HP-UX Java SIP Stack Programmer's Guide (February 2008)

• The network transport layer is the transport layer in the IP stack. It enables transfer
of data between end points by using the services of the network layer. This layer
has two primary protocols, TCP and UDP. The TCP supports reliable and sequential
packet delivery through error recovery and flow control mechanisms. The UDP
is a simple message-based connectionless protocol compared to TCP. The SCTP
is yet another transport layer protocol that application developers can use to
transmit data between end points.
• The network layer routes data packets from the sender to the receiver in the
network. The most common network layer protocol is IP.
SIP Messages
A SIP message is a simple text message that is similar to an HTTP message. It is either
a request from a client to a server, or a response from a server to a client. It uses the
client-server model to send a request and to receive a response. The request messages
are in the form of nouns, for example, INVITE, BYE, and so on. The response messages
are always in the numerical form, for example, 180, 200, or 404. Following is the
format of a SIP message:
<Header_name> : <Description>
This section briefly describes the different types of SIP messages, parts of a message,
and sample messages.
This section addresses the following topics:
• “Message Types” (page 22)
• “Message Parts” (page 28)
Message Types
Following are the SIP message types:
• Request
• Response
The request and response messages use the same basic format. Both types of messages
consist of a start-line, one or more header fields, an empty line indicating the end of
header fields, and an optional message-body. However, the syntax differs in character
set and syntax specifics.
A Request Message
A request is a message sent from the client to the server. Following is a sample request
message:
INVITE sip:[e-mail] SIP/2.0
Via: SIP/2.0/UDP local.hp.com
From: OC <sip:[e-mail]>
To: TPU <sip:[e-mail]>
Subject: Confcall
22 Introduction