1. Lecture 12 Overview

2. TELNET TELNET is a protocol that provides Over TCP connection
general,
bi-directional,
eight-bit byte oriented communications
Over TCP connection
data and control over the same connection
Many application protocols are built upon the TELNET protocol
Used to test HTTP, FTP, SMTP, POP3, IRC, etc.
CPE 401/601 Lecture 12 : TELNET protocol

3. Network Virtual Terminal
intermediate representation of a generic terminal
provides a standard language for communication of terminal control functions
Server
Process
NVT
NVT
TCP
TCP
CPE 401/601 Lecture 12 : TELNET protocol

4. Control Functions
TELNET includes support for a series of control functions commonly supported by servers
This provides a uniform mechanism for communication of (the supported) control functions
CPE 401/601 Lecture 12 : TELNET protocol

5. Command Structure
Commands start with a special character called the Interpret as Command escape character
The IAC code is 255
If a 255 is sent as data, it must be followed by another 255
If IAC is found and the next byte is IAC
a single byte is presented to application/terminal
If IAC is followed by any other code
the TELNET layer interprets this as a command
CPE 401/601 Lecture 12 : TELNET protocol

6. A Typical Mail Environment
Internet
Interactive Mail Access Protocol
SMTP
Mail Server
Mail Server
IMAP
Mail
Client
IMAP
store
POP
Store
SMTP
POP
SMTP
Mail
Client
CPE 401/601 Lecture 12 : protocols

7. Terminology User Agent Message Transfer Agent Mail Exchanger
end-user mail program
Message Transfer Agent
responsible for communicating with remote hosts and transmitting/receiving
both a client and server
Mail Exchanger
host that takes care of for a domain
CPE 401/601 Lecture 12 : protocols

8. SMTP
Used to exchange mail messages between mail servers (Message Transfer Agents)
MTA
MTA
MTA
SMTP
SMTP
File
System UA UA
CPE 401/601 Lecture 12 : protocols - SMTP

9. SMTP Protocol SMTP sender is the client SMTP receiver is the server
Alternating dialogue:
client sends command and server responds with command status message
Order of the commands is important!
Status messages include
ASCII encoded numeric status code (like HTTP,FTP) and
text string
CPE 401/601 Lecture 12 : protocols - SMTP

10. Data Format
ASCII only
must convert binary to an ASCII representation to send via
What if we want to send a line containing only a period?
Sender prepends a period to any line staring with a period (in the message)
Receiver strips the leading period in any line that starts with a period and has more stuff
CPE 401/601 Lecture 12 : protocols - SMTP

11. TCP Connection Termination
Message Progress
TCP Connection Establishment
TCP Connection Termination
CPE 401/601 Lecture 12 : protocols - SMTP

12. Reply Codes 354 Contain a Lot of Information
Only the reply codes count
Other information in a reply is purely for humans
The first digit indicates success, failure, or incomplete
354
The third digit is the specific message for that category
The second digit is the category of error message
CPE 401/601 Lecture 12 : protocols - SMTP

13. SMTP Extensions (EHLO)
A mailer supporting extensions uses EHLO instead of HELO in the greeting
$ telnet mail.unr.edu 25
Trying
220 smtp.unr.edu ESMTP Postfix
EHLO
250-smtp.unr.edu
250-PIPELINING
250-SIZE
250-VRFY
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DNS
A mailer that supports extensions responds with a list of which ones it can do
CPE 401/601 Lecture 12 : protocols - SMTP

14. Mail Headers Email messages contain many headers
some headers are created by the UA
some are automatically added by the MTA
Every MTA adds (at least) a “Received:” header
Some of the headers are parsed by intermediate MTAs
but the message content is ignored and passed on transparently
CPE 401/601 Lecture 12 : protocols - SMTP

15. Multipurpose Internet Mail Extensions
Defines extensions to support binary attachments of arbitrary format
Images, Audio, Video and multi-media messages
Text having unlimited line length or overall length
Multiple objects in a single message
Character sets other than ASCII
Multi-font messages
Requires more capable UAs to interpret messages
CPE 401/601 Lecture 12 : protocols - MIME

