Re: IPAE problems
Christian Huitema <Christian.Huitema@sophia.inria.fr> Fri, 11 June 1993 08:54 UTC
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To: Eric Fleischman <ericf@atc.boeing.com>
Cc: ip-encaps@sunroof.eng.sun.com, sip@caldera.usc.edu
Subject: Re: IPAE problems
In-Reply-To: Your message of "Thu, 10 Jun 93 15:21:30 PDT." <9306102221.AA29171@atc.boeing.com>
Date: Fri, 11 Jun 1993 10:45:16 +0200
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From: Christian Huitema <Christian.Huitema@sophia.inria.fr>
Eric, The IPAE/SIP questions are indeed very interesting, as they deal with the "contamination" scenario -- how to progressively spread the SIP virus in the Internet. Note that the scenario also applies to any other IPv7 that would follow the IPAE transition, by opposition to "dual stack" transition. Dual stack has it own set of problems... A few responses (not to all questions, I am merely trying to progress the spec, not to convince the anonymous sender of the questions). - How do IPAE hosts whose local routers are IP-only find the IPAE routers? My tentative reply would be that "they don't". SIP hosts need at least one adjacent SIP router to do SIP. Indeed, all SIP/IPAE router may also behave like IPv4 routers, which branches us into dual routing, etc. - How do IPAE routers find other IPAE routers? - How to deal with the "enormous branchiness" problem? I would expect that the initial deployment follows the "MBONE" scenario. In fact, one could do more work on BGP compatibility, in order to avoid the "overlay" pitfall -- poor usage of topology. Maybe we should be more agressively looking for proposals here! - When translating from IP to SIP, how do you set the high order part of the address -- especially the C-bit? My personal opinion is "use DNS and caches". An "IPAE.TXT" solution would not scale -- I dislike centralisation. Sending a DNS query has exactly the same dynamics (well, longer time-outs) as sending an ARP request. Packets have to be queued when waiting for the response. Note that "DNS" is only one idea. Another possibility is to use a fixed prefix, and rely on some ICMP "change prefix" to get the translation and update the local cache. That would be somewhat similar to the three legged routing of mobile IP. If there is interest in such a scheme, maybe someone should write it up. Also note that the "prefix per host" affair is a red herring. One actually needs a "prefix per network". - When translating from SIP to IP, how to set the DU ID? This has been discussed in great details on the IPAE list. There are in fact two cases: 1) If a segmentation header is present, then one will very naturally use the infos present in that segmentation header. 2) If the SIP packet is larger than the minimal guaranteed MTU, then the DU-ID should be set to null, and segmentation should be prohibited. 3) In the other case (small packet, no segmentation header), one must allow segmentation in order to pass over some IP links that require it. Using a local random number is bad -- does not allow to detect duplicates. One way to cope with the problem is to use a "one way function" to derive a unique ID from the SIP packet. One way to derive this unique ID is to take a checksum of the whole packet (that is long). In fact, one can assume that unique informations are present in the first bytes of the payload: port numbers, TCP checksum and sequence number, UDP checksum. Thus, checksumming the SIP header + the first 20 bytes of the payload will result in a unique ID. If all routers apply the same strategy, then all routers derive the same ID. In fact, one does not have to checksum the whole SIP header -- one should ignore all informations which are not copied in the IP header; that is, one only has to checksum the lower 32 bits of the addresses, the payload type and the payload length. Cost of doing this is approx same as cost of recomputing the IP checksum hop by hop -- probably acceptable! Maybe that algorithm should be part of the SIP spec! Christian Huitema
- IPAE problems Eric Fleischman
- Re: IPAE problems Christian Huitema