08 Application protocols

Transcription

1 Politecnico di Milano Scuola di Ingegneria Industriale e dell Informazione 08 Application protocols Fundamentals of Communication Networks 1

2 Topics o Client-Server and Peer-to-Peer Paradigms o HTTP: Web Surfing o FTP: remote connectivity o SMTP: ing o DNS: symbolic addressing o P2P: file sharing 2

3 Some Applications running in the INTERNET o o Web o Instant messaging o Remote login o P2P file sharing o Network games o Video streaming o Internet telephony o Real-time video conference o Massive parallel computing o 3

4 Designing network applications Write programs that n run on different end systems and n communicate over a network. n e.g., Web: Web server software communicates with browser software application transport network data link physical application transport network data link physical application transport network data link physical 4

5 Designing network applications little software written for devices in network core n n network core devices do not run user application code application on end systems allows for rapid app development, propagation application transport network data link physical application transport network data link physical application transport network data link physical 5

6 Communications among Processes Process: program running within a host. o within same host, two processes communicate using inter-process communication (defined by OS). o processes in different hosts communicate by exchanging messages 6

7 Processes and Protocols o Processes running on remote host may exchange messages and services through the network o The application protocols define the rules and the formats of the communication between remote processes Application Web (web server, browser, HTML) (mail server, mail client, MIME) Protocols HTTP SMTP 7

8 Lower layers interaction o Application protocols use the services provided by lower layers through the SAPs (Service Access Point) o Each application process is associated to a SAP o OSI Stack: WEB FTP Mail Application controlled Layer Presentation Session OS controlled layers Transport 8

9 Lower layers interaction o The application protocols directly communicate with the transport layer WEB FTP Mail Application controlled Layer OS controlled layers TCP/UDP 9

10 Sockets o process sends/receives messages to/from its socket o socket analogous to door n sending process shoves message out door n sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process host or server process socket TCP with buffers, variables controlled by app developer controlled by OS Internet host or server process socket TCP with buffers, variables o Sockets equivalent to SAPs between application and transport layers 10

11 Addressing Processes o to receive messages, process must have identifier o host device has unique 32-bit IP address o Q: does IP address of host on which process runs suffice for identifying the process? 11

12 Addressing Processes n Answer: NO, many processes can be running on same host n identifier includes both IP address and port number associated with process on host n Example port numbers (HTTP server: 80, Mail server: 25) n to send HTTP message to o IP address: o Port number: 80 n The transport layer multiplexes several flows coming from the application layer 12

13 App-layer protocol defines o Types of messages exchanged, n e.g., request, response o Message syntax: n what fields in messages & how fields are delineated o Message semantics n meaning of information in fields o Rules for when and how processes send & respond to messages Public-domain protocols: o defined in RFCs o allows for interoperability o e.g., HTTP, SMTP Proprietary protocols: o e.g., KaZaA 13

14 What transport service does an app need? Data loss o some apps (e.g., audio) can tolerate some loss o other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing o some apps (e.g., Internet telephony, interactive games) require low delay to be effective Bandwidth o some apps (e.g., multimedia) require minimum amount of bandwidth to be effective o other apps ( elastic apps ) make use of whatever bandwidth they get 14

15 Transport service requirements of common apps Application Data loss Bandwidth Time Sensitive file transfer Web documents real-time audio/video stored audio/video interactive games instant messaging no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5mbps same as above few kbps up elastic no no no yes, 100 s msec yes, few secs yes, 100 s msec yes and no 15

16 Internet transport protocols services TCP service: o connection-oriented: setup required between client and server processes o reliable transport between sending and receiving process o flow control: sender won t overwhelm receiver o congestion control: throttle sender when network overloaded o does not provide: timing, minimum bandwidth guarantees UDP service: o unreliable data transfer between sending and receiving process o does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Q: why bother? Why is there a UDP? 16

17 Applications vs Transport Protocols Application 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] proprietary (e.g. RealNetworks) proprietary (e.g., Vonage,Dialpad) Underlying transport protocol TCP TCP TCP TCP TCP or UDP typically UDP 17

