Sunday, July 05, 2009

Programmable SIP server

I had posted an article on generic SIP API earlier. I implemented a first version in my 39 peers project over the weekend. I also used the API to implement a simple SIP proxy and registrar server. I tested the server using X-Lite clients in an intra-net. The basic SIP registration and call routing with record-route seems to work well.

There are precisely two modules in the implementation: sipapi to implement the core of the API and sipd to implement the server, with 252 and 110 lines, respectively, of Python. The first version only supports simple call routing as needed for a server, without any advanced features such as NAT traversal. It also supports fail-over and load sharing models using the two stage SIP server farm as described in my PhD thesis (although with same set of servers performing both first and second stage for different set of users).

In the implementation, the API exposes an Agent class that represents a listening endpoint. The agent dispatches various events such as "incoming" to signal an incoming message. The application, or SIP server in this case, attaches a local function to handle the "incoming" event and process the event. The processing logic is inspired by SIP Express Router (SER)'s config file. The following command creates the listening agent on the given listening IP and port using UDP transport.

from app import sipapi
agent = sipapi.Agent(sipaddr=('192.168.1.3', 5060), transports=('udp',)).start()


Once created you can attach your function (say "route") to handle incoming messages. The handler function gets an event representing the incoming message, and acts on it using the methods available on "event.action" property.

def route(event):
if len(str(event)) > 8192: return event.action.reject(513, 'Message Overflow')
if event.method == 'INVITE' and event.uri.user == 'anyone':
event.location = URI('sip:someone@somewhere.com')
return event.action.proxy()
...
agent.attach("incoming", route)
sipapi.run()


The event object available to the incoming message handler has an agent property representing the original Agent object. This allows you to access state and configuration elements on the agent. The API also defines a Location class to store contact locations from an event, and retrieve contact locations for a URI. The incoming event has several action methods such as accept, reject, proxy, redirect, challenge. If an action is not invoked, then it calls the default action method that responds with '501 Not Implemented' response to the incoming message.

The API and server can be extended to implement additional features such as NAT traversal, presence server, etc. For example, new event types can be defined to indicate presence change and allow the user to take action on these events. Alternatively, additional modules can define methods to modify SDP or SIP request to handle NAT traversal similar to how SER's nathandler module works. If you would like to work on these server extensions, do let me know.

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