CSCI 233
Internet Protocols
Class 8
David C. Roberts
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Agenda
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An Interesting Internet Product
Remote Login Applications--TELNET
File Transfer Applications—FTP, NFS
Mail Applications—SMTP, POP, IMAP
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New Product
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Scanning Internet Camera
Pans, zooms, produces Internet video
Connects directly to the Internet
Doesn’t need a server
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D-Link DCS-930L Wireless N Network Camera
The D-Link DCS-930L Wireless N Network Camera comes with
everything you need to quickly add a surveillance camera to your home
or small office network. It works right out of the box. Simply connect
the cables, plug in the camera, run the short installation wizard and
setup is complete. To view what the D-Link DCS-930L Wireless N
Network Camera is seeing, simply log on to mydlink.com, choose your
device, and start viewing - there is no need to configure your router to
open up ports or remember hard-to-memorize Internet addresses.
Order the D-Link DCS-930L Wireless N Network Camera today!
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What It Can Do
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DDNS
• What is DDNS?
• How do you think it works?
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DDNS
• Runs as DDNS server
• Works with dynamic IP addresses
• Software is added to host machine to provide IP address to
DDNS at startup
• DDNS service maps DDNS name to dynamic IP address
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Uses of The Product
• Monitor any space; just connect it to the Internet
• Control, monitor the camera from anywhere you have Web
access
• Shows the power, capability of the dumb network that is
available everywhere
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AND NOW. ON TO TELNET
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Remote Login
• TELNET is one of the Internet’s oldest protocols—defined in
the 1960s
• Each computer used to have a terminal wired to it to control it
• Hard to control a large complex of computers, and lots more
terminals than desired
• TELNET allows user to establish login session on remote
machine and execute commands
• Client-server model can provide services to multiple machines
• Using TCP, interactive use of remote machines can also be
provided
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TELNET Protocol
• TELNET establishes TCP connection, passes
keystrokes from user’s keyboard to the remote
computer, brings back output to the user’s
screen
• Services:
• Network virtual terminal—standard interface
• Client and server negotiate options
• Both ends of connection treated symmetrically
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TELNET Virtual Server (NVT)
• TELNET client translates from user terminal format into NVT
format
• Server translates NVT into local OS requirements
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TELNET Client and Server
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TELNET NVT Control
Characters
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Some TELNET Commands
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Current Uses
• Today TELNET is used mostly within an installation for control
of computer systems
• Largely replaced for general use by end users for remote
access to desktops
• However, it’s the foundation of my ISP’s business!
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Summary
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High-level services provide important functionality to TCP/IP
TELNET is one of those services
TELNET provides a remote control console
Implemented across many operating systems
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Methods for File Sharing
• Remote file access
• File transfer
• Peer to peer
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File Transfer Protocol
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File transfer
Interactive access
Format specification
Authentication control
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Process Model
• Server allows concurrent access by multiple
clients, using TCP/IP
• Control and data transfer connections are (often)
separate
• Client and server usually each have separate processes
for control, data transfer
• Data transfer processes and connections are created
and dropped as needed, but control connection
persists during a session
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FTP Client and Server
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FTP Port Number Assignment
• Client uses random, locally determined port to
connect to server
• Client connects to port 21 at the server (a wellknown port)
• Client gets (another) unused local port number,
connects to port 20 at server for data transfer
connection
• TELNET network virtual terminal protocol is used
for the control connection; but only basic NVT
definition.
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FTP Session Establishment
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Modes
• Active mode: client sends the server the IP
address and port number on which client will
listen, and server initiates TCP connection.
• Passive mode: (used if client is behind firewall
and unable to accept incoming TCP connections)
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.
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Active Connection
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Passive Connection
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Representations
• ASCII mode: for text. Data converted from the sending host's
character representation to 8-bit ASCII before transmission,
and to the receiving host's character representation.
Inappropriate for files that contain data other than plain text.
• Image (aka Binary) mode: sending machine sends each file
byte for byte, and recipient stores the byte stream as
received.
• EBCDIC mode: use for plain text between hosts using IBM’s
EBCDIC character set. This mode is otherwise like ASCII mode.
• Local mode: Allows two computers with identical setups to
send data in a proprietary format without the need to convert
it to ASCII
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Trivial File Transfer Protocol
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Inexpensive, unsophisticated service
Simple file transfer, no authentication
Runs over any unreliable packet system
Sending side sends 512 byte file, waits for ACK for each block,
then sends next one
• First packet specifies the transfer
• Block of less than 512 octets indicates end of file
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Question: Why would anyone use TFTP when there is also FTP?
NFS—Network File System
• Originally developed by Sun Microsystems
• Allows computer to access a remotely stored file over an IP
network or the Internet
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NFS Implementation
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How NFS Fits
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Peer to Peer
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Peer to Peer File Sharing
• No notion of clients and servers
• “peer” nodes function as both client and server to one
another
• Foldershare uses peer-to-peer protocols
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Peer to peer
Client-server
Types of Peer to Peer Networks
• Structured:
• Peers are organized following specific criteria and algorithms, which lead to
overlays with specific topologies and properties. They typically use distributed
hash table-based (DHT) indexing.
• Unstructured:
• No algorithm for organization or optimization of network connections
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Pure peer-to-peer systems: entire network consists solely of equipotent peers. There is
only one routing layer, as there are no preferred nodes with any special infrastructure
function.
