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Towards Adaptive Congestion Management for Interactive Real-Time Communications. Dirk Kutscher Mirja Kühlewind Bob Briscoe IAB/IRTF Congestion Control Workshop July 2012. Interactive Real-Time Communication on the Internet. SIP. RTP. RTCP. RTP. RTCP. We started here.

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towards adaptive congestion management for interactive real time communications

Towards Adaptive Congestion Management for Interactive Real-Time Communications

Dirk Kutscher Mirja Kühlewind Bob Briscoe

IAB/IRTF Congestion Control Workshop

July 2012

interactive real time communication on the internet
Interactive Real-Time Communication on the Internet

SIP

RTP

RTCP

RTP

RTCP

We started here...

arrived at systems
...Arrived at Systems
  • IMS: SIP-based control framework in UMTS and LTE
  • Using SIP to trigger QoS reservation
  • Dedicated bearers for real-time traffic
  • Extensive QoS and control infrastructure (“carrier grade”)
rtcweb
RTCWEB
  • Real-time communication without the telco framework
    • No ossified control architecture
    • Web-based model (browser programmability) to foster innovation
    • But obviously no QoS framework
    • Congestion control essentially missing
essentially two problems
Essentially: Two Problems
  • Lack of congestion control of IRTC flows
    • Anticipated usage scenarios of RTCWEB to impact TCP and other flows
      • A perceived threat may be enough to trigger operator actions (or to make users abandon it)
    • Uptake of RTCWEB endangered
  • Impact of TCP-based flows on IRTC
    • Poor queue management & loss-based congestion control to induce delays
    • Unhappy RTCWEB users
challenge
Challenge
  • Reasonable large-scale IRTC performance over the Internet?
    • Without having to resort to closed control frameworks?
    • Without needing separate queues for IRTC and TCP?
way forward
Way Forward
  • Re-think fairness
    • Per flow fairness not helpful
      • Cf. current usage of multi-TCP flows by applications
    • Especially not useful for IRTC
      • E.g., video codecs creating key frames => spikes in sending rates
    • Enable flows to take a bigger share
      • When needed – and when harm to others is acceptable
  • Promote, incentivize and ultimately enforce
    • Congestion responsiveness and minimisation
    • For all traffic
needed elements
Needed Elements
  • Better queue management and better congestion signals
    • AQM
  • Fast and accurate feedback
    • ECN for RTP (& deploy ECN for TCP too!)
  • Adaptiveness on transport and application layers
    • Codec support is there – need profiles and usable APIs
  • Incentives to adapt
    • And a suitable fairness model
incentives
Incentives
  • Making 'cost' of congestion visible to OS on RTT scales
    • Cost: congestion contribution per flow
  • Making 'cost' of congestion visible to network operators
  • Example: User A running multiple applications
    • Multiple flows per application session
    • Receiving congestion signals on all of them
    • Only user/application knows what is important
    • Decision on how to re-act should be based on app profiles and user prefs
    • If important video stream cannot be adapted, some TCP flow should (disproportionally)
    • Same model for competing flows of different users / hosts
congestion exposure conex
Congestion Exposure (ConEx)
  • Making congestion visible in the network
    • At different time scales, for different purposes
  • Senders know their current congestion contribution and declare it to network
  • Operator policy can provide necessary incentives to „act responsibly“ and otherwise enforce
  • Other usages (on larger time scales)
    • Capacity Provisioning and Traffic engineering according to observed congestion...
summary
Summary
  • Want to enable viable IRTC with minimized control plane
  • Have to bring down delay and promote adaptiveness
  • Many elements already there
  • Incentives model based on congestion exposure one of them
  • Experiments needed