1dt066 distributed information systems chapter 2 applications

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1DT066Distributed Information Systems

Chapter 2Applications

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Applications: outline

2.1 principles of network applications

2.2 Web and HTTP2.5 Domain Name

Server2.6 P2P applications

Adapted from: Computer Networking, Kurose/Ross

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Some network apps e-mail web text messaging remote login P2P file sharing multi-user network

games streaming stored

video (YouTube, Hulu, Netflix)

voice over IP (e.g., Skype)

real-time video conferencing

social networking search … …

Adapted from: Computer Networking, Kurose/Ross

write programs that: run on (different) end

systems communicate over

networks 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

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Creating a network appapplication

transportnetworkdata linkphysical

application


application


Adapted from: Computer Networking, Kurose/Ross, pp 109-110

Main reason for the

success of Internet

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App processes communicatingprocess: runs a

program within a host Operating System, OS.

within same host, two processes communicate using inter-process communication (defined by OS)

Processes on different hosts communicate by exchanging messages

client process: process that initiates communication

server process: process that waits to be contacted

clients, servers


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Addressing processes

host machine/device has unique 32-bit IP address

to receive messages, process must have identifier that is known to the sender

Q: Why is not the IP address enough for identifying the process?

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

A: no, many processes can be running on same host

Adapted from: Computer Networking, Kurose/Ross, pp 116

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Application architectures

Two dominating architecture/structures of applications:

client-server peer-to-peer (P2P)


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Client-server architectureserver: always-on host permanent IP address (i.e. “ I

know where you are”) often at a data center for scaling

capacity with demand

client: initiates the contact and

communicates with server may be intermittently connected may have dynamic IP addresses.

(Why not permanent?) clients do not communicate

directly with each other

client/server


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P2P architecture end systems (called

peers) communicate directly

peers request service from other peers and provide services in return to the others self scalability – new

peers bring new service capacity, (as well as new service demands)

not always-on server. May be switched off and may change IP addresses complex management.

Why?

peer-peer


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App-layer protocol defines types of messages

exchanged, e.g., request, response

message syntax: different fields in

messages, e.g. for application data and control information to other party

message semantics meaning of information in

control fields (not app data)

rules for when and how processes send & respond to messages

“open”/standard protocols:

defined in RFCs allows for

interoperability e.g., HTTP, SMTP“closed”/proprietary

protocols: e.g., Skype


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What transport service does an app need?data integrity (i.e. no lost or

modified data) some apps (e.g., file

transfer, web transactions) require 100% reliable data transfer

other apps (e.g., audio) can tolerate some loss

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, …


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Transport service requirements: common apps

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


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Web and HTTP

First, a review… web page consists of objects object can be HTML file, JPEG image,

Java applet, audio file,… web page consists of a base HTML-file

which includes several referenced objects

each object is addressable by a URL, e.g.,www.someschool.edu/someDept/pic.gif

host name path name


Universal

Resource

Locator

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HTTP overview

HTTP: hypertext transfer protocol

Web’s application layer protocol

client/server model client: browser that

requests, receives, (using HTTP protocol) and “displays” Web objects

server: Web server sends (using HTTP protocol) objects in response to requests

PC runningFirefox browser

server running

Apache Webserver

iphone runningSafari browser

HTTP requestHTTP response

HTTP request

HTTP resp

onse

Adapted from: Computer Networking, Kurose/Ross, 126-129

Application Layer 2-15

HTTP overview (continued)

Uses Internets (reliable) transport protocol, TCP:

client initiates TCP connection to web server process, usually port 80.

server accepts TCP connection from client

HTTP messages are exchanged between browser (HTTP client) and Web server (HTTP server)

TCP connection closed

HTTP is “stateless” HTTP protocol at server

do not maintain any information about previous requests from client

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.

Why inconsistent?

aside


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HTTP request message

two fundamental types of HTTP messages: request, response

Format of a HTTP request message: Uses ASCII (human-readable format)

request line(GET, POST, HEAD commands)

header lines

carriage return, line feed at startof line indicatesend of header lines

GET /index.html HTTP/1.1\r\nHost: www-net.cs.umass.edu\r\nUser-Agent: Firefox/3.6.10\r\nAccept: text/html,application/xhtml+xml\r\nAccept-Language: en-us,en;q=0.5\r\nAccept-Encoding: gzip,deflate\r\nAccept-Charset: ISO-8859-1,utf-8;q=0.7\r\nKeep-Alive: 115\r\nConnection: keep-alive\r\n\r\n

carriage return character

line-feed character

Adapted from: Computer Networking, Kurose/Ross pp 126-129

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HTTP request message: general format

requestline

headerlines

Body == data

method sp sp cr lfversionURL

cr lfvalueheader field name

cr lfvalueheader field name

~~ ~~

cr lf

entity body~~ ~~


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HTTP response message

status line(protocolstatus codestatus phrase)

header lines

data, e.g., requestedHTML file

HTTP/1.1 200 OK\r\nDate: Sun, 26 Sep 2010 20:09:20 GMT\r\nServer: Apache/2.0.52 (CentOS)\r\nLast-Modified: Tue, 30 Oct 2007 17:00:02

GMT\r\nETag: "17dc6-a5c-bf716880"\r\nAccept-Ranges: bytes\r\nContent-Length: 2652\r\nKeep-Alive: timeout=10, max=100\r\nConnection: Keep-Alive\r\nContent-Type: text/html; charset=ISO-8859-1\

r\n\r\ndata data data data data ...


