RE: mss negotiation and path mtu discovery mostly broken?

From: Ristuccia, Brian
Date: Wed Apr 25 2007 - 10:31:50 EST

> > 08:39:55.493029 IP > S
> > 2768979373:2768979373(
> > 0) win 5840 <mss 1460,sackOK,timestamp 3873837730 0,nop,wscale 2>
> > 08:39:55.493119 IP > S
> > 963242385:963242385(0)
> > ack 2768979374 win 17896 <mss 8960,sackOK,timestamp 413751
> The MSS clamp for sending to is 8960. MSS is
> only one way -- each uses what the other tells it.

Right - except that keeps sending to with an
increasingly large MSS, even though has asked for no larger
than 1460.

> > In the following dump, the system eventually gets in a
> state where it
> > oscillates between sendng undeliverable 2896 byte packets and
> > deliverable 1448 byte ones.
> This should only happen on PMTU expire, which is normally ~15mins.
> Perhaps you misconfigured it manually using sysctl.

This is /proc/sys/net/ipv4/route/mtu_expires? It's 600.

Brian Ristuccia

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