Internet Engineering Task Force S. Floyd
Internet-Draft M. Allman
Intended status: Best Current Practice ICIR / ICSI
Expires: September 2007 March 2007
Specifying New Congestion Control Algorithms
draft-ietf-tsvwg-cc-alt-00.txt
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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 ([RFC3649], [HTCP], [FAST], [BIC], [CompoundTCP],
[XCP], and many more). 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-floyd-tsvwg-cc-alt-00.txt:
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* 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.
* 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.
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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.
This document does not give hard-and-fast rules for what makes for
an appropriate congestion control scheme. Rather, the document
provides a set of criteria that should be considered and weighed by
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. Status
Following the lead of HighSpeed TCP, 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*
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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 researchers who are not ready to bring their congestion control
mechanisms to the IETF for 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.
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.
(1) Impact on Standard TCP, SCTP [RFC2960], and DCCP [RFC4340].
Proposed congestion control mechanisms should be evaluated when
competing with standard IETF congestion control. 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.
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As an example from an Experimental RFC, fairness with standard
TCP is discussed in Sections 4 and 6 of RFC 3649 (High-Speed
TCP) and using spare capacity is discussed in Sections 6, 11.1,
and 12 of RFC 3649 (High-Speed TCP).
(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.
As an example from an Experimental RFC, performance in difficult
environments is discussed in Sections 6, 9.2, and 10.2 of
RFC 4782 (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 RFC 3649
(High-Speed TCP) and Section 9.7 of RFC 4782 (Quick-Start).
(4) Protection Against Congestion Collapse.
The alternate congestion control mechanism should either
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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. 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 RFC 4782 (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 RFC 4782 (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 RFC 4782 (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.
Minimum requirements for approval for widespread deploy include
guideline (1) on assessing the impact on standard congestion
control. Minimum requirements also include guideline (3) on
investigation of the proposed mechanism in a range of environments,
and guideline (4) on protection against congestion collapse. In
order to be approved for widespread deployment, the proposed
mechanism will also have to meet 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 Internet.
5. Conclusions
This document is intended as a guideline for researchers
in bringing congestion control mechanisms to the IETF to
be considered for Experimental status, and also as a
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guideline to the IETF in evaluating such proposals.
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
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
Informative References
[BIC] L. Xu, K. Harfoush, and I. Rhee, Binary Increase Congestion
Control for Fast Long-Distance Networks, Infocom 2004.
[CompoundTCP] K. Tan, J. Song, Q. Zhang, and M. Sridharan, A
Compound TCP Approach for High-speed and Long Distance Networks,
Infocom 2006.
[FAST] C. Jin, D. Wei and S. Low, FAST TCP: Motivation,
Architecture, Algorithms, Performance, Infocom 2004.
[FJ03] Floyd, S., and Jacobson, V., Random Early Detection
Gateways for Congestion Avoidance, IEEE/ACM Transactions on
Networking, V.1 N.4, August 1993.
[HTCP] Shorten, R.N. and Leith, D.J., H-TCP: TCP for High-speed
and Long-distance Networks. PFLDnet, 2004.
[Metrics] S. Floyd, Metrics for the Evaluation of Congestion
Control Mechanisms. Internet-draft draft-irtf-tmrg-metrics-07,
work in progress, February 2007.
[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.
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[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.
[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
[RFC4340] Kohler, E., Handley, M., and S. Floyd, Datagram
Congestion Control Protocol (DCCP), RFC 4340, March 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.
[XCP] D. Katabi, M. Handley, and C. Rohrs, Congestion Control
for High Bandwidth-Delay Product Networks, Sigcomm 2002.
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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