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Draft Charter - December 2011

Based on draft at http://www.ietf.org/iesg/evaluation/rmcat-charter.txt

RTP Media Congestion Avoidance Techniques (rmcat)
Status: Proposed Working Group
Last Updated: 2011-12-23


Transport Area Director(s):
 David Harrington <ietfdbh@comcast.net>
 Wesley Eddy <wes@mti-systems.com>

Transport Area Advisor:
 Wesley Eddy <wes@mti-systems.com>

Mailing Lists: TBD

Description of Working Group:

Any application sending significant amounts of data over the Internet
is expected to implement reasonable congestion control principles.
For transport protocols such as TCP, SCTP, and DCCP, there are well-
defined algorithms. UDP is not a complete transport solution and does
not include any mechanisms for detecting or responding to congestion.
Though RFC 5405 provides rough guidelines for UDP flows, it does not
specifically consider flows where RTP is layered over UDP, and RTCP
feedback is available, and there are not concrete mechanisms described
for media flows. Techniques such as HTTP "adaptive bitrate streaming"
or "HTTP Live Streaming" are widely-used but are not compatible with
RTP applications.

Sets of media flows over RTP are expected to become more common;
potentially due to the work of the IETF's RTCWEB and CLUE Working
Groups. As such flows may be high-rate, high in number, and sharing
links with other flows, they require implementation of high-quality
congestion control algorithms. While the AVTCORE Working Group will
specify "circuit breaker" mechanisms for detecting and responding to
congestion at a high-level, these are intended to be rough-grained
failsafes, and not well-performing congestion control mechanisms.

TFRC is an abstract set of mechanisms that are intended to provide
congestion control for media flows. There is not yet a standard way
of mapping the TFRC algorithm onto RTP and RTCP. Currently, TFRC is
perceived as having deficiencies that make it an incomplete or
insufficient solution for the envisioned RTCWEB media flows. Other
algorithms developed in light of experience with TFRC are felt to be

Media flows may wish to employ algorithms that attempt to avoid
queueing delay, which goes beyond simply detecting and reacting to
losses or ECN marks. Media flows may also wish to perform congestion
control algorithms that operate with feedback and control shared across
a set of flows, rather than within a single flow. Additionally,
startup considerations for media flows may also be significantly
different than for TCP, and require alternative algorithms to be

The working group is chartered to:
- Develop a clear understanding of the congestion control requirements
for RTP flows, and document deficiencies of existing mechanisms such
as TFRC with regards to these requirements
- Determine if there is an appropriate means to define standard
RTP/RTCP extensions for carrying congestion control feedback, similar
to how DCCP defines CCIDs.
- Publish Experimental RFCs for proposed algorithms
- Document the results of Experimental algorithms deployed in the
- Publish Standards Track RFCs for mature algorithms

Any algorithm published by the working group as an Experimental RFC
must achieve working group consensus that the algorithm is "safe" for
operation on the Internet. Any algorithm published as a Standards
Track RFC must have demonstrated its safety through significant
experimentation and analysis. All algorithms must be described in a
way that permits multiple independent implementations having the same

Recognizing that per-flow QoS mechanisms to reserve bandwidth or
minimize delay do not exist across the operational Internet, the
working group will focus on end-to-end algorithms implemented on
media senders and receivers rather than IP routers. Network QoS
mechanisms are out of scope for this working group.

The working group is expected to initially work closely with the RAI
Area, including (but not limited to) the AVTCORE, AVTEXT, and RTCWEB
working groups while establishing requirements, and likely will have
a strong means to test its algorithms in RTCWEB deployments. The
working group may also need to coordinate with other TSV area working
groups with congestion control expertise, and additionally with the
IRTF's Internet Congestion Control Research Group on some topics.

Goals and Milestones:

Nov 2012 Submit requirements and analysis of existing media flow
         congestion control mechanisms applicable to RTP to the
         IESG as proposal for an Informational RFC
Nov 2012 Submit requirements for standard congestion control
         signalling for RTP/RTCP to the IESG as proposal for an
         Informational RFC
Dec 2012 Determine whether to adopt any proposals as Working Group
         items for intended publication as Experimental RFCs