• Hybrid peer-to-peer systems: may have infrastructure nodes to exist, often called
supernodes.
• Centralized peer-to-peer systems: central server is used for indexing functions and to
bootstrap the entire system. Connections between peers are not determined by any
algorithm.
First prominent and popular peer-to-peer file sharing system,
Napster, was centralized. Gnutella and Freenet are decentralized.
model. Kazaa is hybrid.
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Advantages of Peer to Peer
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No single point of computer failure
No single point of network failure
No traffic congestion at servers
No need for high-capacity server
Uses available capacity of client machines
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Disadvantages of Peer to Peer
• Near-simultaneous updates of the same file can cause lots of
network traffic
• No guarantee of timeliness
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Summary
• Forms of access to remote files:
• Whole-file copying
• Shared on-line access
• FTP uses whole-file copying, allows users to list
directories as well as transfer files in either
direction
• TFTP provides a simple means for file transfer
• NFS provides file system emulation
• Peer to peer provides file transfer without
servers, but with potential timeliness problems
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Electronic Mail
Email allows users to send memos across the Internet.
• Notes can be short or quite large
• Notes can have multiple attachments
• Must work when remote machine is unreachable
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Actors in the Email System
• Post Office
• SMTP servers move email between each other
• SMTP servers store email for delivery to end users
• Users
• POP clients pick up email from SMTP servers
• POP clients hand outgoing mail to SMTP servers
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Delayed Delivery
Sender and receiver do not need to be connected to
the server at the same time
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Alias Expansion, Mail
Forwarding
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Internetworking and Email
• TCP/IP internet makes universal delivery service
possible
• Mail systems built on TCP/IP are inherently
reliable because of end-to-end delivery
• Alternatively, mail gateways are used
• Allow mail transfer between different systems
• When gateway has a message, sender discards it
Question: Is email through a gateway better or worse than a direct
SMTP transfer?
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TCP/IP Email Standards
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Format and computer interaction are specified separately
Format: header, blank line, body
Body unspecified
Header is key word, colon, value
Some keywords required, others optional
Header is readable
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Email Addresses
local-part @ domain-name
• domain-name: name of mail destination
• local-part: address of a mailbox at destination
Note: when gateways are employed, mail addresses are sitedependent
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SMTP
• Simple Mail Transport Protocol
• Focuses on how mail delivery system passes messages from
one machine to a server on another machine
• Does not specify anything about user interface
• Does not specify how mail is stored
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Assured Delivery
• SMTP server forms TCP/IP connection with
receiving server
• Once receiving server has put message into safe
store, then it acknowledges and sender discards
message
• If SMTP can’t transfer message on the first try, it
keeps trying
• After several days of failure, SMTP reports failure
to deliver.
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SMTP Highlights
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All communications is readable ASCII text
Transcript of interactions is readable
Each message is acknowledged separately
Addresses of the form local-part@domain-name
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Example Transfer
S: 220 Beta.GOV SMPT ready
C: HELO Alpha.EDU
S: 20 Beta.GOV
C: MAIL FROM: Smith@Alpha.EDU
S: 250 OK
C: RCPT TO: Green@Beta.GOV
550 No such user here
RCPT TO: Brown@Beta.GOV
250 OK
C: DATA
S: 354 Start mail input;end with <CR><LF>.<CR><LF>
C: ..sends body of mail message
C: <CR><LF>.<CR><LF>
S: 250 OK
C: QUIT
S: 221 Beta.GOV Service closing transmission channel
Client establishes connection
Sent by server
Sent by client
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Mail Retrieval and Mailbox
Manipulation
• POP3—Post Office Protocol
• POP3 client creates TCP connection to POP3
server on mailbox computer
• Mailbox computer runs two servers:
• SMTP to place mail into user mailboxes
• POP3 server to allow user to extract messages from
user mailbox
• POP3 retrieves messages, deletes from user
mailbox
• Two servers must coordinate use of the user
mailbox
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MIME Extension for Non-ASCII
Data
MIME—Multipurpose Internet Mail Extensions
• Defined to allow transmission of non-ASCII data
through mail
• MIME allows arbitrary data to be encoded in
ASCII, transmitted as standard email message
• MIME message tells recipient type of data, type
of encoding used
• Data type and subtype is specified
• MIME information is in 822 mail header
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MIME Content Types
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MIME Example
From: bill@acollege.edu
To: john@example.com
MIME-Version: 1/0
Content-Type: image/gif
Content-Transfer Encoding: base64
….data for the image….
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Mail Retrieval, Mailbox
Protocols
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Post Office Protocol—client login with userid/password
Client can then retrieve, delete messages
Server computer must run two servers: POP and SMTP
POP and SMTP must coordinate use of mailbox
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Internet Message Access Protocol
• POP stores messages offline; not compatible with the use of
multiple computers
• IMAP allows message access, manipulation from multiple
computers
• Platform-independent access to mail
Question: How does server resource usage of IMAP compare with POP?
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Summary
• Email is a very important Internet service
• Separate standards are used for message format and
transfer
• SMTP—how a mail system on one machine transfers to a
server on another
• POP3—how a user can retrieve contents of a mailbox
• IMAP—user protocol for use from multiple computers
• MIME allows arbitrary data to be exchanged using SMTP
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Thank you!