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Chapter 2Application LayerComputer Networking:A Top Down Approach,4th edition.Jim Kurose, Keith RossAddison-Wesley, July2007.

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Last Lecture

HTTP Request and Response

MessagesHTTP state aware mechanism

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Todays Lecture

Electronic mail (Email)

SMTPDomain Name System (DNS)

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HTTP Request MessageTwo types of HTTP messages:

Request, ResponseHTTP request message:

Human-readable format

A typical HTTP request message

GET /somedir/page.html HTTP/1.1Host: www.someschool.edu

User-agent: Mozilla/4.0Connection: closeAccept-language:fr

(Blank Line)

Body

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HTTP Request Message

Request Line

First Line of HTTP request message

The request line has three fields

Method field can take on several values

GET,POST, PUT, DELETE etcMajority of HTTP request messages use GET

URL field

HTTP version field

Header Lines Host: Host on which the object resides

User-agent: Identifies the client program e.g. Mozilla/4.0

Connection: Shows whether the connection should be closedor not

Accept-language: Shows the language the client prefers

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HTTP Request Message

Body is empty withGET method

Used with the POSTmethod

Web page oftenincludes form input

Input is sent toserver in body

PUT method:

To upload objects toweb server

DELETE method Allows a user or an

application to deletean object on web

server

Find out about OPTIONS, TRACE, HEAD?

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HTTP Response Message

HTTP/1.1 200 OKConnection: closeDate: Thu, 06 Aug 1998 12:00:15

GMTServer: Apache/1.3.0 (Unix)Last-Modified: Mon, 22 Jun 1998Content-Length: 6821Content-Type: text/html

Body contains Requested Object

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HTTP Response Message

Status Line Protocol Version

Status Code and Status MessageThe status code and associated phrase indicate the result of

the request Header Line Connection: Shows whether the connection should be closed

or not

Date and time: When the HTTP response was created Server: Analogous to user-agent in request message

Last-modified: When the Object was last created ormodified

Content-length: Number of bytes in object being sent

Content-Type: Indicates the object type (HTML text etc.)

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HTTP Response Status Codes

200 OK Request succeeded and information is returned in

response301 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 Version not supported HTTP version is not supported by the server

The status code and associated phrase indicate the result ofthe request

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Electronic Mail

Asynchronous CommunicationMedium

Three major components: User Agents Mail Servers Simple Mail Transfer Protocol:

SMTP

User Agent a.k.a. mail reader composing, editing, reading

mail messages e.g., Eudora, Outlook, Mozilla

Thunderbird Sends message to the mail

server

user mailboxoutgoing

message queue

mail

server

user

agent

user

agent

user

agentmailserver

user

agentuser

agent

mail

server

user

agent

SMTP

SMTPSMTP

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Electronic Mail

Mail Servers

Each recipient has a mailboxlocated in one of the mail servers

Mailbox manages and maintains themessages that have been sent tohim.

Messagequeue of outgoing (to besent) mail messages

Simple Mail Transfer Protocol(SMTP) between mail servers tosend email messages

Client and Server Both client and sever sides of

SMTP run on every mail server.

Runs on TCP at port 25.

RFC 5321

mailserver

useragent

useragent

user

agentmail

serveruseragent

useragent

mailserver

useragent

SMTP

SMTPSMTP

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Scenario: Alice sends message to Bob

1) Alice uses UA to compose

message tobob@someschool.edu

2) Alices UA sends messageto her mail server; messageplaced in message queue

3) Client side of SMTP opensTCP connection with Bobsmail server

4) SMTP client sends Alices

message over the TCPconnection

5) Bobs mail server places themessage in Bobs mailbox

6) Bob invokes his user agent

to read message

useragent

mailserver

mailserver user

agent1

2 3 4 56

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Sample SMTP InteractionClient SMTP establishes a TCP connection with ServerSMTP

Application Layer handshaking to introduce themselves

Sample SMTP InteractionS: 220 hamburger.edu (Service Ready)C: HELO crepes.frS: 250 Hello crepes.fr, pleased to meet you

C: MAIL FROM: S: 250 alice@crepes.fr... Sender okC: RCPT TO: S: 250 bob@hamburger.edu ... Recipient okC: DATA (All lines after DATA command are treated as the

mail message)

