[c-nsp] MTU Question on T1

[Richard J. Sears]

Hey Gert - 

On the dual T1 issue - correct me if I am wrong please but with ppp
multilink interleaving turned on, won't it take that single 40000 byte
packet and shove it down one T1 because it came in as a single packet..?


?


Thanks


On Mon, 24 Jan 2005 18:54:23 +0100
Gert Doering <gert at greenie.muc.de> wrote:

> 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
> -- 
> USENET is *not* the non-clickable part of WWW!
>                                                            //www.muc.de/~gert/
> Gert Doering - Munich, Germany                             gert at greenie.muc.de
> fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de


******************************************
Richard J. Sears
Vice President         
American Internet Services                          
----------------------------------------------------
rsears at adnc.com
http://www.adnc.com
----------------------------------------------------
858.576.4272 - Phone
858.427.2401 - Fax
INOC-DBA - 6130
----------------------------------------------------

I fly because it releases my mind 
from the tyranny of petty things . . 


"Work like you don't need the money, love like you've
never been hurt and dance like you do when nobody's
watching."