18 Client-Server Architecture o The main target of the communication between remote processes is the provision of services o Two functionalities can be accomplished by a process: n Request for services n Provide services o If a given process accomplishes just one of the above functionalities, the communication is a client-server one 18

19 Client-Server Architecture o Client processes make requests and interpret responses o Server processes interpret requests and provide the responses o If the same host needs to issue requests and provide responses two processes are needed request client response server 19

20 Client/Server Paradigm o Differences between program and process n Program: software n Process: instance of the program being executed o A server process is continuously executed on the host (daemon) and is activated through a passive open o A client process is activated when needed only (by the user or by some other process) through an active open o After the passive open the server is able to handle requests from clients o The active open requires the indication of the IP address and the port of the server 20

21 Client/Server Paradigm o Multiple clients can issues requests to a single server o Clients may also issue multiple requests at the same time request response request responses response requests

22 Client/Server Paradigm o A client may implement both serial and parallel operation modes n Example: multiple requests can be issued for all the files contained in a web page o Even a server may implement both serial and parallel operation modes o Usually, the applications using UDP are handled serially 22

23 Client/Server Paradigm o Usually, the servers using TCP implement parallel operation mode o A TCP connection towards all the clients is opened for all the time needed to exchange requests/responses o The procedure handling each client is handled via multi-threading, using fork operations 23

24 Application Protocols: Possible Architectures o Client-server n Terminals act as clients OR as servers n Client hosts and server hosts may have different features o Peer-to-peer (P2P) n All the terminals can implement the client process AND the server one o Hybrid 24

25 Client-server architecture server: n always-on host n permanent IP address n server farms for scaling clients: n communicate with server n may be intermittently connected n may have dynamic IP addresses n do not communicate directly with each other 25

26 Pure P2P architecture o no always-on server o arbitrary end systems directly communicate o peers are intermittently connected and change IP addresses o example: Gnutella Highly scalable but difficult to manage 26

27 Hybrid of client-server and P2P Skype n Internet telephony app n Finding address of remote party: centralized server(s) n Client-client connection is direct (not through server) Instant messaging n Chatting between two users is P2P n Presence detection/location centralized: o User registers its IP address with central server when it comes online o User contacts central server to find IP addresses of buddies 27

28 Politecnico di Milano Scuola di Ingegneria Industriale e dell Informazione Web Browsing Hyper Text Transfer Protocol (HTTP) 28

29 HyperText Tranfer Protocol (HTTP) "Hypertext Transfer Protocol -- HTTP/1.0," RFC 1945, May "Hypertext Transfer Protocol -- HTTP/1.1," RFC 2068, January 1997 o client-server architecture o clients request objects (files) identified through a URL o servers send back the files to the clients o Stateless operation (no memory on previous requests is mantained) client HTTP request HTTP response server 29

30 Message transfer o o o HTTP relies upon TCP for message transfer Usually a web page is composed of a main document (HTML) and multiple linked objects Requests use the URL (Uniform Resource Locator) Method :// Host : Port / Path Type of protocol Symbolic address of the server Document on the server TCP assigns port number 80 to HTTP servers 30

31 Message transfer o Suppose a client requests a composite web page (1 main HTML document + 10 figures) Other Objects : HTML Text Two operation modes can be adopted Non-persistent connection (default mode of HTTP 1.0) Persistent connection (default mode of HTTP 1.1) 31

32 Non persistent o One TCP connection for each request-response cycle. The server closes the TCP connection once it has sent the requested object o The same procedure is adopted for all the docs within the required web page o Multiple TCP connections can be opened in parallel o The maximum number of connections can be set in the browser configuration options Request (index.html) Response (index.html file) Request (image1.jpg) Request (image2.jpg) 32

33 Persistent connection o o o o The server does not close the connection after the response The same connection can be used to transfer other objects within the same page or even other web pages The server closes the connection on a timeout basis Two Flavors: n without pipelining: the client issues a new request only upon reception of the previous response n with pipelining: multiple requests can be issued at the same time (default mode HTTP v1.1) 33

