bsc
Premium
join:2003-03-11
Lexington, KY
 ·Windstream
| reply to evilghost
Re: MTU Settings behind router and modem
said by evilghost  :Sorry you misread my post, I really wasn't trying to be a jerk, I'm not very strong with some of the aspects of networking so I was trying to wrap my head around the MTU and weight setting local MTU to 1492. In testing it apperas I fragment with a packet size greater than 1450 bytes. How could I use tcpdump to detect fragmentation on live traffic flows? I apologize then, I definitely did snap there. I thought you were telling me I was wrong. We can still be friends!
I will elaborate a little.. the number you put in (1450) is essentially the raw data. This is going to be the SEGMENT size. This is a part of Layer 4 in the OSI model, where TCP and UDP live. Before the data gets sent out it gets headers added from Layer 3 (ICMP for a ping) and Layer 2 (Ethernet in most cases and in the case of DSL PPPoE headers get added here).
All in all in a PPPoE environment, you are going to add 48 (40 for ethernet and other header data and 8 for PPPoE) bytes to the SEGMENT SIZE before you send the data out. So the largest SEGMENT SIZE that will not fragment is 1452. |
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GVG
join:2006-09-19
Charlotte, NC | Now I am confused. Should I set MTU to 1492 or 1452? |
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evilghost
Premium
join:2003-11-22
Springville, AL
·Windstream
| reply to bsc
said by bsc :said by evilghost :Sorry you misread my post, I really wasn't trying to be a jerk, I'm not very strong with some of the aspects of networking so I was trying to wrap my head around the MTU and weight setting local MTU to 1492. In testing it apperas I fragment with a packet size greater than 1450 bytes. How could I use tcpdump to detect fragmentation on live traffic flows? I apologize then, I definitely did snap there. I thought you were telling me I was wrong. We can still be friends! I will elaborate a little.. the number you put in (1450) is essentially the raw data. This is going to be the SEGMENT size. This is a part of Layer 4 in the OSI model, where TCP and UDP live. Before the data gets sent out it gets headers added from Layer 3 (ICMP for a ping) and Layer 2 (Ethernet in most cases and in the case of DSL PPPoE headers get added here). All in all in a PPPoE environment, you are going to add 48 (40 for ethernet and other header data and 8 for PPPoE) bytes to the SEGMENT SIZE before you send the data out. So the largest SEGMENT SIZE that will not fragment is 1452. No problem with the misunderstanding, thanks for your elaboration. I know you're very knowledgeable about the networks sections that I'm strongly deficient in so I appreciate tapping your brain.
I assume setting the LAN to 1492 does nothing if the router's (Tomato GNU/Linux in this case) LAN interface is set to 1500 or does this not matter since fragmentation should not occur since the largest packet will be 1492?
Example; WAN 1492, all LAN interfaces 1492 on client machines, 1500 on LAN interface on router. Fragmentation will not occur since all traffic to/from clients will be MTU 1492 matching WAN MTU? |
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bsc
Premium
join:2003-03-11
Lexington, KY
·Windstream
| Correct, fragmentation will not occur because the clients are sending 1492.
As you can imagine you are taking a slight hit on LAN transfer efficiency in order to maximize your WAN transfers. I think it is more than fair due to the high throughput of a LAN anyway. |
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evilghost
Premium
join:2003-11-22
Springville, AL
·Windstream
| said by bsc :Correct, fragmentation will not occur because the clients are sending 1492. As you can imagine you are taking a slight hit on LAN transfer efficiency in order to maximize your WAN transfers. I think it is more than fair due to the high throughput of a LAN anyway. Thanks, great explanation. |
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