Application  Level  o Client-Server and Peer-to-Peer Paradigmso HTTP: Web Surfingo FTP: remote connectivityo SMTP: emailingo DNS: symbolic addressingo P2P: file sharing

Some network applications■World Wide Web

■HTTP ■E-Mail:

■SMTP, Gmail ■Social networking:

■Facebook, Twitter, Instagram, Snapchat, ecc..

■P2P file sharing: BitTorrent, eMule, etc.. ■Video streaming:

■ NetFlix, YouTube, Hulu■IP Telephony:

■Skype, Hangout, etc… ■ Network games■Video conference ■Massive parallel computing■ Instant messaging ■Remote login:

■TELNET■…

Downlink traffic (North America, March 2016)

Source: www.statista.com (2016)

Designing network applications

Write programs thatn run on different end

systems, andn communicate over a

network.n e.g., Web: Web server

software communicates with browser software (FireFox, Safari, Chrome …)

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

Designing network applicationsapplicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

Little software written for devices in network coren network core devices do

not run user application code

n applications on end systems allow for rapid app development, propagation etc. (no need to change network software, only end-users/servers)

Communications among Processes

Process: program running within a host.o within same host, two processes

communicate using inter-process communication (defined by OS).

o processes in different hosts communicate by exchanging messages

Processes and Protocols

o Processes running on remote host may exchange messages and services through the network

o The application protocols define the rules and the formats of the communication between remote processes

Application Protocols

Web  (web  server,  browser,  HTML) HTTP

E-­mail  (mail  server,  mail  client,  MIME) SMTP

Lower layers interactiono Application protocols use the services provided

by lower layers through the SAPs (Service Access Point)

o Each application process is associated to a SAPo OSI Stack:

Presentation

Session

WEB FTP Mail

Transport

Application controlled Layer

OS controlled layers

Lower layers interactiono The application protocols communicate

directly with the transport layer

WEB FTP Mail

TCP/UDP

Application controlled Layer

OS controlled layers

o process sends/receives messages to/from its socket

o socket analogous to doorn sending process shoves

message out of the door

n sending process relies on 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

Sockets

o Sockets equivalent to SAPs between application and transport layers

Addressing Processes o To receive messages, a process must

have an identifiero Each host device has unique 32-bit IP

addresso Q: does IP address of host on which

process runs suffice for identifying the process?

Addressing Processes

n Answer: NO, many processes can be running on the same host

n identifier includes both IP address and port number associated with process on host

n Example port numbers (HTTP server: 80, Mail server: 25)

n to send HTTP message to www.polimi.it:o IP address: 131.175.12.34o Port number: 80

n The transport layer multiplexes several flows coming from the application layer

App-layer protocol defineso Types of messages

exchanged, n e.g., request, response

o Message syntax:n what fields in messages

& how fields are delineated

o Message semantics n meaning of information

in fieldso Rules for when and how

processes send & respond to messages

Public-domain protocols:

o defined in RFCso allows for

interoperabilityo e.g., HTTP, SMTPProprietary protocols:o e.g., KaZaA

What transport service does an app need?

Data losso some apps (e.g.,

audio) can tolerate some loss

o other apps (e.g., file transfer, telnet) require 100% reliable data transfer

Timingo some apps (e.g.,

Internet telephony, interactive games) require low delay to be “effective”

Bandwidtho some apps (e.g.,

multimedia) require minimum amount of bandwidth to be “effective”

o 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:o connection-oriented:

setup required between client and server processes

o reliable transport between sending and receiving process

o flow control: sender won’t overwhelm receiver

o congestion control:throttle sender when network overloaded

o does not provide: timing, minimum bandwidth guarantees

UDP service:o unreliable data

transfer between sending and receiving process

o does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee

Q: why bother? Why is there a UDP?

