Intro Wireless Application Protocol WebTP Meeting H. Wilson So 14 Feb, 2000.

Outline zWhat is Wireless Application Protocol? zArchitecture Overview (layers) zBottom up description of the stack zRelation to WebTP and differences

What is WAP? z"WAP specifies an application framework and network protocols for wireless devices such as mobile telephones, pagers, and personal digital assistants (PDAs)." zWAP is the mobile phone industry’s answer to interactive web applications. zWAP defines its own set of protocols but models after existing web protocols.

WAP and the Web zGSM, CDMA, etc. zWDP zWTLS zWTP zWML zWML Script z IP z UDP z TLS (from SSL) z no counterpart z HTML z JavaScript Rationale: reuse as much as possible from IP world, but optimize for the wireless world (i.e. compression, adapt to high-loss rate.)

Accessing Web from Cell Phones

WAP Protocol Architecture

Protocol Features

WAP Reference Model

1. Datagram Protocol (WDP) zGoal: To allow transport, security, and session protocols to operate independent of the underlying bearer (e.g. GSM, CDMA, etc.) zTo provide a UDP-like interface

WDP Basic Features zPort Numbers: Demux lower layer packets to different higher layers zSegmentation/Reassembly: only present if underlying bearer does not support it already zDetails: More than you ever want to know about the differences between some 30 kinds of cellular data network.

WDP Summary zIf bearer (e.g. GSM USSD) speaks IP, WDP equals UDP. zOtherwise, WDP adapts to the underlying network and provides Demux & Segmentation normally provided by UDP/IP. zReliable transport builds on top of WDP, not the underlying bearer even if bearer speaks IP

2. WTLS (Transport Layer Security) zGoal: provides the upper-level layer of WAP with a secure transport service interface that preserves the transport service interface (datagram) below it. zSecurity level of WTLS depends on the requirements of the given application

WTLS Handshake

Handshake Simplified Client says Hello Server says Hello Server sends Public Key Initiates key exchange Client generates Session Key Server acks session key Encrypted Data

WTLS Summary zVery similar to SSL, but SSL is usually implemented on top of TCP; WTLS includes retransmission mechanism zClient/Server can authenticate each other (optional) zClient/Server can negotiate ciphers to be used zClient/Server can negotiate a “datagram mode” that has no sequence numbers

3. WTP (Transaction Protocol) zGoal: A light-weight transaction protocol necesaary for interactive "browsing" applications. zT stands for Transaction, not transport. zWTP is message-oriented protocol, not stream-based. zEach new transaction has a new Transaction ID (~ seq no.) zProvides 3 classes of service

Class 0 (Unreliable 1-way) zSender sends a message but does not wait for an ack zReceiver delives message to application w/o checking for duplicates zNo acks are sent zSample app: unreliable weather update (push)

Class 1 (Reliable 1-way) zSender sends a message and waits for an ack (retx if necessary) zReceiver checks for duplicates before delivering message to application zAcks are sent and retransmitted if client retransmits request erroneously

Class 2 (Reliable 2-way) zSender sends a message and waits for an ack (retx if necessary) zReceiver checks for duplicates before delivering message to application zAck for request is piggy-backed on top of the reply from the server application zClient acks the receipt of the result

WTP Duplicate Detection zSince there is no handshake, how does it detect duplicates? zFor class 0, duplicate packets are delivered with checking. zServer initiates handshake to verify (and resynchronize TrasactionID if necessary)

WTP Duplicate Detection ClientServer Class 1, TID =1 Timeout Ack TID = 1 Verify TID = 1 Abort

WTP Duplicate Detection ClientServer 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

WTP: Other interesting features zUser-level acks (more precisely, application-level acks) zMay do Segmentation and Reassembly zAllow multiple messages (PDUs) to be concatenated into one SDU (link-layer frames) zRe-transmit bit to distinguish fresh vs. retransmitted packets

Relation to WebTP zWAP uses a transaction oriented protocol, WebTP advocates ADUs zWAP allows per PDU-level reliability over same connection zWAP does not require handshake, WebTP has a Fast WebTP option. zUnclear: congestion / flow control in WAP

Areas need to be improved zA tradeoff between connection setup and duplicate detection is required, but analysis is missing. zWAP often talks about “keeps states for some time” but never really discuss why their system is correct.