Re: PQC and QOSPF

[Yao-Min CHEN <[e-mail]>]

Masataka Ohta wrote:
> 
> Yao-Min;
> 
> > > > these messages follow the direction of a flow,
> > > > only downstream routers receive resource reservation
> > > > info of upstream routers but not the other way
> > > > around.  The problem with this is that upstream
> > >
> > > Yes, it makes the QOS routing a little more stable and is not a problem.
> >
> > Could you elaborate on the stability part?
> 
> Underestimation of down stream resource makes the current path
> (up stream) preffered.
> 
> > As for "not a problem" part,
> > I hope I understand the protocol correctly but it
> > seems there could be a potential problem.  Consider
> > the following example: a bandwidth of b1 is reserved
> > for a multicast flow on the path S--A--B--C--R1.
> 
> OK. But note that you can't scalably propagate resource information
> about all the receivers of the multicast.
> 
> > Using PQC, router A considers link (B,C)
> > has only available bandwidth b2.  On the
> > other hand, router C can
> > "correct" the link bandwidth to b1+b2 as far
> > as computing the route for the flow is concerned.
> > Now suppose a new receiver R2 would like to join
> > the multicast tree and links (A,R2) and (C,R2)
> > both exist (see Figure below).
> >
> > S--A--B--C--R1
> >     \   /
> >      \ /
> >       R2
> 
> OK.
> 
> > Since router A thinks the available bandwidth b2
> > of (B,C) is insufficient (recall b2<b1), it
> > adds branch (A,R2) for the new multicast tree
> > but router C may compute (C,R2) to reach
> > the new receiver.  So we have an inconsistent
> > calculation of the new multicast tree.
> 
> Wrong.
> 
> As the title of the paper says, routing is hop by hop.
> 
> Neither A nor C are volunteers to calculate the tree. They just
> route RESV requests from down stream to upstream.
> 
> So, first, R2 joins a group with best effort. Then, PATH will
> come and R2 can now QoS route RESV to A or C and a branch is added.
> 
> That's all.
> 

Interesting ... what you proposed is:
- resource reservation info is PUSHed to downstream nodes through 
  PATH messages; then
- QoS path is PULLed by downstream nodes through RESV messages
  (after each node computes its previous hop on this path)

>From another perspective, the proposed scheme separates
1) reachability calculation, and 2) path selection.  A best
effort routing protocol is used only to maintain reachability.
The above PUSH-PULL model is the one actually used for QoS 
path selection.

This model seems also suitable for "traffic engineering."  
Congestion info can be pushed to downstream nodes who then
pull an uncongested path.

Yao-Min