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MTU and MRU

I'm looking for a little bit of help.

I have just been looking on the eclipse website and in their knowledgebase they suggest that I set my MTU at 1458, at the moment my router has it set at 1400 and the MRU at 1492. Should both the MTU *and* MRU be set at the advised number, or just the MTU?

Also.....It states that Windows sets it at 1500, how do I alter windows to the new number or is it just my router I need to change?

Just in case. I am using a D-Link DSL-504T router with 2 WinXP machines plugged into it.

Thank you
 
to change MTU on windows use DrTCP AFAIK u dont have to chnage the MRU setting just set the MTU seetting same for all or some sites might now work
 
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Thanks guys......Done it now...
Not noticed a change....But then I don't really think I will to any great drgree.....Will I?

Also.....As a side note...If 1458 is a standard or whatever....Why would my router come with something else preset?
Does it change from network to network?
 
Trikky said:
Thanks guys......Done it now...
Not noticed a change....But then I don't really think I will to any great drgree.....Will I?

Also.....As a side note...If 1458 is a standard or whatever....Why would my router come with something else preset?
Does it change from network to network?

This link may throw some light on MTU for you Trikky. :)

http://www.dslzoneuk.net/tweaks.php

HTH
 
The advice on the eclipse site *may* be out of date anyway, back in 2002 due to a limitation of BT's network, BT used to advice reducing the MTU to 1458 http://www.adslguide.org.uk/newsarchive.asp?item=1134

BT fixed this a long time ago, although Eclipse may still have a limit.


There's a "tweak tester" here http://www.dslreports.com/tweak that reports the MTU it receives, if it is lower than the limit set on your PC you may have problems accessing some sites.

Note that some routers also have a setting for requested MSS, if present this should be set at 40 bytes less than the desired MTU.

For PPPoA connections an MTU of either 1478 or 1430 should be the most efficient and should give slightly better speeds, although not by much - an MTU of 1478 would only give about 1% improvement over an MTU of 1458. (If I calculated it correclty, an MTU of 1430 is only 0.09% less efficient than 1478)
 
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Trikky said:
Well I did that test...


And at 97% I really don't think I am going to see any kind of improvement. Especially as i am only on a 512 connection! ;)

Now all i need to work on is my ping times! :laugh:

Thanks

Actually that just indicates that there was some minimal packet loss (your 1458 MTU would not have caused this), it is not a speed test
Retransmitted data packets: 21

If you want to check the speed use ADSLguide's speed test http://www.adslguide.org.uk/tools/speedtest.asp

Note that you ISPs normal contention will have far more effect than the differences caused by tweaking the MTU so you would have to do a comparison off-peak.

I use an MTU of 1430 increasing it by one to 1431 makes it about 2% less efficient which results in a reduction in speed by about 20 - 30Kb/s on my connection (30Kb/s is about what I used to get using a dial-up connection - as I said the improvement is fairly minimal :laugh: )

This is caused because PPPoA transmits packets in ATM cells, each cell is a fixed length and carries 48 bytes of data, a packet size of 1430 (+ 10 bytes of added header and trailer) exactly fits 30 ATM cells whereas a packet size of 1431 requires 31 ATM cells the last of which will only have one byte of useful data and 47 bytes of padding.
 
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Off the speedtest I get
Downstream 466 Kbps (58.3 KB/sec) 503 Kbps (inc. overheads)
Upstream 252 Kbps (31.5 KB/sec) 272 Kbps (inc. overheads)

And thats at 10pm......
Not bad at all.....I wouldn't expect too much more than that from a 512 (It would be nice). But in reality I am getting near enough what I am paying for.
 
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Eclipse said to set the router (DG834Gv1) to MTU of 1458 (that was in apr-2004).

But is that still valid?

Also, why do people say the NIC's MTU needs changing. If the router is set to 1458, surely it does not matter that the PC's NIC is 1500, just as long as its >= router mtu?
 
I suspect it may be out of date. One way to find out is to temporarily set the router and network card to use an MTU of 1500 so that you can test it.

Then ping a website (one that will reply to large pings) with a 1500 byte packet with the do not fragment flag set, and see if you get a reply.

Ping packets have a 28 byte header, so to send a 1500 byte packet, you need to specify 1472 bytes of data, for example:-

ping adslguide.org.uk -f -l 1472



You can also try visiting http://www.netmonitor.org/tools/ttest.php with your MTU set to 1500. This site reports the MSS requested and Maximum packet size sent and received.

Providing you have your router configured to use PPPoA VC-Mux encapsulation you should actually get marginally better performance with an MTU of 1478 (or an MTU of 1430 if there really is a 1458 limit)

I think the reason usually given to reduce the MTU on the PC's NIC and leave the router's set to 1500, is to avoid the router fragmenting the packets it receives from the PC.

However, I have the MTU configured on my router to 1430 with the NIC set to 1500, I don't get any fragmentation because my router reduces the "MSS requested" in syn packets sent across the WAN, this results in my PC sending and receiving 1430 packets, hence no fragmentation.
 
nordle said:
Eclipse said to set the router (DG834Gv1) to MTU of 1458 (that was in apr-2004).

But is that still valid?

Also, why do people say the NIC's MTU needs changing. If the router is set to 1458, surely it does not matter that the PC's NIC is 1500, just as long as its >= router mtu?

Its to stop the router doing much work as if you have a MTU of 1500 on the PC it will send a packet to the router of 1500 then the router has to split that into a packet of 1458 and 42 putting its own headers on aswell, so if you put the MTU on the PC to lower than it is on the router the router doesnt have to split the packets and give a higher payload allowing for more data to be carried overall than 1458, 42, 1458, 42 bit packets would be 1458 1458 1458 packet sizes
 
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