Optimum MTU etc settings for Teksavvy PPPOE ADSL
I'm on Teksavvy's 6 megabps service. Speedtest.net consistently shows 4.97 or 4.98 net megabps down. I have a couple of desktops and notebooks behind an ST546 modem-router. According to »www.mynetwatchman.com/kb/ADSL/pppoemtu.htm the optimum MTU setting is 1454, which is what I use. The DSLReports FAQ at »DSL FAQ »What is the MTU? isn't sure, and also suggests checking with your ISP in case there are any special circumstances.
Anyone from Teksavvy have any info on this? Are there other settings to look at as well? I'm running Gentoo linux, so everything is configurable.
I use 1492 I think because of a recommendation I got here a few years ago. I can't recall the technical reason why that was the ideal number (it had to do with headers) or did I test if it made a difference. But I was convinced at the time it was a good idea. Hopefully one of the more knowledgeable members will jump in and enlighten us. I'd be interested to know if you would get slower results if you changed your MTU to 1492 like that article reports.
Edit: ignore this space.
|reply to Walter Dnes |
OK, I tried 4 tests to the Teksavvy server in Toronto, alternating between 1454 and 1492. There appears to be a very slight upload advantage to 1492, although it may be margin of error...
MTU Ping Down Up
•1454 15 4.97 0.65
•1492 16 4.98 0.66
•1454 16 4.98 0.65
•1492 16 4.98 0.66
I'll leave it at 1492 for the time being.
Ethernet MTU = 1500. PPPoE packet header = 8. To fit a PPPoE packet inside an Ethernet frame the MTU should be set to 1500 - 8 = 1492.
If your traffic gets encapsulated in another protocol, such as L2TP, and that protocol has to traverse links that have a 1500 byte MTU, you would subtract that protocol's over head from 1500 to obtain the correct MTU.
In general, L2TP over regular Ethernet should use an MTU of 1454. Most ISPs use Ethernet links that support more than 1500 byte MTUs (i.e. baby jumbo frames) which is sufficient to carry a L2TP tunnel with a full 1492 byte payload (the maximum supported by regular PPPoE).
If you're doing MLPPP, I've seen a MTU of 1486 used to account for the additional 6 bytes that Multilink adds in overhead.
The real overhead is not in the PPPoE or MLPPP encapsulation - its mostly in the aal5/ATM encapsulation that is still used on the wire between the ATU-R and ATU-C (Modem and DSLAM). This adds roughly 15% to 17% overhead depending on your packet mix, so a 5056/800 connection really performs at 4200/640.
MNSi Internet - »www.mnsi.net
|reply to TSI Gabe |
Gabe any reason why a lower MTU would result in a more stable connection?
For the life of me I can't keep PPPoE running more than about 10 days. VDSL2 stays sync'd but Tomato, Airport, etc all have a hard time holding PPPoE (eventually all disconnect with LCP-Echo failure).