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Overview of SIP Servlets

What is a SIP Servlet?

The SIP Servlet API is a part of JAIN APIs and being standardized as JSR116 of JCP (Java Community Process). The SIP Servlet API version 1.0 was published in February, 2003.

Note: In this document, the term "SIP Servlet" is used to represent the API, and "SIP servlet" is used to represent an application created with the API.

J2EE provides Java Servlet that is a main technology of building Web applications. Although Java Servlet is used only to develop HTTP protocol-based applications on a Web application server, it basically has functions as a generic API for server applications. SIP Servlet is defined as the generic servlet API with SIP-specific functions added.


Figure 1. Servlet API and SIP Servlet API

SIP Servlets are very similar to HTTP Servlets, and HTTP servlet developers will quickly adapt to the programming model. The service level defined by both HTTP and SIP Servlets is very similar, and you can easily design applications that support both HTTP and SIP. Listing 1 shows an example of a simple SIP servlet.

List 1: SimpleSIPServlet.java

package com.bea.example.simple;

import java.io.IOException;
import javax.servlet.*;
import javax.servlet.sip.*;

public class SimpleSIPServlet extends SipServlet {

    protected void doMessage(SipServletRequest req)
        throws ServletException, IOException
    {
        SipServletResponse res = req.createResponse(200);
        res.send();
    }
}
    

The above example shows a simple SIP servlet that sends back a 200 OK response to the SIP MESSAGE request. As you can see from the list, SIP Servlet and HTTP Servlet have many things in common:

  1. Servlets must inherit the base class provided by the API.
    HTTP servlets must inherit HttpServlet, and SIP servlets must inherit SipServlet.
  2. Methods doXxx must be overridden and implemented.
    HTTP servlets have doGet/doPost methods corresponding to GET/POST methods. Similarly, SIP servlets have doXxx methods corresponding to the method name (in the above example, the MESSAGE method). Application developers override and implement necessary methods.
  3. The lifecycle and management method (init, destroy) of SIP Servlet are exactly the same as HTTP Servlet. Manipulation of sessions and attributes is also the same.
  4. Although not appeared in the API, there is a deployment descriptor called sip.xml for a SIP servlet, which corresponds to web.xml. Application developers and service managers can edit this file to configure applications using multiple SIP servlets.

However, there are several differences between SIP and HTTP servlets. A major difference comes from protocols. The next section describes these differences as well as features of SIP servlets.

Differences from HTTP Servlets

a. Multiple Responses

You might notice from the List 1 that the doMessage method has only one argument. In HTTP, a transaction consists of a pair of request and response, so arguments of a doXxx method specify a request (HttpServletRequest) and its response (HttpServletResponse). An application takes information such as parameters from the request to execute it, and returns its result in the body of the response.

protected void doGet(HttpServletRequest req, HttpServletResponse res)
    throws ServletException, IOException
    

For SIP, more than one response may be returned to a single request.


Figure 2: Example of Request and Response in SIP

The above figure shows an example of a response to the INVITE request. In this example, the server sends back three responses 100, 180, and 200 to the single INVITE request. To implement such sequence, in SIP Servlet, only a request is specified in a doXxx method, and an application generates and returns necessary responses in an overridden method.

Currently, SIP Servlet defines the following doXxx methods:

protected void doInvite(SipServletRequest req);

protected void doAck(SipServletRequest req);

protected void doOptions(SipServletRequest req);

protected void doBye(SipServletRequest req);

protected void doCancel(SipServletRequest req);

protected void doSubscribe(SipServletRequest req);

protected void doNotify(SipServletRequest req);

protected void doMessage(SipServletRequest req);

protected void doInfo(SipServletRequest req);

protected void doPrack(SipServletRequest req);
    

b. Receiving Responses

One of the major features of SIP is that roles of a client and server are not fixed. In HTTP, Web browsers always send HTTP requests and receive HTTP responses: They never receive HTTP requests and send HTTP responses. In SIP, however, each terminal needs to have functions of both a client and server.

For example, both of two SIP phones must call to the other and disconnect the call.


