Design and Implementation of a WAP Gateway

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Design and Implementation of a WAP Gateway

• A Masters thesis
• by
• Lars Wirzenius
• CSCI 5939.02 Independent study
• Fall 2002
• Presented by Obaidullah Khan

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Chapter 1 Introduction

• Catching up with science fiction Star treck,
  Imperial Earth
• Evolution of mobile phone, laptop, and then
  palmtops
• Vision of mobile phone as web browsers
• WAP technology enable mobile phone acts as
  hypertext browsers

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What is covered in this masters thesis

• Kannel an open source WAP gateway
• Problems
• Design
• Technical issues
• Project management issues
• Kannel project started in June 1999, by Wapit Ltd

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Chapter 2

• Problems in Implementing Services for Mobile
  Phones

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Technical issues

• Low battery capacity
• Small screen and keyboard
• Limited bandwidth and high error rate

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Business issues

• Leverage on existing mobile phone networks
• Must be adaptable to future mobile networks
• Standardized for interoperability and mass market
• Allows easy user interface
• Allow billing

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Pre-WAP solution

• Normal voice calls
• Short textual messages (SMS)
• These two works fine in current networks and
  billing infrastructure exists.
• Fetching normal HTTP pages on GSM phone simply
  not feasible.

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Chapter 3 The Wireless Application Protocol

• History of WAP
• The goals of WAP
• Introduction to WML and WMLScript
• Current status of WAP

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WAP and WAP Forum

• History
• WAP Forum founded in June 1997 by Nokia,
  Ericsson, Motorola, and Unwired Planet
• 1.0 specification in April 1998, but not
  implemented
• 1.1 specification in July 1999, first to be
  implemented in phones

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WAP and WAP Forum (contd)

• Goals
• Develop an open, freely licensed specification
• The specification not tied to any network
  technology or any specific device

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The WAP architecture

• Figure 3.1. WAP architecture

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The WAP architecture (contd)

• WAP Protocol
• In binary and compressed form
• Reduces the protocol overheads
• WAP Gateway
• Protocol translation between phone and server
• Compresses WML pages to save bandwidth

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WML and WMLScript

• WML
• A simple mark up language defined in XML
• A page is a deck of cards
• WMLScript
• Based on ECMAScript or JavaScript
• Makes WAP more dynamic
• Provides libraries for controlling phone
  functionalities

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WML and WMLScript (contd)

•  

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The WAP protocol stack

• Consist of three layers
• Wireless Datagram Protocol (WDP)
• Wireless Transaction Protocol (WTP)
• Wireless Session Protocol (WSP)

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The WAP protocol stack (contd)

• Figure 3.2. A representative WAP session

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The duties of a WAP gateway

• Implement WAP protocol stack
• User authentication and billing
• Optimize WAP usage keep cost down while
  utilizing bandwidth wisely

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Chapter 4 The Kannel Open Source WAP Gateway

• This section covers the design and implementation
  of Kannel WAP gateway

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Introduction and status of the project

• Why it was started?
• Wapit needed a gateway
• When?
• July 1999 at WAP Forum in San Francisco

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Introduction and status of the project (contd)

• Goals?
• A technically good enough gateway for small
  operator / service providers
• Serve thousand of concurrent users at reasonable
  price
• Kannel to be a WAP gateway as well as an SMS
  gateway
• Be scalable
• Crash or failure of one node should not affect
  others

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Gateway architecture

• Discusses requirements
• Division of gateway duties to processes
• - bearer, wap, sms boxes
• Duties of each process

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Gateway architecture (contd)

• External interface of the gateway
• SMS center, using various protocols
• HTTP servers, to fetch WAP / SMS content
• WAP phones, using WAP protocol stack
• Figure 4.1. External interfaces of Kannel

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Gateway architecture (contd)

• SMS protocol essentials
• Client logs into SMS center
• SMS center sends a message as it arrives, client
  should acknowledge it
• Client send a request for message and SMS center
  acknowledge it
• When client is done, it logs out
• Only one connection to an account at a time

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Gateway architecture (contd)

• HTTP protocol essentials
• Client connect to server
• Client sends a request
• Server respond and complete transaction
• Multiple request over same connection are
  possible

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Gateway architecture (contd)

• Achieve maximum throughput
• By Multitasking internally
• By using internal queues
• Reliability problem
• What if Kannel crashes when there are request in
  internal queues?
• Solution
• Kannel should keep the lists on disc, but it does
  not do so

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Gateway architecture (contd)

• Division of duties to processes the boxes
• The bearerbox Implements the WDP layer, and
  connect to SMS center
• The smsbox Implements the SMS gateway
• The wapbox Implements the WAP stack

