Subject: Re: TCP_MSS to 1460 in tcp.h or /etc/netstart?
To: Wolfgang Rupprecht <>
From: Randolph C. Nicklas <>
List: tech-net
Date: 10/28/1997 07:56:41
At 09:42 AM 10/25/97 -0800, Wolfgang Rupprecht wrote:
> (Erik E. Fair  , Time Keeper) writes:
>> 1. change sys/netinet/tcp.h TCP_MSS to 1460
>I've run this way for 5 years now.  First under SunOS, lately under
>NetBSD.  In practice it works just fine.
>It would be great if I didn't have to run a sed script on tcp.h before
>each build.
>> The Ethernet MTU has been the standard MTU for DS0 (64Kb/s) and T1 leased
>> lines in cisco equipment for a long time, and they're still the majority of
>> the backbone last I checked. MTU for FDDI and T3 is even higher.
>I know that FDDI is usually run with 4kB packets.  Out of curiosity,
>what do folks use on T3?

	Cisco's default MTU on its HSSI ports (HSSI-DSU-DS3) is 4470,
	same as the default on its FDDI ports.

>And I almost dread posting this, each time I mention a more modern
>MTU/MSS I get tons of mail from both sides of the cluefullness scale.
>(It seems to be a common misconseption for folks setting up their
>SLIP/PPP links to request a really small MTU in an attempt to get a
>better interactive response.  In the absense of TOS queuing, I don't
>see how it matters much if an interactive packet is stuck behind 3 500
>byte packets or one 1500 byte one.)