1. 2: Application Layer 1 Chapter 2: Application Layer Chapter goals: r conceptual + implementation aspects of network application protocols m client server paradigm m service models r learn about protocols by examining popular application-level protocols More chapter goals r specific protocols: m http m ftp m smtp m pop m dns r programming network applications m socket programming

2. 2: Application Layer 2 Applications and application-layer protocols Application: communicating, distributed processes m running in network hosts in “user space” m exchange messages to implement app m e.g., email, file transfer, the Web Application-layer protocols m one “piece” of an app m define messages exchanged by apps and actions taken m user services provided by lower layer protocols application transport network data link physical application transport network data link physical application transport network data link physical

3. 2: Application Layer 3 Client-server paradigm Typical network app has two pieces: client and server application transport network data link physical application transport network data link physical Client: r initiates contact with server (“speaks first”) r typically requests service from server, r e.g.: request WWW page, send email Server: r provides requested service to client r e.g., sends requested WWW page, receives/stores received email request reply

4. 2: Application Layer 4 Application-layer protocols (cont). API: application programming interface r defines interface between application and transport layer r socket: Internet API m two processes communicate by sending data into socket, reading data out of socket Q: how does a process “identify” the other process with which it wants to communicate? m IP address of host running other process m “port number” - allows receiving host to determine to which local process the message should be delivered … lots more on this later.

5. 2: Application Layer 5 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” Bandwidth r some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective” r other apps (“elastic apps”) make use of whatever bandwidth they get

6. 2: Application Layer 6 Transport service requirements of common apps Application file transfer e-mail Web documents real-time audio/video stored audio/video interactive games financial apps Data loss no loss loss-tolerant no loss Bandwidth elastic audio: 5Kb-1Mb video:10Kb-5Mb same as above few Kbps up elastic Time Sensitive no yes, 100’s msec yes, few secs yes, 100’s msec yes and no

7. 2: Application Layer 7 Services provided by Internet transport protocols TCP service: r connection-oriented: setup required between client, server r reliable transport between sending and receiving process r flow control: sender won’t overwhelm receiver r congestion control: throttle sender when network overloaded r does not providing: timing, minimum bandwidth guarantees UDP service: r unreliable data transfer between sending and receiving process r does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Q: Why is there a UDP?

8. 2: Application Layer 8 Internet apps: their protocols and transport protocols Application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony Application layer protocol smtp [RFC 821] telnet [RFC 854] http [RFC 2068] ftp [RFC 959] HTTP (e.g. YouTube) proprietary (e.g. RealNetworks) SIP[RFC 3261] RTP [RFC 3550] proprietary (e.g., Vocaltec, Skype) Underlying transport protocol TCP TCP or UDP typically UDP or TCP

9. 2: Application Layer 9 WWW: the http protocol http: hypertext transfer protocol r WWW’s application layer protocol r client/server model m client: browser that requests, receives, “displays” WWW objects m server: WWW server sends objects in response to requests r http1.0: RFC 1945 r http1.1: RFC 2068 PC running Explorer Server running NCSA Web server Mac running Navigator http request http response NCSA (National Center for Supercomputing Applications) 는 슈퍼컴퓨터망에서 이용하게 될 각종 프로그램과 통신 규약을 연구하는 곳으로 1986 년에 설립되었으며 NSF 의 5 개 슈퍼컴퓨터 센터 중의 하나이다. 슈퍼컴퓨터 프로그램NSF

10. 2: Application Layer 10 The http protocol: more http: TCP transport service: r client initiates TCP connection (creates socket) to server, port 80 r server accepts TCP connection from client r http messages (application- layer protocol messages) exchanged between browser (http client) and WWW server (http server) r TCP connection closed http is “stateless” r server maintains no information about past client requests 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 aside

11. 2: Application Layer 11 http example Suppose user enters URL www.someSchool.edu/someDepartment/home.index 1a. http client initiates TCP connection to http server (process) at www.someSchool.edu. Port 80 is default for http server. 2. http client sends http request message (containing URL) into TCP connection socket 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 ( someDepartment/home.index ), sends message into socket time (contains text, references to 10 jpeg images)

