An Example Scenario

As an example, consider successive accesses of http://S/path via an HTTP proxy P:

Client C1 sends a request req1 for r=http://S/path to P. addr(req2) = addr(req1) \forall e:Entity penalty(req2, e) = penalty(req1, e) date(req2) >= date(req1) resp1 = get(req2)

P contacts S, makes a GET request for /path, and receives a response resp1:

date(resp1) =	Date: Mon Dec 12 19:30:39 CST 1994
lastMod(addr(req2), date(resp1)) = Last-Modified: Fri Dec 9 19:30:39 CST 1994
expires(addr(req2), date(resp1)) = Expires: Fri Dec 16 19:30:39 CST 1994
type(object(resp1)) = Content-Type: text/plain
body(object(resp1)) = blah blah blah...

Let

e0 = (text/plain, "blah blah blah...") = object(resp1)
t0 = Mon Dec 12 19:30:39 CST 1994
t1 = Fri Dec 9 19:30:39 CST 1994
t2 = Expires: Fri Dec 16 19:30:39 CST 1994

Then at this point, the proxy knows:

	e0 = HTTPGet(req1, t0), i.e.
		e0 in Represent(r, t0)
		and e0 minimizes AcceptPenalty(req1, Represent(r, t0))
			    over Represent(r, t0)
	Last-Modified(r, t0) = t1
	Expires(r, t0) = t2

Since there are no URI: headers in the response, the proxy also knows:

	Represent(r, t0) = {e0}

The proxy passes resp1, containing e0 on to the client C1. It updates its cache so that Pcache[r] = resp1.
Client C2 makes a request req2 via the proxy P. req2 has the same URI as req1.
P examines its cache and finds Pcache[r] = resp1.
- If the proxy is willing to make the hueristic assumption that its clock is in sync with S's clock, then it can test whether the current local time t3 satisfies t0 <= t3 <= t2. If this is the case, then by the definition of Expires, the proxy can conclude that Represent(r, t3) = Represent(r, t0).
  Since it knows that e0 is the only element of Represent(r, t0) and hence of Represent(r, t3), it can conclude that e0 = HTTPGet(req2, t3), and it can return resp1 to C2.
- If the t3 <= t2 test fails, or if the proxy wants to be strict about distributed notions of time, it can issue a "conditional get," using t0 in the request header:
```
	 If-Modified-Since: Mon Dec 12 19:30:39 CST 1994
	 
```
  If the document has not been modified, the server S will indicate this via resp2 at t4:
```
	 304 Not Modified
	 Date: Mon Dec 12 15:30:39 CST 1994
	 
```
  The proxy can conclude that Expires(r, t4) = Expires(r, t0), and proceed as above.

Future Discussion

Other attributes of references

There are certain attributes commont to many information resources. For the resources to which some of these attributes apply, we should develop models and mechanism to make it possible compute the attributes reliably:

<attribute> (uses)
Author (identification, navigation, relavence, reply)
Date of publication (identifying, navigation, relavence)
MD5 signature (identification)
Message-ID, Content-ID (identification)
Maintainer (problem reporting, replying, fallback for annotation)
Abstract (indexing, relavence)
Date of last revision (identification, navigation, relavence)
Expiration (resource migration, traffic optimization)
Translations (to various formats/languages)
Versions
Copies (resource migration, traffic optimization)
Parent, Children (navigation, relavence)
Previous, Next (navigation, relavence)
TOC, Index (navigation)
Back-links (navigation, relavence, maintenance)

An Example Scenario

Future Discussion

Other attributes of references

Other Topics