Reading:

Tanenbaum chapter 1 up to section 1.5 (inclusive)

Application level communications are nearly always asymmetrical between two communicating entities: one is a client of the other's services -
client/server architecture

Tanenbaum fig 1-1

Client-server architecture

• a server process "listens" at a known endpoint address on the server machine
• a client sends a message to this address to request a service
• the server accepts the request, processes it, and replies to the client

A server may serve many client simultaneously.

A service may

• retain state
  - retain memory of a client between request
• be stateless
  - have no memory of a client between requests.

File transfer application

File transfer applications are very old and very stable

• FTP early Internet protocol -> rfc 959
• TFTP later simplification of FTP -> rfc1350
• FTAM OSI much more complex version

The client requests

• - connection authentication
• - directory information
• - file transfers (get and put)

The server supplies

• - authentication permission
• - directory listings
• - file transfer activity (get/put)

Also: the client can set
file transfer mode (binary or ASCII)
timeout values for recovery of lost data

Protocols

The client and server communicate by a protocol.

protocol A strict set of rules that govern the exchange of information between computer devices.
[Nader, Prentice-Hall Dictionary of Computing]

A set of semantic and syntactic rules that determines the behaviour of functional units in achieving communication. [ISO]

Any agreement that governs the procedures used to exchange information between cooperating entities. The agreement usually includes how much information is to be sent, how often it is sent, how to recover from transission errors, and who is to receive the information.
In general a protocol definition will include definitions of
message formats,
sequencing rules concerning messages, and
interpretation rules concerning messages transferred in proper sequence.
[Illingworth, Oxford Dictionary of Computing]

Trivial File Transfer Protocol

Uses 5 types of messages:

• read request sent by client
  file name, file type
• write request sent by client
  file name, file type
• data block sent by client or server
  block number, data
• acknowledgment sent by client or server
  block number
• error sent by client or server

The different types are distinguished by data values in the header of each message.

Uses: very simple, no-authentication, LAN file transfer e.g. bootstrapping over local network, loading fonts etc into Xterminals.

User interface: there is a user interface program called tftp with more functions that sit above this: mode setting, timeout values, remembering host/port number between requests, etc

Client-server issues

1. authentication
   • security of access
2. trust
   • server trust for client
   • client trust for server
3. data representation
4. connection/connectionless protocol
5. stateless/statefull protocol

Tanenbaum fig. 1-19

Chris Johnson
Last modified: Tue Mar 30 11:25:42 EST 1999
Queries to : infs2052@iwaki.anu.edu.au