Figure 3: Relationship between Client and Server in SIP

The above example indicates that a calling or disconnecting terminal acts as a client. In SIP, roles of a client and server can be changed in one dialog. This client function is called UAC (User Agent Client) and server function is called UAS (User Agent Server), and the terminal is called UA (User Agent). SIP Servlet defines methods to receive responses as well as requests.

protected void doProvisionalResponse(SipServletResponse res);

protected void doSuccessResponse(SipServletResponse res);

protected void doRedirectResponse(SipServletResponse res);

protected void doErrorResponse(SipServletResponse res);
    

These doXxx response methods are not the method name of the request. They are named by the type of the response as follows:

doProvisionalResponse A method invoked on the receipt of a provisional response (or 1xx response).
doSuccessResponse A method invoked on the receipt of a success response.
doRedirectResponse A method invoked on the receipt of a redirect response.
doErrorResponse A method invoked on the receipt of an error response (or 4xx, 5xx, 6xx responses).

Existence of methods to receive responses indicates that in SIP Servlet requests and responses are independently transmitted an application in different threads. Applications must explicitly manage association of SIP messages. An independent request and response makes the process slightly complicated, but enables you to write more flexible processes.

Also, SIP Servlet allows applications to explicitly create requests. Using these functions, SIP servlets can not only wait for requests as a server (UAS), but also send requests as a client (UAC).

c. Proxy Functions

Another function that is different from the HTTP protocol is "forking." Forking is a process of proxying one request to multiple servers simultaneously (or sequentially) and used when multiple terminals (operators) are associated with one telephone number (such as in a call center).


Figure 4: Proxy Forking

SIP Servlet provides a utility to proxy SIP requests for applications that have proxy functions.

d. Message Body

As the figure below, the structure of SIP messages is the same as HTTP.


Figure 5: SIP Message Example

HTTP is basically a protocol to transfer HTML files and images. Contents to be transferred are stored in the message body. HTTP Servlet defines stream manipulation-based API to enable sending and receiving massive contents.

ServletRequest

ServletInputStream getInputStream()
BufferedReader     getReader()
    

ServletResponse

ServletOutputStream getOutputStream()
PrintWriter         getWriter()
int  getBufferSize()
void setBufferSize(int size)
void resetBuffer()
void flushBuffer()
    

In SIP, however, only low-volume contents are stored in the message body since SIP is intended for real-time communication. Therefore, above methods are provided only for compatibility, and their functions are disabled.

In SIP, contents stored in the body include:

SDP (Session Description Protocol) - A protocol to define multimedia sessions used between terminals. This protocol is defined in RFC2373.

Presence Information - A message that describes presence information defined in CPIM.

IM Messages - IM (instant message) body. User-input messages are stored in the message body.

Since the message body is in a small size, processing it in a streaming way increases overhead. SIP Servlet re-defines API to manipulate the message body on memory as follows:

SipServletMessage

void   setContent(Object content, String contentType)
Object getContent()
byte[] getRawContent()
    

Roles of a Servlet Container

This section describes major functions provided by WebLogic SIP Server as a SIP servlet container.

Application Management - Describes functions such as application management by servlet context, lifecycle management of servlets, application initialization by deployment descriptors.

SIP Massaging - Describes functions of parsing incoming SIP messages and delivering appropriate SIP servlets, sending messages created by SIP servlets to appropriate UAS, and automatically setting SIP header fields.

Utility Functions - Describes functions such as sessions, factories, and proxying that are available in SIP servlets.

Application Management

Like HTTP servlet containers, SIP servlet containers manage applications by servlet context (see Figure 6). Servlet contexts (applications) are normally archived in a WAR format and deployed in each application server.

Note: The method of deploying in application servers varies depending on your product. Refer to the documentation of your application server.


Figure 6: Servlet Container and Servlet Context

A servlet context for a converged SIP and Web application can include multiple SIP servlets, HTTP servlets, and JSPs.

    

WebLogic SIP Server can deploy applications using the same method as the application server you use as the platform. However, if you deploy applications including SIP servlets, you need a SIP specific deployment descriptor (sip.xml) defined by SIP servlets. The table below shows the file structure of a general converged SIP and Web application.

