Wireless Adapter Seems Capped at 1300 MTU

J

Jim Yohn

I'm running a Windows XP Pro desktop and I have a circumstance where the
wireless connection is getting capped at 1300 MTU. Assuming wireless
adapters are not supposed to be capped at 1300 MTU, I'd like to increase it
to 1500 and improve transmissions.

The other (wired) PC in my home network can get 1500 MTU, so I don't believe
the local router is limiting it. I verified the wireless adapter's max MTU
is 1300 by pinging the router/AP and external IP addresses with this
command:

ping <dest> -f -l 1272. where <dest> is any of the addresses I've tried.

Any packet size higher than 1272 gets a response "Packet needs to be
fragmented but DF set." With the 28 bits Windows tcp/ip adds for error
checking, etc., 1272 + 28 = 1300 MTU.

I have no MTU values set in the registry for the interface (or anywhere).
I've tweaked tcp/ip settings with cablenut to optimize the transmit/receive
window sizes (only after identifying the 1300 MTU cap). The wireless link to
the router/AP is where the MTU limit appears to be in effect (the 1st hop).

It is a US Robotic 5410 adapter connecting to a DSL modem via a USR 8054
Router/AP. Any ideas for a way to increase the MTU using this adapter?

Thanks,
Jim
 

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