RE: X.25 Call User Data for higher level protocols
4 Feb 88 10:17:00 PST

>I am in need of the protocol identifiers used in the CALL USER DATA
>field of the X.25 CALL REQUEST packet for various higher level
>protocols. I believe I know the following information but need to be
>able to fill in the blanks. Can anyone help or provide a reference as
>to where I might find this data?
>Who supplies these numbers? ISO? I would like to define an ID for a
>virtual circuit that includes an extra 16 bit data field that could be
>used like the ethernet type field. Has something like this already
>been defined? Should I be using 802.2? Comments?
> Protocol Call User Data (byte in hex)
> -------- --------------
> PAD 0x01
> IP 0xCC
> XNS ??
> PUP ??
> Chaos ??


CCITT defines the high order two bits of the first octet of call user data
as follows:

        00 - Used for other CCITT recommendations (such as X.29)
        01 - Reserved for use by "national" administrative authorities
        10 - Reserved for use by international administative authorities
        11 - Reserved for arbitrary use between consenting DTEs

I believe (they can correct me if I'm wrong) that BBN chose 0xCC and 0xCD
for use on the DDN. If you want to establish a standard for use in the
Internet I would suggest contacting both DCA and ANSI to see if there is

I would appreciate hearing any results you find.

                                                Art Berggreen


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