Chapter 2 Application Layer Computer Networking A Top

Chapter 2 Application Layer Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. Courtesy of J. F Kurose and K. W. Ross (All material copyright 1996 -2006) 2: Application Layer 1

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v SMTP, POP 3, IMAP r 2. 5 DNS r 2. 6 P 2 P file sharing 2: Application Layer 2

Chapter 2: Application Layer Our goals: r conceptual, implementation aspects of network application protocols v transport-layer service models v client-server paradigm v peer-to-peer paradigm r learn about protocols by examining popular application-level protocols v v HTTP FTP SMTP / POP 3 / IMAP DNS 2: Application Layer 3

Some network apps r E-mail r Internet telephone r Web r Real-time video r Instant messaging r Remote login r P 2 P file sharing conference r Massive parallel computing r Multi-user network games r Streaming stored video clips 2: Application Layer 4

Creating a network app Write programs that v v v run on different end systems and communicate over a network. e. g. , Web: Web server software communicates with browser software little software written for devices in network core v v 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 2: Application Layer 5

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v SMTP, POP 3, IMAP r 2. 5 DNS r 2. 6 P 2 P file sharing 2: Application Layer 6

Application architectures r Client-server r Peer-to-peer (P 2 P) r Hybrid of client-server and P 2 P 2: Application Layer 7

Client-server architecture server: v v v always-on host permanent IP address server farms for scaling clients: v v communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other 2: Application Layer 8

Pure P 2 P architecture r no always-on server r arbitrary end systems directly communicate r peers are intermittently connected and change IP addresses Highly scalable but difficult to manage 2: Application Layer 9

Hybrid of client-server and P 2 P Skype v v v Internet telephony app Finding address of remote party: centralized server(s) Client-client connection is direct (not through server) Instant messaging v v Chatting between two users is P 2 P Presence detection/location centralized: • User registers its IP address with central server when it comes online • User contacts central server to find IP addresses of buddies 2: Application Layer 10

Processes communicating Process: program running within a host. r within same host, two processes communicate using inter-process communication (defined by OS). r processes in different hosts communicate by exchanging messages Client process: process that initiates communication Server process: process that waits to be contacted r Note: applications with P 2 P architectures have client processes & server processes 2: Application Layer 11

Sockets r process sends/receives messages to/from its socket r socket analogous to door v v sending process shoves message out door sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process host or server process controlled by app developer process socket TCP with buffers, variables Internet TCP with buffers, variables controlled by OS r API: (1) choice of transport protocol; (2) ability to fix a few parameters 2: Application Layer 12

Addressing processes r to receive messages, process must have identifier r host device has unique 32 -bit IP address r Q: does IP address of host on which process runs suffice for identifying the process? 2: Application Layer 13

Addressing processes r to receive messages, process must have identifier r host device has unique 32 -bit IP address r Q: does IP address of host on which process runs suffice for identifying the process? v Answer: NO, many processes can be running on same host r identifier includes both IP address and port numbers associated with process on host. r Example port numbers: v v HTTP server: 80 Mail server: 25 r to send HTTP message to gaia. cs. umass. edu web server: v v IP address: 128. 119. 245. 12 Port number: 80 r more shortly… 2: Application Layer 14

App-layer protocol defines r Types of messages exchanged, v e. g. , request, response r Message syntax: v what fields in messages & how fields are delineated r Message semantics v meaning of information in fields Public-domain protocols: r defined in RFCs r allows for interoperability r e. g. , HTTP, SMTP Proprietary protocols: r e. g. , Skype r Rules for when and how processes send & respond to messages 2: Application Layer 15

What transport service does an app need? Data loss r some apps (e. g. , audio) can tolerate some loss r other apps (e. g. , file transfer, telnet) require 100% reliable data transfer Timing r some apps (e. g. , Internet telephony, interactive games) require low delay to be “effective” Throughput r some apps (e. g. , multimedia) require minimum amount of throughput to be “effective” r other apps (“elastic apps”) make use of whatever throughput they get Security r Encryption, data integrity, … 2: Application Layer 16

Transport service requirements of common apps Data loss Throughput Time Sensitive file transfer e-mail Web documents real-time audio/video no loss-tolerant no no no yes, 100’s msec stored audio/video interactive games instant messaging loss-tolerant no loss elastic audio: 5 kbps-1 Mbps video: 10 kbps-5 Mbps same as above few kbps up elastic Application yes, few secs yes, 100’s msec yes and no 2: Application Layer 17