16. A sample MIME message
From: (Jan Trumbo) Subject: small message with Word attachment To: MIME-version: 1.0 Content-type: MULTIPART/MIXED; BOUNDARY="Boundary_[ID_nf99lkyavAuSoClF/HeK0Q]" --Boundary_[ID_nf99lkyavAuSoClF/HeK0Q] Content-type: text/plain; charset=us-ascii Joel, attached is a Word document. - Jan Date: Thu, 19 Sep :49: Content-type: application/mac-binhex40; name=tiny_text_Word_doc.doc Content-disposition: attachment; filename=tiny_text_Word_doc.doc <Word document and more stuff below here>
This identical boundary marker separates the parts of the mail message
Return-path:
Received: from [ ] ( ) by Opus1.COM (PMDF V5.1-2 #9830)
with SMTP id for trumbo; Thu,
19 Sep :50:39 MST
From: (Jan Trumbo)
Subject: small message with Word attachment
X-Sender:
To:
Message-id:
MIME-version: 1.0
Content-type: MULTIPART/MIXED; BOUNDARY="Boundary_[ID_nf99lkyavAuSoClF/HeK0Q]"
--Boundary_[ID_nf99lkyavAuSoClF/HeK0Q]
Content-type: text/plain; charset=us-ascii
Attached is a Word document.
Date: Thu, 19 Sep :49:
Content-type: application/mac-binhex40; name=tiny_text_Word_doc.doc
Content-disposition: attachment; filename=tiny_text_Word_doc.doc
(This file must be converted with BinHex 4.0)
:%R4TERNJG'9iG#"AEh*N)'4[B`"A4%*1690A4!!!!!!)!!!!!8b!2rih!#-!!!!
!!!!%!!!C!!!!!!!!!!!"!!!!!8d!!!I"!!!!!!!!!%d!!!!!!!!!!!!!!!!!!!!
CPE 401/601 Lecture 12 : protocols - MIME

17. Pulling Emails Offline (POP3 model) Online (Original IMAP model)
Client connects to server and pulls all down to client
Everything is stored on the client
Online (Original IMAP model)
Client connects to server for every transaction
Everything is stored on the server
Disconnected (Later IMAP model)
Client and server share storage burden
Server is always authoritative and client must synchronize to server
CPE 401/601 Lecture 12 : protocols

18. POP – Post Office Protocol
Used to transfer mail from a mail server to a User Agent
Mail
Server
POP UA
File
System
CPE 401/601 Lecture 12 : protocols - POP3

19. POP (version 3) Similar to SMTP command/reply lockstep
Minimizes server resources
Used to retrieve mail for a single user
requires authentication
Commands and replies are ASCII lines
Replies start with “+OK” or “-ERR”
Replies may contain multiple lines
POP has no security
Except when used with SSL, TLS
CPE 401/601 Lecture 12 : protocols - POP3

20. A POP3 Exchange
> telnet monte pop3 Trying Connected to monte.cs.rpi.edu ( ). Escape character is '^]'. +OK POP3 monte.cs.rpi.edu v7.59 server ready user joe +OK User name accepted, password please pass joepw +OK Mailbox open, 1 messages stat +OK list +OK Mailbox scan listing follows
telnet plus.pop.mail.yahoo.com 110
telnet pop.gmail.com 995
CPE 401/601 Lecture 12 : protocols - POP3

21. POP3 Example Continued
retr 1 +OK 412 octets Return-Path: <hollingd> Received: (from by monte.cs.rpi.edu (8.9.3/8.9.3) id NAA06943 for joe; Mon, 20 Mar :49: Date: Mon, 20 Mar :49: From: Dave Hollinger Message-Id: To: Status: O blah .
CPE 401/601 Lecture 12 : protocols - POP3

22. Internet Message Access Protocol
On-line, off-line, or disconnected mode operation
Server-side Mailbox Manipulation
Multiple mailbox support
Control of all folders everywhere
Online performance optimization
User can check header or search a string before downloading
Real authentication
CPE 401/601 Lecture 12 : protocols - IMAP

23. POP vs. IMAP POP3: IMAP: …. Internet Whole message headers Internet
All Messages
Internet
Whole message
IMAP:
Dr.Amer
Friends ….
headers
Internet
CPE 401/601 Lecture 12 : protocols

24. WebMail Web-base email access User agent -> web browser
User agent-mail server communication based on HTTP
HTTP to send messages from user to the mail server of the user
HTTP to get mail from the mail server of the destination
CPE 401/601 Lecture 12 : protocols - WebMail

