MTU to CDN's

Jared Mauch jared at puck.nether.net
Fri Jan 19 00:14:00 UTC 2018


> On Jan 18, 2018, at 5:53 PM, George Michaelson <ggm at algebras.org> wrote:
> 
> if I was an ISP (Im not) and a CDN came and said "we want to be inside
> you" (ewww) why wouldn't I say "sure: lets jumbo"
> 
> not even "asking for a friend" I genuinely don't understand why a CDN
> who colocates and is not using public exchange, but is inside your
> transit boundary (which I am told is actually a bit thing now) would
> not drive to the packet size which works in your switching gear.
> 
> I understand that CDN/DC praxis now drives to cheap dumb switches, but
> even dumb switches like bigger packets dont they? less forwarding
> decision cost, for more throughput?

The reason is most customers are at a lower MTU size.  lets say i can
send you a 9K packet.  If you receive that frame, and realize you need
to fragment, then it’s your routers job to slice 9000 into 5 x 1500.
I may have caused you to hit your exception path (which could be expensive)
as well as made your PPS load 5x larger downstream.

This doesn’t even account for the fact that you may need to have a speed
mismatch, whereby I am sending 100Gb+ and your outputs may be only 10G.

If you’re then doing DSL + PPPoE and your customers really see a MTU
of 1492 or less, then another device has to fragment 5x again.

For server to server, 9K makes a lot of sense, it reduces the packet processing
and increases the throughput.  If your consumer electronic wifi gear or switch
can’t handle >1500, and doesn’t even have a setting for layer-2 > 1500, the
cost is just too high.  Much easier for me to send 5x packets in the first place
and be more compatible.

Like many things, I’d love for this to be as simple and purist as you
purport.  I might even be willing to figure out if at $DayJob we could see
a benefit from doing this, but from the servers to switches to routers then
a partner interface.. it’s a lot of things to make sure are just right.

Plus.. can your phone do > 1500 MTU on the Wifi?  Where’s that setting?

(mumbling person about CSLIP and MRUs from back in the day)

- Jared




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