RE: TCP/IP CRC question

From: Shaineel Singh (ssingh_at_kbs.net.au)
Date: 10/07/04

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    To: "'Jorge Mendez Bonini'" <jlmb@cableonda.net>, <security-basics@securityfocus.com>
Date: Thu, 7 Oct 2004 14:55:22 +1000

     
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    While the CRC in the data field would be to ensure integrity the FCS
    field is there to safeguard against collision, etc

    Have a read of this for a better explanation
    http://support.3com.com/infodeli/tools/netmgt/tncsunix/product/091500/
    c11ploss.htm

    Shaineel Singh
    p: +61 422 921 951
    e: mailto:ssingh@kbs.net.au
    w: http://203.220.32.126/~ssingh/

    pgp: 91AC 0F02 B206 36F9 F414 D1EE 8545 94F0 7804 A1EF

    - -----Original Message-----
    From: Jorge Mendez Bonini [mailto:jlmb@cableonda.net]
    Sent: Wednesday, 6 October 2004 4:15 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.

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