1. Computer Science 457 Networking and the Internet
Welcome to:
Computer Science 457 Networking and the Internet
Fall 2011
Dan Massey

2. Chapter 1: Foundations Networking is a very broad topic
How does a wireless card talk to a base station?
How does a message get from ColoState to Amazon?
How does an application (web browser, IM, P2P) work?
What about reliability, resource constraints, security?
First Goal: Learn Big Picture and Terminology
What are the basic network components?
What are network layers?
What is a protocol?

3. Use web browser to lookup www.cnn.com
A Sample Application
Use web browser to lookup
My Laptop -
Running web
browser
Internet
Web Server
My laptop and the web server are both End Systems = Hosts
End systems can also include PDAs, sensors, cell phones, and generally any device using the network to communicate
End systems are located at the network edge and connected to the network using communication links

4. Simple Example: Use web browser to lookup www.cnn.com
Clients and Servers
Simple Example: Use web browser to lookup
My Laptop -
Running web
browser
Internet
Web Server
End systems may be classified as client, a server, both, or neither.
Client - runs some program that requests services: web browser requests a page, reader requests messages, ftp program requests files, etc.
Server - runs some program that listens for requests and provides services web server, server, ftp server, etc.
Client vs. server depends on what programs the end system is running.

5. Networks
Point-to-Point …
Multiple Access …
Wireless

6. Switched Networks A network can be defined recursively as...
Two or more nodes connected by a link, or
Two or more networks connected by two or more nodes

7. Logical Network View fixed size pipe from her to him perfect for voice
reliable conversations (QoS)
provisioning, good engineering
dumb end points, smart network
evolved for 100 years (analog to digital)
Quality of Service

8. Back in the Old Days... (before packet switching)
the wire
the “router”
(Aunt Mable)
1920s telephony: circuits---a physical wire
from one end to the other

9. Then Came TDM... Time Division Multiplexing mux demux
… but keeps the idea of a
fixed pipe (circuit) the right
size for a telephone conversation

10. And FDM and CDM... Frequency Division Multiplexing
a a a a a a a a a a a
a a a a a a a a a a a
Code Division Multiplexing

11. Packet Switching Interleave packets from different sources
Efficient: resources used on demand
Statistical multiplexing
Multiple applications, bursty traffic

12. What is a Packet? Self-contained set of bits
Includes a header and (in most cases) user data (payload)
Header: needed by the network - contains control information needed to deliver the packet to the destination
User data: can be anything – network does not care

13. Example: IP Packet 20-byte header 4-bit 4-bit 8-bit Type of Header
Length
4-bit
Version
8-bit Type of
Service (TOS)
16-bit Total Length (Bytes)
3-bit
Flags
16-bit Identification
13-bit Fragment Offset
8-bit Time to
Live (TTL)
20-byte
header
8-bit Protocol
16-bit Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options (if any)
Payload

14. Packet Switching (Internet is just ONE network to use it)
differences:
packets as low-level component
multiple kinds of traffic
smart edges, dumb network
but:
QoS is much harder
end-points are more expensive

15. Statistical Multiplexing Gain
Problem: Link is 1 Mbps; users require 0.1 mbps when transmitting; users active only 10% of the time. How many users can we support?
Answer:
Circuit switching: can support 10 users.
Packet switching: with 35 users, probability that >=10 are transmitting at the same time =

16. Networking with Layers
Sub-divide the problem
Each layer relies on services from layer below
Each layer exports services to layer above
Interface between layers defines interaction
Hides implementation details
Layers can change without disturbing other layers

17. The Internet Hourglass Design
Application Layer
Protocol Layer
Transport Layer
Network Layer
Link Layer
Data Communication
Physical Layer
[Deering98]

18. IP Suite: End Hosts vs. Routers
HTTP message
HTTP
HTTP
TCP segment
TCP
TCP
router
router IP
IP packet IP
IP packet IP
IP packet IP
Ethernet
interface
Ethernet
interface
SONET
interface
Ethernet
interface
SONET
interface
Ethernet
interface

19. Characteristics of Packet Switching
Store and forward
Packets are self contained units
Can use alternate paths - reordering
Contention
Congestion
Delay

20. Putting the Network Together
The network is composed of:
Hosts or endpoints (PCs, PDAs, cell-phones, laptops, etc.)
Routers (specialized machines that route packets)
The distinction is often blurred (caches, content servers, etc.)

21. Host Configuration Host configuration needs:
a physical network cable (Ethernet, etc.)
an IP address (often automatic with DHCP)
a network mask
a gateway
a DNS server (and other servers)

22. An ISP ISP needs: a (big?) block of addresses
connections to one or more other ISPs, peerings
multiple routers, probably at exchange points
servers for your users: mail, web, etc.
servers for you: monitoring, etc.
competent network admins (recommended)
an AUP (Acceptable Use Policy)
a lawyer

23. Idealized Network Structure
Backbones, Tier 1
(national, global)
Regional, Tier2
Campus LANs,
Business

24. How Do Computers Find Each Other?
Internet

25. What Are the Different Kinds of Addresses?
Have domain name (e.g.,
Global, human readable name
DNS translates name to IP address (e.g )
Global, understood by all networks
Finally, we need local net address
e.g., Ethernet ( c-19-dc-45)
Local, works only on a particular type of link

26. Domain Naming System (DNS)
Local
DNS server
Computer 1
What’s the IP address for
It is
DNS address manually configured into OS

27. Finding Ethernet Address: Address Resolution (ARP)
Broadcast: who knows the
Ethernet address for ?
(gateway address)
Ethernet
Broadcast: I do, it is
c-19-dc-45
Ethernet

28. Sending a Packet Through the Internet
Routers send packet to next closest point H R H R H H R R R R R H R
H: Hosts
R: Routers H

29. How Do the Routers Know Where to Send Data?
Forwarding tables at each router populated by routing protocols.
Original Internet: manually updated.
Routing protocols update tables based on “cost.”
Exchange tables with neighbors or everyone.
Use neighbor leading to shortest path.