Mobility and Routing
Finding a path from a source to destination
Issues
- Frequent route changes: amount of data transferred between route changes may be much smaller than traditional networks
- Route changes related to host movement
Goal of routing protocols
- decrease routing-related overhead
- find short routes
- find stable routes
- Mobile IP would need to modify the previous hand-off procedure to inform the home agent the identity of the new foreign agent
- Triangular optimization can reduce the routing delay
route directly to foreign agent, instead of via home agent
- Transport protocols typically designed for fixed end-systems, wired networks.
Issues - packet loss due to wireless characteristics
- packet loss due to mobility
- TCP assumes congestion if packet dropped
- acks, retransmissions and performance
- TCP cannot be changed fundamentally
I-TCP segments the connection:
- no changes to the TCP protocol for hosts connected to the wired Internet
- optimized TCP protocol for mobile hosts
- splitting of the TCP connection at, e.g., the foreign agent into 2 TCP connections, no real end-to-end connection any longer
- hosts in the fixed part of the net do not notice the characteristics of the wireless part
- no changes in the fixed network necessary
- transmission errors on the wireless link do not propagate into the fixed network
- simple to control, mobile TCP is used only for one hop between, e.g., a foreign agent and mobile host
- loss of end-to-end semantics
- higher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agent
Application Adaptations for Mobility:
System-transparent, application-transparent
- the conventional, unaware client/server model
System-aware, application-transparent
- the client/proxy/server model
- the disconnected operation model
System-transparent, application-aware
- dynamic client/server model
System-aware, application-aware
- the mobile agent model
The Client/Proxy/Server Model :
- Proxy functions as a client to the fixed network server, and as a mobility-aware server to the mobile client
- Proxy may be placed in the mobile host (Coda), or the fixed network, or both (WebExpress)
- Enables thin client design for resource-poor mobile computers
The Mobile Agent Model
- Mobile agent receives client request and moves into fixed network
- Mobile agent acts as a client to the server
- Mobile agent performs transformations and filtering
- Mobile agent returns back to mobile platform, when the client is connected
Mobile Data Management
- Pull data delivery: clients request data by sending uplink msgs to server
- Push data delivery: servers push data (and validation reports) through a broadcast channel,to a community of clients
- Client caching strategies and cache invalidation algorithms are critical
World Wide Web and Mobility:
§HTTP and HTML have not been designed for mobile applications/devices
HTTP Characteristics
stateless, client/server, request/response
connection oriented, one connection per request
primitive caching and security
HTML Characteristics
designed for computers with high performance, color high-resolution display, mouse, hard disk
typically, web pages optimized for design, not for communication; ignore end-system characteristics
HTTP and Mobility:
§HTTP:
designed for large bandwidth and low delay
big protocol headers (stateless, ASCII)
uncompressed content transfer
TCP 3-way handshake, DNS lookup overheads
Caching:
often disabled by information providers
dynamic objects, customized pages, generated on request via CGI
Security problems
how to use SSL/TLS together with proxies?
System Support for Mobile WWW:
Enhanced browsers
Client proxy
pre-fetching, caching, off-line use
Network proxy a
-adaptive content transformation for connections
Client and network proxy
Enhanced servers
HDML (handheld device markup language)
HDTP (handheld device transport protocol)
WAP - Wireless Application Protocol:
§Forum: wapforum.org
co-founded by Ericsson, Motorola, Nokia, Unwired Planet
Goals
deliver Internet services to mobile devices
independence from wireless network standards
Platforms
e.g., GSM (900, 1800, 1900), CDMA IS-95, TDMA IS-136, 3rd generation systems (IMT-2000, UMTS, W-CDMA)
WAP Overview:
Browser
micro browser, similar to existing web browsers
Script language
similar to Java script, adapted to mobile devices
Gateway
transition from wireless to wired world
Server
wap server, similar to existing web servers
Protocol layers
transport layer, security layer, session layer etc.
Wireless Markup Language (WML):
§Cards and Decks
WML document consists of many cards, cards are grouped to decks
a deck is similar to an HTML page, unit of content transmission
WML describes only intent of interaction in an abstract manner
presentation depends on device capabilities
Features
text and images
user interaction
navigation
context management
Limitations of Mobile Environment:
- ·Limitations of the Wireless Network
- ·heterogeneity of fragmented networks
·frequent disconnections ·limited communication bandwidth - ·Limitations Imposed by Mobility
- ·lack of mobility awareness by system/applications
- ·Limitations of the Mobile Computer
- ·short battery lifetime
·limited capacities
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