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.,  Vonage,Dialpad)

Underlyingtransport  protocol

TCPTCPTCPTCPTCP  or  UDP

typically  UDP

Applications vs Transport Protocols

Client-Server Architecture

o The main target of the communication between remote processes is the provisioning of services

o Two functionalities can be accomplished by a process:n Request for servicesn Provide services

o If a given process accomplishes just one of the above functionalities, the communication is a client-server one

client serverrequest

response

Client-Server Architecture

o Client processes make requests and interpret responses

o Server processes interpret requests and provide the responses

o If the same host needs to issue requests and provide responses two processes are needed

Client/Server Paradigmo Differences between program and process

n Program: softwaren Process: instance of the program being executed

o A server process is continuously executed on the host (daemon) and is activated through a passive open

o A client process is activated when needed only (by the user or by some other process) through an active open

o After the passive open the server is able to handle requests from clients

o The active open requires the indication of the IP address and the port of the server

request

response

... ...

Client/Server Paradigmo Multiple clients can issues requests to a

single servero Clients may also issue multiple requests at

the same time

Client/Server Paradigm

o A client may implement both serial and parallel operation modesn Example: multiple requests can be issued

for all the files contained in a web pageo Even a server may implement both

serial and parallel operation modeso Usually, the applications using UDP are

handled serially

Client/Server Paradigm

o Usually, the servers using TCP implement parallel operation mode

o A TCP connection towards all the clients is opened for all the time needed to exchange requests/responses

o The procedure handling each client is handled via multi-threading, using fork operations

Application Protocols: Possible Architectures

o Client-servern Terminals act as clients OR as serversn Client hosts and server hosts may have

different featureso Peer-to-peer (P2P)

n All the terminals can implement the client process AND the server one

o Hybrid

Client-server architectureServer:

n always-on hostn permanent IP

addressn server farms for

scalingClients:

n communicate with server

n may be intermittently connected

n may have dynamic IP addresses

n do not communicate directly with each other

Pure P2P architectureo no always-on servero arbitrary end

systems directly communicate

o peers are intermittently connected and change IP addresses

o example: Gnutella

Highly scalable but difficult to manage

Hybrid of client-server and P2P

Skypen Internet telephony appn Finding address of remote party: centralized

server(s)n Client-client connection is direct (not through

server) Instant messaging

n Chatting between two users is P2Pn Presence detection/location centralized:

o User registers its IP address with central server when it comes online

o User contacts central server to find IP addresses of buddies

Web  BrowsingHyper Text Transfer Protocol (HTTP)

HyperText Tranfer Protocol (HTTP)

o client-server architectureo clients request objects (files) identified

through a URLo servers send back the files to the clientso Stateless operation (no memory on

previous requests is mantained)

client serverHTTP request

HTTP response

• "Hypertext Transfer Protocol -- HTTP/1.0," RFC 1945, May 1996. • "Hypertext Transfer Protocol -- HTTP/1.1," RFC 2616, June 1999.• “Hypertext Transfer Protocol” Version 2 (HTTP/2), RFC 7540, May 2015.•

Message transfero HTTP relies upon TCP for message transfer

o Usually a web page is composed of a main document (HTML) and multiple linked objects

o Requests use theURL (Uniform Resource Locator)

http://www.u-psud.fr/index.html

Type of protocol

Symbolic address of theserver

Document on the server

TCP assigns port number 80 to HTTP servers

Method Host:// : Port Path/

Message transfero Suppose a client requests a composite web page

(1 main HTML document + 10 figures)

:

■Two operation modes can be adopted

■Non-persistentconnection (default mode of HTTP 1.0)

■Persistent connection(default mode of HTTP 1.1)HTML Text

Other Objects

Non-persistento One TCP connection for each request-response cycle.