25. Lecture 13 File Transfer Protocols
CPE 401 / 601
Computer Network Systems
slides are modified from Dave Hollinger
slides are modified from Dave Hollinger and Michael

26. File Transfer Protocols
File Transfer Protocol (RFC 959)
Why FTP?
FTP’s connections
FTP in action
FTP commands/responses
Trivial File Transfer Protocol (RFC 1350)
TFTP and TFTP’s message formats
FTP and TFTP compared
CPE 401/601 Lecture 13 : File Transfer Protocols

27. Why do we need a FTP Service?
Purpose: To Transfer files between two computers
Goals of FTP Service
Promote sharing of files (programs and/or data)
Encourage indirect/implicit use of remote computers
Shield users from variations in file storage among hosts
Transfer data reliably and efficiently
CPE 401/601 Lecture 13 : File Transfer Protocols

28. Problems of File Transfer
At first, file transfer may seem simple
Heterogeneous systems use different:
Operating Systems
Character Sets
Naming Conventions
Directory Structures
File Structures and Formats
FTP need to address and resolve these problems
CPE 401/601 Lecture 13 : File Transfer Protocols

29. File Transfer Protocol
Exchange and manipulate files over TCP
Separate control and data connections between the client and server applications
RFC 959 includes information and details
parameters for commands
lists of reply status codes
protocol state diagrams
support for a variety of file structures
sample sessions
CPE 401/601 Lecture 13 : FTP

30. The FTP Model PI: Protocol Interpreter DTP: Data Transfer Protocol
User Interface
User
Control
Server PI
User PI
File
System
Data
File
System
Server DTP
User DTP
CPE 401/601 Lecture 13 : FTP

31. Control and Data Connections
Control functions (commands) and reply codes are transferred over the control connection.
All data transfer takes place over the data connection.
The control connection must be “up” while data transfer takes place.
CPE 401/601 Lecture 13 : FTP

32. Control Connection The control connection is the “well known” service.
The control connection uses the TELNET protocol.
Commands and replies are all line oriented text (default is ASCII).
CPE 401/601 Lecture 13 : FTP

33. FTP’s Connections * Insulates users from “raw” FTP commands
* Persistent command
and reply connection
Non-persistent data
connection
client
* Routes “raw” FTP commands
* Receives server’s replies
*Server uses port 20
for data connections
Server is listening on port 21
for connection requests
User
Interface
server
User
Protocol
Interpreter
Control
Connection
Server
Protocol
Interpreter 21
User
Data Transfer
Function
Data
Connection
Server
Data Transfer
Function 20
CPE 401/601 Lecture 13 : FTP

34. FTP - Connection Establishment
ftp> open cse.unr.edu
Connected to cse.unr.edu
220 cse FTP server ready.
530 Please login with USER and PASS
client
331 Password req for mgunes.
Password:
User
Interface
230 User mgunes logged in.
ftp>
USER mgunes
PASS mypass
server
User
Protocol
Interpreter
Server
Protocol
Interpreter
Control
Connection
User
Data Transfer
Function
Data
Connection
Server
Data Transfer
Function
CPE 401/601 Lecture 13 : FTP

35. FTP - Data Transfer Control Connection Data Connection client
ls client.txt
-rw-r--r-- mgunes client.txt
User
Interface
226 Closing Data Connection
PORT 192,168,100,173,19,137
150 Data Connection will be open shortly
200 Port Command Sucessful
LIST client.txt
server
User
Protocol
Interpreter
User
Protocol
Interpreter
Server
Protocol
Interpreter
Server
Protocol
Interpreter
Control
Connection
Passive open on
Port 5001
User
Data Transfer
Function
Data
Connection
Server
Data Transfer
Function
CPE 401/601 Lecture 13 : FTP
Establish Data Connection

36. FTP - Connection Closing
bye
client
User
Interface
221 Service Closing
QUIT
server
User
Protocol
Interpreter
Server
Protocol
Interpreter
Control
Connection
User
Data Transfer
Function
Data
Connection
Server
Data Transfer
Function
CPE 401/601 Lecture 13 : FTP

