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Application Layer

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Chapter 2�Application Layer

Computer Networking: A Top Down Approach �6th edition �Jim Kurose, Keith Ross�Addison-Wesley�March 2012

A note on the use of these ppt slides:

We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:

​

​

  • If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!)
  • If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

​

Thanks and enjoy! JFK/KWR

​

All material copyright 1996-2012

J.F Kurose and K.W. Ross, All Rights Reserved

2 of 109

Chapter 2: outline

2.1 principles of network applications

2.2 Web and HTTP

2.3 FTP

2.4 electronic mail

    • SMTP, POP3, IMAP

2.5 DNS

​

2.6 P2P applications

2.7 socket programming with UDP and TCP

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Chapter 2: application layer

our goals:

  • conceptual, implementation aspects of network application protocols
    • transport-layer service models
    • client-server paradigm
    • peer-to-peer paradigm
  • learn about protocols by examining popular application-level protocols
    • HTTP
    • FTP
    • SMTP / POP3 / IMAP
    • DNS
  • creating network applications
    • socket API

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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
  • …
  • …

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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

​

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application

transport

network

data link

physical

application

transport

network

data link

physical

application

transport

network

data link

physical

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Application architectures

possible structure of applications:

  • client-server
  • peer-to-peer (P2P)

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Client-server architecture

server:

  • always-on host
  • permanent IP address
  • data centers for scaling

clients:

  • communicate with server
  • may be intermittently connected
  • may have dynamic IP addresses
  • do not communicate directly with each other

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client/server

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P2P architecture

  • no always-on server
  • arbitrary end systems directly communicate
  • peers request service from other peers, provide service in return to other peers
    • self scalability – new peers bring new service capacity, as well as new service demands
  • peers are intermittently connected and change IP addresses
    • complex management

​

​

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peer-peer

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Processes communicating

process: 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

​

Application Layer

2-9

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

clients, servers

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Sockets

  • process sends/receives messages to/from its socket
  • socket analogous to door
    • sending process shoves message out door
    • sending process relies on transport infrastructure on other side of door to deliver message to socket at receiving process

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2-10

Internet

controlled

by OS

​

controlled by

app developer

transport

application

physical

link

network

process

transport

application

physical

link

network

process

socket

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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?
  • 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…

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

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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

open protocols:

  • defined in RFCs
  • allows for interoperability
  • e.g., HTTP, SMTP

proprietary protocols:

  • e.g., Skype

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What transport service does an app need?

data integrity

  • 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”

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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

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application

​

file transfer

e-mail

Web documents

real-time audio/video

​

stored audio/video

interactive games

text messaging

data loss

​

no loss

no loss

no loss

loss-tolerant

​

loss-tolerant

loss-tolerant

no loss

throughput

​

elastic

elastic

elastic

audio: 5kbps-1Mbps

video:10kbps-5Mbps

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

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Internet transport protocols services

TCP service:

  • 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 guarantee, security
  • connection-oriented: setup required between client and server processes

​

UDP service:

  • unreliable data transfer between sending and receiving process
  • does not provide: reliability, flow control, congestion control, timing, throughput guarantee, security, orconnection setup,

​

Q: why bother? Why is there a UDP?

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Internet apps: application, transport protocols

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application

​

e-mail

remote terminal access

Web

file transfer

streaming multimedia

​

Internet telephony

​

application

layer protocol

​

SMTP [RFC 2821]

Telnet [RFC 854]

HTTP [RFC 2616]

FTP [RFC 959]

HTTP (e.g., YouTube), �RTP [RFC 1889]

SIP, RTP, proprietary

(e.g., Skype)

underlying

transport protocol

​

TCP

TCP

TCP

TCP

TCP or UDP

​

​

TCP or UDP

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Securing TCP

TCP & UDP

  • no encryption
  • cleartext passwds sent into socket traverse Internet in cleartext

SSL

  • provides encrypted TCP connection
  • data integrity
  • end-point authentication

SSL is at app layer

  • Apps use SSL libraries, which “talk” to TCP

SSL socket API

    • cleartext passwds sent into socket traverse Internet encrypted
    • See Chapter 7

​

​

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Chapter 2: outline

2.1 principles of network applications

    • app architectures
    • app requirements

2.2 Web and HTTP

2.3 FTP

2.4 electronic mail

    • SMTP, POP3, IMAP

2.5 DNS

​

2.6 P2P applications

2.7 socket programming with UDP and TCP

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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 base HTML-file which includes several referenced objects
  • each object is addressable by a URL, e.g.,

​

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www.someschool.edu/someDept/pic.gif

host name

path name

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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

