Chapter 2 Application Layer Course on Computer Communication

Chapter 2: Application Layer Course on Computer Communication and Networks, CTH/GU The slides are adaptation of the slides made available by the authors of the course’s main textbook: Computer Networking: A Top Down Approach, 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, 2009. copyright 1996 -2009, J. F Kurose and K. W. Ross 2: Application Layer 1

Chapter 2: Application Layer Chapter goals: r conceptual + implementation aspects of network application protocols m client server, p 2 p paradigms (we will study the latter seperately) m service models r learn about protocols by examining popular application-level protocols (more will come later, when studying realtime traffic aspects) r specific protocols: m http, (ftp), smtp, pop, dns, p 2 p file sharing r programming network applications m socket programming 2: Application Layer 2

Applications and application-layer protocols Application: communicating, distributed processes m running in network hosts in “user space” m exchange messages m e. g. , email, file transfer, the Web Application-layer protocols m one “piece” of an application others are e. g. user agents. • Web: browser • E-mail: mail reader • streaming audio/video: media player m m define messages exchanged by apps and actions taken use services provided by lower layer protocols application transport network data link physical 2: Application Layer 3

Client-server paradigm Typical network app has two pieces: client and server Client: r initiates contact with server (“speaks first”) r typically requests service from server, r for Web, client is implemented in browser; for e-mail, in mail reader Server: r provides requested service to client r e. g. , Web server sends requested Web page, mail server delivers e-mail application transport network data link physical request reply application transport network data link physical 2: Application Layer 4

Auxiliary terms ++ Q: how does a process socket: Internet “identify” the other application programming process with which it interface wants to communicate? m 2 processes communicate by sending data into socket, reading data out of socket (like sending out, receiving in via doors) m m IP address of host running other process “port number” - allows receiving host to determine to which local process the message should be delivered … more: cf programming project guidelines 2: Application Layer 5

Properties of transport service of interest to the app Data loss Bandwidth, Timing tolerate some loss r other apps (e. g. , file transfer, telnet) require 100% reliable data transfer r Connection-oriented vs connectionless services multimedia) require minimum amount of bandwidth r some apps (e. g. , Internet telephony, interactive games) require low delay and/or low jitter r other apps (elastic apps, e. g. file transfer) make use of whatever bandwidth, timing they get r some apps (e. g. , audio) can r some apps (e. g. , 2: Application Layer 6

Transport service requirements of common apps Data loss Bandwidth Time Sensitive file transfer e-mail Web documents real-time audio/video no loss No-loss-tolerant no no no yes, 100’s msec stored audio/video interactive games financial apps loss-tolerant no loss elastic audio: 5 Kb-1 Mb video: 10 Kb-5 Mb same as above few Kbps up elastic Application yes, few secs yes, 100’s msec yes and no 2: Application Layer 7

Services provided by Internet transport protocols TCP service: r connection-oriented: setup r r required between client, server 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 bandwidth guarantees UDP service: r connectionless r unreliable transport between sending and receiving process r does not provide: flow control, congestion control, timing, or bandwidth guarantee Q: why bother? Why is there a UDP? 2: Application Layer 8

Internet apps: their protocols Application e-mail remote terminal access Web file transfer streaming multimedia remote file server Internet telephony nslookup and many others Application layer protocol Underlying transport protocol » smtp [RFC 821] TCP telnet [RFC 854] TCP » http [RFC 2068] TCP ftp [RFC 959] TCP proprietary TCP or UDP (e. g. Real. Networks) NSF TCP or UDP SIP, RTP, typically UDP, TCP proprietary (e. g. , Skype) also possible » DNS [RFC 882, 883, 1034, 1035] UDP 2: Application Layer 9

The Web: some jargon r Web page: m consists of “objects” m addressed by a URL r Most Web pages consist of: m m base HTML page, and several referenced objects. r URL has two components: host name and path name: r User agent for Web is called a browser: m m MS Internet Explorer Netscape Communicator r Server for Web is called Web server: m m m Apache (public domain) MS Internet Information Server Netscape Enterprise Server www. some. School. edu/some. Dept/pic. gif 2: Application Layer 10

