Application Layer2-1 Chapter 2 Application Layer Part of slides provided by J.F Kurose and K.W. Ross, All Rights Reserved Communication Networks P. Demeester.

Application Layer 2-1

Chapter 2Application Layer

Part of slides provided by J.F Kurose and K.W. Ross, All Rights Reserved

Communication NetworksP. Demeester

Computer networking -A top-down approach featuring the internet4th Edition, 2008Addison WesleyJames F. Kurose, Keith W. RossISBN 0-321-49770-8


TCP-Connection

Internet Protocol

10 Mbit/s local area network

router

100 Gbit/s WDM transatlantic optical cable

2 Mbit/sISDN linklocal telephone

switch

56.6 kbit/smodem connection

140 Mbit/sPDH link

IP-datagram

client server

RFC 1011 : “Internet Protocols”

URL: www.ietf.org

IETF webpage

find info about Internet on the Web !

Remote login : TelnetWeb access : HTTPFile transfer : FTP

E-mail : SMTP/POP/IMAPAddress resolution : DNS

Focus : Application Layer


Chapter 2 outline

2.1 Principles of app layer protocols

2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail 2.5 DNS


Application : communicating, distributed processes (e.g. e-mail, Web, P2P file sharing, …)

Applications offer services to human users (or other applications) via a User Interface: Text-oriented User Interface (Telnet, FTP)

Graphical User Interface (GUI) (WWW)

Applications use an application-layer protocol to implement these services(protocol commands “independent” of user interface)

The application layer is the collection of these protocols (FTP, SMTP, POP, IMAP, HTTP, etc.)

Example : FTP user interface : PUT and GETor drag-and-drop

FTP protocol : STOR and RETR

Applications and application-layer protocols

Application Layer 2-5Note : server program <> server-class computer

APPLICATIONCLIENT

HTMLdocuments

Application Layer Protocol

...

Webbrowser GUI

communicatingprocesses

transport

network

data link

physical

terminal

network

data link

physical

data link

physical

router

network

data link

physical

data link

physical

router

transport

network

data link

physical

terminal

APPLICATIONSERVER

HTMLdocuments

Application Layer Protocol

...

Web server GUI

API:socket interface

Applications and application-layer protocols


Processes communicating across network

process sends/receives messages to/from its socket (=SAP)

socket analogous to door sending process shoves

message out door sending process assumes

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

process

TCP withbuffers,variables

socket

host orserver

process

TCP withbuffers,variables

socket

host orserver

Internet

controlledby OS

controlled byapp developer

API or socket : (1) choice of transport protocol; (2) ability to fix a few parameters


Application-layer protocol defines:

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

Syntax of message types: what fields in messages & how fields are delineated

Semantics of the fields, i.e., meaning of information in fields

Rules for when and how processes send & respond to messages

Public-domain protocols:

defined in RFCs allows for

interoperability eg, HTTP, SMTPProprietary protocols: eg, KaZaA


Application Client - Server

CLIENT : “active open”• invoked directly by user• local on user’s personal computer• actively initiates contact with server• one session at a time• access multiple services as needed• simple hardware and software• e.g. : Eudora, Internet Explorer

SERVER : “passive open”• special purpose program for one service• can handle multiple remote clients at same time• runs on a shared computer• waits passively for contact from arbitrary remote client• powerful hardware and sophisticated operating system• server program also called daemon (e.g. FTP daemon)• e.g.: Apache, Sendmail

• Information can flow in both directions between client and server• An application program can act as client and server

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

reply

request


CLIENT 2CLIENT N

server

request reception and thread creation

request handler 1

network

request handler 2 ... request

handler N

IP address: 157.193.122.12TCP protocol: 6HTTP port: 1173



CLIENT 1

IP address: 148.35. 2.205TCP protocol: 6HTTP port: 80

thre

ad

pool

Dynamic Server Process creation


What transport service does an app need?

