Mobile Web Mechanics

Mobile Application Design and Development [./]
Spring 2010 — INFO 152 (CCN 42504)

Erik WildeUC Berkeley School of Information, UC Berkeley School of Information
2010-01-27

[http://creativecommons.org/licenses/by/3.0/]

This work is licensed under a CC
Attribution 3.0 Unported License [http://creativecommons.org/licenses/by/3.0/]

Contents Erik Wilde: Mobile Web Mechanics

Contents

2Abstract
1 Current Events
- 4iPad or iSlate?
- 5Bigger is Better
2 Connectivity
- 7Networking Capabilities
- 8Cell Phone Tower
- 9Cell Phone Traffic
- 10Internet Connectivity
- 11Where Does It Go?
- 12Static Internet Configuration
3 Transport
- 14Web Communications
- 3.1 End-to-End Transport
  - 16Establishing Connectivity
  - 17Secure Connections
- 3.2 Intermediaries
  - 19Proxies
  - 20User Agent Proxies
  - 21Reverse Proxies
  - 22Provider Proxies
23Conclusions

Erik Wilde: Mobile Web Mechanics

(2) Abstract

In this lecture, we look at the mechanics that are required to make Web-based mobile applications possible. This starts with basic networking capabilities, the question how individual devices establish Internet connectivity, and how the devices then communicate with the next hop in the communications chain. Based on this connectivity, we look at what it takes to establish a Web connection, how clients and servers are connected (how the data is actually routed), and how this can be influenced by intermediaries. Finally, we look in more detail at the endpoints of such a conversation, the browser and the server. How does a browser on a mobile device access the network, and how does a server receiving a request route it to the appropriate logic for processing it? We will discuss many of these in more detail later, but this lecture presents the big picture of how they all work together to make the mobile Web work.

Current Events

Outline (Current Events)

Current Events [2]
Connectivity [6]
Transport [7]
1. End-to-End Transport [2]
2. Intermediaries [4]

Current Events Erik Wilde: Mobile Web Mechanics

(4) iPad or iSlate?

Current Events Erik Wilde: Mobile Web Mechanics

(5) Bigger is Better

Connectivity

Outline (Connectivity)

Current Events [2]
Connectivity [6]
Transport [7]
1. End-to-End Transport [2]
2. Intermediaries [4]

Connectivity Erik Wilde: Mobile Web Mechanics

(7) Networking Capabilities

Mobile/Cell phone technologies
- CDMA [http://en.wikipedia.org/wiki/CDMA] as the main technology in the U.S.
- GSM [http://en.wikipedia.org/wiki/GSM] as the main technology everywhere else
- 3G [http://en.wikipedia.org/wiki/3G] for faster speeds (W-CDMA/UMTS [http://en.wikipedia.org/wiki/W-CDMA_%28UMTS%29] or CDMA2000 [http://en.wikipedia.org/wiki/CDMA2000])
Several sub-technologies and sub-generations developed
- GPRS [http://en.wikipedia.org/wiki/General_Packet_Radio_Service] for GSM networks (often referred to as 2.5G)
- EDGE [http://en.wikipedia.org/wiki/Enhanced_Data_Rates_for_GSM_Evolution] for GSM networks (often referred to as 2.75G)
- EV-DO [http://en.wikipedia.org/wiki/Evolution-Data_Optimized] for CDMA2000 networks
Devices often are limited to one family of standards
- iPhone started as a GSM phone and there is no CDMA version
- Kindle started with EV-DO and needed to be adapted for Europe
Internet Protocols are able to bridge the network gaps
- that's the reason why it is called Internet
- Internet protocols work in (almost all) possible network worlds

Connectivity Erik Wilde: Mobile Web Mechanics

(8) Cell Phone Tower

Connectivity Erik Wilde: Mobile Web Mechanics

(9) Cell Phone Traffic

Cell sizes vary between less than a mile and 30-45 miles
- sometimes limited by technology design (timing issues)
- always limited by the low-power signals emitted by devices
Cell towers forward traffic into the carrier network
- in many cases using land lines (fiber optics)
- in remote locations via specialized radio relays
Uncovered areas can be covered by satellite service
- world-wide network coverage for mobile devices
- expensive and requires dedicated satellite equipment

Connectivity Erik Wilde: Mobile Web Mechanics

(10) Internet Connectivity

Mobile carriers handle traffic inside their networks
- network call traffic never has to leave the carrier network
- data traffic and inter-network calls have to leave the network
Joining the Internet means to connect to a connected gateway
- mobile carriers often have few central gateway locations
- all mobile Internet traffic passes through these locations
Technically, mobile networks work very much like company Intranets
- a centrally managed and organized infrastructure
- carriers can enforce their own rules and regulations within this network

Connectivity Erik Wilde: Mobile Web Mechanics

(11) Where Does It Go?