12. 2: Application Layer 12 http example (cont.) r non-persistent connection: one object in each TCP connection m some browsers create multiple TCP connections simultaneously - one per object r persistent connection: multiple objects transferred within one TCP connection 5. http client receives response message containing html file, displays html. Parsing html file, finding 10 referenced jpeg objects 6. Steps 1-5 repeated for each of 10 jpeg objects 4. http server closes TCP connection. time

13. 2: Application Layer 13 http message format: request r two types of http messages: request, response r http request message: m ASCII (human-readable format) GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu Connection: close User-agent: Mozilla/5.0 Accept: text/html, image/gif,image/jpeg Accept-language:fr (extra carriage return, line feed) request line (GET, POST, HEAD commands) header lines Carriage return, line feed indicates end of message

14. 2: Application Layer 14 http request message: general format

15. 2: Application Layer 15 http message format: reply HTTP/1.1 200 OK Connection: close Date: Thu, 09 Aug 2011 12:00:15 GMT Server: Apache/2.2.3 (CentOS) Last-Modified: Tue, 09 Aug 2011 15. Content-Length: 6821 Content-Type: text/html data data data data data... status line (protocol status code status phrase) header lines data, e.g., requested html file

16. 2: Application Layer 16 http reply status 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 In first line in server->client response message. A few sample codes:

17. 2: Application Layer 17 Web Caches (proxy server) r user sets browser: WWW accesses via web cache r client sends all http requests to web cache m if object at web cache, web cache immediately returns object in http response m else requests object from origin server, then returns http response to client Goal: satisfy client request without involving origin server client Proxy server client http request http response http request http response http request http response origin server origin server

18. 2: Application Layer 18 Why WWW 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 origin servers public Internet institutional network 10 Mbps LAN 1.5 Mbps access link institutional cache

19. 2: Application Layer 19 ftp: the file transfer protocol r transfer file to/from remote host r client/server model m client: side that initiates transfer (either to/from remote) m server: remote host r ftp: RFC 959 r ftp server: port 21 file transfer FTP server FTP user interface FTP client local file system remote file system user at host

20. 2: Application Layer 20 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 FTP client FTP server TCP control connection port 21 TCP data connection port 20

21. 2: Application Layer 21 ftp commands, responses Sample commands: r sent as ASCII text over control channel  USER username  PASS password  LIST return list of file in current directory  RETR filename retrieves (gets) file  STOR filename stores (puts) file onto remote host Sample return codes r status code and phrase (as in http) r 331 Username OK, password required r 125 data connection already open; transfer starting r 425 Can’t open data connection r 452 Error writing file

22. 2: Application Layer 22 Electronic Mail Three major components: r user agents r mail servers r simple mail transfer protocol: smtp User Agent r a.k.a. “mail reader” r composing, editing, reading mail messages r e.g., Eudora, pine, elm, Netscape Messenger r outgoing, incoming messages stored on server user mailbox outgoing message queue mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP

23. 2: Application Layer 23 Electronic Mail: mail servers Mail Servers r mailbox contains incoming messages (yet to be read) for user r message queue of outgoing (to be sent) mail messages r smtp protocol between mail server to send email messages m client: sending mail server m “server”: receiving mail server mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP

24. 2: Application Layer 24 Electronic Mail: smtp [RFC 821] 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 m closure r command/response interaction m commands: ASCI text m response: status code and phrase

25. 2: Application Layer 25 Message format: multimedia extensions r MIME: multimedia mail extension, RFC 2045, 2056 r additional 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 data type, subtype, parameter declaration method used to encode data MIME version encoded data

26. 2: Application Layer 26 MIME types Text  example subtypes: plain, html Image  example subtypes: jpeg, gif Audio  exampe subtypes: basic (8-bit mu-law encoded), 32kadpcm (32 kbps coding) Video  example subtypes: mpeg, quicktime Application r other data that must be processed by reader before “viewable”  example subtypes: msword, octet-stream