The server closes the TCP connection once it has sent the requested object

o The same procedure is adopted for all the docs within the required web page

o Multiple TCP connections can be opened in parallelo The maximum number of connections can be set in the

browser configuration options

Request (index.html)

Response (index.html file)Request (image1.jpg)

Request (image2.jpg)

Persistent connectiono The server does not close the connection after the responseo The same connection can be used to transfer other objects

within the same page or even other web pageso The server closes the connection on a timeout basiso Two Flavors:

n without pipelining: the client issues a new request only upon reception of the previous response

n with pipelining: multiple requests can be issued at the same time (default mode HTTP v1.1)

Requests

Some Methods

GET To get a doc from the server. The doc is specified by the URL. The server answers with the required doc in the body of the response message

HEAD To get info on a specified doc. The server answers with the requested information

POST To post some input to the server regarding a given object identified by the URL

PUT To store a doc on the server. The doc is carried by the request message. The URL specifies the position for the doc to be stored.

■Other Methods:■ PATCH, COPY, MOVE, DELETE, LINK, UNLINK, OPTIONS.

Responses

Messages

100 Continue: Go On

.

500 Internal server error

Server failure

501 Not implemented Required functionality not supported

503 Service unavailable

Unavailable service

200 OK: Request OK, the required info is in the field of this message

302 Moved Permanently:

The required object has been moved (perm)

304 Moved Temporalily:

The required object has been moved (temp)

400 Bad Request: Generic error401 Unauthorized: Access failed due to userID or

password error404 Not Found: File not found

■1xx Informational

■2xx Success

■3xx Redirection

■4xx Client error

■5xx Server error

Headers

o headers are used to exchange further service information

o A message can carry multiple headerso Examples

Header name Header value:

Cache-control Cache infoAccept Supported formatsAccept-language Supported languagesAuthorization Client permitsIf-modified-since send doc. only if modified

User-agent user agent type

Message Exchangeo Example: request

GET /ntw/index.html HTTP/1.1 Connection: close User-agent: Mozilla/4.0 Accept: text/html, image/gif, image/jpeg Accept-language:it

HTTP/1.1 200 OK Connection: close Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 09:23:24 GMT Content-Length: 6821 Content-Type: text/html data data data data data ...

o Example: response

Conditional get

Client:GET /fruit/kiwi.gif HTTP/1.0 User-agent: Mozilla/4.0 Accept: text/html, image/gif, image/jpeg If-modified-since: Mon, 22 Jun 1998 09:23:24

Server:HTTP/1.0 304 Not Modified Date: Wed, 19 Aug 1998 15:39:29 Server: Apache/1.3.0 (Unix) (empty entity body)

o Also method HEAD can be used

Network caching and proxy

o Main duty of a proxy is to provide a distributed cache memory

o If a doc is stored in a proxy near the client the download time can be reduced

Proxy

o A proxy is an application gateway, that is, it implements up to the application layer

o It must act both as a client and as a servero The final server speaks with the client on

the proxy (hiding of users)

LL

IP

TCP

HTTPUser Agent

LL

IP

TCP

HTTP

Server

LL

IP

TCP

HTTPProxy

Authenticationo HTTP is statelesso Consecutive requests from the same user

cannot be recognizedo Very simple authentication procedure based

on userID and password to be inserted in the requests

GET /ntw/index.html HTTP/1.1

401 Authorization RequiredWWW-Authenticate:[authentication type]

GET /ntw/index.html HTTP/1.1Authorization: account, passwd

GET image.gif HTTP/1.1Authorization: account, passwd

...

Cookieso The server can assign to each client a cookie number

which identifies the client in future transactionso The cookie number is stored by the client and used in

the following requests towards the same servero Used in e-commerce, among others

GET /ntw/index.html HTTP/1.1

200 OKSet-cookie:18988466

GET /ntw/basket/index.html HTTP/1.1Cookie: 18988466

GET image.gif HTTP/1.1Cookie: 18988466

...

