Re: exchange point media

[Mikael Abrahamsson <swmike () swm pp se>]

On Mon, 17 Jul 2000, Richard A. Steenbergen wrote:

> A Foundry BigIron doing L3 should, exactly as if it was a router and not a
> switch, I believe. At that point there is no real technical distinction
> between it and a router with lots of ethernet ports however. I'm not aware
> of any exchanges doing L3...

Well, the device should do no routing in the classical meaning of the
word, not on L3 anyway. To do fragmentation it has to be semi-L3-aware
though. It also needs an IP adress to send the needtofrag-ICMPs from.
 
> FreeBSD lets you set the MTU based on the route... You could do something
> like this, enabling a larger MTU for specific targets, I suppose. I'm not
> aware of anyone who is doing this (or probably anyone who would,
> especially at L2, without a good reason). This assumes the exchange point
> has a switch capable of it.

We're talking L3 here (routers). Normally the L3 MTU is derived from the
L2 MTU, here we would need to derive it from either static configuration
or from the below MSS/MTU mechanism (which I don't think will happen as it
has too much of a "hack" in it).
   
> The TCP MSS is negiotated based off the MTU, so yo cannot base the MTU off
> the MSS, circular logic. I highly doubt you will ever get support for
> jumbo frames auto-negotiated without first standarding the jumbo-frames.   

Yes, it can. Router1 should be able to figure out router2:s MTU from the
MSS of its TCP session with router2. Router2 has no problem here since
it's MTU is the lowest one anyway.

I was under the impression that there is nothing magical about jumbo
frames and that there are no interoperational problems with them as long
as they're supported at all. Please correct me if I am wrong.
 
> I for one would love to see an intelligent standard realizing that 1500 is
> a remarkably stupid and limiting number, and enabling us to bring new life
> to public exchange point peering.

I think any new exchange point technology needs to have an MTU > 1500.

-- 
Mikael Abrahamsson    email: swmike () swm pp se