Post on 06-Jan-2020
2: Application Layer 1
Chapter 2Application Layer
Computer Networking: A Top Down Approach, 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.
A note on the use of these ppt slides:The notes used in this course are substantially based on powerpoint slides developed and copyrighted by J.F. Kurose and K.W. Ross, 2007
2: Application Layer 2
Chapter 2: Application Layer
2.1 Principles of network applications2.2 Web and HTTP2.3 FTP 2.4 Electronic Mail
SMTP, POP3, IMAP2.5 DNS
2.6 P2P Applications2.7 Socket programming with TCP2.8 Socket programming with UDP
2: Application Layer 3
Chapter 2: Application LayerOur goals: Learn about protocols
by examining popular application-level protocols
HTTPFTPSMTP / POP3 / IMAPDNS
Programming network applications
Socket API
Conceptual, implementation aspects of network application protocols
Transport-layer service modelsClient-server paradigmPeer-to-peer paradigm
2: Application Layer 4
Some Network Apps
e-mailwebinstant messagingremote loginP2P file sharingmulti-user network gamesstreaming stored video clips
voice over IPreal-time video conferencinggrid computing
2: Application Layer 5
Creating a Network App applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
Write programs thatRun on (different) end systemsCommunicate over networkE.g., web server software communicates with browser software
Little software written for devices in network core
Network core devices do not run user applications Applications on end systems allows for rapid app development, propagation
2: Application Layer 6
Chapter 2: Application Layer
2.1 Principles of network applications2.2 Web and HTTP2.3 FTP 2.4 Electronic Mail
SMTP, POP3, IMAP2.5 DNS
2.6 P2P file sharing2.7 Socket programming with TCP2.8 Socket programming with UDP2.9 Building a Web server
2: Application Layer 7
Application Architectures
Client-server
Peer-to-peer (P2P)
Hybrid of client-server and P2P
clientclient
client
2: Application Layer 8
Client-Server ArchitectureServer:
Always-on hostPermanent IP addressServer farms for scaling
Clients:Communicate with serverMay be intermittently connectedMay have dynamic IP addressesDo not communicate directly with each other
client/server
2: Application Layer 9
Pure P2P Architecture
No always-on serverArbitrary end systems directly communicatePeers are intermittently connected and change IP addressesExample: Gnutella
Highly scalable but difficult to manage
peer-peer
2: Application Layer 10
Hybrid of Client-Server and P2PSkype
Voice-over-IP P2P applicationCentralized server: finding address of remote party Client-client connection: direct (not through server)
Instant messagingChatting between two users is P2PCentralized 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
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
Note: applications with P2P architectures have client processes & server processes
2: Application Layer 11
2: Application Layer 12
Sockets
Process sends/receives messages to/from its socketSocket analogous to door
Sending process shoves message out doorSending process relies on transport infrastructure on other side of door which brings message to socket at receiving process
controlled byapp developer
Internet
controlledby OS
API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later)
process
TCP withbuffers,variables
socket
host orserver
process
TCP withbuffers,variables
socket
host orserver
2: Application Layer 13
Addressing ProcessesTo receive messages, process must have identifierHost device has unique 32-bit IP addressQ: does IP address of host on which process runs suffice for identifying the process?Answer: NO, many processes can be running on same host
Instant Messenger
Web Browsing
Packet X
2: Application Layer 14
Addressing ProcessesTo receive messages, process must have identifierHost device has unique 32-bit IP addressQ: does IP address of host on which process runs suffice for identifying the process?Answer: NO, many processes can be running on same host
Identifier includes both IP address and port numbersassociated with process on hostExample port numbers:
HTTP server: 80Mail server: 25
To send HTTP message to www.cs.ust.hk web server:
IP address: 143.89.40.4Port number: 80
2: Application Layer 15
App-Layer Protocol Defines
Public-domain protocols:Defined in RFCsAllows for interoperabilitye.g., HTTP, SMTP
Proprietary protocols:e.g., KaZaA
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
2: Application Layer 16
What Transport Service does an App Need?
Data lossSome apps (e.g., audio, video) can tolerate some lossOther apps (e.g., file transfer, telnet) require 100% reliable data transfer
TimingSome apps (e.g., Internet telephony, interactive games) require low delay to be “effective”
BandwidthSome apps (e.g., multimedia) require minimum amount of bandwidth to be “effective”Other apps (“elastic apps”) make use of whatever bandwidth they get (Best-Efforts)
2: Application Layer 17
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
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
2: Application Layer 18
Internet Transport Protocols Services
high capacity receiver
congested network
congestion control problem
low capacity receiver
fast network
flow control problem
TCP service:Connection-oriented: setup required between client and server processesReliable transport between sending and receiving processFlow control: sender won’t overwhelm receiver Congestion control: throttle sender when network overloadedDoes not provide: timing, minimum bandwidth guarantees
2: Application Layer 19
Internet Transport Protocols Services
TCP service:Connection-oriented: setup required between client and server processesReliable transport between sending and receiving processFlow control: sender won’t overwhelm receiver Congestion control: throttle sender when network overloadedDoes not provide: timing, minimum bandwidth guarantees
UDP service:Unreliable data transfer between sending and receiving processDoes not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee
Q: Why bother? Why is there a UDP?