HTML (HyperText Markup Language)

o HTTP handles the object transfer and does not account for the object format

o The visualization of the object is done through interpreter programs (browsers)

o Formatted text pages are transferred in ASCII files and are interpreted according to formatting instructions written in HTML

o HTML pages may contain references to other objects which need to be interpreted by the browser as n Part of the document to visualizen Links to other pages

o If an HTML page is stored on the server and is sent upon request, this is a static page

Dynamic WEB Pages

o If a page is created on-the-fly upon reception of a request, this is a dynamic page

o The server examines the request, executes a program associated to the request and generates the HTML page to be sent back

GET /cgi-bin/prog.pl HTTP/1.1

prog.pl

200 OK Dynamic Page

Active Web Pageso A web page may contain a program to

be executed by the cliento The program is downloaded and

executed locally by the cliento This can be used to set up interactive

pages, moving graphs, etc.

GET /java/applet HTTP/1.1

200 OK program

Page download with HTTP 1.0

o We can download an HTTP page using telnet

o 1) We just need to connect to the web server, port 80telnet www.u-psud.fr 80Trying 129.175.212.14...Connected to cmspub2.di.u-psud.fr.Escape character is '^]'.

o 2) We request (GET) a webpageGET /fr/index.html HTTP/1.0

o 3) The page is downloaded, the connection is closed

cURL

o cURL is a command-line tool for manipulating and transfer data with URLs (see http://curl.haxx.se/)

o Let’s try something:n Obtain a webpage:

o curl www.u-psud/fr/index.htmln If we only want the header of this page:

o curl –-head www.u-psud/fr/index.htmlHTTP/1.1 200 OKDate: Wed, 22 Mar 2017 08:15:53 GMTServer: Apache/2.2.15 (CentOS)Accept-Ranges: bytesX-Mod-Pagespeed: 1.12.34.1-0Vary: Accept-Encoding,User-AgentCache-Control: max-age=0, no-cacheContent-Length: 164349Connection: closeContent-Type: text/html; charset=UTF-8

cURL

o Let us monitor the exchange of files between the client and the servern curl -–trace-ascii filename.txt www.u-psud.fr/fr/index.html

o If we are interested in the request/ response time (cURLcan be used to measure the loading time of webpages):n curl -–trace-ascii filename.txt -–trace-time www.u-

psud.fr/fr/index.html

09:24:13.149793 == Info: Trying 129.175.212.14...09:24:13.151325 == Info: Connected to www.u-psud.fr (129.175.212.14) port 80 (#0)09:24:13.151396 => Send header, 90 bytes (0x5a)0000: GET /fr/index.html HTTP/1.1001d: Host: www.u-psud.fr0032: User-Agent: curl/7.43.0004b: Accept: */*0058:09:24:13.197224 <= Recv header, 17 bytes (0x11)0000: HTTP/1.1 200 OK…

File  Transfer  Protocol  (FTP)

File Transfer Protocol (FTP)

o Used to transfer files between two remote hosts

o The application operates directly on the file system (both at the server and at the client side)

"File Transfer Protocol”, RFC 959, October 1985.

File Transfer Protocol (FTP)o Uses TCP for the transfero Two TCP connections are used for the

transfer of data and control

User Interface

Control process

Data tranfer process

LFS

Control process

Data transfer process

LFS

client

server

Port 21

Port 20

FTP: user interface

FTP: control connectiono It is opened in the usual way

n The server issues a passive open with port number 21 and waits for requests

n The client issues an active open with a dynamic port number every time it needs to transfer files

o The control connection is persistent, and remains open for the whole data transfer time

Control process

Data transfer process

LFS LFS

client serverPassive openPort 21 Control

process

Data transfer process

Active openPort 66778

FTP: Data connections

o Data connections are non-persistent, n one connection for each file to transfer, n connection closed upon completion of file transfer

o To open a data connection:1st Way:n The client issues a passive open with a dynamic port

numbern The client notifies the port number to the server on the

control connection through the PORT commandn The server issues an active open towards the specified

port of the client, using 20 as local port number2nd Way:n The client sends the PASV command to the servern The server chooses a dynamic port number, issues a

passive open and communicate the chosen port number to the client

n The client issues an active open using the port number received from the server

