4-Traditional Internet Applications

1. 4-Traditional Internet Applications Dr. John P. Abraham Professor UTPA

2. External references • Your book provides limited amount of information. Please note that I have used Forouzan to supplement this chapter. Refer to the syllabus.

3. Application Layer Protocols • Two broad types of protocols for this layer: • Private communication: programmer creates a pair of applications that communicate over the internet with intention it is for private use. • Standardized service. Example is a server service. All types of users will use it. The specification must be precise and unambiguous so that all can interoperate correctly.

4. Representation and Transfer • Data Representation: Syntax of data items that are exchanged. Specific form used during transfer, translation of integers, characters, and files between computers. • Data transfer. Interaction between client and server, message syntax and semantics, valid and invalid exchange error handling, termination of interaction.

5. Web protocols • HTML – a representation standard of a web page. • URL – specify the location of a page. • HTTP – transfer protocol that browser uses to communicate with the webserver. • Request types: get, head, post, put. • I leave detailed description because it is pretty elementary.

6. HTTP Requests • GET: requests a document. Server send status information and the document. • HEAD: Requests status information. Server sends just the status not the document. • POST: Sends data to the server. The server appends the data to a specified item. • PUT: Same as above, except, instead of appending it replaces previous data.

7. Response Header • Version of the HTTP protocol. • Status code (whether the server handled the request – code 200). Code 404 means item can’t be found, 400 means bad request. • Other information. Server identification. Last Modified. Content length and Content Type. All followed by CRLF.

8. Caching In Browsers • Important optimization technique to reduce download times by saving a copy of each image in a cache on the user’s disk. • If the document changes the ‘Head request to server’ will sent last modified date and time which is compared with the cached copy. If the local copy is stale a new one is downloaded.

9. Browser Architecture p 59.

10. FTP • Can transfer any type of data • Bidirectional transfer • Supports access restrictions • Ability to browse folders • Control messages are exchanged in ASCII • Supports heterogeneity

11. FTP connections • Two connections • Control • Data • Use different port numbers: Server uses port 21 and waits for the client (Passive open). Client uses ephemeral port and issues an active open. The server then issues an active open (client takes and active role by sending a TCP message to start the connection – SYN) using port 20 and the received port from the client and data transfer begins. • Transmission modes: stream, block and compressed.

12. The following two slides are very important. Please know the contents. Figure 19.1FTP

13. FTP Active Passive Opens • FTP can be run in active or passive mode, which determine how the data connection is established. In active mode, the client sends the server the IP address and port number on which the client will listen, and the server initiates the TCP connection. In situations where the client is behind a firewall and unable to accept incoming TCP connections, passive mode may be used. In this mode the client sends a PASV command to the server and receives an IP address and port number in return. The client uses these to open the data connection to the server.

14. FTP active passive contd. • A client makes a TCP connection to the server's port 21. This connection, called the control connection, remains open for the duration of the session, with a second connection, called the data connection, either opened by the server from its port 20 to a negotiated client port (active mode) or opened by the client from an arbitrary port to a negotiated server port (passive mode) as required to transfer file data. The control connection is used for session administration (i.e., commands, identification, passwords) exchanged between the client and server using a telnet-like protocol. Due to this two-port structure, FTP is considered an out-of-band, as opposed to an in-band protocol such as HTTP.

15. Table 19.1 Access commands. \ The following Slides are good references. If you are either programming FTP, or a heavy user of FTP, you need to have this info.

16. Table 19.2 File management commands

17. Table 19.3 Data formatting commands

18. Table 19.4 Port defining commands

19. Table 19.5 File transfer commands

20. Table 19.5 File transfer commands (continued)

21. Table 19.6 Miscellaneous commands

22. Table 19.7 Responses

23. Table 19.7 Responses (continued)

24. Table 19.7 Responses (continued)

25. Table 19.7 Responses (continued) Table 19.7 Responses (continued)

26. Table 19.7 Responses (continued) Table 19.7 Responses (continued)

27. Example 1 Figure 19.8 shows an example of using FTP for retrieving a list of items in a directory. 1. After the control connection to port 21 is created, the FTP server sends the 220 (service ready) response on the control connection.2. The client sends the USER command.3. The server responds with 331 (user name is OK, password is required).4. The client sends the PASS command. 5. The server responds with 230 (user login is OK) See Next Slide

28. Example 1 (cONTINUED) 6. The client issues a passive open on an ephemeral port for the data connection and sends the PORT command (over the control connection) to give this port number to the server.7. The server does not open the connection at this time, but it prepares itself for issuing an active open on the data connection between port 20 (server side) and the ephemeral port received from the client. It sends response 150 (data connection will open shortly).8. The client sends the LIST message.9. Now the server responds with 125 and opens the data connection. See Next Slide

