlled
    environments.  The second class can also include algorithms where
    the IETF does not yet have sufficient understanding to decide if the
    algorithm is or is not safe for deployment on the Internet.

    Each alternate congestion control algorithm published is required to
    include a statement in the abstract indicating whether or not the
    proposal is considered safe for use on the Internet.  Each alternate
    congestion control algorithm published is also required to include a
    statement in the abstract describing environments where the protocol
    is not recommended for deployment.  There may be environments where
    the protocol is deemed *safe* for use, but still is not
    *recommended* for use because it does not perform well for the user.

    As examples of such statements, [RFC3649] specifying HighSpeed TCP
    includes a statement in the abstract stating that the proposal is
    Experimental, but may be deployed in the current Internet.  In
    contrast, the Quick-Start document [RFC4782] includes a paragraph in
    the abstract stating the the mechanism is only being proposed for
    controlled environments.  The abstract specifies environments where
    the Quick-Start request could give false positives (and therefore
    would be unsafe to deploy).  The abstract also specifies
    environments where packets containing the Quick-Start request could
    be dropped in the network; in such an environment, Quick-Start would
    not be unsafe to deploy, but deployment would still not be
    recommended because it could cause unnecessary delays for the
    connections attempting to use Quick-Start.

    For authors of alternate congestion control schemes who are not
    ready to bring their congestion control mechanisms to the IETF for

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    standardization (either as Experimental or as Proposed Standard),
    one possibility would be to submit an internet-draft that documents
    the alternate congestion control mechanism for the benefit of the
    IETF and IRTF communities.  This is particularly encouraged in order
    to get algorithm specifications widely disseminated to facilitate
    further research.  Such an internet-draft could be submitted to be
    considered as an Informational RFC, as a first step in the process
    towards standardization.  Such a document would also be expected to
    carry an explicit warning against using the scheme in the global
    Internet.

    Note: we are not changing RFC publication process for non-IETF
    produced documents (e.g., those from the IRTF or independent
    RFC-Editor submissions).  However, we would hope the guidelines in
    this document inform the IESG as they consider whether to add a note
    to such documents.

3.  Guidelines

    As noted above, authors are expected to do a well-rounded evaluation
    of the pros and cons of proposals brought to the IETF.  The
    following are guidelines to help authors and the IETF community.
    Concerns that fall outside the scope of these guidelines are
    certainly possible; these guidelines should not be considered as an
    all-encompassing check-list.

    (0) Differences with Congestion Control Principles [RFC2914]

        Proposed congestion control mechanisms should include a clear
        explanation of the deviations from [RFC2914].

    (1) Impact on Standard TCP, SCTP [RFC2960], and DCCP [RFC4340].

        Proposed congestion control mechanisms should be evaluated when
        competing with standard IETF congestion control
        [RFC2581,RFC2960,RFC4340].  Alternate congestion controllers
        that have a significantly negative impact on traffic using
        standard congestion control may be suspect and this aspect
        should be part of the community's decision making with regards
        to the suitability of the alternate congestion control
        mechanism.

        We note that this bullet is not a requirement for strict
        TCP-friendliness as a prerequisite for an alternate congestion
        control mechanism to advance to Experimental.  As an example,
        HighSpeed TCP is a congestion control mechanism that is
        Experimental, but that is not TCP-friendly in all environments.
        We also note that this guideline does not constrain the fairness
        offered for non-best-effort traffic.

        As an example from an Experimental RFC, fairness with standard
        TCP is discussed in Sections 4 and 6 of [RFC3649] (HighSpeed
        TCP) and using spare capacity is discussed in Sections 6, 11.1,
        and 12 of [RFC3649].

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    (2) Difficult Environments.

        The proposed algorithms should be assessed in difficult
        environments such as paths containing wireless links.
        Characteristics of wireless environments are discussed in
        [RFC3819] and in Section 16 of [Tools].  Other difficult
        environments can include those with multipath routing within a
        connection.  We note that there is still much to be desired in
        terms of the performance of TCP in some of these difficult
        environments.  For congestion control mechanisms with explicit
        feedback from routers, difficult environments can include paths
        with non-IP queues at layer-two, IP tunnels, and the like.  A
        minimum goal for experimental mechanisms proposed for widespread
        deployment in the Internet should be that they do not perform
        significantly worse than TCP in these environments.

        While it is impossible to enumerate all possible "difficult
        environments", we note that the IETF has previously grappled
        with paths with long delays [RFC2488], high delay bandwidth
        products [RFC3649], high packet corruption rates [RFC3155],
        packet reordering [RFC4653] and significantly slow links
        [RFC3150].  Aspects of alternate congestion control that impact
        networks with these characteristics should be detailed.

