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Posted
on July 02, 2001 by Toby
Crundwell & Thomas
McGuire
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 www.yourisp.com. Replace www.yourisp.com
with whatever address your ISP’s 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 Don’t Fragment flag in packet.
–l
xxxx. This sends a buffer
sized xxxx.
E.g. ping
–f –l 1472 194.145.128.1
1472
+ 28 = 1500. As a result I’d set my MTU to 1500 as higher values return the fragment message.
Setting
MTU/TTL/RWIN
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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