Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!CISCO.COM!satz
From: satz@CISCO.COM
Newsgroups: comp.protocols.tcp-ip
Subject: Re: X.25 and IP
Message-ID: <8902062326.AA23640@ucbvax.Berkeley.EDU>
Date: 6 Feb 89 18:04:31 GMT
References: <[A.ISI.EDU].5-Feb-89.15:37:14.CERF>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 31

Vint,

>> Now I am confused. Two cases: 
>> 
>> 1. no gateways intervene
>> 2. one or more gateways intervene between source and destination.

I would relabel these two cases as

1. packet sourced by host on connected subnet
2. packet sourced by host behind connected subnet (behind a router)
3. packet destined for host on connected subnet
4. packet destined for host behind connected subnet (behind a router).

And, I am not sure the distinction is necessary.

Since the X.25 virtual circuit source may not be from the originator but
from the previous hop, you can't necessarily believe the IP source address.
Also security and configuration reasons may require you to enumerate the
complete list of connected hosts you will speak with. For example if you
want to verify that an X.25 VC can Reverse Charge, some neighbor
information will need to be preconfigured. This information is needed
before the first X.25 DATA packet arrives to determine whether the VC can
be accepted.

The cisco implementation requires preconfigured table entries about other
entities sharing the same connected subnet unless a mapping function exists
(DDN or BFE). Preconfigured entries can always be used to provide
additional information (such as subnet multicast).

Greg