A: Low latency, low overhead, simple
2: Application Layer 20
Internet Apps: Application, Transport Protocols
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
2: Application Layer 21
Chapter 2: Application Layer
2.1 Principles of network applications
app architecturesapp requirements
2.2 Web and HTTP2.4 Electronic Mail
SMTP, POP3, IMAP2.5 DNS
2.6 P2P file sharing2.7 Socket programming with TCP2.8 Socket programming with UDP
2: Application Layer 22
Web and HTTP
First some jargonWeb page consists of objectsObject can be HTML file, JPEG image, Java applet, audio file,…Web page consists of base HTML-file which includes several referenced objectsEach object is addressable by a URLExample URL:www.someschool.edu/someDept/pic.gif
host name path name
2: Application Layer 23
Web and HTTP
First some jargon
User agent for Web is called a browser:MS Internet ExplorerNetscape CommunicatorFirefox web browser
Server for Web is called Web server:Apache (public domain)MS Internet Information Server
2: Application Layer 24
HTTP Overview
HTTP: hypertext transfer protocolWeb’s application layer protocolClient/server model
Client: browser that requests, receives, “displays” Web objectsServer: web server sends objects in response to requests
HTTP 1.0: RFC 1945HTTP 1.1: RFC 2068
HTTP request
HTTP request
HTTP response
HTTP response
PC runningExplorer
Server running
Apache Webserver
Mac runningNavigator
2: Application Layer 25
HTTP Overview (cont.)
HTTP is “stateless”Server maintains no information about past client requests
Uses TCP:Client initiates TCP connection (creates socket) to server, port 80Server accepts TCP connection from clientHTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)TCP connection closed
Protocols that maintain “state” are complex!Past history (state) must be maintainedIf server/client crashes, their views of “state” may be inconsistent, must be reconciled
aside
2: Application Layer 26
HTTP Connections
Nonpersistent HTTPAt most one object is sent over a TCP connectionEach TCP connection is closed after the server sends the objectHTTP/1.0 uses nonpersistent HTTP
Persistent HTTPMultiple objects can be sent over single TCP connection between client and serverHTTP/1.1 uses persistent connections in default mode
2: Application Layer 27
Nonpersistent HTTP(contains text,
references to 10 jpeg images)
Suppose user enters URL www.someSchool.edu/someDepartment/home.index
1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80
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
2: Application Layer 28
Nonpersistent HTTP (cont.)
time
4. HTTP server closes TCP connection.
5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects
6. Steps 1-5 repeated for each of 10 jpeg objects
2: Application Layer 29
Non-Persistent HTTP: Response Time
Definition of RTT: time to send a small packet to travel from client to server and back
Response time:
time to transmit file
initiate TCPconnection
RTT
requestfile
RTT
filereceived
time
One RTT to initiate TCP connectionOne RTT for HTTP request and first few bytes of HTTP response to returnFile transmission time
total = 2RTT+transmit timetime
2: Application Layer 30
Persistent HTTP
Nonpersistent HTTP issues:Requires 2 RTTs per objectOS overhead for each TCP connectionBrowsers often open parallel TCP connections to fetch referenced objects
Persistent HTTPServer leaves connection open after sending responseSubsequent HTTP messages between same client/server sent over open connection
Persistent without pipelining:Client issues new request only when previous response has been receivedOne RTT for each referenced object
Persistent with pipelining:Default in HTTP/1.1Client sends requests as soon as it encounters a referenced objectAs little as one RTT for all the referenced objects
2: Application Layer 31
HTTP Request Message
Two types of HTTP messages: request, responseHTTP request message:
ASCII (human-readable format)
GET /somedir/page.html HTTP/1.1Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:fr
(extra carriage return, line feed)
request line(GET, POST,
HEAD commands)
headerlines
Carriage return, line feed
indicates end of message
2: Application Layer 32
HTTP Request Message: General Format
Empty with GET method
Is used with POST method
cr: carriage returnlf: line feed
2: Application Layer 33
Uploading Form Input
Post method:Web page often includes form inputInput is uploaded to server in entity body
URL method:Uses GET methodInput is uploaded in URL field of request line:
www.somesite.com/animalsearch?monkeys&banana
2: Application Layer 34
Method Types
HTTP/1.0GETPOSTHEAD
Similar to GET methodAsks server to leave requested object out of responseOften being used for debugging
HTTP/1.1GET, POST, HEADPUT
In conjunction with Web publishing toolUploads file in entity body to path specified in URL field
DELETEDeletes file specified in the URL field
2: Application Layer 35
HTTP Response Message
HTTP/1.1 200 OK Connection: closeDate: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html
data data data data data ...
