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Old 04-03-2004, 12:52   #22
DrAwesome
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Smile PPPoA MTU(Locked?)

While reading through some of the ADSL forums i came across a very interesting post that had me wondering if the poster/poste assumption that some ADSL isp's lock the MTU value to control bandwidth/speed.
As the poster could not alter his/her mtu from 1448 to 1500 (checking each time after using various utils such as DRTCP021 also the util/patches that are on Speedguide.net) which increases or decreases the mtu.

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Speedguide TCP/IP Analyzer
(test your own TCP/IP see what your MTU is running at)


TCP options string = 020405800103030201010402
MTU = 1448
MTU is not fully optimized for broadband. Consider increasing your MTU to 1500 for better throughput.
MSS = 1408
MSS is not fully optimized for broadband (although it might work well for slower connections). Consider increasing your MTU value.

Default Receive Window (RWIN) = 256960
RWIN Scaling (RFC1323) = 2 bits (scale factor of 4)
Unscaled Receive Window = 64240

For optimum performance, consider changing RWIN to a multiple of MSS.
Other values for RWIN that might work well with your current MTU/MSS:
518144 (MSS x 46 * scale factor of 8
259072 (MSS x 46 * scale factor of 4
129536 (MSS x 46 * scale factor of 2
64768 (MSS x 46)

TCP Optimizer is very easy to use http://www.speedguide.net/downloads.php but after setting to optimal mtu1500, apply, and then XP Pro re-boots my mtu is still at 1448 the only conclusion i can come up with (after re-installing XP Pro several times and trying to increase my MTU to 1500) is that my present isp runs their dhcp pppoa service where the MTU setting must be 1448 i wonder if they have deliberately done this to save on bandwidth?

I guess when you find that your connection is slow and you take into account that the internet does slow down from time to time and the possibility that your local BT exchange trigger level may be full but when you test your connection at non peak time 3-4am in the morning and you cant achieve a sensible speed even to your isp's web page or the bbc web page i guess its time to look at other areas of where a problem may lie?

I know adsl bandwidth is contend service/ratio is setting/locking the mtu value of ensuring that bandwidth is evenly distributed?

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Hmmm interesting i know from my XBL & PS2 days of gaming setting a lower mtu value than 1500 was better for my gaming needs especially at peak time & Sat/Sundays when i was using NTL BB... I wonder if anyone else has any thoughts on the above has the users isp locked the mtu so he/she cannot change it, another thing i noticed was that the user provider uses PPPOA as their DHP which is an oddball? as i always thought PPPOE is the most common used.
PPPoE stands for point to point protocol over ethernet. It is a method of encapsulating the data for transmission to a far point. PPPoA is PPP over ATM.. from a software point of view, this is very similar to PPPoE .. in this case though, a DSL modem pumping ATM is internal to the computer, rather than being a short ethernet cable away.

If the isp cannot control/limit the bandwidth through lowering/locking the mtu i wonder why the isp decided to favour PPPoA over PPPoE?
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