Internet transport protocols services TCP service: r connection-oriented: setup r r required between client and server processes reliable transport between sending and receiving process flow control: sender won’t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum throughput guarantees, security UDP service: r unreliable data transfer between sending and receiving process r does not provide: connection setup, reliability, flow control, congestion control, timing, throughput guarantee, or security Q: why bother? Why is there a UDP? 2: Application Layer 18

Internet apps: application, transport protocols Application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony Application layer protocol Underlying transport protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] HTTP (eg Youtube), RTP [RFC 1889] SIP, RTP, proprietary (e. g. , Skype) TCP TCP TCP or UDP typically UDP 2: Application Layer 19

Chapter 2: Application layer r 2. 1 Principles of network applications v app architectures v app requirements r 2. 2 Web and HTTP r 2. 4 Electronic Mail v SMTP, POP 3, IMAP r 2. 5 DNS r 2. 6 P 2 P file sharing 2: Application Layer 20

Web and HTTP First some jargon r Web page consists of objects r Object can be HTML file, JPEG image, Java applet, audio file, … r Web page consists of base HTML-file which includes several referenced objects r Each object is addressable by a URL r Example URL: www. someschool. edu/some. Dept/pic. gif host name path name 2: Application Layer 21

HTTP overview HTTP: hypertext transfer protocol r Web’s application layer protocol r client/server model v client: browser that requests, receives, “displays” Web objects v server: Web server sends objects in response to requests r HTTP 1. 0: RFC 1945 r HTTP 1. 1: RFC 2616 HT TP req ues PC running HT t TP res Explorer pon se st ue q e r P nse Server T o p running HT es r P T Apache Web HT server Mac running Navigator 2: Application Layer 22

HTTP overview (continued) Uses TCP: r client initiates TCP connection (creates socket) to server, port 80 r server accepts TCP connection from client r HTTP messages (applicationlayer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) r TCP connection closed HTTP is “stateless” r server maintains no information about past client requests aside Protocols that maintain “state” are complex! r past history (state) must be maintained r if server/client crashes, their views of “state” may be inconsistent, must be reconciled 2: Application Layer 23

HTTP connections Nonpersistent HTTP r At most one object is sent over a TCP connection. r HTTP/1. 0 uses nonpersistent HTTP Persistent HTTP r Multiple objects can be sent over single TCP connection between client and server. r HTTP/1. 1 uses persistent connections in default mode 2: Application Layer 24

Nonpersistent HTTP (contains text, Suppose user enters URL references to 10 www. some. School. edu/some. Department/home. index jpeg images) 1 a. HTTP client initiates TCP connection to HTTP server (process) at www. some. School. edu on port 80 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object some. Department/home. index 1 b. HTTP server at host www. some. School. 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 25

Nonpersistent HTTP (cont. ) 4. HTTP server closes TCP 5. HTTP client receives response connection. message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects time 6. Steps 1 -5 repeated for each of 10 jpeg objects 2: Application Layer 26

Non-Persistent HTTP: Response time Definition of RTT: time to send a small packet to travel from client to server and back. Response time: r one RTT to initiate TCP connection r one RTT for HTTP request and first few bytes of HTTP response to return r file transmission time total = 2 RTT+transmit time initiate TCP connection RTT request file RTT file received time to transmit file time 2: Application Layer 27

Persistent HTTP Nonpersistent HTTP issues: r requires 2 RTTs per object r OS overhead for each TCP connection r browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP r server leaves connection open after sending response r subsequent HTTP messages between same client/server sent over open connection Persistent without pipelining: r client issues new request only when previous response has been received r one RTT for each referenced object Persistent with pipelining: r default in HTTP/1. 1 r client sends requests as soon as it encounters a referenced object r as little as one RTT for all the referenced objects 2: Application Layer 28

HTTP request message r two types of HTTP messages: request, response r HTTP request message: v ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page. html HTTP/1. 1 Host: www. someschool. edu User-agent: Mozilla/4. 0 header Connection: close lines Accept-language: fr Carriage return, line feed indicates end of message (extra carriage return, line feed) 2: Application Layer 29