​

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PC running

Firefox browser

server

running

Apache Web

server

iphone running

Safari browser

HTTP request

HTTP response

HTTP request

HTTP response

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HTTP overview (continued)

uses TCP:

  • client initiates TCP connection (creates socket) to server, port 80
  • server accepts TCP connection from client
  • HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)
  • TCP connection closed

HTTP is “stateless”

  • server maintains no information about past client requests

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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

​

aside

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HTTP connections

non-persistent HTTP

  • at most one object sent over TCP connection
    • connection then closed
  • downloading multiple objects required multiple connections

​

persistent HTTP

  • multiple objects can be sent over single TCP connection between client, server

​

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Non-persistent HTTP

suppose user enters URL:

1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80

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2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.index

1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client

3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket

time

(contains text,

references to 10

jpeg images)

www.someSchool.edu/someDepartment/home.index

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Non-persistent HTTP (cont.)

5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects

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6. Steps 1-5 repeated for each of 10 jpeg objects

4. HTTP server closes TCP connection.

time

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Non-persistent HTTP: response time

RTT (definition): time for a small packet to travel from client to server and back

HTTP response time:

  • one RTT to initiate TCP connection
  • one RTT for HTTP request and first few bytes of HTTP response to return
  • file transmission time
  • non-persistent HTTP response time =

2RTT+ file transmission time

​

Application Layer

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time to

transmit

file

initiate TCP

connection

RTT

request

file

RTT

file

received

time

time

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Persistent HTTP

non-persistent HTTP issues:

  • requires 2 RTTs per object
  • OS overhead for each TCP connection
  • browsers often open parallel TCP connections to fetch referenced objects

​

​

​

persistent HTTP:

  • server leaves connection open after sending response
  • subsequent HTTP messages between same client/server sent over open connection
  • client sends requests as soon as it encounters a referenced object
  • as little as one RTT for all the referenced objects

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HTTP request message

  • two types of HTTP messages: request, response
  • HTTP request message:
    • ASCII (human-readable format)

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request line

(GET, POST,

HEAD commands)

header

lines

carriage return,

line feed at start

of line indicates

end of header lines

GET /index.html HTTP/1.1\r\n

Host: www-net.cs.umass.edu\r\n

User-Agent: Firefox/3.6.10\r\n

Accept: text/html,application/xhtml+xml\r\n

Accept-Language: en-us,en;q=0.5\r\n

Accept-Encoding: gzip,deflate\r\n

Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n

Keep-Alive: 115\r\n

Connection: keep-alive\r\n

\r\n

carriage return character

line-feed character

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HTTP request message: general format

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request

line

header

lines

body

method

sp

sp

cr

lf

version

URL

cr

lf

value

header field name

cr

lf

value

header field name

~

~

~

~

cr

lf

entity body

~

~

~

~

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Uploading form input

POST method:

  • web page often includes form input
  • input is uploaded to server in entity body

URL method:

  • uses GET method
  • input is uploaded in URL field of request line:

​

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www.somesite.com/animalsearch?monkeys&banana

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Method types

HTTP/1.0:

  • GET
  • POST
  • HEAD
    • asks server to leave requested object out of response

HTTP/1.1:

  • GET, POST, HEAD
  • PUT
    • uploads file in entity body to path specified in URL field
  • DELETE
    • deletes file specified in the URL field

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HTTP response message

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2-31

status line

(protocol

status code

status phrase)

header

lines

data, e.g.,

requested

HTML file

HTTP/1.1 200 OK\r\n

Date: Sun, 26 Sep 2010 20:09:20 GMT\r\n

Server: Apache/2.0.52 (CentOS)\r\n

Last-Modified: Tue, 30 Oct 2007 17:00:02 GMT\r\n

ETag: "17dc6-a5c-bf716880"\r\n

Accept-Ranges: bytes\r\n

Content-Length: 2652\r\n

Keep-Alive: timeout=10, max=100\r\n

Connection: Keep-Alive\r\n

Content-Type: text/html; charset=ISO-8859-1\r\n

\r\n

data 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

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  • status code appears in 1st line in server-to-client response message.
  • some sample codes:

33 of 109

Trying out HTTP (client side) for yourself

1. Telnet to your favorite Web server:

​

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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/1.1

Host: cis.poly.edu

by typing this in (hit carriage

return twice), you send

this 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)

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User-server state: cookies

many Web sites use cookies

four components:

1) cookie header line of HTTP response message

2) cookie header line in next HTTP request message

3) cookie file kept on user’s host, managed by user’s browser

4) back-end database at Web site

example:

  • Susan always access Internet from PC
  • visits specific e-commerce site for first time
  • when initial HTTP requests arrives at site, site creates:
    • unique ID
    • entry in backend database for ID

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Cookies: keeping “state” (cont.)

