Path: utzoo!mnetor!uunet!husc6!purdue!riedl
From: riedl@cs.purdue.EDU (John T Riedl)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: CRC calculation costs (was Re: SLIP working group? )
Message-ID: <3765@medusa.cs.purdue.edu>
Date: 5 Apr 88 19:47:39 GMT
Sender: news@cs.purdue.EDU
Organization: Department of Computer Science, Purdue University
Lines: 26

Tom Mueller and I did some experiments [1] on measuring the costs of
various components of UDP on Suns (3/50s), using *very small packets*.
We measured UDP round-trips of 7.2ms, with checksuming being 0.12ms of
that, and mbuf manipulation about 0.5ms.  1.6ms was spent in the
socket layering (just the socket abstraction code), 0.5ms in the
context switch on receive, and 0.4ms in copying from kernel to user
space.  Note that these are the cheap 1s-complement bitsums that have
been under discussion recently and that only the headers are being
checksumed.

Our data compare well with Watson and Mamrak's work on VMS [2].  They
report on transmitting 1024 byte packets.  The total transmission time
(one-way) was 4.2ms, of which 0.5 ms was data checksuming vs. 0.8ms
for the context switch and system call overhead.  So at least in some
environments, checksuming is relatively cheap.

[1] Bharat Bhargava, Tom Mueller, and John Riedl, "Experimental
Analysis of Layered Ethernet Software".  In Proceedings of the
ACM-IEEE Computer Society 1987 Fall Joint Computer Conference.

[2] Richard Watson and Sandy Mamrak, "Gaining Efficiency in Transport
Services by Appropriate Design and Implementation Choices".  ACM
Transactions on Computer Systems, May 1987.
-- 
John Riedl
{ucbvax,decvax,hplabs}!purdue!riedl  -or-  riedl@mordred.cs.purdue.edu