HTTP request message: general format 2: Application Layer 30

Uploading form input Post method: r Web page often includes form input r Input is uploaded to server in entity body URL method: r Uses GET method r Input is uploaded in URL field of request line: www. somesite. com/animalsearch? monkeys&banana 2: Application Layer 31

Method types HTTP/1. 0 r GET r POST r HEAD v asks server to leave requested object out of response HTTP/1. 1 r GET, POST, HEAD r PUT v uploads file in entity body to path specified in URL field r DELETE v deletes file specified in the URL field 2: Application Layer 32

HTTP response message status line (protocol status code status phrase) header lines data, e. g. , requested HTML file HTTP/1. 1 200 OK Connection close Date: 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. . . 2: Application Layer 33

HTTP response status codes In first line in server->client response message. A few sample codes: 200 OK v request succeeded, requested object later in this message 301 Moved Permanently v requested object moved, new location specified later in this message (Location: ) 400 Bad Request v request message not understood by server 404 Not Found v requested document not found on this server 505 HTTP Version Not Supported 2: Application Layer 34

Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: telnet cis. poly. edu 80 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 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! 2: Application Layer 35

User-server state: cookies Many major Web sites use cookies Four components: 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 Example: v v v Susan access Internet always from same PC She visits a specific ecommerce site for first time When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID 2: Application Layer 36

Cookies: keeping “state” (cont. ) client ebay: 8734 Cookie file amazon: 1678 ebay: 8734 usual http request msg usual http response + Set-cookie: 1678 usual http request msg cookie: 1678 usual http response msg Cookie file amazon: 1678 ebay: 8734 cookiespecific action ss acce ac ce one week later: e n server da try i tab n b creates ID as ac e ke nd 1678 for user ss Cookie file server usual http request msg cookie: 1678 usual http response msg cookiespectific action 2: Application Layer 37

Cookies (continued) What cookies can bring: r authorization r shopping carts r recommendations r user session state (Web e- mail) aside Cookies and privacy: r cookies permit sites to learn a lot about you r you may supply name and e-mail to sites How to keep “state”: r Protocol endpoints: maintain state at sender/receiver over multiple transactions r cookies: http messages carry state 2: Application Layer 38

Web caches (proxy server) Goal: satisfy client request without involving origin server r user sets browser: Web accesses via cache r browser sends all HTTP requests to cache v v object in cache: cache returns object else cache requests object from origin server, then returns object to client origin server HT client. HTTP TP req Proxy server ues t res pon se t s ue q re P nse o T p HT es r TP T H client est u q e Pr T nse o p HT res P T HT origin server 2: Application Layer 39

More about Web caching r Cache acts as both client and server r Typically cache is installed by ISP (university, company, residential ISP) Why Web caching? r Reduce response time for client request. r Reduce traffic on an institution’s access link. r Internet dense with caches: enables “poor” content providers to effectively deliver content (but so does P 2 P file sharing) 2: Application Layer 40

Caching example Assumptions r average object size = 100, 000 bits r avg. request rate from institution’s browsers to origin servers = 15/sec r delay from institutional router to any origin server and back to router = 2 sec Consequences origin servers public Internet 1. 5 Mbps access link institutional network 10 Mbps LAN r utilization on LAN = 15% r utilization on access link = 100% r total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds institutional cache 2: Application Layer 41

Caching example (cont) Possible solution r increase bandwidth of access link to, say, 10 Mbps Consequences origin servers public Internet r utilization on LAN = 15% r utilization on access link = 15% = Internet delay + access delay + LAN delay = 2 sec + msecs r often a costly upgrade 10 Mbps access link r Total delay institutional network 10 Mbps LAN institutional cache 2: Application Layer 42

Caching example (cont) origin servers Install cache r suppose hit rate is. 4 Consequence public Internet r 40% requests will be satisfied almost immediately r 60% requests satisfied by origin server r utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec) r total avg delay = Internet delay + access delay + LAN delay =. 6*(2. 01) secs +. 4*milliseconds < 1. 4 secs 1. 5 Mbps access link institutional network 10 Mbps LAN institutional cache 2: Application Layer 43