S: 354 Enter mail, end with "." on a line by itselfC: Do you like ketchup?C: How about pickles?C: .S: 250 Message accepted for deliveryC: QUITS: 221 hamburger.edu closing connection

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SMTP and HTTP SMTP uses persistent connections

Can send all of the messages over the same TCP connection

Comparison with HTTP: HTTP transfer files from web server to web client

SMTP from one mail server to another

Both use Persistent Connections HTTP: pull

Someone loads information on a web server and users useHTTP to pull this info

TCP connection is initiated by the machine that wants toreceive the file

SMTP: push

Sending mail sever pushes the file to the receiving mail server

TCP connection is initiated by the machine that wants to sendthe file

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Mail Access Protocols

SMTP: delivery/storage to receivers server

Mail access protocol: retrieval from server

POP: Post Office Protocol [RFC 1939]

IMAP: Internet Mail Access Protocol [RFC 1730]

HTTP: Gmail, Hotmail, Yahoo! Mail, etc. User agent is ordinary web browser

HTTP to send email to web server

HTTP to access mail box

useragent

senders mailserver

useragent

SMTP SMTP accessprotocol

receivers mailserver

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DNS: Domain Name System

People: many identifiers: SSN, name, passport #

Internet hosts: IP address (32 bit)

name, e.g.,ww.yahoo.com - used byhumans

Q: map between IPaddresses and name ?

Domain Name System: Distributed Database

implemented in hierarchy ofmany DNS servers

An application-layer protocol

that allows hosts to query thedistributed database

DNS uses UDP over portnumber 53.

RFC 1034 and RFC 1035

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DNS

Simple design would have one DNS serverthat contains all the mappings

Why not centralize DNS?

Single point of failure

Traffic volume Distant centralized database

Maintenance

A centralized database in a single DNSserver doesnt scale!

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Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.edu

DNS servers

umass.edu

DNS servers

yahoo.com

DNS serversamazon.com

DNS servers

pbs.org

DNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com:

Client first queries a root server

The root server returns the IP addresses for TLD servers forthe top level domain com

The client then contacts one of the TLD servers which returnsthe IP address of an authoritative server for amazon.com

The authoritative server then returns the IP address for thehost name www.amazon.com

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DNS: Root Name Servers

13 root DNS servers world wide

Each server is a cluster of replicated servers security and reliability purposes.

For more information see www.root-servers.org

b USC-ISI Marina del Rey, CA

l ICANN Los Angeles, CA

e NASA Mt View, CA

f Internet Software C. Palo Alto,CA (and 36 other locations)

i Autonomica, Stockholm (plus

28 other locations)

k RIPE London (also 16 other locations)

m WIDE Tokyo (also Seoul,

Paris, SF)

a Verisign, Dulles, VA

c Cogent, Herndon, VA (also LA)

d U Maryland College Park, MD

g US DoD Vienna, VA

h ARL Aberdeen, MDj Verisign, ( 21 locations)

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TLD and Authoritative Servers

Top-level Domain (TLD) Servers: Responsible for com, org, net, edu, etc, and all

top-level country domains uk, fr, jp. Network Solutions maintains servers for com TLD

Educause for edu TLD ICANN: Internet Corporation for Assigned Names

and Numbers

Authoritative DNS Servers:

Every organization with publicly accessible hostsprovide accessible DNS records.That maps the names of those hosts to IP addresses

Authoritative DNS servers houses these DNS records

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Local Name Server

Does not strictly belong to hierarchy

Each company, university has one. Also called default name server

When host makes DNS query, query is sentto its local DNS server acts as proxy, forwards query into hierarchy

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requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

2 345

6

authoritative DNS serverdns.cs.umass.edu

78

TLD DNS server

DNS nameresolution example

Host at cis.poly.eduwants IP address forgaia.cs.umass.edu

Iterated query: Contacted server

replies with name ofserver to contact

I dont know this

name, but ask thisserver

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requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

2

45

6

authoritative DNS serverdns.cs.umass.edu

7

8

TLD DNS server

3Recursive Query: Puts burden of name

resolution on otherserver

DNS Caching Extensively used Cache entries timeout

(disappear) after sometime

TLD servers typicallycached in local nameservers Thus root name

servers not oftenvisited

DNS nameresolution example