status line(protocol
status codestatus phrase)
headerlines
data, e.g., requestedHTML file
2: Application Layer 36
HTTP Response Status CodesIn first line in server->client response messageA few sample codes:
200 OKRequest succeeded, requested object later in this message
301 Moved PermanentlyRequested object moved, new location specified later in this message (Location:)
400 Bad RequestRequest message not understood by server
404 Not FoundRequested document not found on this server
505 HTTP Version Not Supported
2: Application Layer 37
Trying out HTTP (Client Side) for Yourself
1. 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:By typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to HTTP server
GET /~ross/ HTTP/1.1Host: cis.poly.edu
3. Look at response message sent by HTTP server!
2: Application Layer 38
User-Server State: Cookies
Why need cookies?HTTP server is stateless
• Simplify server design and handle thousands of simultaneous TCP connections
It is often desirable to identify users• Restrict user access• Serve content as a function of the user identity
Cookies [RFC 2109]Allow sites to keep track of users
2: Application Layer 39
User-Server State: Cookies
Many major Web sites use cookies
Four components:
Example:Susan access Internet always from same PCShe visits a specific e-commerce site for first timeWhen initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID
1) Cookie header line of HTTP response message
2) Cookie header line in HTTP request message
3) Cookie file kept on user’s host, managed by user’s browser
4) Back-end database at Web site
2: Application Layer 40
Cookies: Keeping “State” (cont.)
client serverusual http request msg
usual http response +Set-cookie: 1678
usual http request msgcookie: 1678
usual http response msg
usual http request msgcookie: 1678
usual http response msg
servercreates ID
1678 for user
Cookie file
ebay: 8734
entry in backend database
access
access
Cookie file
amazon: 1678ebay: 8734
cookie-specificaction
one week later:
Cookie file
amazon: 1678ebay: 8734
cookie-spectific
action
Cookies (continued)What cookies can bring:
AuthorizationShopping cartsRecommendationsUser session state (Web e-mail) Cookies and privacy:
Cookies permit sites to learn a lot about youYou may supply name and e-mail to sites
aside
How to keep “state”:Protocol endpoints: maintain state at sender/receiver over multiple transactionsCookies: http messages carry state
2: Application Layer 41
2: Application Layer 42
Cookie Exampletelnet www.google.com 80
Trying 216.239.33.99...Connected to www.google.com.Escape character is '^]'.
GET /index.html HTTP/1.0
HTTP/1.0 200 OKDate: Wed, 10 Sep 2004 08:58:55 GMTSet-Cookie:
PREF=ID=43bd8b0f34818b58:TM=1063184203:LM=1063184203:S=DDqPgTb56Za88O2y; expires=Sun, 17-Jan-203819:14:07 GMT; path=/; domain=.google.com
Web Caches (Proxy Server)
User sets browser: Web accesses via cacheBrowser 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
clientorigin server
origin server
HTTP request
HTTP request
HTTP response
HTTP response
HTTP request
HTTP response
2: Application Layer 43
2: Application Layer 44
More about Web Caching
Cache acts as both client and serverTypically 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)
2: Application Layer 45
Caching Example origin
serversPublic
Internet
institutionalnetwork 10 Mbps LAN
1.5 Mbps access link
Traffic density = La/R
AssumptionsAverage object size = 100,000 bitsAvg. request rate from institution’s browsers to origin servers = 15/secDelay from institutional router to any origin server and back to router = 2 sec
ConsequencesUtilization on LAN = 15%Utilization on access link = 100%Total delay = Internet delay + access delay + LAN delay
= 2 sec + minutes + milliseconds
2: Application Layer 46
Caching Example (cont.)origin
serversPublic
Internet
institutionalnetwork 10 Mbps LAN
10 Mbps access link
Possible solutionIncrease bandwidth of access link to, say, 10 Mbps
ConsequencesUtilization on LAN = 15%Utilization on access link = 15%Total delay = Internet delay + access delay + LAN delay
= 2 sec + msecs + msecsOften a costly upgrade
2: Application Layer 47
Caching Example (cont.)origin
serversPublic
Internet
institutionalnetwork 10 Mbps LAN
1.5 Mbps access link
institutionalcache
Install cacheSuppose hit rate is 0.4
Consequence40% requests will be satisfied almost immediately60% requests satisfied by origin serverUtilization of access link reduced to 60%, resulting in negligible delays (say 10 msec)Total avg delay = Internet delay + access delay + LAN delay = 0.6*(2.01) secs + 0.4*0.01seconds < 1.4 secs
2: Application Layer 48
Conditional GETThe copy of an object residing in the cache may be stale cache server
HTTP request msgIf-modified-since:
<date>
HTTP responseHTTP/1.0
304 Not Modified
object not
modified
HTTP request msgIf-modified-since:
<date>
HTTP responseHTTP/1.0 200 OK
<data>
object modified
Goal: don’t send object if cache has up-to-date cached versionCache: specify date of cached copy in HTTP requestIf-modified-since:
<date>
Server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not
Modified
2: Application Layer 49
Chapter 2: Application Layer
2.1 Principles of network applications2.2 Web and HTTP2.3 FTP2.4 Electronic Mail
SMTP, POP3, IMAP2.5 DNS
2.6 P2P file sharing2.7 Socket programming with TCP2.8 Socket programming with UDP2.9 Building a Web server
2: Application Layer 50
FTP: the File Transfer Protocol
Transfer file to/from remote hostClient/server model
Client: side that initiates transfer (either to/from remote)Server: remote host
Ftp: RFC 959Ftp server: port 21
file transferFTP
serverFTPuser
interface
FTPclient
local filesystem
remote filesystem
user at host
2: Application Layer 51
FTP: Separate Control, Data Connections
FTP client contacts FTP server at port 21, specifying TCP as transport protocolClient obtains authorization over control connectionClient browses remote directory by sending commands over control connection.When server receives file transfer command, server opens 2nd TCP connection (for file) to clientAfter transferring one file, server closes data connection.
