Re: Submitted new I-D: Cache Digests for HTTP/2

[Alcides Viamontes E <[e-mail]>]

Hello,

Thanks for your response Kazuho. I need to do some thinking before
fully addressing your comments, since you add valuable information
that I didn't consider before. At the risk of adding some noise
(please forgive me for that), I will write a few quick remarks:

> Your calculation is wrong.  A 200-entry GCS (with 1/512 false positive
> rate) will be slightly larger than 225 bytes (log2(512) * 200 bits) in
> binary form.

That's good news! My Google's cookie is 1246 bytes long. If we are not
talking about several kilobytes, then the restrictions are less.

I would like to know more about the expectations for intermediaries.
As of today, HTTP/2 is working with TLS, therefore the website
operator needs to bless the HTTP/2 edge server with the SSL
certificate's private key. Maybe we can hope that website operators
will choose an HTTP/2 stack that does what he/she intends? In other
words, I think we shouldn't make the spec more difficult to use just
to accommodate potential issues with intermediaries. In that light, we
could perhaps require intermediaries to use a cache digest mindfully.
In any case please forgive me for my lack of data, this is just food
for thoughts and I will be glad to know more about how things look
right now with HTTP/2 intermediaries and caches.

I will write a more detailed follow-up a few weeks from now. I will
also try to make a polyfill implementation using service workers and
ShimmerCat to learn how this looks in practice.

Bests,

./Alcides.