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Gateway architecture (contd)

• Figure 4.2. Boxes of Kannel
• Only one bearerbox but multiple smsboxses
  wapboxes
• Each box is internally multithreaded
• Static thread structure

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Gateway architecture (contd)

• Heartbeats
• Each box sends I am still alive messages
• The bearerbox keeps track of heartbeats of all
  boxes
• Bearerbox closes connection, if heartbeat is not
  receives for long time
• Heartbeat serves as load factor, which helps
  bearerbox in routing packets among boxes

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The Bearer Box

• What it does?
• Receives UDP messages from phones
• Send these messages to wapboxes
• Receives reply from wapboxes
• Sends the UDP message back to phones
• Only UDP bearer for WDP is supported

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The Bearer Box (contd)

• Figure 4.3. Bearerbox architecture

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The Bearer Box (contd)

• Bearerbox routes the UDP packets to wapboxes
• Phones are allocated IP dynamically
• All messages from same IP are sent to same wapbox
• The bearerbox must know when a session or
  transaction finishes
• Multiple queues within the bearerbox
• Bearerbox balance the load among wapboxes using
  load factor

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The WAP Box

• What it does?
• Wapbox reads messages from bearerbox, maintains
  internal states for each client, and makes HTTP
  requests for clients
• Only WTP and WSP are implemented
• WTLS is not implemented in this thesis

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The WAP Box (contd)

• Figure 4.4. Wapbox thread structure

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The WAP Box (contd)

• WAP protocol stack layer is implemented in its
  thread
• Communication between threads is via message
  queues
• Each layer exposes only event data structures
  simplified code, execution is faster
• Static thread structure starts at program
  startup and keeps running
• Each layer extracts the event, process it, and
  sends other events to other layers
• Locking is needed only when accessing the event
  queue for each layer

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Implementation of protocol state machine

• WTP and WSP ( connection mode) are implemented in
  term of state machine
• Macro language ( the C processor) is used to
  describe state machine in source code

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Efficient implementation of HTTP request

• WAP gateway must implement HTTP request
  efficiently at high usage level
• Implementing each HTTP request in a separate
  thread would be expensive
• Implement HTTP as a static number of threads
  for performance

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Efficient implementation of HTTP request (contd)

• Basic steps in HTTP request
• IP number look-up for the HTTP server delay in
  DNS query
• Open a TCP connection may takes some time if
  network is congested or due to slow server
• Write the request if the request is large it may
  takes more than one TCP packet
• Read the reply it can takes several seconds
• Close the connection this should be fast

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Efficient implementation of HTTP request (contd)

• More than one thread requires scheduling, and
  context switching at wake-up
• Having one thread eliminate context switching
  overhead
• Unix system call select and poll are used, in
  single thread mode, to wait for input from
  several sources

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Efficient implementation of HTTP request (contd)

• What a thread does?
• Wait until there is something to read
• Read it into a connection specific buffer
• If the buffer has a complete HTTP reply, return
  it to whoever made the request
• Repeat forever

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Making concurrent domain name lookups

• DNS maps textual domain name into IP number
• Problem
• The gethostbyname function in C does not support
  concurrency
• Solution
• Implement DNS protocol within gateway Or
• Run sub-processes to do gethostbyname calls
• Single calls at a time, but multiple
  sub-processes provides concurrency
• Security risk

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Converting WML and WMLScript to binary

• Used two simple application of complier theory
  for conversion to binary form
• Compiler gets as input the WML source code and
  character set from HTTP header
• Return a binary form of WML deck or error message
• WML complier utilizes a string table to compress
  the output
• WMLScript compiler parse the input, form a parse
  tree, optimize the tree, then generate byte code,
  and optimize again

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Chapter 5 Experiences From this Implementation

• Subjective evaluation
• First installation in Sep. 1999, with some bugs
  to fix
• Fixing the bugs slowed the development but
  improved the quality
• Sufficient quality in order to succeed
• Setup a nag script successfully compile
  remotely and report errors to developers
• Automatic test cases, added later

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Experiences From this Implementation (contd)

• Effect of open source project
• Ever changing group of developers
• More varied testing more compatible
• More people doing debugging
• Simple source code and program structure
• More time spent on email communication

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Chapter 6 Plans for the Future

• New features
• WAP security layer (WTLS)
• WAP push technology
• Using SMS messages as a bearer for WAP

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Plans for the Future (contd)

• Better quality
• Improved security, reliability, and speed
• Over feeding of data causes it to run out of
  memory and crash
• Store SMS messages in persistent memory
• Should allow push feature
• Migrating job between SMS box or WAP box might
  improve performance