FTP: Data connections

o The data transfer can be accomplished in different ways and using different formats:

o File types:n ASCII n Binary:

o Transmission modes:n Stream mode: the file is sent down to TCP as a

stream of unstructured bytesn Block mode: the file is structured in blocks with

a header each, and sent down to TCP

FTP: commandso Commands are transferred in ASCII

USER usernamePASS password QUIT log out

Access CommandsCWD change directoryDELE delete fileLIST list filesRETR retrieve fileSTOR store file

File Management

TYPE file typeMODE transfer mode

PORT client portPASV server choose port

Transfer Management

Port Management

FTP: Responses125 Data connection already open; transfer starting 200 Command OK225 Data connection open226 Closing data connection227 Entering passive mode; srv. sends Ip_add.,port230 User login OK331 Username OK, password required 425 Can't open data connection 426 Connection closed; tranfer aborted452 Error writing file 500 Syntax error; unrecognized command501 Syntax error in parameters or arguments502 Command not implemented

FTP: Transfer Example

Client Server220 service ready

USER fabio

331 username OK; password ?

PASS pippo123

230 user login OK

PORT 65667

150 opening data connection

LIST /usr/pub

125 data connection OK

226 closing data connection

Data

E-­mail  ServiceSimple Mail Transfer Protocol (SMTP)

E-mailo Service to send textual messages in an

asynchronous way

o It is implemented through a network of mail servers using the SMTP (Simple Mail Transfer Protocol)

SMTP

o Textual protocolo Also the body of the messages needs to be

ASCIIn Binaries must be converted to ASCII

o Once a server receives a message from a user agentn Stores the message in a queuen Opens a TCP connection (port 25) with the

destination servern Sends the message

J.B. Postel, "Simple Mail Transfer Protocol," RFC 821, August 1982.

Client/Server Message exchange

S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr>S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu>S: 250 bob@hamburger.edu ... Recipient ok C: DATAS: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: .S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection

Han

dsha

ke

Message Format

o The message format is specified (command DATA)

o Some headers are added to the message

D.H. Crocker, "Standard for the Format of ARPA Internet Text Messages," RFC 822, August 1982.

From: alice@crepes.fr To: bob@hamburger.edu Subject: Request of information<black line><Body>.

Multipurpose Internet Mail Extensions (MIME)

o MIME is used to allow the transfer of non-ASCII messages

•"Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies," RFC 2045, Nov. 1996. •"Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types," RFC 2046, Nov. 1996.

From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ........................ .....base64 encoded data.

Multipurpose Internet Mail Extensions (MIME)

o Coding techniques:n Base64:

o The flow of bits is divided into chunks of 24 bits eacho Each chunk is divided into 4 groups of 6 bits eacho Each chunk is interpreted as a character according

to a conversion table

11001100 10000001 00111001

110011 (51) 001000 (8) 000100 (4) 111001 (57)

01111010 01001001 01000101 00110101

Z I E 5

base64

Multipurpose Internet Mail Extensions (MIME)

o Quoted-printableo The flow of bits is divided into chunks of 8 bits eacho If a sequence corresponds to a ASCII character, it is sent

straight awayo Otherwise it is sent as three characters: “=“ followed by

the hexadecimal representation of the byte

00100110&

01001100L

10011101Non-ASCII

Quotable-printable

001110019

00100110&

01001100L =

001110019

1001 9

1101D

Multipurpose Internet Mail Extensions (MIME)

o MIME allows the transfer of multiple objects within the same message:

From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe with commentary MIME-Version: 1.0 Content-Type: multipart/mixed; Boundary=StartOfNextPart --StartOfNextPart Dear Bob, Please find a picture of an absolutely scrumptious crepe.

--StartOfNextPart Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data .....

--StartOfNextPart Let me know if you would like the recipe..