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2-35

client

server

usual http response msg

usual http response msg

cookie file

one week later:

usual http request msg

cookie: 1678

cookie-

specific

action

access

ebay 8734

usual http request msg

Amazon server

creates ID

1678 for user

create

entry

usual http response

set-cookie: 1678

ebay 8734

amazon 1678

usual http request msg

cookie: 1678

cookie-

specific

action

access

ebay 8734

amazon 1678

backend

database

36 of 109

Cookies (continued)

what cookies can be used for:

  • authorization
  • shopping carts
  • recommendations
  • user session state (Web e-mail)

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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

37 of 109

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

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goal: satisfy client request without involving origin server

client

proxy

server

client

HTTP request

HTTP response

HTTP request

HTTP request

origin

server

origin

server

HTTP response

HTTP response

38 of 109

More about Web caching

  • cache acts as both client and server
    • server for original requesting client
    • client to origin 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 (so too does P2P file sharing)

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Caching example:

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2-39

origin

servers

public

Internet

institutional

network

1 Gbps LAN

1.54 Mbps

access link

assumptions:

  • avg object size: 100K bits
  • avg request rate from browsers to origin servers:15/sec
  • avg data rate to browsers: 1.50 Mbps
  • RTT from institutional router to any origin server: 2 sec
  • access link rate: 1.54 Mbps

consequences:

  • LAN utilization: 15%
  • access link utilization = 99%
  • total delay = Internet delay + access delay + LAN delay

= 2 sec + minutes + usecs

​

​

problem!

40 of 109

Caching example: fatter access link

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assumptions:

  • avg object size: 100K bits
  • avg request rate from browsers to origin servers:15/sec
  • avg data rate to browsers: 1.50 Mbps
  • RTT from institutional router to any origin server: 2 sec
  • access link rate: 1.54 Mbps

consequences:

  • LAN utilization: 15%
  • access link utilization = 99%
  • total delay = Internet delay + access delay + LAN delay

= 2 sec + minutes + usecs

​

​

origin

servers

1.54 Mbps

access link

154 Mbps

154 Mbps

msecs

Cost: increased access link speed (not cheap!)

9.9%

public

Internet

institutional

network

1 Gbps LAN

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Caching example: install local cache

institutional

network

1 Gbps LAN

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origin

servers

1.54 Mbps

access link

local web

cache

assumptions:

  • avg object size: 100K bits
  • avg request rate from browsers to origin servers:15/sec
  • avg data rate to browsers: 1.50 Mbps
  • RTT from institutional router to any origin server: 2 sec
  • access link rate: 1.54 Mbps

consequences:

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

= 2 sec + minutes + usecs

​

​

?

?

How to compute link

utilization, delay?

Cost: web cache (cheap!)

public

Internet

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Caching example: install local cache

Calculating access link utilization, delay with cache:

  • suppose cache hit rate is 0.4
    • 40% requests satisfied at cache, 60% requests satisfied at origin

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origin

servers

1.54 Mbps

access link

  • access link utilization:
    • 60% of requests use access link
  • data rate to browsers over access link = 0.6*1.50 Mbps = .9 Mbps
    • utilization = 0.9/1.54 = .58
  • total delay
    • = 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache)
    • = 0.6 (2.01) + 0.4 (~msecs)
    • = ~ 1.2 secs
    • less than with 154 Mbps link (and cheaper too!)

public

Internet

institutional

network

1 Gbps LAN

local web

cache

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Conditional GET

  • Goal: don’t send object if cache has up-to-date cached version
    • no object transmission delay
    • lower link utilization
  • cache: specify date of cached copy in HTTP request

If-modified-since: <date>

  • server: response contains no object if cached copy is up-to-date:

HTTP/1.0 304 Not Modified

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HTTP request msg

If-modified-since: <date>

HTTP response

HTTP/1.0

304 Not Modified

object

not

modified

before

<date>

HTTP request msg

If-modified-since: <date>

HTTP response

HTTP/1.0 200 OK

<data>

object

modified

after

<date>

client

server

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Chapter 2: outline

2.1 principles of network applications

    • app architectures
    • app requirements

2.2 Web and HTTP

2.3 FTP

2.4 electronic mail

    • SMTP, POP3, IMAP

2.5 DNS

​

2.6 P2P applications

2.7 socket programming with UDP and TCP

Application Layer

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FTP: the file transfer protocol

