RINA - scott whaps at the nanog hornets nest :-)

Jack Bates jbates at brightok.net
Mon Nov 8 14:29:44 CST 2010

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?


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