Security Basics: Re: TCP/IP CRC question

[Don Parker <dparker () bridonsecurity com>]

Hi Jorge, the checksum is used for all four core protocls. It is also calculated 
by the sending computer. Each time it traverses a router it is then recalculated 
as the ttl value changes. Please bear in mind though that the checksum is only for 
the header itself.

This checksum is there so that the destination computer once it receives the 
packet will recompute the checksum to confirm the packet was not corrupted from 
the last hop. Hopefully this answers your questions.

Cheers,

Don

--------------------------------------------------------------
Don Parker, GCIA GCIH
Intrusion Detection & Incident Handling Specialist
Bridon Security & Training Services
http://www.bridonsecurity.com
voice: 1-613-302-2910
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On Wed, 06 Oct 2004 01:15 , Jorge Mendez Bonini <jlmb () cableonda net> sent:

> 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.