Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!SPIDER.CO.UK!keith From: keith@SPIDER.CO.UK (Keith Mitchell) Newsgroups: comp.protocols.tcp-ip Subject: Re: IP over X.25 (request for info) Message-ID: <8902071459.AA22881@redrump.spider.co.uk> Date: 7 Feb 89 14:59:55 GMT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 56 I am nearing completion of a Unix V.3 STREAMS-based implementation of IP over X.25, which is part of our SpiderTCP and SpiderX.25 protocol software products. We call it IXE (IP/X.25 Encapsulation). It fits into the STREAMS protocol stack as a multiplexing driver underneath IP and above our X.25 stack. It conforms fully to the rather skimpy RFC 877 spec, and although at present it will only work with CCITT-flavour X.25 PDNs, DDN support is on the way. (CCITT is a lot more in demand in Europe than DDN, obviously). On the other hand, it *does* work with both the 1980 and 1984 flavours of X.25. Clearing down of idle X.25 calls and pre-emption of these when resources are scarce are implemented. Address mapping for this stuff is something of an issue. At present we use a lookup table, which is the only way of really doing the job, but if you have a lot of WAN destinations to talk to then there is a danger of guzzling up lots of kernel memory. Some kind of address resolution protocol would be nice, but another way to look at it is from a security point of view. If you are connected to a public network, then anyone could call you up and claim to be some host with a remote IP address that higher- level software trusts. This leaves you wide open to spoofing by Public Data Network hackers. On the other hand, PDNs are usually set up so you can trust the X.25 calling address, so we use this to look back into the table and check that the remote IP host is who it claims to be. Using the CUDF field for address resolution does not seem like a good idea to me. I would prefer the use of X.25 facilities for this, most specifically the extended addressing one of X.25(1984). When used for the ISO CONS, this carries an NSAP. It is interesting to note that a scheme for encoding IP addresses into NSAPs already exists (RFC 986), at least for the connection-less world. Some scheme along these lines would thus bypass the need for table lookup, and I think is probably the right approach. (Again you have to trust the address you get from the PDN). It strikes me that generally RFC 877 leaves a lot of issues unanswered, and the introduction of the 1984 (and now 1988) standards for X.25 have aggravated this. Is is perhaps time for a new standard in this area ? Any such work probably ought to take into account using ISO IP (or CLNP) over X.25 circuits as well. Does anyone have any thoughts on this ? Keith Mitchell Spider Systems Ltd. Spider Systems Inc. 65 Bonnington Road 12 New England Executive Park Edinburgh, Scotland Burlington, MA 01803 +44 31-554 9424 +1 (617) 270-3510 keith@spider.co.uk keith%spider.co.uk@uunet.uu.net keith@uk.co.spider ...!uunet!ukc!spider!keith