MailBox Access Protocols

n POP3 (Post Office Protocol version 3)n IMAP (Internet Mail Access Protocol)n HTTP

n Security Issue: the protocols can run over TLS/SSL (Transport Layer Security, Secure Sockets Layer)

ESMTP

POP3

Authorization Phaseo Client Commands:

n user: usernamen pass: password

o Server Responses:n +OKn -ERR

Transaction Phase, client:o list: list mess. numbero retr: get messageo dele: delete messageo quit

C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents>S: . C: dele 1 C: retr 2 S: <message 1 contents>S: . C: dele 2 C: quit S: +OK POP3 server signing off

S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on

Case History

o December 1995, S. Bhatia and J. Smith propose the first web based e-mail service (Hotmail)

o In 1 month, 100K userso In 1 year, 12M userso December 1997, Hotmail is acquired

by Microsoft for $400Mo Example of “first mover advantage”

and “viral marketing”

Remote  TerminalTELNET

TELNET (TErminaL NETwork)o Remote terminal applicationo The commands are transferred through a TCP

connection

Terminaldriver

TCP

IP

LL

Telnetclient

Pseudo-terminaldriver

TCP

IP

LL

Telnetserver

Internet

TELNET (TErminaL NETwork)o TELNET transfers characters

n Data characters:o ASCII with the first byte “0”o ASCII characters with the first bite “1” (preceded

by a special control byte)n Control characters:

o 8 bit commands (first bit “1”)o examples

n IAC (255): next one is a control charactern EC (247): erase character

c a t f i l e a IAC EC 1

IAC = Interpret As CommandEC = Erase Character

TELNET (TErminaL NETwork)

Domain  Name  System  (DNS)

Domain Name System (DNS)

o IP addresses are not suited to be used by applications

o Symbolic addresses are more convenientn Hierarchical (street, city, state)

n Independent from layer 3

o Binding is needed

131.175.21.1 morgana.elet.polimi.it

Domain Name System (DNS)

o IP networks provide a symbolic addressing service …

o … supported by a distributed database service which handles the binding: DNS (Domain Name System)

o DNS is an application layer protocol which uses UDP/IP to transfer its messages

o DNS is currently used also forn Host aliasingn Mail server aliasingn Load distribution

"Domain Names - Concepts and Facilities," RFC 1034, Nov. 1987. "Domain Names - Implementation and Specification," RFC 1035, Nov. 1987.

Symbolic Addressing

o Hierarchical Addressingo Each branch is controlled by a known

authorityo To get a symbolic address you must go

through these authorities

com edu org gov mil it fr jp de ...

ucla columbia polimi

eletcs

virgilio

rett

yahoo

morgana

morgana.elet.polimi.it

Types of Name Serverso Local Name Servers

n Directly connected to the hostsn Each ISP (residential, university, industry, etc.)

has a LNSn Talks with the Root NS

o Root Name Serversn Stores info on the addressing of big groups of

hosts and domainsn Stores info on the authoritative NS for a given

domainn Talks with the Authoritative NS

o Authoritative Name Serversn NS responsible for a specific hostname

Hierarchical DNS

Every level in the hierarchy has a different “depth” of information

Example: a user wants the IP address of www.google.com

– Root Name Servers know how to “find” the name servers that manage “.com” domains

– .com servers know how to find NS that manage domain “google.com”

– google.com Name Servers know the correspondence “symbolic name www.google.com<--->IP Address”

Root NS

How To resolve a binding

o Every host knows (has configured) the Local NS (LNS) address

o Each request for resolving a binding is sent to the local NS using UDP

o The LNS gets the info and answers

HOST Local NS

DNS request

DNS response DNS Network

Stored Infoo Type

n A: Name is a host name and Value is the IP address(morgana.elet.polimi.it, 131.175.21.1, A, TTL)

n NS: Name is a domain and Value is the symbolic name of a server which knows how to resolve the name(elet.polimi.it, morgana.elet.polimi.it, NS, TTL)

n CNAME: Name is an alias and Value is the real name(www.polimi.it, zephyro.rett.polimi.it, CNAME, TTL)

n MX: Name is a mail domain or a mail alias and Value is the name of the mail server(elet.polimi.it, mailserver.elet.polimi.it, MX,TTL)

