What is SIP Network Entities and Message?
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Let’s learn about the SIP Network Entities and Message
SIP Network Entities
Various logical entities exist on the SIP network to play different roles. Major SIP entities are SIP user agent and SIP network server, as shown in Figure 1.
Figure 1-21 SIP network entities
User agent (UA)
The UA sends or receives SIP requests and processes these requests. The UA is logically classified into user agent client (UAC) and user agent server (UAS). The UAC sends requests to UAS, and the UAS responds to these requests. The responses can be acceptance, rejection, or redirection.
Proxy server
The proxy server, a logical network entity, forwards requests or responses on behalf of a client. The proxy server can also function as a server. The proxy server provides the following functions: routing, authentication, accounting control, call control, and service providing. The proxy server attempts to forward requests to multiple addresses in multiple modes, such as branching, cycling, and recursively querying.
NOTE:
The IMS network or softswitch functions as the redirection server, proxy server, and registration server, and the DNS server functions as the location server.
SIP Message
Format
The SIP message is encoded in the text format, each line ending with CR or LF. The SIP message has two types, the request message and the response message. The general message format is as follows:
| SIP message = | Start line |
| *Message header field | |
| Empty line (CRLF) | |
| [Message body] |
A SIP message consists of a start-line, one or more header fields, and a message body. The request and response messages are the same in the format and only differ in the start-line. The request message has a request-line as the start-line and the response message has a status-line as the start-line.
Request Message
Request messages are sent from the client to the server. SIP request messages include INVITE, ACK, OPTIONS, BYE, CANCEL, REGISTER, PRACK, and UPDATE. Table 1 describes these SIP request messages.
Table 1 SIP request messages
| Request Message | Function |
|---|---|
| INVITE | Invites a user to join a call |
| ACK | Acknowledges the response message of the request |
| OPTIONS | Requests for querying capability information |
| BYE | Releases an established call |
| CANCEL | Releases an unestablished call |
| REGISTER | Registers the user location information on the SIP network server |
| PRACK | Acknowledges a reliable provisional response message |
| UPDATE | Updates the session |
Response Message
The SIP response message is used for responding to the SIP request message, indicating whether the call is successful or fails. Different from request messages, the start-line of response messages is also called status-line, which consists of SIP-Version, Status-Code, and Reason-Phrase.
- SIP-Version: indicates the used SIP version.
- Status-Code: identifies the response message type. The status-code is a 3-digit integer. The first digit of the status-code defines the response type. The other two digits provide detailed descriptions about the response. Table 2 describes response messages.
- Reason-Phrase: provides descriptions about the status code. This field is optional.
Table 2 SIP response messages
| Status Code | Meaning | Function |
|---|---|---|
| 1XX | Provisional | The request has been received and is being processed. |
| 2XX | Success | The action was successfully received, understood, and accepted. |
| 3XX | Redirection | Further action needs to be taken in order to complete the request. |
| 4XX | Client Error | The request contains bad syntax or cannot be fulfilled at this server. |
| 5XX | Server Error | The server failed to fulfill an apparently valid request |
| 6XX | Global Failure | The request cannot be fulfilled at any server. |
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