Bài giảng Mạng máy tính - Chapter 2: Application Layer

Computer Networks

Lectured by:

Nguyen Le Duy Lai

(lai@hcmut.edu.vn)

Computer

Networking: A Top

Down Approach

7^thEdition, Global Edition

Jim Kurose, Keith Ross

Pearson

April 2016

Introduction 1-1

Chapter 2

Application Layer

Computer

Networking: A Top

Down Approach

7^thEdition, Global Edition

Jim Kurose, Keith Ross

Pearson

April 2016

Application Layer

2-2

Chapter 2: outline

2.5 P2P applications

2.1 principles of network

applications

2.6 video streaming and

Content Distribution

Networks (CDNs)

2.2 Web and HTTP

2.3 electronic mail

• SMTP, POP3, IMAP

2.4 DNS

2.7 socket programming

with UDP and TCP

Application Layer 2-3

Chapter 2: application layer

our goals:

▪ conceptual,

▪ learn about protocols by

examining popular

application-level

protocols

implementation aspects

of network application

protocols

• HTTP

• FTP

• SMTP / POP3 / IMAP

• DNS

• transport-layer

service models

• client-server

▪ creating network

paradigm

applications

• peer-to-peer (P2P)

• socket API

paradigm

• content distribution

networks (CDNs)

Application Layer 2-4

Some network apps

▪ e-mail

▪ web

▪ text messaging

▪ remote login

▪ P2P file sharing

▪ voice over IP (e.g.,

Skype)

▪ real-time video

conferencing

▪ social networking

▪ search

▪ …

▪ multi-user network

games

▪ …

▪ streaming stored

video (YouTube, Hulu,

Netflix)

Application Layer 2-5

application

transport

network

data link

physical

Creating a network app

write programs that:

▪ run on (different) end systems

▪ communicate over network

▪ e.g., web server software

communicates with browser

software

application

transport

network

data link

physical

no need to write software

for network-core devices

application

transport

network

data link

physical

▪ network-core devices do not

run user applications

▪ applications on end systems

allows for rapid app

development, propagation

Application Layer 2-6

Application architectures

possible structure of applications:

▪ client-server

▪ peer-to-peer (P2P)

Application Layer 2-7

Client-server architecture

server:

▪ always-on host

▪ permanent IP address

▪ data centers for scaling

clients:

▪ communicate with server

▪ may be intermittently

client/server

connected

▪ may have dynamic IP

addresses

▪ do not communicate directly

with each other

Application Layer 2-8

P2P architecture

▪ no always-on server (peer)

peer-peer

▪ arbitrary end systems

directly communicate

▪ peers request service from

other peers, and provide

service in return to other

peers

• self scalability – new

peers bring new service

capacity, as well as new

service demands

▪ peers are intermittently

connected and change IP

addresses

• complex management

Application Layer 2-9

Processes communicating

clients, servers

process: program running

within a host

client process: process that

▪ within same host, two

processes communicate

using inter-process

communication (defined by

OS)

initiates communication

server process: process that

waits to be contacted

▪ processes in different hosts

communicate by exchanging

messages

▪ aside:applications with P2P

architectures have client

processes & server

processes

Application Layer 2-10

Sockets

▪ process sends/receives messages to/from its socket

▪ socket analogous to door

• sending process shoves message out door (or socket)

• sending process relies on transport infrastructure on

other side of door to deliver message to socket at

receiving process

application

socket

controlled by

app developer

process

transport

controlled

by OS

network

link

network

link

Internet

physical

Application Layer 2-11

Addressing processes

▪ identifier includes both IP

address and port numbers

associated with process on

host.

▪ example of port numbers:

• HTTP server: 80

▪ 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 for

identifying the process?

• SMTP mail server: 25

▪ E.g., to send HTTP message

to gaia.cs.umass.edu web

server:

▪ A: no, many processes can

be running on same host

• IP address: 128.119.245.12

• port number: 80

▪ more shortly…

Application Layer 2-12

App-layer protocol defines

▪ types of messages

open protocols:

exchanged,

▪ defined in RFCs

▪ allows for interoperability

▪ e.g., HTTP, SMTP

proprietary protocols:

▪ e.g., Skype

• 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

Application Layer 2-13

What transport service does an app need?

data integrity

▪ some apps (e.g., file transfer,

throughput

▪ some apps (e.g.,

multimedia) require

minimum amount of

throughput to be

web transactions) require

100% reliable data transfer

▪ other apps (e.g., audio) can

“effective”

tolerate some loss

▪ other apps (“elastic apps”)

make use of whatever

throughput they get

timing

▪ some apps (e.g., Internet

telephony, interactive

games) require low delay

to be “effective”

security

▪ encryption, data integrity,

…

Application Layer 2-14

Transport service requirements: common apps

application

throughput

data loss

time sensitive

file transfer

e-mail

Web documents

real-time audio

/video

elastic

no loss

no

no loss

elastic

no

no loss

loss-tolerant

elastic

no

audio: 5kbps-1Mbps

video:10kbps-5Mbps

same as above

few kbps up

elastic

yes, 100’s msec

stored audio/video

interactive games

text messaging

loss-tolerant

no loss

yes, few secs

yes, 100’s msec

yes and no

Application Layer 2-15

Internet transport protocols services

TCP service:

▪ connection-oriented: setup

required between client and

server processes

▪ reliable transport between

sending and receiving

processes

UDP service:

▪ connectionless

▪ unreliable data transfer

between sending and

receiving process

▪ does not provide: reliability,

flow control, congestion

control, timing,

▪ flow control: sender won’t

overwhelm receiver

throughput guarantee,

security, or connection

setup,

▪ congestion control: throttle

sender when network

overloaded

▪ does not provide: timing,

minimum throughput

guarantee, security

Q: why bother? Why is

there a UDP?

Application Layer 2-16

Internet apps: application, transport protocols

application

layer protocol

underlying

transport protocol

application

SMTP [RFC 2821]

Telnet [RFC 854]

HTTP [RFC 2616]

FTP [RFC 959]

HTTP (e.g., YouTube),

RTP [RFC 1889]

SIP, RTP, proprietary

(e.g., Skype)

e-mail

remote terminal access

Web

TCP

TCP or UDP

file transfer

streaming multimedia

Internet telephony

TCP or UDP

Application Layer 2-17

Securing TCP

TCP & UDP

SSL is at app layer

▪ no encryption

▪ apps use SSL libraries, that

“talk” to TCP

SSL socket API

▪ E.g., cleartext passwords

sent into socket traverse

Internet encrypted

▪ E.g., Passwords sent into

socket traverse Internet in

cleartext

SSL

▪ provides encrypted TCP

connection

▪ see Chapter 8

▪ data integrity

▪ end-point authentication

Application Layer 2-18

Chapter 2: outline

2.5 P2P applications

2.1 principles of network

applications

2.6 video streaming and

content distribution

networks

2.2 Web and HTTP

2.3 electronic mail

• SMTP, POP3, IMAP

2.4 DNS

2.7 socket programming

with UDP and TCP

Application Layer 2-19

Web and HTTP

First, a review…

▪ 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, e.g.,

www.someschool.edu/someDept/pic.gif

path name

host name

Application Layer 2-20

Bài giảng Mạng máy tính - Chapter 2: Application Layer - Nguyễn Lê Duy Lai