Network Layer: Delivery, Forwarding, and Routing Study Notes

  • The delivery of a packet is called direct if the deliverer (host or router) and the destination are on the same network; the delivery of a packet is called indirect if the deliverer (host or router) and the destination are on different networks.
  • In the next-hop method, instead of a complete list of the stops the packet must make, only the address of the next hop is listed in the routing table; in the networkspecific method, all hosts on a network share one entry in the routing table.
  • In the host-specific method, the full IP address of a host is given in the routing table.
  • In the default method, a router is assigned to receive all packets with no match in the routing table.
  • The routing table for classless addressing needs at least four columns.
  • Address aggregation simplifies the forwarding process in classless addressing.
  • Longest mask matching is required in classless addressing.
  • Classless addressing requires hierarchical and geographical routing to prevent immense routing tables.
  • A static routing table’s entries are updated manually by an administrator; a dynamic routing table’s entries are updated automatically by a routing protocol.
  • A metric is the cost assigned for passage of a packet through a network.
  • An autonomous system (AS) is a group of networks and routers under the authority of a single administration.
  • RIP is based on distance vector routing, in which each router shares, at regular intervals, its knowledge about the entire AS with its neighbors.
  • Two shortcomings associated with the RIP protocol are slow convergence and instability. Procedures to remedy RIP instability include triggered update, split horizons, and poison reverse.
  • 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 routing tables are calculated by using Dijkstra’s algorithm.
  • BGP is an interautonomous system routing protocol used to update routing tables.
  • BGP is based on a routing protocol called path vector routing. In this protocol, the ASs through which a packet must pass are explicitly listed.
  • In a source-based tree approach to multicast routing, the source/group combination determines the tree; in a group-shared tree approach to multicast routing, the group determines the tree.
  • MOSPF is a multicast routing protocol that uses multicast link state routing to create a source-based least-cost 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.
  • DVMRP is a multicast routing protocol that uses the distance routing protocol to create a source-based tree.
  • The Core-Based Tree (CBT) protocol is a multicast routing protocol that uses a router as the root of the tree.
  • PIM-DM is a source-based tree routing protocol that uses RPF and pruning and grafting strategies to handle multicasting.
  • PIM-SM is a group-shared tree routing protocol that is similar to CBT and uses a rendezvous router as the source of the tree.
  • For multicasting between two noncontiguous multicast routers, we make a multicast backbone (MBONE) to enable tunneling.

 

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