27. 2: Application Layer 27 Mail access protocols r SMTP: delivery/storage to receiver’s server r Mail access protocol: retrieval from server m POP: Post Office Protocol [RFC 1939] authorization (agent server) and download m IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server user agent sender’s mail server user agent SMTP POP3 or IMAP receiver’s mail server

28. 2: Application Layer 28 DNS: Domain Name System People: many identifiers: m SSN, name, Passport # Internet hosts, routers: m IP address (32 bit) - used for addressing datagrams m “name”, e.g., gaia.cs.umass.edu - 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) m note: core Internet function implemented as application-layer protocol m complexity at network’s “edge”

29. 2: Application Layer 29 DNS name servers r no server has all name- to-IP address mappings local name servers: m each ISP, company has local (default) name server m host DNS query first goes to local name server authoritative name server: m for a host: stores that host’s IP address, name m can perform name/address translation for that host’s name Why not centralize DNS? r single point of failure r traffic volume r distant centralized database r maintenance

30. 2: Application Layer 30 DNS: Root name servers r contacted by local name server that can not resolve name r root name server: m contacts authoritative name server if name mapping not known m gets mapping m returns mapping to local name server

31. 2: Application Layer 31 Simple DNS example host surf.eurecom.fr wants IP address of gaia.cs.umass.edu 1. Contacts its local DNS server, dns.eurecom.fr 2. dns.eurecom.fr contacts root name server, if necessary 3. root name server contacts authoritative name server, dns.umass.edu, if necessary requesting host surf.eurecom.fr gaia.cs.umass.edu root name server authorititive name server dns.umass.edu local name server dns.eurecom.fr 1 2 3 4 5 6

32. 2: Application Layer 32 DNS example Root name server: r may not know authoratiative name server r may know intermediate name server: who to contact to find authoritative name server requesting host surf.eurecom.fr gaia.cs.umass.edu root name server local name server dns.eurecom.fr 1 2 3 4 5 6 authoritative name server dns.cs.umass.edu intermediate name server dns.umass.edu 7 8

33. 2: Application Layer 33 DNS: iterated queries recursive query: r puts burden of name resolution on contacted name server r heavy load? iterated query: r contacted server replies with name of server to contact r “I don’t know this name, but ask this server” requesting host surf.eurecom.fr gaia.cs.umass.edu root name server local name server dns.eurecom.fr 1 2 3 4 5 6 authoritative name server dns.cs.umass.edu intermediate name server dns.umass.edu 7 8 iterated query

34. 2: Application Layer 34 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 under design by IETF m RFC 2136 m http://www.ietf.org/html.charters/dnsind-charter.html

35. 2: Application Layer 35 DNS records DNS: distributed db storing resource records (RR) r Type=NS  name is domain (e.g. foo.com)  value is IP address of authoritative name server for this domain RR format: (name, value, type,ttl) r Type=A  name is hostname  value is IP address r Type=CNAME  name is an alias name for some “cannonical” (the real) name  value is cannonical name r Type=MX  value is hostname of mailserver associated with name

36. 2: Application Layer 36 Chapter 2: Summary r application service requirements: m reliability, bandwidth, delay r client-server paradigm r Internet transport service model m connection-oriented, reliable: TCP m unreliable, datagrams: UDP Our study of network apps now complete! r specific protocols: m http m ftp m smtp, pop3 m dns r socket programming m client/server implementation m using tcp, udp sockets

37. 2: Application Layer 37 Chapter 2: Summary r typical request/reply message exchange: m client requests info or service m server responds with data, status code r message formats: m headers: fields giving info about data m data: info being communicated Most importantly: learned about protocols r control vs. data msgs m in-based, out-of-band r centralized vs. decentralized r stateless vs. stateful r reliable vs. unreliable msg transfer r “complexity at network edge” r security: authentication