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DATA COMMUNICATON 2

Common Network Common Network ServicesServices

BY;Muhammad Aurangzeb12BS(cs)27

Departament of Computer sciences-Quest

Network ServicesNetwork Services

• GOALS;

• understand most common data services in networks:o Domain Name Servers (DNS)o Remote access serviceso File transfer serviceso e-mail serviceso Streaming services

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IntroductionIntroduction

• Network Services -> key in companies and organizations

• Work based in shared network resources & distributed services

• Great diversity of services

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Types of Network ServicesTypes of Network ServicesConfiguration and management

o Management of hosts.o e.g. DHCP.

Remote Accesso Remote hosts are allowed to access to a network. o e.g. SSH

File managemento File transfer, storage and management o e.g. FTP.

Print serviceso Printer share.

Informationo Information sharing and queryingo e.g. WWW, video sharing, IPTV

Communicationo User communication by means of text, audio and/or video

messageso e.g. e-mail, chat, videoconference, IP telephony, games

online.5

Architecture of Network ServicesArchitecture of Network Services

• ApproachesApproaches

o Client-serverClient-server

o Peer-to-peer (P2P)Peer-to-peer (P2P)

o Hybrid of client-server and P2PHybrid of client-server and P2P

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Client/Server architectureClient/Server architecture

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server: always-on host permanent IP

address server farms for

scalingclients:

communicate with server

may be intermittently connected

may have dynamic IP addresses

do not communicate directly with each other

client/server

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P2P ARCHITECTUREP2P ARCHITECTURE

• no always-on server• arbitrary end systems

directly communicate• peers are

intermittently connected and change IP addresses

• Highly scalable but difficult to manage

peer-peer

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Hybrid of client-server and Hybrid of client-server and P2PP2PSkype

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

detection/location• user registers its IP address with central

server when it comes online• user contacts central server to find IP

addresses of buddies

DNS: Domain Name DNS: Domain Name SystemSystem

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Problem: Web browsing

Resource holder’s host

User’s host

Internet

DNS: Domain Name SystemDNS: Domain Name System

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Approach: From the user’s point of view

o Identifies resource holder by means of an address (www.dte.us.es)

Cont:Cont:

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Approach: May www.dte.us.es be used as an identifier of the

queried resource?

User’s host

Internet

www.dte.us.es

Cont:Cont:

Resource holder’s host

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Approach: From the network’s point of view

o IP addresses are used (routing & addressing)

Internet

IP

Cont:Cont:

User’s hostResource

holder’s host

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Approach: A mechanism to translate names into IP addresses is

necessary

Internet

www.dte.us.es

IP

DNS

Cont:Cont:

User’s hostResource

holder’s host

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Hosts, routers: IP address (32 bits) – used to address datagrama “name”, e.g.: www.google.es – used by human beings

130.213.40.3

Myserver.dte.us.es

Cont:Cont:

Resource holder’s host

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Name space: my-pc.cont.fake.es.

Host name

domain

Cont:Cont:

DNS: Domain Name SystemDNS: Domain Name System

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IP Address 4 bytes in decimal format (69.146.202.8) Hierarchical structure -> precise information about host

location

Name No host location information except maybe the country

DNS: Domain Name SystemDNS: Domain Name System

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Domain Name System: distributed database

implemented in hierarchy of many name servers

application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) DNS uses UDP services

DNS services hostname to IP

address translation host aliasing

Canonical, alias names

mail server aliasing load distribution

replicated Web servers: set of IP addresses for one canonical name

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Basic foundations

1. App needs to know a remote IP address associated to a name

2. App requests IP address to DNS client

3. DNS client sends a request to the network

4. DNS client rcvs a reply including IP addr

5. DNS client gives IP address to app

DNS: Domain Name SystemDNS: Domain Name System

Why not centralize DNS? single point of failure traffic volume distant centralized

database maintenance

doesn’t scale!