TCP control connectionport 21
TCP data connectionport 20
Server opens another TCP data connection to transfer another file.Control connection: “out of band”FTP server maintains “state”: current directory, earlier authentication
FTPclient
FTPserver
2: Application Layer 52
FTP Commands, Responses
Sample commands:Sent as ASCII text over control channelUSER usernamePASS password
LIST return list of file in current directoryRETR filename retrieves (gets) fileSTOR filename stores (puts) file onto remote host
Sample return codesstatus code and phrase (as in HTTP)331 Username OK, password required125 data connection already open; transfer starting425 Can’t open data connection452 Error writing file
2: Application Layer 53
Chapter 2: Application Layer
2.1 Principles of network applications2.2 Web and HTTP2.3 FTP 2.4 Electronic Mail
SMTP, POP3, IMAP2.5 DNS
2.6 P2P file sharing2.7 Socket programming with TCP2.8 Socket programming with UDP2.9 Building a Web server
2: Application Layer 54
Electronic Mailuser mailbox
outgoing message queue
useragent
useragent
useragent
useragent
useragent
useragent
mailserver
mailserver
mailserver
SMTP
SMTP
SMTP
Three major components:User agents Mail servers Simple mail transfer protocol: SMTP
User Agenta.k.a. “mail reader”Composing, editing, reading mail messagese.g., Eudora, Outlook, elm, Netscape MessengerOutgoing, incoming messages stored on server
2: Application Layer 55
Electronic Mail: Mail Servers
Mail ServersMailbox contains incoming messages for userMessage queue of outgoing (to be sent) mail messagesSMTP protocol between mail servers to send email messages
Client: sending mail serverServer: receiving mail server
useragent
useragent
useragent
useragent
useragent
useragent
mailserver
mailserver
mailserver
SMTP
SMTP
SMTP
user mailbox
outgoing message queue
2: Application Layer 56
Electronic Mail: SMTP [RFC 2821]
Uses TCP to reliably transfer email message from client to server, port 25Direct transfer: sending server to receiving serverThree phases of transfer
Handshaking (greeting)Transfer of messagesClosure
Command/response interactionCommands: ASCII textResponse: status code and phrase
Messages must be in 7-bit ASCII
2: Application Layer 57
Scenario: Alice sends Message to Bob4) 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
1) Alice uses UA to compose message and “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
mailserver
mailserver user
agent
1
2 3 4 56
useragent
2: Application Layer 58
Sample SMTP InteractionS: 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
2: Application Layer 59
Try SMTP Interaction for Yourself:
telnet servername 25See 220 reply from serverEnter HELO, MAIL FROM, RCPT TO, DATA, QUIT commandsabove lets you send email without using email client (reader)
2: Application Layer 60
SMTP: Final Words
SMTP uses persistent connectionsSMTP requires message (header & body) to be in 7-bit ASCIISMTP server uses CRLF.CRLF to determine end of message
Comparison with HTTP:HTTP: pullSMTP: push
Both have ASCII command/response interaction, status codes
HTTP: each object encapsulated in its own response msgSMTP: multiple objects sent in multipart msg
2: Application Layer 61
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!
BodyThe “message”, ASCII characters only
header
body
blankline
2: Application Layer 62
Message Format: Multimedia Extensions
MIME: multimedia mail extension, RFC 2045, 2056Additional lines in msg header declare MIME content type
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
multimedia datatype, subtype,
parameter declaration
encoded data
MIME version
method usedto encode data
2: Application Layer 63
MIME TypesContent-Type: type/subtype; parameters
TextExample subtypes: plain, html
ImageExample subtypes: jpeg, gif
AudioExampe subtypes: basic(8-bit mu-law encoded), 32kadpcm (32 kbps coding)
VideoExample subtypes: mpeg, quicktime
ApplicationOther data that must be processed by reader before “viewable”Example subtypes: msword, octet-stream
2: Application Layer 64
Multipart TypeFrom: alice@crepes.frTo: bob@hamburger.eduSubject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=StartOfNextPart
--StartOfNextPartDear Bob, Please find a picture of a crepe.--StartOfNextPartContent-Transfer-Encoding: base64Content-Type: image/jpegbase64 encoded data ..... ......................... ......base64 encoded data --StartOfNextPartDo you want the recipe?