The Web: the http protocol client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection r http messages (application-layer 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 htt PC running Explorer htt pr equ pr esp est ons e t es u q re tp ht tcp socket port 80 se n po s re Protocols that maintain “state” are complex! Mac running r past history must be maintained Navigator r if server or client crashes, their views r http 1. 0: RFC 1945 of “state” may be inconsistent, must be reconciled r http 1. 1: RFC 2068 Server running NCSA Web server 2: Application Layer 11

http example Suppose user enters URL www. some. School. edu/some. Department/home. index (contains text, references to 10 jpeg images) 1 a. http client initiates TCP connection to http server (process) at www. some. School. edu. Port 80 is default for http server. 2. http client sends http request message (containing URL) into TCP connection socket time 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 (some. Department/home. index), sends message into socket 2: Application Layer 12

http example (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 13

Non-persistent and persistent connections Non-persistent r HTTP/1. 0 r server parses request, responds, and closes TCP connection r new TCP connection for each object => extra overhead per object But most 1. 0 browsers use parallel TCP connections. Persistent r default for HTTP/1. 1 r on same TCP connection: server, parses request, responds, parses new request, . . r Client sends requests for all referenced objects as soon as it receives base HTML; r Less overhead per object r Objects are fetched sequentially But can also pipeline requests (resembles non-persistent optimised behaviour) 2: Application Layer 14

http message format: request ASCII (human-readable format; try telnet to www server, port 80) request line (GET, POST, HEAD (PUT, DELETE in v 1. 1. ) GET /somedir/page. html HTTP/1. 0 commands) User-agent: Mozilla/4. 0 Accept: text/html, image/gif, image/jpeg header Accept-language: fr lines Carriage return, line feed indicates end of message (extra carriage return, line feed) 2: Application Layer 15

http request message: general format 2: Application Layer 16

http message format: respone status line (protocol status code status phrase) header lines data, e. g. , requested html file HTTP/1. 0 200 OK 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 17

http response status codes In first line in server->client response message. A few sample codes: 200 OK m request succeeded, requested object later in this message 301 Moved Permanently m requested object moved, new location specified later in this message (Location: ) 400 Bad Request m request message not understood by server 404 Not Found m requested document not found on this server 505 HTTP Version Not Supported 2: Application Layer 18

Trying out http (client side) for yourself 1. Telnet to your favorite Web server: telnet www. eurecom. fr 80 Opens TCP connection to port 80 (default http server port) at www. eurecom. fr. Anything typed in sent to port 80 at www. eurecom. fr 2. Type in a GET http request: GET /~ross/index. html HTTP/1. 0 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 19

User-server interaction: authentication server client Authentication goal: control access to server documents usual http request msg r stateless: client must present 401: authorization req. authorization in each request WWW authenticate: r authorization: typically name, password usual http request msg m authorization: header + Authorization: line in request usual http response msg m if no authorization presented, server refuses usual http request msg access, sends WWW authenticate: header line in response + Authorization: line usual http response msg Browser caches name & password so that user does not have to repeatedly enter it. time 2: Application Layer 20

Cookies: keeping “state” 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 21

Cookies (continued) What cookies can bring: r authorization r shopping carts r recommendations r user session state aside Cookies and privacy: r cookies permit sites to learn a lot about you r you may supply name and e-mail to sites r search engines use cookies to learn yet more r advertising companies obtain info across sites 2: Application Layer 22

Conditional GET: client-side caching r Goal: don’t send object if server client has up-to-date stored (cached) version r client: specify date of cached copy in http request If-modified-since: <date> http request msg If-modified-since: <date> http response HTTP/1. 0 304 Not Modified object not modified r server: response contains no object if cached copy up-todate: HTTP/1. 0 304 Not Modified http request msg If-modified-since: <date> http response object modified HTTP/1. 1 200 OK … <data> 2: Application Layer 23

Web Caches (proxy server) Goal: satisfy client request without involving origin server r user configures browser: Web accesses via web cache origin server r client sends all http requests to web cache Proxy h st t m if object at web cache, web tp e u server req ues se cache immediately returns client http p n t t o p ht res es object in http response r pon se ttp h m else requests object from t es htt u q pr origin server (or from next e r se equ p n t o h est p t cache), then returns http ht tp s e r res pon tp response to client t h se r Hierarchical, cooperative caching, ICP: Internet Caching client origin Protocol server 2: Application Layer 24

