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OSI Networking Model Layer 3


3 - NETWORK - Path determination (routing) and logical addressing (IP)


Function: Routes data from one network node to another. This layer translates "logical" device names and addresses into their network hardware equivalents, and does Routing, if necessary, for devices that are more than one network link away. Used in Wide Area Networks WAN applications.


Data unit: Datagrams & Packets.


Protocols (examples): IP, ARP, RARP, ICMP, RIP, OSFP, IGMP; IPX, NWLink, NetBEUI, OSI, DDP, DECnet


Devices: Brouter, Router, Frame Relay Device, ATM Switch, Advanced Cable Tester.


Topics & Methods Explained


Logical network:
The identifier used to logically distinguish two different networks in an internetwork. Routers are internetwork connectivity devices that connect two networks with different logical network addresses.


A service address (called a port or a socket by specific protocols) identifies a specific upper-layer software process or protocol. Multiple service addresses can be assigned to any computer on which several network apps are running.


A technique that connects the sender and the receiver by a single path for the duration of a conversation. Same as in the telephone system, a complete (dedicated) path, end-to-end, must exist before communication can take place.


A dedicated path for an entire conversations is NOT established. Rather, conversations are divided into messages. Each msg is packed with its own destination address & then transmitted. Sometimes called store-n-forward network.


Combines the advantages of message and circuit switching. Messages are broken into packets tagged with source, destination, and intermediary node addresses as appropriate. Packets can be stored in RAM instead of on harddisk

Route discovery

Distance vector:   (RIP, RIP II)
Routers compile and send network route tables to other routers. Each router builds its own route table by constantly broadcasting/combining tables from nearby routers. Complete tables, incl. the changes, are passed between routers.


Link-state:   (OSPF, NLSP)
Routers (or entities that wish to discover a route) identify the networks they are attached to, receiving an initial route table from a local router. After an initial message is sent out, the router only notifies the others when changes occur.

Route selection

Static route selection is based upon a path designated by the network administrator or by an assigned network device. Routers are not allowed to make route selection decisions. Data packets always follow a predetermined path.


Cost information is continually gathered. Every packet is assigned a route depending upon the latest route discovery costs. Each stop along the path can cause the selection of the next stop for every packet.

Connection services

Network-layer flow control:
Uses acknowledgments to control data flow based upon internetwork's capabilities. May also involve intelligent path selection. Can be performed by the negotiation of a guaranteed rate, or through static and dynamic windows.


Error control:
Concerned with packet loss, duplicate packets, & altered data. Altered data are typically detected by appending a CRC or other checksum to the packet. Checksums must be recalculated at every hop. (header's address is changed).


Packet sequence control:
Used to put the arriving packets into proper sequence in order to rebuild upper-layer messages. Required for datagram networks where packets routinely arrive out of order. It may also be necessary with large virtual circuit networks.

Gateway services

Network-layer translation:
Independent networks use different address, route discovery, route selection, and connection services rules. In order for these networks to be combined on an internetwork, they must resolve their differences. The gateway translates.



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