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From: opnrhen@acsu.buffalo.edu (Steve Rhen)
Newsgroups: comp.protocols.tcp-ip,comp.protocols.tcp-ip.ibmpc,comp.unix.xenix.sco,comp.unix.xenix.misc,comp.unix.sysv386,comp.unix.sysv286
Subject: Re: 3c501 users please read this
Message-ID: <58278@eerie.acsu.Buffalo.EDU>
Date: 7 Feb 91 00:08:13 GMT
References: <NELSON.91Feb5145001@sun.clarkson.edu>
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In article <NELSON.91Feb5145001@sun.clarkson.edu> nelson@clutx.clarkson.edu (aka NELSON@CLUTX.BITNET) writes:
>Sorry to post this so widely, but since the question has come up
>recently twice on the net, and twice locally to Clarkson, people
>should be aware of this problem.
>
>The 3c501 cannot receive back-to-back packets (two packets addressed
>to the same host with no intervening packet).  BSD Unix (well, SunOS
>and NeXTos at least) will attempt to fill your TCP window by sending
>back-to-back packets.  Therefore, your TCP Window must not be larger
>than your TCP MSS.
>
>  o Under KA9Q, use "tcp window 1024" and "tcp mss 1024".
>  o Under SCO Xenix, use the UNDOCUMENTED ifconfig option -onepacket.
>  o Under NCSA Telnet, use "maxseg=1024" and "rwin=1024".
>  o Under FTP Software's PC/TCP, use "ipconfig window 1024".
>

	I've run into situations where it may be necessary to reduce
  the tcp and mss window sizes down as low as 512 to prevent excessive
  retransmits. Many TCP implimentations will use 512 mss packets if they
  think that the packet is leaving the local network. (This is recommended
  based on 576 MTU in gateway requirements in the absense of MTU discovery.)
  With local subnetting and high performance routers two short back to back
  back packets can be generated within the available the the receive window, 
  causing the same poor performance.


Stephen Rhen
University at Buffalo
opnrhen@ubvms.cc.buffalo.edu