2: Application Layer 65
Mail Access Protocols
SMTP: delivery/storage to receiver’s serverMail access protocol: retrieval from server
POP: Post Office Protocol [RFC 1939]• Authorization (agent <-->server) and download
IMAP: Internet Mail Access Protocol [RFC 1730]• More features (more complex)• Manipulation of stored msgs on server
HTTP: Hotmail , Yahoo! Mail, etc.
SMTP SMTP accessprotocol
sender’s mail server
useragent
receiver’s mail server
useragent
2: Application Layer 66
POP3 Protocol
Authorization phaseClient commands:
user: declare usernamepass: password
Server responses+OK
-ERR
Transaction phase, client:list: list message numbersretr: retrieve message by numberdele: deletequit
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
2: Application Layer 67
POP3 (more) and IMAPMore about POP3
Previous example uses “download and delete”modeBob cannot re-read e-mail if he changes client“Download-and-keep”: copies of messages on different clientsPOP3 is stateless across sessions
IMAPKeep all messages in one place: the serverAllows user to organize messages in foldersIMAP keeps user state across sessions:
names of folders and mappings between message IDs and folder name
2: Application Layer 68
Chapter 2: Application Layer
2.1 Principles of network applications2.2 Web and HTTP2.3 FTP 2.4 Electronic Mail
SMTP, POP3, IMAP2.5 DNS
2.6 P2P file sharing2.7 Socket programming with TCP2.8 Socket programming with UDP2.9 Building a Web server
2: Application Layer 69
DNS: Domain Name System
People: many identifiers:SSN, name, passport #
Internet hosts, routers:IP address (32 bit) -used for addressing datagrams“Name”, e.g., ww.yahoo.com - used by humans
Q: map between IP addresses and name ?
Domain Name System:Distributed databaseimplemented in hierarchy of many name serversApplication-layer protocolhost, routers, name servers to communicate to resolve names (address/name translation)
Note: core Internet function, implemented as application-layer protocolComplexity at network’s “edge”
2: Application Layer 70
DNS Why not centralize DNS?DNS services
Single point of failureTraffic volumeDistant centralized databaseMaintenance
Doesn’t Scale!
Hostname to IP address translationHost aliasing
Canonical and alias names
Mail server aliasingLoad distribution
Replicated Web servers: set of IP addresses for one canonical name
2: Application Layer 71
Distributed, Hierarchical DatabaseRoot DNS Servers
com DNS servers org DNS servers edu DNS servers
poly.eduDNS servers
umass.eduDNS serversyahoo.com
DNS serversamazon.comDNS servers
pbs.orgDNS servers
Client wants IP for www.amazon.com; 1st approx:Client queries a root server to find com DNS server (top-level domain)Client queries com DNS server to get amazon.com DNS server (authoritative)Client queries amazon.com DNS server to get IP address for www.amazon.com
2: Application Layer 72
DNS: Root Name ServersContacted by local name server that can not resolve nameRoot name server:
Contacts authoritative name server if name mapping not knownGets mappingReturns mapping to local name server
13 root name servers worldwide
b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA
e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 36 other locations)
i Autonomica, Stockholm (plus 3 other locations)
k RIPE London (also Amsterdam, Frankfurt)
m WIDE Tokyo
a Verisign, Dulles, VAc Cogent, Herndon, VA (also Los Angeles)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)
2: Application Layer 73
TLD and Authoritative Servers
Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp
Network solutions maintains servers for com TLDEducause for edu TLD
Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail)
Can be maintained by organization or service provider
2: Application Layer 74
Local Name Server
Does not strictly belong to hierarchyEach ISP (residential ISP, company, university) has one
Also called “default name server”When a host makes a DNS query, query is sent to its local DNS server
Acts as a proxy, forwards query into hierarchy
2: Application Layer 75
requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
23
4
5
6781
authoritative DNS serverdns.cs.umass.edu
TLD DNS server
Example
Host at cis.poly.edu wants IP address for gaia.cs.umass.edu
Recursive query
Iterated query
2: Application Layer 76
Recursive QueriesRecursive query:
Puts burden of name resolution on contacted name serverHeavy load?
