SIP Servlets

1

• SIP Servlets

2
Problem Statement

• Want to enable construction of a wide variety of
  IP telephony applications
• Traditional voice features
• IN features
• Converged applications integration with IM,
  Web, Presence
• Generic call model based APIs not sufficient!
• Dont expose features of SIP needed for converged
  services
• Call model not central other events drive apps!
• API should appeal to a wide developer community
• Want to support hosting model
• Developer is not the deployer
• Carrier grade applications

3
Appealing to the Web Development Community

• Number of web developers to telecom developers is
  on order of ?1
• Most CS graduates today know servlets, CGI, XML,
  and web development servers
• Goal is to create lots of services by appealing
  to the masses
• Great ideas will come from two hackers in a
  garage
• Therefore,
• Want a SIP services API that is familiar to web
  developers
• Leverage the HTTP servlet model

4
HTTP Servlets

• HTTP Java Servlets Widely Used in Web Application
  Development
• Applications Consist of Sets of HTTP Servlets,
  Each of Which Processes a Single Web Request in
  the Application
• HTTP Servlets Return Web Pages to Display
• HTTP Servlets Can Create Session Data
• e.g., shopping cart, that spans multiple requests
• Container Manages HTTP Servlet Lifecycles,
  Fault Tolerance, Session State
• HTTP Servlets Collected into a War File Web
  Archive

HTTP Servlets
War File
Developer
Deployer
Web Server
5
SIP Servlets

• Direct Application of HTTP Servlet Model to SIP
• Java-based API
• Telecommunications application is a set of SIP
  (and HTTP!) servlets
• SIP servlets process a particular SIP request or
  response
• Lifecycle managed by container
• SIP servlets can create and access session data,
  call data, transaction data
• SIP servlet container provides same functions as
  http container
• CAR file equivalent of WAR file

SIP Servlets
Car File
Developer
SIP Server
6
What is a SIP Servlet?

• Java interface
• Defines methods that are callbacks when certain
  events occur
• doInvte()
• doBYE()
• doResponse()
• Application writer implements servlet class,
  fills in methods with own code
• Servlets dont store state domain objects are
  used (later)
• Servlet can instruction container to
• Proxy a request
• Initiate a new request
• Forward a response
• Generate a response

• Servlet engine handles the messy details of SIP
• Call-Ids, tags, retransmissions, record-routes,
  vias
• Servlet has access to important fields of SIP
  messages
• To, From, Request-URI, Contact, body

7
Example SIP Servlet
public class MyServlet implements SipServlet
public void doInvite(SipServletRequest
req,

SipServletResponse res)
req.getProxy(true).proxy(sipuser_at_host
)
8
Definition of Servlet Mappings

• Single server supports many applications
• When a SIP INVITE arrives, which one (or ones)
  process the request??
• Servlet mappings are rules that create bindings
  from SIP messages to servlet classes
• Based on expression matching in fields of message
• Servlet mappings can be
• Set up by application deployer
• Set by application writer

Class 1
Rule Match
Class 2
INVITE
Class 3
Class 4
Rule DB
9
Deployment Descriptors

• Third party model requires information to be
  conveyed from writer to deployer beyond just code
• Deployment descriptor fills this need
• Descriptive names and usage of classes
• Name and usage of entire application
• Servlet mappings
• Context parameters
• References to resources needed by applications
• EJB Homes
• JNDI contexts
• Session timeouts

• Converged Archive (CAR) File
• JAR file with specific structure
• Used to package entire application into one
  bundle
• Contains
• Servlet classes
• Deployment descriptor
• Static content
• HTTP Servlets use WAR file (Web Archive)
• CAR file is superset of WAR

10
Storing application state

• In most applications, state is distributed
  throughout the objects it uses
• Troublesome for fault tolerance
• The data must be replicated
• Data might need to be distributed across a set of
  homogeneous servers
• The data might need to be stored on reliable
  media to ensure recovery
• If data is distributed, container has no easy way
  to store and manage it for the application

• Result application needs to handle its own data
  management. Bad.
• Alternative container provides domain objects in
  which application can place all state
• All state in one place
• Container can easily manage fault tolerance,
  reliability, etc.
• In HTTP Servlets, the domain objects are Session
  and ServletContext
• Session are things like shopping carts, span
  multiple HTTP requests

11
SIP Domain Objects
ApplicationSession
HttpSession
Unification
SipSession
SipTransaction
ServletContext
12
Relationship to JAIN SIP

• JAIN SIP is a generic, low-level interface for
  accessing SIP services
• Can be used in
• Clients
• Servers
• Gateways
• Focuses purely on the protocol
• Complete access to SIP capabilities
• Supports transactions only
• SIP Servlet Container is a particular application
  of JAIN SIP

Servlet
Servlet
SIP Servlet API
SIP Servlet Container
JAIN SIP
SIP Protocol
13
Relationship to JAIN SIP

• Servlets focus on high volume carrier grade
  servers
• Add significant, non-SIP protocol functions
• Lifecycle management
• Domain objects
• Context and configuration
• Deployment descriptors
• Archive files
• Synchronization primitives
• Security
• Add significant SIP protocol functions
• Construction of requests and responses from
  domain objects

• Hide many parts of JAIN SIP
• Direct access to many headers is not provided
• Write access to most everything is often
  restricted
• Servlets should be defined to allow a SIP
  container to be built using JAIN SIP
• SIP Objects in Servlet API defined with
  interfaces that match JAIN SIP signatures
• Cannot directly expose JAIN SIP objects, though

14
Information Resource
Jonathan Rosenberg 1.973.952.5000jdrosen_at_dynamic
soft.com