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file transfer

FTP

server

FTP

user

interface

FTP

client

local file

system

remote file

system

user

at host

  • 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

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FTP: separate control, data connections

  • FTP client contacts FTP server at port 21, using TCP
  • client authorized over control connection
  • client browses remote directory, sends commands over control connection
  • when server receives file transfer command, server opens 2nd TCP data connection (for file) to client
  • after transferring one file, server closes data connection

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FTP

client

FTP

server

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

​

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FTP 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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Chapter 2: outline

2.1 principles of network applications

    • app architectures
    • app requirements

2.2 Web and HTTP

2.3 FTP

2.4 electronic mail

    • SMTP, POP3, IMAP

2.5 DNS

​

2.6 P2P applications

2.7 socket programming with UDP and TCP

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Electronic mail

Three 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, Thunderbird, iPhone mail client
  • outgoing, incoming messages stored on server

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user mailbox

outgoing

message queue

mail

server

mail

server

mail

server

SMTP

SMTP

SMTP

user

agent

user

agent

user

agent

user

agent

user

agent

user

agent

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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

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mail

server

mail

server

mail

server

SMTP

SMTP

SMTP

user

agent

user

agent

user

agent

user

agent

user

agent

user

agent

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Electronic Mail: SMTP [RFC 2821]

  • uses TCP to reliably transfer email message from client to server, port 25
  • direct transfer: sending server to receiving server
  • three phases of transfer
    • handshaking (greeting)
    • transfer of messages
    • closure
  • command/response interaction (like HTTP, FTP)
    • commands: ASCII text
    • response: status code and phrase
  • messages must be in 7-bit ASCI

​

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Scenario: Alice sends message to Bob

1) Alice uses UA to compose message “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

​

Application Layer

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user

agent

mail

server

mail

server

1

2

3

4

5

6

Alice’s mail server

Bob’s mail server

user

agent

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Sample SMTP interaction

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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: 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

S: 221 hamburger.edu closing connection

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Try SMTP interaction for yourself:

  • telnet servername 25
  • see 220 reply from server
  • enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands

​

above lets you send email without using email client (reader)

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2-54

55 of 109

SMTP: final words

  • SMTP uses persistent connections
  • SMTP requires message (header & body) to be in 7-bit ASCII
  • SMTP server uses CRLF.CRLF to determine end of message

comparison with HTTP:

  • HTTP: pull
  • SMTP: push
  • both have ASCII command/response interaction, status codes
  • HTTP: each object encapsulated in its own response msg
  • SMTP: multiple objects sent in multipart msg

Application Layer

2-55

56 of 109

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 MAIL FROM, RCPT TO: commands!

  • Body: the “message”
    • ASCII characters only

Application Layer

2-56

header

body

blank

line

57 of 109

Mail access protocols

  • SMTP: delivery/storage to receiver’s server
  • mail access protocol: retrieval from server
    • POP: Post Office Protocol [RFC 1939]: authorization, download
    • IMAP: Internet Mail Access Protocol [RFC 1730]: more features, including manipulation of stored msgs on server
    • HTTP: gmail, Hotmail, Yahoo! Mail, etc.

​

Application Layer

2-57

sender’s mail

server

SMTP

SMTP

mail access

protocol

receiver’s mail

server

(e.g., POP,

IMAP)

user

agent

user

agent

58 of 109

POP3 protocol

authorization phase

  • client commands:
    • user: declare username
    • pass: password
  • server responses
    • +OK
    • -ERR

transaction phase, client:

  • list: list message numbers
  • retr: retrieve message by number
  • dele: delete
  • quit

Application Layer

2-58

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

59 of 109

POP3 (more) and IMAP

more about POP3

  • previous example uses POP3 “download and delete” mode
    • Bob cannot re-read e-mail if he changes client
  • POP3 “download-and-keep”: copies of messages on different clients
  • POP3 is stateless across sessions

IMAP

  • keeps all messages in one place: at server
  • allows user to organize messages in folders
  • keeps user state across sessions:
    • names of folders and mappings between message IDs and folder name

Application Layer

2-59

60 of 109

Chapter 2: outline

2.1 principles of network applications

    • app architectures
    • app requirements

2.2 Web and HTTP

2.3 FTP

2.4 electronic mail

    • SMTP, POP3, IMAP

2.5 DNS

​

2.6 P2P applications

2.7 socket programming with UDP and TCP

Application Layer

2-60

61 of 109

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

Domain Name System:

  • distributed database implemented in hierarchy of many name servers
  • application-layer protocol: hosts, name servers communicate to resolve names (address/name translation)
    • note: core Internet function, implemented as application-layer protocol
    • complexity at network’s “edge”

Application Layer

2-61

62 of 109

DNS: services, structure

why not centralize DNS?

