[PLUG] ICMP ok, TCP fails

[Greg Lopp <lopp@pobox.com>]

Here's the problem: I'm trying to teach a USB device how to look like 
and behave like a CDC ethernet device (i.e. pass ethernet frames across 
USB, USB is my datalink layer).

I have the "look like" part working, as linux 2.6.10 and windows both 
recognize my descriptors.  I type "ifup usb0" and can pass packets back 
and forth....up to a point.

I can ping in either direction, so ICMP, wrapped inside IP, wrapped 
inside ethernet works in both directions.  But when I fire up ethereal 
and try wget or ftp to the device, I see linux send a SYN and the device 
returns it's SYN/ACK, but linux never sends back it's ACK.  It sits 
there, times out and sends its SYN again.  When I use strace, the 
process never returns from connect().

What could possibly be going on here?  I've used ethereal and tcpdump to 
witness the SYN/ACK packet return...so it shouldn't be a problem with my 
device.  Are there any other tools I can use to watch these packets work 
their way further up the stack? I'd rather not have to hack around in 
libc.so.6   Have have been inside the usbnet module, but that only 
covers the DL layer.


Additional details:
The target is running a third party "embedded TCP/IP" stack.
There are no such problems when using a conventional ethernet connection.
I can provide dumps and capture files upon request.

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