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Re: [RRG] getting rid of longest match




On Feb 25, 2008, at 3:06 PM, Iljitsch van Beijnum wrote:

On 25 feb 2008, at 23:43, Scott Brim wrote:

What I'm thinking is that for any given part of the address space, there would be a fixed prefix length. So for 192.0.0.0/8 / 24, for 64.0.0.0/8 /20, for 17.0.0.0/8 /8... So in theory, there would be no overlap. There could still be if the filters weren't set up properly, which would probably have to resolved when translating the RIB into the FIB.

Isn't this just what we did pre-CIDR?

Yes, except that it wouldn't be hardcoded into the IP stack but enforced through filters. So it could be /24 for 193.0.0.0/9 and / 19 for 193.128.0.0/9.

The issue with overlap is: which one do you choose? Longest match says the smallest block is the good one, but I don't think YouTube appreciated that logic yesterday. If we ditch longest match we need another rule. Allowing only one prefix size for a certain address range is one way to resolve this, but others are also possible. For instance, S-BGP or soBGP-like security mechanisms.


If I understand this proposal, then for any given prefix, the entire net would agree on a prefix length and only propagate paths for this prefix length. This would imply that there would actually be no required hardware change, as all of this filtering could (and should) happen during BGP processing (specifically during UPDATE parsing) and prior to RIB insertion and thus prior to the FIB insertion.

Thus, from a hardware perspective, this is largely a NOP.

Note that this could somewhat decrease the complexities of future hardware designs. Since we now guarantee that there are no overlapping prefixes, the table now effectively forms a tree of variable prefix lengths, but now the first match found in the tree suffices. In the grand scheme of things I would judge this not to have a significant impact, but it is arguably simpler.

Tony


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