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resources should be reserved or how security
should be implemented. This is left for other
protocols. In this respect, SIP is more limited
in scope than for example the architecture
described by ITU-T H. 323.

A client – server architecture is used. The main
entities are the User Agent, the SIP Proxy
Server, the SIP Redirect Server and the Regis-
trar, some illustrated in Figure 26.

The User Agents (also referred to as SIP end-
points), work as clients (UACs) when initiating
requests and as servers (UASs) when responding
to requests. User Agents communicate with
other User Agents either directly or through
intermediate servers. The User Agent also stores
and manages call states.

Intermediate servers may behave as proxies
or redirect servers. A Proxy Server forwards
requests from a User Agent to another SIP
server, a User Agent within its network domain,
and may collect information for accounting and
charging. A Redirect Server responds to client
requests and informs of the address of the next
server requested. Several hops can be made until
the final destination is reached. Basically a
server can either maintain the state information
or forward request in a stateless manner.

As a general remark, a provisional response
(1xx) should be sent as soon as possible when
a final response cannot be sent within 200 ms.
Algorithms are given in [RFC2543] for how to
calculate re-transmission timers for the different
SIP messages.

7.4.2 Session Description Protocol
The Session Description Protocol (SDP) is used
for describing sessions consisting of audio,
video, or multimedia in general. A basic idea
was to use it for announcing multicast sessions
as described in [RFC2327]. Then session direc-
tories may be used to advertise and convey the
relevant set-up information to the recipients.
SDP, being a format for describing sessions, is
independent of the protocol used for carrying
this information.

Related to SDP, a few key terms are defined:

• Conference: A multimedia conference is a set
  of two or more communicating users along
  with the software they are using to communi-
  cate.


• Session: A multimedia session is a set of mul-
  timedia senders and receivers and the data
  streams flowing from senders to receivers.
  A multimedia conference is an example of
  a multimedia session.
  
  
  • Session announcement: A session announce-
    ment is a mechanism by which a session
    description is conveyed to users in a proactive
    fashion, i.e. the session description was not
    explicitly requested by the user.
  
  
  
  

SDP carries the following information:

• Session name and purpose;


• Time(s) the session is active (start/stop times,
  repeat times);


• Media included in the session (type of media:
  video, audio, etc., transport protocol, format
  of the media; H.261, MPEG, etc.);


• Information for receiving/accessing the infor-
  mation (address, port, etc.);


• Information about bandwidth used (e.g. for a
  conference);


• Contact information for session-responsible
  person.

The two last ones may be included if found

desirable.

The session descriptions are textual, only using

ASCII coding. Each line specifies a characteris-

tic in the form: <type> = <value>, where

<type> is one character, see Table 2.

As an example the bandwidth is specified as

b=<modifier>:<bandwidth-value>

where the bandwidth value is given in kbit/s.

Two modifiers are possible; either the total

bandwidth for all media flows at all sites (called

conference total), or bandwidth for a single

media flow at a single site (called application-

specific maximum).

User Agent

Proxy

Server

Proxy

Server

User Agent

Redirect

Server

1

8

2

3

4

7 5

6

Figure 26 SIP architecture

and entities