Re: TCP/IP CRC question

From: Ansgar -59cobalt- Wiechers (bugtraq_at_planetcobalt.net)
Date: 10/07/04

  • Next message: Josh Mills: "Re: Auditing a Win2K box" 
    Date: Thu, 7 Oct 2004 11:43:42 +0200
To: security-basics@securityfocus.com

    On 2004-10-06 Jorge Mendez Bonini wrote:
    > 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..)?

    Not sure if I understand your question correctly. The FCS is a checksum
    on the entire Ethernet frame. Its purpose is to detect transmission
    failures on the link-layer (and discard erroneous frames so they don't
    have to be processed by upper layers).

    Regards
    Ansgar Wiechers 

    -- 
"Those who would give up liberty for a little temporary safety
deserve neither liberty nor safety, and will lose both."
--Benjamin Franklin
  • Next message: Josh Mills: "Re: Auditing a Win2K box"

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