Re: CT Proxies and Forward Caches

Thanks for the clarification, Umesh.

Very good point.

The Content-Location header would probably have deserved a mention in 
the TAG Finding I mentioned at the beginning of the thread and in 
particular in 2.1.1 section [1], third item, since the Vary header makes 
things work, and the Content-Location header makes things 
cache-friendly. It saves the redirection, and makes groups used by the 
server available to caches without revealing how they were built.

As far as content-transformation is concerned, there may not be much to 
say though as it's a rather generic caching issue. The need to use a 
"Vary" on the "User-Agent" header is yet typical of the Mobile world, so 
we probably should emphasize this point somewhere. I'm not sure the 
Content Transformation guidelines document is the right place for it, 
but since Content-Location sounds like a "natural" companion for the 
Vary header, we could add a note, next to the guideline that says that 
the server MUST add a "Vary" HTTP header when varying representations, 
along the lines of:

"When varying representations based on received HTTP headers, 
cache-efficient techniques should be used. For example, if the total 
number of representations is limited whereas the number of values for a 
HTTP header used for varying representation is high [typically the case 
when varying representations based on the User-Agent string], the 
different representations should be made available at specific URIs and:
a) the request to the generic resource should return the specific 
representation along with a Content-Location header that identifies the 
representation being served.
or b) the request to the generic resource should return a redirection to 
the specific representation."

Any other view on that?

