[e2e] purpose of pseudo header in TCP checksum

Alok alokdube at hotpop.com
Tue Feb 15 02:27:48 PST 2005


As per RFC 793,

    This pseudo header contains the Source
    Address, the Destination Address, the Protocol, and TCP length.
    This gives the TCP protection against misrouted segments.  This
    information is carried in the Internet Protocol and is transferred
    across the TCP/Network interface in the arguments or results of
    calls by the TCP on the IP.

                     |           Source Address          |
                     |         Destination Address       |
                     |  zero  |  PTCL  |    TCP Length   |

      The TCP Length is the TCP header length plus the data length in
      octets (this is not an explicitly transmitted quantity, but is
      computed), and it does not count the 12 octets of the pseudo

what is the purpose of the using SA and DA for checksum calculations for the
TCP header?
Is it because that the SA/DA may be corrupted as the IP header checksum does
not run on SA/DA ?

I mean I dont understand what "This gives the TCP protection against
misrouted segments" means, do intermediate routing nodes look at TCP


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