RE: Hop

["Ian Lowe" <ianlowe@xxxxxxxxxxxxxxxxx>]

Heh - it's a bit of a mindjob, isn't it :D

The hop exists for the situation of linked transports (rather than networks
per se) - it's actually used to prevent loops (or rather, excessive
looping)

If you have an Ethernet bus, an RS232 bus, some RS485, and someone
mistakenly creates a loop.. The hop count is intended to say "no more
of
this, young man!" and stop the packets from going more than 2/3 times
round
the track.

The reason for the hashing etc, was to avoid a repeated storming of packets
we saw when developing the app - if you had two packets from the same
source
in a row, it would bounce them back and forward until the hopcount maxed
out..

With hindsight, I think this may have been down to the whole "listen
on"
thing - I couldn't think of a good reason why the looping happened, but
building a hash of the message seemed to do the trick.

As I recall, Tony and I had a wee debate about "hop=" in the
first place -
it's not *really* needed... But it's there are a safety feature given that
xpl was always intended to run across multiple network systems (and
therefore encounter possible bridging issues)

Ian.

-----Original Message-----
From: ukha_xpl@xxxxxxx [mailto:ukha_xpl@xxxxxxx] On Behalf
Of Mal Lansell
Sent: 14 October 2005 00:16
To: ukha_xpl@xxxxxxx
Subject: [ukha_xpl] Hop

So I was looking at the idea of an xPL Router, when Ian pops up and says
"hey, we wrote one", but I the idea still rumbled around my brain
for a bit.
After a bit of design, I was left wondering how to stop messages looping
around between routers.

Looking at the existing app, it appears to hash the received message, and
then not retransmit any that match any of the hashes in its list.
But I thought that was what the hop was for - if it is > 1 then don't
retransmitt.

Then I realised, what if there is an rs232 network connected? - the xPL
messages from that will have a hop of 2 (1 when sent, +1 when moving from
rs232 ro udp).  That led me to wonder just what is the point of the hop
count - if there is more than one app on the PC that can increment a hop,
then without knowing who did it, how it can be of any use...?

Mal




xPL Links: http://www.xplproject.org.uk http://www.xplhal.com
http://www.xpl.myby.co.uk
To Post a Message: ukha_xpl@xxxxxxx
To Subscribe:  ukha_xpl-subscribe@xxxxxxx
To Unsubscribe:  ukha_xpl-unsubscribe@xxxxxxx