Overview of the handling of SIP sessions

The SIP channel handles several types of SIP sessions, or dialogs, not all of them being "telephone calls".

Incoming calls that will be sent to the PBX core
Outgoing calls, generated by the PBX
SIP subscriptions and notifications of states and voicemail messages
SIP registrations, both inbound and outbound
SIP peer management (peerpoke, OPTIONS)
SIP text messages

In the SIP channel, there's a list of active SIP dialogs, which includes all of these when they are active. "sip show channels" in the CLI will show most of these, excluding subscriptions which are shown by "sip show subscriptions"

incoming packets: Incoming packets are received in the monitoring thread, then handled by sipsock_read(). This function parses the packet and matches an existing dialog or starts a new SIP dialog.

sipsock_read sends the packet to handle_request(), that parses a bit more. if it's a response to an outbound request, it's sent to handle_response(). If it is a request, handle_request sends it to one of a list of functions depending on the request type - INVITE, OPTIONS, REFER, BYE, CANCEL etc sipsock_read locks the ast_channel if it exists (an active call) and unlocks it after we have processed the SIP message.

A new INVITE is sent to handle_request_invite(), that will end up starting a new channel in the PBX, the new channel after that executing in a separate channel thread. This is an incoming "call". When the call is answered, either by a bridged channel or the PBX itself the sip_answer() function is called.

The actual media - Video or Audio - is mostly handled by the RTP subsystem in rtp.c

Outbound calls: Outbound calls are set up by the PBX through the sip_request_call() function. After that, they are activated by sip_call().

Hanging up: The PBX issues a hangup on both incoming and outgoing calls through the sip_hangup() function

Dialogs, packets, transactions

This version of chan_sip has a concept of a SIP dialog (sip_dialog) that lives throughut a session - a call (INVITE), a registration (REGISTER) or a subscription for status (SUBSCRIBE)... We do save new requests we send in a queue (sip_dialog->packets) until they're acknowledged by the remote party, when we remove them (sip_ack() )

For requests and responses, we store them in a structure called sip_request and process them. The matching between a response or a new request and an existing dialog is done in match_or_create_dialog().

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Chan_SIP3:: Overview

Dialogs, packets, transactions