1. Lecture 2 Dr. Richard Spillman Fall 2009
NETWORKS
Lecture 2
Dr. Richard Spillman
Fall 2009

2. OUTLINE
Network Structures
Application Layer

3. Network Structures

4. The Internet For this all to work everyone must be on the same page
Local ISP
Server
Router
millions of connected computing devices: hosts, end-systems running network apps
communication links
routers: forward packets (chunks) of data thru network
protocols: control sending, receiving of messages
TCP, IP, HTTP, FTP, PPP
Home systems
Regional ISP
Company Network
For this all to work
everyone must be
on the same page

5. The Enormity of the Problem
Home systems
Local ISP
Server
Router
Regional ISP
Company Network
SUN
IBM
What we have going is a vast network consisting of different hardware and varied software and they all have to transparently communicate with each other
Lucent
MAC
Cisco
Cisco
Cisco
Gateway
Cisco
Nortel
Nortel
Dell
Dell
Silicon
Graphics HP

6. Protocols
To make this all work, everything must follow a common protocol
Define the format
Order of messages sent and received
Actions taken on message transmission or receipt
Request connection
Request response

7. The Network Core
Home systems
Local ISP
Server
Router
Regional ISP
Company Network
The first question is: How is the data transmitted through the network?
This is the function of the network core, the set of routers
There are two modes of operation
Circuit Switching: set up a dedicated path
Packet Switching: data sent through the net in “discrete” packages

8. Circuit Switching Set a fixed path through the routers:
Home systems
Local ISP
Server
Router
Regional ISP
Company Network
Source
Set a fixed path through the routers:
Destination

9. Packet Switching Each data stream is divided into packets
Home systems
Local ISP
Server
Router
Regional ISP
Company Network
Each data stream is divided into packets
Packets travel in a store and forward mode
Packets hop from one router to another
Wait their turn at each router before making a hop

10. Where did this come from?
Protocol Layers
Home systems
Local ISP
Server
Router
Regional ISP
Company Network
Where did this come from?
Where does
It go?
What does
It need?
Networks are extremely complex and there is little hope that their structure could be organized so it is not feasible to just pass data between hosts
Want a
communication link
These questions
can only be answered
by the packet itself

11. Why Layers? Application Transport Network Link Physical
All the answers are added to the data to form the required packet as it is sent out onto the internet
The way in which the information is added is modeled by a layer process
The Internet Layer Model has 5 segments
Application
Transport
Network
Link
Physical
The model makes it easier to discuss the different actions and makes it easier to modify one layer without affecting the others

12. Layer Tasks application: supporting network applications
ftp, smtp, http
transport: host-host data transfer
tcp, udp (reliable delivery, rate regulation)
network: routing of datagrams from source to destination
ip, routing protocols
link: data transfer between neighboring network elements
ppp, ethernet, wireless, multiple access protocols
physical: bits “on the wire”

13. Logical Communication
Each node interacts with components of the layers D
Physical
Link
Network
Transport
Application
Physical
Link
Network T D
Physical
Link
Network
Transport
Application
For example consider the transport layer
Ack
Accept data from the application layer
Add addressing & reliability check information
Send a “datagram” to its peer
Wait for ack

14. Physical Communication
What is really happening:
Physical
Link
Network
Transport
Application D T D
Physical
Link
Network T D N T D N T D L N T D N T D L N T D L N T D D
Physical
Link
Network
Transport
Application T D N T D L N T D

15. Layer Structures
Each layer has a specific application and carries specific information to communicate with a peer layer on another system
As a result of this configuration model, it is possible to discuss each layer separately
New features can be added to a layer without affecting other layers
The operation of a layer can be understood without involving the other layers
Application
Transport
Network
Link
Physical

16. Application Layer

17. Application Layer Application Layer Standards
allow two application programs to work together, even if they come from different vendors
For example, browser on user PC and webserver application program on webserver
Common Application Layer programs
Hypertext Transfer Protocol (HTTP)
File Transfer Protocol (FTP)
Name service (DNS)
Telnet
Rlogin
Network File System (NFS)
Simple Mail Transfer Protocol (SMTP)
Post Office Protocol 3 (POP3)
News

18. Internet Applications
Common Internet applications have defined protocols:
Application
remote terminal access
Web
file transfer
streaming multimedia
remote file server
layer protocol
smtp [RFC 821]
telnet [RFC 854]
http [RFC 2068]
ftp [RFC 959]
proprietary
(e.g. RealNetworks)
NSF
Underlying
transport protocol
TCP
TCP or UDP

19. http Application Protocol

20. www: the http protocol client/server model
PC running
Explorer
Server
running
NCSA Web
server
Mac running
Navigator
http request
http response
client/server model
client: browser that requests, receives, “displays” WWW objects
server: WWW server sends objects in response to requests
http request
http response

21. What really happens . . .
client initiates TCP connection (creates socket) to server, port 80
server accepts TCP connection from client
http messages (application-layer protocol messages) exchanged between browser (http client) and WWW server (http server)
TCP connection closed

22. www.someSchool.edu server
Example
Suppose the user enters the URL
server
Sends http request message
TCP connection request on Port 80
TCP connection accepted
Response message including requested object
Server closes
TCP port

23. http Message Format two types of http messages: request, response
http request message:
ASCII (human-readable format)
request line
(GET, POST,
HEAD commands)
GET /somedir/page.html HTTP/1.1
Connection: close
User-agent: Mozilla/4.0
Accept: text/html, image/gif,image/jpeg
Accept-language:fr
(extra carriage return, line feed)
header
lines
Carriage return,
line feed
indicates end
of message

24. http Request Header
The application layer adds the http request header:

25. http Reply Format status line (protocol status code status phrase)
HTTP/ OK
Connection: close
Date: Thu, 06 Aug :00:15 GMT
Server: Apache/1.3.0 (Unix)
Last-Modified: Mon, 22 Jun 1998 …...
Content-Length: 6821
Content-Type: text/html
data data data data data ...
header
lines
data, e.g.,
requested
html file

26. http Reply Codes In the first line of the server to client reply
A few sample codes:
200 OK
request succeeded, requested object later in this message
301 Moved Permanently
requested object moved, new location specified later in this message (Location:)
400 Bad Request
request message not understood by server
404 Not Found
requested document not found on this server
505 HTTP Version Not Supported

27. Try it for yourself . . . 1. Telnet to your favorite WWW server:
telnet 80
Opens TCP connection to port 80
(default http server port) at
2. Type in a GET http request:
By typing this in (hit carriage
return twice), you send
this minimal (but complete)
GET request to http server
GET /~spillmrj HTTP/1.1
3. Look at response message sent by http server!

28. http Cookies server sends “cookie” to client in response Set-cookie: #
client present cookie in later requests
cookie: #
server matches presented-cookie with server-stored cookies
authentication
remembering user preferences, previous choices

29. http Conditional Interaction
Goal: don’t send object if client has up-to-date stored (cached) version
client: specify date of cached copy in http request
If-modified-since: <date>
server: response contains no object if cached copy up-to-date:
HTTP/ Not Modified

30. Web Cache
client
Proxy
server
http request
http response
origin
Goal: Satisfy client request without involving the original server
user sets browser: WWW accesses via web cache
client sends all http requests to web cache
if object at web cache, web cache immediately returns object in http response
else requests object from origin server, then returns http response to client

31. Why Web Caching?
origin
servers
public
Internet
institutional
network
10 Mbps LAN
1.5 Mbps
access link
cache
Assume: cache is “close” to client (e.g., in same network)
smaller response time: cache “closer” to client
decrease traffic to distant servers
link out of institutional/local ISP network often bottleneck