Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!tut.cis.ohio-state.edu!ucbvax!PESCADERO.STANFORD.EDU!deering
From: deering@PESCADERO.STANFORD.EDU (Steve Deering)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: more on Fletcher
Message-ID: <89.12.09.1713.750@pescadero.stanford.edu>
Date: 10 Dec 89 01:13:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 22

Keith,

My reason for wanting the checksum at the end of the packet is to allow
a high-performance (hardware-assisted) implementation to compute the
checksum on the fly as it copies a packet onto the wire, tacking it
on at the end.  This eliminates one expensive pass over the packet.  I
believe that Ultra's speedy TP-4 implementation places the checksum at
the end for the same reason.

> 	It wouldn't be hard to come up with a minor modification to
> the VMTP checksum having the same property of position independence,
> which would be almost as fast to compute as the TCP checksum.

We'd welcome any suggestions.  We did consider using 16-bit Fletcher
(i.e., two 16-bit accumulators, yielding a 32-bit result), but as you
mentioned, it would be several times slower than the TCP checksum.
The TCP checksum takes basically one instruction per 32-bit word on
common processors (using add-with-carry and loop unrolling); it's
pretty hard to beat that.  VMTP's checksum is a few percent slower
than TCP's; it's not as strong as we would like.

Steve