Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!ucsd!rutgers!aramis.rutgers.edu!geneva.rutgers.edu!hedrick From: hedrick@geneva.rutgers.edu (Charles Hedrick) Newsgroups: comp.protocols.iso Subject: Re: TCP/IP vs. OSI Performance Message-ID: Date: 27 Mar 89 02:09:13 GMT References: <529@skep2.ATT.COM> <9882@megaron.arizona.edu> <14957@bellcore.bellcore.com> <9896@megaron.arizona.edu> Organization: Rutgers Univ., New Brunswick, N.J. Lines: 69 Technically you're right that there is no difference between ISO and IP as to the relationship between addresses and routing. But practically there is. While IP may not specify an official routing strategy based on addresses, there is a de facto one, and given the addressing structure, it's hard to see any practical alternative. We have basically two-level hierarchical routes, with a table indexed by network number doing routing between institutions and a table indexed by subnet doing routing within institutions. The network number allocation procedures guarantee that there isn't any more structure in the network numbers that we could use. ISO has the possibility of more structure. It allows longer addresses, and it allows for multiple address allocation authorities. This allows for more levels of hierarchy, and it supplies a builtin top level to the hierarchy (authority responsible for the address format) that is not present in IP. Thus in theory we can hope that gateways won't have to have routing tables that list the entire world. There is a cost for this, however, which is that addresses are harder to parse, and that the structure we are trying to use for routing is different for different addresses. I don't think we'll know which approach is better for a number of years. Basically I expect the Internet to develop routing technology that uses knowledge outside the address. Until we get the equivalent of the Internet using ISO technology, we simply won't know whether the flexibility inherent in the ISO addresses is worth the overhead of handling variable address formats. There are several different ways of getting past the current Internet routing problem. (The problem being that at the moment every gateway has to compute routes to every network in the Internet.) The most promising seem to involve additional routing technology other than the conventional IP route table. The following is a straw man that has the same flavor as what seems to be floating around in the IP community. Suppose we no longer keep a complete routing table, i.e. one that lists every possible network. Instead we have several levels of hierarchy. E.g. the world is divided into USnet, Euronet, and Japannet. We don't care about routing within Euronet and Japannet. USnet is divided into NSFnet, NASAnet, and ARPAnet. We are a member of NSFnet, so we don't bother to keep track of the others. etc. The routing table at Rutgers looks sort of like this: Euronet -> jvnc-euro-gw Japannet -> sri-japan-gw NASAnet -> ames-nasa-gw ARPAnet -> mills-1200baud-fuzzball-link nsfnet -> jvnc-nsfnet-gw [default used for NSFnet nets we don't know] nysernet -> columbia-gw In addition to this we have a conventional routing table based on specific network numbers. But that's just a cache. When we get a packet, we look for its network number in the conventional routing table. If it's there, we route it. Otherwise, we go through the routing equivalent of the domain name system. If the root route server tells us that the network in question is part of euronet, we stop there and just send it to a European gateway. If it's part of USnet, we ask the USnet route server for more information, etc. What gets circulated in routing updates within NSFnet is then routes for these higher-level entities, not for specific networks. The current NSFnet backbone routing is in effect a one-level version of this, with fixed tables giving the membership of various networks in the higher-level entities, rather than using route servers. This sort of scheme, or various other similar suggestions, uses "route servers" as an alternative to additional structure in the addresses. My guess is that in the long run depending entirely upon address structure is going to turn out not to be enough, and we're going to have to go to a scheme like this. But we probably won't know for sure what the right tradeoffs are until 10 years of so from now, when we have very large international networks that use routing technology more complex than the current IP technology.