ISDN tweak guide (Windows 98 & ME)

MTU, TTL & More

First a few definitions...

MTU. Maximum Transmission Unit. This specifies the maximum transmission unit size of a packet. A packet larger than the MTU size will be fragmented into smaller packets. Using the (Maximum) MTU your ISP allows can result in less fragmentation of packets.

TTL. Time To Live. This sets the number of hops a packet is allowed to pass between "you" & it's destination.

DefaultRcvWindow. This sets the size of the TCP send & receives windows, which is the amount of data that can be accepted in a single transaction.

Before setting your MTU you must find it. To start with you should set your MTU to a high value. This way the following procedure will be accurate, i.e. If you MTU is currently 576 then the procedure won't find an MTU an larger than 576. Basically skip to the Setting MTU/TTL/RWIN section & set your MTU to 1500 or greater. Then Reboot for the changes to take effect, then return to this section. You will now be able to get an accurate indication of your MTU.

Click on Start, Programs, Accessories, MS-DOS prompt once connected to the Internet. Type in ping f l xxxx Replace with whatever address your ISPs is (Or the IP address). Replace xxxx with numeric values until you get the message Packet needs to be fragmented by DF set. Use whatever the largest value is before that error message & add 28 to it to find your MTU.

f. Sets a Dont Fragment flag in packet.

l xxxx. This sends a buffer sized xxxx.

E.g. ping f l 1472

1472 + 28 = 1500. As a result Id set my MTU to 1500 as higher values return the fragment message.


Once the value is discovered click on Start, Run, type in regedit & hit Enter. Open the following registry key [HKEY_LOCAL_MACHINE\System\CurrentControlSet\ Services\VxD\MSTCP].

Right click in the right hand pane. Select New & String value. Make 2, one called DefaultRcvWindow & the other DefaultTTL. Once they have been created right click on each of them select Modify. For DefaultTTL I'd recommend selecting 32 or 64, if you cannot reach a server with 64 hops it is futile to waste anymore on trying. For DefaultRcvWindow I suggest using the following formula to get a value for it. Ideally the factor of multiplication (X) should result in: MTU 40 * X = Connect speed/10 or thereabouts, e.g. In my case that is 1500-40 * 3 = (circa) 45333/10. 4380 = (circa) 4533.


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