29. Example 1 (cONTINUED) 10. The server then sends the list of the files or directories (as a file) on the data connection. When the whole list (file) is sent, the server responds with 226 (closing data connection) over the control connection.11. The client now has two choices. It can use the QUIT command to request the closing of the control connection or it can send another command to start another activity (and eventually open another data connection). In our example, the client sends a QUIT command.12. After receiving the QUIT command, the server responds with 221 (service closing) and then closes the control connection. See Next Slide

30. Figure 19.8Example 1

31. Example 2 The following shows an actual FTP session that parallels Example 1. The colored lines show the responses from the server control connection; the black lines show the commands sent by the client. The lines in white with black background shows data transfer. $ ftp voyager.deanza.fhda.eduConnected to voyager.deanza.fhda.edu.220 (vsFTPd 1.2.1)530 Please login with USER and PASS.Name (voyager.deanza.fhda.edu:forouzan): forouzan331 Please specify the password. See Next Slide

32. Example 2 Password:230 Login successful.Remote system type is UNIX.Using binary mode to transfer files.ftp> ls reports227 Entering Passive Mode (153,18,17,11,238,169)150 Here comes the directory listing. drwxr-xr-x 2 3027 411 4096 Sep 24 2002 businessdrwxr-xr-x 2 3027 411 4096 Sep 24 2002 personaldrwxr-xr-x 2 3027 411 4096 Sep 24 2002 school 226 Directory send OK.ftp> quit221 Goodbye.

33. Example 3 Figure 19.9 shows an example of how an image (binary) file is stored.1. After the control connection to port 21 is created, the FTP server sends the 220 (service ready) response on the control connection.2. The client sends the USER command.3. The server responds with 331 (user name is OK, a password is required).4. The client sends the PASS command.5. The server responds with 230 (user login is OK).6. The client issues a passive open on an ephemeral port for the data connection and sends the PORT command (over the control connection) to give this port number to the server. See Next Slide

34. Example 3 (cONTINUED) 7. The server does not open the connection at this time, but prepares itself for issuing an active open on the data connection between port 20 (server side) and the ephemeral port received from the client. It sends the response 150 (data connection will open shortly).8. The client sends the TYPE command.9. The server responds with the response 200 (command OK).10. The client sends the STRU command.11. The server responds with 200 (command OK). 12. The client sends the STOR command.13. The server opens the data connection and sends the response 250. See Next Slide

35. Example 3 (cONTINUED) 14. The client sends the file on the data connection. After the entire file is sent, the data connection is closed. Closing the data connection means end-of-file.15. The server sends the response 226 on the control connection.16. The client sends the QUIT command or uses other commands to open another data connection for transferring another file. In our example, the QUIT command is sent.17. The server responds with 221 (service closing) and it closes the control connection. See Next Slide

36. Figure 19.9Example 3

37. Example 4 We show an example of anonymous FTP. We assume thatsome public data are available at internic.net. $ ftp internic.netConnected to internic.net220 Server readyName: anonymous331 Guest login OK, send “guest” as passwordPassword: guestftp > pwd257 ’/’ is current directory See Next Slide

38. Example 4 bin. . .. . .. . . ftp > close221 Goodbyeftp > quit

39. TFTP • Simply copy a file without all the features of FTP. Mostly used now for flashing routers, diskless workstations, etc. that are directly connected. • Uses IP and UDP. Uses port number 69 initially, once it gets the ephemeral port from the client, the server opens its own ephemeral port to communicate, freeing port 69 to be used by someone else.

40. Email • MIME • SMTP • POP • IMAP Main components: User agents, message access agent, and transfer agents

41. Components of email User agent: used to compose mail, read mail, store in local computer (if two users are on the same LAN, we only need two user agents). Eg. Eudora, Outlook, Netscape. MTAs (message transfer agents) to transfer from local machine to a server, server to another server and so on. SMTP Message Access agent – to retrieve from the local server to the local recipient computer. Pop and IMAP, MIME

42. Mime (multipurpose internet mail extenstion) • Allows non-ASCII characters. Used for all languages, video, and audio.

43. Figure 20.9MIME

44. Figure 20.10MIME header

45. Table 20.1 Data types and subtypes in MIME

46. Table 20.1 Data types and subtypes in MIME (Continued)

47. Table 20.2 Content-transfer-encoding

48. SMTP (simple mail transfer protocol) • Uses commands and responses to transfer messages. Each command or reply is terminated by carriage return and linefeed.

49. 20.3 MESSAGE TRANSFER AGENT: SMTP The actual mail transfer requires message transfer agents (MTAs). The protocol that defines the MTA client and server in the Internet is called Simple Mail Transfer Protocol (SMTP). The topics discussed in this section include: Commands and Responses Mail Transfer Phases

50. Figure 20.13SMTP range