From: Peter van Oene (pvo@usermail.com)
Date: Mon Oct 21 2002 - 13:31:19 GMT-3
In keeping with Howard's OSPF vs ISIS variances thread, I thought I would
post this gem from Radia Perlman which she posted recently to the OSPP WG
in the IETF. Interesting historical points
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>From: Radia Perlman - Boston Center for
>Networking <Radia.Perlman@SUN.COM>
>Subject: Re: what is the fundamental difference between OSPF and IS-IS?
>To: OSPF@DISCUSS.MICROSOFT.COM
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> From: "Liu B." <binl@EEE-FS7.BHAM.AC.UK>
> >>what is the fundamental difference (or
> >>improvement?) between OSPF and IS-IS?
>
> >>Bin Liu
>
>Several people mentioned my paper from 1991, and since it will
>probably be hard to get (I don't have a copy, and haven't seen
>it in years), I thought I'd mention it's probably not worth
>worrying about. I'd be surprised if it would
>be too helpful today. Both protocols have changed since then, so I'd think
>the paper would be mostly interesting, if at all, for historical reasons.
>
>So back to "what is the difference between OSPF and IS-IS?"...
>
>This sort of question should be asked more often, especially where there
>are "foo" vs "bar" debates. As I said in my "Miss Manners meets the IETF"
>talk, unfortunately such questions often lead to err..nontechnical
>responses. I'm glad that all the responses to this note have been
>thoughtful and technical.
>
>And since people will periodically wonder about the real technical
>differences between foo and bar, it is useful to write the stuff down.
>
>This particular question (differences between IS-IS and OSPF)
>is a topic that is asked about sufficiently often that it would
>probably be worth writing an updated paper, based on the most recent
>versions of the two protocols. My book (Interconnections) discusses
>some more differences, and Dave Katz's presentation is also
>helpful. But I think it would be nice to have some set of people
>carefully compare both specs, and
>calmly write down the differences and the pros and cons of the differences.
>And given that it seems like both protocols will persist, if there
>are cases where one scheme is significantly
>better, it ought to be folded into the
>other protocol if possible.
>
>As people on the list have pointed out, there is a lot of similarity between
>OSPF and IS-IS. To understand why there are two protocols, it helps
>to know some of the history.
>
>Basically, the first link state protocol was for the ARPANET. It
>introduced the idea of link state protocols, with the major
>ideas being:
> . flood link state information to everyone
> . use Dijkstra's algorithm on the link state database to
> compute paths
> . an algorithm for incrementally updating the Dijkstra tree
> when there are just a few link changes.
>
>The improvements (that I can think of off the top of my head)
>introduced by IS-IS were:
> a) making link state distribution robust (the ARPANET had an amusing
> failure mode)
> b) efficient incorporation of LANs, including the concept of Designated
> Routers, LANs as pseudonodes, and link state syncronization over
> a LAN using CSNPs. (ARPANET was just point-to-point links)
> c) exchange of parameter information (such as how long to wait before
> declaring neighbor down), so that parameters can be set independently
> and differently, and still interwork
>
>IS-IS was originally designed for CLNP, which had two forms of routing:
> . exact match of bottom 6 bytes, or
> . shortest prefix of top part
>That's where the "two levels" came about, though the "area routing"
>stuff could in theory be multilevel. IP only has one type of routing, which
>is like the shortest prefix, area routing in CLNP.
>IS-IS would have looked a little different if it had originally
>been designed for IP.
>
>OSPF was designed specifically for IP and was loosely based
>on IS-IS. At the time OSPF was beginning
>to be designed, it didn't occur to anyone that IS-IS could be easily
>adapted to route
>IP. Ross Callon noticed that once a routing algorithm existed, it was
>a mere detail to add reachability information for a different data
>packet format, and he wrote RFC 1195. (Interestingly, the concept
>of integrated routing wasn't a new idea. I realized years later, when
>looking at RIP's packet format, that RIP, deployed years before
>integrated IS-IS was conceived, was designed for routing multiple
>address families).
>
>Perhaps if the idea of using IS-IS for IP was thought of before
>work on OSPF had started, there wouldn't be two protocols. But once
>a group starts on something it's hard to stop. So there wound up
>being two protocols.
>
>NLSP was a version of IS-IS for IPX, and introduced some improvements,
>my favorite being DR election (see below).
>
>Differences I can remember (off the top of my head).
>
>a) Designated Router election: it's "deterministic" in IS-IS, which means,
> given the same set of routers, the same router will be elected. It's
> "sticky" in OSPF, meaning that once you get to be DR, you stay DR.
> This makes things more stable...if the highest priority router is
> flaky in IS-IS, every time it goes up it takes over, only to crash again.
> But I was told when designing IS-IS that determinism was important, which
> requires the behavior in IS-IS.
>
> For DR election, if I had to choose between OSPF's way and IS-IS's
> way, I'd choose OSPF's way, but NLSP (IS-IS for IPX plus
> some improvements) had a method
> that gave the best of both worlds.
> It has nodes change their priority after becoming DR,
> which allows you through astute priority settings, to
> choose deterministic or sticky behavior, or anything
> in between.
>
>b) LSP distribution on a LAN: IS-IS does it with CSNP's. OSPF with
> explicit acks. I believe both ways are just fine.
>
>c) Originally IS-IS passed no upper layer information into areas, and
> you just exited via the nearest level 2 router, whereas OSPF always
> fed all information into the area so you could choose the optimal
> exit point. Both have now been modified so you can choose any
> point on the tradeoff between extra routing info and optimal routing.
>
>d) parameter synchronization: IS-IS allows neighbors to have different
> values for things like Hello Timer, and still interwork.
>
>
>So anyway, there are a bunch of little nerdy differences, some of
>which might matter and some of which are just different because different
>groups did them. For the ones that matter, mostly the protocols have
>evolved to take advantages of features in the other protocol.
>
>I assume if there was an updated IS-IS vs OSPF document, someone would
>have mentioned it in response to Bin Liu's post. So assuming there
>isn't such, if someone wanted to try to do it, I (and I'm sure lots
>of other people) would be happy to help.
>
>Radia
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