RE: TCP/IP CRC question

["David Gillett" <gillettdavid () fhda edu>]

1.  The upper-layer protocols are end-to-end; the link layer
gets rebuilt with every hop.

2.  The FCS is allowed to be left as zero by the sender, and/or
ignored by the recipient.

David Gillett


> -----Original Message-----
> From: Jorge Mendez Bonini [mailto:jlmb () cableonda net]
> Sent: Tuesday, October 05, 2004 11:15 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.