W3C home > Mailing lists > Public > ietf-http-wg-old@w3.org > September to December 1995

Re: Byte ranges (was Re: Logic Bag concerns)

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Thu, 07 Dec 95 15:10:55 PST
Message-Id: <9512072310.AA02833@acetes.pa.dec.com>
To: Mike Braca <mb@ebt.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
> Maybe it's simpler to think about this if you pronounce
> "GET" as "Make sure my cache for this object is up to date".

    Well, yes, but an empty cache is just as valid as one full of
    up-to-date objects :)

    I hate the notion of automatically dumping the whole file onto the
    net.  First of all, it is not intuitive to an ex-OS hacker like
    myself that if something in the cache goes stale, I reload the
    whole (file,...) instead of simply flushing the object from the
    cache.

As a current OS hacker, I can see the point you are trying to make,
but I think you are drawing a false analogy.

Most caches used in CPU hardware and operating systems are not
explicitly checked for validity on each use, against a "master"
copy of the data.  This means that if the hardware or OS suspects
that a cached item is not valid, it must either remove it from
the cache or mark it with a valid bit.

Also, in the case of a CPU or OS, the invalidation of an item
is "local" to the system, and so can normally be detected immediately.

To be more concrete: when a CPU loads a line from its cache, it
doesn't have the time to ask the main memory controller to tell it
if the cache line is still valid.  It has to have a fast, local
check (i.e., a valid bit) or the cache would be pretty useless.
Also, since (especially in a uniprocessor) the CPU itself is
responsible for most changes to memory locations, it can easily
clear the valid bit synchronously on a write.  The situation becomes
more complex with DMA and/or multiple processors, of course.

In our situation, on the other hand, the system containing the
cache (client or proxy) is quite remote from the system that
modifies objects (the origin server) and so it's not at all
reasonable to expect the client to know when to flush the cache.
This is why we have already (implicitly) agreed on a caching
model for HTTP in which each cache entry is validated on demand,
rather than when the original object is changed.  This is a
form of "late binding" or "lazy evaluation": don't do work up
front if you might not need to do it at all.

    More pragmatically, we are working on displaying potentially huge
    SGML documents via HTTP, and if a 20 MB file that I am pulling
    subtrees out of gets changed, I don't want to trigger a send of the
    whole file.  Perhaps Ari can tell me how they plan for this to work
    in the PDF case -- I may be missing something obvious here!

    What would make sense to me is to define something like a "305
    Modified" response that passes me back the new cache-validator.
    Then _I_ can decide whether to fetch the whole file, or an index or
    whatever. What are the drawbacks to such a scheme?
    
Doesn't the HEAD method do what you want?  That is, if you really
want to cheat on the principle that retrieved byte ranges should
be consistent with the version in the client's cache, then you
could do
	HEAD <url>
which should return exactly the same entity-headers as GET (according
to the current draft of the spec), including the Cache-validator.

Once you have the Cache-validator, you could then use the
GET method with Range: and Cache-validator: headers to load
specific parts of the 20 MB file.

I would make one concession to make this work a little better.
If the underlying 20 MB file changes between retrievals of
several Ranges, you probably want to know that (right?) but
you clearly don't want to get back the entire document in
response to Get+Range+Cache-validator request.  Therefore,
I propose a modification of the Range: header to be
	Range = "Range" ":" byte-range ["unconditional"]
Actually, the word "unconditional" is too long; "U" would
suffice.

So
	GET url
	Range: 0-60
	Cache-validator: XYZZY
would still mean
	if (cache-validator == "XYZZY")	then
	    send bytes 0-60
	else
	    send whole file

but
	GET url
	Range: 0-60 U
	Cache-validator: XYZZY
would still mean
	if (cache-validator == "XYZZY")	then
	    send bytes 0-60
	else
	    send 305 Modified

Finally (and this is the original reason I was going to propose
the unconditional Range: header, but your message arrived first),
this
	GET url
	Range: 0-60 U
(i.e., no Cache validator supplied by the client) would simply
mean
	send bytes 0-60
I think this would be a great solution to the problem of
how to implement Netscape's early rendering of bounding boxes
with the single-persistent-TCP-connection model.

Here is how that would work (assuming that the TCP connection
is already open)
Step 1:
    do
	GET file.html
    then parse out a list of image files.

Step 2:
    for each GIF file do
	GET fileN.gif
	Range 0-<size of GIF header> U
    for each JPEG file do
	GET fileN.jpeg
	Range 0-<size of JPEG header> U
once you have all the responses to step 2, you can render *all*
the image bounding boxes, not just the first 4.

Step 3:
Compare the Cache-Validators returned with the responses from
step 2 to the cache-validators stored with the cached images,
if any.  Since these are opaque values (in my model), this is a
simple equality check.  You now not only know the correct bounding 
boxes for each image file, you also know which files are valid
in your cache.  (Note that if you have cached image files with
future Expires: dates, then you don't need to include them in
steps 2 or 4.)

Step 4:
    for each GIF file do
	GET fileN.gif
	Range <size of GIF header>- U
    for each JPEG file do
	GET fileN.jpeg
	Range <size of JPEG header>- U
Note that you can issue the GETs for step 4 before receiving
the responses from step 2.

This has these advantages:
	(1) works fine with just one persistent TCP connection
	(2) renders any number of bounding boxes, not just 4
	(3) does not require the HTTP server to understand
	image formats, and does not require the creator of the
	HTML file to know what the image sizes are.
	(4) does not cost very much, since each image byte
	is only retrieved once, and no unnecessary round trips
	are required.

-Jeff

P.S.: If people don't like the assymetry between the semantics
of the unconditional Range header with and without the Cache
validator, then here's a somewhat clearer proposal:

	Range = "Range" ":" byte-range ["U" | "V"]

U = unconditional, V = unconditional if valid

Semantics:
	GET url
	Range: 0-60
	Cache-validator: XYZZY
would still mean
	if (cache-validator == "XYZZY")	then
	    send bytes 0-60
	else
	    send whole file

	GET url
	Range: 0-60 V
	Cache-validator: XYZZY
would mean
	if (cache-validator == "XYZZY")	then
	    send bytes 0-60
	else
	    send 305 Modified

	GET url
	Range: 0-60 U
would ignore the cache validator and would simply mean
	send bytes 0-60

Combining these all into one piece of server pseudo-code:

	GET url
	[Range: range-value [range-modifier]]
	[Cache-validator: validator-value]

would mean
	/* set up defaults */
	if (Range: not present) {
		range-value = empty set;
		range-modifier = "None";
	}
	if (Range: present but range-modifier not present)
		range-modifier = "None";
	if (Cache-validator: not present) {
		validator-value = null value;
	}

	/* compute what to send */
	if (range-modifier == "U" 
	     or
		actual-cache-validator == validator-value) {
	    send bytes described by range-value;
	}
	else {
	    if (range-modifier == "V")
		send 305 modified;
	    else
		send whole file;
	}

-Jeff
Received on Thursday, 7 December 1995 15:21:49 EST

This archive was generated by hypermail pre-2.1.9 : Wednesday, 24 September 2003 06:31:36 EDT