1. EE 122: Network Applications
Kevin Lai
Aug 28, 2002

2. Network Functionality
Network functionality is only useful if it benefits users
also applies to any computer system, but easier to forget in networking
e.g., Is it worth it to upgrade a 56Kb/s modem link in your network to 100Mb/s Ethernet? Only if it significantly improves user experience
Generalization of Amdahl’s Law from computer architecture
Also applies to metrics other than time

3. Network Applications how networks affect users
buy stock in networking companies, work for networking companies
use network applications
need to understand application requirements
to build useful networks
to understand why networks are built the way they are
to understand why some ideas failed

4. Different Network Applications
telephony (making a phone call)
sending
web browsing
buying something
reading news
sending instant messages
file sharing
playing a distributed game
watching/listening to streaming media

5. Requirements of the Network
throughput
latency
cost
compatibility with existing equipment usually lowers cost
qualifications
distribution: mean, worst-case
in small bursts / all the time
ease of programmability
routing
unicast, multicast, broadcast

6. Web Browsing Network Requirements
Transfer 1KB-100KB files
e.g., HTML file, JPEG image
Depends on size of content at sites visited
News oriented sizes (e.g.,
Average throughput 32Kb/s – 1Mb/s
Average latency 100 – 250ms
Average loss < 10%
Flat rate $20-$50/month

7. Telephony Network Requirements
Stream data at 9.6Kb/s – 128Kb/s
an encoded audio signal
Wired
Worst case throughput 64Kb/s – 128Kb/s
Worst case latency 100ms – 125ms
Loss < 1%
$.01-$.10/minute
Mobile
Worst case throughput 9.6Kb/s – 128Kb/s
Worst case latency 100ms – 250ms
Loss < 10%
$.10-$1/minute

8. File Sharing Requirements
Transfer 3MB – 600MB file
MP3 audio file, MPEG video file
Inbound throughput 128Kb/s – 10Mb/s
Outbound throughput 128Kb/s – ? Mb/s
Flat rate $20-$50/month

9. Distributed Game Requirements
e.g., WarCraft III, Quake III, EverQuest
unlimited number of players
stream data about player’s state
e.g., location in the world, appearance, items carried, units controlled, etc.
Throughput 128Kb/s – ? Mb/s
Latency 0ms – 250ms
Flat rate $20-$50/month

10. Comparing Applications
Research community consistently failed to predict next popular application
predicted: telnet actually:
predicted: ftp actually: WWW
predicted: IP telephony actually: instant messaging
predicted: streaming actually: file sharing
predicted: streaming maybe: distributed gaming
Otherwise, little uniformity in application requirements

11. Designing for Requirements
Given different application requirements, how to design network that can run different applications well?
define network service model
implement service model on physical technology
satisfy service model when multiple applications and users are sharing network
replace obsolete service model

12. Network Service Model
Specifies what the network does for an application
leaves the rest to the application
Examples
Establishes a circuit that “guarantees” 1Mb/s from one node to another for $.10/s
regardless of other traffic
Deliver a packet from one node to another with a delay of at most 100ms for $.10/Mb
Deliver a packet with unknown delay for $.10/Mb, but less delay than people paying $.05/Mb
Deliver a packet with unknown delay, maybe
Which model satisfies which application?
What model do existing networks provide?

13. Service Model Philosophies
More
Functionality
Less
Functionality
Provide functionality for existing popular application
e.g., telephone network
less work for application developers
some functionality can only be provided by the network
more expensive network
per flow state
not all applications may use functionality
target applications may be replaced with new applications
Put only the most basic functionality in
e.g., internet
invert above properties

14. Service Model Philosophies
More
Functionality
Less
Functionality
-Integrated
Services
-Asynchronous
Transfer Mode (ATM)
-Best effort
delivery
-Ethernet
-Unicast
-Differentiated
Services
-Multicast
-Explicit Congestion
Notification (ECN)
Simpler has predominated (so far)
Reduced Instruction Set Computing (RISC) analogy

15. Implementing service model
How to implement service model on physical medium?
e.g., Category 5 cabling, optical fiber, radio waves
analog  digital, digital  analog
encoding
how to separate packets
framing
physical medium is subject to errors
e.g., fading, multipath, microwave ovens, sun spots, etc.
service model may specify higher reliability than physical medium provides
reliability, error detection

16. Sharing Many users of same application using network
Users of different applications using network
How to do resource allocation?
resources: bandwidth, memory, CPU cycles
Not all packets are created equal
IP telephony packet must be delivered with low delay
File sharing packet can be delayed
Not all users created equal
some users pay more
some users follow rules
some do not for personal gain (selfish users)
some do not just to be mean (malicious users)

17. Replacing Network Model
Every service model developed so far has eventually become obsolete
service model optimizes for a particular application mix
application mix changes
Network is much harder to upgrade than other systems
bad news: 100’s M nodes  full upgrade takes decades
good news: use old network to bootstrap new network
e.g., first Internet nodes used phone lines
How to implement new network service model on top of existing network
solution: overlay networks