Data loss 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


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

Transport service requirements of common apps


Internet transport protocols services

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


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., Dialpad)

Underlyingtransport protocol

TCPTCPTCPTCPTCP or UDP

typically UDP

Internet apps: application, transport protocols


TCP connectionTCP

Telnetclient

TCP

Telnetserver

Login shell(ksh, csh, command.com)

‘a’‘a’

‘a’‘a’

‘a’‘a’

ACK

‘a’

ACK

‘a’

‘a’

I/O : keyboard

screen

Telnetclient

program

socket interface

login shellTelnetserver

program

socket interface

‘a’

a

login on a remote machine(different location, different operating system)

Note : a socket connection has to be set up before info can be exchanged

Simple example : Telnet


Echo example$ telnet eduserv2 7Trying 157.193.40.10...Connected to eduserv2.rug.ac.be.HalloHallo^C

Daytime example$ telnet eduserv2 13Trying 157.193.40.10...Connected to eduserv2.rug.ac.be.Mon Oct 2 14:27:43 1998Connection closed by foreign host.

• standard Telnet port : 23==> in general response is prompt for “login: ” and “password: ”

(does not belong to Telnet protocol)• also other ports possible (server side should not run Telnet deamon)

example : ECHO (port 7), DAYTIME (port 13), HTTP (80), …

Telnet client : - send characters from keyboard- show received characters on display

Ask creation of a socket to eduserv2 at port 7first name to address translation is done : 157.193.40.10

then connection is set-up

We type Hallo and see it on the display (Telnet operation)We receive it a second time on the display (Echo operation)

^C will close the connection

Simple example : Telnet


Chapter 2 outline

2.1 Principles of app layer protocols 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail 2.5 DNS


First some jargon 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:

www.ibcn.intec.UGent.be/courses/CommNet.html

host name path name

Web and HTTP


HTTP: hypertext transfer protocol

Web’s application 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

HTTP requestHTTP response

Mac runningNavigator

HTTP request

HTTP response

HTTP overview


HTTP overview (continued)

Uses TCP: server is listening (open

socket) client initiates TCP connection

(creates socket) to server, port 80

server accepts TCP connection from client

HTTP messages (application-layer protocol 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


Web page example CommNet.html

ietf.gif

rug.gif

atlantis.gif

intec.gif

(contains text, references to 4

gif images)


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


Suppose user enters URL www.ibcn.intec.UGent.be/courses/CommNet.html

1a. HTTP client initiates TCP connection to HTTP server (process) at www.ibcn.intec.UGent.be on port 80

1b. HTTP server at host www.ibcn.intec.UGent.be 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 sockettime

2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object courses/CommNet.html

Nonpersistent HTTP


5. HTTP client receives response message containing html file, displays html and closes TCP connection. Parsing html file, finds 4 referenced .gif objects

6. Steps 1-5 repeated for each of 4 gif objects

time

4. HTTP server closes TCP connection.

Nonpersistent HTTP (cont.)


Response time modeling

one RTT to initiate TCP connection

one RTT for HTTP request and first few bytes of HTTP response to return

file transmission timetotal = 2RTT+transmit

time

initiate TCPconnection

RTT

requestfile

RTT time to transmit filefile

received

time time

Definition of RTT (Round Trip Time) : time to send a small packet to travel from client to server and back.

e.g.: 50 msec RTT, 1 kB (1000 x 8)/0.1 = 80 kb/s


Nonpersistent HTTP issues: requires 2 RTTs per object OS must work and allocate

host resources for each TCP connection

but browsers often open parallel TCP connections to fetch referenced objects

Persistent HTTP server leaves connection

open after sending response

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

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

Persistent HTTP


HTTP : Request Messagemethod URL versionSP CR/LFSP

header field name valueCO CR/LF

CR/LF

Entity Body


...request line

new line

header lines

method : GET, POST, HEAD (e.g.: GET http://www.ietf.org/ HTTP/1.0)

header : connection : indicate if the TCP connection should be closed after the transfer

user-agent : software used on client side (e.g. Mozilla/4.0)

accept : list of what formats are acceptable (e.g. text/html, image/gif, image/jpeg)

accept-language : preferred language to receive information (e.g.: fr)

used with POST(e.g. when a form is filled and transferred with the request)

GET http://www.ietf.org/ HTTP/1.0Connection: closeUser-agent: Mozilla/4.0Accept: text/html, image/gif, image/jpegAccept-language: fr


Uploading form input

Post method: Web page often

includes form input Input is uploaded to

server in entity body

URL method: Uses GET method Input is uploaded in

URL field of request line:

www.somesite.com/animalsearch?monkeys&banana


Method types

HTTP/1.0 GET POST HEAD

asks server to leave requested object out of response

HTTP/1.1 GET, POST, HEAD PUT

uploads file in entity body to path specified in URL field

DELETE deletes file specified

in the URL field


HTTP : Response Message

version : HTTP/1.0 or HTTP/1.1status code : numberphrase : explanation of status codeheader : Connection: , Date: , Server: , Last-Modified: ,

