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From: rbraun@spdcc.COM (Rich Braun)
Newsgroups: comp.dcom.lans
Subject: Re: SLIP/PPP perf. differences
Message-ID: <7620@spdcc.SPDCC.COM>
Date: 23 May 91 21:00:34 GMT
References: <1104@venice.SEDD.TRW.COM> <7611@spdcc.SPDCC.COM> <1991May23.023743.3969@searchtech.com>
Organization: Kronos Inc., Waltham, Mass.
Lines: 22

I wrote:
>>Header compression brings your 41-byte echo packet down to something
>>like 3 bytes ...

mra@searchtech.com (Michael Almond) writes:
>Why doesn't regular TCP/IP use this?

Because Ethernet packets not only have a maximum size (1536 bytes), but
they also have a minimum size (64 bytes, I think).  The reason for this
is to ensure that the collision-detection logic has enough time during
a packet transmission to work throughout an Ethernet segment, which has
a maximum physical length related to the speed of light divided by the
transmission time of said minimum-size packet.  (This is a very simplified
explanation.)  If you look at a typical Ethernet datagram which is shorter
than the minimum, you will find it padded out to the minimum length (with
zeros, typically).

Because of this, there was no incentive to implement header compression
on Ethernet implementations.  Serial protocols are obviously another
matter entirely.

-rich