Subject: Re: question on link state routing protocols

From: [email-masked] (Noel Chiappa)
Date: Wed, 8 May 2013 17:34:59 -0400 (EDT)

> From: Tony Li <[email-masked]>

    > The CSNP mechanism is FAR more robust than a LSP-by-LSP
    > acknowledgement. It is a continuing and ongoing cross-checksum between
    > adjacencies. With IS-IS, if an LSP is lost in flight, then it is caught
    > in the next CSNP cycle and retransmitted.

Perhaps "robust" was poor word choice on my part. Let's substitute "timely".

A lost LSP is not caught until the next CSNP cycle, right? Why is it OK to
wait until then?

	Noel

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From: Tony Li <[email-masked]>
Date: Wed, 8 May 2013 14:53:08 -0700

> A lost LSP is not caught until the next CSNP cycle, right? Why is it OK to

P[packet loss] ~= 10^-18

Doing more would be over-design.

Tony

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From: "Dave Oran (oran)" <[email-masked]>
Date: Wed, 8 May 2013 21:57:24 +0000

> A lost LSP is not caught until the next CSNP cycle, right? Why is it OK to
Because a timeout is a timeout is a timeout. If you want faster responsiveness you can crank down the CSNP interval. CSNPs aren't that big (compared to actual LSPs) and are multicast so they don't use O(numberofroutersonthe link) of bandwidth. If you want to be especially clever you can use different CSNP intervals for bad news and good news.

Transitive closure on LANs is pretty important, so just comparing the 2-party responsiveness doesn't tell the whole story.

> 	Noel
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From: Mike Shand <[email-masked]>
Date: Mon, 13 May 2013 18:38:50 +0100

What is important here is that for correctness the databases MUST 
eventually be synchronized. However, as has been pointed out, LSP loss 
is actually a low probability event (mostly due to overloading receivers 
with too much information... certainly in the old days, when this was 
designed), and timeliness of retransmission is not particularly 
important, since in most cases, there will be multiple paths for the 
LSPs to arrive over, so temporary loss of a single LSP is rarely an 
issue. In particular, in the cases were there IS only a single path on 
which it can be received, it tends to indicate that the temporary 
mis-synchronization is even less important, since there is probably not 
much you could do about it, even if you DID get the timely information.

As Tony says, there is a danger of over-engineering here.

     Mike