CHAPTER 2. Creating a network app write programs that – run on (different) end systems – communicate over network – e.g., web server software communicates.

CHAPTER 2

Creating a network app

write programs that– run on (different) end systems– communicate over network– e.g., web server software

communicates with browser software

No need to write software for network-core devices– network-core devices do not

run user applications – applications on end systems

allows for rapid app development, propagation

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

Application 2-2

Application architectures

• client-server• peer-to-peer (P2P)• hybrid of client-server and P2P

Application 2-3

Client-server architectureserver:

– always-on host– permanent IP address– server farms for scaling

clients:– communicate with server– may be intermittently connected– may have dynamic IP addresses– do not communicate directly

with each other

client/server

Application 2-4

Pure P2P 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

Application 2-5

Hybrid of client-server and P2PSkype

– 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

Application 2-6

Processes communicatingprocess: program running

within a host.• within same host, two

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

• processes in different hosts communicate by exchanging messages

client process: process that initiates communication

server process: process that waits to be contacted

aside: applications with P2P architectures have client processes & server processes

Application 2-7

Addressing processes• to receive messages, process

must have identifier• host device has unique 32-bit

IP address• Q: does IP address of host

on which process runs suffice for identifying the process?

Application 2-8

Addressing processes• to receive messages,

process must have identifier

• host device has unique 32-bit IP address

• Q: does IP address of host on which process runs suffice for identifying the process?– A: No, many processes

can be running on same host

• identifier includes both IP address and port numbers associated with process on host.

• example port numbers:– HTTP server: 80– Mail server: 25

• to send HTTP message to gaia.cs.umass.edu web server:– IP address: 128.119.245.12– Port number: 80

• more shortly…

Application 2-9

App-layer protocol defines

• types of messages exchanged, – e.g., request, response

• message syntax:– what fields in messages &

how fields are delineated

• message semantics – meaning of information in

fields

• rules for when and how processes send & respond to messages

public-domain protocols:• defined in RFCs• allows for interoperability• e.g., HTTP, SMTPproprietary protocols:• e.g., Skype

Application 2-10

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”

Throughput some apps (e.g., multimedia) require

minimum amount of throughput to be “effective”

other apps (“elastic apps”) make use of whatever throughput they get

Security encryption, data integrity, …

Application 2-11

Transport service requirements of common apps

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

Throughput

elasticelasticelasticaudio: 5kbps-1Mbpsvideo:10kbps-5Mbpssame as above few kbps upelastic

Time Sensitive

nononoyes, 100’s msec

yes, few secsyes, 100’s msecyes and no

Application 2-12

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 throughput guarantees, security

UDP service:• unreliable data transfer

between sending and receiving process

• does not provide: connection setup, reliability, flow control, congestion control, timing, throughput guarantee, or security

Q: why bother? Why is there a UDP?

Application 2-13

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

aside

how to keep “state”: protocol endpoints: maintain state at sender/receiver

over multiple transactions cookies: http messages carry state

Application 2-14

Web caches (proxy server)

• user sets browser: Web accesses via cache

• browser sends all HTTP requests to cache– object in cache: cache

returns object – else cache requests object

from origin server, then returns object to client

Goal: satisfy client request without involving origin server

client

Proxyserver

client

HTTP request

HTTP response

HTTP request HTTP request

origin server

origin server

HTTP response HTTP response

Application 2-15

More about Web caching

• cache acts as both client and server

• typically cache is installed by ISP (university, company, residential ISP)

why Web caching?• reduce response time for

client request• reduce traffic on an

institution’s access link.• Internet dense with caches:

enables “poor” content providers to effectively deliver content (but so does P2P file sharing)

Application 2-16

Caching example assumptions• average object size = 100,000 bits• avg. request rate from institution’s

browsers to origin servers = 15/sec• delay from institutional router to

any origin server and back to router = 2 sec

consequences• utilization on LAN = 15%• utilization on access link = 100%• total delay = Internet delay + access

delay + LAN delay = 2 sec + minutes + milliseconds

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

Application 2-17

Caching example (cont)possible solution• increase bandwidth of access link

to, say, 10 Mbpsconsequence• utilization on LAN = 15%• utilization on access link = 15%• Total delay = Internet delay +

access delay + LAN delay = 2 sec + msecs + msecs• often a costly upgrade

originservers

public Internet

institutionalnetwork 10 Mbps LAN

10 Mbps access link

institutionalcache

Application 2-18

Caching example (cont)possible solution: • install cache

consequence• suppose hit rate is 0.4

– 40% requests will be satisfied almost immediately

– 60% requests satisfied by origin server

• utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec)

• total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

Application 2-19

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

file transfer FTPserver

FTPuser

interface

FTPclient

local filesystem

remote filesystem

user at host

Application 2-20

FTP: separate control, data connections

• FTP client contacts FTP server at port 21, TCP is transport protocol

• client authorized over control connection

• client browses remote directory by sending commands over control connection.

• when server receives file transfer command, server opens 2nd TCP connection (for file) to client

• after transferring one file, server closes data connection.

FTPclient

FTPserver

TCP control connection,server port 21

TCP data connection,server port 20

server opens another TCP data connection to transfer another file.

control connection: “out of band” FTP server maintains “state”:

current directory, earlier authentication

Application 2-21

Electronic MailThree major components: • user agents • mail servers • simple mail transfer protocol:

SMTP

User Agent• a.k.a. “mail reader”• composing, editing, reading mail

messages• e.g., Outlook, elm, Mozilla

Thunderbird, iPhone mail client• outgoing, incoming messages

stored on server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Application 2-22

Electronic Mail: mail servers

Mail Servers • mailbox contains incoming

messages for user• message queue of outgoing (to

be sent) mail messages• SMTP protocol between mail

servers to send email messages– client: sending mail server– “server”: receiving mail

server

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Application 2-23

DNS: Domain Name System

people: many identifiers:– SSN, name, passport #

Internet hosts, routers:– IP address (32 bit) - used

for addressing datagrams– “name”, e.g.,

www.yahoo.com - used by humans

Q: map between IP address and name, and vice versa ?

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)– note: core Internet function,

implemented as application-layer protocol

– complexity at network’s “edge”

Application 2-24

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

doesn’t scale!

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

Application 2-25

Formulas

• Dcs = max { NR/Us , F/dmin }• N = Number of clients• Us= Upload capacity of server• F = file size• Dmin = minimum distribution time.

• Dp2p = max { F / Us , F / dmin, NF / us + i=1 to n ui}•

Problem 1

• P1: Consider distributing a file of F=10 Gbits to N peers. The server has an upload rate of Us = 20 Mbps and each peer has a download rate of di = 1 Mbps and upload rate of u. for N = 10, 100 and 1000 and u = 200 Kbps, 600 kbps and 1 Mbs, prepare a chart giving the minimum distribution time for each of the combination of N and u for both client server distribution and P2P distribution.