Content-Length: , Content-Type: , ...

version Status code phraseSP CR/LFSP response line

CR/LF new line



... header lines

Entity Body the requested information

HTTP/1.1 200 OKDate: Fri, 18 Feb 2000 15:46:11 GMTServer: Apache/1.3.6 (Unix)Last-Modified: Tue, 21 Dec 1999 13:44:47 GMTETag: "a-8ae-385f844f"Accept-Ranges: bytesContent-Length: 2222Connection: closeContent-Type: text/html


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:


HTTP example$ telnet www.UGent.be 80Trying 157.193.40.33...Connected to sangoku.ugent.be.GET HTTP://www.UGent.be/ HTTP/1.0 HTTP/1.1 200 OKDate: Fri, 18 Feb 2000 15:46:11 GMTServer: Apache/1.3.6 (Unix)Last-Modified: Tue, 21 Dec 1999 13:44:47 GMTETag: "a-8ae-385f844f"Accept-Ranges: bytesContent-Length: 2222Connection: closeContent-Type: text/html <html> <head> <title>UNIVERSITEIT GENT - UNIVERSITY OF

GHENT</title> <style type="text/css"> ... </style></head>

<body bgcolor="#000066" link="#cccccc" vlink="#cccccc" alink="#666666">

...</body></html>Connection closed by foreign host

connection set-up

request to get UGent homepage

reply header

HTML documentUGent homepage

close connection


User-server interaction: authorization

Authorization : control access to server content

authorization credentials: typically name, password

stateless: client must present authorization in each request authorization: header line in

each request if no authorization: header,

server refuses access, sends WWW authenticate:

header line in response

usual http request msg401: authorization req.

WWW authenticate:

usual http request msg

+ Authorization: <cred>usual http response

msg


+ Authorization: <cred>usual http response

msg

client server

time


Cookies: keeping “state”

Many major Web sites use cookies

Four components:1) cookie header line in

the HTTP response message

2) cookie header line in HTTP request message

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

4) back-end database at Web site

Example: Susan access Internet

always from same PC She visits a specific e-

commerce site for first time

When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID


Cookies: keeping “state” (cont.)

client server

usual http request msgusual http response

+Set-cookie: 1678


cookie: 1678usual http response

msg


cookie: 1678usual http response msg

cookie-specificaction

cookie-spectificaction

servercreates ID

1678 for user

access

acce

ss

Cookie file

amazon: 1678ebay: 8734

Cookie file

ebay: 8734

Cookie file

amazon: 1678ebay: 8734

one week later:

entry in backend

database

CP_GUTC128.107.241.139.211311061359797259cisco.com/0203749184031418710307135939229583073*


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 search engines use

redirection & cookies to learn yet more

advertising companies obtain info across sites

aside


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

client: specify 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

client 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

Conditional GET: client-side caching


Chapter 2 outline



FTP: the file transfer protocol

transfer file to/from remote host client/server model

client: side that initiates transfer (either to/from remote)

server: remote host ftp: RFC 959 ftp server: port 21 (control), port 20 (data)

file transfer FTPserver

FTPclient

local filesystem

remote filesystem

FTPuser

interfaceuser

at host


FTP: separate control, data connections FTP client contacts FTP server

at port 21, specifying TCP as transport protocol

Client obtains authorization over control connection

Client browses remote directory by sending commands over control connection.

When server receives a command for a file transfer, the server opens a TCP data connection to client

After transferring file, server closes data connection

Server opens a new TCP data connection to transfer another file.

