Chapter 21
Instant Messaging on the Internet
Instant messaging is one of today’s most popular services. Many youngsters (and not-so-young people) use the service to keep in touch with their relatives, friends, co-workers, etc. Millions of instant messages are sent every day. So, it will come as no surprise that such a popular service is already supported in the IMS.
Instant messaging is the service that allows a user to send some content to another user in near-real time. Because of the real-time characteristics of instant messages the content is typically not stored in network nodes, as often happens with other services such as email.
The content in an instant message is typically a text message, but can be an HTML page, a picture, a file containing a song, a video clip, or any generic file.
The instant messaging service combines perfectly with the presence service, since presence allows a user to be informed when other users become available (e.g., connect to the network). Then, users can send instant messages to their friends and start some sort of messaging conversation.
21.1 The im URI
Like presence, mail, or AAA functions, an instant messaging service can be identified by an im URI. Like the pres URI the im URI does not define the protocol used to access an instant message resource. So, whenever SIP is the protocol used to send the instant message it is recommended to use sip or sips URIs.
The syntax of the im URI is
IM-URI = "im:" [to] [headers]
to = mailbox
headers = "?" ...