Francois


[1] http://www.w3.org/2001/tag/doc/alternatives-discovery.html#id2261787



Umesh Sirsiwal wrote:
> Hi Fancois,
> Sorry for the confusion. Based on my understanding of the Link element,
> I can further clarify difference between the Link element and the
> Presentation-URI. 
> 
> My understanding is that the Link header provides a method of
> advertising available alternatives for the page being served. On the
> other hand the Presentation-URI provides a method to identify the
> alternative included in the response. In case of the deployment case you
> mentioned below, once the CT proxy has identified the page to be served
> it will include a Presentation-URI header identifying the selected URI.
> 
> Using this the Vary header will be able to identify the criteria on
> which the server varied its response, while the Presentation-URI will be
> able to identify which of the several alternatives was served. 
> 
> Rereading HTTP specification, the Presentation-URI is the same as
> Content-Location header field. I am proposing that the CP or the CT
> proxy which can serve multiple presentation of the content for the same
> URI, should include Content-Location header to identify the entity it is
> serving.
> 
> -Umesh
> 
>> -----Original Message-----
>> From: Francois Daoust [mailto:fd@w3.org]
>> Sent: Monday, May 26, 2008 11:31 AM
>> To: Umesh Sirsiwal
>> Cc: Jo Rabin; Sullivan, Bryan; public-bpwg-ct@w3.org
>> Subject: Re: CT Proxies and Forward Caches
>>
>> Hi Umesh,
>>
>> I'm not sure I completely follow your point here, feel free to correct
>> me.
>>
>> The Presentation-URI header you mention to identify alternative
>> representations being served looks like the "Link" element we're
>> currently discussing in another thread, see:
>>   http://lists.w3.org/Archives/Public/public-bpwg-ct/2008May/0021.html
>> and replies.
>>
>> In the case of the Link element, we're currently trying to see when it
>> makes sense to use it, and how it could be used in practice. This
> would
>> indeed avoid the extra round trip in the sense that the CT-proxy would
>> be able to do the redirection for the user and so the "redirect" would
>> not reach the high-latency network the end-user is connected to.
>>
>> Now, obviously, the problem with the "Link" element is that it is at
>> the
>> markup level, and not at the HTTP level. It would be cool to have a
>> "Link" HTTP header, typically for images and more generally for all
>> non-HTML content. We're not the only ones who want the "Link" header
>> back to life ("back" since it previously existed but disappeared for
>> lack of use, how ironic ;-)), and there are many on-going discussions
>> within W3C and IETF about that. If it ever becomes a reality, it would
>> indeed be useful to serve multiple representations of a resource.
>>
>> Note that it's not directly related to content transformation in
>> itself.
>> The presence of a content transformation proxy merely adds to the
> case.
>> Did I get you right?
>>
>> Francois.
>>
>>
>> Umesh Sirsiwal wrote:
>>> Jo, Francois, Bryan,
>>> Thanks for the responses. IMO absence of standardization in this
>> space
>>> will cause caches built in CT or otherwise to implement heuristics
>> based
>>> solutions to deduce intent of CP or CT. That is less then desirable.
>>>
>>> To avoid the extra round trip Francois pointed out, the CP can
>> possible
>>> serve an HTTP header (let us call it Presentation-URI) identifying
>>> alternative representation served. The CT proxy or other caches will
>>> need to pay attention to this new header. But, as long as Via header
>> is
>>> always included, they will be able to correctly cache and serve the
>>> content.
>>>
>>> The Presentation-URI does not have to be limited to the three
> groups.
>> In
>>> some cases the Presentation-URI can be very specific and say
>> something
>>> like www.example.com/Device_a. Won't that work?
>>>
>>> -Umesh
>>>
>>>
>>>> -----Original Message-----
>>>> From: Jo Rabin [mailto:jrabin@mtld.mobi]
>>>> Sent: Thursday, May 22, 2008 6:16 AM
>>>> To: Francois Daoust
>>>> Cc: Umesh Sirsiwal; Sullivan, Bryan; public-bpwg-ct@w3.org
>>>> Subject: Re: CT Proxies and Forward Caches
>>>>
>>>> Aside from the redirect cost that Francois mentions, I am not sure
>>> that
>>>> having separate URIs to allow caching of the "high" "medium" and
>> "low"
>>>> cases is the whole answer, since the response may still vary within
>>>> those groups depending on work-arounds to the quirks of any
>> particular
>>>> device within the grouping.
>>>>
>>>> As Francois points out, this relates to the "long-running" ISSUE-
>> 222,
>>>> and it's down to me to try to make sure that it doesn't run much
>>> longer
>>>> :-(
>>>>
>>>> Jo
>>>>
>>>> On 21/05/2008 09:34, Francois Daoust wrote:
>>>>> Indeed, the use of a "Vary: User-Agent" header generates much more
>>>>> entries than a more typical use of Vary such as "Vary: Accept-
>>>> Language",
>>>>> and is thus not a really cache-friendly directive.
>>>>>
>>>>> The solution Bryan suggested to create representation-specific
> URIs
>>>> for
>>>>> each UA group, coupled with a redirect response from a canonical
>>>>> representation is much better from a cache perspective but it has
> a
>>>>> cost: that of a round-trip between the server and the client to
>>> serve
>>>>> the redirect response to the representation-specific URI. This
>>>> solution
>>>>> is recommended by the W3C Technical Architecture Group in a
> finding
>>>> "On
>>>>> Linking Alternative Representations To Enable Discovery And
>>>> Publishing"
>>>>> [1].
>>>>>
>>>>> We only mention the use of the "Vary" header in current version of
>>>> the
>>>>> Content Transformation Guidelines document, but we have a long-
>>>> running
>>>>> discussion (internally named ISSUE-222) on the above mentioned TAG
>>>>> finding. We may include that possibility in the document as well.
>>>>>
>>>>> [1] http://www.w3.org/2001/tag/doc/alternatives-
>>>> discovery.html#id2261672
>>>>> Sullivan, Bryan wrote:
>>>>>> Hi Umesh,
>>>>>> As you mention, meta-group assignment (e.g. good/better/best) is
> a
>>>>>> deployment-specific function, i.e. one Content Provider (CP) may
>>>>>> choose a different set of groups and UA assignment as compared to
>>>>>> another. Without the direct involvement of the CT proxy in group
>>>>>> selection, the only way I see to reduce the cached
> representations
>>>> is
>>>>>> for the CP to provide a distinct URI to UA's in a group (e.g. a
>> URI
>>>>>> parameter or unique path), so the various UA's naturally get
>> served
>>>>>> one of a fewer variations of the page from the cache.
>>>>>>
>>>>>> "direct involvement of the CT proxy in group selection" implies
>>> some
>>>>>> kind of metadata exchange between CP and CT proxy, through which
>>>>>> group-related pages can be indicated, and maybe a tighter
>>>> integration
>>>>>> of the CT proxy and cache. Both appear (to me) to be less
>> desirable
>>>> to
>>>>>> standardize, and at least more complex to consider.
>>>>>>
>>>>>> Best regards,
>>>>>> Bryan Sullivan | AT&T
>>>>>>
>>> --------------------------------------------------------------------
>>>> ----
>>>>>> *From:* public-bpwg-ct-request@w3.org
>>>>>> [mailto:public-bpwg-ct-request@w3.org] *On Behalf Of *Umesh
>>> Sirsiwal
>>>>>> *Sent:* Monday, May 19, 2008 8:12 AM
>>>>>> *To:* public-bpwg-ct@w3.org
>>>>>> *Subject:* CT Proxies and Forward Caches
>>>>>>
>>>>>> Several content transformation proxies and the Internet in
> general
>>>>>> includes forward caches. Current definition of HTTP includes
>>>>>> indication of transformation using Vary header. In most cases the
>>>>>> Content Transformation proxies and servers vary their responses
>>>> based
>>>>>> on User-Agent header. The number of User-Agent string in is very
>>>> high
>>>>>> and caches cannot possibly store these mean copies of the
>> response.
>>>>>> Most servers are likely to classify the devices in certain meta-
>>>> groups
>>>>>> for the purpose of content transformation. However, this meta-
>> group
>>>> is
>>>>>> expected to be server specific. In absence of formal method, the
>>>>>> caches will be left to guess the meta-group. What will be the
>>> method
>>>>>> to solve this?
>>>>>>
>>>>>>
>>>>>>
>

Received on Friday, 30 May 2008 14:51:05 UTC