34 Requests 34

35 Some Methods GET HEAD POST PUT To get a doc from the server. The doc is specified by the URL. The server answers with the required doc in the body of the response message To get info on a specified doc. The server answers with the requested information To post some input to the server regarding a given object identified by the URL To store a doc on the server. The doc is carried by the request message. The URL specifies the position for the doc to be stored. Other Methods: PATCH, COPY, MOVE, DELETE, LINK, UNLINK, OPTIONS. 35

36 Responses 36

37 Messages 1xx Informational 2xx Success 3xx Redirection 4xx Client error 5xx Server error 100 Continue: Go On. 200 OK: Request OK, the required info is in the field of this message 302 Moved Permanently: The required object has been moved (perm) 304 Moved Temporalily: The required object has been moved (temp) 400 Bad Request: Generic error 401 Unauthorized: Access failed due to userid or password error 404 Not Found: File not found 500 Internal server error Server failure 501 Not implemented Required functionality not supported 503 Service unavailable Unavailable service 37

38 Headers Header name : Header value o headers are used to exchange further service information o A message can carry multiple headers o Examples Cache-control Accept Accept-language Authorization If-modified-since User-agent Cache info Supported formats Supported languages Client permits send doc. only if modified user agent type 38

39 Message Exchange o Example: request GET /ntw/index.html HTTP/1.1 Connection: close User-agent: Mozilla/4.0 Accept: text/html, image/gif, image/jpeg Accept-language:it o Example: response HTTP/ OK Connection: close Date: Thu, 06 Aug :00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun :23:24 GMT Content-Length: 6821 Content-Type: text/html data data data data data... 39

40 Conditional get Client: GET /fruit/kiwi.gif HTTP/1.0 User-agent: Mozilla/4.0 Accept: text/html, image/gif, image/jpeg If-modified-since: Mon, 22 Jun :23:24 Server: HTTP/ Not Modified Date: Wed, 19 Aug :39:29 Server: Apache/1.3.0 (Unix) (empty entity body) o Also method HEAD can be used 40

41 Network caching and proxy o Main duty of a proxy is to provide a distributed cache memory o If a doc is stored in a proxy near the client the download time can be reduced 41

42 Proxy o A proxy is an application gateway, that is, it implements up to the application layer o It must act both as a client and as a server o The final server speaks with the client on the proxy (hiding of users) User Agent HTTP TCP IP LL Proxy HTTP TCP IP LL Server HTTP TCP IP LL 42

43 Authentication o HTTP is stateless o Consecutive requests from the same user cannot be recognized o Very simple authentication procedure based on userid and password to be inserted in the requests GET /ntw/index.html HTTP/ Authorization Required WWW-Authenticate:[tipo di autenticazione] GET /ntw/index.html HTTP/1.1 Authorization: account, passwd. GET image.gif HTTP/1.1 Authorization: account, passwd 43

44 Cookies o o o The server can assign to each client a cookie number which identifies the client in future transactions The cookie number is stored by the client and used in following requests towards the same server Used in e-commerce GET /ntw/index.html HTTP/ OK Set-cookie: GET /ntw/carrello/index.html HTTP/1.1 Cookie: GET image.gif HTTP/1.1 Cookie:

45 HTML (HyperText Markup Language) o HTTP handles the object transfer and does not account for the object format o The visualization of the object is done through interpreter programs (browsers) o Formatted text pages are transferred in ASCII files and are interpreted according to formatting instructions written in HTML o HTML pages may contain references to other objects which need to be interpreted by the browser as n Part of the document to visualize n Links to other pages o If a HTML page is stored on the server and is sent upon request, this is a static page 45

46 Dynamic WEB Pages o If a page is created on the fly upon reception of a request, this is a dynamic page o The server examines the request, executes a program associated to the request and generates the HTML page to be sent back GET /cgi-bin/prog.pl HTTP/1.1 prog.pl 200 OK Dynamic Page 46