Conditional GET r Goal: don’t send object if cache has up-to-date cached version r cache: specify date of cached copy in HTTP request If-modified-since: <date> r server: response contains no object if cached copy is up-to -date: HTTP/1. 0 304 Not Modified server cache HTTP request msg If-modified-since: <date> HTTP response object not modified HTTP/1. 0 304 Not Modified HTTP request msg If-modified-since: <date> HTTP response object modified HTTP/1. 0 200 OK <data> 2: Application Layer 44

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v SMTP, POP 3, IMAP r 2. 5 DNS r 2. 6 P 2 P file sharing 2: Application Layer 45

FTP: the file transfer protocol user at host FTP user client interface file transfer local file system FTP server remote file system r transfer file to/from remote host r client/server model client: side that initiates transfer (either to/from remote) v server: remote host r ftp: RFC 959 r ftp server: port 21 v 2: Application Layer 46

FTP: separate control, data connections TCP control connection port 21 r FTP client contacts FTP r r server at port 21, specifying TCP as transport protocol Client obtains authorization over control connection Client browses remote directory by sending commands over control connection. When server receives file transfer command, server opens 2 nd TCP connection (for file) to client After transferring one file, server closes data connection. FTP client TCP data connection port 20 FTP server r Server opens another TCP data connection to transfer another file. r Control connection: “out of band” r FTP server maintains “state”: current directory, earlier authentication 2: Application Layer 47

FTP commands, responses Sample commands: Sample return codes r sent as ASCII text over r status code and phrase (as control channel r USER username r PASS password r LIST return list of file in r r current directory r RETR filename retrieves r r STOR filename stores r (gets) file (puts) file onto remote host in HTTP) 331 Username OK, password required 125 data connection already open; transfer starting 425 Can’t open data connection 452 Error writing file 2: Application Layer 48

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v SMTP, POP 3, IMAP r 2. 5 DNS r 2. 6 P 2 P file sharing 2: Application Layer 49

Electronic Mail outgoing message queue user mailbox user agent Three major components: r user agents r mail servers mail server r simple mail transfer SMTP protocol: SMTP User Agent r a. k. a. “mail reader” r composing, editing, reading mail messages r e. g. , Eudora, Outlook, elm, Mozilla Thunderbird r outgoing, incoming messages stored on server SMTP mail server user agent SMTP user agent mail server user agent 2: Application Layer 50

Electronic Mail: mail servers user agent Mail Servers r mailbox contains incoming messages for user r message queue of outgoing (to be sent) mail messages r SMTP protocol between mail servers to send email messages v client: sending mail server v “server”: receiving mail server SMTP mail server user agent SMTP user agent mail server user agent 2: Application Layer 51

Electronic Mail: SMTP [RFC 2821] r uses TCP to reliably transfer email message from client to server, port 25 r direct transfer: sending server to receiving server r three phases of transfer v handshaking (greeting) v transfer of messages v closure r command/response interaction v commands: ASCII text v response: status code and phrase r messages must be in 7 -bit ASCII 2: Application Layer 52

Scenario: Omar sends message to Ali 4) SMTP client sends Omar’s message over the TCP connection 5) Ali’s mail server places the message in Ali’s mailbox 6) Ali invokes his user agent to read message 1) Omar uses UA to compose message and “to” ali@someschool. edu 2) Omar’s UA sends message to his mail server; message placed in message queue 3) Client side of SMTP opens TCP connection with Ali’s mail server 1 user agent Omar 2 mail server 3 mail server 4 5 6 user agent Ali 2: Application Layer 53

Sample SMTP interaction S: C: S: C: C: C: S: 220 hamburger. edu HELO crepes. fr 250 Hello crepes. fr, pleased to meet you MAIL FROM: <omar@crepes. fr> 250 omar@crepes. fr. . . Sender ok RCPT TO: <ali@hamburger. edu> 250 ali@hamburger. edu. . . Recipient ok DATA 354 Enter mail, end with ". " on a line by itself Do you like ketchup? How about pickles? . 250 Message accepted for delivery QUIT 221 hamburger. edu closing connection 2: Application Layer 54

Try SMTP interaction for yourself: r telnet servername 25 r see 220 reply from server r enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send email without using email client (reader) 2: Application Layer 55