Internet connectivity needs a few essential configurations
- IP address [Geolocation; Internet Addressing (1)] assigned to the device (global or local/NAT)
- subnet mask (identification of directly reachable hosts)
- gateway to be addressed for outgoing traffic
- DNS server(s) for domain name lookups
- optionally, other servers such as time servers
These configurations can be made dynamically or statically
- static configuration often is used for desktop/server computers
- DHCP [http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol] is used for most other computers/devices

Connectivity Erik Wilde: Mobile Web Mechanics

(12) Static Internet Configuration

Transport

Outline (Transport)

Current Events [2]
Connectivity [6]
Transport [7]
1. End-to-End Transport [2]
2. Intermediaries [4]

Transport Erik Wilde: Mobile Web Mechanics

(14) Web Communications

All Web traffic uses HTTP as application protocol
- a common language for request/response interactions
Some Web traffic needs an additional layer of security
- most Web traffic uses unencrypted HTTP traffic
- some applications need security/privacy and use a security layer (SSL/TLS)
HTTP as the common language allows for various optimizations
- traffic can be inspected and handled by Intermediaries [Intermediaries (1)]
- intermediaries can to inspection, optimization, or even conversion

End-to-End Transport

Outline (End-to-End Transport)

Current Events [2]
Connectivity [6]
Transport [7]
1. End-to-End Transport [2]
2. Intermediaries [4]

End-to-End Transport Erik Wilde: Mobile Web Mechanics

(16) Establishing Connectivity

HTTP traffic need a transport infrastructure
- clients open connections for sending HTTP traffic
- HTTP is a set of conventions regulating traffic over the transport
Any Internet host can establish Internet connections
- IP is responsible for sending packets from and to computers
- TCP is responsible for handling connections between programs
Routing on the Internet is a complex process
- backbone routers routinely exchange large routing tables
- Internet traffic always is packet-based, each IP packet travels alone
- IP is packet-based for robustness

End-to-End Transport Erik Wilde: Mobile Web Mechanics

(17) Secure Connections

Intermediaries

Outline (Intermediaries)

Current Events [2]
Connectivity [6]
Transport [7]
1. End-to-End Transport [2]
2. Intermediaries [4]

Intermediaries Erik Wilde: Mobile Web Mechanics

(19) Proxies

HTTP is the language of the Web
Clients may want to use proxies for a variety of reason
- proxies can make the Internet a more secure place (content filtering)
- proxies can make the Internet a safer place (content filtering)
- proxies can make the Internet faster (caching)
- proxies can allow access to controlled content (library proxy)
- proxies can be required by company policies (firewalls/logging)
Proxies are HTTP intermediaries and exist in different configurations

Intermediaries Erik Wilde: Mobile Web Mechanics

(20) User Agent Proxies

Configured by the client
Useful if a client knowingly wants to delegate HTTP requests
Can be global or for certain domains only
- library proxies must be used for accessing publisher sites

Intermediaries Erik Wilde: Mobile Web Mechanics

(21) Reverse Proxies

Configured by service providers
Clients connect to the server (they think)
- server-side configuration redirects traffic to the reverse proxy
Allows sophisticated server-side load balancing
Similar goals can be attained with advanced DNS resolution
- resolve the same DNS name to different IP address [Geolocation; Internet Addressing (1)]es
- allows completely different IP addresses for the same server name

Intermediaries Erik Wilde: Mobile Web Mechanics

(22) Provider Proxies

Configured by Internet providers
- intercepting traffic on various levels
- useful for monitoring/filtering/augmentation/transcoding
Monitoring allows providers to track Internet traffic
Filtering can be based on connections or content
Augmentation can add information (e.g., user location)
Transcoding can change content (e.g., adapting to mobile browsers)

Erik Wilde: Mobile Web Mechanics

(23) Conclusions

Mobile Web requests need a complex infrastructure
We will look at some aspects in greater detail
Understanding the general mechanics is important
Layering allows us to hide certain details
- Web developers should understand things down to TCP/SSL

2010-01-27 Mobile Application Design and Development [./]
Spring 2010 — INFO 152 (CCN 42504)