  • single point of failure
  • traffic volume
  • distant centralized database
  • maintenance

​

DNS services

  • hostname to IP address translation
  • host aliasing
    • canonical, alias names
  • mail server aliasing
  • load distribution
    • replicated Web servers: many IP addresses correspond to one name

​

Application Layer

2-62

A: doesn’t scale!

63 of 109

DNS: a distributed, hierarchical database

client wants IP for www.amazon.com; 1st approx:

  • client queries 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

Application Layer

2-63

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

…

…

64 of 109

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

Application Layer

2-64

13 root name “servers” worldwide

a. Verisign, Los Angeles CA

(5 other sites)

b. USC-ISI Marina del Rey, CA

l. ICANN Los Angeles, CA

(41 other sites)

e. NASA Mt View, CA

f. Internet Software C.

Palo Alto, CA (and 48 other sites)

i. Netnod, Stockholm (37 other sites)

k. RIPE London (17 other sites)

m. WIDE Tokyo

(5 other sites)

c. Cogent, Herndon, VA (5 other sites)

d. U Maryland College Park, MD

h. ARL Aberdeen, MD

j. Verisign, Dulles VA (69 other sites )

g. US DoD Columbus, OH (5 other sites)

65 of 109

TLD, authoritative servers

top-level domain (TLD) servers:

    • responsible for com, org, net, edu, aero, jobs, museums, and all top-level country domains, e.g.: uk, fr, ca, jp
    • Network Solutions maintains servers for .com TLD
    • Educause for .edu TLD

authoritative DNS servers:

    • organization’s own DNS server(s), providing authoritative hostname to IP mappings for organization’s named hosts
    • can be maintained by organization or service provider

​

Application Layer

2-65

66 of 109

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

​

Application Layer

2-66

67 of 109

DNS name �resolution example

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

Application Layer

2-67

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

iterated query:

  • contacted server replies with name of server to contact
  • “I don’t know this name, but ask this server”

68 of 109

Application Layer

2-68

4

5

6

3

recursive query:

  • puts burden of name resolution on contacted name server
  • heavy load at upper levels of hierarchy?

requesting host

cis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS server

dns.poly.edu

1

2

7

authoritative DNS server

dns.cs.umass.edu

8

DNS name �resolution example

TLD DNS

server

69 of 109

DNS: caching, updating records

  • once (any) name server learns mapping, it caches mapping
    • cache entries timeout (disappear) after some time (TTL)
    • TLD servers typically cached in local name servers
      • thus 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
  • update/notify mechanisms proposed IETF standard
    • RFC 2136

Application Layer

2-69

70 of 109

DNS records

DNS: distributed db storing resource records (RR)

type=NS

    • name is domain (e.g., foo.com)
    • value is hostname of authoritative name server for this domain

​

Application Layer

2-70

RR format: (name, value, type, ttl)

type=A

    • name is hostname
    • value is IP address

​

type=CNAME

    • name is alias name for some “canonical” (the real) name
    • www.ibm.com is really

servereast.backup2.ibm.com

    • value is canonical name

​

type=MX

    • value is name of mailserver associated with name

​

71 of 109

DNS protocol, messages

  • query and reply messages, both with same message format

Application Layer

2-71

msg header

  • identification: 16 bit # for query, reply to query uses same #
  • flags:
    • query or reply
    • recursion desired
    • recursion available
    • reply is authoritative

identification

flags

# questions

questions (variable # of questions)

# additional RRs

# authority RRs

# answer RRs

answers (variable # of RRs)

authority (variable # of RRs)

additional info (variable # of RRs)

2 bytes

2 bytes

72 of 109

Application Layer

2-72

name, type fields

for a query

RRs in response

to query

records for

authoritative servers

additional “helpful”

info that may be used

identification

flags

# questions

questions (variable # of questions)

# additional RRs

# authority RRs

# answer RRs

answers (variable # of RRs)

authority (variable # of RRs)

additional info (variable # of RRs)

DNS protocol, messages

2 bytes

2 bytes

73 of 109

Inserting records into DNS

  • example: new startup “Network Utopia”
  • register name networkuptopia.com at DNS registrar (e.g., Network Solutions)
    • provide names, IP addresses of authoritative name server (primary and secondary)
    • registrar inserts two RRs into .com TLD server:�(networkutopia.com, dns1.networkutopia.com, NS)

(dns1.networkutopia.com, 212.212.212.1, A)

  • create authoritative server type A record for www.networkuptopia.com; type MX record for networkutopia.com

