21 Jan
2021
21 Jan
'21
2:37 a.m.
On Jan 20, 2021, at 5:54 PM, Johnny Billquist <bqt
at softjar.se> wrote:
Hey, speaking of this...
I decided to go and check how RSX actually works. And that have led to some interesting
observations on my side.
1. RSX actually do respect the maximum data layer block size the other end sends in its
init message. So even if I have a large size defined in RSX, it should be working correct
against another end with a smaller block size.
Yes, and that behavior is exactly what the spec intended.
2. With a block size of 576 (either local or remote),
the actual packets being sent are 578 bytes. Seems there is a 2 byte checksum that is in
addition to the actual payload. I found this a bit surprising, but it seems this might be
very intentional. So I'm trying to see if this also is true for other systems. If
someone else can do some checking that would also be interesting.
You mean the routing messages? Yes, they end in a 2 byte checksum, but that is part of
the message and is required to be included in the size limit. If an implementation is
told block size 576 and it sends 578 byte routing messages, that's a bug. Apparently
one it gets away with, but it's not supposed to do that.
Is this on Multinet? I would expect this sort of thing to be caught on DDCMP. For
example, DECnet/E allocates DMC buffers of the precise size you ask for, so if you were to
send it a routing message 2 bytes longer than expected, DDCMP would reject it as an
oversized packet and the link would go down due to excessive retransmits.
3. VMS V5.4 seem to not be happy at all if the other
end declares a largee maximum data layer block size that the local one. This is the
problem I had when talking to a VAX at Peter, which is running V5.4. I have no idea if
that problems is still there in V7...
That's ugly and a definite bug.
It would be really interesting to have a Multinet VMS
machine setup in the same area as me, with a link to RSX, so I could do some more
testing...
paul