47 Active Web Pages o A web page may contain a program to be executed by the client o The program is downloaded and executed locally by the client o This can be used to set up interactive pages, moving graphs, etc. GET /java/applet HTTP/ OK program 47

48 Politecnico di Milano Scuola di Ingegneria Industriale e dell Informazione File Transfer Protocol (FTP) 48

49 File Transfer Protocol (FTP) "File Transfer Protocol, RFC 959, October o Used to transfer files between two remote hosts o The application operates directly on the file system (both at server and at client side) 49

50 File Transfer Protocol (FTP) o Uses TCP for the transfer o Two TCP connections are used for the transfer of data and control client User Interface Control process Data tranfer process Port 21 Port 20 server Control process Data tranfer process LFS LFS 50

51 FTP: user interface 51

52 FTP: control connection o It is opened in the usual way n The server issues a passive open with port number 21 and waits fir requests n The client issues an active open with a dynamic port number every time it needs to transfer files o The control connection is persistent, and remains open for all the data transfer time client Control process Active open Port Passive open Port 21 server Control process LFS Data tranfer process LFS Data tranfer process 52

53 FTP: Data connections o Data connections are non-persistent, n one connection for each file to transfer, n connection closed upon completion of file transfer o To open a data connection: 1st Way: n The client issues a passive open with a dynamic port number n The client notifies the port number to the server on the control connection through the PORT command n The server issues an active open towards the specified port of the client using 20 as local port number 2nd Way: n The client sends the PASV command to the server n The server chooses a dynamic port number, issues a passive open and communicate the chosen port number to the client n The client issues an active open using the port number received from the server 53

54 FTP: Data connections o The data transfer can be accomplished in different ways and using different formats: o File types: n ASCII n Binary: o Transmission modes: n Stream mode: the file is sent down to the TCP as a stream of unstructured bytes n Block mode: the file is structured in blocks with a header each and sent down to the TCP 54

55 FTP: commands o Commands are transferred in ASCII Access Commands USER username PASS password QUIT log out Transfer Management TYPE file type MODE transfer mode File Management CWD change directory DELE delete file LIST list files RETR retrieve file STOR store file Port Management PORT client port PASV server choose port 55

56 FTP: Responses 125 Data connection already open; transfer starting 200 Command OK 225 Data connection open 226 Closing data connection 227 Entering passive mode; srv. sends Ip_add.,port 230 User login OK 331 Username OK, password required 425 Can't open data connection 426 Connection closed; tranfer aborted 452 Error writing file 500 Syntax error; unrecognized command 501 Syntax error in parameters or arguments 502 Command not implemented 56

57 FTP: Transfer Example Client 220 service ready USER matteo 331 username OK; password? PASS pippo user login OK PORT opening data connection LIST /usr/pub 125 data connection OK Data 226 closing data connection Server 57

58 Politecnico di Milano Scuola di Ingegneria Industriale e dell Informazione Service Simple Mail Transfer Protocol (SMTP) 58

59 o Service to send textual messages in an asynchronous way o It is implemented through a network of mail servers using the SMTP (Simple Mail Transfer Protocol) 59

60 SMTP J.B. Postel, "Simple Mail Transfer Protocol," RFC 821, August o Textual protocol o Also the body of the messages needs to be ASCII n Binaries must be converted to ASCII o Once a server receives a message from a user agent n Stores the message in a queue n Opens a TCP connection (port 25) with the destination server n Sends the message 60

61 Client/Server Message exchange Handshake S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 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 61

62 Message Format D.H. Crocker, "Standard for the Format of ARPA Internet Text Messages," RFC 822, August o The message format is specified (command DATA) o Some headers are added to the message From: alice@crepes.fr To: bob@hamburger.edu Subject: Request of information <black line> <Body>. 62

63 Multipurpose Internet Mail Extensions (MIME) "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies," RFC 2045, Nov "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types," RFC 2046, Nov o MIME is used to allow the transfer of non-ascii messages 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. 63

