Payload Carrier

Earlier in this chapter we saw how a basic message was embellished with a structured and parseable “attachment” in the form of a jabber:x:oob-qualified <x/> tag and its child tags: 

<x xmlns='jabber:x:oob'>
  <url>http://www.megacorp.co.uk/earnings3q.html</url>
  <desc>Third Quarter Earnings for Megacorp</desc>
</x>

How does this actually work in practice? Well, partly by requirement, partly by convention:

  • All three of the elements, <message/>, <iq/>, and <presence/>, can carry these attachments.

  • These attachments usually serve one of two purposes:

  • To bring primary (or secondary) meaning to the element that contains it.

  • To act as the driving force behind the element, such as that the element really exists only to carry the attachment.

This distinction is rather subtle, so let’s look at a couple of examples.

Example 2-2 shows an attachment in the jabber:x:delay namespace. This works like a timestamp and in this context indicates when that presence element appeared—in other words, from what time Jim started on his break.

Example 2-2. A jabber:x:delay extension adds meaning to a <presence/> element

<presence from='jim@company-a.com/home' to='john@company-b.com'>
  <show>chat</show>
  <status>having a break from work</status>
  <priority>1</priority>
  <x xmlns='jabber:x:delay' from='jim@company-a.com/home'
     stamp='20010611T13:13:04'/>
</presence>

Example 2-3 shows us a message element containing a URL attachment. In this case, the element serves to carry the URL attachment; ...

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