Network Working Group S. Floyd
Internet-Draft M. Allman
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December 2006
Specifying New Congestion Control Algorithms
draft-floyd-tsvwg-cc-alt-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
The IETF's standard congestion control schemes have been widely
shown to be inadequate for various environments (e.g., high-speed or
wireless networks). Recent research has yielded many alternate
congestion control schemes. Using these new congestion control
schemes in the global Internet has possible ramifications to both
the network 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-cc-alt-00.txt:
* Changed the name to draft-floyd-tsvwg-cc-alt-00.txt.
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* 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.
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
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the "Experimental" status. The first class is algorithms that are
judged to be safe to deploy in the global Internet and further
investigated in that environment. The second class is algorithms
that, while promising, are not deemed safe enough for deployment on
the Internet, but are being specified to facilitate investigations
via simulation and testbeds.
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 even though it may not be unsafe
for use.
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 [QuickStart] 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 would 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 Informational,
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
evaluations 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 and these guidelines should not be used as a
check-list.
(1) Fairness to Standard TCP, SCTP, and DCCP.
In environments where standard congestion control is able to
make reasonable use of the available bandwidth the proposed
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change should not significantly change this state.
For instance, in a situation where each of N flows uses 1/N of
the network capacity, a new congestion control scheme should not
significantly deviate from this state. For instance, a flow
using an alternate congestion controller that took half the
capacity and left each of the remaining N flows with 1/2N of the
capacity would be suspect.
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.
(2) Using Spare Capacity.
Similar to point (1), alternate congestion control algorithms
may take up spare capacity in the network, but may not steal
significant amounts of capacity from flows using currently
standardized congestion control.
For instance, consider the case where N flows cannot naturally
use all the capacity and equally share one-third of the capacity
(with each flow using 1/3N of the capacity). A single flow
using an alternate congestion control scheme could use the
unused two-thirds of capacity. However, the flow using
alternate congestion control should not steal significant
amounts of additional capacity from the N standard flows.
(3) Difficult Environments.
An assessment of proposed algorithms in difficult environments
such as paths containing wireless links and paths with
reverse-path congestion. In addition, proposed algorithms
should be evaluated in situations where the bottleneck has high
and low levels of statistical multiplexing.
(4) 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 and round-trip times. 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 extending the
investigation into areas where the proposal does not perform
well.
(5) Protection Against Congestion Collapse.
The alternate congestion control mechanism should either
stop sending when the packet drop rate exceeds some threshold
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[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, 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.
(6) Fairness within the Alternate Congestion Control Algorithm.
In environments with multiple competing flows using the
alternate congestion control algorithm, the proposal should
explore how bandwidth is shared among the competing flows.
(7) 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.
(8) 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.
(9) Incremental Deployment.
The proposal should discuss whether the alternate congestion
control mechanism allows for incremental deployment in the
targeted environment. If the alternate congestion control
mechanism is intended only for specific environments, the
proposal should consider how this intention is to be
carried out.
4. 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
guideline to the IETF in evaluating such proposals.
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5. 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.
6. IANA Considerations
This document does not require any IANA action.
Acknowledgments
Discussions with Lars Eggert and Aaron Falk seeded this document.
Thanks to Colin Perkins for feedback. This document also draws
from [Metrics]. Thanks to Colin Perkins for feedback.
Normative References
Informative 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.
[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.
[Metrics] S. Floyd, Metrics for the Evaluation of Congestion
Control Mechanisms. Internet-draft draft-irtf-tmrg-metrics-04,
work in progress, August 2006.
[QuickStart] S. Floyd, M. Allman, A. Jain, and P. Sarolahti,
Quick-Start for TCP and IP. Internet-draft
draft-ietf-tsvwg-quickstart-07.txt, work in progress, October
2006. Approved for Experimental.
Authors' Addresses
Sally Floyd
Phone: +1 (510) 666-2989
ICIR (ICSI Center for Internet Research)
Email: floyd@icir.org
URL: http://www.icir.org/floyd/
Mark Allman
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ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704-1198
Phone: (440) 235-1792
Email: mallman@icir.org
URL: http://www.icir.org/mallman/
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