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12 messages in net.nether.puck.cisco-nsp[c-nsp] MTU Question on T1| From | Sent On | Attachments |
|---|---|---|
| Richard J. Sears | Jan 24, 2005 12:33 pm | |
| Gert Doering | Jan 24, 2005 12:54 pm | |
| Richard J. Sears | Jan 24, 2005 1:13 pm | |
| Gert Doering | Jan 24, 2005 1:28 pm | |
| Joe Maimon | Jan 24, 2005 1:28 pm | |
| Jon Lewis | Jan 24, 2005 1:39 pm | |
| Rodney Dunn | Jan 24, 2005 2:39 pm | |
| Bill Wichers | Jan 24, 2005 7:46 pm | |
| Richard J. Sears | Jan 24, 2005 8:15 pm | |
| Jon Lewis | Jan 24, 2005 8:18 pm | |
| Jay Hennigan | Jan 24, 2005 8:27 pm | |
| Gert Doering | Jan 25, 2005 3:08 am |
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| Subject: | [c-nsp] MTU Question on T1 | Actions... |
|---|---|---|
| From: | Joe Maimon (jmai...@ttec.com) | |
| Date: | Jan 24, 2005 1:28:29 pm | |
| List: | net.nether.puck.cisco-nsp | |
Its already fragmented at 1500.....but still?
Richard J. Sears wrote:
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
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****************************************** Richard J. Sears Vice President American Internet Services
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