64 Multipurpose Internet Mail Extensions (MIME) o Coding techniques: n Base64: o The flow of bits is divided into chunks of 24 bits each o Each chunk is divided into 4 groups of 6 bits each o Each chunk is interpreted as a character according to a conversion table base (51) (8) (4) (57) Z I E

65 Multipurpose Internet Mail Extensions (MIME) o Quoted-printable o The flow of bits is divided into chunks of 8 bits each o If a sequence corresponds to a ASCII character is sent straight away o Otherwise is sent as three characters: = followed by the hexadecimal representation of the byte & L Non-ASCII Quotable-printable & L = D

66 Multipurpose Internet Mail Extensions (MIME) o MIME allows the transfer of multiple objects within the same message: From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe with commentary MIME-Version: 1.0 Content-Type: multipart/mixed; Boundary=StartOfNextPart --StartOfNextPart Dear Bob, Please find a picture of an absolutely scrumptious crepe. --StartOfNextPart Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data... --StartOfNextPart Let me know if you would like the recipe.. 66

67 MailBox Access Protocols ESMTP n POP3 (Post Office Protocol version 3) n IMAP (Internet Mail Access Protocol) n HTTP n Security Issue: the protocols can run over TLS/SSL 67

68 POP3 Authorization Phase o Client Commands: o n user: username n pass: password Server Responses: n +OK n -ERR Transaction Phase, client: o o o o list: list mess. number retr: get message dele: delete message quit S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on C: list S: S: 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 68

69 Case History o December 1995, S. Bhatia and J. Smith propose the first web based e- mail service (Hotmail) o In 1 month, 100K users o In 1 year, 12M users o December 1997, Hotmail is acquired by Microsoft for $400M o Example of first mover advantage and viral marketing 69

70 Politecnico di Milano Scuola di Ingegneria Industriale e dell Informazione Remote Terminal TELNET 70

71 TELNET (TErminaL NETwork) o Remote terminal application o The commands are transferred through a TCP connection Telnet client Telnet server Terminal driver TCP IP LL Internet TCP IP LL Pseudo-terminal driver 71

72 TELNET (TErminaL NETwork) o TELNET transfers characters n Data characters: o ASCII with the first byte 0 o ASCII characters with the first byte 1 (preceded by a special control byte) n Control characters: o 8 bit commands (first bit 1 ) o examples n n IAC (255): next one is a control character EC (247): erase character c a t f i l e a IAC EC 1 72

73 TELNET (TErminaL NETwork) 73

74 Politecnico di Milano Scuola di Ingegneria Industriale e dell Informazione Domain Name System (DNS) 74

75 Domain Name System (DNS) o IP addresses are not suited to be used by applications o Symbolic addresses are more convenient n Hierarchical (street, city, state) n Independent from layer 3 o Binding is needed morgana.elet.polimi.it 75

76 Domain Name System (DNS) "Domain Names - Concepts and Facilities," RFC 1034, Nov "Domain Names - Implementation and Specification," RFC 1035, Nov o IP networks provide a symbolic addressing service o Supported by a distributed database service which handles the binding: DNS (Domain Name System) o DNS is an application layer protocol which uses UDP/IP to transfer its messages o DNS is currently used also for n Host aliasing n Mail server aliasing n Load distribution 76

77 Symbolic Addressing com edu org gov mil it fr jp de... yahoo ucla columbia polimi virgilio morgana.elet.polimi.it cs rett elet morgana o Hierarchical Addressing o Each branch is controlled by a known authority o To get a symbolic address you must go through these authorities 77

78 Types of Name Servers o Local Name Servers n Directly connected to tyhe hosts n Each ISP (residential, university, industry, etc.) has a LNS n Talks with the Root NS o Root Name Servers n Stores info on the addressing of big groups of hosts and domains n Stores info on the authoritative NS for a given domain n Talks with the Authoritative NS o Authoritative Name Servers n NS responsible for a specific hostname 78