SMTP: final words r SMTP uses persistent connections r SMTP requires message (header & body) to be in 7 bit ASCII r SMTP server uses CRLF to determine end of message Comparison with HTTP: r HTTP: pull r SMTP: push r both have ASCII command/response interaction, status codes r HTTP: each object encapsulated in its own response msg r SMTP: multiple objects sent in multipart msg 2: Application Layer 56

Mail message format SMTP: protocol for exchanging email msgs RFC 822: standard for text message format: r header lines, e. g. , To: v From: v Subject: different from SMTP commands! v header blank line body r body v the “message”, ASCII characters only 2: Application Layer 57

Message format: multimedia extensions r MIME: multimedia mail extension, RFC 2045, 2056 r additional lines in msg header declare MIME content type MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data From: omar@crepes. fr To: ali@hamburger. edu Subject: Picture of yummy crepe. MIME-Version: 1. 0 Content-Transfer-Encoding: base 64 Content-Type: image/jpeg base 64 encoded data. . . . . base 64 encoded data 2: Application Layer 58

Mail access protocols user agent SMTP sender’s mail server access protocol user agent receiver’s mail server r SMTP: delivery/storage to receiver’s server r Mail access protocol: retrieval from server v v v 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: gmail, Hotmail , Yahoo! Mail, etc. 2: Application Layer 59

POP 3 protocol authorization phase r client commands: v v user: declare username pass: password r server responses v v +OK -ERR transaction phase, client: r list: list message numbers r retr: retrieve message by number r dele: delete r quit S: C: S: +OK POP 3 server ready user ali +OK pass hungry +OK user successfully logged C: S: S: S: C: C: S: list 1 498 2 912. retr 1 <message 1 contents>. dele 1 retr 2 <message 1 contents>. dele 2 quit +OK POP 3 server signing off 2: Application Layer 60 on

POP 3 (more) and IMAP More about POP 3 r Previous example uses “download and delete” mode. r Ali cannot re-read email if he changes client r “Download-and-keep”: copies of messages on different clients r POP 3 is stateless across sessions IMAP r Keep all messages in one place: the server r Allows user to organize messages in folders r IMAP keeps user state across sessions: v names of folders and mappings between message IDs and folder name 2: Application Layer 61

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v SMTP, POP 3, IMAP r 2. 5 DNS r 2. 6 P 2 P file sharing 2: Application Layer 62

DNS: Domain Name System People: many identifiers: v Government ID, name, passport # Internet hosts, routers: v v IP address (32 bit) used for addressing datagrams “name”, e. g. , www. yahoo. com - used by humans Q: map between IP addresses and name ? Domain Name System: r distributed database implemented in hierarchy of many name servers r application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) v note: core Internet function, implemented as application-layer protocol v complexity at network’s “edge” 2: Application Layer 63

DNS services r Hostname to IP address translation r Host aliasing v Canonical and alias names r Mail server aliasing r Load distribution v Replicated Web servers: set of IP addresses for one canonical name Why not centralize DNS? r single point of failure r traffic volume r distant centralized database r maintenance doesn’t scale! 2: Application Layer 64

Distributed, Hierarchical Database Root DNS Servers com DNS servers yahoo. com amazon. com DNS servers org DNS servers pbs. org DNS servers edu DNS servers poly. edu umass. edu DNS servers Client wants IP for www. amazon. com; 1 st approx: r Client queries a root server to find com DNS server r Client queries com DNS server to get amazon. com DNS server r Client queries amazon. com DNS server to get IP address for www. amazon. com 2: Application Layer 65

DNS: Root name servers r contacted by local name server that can not resolve name r root name server: v v v contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD k RIPE London (also Amsterdam, Frankfurt) g US Do. D Vienna, VA i Autonomica, Stockholm (plus 3 h ARL Aberdeen, MD j Verisign, (11 locations) other locations) m WIDE Tokyo e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) 13 root name servers worldwide b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA 2: Application Layer 66

TLD and Authoritative Servers r Top-level domain (TLD) servers: v responsible for com, org, net, edu, etc, and all toplevel country domains uk, fr, ca, jp. v Network solutions maintains servers for com TLD v Educause for edu TLD r Authoritative DNS servers: v organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e. g. , Web, mail). v Can be maintained by organization or service provider 2: Application Layer 67

Local Name Server r Does not strictly belong to hierarchy r Each ISP (residential ISP, company, university) has one. v Also called “default name server” r When a host makes a DNS query, query is sent to its local DNS server v Acts as a proxy, forwards query into hierarchy. 2: Application Layer 68

