Subject: Re: heterogeneity

From: Scott Shenker <[email-masked]>
Date: Thu, 5 Jan 1995 10:55:23 PST

Steve,

> 
> I find myself questioning the argument that lack of availability of
> a particular service level will reduce the viability of the model.  I'd
> like to think that if a user asks for Guaranteed, he wants Guaranteed. 
> To respond with possible alternatives rather than denying the request
> seems like it is going to add unnecessary setup time and traffic, 
> particularly if the user did mean he needed the specified level of 
> service.  As a simple alternative, you might have the user specify
> a 'desired' level of service (say Guaranteed), but also specify what
> he would accept if that were not available (e.g. ASAP), or nothing
> if he really knew what he needed.  

I am not sure I understand your comment, so please correct me if my
reply misses the mark.   There seem to be two separate issues.

Advertising allows the receiver to see whether or not Guaranteed
service is available end-to-end.  If a user really wants guaranteed
service end-to-end and it isn't available on this path then either the
user asks routing for another path or the user gives up.  I think some
users will want replacements, and so they would be the only ones to
issue requests along a path that did not offer guaranteed service
end-to-end.

Your proposed alternative is really a variant on the scripts, except
you want the user to supply the replacement service explicitly (e.g.,
ASAP) rather than the degree of reliability of the replacement.  The
reason I had the router choose the service is that in come cases ASAP
service is a very good replacement for Guaranteed service (e.g., a very
overprovisioned link) and other times it isn't (e.g., a very congested
link) and there is no way for the user to tell the difference.  The
router can, and so I let the router choose the service.

> 
> Putting that aside though, I had a question on your proposal.  If 
> a particular router makes a replacement, do all subsequent routers
> attempt to provide the replacement service level or do they still
> try to meet the original request?  I believe that this has some
> implications in trying to meet the original requests, particularly if
> the non-complying router should eventually be bypassed.

All routers respond the original request.  So if only a single
router along the path does not offer guaranteed service (say, for
example, the ethernet at the first hop) then all other routers
give the flow guaranteed service.

--Scott