Chapter 2, slide: 1 ECE/CS 372 – introduction to computer networks Lecture 5 Announcements: r Lab1...

Chapter 2, slide: 1

ECE/CS 372 – introduction to computer networks

Lecture 5

Announcements: Lab1 is due today

Lab2 is posted and is due next Tuesday

Acknowledgement: slides drawn heavily from Kurose & Ross

Chapter 2, slide: 2

Chapter 1: recapBy now, you should know:

the Internet and its components

circuit-switching networks vs. packet-switching networks

different network access technologies

the three Tiers 1, 2, and 3

layered architecture of networks

types of delays and throughput analysis

Chapter 2, slide: 3

Chapter 2: Application LayerOur goals: aspects of network

application protocols transport-layer

service models client-server

paradigm peer-to-peer

paradigm

learn about protocols by examining popular application-level protocol: HTTP

Chapter 2, slide: 4

Some network apps

e-mail web instant messaging remote login P2P file sharing multi-user network

games streaming stored

video clips

voice over IP real-time video

conferencing grid computing

Chapter 2, slide: 5

Creating a network app

write programs that run on (different) end

systems communicate over

network e.g., web server software

communicates with browser software

little software written for devices in network core network core devices do

not run user applications

application

transportnetworkdata linkphysical

application


application


Chapter 2, slide: 6

Chapter 2: Application layer

Principles of network applications app architectures app requirements

Web and HTTP

P2P file sharing

Chapter 2, slide: 7

Application architectures

There are 3 types of architectures:

Client-server Peer-to-peer (P2P) Hybrid of client-server and P2P

Chapter 2, slide: 8

Client-server architectureserver:

always-on fixed/known IP address serves many clients at same time

clients: communicate with server only may be intermittently connected may have dynamic IP addresses do not communicate directly with

each other

E.g., of client-server archit.: Google, Amazon, MySpace,

YouTube,

client/server

Chapter 2, slide: 9

Pure P2P architecture no always-on server arbitrary end systems

directly communicate peers are intermittently

connected and change IP addresses

example: BitTorrent

Pros and cons: scalable and distributive difficult to manage not secure

peer-peer

Chapter 2, slide: 10

Hybrid of client-server and P2PSkype

voice-over-IP P2P application centralized server: finding address of remote party client-client connection: direct (not through server)

Instant messaging chatting between two users is P2P centralized service: client presence location

• user registers its IP address with central server when it comes online

• user contacts central server to find IP addresses of buddies


Processes communicating

Process: is program running within a host.

processes in same host communicate using inter-process communication (managed by OS).

processes in different hosts communicate by exchanging messages

Client process: process that initiates communication

Server process: process that waits to be contacted

Note: applications with P2P architectures have client processes & server processes


Sockets

process sends/receives messages to/from its socket

socket analogous to door sending process shoves

message out door sending process relies on

transport infrastructure on other side of door which brings message to socket at receiving process

controlledby OS

process

TCP withbuffers,variables

socket

host orserver

process

TCP withbuffers,variables

socket

host orserver

Internet

controlled byapp developer

App Program Interface (API): (1) choice of transport protocol; (2) ability to fix a few parameters


Addressing processes to receive messages,

process must have identifier

host device has unique 32-bit IP address

Q: does IP address of host on which process runs suffice to identify the process?

identifier consists of: IP address (host) port numbers (process)

Example port numbers: HTTP server: 80 Mail server: 25

to send HTTP message to gaia.cs.umass.edu web server: IP address:

128.119.245.12 Port number: 80

more shortly…

A: No, many processes can be running on same host


App-layer protocol defines

Types of messages exchanged, e.g., request, response

Message syntax: what fields in messages

& how fields are delineated

Message semantics meaning of information

in fields

Rules for when and how processes send & respond to messages

Public-domain protocols:

defined in RFCs allows for

interoperability e.g., HTTP, SMTPProprietary protocols: e.g., Skype

Question: why do we need an “App-layer protocol” ?


What transport service does an app need?Data loss/reliability some apps (e.g., audio)

can tolerate some loss other apps (e.g., file

transfer, telnet) require 100% reliable data transfer

Timing some apps (e.g.,

Internet telephony, interactive games) require low delay to be “effective”

Bandwidth some apps (e.g.,

multimedia) require minimum amount of bandwidth to be “effective”

other apps (“elastic apps”) make use of whatever bandwidth they get

Security what about it !!!


Transport service requirements of common apps

Application

file transfere-mail

Web documentsreal-time audio/video

stored audio/videointeractive gamesinstant messaging

Data loss

no lossno lossno lossloss-tolerant

loss-tolerantloss-tolerantno loss

Bandwidth

elasticelasticelasticaudio: 5kbps-1Mbpsvideo:10kbps-5Mbpssame as above few kbps upelastic

Time Sensitive

nononoyes, 100’s msec

yes, few secsyes, 100’s msecyes and no


What services do Internet transport protocols provide?TCP service: connection-oriented: setup

required between client and server processes

reliable transport between sending and receiving process

flow control: sender won’t overwhelm receiver

congestion control: throttle sender when network overloaded

does not provide: timing, minimum bandwidth guarantees

UDP service: unreliable data transfer

between sending and receiving process

does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee

Q: why bother? Why is there a UDP?


Internet apps: application, transport protocols

Application

e-mailremote terminal access

Web file transfer

streaming multimedia

Internet telephony

Applicationlayer protocol

SMTP [RFC 2821]Telnet [RFC 854]HTTP [RFC 2616]FTP [RFC 959]proprietary(e.g. RealNetworks)proprietary(e.g., Vonage, Skype)

Underlyingtransport protocol

TCPTCPTCPTCPTCP or UDP

typically UDP




Web and HTTP

P2P file sharing


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


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


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


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


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)


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


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


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




RTT

requestfile

RTT

filereceived

time time

A HTTP request consists of: 1 basic html object 2 referenced JPEG objects Each object is 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 Each object is 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


ECE/CS 372 – introduction to computer networks

Lecture 6

Announcements: Lab2 is due next Tuesday

Acknowledgement: slides drawn heavily from Kurose & Ross


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 + sec + milliseconds unacceptable delay!

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 total delay still dominated by

Internet delay

originservers

public Internet


10 Mbps access link

institutionalcache


Caching example (cont)2nd possible sol: web cache suppose hit rate is 0.4 (typically, between 0.3 & 0.7)

consequence 40% requests will be satisfied

almost immediately and 60% requests satisfied by origin server

utilization of access link reduced by 40%, giving an access delay in the order of milliseconds; say 10 millisec

originservers

public Internet


1 Mbps access link

institutionalcache

Total delay = 0.4x(0.1) (LAN) + 0.6x(0.1+0.1+2) (LAN + access + Internet) = (about) 1.3

second

total avg delay reduced by about 40%


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




Web and HTTP

P2P file sharing


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

= 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)


Client-server: file distribution time

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

We now show that the distribution time is actually equal to max{NF/us, F/min(di) }

See board notes

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


Chapter 2: Summary

application architectures client-server, P2P

application service requirements: reliability, bandwidth, delay

Web and HTTP Non-Persistent, persistent, web

cache

Distribution time Client-server, P2P

We covered general concepts, like:

Chapter 2, slide: 1 ECE/CS 372 – introduction to computer networks Lecture 5 Announcements: r Lab1...

Documents

Transcript of Chapter 2, slide: 1 ECE/CS 372 – introduction to computer networks Lecture 5 Announcements: r Lab1...