Session Routing?

[K Sun <ksun@essex.ac.uk>]

Hello,
    I have a little confusion about Session Routing.
In Page 147 (Ns Notes and Documentation), there are some words about 
Sesssion Routing:
1. "If the topology changes while the simulation is in progress, then 
some sources and destinations may become unreachable from each other".
2. "However,  it will also run th same algorithm to recompute routes in 
the event that the topology changes during the course of simulation."

>From 1, it implies, just because the routes can not be changed in time, 
there are some sources and destinations unreachable.
>From 2, it implies the routes can be changed while the topology is 
changed. 
I do not know it clearly. 
Or it means, there is only a very small interval in which there are 
some nodes unreachable?

Thanks,

Sun Kai