37. FTP Connections
CPE 401/601 Lecture 13 : FTP

38. FTP Client Commands Command Description get filename mget filename*
Retrieve file from server
mget filename*
Retrieve multiple files from server*
put filename
Copy local file to server
mput filename*
Copy multiple local files to server*
open server
Begin login to server
bye / close / exit
Logoff server
ls / dir
List files in current remote dir on server
lcd
Change local directory cd
Change remote directory
rhelp / remotehelp
Lists commands the server accepts
* Sent to server as multiple command by User Protocol Interpreter
CPE 401/601 Lecture 13 : FTP

39. Data Transfer Modes STREAM: BLOCK: COMPRESSED:
file is transmitted as a stream of bytes
BLOCK:
file is transmitted as a series of blocks preceded by headers containing count and descriptor code
COMPRESSED:
uses a simple compression scheme on transmitted blocks
CPE 401/601 Lecture 13 : FTP

40. FTP Replies All replies are sent over control connection.
Replies are a single line containing
3 digit status code (sent as 3 numeric chars).
text message.
The FTP spec. includes support for multiline text replies.
CPE 401/601 Lecture 13 : FTP

41. FTP Reply Status Code
First digit of status code indicates type of reply:
‘1’: Positive Preliminary Reply (got it, but wait).
‘2’: Positive Completion Reply (success).
‘3’: Positive Intermediate Reply (waiting for more information).
‘4’: Transient Negative Completion (error - try again).
‘5’: Permanent Negative Reply (error - can’t do).
CPE 401/601 Lecture 13: FTP

42. FTP Reply Status Code 2nd digit indicates function groupings.
‘0’: Syntax (problem with command syntax).
‘1’: Information (reply to help or status cmds).
‘2’: Connections (problem with a connection).
‘3’: Authentication (problem with login).
‘4’: Unspecified.
‘5’: File system (related to file system).
3rd digit indicates specific problem within function group.
CPE 401/601 Lecture 13 : FTP

43. Example FTP Responses 120 Service will be ready shortly 200 Command OK
User login OK
User name OK; password is needed
Service not available
User not logged in
Requested action aborted; exceeded storage allocation
CPE 401/601 Lecture 13 : FTP

44. Trivial FTP

45. Trivial FTP (TFTP) Simple and small:
5 message formats
Runs on UDP
Designed to fit in ROM
Uses a “stop and wait” protocol
NO BUILT IN SECURITY FEATURES (login)
Used only to read and write files from/to a server
Cannot list directories
Useful for bootstrapping diskless systems
Workstations
X terminals
TFTP uses the services of UDP on well known port 69.
CPE 401/601 Lecture 13 : TFTP

46. Diskless Workstation Booting 1
The call for help
Help! I don't know who I am!
My Ethernet address is:
4C:23:17:77:A6:03
Diskless
Workstation
RARP
CPE 401/601 Lecture 13 : TFTP

47. The answer from the all-knowing
RARP
Server
I know all! You are to be know as:
Diskless
Workstation
RARP REPLY
CPE 401/601 Lecture 13 : TFTP

48. The request for instructions
I need the file named
boot
Diskless
Workstation
TFTP Request (Broadcast)
CPE 401/601 Lecture 13 : TFTP

49. The dialog here is part 1 TFTP Server I got part 1 here is part 2
Diskless
Workstation
boot file
TFTP File Transfer
CPE 401/601 Lecture 13 : TFTP

50. TFTP Protocol 5 message types: Each is an independent UDP Datagram
Read request
Write request
Data
ACK (acknowledgment)
Error
Each is an independent UDP Datagram
Each has a 2 byte opcode (1st 2 bytes)
The structure of the rest of the datagram depends on the opcode
CPE 401/601 Lecture 13 : TFTP

51. TFTP Message Formats FILENAME OPCODE MODE BLOCK# DATA ERROR MESSAGE
MODE
BLOCK#
DATA
ERROR MESSAGE
2 bytes
RRQ: client to establish a connection for reading data from the server.
WRQ: write data from the client to the server
DATA: All data bytes are exactly 512 bytes long except the last block which is between 0 and 511 bytes…EOF indicator – if data is in exact multiples of 512, sender must send one extra block of zero bytes. DATA can be in NVT ASCII or octet (binary format)
ACK: Block number is a 2byte field containing the number of the block received
CPE 401/601 Lecture 13 : TFTP