On Tue, Jan 12, 2016 at 2:04 AM, Kazuho Oku <[e-mail]> wrote:
> 2016-01-11 2:11 GMT+09:00 Alcides Viamontes E <[e-mail]>:
>> Hello,
>>
>> My interest in the draft "Cache Digests for HTTP/2"
>> https://datatracker.ietf.org/doc/draft-kazuho-h2-cache-digest/
>> concerns the original, intended use case that Mr. Kakuho Oku and Mr.
>> M. Nottingham cited. As the authors, I would like very much like to
>> see this made a standard and implemented in browsers. However, I
>> perceive a few issues. Beforehand, I apologize for this long email,
>> for any gaps in my understanding of the subject, and for not being
>> familiar with the language and procedures used in this list.
>
> Thank you for your feedback!
>
>> Here are the issues that I see:
>>
>> 1.- In its current wording, no information about which version of a
>> representation the browser already has is present in the cache digest.
>> That information can be included in the URL itself (cache busting),
>> but then it becomes a concern for web-developers, adds complexity to
>> their work, and bypasses the mechanisms that HTTP has in place for
>> maintaining cache state.  It also increases space pressure in the the
>> browser's cache as the server is left with no means to expire old
>> cached contents in the browser.
>
> That is a very good point.
>
> Let me first discuss the restrictions of the cache model used by HTTP,
> and then go on to discuss what we should do if we are to fix the point
> you raised.
>
> First about the restriction; the resources in the cache can be divided
> into two groups: fresh and non-fresh.  A server should never push a
> resource that is considered as fresh in the client's cache.  Clients
> will not notice the push / the HTTP/2 allows client to discard such
> push.  Therefore, a CACHE_DIGEST frame
> must include a filter that marks the resources that are marked as
> being fresh.  That is what the current draft specifies.
>
> Next about the point of including version information (e.g.
> Last-Modified, ETag) in the cache digest.  I believe we can add a
> second Golomb-coded set to the frame that uses hash(URI + version
> information) as the key.  A server can refer to the information to
> determine whether if it should push a 304 response or a 200 response.
>
> The downside is that the CACHE_DIGEST frame may become larger (if the
> server sends many responses that would become non-fresh), so it might
> be sensible to allow the client to decide if it should send the second
> Golomb-coded set.
>
> In addition, we should agree on how to push 304 response.  My
> understanding is that HTTP/2 spec., is vague on this, and that there
> has not yet been an agreement  between the client developers on how it
> should be done.
>
> Once that is solved, I think we should update the I-D to cover the
> version information as well.
>
>> 2.- There is no way for the server to know that a CACHE_DIGEST frame
>> is coming immediately after a HEADERS frame. A server may trigger some
>> processing already after the end of headers has been received, while
>> making  further DATA frames available as a stream of data to the
>> application. With CACHE_DIGEST frames, the cache aware server will
>> have to delay processing until the end of the stream has been seen to
>> be sure that no CACHE_DIGEST frame is coming, or would have to
>> re-start processing on seeing the frame. Arguably this is not a big
>> problem for GET requests with an empty body, but it would be nice if
>> the spec didn't force the server to wait for the end of the stream.
>
> Agreed.
>
> There are three options here (the draft adopts option C):
>
> a) send CACHE_DIGEST frame right before HEADERS
> b) send CACHE_DIGEST frame at stream_id=zero, with the value of the
> authority that should be associated to the digest included within the
> frame
> c) send CACHE_DIGEST frame right after HEADERS
>
> B is clearly the easiest but would have a small impact on the consumed
> bandwidth, since the authority needs to be sent separately.
>
> In A, the server does not need to delay the processing of the request,
> but needs to cache the value of the digest.
>
> It would be great to discuss which of the three approach will be the
> best solution in general (or if there could be other approaches).
>
>> 3. - Traditionally, cache state information has been placed in HTTP
>> header fields. A CACHE_DIGEST frame puts some of that information in a
>> new place, which is sure to cause some pain to web developers and
>> sys-admins trying to understand the behavior of their applications.
>
> CACHE_DIGEST frame should not be a HTTP header, since including the
> value in every HTTP request (as a header) will make the HTTP requests
> huge.  Since the client's cache state changes as the server sends
> responses, we cannot expect HPACK to effectively compress the
> requests.  We should send cache digest only once per HTTP/2
> connection.
> (note that intermediaries are allowed to re-order the HTTP requests
> sent from a client, so it is impossible to include the digest only in
> the first HTTP request as a header).
>
> The other reason is that the digest should be hop-by-hop.  The default
> behavior of a proxy (that do not understand the extension) should be
> to drop the digest.
>
>> 4.- The draft assumes a somewhat more restricted scope of Push than
>> allowed by the HTTP/2 spec, RFC7540, and to some extent, goes against
>> current practice. Section 8.2 of RFC7540, "Server Push", says "The
>> server MUST include a value in the :authority" pseudo-header field for
>> which the server is authoritative". Section 10.1 defines server
>> authority by referring to [RFC7230], Section 9.1. For the HTTPS case,
>> a server is authoritative for a domain if it can present a certificate
>> that covers that domain. To the point, RFC7540 does not forbids a
>> server to push resources for different domains, provided that it has
>> the right credentials. Pushing assets for a domain different than the
>> one where the request is received is useful when considering the way
>> web applications are structured today: many serve their application
>> logic using a www.example.com domain, while serving their static
>> assets at static.example.com . Therefore, upon receiving a request to
>> www.example.com, a server may want to push resources for
>> static.example.com. However, section 2.1 of the draft works against
>> that use case.
>
> Thank you for pointing that out.
>
> I think that for plaintext HTTP we agree that the client needs to
> associate the name of the authority to the digest that it sends
> (including one of the three options discussed above).
>
> Considering the case for HTTPS, may be we should better allow the
> client whether or not to associate an authority.
>
>> 5.- A last issue has to do with what to include in the cache digest.
>> Mr.  Oku proposes to only push resources which are in the critical
>> render path in his article at [1]. Correspondingly, the cache digest
>> would only need to include those resources. Can we have a simple
>> mechanism to control the cache digest contents?
>
> It is obvious that providing a way to specify the resources that
> should be included in the cache digest will let clients generate more
> compact digest values.
>
> The downside is that it would be difficult for server administrators
> to _change_ a resource to become part of the digest.  Consider the
> case where a server has send resource A that is not being marked as
> part of the digest, and then the server administrator then changes the
> configuration for resource A to be included in part of the digest.
> The client will not include A in the digest it sends, since it is not
> marked.  The server will push the A to the client since it is not
> included in the digest.  (As discussed above) a client may discard the