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Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS servers

yahoo.comDNS servers

amazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Distributed, Hierarchical DatabaseDatabase

Client wants IP for www.amazon.com; 1st approx: client queries a root server to find com DNS

server client queries com DNS server to get

amazon.com DNS server client queries amazon.com DNS server to get IP

address for www.amazon.com

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DNS: Root name serversDNS: Root name servers

contacted by local name server that can not resolve name root name server:

contacts authoritative name server if name mapping not known

gets mapping returns mapping to local name server

13 root name servers worldwide

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

e NASA Mt View, CAf 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, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)

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Local Name ServerLocal Name Server

does not strictly belong to hierarchy each ISP (residential ISP, company,

university) has one. also called “default name server”

when host makes DNS query, query is sent to 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

authoritative DNS serverdns.cs.umass.edu

TLD DNS server

DNS name DNS name resolution exampleresolution example

Host at cis.poly.edu wants IP address for gaia.cs.umass.edu

iterated query:

contacted server replies with name of server to contact

“I don’t know this name, but ask this server”

recursive query:

puts burden of name resolution on contacted name server

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DNS name resolution exampleDNS name resolution example

recursive queryiterative query

requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

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23 4

5

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authoritative DNS serverdns.cs.umass.edu

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TLD DNS server

requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

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2

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6

authoritative DNS serverdns.cs.umass.edu

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8

TLD DNS server

3

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DNS protocol, messagesDNS protocol, messages

Name, type fields for a query

RRs in responseto query

records forauthoritative servers

additional “helpful”info that may be used

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DNS protocol, messagesDNS protocol, messages

Header, 6 fields, 2 bytes each

Access remote Access remote servicesservices

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Access remote servicesAccess remote services

Virtual connection to a remote terminal

“Substitutes” a direct cable

Access remote services Telnet SSH

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TelnetTelnet

Basically -> TCP connection (port 23) Some other features negotiated in handshake

TCP/ IPInternet

Telnetclient

Operative system

Telnetserver

Operative system

Client readsfrom terminal

Client sends to server

Server receives from client

Server sends to pseudo-terminal

No security –> even passwords are visible Solution: encryptation -> SSH Telnet is now quite obsolete

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SSHSSH

SSH (Secure Shell): RFC 4251 Aims similar to telnet -> provides security! TCP connection (port 22) Other additional functions

o Secure FTPo support any insecure protocol ->

tunnelingo secure connections for X Windows

servers -> graphic apps

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SSH. FeaturesSSH. Features

Reduces security menaces:o Man in the middle -> sniffer o Spoofing

Cyphered data -> 128-bit encryptation Security scheme -> public/private key

(RSA) Client-server arquitecture. Two versions

o SSHv1 (1995) -> vulnerabilities

o SSHv2 (1997) Improvements over SSHv1 Current standard

File Transfer ServicesFile Transfer Services

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File Transfer ServicesFile Transfer Services

File transfer between remote hosts Aims:

Remote hosts may share files Client & server file systems are independent Efficient data transfer

Two main protocols FTP (File Transfer Protocol): uses TCP -> reliable

• RFC 959 TFTP (Trivial File Transfer Protocol): uses UDP ->

more simple• RFC 1350

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TFTPTFTP

Trivial File Transfer Protocol File transfer Very simple protocol Non reliable -> UDP (port 69) No folders; no encryptation For transfering small files

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TFTPTFTP TFTP messages

Opcode(2 bytes)

Variable length data

Opcode: type of message• 01: RRQ (Read Request) • 02: WRQ (Write Request)• 03: DATA• 04: ACK• 05: Error message

Variable length data: depend on the opcode

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FTP: the file transfer protocolFTP: 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 Uses TCP: ports 20, 21 -> reliable transfer

file transfer FTPserver

FTPuser

interface

FTPclient

local filesystem

remote filesystem

user at host

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FTP Client FTP Server

Data transfer

Data transfer

Control dialogue

Controldialogue

TCP protocol

Two TCP connections Data: transferred data (port 20) Control: allows the user moving through the

directory structure and downloading & uploading files (port 21)

FTPFTP

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FTP Client/Server modelFTP Client/Server model

FTP Client FTP Server

Data transfer

Data transfer

Control dialogue

Controldialogue

TCP protocol

FTP Client initiates connection (server’s port 21) Connection parameters are negotiated in

handshake Data port Connection mode: active/pasive Transfer mode: ASCII/binary

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FTP ServersFTP Servers

Configuration parameters Control port (default: port 21) Max # of connections to server & max # of connections

per IP Connection timeout Welcome & goodbye msgs Passive mode port numbers

Users & groups Authenticated users: have login & passwd -> registered

in server Anonymous users Groups: share the same properties in FTP server

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FTP ServersFTP Servers

Permissions Read, write, execution (rwx) Permissions for the owner, for groups & for the rest of

users

Bandwidth limit Server may limit transfer rate for the users

Logs Register data or any other info about user connections

& errors

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FTP ClientsFTP Clients

ftp <ip_addr> Commands

cd get put mkdir exit … Do not mistake FTP

commands typed by client for FTP control commands

FTP commands, responsesFTP 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

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E-mail servicesE-mail services