52. TFTP transfer modes octet : for transferring binary files.
no translation done.
netascii : for transferring text files.
all lines end with \r\n (CR,LF).
provides standard format for transferring text files.
both ends responsible for converting to/from netascii format.
CPE 401/601 Lecture 13 : TFTP

53. NetAscii Transfer Mode
Unix - end of line marker is just '\n'
receiving a file
you need to remove '\r' before storing data.
sending a file
you need to replace every '\n' with "\r\n" before sending
CPE 401/601 Lecture 13 : TFTP

54. Read Request variable length fields! 01 filename mode
mode
null terminated ascii string
containing name of file
null terminated ascii string
containing transfer mode
2 byte opcode
network byte order
variable length fields!
CPE 401/601 Lecture 13 : TFTP

55. Write Request variable length fields! 02 filename mode
mode
null terminated ascii string
containing name of file
null terminated ascii string
containing transfer mode
2 byte opcode
network byte order
variable length fields!
CPE 401/601 Lecture 13 : TFTP

56. TFTP Data Packet 03 block # data 0 to 512 bytes 2 byte block number
network byte order
2 byte opcode
network byte order
all data packets have 512 bytes
except the last one.
CPE 401/601 Lecture 13 : TFTP

57. TFTP Acknowledgment 04 block # 2 byte block number network byte order
2 byte opcode
network byte order
CPE 401/601 Lecture 13 : TFTP

58. TFTP Error Packet 05 errcode errstring
null terminated ascii error string
2 byte opcode
network byte order
2 byte error code
network byte order
CPE 401/601 Lecture 13 : TFTP

59. TFTP Error Codes (16 bit int)
0 - not defined
1 - File not found
2 - Access violation
3 - Disk full
4 - Illegal TFTP operation
5 - Unknown port
6 - File already exists
7 - No such user
CPE 401/601 Lecture 13 : TFTP

60. TFTP Connection Establishment
CPE 401/601 Lecture 13 : TFTP

61. TFTP Session
CPE 401/601 Lecture 13 : TFTP

62. Lost Data Packets Original Protocol Specification
Sender uses a timeout with retransmission.
sender could be client or server.
Duplicate data packets must be recognized and ACK retransmitted.
This original protocol suffers from the "sorcerer’s apprentice syndrome".
CPE 401/601 Lecture 13 : TFTP

63. Sorcerer’s Apprentice Syndrome
send DATA[n]
(time out)
retransmit DATA[n]
receive ACK[n]
send DATA[n+1]
receive ACK[n] (dup)
send DATA[n+1](dup)
...
receive DATA[n]
send ACK[n]
receive DATA[n] (dup)
send ACK[n] (dup)
receive DATA[n+1]
send ACK[n+1]
receive DATA[n+1] (dup)
send ACK[n+1] (dup)
CPE 401/601 Lecture 13 : TFTP

64. The Fix
Sender should not resend a data packet in response to a duplicate ACK.
If sender receives ACK[n]
don’t send DATA[n+1] if the ACK was a duplicate.
CPE 401/601 Lecture 13 : TFTP

65. Concurrency TFTP servers use a "well known address"
UDP port number
How would you implement a concurrent server?
forking (alone) may lead to problems!
Can provide concurrency without forking, but it requires lots of bookkeeping.
CPE 401/601 Lecture 13 : TFTP

66. TFTP Concurrency
According to the protocol, the server may create a new udp port and send the initial response from this new port.
The client should recognize this, and send all subsequent messages to the new port.
CPE 401/601 Lecture 13 : TFTP

67. When is it over? There is no length of file field sent!
All data messages except the last one contain 512 bytes of data.
message length is = 516
The last data message might contain 0 bytes of data!
CPE 401/601 Lecture 13 : TFTP

68. Issues What if more than 65535 chunks are sent?
65536 blocks x 512 bytes/block = 33,554,432 bytes.
The RFC does not address this issue!
Remember that the network can duplicate packets!
CPE 401/601 Lecture 13 : TFTP

69. FTP vs. TFTP FTP provides (minimal) security through login procedure
TFTP has NO login procedure
FTP Provides a reliable service through its use of TCP
TFTP must handle its own retransmissions since it uses UDP
FTP uses two connections
TFTP uses one connection (stop and wait)
FTP provides many commands
TFTP can only read and write files
CPE 401/601 Lecture 13 : TFTP