Communicating Applications

(1)

(2)

Network Applications

The

raison d'être

of computer networks

Innovation happens in the application layer

– There is always a “killer application” – Remote login -> Email -> Web -> P2P

– Easy to add new protocols – only end-hosts need to install application program

Lower layers “fixed”

(3)

Applications Exist in End-points

Application Transport Network Link Network Link Application Transport Network Link API API

(4)

What is in an Application?

Application process App. Protocol BSD Socket API Application process App. Protocol BSD Socket API HTTP,SMTP,FTP, etc. OS Stack OS Stack

(5)

Application Architectures

(6)

Client-Server Architecture

Applications: Web, Email, File Transfer

Server:

– provides a service, e.g., content

– always on, passive, fixed address/name – single point of failure

– server farms, load balancing, provide robustness and scalability

Client:

– service consumer

(7)

Peer-to-Peer Architecture

Applications: BitTorrent, Gnutella, Skype, etc.

No “infrastructure”

– Hosts/peers communicate directly and are “equal” – self-scalability

– often requires boot-strap server (entry into network)

Applications often form a logical

Overlay Network

physical network

(8)

Client/Server Architectures with

Overlay Aspects

DNS (Domain Name System)

– Hierarchical network of DNS servers

Akamai

– Content Delivery Network (CDN) for Web content

– Clever use of DNS to redirect your request to nearest server

Email

– MTAs communicate via SMTP to deliver emails

Routing Protocols (RIP, OSPF)

– Routers communicate to exchange path information

(9)

Where to Draw the Line

No strict definition of peer-to-peer and overlay

“General” definition

– p2p = many-to-many (client/server = many-to-one)

– Includes distributed client/server (infrastructure in p2p mode)

“Pure/strict” definition

– No infrastructure – Entirely user driven

– Often associated with specific applications (e.g, KaZaa, Gnutella, BitTorrent, Skype)

(10)

Internet Traffic Trends

(11)

Application Service

Requirements

Bandwidth elastic elastic elastic audio: 5Kb-1Mb video:10Kb-5Mb same as above few Kbps up elastic Time Sensitive no no no yes, 100’s msec yes, few secs yes, 100’s msec yes and no Application file transfer e-mail web documents real-time audio/video stored audio/video interactive games financial apps Data loss no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss

(12)

Transport Services Available to

Applications

TCP (Transport Control Protocol):

– full duplex byte stream connection

– reliable data transfer (loss-free, in order delivery) – congestion control

UDP (User Datagram Protocol):

– lightweight, connectionless datagram delivery service – unreliable (losses, out-of-order delivery)

– no congestion control

DCCP (Datagram Congestion Control Protocol):

– a mix between TCP and UDP

(13)

Addressing Applications

In packet switched networks all packets carry the

end-host address of the destination

– e.g., IP address in the Internet Protocol

But, once at the destination, how does the host

know which application the packet is for?

– The transport protocol specifies a 16-bit port number – A port maps to a process on the host

– The task of mapping the incoming packets to specific applications is called demultiplexing

p1 p2 p3

Network stack p1 p2 p3

Network stack

(14)

Port Numbers

Port numbers assigned by IANA/ICANN

Well-known port numbers (0-1023)

– 80 = HTTP, 21 = FTP, 25 = SMTP

– Restricted access in OS

Registered port numbers (1024-49151)

– Application ports that can be registered by e.g., companies

– 1214 = KaZaa, 26000 = Quake

Dynamic port numbers (49152-65535)

(15)

Writing Applications: What is a

socket?

Operating system handle for identifying the

end-points of communication

Berkeley sockets (BSD)

de facto

standard

To an application process a socket is just a number

Internally (in the OS), a socket is a “struct”:

– Local IP and port number – Remote IP and port number – State

sock = socket(PF_INET, SOCK_STREAM, 0); bind(sock, &sock_addr, addr_length);

(16)

(17)

The World Wide Web (1990s)

Four main components

– Server (e.g., Apache)

– Client (Firefox, IE, Safari)

– Application protocol (HTTP=stateless protocol) – Objects/data (HTML document, picture)

Persistent and non-persistent connections

Parallel transfers

Cookies

– state on clients

(18)

HyperText Transfer Protocol

<title>UU/Department of Information Technology</title>

</head>

<table style='height:100%;overflow:visible;' cellspacing

GET / HTTP/1.1 Host: www.it.uu.se Connection: close User-agent: Mozilla/4.0 Accept-language: sv HTTP/1.1 200 OK Connection: close

Date: Mon, 03 Nov 2008 12:00:01 GMT Server: Apache/1.3.0 (Unix)

Last-Modified: Sun, 2 Nov 2008 15.34:23 GMT Content-Length: 8736

(19)

File Transfer Protocol - FTP

“Killer application” of the 80ies

Separate channels for control traffic and data traffic

– Commands do not have to wait for data (sent out-of-band)

Authentication

Server keeps state

– e.g., client's current directory

File system FTP client

FTP

FTP Server

FTP

TCP data connection (port 20) TCP control connection (port 21)

(20)

Email

Asynchronous communication service

One of the first applications of the Internet

• Remains one of the most important applications to this day

Components:

• Mail User Agents (MUAs):

• Outlook, Thunderbird

• Mail Transfer Agents (MTAs)

• Sendmail, Postfix, Microsoft Exchange server

• Simple Mail Transfer Protocol (SMTP)

• Push protocol

• 7-bit ASCII encoding (headers and body)

(21)

E-mail client

Sending and Receiving Email

E-mail client SMTP server SMTP server POP server SMTP server SMTP SMTP SMTP SMTP POP POP

(22)

SMTP Interaction

S: 220 hamburger.edu C: HELO crepes.fr

S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]>

S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]>

S: 250 [email protected] ... Recipient ok C: DATA

S: 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

(23)

SMTP vs. HTTP

HTTP is a pull protocol

SMTP is a push protocol

Both have ASCII command/response interaction

HTTP encapsulates each object in its own response

message

SMTP sends multiple objects in multipart message

SMTP uses persistent connections (optional for

(24)

24

Mail message format

SMTP: protocol for exchanging email msgs

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

blank line

(25)

25

Message format: multimedia extensions

MIME: 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

multimedia data type, subtype, parameter declaration method used to encode data MIME version encoded data

(26)

26

POP3 and IMAP

POP3

“Download and delete” mode.

Cannot re-read e-mail if one changes client

“Download-and-keep”: copies of messages on different clients

POP3 is stateless across sessions

IMAP

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

(27)

Domain Name System (DNS)

Problem:

It is difficult to remember addresses based

on numbers

Name hosts with human readable names

– e.g. hamberg.it.uu.se

Translate between address and name

– Initially done by editing /etc/hosts file

• Not really scalable

– DNS developed in the late 80ies

(28)

28

Root DNS Servers

com DNS servers org DNS servers edu DNS servers poly.edu DNS servers umass.edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers

Distributed, Hierarchical Database

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

(29)

2: Application Layer 29 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu 1 2 3 4 5 6 authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server

DNS name

resolution 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”

(30)

2: Application Layer 30

DNS: 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, 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, MD