Network Layers

 Network Layer: Address Mapping, Error Reporting, and Multicasting

* A metric is the cost assigned for passage of a packet through a network.

* A router consults its routing table to determine the best path for a packet.

* An autonomous system (AS) is a group of networks and routers under the authority of a single administration.

* RIP and OSPF are popular interior routing protocols used to update routing tables in an AIS.

* RIP is based on distance vector routing, in which each router shares, at regular intervals, its knowledge about the entire AS with its neighbor.

* A RIP routing table entry consists of a destination network address, the hop count to that destination, and the IP address of the next router.

* OSPF divides an AS into areas, defined as collections of networks, hosts, and routers.

* OSPF is based on link state routing, in which each router sends the state of its neighborhood to every other router in the area. A packet is sent only if there is a change in the neighborhood.

* OSPF defines four types of links (networks): point-to-point, transient, stub, and virtual.

* Five types of link state advertisments (LSAs) disperse information in OSPF: router link, network link, summary link to network, summary link to AS boundary router, and external link.

* A router compiles all the information from the LSAs it receives into a link state database. This database is common to all routers in an area.

* An LSA is a mutifield entry in a link state update packet.

* BGP is an interautonomous system routing protocol used to upadate routing tables.

* BGP is based on a routing method called path vector routing. In this method, the ASs through which a packet must pass are explicitly listed.

* There are four types of BGP messages: open, update, keep-alive, and notification.

* The Internet Group Management Protocol (IGMP) helps multicast routers create and update a list of loyal members related to a router interface.

* The three IGMP message types are the query message, the membership report, and the leave report.

* A host or router can have membership in a group.

* A host maintains a list of processes that have membership in a group.

* A router maintains a list of groupids that shows group membership for each interface.

* Multicasting applications include distributed databases, information dissemination, teleconferencing, and distance learning.

* For efficient multicasting we use a shortest-path spanning tree to represent the communication path.

* In a source-based tree approach to multicast routing, the source--group combinations determines the tree.

* In a group-based tree approach to multicast routing, the group determines the tree.

* DVRMP is a multicast routing protocol that uses the distance routing protocol to create a source-based tree.

* In reverse path forwarding (RPF), the router forwards only the packets that have traveled the shortest path from the source to the router.

* Reverse path broadcasting (RPB) creates a shortest-path broadcast tree from the source to each destination. It guarantees that each destination receives one and only one copy of the packet.

* Reverse path multicasting (RPM) adds pruning and grafting to RPB to create a multicast shortest-path tree that supports dynamic membership changes.

* MOSPF is a multicast protocol that uses multicast link state routing to create a source-based least-cost tree.

* The Core-Based Tree (CBT) protocol is a multicast routing protocol that uses a core as the root of the tree.

* PIM-DM is a source-based routing protocol that uses RPF and pruning and grafting strategies to handle multicasting.

* PIM-SM is a group-shared routing protocol that is similar to CBT and u ses a rendzvous point as the source of the tree.

* For multicasting between two noncontiguous multicast routers, we make a multicast backbone (MBONE) to enable tunneling.