🚀 go-pugleaf

This may be an FAQ (though I've looked in the MacTCP information
produced by Eric Behr at spider.math.ilstu.edu but can't find anything
relevant) and if so I apologise.

We have a number of Macs running various versions of NCSA Telnet (with
and without MacTCP), Eudora etc..  We have one particular one, a IIfx
with an Apple ethernet card, which occasionally fails to make IP
connections to other hosts; not all the time, just sometimes.  It is
using MacTCP and the problem occurs with both Telnet and Eudora.  For
the interested what actually happens is that the host accepts the TCP
connection and then resets it.  The sequence is as follows:

1) Mac sends initial TCP connect packet (SYN) offering window of 1218
   bytes and TCP option of MSS of 536. 

2) Host responds (ACK and SYN) with its own window size of 4096 and MSS
   of 1480. 

3) Mac then ACKs this packet with its window still set at 1218 and no
   further TCP options. 

4) Host sends RST flag to reset the call.

I presume the host has decided that it can't cope with the combination
of a TCP window size of 1218 and an MSS of 1480.  We get exactly the
same behaviour when using either NCSA Telnet or Eudora.  On other
occasions, i.e.  when the thing works, the same Mac advertises a window
of 4221, as do other Macs using MacTCP, and everything works
beautifully. 

Since this seems to be independent of the specific application I assume
it's a MacTCP problem, possibly a shortage of memory.  Unfortunately I
don't know of any way of configuring memory for MacTCP (presumably it
uses the System heap) and can only guess that there's a general shortage
of System memory at the moments when this problem happens.  I'll
continue to investigate (though the machine is actually 2 miles away
from my office!) but if anyone has come across this sort of behaviour
I'd be interested in knowing. 

As I said it's a IIfx and Inter*Poll says System B1-7.0.1, Finder
B2-7.0, LaserWriter 7.1.1, Responder 201, AppleTalk 56, AppleShare 7.0. 
I don't know which version of MacTCP.


Sam Wilson
Network Services Division
Computing Services, The University of Edinburgh
Edinburgh, Scotland, UK