FTPclient FTP

server

TCP control connection

port 21

TCP data connection

port 20

Control connection: “out of band”

FTP server maintains “state”: current directory, earlier authentication

USER, PASS, PWD, CWD, LIST, PORT, RETR, STOR, QUIT


FTP commands, responses

Sample commands: sent as ASCII text over

control channel USER username PASS password LIST return list of file in

current directory RETR filename retrieves

(gets) file STOR filename stores

(puts) file onto remote host

Sample return codes status code and phrase

(as in HTTP) 331 Username OK,

password required 125 data connection

already open; transfer starting

425 Can’t open data connection

452 Error writing file


FTP example

$ Telnet ftp.microsoft.com 21220 CPMSFTFTPA06 Microsoft FTP Service (Version 5.0).Connected to: MicrosoftUSER anonymous331 Anonymous access allowed, send identity (e-mail name) as password.PASS230-This is FTP.MICROSOFT.COM Please see the dirmap.txt230-file for more information.230 Anonymous user logged in.SYST215 Windows2000PWD257 "/" is current directory.TYPE A200 Type set to A.PORT 157,193,122,155,4,18200 PORT command successful.LIST150 Opening ASCII mode data connection for /bin/ls.226 Transfer complete.

CWD /products/250 CWD command successful.TYPE A200 Type set to A.PORT 157,193,122,155,4,19200 PORT command successful.LIST150 Opening ASCII mode data connection for /bin/ls.226 Transfer complete.


FTP example


Chapter 2 outline

2.1 Principles of app layer protocols 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail

SMTP, POP3, RFC822, IMAP 2.5 DNS


Electronic mail

SMTPPort 25

SMTPPort 25

POP3Port 110

User agent (UA)Email client

(e.g. Pine, Eudora)

Internet SMTP

Simple Mail Transfer Protocol

Post Office Protocol

there may be intermediate

MTA’s

Message Transfer Agent (MTA)Email server, SMTP server(e.g. sendmail)

mailserver

mailserver

user mailbox

outgoing message queue

SMTP

POP3


SMTP (Simple Mail Transfer Protocol) : transfer e-mail message from UA to MTA or between MTAs

POP3 (Post Office Protocol 3)retrieve e-mail from MTA

IMAP (Internet Message Access Protocol)advanced retrieve of e-mail from MTAintelligence in MTA (also advanced database structure)

RFC 822 (message format)format of a plain text message

MIME (Multipurpose Internet Mail Extensions)format and coding of non plain text messages (e.g. JPEG, Word) and split into several sub-messages (e.g. attachments)

Electronic mail protocols/formats


telnet mailserver 25

Trying 157.193.84.4...Connected to mailserver.intec.rug.ac.be.Escape character is '^]'.220 mailserver.intec.rug.ac.be ESMTP Sendmail 8.9.0/8.9.0; Fri, 11 Feb 2000 10:36:18 +0100 (MET)

acnet0:/home/staff/janedoe$

HELO intec.rug.ac.be250 mailserver.intec.rug.ac.be Hello acnet0.intec.rug.ac.be [157.193.84.63], pleased to meet you

MAIL FROM:<[email protected]>

250 <[email protected]>... Sender ok

RCPT TO:<[email protected]>

250 <[email protected]>... Recipient ok

DATA

354 Enter mail, end with "." on a line by itself

Subject: example messageJohn,hier een kleine test-boodschap..250 KAA29013 Message accepted for delivery

Electronic mail : SMTP


intec2:home/staff/johndoe$ telnet allserv 110

Trying 157.193.40.42...Connected to allserv.rug.ac.be.Escape character is '^]'.+OK QPOP (version 2.2) at allserv.rug.ac.be starting. <[email protected]>USER johndoe

+OK Password required for johndoe.

PASS <right password>

+OK johndoe has 145 messages (8421978 octets).

STAT

+OK 145 8421978

LIST 125

+OK 125 1596

RETR 125

+OK 1596 octets< RFC822-headers + MIME-headers + message contents> (see next slide)

POP3 (also HTTP) : pull protocol <> SMPT : push protocol

Electronic mail : POP3


IMAP (Internet Message Access Protocol)

Keep all messages in one place: the server

Allows user to organize messages in folders

IMAP keeps user state across sessions: names of folders and mappings between

message IDs and folder name

IMAP3 (port 220) SMTP(port 25)IMAPmail

server+ archive

Web based e-mail access : HTTP protocol


Mail message format

RFC 822: standard for text message format:

header lines, e.g., To: From: Subject:different from SMTP commands

! body

the “message”, ASCII characters only

header

body

blankline


MIME (Multipurpose Internet Mail Extensions) : multimedia mail extension, RFC 2045, 2056

additional lines in msg header declare MIME content type

From: [email protected] To: [email protected] Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data

