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Re: Confusion about Age: accuracy vs. safety

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Tue, 20 Aug 96 17:46:55 MDT
Message-Id: <9608210046.AA21591@acetes.pa.dec.com>
To: "Roy T. Fielding" <fielding@liege.ICS.UCI.EDU>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Roy and I were having a debate about the proper specification
of the Age header, about 6 weeks ago.  I dropped the ball on
this for various reasons, but Larry Masinter has asked us to
settle this issue.

Roy argues that the current language in section 14.6,
  If a cache receives a value larger than the largest positive integer it
  can represent, or if any of its age calculations overflows, it MUST
  transmit an Age header with a value of 2147483648 (2^31). HTTP/1.1
  caches MUST send an Age header in every response. [...]

should be changed so that the last sentence there says:
  [An] HTTP/1.1
  [cache] MUST send an Age header field in any response obtained from
  its own cache.

He argues that this is simply removing an ambiguity, because we
can't possibly mean that a cache must include the Age header when
it is simply passing along a response that it has just received
from an inbound server, because that would lead to an overestimation
of the Age value:

    What you are saying is that every step along the way should add the amount
    of time it took to satisfy the request EVEN IF THE REQUEST CAME DIRECTLY
    FROM THE ORIGIN.  That means that if the request passes through three
    caches (A, B, C) with each segment taking (a, b, c, d) amount of time to
    satisfy the request
    
	 UA  ------->  A  ------->  B  --------->  C  ------->  OS
		a           b              c             d
    
    then the Age will be calculated as follows:
    
	 At  C:  age=d
	     B:  age=d+(c+d)
	     A:  age=d+(c+d)+(b+c+d)
	    UA:  age=d+(c+d)+(b+c+d)+(a+b+c+d)
    
    when we all know that the REAL age at UA must be less than (a+b+c+d).
    Note that (a+b+c+d) will always be added by UA.

    The wording that I submitted would have corrected that error in the
    specification.  Fortunately, the existing wording of the spec is only
    ambiguous -- we'll just have to interpret "cache" to mean that it only
    takes effect on retrieval from the caching mechanism.

Roy is wrong; this is not an ambiguity: the current spec language
means what it says, and this is the right thing for it to say.

Roy is right that, as written, the spec causes the reported Age
value to be larger than the actual time since the response was
generated at the origin server.  THIS IS GOOD.  THIS IS INTENTIONAL.
Moreover, because we are not relying on mandatory clock synchronization,
THIS IS THE ONLY SAFE APPROACH.  The alternative is a specification
which would increase the chances of underestimating the Age of a
response.  Underestimation is SERIOUSLY BAD, because it will lead
to a cache believing that a response is fresh when it is, in fact,
stale.

Overestimation of the Age can lead to a cache treating a fresh
response as stale, which can cause extra revalidation messages.
This is somewhat inefficient, but will never lead to a client
inadvertently seeing an expired cache entry.  Underestimation
is thus a much worse error than overestimation, and so the 
spec is designed to avoid underestimation as assiduously as
possible.

Now let's look at how badly the specified behavior can overestimate
Age.  The algorithm in 13.2.3 allows (implicitly) the receiving
cache to calculate the retrieval delay based on when the beginning
of the response is received; it doesn't have to wait for the entire
response.  Therefore, the magnitude of this delay is several RTTs
through each hop of the network.  (It would be just one RTT if the
connections are already open.)  Roy's formulas:

	 At  C:  age=d
	     B:  age=d+(c+d)
	     A:  age=d+(c+d)+(b+c+d)
	    UA:  age=d+(c+d)+(b+c+d)+(a+b+c+d)

generalize to
	Age = Mean_RTT * N * (N + 1)/2
for N hops (N - 1 proxies)

The "correct" Age would be Mean_RTT * N, so the size of the overestimate
(the "error) is
	Excess_age = Mean_RTT * N * (N - 1)/2

Here are some sample values for a Mean_RTT of 1 second (which
is a relatively high value):

	Number of proxies	N	Excess_age (seconds)

		0		1	0
		1		2	1
		2		3	3
		3		4	6
		4		5	10
		5		6	15
		6		7	21

OK, so if the request chain includes 6 or more proxies, the overestimate
just might start to change the caching behavior for responses with
unusually short maximum ages.  (I'd be surprised if people sent
max-age values under 1 minute, but perhaps someone can provide a
counterexample).  Bottom line: this "error" is not really worth
getting excited about.

Let's now look at what kinds of errors (underestimations of Age)
could arise if we followed Roy's proposal: a proxy only sends an
Age header if the response comes from its own cache.

Consider this not-very-contrived request-chain:

   client C => HTTP/1.1 proxy P1 => HTTP/1.0 proxy P0 => Origin server S

Now let us suppose that client C makes a request for resource
http://S/foo.html, and that proxy P0 has had a copy of this
resource in its cache for, say, 30 minutes.  Let's also suppose
that the clock on C (perhaps someone's PC) was set accidentally
to the wrong time zone, and it's an hour slow.

So when the response arrives at C, under Roy's proposal, there
is no Age header attached and so the client starts the Age
ticker running at that point.  I.e., the client will underestimate
the Age by the 30 minutes that the response was sitting in the
cache at P0.  This might well exceed the max-age value sent by
the origin server, but C would be ignorant of the expiration
of its cache entry for perhaps a significant amount of time.

Under the specification as it currently exists, P1 (the first
HTTP/1.1 cache to see the response) would construct an Age
value.  If its own clock is set wrong, we are no better off.
But if its clock is correct, then it will observe the discrepancy
between its current clock and the Date value on the response,
and will set the Age accordingly (to 30 minutes).

Thus, the specification as written will prevent Age underestimation
resulting from end-client clock skew as long as at least one of
the HTTP/1.1 proxies in the chain has a nearly-correct clock.

Summary: the specification, as written, does somewhat overestimate
the Age, but not by a tremendous amount, and is intended to reduce
as much as possible the probability of inadvertently delivering a
stale response to a user.  Roy's proposed change would give slightly
more accurate Age estimates, but could cause the undected delivery
of stale responses in the presence of clock skew.

-Jeff
Received on Tuesday, 20 August 1996 17:53:32 EDT

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