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1. SIP is a signaling protocol normally used to create, modify, and terminate a multimedia session over the Internet Protocol. A session is a simple call between two endpoints. An endpoint can be a Smartphone, a laptop, or any device that can receive and transmit multimedia content over the Internet.

2. SIP is an application layer protocol defined by IETF (Internet Engineering Task Force) standard. It is defined in RFC 3261.

3. SIP is comprised with two widely used internet protocols: HTTP for web browser and SMTP used for email. From HTTP, SIP makes use of the client-server architecture and the use of URL and URI. From SMTP, it borrowed a text encoding scheme and a header style.

4. SIP makes use of the help of SDP (Session Description Protocol) which describes a session and RTP (Real Time Transport Protocol) generally used for delivering voice and video over IP network.

5. SIP can be used for two-party (unicast) or multiparty (multicast) sessions.

6. Other SIP applications consist of file transfer, instant messaging, video conferencing, online games, and steaming multimedia distribution.

SIP System Architecture

SIP is structured as a layered protocol, which means its behavior is described in terms of a set of fairly independent processing stages with only a loose coupling between each stage.

1. The lowest layer of SIP is its syntax and encoding. Its encoding is specified using an augmented Backus-Naur Form grammar (BNF).

2. At the second level is the transport layer. It explains how a Client sends requests and receives responses and how a Server receives requests and sends responses over the network. All SIP elements include a transport layer.

3. The next layer is transaction layer. A transaction is a request sent by a Client transaction (using the transport layer) to a Server transaction, along with all responses to that request sent from the server transaction back to the client. Any task that a user agent client (UAC) executes takes place using a series of transactions. Stateless proxies do not contain a transaction layer.

4. The layer above the transaction layer is called the transaction user. Each of the SIP entities, except the stateless proxy, is a transaction user.

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