Implementing a SIP Back-To-Back User Agent using javax.servlet.sip.B2buaHelper

Let us take the following Back-To-Back UA scenario which is typically used in the call setup. Keep this scenario in mind (or refer to it) as you go through the code in the rest of this blog.

   Client                SailFin               Server

                                       INVITE -------->

                                       <-- 200OK (sdp1)
    <-- INVITE (sdp1)

    200OK (sdp2) --->

    <------------ ACK

                                       ACK (sdp2) ---->

   <------------------ Media session ----------------->


   BYE ------------->

                                       BYE ----------->

                                       <-------- 200 OK

   <---------- 200 OK

Let us assume the following sip addresses for each SIP entities :

SailFin	: sip:127.0.0.1:5060
Client 	: sip:127.0.0.1:5070
Server	: sip:127.0.0.1:5080

The initial INVITE request will be sent from web/sip/ejb component depending on the type of the application. The code below shows how to create such a request and send it out to the server. I assume that the sipFactory is injected using @Resource SipFactory sipFactory;

SipServletRequest inviteRequest = sipFactory.createRequest(
					sipFactory.createApplicationSession(),"INVITE", client, server);
inviteRequest.send();

When the server responds with 200 OK with its SDP (lets call it "sdp1") for the above INVITE request, then we should create the second INVITE request and send it to the client. The second INVITE request should contain the "sdp1" of the server. 

This is done in the doSuccessResponse method of B2BUA like this:

    @Override
    protected void doSuccessResponse(SipServletResponse resp)
            throws ServletException, IOException {
        if (resp.getMethod().equals("INVITE")) {
            List<SipSession> sipSessions =
                    getSipSessions(resp.getApplicationSession());
            if (sipSessions.size() == 1) { // 200 OK response received from the server
                sipSessions.get(0).setAttribute("ACK", resp.createAck()); // we need ACK for later use.
                sendInviteToClient(resp);
            }
        }
    }

    private void sendInviteToClient(SipServletResponse serverResp)
            throws ServletException, IOException {
        SipServletRequest serverReq = serverResp.getRequest();
        B2buaHelper b2buaHelper = serverReq.getB2buaHelper();

        // Swap To & From headers.
        Map<String, Set<String>> headerMap = new HashMap<String, HashSet<String>>();
        Set<String> from = new HashSet<String>();
        from.add(serverResp.getHeader("From"));
        headerMap.put("To", from);
        Set<String> to = new HashSet<String>();
        to.add(serverResp.getHeader("To"));
        headerMap.put("From", to);

        SipServletRequest clientRequest =
                b2buaHelper.createRequest(serverReq, true, headerMap);
        clientRequest.setRequestURI(
                clientRequest.getAddressHeader("To").getURI());
        if (serverResp.getContent() != null) { // set "sdp1"
            clientRequest.setContent(serverResp.getContent(),
                    serverResp.getContentType());
        }
        clientRequest.send();
    }

    // Helper method.
    private List<SipSession> getSipSessions(SipApplicationSession sas) {
        List<SipSession> sipSessions = new ArrayList<SipSession>();
        Iterator<SipSession> iter =
                (Iterator<SipSession>) sas.getSessions("SIP");
        while (iter.hasNext()) {
            sipSessions.add(iter.next());
        }
        return sipSessions;
    }

Now when the client responds with 200 OK with its SDP (lets call it "sdp2"), then we should send ACK to both the parties. The ACK to the server should contain the "sdp2" of the client.

This is done again in the doSuccessResponse method of the B2BUA like this:

    @Override
    protected void doSuccessResponse(SipServletResponse resp)
            throws ServletException, IOException {
        if (resp.getMethod().equals("INVITE")) {
            List<SipSession> sipSessions =
                    getSipSessions(resp.getApplicationSession());
            if (sipSessions.size() == 1) { // 200 OK response received from the server
                ......
            } else { // 200 OK response received from the client
                sendAckToClient(resp);
                sendAckToServer(resp);
            }
        }
    }

    private void sendAckToClient(SipServletResponse clientResp)
            throws ServletException, IOException {
        SipServletRequest ack = clientResp.createAck();
        ack.send();
    }

    private void sendAckToServer(SipServletResponse clientResp)
            throws ServletException, IOException {
        B2buaHelper b2buaHelper = clientResp.getRequest().getB2buaHelper();
        SipSession clientSession = clientResp.getSession();
        SipSession serverSession = b2buaHelper.getLinkedSession(clientSession);
        SipServletRequest ack = (SipServletRequest) serverSession.getAttribute("ACK");
        serverSession.removeAttribute("ACK");
        if (clientResp.getContent() != null) { // set "sdp2"
            ack.setContent(clientResp.getContent(), clientResp.getContentType());
        }
        ack.send();
    }

With this, we are done with successfully setting up the "Media session" between the client and the server.

    @Override
    protected void doBye(SipServletRequest clientBye)
            throws ServletException, IOException {
            sendByeToServer(clientBye);
            clientBye.getSession().setAttribute("BYE",clientBye.createResponse(200)); // for later use.
        }
    }

    private void sendByeToServer(SipServletRequest clientBye)
            throws ServletException, IOException {
        B2buaHelper b2buaHelper = clientBye.getB2buaHelper();
        SipSession serverSession = b2buaHelper.getLinkedSession(clientBye.getSession());
        SipServletRequest serverBye = serverSession.createRequest("BYE");
        serverBye.send();
    }

    @Override
    protected void doSuccessResponse(SipServletResponse resp)
            throws ServletException, IOException {
        if (resp.getMethod().equals("INVITE")) {
            .....
        } else if (resp.getMethod().equals("BYE")) { // 200 OK response received from server.
            send200OKToClient(resp);
        }
    }

    private void send200OKToClient(SipServletResponse serverResp)
            throws ServletException, IOException {
        B2buaHelper b2buaHelper = serverResp.getRequest().getB2buaHelper();
        SipSession clientSession =
                b2buaHelper.getLinkedSession(serverResp.getSession());
        SipServletResponse clientResp =
                (SipServletResponse) clientSession.getAttribute("BYE");
        clientSession.removeAttribute("BYE");
        clientResp.setStatus(serverResp.getStatus(), serverResp.getReasonPhrase());
        log("Sending response.\n" + clientResp);
        clientResp.send();
        clientSession.getApplicationSession().invalidate();
    }