Iterated query:Contacted server replies with name of server to contact“I don’t know this name, but ask this server”
requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
1
2
45
6
authoritative DNS serverdns.cs.umass.edu
7
8
TLD DNS server
3
Recursive query
2: Application Layer 77
DNS: Caching and Updating Records
Once (any) name server learns mapping, it cachesmapping
Cache entries timeout (disappear) after some timeTLD servers typically cached in local name servers
• Thus root name servers not often visitedUpdate/notify mechanisms under design by IETF
RFC 2136 (Dynamic Updates in the Domain Name System (DNS UPDATE))http://www.ietf.org/html.charters/dnsext-charter.html (DNS Extensions)
2: Application Layer 78
DNS RecordsDNS: distributed db storing resource records (RR)
RR format: (name, value, type, ttl)
Type=NSname is domainvalue is hostname of authoritative name server for this domainE.g., (ibm.com,dns.ibm.com, NS)
Type=Aname is hostnamevalue is IP addressE.g., (servereast.backup2. ibm.com,129.42.60.212,A)
Type=CNAMEname is alias name for some “canonical” (the real) namevalue is canonical nameE.g., (ibm.com,servereast. backup2.ibm.com,CNAME)
Type=MXvalue is name of mailserver associated with nameE.g., (ibm.com, mail. backup2.ibm.com, MX)
2: Application Layer 79
DNS Protocol, MessagesDNS protocol : query and reply messages, both with
same message format
Msg headeridentification: 16 bit # for query, reply to query uses same #flags:
query or reply indicationrecursion desired (client)recursion available (DNS server)reply is authoritative
2: Application Layer 80
DNS Protocol, Messages
Name, type fieldsfor a query
RRs in responseto query
records forauthoritative servers
additional “helpful”info that may be used
Try nslookuphttp://network-tools.com/nslook/Default.asp
2: Application Layer 81
Inserting Records into DNS
Example: just created startup “Network Utopia”Register name networkuptopia.com at a registrar (e.g., Network Solutions)
Need to provide registrar with names and IP addresses of your authoritative name server (primary and secondary)Registrar inserts two RRs into the com TLD server:
(networkutopia.com, dns1.networkutopia.com, NS)(dns1.networkutopia.com, 212.212.212.1, A)
Put in authoritative server Type A record for www.networkuptopia.com and Type MX record for networkutopia.comHow do people get the IP address of your Web site?
2: Application Layer 82
Get IP address of your Web Site
www.networkutopia.com
Local DNS server
TLD com server(networkutopia.com, dns1.networkutopia.com, NS)(dns1.networkutopia.com, 212.212.212.1, A)
212.212.212.1212.212.71.4
212.212.71.4
2: Application Layer 83
Chapter 2: Application layer
2.1 Principles of network applications
app architecturesapp requirements
2.2 Web and HTTP2.4 Electronic Mail
SMTP, POP3, IMAP2.5 DNS
2.6 P2P file sharing2.7 Socket programming with TCP2.8 Socket programming with UDP2.9 Building a Web server
2: Application Layer 84
P2P File Sharing
ExampleAlice runs P2P client application on her notebook computerIntermittently connects to Internet; gets new IP address for each connectionAsks for “Hey Jude”Application displays other peers that have copy of Hey Jude
Alice chooses one of the peers, Bob.File is copied from Bob’s PC to Alice’s notebook: HTTPWhile Alice downloads, other users uploading from AliceAlice’s peer is both a Web client and a transient Web server
All peers are servers = highly scalable!
2: Application Layer 85
P2P: Centralized Directory
Original “Napster” design1) When peer connects, it
informs central server:IP addressContent
2) Alice queries for “Hey Jude”
3) Alice requests file from Bob
centralizeddirectory server
peers
Alice
Bob
1
1
1
12
3
2: Application Layer 86
P2P: Problems with Centralized Directory
Single point of failurePerformance bottleneckCopyright infringement
File transfer is decentralized, but locating content is highly centralized
2: Application Layer 87
Query Flooding: Gnutella
Overlay network: graphEdge between peer X and Y if there’s a TCP connectionAll active peers and edges is overlay netEdge is not a physical linkGiven peer will typically be connected with < 10 overlay neighbors
Fully distributedNo central server
Public domain protocolMany Gnutella clients implementing protocol
2: Application Layer 88
Gnutella: Protocol
Query
QueryHit
Query
Query
QueryHit
Query
Query
QueryH
it
File transfer:HTTPQuery message
sent over existing TCPconnections
Peers forwardQuery message
QueryHit sentover reversepath
Scalability:limited scope flooding
2: Application Layer 89
Gnutella: Peer Joining
1. Joining peer X must find some other peer in Gnutella network: (bootstrap problem)use list of candidate peers (maintain in X or Gnutella site)
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
Peer leaving: see homework problem!
2: Application Layer 90
Hierarchical Overlay
ordinary peer
group-leader peer
neighoring relationshipsin overlay network
Between centralized index, query flooding approachesEach peer is either a group leader or assigned to a group leader.
TCP connection between peer and its group leader.TCP connections between some pairs of group leaders.
