Application layer

1. Principles of network applications Web and HTTP FTP Electronic Mail SMTP, POP3, IMAP DNS Application layer

2. Figure 2.16 TCP/IP and OSI model

3. Figure 2.14 Application layer The application layer is responsible for providing services to the user.

4. Figure 2.15 Summary of layers

5. Application Layer • Our goals: • conceptual, implementation aspects of network application protocols • transport-layer service models • client-server paradigm • peer-to-peer paradigm • learn about protocols by examining popular application-level protocols • HTTP • FTP • SMTP / POP3 / IMAP • DNS

6. Some network apps • E-mail • Web • Instant messaging • Remote login • P2P file sharing • Multi-user network games • Streaming stored video clips • Internet telephone • Real-time video conference • Massive parallel computing

7. application transport network data link physical application transport network data link physical application transport network data link physical Creating a network app • Write programs that • run on different end systems and • communicate over a network. • e.g., Web: Web server software communicates with browser software

8. Application architectures • Client-server • Peer-to-peer (P2P) • Hybrid of client-server and P2P

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

10. TCP service: connection-oriented: setup 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 providing: timing, minimum bandwidth guarantees UDP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Internet transport protocols services

11. www.someschool.edu/someDept/pic.gif path name host name Web and HTTP: 1st application protocol First some jargon • Web page consists of objects • Object can be HTML file, JPEG image, Java applet, audio file,… • Web page consists of base HTML-file which includes several referenced objects • Each object is addressable by a URL • Example URL:

12. HTTP: hypertext transfer protocol Web’s application layer protocol client/server model client: browser that requests, receives, “displays” Web objects server: Web server sends objects in response to requests HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068 HTTP overview HTTP request PC running Explorer HTTP response HTTP request Server running Apache Web server HTTP response Mac running Navigator

13. Uses TCP: client initiates TCP connection (creates socket) to server, port 80 (well-known) server accepts TCP connection from client HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed HTTP is “stateless” server maintains no information about past client requests (we can use cookies for implementing state) HTTP overview (continued)

14. Nonpersistent HTTP At most one object is sent over a TCP connection. HTTP/1.0 uses nonpersistent HTTP Persistent HTTP Multiple objects can be sent over single TCP connection between client and server. HTTP/1.1 uses persistent connections in default mode HTTP connections

15. Suppose user enters URL www.cse.cuhk.edu.hk/index.html 1a. HTTP client initiates TCP connection to HTTP server (process) at www.cse.cuhk.edu.hk on port 80 Non-persistent HTTP (contains text, references to 10 jpeg images) Server CLIENT 1b. HTTP server at host www.cse.cuhk.edu.hk waiting for TCP connection at port 80. “accepts” connection, notifying client 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object index.html 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket (we don’t send jpeg here) time

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

17. Web caches (proxy server) Goal: satisfy client request without involving origin server • user sets browser: Web accesses via cache • browser sends all HTTP requests to cache • object in cache: cache returns object • else cache requests object from origin server, then returns object to client origin server Proxy server HTTP request HTTP request client HTTP response HTTP response HTTP request HTTP response client origin server

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

19. FTP client contacts FTP 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 a command for a file transfer, the server opens a TCP data connection to client After transferring one file, server closes connection. TCP control connection port 21 TCP data connection port 20 FTP client FTP server FTP: separate control, data connections • Server opens a second TCP data connection to transfer another file. • FTP server maintains “state”: current directory, earlier authentication(unlike http which doesn’t maintain state)

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

21. Mail Servers mailbox contains incoming messages for user messagequeue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send email messages client: sending mail server “server”: receiving mail server user agent user agent user agent user agent user agent user agent SMTP SMTP SMTP mail server mail server mail server Electronic Mail: mail servers

22. uses TCP to reliably transfer email message from client to server, port 25 direct transfer: sending server to receiving server three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII (problem when we send multimedia files) Electronic Mail: SMTP [RFC 2821]

23. 1) Alice uses pine to compose message and “to” bob@cs.ucla.edu 2) Alice’s pine sends message to her mail server; message placed in message queue 3) Client side of SMTP opens TCP connection with Bob’s mail server 4) SMTP client sends Alice’s message over the TCP connection 5) Bob’s mail server places the message in Bob’s mailbox 6) Bob invokes his user agent to read message user agent user agent mail server mail server Scenario: Alice sends message to Bob 1 2 6 3 4 5

24. SMTP: protocol for exchanging email messages RFC 822: standard for text message format: header lines, e.g., To: From: Subject: differentfrom SMTP commands! body the “message”, ASCII characters only This doesn’t work for multimedia (e.g, video/audio) data. Mail message format Define what is a message. In other words, the data format header blank line body (????????)

25. MIME: multimedia mail extension, RFC 2045 (audio),2056 (video) 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 Message format: multimedia extensions MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data

26. 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 other data that must be processed by reader before “viewable” example subtypes: msword, octet-stream (pdf format) MIME typesContent-Type: type/subtype; parameters

27. SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download (provide userid and password) IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server HTTP: Hotmail , Yahoo! Mail, etc. user agent user agent sender’s mail server SMTP Mail access protocols SMTP access protocol receiver’s mail server

28. People: many identifiers: HKID#, name, passport #, CUHK ID #...etc Internet hosts, routers: IP address (32 bit) - used for addressing datagrams “name”, e.g., idiotville.cse.cuhk.edu.hk - used by humans Q: map between IP addresses and name ? Domain Name System: distributed database implemented in hierarchy of many name servers application-layer protocol: host, routers, name servers to communicate to resolvenames (address/name translation) DNS: Domain Name System

29. DNS • Why not centralize DNS? • single point of failure • traffic volume • distant centralized database • maintenance • DNS services • Hostname to IP address translation • Host aliasing • Canonical and alias names • Mail server aliasing • IP to hostname mapping

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

31. DOMAIN NAME SPACE To have a hierarchical name space, a domain name space was designed. In this design the names are defined in an inverted-tree structure with the root at the top. The tree can have only 128 levels: level 0 (root) to level 127.

32. Figure 25.2 Domain name space

33. Figure 25.3 Domain names and labels

34. Figure 25.8 DNS IN THE INTERNET

35. a NSI Herndon, VA c PSInet Herndon, VA d U Maryland College Park, MD g DISA Vienna, VA h ARL Aberdeen, MD j NSI (TBD) Herndon, VA k RIPE London i NORDUnet Stockholm m WIDE Tokyo e NASA Mt View, CA f Internet Software C. Palo Alto, CA b USC-ISI Marina del Rey, CA l ICANN Marina del Rey, CA DNS: Root name servers • contacted by local name server that can not resolve name • root name server: • contacts authoritative name server if name mapping not known • gets mapping • returns mapping to local name server 13 root name servers worldwide

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

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

38. Host at cis.poly.edu wants IP address for gaia.cs.umass.edu local DNS server dns.poly.edu Example root DNS server 2 3 TLD DNS server 4 5 6 7 1 8 authoritative DNS server dns.cs.umass.edu requesting host cis.poly.edu gaia.cs.umass.edu

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

40. Figure 25.12 Recursive resolution

41. Figure 25.13 Iterative resolution

42. Figure 25.14 Query and response messages