Table 1: File Structure Example of Application

File Description
WEB-INF/ Place your configuration and executable files of your converged SIP and Web application in the directory. You cannot directly refer to files in this directory on Web (servlets can do this).
WEB-INF/web.xml The J2EE standard configuration file for the Web application.
WEB-INF/sip.xml The SIP Servlet-defined configuration files for the SIP application.
WEB-INF/classes/ Store compiled class files in the directory. You can store both HTTP and SIP servlets in this directory.
WEB-INF/lib/ Store class files archived as Jar files in the directory. You can store both HTTP and SIP servlets in this directory.
*.jsp, *.jpg Files comprising the Web application (e.g. JSP) can be deployed in the same way as J2EE.

Information specified in the sip.xml file is similar to that in the web.xml except <servlet-mapping> setting that is different from HTTP servlets. In HTTP you specify a servlet associated with the file name portion of URL. But SIP has no concept of the file name. You set filter conditions using URI or the header field of a SIP request. The following example shows that a SIP servlet called "register" is assigned all REGISTER methods.

List 1: Filter Condition Example of sip.xml

 <servlet-mapping>
   <servlet-name>registrar</servlet-name>
   <pattern>
     <equal>
       <var>request.method</var>
       <value>REGISTER</value>
     </equal>
   </pattern>
 </servlet-mapping>
    

Once deployed, lifecycle of the servlet context is maintained by the servlet container. Although the servlet context is normally started and shutdown when the server is started and shutdown, the system administrator can explicitly start, stop, and reload the servlet context.

SIP Massaging

SIP messaging functions provided by a SIP servlet container are classified under the following four types:

All SIP messages that a SIP servlet handles are represented as a SipServletRequest or SipServletResponse object. A received message is first parsed by the parser and then translated to one of these objects and sent to the SIP servlet container.

A SIP servlet container receives the following three types of SIP messages, for each of which you determine a target servlet.

First SIP Request - When the SIP servlet container received a request that does not belong to any SIP session, it uses filter conditions in the sip.xml file (described in the previous section) to determine the target SIP servlet. Since the container creates a new SIP session when the initial request is delivered, any SIP requests received after that point are considered as subsequent requests.

Note: Filtering should be done carefully. In WebLogic SIP Server 2.0, when the received SIP message matches multiple SIP servlets, it is delivered only to any one SIP servlet.

Subsequent SIP Request - When the SIP servlet container received a request that belongs to any SIP session, it delivers the request to a SIP servlet associated with that session. Whether the request belongs to a session or not is determined using dialog ID.

SIP Response - When the received response is to a request that a SIP servlet proxied, the response is automatically delivered to the same servlet since its SIP session had been determined. When a SIP servlet sends its own request, you must first specify a servlet that receives a response in the SIP session. For example, if the SIP servlet sending a request also receives the response, the following handler setting must be specified in the SIP session.

SipServletRequest req = getSipFactory().createRequest(appSession, ...);
req.getSession().setHandler(getServletName());
  

Normally, in SIP a "session" means a real-time session by RTP/RTSP. On the other hand, in HTTP Servlet a "session" refers to a way of relating multiple HTTP transactions. In this document, session-related terms are defined as follows:

Realtime Session A realtime session established by RTP/RTSP.
HTTP Session A session defined by HTTP Servlet. A means of relating multiple HTTP transactions.
SIP Session A means of implementing the same concept as in HTTP session in SIP. SIP (RFC3261) has a similar concept of "dialog," but in this document this is treated as a different term since its lifecycle and generation conditions are different.
Application Session A means for applications using multiple protocols and dialogs to associate multiple HTTP sessions and SIP sessions. Also called "AP session."

WebLogic SIP Server automatically execute the following response and retransmission processes.

Sending "100 Trying" - When WebLogic SIP Server receives an INVITE request, it automatically creates and sends "100 Trying."

Response to CANCEL - When WebLogic SIP Server receives a CANCEL request, it executes the following processes if the request is valid.

