sendmail bad file problem

[Web Dove]

The problem was that the ip layer was breaking up 512 byte tcp packets
into uncontrolled 488 byte chaos packets.

For big correspondence 4 tcp packets (2048b) would be queued at a time
so each time a retransmit was tried, they would be broken up into 5
uncontrolled chaos packets and sent out back to back.  Since the packets
were uncontrolled, and possibly because the gateway was a lowly lsi-11,
5 packets back to back were too much to accept at once.  Unfortunately,
if one of the early packets is lost (say the second), then no tcp
packets can be rebuilt from them.  Thus, the connection suddenly drops
to 0 throughput.  If the second packet is consistantly dropped, then
eventually the connection times out.

It is unfortunate that the system error for such an event is EBADF as
opposed to ECONNABBORTED (which would have led to a more easily
deciphered error message.  I don't know where that error was generated.

The solution was to change tcp_{input,output,subr} to enforce a 400 byte
maxseg size.  This means that the loss of an individual chaos packet
leads to the loss of a single tcp packet, rather than many of them.