1

ELEN602 Lecture 2

• Review of Last Lecture

• Layering

2

Communication Networks

• Many forms of communication networks around

– Postal mail, telephone, Internet etc.

• Generic steps involved in communication

– Addressing, name-to-address translation, transport, routing, delivery

• Forms of delivery

– Reliable, in-order

– Unreliable, no acknowledgement

3

The user clicks on a link to indicate which document is to be retrieved.

The browser must determine the address that contains the document. It does this by sending a query to its local name server.

Once the address is known the browser establishes a connection to the specified machine, usually a TCP connection. In order for the connection to be successful, the specified machine must be ready to accept TCP connections.

The browser runs a client version of HTTP, which issues a request specifying both the name of the document and the possible document formats it can handle.

The machine that contains the requested document runs a server version of HTTP. It reacts to the HTTP request by sending an HTTP response which contains the desired document in the appropriate format.

The TCP connection is then closed and the user may view the document.

1.

2.

3.

4.

5.

6.

Steps in Web Surfing

4

HTTPserver

HTTPclient

Request

Response

Application level View

5

Steps in HTTP Transfer

• User selects document• Networking software locates the remote machine address &

establishes a 2-way connection• HTTP client sends request for document (GET )• HTTP daemon on remote machine interprets the GET message • Remote HTTP daemon sends the result of GET operation

– Result code, Description of the information, requested file• Remote HTTP daemon disconnects the connection• HTTP client displays the HTML content in the browser.

6

Domain Name Service

• DNS servers help resolve name-to-address translation

– Given a name such as ee.tamu.edu, find its IP address

• IP addresses = telephone numbers in phone network

• Machines (computers/network elements) know each other by IP addresses

• IP addresses are 32bits in IPv4.

• DNS protocol allows name-to-IP address resolution

– Similar to phone directory assistance (411 or 555-1212)

7

DNS Example (cont’d)

• QNAME = ee.tamu.edu, QCLASS = IN QTYPE = A

• QNAME = ee.tamu.edu QCLASS = IN QTYPE = A

– IN A 165.91.241.199

• The application will now use 165.91.241.199 whenever it needs to contact ee.tamu.edu

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HTTPserver

HTTPclient

TCP TCP

GET 80, #

#, 80 STATUS

Port 80

Ephemeral

Port #

A more detailed view - Transport protocol

9

Email and SMTP

• Web transfers use HTTP, email uses SMTP (Simple Mail Transfer Protocol) - Application level protocols

• Sender - Local SMTP Server -- Remote SMTP Server - Receiver

• SMTP uses Port 25, HTTP uses port 80

• POP (Post Office Protocol) may be used if receiver not on all the time.

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• Sender establishes TCP Connection to SMTP server

• Sends HELO message -identifying itself to SMTP server

• Client sends sender’s email address

• SMTP server acknowledges with 250 if successful

• Client sends receiver’s email address

• SMTRP server acknowledges with 250

• Client sends a DATA message - requesting permission to send email message

• SMTP server sends OK-to-send message

• Client transfers the email message

• SMTP server returns ack with a message ID

• Client indicates that the transfer is complete.

• SMTP confirms the receipt of End-of-transfer message

11

nentity

n-PDUs

nentity

Abstract View

12

n+1entity

n-SAP

n+1entity

n-SAP

n entity n entity

n-SDU

n-SDU

n-SDU

H

H n-SDU

n-PDU

Layering in General

13

n-SDU

n-PDU n-PDU n-PDU

n-SDU

n-PDU n-PDU n-PDU

Segmentation Reassembly

n-PDU

n-SDU

n-PDU

Blocking Unblocking

n-SDUn-SDUn-SDUn-SDUn-SDU

(a)

(b)

Data Units at different layers

14

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

NetworkLayer

Electrical and/or Optical Signals

Application A Application B

Data LinkLayer

PhysicalLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

Communication Network

OSI Seven-layer Model

15

C

PS

PS

C

PSPS

C

C

C

PS = packet switch

C = computer

Packet-Switched Network

16

G

G

G

G

G

net 1

net 2

net 3

net 4

net 5

G = gateway/router

G

An Internetwork

17

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

Application A Application Bdata

data

data

data

data

data

data

ah

ph

sh

th

nh

dh

bits

dt

Headers and Trailers

18

ApplicationLayer

TransportLayer

InternetLayer

NetworkInterface

ApplicationLayer

TransportLayer

InternetLayer

NetworkInterface

(a) (b)

TCP/IP Network Architecture

19

Application

Transport

Internet

Network Interface

Application

Transport

Internet

Network Interface

Internet

Network Interface

Network 1 Network 2

Machine A Machine B

Router/Gateway

IP Layer and Network Interface Layer

20

HTTP SMTP RTP

TCP UDP

IP

Network

Interface 1

Network

Interface 3

Network

Interface 2

DNS

IP Hour-Glass Protocol Graph

21

Net Interface

IP

TCP

HTTP

Net Interface

IP

Net Interface

IP

TCP

HTTP

Ethernet PPP

Router

router

(1,1)

s

(1,2)

w

(2,1)

(1,3) r

(2,2)

PPP

Ethernet

(a)

(b)

Server PC

22

IP Header

Header containssource and destination physical addresses; network protocol type

Frame Check Sequence

Ethernet Header

IP Datagram on an Ethernet

23

HTTP Request

TCP Header

Header contains source and destination port numbers

Header contains source and destination IP addresses; transport protocol type

IP Header

Header contains source and destination physical addresses; network protocol type

Frame Check Sequence

Ethernet Header

24

Application 1

Socket

socket interface

user

kernel

Application 2

user

kernel

Underlying communication

Protocols

Underlying communication

Protocols

Communications network

Socket

socket interface

25

socket()

bind()

listen()

read()

close()

socket()

connect()

read()

write()

close()

blocks until server receivesa connect request from client

data

data

Server

Clientaccept()

write()

connect negotiation

26

socket()

bind()

sendto()

close()

socket()

bind()

recvfrom()

sendto()

close()

blocks until serverreceives data from client data

data

Server

Client

recvfrom()

27

User Interface

User PI

User DTP

PI = Protocol interpreter

DTP = Data transfer process

User FTP

Server PI

Server DTP

Server FTP

Control

Connection

Data

Connection

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C:\WINDOWS>ping nal.toronto.edu

Pinging nal.toronto.edu [128.100.244.3] with 32 bytes of data:

Reply from 128.100.244.3: bytes=32 time=118ms TTL=243Reply from 128.100.244.3: bytes=32 time=118ms TTL=243Reply from 128.100.244.3: bytes=32 time=118ms TTL=243Reply from 128.100.244.3: bytes=32 time=118ms TTL=243

C:\WINDOWS>