Path: utzoo!attcan!uunet!husc6!bloom-beacon!apple!bionet!agate!ucbvax!SEISMO.CSS.GOV!rick
From: rick@SEISMO.CSS.GOV (Rick Adams)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: SLIP MTU "violation" - does anyone? does anyone care?
Message-ID: <8810031741.AA04998@beno.CSS.GOV>
Date: 3 Oct 88 17:41:38 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 21

When I was running 4.2bsd (or maybe it was 4.1C?) on a vax connected to
an IMP via 1822 (circa 1983, when the code was written), "netstat -i"
showed an MTU of 1006 for the IMP interface.

I believe it is from the 1007 bytes rounded down to an even number
because of something that didn't work right with odd numbered MTUs.

As I said, it's an historical relic that there is no good reason for
perpetuating.

The "correct" MTU should really be something like 1024 data bytes + IP
headers + TCP headers.

Please keep in mind that SLIP was a hack. There was no real design to
it. It was a quick solution to an immediate problem. The framing scheme
was chosen to be compatible with 3Com's UNET (anyone still remember
that?). The MTU was chosen to avoid fragmenting when passing through
IMPS. Any other parameter you might find was chosen in a similar
manner. I.e. get it to work fast.

---rick