MIME version

method usedto encode data

multimedia datatype, subtype,

parameter declaration

encoded data

Message format: multimedia extensions


Mail message format exampleReturn-Path: <[email protected]>Delivered-To: [email protected]: from mserv.rug.ac.be (mserv.rug.ac.be [157.193.40.37]) by allserv.rug.ac.be (8.9.3/8.9.3) with ESMTP id RAA19192 for <[email protected]>; Fri, 11 Feb 2000 10:39:45 +0100

(MET)Received: from mailserver.intec.rug.ac.be (mailserver.intec.rug.ac.be

[157.193.84.3]) by mserv.rug.ac.be (8.9.3/8.9.3) with ESMTP id RAA21860 for <[email protected]>; Fri, 11 Feb 2000 10:39:19 +0100 (MET)Received: from acnet0.intec.rug.ac.be (acnet0.intec.rug.ac.be

[157.193.84.63]) by mailserver.intec.rug.ac.be (8.9.3/8.9.3) with SMTP id RAA19039 for <[email protected]>; Fri, 11 Feb 2000 10:38:41 +0100 (MET)Date: Fri, 11 Feb 2000 10:38:41 +0100 (MET)From: Jane Doe <[email protected]>Subject: example messageMessage-Id: <[email protected]>MIME-Version: 1.0Content-Type: textContent-Length: 34

John,hier een kleine test-boodschap..

RFC 822headers

MIMEheaders

Message


<RFC822 headers left away>Mime-Version: 1.0Content-Type: multipart/mixed; boundary="=====================_909671503==_"X-UIDL: 8adae81620fdf73614975fcaa08a3ed5Status: OX-Status: --=====================_909671503==_Content-Type: text/plain; charset="us-ascii"

John,This is an email message with two attached MS-Word documents.

--=====================_909671503==_Content-Type: application/msword; name="MIMEtest1.doc"; x-mac-type="42494E41"; x-mac-creator="4D535744"Content-Transfer-Encoding: base64Content-Disposition: attachment; filename="MIMEtest1.doc"

0M8R4KGxGuEAAAAAAAAAAAAAAAAAAAAAPgADAP7/CQAGAAAAAAAAAAAAAAABAAAAIQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA...AAAAAAAAAAAAAAAAAAAAAAAAAA==--=====================_909671503==_Content-Type: application/msword; name="MIMEtest2.doc"; x-mac-type="42494E41"; x-mac-creator="4D535744"Content-Transfer-Encoding: base64Content-Disposition: attachment; filename="MIMEtest2.doc"

0M8R4KGxGuEAAAAAAAAAAAAAAAAAAAAAPgADAP7/CQAGAAAAAAAAAAAAAAABAAAAIQAAAAAAAAAAEAAAIwAAAAEAAAD+////AAAAACAAAAD/////////////////////////////////////////////...AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA==--=====================_909671503==_--.

RFC 822 headers

MIMEheader

Message

Attachment1

Attachment2

MIMEheader

MIMEheader

End Of Message

Mail message format example

Les 3-4


Chapter 2 outline



DNS : application layer protocol using distributed database to provide name to address translation

• Two possible identifications : name or address• Name : used by Internet users• Address : used by Internet hardware

Address : 4 bytes (4 numbers with values between 0 and 255)example : 157.193.40.7 (corresponding to inf6serv.ugent.be server)

advantage : fixed limited length, hierarchical, easy to handle in the network, physical structure of the network

Name : mnemonic : xxx.xxx. … .xxx- example : intec.ugent.be- advantage : readable, independence of name and address,

logical structure of an organization

Some examples (at INTEC): www.intec.ugent.be <==> 157.193.173.228 (Web server) [<==> webserver3.intec.ugent.be]mail.intec.ugent.be <==> 157.193.172.4 (Mail server) [<==> mail-tech.intec.ugent.be]mail2.intec.ugent.be <==> 157.193.214.4 (Mail server) [<==> plinius.intec.ugent.be]plato.intec.ugent.be <==> 157.193.173.140 (a host)

DNS : Domain Name System


rug.ac.be

• hierarchical• distributed assignment• relative distinguished names• logical structure of organization

root

com edu org fr... be

ac alcatelgov

rug vub kuleuven

intec2 elistelin

ac comlucent alcatel

beckket ovidius

dom

ain

host

ovidius.intec2.rug.ac.be

recently : rug.ac.be ugent.be



• different names for same IP address : aliaseswww.intec.ugent.be and webserver3.intec.ugent.be

• different IP addresses for one namewww.cnn.com : Addresses: 64.236.24.20, 64.236.24.28, 64.236.16.20, 64.236.16.52,

64.236.16.84, 64.236.16.116, 64.236.24.4, 64.236.24.12

• absolute name : intec.ugent.be.full name specification (up to highest level) : end with a “.”

