CS 372 – introduction to computer networks* Wednesday June 30

Chapter 2, slide: 1

CS 372 – introduction to computer networks*Wednesday June 30

Announcements: Assignment 2 is posted.

Acknowledgement: slides drawn heavily from Kurose & Ross

* Based in part on slides by Bechir Hamdaoui and Paul D. Paulson.

Chapter 2, slide: 2

Chapter 2: Application layer

Principles of network applications app architectures app requirements

Web and HTTP File transfer: FTP

P2P file sharing Socket programming

with TCP

Chapter 2, slide: 3

Web and HTTPFirst some terminologies:

Web page consists of objects Object can be HTML file, JPEG image, Java

applet, audio file,… Web page consists of base HTML-file which

includes several referenced objects Each object is addressable by a URL Example URL (Uniform Resource Locator):

www.someschool.edu/someDept/pic.gif

host name path name

Chapter 2, slide: 4

HTTP overview: app architecture

HTTP: hypertext transfer protocol

Web’s appl-layer protocol client/server model

client: browser that requests, receives, “displays” Web objects

server: Web server sends objects in response to requests

HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068

PC runningExplorer

Server running

Apache Webserver

Mac runningNavigator

HTTP request

HTTP request

HTTP response

HTTP response

Chapter 2, slide: 5

HTTP overview (continued)

Uses TCP: client initiates TCP

connection (creates socket) to server, port 80

server accepts TCP connection from client

HTTP messages exchanged between browser (HTTP client) and Web server (HTTP server)

TCP connection closed

HTTP is “stateless” server maintains no

information about past client requests

Protocols that maintain “state” are complex!

past history (state) must be maintained

if server/client crashes, their views of “state” may be inconsistent, must be reconciled

aside

Chapter 2, slide: 6

HTTP connections

Nonpersistent HTTP At most one object is

sent over a TCP connection.

HTTP/1.0 uses nonpersistent HTTP

Persistent HTTP Multiple objects can

be sent over single TCP connection between client and server.

HTTP/1.1 uses persistent connections in default mode

Chapter 2, slide: 7

Nonpersistent HTTPSuppose user enters URL www.someSchool.edu/someDepartment/home.index

1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80

2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.index

1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client

3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket

time

(contains text, references to 10

jpeg images)

Chapter 2, slide: 8

Nonpersistent HTTP (cont.)

5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects

6. Steps 1-5 repeated for each of 10 jpeg objects

4. HTTP server closes TCP connection.

time

Chapter 2, slide: 9

Non-Persistent HTTP: Response timeDefinition of RTT (round trip

time): time to send a small packet to travel from client to server and back.

Response time: one RTT to initiate TCP

connection one RTT for HTTP request

and first few bytes of HTTP response to return

file transmission time = 2RTT + transmit time

time to transmit file

initiate TCPconnection

RTT

requestfile

RTT

filereceived

time time

Chapter 2, slide: 10

Non-Persistent HTTP: issues



RTT

requestfile

RTT

filereceived

time time

Nonpersistent HTTP: Name some issues?? requires 2 RTTs per

object

E.g., a 10 object-page needs ~ 20 RTTs

Open/close TCP connection for each object => OS overhead

Any ideas for improvement?


Persistent HTTP

Persistent HTTP server leaves

connection open after sending response

subsequent HTTP messages between same client/server sent over open connection

reduces response time

Persistent without pipelining:

client issues new request only when previous response has been received

one RTT for each referenced object

Persistent with pipelining: default in HTTP/1.1 client sends requests as

soon as it encounters a referenced object

as little as one RTT for all the referenced objects


HTTP request message

two types of HTTP messages: request, response

HTTP request message: ASCII (human-readable format)

GET /somedir/page.html HTTP/1.1Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:fr

(extra carriage return, line feed)

request line(GET, POST,

HEAD commands)

header lines

Carriage return, line feed

indicates end of message


HTTP response message

HTTP/1.1 200 OK Connection closeDate: Thu, 06 Aug 1998 12:00:15 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 ...

status line(protocol

status codestatus phrase)

header lines

Entity body:data, e.g., requestedHTML file


HTTP response status 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

In first line in server -> client response message.

