CS230 Information Processing Section 2 Data Transfer

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CS230 Information Processing Section 2 Data
Transfer

• 2004
• Mr Bart Busschots, BSc, AMInstP.

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Data Transfer

• Data transfer takes place at all levels of a
  computer system
• Low-level data transfer
• between CPU and memory
• between memory and devices - disks, modems,
  graphics, sound
• between modem/NIC and remote modem/NIC
• High-level data transfer
• communicating processes on the same machine (e.g.
  IPC)
• communicating processes on separate machines
  (e-mail, web, file sharing, etc.)
• Data transfer requires a proper communications
  architecture

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Peer To Peer Communications

• The simplest form of communications between
  parties is peer to peer communications
• Peer to peer communication involves
• Two parties between whom data is to be exchanged
• Some form of communications service available to
  both parties (i.e. network service)
• A mutually understood set of rules regarding
  communications - a protocol
• A communications protocol specifies
• the ordering of messages between parties
• the structure and format of messages

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Network Reference Models

• Networking models are based on a number of
  conceptual layers of abstraction
• Communications between two parties take place at
  several layers, each layer using the services of
  the layer beneath it
• The most popular networking reference model is
  the TCP/IP model used on the Internet

Application Layer
Transport Layer
Network Layer
Data Link/Physical Layer
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Physical/Data Link Layer

• Data link/physical layer provides the ability to
  send raw byte sequences over the wire to
  another physical machine/router
• Physical
• Modem communications standards - V.42, V.90, DSL,
  ISDN, etc.
• Mobile communications - GSM, D-AMPS, GPRS, UMTS,
  etc.
• LAN communications - Ethernet 802.3, 802.4, etc.
• Internet backbones - T1, T3, ATM, etc.
• Data link
• SLIP - Serial Link Internet Protocol
• PPP - Point to Point Protocol

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Network and Transport Layers

• Network layer protocols provide host-to-host
  communications over the internet
• IP - Internet Protocol (Host to Host
  communications)
• ICMP and IGMP - Network-layer control protocols
• Transport layer provides connection-oriented and
  connection-less transport services from
  application to application over IP
• TCP - Transport Control Protocol for
  connection-oriented transport
• UDP - User Datagram Protocol for connection-less
  transport

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Application Layer Protocols

• Application layer protocols transfer data between
  application programs residing on separate hosts
• Application protocols include
• SMTP/POP3 for sending/downloading e-mail
• FTP for file transfers
• NNTP for newsgroup access
• NFS and SMB for distributed file systems
• HTTP for hypermedia documents
• Well look at peer to peer communications between
  programs at the application layer over the
  Internet

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Client-Server Architecture

• The client-server model of communications
  specifies
• a server application that provides a service upon
  request
• a (number of) client applications that request
  service from the remote servers
• server applications are known as daemons, which
  listen on a port for incoming connections
• client applications establish connections to
  server applications and request a service to be
  performed
• The client-server model works by
• client establishes connection with server
• client makes a request
• server processes the request
• server sends response back to client

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Sample Application Protocol - SMTP

• Simple Mail Transfer Protocol (SMTP), specified
  in RFC821 (www.rfc-editor.org) is used to send
  e-mail
• SMTP is a simple request-response protocol that
  uses text strings to communicate between client
  and server
• SMTP operates as follows
• Mail client establishes a connection with the
  SMTP server
• After an initial handshake, the client states the
  sender and recipient(s) of the message
• If the recipients exist at that server, the
  server notifies the client to proceed
• The client uploads the mail message
• When finished, the client closes the connection

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SMTP Commands and Response Codes

• Commands
• HELO Initial greeting sent by client
• MAIL FROM ltsendergt Specifies sender of message
• RCPT TO ltrecipientgt Specifies recipient of
  message
• DATA Message body follows
• QUIT Close connection
• Some SMTP Response Codes
• 220 Service available
• 250 Mail action successful
• 500 Command unrecognised
• 550 Mail not sent

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SMTP Messages
Connect to TCP port 25
Mail Client
SMTP Server
220 mailserver.cs.may.ie Welcome
HELO mailclient.cs.may.ieltCRLFgt
250 Hello mailclient.cs.may.ie. Proceed
MAIL FROM adowling_at_cs.may.ie ltCRLFgt
250 adowling_at_cs.may.ie... Sender ok
RCPT TO dip01100_at_cs.may.ie ltCRLFgt
250 dip01100_at_cs.may.ie... Recipient ok
DATAltCRLFgt
354 Enter mail, end with "." on a line by itself
This is a test!ltCRgtltLFgt.ltCRgtltLFgt
250 Message accepted for delivery . . .
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Sample Application Protocol - POP3

• Post Office Protocol is used to download mail
  that is remotely stored on a server
• Like SMTP, POP3 is a simple, text-based
  request-response protocol
• POP3 operates as follows
• Mail client establishes a connection with the
  POP3 server
• The client supplies a username and password in
  order to log in
• The server checks the username/password
  combination. If they are valid, the client is
  allowed to proceed
• The client retrieves/deletes messages from the
  mailbox
• When finished, the client closes the connection
  with the QUIT command

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POP3 Commands and Response Codes

• Commands
• USER ltusernamegt Log in as ltusergt
• PASS ltpasswordgt Password for ltusergt is
  ltpasswordgt
• STAT Status of mailbox
• RETR ltngt Retrieve message n
• DELE ltngt Delete message n
• QUIT Close connection
• Some POP3 Response Types
• OK ltmsggt Command succeeded. ltmsggt has
  reason.
• -ERR ltmsggt Command failed. ltmsggt has reason.

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A Simple E-mail System
Senders Mail Client (e.g. Netscape)
SMTP Server
Write to file
SMTP
Mail Store
Recipients Mail Client (e.g. Outlook)
POP3 Server
POP3
Read from file
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Distributed File Sharing

• Files residing on one machines disk may be
  exported or shared to client machines
• the machine exporting the files is a file server
• entire directory structures are exported from a
  mount point
• the file server runs a file server daemon process
  to handle requests
• Microsoft has SMB running for file shares
• UNIX supports NFS file servers
• Client machines mount remote file shares and map
  them onto parts of the local file system, or onto
  virtual devices
• accesses to remote file systems cause the client
  operating system to communicate with the remote
  file server
• communications between client and server are
  performed automatically and transparently to the
  user

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End