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

Today's Internet traffic includes interactive real-time media, which is often carried
via sets of 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.  This must be completed prior to finishing any
      Experimental algorithm specifications.
    - Identify interactions between applications and RTP flows to enable conveying
      helpful cross-layer information such as per-packet priorities, flow elasticity, etc.
      This information might be used to populate an API, but the WG will not define a
      specific API itself.
    - Determine if extensions to RTP/RTCP are needed for carrying congestion
      control feedback, using DCCP as a model.  If so, provide the requirements
      for such extensions to the AVTCORE working group for standardization there.
    - 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 shared bottlenecks between groups of
      flows, and means to flexibly allocate their rates within the aggregate hitting
      the shared bottleneck.
    - Define evaluation criteria for proposed congestion control mechanisms, and
      publish these as an Informational RFC.  This must be completed prior to
      finishing any Proposed Standard algorithm specifications.
    - 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.  This must be completed prior to
      finishing any Proposed Standard algorithm specifications.
    - 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 algorithm; in this case it will be one of the objects of the
      experimentation to determine the applicabilities and relative merits of the
      algorithms.
    - For each of the Experimental algorithms that have not been
      selected for the Standards Track, the working group will review
      the algorithm and determine whether there are significant flaws,
      such as ones that turn out to be harmful to flows using or
      competing with them. If so, the WG will write a document
      describing the issues encountered and recommending to the IESG
      to move the specification to Historic status. 

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 algorithms or modifications to TCP of any
      kind
    - 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 coordinate 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 as
      an Informational RFC
    - Evaluation criteria for congestion control algorithms for interactive real time media
      as an Informational RFC
    - Requirements for RTCP extensions for use with congestion control algorithms as
      an input to the AVTCORE WG.
    - Interactions between applications and RTP flows as an Informational RFC
    - Identifying and controlling groups of flows as a Proposed Standard RFC
    - Techniques to detect, instrument or diagnose failing to meet RT schedules as
      either an Informational RFC or on the Standards Track if needed for
      interoperability or other aspects that would justify it.
    - Candidate congestion control algorithm for interactive real time media as
      Experimental RFCs (likely more than one)
    - Experimentation and evaluation results for candidate congestion control
      algorithms as an Informational RFC
    - One or more recommended congestion control algorithms for interactive real time
      media as Proposed Standard RFCs