​

Application Layer

2-73

74 of 109

Attacking DNS

DDoS attacks

  • Bombard root servers with traffic
    • Not successful to date
    • Traffic Filtering
    • Local DNS servers cache IPs of TLD servers, allowing root server bypass
  • Bombard TLD servers
    • Potentially more dangerous

​

Redirect attacks

  • Man-in-middle
    • Intercept queries
  • DNS poisoning
    • Send bogus relies to DNS server, which caches

Exploit DNS for DDoS

  • Send queries with spoofed source address: target IP
  • Requires amplification

Application Layer

2-74

75 of 109

Chapter 2: outline

2.1 principles of network applications

    • app architectures
    • app requirements

2.2 Web and HTTP

2.3 FTP

2.4 electronic mail

    • SMTP, POP3, IMAP

2.5 DNS

​

2.6 P2P applications

2.7 socket programming with UDP and TCP

Application Layer

2-75

76 of 109

Pure P2P architecture

  • no always-on server
  • arbitrary end systems directly communicate
  • peers are intermittently connected and change IP addresses

examples:

    • file distribution (BitTorrent)
    • Streaming (KanKan)
    • VoIP (Skype)

​

​

Application Layer

2-76

77 of 109

File distribution: client-server vs P2P

Question: how much time to distribute file (size F) from one server to N peers?

    • peer upload/download capacity is limited resource

Application Layer

2-77

us

uN

dN

server

network (with abundant

bandwidth)

file, size F

us: server upload capacity

ui: peer i upload capacity

di: peer i download capacity

u2

d2

u1

d1

di

ui

78 of 109

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

Application Layer

2-78

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

79 of 109

File distribution time: P2P

  • server transmission: must upload at least one copy
    • time to send one copy: F/us

Application Layer

2-79

time to distribute F

to N clients using

P2P approach

us

network

di

ui

F

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

  • 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 + Σui

… but so does this, as each peer brings service capacity

increases linearly in N …

80 of 109

Application Layer

2-80

Client-server vs. P2P: example

client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us

81 of 109

P2P file distribution: BitTorrent

Application Layer

2-81

tracker: tracks peers

participating in torrent

torrent: group of peers exchanging chunks of a file

Alice arrives …

  • file divided into 256Kb chunks
  • peers in torrent send/receive file chunks

… obtains list

of peers from tracker

… and begins exchanging

file chunks with peers in torrent

82 of 109

  • peer joining torrent:
    • has no chunks, but will accumulate them over time from other peers
    • registers with tracker to get list of peers, connects to subset of peers (“neighbors”)

Application Layer

2-82

P2P file distribution: BitTorrent

  • while downloading, peer uploads chunks to other peers
  • peer may change peers with whom it exchanges chunks
  • churn: peers may come and go
  • once peer has entire file, it may (selfishly) leave or (altruistically) remain in torrent

83 of 109

BitTorrent: requesting, sending file chunks

requesting chunks:

  • at any given time, different peers have different subsets of file chunks
  • periodically, Alice asks each peer for list of chunks that they have
  • Alice requests missing chunks from peers, rarest first

Application Layer

2-83

sending chunks: tit-for-tat

  • Alice sends chunks to those four peers currently sending her chunks at highest rate
    • other peers are choked by Alice (do not receive chunks from her)
    • re-evaluate top 4 every10 secs
  • every 30 secs: randomly select another peer, starts sending chunks
    • “optimistically unchoke” this peer
    • newly chosen peer may join top 4

​

84 of 109

BitTorrent: tit-for-tat

Application Layer

2-84

(1) Alice “optimistically unchokes” Bob

(2) Alice becomes one of Bob’s top-four providers; Bob reciprocates

(3) Bob becomes one of Alice’s top-four providers

higher upload rate: find better trading partners, get file faster !

85 of 109

Distributed Hash Table (DHT)

  • Hash table

​

  • DHT paradigm�
  • Circular DHT and overlay networks�
  • Peer churn

86 of 109

Simple Database

Key

Value

John Washington

132-54-3570

Diana Louise Jones

761-55-3791

Xiaoming Liu

385-41-0902

Rakesh Gopal

441-89-1956

Linda Cohen

217-66-5609

…….

………

Lisa Kobayashi

177-23-0199

Simple database with(key, value) pairs:

    • key: human name; value: social security #

​

​

    • key: movie title; value: IP address

​

87 of 109

Hash Table

Original Key

Key

Value

John Washington

8962458

132-54-3570

Diana Louise Jones

7800356

761-55-3791

Xiaoming Liu

1567109

385-41-0902

Rakesh Gopal

2360012

441-89-1956

Linda Cohen

5430938

217-66-5609

…….

