Re: Hop

["Mal Lansell" <mal@xxxxxxxxxxx>]

I presumed the hash was to detect messages broadcast by the router
when they were received back from its local hub - if they're not
filtered out then they would bounce back and forth between the
routers at each end of the connection.  I thought the hop might be
able to help here instead, but it sounds like its not used like that.

Out of interest, do any of our apps look at the hop?  I know my hub
doesn't - but should it?

Mal



--- In ukha_xpl@xxxxxxx, "Ian Lowe" <ianlowe@x...> wrote:
>
> 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
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