Security Basics: Re: TCP/IP CRC question

[Alex V.Lukyanenko <y_avenger_y () ua fm>]

Hello Jorge,

Wednesday, October 6, 2004, 9:15:19 AM, you wrote:

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

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.:

1. Ethernet can be used for the transmission of higher-level protocols
that do not perform CRC checking
2. IP can go over a protocol that does not perform CRC checking.

It's better to be safe than sorry.

-- 
Best regards, mailto:y_avenger_y () ua fm