- Building Telephony Systems with OpenSER
- Flavio E. Goncalves
- 276字
- 2021-07-02 11:38:34
Stateful Operation

This is a simplified description of the stateful operation. You will find a complete and more detailed description in the RFC3261 text. There is a close resemblance between the openser.cfg
sections and the figure above. However some processes are manual, such as to check the Max-forwards header, while others are encapsulated in a single command. To illustrate, when you call t_relay()
all the forward request processing as described is done automatically.
When operating in stateful mode, a proxy is simply a SIP transaction processor and all these processing steps are required:
- Validate the request
- Pre-process the routing information
- Determine the request's target
- Forward the request to the target
- Process all responses
A stateful proxy creates a new server transaction for each new request received. Any retransmissions of the request will then be handled by that server transaction.
Example: For each request traversing our SIP proxy we will:
Step 1: Request validation
- Check the message size to avoid buffer overflows.
- Check the Max-forwards header to detect loops.
Step 2: Routing information pre-processing
- If there is a record-route header, process it.
Step 3: Determine the request target
- Is it in the location database (registered users)?
- Is there a route to the destination (gateway destinations)?
- Is it directed to an external domain (external addresses)?
Step 4: Request forwarding
- Call the function
t_relay()
and OpenSER will do all the job for you statefully.
Step 5: Response processing
- Usually this is done automatically by OpenSER. Sometimes you can use the
onreply_route[]
section to handle some response. Example: in a "call forward on busy" scenario, we could use the response 486 (Busy) to direct the call to a voicemail server.
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