2: Application Layer1 ECE5650 FTP, , DNS, and P2P

2: Application Layer2 Recap: HTTP and Web r HTTP request msg format and method types:  GET, POST, HEAD, PUT, DELATE r HTTP response msg format and status codes r Cookies and their usage:  Persistent vs Non-Persistent cookies r Web cache or proxy server:  Conditional GET ( If-modified-since: ) in HTTP header

2: Application Layer3 Examples ofInternet Services r 2.1 Principles of network applications r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail  SMTP, POP3, IMAP r 2.5 DNS r 2.6 P2P file sharing r 2.7 Socket programming with TCP r 2.8 Socket programming with UDP r 2.9 Building a Web server

2: Application Layer4 FTP: the file transfer protocol r transfer file to/from remote host r client/server model  client: side that initiates transfer (either to/from remote)  server: remote host r ftp: RFC 959 r ftp server: port 21 for control, port 20 for data file transfer FTP server FTP user interface FTP client local file system remote file system user at host

2: Application Layer5 SFTP: secure file transfer protocol r All communication (login, control and data are secured) r transfer file to/from remote host r same as FTP client/server model r network protocol designed by the IETF to provide secure file transfer and manipulation facilities over the secure shell (SSH) protocol. file transfer over SSH SFTP server SFTP user interface SFTP client local file system remote file system user at host

2: Application Layer6 FTP: separate control, data connections r FTP client contacts FTP server at port 21, specifying TCP as transport protocol r Client obtains authorization over control connection r Client browses remote directory by sending commands over the persistent control connection. r When server receives a command for a file transfer or directory listing, the server opens a TCP data connection to client r After transferring one file, server closes connection. FTP client FTP server TCP control connection port 21 TCP data connection port 20 r Server opens a second TCP data connection to transfer another file. r Control connection: “out of band”  HTTP sends control info in-band r FTP server maintains “state”: current directory, earlier authentication

2: Application Layer7 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 (Get) filename retrieves (gets) file  STOR (Put) 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

2: Application Layer8 All FTP commands (RFC 959) r Access control commands:  USER, PASS, ACT, CWD, CDUP, SMNT, REIN, QUIT. r Transfer parameter commands:  PORT, PASV, TYPE STRU, MODE. r Service commands:  RETR, STOR, STOU, APPE, ALLO, REST, RNFR, RNTO, ABOR, DELE, RMD, MRD, PWD, LIST, NLST, SITE, SYST, STAT, HELP, NOOP. r

2: Application Layer9 FTP Summary r FTP/SFTP is used to transfer files between hosts r FTP is an out-of-band protocol: control is sent over server port 21 while data is sent over server port 20. r Control connection is persistent and the FTP server must maintain the state of the user. r Data connection is non-persistent and initiated by FTP server.

2: Application Layer10 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, Outlook, 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

2: Application Layer11 Electronic Mail: mail servers 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 messages  client: sending mail server  “server”: receiving mail server mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP

2: Application Layer12 Electronic Mail: SMTP [RFC 2821] r uses TCP to reliably transfer message from client to server, port 25 r direct transfer: sending server to receiving server r three phases of transfer  handshaking (greeting)  transfer of messages  closure r command/response interaction  commands: ASCII text  response: status code and phrase r messages must be in 7-bit ASCII

2: Application Layer13 Scenario: Alice sends message to Bob 1) Alice uses UA to compose message and “to” 2) Alice’s UA 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 mail server mail server user agent

2: Application Layer14 Sample SMTP interaction C: telnet smtp.wayne.edu 25 S: 220 mirapointmr3.wayne.edu C: HELO alice S: 250 Hello alice, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C:. S: 250 Message accepted for delivery C: QUIT S: 221 wayne.edu closing connection

2: Application Layer15 Try SMTP interaction for yourself:  telnet ece.eng.wayne.edu 25 r see 220 reply from server r enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send without using client (reader)

2: Application Layer16 SMTP Mail message format SMTP: protocol for exchanging msgs RFC 822: standard for text message format: r header lines, e.g.,  To:  From:  Subject: different from SMTP commands. SMTP msg goes into the DATA command r body  the “message”, 7-bit ASCII characters only header body blank line

2: Application Layer17 Message format: multimedia extensions r MIME: multimedia mail extension, RFC 2045, 2056 r additional lines in msg header declare MIME content type From: To: 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, quoted- printable is another method MIME version encoded data where each line is 78 7-bit ASCII characters including CRLF

2: Application Layer18 Base64 encoding scheme (RFC 2045) r Encoding (not encrypting) method r input: 8 bit ASCII data r output: one of the 64 values listed in the Base64 encoding table and the “=“ character r Process:  group the input data in chunks of 3-bytes or 24-bits each  break each 24-bit chunk into 4 values that are 6-bits each  use the Base64 encoding table to find the character of each value  if any chunk is less than 3 bytes then pad it with 0s and use the “=“ for any of these 6 0s. A 6-bit value with all 0s and at least 1 non-padded 0 will be an “A”  Smallest base64 encoded output is 4 characters. Example: input is AB A ASCII hex=0x41 B ASCII hex=0x42 16-bits + 8 padded 0s: values that are 6-bits each: padded 0s Base64 output is QUI=

2: Application Layer19 Mail access protocols r SMTP: delivery/storage to receiver’s server r Mail access protocol: retrieval from server  POP3: Post Office Protocol version 3 [RFC 1939] uses port 110 authorization (agent server) and download  IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server  HTTP: Hotmail, Yahoo! Mail, etc. user agent sender’s mail server user agent SMTP access protocol receiver’s mail server

2: Application Layer20 POP3 protocol authorization phase r client commands:  user: declare username  pass: password r server responses  +OK (Server accepted prior command)  -ERR (server rejected prior command) transaction phase, client:  list: list message numbers  retr: retrieve message by number  dele: delete r quit r UIDL: “unique-ID listing” list unique message ID for all of the messages present in the users mailbox. Useful for download- and-keep by keeping a file that lists the messages retrieved in earlier sessions, the client can use the UIDL command to determine which messages on the server have already been seen. r “Received:” in the msg indicates the SMTP servers that forwarded the msg C: list S: S: S:. C: retr 1 S: Return-Path: S: Received: from b1 by d1... S: more data.. S:. C: dele 1 C: retr 2 S: S:. C: dele 2 C: quit S: +OK POP3 server signing off C: telnet mail.wayne.edu 110 S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on

2: Application Layer21 POP3 (more) and IMAP More about POP3 r Previous example uses “download-and-delete” mode. r User cannot re-read the deleted . r “Download-and-keep”: copies of messages on different clients r POP3 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:  names of folders and mappings between message IDs and folder name

2: Application Layer22 Summary r SMTP and POP3 uses persistent connections r SMTP requires message (header & body) to be in 7- bit ASCII  SMTP server uses CRLF.CRLF to determine end of message r download-and-delete vs download-and-keep in POP3 r All data communications are insecure by default Comparison with HTTP: r HTTP: pull data from web server r SMTP: push data to mail server r both have command/response interaction, status codes r HTTP: each object encapsulated in its own response msg r SMTP: multiple objects sent in one multipart msg r SMPT msg must be in 7-bit ASCII while HTTP has no restriction

2: Application Layer23 Examples ofInternet Services r 2.1 Principles of network applications r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail  SMTP, POP3, IMAP r 2.5 DNS r 2.6 P2P file sharing r 2.7 Socket programming with TCP r 2.8 Socket programming with UDP r 2.9 Building a Web server