Re: [Standards] XEP-0231 (Data Element) - local caching

Tue, 29 Jul 2008 18:53:42 -0700 

Ahoj Pavle!

Pavel Simerda wrote:

Hello,

I have some suggestions for XEP-0231 (Data Element).


Thanks for looking at this spec so thoroughly.


Right now, as the example shows:

<message from='[EMAIL PROTECTED]/castle'
         to='[EMAIL PROTECTED]'
         type='groupchat'>
  <body>Yet here's a spot.</body>
  <html xmlns='http://jabber.org/protocol/xhtml-im'>
    <body xmlns='http://www.w3.org/1999/xhtml'>
      <p>
        Yet here's a spot.
        <img alt='A spot'
             src='cid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6@shakespeare.lit'/>
      </p>
    </body>
  </html>
<data xmlns='urn:xmpp:tmp:data-element' alt='A spot' 
        cid='[EMAIL PROTECTED]'
        type='image/png'>
    iVBORw0KGgoAAAANSUhEUgAAAAoAAAAKCAYAAACNMs+9AAAABGdBTUEAALGP
    C/xhBQAAAAlwSFlzAAALEwAACxMBAJqcGAAAAAd0SU1FB9YGARc5KB0XV+IA
    AAAddEVYdENvbW1lbnQAQ3JlYXRlZCB3aXRoIFRoZSBHSU1Q72QlbgAAAF1J
    REFUGNO9zL0NglAAxPEfdLTs4BZM4DIO4C7OwQg2JoQ9LE1exdlYvBBeZ7jq
    ch9//q1uH4TLzw4d6+ErXMMcXuHWxId3KOETnnXXV6MJpcq2MLaI97CER3N0
    vr4MkhoXe0rZigAAAABJRU5ErkJggg==
  </data>
</message>

Note: in this particular example the data is very short, this may not
be the case in real world where people tend to ignore the size of data
they send.
Yes, that's just about the smallest image I could find. The spec says that the image should not be more than 8k (which is twice the suggested size of an IBB chunk) but we don't know if people will typically send images that are smaller or larger than 8k -- I think smaller but I don't know that yet. 


We send data once for every session (and omit for subsequent messages).
In this case it's important to define "session" (see rfc321bis). Is it a chat session, a presence session, or something else? 


This has two important implications:

1) The other entity may or may not cache it for the session and reuse
it. That is good.

2) If an entity keeps the data for a longer time (e.g. for weeks
or even permanently), this cache will never be used. As the sending
entity always resends the data for a new session.

What I propose is:

 * By default the sending entity would not send the data. It would
   merely reference it by its cid url.
 * Let the recieving client follow "3.4 Retrieving Uncached Media Data"
   if the data is not cached (no real change, this is already being
   done).
I think I like that approach. It introduces a round trip for the IQ, which might introduce some latency. But it puts the burden for "storing" and "serving" the image on the sender, which might discourage abuse of in-band images. 


 * Reserve the possibility of sending the data immediately with the
   message for the *specific* case that the sending client actually
   knows the recieving party cannot have the data cached (e.g. the
   data was never sent before). This behavior should be considered
   optional.
In that case the sender needs to keep a list of every JID to which it has ever sent the image. That seems suboptimal.
And I suppose the recipient might have received the image from another sender at some point, or might have received the image through other means (e.g., an emoticon "bundle"). 


I further propose we add some informational section about generation
of CIDs. Although it's specified elsewhere, I believe this XEP will be
very useful and will be referenced from many future XEPs (and maybe
improved as well - possibly some server caching etc). I think the
informational section could suggest UUIDs generated by hashing the
actual content.


Yes I think that would be helpful.


Another thing that could be considered... is to add some sort of
caching hint attribute that would suggest how long its reasonable to
cache a particular resource. 

Do you think that would really be helpful? I'm still thinking about it...


Maybe we could borrow from HTTP Cookies
but allow (suggest) the clients to have some mechanisms for limiting the
time, size and number of cached objects.

There are many possibilities, I will just describe one of them.


Do you have examples of these?


cache="no"
 - no reason for caching the file will not be used again


Perhaps a thumbnail related to file transfer or some other ephemeral image?


cache="session"
 - we suggest the recieving party only caches for this
   particular session


Perhaps also a thumbnail, or an image related to a whiteboarding session?


cache="12"
 - we suggest caching for twelve days from the last use of this cid (!)
 - for every use (recieved reference) the recieving client should reset
   the date we count from
Perhaps images included in an XHTML notification from a blogging service or somesuch? 


cache="unlimited"
 - we suggest the client picks the longest time it allows (it could
   possibly cache some small pieces of data permanenty)


Perhaps a commonly-used emoticon?


Of course, the client MAY ignore the caching hit. In this case it
SHOULD NOT cache at all.
Why not? My client could ignore caching hints because it has its own local policy (e.g. cache images only from people in my "Friends" group, but cache those forever because I want to keep them in message history). Or my client could ignore caching hints because it simply can't cache images (no room on the device, web client, etc.). 


If the cache attribute is not specified, we should decide on a
reasonable default value ('session' or '1' day both seem good to me).


I think that's up to the client.


Cheers,
Pavel


Thanks!

/psa

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