Loss of RFNM (w/fix)

[Clyde W. Hoover]

Subject: Loss of RFNMs on ARPAnet hosts
Index:	/sys/netimp/if_imp.h 4.2BSD

NOTE:	This is **not applicable** unless the modifications from Chris Kent
	(cak at purdue.ARPA, posted 21 March 1984) have been made to
	/sys/netinet/tcp_output.c.  These modifications advertise a
	maximum TCP segment size that is tuned per network interface.

Description:
	Connections to certain hosts on the ARPAnet will start failing with
	"out of buffer space" messages.  Doing a 'netstat -h' shows
	that the host (or the gateway to it) has a RFNM count of 8.

	The RFNM count never drops below 8 and so the network path is
	unusable until the system is rebooted.

	The problem lies in the LH/DH-11 IMP interface.
	Sometimes, most likely always, it will not set the <END OF MESSAGE>
	flag in the control & status register if the input buffer is filled  
	at the same time that <LAST BIT SIGNAL> from the 
	IMP comes up.

	This causes the LH/DH driver to append the next 
	incoming message from the IMP to the the previous message.
	This process (appending of messages) will continue until
	a message SHORTER then the input buffer size is sent --
	a RFNM response does nicely.

	This results in the LOSS of the succeeding messages (e.g. RFNMs)
	since the 1822 protocol handling code expects to get only
	<ONE> message from the LH/DH at a time.

	This problem happens when the IMP MTU is advertised as the TCP
	maximum segment size (as is done by the TCP changes from cak at purdue).
	This allows an incoming message to be 1006 + 12 bytes long, which
	equals the size of the 1018 byte input buffer in
	the IMP (I believe) and so exercises the bug in the LH/DH.

Fix:
	/sys/netimp/if_imp.h:
/*
 * When IMPMTU based on 8159 is advertised as the TCP maximum
 * segment size, it exercises a bug in the LH/DH-11 that causes small
 * messages (e.g. RFNMs) to be dropped.  So we just arbitrarily
 * shrink this number until it falls below the bug threshold.
 * #define      IMPMTU          ((8159 / NBBY) & ~01)
 *                                ^^^^
 */
#define IMPMTU          ((8000 / NBBY) & ~01)

	As long as all incoming messages are less than or
	equal to  576 + 12 bytes long. Since 576 + 12 bytes is less
	then 1018 bytes, the size of the input driver buffer,
	the end of message flag in the control&status 
	register is always set correctly.

-- 
Shouter-To-Dead-Parrots @ Univ. of Texas Computation Center; Austin, Texas  

"All life is a blur of Republicans and meat." -Zippy the Pinhead

	clyde at ngp.UTEXAS.EDU, clyde at sally.UTEXAS.EDU
	...!ihnp4!ut-ngp!clyde, ...!allegra!ut-ngp!clyde