Why Web Caching? Assume: cache is “close” to client (e. g. , in same network) r smaller response time: cache “closer” to client r decrease traffic to distant servers m link out of institutional/local ISP network often bottleneck r Important for large data applications (e. g. video, …) r Performance effect: origin servers public Internet 1. 5 Mbps access link institutional network 10 Mbps LAN institutional cache E(delay)=hit. Ratio*Local. Acc. Delay + (1 -hit. Ratio)*Remote. Acc. Delay 2: Application Layer 25

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) m server: remote host r ftp: RFC 959 r ftp server: port 21 m 2: Application Layer 26

ftp: separate control, data connections r ftp client contacts ftp server at port 21, specifying TCP as transport protocol r two parallel TCP connections opened: m control: exchange commands, responses between client, server. “out of band control” m data: file data to/from server r ftp server maintains “state”: current directory, earlier authentication TCP control connection port 21 FTP client TCP data connection port 20 FTP server 2: Application Layer 27

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 28

Electronic Mail User Agent r a. k. a. “mail reader: composing, editing, reading mail messages -e. g. , Outlook, Mozzila messenger Mail Servers r Mailbox: incoming messages (yet to be read) for user r message queue of outgoing (to be sent) mail messages r SMTP protocol between mail servers to send email messages m client: sending mail server m “server”: receiving mail server outgoing message queue user mailbox user agent mail server SMTP mail server user agent SMTP user agent mail server user agent 2: Application Layer 29

Electronic Mail: smtp [RFC 821, 2821] r uses TCP to reliably transfer email msg from client to server, port 25 r direct transfer: sending server to receiving server r three phases of transfer m handshaking (greeting) m transfer of messages m closure r command/response interaction m commands: ASCII text m response: status code and phrase r messages must be in 7 -bit ASCII 2: Application Layer 30

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: <alice@crepes. fr> 250 alice@crepes. fr. . . Sender ok RCPT TO: <bob@hamburger. edu> 250 bob@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 31

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 32

Mail message format smtp: protocol for exchanging email msgs RFC 822: standard for text message format: r header lines, e. g. , To: m From: m Subject: different from smtp commands! m header blank line body r body m the “message”, ASCII characters only 2: Application Layer 33

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 (base 64: encode everything in A-Z, a-z, 0 -9, +, /; good for binary quoted-printable: 8 -bit chars = “= [hd hd]” (hd= hexadecimal digit); good for ascii extensions From: alice@crepes. fr To: bob@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 34

MIME types Content-Type: type/subtype; parameters Text r example subtypes: plain, html Image r example subtypes: jpeg, gif Audio r exampe subtypes: basic (8 -bit mu-law encoded), 32 kadpcm (32 kbps coding) Video r example subtypes: mpeg, quicktime Application r other data that must be processed by reader before “viewable” r example subtypes: msword, octet-stream 2: Application Layer 35

Multipart Type From: alice@crepes. fr To: bob@hamburger. edu Subject: Picture of yummy crepe. MIME-Version: 1. 0 Content-Type: multipart/mixed; boundary=98766789 --98766789 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain Dear Bob, Please find a picture of a crepe. --98766789 Content-Transfer-Encoding: base 64 Content-Type: image/jpeg base 64 encoded data. . . . . base 64 encoded data --98766789 -- 2: Application Layer 36

Mail access protocols user agent SMTP sender’s mail server POP 3 or IMAP user agent receiver’s mail server r SMTP: delivery/storage to receiver’s server r Mail access protocol: retrieval from server m m m POP: Post Office Protocol [RFC 1939] • authorization (agent <-->server) and download • cannot re-read e-mail if he changes client IMAP: Internet Mail Access Protocol [RFC 1730] • Manipulation, organization (folders) of stored msgs (folders, etc) on one place: the IMAP server • keeps user state across sessions: HTTP: Hotmail , Yahoo! Mail, etc. 2: Application Layer 37

