MTU - Maximum Transmission Unit

[Mike8]

Are all you techies working out your MTU to see if you can do a little fine tuning.

Networks use certain protocols to send and receive data TCP (transmission control protocol) being one of the main ones for Internet/Ethernet it provides a reliable method of sending data between two points and uses acknowledgements (ACK) to know that the Tx has received an acknowledgement from the Rx. When you are sent data, this is broken up into smaller packets, and when an acknowledgement is received the next packet is sent and so on. If the ACK is not received the host computer will wait a short time before trying to retransmit. (Hence avoiding this is useful.)

The MTU is the maximum packet size that can be transmitted in one go before it has to be cut up into smaller chunks. Each network has a maximum limit of data size it can cope with before data fragments into smaller packets. This figure may vary from network to network.

I have always used 1458 (the windows default is 1500) and as O2 couldnt advise I have just done a ping exercise, to find out the typical MTU for O2's network. I would be interested to know of anyone elses calculations.

Using (in XP) run/start/cmd [enter], is the 'dos style box' I am entering the following:


ping -f -l 1500 82.132.141.30 (IP for o2.co.uk)
and get a return of packet needs to be fragmented but DF set.
going all the way down in increments of 10, I get to 1430 then it times out (although I should get a normal response). Then going up again in 1 step increments till I hit fragmentation required.

So I have set my MTU to 1430 + 28 = 1458, which I was using before.
(1430 + 28 which is the header size for IP +ICMP)

Another parameter is RWIN, the receiving window buffer - the max amount of data that can be accepted before having to send an ACK.

RWIN is normally adjusted dynamically in Windows XP.

Of course with the speeds of ADSL2+ it might all be for the birds.

Any comments.

[Saturday]

Originally Posted by Mike8

I have always used 1458

And I think maybe I can now guess why you then went on to determine that 1458 is the right MTU for O2 (LLU) - 'cause it isn't

The MTU can be set in the router AND the PC. In one or the other, I think you still had 1458 set, hence why you got the same result again.

Try setting your MTU back to 1500 in your PC and reboot it. Then ensure it is also set at 1500 in your router. If you now run the test again you'll find that the largest possible non-fragmented packet is 1472 (1500 - 28 ICMP & IP headers) i.e the correct MTU to set is 1500.

If you're still getting 1458 then I'm baffled as I can assure you 1500 is the correct figure for O2 LLU.

Originally Posted by Mike8

RWIN is normally adjusted dynamically in Windows XP.

Umm - I'm not sure what you mean by that because XP (unlike Vista) does not have the correct RWIN figure by default for a fast broadband connection. It is much too small and needs to be increased. And it's definitely not dynamic in XP. It is in Vista though

There are a number of tools available to help set the correct RWIN (and MTU etc) for XP. A very popular one is TCPOptimizer.

Personally I prefer to do things manually so I favour DrTCP in conjunction with DSLreports.

HTH

[Mike8]

Thanks for that, I have just done the DSL reports test, as below:

Service: dsl
Dn speed: 16376
O/S: winXP
Connection: router pppoe


Tweakable Settings:

Receive Window (RWIN): 521824
Window Scaling: 3
Path MTU Discovery: ON
RFC1323 Window Scaling: ON
RFC1323 Time Stamping: OFF
Selective Acks: ON
MSS requested: 1418
TTL: unknown
TTL remaining: 55
TOS flags: none set


Test Download:

Test DownloadActual data bytes sent: 1024001
Actual data packets: 731
Max packet sent (MTU): 1458
Max packet recd (MTU): 1458
Retransmitted packets: 0
sacks you sent: 0
pushed data pkts: 26
data transmit time: 2.056 secs
our max idletime: 379.5 ms
transfer rate: 401415 bytes/sec
transfer rate: 3211 kbits/sec
transfer efficiency: 100%


I'm still on 1458 router and pc?

[Saturday]

Originally Posted by Mike8

I'm still on 1458 router and pc?

You are on one or both of them. If you have 1500 set in your PC and 1458 set in the router then the router "wins". If you have 1458 set in your PC and 1500 set in the router then the PC "wins". i.e. one or other restricts what is getting through.

So, if you haven't set 1500 in both (and rebooted the PC, shouldn't need to reboot the router) i.e. one of them is still set to 1458, when you run the test at dslreports it will show your MTU as 1458 as that what it was for the download test.

[Mike8]

At least we get some interesting conversations going on this forum. I will put 1500 in the PC and 1500 in the router and do my trial again. But when you are looking at around 13500Mbps download average thruput, would you notice the difference?

[Saturday]

No, you probably wouldn't notice much difference if the MTU is a little too small - that's much better than being a little too big.

RWIN on the other hand matters a lot, though, as you're getting in excess of 80% throughput to sync I guess your RWIN can't be out by much - or you're on Vista.

[Mike8]

RWIN

TCP Optimiser reports my existing RWIN as 521824 when pressing
the current settings radio button.

When pressing the optimum radio button, it suggests 513920.

Would you change it.

[Saturday]

I would - doesn't mean it has to be changed though. Do it if you can be bothered, otherwise leave it. It won't make a noticeable difference as it is pretty close.

[Mike8]

Thanks Saturday. Will try when I have time to play, but I think my O2 is looking pretty hot, please my exchange was enabled in May and I can have real bb.

[Undecided adrian]

as far as MTU goes, if you use an xbox 360 the MTU needs to be less than 1500, although I just use the defaults in my router and everything seems to work ok.