        As an example from an Experimental RFC, performance in difficult
        environments is discussed in Sections 6, 9.2, and 10.2 of
        [RFC4782] (Quick-Start).

    (3) Investigating a Range of Environments.

        Similar to the last criteria, proposed alternate congestion
        controllers should be assessed in a range of environments.  For
        instance, proposals should be investigated across a range of
        bandwidths, round-trip times, levels of traffic on the reverse
        path, and levels of statistical multiplexing at the congested
        link.  Similarly, proposals should be investigated for robust
        performance with different queueing mechanisms in the routers,
        especially Random Early Detection (RED) [FJ03] and Drop-Tail.
        This evaluation is often not included in the internet-draft
        itself, but in related papers cited in the draft.

        A particularly important aspect of evaluating a proposal for
        standardization is in understanding where the algorithm breaks
        down.  Therefore, particular attention should be paid to
        characterizing the areas where the proposed mechanism does not
        perform well.

        As an example from an Experimental RFC, performance in a range
        of environments is discussed in Section 12 of [RFC3649]
        (HighSpeed TCP) and Section 9.7 of [RFC4782] (Quick-Start).

    (4) Protection Against Congestion Collapse.


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        The alternate congestion control mechanism should either stop
        sending when the packet drop rate exceeds some threshold
        [RFC3714], or should include some notion of "full backoff".  For
        "full backoff", at some point the algorithm would reduce the
        sending rate to one packet per round-trip time and then
        exponentially backoff the time between single packet
        transmissions if congestion persists.  Exactly when either "full
        backoff" or a pause in sending comes into play will be
        algorithm-specific.  However, as discussed in [RFC2914], this
        requirement is crucial to protect the network in times of
        extreme congestion.

        If "full backoff" is used, this bullet does not require that the
        full backoff mechanism must be identical to that of TCP
        [RFC2988].  As an example, this bullet does not preclude full
        backoff mechanisms that would give flows with different
        round-trip times comparable bandwidth during backoff.

    (5) Fairness within the Alternate Congestion Control Algorithm.

        In environments with multiple competing flows all using the same
        alternate congestion control algorithm, the proposal should
        explore how bandwidth is shared among the competing flows.

    (6) Performance with Misbehaving Nodes and Outside Attackers.

        The proposal should explore how the alternate congestion control
        mechanism performs with misbehaving senders, receivers, or
        routers.  In addition, the proposal should explore how the
        alternate congestion control mechanism performs with outside
        attackers.  This can be particularly important for congestion
        control mechanisms that involve explicit feedback from routers
        along the path.

        As an example from an Experimental RFC, performance with
        misbehaving nodes and outside attackers is discussed in Sections
        9.4, 9.5, and 9.6 of [RFC4782] (Quick-Start).  This includes
        discussion of misbehaving senders and receivers; collusion
        between misbehaving routers; misbehaving middleboxes; and the
        potential use of Quick-Start to attack routers or to tie up
        available Quick-Start bandwidth.

    (7) Responses to Sudden or Transient Events.

        The proposal should consider how the alternate congestion
        control mechanism would perform in the presence of transient
        events such as sudden congestion, a routing change, or a
        mobility event.  Routing changes, link disconnections,
        intermittent link connectivity, and mobility are discussed in
        more detail in Section 17 of [Tools].

        As an example from an Experimental RFC, response to transient
        events is discussed in Section 9.2 of [RFC4782] (Quick-Start).


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    (8) Incremental Deployment.

        The proposal should discuss whether the alternate congestion
        control mechanism allows for incremental deployment in the
        targeted environment.  For a mechanism targeted for deployment
        in the current Internet, it would be helpful for the proposal to
        discuss what is known (if anything) about the correct operation
        of the mechanism with some of the equipment installed in the
        current Internet, e.g., routers, transparent proxies, WAN
        optimizers, intrusion detection systems, home routers, and the
        like.

        As a similar concern, if the alternate congestion control
        mechanism is intended only for specific environments, the
        proposal should consider how this intention is to be carried
        out.  For example, if a proposed congestion control scheme is
        deemed suitable for deployment in controlled environments but
        unsafe for widespread deployment in the Internet, is it
        sufficient just to have a sentence in the Abstract of the
        document stating this, or are some additional mechanisms needed
        as well?

        As an example from an Experimental RFC, deployment issues are
        discussed in Sections 10.3 and 10.4 of [RFC4782] (Quick-Start).

4.  Minimum Requirements

    This section suggests minimum requirements for a document to be
    approved as Experimental with approval for widespread deployment in
    the global Internet.  We note that this is not a binding document
    with fixed and unchanging requirements, but simply a document
    targeted for approval as Best Current Practice.

