[c-nsp] MTU Question on T1

[Gert Doering]

Hi,

On Mon, Jan 24, 2005 at 09:21:56AM -0800, Richard J. Sears wrote:
> Their entire claim is that because they cannot ping the other router
> with a 40000 byte packet with less than 50ms latency, that is the entire
> problem and until we fix that issue - don't call them back.
> 
> So - my brain begins to think and I go to my 7500s, ping any number of
> customers with T1s with no traffic on them with Cisco's max packet size of
> 18024 - I see around 225ms. 

Pure physics will make this impossible.

A T1 is 1,500,000 bits per second, which is about 190,000 bytes/second.

If you send 40,000 bytes through that channel, just moving the bits one-way
will take 40/190 = 210 ms.  No fragmentation and reassembly involved,
just pure bits clocked at 1.5 Mbit/second.

For the RTT, you have the data going back and forth, so the time will 
go up to 420 ms minimum.   Add actual latency to this (like "40ms" for
a longish T1).

If your gear will distribute the packets 1:1 onto both T1s, you can 
speed up things a lot, because you can effectively pump with 3 Mbit/s.
- it won't be twice the rate due to more effort in reordering things,
but nearly so.

In any case, there is no way to get a packet with 40 kbyte across a T1
in 50 ms.   So either the vendor in question is saying "our application
will not work over T1 links" (without admitting it), or they are just too
stupid to do the math.

> Each T1 is set with an mtu of 1500 bytes, which means that every 40000
> byte packet have to be broken up into almost 27 packets, get sent across
> the T1, be put back together and then reverse that process in order to
> get the RTT.
> 
> I do not see any place to increase the MTU on a Cisco T1 WIC card (if
> even we would want to), nor do I see errors.

Increasing the MTU won't help you there, as the packet is already
fragmented by the sending host.  Ethernet MTU is 1500...

> After looking at about 20 different T1s - I get the sense that I won't
> see anything less than 300 to 400ms doing a 40000 byte ping test across
> two Cisco 1720's.

Correct conclusion.

gert
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de