netdemo: TCP locks up after echoing 11K

[Arnout Vandecappelle]

 Hoi,

 I'm developing an Ethernet driver and testing it with the netdemo.  It
seems to work OK, except that the echo servers hang when data is sent
too fast.

 I use netcat to send a large file to the echo server.  I use tcpdump to
watch the actual data sent.

 The client (a PC) dumps the first 57344 bytes of data very quickly on
the network.  Of these, 36376 are actually sent out by TCP and
acknowledged by the RTEMS board.  The echo server sends back 18488 bytes
of data.  Of these, only 1536 bytes are actually sent on the network
(and acknowledged): the echo server blocks when trying to send the next
512 bytes.  At this point, the two TCP end point continue exchanging ACK
packets indefinitely.  The client doesn't send anything more since the
receive window is full.  Why the server doesn't continue sending, I
don't know...

 Note that the communication between the ethernet chip and the CPU is
very slow, which leads to some packets being dropped.  However, I don't
think that should be an issue: the missing data is retransmitted.

 My guess is that the RTEMS TCP stack has consumed all its transmission
buffer space, but still needs to allocate an extra mbuf to start the
next transmission.  Can anyone confirm this theory?  Can anyone think of
a fix?

 By the way, I'm using 4.6.99.2, but I've looked at the CVS version and
don't see any major differences with this respect.

 Regards, and thanks,
 Arnout

-- 
Arnout Vandecappelle                     <arnout at mind.be>
Senior Embedded Software Architect       +32-16-286540
Mind Embedded Development (an Essensium division)