DNS name resolution example root DNS server 2 r Host at cis. poly. edu wants IP address for gaia. cs. umass. edu iterated query: r contacted server replies with name of server to contact r “I don’t know this name, but ask this server” 3 4 TLD DNS server 5 local DNS server dns. poly. edu 1 8 requesting host 7 6 authoritative DNS server dns. cs. umass. edu cis. poly. edu gaia. cs. umass. edu 2: Application Layer 69

DNS name resolution example recursive query: 2 r puts burden of name resolution on contacted name server r heavy load? root DNS server 3 7 local DNS server dns. poly. edu 1 6 TLD DNS server 5 4 8 requesting host authoritative DNS server dns. cs. umass. edu cis. poly. edu gaia. cs. umass. edu 2: Application Layer 70

DNS: caching and updating records r once (any) name server learns mapping, it caches mapping v cache entries timeout (disappear) after some time v TLD servers typically cached in local name servers • Thus root name servers not often visited r update/notify mechanisms under design by IETF v RFC 2136 v http: //www. ietf. org/html. charters/dnsind-charter. html 2: Application Layer 71

DNS records DNS: distributed db storing resource records (RR) RR format: (name, value, type, ttl) r Type=A v name is hostname v value is IP address r Type=CNAME v name is alias name for some “canonical” (the real) name www. ibm. com is really r Type=NS servereast. backup 2. ibm. com v name is domain (e. g. v value is canonical name foo. com) v value is hostname of r Type=MX authoritative name server v value is name of mailserver for this domain associated with name 2: Application Layer 72

DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header r identification: 16 bit # for query, reply to query uses same # r flags: v query or reply v recursion desired v recursion available v reply is authoritative 2: Application Layer 73

DNS protocol, messages Name, type fields for a query RRs in response to query records for authoritative servers additional “helpful” info that may be used 2: Application Layer 74

Chapter 2: Application layer r 2. 1 Principles of network applications v app architectures v app requirements r 2. 2 Web and HTTP r 2. 4 Electronic Mail v SMTP, POP 3, IMAP r 2. 5 DNS r 2. 6 P 2 P file sharing 2: Application Layer 75

Pure P 2 P architecture r no always-on server r arbitrary end systems directly communicate peer-peer r peers are intermittently connected and change IP addresses r Three topics: v File distribution v Searching for information v Case Study: Skype 2: Application Layer 76

File Distribution: Server-Client vs P 2 P Question : How much time to distribute file from one server to N peers? us: server upload bandwidth Server us File, size F d. N u 1 d 1 u 2 ui: peer i upload bandwidth d 2 di: peer i download bandwidth Network (with abundant bandwidth) 2: Application Layer 77

File distribution time: server-client r server sequentially sends N copies: v NF/us time r client i takes F/di time to download Server F us d. N u 1 d 1 u 2 d 2 Network (with abundant bandwidth) Time to distribute F to N clients using = dcs = max { NF/us, F/min(di) } i client/server approach increases linearly in N (for large N) 2: Application Layer 78

File distribution time: P 2 P r server must send one Server F u 1 d 1 u 2 d 2 copy: F/us time us r client i takes F/di time Network (with d. N to download abundant bandwidth) u. N r NF bits must be downloaded (aggregate) r fastest possible upload rate: us + Sui d. P 2 P = max { F/us, F/min(di) , NF/(us + i S ui ) } 2: Application Layer 79

Server-client vs. P 2 P: example Client upload rate = u, F/u = 1 hour, us = 10 u, dmin ≥ us 2: Application Layer 80

File distribution: Bit. Torrent r P 2 P file distribution tracker: tracks peers participating in torrent: group of peers exchanging chunks of a file obtain list of peers trading chunks peer 2: Application Layer 81

Bit. Torrent (1) r file divided into 256 KB chunks. r peer joining torrent: has no chunks, but will accumulate them over time v registers with tracker to get list of peers, connects to subset of peers (“neighbors”) r while downloading, peer uploads chunks to other peers may come and go r once peer has entire file, it may (selfishly) leave or (altruistically) remain v 2: Application Layer 82