    The minimum requirements for approval for widespread deployment in
    the global Internet include the following guidelines (1) on
    assessing the impact on standard congestion control, (3) on
    investigation of the proposed mechanism in a range of environments,
    guideline (4) on protection against congestion collapse and
    guideline (8), discussing whether the mechanism allows for
    incremental deployment.

    For other guidelines, i.e., (2), (5), (6), and (7), evidence that
    the proposed mechanism has significantly more problems than those of
    TCP should be a cause for concern in approval for widespread
    deployment in the global Internet.

6.  Security Considerations

    This document does not represent a change to any aspect of the
    TCP/IP protocol suite and therefore does not directly impact
    Internet security.  The implementation of various facets of the
    Internet's current congestion control algorithms do have security
    implications (e.g., as outlined in [RFC2581]).  Alternate congestion
    control schemes should be mindful of such pitfalls, as well, and

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    should examine any potential security issues that may arise.

7.  IANA Considerations

    This document does not require any IANA action.

Acknowledgments

    Discussions with Lars Eggert and Aaron Falk seeded this document.
    Thanks to Bob Briscoe, Gorry Fairhurst, Doug Leith, Jitendra Padhye,
    Colin Perkins, members of TSVWG, and participants at the TCP
    Workshop at Microsoft Research for feedback and contributions.  This
    document also draws from [Metrics].

Normative References

    [RFC2581] M. Allman, V. Paxson, and W. Stevens, TCP Congestion
    Control, RFC 2581, Proposed Standard, April 1999.

    [RFC2914] S. Floyd, Congestion Control Principles, RFC 2914, Best
    Current Practice, September 2000.

    [RFC2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
    Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang, L.,
    and V. Paxson, Stream Control Transmission Protocol, RFC 2960,
    October 2000.

    [RFC4340]  Kohler, E., Handley, M., and S. Floyd, Datagram
    Congestion Control Protocol (DCCP), RFC 4340, March 2006.

Informative References

    [FJ03] Floyd, S., and Jacobson, V., Random Early Detection
    Gateways for Congestion Avoidance, IEEE/ACM Transactions on
    Networking, V.1 N.4, August 1993.

    [Metrics] S. Floyd, Metrics for the Evaluation of Congestion
    Control Mechanisms.  Internet-draft draft-irtf-tmrg-metrics-07,
    work in progress, February 2007.

    [RFC2488] M. Allman, D. Glover, and L. Sanchez. Enhancing TCP Over
    Satellite Channels using Standard Mechanisms. RFC 2488. January
    1999.

    [RFC2988] Vern Paxson, Mark Allman.  Computing TCP's Retransmission
    Timer, November 2000.  RFC 2988.

    [RFC3150] S. Dawkins, G. Montenegro, M . Kojo, V. Magret, End-to-end
    Performance Implications of Slow Links, RFC 3150, July 2001.

    [RFC3155] S. Dawkins, G. Montenegro, M. Kojo, V. Magret, N. Vaidya,
    End-to-end Performance Implications of Links with Errors, RFC 3155,
    August 2001.


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    [RFC3649] S. Floyd, HighSpeed TCP for Large Congestion Windows,
    RFC 3649, September 2003.

    [RFC3714] S. Floyd and J. Kempf, IAB Concerns Regarding Congestion
    Control for Voice Traffic in the Internet, RFC 3714, March 2004.

    [RFC3819] P. Karn, C. Bormann, G. Fairhurst, D. Grossman, R. Ludwig,
    J. Mahdavi, G. Montenegro, J. Touch, and L. Wood, Advice for Internet
    Subnetwork Designers, RFC 3819, July 2004

    [RFC4653] Sumitha Bhandarkar, A. L. Narasimha Reddy, Mark Allman,
    Ethan Blanton, Improving the Robustness of TCP to Non-Congestion
    Events, RFC 4653, August 2006.

    [RFC4782] S. Floyd, M. Allman, A. Jain, and P. Sarolahti,
    Quick-Start for TCP and IP.  RFC 4782, Experimental, January
    2007.

    [Tools] S. Floyd and E. Kohler, Tools for the Evaluation of
    Simulation and Testbed Scenarios, Internet-draft
    draft-irtf-tmrg-tools-03.txt, work in progress, December 2006.

Authors' Addresses

    Sally Floyd
    ICIR (ICSI Center for Internet Research)
    1947 Center Street, Suite 600
    Berkeley, CA 94704-1198
    Phone: +1 (510) 666-2989
    Email: floyd at icir.org
    URL: http://www.icir.org/floyd/

    Mark Allman
    ICSI Center for Internet Research
    1947 Center Street, Suite 600
    Berkeley, CA 94704-1198
    Phone: (440) 235-1792
    Email: mallman at icir.org
    URL: http://www.icir.org/mallman/

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