Subject: Re: Very slow pipe/TCP connection in 1.6_BETA4
To: None <>
From: Havard Eidnes <>
List: current-users
Date: 07/16/2002 11:19:07
> I am running a Amanda amrecover for a single file on a i386/1.6_BETA4=

> system and the througput through /dev/nrst0 >> amrestore >pipe> gzip
> -d >TCP> /sbin/restore manages less than 10Kbytes/sec.
> A tcpdump shows that the ACK is sent 0.2 seconds after the packet it
> acknowledges:

This indicates that you are only getting ACKs on timeouts for this TCP

> netstat shows the Send-Q of the gzip end of the connection
> always at 16384 byes, the Recv-Q of the restore side always
> at 0 bytes.

This tends to indicate that the TCP window sizes are 16KB, at least on
the sender side.  Now, I wonder how this interacts with the lo0 MTU of
33220 bytes.  Badly, is my best guess...  If your TCP window size is
16384 bytes, no TCP segment sent across this interface will ever be
MSS-sized.  If you have Nagle's algorithm enabled on the connection it
will interfere (it'll tend to hold on to segments), as will probably
the ack strategy, as the general rule for sending ACKs is, I think, one=

ACK per 2 MSS or on timeout, and you're seeing the latter.

This could be some extreme form of the TCP performance anomalies
earlier described in a paper by Kjersti Moldeklev (a fellow Norwegian),=

a pointer can be found on

These problems are triggered when there is a mismatch between the MTU
and the TCP window sizes.

However, simplistic testing on an i386/1.6B SMP system with ttcp, 16KB
receive/send space with a connection across lo0 does not appear to
trigger the problem, so I'm slightly puzzled.

If my theory holds some water, a possible short-term workaround could
be to tweak /sys/net/if_loop.c to define LOMTU to something a little
less than 1/3 of 16KB, e.g. (4096 + MHLEN + MLEN).  (Apparently the MTU=

of the loopback interface can not be reconfigured with ifconfig on the

I'm unsure of what the correct longer-term fix would be.


- H=E5vard