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Low Extra Delay Background Transport (ledbat) (WG)

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Transport Area Director(s):

Transport Area Advisor:

Meeting Slides


No Request For Comments

Charter (as of 2011-04-07)

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.

Goals and Milestones:

Oct 2009  Submit 'Multiple Transport Connections in Applications Design' to the IESG for consideration as an Informational RFC
Oct 2009  Submit 'Low Extra Delay Background Transport (LEDBAT)' to the IESG for consideration as an Experimental RFC
Feb 2010  Submit 'A Survey of Lower-than-Best Effort Transport Protocols' to the IESG for consideration as an Informational RFC