Group leader tracks content in its children
2: Application Layer 91
Comparing Client-Server, P2P ArchitecturesQuestion : How much time distribute file
initially at one server to N other computers?us: server upload bandwidth
us
u2d1 d2u1
Serverui: client/peer i upload bandwidth
uN
dN Network (with abundant bandwidth)
di: client/peer i download bandwidthFile, size F
2: Application Layer 92
Client-Server: File Distribution Time
us
u2d1 d2u1
uN
dN
Server
Network (with abundant bandwidth)
FServer sequentially sends N copies:
NF/us time Client i takes F/ditime to download
= dcs = max { NF/us, F/min(di) }i
Time to distribute Fto N clients using
client/server approach
increases linearly in N(for large N)
2: Application Layer 93
P2P: File Distribution Time
us
u2d1 d2u1
uN
dN
Server
Network (with abundant bandwidth)
FServer must send one copy: F/us time Client i takes F/di time to downloadNF bits must be downloaded (aggregate)
Fastest possible upload rate (assuming all nodes sending file chunks to same peer): us + Σuii=1,N
dP2P = max { F/us, F/min(di) , NF/(us + Σui) }i i=1,N
2: Application Layer 94
Comparing Client-Server, P2P Architectures
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20 25 30 35
N
Min
imum
Dis
tribu
tion
Tim
e P2PClient-Server
2: Application Layer 95
P2P Case Study: BitTorrent
tracker: tracks peers participating in torrent
torrent: group of peers exchanging chunks of a file
obtain listof peers
trading chunks
peer
P2P file distribution
2: Application Layer 96
BitTorrent (1)
File divided into 256KB chunksPeer joining torrent:
Has no chunks, but will accumulate themover timeRegisters with tracker to get list of peers, connects to subset of peers (“neighbors”)
While downloading, peer uploads chunks to other peersPeers may come and goOnce peer has entire file, it may (selfishly) leave or (altruistically) remain
2: Application Layer 97
BitTorrent (2)Pulling Chunks
At any given time, different peers have different subsets of file chunksPeriodically, a peer (Alice) asks each neighbor for list of chunks that they have.Alice issues requests for her missing chunks
rarest first
Sending Chunks: tit-for-tatAlice sends chunks to four neighbors currently sending her chunks at the highest rate
re-evaluate top 4 every 10 secs
every 30 secs: randomly select another peer, starts sending chunks
newly chosen peer may join top 4
2: Application Layer 98
P2P Case Study: SkypeSkype clients (SC)
Supernode(SN)
Skype login server
P2P (pc-to-pc, pc-to-phone, phone-to-pc) Voice-Over-IP (VoIP) application
also IMProprietary application-layer protocol (inferred via reverse engineering) Hierarchical overlay
2: Application Layer 99
Skype: Making a Call
User starts Skype
Skype login server
SC registers with SNlist of bootstrap SNs
SC logs in (authenticate)Call: SC contacts SN will callee ID
SN contacts other SNs(unknown protocol, maybe flooding) to find addr of callee; returns addr to SC
SC directly contacts callee, overTCP
2: Application Layer 100
Chapter 2: Application layer
2.1 Principles of network applications2.2 Web and HTTP2.3 FTP 2.4 Electronic Mail
SMTP, POP3, IMAP2.5 DNS
2.6 P2P file sharing2.7 Socket programming with TCP2.8 Socket programming with UDP
2: Application Layer 101
Socket ProgrammingGoal: learn how to build client/server application that
communicate using sockets
Socket APIIntroduced in BSD4.1 UNIX, 1981Explicitly created, used, released by apps Client/server paradigm Two types of transport service via socket API:
Unreliable datagram Reliable, byte stream-oriented
a host-local, application-created,
OS-controlled interface (a “door”) into which
application process can both send and
receive messages to/from another application
process
socket
2: Application Layer 102
Socket-Programming using TCPSocket: a door between application process and end-
end-transport protocol (UCP or TCP)TCP service: reliable transfer of bytes from one
process to another
controlled byapplicationdeveloper
controlled byapplicationdeveloper
process
TCP withbuffers,variables
socketprocess
TCP withbuffers,variables
socket
Internetcontrolled byoperatingsystem
controlled byoperating
system
host orserver
host orserver
2: Application Layer 103
Socket Programming with TCPClient must contact server
Server process must first be runningServer must have created socket (door) that welcomes client’s contact
Client contacts server by:Creating client-local TCP socketSpecifying IP address, port number of server processWhen 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 client
Allows server to talk with multiple clientsSource port numbers used to distinguish clients (more in Chap 3)
TCP provides reliable, in-ordertransfer of bytes (“pipe”) between client and server
application viewpoint
2: Application Layer 104
Stream Jargon
A stream is a sequence of characters that flow into or out of a process
An input stream is attached to some input source for the process, e.g., keyboard or socket
An output stream is attached to an output source, e.g., monitor or socket
2: Application Layer 105
Socket Programming with TCP
outT
oSer
ver
to network from network
inFr
omS
erve
r
inFr
omU
ser
keyboard monitor
Process
clientSocket
inputstream
inputstream
outputstream
TCPsocket
Clientprocess
client TCP socket
Example client-server app:1) Client reads line from
standard input (inFromUserstream) , sends to server via socket (outToServerstream)
2) Server reads line from socket3) Server converts line to
uppercase, sends back to client
4) Client reads, prints modified line from socket (inFromServer stream)
2: Application Layer 106
Client/Server Socket Interaction: TCPServer (running on hostid) Client
wait for incomingconnection requestconnectionSocket =welcomeSocket.accept()
create socket,port=x, forincoming request:welcomeSocket =
ServerSocket()
create socket,connect to hostid, port=xclientSocket =
Socket()
closeconnectionSocket
read reply fromclientSocket
closeclientSocket
send request usingclientSocketread request from
connectionSocket
write reply toconnectionSocket
TCP connection setup
Example: Java Client (TCP)
import java.io.*; import java.net.*; class TCPClient {
public static void main(String argv[]) throws Exception {
String sentence; String modifiedSentence;
BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname", 6789);
DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
Createinput stream
Create client socket,
connect to serverCreate
output streamattached to socket
2: Application Layer 107
Example: Java Client (TCP), cont.