Name, Value, Type, TTL

Database Organizationo ARPANET was using a central databaseo Internet uses a distributed database structureo Branches are divided into zones and each zone is

associated to a DNSo The server of a zone is authoritative for that zone

com edu org gov mil it fr jp de ...

ucla columbia polimi

eletcs

virgilio

rett

yahoo

morgana

How to get Infoo Recursive Way:

n Requests travel along the hierarchy

n Responses travel the opposite direction

Source: Computer Networking, J. Kurose

How to get Infoo Iterative Way:

o A server can notify the name of another server where to get the info from

Caching

o A server can cache a info temporarily

o If a request is issued regarding cached info, the server can answer even if it is not authoritative for that specific info

o TTL is set by the authoritative server to advertise the “freshness” of a piece of info

o The non-authoritative server uses the TTL to set a validity timer for the cached info

DNS Messages

o identification: identifies the couple request/response

o flag: request/response, authoritative/non auth., iterative/recursive

o number of: field sin the following header sections

o questions: name to resolve (usually A or MX)

o answers: complete resource records

o authority: contains other record provided by other servers

o additional info

Binary Format (not ASCII)

How to add a new domain to the Internet

o A new startup, I-Like-Networking wants to register the domain I-Like-Networking.com(let us suppose this domain is free)

o The startup will register this domain in one of the DNS Registrarsn I-Like-Networking must give to the DNS registrar

the symbolic name and the corresponding IP address of the authoritative Name Server(s)

n The DNS registrar inserts two Resource Records in the TLD (Top Level Domain) server .com

I-­‐Like-­‐Networking,  dsn1.I-­‐Like-­‐Networking.com,  NS  dns1.I-­‐Like-­‐Networking.com,  212.212.212.1,  A  

n The DNS registrar eventually writes a record (of type MX) for I-Like-Networking.com

Simple DNS experimentso Using the command nslookup which permits

to send requests to a given DNS servern man nslookup

o To find the correspondence of a symbolic name:n nslookup www.u-psud.fr

o To find authoritative Name Servers for a given name:n nslookup –type=NS www.u-psud.fr

Simple DNS experimentsnslookup

o To find authoritative Name Servers for a given name:n nslookup –type=NS www.u-psud.fr

Server: 129.175.3.2Address: 129.175.3.2#53

Non-authoritative answer:www.u-psud.fr canonical name = cmspub2.di.u-psud.fr.

Authoritative answers can be found from:di.u-psud.frorigin = dns1.u-psud.frmail addr = dns.u-psud.frserial = 2009095455refresh = 1200retry = 3600expire = 1209600minimum = 3600

Simple DNS experimentsdig command

o The “dig” command (similar to nslookup) gives more detail on the DNS protocoln Let us try a simple query: dig www.u-sud.fr

; <<>> DiG 9.8.3-P1 <<>> www.u-psud.fr;; global options: +cmd;; Got answer:;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 15084;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 3, ADDITIONAL: 6

;; QUESTION SECTION:;www.u-psud.fr. IN A

;; ANSWER SECTION:www.u-psud.fr. 2235 IN CNAME cmspub2.di.u-psud.fr.cmspub2.di.u-psud.fr. 2235 IN A 129.175.212.14

;; AUTHORITY SECTION:di.u-psud.fr. 1494 IN NS ext1.u-psud.fr.di.u-psud.fr. 1494 IN NS dns1.u-psud.fr.di.u-psud.fr. 1494 IN NS dns2.u-psud.fr.

