On 11/8/2010 12:36 PM, Mans Nilsson wrote:
I'd concur that links where routers exchange very large routing tables benefit from PMTUD (most) and larger MTU (to some degree), but I'd argue that most IXPen see few prefixes per peering, up to a few thousand max. The large tables run via PNI and paid transit, as well as iBGP. There, I've seen drastical improvements in convergence time once PMTUD was introduced and arcane MSS defaults dealt with. MTU mattered not much.
Given this empirical data, clearly pointing to the fact that It Does Not Matter, I think we can stop this nonsense now.
His point wasn't to benefit the BGP routers at the IX, but to support those who need to transmit > 1500 size packets and have the ability to create them on the edge. In particular, the impact of running long distances (high latency) with higher packet drop probability. In such a scenario, it does matter. Even if you don't see that many > 1500 byte packets, doesn't imply that it doesn't matter. I have v6 peerings and see very little traffic on them compared to v4. Should I then state that v6 doesn't matter? If people have an expectation of not making it through core networks at
1500, they won't bother trying to send >1500. If the IX doesn't support >1500, why would people connecting to the IX care if their backbones support >1500?
Jack