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developed. Whenever an RTP flow is used, cor-

responding out-of-band RTCP flows are estab-

lished, enabling the sender and receiver to ex-

change information on the performance of the

traffic flow and may be used for higher level

application control functions. Commonly, RTP

is carried in UDP packets, and then the RTP ses-

sion is mostly associated with an even numbered

port and its corresponding RTCP associated with

the next higher odd numbered port.

Six SIP messages have been specified: i) invite –

to begin an SIP dialogue; ii) ack – to respond to

an SIP request; iii) cancel – to reject the session;

iv) bye – to disconnect a session after it has been

established; v) options – to discover user’s

response without actually sending an invitation;

and vi) register – to register by a location data

base. In reply to a SIP message a response is

generated, of which there are six types: 1xx

informational, 2xx success, 3xx redirection, 4xx

client error, 5xx server error, 6xx global failure.

These response messages follow a format similar

to the ones used for HTTP.

The format of SIP messages comprises two

parts; a header consisting of SIP fields, and a

body. Header fields contain parameters such

as the identity of the caller, the identity of the

receiver, a unique call identity, sequence num-

ber, etc.; see Figure 24. The body typically uses

the Session Description Protocol (SDP) to

describe the session. SIP messages are coded in

plain text.

A short form (compact form) can be used to

identify the fields, using a single letter coding

(c – Content-Type; e – Content-Encoding; f –

From; i – Call-ID; m – Contact; l – Content

Length; s – Subject; t – To; v – Via).

In order to avoid that the calling user has to

know the whereabouts of the called user in

advance, a number of elements are included in

the SIP architecture:

• Proxy servers: An intermediary program that
  acts as both a server and a client for the purpose
  of making requests on behalf of other clients.
  Requests are served internally or passed on,
  possibly after translation, to other servers. A
  proxy interprets, and, if necessary, rewrites a
  request message before forwarding it.


• Location servers: Location server may be co-
  located with another SIP server.


• Registration servers: A registrar is a server
  that accepts Register requests. A registrar is
  typically co-located with a proxy or redirect
  server and may offer location services.


• Redirect servers: A redirect server accepts a
  SIP request, maps the address into zero or
  more new addresses and returns these add-
  resses to the client. Unlike a proxy server, it
  does not initiate its own SP request. Unlike a
  user agent server, it does not accept calls.


• User agent: A user agent is an application that
  contains both a user agent client (initiate a ses-
  sion) and a user agent server (receives a ses-
  sion request from the network)

Here, a client is an application program that

sends SIP requests. The client may or may not

interact directly with a human user. A server is

an application program that accepts requests in

order to serve requests and sends back responses

to those requests.

Every SIP user has a SIP URL. They are similar

to e-mail addresses, for instance

sip:[email protected]. There is a registra-

tion server within a user’s home domain. When

a user is started, it sends a register message to

this registration server which typically contains

the URL of the user, that user’s actual terminal

address, port number, transport protocol (TCP,

UDP), time stamp for duration of registration.

The registration server authenticates the user and

inserts the mapping between URL and the termi-

nal address in the location data base. This allows

users to be reached irrespective of their actual

point of contact, similar to mobile IP.

So, when a calling user wants to reach the called

user, assistance of a proxy or redirect server.

Such servers would request mapping from URL

to terminal address from the location server.

Naturally, when the users already know the ter-

minal addresses, such requests are not needed.

A simplified message sequence chart is depicted

in Figure 25.

SIP is a protocol that deals with session initia-

tion and does not explicitly describe how

Figure 25 Example of

registration and set-up

using SIP

registration
phase

call set-up
phase

calling user SIP server called user

register

register

100 ok

100 ok

invite

invite

180 ringing 180 ringing

200 ok 200 ok

ack

ack