SIP

The Session Initiation Protocol (SIP) is a signaling protocol used for initiating, maintaining, and terminating real-time sessions that include voice, video, and messaging applications. SIP is widely used in Internet telephony for voice and video calls, as well as in private IP telephone systems.

Key Concepts of SIP

User Agents (UA): The endpoints in a SIP network, which can be clients (UAC) or servers (UAS).
Registrar: A server that accepts REGISTER requests and updates the location database with the contact information of the user.
Proxy Server: An intermediary entity that routes SIP requests to the user agent server.
Redirect Server: A server that provides the client with information about the next hop or hops that a message should take.
Session Description Protocol (SDP): A protocol used to describe multimedia communication sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation.

SIP Methods

INVITE: Initiates a call by inviting the user to participate in a session.
ACK: Confirms that the client has received a final response to an INVITE request.
BYE: Terminates a session between two users.
CANCEL: Cancels any pending request.
REGISTER: Registers the user agent with a SIP server.
OPTIONS: Queries the capabilities of servers.
INFO: Sends mid-session information that does not modify the session state.

SIP Responses

1xx (Provisional): Informational responses
- 100 Trying
- 180 Ringing
- 181 Call Is Being Forwarded
- 182 Queued
- 183 Session Progress
2xx (Success): Indicates that the request was successfully received, understood, and accepted
- 200 OK
- 202 Accepted
3xx (Redirection): Further action needs to be taken to complete the request
- 300 Multiple Choices
- 301 Moved Permanently
- 302 Moved Temporarily
- 305 Use Proxy
- 380 Alternative Service
4xx (Client Error): The request contains bad syntax or cannot be fulfilled
- 400 Bad Request
- 401 Unauthorized
- 402 Payment Required
- 403 Forbidden
- 404 Not Found
- 405 Method Not Allowed
- 406 Not Acceptable
- 407 Proxy Authentication Required
- 408 Request Timeout
- 410 Gone
- 413 Request Entity Too Large
- 414 Request-URI Too Long
- 415 Unsupported Media Type
- 416 Unsupported URI Scheme
- 420 Bad Extension
- 421 Extension Required
- 423 Interval Too Brief
- 480 Temporarily Unavailable
- 481 Call/Transaction Does Not Exist
- 482 Loop Detected
- 483 Too Many Hops
- 484 Address Incomplete
- 485 Ambiguous
- 486 Busy Here
- 487 Request Terminated
- 488 Not Acceptable Here
- 491 Request Pending
- 493 Undecipherable
5xx (Server Error): The server failed to fulfill a valid request
- 500 Internal Server Error
- 501 Not Implemented
- 502 Bad Gateway
- 503 Service Unavailable
- 504 Server Time-out
- 505 Version Not Supported
- 513 Message Too Large
6xx (Global Failure): The request cannot be fulfilled at any server
- 600 Busy Everywhere
- 603 Decline
- 604 Does Not Exist Anywhere
- 606 Not Acceptable

Example: Implementing a Basic SIP Client Using `pjsip` Library

Here is an example of implementing a basic SIP client using the pjsip library in Python:

import pjsua as pj

# Callback to receive events from the library
class MyAccountCallback(pj.AccountCallback):
    def __init__(self, account):
        pj.AccountCallback.__init__(self, account)

    def on_reg_state(self):
        print("Registration status: ", self.account.info().reg_status)

# Create a library instance
lib = pj.Lib()

try:
    # Initialize the library
    lib.init(log_cfg=pj.LogConfig(level=3))

    # Create a UDP transport which listens to any available port
    transport = lib.create_transport(pj.TransportType.UDP, pj.TransportConfig(5060))

    # Start the library
    lib.start()

    # Create an account
    acc_cfg = pj.AccountConfig("sip.example.com", "username", "password")
    acc_cb = MyAccountCallback(acc_cfg)
    acc = lib.create_account(acc_cfg, cb=acc_cb)

    # Wait for user input to end the program
    input("Press <ENTER> to quit\n")

    # Shutdown the library
    lib.destroy()
    lib = None

except pj.Error as e:
    print("Exception: " + str(e))
    lib.destroy()
    lib = None

Relevant Switches and Parameters

Common `pjsua` Methods

Lib(): Initializes a new library instance.
init(log_cfg): Initializes the library with optional logging configuration.
create_transport(type, config): Creates a transport of the specified type (e.g., UDP).
start(): Starts the library.
AccountConfig(domain, username, password): Configures an account with the specified domain, username, and password.
create_account(config, cb): Creates an account with the specified configuration and callback.
destroy(): Shuts down the library and releases resources.

Understanding SIP and its associated methods and responses is crucial for implementing and troubleshooting real-time communication services.

Last modified: 14 December 2024

SIP