Presence – from the other site

You know it. When you see a new smart phone/navigation/camera or any other gadget for the first time, it seems to be complex. It takes some time and you get used to. And then finally you get bored. It’s the same with the IMS. The VoLTE flows are not challenging for you any more. Maybe the conferencing or SRVCC – a bit. But in general there isn’t anything really complex. That’s why I like presence. There are many various flows and use-cases. (And I don’t deliver these trainings that often as VoLTE :)).

Sure, we’ve went through a couple of presence topics and flows (Is the Presence Social?, Presence – More than you wanted to know, What are you capable of?, Someone is watching). This time we can do it slightly more complex and check how it looks like from multi-site perspective.

Presence - two sites

Presence – two sites

We’ll go through the most basic flows and we’ll focus on what service is originating and what service is terminating. Each site can belong to a different operator or it can be just one network. For simplicity the DNS will assign the primary AS based on geolocation. Geo-redundancy scenarios will be described some other time.

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Someone’s Watching

The quantum physics says that some events will happen differently if someone is watching. And one doesn’t need to be a physics to have that experience. At least my performance is always worst when someone is watching me playing piano.

Authors of IMS Presence System were either aware of it or they knew that there can be a thin line between being a watcher and being a stalker. Either way IMS allows to inform a presentity about watchers and authorize their subscription.

The key specification is the RFC 3857 which defines so called watcher info (winfo) event “template-package”. Event template-packages are event packages that can be applied to any other event package (so not only presence). The winfo provides the users with information about who is watching the subscribed resource. If the winfo is applied to presence (see previous posts Presence – More than you wanted to know and Is the Presence Social?) the value of the Event header field is set to presence.winfo.

Event: presence.winfo

The package defines a new MIME type application/watcherinfo+xml. When a SIP request or response contains a Content-Type header field set to application/watcherinfo+xml, it is indicating that the body is an XML document that contains watcher information.

Content-Type: application/watcherinfo+xml
<?xml version="1.0"?>
   <watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
                version="0" state="full">
      <watcher-list resource=""
        <watcher id="23456abcd" event="subscribe"

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