A few sample codes:


Trying out HTTP (client side) for yourself

1. Telnet to your favorite Web server:

Opens TCP connection:• to port 80 (default HTTP port)• at cis.poly.edu (~ IP address)• anything typed is sent there

telnet cis.poly.edu 80

2. Type in a GET HTTP request:

GET /~ross/ HTTP/1.1Host: cis.poly.edu

By typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to HTTP server

3. Look at response message sent by HTTP server!Try it at home or lab!




RTT

requestfile

RTT

filereceived

time time

A HTTP request consists of: 1 basic html object 2 referenced JPEG objects All objects of size = 106bits

RTT = 1 second Transmission rate = 1Mbps Consider transmission delay of

objects only

Question: how long it takes to receive the entire page:a) Non-persistent connectionb) Persistent without pipeliningc) Persistent with pipelining

Web and HTTP: Review Question




RTT

requestfile

RTT

filereceived

time time

A HTTP request consists of: 1 basic html object 2 referenced JPEG objects All objects of size = 106bits

RTT = 1 second Transmission rate = 1Mbps Consider transmission delay of

objects only

Answer: (transmit time = 1 sec)a) 3+3+3=9 sec

initiate + request + transmit for each of all 3

b) 1+2+2+2=7 sec initiate + (request + transmit for each of all 3)

c) 1+2+3=6 sec initiate + (request + transmit for basic) + (one request for 2 + two transmits, one for each of the 2 objects)

Web and HTTP: Review Question

User-server state: cookies

Many major Web sites use cookies

Four components:1) cookie header line of

HTTP response message

2) cookie header line in HTTP request message

3) cookie file kept on user’s host, managed by user’s browser

4) back-end database at Web site

Example: Susan always access

Internet always from PC visits specific e-

commerce site for first time

when initial HTTP requests arrives at site, site creates: unique ID entry in backend

database for ID

Cookies: keeping “state” (cont.)client server

usual http response msg

usual http response msg

cookie file

one week later:

usual http request msg

cookie: 1678cookie-specificaction

access

ebay 8734usual http request

msgAmazon server

creates ID1678 for user

create entry

usual http response Set-cookie: 1678

ebay 8734amazon 1678

usual http request msg

cookie: 1678cookie-spectificaction

accessebay 8734amazon 1678

backenddatabase

Cookies (continued)

What cookies can bring: authorization shopping carts recommendations user session state

(Web e-mail)

Cookies and privacy:• cookies permit sites

to learn a lot about you

• you may supply name and e-mail to sites

aside

How to keep “state”:• protocol endpoints: maintain

state at sender/receiver over multiple transactions

• cookies: http messages carry state


Web caches (or proxy server)

If page is needed, browser requests it from the Web cache

Q: what if object not in cache??

cache requests object from origin server, then returns object to client

Goal: satisfy client request without involving origin server

client

Proxyserver

client

HTTP request

HTTP response

HTTP request HTTP request

origin server

origin server

HTTP response HTTP response


More about Web caching

cache acts as both client and server

typically cache is installed by ISP (university, company, residential ISP)

Why Web caching? reduce response time

for client request

reduce traffic on an institution’s access link.


Caching example

Assumptions avg. object size =

0.1x106bits avg. request rate from

institution to origin servers = 10/sec

Internet delay = 2 sec

Consequences utilization on LAN = 10%

(LAN: local area network) utilization on access link =

100% total delay = Internet delay +

access delay + LAN delay = 2 sec + minutes +

milliseconds

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1 Mbps access link

institutionalcache


Caching example (cont)

possible solution increase bandwidth of

access link to, say, 10 Mbps

consequence utilization on LAN = 10% utilization on access link =

10% Total delay = Internet delay +

access delay + LAN delay = 2 sec + msecs + msecs often a costly upgrade

originservers

public Internet


10 Mbps access link

institutionalcache


Caching example (cont)

possible solution: web cache

suppose hit rate is 0.4

consequence 40% requests will be satisfied

almost immediately

60% requests satisfied by origin server

utilization of access link reduced to 60%

total avg delay will also be reduced

originservers

public Internet


1 Mbps access link

institutionalcache


Web cache (cont)

Advantages are obvious: Reduce response time Reduce internet traffic

Any problems with caches??