BufferedReader inFromServer = new BufferedReader(newInputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
outToServer.writeBytes(sentence + '\n');
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER: " + modifiedSentence);
clientSocket.close();
} }
Createinput stream
attached to socket
Send lineto server
Read linefrom server
2: Application Layer 108
2: Application Layer 109
Example: Java Server (TCP)import java.io.*; import java.net.*;
class TCPServer {
public static void main(String argv[]) throws Exception {
String clientSentence; String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789);
while(true) {
Socket connectionSocket = welcomeSocket.accept();
BufferedReader inFromClient = new BufferedReader(newInputStreamReader(connectionSocket.getInputStream()));
Createwelcoming socket
at port 6789
Wait, on welcomingsocket for contact
by client
Create inputstream, attached
to socket
Example: Java Server (TCP), cont
DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
outToClient.writeBytes(capitalizedSentence); }
} }
Read in linefrom socket
Create outputstream, attached
to socket
Write out lineto socket
End of while loop,loop back and wait foranother client connection
2: Application Layer 110
2: Application Layer 111
Chapter 2: Application Layer
2.1 Principles of network applications2.2 Web and HTTP2.3 FTP 2.4 Electronic Mail
SMTP, POP3, IMAP2.5 DNS
2.6 P2P file sharing2.7 Socket programming with TCP2.8 Socket programming with UDP2.9 Building a Web server
2: Application Layer 112
Socket Programming with UDP
UDP: no “connection” between client and serverNo handshakingSender explicitly attaches IP address and port of destination to each packetServer must extract IP address, port of sender from received packet
UDP: transmitted data may be received out of order, or lost
application viewpoint
UDP provides unreliabletransfer of groups of bytes (“datagrams”)
between client and server
2: Application Layer 113
Client/Server Socket Interaction: UDP
closeclientSocket
Server (running on hostid)
read reply fromclientSocket
create socket,clientSocket = DatagramSocket()
Client
Create, address (hostid, port=x,send datagram request using clientSocket
create socket,port=x, forincoming request:serverSocket = DatagramSocket()
read request fromserverSocket
write reply toserverSocketspecifying clienthost address,port number
2: Application Layer 114
Example: Java Client (UDP)
send
Pac
ket
to network from network
rece
iveP
acke
t
inFr
omU
ser
keyboard monitor
Process
clientSocket
UDPpacket
inputstream
UDPpacket
UDPsocket
Output: sends packet (recallthat TCP sent “byte stream”)
Input: receives packet (recall thatTCP received “byte stream”)
Clientprocess
client UDP socket
Example: Java Client (UDP)
import java.io.*; import java.net.*;
class UDPClient { public static void main(String args[]) throws Exception {
BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
Createinput stream
Create client socket
Translatehostname to IP
address using DNS
2: Application Layer 115
Example: Java Client (UDP), cont.
DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
clientSocket.send(sendPacket);
DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
clientSocket.receive(receivePacket);
String modifiedSentence = new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); }
}
Create datagram with data-to-send,
length, IP addr, port
Send datagramto server
Read datagramfrom server
2: Application Layer 116
2: Application Layer 117
Example: Java Server (UDP)
import java.io.*; import java.net.*;
class UDPServer { public static void main(String args[]) throws Exception
{
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024];
while(true) {
DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
Createdatagram socket
at port 9876
Create space forreceived datagram
Receivedatagram
Example: Java Server (UDP), cont
2: Application Layer 118
String sentence = new String(receivePacket.getData());
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress,
port);
serverSocket.send(sendPacket); }
}
}
Get IP addrport #, of
sender
Write out datagramto socket
End of while loop,loop back and wait foranother datagram
Create datagramto send to client
2: Application Layer 119
Chapter 2: Summaryour study of network apps now complete!
Application architecturesClient-serverP2PHybrid
Application service requirements:
Reliability, bandwidth, delayInternet transport service model
Connection-oriented, reliable: TCPUnreliable, datagrams: UDP
Specific protocols:HTTPFTPSMTP, POP, IMAPDNSP2P: BitTorrent, Skype
Socket programming
2: Application Layer 120
Chapter 2: SummaryMost importantly: learned about protocols
Typical request/reply message exchange:
Client requests info or serviceServer responds with data, status code
Message formats:Headers: fields giving info about dataData: info being communicated
Important themes: Control vs. data msgs
In-band, out-of-bandCentralized vs. decentralized Stateless vs. statefulReliable vs. unreliable msg transfer “Complexity at network edge”