Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service: Architecture
Transport Area Working Group B. Briscoe, Ed.
Intended status: Informational K. De Schepper
Expires: August 23, 2020 Nokia Bell Labs
M. Bagnulo Braun
Universidad Carlos III de Madrid
February 20, 2020
Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service:
This document describes the L4S architecture, which enables Internet
applications to achieve Low Latency, Low Loss, and Scalable
throughput (L4S), while coexisting on shared network bottlenecks with
existing Internet applications that are not built to take advantage
of this new technology.
In traditional bottleneck links that utilize a single, shared egress
queue, a variety of application traffic flows can share the
bottleneck queue simultaneously. As a result, each sender's behavior
and its response to the congestion signals (delay, packet drop, ECN
marking) provided by the queue can impact the performance of all
other applications that share the link. Furthermore, it is
considered important that new protocols coexist in a reasonably fair
manner with existing protocols (most notably TCP and QUIC). As a
result, senders tend to normalize on behaviors that are not
significantly different than those in use by the majority of the
existing senders. For many years, the majority of traffic on the
Internet has used either the Reno AIMD congestion controller or the
Cubic algorithm, and as a result any newly proposed congestion
controller needs to demonstrate that it provides reasonable fairness
when sharing a bottleneck with flows that use Reno or Cubic. This
has led to an ossification in congestion control, where improved
congestion controllers cannot easily be deployed on the Internet.
It is well known that the common existing congestion controllers
(e.g. Reno and Cubic) increase their congestion window (the amount
of data in flight) until they induce congestion, and they respond to
the congestion signals of packet loss (or equivalently ECN marks) by
significantly reducing their congestion window. This leads to a
large sawtooth of the congestion window that manifests itself as a
combination of queue delay and/or link underutilization.
Briscoe, et al. Expires August 23, 2020 [Page 1]
Internet-Draft L4S Architecture February 2020
Meanwhile, in closed network environments, such as data centres, new
congestion controllers (e.g. DCTCP [RFC8257]) have been deployed
that significantly outperform Reno and Cubic in terms of queue delay
and link utilization across a much wider range of network conditions.
The L4S architecture provides an approach that allows for the
deployment of next generation congestion controllers while preserving
reasonably fair coexistence with Reno and Cubic.
The L4S architecture consists of three components: network support to
isolate L4S traffic from other traffic and to provide appropriate
congestion signaling to both types; protocol features that allow
network elements to identify L4S traffic and allow for communication
of congestion signaling; and host support for immediate congestion
signaling and an appropriate congestion response that enables
Status of This Memo
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