Path: utzoo!attcan!uunet!ogicse!ucsd!ucbvax!BBN.COM!malis From: malis@BBN.COM (Andy Malis) Newsgroups: comp.protocols.tcp-ip Subject: Re: GOSIP and X.25 Message-ID: <9007190054.AA07340@ucbvax.Berkeley.EDU> Date: 16 Jul 90 21:04:21 GMT References: Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 65 Darrel, > Thanks to everyone who responded to my last inquiry on GOSIP. I'm > still not very knowledgeable about the OSI suite. I have a couple more > simple (I hope) questions. I think I can help. > The first one has to do with X.25. Is it possible or practical to > run both TCP/IP and the GOSIP suite over a single X.25 interface into a > network?? and ditto for ethernets?? I suppose ether is easy relative > to X.25. X.25 is actually easy, at least on the DDN. I can't speak for ether. > I'm most familiar with the milnet (standard) brand of TCP/IP > over X.25, where CC is the first byte of call user data in call request > packets, and where datagrams are sent as complete packet sequences, > etc. It would seem that in order to support BOTH suites over a single > X.25 interface, then it would be necessary to parse which protocol is > being used, IP or ISO8473, after the PDU is received. That doesn't > seem too practical. You're right, that's not the best way to do it. If you check RFC 1060, p. 48 (X.25 type numbers), you'll see that ISO IP has its own protocol ID (first byte of CUD in Call Request packets), CD. So, to use ISO-IP, you just open an X.25 VC to another consenting host using the ISO IP protocol ID. You can even run IP and ISO-IP in parallel between two hosts, over separate VCs of course. I have the impression that on the Milnet, ISO-IP usually runs over Basic X.25, rather than Standard X.25, since there is no need to interoperate with AHIP (1822) hosts, but either should work as long as the two hosts agree. > Would a host now have to maintain a virtual > circuit table that includes info on what kinds of PDUs can be sent on > that vc (IP or 8473). This assumes the X.121 addresses for a > dual protocol hosts is the same regardless of which suite is being > called. Yes, you should keep a VC table that lists the X.121 address on the other end of the VC, and the protocol being used on the VC. But you really have to do that anyway. Yes, on the DDN (Milnet), at least, the X.121 address is independent of the protocol, since the PSNs don't care what protocol you're running above X.25; they just need the address to get the data to the right place. The physical address is the same. > In the case of Milnet, does anybody know how to address OSI suite > hosts? Right now it looks like I just build a table of X.121 > addresses. Will there ever be an algorithmic derivation to get the > X.121 addresses?? Seems to be a knotty little topic, but maybe its > easier than it seems. If I understand your question correctly, I think the answer lies in RFC 1069, which defines ISO-IP addresses for use in the Internet. Figure 3, on p. 7, shows the regular IP address residing in octets 16-19 of the ISO-IP address. Once you have the regular IP address, you can use the regular algorithmic derivation to get the Milnet X.121 address. Andy Malis BBN Communications