​

………

Lisa Kobayashi

9290124

177-23-0199

  • More convenient to store and search on numerical representation of key
  • key = hash(original key)

​

88 of 109

Distributed Hash Table (DHT)

  • Distribute (key, value) pairs over millions of peers
    • pairs are evenly distributed over peers
  • Any peer can query database with a key
    • database returns value for the key
    • To resolve query, small number of messages exchanged among peers
  • Each peer only knows about a small number of other peers
  • Robust to peers coming and going (churn)

​

89 of 109

Assign key-value pairs to peers

  • rule: assign key-value pair to the peer that has the closest ID.
  • convention: closest is the immediate successor of the key.
  • e.g., ID space {0,1,2,3,…,63}
  • suppose 8 peers: 1,12,13,25,32,40,48,60
    • If key = 51, then assigned to peer 60
    • If key = 60, then assigned to peer 60
    • If key = 61, then assigned to peer 1

​

​

90 of 109

Circular DHT

  • each peer only aware of �immediate successor and �predecessor.

​

1

12

13

25

32

40

48

60

“overlay network”

91 of 109

Resolving a query

1

12

13

25

32

40

48

60

What is the value�associated with key 53 ?

value

O(N) messages

on avgerage to resolve

query, when there

are N peers

92 of 109

Circular DHT with shortcuts

  • each peer keeps track of IP addresses of predecessor, successor, short cuts.
  • reduced from 6 to 3 messages.
  • possible to design shortcuts with O(log N) neighbors, O(log N) messages in query

1

12

13

25

32

40

48

60

What is the value for

key 53

value

93 of 109

Peer churn

example: peer 5 abruptly leaves

​

1

3

4

5

8

10

12

15

handling peer churn:

  • peers may come and go (churn)
  • each peer knows address of its two successors
  • each peer periodically pings its �two successors to check aliveness
  • if immediate successor leaves, choose next successor as new immediate successor

​

94 of 109

Peer churn

example: peer 5 abruptly leaves

  • peer 4 detects peer 5’s departure; makes 8 its immediate successor
  • 4 asks 8 who its immediate successor is; makes 8’s immediate successor its second successor.

​

1

3

4

8

10

12

15

handling peer churn:

  • peers may come and go (churn)
  • each peer knows address of its two successors
  • each peer periodically pings its �two successors to check aliveness
  • if immediate successor leaves, choose next successor as new immediate successor

​

95 of 109

Chapter 2: outline

2.1 principles of network applications

    • app architectures
    • app requirements

2.2 Web and HTTP

2.3 FTP

2.4 electronic mail

    • SMTP, POP3, IMAP

2.5 DNS

​

2.6 P2P applications

2.7 socket programming with UDP and TCP

Application Layer

2-95

96 of 109

Socket programming

goal: learn how to build client/server applications that communicate using sockets

socket: door between application process and end-end-transport protocol

Application Layer

2-96

Internet

controlled

by OS

​

controlled by

app developer

transport

application

physical

link

network

process

transport

application

physical

link

network

process

socket

97 of 109

Socket programming

Two socket types for two transport services:

    • UDP: unreliable datagram
    • TCP: reliable, byte stream-oriented

​

Application Layer

2-97

Application Example:

  1. Client reads a line of characters (data) from its keyboard and sends the data to the server.
  2. The server receives the data and converts characters to uppercase.
  3. The server sends the modified data to the client.
  4. The client receives the modified data and displays the line on its screen.

98 of 109

Socket programming with UDP

UDP: no “connection” between client & server

  • no handshaking before sending data
  • sender explicitly attaches IP destination address and port # to each packet
  • rcvr extracts sender IP address and port# from received packet

UDP: transmitted data may be lost or received out-of-order

Application viewpoint:

  • UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

Application Layer

2-98

99 of 109

Client/server socket interaction: UDP

close

clientSocket

read datagram from

clientSocket

create socket:

​

clientSocket =

socket(AF_INET,SOCK_DGRAM)

Create datagram with server IP and

port=x; send datagram via�clientSocket

create socket, port= x:

serverSocket =

socket(AF_INET,SOCK_DGRAM)

read datagram from

serverSocket

write reply to

serverSocket

specifying �client address,

port number

Application 2-99

server (running on serverIP)

client

100 of 109

Application Layer

2-100

Example app: UDP client

from socket import *

serverName = ‘hostname’

serverPort = 12000

clientSocket = socket(socket.AF_INET,

socket.SOCK_DGRAM)

message = raw_input(’Input lowercase sentence:’)

clientSocket.sendto(message,(serverName, serverPort))

modifiedMessage, serverAddress =

clientSocket.recvfrom(2048)

print modifiedMessage

clientSocket.close()