Header of the DNS message

Description of the request

Answer

Authoritative servers for the request

Simple DNS experimentsdig command

;; ADDITIONAL SECTION:dns1.u-psud.fr. 583 IN A 129.175.34.24dns1.u-psud.fr. 583 IN AAAA 2001:660:3201::2dns2.u-psud.fr. 583 IN A 129.175.36.26dns2.u-psud.fr. 583 IN AAAA 2001:660:3201::3ext1.u-psud.fr. 285 IN A 91.121.18.203ext1.u-psud.fr. 3285 IN AAAA 2001:41d0:1:65cb::1

;; Query time: 9 msec;; SERVER: 129.175.3.2#53(129.175.3.2);; WHEN: Wed Mar 22 11:36:27 2017;; MSG SIZE rcvd: 261

Additional information

Performance information on the request

CDNs – Content Distribution Networks

o Problem: how to efficiently distribute content, at the same time, to several users very far from each oterh

o Solution: create a network of geographically distributed servers that “host” a copy of the request content (a big “distributed cache”) –

o The network of servers (CDN) can be owned by the service provider (Google, Netflix, Facebook) or by third parts (Akamai, Limelight …)

CDNs – Content Distribution Networks

o DNS is used to redirect users’ requests to the “best” server, in terms of:

o Closest: choose the server geographically closest to the client

o Shortest path: choose the server reachable with the least number of hops towards the client

o Let the client choose: the service gives a list of possible servers and the client chooses the “best” (Netflix)

Peer-­to-­Peer  Exchange

P2P file sharingExampleo Alice runs P2P client

application on her notebook computer

o Intermittently connects to Internet; gets new IP address for each connection

o Asks for “Hey Jude”o Application displays

other peers that have copy of Hey Jude.

o Alice chooses one of the peers, Bob.

o File is copied from Bob’s PC to Alice’s notebook: HTTP

o While Alice downloads, other users uploading from Alice.

o Alice’s peer is both a Web client and a transient Web server.

All peers are servers = highly scalable!

P2P: centralized directory

Original “Napster” design

1) when peer connects, it informs the central server:n IP addressn content

2) Alice queries for “Hey Jude”

3) Alice requests file from Bob

centralizeddirectory server

peers

Alice

Bob

1

1

1

12

3

P2P: problems with centralized directory

o Single point of failure

o Performance bottleneck

o Copyright infringement

file transfer is decentralized, but locating content is highly centralized

Query flooding: Gnutellao Fully distributed

n no central servero public domain

protocolo many Gnutella

clients worldwide

Overlay network: grapho edge between peer X

and Y if there’s a TCP connection

o all active peers and edges are overlay net

o Edge is not a physical link

o Given peer will typically be connected with < 10 overlay neighbors

Gnutella: protocol

QueryQueryHit

QueryQueryHit

File transfer:HTTP❒ Query message

sent over existing TCPconnections❒ peers forwardQuery message❒ QueryHit sent over reversepath

Scalability:limited scopeflooding

Gnutella: Peer joining

1. Joining peer X must find some other peer in Gnutella network: to use list of candidate peers

2. X sequentially attempts to make TCP with peers on list until connection setup with Y

3. X sends Ping message to Y; Y forwards Ping message.

4. All peers receiving Ping message respond with Pong message

5. X receives many Pong messages. It can then setup additional TCP connections

Exploiting heterogeneity: KaZaA

o Each peer is either a group leader or assigned to a group leader.n TCP connection

between peer and its group leader.

n TCP connections between some pairs of group leaders.

o Group leader tracks the content in all its children.

ordinary  peer

group-­leader  peer

neighoring  relationshipsin  overlay  network

KaZaA: Querying

o Each file has a hash and a descriptoro Client sends keyword query to its group

leadero Group leader responds with matches:

n For each match: metadata, hash, IP addresso If group leader forwards query to other

group leaders, they respond with matcheso Client then selects files for downloading

n HTTP requests using hash as identifier sent to peers holding desired file