The process by which an underlying transport is chosen for sending of a message is broken up into different steps depending on the type of message.
SIP Request Handling
1. URI Parsing
The PJSIP stack fundamentally acts on URIs. When sending to a URI it is parsed into the various parts (user, host, port, user parameters). For the purposes of transport selection the transport parameter is examined. This specifies the type of transport. If this parameter is not present it is assumed to be UDP. This is important because it is used in DNS resolution. If a "sips" URI scheme is used an automatic switchover to TLS will occur.
2. DNS SRV Resolution (If host portion is not an IP address and no port is present in the URI)
The transport type from above is used to determine which SRV record to look up. This means that the original URI must include the transport type for TCP and TLS types UNLESS the "sips" URI scheme is used which automatically switches to TLS.
3a. Transport Selection (No explicit transport provided)
Now that the underlying type of transport is known and the resolved target exists the transport selection process can begin.
Connection-less protocols (such as UDP)
A transport, decided upon by a hashing mechanism, matching the transport type and address family is selected.
Connection-oriented protocols (such as TCP or TLS)
An already open connection to the resolved IP address and port is searched for. If the connection exists it is reused for the request. If no connection exists the first transport matching the transport type and address family as configured in pjsip.conf is chosen. It is instructed to establish a new connection to the resolved IP address and port.
The provided transport is instructed to establish a new connection to the resolved IP address and port.
This does NOT currently attempt to reuse any existing connections. A new one will always be created. This is an issue being tracked under issue ASTERISK-22658
If an existing connection exists to the IP address and port using the specific transport type then it is reused and a new one is not established.
4. Multihomed Transport Selection (Connection-less protocols)