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HTTP response status codes

200 OK request succeeded, requested object later in this msg

301 Moved Permanently requested object moved, new location specified later

in this msg (Location:)

400 Bad Request request msg not understood by server

404 Not Found requested document not found on this server

505 HTTP Version Not Supported

status code appears in 1st line in server-to-client response message.

some sample codes:


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Trying out HTTP (client side) for yourself1. Telnet to your favorite Web server:

opens TCP connection to port 80(default HTTP server port) at cis.poly.edu.anything typed in sent to port 80 at cis.poly.edu

telnet cis.poly.edu 80

2. type in a GET HTTP request:

GET /~ross/ HTTP/1.1Host: cis.poly.edu

by typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to HTTP server

3. look at response message sent by HTTP server!

(or use Wireshark to look at captured HTTP request/response)Adapted from: Computer Networking, Kurose/Ross, pp 131-134

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Cookies: keeping “state” between requests

client Amazonserver

usual http response msg

usual http response msg

cookie file

one week later:

usual http request msg

cookie: 1678cookie-specificaction

access

ebay 8734usual http request

msgAmazon server

creates ID1678 for user create

entry

usual http response

set-cookie: 1678

ebay 8734amazon 1678

usual http request msg

cookie: 1678cookie-specificaction

access

ebay 8734amazon 1678

backenddatabase


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Cookies (continued)what cookies can

be used for: 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


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DNS: domain name system

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

Internet hosts, routers: IP address (32 bit)

- used for addressing datagrams

“name”, e.g., www.yahoo.com - used by humans

Domain Name System: application-layer protocol:

hosts, name servers communicate to resolve names (address/name translation) Note: This is a core

Internet function, implemented as application-layer protocol

Again - complexity at network’s “edge”

distributed database implemented in hierarchy of many name servers


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DNS: services, structure It doesn’t scale? single point of failure traffic volume distant centralized

database, long delay. maintenance

DNS services hostname to IP address

translation – demo! host aliasing

canonical, alias names mail server aliasing load distribution

replicated Web servers: many IP addresses correspond to one name. Why and when?

why not centralize DNS


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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

DNS: a distributed, hierarchical database

client wants IP for www.amazon.com; 1st approx: client queries Root DNS server, to get .com server client queries .com DNS server to get the amazon.com

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

address for www.amazon.com

… …


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requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

6

authoritative DNS serverdns.cs.umass.edu

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TLD DNS server

DNS name resolution bottom up 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”


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Local DNS 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 has local cache of recent name-to-address

translation pairs (but may be out of date!) acts as proxy, forwards query into hierarchy


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DNS: caching, updating records once (any) name server learns mapping,

it caches mapping cache entries timeout (disappear) after some

time (TTL) root name servers not often visited

cached entries may be out-of-date (best effort name-to-address translation!) if name host changes IP address, may not be

known Internet-wide until all TTLs expire


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Pure P2P architecture no always-on server arbitrary end systems

communicate directly peers are

intermittently connected and may change IP addresses. Still participating.

examples: file distribution

(BitTorrent) Streaming (Spotify) VoIP (Skype)


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File distribution time: client-server server transmission: must

sequentially send (upload) N file copies: time to send one copy: F/us

time to send N copies: NF/us

increases linearly in N

time to distribute F to N clients using

client-server approach Dc-s > max{NF/us,,F/dmin}

client: each client must download file copy dmin = min client download

rate min client download time:

F/dmin

us

network

di

ui

F


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File distribution time: P2P server transmission:

must upload at least one copy time to send one copy:

F/us

time to distribute F to N clients using

P2P approach

us

network

di

ui

F

DP2P > max{F/us,,F/dmin,,NF/(us + ui)}

client: each client must download file copy min client download time:

F/dmin clients: as aggregate must download NF bits max upload rate (limting max download

rate) is us + ui

… but so does this, as each peer brings service capacityincreases linearly in N …


1dt066 distributed information systems chapter 2 applications

Documents

computer networking

process communication

messagesclient process

communicationserver

ip addressto

hostpermanent ip address

user applications applications

http server