mtu unchange

[Spherion]

how come i keep setting my mtu to 1492 but everytime i do a TCP/IP Analyzer it show i only set it to 1454 ?

[Spherion]

well i found out why every time i took the tci/ip analysis test my mtu always show 1454 when i keep setting it to 1492.
it was because my stupid isp ethernet card they gave me got it setting to go only 1454 max for mtu.
is there anyway i can get around this setting to set it to 1492 ?

[Damienb123]

Maybe this wont be of interest to you or maybe it will...

Your calculations for packet MTU size does not seem to include calculations for ATM cell sizes. 1454 is an optimal value for DSL with PPPoE in most cases due to the ATM cell sizes that are fixed at 53 bytes.

ATM breaks up IP packet into 48 byte chunks then puts its own 5 byte header on each cell. The final cell for each ATM transmission can contain a maximum of 40 bytes data as it has to include 8 bytes for SAR data (Segmentation and Reassembly).

Thus: (Allways wanted to use that)
Initial cells: 5 bytes header + 48 bytes data
Final cell: 5 bytes header + x bytes (remainder) data + cell padding (slack space) + 8 bytes SAR

Therefore: (Allways wanted to use that too)
If you choose say the maximum allowable MTU size for PPPoE of 1492 bytes then you will have some slack space in the final ATM packet.

ie: 1492 + 26 bytes header = 1518 bytes
1518 / 48 = 31 cell + 30 bytes for final cell
Final cell 5 bytes ATM header +
30 bytes payload +
8 bytes SAR = 43 bytes
This leaves an additional 10 bytes for ATM cell padding to make up the 53 bytes required for an ATM cell.

It is true that this is 10 bytes out of each packet (1492 bytes not 1518), but if you do want to start getting rid of overhead, this may be one item worth while starting with.

Routers on the other hand, should never change packet sizes or at least be set at the maximum permissable packet size to reduce fragmentation.

I hope this helps you.

Disclaimer: I dont know what I am talking about! :)

[mnosteele52]

Try this:

Start - run and type in regedit and go here in your registry:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\Net\000n

(Where "n" is a number like 0001, 0002, etc.) Look for the 000? folder, with the "iVasion PoET Adapter" in it. (To the left of which it will say, "DriverDesc".)

Look for the entry TunnelMode = 1
Double click on TunnelMode, and change the 1 to a 0.

:) ;)