• relative name : intec.ugentonly lower level specified (higher level is known from the context)

• location of name server should not be within the domaine.g. ugent.be has a (back-up) name server at ns.belnet.be (193.190.198.10)

• transport layer : UDP for DNS queries and answers• transport layer : TCP for zone transfers

(=exchange of data between name servers)



Questions to be answered : -1- What is the IP address of tacitus.intec.ugent.be ?-2- Who is the name server for the “ugent.be” domain ?-3- Who is the mail server for [email protected] ?-4- What is the canonical name of mail.intec.ugent.be ?



• Resolver : local program (client side) sending out a mapping request

(allserv.ugent.be ?) to local name server

resolver

client

LocalDNS server

16

RootDNS server

2

5

AuthoritativeDNS server

3

recursive

• Local name server (default name server) : handles request from client

contacts other name server(s) to resolve the name• Root name server : top level root server (13 in total)• Authoritative name server : where host (requested name) is registered

(at least two authoritative name servers for each host)

allserv.ugent.be ?

4157.193.40.42

b USC-ISI Marina del Rey, CAl ICANN Marina del Rey, CA

e NASA Mt View, CAf Internet Software C. Palo Alto, CA

i NORDUnet Stockholm

k RIPE London

m WIDE Tokyo

a NSI Herndon, VAc PSInet Herndon, VAd U Maryland College Park, MDg DISA Vienna, VAh ARL Aberdeen, MDj NSI (TBD) Herndon, VA

DNS : mapping name to address


resolver

client

LocalDNS server

18

RootDNS server2

3


67

iterative

IntermediateDNS server

4

5



resolver

client

LocalDNS server

18

RootDNS server2

3


56

iterative + recursive

IntermediateDNS server

4

7

caching of name/address translation pairs• caching in intermediate name servers• improve delay performance of name/address translation• reduce number of DNS queries on the network• cached record is valid limited in time (few days)



How to store information in the DNS databases ? Resource Record (RR) :

[name], [TTL], [class], record-type, record-data

name : name to be resolvedTTL : how long record may be cachedclass : IN (for Internet)record-type : e.g.: NS, A, MX, CNAMErecord-data : e.g.: IP address

What to store in a DNS :- list of the worldwide root servers - list of names (host, name server, …) and their corresponding address- alias names and their canonical name- list of addresses and their corresponding names (for inverse look-up)- ...

DNS : Resource Records (RR)


Record-Types / Record-Data :

A : the name is a hostname and the record-data is the IP-addressplinius.intec.ugent.be IN A 157.193.214.4

NS : the name is a domain and the record-data is the hostname of a server that knows how to obtain the IP addresses in that domain

ugent.be IN NS ugdns1.ugent.be (authoritative name server for ugent.be)

intec2.ugent.be IN NS ugdns1.ugent.be (intermediate name server for intec2.ugent.be)

CNAME : the name is an alias for a hostname and the record-data is the corresponding canonical hostname

mail2.intec.ugent.be IN CNAME plinius.intec.ugent.be

MX : the name is a domain name and the record-data is the corresponding name of a mail server (MTA), preference indicates the primary, secondary, … mail servers for the domain

intec.ugent.be IN MX preference=10 mail-tech.intec.ugent.be

preference=30 cedar.ugent.be

preference=30 pecan.ugent.be

DNS : Resource Records (RR)


header

questions

answers

authority

additional

QR|Op Code|AA|TC|RD|RA|Z|RCODE

ID

number of questions

number of answers

number of authority records

number of additional records

DNS : Messages


$ nslookup> set type=A> set d2> plinius.intec2.ugent.beServer: dns5.intec.ugent.beAddress: 157.193.122.2

