How to set up my MTU
It's worth saying that if you have either a Windows Vista (but not XP etc) or a Linux OS then they will try to choose optimum network parameters for you, including MTU size.
look up TCP Optimizer on Google 
Thank you all for your advice and links!
Some Qs:
I am not at home to try my ADSL-Max right now, but at work where we have a fixed speed 1152/288 kbps BT ADSL line I could not see a discernible difference between different settings. One thing was sure out of my MTU ping tests: the MTU value set at the PC determines the MTU Ping test result (i.e. the MTU test value, above which fragmentation occurs). Add to that 28 for the IP header and you're back at the MTU value I had set at the NIC.
Quote from: Jameseh I noticed a considerable change when changing my router MTU from 1500 to 1430.
To the tune of about 3Mbps.
$ ping -c 3 -v -M do -s 1472 212.58.251.201In this case I add 28 and arrive at the value of 1500, which is exactly what I set the interface card at. I tried values of 1500, 1490, 1480 and 1430 at the PC and router and could not see much of a difference. If anything I would say that an MTU of 1500 gave the best result, not in terms of ping round trip time but as shown by www.speedtest.net).
PING 212.58.251.201 (212.58.251.201) 1472(1500) bytes of data.
1480 bytes from 212.58.251.201: icmp_seq=1 ttl=241 time=78.8 ms
1480 bytes from 212.58.251.201: icmp_seq=2 ttl=241 time=78.6 ms
1480 bytes from 212.58.251.201: icmp_seq=3 ttl=241 time=77.5 ms
--- 212.58.251.201 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2000ms
rtt min/avg/max/mdev = 77.548/78.337/78.863/0.653 ms
Some Qs:
- Should I expect something different when I try this test at the ADSL-Max line?
- Is the MTU ping test merely reflecting what the PC MTU has been set at?
- Is there another test (that doesn't involve a Java applet) I could try?
I did some more tests and research and can report the following:
1. Path MTU discovery on a server will test what is the maximum packet that a client connected to that server will support and then send packets of that size back to the client. If any connection on the way (firewall(s), etc) are blocking ICMP packets, or the server has been configured not to accept ICMP (type 3, subtype 4) packets, PMTU will not work. In that case certain connections will be troubled with high fragmentation and therefore slower than possible speeds and high latency.
2. To overcome the above it may pay to set the MTU manually (rather than rely on the PMTU mechanism). Given that BT based Broadband uses ATM encapsulation (PPPoA) and this requires 42 bytes, we are left with 1500-42=1458. Therefore the MTU for BT based PPPoA Broadband is 1458 (while for PPPoE is 1492 and for cable/LAN I believe it is 1500). Any packets above that size will incur fragmentation. Metronet ADSL support also advised that this is what BT recommends for their network and it makes sense.
3. Using something like www.speedtest.net to measure your connection speed is probably not very accurate. This is because the download is not particularly large. Try instead to download an iso package (e.g. for Debian, Ubuntu, SUSE, etc. or your local favourite software supplier) from your nearest mirror. After a minute or so the download would have stabilised and then you'll know what your speed under load is (please don't abuse the mirrors).
4. Don't forget to set the MTU for all computers on your network and all the network devices; e.g. router *and* modem. Using the MTU discovery ping test I found that my modem was set to a different figure than my router.
5. None of the above is going to cure pipe contention and ToS throttling. In busy times, or if you have exceeded your monthly bandwidth allowance (for those of us who are on PAYG service) expect speeds to be lower. Unless you can afford dedicated fiber to the desktop, you ain't gonna get 8mbps any time soon.
Hope this helps others to solve their MTU problems.
--
Regards,
Mick
1. Path MTU discovery on a server will test what is the maximum packet that a client connected to that server will support and then send packets of that size back to the client. If any connection on the way (firewall(s), etc) are blocking ICMP packets, or the server has been configured not to accept ICMP (type 3, subtype 4) packets, PMTU will not work. In that case certain connections will be troubled with high fragmentation and therefore slower than possible speeds and high latency.
2. To overcome the above it may pay to set the MTU manually (rather than rely on the PMTU mechanism). Given that BT based Broadband uses ATM encapsulation (PPPoA) and this requires 42 bytes, we are left with 1500-42=1458. Therefore the MTU for BT based PPPoA Broadband is 1458 (while for PPPoE is 1492 and for cable/LAN I believe it is 1500). Any packets above that size will incur fragmentation. Metronet ADSL support also advised that this is what BT recommends for their network and it makes sense.
3. Using something like www.speedtest.net to measure your connection speed is probably not very accurate. This is because the download is not particularly large. Try instead to download an iso package (e.g. for Debian, Ubuntu, SUSE, etc. or your local favourite software supplier) from your nearest mirror. After a minute or so the download would have stabilised and then you'll know what your speed under load is (please don't abuse the mirrors).
4. Don't forget to set the MTU for all computers on your network and all the network devices; e.g. router *and* modem. Using the MTU discovery ping test I found that my modem was set to a different figure than my router.
5. None of the above is going to cure pipe contention and ToS throttling. In busy times, or if you have exceeded your monthly bandwidth allowance (for those of us who are on PAYG service) expect speeds to be lower. Unless you can afford dedicated fiber to the desktop, you ain't gonna get 8mbps any time soon.
Hope this helps others to solve their MTU problems.
--
Regards,
Mick