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From: smb@ulysses.homer.nj.att.com (Steven M. Bellovin)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Source Quench & Jacobson's Alg.
Message-ID: <11349@ulysses.homer.nj.att.com>
Date: 21 Mar 89 18:44:00 GMT
References: <36540001@hpindwa.HP.COM>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 38

RFC 1009 requires that source quench be sent; this statement should be
taken as authoritative:

	      2.2.3.  Source Quench

	 All gateways must contain code for sending ICMP Source Quench
	 messages when they are forced to drop IP datagrams due to
	 congestion.  Although the Source Quench mechanism is known to
	 be an imperfect means for Internet congestion control, and
	 research towards more effective means is in progress, Source
	 Quench is considered to be too valuable to omit from production
	 gateways.

	 There is some argument that the Source Quench should be sent
	 before the gateway is forced to drop datagrams [62].  For
	 example, a parameter X could be established and set to have
	 Source Quench sent when only X buffers remain.	 Or, a parameter
	 Y could be established and set to have Source Quench sent when
	 only Y per cent of the buffers remain.

	 Two problems for a gateway sending Source Quench are: (1) the
	 consumption of bandwidth on the reverse path, and (2) the use
	 of gateway CPU time.  To ameliorate these problems, a gateway
	 must be prepared to limit the frequency with which it sends
	 Source Quench messages.  This may be on the basis of a count
	 (e.g., only send a Source Quench for every N dropped datagrams
	 overall or per given source host), or on the basis of a time
	 (e.g., send a Source Quench to a given source host or overall
	 at most once per T millseconds).  The parameters (e.g., N or T)
	 must be settable as part of the configuration of the gateway;
	 furthermore, there should be some configuration setting which
	 disables sending Source Quenches.  These configuration
	 parameters, including disabling, should ideally be specifiable
	 separately for each network interface.

	 Note that a gateway itself may receive a Source Quench as the
	 result of sending a datagram targeted to another gateway.  Such
	 datagrams might be an EGP update, for example.