July 17-19 overheads

Internet layering

Application programs access services available across the internet through on e of the transport level protocols.

Transport layer protocol provides reliable communication between application software on different hosts.

internet layer handles communications between machines: encapsulates data, routes packets, assembles datagrams, sends error control messeges.

Network interface layer is responsible for accepting datagrams and transmitting them over phusical networks.

>IP protocol stacks

Protocol layering

• Tasks on different levels of abstraction are handled by software modules that belong to different layers (model levels)
  • Absence of data anticipated by an application
  • Incompatability of protocols in data presentation
  • Network congestion
  • Data transmission error
  • Hardware failure
  ISO/OSI standard seven layer model of networks.

	7. Application	X400
	6. Presentation	Abstract syntax notation (ASN.1)
	5. session	Packet assemble/disassemble pad
	4. Transport	End.to.end reliability
X.25	3. Network	Response to congestion
X.25	2. Data link	High level datalink communication (HDLC)
X.25	1. Physical	X.21 : Public networks

An example of how the OSI model is used. Some of the headers may be null.

The layering principle when a router is used. The frame delivered to router R is exactly the frame sent from host A, but differs from the frame sent between R and B

Protocol models and layers