1 Design and Implementation of a WAP Gateway A Master’s thesis by Lars Wirzenius CSCI – Independent study Fall 2002 Presented by: Obaidullah Khan

2 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

3 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

4 Chapter 2: Problems in Implementing Services for Mobile Phones

5 Technical issues Low battery capacity Small screen and keyboard Limited bandwidth and high error rate

6 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

7 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.

8 Chapter 3: The Wireless Application Protocol  History of WAP  The goals of WAP  Introduction to WML and WMLScript  Current status of WAP

9 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

10 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

11 The WAP architecture Figure 3.1. WAP architecture

12 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

13 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

14 WML and WMLScript (cont’d) WML pages at content server WAP Gateway Phonetextual form Source code binary form

15 The WAP protocol stack  Consist of three layers Wireless Datagram Protocol (WDP) Wireless Transaction Protocol (WTP) Wireless Session Protocol (WSP)

16 The WAP protocol stack (cont’d) Figure 3.2. A representative WAP session

17 The duties of a WAP gateway  Implement WAP protocol stack  User authentication and billing  Optimize WAP usage – keep cost down while utilizing bandwidth wisely

18 Chapter 4: The Kannel Open Source WAP Gateway This section covers the design and implementation of Kannel WAP gateway

19 Introduction and status of the project  Why it was started? Wapit needed a gateway  When? July 1999 at WAP Forum in San Francisco

20 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

21 Gateway architecture  Discusses requirements  Division of gateway duties to processes - bearer, wap, sms boxes  Duties of each process

22 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

23 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

24 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

25 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

26 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

27 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

28 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

29 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

30 The Bearer Box (cont’d) Figure 4.3. Bearerbox architecture

31 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

32 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

33 The WAP Box (cont’d) Figure 4.4. Wapbox thread structure

34 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

35 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

36 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

37 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

38 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

39 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

40 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

41 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

42 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

43 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 communication

44 Chapter 6: Plans for the Future  New features WAP security layer (WTLS) WAP push technology Using SMS messages as a bearer for WAP

45 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