INFS2052
Internet and Intranet Information Systems

lecture notes 1-3

OSI and Internet protocol layers

Lecture 1-3
OSI and Internet protocol layers

Reading:

Tanenbaum sections 1.3, 1.4

TFTP - PDUs and their sequencing

The Trivial File Transfer Protocol contains 5 types of PDUs.

PDU means protocol data unit
- the generic term unit of data exchanged in any protocol [OSI]

RFC 1350          TFTP Revision 2           July 1992

Appendix

TFTP Formats

 Type   Op #     Format without header

        2 bytes    string   1 byte     string   1 byte

        ---------------------------------------------

 RRQ/  | 01/02 |  Filename  |   0  |    Mode    |  0 |

 WRQ    ---------------------------------------------

        2 bytes    2 bytes       n bytes

        ---------------------------------

 DATA  | 03    |   Block #  |    Data    |

        ---------------------------------

        2 bytes    2 bytes

        -------------------

 ACK   | 04    |   Block #  |

        --------------------

        2 bytes  2 bytes        string    1 byte

        ----------------------------------------

 ERROR | 05    |  ErrorCode |   ErrMsg   |   0  |

        ----------------------------------------

TFTP protocol

The exchange of PDUs follows sequences defined by the protocol.

For example

Read request (between 512 and 1024 bytes)

client  RRQ           *ACK.1       *ACK.2

         \           /  \         /  \

          \         /    \       /    \

           \       /      \     /      \

            \     /        \   /        \

server       * DATA.1       * DATA.2     *

Write request (less than 512 bytes)

client  WRQ           *DATA.1        * 

         \           /   \          /

          \         /     \        /

           \       /       \      /

            \     /         \    /

server       *ACK.0          *ACK.2

Functions required for client-server communication

Some of the functions required for client and server to communicate over an internetwork:

• to send over local network - manage shared medium
• to send long-distance - select route through networks
• detect and recover from local transmission errors
• detect and recover from lost messages
• find "address" from host name
• handle different data representations at endpoints
• manage "whose turn next" in interactions
• allow sending to and from multiple addresses on one computer
• allow choice of diferent qualities of service
• manage over-crowding - congestion - in networks

Combining these and many other functions into one mass of software is very difficult.

The software functions are best structured in layers.

Layered protocols -
reference models

The OSI reference model defines 7 layers.

Fig 12.1 from Comer Computer Networks and Internets.

Each layer has a well defined function.

The layer boundaries are chosen to minimise information flow across the boundaries.

A layer is used where a different level of abstraction is required.

The 7 OSI layers

1 physical layer

Transmit stream of bits over one link: volts, microseconds
- includes electrical/mechanical interfaces to comms media.

2 datalink layer

Data-flow division into frames; error detection and recovery, flow control.

3 network layer

Form and route packets across internetworks between host computers (understand addresses); manage congestion.

4 transport layer

Transparent, reliable transfer of data between multiple endpoints - truly end to end.

5 session layer

Session administration of "conversation"; managing "turns"; checkpoint recovery.

6 presentation layer

Enabling exchange of data with possibly different endpoint representations.

7 application (services) layer

Support for application services; remote procedure call; file transfer, email exchange, virtual terminals etc.

General model of layers in OSI

Tanenbaum fig 1-16

The implementation of a combination of particular protocols on a particular computer is called a protocol stack.

Internetworking requires a short stack of lower layers at the intermediate gateway computers between networks.

They may provide protocol translation as well as routing.

TCP/IP layers

The OSI reference model and the 7 layers are rarely used in practice.

The alternative model is derived from the earlier implementation of the Dept of Defense (USA) ARPANET

• is simpler to describe
• is very well founded in practice
• exists in very scalable, efficient implementations for many operating systems

Called

• TCP/IP suite
• Internet protocols (the Internet)

Tanenbaum fig 1-18

Chris Johnson
Last modified: Tue Mar 30 11:25:57 EST 1999
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