Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!ucsd!ucbvax!NNSC.NSF.NET!craig
From: craig@NNSC.NSF.NET (Craig Partridge)
Newsgroups: comp.protocols.tcp-ip
Subject: more on Fletcher
Message-ID: <8911301916.AA14465@ucbvax.Berkeley.EDU>
Date: 30 Nov 89 17:13:52 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 62


Hey folks -- let's not make using a different checksum harder than it
needs to be.  It is very simple to do.  A one-page RFC explaining how
is appended.

Please note I'm not endorsing the use of alternate checksums (although
it seems harmless enough).

Craig

Internet Engineering Task Force                           C. Partridge
Request for Comments: XXXX                                BBN

                     TCP Alternate Checksum Option

Status of This Memo

    This memo is a draft RFC that proposes a TCP option to allow
    use of alternate 16-bit checksums in the TCP header.
    Distribution of this memo is unlimited.

Introduction

    Some members of the networking community have expressed
    interest in using checksums with different error detection and
    correction properties than the standard TCP checksum.  This
    option makes use of such checksums feasible, provided the
    checksums are 16-bit quantities.

Definition of the Option

    This three-byte option may be sent in a SYN segment by a TCP
    to indicate that it is prepared to both and receive an
    alternate checksum.  The alternate checksum replaces the
    regular TCP checksum in the checksum field of the TCP header.
    It is computed over the same data as the regular TCP checksum.

    TCP Alternate Checksum Option:


             +---------+---------+---------+
             | Kind=X  |Length=3 |  cksum  |
             +---------+---------+---------+

    Here cksum is a number identifying the type of checksum
    to be used.  The currently defined values of cksum are:

		    0  -- TCP checksum
		    1  -- 16-bit Fletcher's checksum

    To use an alternate checksum, a TCP must both send and receive
    a SYN segment specifying that the same alternate checksum
    is to be used.  Note that SYN segments must always use the
    TCP checksum.

    If a TCP receives an option containing different checksum
    type value from the one it sent or does not receive an option
    from the remote TCP, it must use the TCP checksum on the connection.
    Observe that in practice this means that the initiating TCP
    determines whether an alternate checksum is used, and the
    listening TCP can only choose to accept or reject the
    proposed alternate checksum.


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