Security Basics: Re: TCP/IP CRC question

[Ansgar -59cobalt- Wiechers <bugtraq () planetcobalt net>]

On 2004-10-06 Jorge Mendez Bonini wrote:
> 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..)?
Not sure if I understand your question correctly. The FCS is a checksum
on the entire Ethernet frame. Its purpose is to detect transmission
failures on the link-layer (and discard erroneous frames so they don't
have to be processed by upper layers).

Regards
Ansgar Wiechers
-- 
"Those who would give up liberty for a little temporary safety
deserve neither liberty nor safety, and will lose both."
--Benjamin Franklin