Local cache copies of web pages may not be up-to-date??

What do we do then?

Solution Upon receiving a web

request, a cache must consult origin server to check whether the requested page is up-to-date

Conditional GET method What: Sent by cache to origin

server: check page status

When:For each new request: client checks with cache


Conditional GET

Goal: don’t send object if cache has up-to-date version

How: cache specifies date of cached copy in HTTP requestIf-modified-since: <date>

Server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified

cache server

HTTP request msgIf-modified-since: <date>

HTTP responseHTTP/1.0

304 Not Modified

object not

modified

HTTP request msgIf-modified-since: <date>

HTTP responseHTTP/1.0 200 OK

<data>

object modified



Principles of network applications

Web and HTTP FTP


with TCP


FTP: the file transfer protocol

transfer file to/from remote host client/server model

client: that initiates transfer (to or from remote) server: remote host

ftp: RFC 959

file transfer FTPserver

FTPuser

interface

FTPclient

local filesystem

remote filesystem

user at host


FTP: separate control, data connections

FTP client contacts FTP server to open TCP control connection

client browses remote directory by sending commands over the TCP control connection.

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionport 20

when server receives file transfer command, server opens 2nd TCP data connection (for file) to client

after transferring one file, server closes TCP data connection.


FTP: separate control, data connections

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionport 20

More on FTP server opens a

separate TCP data connection for each file to transfer

TCP control stays on

control connection: called: “out of band”

FTP server maintains “state”: current directory, earlier authent. => limit on concurrent connections



Principles of network applications

Web and HTTP FTP


with TCP


File sharing approaches

There are 2 approaches

Centralized: Client-server architecture

Distributed: P2P architecture (e.g., BitTorrent)

obtain listof peers

trading chunks

peer

server

server

peers

Alice

Bob


File sharing: P2P vs. client-server architectures

Robustness to failure

Scalability

Security

Performance

Client-Server

Single point of failure

Not scalable

More secure

Bottleneck

P2P

Fault-tolerant

Scalable

Less secure

Better


Comparing Client-Server, P2P architecturesQuestion : What is the file distribution time:

from one server to N hosts?

us

u2d1 d2u1

uN

dN

Server

Network (with abundant bandwidth)

File, size F

us: server upload bandwidth

ui: client/peer i upload bandwidth

di: client/peer i download bandwidth


Client-server: file distribution time

us

u2d1 d2u1

uN

dN

Server


F server

sequentially sends N copies: NF/us time

client i takes F/di

time to download

lower bound is alsoachievable

= dcs > max { NF/us, F/min(di) }i

Time to distribute F to N clients using

client/server approach

increases linearly in N(for large N)

uN NF bits must be downloaded- NF bits must be uploaded- Fastest possible upload rate (assuming all nodes sending file chunks to same peer) is:

us + ui


P2P: file distribution time

us

u2d1 d2u1

dN

Server


F

server must send one copy: F/us time

client i takes F/di time to download

i=1,N

dP2P > max { F/us, F/min(di) , NF/(us + ui) }i=1,N


0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35

N

Min

imum

Dis

trib

utio

n T

ime P2P

Client-Server

Comparing Client-server, P2P architectures



Principles of network applications app architectures app requirements

Web and HTTP FTP


with TCP (assignment 2)


Assignment 2- programming partBuilding a Multi-Threaded Web Server

Read Chapter 2, Section 7 first• Good examples on how to write socket-programs

in Java using TCP

Then do your assignment• Socket/Java programming: relatively

simple and easy to do


Chapter 2: Summary

application architectures client-server P2P

application service requirements: reliability, bandwidth, delay

Internet transport service model connection-oriented, reliable:

TCP unreliable, datagrams: UDP

our study of network apps now complete!

specific protocols: HTTP FTP P2P

socket programming (assignment 2)

Documents

CS 372 – introduction to computer networks* Wednesday June 30