Re: When route is changing....?

[Polly Huang <huang@catarina.usc.edu>]

On Tue, 12 Oct 1999, K Sun wrote:

> Hello,
>     on page 147 in ns Notes and Documentation (July 1, 1999), there are 
> some words about "Session Routing" (the last two lines of 'sesssion 
> rouing'), 
> "
> ....
> However, the user should note that the instantaneous recompute of the 
> routes in the topology can lead to temporary violations of causality 
> around the instant that the topology changes.
> ..
> "
>    I wonder what "temporary violations of causality" means during "the 
> instant that the topology changes"? Under my understanding, it should 
> mean:
> Because some packets has been delivered in the middle way according to 
> the previous route, they might be lost (or ..) after route change has 
> been completed. Anyway, in programs, if we realise some codes to 
> process such a situation, those packets are still under control.
> 
>    I am not sure whether my understanding is right or not. Hope further 
> explanation for it.

Sun,

That means (I think) if new routes happen to be much better than the old
ones (e.g. bringing up a direct link in between src and dst), packets
fired at a later time can arrive before earlier packets, which
might cause events to be triggered in a differenr order (thus called the
causality problem).  SO, if one's design/protocol has strict
ordering requirement, he or she should aovid session routing  or
examine carefully whether session routing is appropriate.

Cheers,
-Polly


>     Regards,
>        Sun Kai 
> 
> 
> 
>