Bit. Torrent (2) Sending Chunks: tit-for-tat r Ali sends chunks to four Pulling Chunks neighbors currently r at any given time, sending him chunks at the different peers have highest rate different subsets of v re-evaluate top 4 every file chunks 10 secs r periodically, a peer (Ali) asks each neighbor for r every 30 secs: randomly select another peer, list of chunks that they starts sending chunks have. v newly chosen peer may r Ali sends requests for join top 4 his missing chunks v “optimistically unchoke” v rarest first 2: Application Layer 83

Bit. Torrent: Tit-for-tat (1) Ali “optimistically unchokes” Omar (2) Ali becomes one of Omar’s top-four providers; Omar reciprocates (3) Omar becomes one of Ali’s top-four providers Omar Ali With higher upload rate, can find better trading partners & get file faster! 2: Application Layer 84

P 2 P: searching for information Index in P 2 P system: maps information to peer location (location = IP address & port number) File sharing (eg e-mule) r Index dynamically tracks the locations of files that peers share. r Peers need to tell index what they have. r Peers search index to determine where files can be found Instant messaging r Index maps user names to locations. r When user starts IM application, it needs to inform index of its location r Peers search index to determine IP address of user. 2: Application Layer 85

P 2 P: centralized index original “Napster” design 1) when peer connects, it informs central server: v v Ali centralized directory server 1 peers IP address content 2) Omar queries for “XYZ” 3) Omar requests file from Ali 1 1 2 3 1 Omar 2: Application Layer 86

P 2 P: problems with centralized directory r Single point of failure r Performance bottleneck r Copyright infringement: “target” of lawsuit is obvious file transfer is decentralized, but locating content is highly centralized 2: Application Layer 87

Query flooding r fully distributed v no central server r used by Gnutella r Each peer indexes the files it makes available for sharing (and no other files) overlay network: graph r edge between peer X and Y if there’s a TCP connection r all active peers and edges are overlay net r Edge: virtual (not physical) link r given peer will typically be connected with < 10 overlay neighbors 2: Application Layer 88

Gnutella: protocol r Query message sent over existing TCP connections r peers forward Query message ry e r Query. Hit it Qu H ry sent over e Qu reverse Query path File transfer: HTTP Query. Hit Qu ery Query. Hit Scalability: limited scope flooding Qu er y 2: Application Layer 89

Gnutella: Peer joining 1. 2. 3. Joining peer X must find some other peer in Gnutella network: use list of candidate peers X sequentially attempts to make TCP connection with peers on list until connection setup with Y Flooding: X sends Ping message to Y; Y forwards Ping message to his overlay neighbors (who then forward to their neighbors…. ) 1. 4. peers receiving Ping message respond to X with Pong message X receives many Pong messages. It can then setup additional TCP connections 2: Application Layer 90

Hierarchical Overlay r Between centralized index, query flooding approaches r Each peer is either a super node or assigned to a super node. v v TCP connection between peer and its super node. TCP connections between some pairs of super nodes. r Super node tracks the content in all its children. 2: Application Layer 91

P 2 P Case study: Skype clients (SC) r inherently P 2 P: pairs of users communicate. r proprietary application Skype login server -layer protocol (inferred via reverse engineering) r hierarchical overlay with SNs r Index maps usernames to IP addresses; distributed over SNs Supernode (SN) 2: Application Layer 92

Peers as relays r Problem when both Alice and Bob are behind “NATs”. v NAT prevents an outside peer from initiating a call to insider peer r Solution: v Using Alice’s and Bob’s SNs, Relay is chosen v Each peer initiates session with relay. v Peers can now communicate through NATs via relay 2: Application Layer 93

Chapter 2: Summary Our study of network apps now complete! r Application architectures v client-server v P 2 P v hybrid r Application service requirements: v reliability, bandwidth, delay r Internet transport service model v v connection-oriented, reliable: TCP unreliable, datagrams: UDP r Specific protocols: v HTTP v FTP v SMTP, POP, IMAP v DNS 2: Application Layer 94

Chapter 2: Summary Most importantly: learned about protocols r typical request/reply message exchange: v v client requests info or service server responds with data, status code r message formats: v headers: fields giving info about data v data: info being communicated r control vs. data msgs in-band, out-of-band centralized vs. decentralized stateless vs. stateful reliable vs. unreliable msg transfer “complexity at network edge” v r r 2: Application Layer 95