SIP Extensions for Instant Messaging
Applications that integrate IM with presence have been in use on the Internet
for a long time, though not in a standard form. SIP, as initially designed, has
useful mechanisms for presence but needs an extension for IM. The SIP exten-
sion for IM is MESSAGE, and MESSAGEclosely resembles the INVITEmethod.
MESSAGErelieves SIP from the session mode and can support conversations
based on independent messages, initially using only text, but as we will see,
multimedia conversations are also supported without requiring the prior set-
ting up of an explicit session. IM has therefore two modes of operation:
■■ Short individual messages, mostly text.
■■ Session mode when the conversation needs to be associated with a SIP
session, for such application as for secure tunneling through NAT and
firewalls between domains from different enterprises or for the transfer
of large audio/video files. The protocol for the session mode is called
the Message Session Relay Protocol (MSRP) [22], [23].
MESSAGEmay traverse a number of SIP proxies, fork into different branches,
and have 2xx response—in short, use the existing SIP infrastructure, with the
caveat that the infrastructure needs to understand the MESSAGEextension as
well. This is not an expensive proposition and saves complete separate systems
for voice and IM and presence. Besides avoiding the cost of acquiring and oper-
ating different systems, users don’t need separate applications for voice, IM,
and presence, and can greatly benefit from their integration.
Sharing the SIP infrastructure for signaling and IM requires congestion con-
trol because of the high-volume usage of IM. For this reason, it may be advan-
tageous to have SIP endpoints exchange MESSAGEdirectly in a peer-to-peer
fashion, similar to RTP media packets. This also has the advantage of IM text
being treated in central conferences in the same manner as RTP media packets
for consistent multipoint centralized conferencing architecture.
An IM inbox has an Instant Message URI in the form im:user@domain.
Figure 13.9 shows an example of the IM message exchange between two SIP
user agents [24].
Note here two interesting properties of IM using SIP:
■■ IM is can be based on the common infrastructure with voice.
■■ The same SIP UA can support voice with one party or parties and while
at the same time, use IM with another party.
Presence and Instant Messaging 239