Network Applications
Outline
File Transfer Protocol
Telnet
Simple Mail Transfer Protocol
Peer-to-peer applications
(Napster,Gnutella)
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File Transfer Protocol
• Most basic file transfer application in the Internet
•
•
•
•
– One of the original client/server applications run on the ARPANET
Runs on top of TCP
Allows for both file transfer and interactive access
Requires authentication via user name and password
Requires that a host system run an FTP server
– Listens for incoming requests on a well known port (21)
– Anonymous/Guest logins are common
• FTP is a two process model
– Control process which communicates with peer control process
• These processes communicate commands/responses as well as port
information
– Data transfer process which actually transfers requested file
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File Transfer Protocol contd.
• Client control process connects to server control
process
– ftp mirror1.cs.wisc.edu
• The client also starts a data transfer process which
listens on a local port
– Communicates this port number to server via control process
• If client requests a file transfer, server initiates
connection to client’s data transfer port
– Server uses well known port for data transfer (20)
• Commands used by FTP are actually a subset of TELNET
protocol NVT ASCII
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FTP example
3-digit
code eases
parsing
5xx means
action
required
1xx=OK, I will
2xx=OK, done.
3xx=OK so far
4xx=NO, temp
tux29(1)% ftp mirror1.cs.wisc.edu
Connected to mirror1.cs.wisc.edu.
220 Welcome to The UW-Madison Computer Sciences mirror site
530 Please login with USER and PASS.
Name (mirror1.cs.wisc.edu:jgast): anonymous
331 Please specify the password.
Password: XXXXXXX
230 Login successful. Have fun.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> cd pub
250 Directory successfully changed.
ftp> ls
200 PORT command successful. Consider using PASV.
150 Here comes the directory listing.
drwxrwxrwx 2 0
0
2048 Aug 23 12:56 mirrors
226 Directory send OK.
ftp> quit
221 Goodbye.
tux29(2)%
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Typical FTP Session
CLIENT
SERVER
listen on port 21
Cntrl
Data
Cntrl
Data
1025
Connect to port 21
21
Cntrl
Cntrl
Data
Data
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Typical FTP Session
CLIENT
Ask for LIST, 1493
1025
21
Cntrl
Cntrl
PORT command OK
Data
1493
SERVER
Data
20
List of files
1025
RETR gnotella.zip, 1522 21
Cntrl
Cntrl
PORT command OK
Data
Data
Lots of data
1522
20
Never talks to an unpriviledged port on an untrusted server
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Passive FTP Session
CLIENT
PASV
1025
21
Cntrl
Cntrl
ask to
LIST files
firewall
Data
Cntrl
SERVER
1025
Passive Mode, 4107
21
1673
4107
Data
Data
Cntrl
Data
The list of files
Use passive mode if the server can’t initiate a connection to the client
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Telnet
• Internet’s most basic network virtual terminal application allowing
users to log into a remote host
• Runs on top of TCP
• Requires authentication via user name and password
• Requires host system to run a telnet server (telnetd)
• Passes keystrokes to remote system and carries output back to
user’s screen
• Offers three basic services
– Network virtual terminal – standard interface to remote system
– Mechanism for negotiating communication options
– Treats both ends of a connection symmetrically
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Telnet
• Client’s TELNET process connects to server’s TELNET
– telnet foo.cs.wisc.edu
• Server listens on well known port (23) for incoming connections
– forks new slave process to handle connection
• Much of the hard work in telnet is to accommodate heterogeneity
of systems
– Control characters, etc.
– Accomplished via network virtual terminal (NVT) specification for
ASCII
• It’s not very secure!
• It can cause a lot of traffic - tinygrams
– Nagle’s algorithm (RFC 896, ‘84)
• Collects and sends groups of data based on ACK process
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Simple Mail Transfer Protocol
• Basic protocol for email exchange over the Internet
• Runs on top of TCP
• Fundamental difference between SMTP and FTP/TELNET is that it
is NOT an interactive protocol
– Messages are queued and spooled by SMTP agent
• Users interact with email application
– Lots!
• Application interfaces with Message Transfer Agent
– Sendmail on UNIX
– Setup and configured by admins.
• SMTP specifies how MTA’s pass email across the Internet
– Also uses NVT commands
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Simple Mail Transfer Protocol
• Client uses email application to construct and send messages
– Addresses consist of machine name and mailbox address
• Mailbox is usually the same as one’s login but you can have aliases
• Destination machine can also be an alias@cs.wisc.edu
• Message is passed to mail spooler which is part of MTA
– Application communicates with MTA via email transfer protocol
• Post Office Protocol (POP3) is common, but not very secure
• Our department uses IMAP
• MTA’s on remote systems listen for incoming mail on well known port
(25)
• Messages are delivered in two parts – header and body
– Header format has exact specification = RFC 822
– Body content types are specified by MIME
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SMTP Example
Jim Gast
UW-madison
Anne Gast
Humko Products
From:jgast@cs.wisc.edu
To: "Anne Gast" <AGast@humko.com>
Subject: I hope you brought a toothbrush
Date: Tues, 11 Sep 2001 10:46:16 -0400
MIME-Version: 1.0
X-Mailer: Internet Mail Service (5.5.1960.3)
Content-Type: text/plain
Content-Transfer-Encoding: 7bit
header
Hi, Anne!
