Subject: I-D ACTION:draft-kksjf-ecn-00.txt

From: [email-masked]
Date: Tue, 18 Nov 1997 10:02:36 -0500

A New Internet-Draft is available from the on-line Internet-Drafts directories.

	Title		: A Proposal to add Explicit Congestion 
                          Notification (ECN) to IPv6 and to TCP
	Author(s)	: K. Ramakrishnan, S. Floyd
	Filename	: draft-kksjf-ecn-00.txt
	Pages		: 11
	Date		: 17-Nov-97
	
   This note describes a proposed addition of ECN (Explicit Congestion
   Notification) to IPv6 and to TCP.  First we describe TCP's use of
   packet drops as an indication of congestion.  Next we argue that with
   the addition of active queue management (e.g., RED) to the Internet
   infrastructure, where routers detect congestion before the queue
   overflows, routers are no longer limited to packet drops as an
   indication of congestion, but could instead set an ECN bit in the
   packet header, for ECN-capable transport protocols.  We describe when
   the ECN bit would be set in the routers, and describe what
   modifications would be needed to TCP to make it ECN-capable.
   Modifications to other transport protocols (e.g., unreliable unicast
   or multicast, reliable multicast, other reliable unicast transport
   protocols) could be considered as those protocols advance through the
   standards process.

   TCP's congestion control and avoidance algorithms are based on the
   notion that the network is a black-box [Jacobson88, Jacobson90].  The
   network's state of congestion or otherwise is determined by end-
   systems probing for the network state, by gradually increasing the
   load on the network (by increasing the window of packets that are
   outstanding in the network) until the network becomes congested and a
   packet is lost.  Treating the network as a "black-box" and treating
   loss as an indication of congestion in the network is appropriate for
   pure best-effort data carried by TCP that has little or no
   sensitivity to delay or loss of individual packets.  In addition,
   TCP's congestion management algorithms have techniques built-in (such
   as fast retransmit and fast recovery) to minimize the impact of
   losses from a throughput perspective.

   However, these mechanisms are not intended to help applications that
   are in fact sensitive to the delay or loss of one or more individual
   packets.  Interactive traffic such as telnet, web-browsing, and
   transfer of audio and video data ("real-audio" and "real-video") can
   be sensitive to packet losses (for unreliable data delivery such as
   UDP) or to the increased latency of the packet caused by the need to
   retransmit the packet after a loss (for reliable data delivery such
   as TCP).
 
   Since TCP determines the appropriate congestion window to use by
   gradually increasing the window size until it experiences a dropped
   packet, this causes the queues at the bottleneck router to build up.
   With most packet drop policies at the router that are not sensitive
   to the load placed by each individual flow, this means that some of
   the packets of latency-sensitive flows are going to be dropped.
   Active queue management mechanisms that detect congestion before the
   queue overflows, and provide an indication of this congestion to TCP,
   is desirable because it avoids some bad properties of dropping on
   queue overflow, especially with drop-tail schemes.  Drop tail
   introduces synchronization of loss across multiple flows which is
   undesirable.  Indicating incipient congestion means that TCP does not
   have to increase its window size up to the point where a router's
   buffer is filled up. This can reduce queuing delays and avoid
   synchronization, which are desirable characteristics.

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