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Intro Wireless Application

Protocol

WebTP Meeting

H. Wilson So14 Feb, 2000.

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Outline

What is Wireless Application Protocol?

Architecture Overview (layers)

Bottom up description of the stack

Relation to WebTP and differences

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What is WAP?

"WAP specifies an application frameworkand network protocols for wireless devices

such as mobile telephones, pagers, andpersonal digital assistants (PDAs)."

WAP is the mobile phone industrys

answer to interactive web applications.WAP defines its own set of protocols but

models after existing web protocols.

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WAP and the Web

GSM, CDMA, etc.

WDP

WTLSWTP

WML

WML Script

IP

UDP

TLS (from SSL)no counterpart

HTML

JavaScript

Rationale: reuse as much as possible from IP world,

but optimize for the wireless world (i.e. compression,

adapt to high-loss rate.)

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Accessing Web from Cell

Phones

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WAP Protocol Architecture

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Protocol Features

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WAP Reference Model

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1. Datagram Protocol

(WDP)

Goal: To allow transport, security, andsession protocols to operate independent

of the underlying bearer (e.g. GSM,CDMA, etc.)

To provide a UDP-like interface

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WDP Basic Features

Port Numbers: Demux lower layer packetsto different higher layers

Segmentation/Reassembly: only present ifunderlying bearer does not support italready

Details: More than you ever want to knowabout the differences between some 30kinds of cellular data network.

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WDP Summary

If bearer (e.g. GSM USSD) speaks IP,WDP equals UDP.

Otherwise, WDP adapts to the underlyingnetwork and provides Demux &Segmentation normally provided by

UDP/IP.Reliable transport builds on top of WDP,

not the underlying bearer even if bearer

speaks IP

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2. WTLS (Transport Layer

Security)

Goal: provides the upper-level layer ofWAP with a secure transport service

interface that preserves the transportservice interface (datagram) below it.

Security level of WTLS depends on the

requirements of the given application

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WTLS Handshake

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Handshake Simplified

Client says HelloServer says Hello

Server sends Public Key

Initiates key exchange

Client generates

Session Key

Server acks session

key

Encrypted Data Encrypted Data

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WTLS Summary

Very similar to SSL, but SSL is usuallyimplemented on top of TCP; WTLS

includes retransmission mechanismClient/Server can authenticate each other

(optional)

Client/Server can negotiate ciphers to beused

Client/Server can negotiate a datagram

mode that has no sequence numbers

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3. WTP (Transaction

Protocol)

Goal: A light-weight transaction protocolnecesaary for interactive "browsing"applications.

T stands for Transaction, not transport.

WTP is message-oriented protocol, notstream-based.

Each new transaction has a newTransaction ID (~ seq no.)

Provides 3 classes of service

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Class 0 (Unreliable 1-way)

Sender sends a message but does notwait for an ack

Receiver delives message to applicationw/o checking for duplicates

No acks are sent

Sample app: unreliable weather update(push)

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Class 1 (Reliable 1-way)

Sender sends a message and waits for anack (retx if necessary)

Receiver checks for duplicates beforedelivering message to application

Acks are sent and retransmitted if client

retransmits request erroneously

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Class 2 (Reliable 2-way)

Sender sends a message and waits for anack (retx if necessary)

Receiver checks for duplicates beforedelivering message to application

Ack for request is piggy-backed on top of

the reply from the server applicationClient acks the receipt of the result

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WTP Duplicate Detection

Since there is no handshake, how does itdetect duplicates?

For class 0, duplicate packets aredelivered with checking.

Server initiates handshake to verify (and

resynchronize TrasactionID if necessary)

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WTP Duplicate Detection

Client Server

Class 1, TID =1

Class 1, TID =1

TimeoutAck TID = 1

Verify TID = 1

Abort

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WTP Duplicate Detection

Client Server

Class 1, TID =1

Class 0, TID =2

Ack TID = 1

Verify TID = 1

Ok

Class 0, TID =3

Class 0, TID =N

Class 1, TID =1

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WTP: Other interesting

features

User-level acks (more precisely,application-level acks)

May do Segmentation and ReassemblyAllow multiple messages (PDUs) to be

concatenated into one SDU (link-layer

frames)Re-transmit bit to distinguish fresh vs.

retransmitted packets

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Relation to WebTP

WAP uses a transaction oriented protocol,WebTP advocates ADUs

WAP allows per PDU-level reliability oversame connection

WAP does not require handshake, WebTP

has a Fast WebTP option.Unclear: congestion / flow control in WAP

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Areas need to be improved

A tradeoff between connection setup andduplicate detection is required, but

analysis is missing.WAP often talks about keeps states for

some time but never really discuss why

their system is correct.