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From: Luigi Rizzo <luigi@labinfo.iet.unipi.it>
Message-Id: <199609201218.OAA23333@labinfo.iet.unipi.it>
Subject: IP queues -- how to limit their size ?
To: isp@freebsd.org
Date: Fri, 20 Sep 1996 14:18:58 +0200 (MET DST)
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Hi,

(with reference to BSD implementations of TCP, but I'd be curious to
know how routers approach this problem):

is there any way to limit the amount of packets queued for delivery
on a given interface ? I am asking because I notice an annoying
problem when using PPP over a 14.4 line (easily reproducible when
using a WWW browser).

When one or more connections are transferring data, queues tend to
pile up segments, up to the maximum window size for each connection
(16K default). Since the bandwidth is so low, the queues are not
very long (10-12 segments per connection) but take a long time to
flush (4..12 sec per 16k, depending on the effectiveness of
compression). The connections end up with an RTT constantly growing,
up to unacceptable values (10s or more), and the RTT is totally
dominated by the queueing delay.

All this is totally pointless (as it delays other data, causes
reduced control over the line, etc.) but since TCP as it is now
does not stop expanding windows until a drop occurs, the only
solution would be to avoid that a queue on an interface contains
more than X seconds of data (related to its speed).

So, any idea on how to set this limitation ?

	Thanks
	Luigi
====================================================================
Luigi Rizzo                     Dip. di Ingegneria dell'Informazione
email: luigi@iet.unipi.it       Universita' di Pisa
tel: +39-50-568533              via Diotisalvi 2, 56126 PISA (Italy)
fax: +39-50-568522              http://www.iet.unipi.it/~luigi/
====================================================================