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SIP performs some functions for managing a VoIP transmission:
-Locate the user, associating the username of a user with the IP address used at that time.
-Defines the means at its disposal a user and which codecs support
-Informs the network of the state in which the user is
-Establishes the parameters of communication
-Manages the additional services, such as adding multiple users to a call, call transfer, ....

Protocols used by SIP
The management of a call takes place with the creation of an audio / video stream next to a control flow, such as the RTCP. To make easier the communication protocol using other protocols.

SDP (Session Description Protocol)
SDP has the task of defining the parameters of multimedia SIP sessions.
With SDP participants in the conversation understand what SIP:
-The number of flows data / control.
-The type of data.
-Codec communication.
-The transport protocol.
-The band to be used.
-Addresses and ports involved.
-The time of start and end of the flow (for streaming).
-Source information.

SIP components
To manage SIP server, you need some accessories:
User-Agent. The client needed to send a call request, the server serves instead to respond to a request. This part can be done by software or by a user terminal with the look of your phone.
Server-Registrar is to gather all the information on users.Associate with a client an IP address. It also serves to manage the authentication mechanisms, in consultation with a AAA Server.
-AAA Server which contains information for authorization, authentication and accounting necessary to manage the network. This server is normally already present on the network for other purposes.
-Location Server is used to try and locate a user, since the user can change IP.
Proxy-Server is used to connect to external domains, because SIP is based on domains. Also addresses issues with NAT.
Redirect-Server is used to redirect traffic depending on certain conditions.
-Media Server serves to contain the mailboxes, so as to be accessible at any time, even when the client is off. It may also contain pre-recorded messages to perform functions.
Mid-Proxy is used to filter traffic A / V or perform transcoding. It is used to simplify the client.
-MCU is used to carry out functions of mixers (for multicast). It is used to simplify the client (the client does not have to manage who enters / exits from the conversations).
-Gateway is used to manage connections to the telephone network.
Very often the servers are collected on a single machine (registrar, SIP Proxy and Redirect, Proxy Media go into the SIP server).

SIP Features
The addresses have the same form of electronic mail (name @ domain).
The difference is that the DNS server is not the IP address of the recipient, but the IP address of the domain of the SIP server. This will then consult the Registrar server, which associates the correct IP address. In the connection phase (log) require that the client is going to record from your SIP server.

SIP messages
They are composed of ASCII strings with the following format:
-Type of the message
-A set of SIP header
-An empty line (CRLF)
-The Payload (SDP), which is optional.
Typical messages are:
-REGISTER need to register an address within a SIP server.
-Invite is the communication request.
-ACK indicates a successful conclusion to the call request and is sent to begin the actual communication.
BYE-means closing the A / V
-CANCEL cancels a pending connection request. Can also be used to close the fork (), in the case where the same user has multiple recordings of different IP addresses, when the call has positive exist on one of the addresses.
-OPTIONS declares the capabilities of user agents.
-SUBSCRIBE indicates they need to want to know the status of a user agent.
-NOTIFY informs you of the status of a UA
MESSAGE-forward a message (text / XML) used for instant messaging.

Resolution of a SIP address
The resolution of a SIP address to an IP address is a necessary step, both
to search for a particular user for an INVITE message, whether to perform a REGISTER. The procedure is:
-Contact a DNS server for a NAPTR query (it discovered the names of services)
-Contact the DNS server for a SRV query (it discovered servers that perform those services).
-A contact server / YYYY to handle authentication.

Security and SIP
Security in SIP is obtained in the following ways:
-Authenticating each user, via the AAA Server.
-Encrypting the body of SIP messages and the list of intermediate nodes on which the message is passed.
-Handling mechanisms to solve the problems of spam and denial of service.

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