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Low Extra Delay Background Transport

Document Charter Low Extra Delay Background Transport WG (ledbat)
Title Low Extra Delay Background Transport
Last updated 2008-11-11
State Approved
WG State Concluded
IESG Responsible AD Wesley Eddy
Charter edit AD (None)
Send notices to (None)

The LEDBAT WG is chartered to standardize a congestion
  control mechanism that should saturate the bottleneck,
  maintain low delay, and yield to standard TCP.
  Applications that transmit large amounts of data for a long
  time with congestion-limited TCP, but without AQM, fill the
  buffer at the head of the bottleneck link. With FIFO queue,
  this increases the delay experienced by other applications.
  With buffer of one RTT, the delay doubles. In some cases,
  the extra delay may be much larger. This is a particularly
  acute and common case is when P2P applications upload over
  thin home uplinks: delays in these cases can sometimes be of
  the order of seconds.
  The IETF's standard end-to-end transport protocols have not
  been designed to minimize the extra delay introduced by them
  into the network. TCP, as a side effect of filling the
  buffer until it experiences drop-tail loss, effectively
  maximizes the delay. While this works well for applications
  that are not delay-sensitive, it harms interactive
  applications that share the same bottleneck. VoIP and games
  are particularly affected, but even web browsing may become
  LEDBAT is a transport-area WG that will focus on broadly
  applicable techniques that allow large amounts of data to be
  consistently transmitted without substantially affecting the
  delays experienced by other users and applications.
  The WG will work on the following:
  (1) An experimental congestion control algorithm for
  less-than-best-effort "background" transmissions, i.e., an
  algorithm that attempts to scavenge otherwise idle bandwidth
  for its transmissions in a way that minimizes interference
  with regular best-effort traffic.
  Desired features of such an algorithm are:
  * saturate the bottleneck
  * eliminate long standing queues and thus keep
  delay low when no other traffic is present
  * quickly yield to traffic sharing the same bottleneck queue
  that uses standard TCP congestion control
  * add little to the queueing delays induced by TCP traffic
  * operate well in networks with FIFO queueing with drop-tail
  * be deployable for popular applications that currently
  comprise noticeable fractions of Internet traffic
  * where available, use explicit congestion notification (ECN),
  active queue management (AQM), and/or end-to-end
  differentiated services (DiffServ).
  Application of this algorithm to existing transport
  protocols (TCP, SCTP, DCCP) is expected to occur in the
  working groups that maintain those protocols.
  Once experience is gained with this congestion control
  algorithm on the Internet, the WG will consider if it is
  appropriate to ask the IESG to advance the document as a
  Proposed Standard.
  (2) A document that clarifies the current practices of
  application design and reasons behind them and discusses the
  tradeoffs surrounding the use of many concurrent TCP
  connections to one destination and/or to different
  Applications routinely open multiple TCP connections. For
  example, P2P applications maintain connections to a number
  of different peers while web browsers perform concurrent
  downloads from the same web server. Application designers
  pursue different goals when doing so: P2P apps need to
  maintain a well-connected mesh in the swarm while web
  browsers mainly use multiple connections to parallelize
  requests that involve application latency on the web server
  side. The IETF transport area community is concerned about
  this practice, because standard Internet congestion control
  results in different transport connections sharing
  bottleneck capacity. When an application uses several
  non-rate-limited transport connections to transfer through a
  bottleneck, it may obtain a larger fraction of the
  bottleneck than if it had used fewer connections. Although
  capacity is the most commonly considered bottleneck
  resource, middlebox state table entries are also an
  important resource for an end system communication. Other
  resource types may exist, and the guidelines are expected to
  comprehensively discuss them.
  Applications use a variety of techniques to mitigate these
  concerns. These techniques have not always been reviewed by
  the IETF and their interaction with TCP dynamics is poorly
  understood. The WG will document the known techniques,
  discussing the consequences and, where appropriate, provide
  guidance to application designers.