Re: [BLISS] [Fwd: I-D Action:draft-roach-sip-http-subscribe-00.txt]

[Mark Nottingham <[e-mail]>]

Three more;

* Does the NOTIFY message include the URI of the resource that has  
changed (didn't see it, but I may be missing something). It should be  
added if not; otherwise, you're baking in a one-to-one relationship  
between monitoring channels and http URIs. There will be use cases  
where the receiver of the NOTIFY doesn't have enough context to  
associate the message body you're sending with a URI.

* The simplest and most efficient notification message to send is  
"this resource has changed"; the client can then decide whether or not  
to fetch an update, and combining any old response with a new one is  
simple. It also assures that the subscribing client isn't required to  
implement a cache. I would very much encourage you to support this  
mode of interaction.

* Conversely, if you are you intent on allowing transfer of an entity  
with a notification, you may want to consider allowing the transfer of  
multiple entities, to allow the use of content negotiation (e.g., both  
an English and French representation under the same URI, or a png and  
a gif). application/http would support this.

Cheers,


On 11/12/2008, at 4:48 PM, Mark Nottingham wrote:

> Sorry for the delay, finally off the road and had a chance to read.
>
> Overall I like this document a lot, and I think it should be easy to  
> align its use of the 'monitor' link relation with the Atom-over-HTTP  
> protocol described in draft-nottingham-http-cache-channels.
>
> A couple of comments / concerns:
>
> * "...it MUST return a Link: header..." seems like it's too strong;  
> servers may want to be selective about when they offer monitoring,  
> and there may be other ways to advertise it that are more appropriate.
>
> * Following on that thought, the idea behind the most recent Link  
> draft is to talk about link relations generically, and make Link  
> headers one use of them. Another use of them is in draft-nottingham- 
> site-meta (which is still under pretty active revision); this would  
> allow you to advertise a monitoring endpoint for your whole site,  
> for example.
>
> * "It MAY return both." is ambiguous; think you mean SIP + SIPS.
>
> * 3.4 Subscription Duration -- just a thought, is it worth tying the  
> default subscription duration to the freshness lifetime of the HTTP  
> response (as defined by HTTP)?
>
> * 3.5.1 message/httpfrag -- did you consider using application/http  
> (defined in RFC2616)? I mention this because HTTP already defines  
> how to update a cache entry's headers based upon a HEAD or 304  
> response; it would be nice to reuse that if possible. Also, what  
> does making the status-line optional bring? The semantics of the  
> message would be clearer if it were required...
>
> * 3.5.2 - Why require an ETag or Content-MD5? Why recommend a Last- 
> Modified?
>
> * 3.10 - limiting notifications to one a second makes sense also  
> because this is the granularity of time in HTTP.
>
> Specific responses below;
>
>
> On 22/11/2008, at 2:53 AM, Adam Roach wrote:
>> On 11/21/08 2:07 AM, Theo Zourzouvillys wrote:
>>> On Fri, Nov 21, 2008 at 4:12 AM, Adam Roach <[e-mail]>  
>>> wrote:
>>>
>>>> Very interesting proposals. Admittedly, for the use cases SIP is  
>>>> most
>>>> interested in, the results are not typically HTML, but the idea of
>>>> communicating these in the body may have merit. I'm curious about  
>>>> which use
>>>> cases you have in mind -- do you imagine that there will be  
>>>> circumstances
>>>> under which the HTTP server will have the ability to notify other  
>>>> nodes of
>>>> changes, but not have the ability to include a Link: header?
>>>>
>>>
>>> absolutely - perhaps a resource hosted on a CDN, where you commonly
>>> can not send custom headers.
>
> You certainly can, in a cache-driven CDN (e.g., Akamai); the cases  
> where you can't aren't really CDNs as the industry understands them,  
> they're Web hosting farms (because they use replication; e.g.,  
> Amazon's new service, if I read it correctly). In that case,  
> providing a channel without coordination with them can be harmful,  
> because you can get into race conditions where you send the  
> notification before the content is fully replicated.
>
> More to the point, if you're using a CDN (of either type), the most  
> likely value-add scenarios are using a monitoring channel to notify  
> the CDN itself of changes, and also the CDN providing a monitoring  
> server as a service.
>
>
>>> or a blog, which may be hosted on some other platform (for example,
>>> blogger.com) where you can edit the HTML but not the headers.
>
> I'm getting somewhat exasperated at how often this argument comes up  
> in specification discussions; to date mostly at the W3C. If we  
> follow it to its conclusion, the only Internet protocols that we can  
> develop are those that work with free hosting sites that only  
> provide FTP access. Surely the Internet and the Web shouldn't be  
> held hostage to them.
>
> That said, I agree that the draft shouldn't preclude other discovery  
> mechanisms, as mentioned above.
>
>>> the Link header as defined in draft-notthingham-http-link-header
>>> itself is only a way of representing the <link> header in HTML and
>>> <atom:link/> header in atom entries outside of the content where  
>>> it is
>>> not possible because the content-type does not have support.  So it
>>> already "just works" by creating a link registry entry, however  
>>> would
>>> be good to explicitly point out that it could be anywhere a link
>>> relation is allowed.
>>
>> That's interesting -- Mark, do you have any input on the topic?
>
> I'm having trouble parsing the above -- is the concern that some  
> people might believe that any link in the headers *has* to be  
> mirrored in the content? If so, that's the first time I've heard  
> this brought up, and it certainly isn't the intent. We can clarify  
> if necessary.
>
> Cheers,
>
>
> --
> Mark Nottingham     http://www.mnot.net/
>


--
Mark Nottingham     http://www.mnot.net/

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