1. Section 4 : The OSI Network Layer CSIS 479R Fall 1999 “Network +” George D. Hickman, CNI, CNE

2. Objectives Identify the basic purpose of the OSI Network layer Identify the uses of the addresses defined at the Network Layer Identify the characteristics of the three switching methods Identify the characteristics of the two route discovery methods

3. Objectives (con’t) Identify the characteristics of the two route selection methods Identify the characteristics of Network layer connection services Identify the basic purpose of gateway services Describe why connectivity devices are needed on networks

4. Objectives (con’t) Describe the function of Ethernet and token ring repeaters Describe the function of bridges Explain how a transparent bridge works Explain how a source-routing bridge works Explain how switching-hubs work

5. Objectives (con’t) Describe how token ring switches work Describe routing protocols Identify nonroutable protocols and how to deal with them Describe combination devices Decide when to use bridges,switches, and routers

6. Basic Purpose of the OSI Network Layer Routing Moving information between multiple independent (often dissimilar) networks called internetworks Routes Data using: Switching Network Layer Addressing (Logical, not physical or MAC) Routing Algorithms

7. Processes and Methods Logically separate networks must have unique network addresses Switching determines how connections are made Implement routing to have “best” data path determined Different levels of connection services can be implemented, based on errors expected

8. Processes and Methods (con’t) Addressing Logical Network Service Switching Packet Message Circuit

9. Processes and Methods (con’t) Route Discovery Distance Vector Link-state Route Selection Static Dynamic

10. Processes and Methods (con’t) Connection services Network-layer flow control Error control Packet sequence control Gateway services Network layer translation

11. Network Layer Addressing Logical network address Logically distinguish two networks in an internetwork Routers connect two networks with different logical network addresses Service address Every entity (hardware or software) needs its own address (port or socket) to send and receive data Ports identity an upper layer software process or protocol. (Computers can run several concurrently) See figure 4-3 on page 4-10

12. Switching Method Characteristics Circuit Switching A Single path is set up for the duration of a single conversation. Message Switching Conversations are divided into messages, each of which MAY take a different path. Packet Switching Combination of both Messages broken into packets small enough to fit in RAM. A sequence number is added to each packet Most LANs use Packet Switching

13. Packet Switching Virtual Circuit Packet Switching Logical connections between sender and receiver Appears as a point-to-point link, but is actually a logical switched path Datagram Packet Switching Used when no logical connection is required In common use on the internet Relies on the network layer to navigate paths and correct errors

14. Route Discovery Terms Hop Count Number of routers data must pass through to reach the destination Tick count Amount of time required to reach the destination network Relative expense An assignable number based on user-defined costs Route discovery uses two methods Distance Vector Link-state

15. Distance Vector Route Discovery Method Routers keep tables of routes to various destinations These tables are communicated to neighboring routers If router A tells router B that it can reach network 100 in 6 hops, router B would show it can reach network 100 in 7 hops Updating routing tables is called convergence

16. Link State Route Discovery Method Considered more intelligent and faster than distance-vector After an initial table is received, routers only broadcast information that has changed (not all routes) Link State Route Discovery requires less network overhead, but can require considerable planning

17. Route Selection Methods Dynamic Routers can change in response to changing network conditions Route selections are made at each router Static Data packets always follow a predetermined path Path defined by network administrator or by an assigned network device

18. Network Layer Connection Services Data Link- LLC sublayer connection services control the amount of data transmitted and notification of bad frames Data Link layer connection services manage communication between source and destination devices Network Layer connection services build on these services, add reliability

19. Connection Services Network Layer Flow Control Similar to Data Link layer flow control Can be guaranteed rate or static/dynamic windows Can involve intelligent path selection (congestion control)

20. Connection Services (Con’t) Network Layer Error Control Primarily concerned with packet loss, duplicate packets, altered packets Lost packets handled by acknowledgments Duplicate packets are handled by dropping all but one of the packets Altered packets are detected (sometimes corrected) by use of a CRC check CRC is calculated at each router, since the address field is changed at each hop

21. Connection Services (Con’t) Network Layer Packet Sequence Control Upper layer messages are rebuilt by putting arrived packets back into the original order Packet sequence control can take place at Network layer, but at the Transport layer is more common.

22. Gateway Services A Gateway interprets and translates the rules on two separate networks Address, Route Discovery, Route Selection, and Connection Service rules may vary from one network to another Book example: Two networks use data units of different sizes. A Network Layer gateway fragments and reassembles data into acceptable sizes for both networks The Default Gateway or Default Router is where all packets are sent that are destined for devices not on the local network segment

23. Connectivity Devices LAN Media limitations Media may limit number of devices, length of cable run, or available bandwidth Repeaters and Bridges address these issues Data destination across multiple networks Routers forward data from network to network Incompatible Systems Two systems without a common protocol Gateway acts as a translator

24. Bridges, Repeaters, Switching Hubs, Routing Protocols Are covered in the book; we will not have class time to cover them

25. Connection Devices Brouters Bridge and Router in a single device Cheaper than both devices, more expensive than either one Switching Hub and Router Combination Leading-edge technology, which means implementation problems

26. Nonroutable Protocols Protocols that do not use Network Layer for routing Protocols that use only static routes that cannot be updated Deal with them by encapsulation (tunneling) or bridging. Routing SNA NetBIOS NetBEUI LAT

27. Network Layer This is a major layer in the OSI model, and has many important features we were unable to discuss in class It is critical that you read the material in the book. Email me if you have any questions about the content.