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E-mail servicesE-mail servicesMain features:• One of the most important services on the

Internet• Allows users to exchange mails in an easy,

fast and cheap way• Multiple receivers• Client-server scheme• Types of client apps

o Graphic interfaces (Microsoft Outlook, Mozilla Thunderbird, Apple Mail)

o Text (pine, elm, mail)o Web (Gmail, Hotmail, SquirrelMail)

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E-mail servicesE-mail services

Related concepts:• Mail account

o Associated to user name & passwduser@domain.com

• Mailbox• Mail alias• Mail list

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E-mail servicesE-mail services

Standards:• SMTP (Simple Mail Transfer Protocol)• IMF (Internet Mail Format)• MIME (Multipurpose Internet Mail

Extensions)• POP (Post Office Protocol)• IMAP (Internet Message Access Protocol)

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E-mail servicesE-mail servicesComponents:• Mail User Agent (MUA)• Mail Transfer Agent (MTA)• Mail Delivery Agent (MDA)

Mail User Agents (MUA)• Mail client• Compose, edit, read mail

messages• Use two mail servers:

o Outgoing mail server (SMTP)

o Incoming mail server (POP or IMAP)

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User mailbox

Outgoing mail

queue

MTA

MUA

MUA

MUAMTA

MUA

MUA

MTA

MUA

SMTP

SMTP

SMTP

POPIMAP

E-mail servicesE-mail servicesMail Agent Transfer (MTA)• Mail server• Stores sender’s mails for

delivery (outgoing queue) • Stores incoming mails for

their users

Mail Delivery Agent (MDA)

• In charge of copying incoming messages to user mailbox

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MTA

MUA

MUA

MUAMTA

MUA

MUA

MTA

MUA

SMTP

SMTP

SMTP

POPIMAP

User mailbox

Outgoing mail

queue

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Scenario: Alice sends message to Bob1) Alice uses UA to

compose message and “to” bob@someschool.edu

2) Alice’s UA sends message to her mail server; message placed in message queue

3) Client side of SMTP opens TCP connection with Bob’s mail server

4) SMTP client sends Alice’s message over the TCP connection

5) Bob’s mail server places the message in Bob’s mailbox

6) Bob invokes his user agent to read message

useragent

mailserver

mailserver user

agent

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2 3 4 56

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Message formatMessage format• IMF (RFC 5322)IMF (RFC 5322)• HeadersHeaders

o To:To:o From:From:o Subject:Subject:o Date: Date:

• BodyBodyo Simple text messages Simple text messages

(no extended ASCII ) up (no extended ASCII ) up to 998 characters(no to 998 characters(no CRLF)CRLF)

Headers

Body

Blank line

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Message formatMessage format

MIME extensions:• Add funcionalityAdd funcionality

o Attached filesAttached fileso Extended ASCII Extended ASCII

• New headersNew headerso Mime-Version:Mime-Version:o Content-Type:Content-Type:

Default -> text/plainDefault -> text/plainAttachments -> Attachments ->

MultipartMultiparto Content-Description:Content-Description:o Content-Transfer-Content-Transfer-

Encoding:Encoding:

Types de encoding 7 bits 8 bits & binary quoted-printable &

base64.

Example quoted-printableF3 = ó & F1 = ñTransmisión de ñ

Transmisi=F3n de =F1

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Web accessWeb accessFeatures:• A browser is used instead of a mail client• MUA is integrated in a web page• Host uses HTTP to communicate with web server• Web server habitually uses IMPA for the access to

the incoming msgs in the mail server

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Problems Problems Main problems:• Unencrypted msgs are transmitted

o Use mechanisms of security (PGP, PEM, s/MIME)

• Bad uses o SPAM

StreamingStreaming

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Streaming Stored Streaming Stored Multimedia Multimedia

Stored streaming: media stored at source transmitted to client streaming: client playout

begins before all data has arrivedo timing constraint for still-to-be transmitted

data: in time for playout

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Streaming Streaming LiveLive Multimedia Multimedia

Examples: Internet radio talk show live sporting event

Streaming (as with streaming stored multimedia)

playback buffer playback can lag tens of seconds after

transmission still have timing constraint

Interactivity fast forward impossible rewind, pause possible!

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