------------SendRequest(), len 41 HEADER: opcode = QUERY, id = 111, rcode = NOERROR header flags: query, want recursion questions = 1, answers = 0, authority records = 0,

additional = 0

QUESTIONS: plinius.intec2.ugent.be, type = A, class = IN

Working on local PC

dns5.intec.ugent.be is the default DNS server

IP address for host plinius.intec2.ugent.be

debug mode

DNS : Message : Request


Got answer (162 bytes): HEADER: opcode = QUERY, id = 111, rcode = NOERROR header flags: response, auth. answer, want recursion, recursion avail. questions = 1, answers = 1, authority records = 1, additional = 1

QUESTIONS: plinius.intec2.ugent.be, type = A, class = IN ANSWERS: -> plinius.intec2.rug.ac.be type = A, class = IN, dlen = 4 internet address = 157.193.122.4 ttl = 86400 (1 day) AUTHORITY RECORDS: -> intec2.ugent.be type = NS, class = IN, dlen = 10 nameserver = tacitus.intec2.ugent.be ttl = 86400 (1 day) ADDITIONAL RECORDS: -> tacitus.intec2.ugent.be type = A, class = IN, dlen = 4 internet address = 157.193.122.1 ttl = 86400 (1 day)

This is the requested answer : plinius has IP address 157.193.122.4

(it is an A record)

This is the nameserver that was able to give the answer(it is an NS record)

This is the IP address of the nameserver(it is an A record)

DNS : Messages : Answer


MTA 4MTA 2

local DNS intec2

root DNS eduDNS

ucsc.eduDNS

cse.ucsc.eduDNS

email clientat intec2

email to jane.doe@

cse.ucsc.edu

.forward : jane.doe@

it.cse.ucsc.edu

resolve (MX)cse.ucsc.edu

resolve (MX)it.cse.ucsc.edu

1

2

34

5 10

6 78

9

11

14

13

2012

17

18

MTA 3

MTA2 = terra.cse.ucsc.eduMTA3 = sundance.cse.ucsc.eduMTA4 = mail.it.cse.ucsc.edu

it.cse.ucsc.eduDNS

15

16

Local MTAplinius.intec2

19

Example : DNS + e-mail


Received: by plinius (mbox pietdm)...Received: from mail-tech.intec.ugent.be

by plinius.intec2.ugent.be for <[email protected]

Received: from allserv.ugent.beby mail-tech.intec.ugent.befor <[email protected]>

Received: from mserv.ugent.beby allserv.ugent.befor <[email protected]>

Received: from plinius.intec2.ugent.beby mserv.ugent.befor <[email protected]>

Received: from ECO.intec2.ugent.beby plinius.intec2.ugent.befor <[email protected]>

SMTP server = mail.intec2.ugent.be(alias for plinius.intec2.ugent.be)

Example : multiple MTA’s


my computereco.intec2.ugent.be

email to piet.demeester@

ugent.be

mail.intec2.ugent.be=plinius.intec2.ugent.be mserv.ugent.be

allserv.ugent.bemail-tech.intec.ugent.be

my local mail serverat INTEC2 (Urbis)

mail server for ugent.be

Split mails to : eduserv1eduserv2allserv

.forwardto intec.ugent.be

mail serverfor intec.ugent.be

Split mails to : INTEC (local)

INTEC2 (Urbis)

Example : multiple MTA’s


application service requirements: reliability, bandwidth,

delay

client-server paradigm Internet transport

service model connection-oriented,

reliable: TCP unreliable, datagrams:

UDP

Our study of network apps now complete!

specific protocols: HTTP FTP SMTP, POP, IMAP DNS

Chapter 2 Summary


Chapter 2 Summary

typical request/reply message exchange: client requests info or

service server responds with

data, status code

message formats: headers: fields giving

info about data data: info being

communicated

Most importantly: learned about protocols

control vs. data msgs in-band, out-of-band

centralized vs. decentralized

stateless vs. stateful reliable vs. unreliable msg

transfer “complexity at network

edge” security: authentication


Table of contents

2.1 Principles of app layer protocols 3 2.2 Web and HTTP 16 2.3 FTP 37 2.4 Electronic Mail 43 2.5 DNS 53 Summary 70 Table of contents 72

Application Layer2-1 Chapter 2 Application Layer Part of slides provided by J.F Kurose and K.W. Ross, All Rights Reserved Communication Networks P. Demeester.

Documents

application layer2

application layer slide

applicationlayer protocols

http r

outline r

rfcs r

interoperability r

r types of messages