Security Basics: RE: TCP/IP CRC question

["Shaineel Singh" <ssingh () kbs net au>]

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While the CRC in the data field would be to ensure integrity the FCS
field is there to safeguard against collision, etc

Have a read of this for a better explanation
http://support.3com.com/infodeli/tools/netmgt/tncsunix/product/091500/
c11ploss.htm

Shaineel Singh
p: +61 422 921 951
e: mailto:ssingh () kbs net au
w: http://203.220.32.126/~ssingh/

pgp: 91AC 0F02 B206 36F9 F414  D1EE 8545 94F0 7804 A1EF



- -----Original Message-----
From: Jorge Mendez Bonini [mailto:jlmb () cableonda net] 
Sent: Wednesday, 6 October 2004 4:15 PM
To: security-basics () securityfocus com
Subject: TCP/IP CRC question


I've been reading about the TCP/IP protocol stack (TCP/Ilustrated
Vol1 
by Richard Stevens) lately and since Ethernet is quite common
nowadays 
almost all link layer examples refer to it (I also checked Douglas
Comer 
  Internetworking with TCP/IP vol1).
I tried searching RFC but found it very time consuming without good 
results (maybe I'm not used to it...yet)

The Ethernet Frame contains a field known as FCS which contains a
CRC. Now, my question is: If the CRC is generated from DATA field
among others, What's the point 
of using checksums on the upper protocolos (IP checksums etc..)?

Thanks for your time.

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