79 Hierarchical DNS Source: Computer Networking, J. Kurose 79

80 Root NS in

81 How To resolve a binding o Every host knows the LNS address o Each request for resolving a binding is sent to the local NS using UDP o The LNS gets the info and answers HOST DNS request DNS response Local NS DNS Network 81

82 Stored Info o Type Name, Value, Type, TTL n A: Name is a host name and Value is the IP address (morgana.elet.polimi.it, , A, TTL) n NS: Name is a domain and Value is the symbolic name of a server which knows how to resolve the name (elet.polimi.it, morgana.elet.polimi.it, NS, TTL) n CNAME: Name is an alias and Value is the real name ( zephyro.rett.polimi.it, CNAME, TTL) n MX: Name is a mail domain or a mail alias and Value is the name of the mail server (elet.polimi.it, mailserver.elet.polimi.it, MX,TTL) 82

83 Database Organization o ARPANET was using a central database o Internet uses a distributed database structure o Branches are divided into zones and each zone is associated a DNS o The server of a zone is authoritative for that zone com edu org gov mil it fr jp de... yahoo ucla columbia polimi virgilio cs rett elet morgana 83

84 How to get Info o Recursive Way: n Requests travel along the hierarchy n Responses travel the opposite direction Source: Computer Networking, J. Kurose 84

85 How to get Info o Iterative Way: o A server can notify the name of another server where to get the info from 85

86 Caching o A server can cache a info temporarily o If a request is issued regarding cached info the server can answer even if it is not authoritative for that specific info o TTL is set by the authoritative server to advertise the freshness of a piece of info o The non-authoritative server uses the TTL to set a validity timer for the cached info 86

87 DNS Messages Binary Format (not ASCII) o o o o o o o identification: identifies the couple request/response flag: request/response, authoritative/non auth., iterative/recursive number of: field sin the following header sections questions: name to resolve (usually A or MX) answers: complete resource records authority: contains other record provided by other servers additional info 87

88 Politecnico di Milano Scuola di Ingegneria Industriale e dell Informazione Peer-to-Peer Exchange 88

89 P2P file sharing Example o Alice runs P2P client application on her notebook computer o Intermittently connects to Internet; gets new IP address for each connection o Asks for Hey Jude o Application displays other peers that have copy of Hey Jude. o Alice chooses one of the peers, Bob. o File is copied from Bob s PC to Alice s notebook: HTTP o While Alice downloads, other users uploading from Alice. o Alice s peer is both a Web client and a transient Web server. All peers are servers = highly scalable! 89

90 P2P: centralized directory original Napster design 1) when peer connects, it informs central server: n IP address n content 2) Alice queries for Hey Jude 3) Alice requests file from Bob centralized directory server Alice 3 Bob peers 90

91 P2P: problems with centralized directory o Single point of failure o Performance bottleneck o Copyright infringement file transfer is decentralized, but locating content is highly centralized 91

92 Query flooding: Gnutella o fully distributed n no central server o public domain protocol o many Gnutella clients worldwide overlay network: graph o edge between peer X and Y if there s a TCP connection o all active peers and edges are overlay net o Edge is not a physical link o Given peer will typically be connected with < 10 overlay neighbors 92

93 Gnutella: protocol Query message sent over existing TCP connections peers forward Query message QueryHit sent over reverse path Query QueryHit File transfer: HTTP Query QueryHit Scalability: limited scope flooding 93

94 Gnutella: Peer joining 1. Joining peer X must find some other peer in Gnutella network: to use list of candidate peers 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 94

95 Exploiting heterogeneity: KaZaA o Each peer is either a group leader or assigned to a group leader. n TCP connection between peer and its group leader. n TCP connections between some pairs of group leaders. o Group leader tracks the content in all its children. ordinary peer group-leader peer neighoring relationships in overlay network 95

96 KaZaA: Querying o Each file has a hash and a descriptor o Client sends keyword query to its group leader o Group leader responds with matches: n For each match: metadata, hash, IP address o If group leader forwards query to other group leaders, they respond with matches o Client then selects files for downloading n HTTP requests using hash as identifier sent to peers holding desired file 96