Re: PQC and QOSPF

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

Jeffrey, 

Thank you for the prompt and authoritative reply from 
the QOSPF point of view. 

Summarizing from your response, RRA flooding is needed
to avoid "stepping over your own shadow" but such 
flooding indeed leads to a scaling problem.  Possible 
approaches to alleviate the problem:
- Let RRA info be propagated only to routers on the data 
  path, possibly using RSVP.  But RSVP camp did not like 
  the idea.  
- Use ER, so that the info is only needed at the first 
  hop routers.  

Using RSVP to carry routing related info does appear
to violate the separation between reservation and 
routing.  Now, if we use ER, still there needs to be a 
way to propagate the resource reservation info to the 
first-hop (source) routers.  For this purpose, 
EROSPF (Explicit Routing OSPF) proposed in in 
draft-zhang-qospf uses RRAs but limits the 
flooding destination only to the first-hop 
routers (but these RRAs have to be propagated 
through intermediate routers anyway).  The 
complexity analysis in the draft seems to indicate 
the same message complexity as PQC.  Let me know 
if this comparison is incorrect. Thanks. 

Yao-Min 




Zhaohui Zhang wrote:
> 
> Yao-Min,
> 
> I have not read the paper yet, so my response is based your email only.
> 
> >
> > A criticism on QOSPF made in the PQC paper
> > (http://www.isoc.org/inet97/proceedings/F4/F4_2.HTM)
> > was the scaling issue with the Resource Reservation
> > Advertisements (RRAs).  RRA, being per-flow
> > information, is flooded throughout an OSPF area.
> >
> > The approach proposed in PQC is to have the resource
> > reservation information for a flow only propagated
> > to routers that are on the path of the flow.
> > The propagation mechanism uses RSVP.
> 
> Flooding RRAs anywhere does cause the scaling problem, but from day one
> we proposed an alleviation - use Explicit Routing.
> 
> With ER, reservation information is sent only to the "first hop"
> routers - even better than sending to routers on the path of the flow.
> 
> We also tried to put it into RSVP so that only routers on the path would
> get the info, but RSVP folks did not think that was a good idea, so we
> had to give up.
> 
> >
> > Is there a compelling reason in the design of
> > QOSPF to have RRAs flooded throughout an area?
> 
> The resource information is needed to deal with "step over your own
> shadow" problem. Any router involved in the calculation of the path for
> a flow needs to know the reservation info for the flow - I believe you
> share the same view.
> 
> Flooding them anywhere does not scale and is not necessary, yet it is easy
> to understand and simple to implement, so we did not put ER up front but
> put it in the appendix instead.
> 
> >
> > --
> > Yao-Min Chen, Fujitsu Labs of America, [e-mail]
> > 595 Lawrence Expressway, Sunnyvale, CA 94086-3922
> > Phone: [phone] Fax: [phone]
> >
> 
> Thanks.
> 
> Jeffrey