Re: TCP/IP CRC question

[Fernando Gont <fernando () gont com ar>]

At 02:11 07/10/2004 +0300, Alex V.Lukyanenko wrote:

> JMB> The Ethernet Frame contains a field known as FCS which contains a CRC.
> JMB> Now, my question is:
> JMB> If the CRC is generated from DATA field among others, What's the point
> JMB> of using checksums on the upper protocolos (IP checksums etc..)?
> JMB> Thanks for your time.
>
> Because the protocols are independant of the underlying/overlying
> protocols, i.e.:

Not completely true: The TCP checksum is computed using a pseudoheader tha 
includes fields (the source and destination IP addresses) from the Internet 
Protocol (IP).


> 1. Ethernet can be used for the transmission of higher-level protocols
> that do not perform CRC checking

No. You still need end-to-end error detection. Google for "end-to-end 
arguments in system design"., a classic paper which includes stuff on this 
issue.


> 2. IP can go over a protocol that does not perform CRC checking.

True for IPv4, although not strictly true for IPv6.


--
Fernando Gont
e-mail: fernando () gont com ar || fgont () acm org