Loss of RFNM (w/fix)

[Ron Natalie <ron>]

> Subject: Loss of RFNMs on ARPAnet hosts
> Index:	/sys/netimp/if_imp.h 4.2BSD
> 
> /*
>  * 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)
> 
This BUG fix, while it will work, is WRONG.  Reading carefully you
will find that the EOM bit may not be set on the 8159/NBBY byte.
You should set the input DMA to IMPMTU+2 to avoid this problem.

-Ron