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Design and Implementation of a WAP Gateway

A Master’s thesisby

Lars Wirzenius

CSCI 5939.02 – Independent studyFall 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 master’s 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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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

Business issues

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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 (cont’d)

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 (cont’d)

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 (cont’d)

 WML pages atcontent server

WAPGateway

Phonetextual form

Source code

binary

form

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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 (cont’d)

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 (cont’d)

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 (cont’d)

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 (cont’d)

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 (cont’d)

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 (cont’d)

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 (cont’d)

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 (cont’d)

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 (cont’d)

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 (cont’d)

Figure 4.3. Bearerbox architecture

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The Bearer Box (cont’d)

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 (cont’d)

Figure 4.4. Wapbox thread structure

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The WAP Box (cont’d)

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 (cont’d)

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 (cont’d)

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 (cont’d)

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 (cont’d)

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 (cont’d)

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