Subdomains & sharing

I’ve been doodling diagrams in notebooks about ad engine integration since I got thinking about it yesterday. In theory, I think much tighter integrations between ad engines is possible, but it requires some coordination.  (Probably more coordination than will ever happen.)

The ‘special sauce’ lies in the ability to share cookies set on a base domain with multiple subdomains. A cookie written to ‘.domain.ext’ is accessible by ‘greg.domain.ext’, ‘mike.domain.ext’, and ‘whatever.domain.ext’, but cookies set on ‘greg.domain.ext’ are NOT readable by ‘mike.domain.ext’.

To make collaboration work, you’d need three things:

1) a server at ‘domain.ext’ that did nothing but check for a cookie with a unique ID, and if absent set a cookie with a unique ID.

2) a unique subdomain at ‘whatever.domain.ext’ for each ad engine, pointed to the ad engine’s servers and used as the ad engine’s cookie space.

3) a call to ‘domain.ext’ on the publisher page, as well as to the unique ‘whatever.domain.ext’ subdomains of whatever partners they wished to give cookie access to.

I still need to think about the corner cases, but I think that’s enough to enable effective integration with third-parties without resorting to browser redirects, while still allowing third-parties to make use of data about an individual user (like a behavioral targeting profile) created when the user was on other sites.

Anyone know of any collaboration schemes that relied on multiple subdomains’ ability to access the same cookies set on the base domain?