    1. Sends a 200 response to the CANCEL request.
    2. Sends a 487 response to the INVITE request to be cancelled.
    3. Invokes a doCancel method on the SIP servlet.
      This allows the application to abort the process within the doCancel method, eliminating the need for explicitly sending back a response.

Sends ACK to an error response to INVITE - When a 4xx, 5xx, or 6xx response is returned for INVITE that were sent by a SIP servlet, WebLogic SIP Server automatically creates and sends ACK. This is because ACK is required only for a SIP sequence, and the SIP servlet does not require it.
When the SIP servlet sends a 4xx, 5xx, or 6xx response to INVITE, it never receives ACK for the response.

Retransmission process when using UDP - SIP defines that sent messages are retransmitted when low-trust transport including UDP is used. WebLogic SIP Server automatically do the retransmission process according to the specification.

Mostly, applications do not need to explicitly set and see header fields In HTTP Servlet since HTTP servlet containers automatically manage these fields such as Content-Length and Content-Type. SIP Servlet also has the same header management function.

In SIP, however, since important information about message delivery exists in some fields, these headers are not allowed to change by applications. Headers that can not be changed by SIP servlets are called "system headers." The table below lists system headers:

Table 2: System Headers

Header Name Description
Call-ID Contains ID information to associate multiple SIP messages as Call.
From, To Contains Information on the sender and receiver of the SIP request (SIP, URI, etc.). tag parameters are given by the servlet container.
CSeq Contains sequence numbers and method names.
Via Contains a list of servers the SIP message passed through. This is used when you want to keep track of the pass to send a response to the request.
Record-Route, Route Used when the proxy server mediates subsequent requests.
Contact Contains network information (such as IP address and port number) that is used for direct communication between terminals. For a REGISTER message, 3xx, or 485 response, this is not considered as the system header and SIP servlets can directly edit the information.

Utility Functions

SIP Servlet defines the following utilities that are available to SIP servlets:

  1. SIP Session, Application Session
  2. SIP Factory
  3. Proxy

SIP Session, Application Session

As stated before, SIP Servlet provides a "SIP session" whose concept is the same as a HTTP session. In HTTP, multiple transactions are associated using information like Cookie. In SIP, this association is done with header information (Call-ID and tag parameters in From and To). Servlet containers maintain and manage SIP sessions. Messages within the same dialog can refer to the same SIP session. Also, For a method that does not create a dialog (such as MESSAGE), messages can be managed as a session if they have the same header information.

SIP Servlet has a concept of an "application session," which does not exist in HTTP Servlet. An application session is an object to associate and manage multiple SIP sessions and HTTP sessions. It is suitable for applications such as B2BUA.

Note: In WebLogic SIP Server 2.0, HTTP sessions are not associated with application sessions.

SIP Factory

A SIP factory (SipFactory) is a factory class to create SIP Servlet-specific objects necessary for application execution. You can generate the following objects:


Table 3: Objects Generated with SipFactory

Class Name Description
URI, SipURI, Address Can generate address information including SIP URI from String.
SipApplicationSession Creates a new application session. It is invoked when a SIP servlet starts a new SIP signal process.
SipServletRequest Used when a SIP servlet acts as UAC to create a request. Such requests can not be sent with Proxy.proxyTo. They must be sent with SipServletRequest.send.

SipFactory is located in the servlet context attribute under the default name. You can take this with the following code.

ServletContext context = getServletContext();
SipFactory factory =
    (SipFactory) context.getAttribute("javax.servlet.sip.SipFactory");
    

Proxy

Proxy is a utility used by a SIP servlet to proxy a request. In SIP, proxying has its own sequences including forking. You can specify the following settings in proxying with Proxy:

Recursive routing (recurse) - When the destination of proxying returns a 3xx response, the request is proxied to the specified target.

Record-Route setting - Sets a <code>Record-Route</code> header in the specified request.

Parallel/Sequential (parallel) - Determines whether forking is executed in parallel or sequentially.

stateful - Determines whether proxying is transaction stateful.

Supervising mode - In the event of the state change of proxying (response receipts), an application reports this.