ECE5650

1
ECE5650

• FTP, Email, DNS, and P2P

2
Recap HTTP and Web

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

3
Examples ofInternet Services

• 2.1 Principles of network applications
• 2.2 Web and HTTP
• 2.3 FTP
• 2.4 Electronic Mail
• SMTP, POP3, IMAP
• 2.5 DNS

• 2.6 P2P file sharing
• 2.7 Socket programming with TCP
• 2.8 Socket programming with UDP
• 2.9 Building a Web server

4
FTP the file transfer protocol
file transfer
user at host
remote file system

• 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 for control, port 20 for data

5
SFTP secure file transfer protocol
file transfer over SSH
user at host
remote file system

• All communication (login, control and data are
  secured)
• transfer file to/from remote host
• same as FTP client/server model
• network protocol designed by the IETF to provide
  secure file transfer and manipulation facilities
  over the secure shell (SSH) protocol.

6
FTP separate control, data connections

• 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 the persistent control connection.
• When server receives a command for a file
  transfer or directory listing, the server opens a
  TCP data connection to client
• After transferring one file, server closes
  connection.

• Server opens a second TCP data connection to
  transfer another file.
• Control connection out of band
• HTTP sends control info in-band
• FTP server maintains state current directory,
  earlier authentication

7
FTP commands, responses

• Sample commands
• 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
• status code and phrase (as in HTTP)
• 331 Username OK, password required
• 125 data connection already open transfer
  starting
• 425 Cant open data connection
• 452 Error writing file

8
All FTP commands (RFC 959)

• Access control commands
• USER, PASS, ACT, CWD, CDUP, SMNT, REIN, QUIT.
• Transfer parameter commands
• PORT, PASV, TYPE STRU, MODE.
• Service commands
• RETR, STOR, STOU, APPE, ALLO, REST, RNFR, RNTO,
  ABOR, DELE, RMD, MRD, PWD, LIST, NLST, SITE,
  SYST, STAT, HELP, NOOP.
• www.faqs.org/rfcs/rfc959.html

9
FTP Summary

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

10
Electronic Mail

• 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, Netscape Messenger
• outgoing, incoming messages stored on server

11
Electronic Mail mail servers

• Mail Servers
• mailbox contains incoming messages for user
• message queue of outgoing (to be sent) mail
  messages
• SMTP protocol between mail servers to send email
  messages
• client sending mail server
• server receiving mail server

12
Electronic Mail SMTP RFC 2821

• 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

13
Scenario Alice sends message to Bob

• 4) SMTP client sends Alices message over the TCP
  connection
• 5) Bobs mail server places the message in Bobs
  mailbox
• 6) Bob invokes his user agent to read message

• 1) Alice uses UA to compose message and to
  bob_at_wayne.edu
• 2) Alices UA sends message to her mail server
  message placed in message queue
• 3) Client side of SMTP opens TCP connection with
  Bobs mail server

1
2
6
3
4
5
14
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 ltalice_at_crepes.frgt S 250
alice_at_crepes.fr... Sender ok C RCPT TO
ltbob_at_wayne.edugt S 250 bob_at_wayne.edu ...
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
15
Try SMTP interaction for yourself

• telnet ece.eng.wayne.edu 25
• see 220 reply from server
• enter HELO, MAIL FROM, RCPT TO, DATA, QUIT
  commands
• above lets you send email without using email
  client (reader)

16
SMTP Mail message format

• SMTP protocol for exchanging email msgs
• RFC 822 standard for text message format
• header lines, e.g.,
• To
• From
• Subject
• different from SMTP
• commands. SMTP msg goes
• into the DATA command
• body
• the message, 7-bit ASCII characters only

header
blank line
body
17
Message format multimedia extensions

• MIME multimedia mail extension, RFC 2045, 2056
• additional lines in msg header declare MIME
  content type

MIME version
method used to encode data, quoted-printable is
another method
multimedia data type, subtype, parameter
declaration
encoded data where each line is 78 7-bit ASCII
characters including CRLF
18
Base64 encoding scheme (RFC 2045)

• Encoding (not encrypting) method
• input 8 bit ASCII data
• output one of the 64 values listed in the Base64
  encoding table and the character
• 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 hex0x41 B ASCII
hex0x42 16-bits 8 padded 0s 0100 0001 0100
0010 0000 0000 4 values that are 6-bits
each 010000-010100-001000-000000 16-20-8-6
padded 0s Base64 output is QUI
19
Mail access protocols
SMTP
access protocol
receivers mail server

• SMTP delivery/storage to receivers server
• Mail access protocol retrieval from server
• POP3 Post Office Protocol version 3 RFC 1939
  uses port 110
• authorization (agent lt--gtserver) and download
• IMAP Internet Mail Access Protocol RFC 1730
• more features (more complex)
• manipulation of stored msgs on server
• HTTP Hotmail , Yahoo! Mail, etc.

20
POP3 protocol
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

• authorization phase
• client commands
• user declare username
• pass password
• 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
• quit
• 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.
• Received in the msg indicates the SMTP servers
  that forwarded the msg

C list S 1 498 S 2 912
S . C retr 1 S Return-Path
ltfromuser_at_..gt S Received from b1 by d1
... S more data.. S . C dele 1
C retr 2 S ltmessage 1 contentsgt
S . C dele 2 C quit S OK
POP3 server signing off
21
POP3 (more) and IMAP

• More about POP3
• Previous example uses download-and-delete mode.
• User cannot re-read the deleted e-mail.
• Download-and-keep copies of messages on
  different clients
• POP3 is stateless across sessions

• IMAP
• Keep all messages in one place the server
• Allows user to organize messages in folders
• IMAP keeps user state across sessions
• names of folders and mappings between message IDs
  and folder name

22
Email Summary

• Comparison with HTTP
• HTTP pull data from web server
• SMTP push data to mail server
• both have command/response interaction, status
  codes
• HTTP each object encapsulated in its own
  response msg
• SMTP multiple objects sent in one multipart msg
• SMPT msg must be in 7-bit ASCII while HTTP has no
  restriction

• SMTP and POP3 uses persistent connections
• SMTP requires message (header body) to be in
  7-bit ASCII
• SMTP server uses CRLF.CRLF to determine end of
  message
• download-and-delete vs download-and-keep in POP3
• All data communications are insecure by default

23
Examples ofInternet Services

• 2.1 Principles of network applications
• 2.2 Web and HTTP
• 2.3 FTP
• 2.4 Electronic Mail
• SMTP, POP3, IMAP
• 2.5 DNS

• 2.6 P2P file sharing
• 2.7 Socket programming with TCP
• 2.8 Socket programming with UDP
• 2.9 Building a Web server