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SIP Protocol Stack


SIP is structured as a layered protocol.
i) Lowest layer is syntax and encoding.
ii) Second layer is transport layer.
iii) Third layer is a transaction layer.
iv) Layer above transaction layer is transaction user.




Syntax Encoding:
  • Responsible for encoding/decoding and syntactical parsing of the messages.
  • Augmented Backus-Naur Form (ABNF) 

Transport Layer:

  • Performs actual transmission of requests and responses over network transports.
  • Responsible for managing persistent connections for transport protocols like TCP, TLS &         SCTP.
  • Performs framing of message oriented transport like UDP.
  • All SIP elements contain a transport layer.

Transaction Layer:
  • Handles application layer retransmissions, matching of responses to requests and                  application layer timeouts.
  • Consists of client/server transaction.
  • Each component is represented by a finite state machine that is constructed to process a        particular request (Please refer RFC 3261 - Transactions for detailed description).
  • Stateless proxies do not contain a transaction layer.
Client Transaction:
  1. Responsible for sending the request.
  2. Responsible for receiving responses from the transport layer and delivering them to the TU (Transaction User).
  3. Filtering out any response re-transmissions or disallowed responses (ex response to ACK).
Server Transaction:
  1. Responsible for sending the response.
  2. Responsible to receive requests from the transport layer and deliver them to the TU.
  3. Filters any request re-transmissions from the network.
  4. Accepts responses from TU and delivers them to the transport layer for transmission over network.

Transaction User:
  • To send a request, TU creates a client transaction instance and passes the request along with the          destination IP address, port and transport to which to send the request.
  • TU that creates a client transaction can also CANCEL it.
  • Each of the SIP entities, except the stateless proxy is a Transaction User.



Basic SIP Call Flow <==                                          ==> SIP Timers & State Machine

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