Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cwjcc!tut.cis.ohio-state.edu!ucbvax!A.ISI.EDU!CERF
From: CERF@A.ISI.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: X.25 and IP
Message-ID: <[A.ISI.EDU].5-Feb-89.15:37:14.CERF>
Date: 5 Feb 89 20:37:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 35

Art,

Now I am confused. Two cases: 

1. no gateways intervene
2. one or more gateways intervene between source and destination.

In the first case, the source needs to initiate an X.25 VC to
the destination. Given the IP destination address, known to
the source, the source must translate from IP destination
to comparable X.121 address. This is going to have to be
either straight table look-up or algorithmic (mapping of
a 32 IP address into an X.121 address - non-trivial in most
cases). The receiving host will get a call set-up packet
at the X.25 level which contains at least the source X.121
address. For this special case, one might be able to map
from the source X.121 address in the X.25 call set-up packet
to the source IP address, but this seems unnecessary since
the arriving IP packet coming on the set-up link will contain
source and destination IP addresses. Are you trying to bind
the call set-up to a particular service process before finding
out the originating IP source address?

In the second case, as you point out, the calling address of the
final X.25 call set-up may bear no relationship to the source
IP address of the caller since the last VC is between a gateway
andthe target host, not between the source host and thetarget
host. That being the case, it doesn't seem possible to try to map
the calling X.121 address into a source IP address at all.

I have the continuing feeling that I have not understood the
problem originally posed since I'm not able to make sense of your
most recent answer.

Vint