RTP Media Congestion Avoidance Techniques
charter-ietf-rmcat-00-01

Description of Working Group

In today's current Internet, part of the traffic is delivery of interactive real time media,
often in the form of sets of media flows using RTP over UDP. There is no generally
accepted congestion control mechanism for this kind of data flow. With the
deployment of applications using the RTCWEB protocol suite, the number of such
flows is likely to increase, especially non-fixed-rate flows such as video or adaptive
audio. There is therefore some urgency in specifying one or more congestion control
mechanisms that can find general acceptance.

Congestion control algorithms for interactive real time media may need to be quite
different from the congestion control of TCP: for example, some applications can be
more tolerant to loss than delay and jitter. The set of requirements for such an
algorithm includes, but is not limited to:
- Low delay and low jitter for the case where there is no competing traffic using
other algorithms
- Reasonable share of bandwidth when competing with RMCAT traffic, other
real-time media protocols, and ideally also TCP and other protocols. A
'reasonable share' means that no flow has a significantly negative impact
[RFC5033] on other flows and at minimum that no flow starves.
- Effective use of signals like packet loss and ECN markings to adapt to
congestion

The working group will:
- 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.
- Define interactions between applications and RTP flows to enable specifying
requirements such as per-packet priorities.
- Determine if there is an appropriate means to define standard RTP/RTCP
extensions for carrying congestion control feedback, similar to how DCCP
defines congestion control mechanisms, and if so, document such extensions
as standards-track RFCs.
- Develop techniques to detect, instrument or diagnose failing to meet RT
schedules due to failures of components outside of the charter scope, possibly
in collaboration with IPPM.
- Develop a mechanism for identifying and controlling groups of flows.
- Define evaluation criteria for proposed mechanisms, and publish these as an
Informational RFCs.
- Find or develop candidate congestion control algorithms, verify that these can be
tested on the Internet without significant risk, and publish one or more of these
as Experimental RFCs.
- Publish evaluation criteria and the result of experimentation with these
Experimental algorithms on the Internet
- Once an algorithm has been found or developed that meets the evaluation
criteria, and has a satisfactory amount of documented experience on the
Internet, publish this algorithm as a Standards Track RFC. There may be more
than one such algorithm.

The work will be guided by the advice laid out in RFC 5405 (UDP usage guidelines),
RFC 2914 (congestion control principles), and RFC5033 (Specifying New Congestion
Control Algorithms).

The following topics are out of scope of this working group, on the assumption that
work on them will proceed elsewhere:
- Circuit-breaker algorithms for stopping media flows when network conditions
render them useless; this work is done in AVTCORE;
- Media flows for non-interactive purposes like stored video playback; those are not
as delay sensitive as interactive traffic;
- Defining active queue management; modifications to TCP of any kind; and
- Multicast congestion control (common control of multiple unicast flows is in
scope).
- Topologies other than point-to-point connections. Implications on multi-hop
connections will be considered at a later stage.

The working group is expected to work closely with the RAI area, including the
underlying technologies being worked on in the AVTCORE and AVTEXT WGs, and
the applications/protocol suites being developed in the CLUE and RTCWEB working
groups. It will also liaise closely with other Transport area groups working on
congestion control, and with the Internet Congestion Control Research Group of the
IRTF.

Deliverables
- Requirements for congestion control algorithms for interactive real time media -
Informational RFC
- Evaluation criteria for congestion control algorithms for interactive real time media
- Informational RFC
- RTCP extensions for use with congestion control algorithms - Standards-track
RFC
- Interactions between applications and RTP flows - Informational RFC
- Identifying and controlling groups of flows - Standards-track RFC
- Techniques to detect, instrument or diagnose failing to meet RT schedules -
Informational RFC
- Candidate congestion control algorithm for interactive real time media -
Experimental RFCs (likely more than one)
- Experimentation and evaluation results for candidate congestion control
algorithms - Informational RFC
- A recommended congestion control algorithm for interactive real time media -
Standards-track RFC