You won’t be flying home today. Hijacked
planes hit both towers of the World Trade
Center!
body
-Jim
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Sample SMTP Session
tux34(21)% telnet smtp.cs.wisc.edu 25
Trying 128.105.6.11...
Connected to schroeder.cs.wisc.edu (128.105.6.11).
220 schroeder.cs.wisc.edu ESMTP Sendmail 8.11.3; Tue, 11 Sep 2001 14:09:52 -0500
HELO jgast.cs.wisc.edu
250 schroeder.cs.wisc.edu Hello tux34.cs.wisc.edu [128.105.111.134], pleased to meet you
MAIL FROM:<jgast@cs.wisc.edu>
250 2.1.0 <jgast@cs.wisc.edu> ... Sender ok
Envelope
RCPT TO:<jgast@tds.net>
250 2.1.5 <jgast@tds.net>... Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
Header
To: jgast@tds.net
Test for CS640
.
250 2.0.0. f8BJAeq14849 Message accepted for delivery
QUIT
221 2.0.0 schroeder.cs.wisc.edu closing connection
Connection closed by foreign host.
tux34(22)%
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13
Peer to Peer Applications
• Applications which facilitate the sharing of data between widely
distributed users
– Clearly the new “killer app” on the Internet
– Napster model
• One central source of meta data
– Gnutella model
• Distributed source of meta data
• This is the way things will go…
• Napster
– Invented by Shawn Fanning at Northeastern U. in 1998-9
– Founded as a company in May, 1999
– Indexes millions of songs on millions of systems world wide
• Well known servers at Abovenet and globalcenter
– Sued by Recording Industry Association of America in Dec. 1999
– Currently in limbo…
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Napster Protocol
• There are three basic entities in Napster
– Client – browses, requests files
– Server – end host system which shares/serves files
– Metaserver/Redirector – file index repository
• Protocol is “private” – has been reverse engineered
• Messages are passed between entities
– Form: <length><type><data>
• Length and type are two bytes
• Length specifies the number of bytes of data
• Data is ASCII string often enclosed in quotes
– There are a large number of possible messages defined for the protocol
• Information – errors, versions, status…
• Control – login, upgrades, add/delete accounts…
• Operation – search, listings, transfer requests, etc.
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Napster Protocol contd.
• “Official” software runs on Windows
– Many other clients available eg. Gnapster
– All you need to access Napster is a client
• Runs on TCP
– Servers (typically) use ports 7777, 8888
– Metaservers run on port 8875
• Protocol
– Connect to a well known metaserver and login
– Can join chat session or search for files (our focus)
– Clients send “search query” request - results in locations of matching
files
– Downloading files happen directly between hosts
• Metaserver is not involved
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Napster Protocol contd.
• File transfers use one of four modes
– Up/download
– Firewall up/download
• Non-firewalled download
– Client sends download (203) request to metaserver
– Metaserver responds with download ack (204) that contains user,
IP address, port … of requested file
– Client makes TCP connection to server
• Server responds with ASCII “1” immediately
– Client sends GET in one packet then username, filename
• Also include byte offset to resume interrupted transfer
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Napster Protocol contd.
• Remote client returns filesize followed by datastream
• Client notifies metaserver transfer has begun (218) and ended (219)
• Firewalled download
– 204 ack from metaserver has port number 0 indicating server is behind a
firewall
– Client sends firewall download request (500) to metaserver requesting
that server upload file to client
• Client then waits for server to upload requested file
– Metaserver then contacts server requesting upload to client
• Done by HTTP tunneling: send message which looks like HTTP request
– Upload continues just like download
• Each upload is tracked
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Gnutella
• Napster’s flaw is that there is a single source for the index
• The simple idea in Gnutella is to distribute the method for finding
data
– Great idea!
– Lots of fun architectural possibilities!
– A few interesting research projects underway
• Gnutella is a distributed search protocol with a decentralized model
– Clients can issue/view query results
– Clients can serve/request data
– Clients accept queries and respond with matches from their local data
store
– Provides for highly reliable system
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Gnutella Protocol
• Protocol defines method of client communication
– Set of descriptors used for communicating data
– Setof rules governing inter-client exchange of descriptors
• Descriptors
–
–
–
–
–
Ping: active discovery of hosts on a network
Pong: response to Ping includes client address and metadata
Query: search mechanism
QueryHit: response to Query includes info necessary to get data
Push: mechanism enabling firewalled clients to be part of network
• A Gnutella client (servent) connects to network by establishing a
connection with another client on the network
– Finding another client is not part of Gnutella spec.
• Host cache services are the typical way this is done
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Gnutella Protocol
• New client then creates connection to the Gnutella client and
thereby becomes part of the network
– Gnutella client can reject the connect request
– Successful new client can then send/receive descriptors
• Pings/pongs are then sent to establish network
– No specification as to how much/often to probe
– Network data can/is cached
• Message routing should be well behaved
– Ping/Query descriptors should be sent to all directly connected clients
– Pong/QueryHit descriptors should be sent back along same path
– TTL is mechanism to limit distance
• File downloads via HTTP/1.0 protocol via direct connect
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Other features
• http://www.KaZaA.com
–
–
–
–
“Distributed, self-organizing”
Only supernodes are search hubs
Intelligent download (Any of several identical items)
Simultaneous download of chunks
• http://www.napigator.com
– Finds and organizes open napster servers
• http://www.filetopia.org
– Adds encryption
• http://www.audiogalaxy.com
• http://www.cubicmetercrystal.com/alpine
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