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Re: [RRG] A data point on transit MTU size



On 24 sep 2008, at 19:15, Templin, Fred L wrote:

I'm not convinced as to how "large" the buffers need to
be? AFAICT, the size depends on the number of simultaneous
reassemblies in progress

Right. But:

1. This could be a large number. Assume every full size packet is fragmented then the number of fragments held at any given time is pretty much number of ITRs we talk to / 2

2. It's possible to exhaust limits in this area for DoS purposes

Having fixed size per standard makes it much simpler. Don't
have that for IP.

The most we will ever ask the ETR to reassemble is 2KB. Ever.

Obviously we want the system to be able to handle packets bigger than 2k if hardware limitations permit, but the only reason for reassembly would be the case where the inner packet is 1500 bytes DF=1 so you would never have to reassemble more than two fragments, can require that those arrive in order, too. Another optimization: require that the first fragment is 1024 or 800 bytes or so rather than 1500, so the reassembly buffer only needs to be 1k per session.

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