Intro%20Wireless%20Application%20Protocol

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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 framework and
  network protocols for wireless devices such as
  mobile telephones, pagers, and personal 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
• WTLS
• WTP
• 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, and session
  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 packets to
  different higher layers
• Segmentation/Reassembly only present if
  underlying bearer does not support it already
• Details More than you ever want to know about
  the differences between some 30 kinds 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 underlying network
  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 of WAP with
  a secure transport service interface that
  preserves the transport service 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 Hello
Server says HelloServer sends Public
KeyInitiates 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 usually
  implemented on top of TCP WTLS includes
  retransmission mechanism
• Client/Server can authenticate each other
  (optional)
• Client/Server can negotiate ciphers to be used
• 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 protocol
  necesaary for interactive "browsing"
  applications.
• T stands for Transaction, not transport.
• WTP is message-oriented protocol, not
  stream-based.
• Each new transaction has a new Transaction ID (
  seq no.)
• Provides 3 classes of service

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Class 0 (Unreliable 1-way)

• Sender sends a message but does not wait for an
  ack
• Receiver delives message to application w/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 an ack (retx
  if necessary)
• Receiver checks for duplicates before delivering
  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 an ack (retx
  if necessary)
• Receiver checks for duplicates before delivering
  message to application
• Ack for request is piggy-backed on top of the
  reply from the server application
• Client acks the receipt of the result

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WTP Duplicate Detection

• Since there is no handshake, how does it detect
  duplicates?
• For class 0, duplicate packets are delivered 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
Ack TID 1
Timeout
Class 1, TID 1
Verify TID 1
Abort
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WTP Duplicate Detection
Client
Server
Class 1, TID 1
Ack TID 1
Class 0, TID 2
Class 0, TID 3
Class 0, TID N
Class 1, TID 1
Verify TID 1
Ok
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WTP Other interesting features

• User-level acks (more precisely,
  application-level acks)
• May do Segmentation and Reassembly
• Allow 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 over same
  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 and duplicate
  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.