distribution layer confusion

[brad cain <bcain@mediaone.net>]

The comments below are really an attempt to better describe what
I believe the distribution architectural layer is really about.  I think
there has been some confusion between what we have been calling
request routing and distribution.  [Note that these comments are not
about protocols but about the architecture]

Please comment as I think we should iron this out soon...

-brad




My belief is that the "distribution layer" is really divided into
two straightforward parts:
	1. content replication
	2. content signaling

Content replication: this is the actual replication of specific
pieces of content between CDNs through distribution peering.
A protocol is used to negotiate and advertise which content
should be replicated between the peers. [note: There may
be a negotiation phase but I'm a little worried about a complicated
request response model.  A simple negotiation may be needed to
negotiate content types.]  The key thing here is that this is
about copying content from one CDN to another.

Example:

CDN A is peered with CDN B.  CDN A advertises a set of URIs
which should be replicated on CDN B.  In this advertisement
a location is sent as to where the content should be obtained
from (i.e. a surrogate or origin server).  A real world example
would be for distribution of streaming media files: CDN A
would advertise to CDN B which files should be copied and
from where.




Content signaling: This is the distribution of content freshness
information between CDNs.  An example would be the WCIP protocol.
These signals are used to purge content or to update the expiration
times of the content.  Content signals may also be used to update 
other metadata associated with the content (this is probably more 
future oriented).

Example:

CDN A is the "root" or main CDN for content provider B.  CDN A is
peered with CDN C.  When content from provider B is changed, CDN A
relays a content signal to CDN C so that it is expired from its
surrogates.