Reference Models

• OSI reference model
  • physical layer
  • data link layer
  • network layer
  • transport layer
  • session, presentation, application layers
• Internet reference model
  • internet
  • transport
  • application and host-to-network
• Example Networks
  • Novell Netware/IPX
  • ARPANET/NFSNet/Internet

OSI Reference Model

• 7-layer model
• protocols associated with it
• (modified) model is in widespread use (esp. Tanenbaum!), protocols are not
• model describes:
  • services: specification, what is provided
  • interfaces: object methods, where to access the services
  • protocols: implementation, how the service is provided

Physical Layer

• Using a wire to transmit bits
• how is a 1-bit represented? how is a 0-bit represented?
  • voltage levels
  • light levels
  • phases
  • frequencies
• how fast are bits sent?
• is transmission bidirectional?

Data-Link Layer

• framing: finding a sequence of frames in a sequence of bits
• detecting and perhaps correcting errors
• detecting and perhaps correcting duplicates
• flow control
• negotiations for access to a shared medium

Network Layer

• routing packets from source to destination
• addressing
• interconnecting heterogeneous networks
• congestion control
• accounting

Transport Layer

• end-to-end
• accept data from the application
• make data acceptable to the network layer (for example, split it into smaller packets)
• management of network connections
• demultiplexing on the end hosts
• optional transport layer functions:
  • error-free point-to-point
  • connectionless datagrams
  • broadcast and multicast

Session, Presentation, Application

• Session Layer:
  • dialog control (taking turns)
  • token management
  • synchronization
• Presentation Layer:
  • standard encoding of numbers, strings, etc
• Application Layer:
  • FTP
  • network virtual terminal
  • SSL (encryption)

In-class exercise

• work in teams of two or three
• 5 minutes
• assign each of the following to an OSI layer:
  • retransmission
  • encryption
  • routing
  • finding the beginning of a frame
  • splitting the data to fit the network

What's wrong with the OSI model?

• not an accurate description of (or prescription for) reality:
  • unused layers (session, presentation)
  • missing layers ("data link" is too simple)
  • one-size-fits-all
• too complicated to understand
• corresponding protocols equally complicated

Internet Model

• "below IP" layer
• Internet layer: IP
• Transport layer: TCP and UDP
• application layer

Internet Layer

• packet-switching, connectionless delivery
• analogous to paper mail:
  • drop letters in box, each in its own envelope
  • letters may take different routes to destination, may be lost, reordered, or unreadable (wet)
• main functions:
  • end-to-end data delivery
  • routing
  • congestion control
• similar to OSI network layer

Transport Layer

• Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
• TCP:
  • reliable
  • connection-oriented
  • stream-oriented
  • flow control
  • (de)multiplexing
• UDP:
  • unreliable
  • connectionless
  • packet-oriented
  • (de)multiplexing

Below IP

• IP requires of the layer(s) below that:
  • they can forward data to an attached host
  • they can broadcast data to all attached hosts
• lower layers could be:
  • Ethernet
  • ATM
  • FDDI
  • Satellite
  • carrier pigeon (RFC 1149 -- http://www.cis.ohio-state.edu/htbin/rfc/rfc1149.html)
• IP does not in any other way describe the lower layers

Application Layer

• Bulk data transfer:
  • files (FTP)
  • email (SMTP)
  • news (NNTP)
• interactive transfer of small amounts of data:
  • telnet
  • chat (IRC, talk)
• interactive transfer of large amounts of data:
  • world-wide web
  • video conferencing

What's wrong with the TCP/IP model?

• only describes TCP/IP (not a model)
• no mention of physical and data link layers (good and bad)
• some weak protocols (e.g. telnet)

Example Networks

• Novell Netware/IPX:
  • proprietary protocols, similar to TCP/IP
  • IPX: Internet Packet eXchange
  • transport layer
    • NCP: Network Core Protocol, services
    • SPX: Sequenced Packet eXchange
  • routers help clients find servers
  • automatic address assignment
• ARPANET/NFSNet/Internet
  • ARPANET first packet switching network
  • IMPs (Interface Message Processors) used as network processors
  • connectionless -- dynamic routes, reliability
  • exponential growth

Homework 2

• Due Wed January 26 at 3pm
• send email to kanadam@hawaii.edu
• Read Chapter 1
• Textbook p. 74:
  • Question 2
  • Question 7
  • Question 10
• Textbook p. 75, Question 18