> resource being pushed.  So A will continued to be pushed every time a
> new request is issued.
>
> Considering such possibility, it would be less troublesome if we could
> go without introducing a way to configure what should be included in
> the digest.
>
>
>> I can provide some data and some rough suggestions to address the issues above.
>>
>> How big would a cache digest be anyway?
>> -------------------------------------------------------
>>
>> To address issues 2 and 3 we need to determine how constrained we are
>> regarding space. We have made a little study[2] across 1300 sites
>> submitted by performance-conscious site operators, and from there we
>> can establish that while 50% of sites fetch between 25 and 110
>> resources, it is not too rare to have sites doing more than 200 HTTP
>> requests. If anything, that number is going to grow. Specially with
>> HTTP/2. Let's then use 200 as a ballpark estimate of the number of
>> items in a cache digest and start from there.
>>
>> The source that the draft includes for Golomb-coded-sets (GCS) hints
>> that it is possible to use the number of bits in a Bloom filter as an
>> upper bound for the size of the corresponding GCS. Therefore, with a
>> digest of size 200, we would be using an upper bound of 200*1.44*512
>> bits, which is around 18 kB is expressed as binary, and around 24 kB
>> if expressed in ascii form, base64-encoded, assuming a false positive
>> probability of 1/512.
>
> Your calculation is wrong.  A 200-entry GCS (with 1/512 false positive
> rate) will be slightly larger than 225 bytes (log2(512) * 200 bits) in
> binary form.
>
>> Notice that by using PUSH the browser may skip many of those requests.
>> In our site (https://www.shimmercat.com), we have measured HTTP/2
>> requests averaging at 60 bytes per request. Therefore, one may end up
>> saving up to 200 * 60 = 12  kB in traffic, bringing down the previous
>> numbers to 18 kB -12 kB =6 kB and 24 kB - 12 kB = 12 kB. I think that
>> 12 kB is acceptable for a site with 200 requests, specially since
>> HTTP/2 PUSH would greatly increase the data transfer density for those
>> sites.
>>
>>
>> Can we embed the cache digest in a header?
>> ------------------------------------------------------------
>>
>> Having 24 kB of cache digest in a header may delay processing the
>> request more than acceptable, since most servers will wait to get the
>> entire header block before starting to create an answer. There is an
>> alternative however, and that would be to put a  field with the cache
>> digest in a request trailer, allowed with chunked transfer under
>> HTTP/1.1 and in all streams with HTTP/2. The pros of having the cache
>> digest in a header or trailer field are the following:  we don't break
>> with the tradition of exchanging cache state through headers,  headers
>> are visible to developers' tools, it would be possible to test things
>> using polyfills and service workers while the browsers catch up with
>> native implementations, no extensions to HTTP/2 are needed, and cache
>> digests would become possible even over plain old HTTP/1.1. It can
>> also be made a little more future-proof:
>>
>> In the headers:
>>
>>        cache-digest: trailers
>>
>> (the indication above is not needed however if the cache-digest-scope
>> is used, see below)
>>
>> In the trailers:
>>
>>          cache-digest: data:application/golomb-coded-set;base64,.....
>>
>> The cons is that ascii is bigger than binary.
>>
>> Even if the CACHE_DIGEST frame is pursued, it would be nice to have
>>
>>          cache-digest: frame
>>
>> as part of the request (and this time in the headers section, not the
>> trailers) for the server to recognize that a cache digest frame is
>> coming and for developers to have a hint that said information is
>> being transmitted between client and server.
>>
>>
>> Distinguishing representation versions in the cache digest (Addressing point 1)
>>
>> ---------------------------------------------------------------------------------------------------------
>>
>> The GCS filter requires the client and the server to be able to
>> compute the same hash key for a given resource and version. As far as
>> I understand, having semantics here similar to if-modified-since would
>> not be possible. But strong etags could be used when computing the
>> key, therefore enabling the equivalent to if-none-match. Step 4 in the
>> algorithm of section 2.1 of the draft could be extended to have the
>> etag used together with the URL when taking the hash.
>>
>>
>> Which representations should be part of the digest? (Addressing point 4 and 5)
>> --------------------------------------------------------------------------------------------------------
>>
>> I suggest to introduce the concept of cache digest scope. Only
>> representations which were given a cache digest scope would be made
>> part of a cache digest. And the set of representations URLs to be
>> included by the client in the digest would be the intersection of:
>>
>> 1. The set of representations that have the same cache digest scope in
>> the browser's cache than the domain of the first request (the
>> document), and
>> 2. The set of representations in the browser's cache for which the
>> server is considered authoritative.
>>
>> The cache digest scope would be unique per domain.
>>
>> In other words, it would look like the following:
>>
>> Client asks for
>>                   https://www.example.com/
>> Server answers, and adds a header
>>   cache-digest-scope: example
>> The server then answers or pushes
>>    https://static.example.com/styles.css ,
>>         it uses the same header
>>            cache-digest-scope: example.
>>
>> The server also answers or pushes
>>    https://media.example.com/hero-1.png,
>>        but no cache-digest-scope is provided.
>>
>>
>> .... some time after, when a new connection is established by the same
>> client to fetch another page from the same domain:
>>
>> Client asks for (a different page)
>>          https://www.example.com/page1.html ,
>>         now the client specifies a header
>>        cache-digest-scope: example
>>         client also provides a cache digest with all
>>         the resources that were assigned
>>         the same cache digest scope by the server.
>>         That digest would include the resource from
>> https://static.example.com/styles.css
>>         but not the one at
>>                  https://media.example.com/hero-1.png
>>
>> The server answers and pushes a 304 not modified for
>> https://static.example.com/styles.css ,
>>         or a 200 with new contents, using a cache contents
>>         aware PUSH_PROMISE frame.
>>
>>
>> This  mechanism addresses 4 by allowing digests to extend over
>> multiple domains, and addresses 5 by allowing the server to control
>> which assets are part of the digest: resources *without* the
>> "cache-digest-scope" header are never made part of the digest. Also,
>> the holder of a wildcard certificate can still use it to host separate
>> multi-domain applications, for example (app1.example.com,
>> static1.example.com with cache digest scope "1") and
>> (app2.example.com, static2.example.com with cache digest scope "2"),
>> without fearing the cache digest to grow too big. Furthermore, if a
>> server doesn't implement PUSH or otherwise doesn't use the cache
>> digest, it implicitly opts out of cache digests,  saving bandwidth.
>>
>> The cache-digest-scope: xxxx header would be idem in most requests and
>> responses, and HPACK in HTTP/2 could compress it to a few bytes by
>> using the dynamic table.
>>
>> Best regards,
>>
>> ----
>> Alcides.
>>
>>
>> [1] http://blog.kazuhooku.com/2015/12/optimizing-performance-of-multi-tiered.html
>> [2] http://nbviewer.ipython.org/github/shimmercat/art_timings/blob/master/TimingsOfResourceLoads.ipynb
>>
>
>
>
> --
> Kazuho Oku