POP 3 protocol S: +OK POP 3 authorization phase r client commands: user: declare username m pass: password r server responses m +OK m -ERR m transaction phase, client: r list: list message numbers r retr: retrieve message by number r dele: delete r Quit C: S: server ready user alice +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 2 contents>. dele 2 quit +OK POP 3 server signing off 2: Application Layer 38 on

DNS: Domain Name System People: many identifiers: m SSN, name, Passport # Internet hosts, routers: IP address (32 bit) - used for addressing datagrams (129. 16. 237. 85) m m “name”, e. g. , (www. cs. chalmers. se)- used by humans name (alphanumeric addresses) hard to process @ router Q: map between IP addresses and name ? 2: Application Layer 39

DNS: 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) m note: core Internet function implemented as application-layer protocol; complexity at network’s “edge” r More services by DNS: m m alias host names, i. e. mnemonic canonical (more complex) name load distribution: different canonical names, depending on who is asking r The Internet Corporation for Assigned Names and Numbers (http: //www. icann. org/) and Domain Name Supporting Organization main coordinators 2: Application Layer 40

DNS name servers Why not centralize DNS? r single point of failure r traffic volume r distant centralized database r maintenance doesn’t scale! local name servers: each ISP, company has one m host DNS query first goes to local name server; acts as proxy/cache root name servers: contacts authoritative name server if name mapping not known (~ dozen root name servers worldwide) m Top-level domain (TLD) servers: responsible for (e. g. knowing the authoritative name servers) com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp. authoritative name server: m for a host: stores that host’s IP address, name 2: Application Layer 41

Distributed, Hierarchical Database Root DNS Servers (Top-level) com DNS servers (authoritative) 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 42

DNS: Root name servers r contacted by local name server that can not resolve name r root name server: m m m 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, g US Do. D Vienna, VA Frankfurt) h ARL Aberdeen, MD i Autonomica, Stockholm j Verisign, ( 11 locations) (plus 3 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 43

Example: recursive query root DNS server 2 r Host at cis. poly. edu 3 7 wants IP address for gaia. cs. umass. edu local DNS server dns. poly. edu 1 6 TLD DNS serve 5 4 8 requesting host authoritative DNS server dns. cs. umass. edu cis. poly. edu gaia. cs. umass. edu 2: Application Layer 44

root DNS server Recursive vs iterative queries r recursive query: r puts burden of name r r 2 resolution on contacted name server local DNS server heavy load? dns. poly. edu iterated query: 1 8 contacted server replies with name of server to contact requesting host “I don’t know this cis. poly. edu name, but ask this server” 3 4 TLD DNS server 5 7 6 authoritative DNS server dns. cs. umass. edu gaia. cs. umass. edu 2: Application Layer 45

DNS: caching and updating records r once (any) name server learns mapping, it caches mapping m cache entries timeout (disappear) after some time r update/notify mechanisms (and more, incl. security) cf. m RFC 2136, 3007 (ddns) m http: //www. ietf. org/html. charters/dnsext-charter. html 2: Application Layer 46

DNS records DNS: distributed db storing resource records (RR) RR format: (name, value, type, ttl) r Type=A m name is hostname m value is IP address r Type=CNAME m name is an alias name m value is canonical name r Type=NS m name is domain (e. g. foo. com) m value is IP address of authoritative name server for this domain r Type=MX m value is hostname of mailserver associated with name ttl = time to live r Type=HINFO m value is host description (CPU, OS) 2: Application Layer 47

DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header r query(reply)-id: 16 bit # for query, reply to query uses same # r flags: m query or reply m recursion desired m recursion available m reply is authoritative 2: Application Layer 48

DNS protocol, messages Name, type fields for a query RRs in reponse to query records for authoritative servers additional “helpful” info that may be used 2: Application Layer 49

Inserting records into DNS r Example: just created startup “Network Utopia” r Register name networkuptopia. com at a registrar (e. g. , Network Solutions) m m 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, dns 1. networkutopia. com, NS) (dns 1. networkutopia. com, 212. 1, A) r Put in authoritative server Type A record for www. networkuptopia. com and Type MX record for networkutopia. com r How do people get the IP address of your Web site? 2: Application Layer 50

To come later on (after all ”layers”) r Peer-to-peer (p 2 p) applications 2: Application Layer 51