Python UDPClient

include Python’s socket

library

create UDP socket for server

get user keyboard

input

Attach server name, port to message; send into socket

print out received string and close socket

read reply characters from

socket into string

101 of 109

Application Layer

2-101

Example app: UDP server

from socket import *

serverPort = 12000

serverSocket = socket(AF_INET, SOCK_DGRAM)

serverSocket.bind(('', serverPort))

print “The server is ready to receive”

while 1:

message, clientAddress = serverSocket.recvfrom(2048)

modifiedMessage = message.upper()

serverSocket.sendto(modifiedMessage, clientAddress)

Python UDPServer

create UDP socket

bind socket to local port number 12000

loop forever

Read from UDP socket into message, getting client’s address (client IP and port)

send upper case string back to this client

102 of 109

Socket programming with TCP

client must contact server

  • server process must first be running
  • server must have created socket (door) that welcomes client’s contact

client contacts server by:

  • Creating TCP socket, specifying IP address, port number of server process
  • when client creates socket: client TCP establishes connection to server TCP

​

  • when contacted by client, server TCP creates new socket for server process to communicate with that particular client
    • allows server to talk with multiple clients
    • source port numbers used to distinguish clients (more in Chap 3)

Application Layer

2-102

TCP provides reliable, in-order

byte-stream transfer (“pipe”)

between client and server

application viewpoint:

103 of 109

Client/server socket interaction: TCP

Application Layer

2-103

wait for incoming

connection request

connectionSocket =

serverSocket.accept()

create socket,

port=x, for incoming request:

serverSocket = socket()

create socket,

connect to hostid, port=x

clientSocket = socket()

server (running on hostid)

client

send request using

clientSocket

read request from

connectionSocket

write reply to

connectionSocket

TCP

connection setup

close

connectionSocket

read reply from

clientSocket

close

clientSocket

104 of 109

Application Layer

2-104

Example app: TCP client

from socket import *

serverName = ’servername’

serverPort = 12000

clientSocket = socket(AF_INET, SOCK_STREAM)

clientSocket.connect((serverName,serverPort))

sentence = raw_input(‘Input lowercase sentence:’)

clientSocket.send(sentence)

modifiedSentence = clientSocket.recv(1024)

print ‘From Server:’, modifiedSentence

clientSocket.close()

Python TCPClient

create TCP socket for server, remote port 12000

No need to attach server name, port

105 of 109

Application Layer

2-105

Example app: TCP server

from socket import *

serverPort = 12000

serverSocket = socket(AF_INET,SOCK_STREAM)

serverSocket.bind((‘’,serverPort))

serverSocket.listen(1)

print ‘The server is ready to receive’

while 1:

connectionSocket, addr = serverSocket.accept()

sentence = connectionSocket.recv(1024)

capitalizedSentence = sentence.upper()

connectionSocket.send(capitalizedSentence)

connectionSocket.close()

Python TCPServer

create TCP welcoming

socket

server begins listening for incoming TCP requests

loop forever

server waits on accept()

for incoming requests, new socket created on return

read bytes from socket (but not address as in UDP)

close connection to this client (but not welcoming socket)

106 of 109

Chapter 2: summary

  • application architectures
    • client-server
    • P2P
  • application service requirements:
    • reliability, bandwidth, delay
  • Internet transport service model
    • connection-oriented, reliable: TCP
    • unreliable, datagrams: UDP

our study of network apps now complete!

Application Layer

2-106

  • specific protocols:
    • HTTP
    • FTP
    • SMTP, POP, IMAP
    • DNS
    • P2P: BitTorrent, DHT
  • socket programming: TCP, UDP sockets

107 of 109

  • typical request/reply message exchange:
    • client requests info or service
    • server responds with data, status code
  • message formats:
    • headers: fields giving info about data
    • data: info being communicated

Application Layer

2-107

important themes:

  • control vs. data msgs
    • in-band, out-of-band
  • centralized vs. decentralized
  • stateless vs. stateful
  • reliable vs. unreliable msg transfer
  • “complexity at network edge”

Chapter 2: summary

most importantly: learned about protocols!

108 of 109

Introduction

1-108

Chapter 1�Additional Slides

109 of 109

Transport (TCP/UDP)

Network (IP)

Link (Ethernet)

Physical

application

(www browser,

email client)

application

OS

packet

capture

(pcap)

packet

analyzer

copy of all Ethernet frames sent/received