Re: hard questions: request routing

[Oliver Spatscheck <spatsch@research.att.com>]

Francis,

	I see the only advantage of multiple levels in the aggregation
of business relationships. I think otherwise you can use a full mesh.
So the question is where is CDI going. If you assume that every ISP
will be a CDN then we would need a few 1000 square business relationships
which seems not to be manageable. If you assume ISPs don't get into
the CDN business we have a top of 10 square business relation ships
and no deeper hierarchy is needed. I don't think that in either
case the protocol could not handle a 2-level system. 

Oliver 


Francis Zane writes:
 > 
 > My new concern is:
 > 
 > * Is this fundamentally a 2-level (content provider CDN -> end CDN) or
 > a multilevel (ie, chains of CDNs inbetween) problem.
 > 
 > (I'm leaving out the producer to the first CDN, as well as
 > distribution within the terminal CDN, since those should be easily
 > loop-free.)
 > 
 > This comes out of Oliver's suggestion of the 3-level mechanism (ie,
 > one forwarding CDN inbetween).  Which begs the question: if we're
 > going to limit the hierarchy depth, why one forwarder and not two or
 > zero?  More specifically, 
 > 
 > Question: What value do intermediate CDNs provide?
 > 
 > The only advantages I see is that they perform some useful
 > aggregations: collecting the metrics, decentralizing the routing
 > decisions, and simplifying the business arrangements.  My thinking is
 > that other than that, they just add complexity to the forwarding.
 > But maybe there are others?  Load-splitting?
 > 
 > On the flip side, this aggregation could be done just as well by
 > filtering the announcement traffic, or even an out-of-band system
 > (think airline travel agents) which CDNs subscribe to which sends them
 > updates on announcements/prices they care about, thus reducing the
 > actual RR to a 2-level system.  The point would be that they're not
 > involved in forwarding at the request level, and resolutions always
 > lead to terminal CDNs, so there aren't loops.
 > 
 > Suppose we go to a 2-level system.  What does it look like?  The one
 > I'm envisioning sounds more like the one Abbie is discussing (I think;
 > I'm still not 100% clear on the picture there, sorry): Each source of
 > content (I forget the word here) collects announcements it cares
 > about, builds a forwarding table based on this, and uses it to direct
 > to terminal CDNs.  Without loops, though, the lack of a consistent
 > global view doesn't matter.  One could even envision pseudo-terminal
 > CDNs, which aggregate terminal CDNs and never redirect to outside
 > CDNs, which would also be loop-free.
 > 
 > In a sense, I'd be sort of sad to go this way, as the forwarding
 > mechanism is somehow more, I don't know, algorithmically sexy, but I
 > don't yet see what the advantages are.
 > 
 > Francis
 >