Re: Multiple inputs Single output Router

[John Heidemann <johnh@ISI.EDU>]

On Thu, 23 Sep 1999 17:02:52 PDT, Frank Chu wrote: 
>Am I correct in the following understandings ?
>
>1. Queueing displine can only be implemented in
>"link"s which simulate output queues
>
>2. If a multiple inputs (say, 2 inputs) and single
>output router, I will have to use this:
>
>  n1---->Link1----\
>                  n3--->Link3--->
>  n2---->Link2----/
>
> Then packets from Link 1 and 2 will arrive in
>classifier of n3 first and then be aggregated into
>Link3's buffer.  But, how does NS do the aggregation ?
> First come first in ?

Aggregation is done based on the queueing model of Link3.
(DropTail by default, but there are also alternatives like RED.)

> If two packets arrive at the same time (simulation
>time), which one will enter the Link3 ? Is it possible
>to have unfair situation that packets from one link
>enter a lot while the other seldom have chances ?

It's not specified in what order concurrent events are handled.
It should be deterministic (even with different schedulers---if
it's not that's a scheduler bug), but which order is chosen
depends on many factors.

It should not be unfair, though.  Concurrent arrival events will
always both be handled before later arrival events.

   -John Heidemann