[Olsr-dev] reliable vs unreliable updates

[Henning Rogge]

Am Donnerstag 29 Oktober 2009 04:44:05 schrieb David Murray:
> Hi,
> 
> I'm writing this because I am interested in understanding why mesh routing
> protocols send unreliable/unacknowledged TC updates. Modern day wired
> routing protocols such as OSPF and EIGRP both use reliable updates. As OLSR
> is designed for unreliable wireless networks it therefore seems a little
> strange that we have moved to unreliable updates.
> 
> It seems that the number one problem for link state routing is having the
>  LS databases desynchronised. I have read that OLSR compensates for its
>  unreliability by sending redundant TC messages. If we could send
> acknowledgements reliably then surely we could reduce the number of TC
> messages that need to be sent. This would mean that the increased overhead
> of acknowledgements, used to provide reliable TC messages, would not be as
> great as they may initially appear.
Theoretical yes... but with the broadcast medium like WLAN you have an ACK 
explosion with this strategy. WLAN links are a lot more unreliable as the 
typical OSPF/EIGRP ethernet links.

> I read on the wikipedia page for OLSR that using reliable updates in
> wireless mesh networks is much more difficult to implement than in
> traditional protocols. Is this the main reason? Why is this?
Protocols with unreliable updates are easier to implement, you just keep a 
"Time to live" field in your database for anything you get. Of course this can 
be trouble, but it's an easy strategy.

Henning
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