Network Layer: Delivery, Forwarding, and Routing - Notes

Network Layer Overview

  • Concepts: Delivery, Forwarding, and Routing of IP packets to their final destinations, primarily by the network layer.

  • Delivery: How packets are handled in underlying networks.

  • Forwarding: The process of directing packets toward their endpoint.

  • Routing: Creating and updating routing tables to assist in forwarding.

22.1 Delivery

  • Direct vs. Indirect Delivery:

    • Direct Delivery: The sender and destination are on the same physical network, or it’s the last router to the destination.

    • Indirect Delivery: Uses a chain of routers to reach its final destination when not on the same network.

    • Every delivery involves at least one direct delivery.

22.2 Forwarding

  • Forwarding Process: A host/router uses its routing table to make delivery decisions when receiving packets.

  • Challenges in Large Networks: Simple lookups in large tables are inefficient due to the number of required entries.

Forwarding Techniques:

  1. Next-Hop Method: Stores only the next-hop address in the routing table rather than full routing paths, simplifying the lookups.

  2. Network-Specific Method: Aggregates hosts on the same network into a single entry rather than individual entries.

  3. Default Method: Directs unspecified packets to a defined router (often 0.0.0.0).

Address Aggregation

  • Groups multiple IP addresses into fewer entries to alleviate routing table size and complexity.

  • Works in conjunction with Classless Inter-Domain Routing (CIDR) to efficiently manage IP addresses.

22.2 Forwarding Process Details:

  • In classless addressing, a routing table must have a corresponding mask for every destination, improving lookup accuracy.

  • Examples:

    • Destination 180.70.65.140 resolved with relevant masks in routing table.

    • Successive application of masks determines correct forwarding.

Hierarchical Routing

  • Utilizes the ISP structure to manage routing table size efficiently; divided into autonomous systems (AS).

Routing Tables

  • Static vs. Dynamic Routing Tables: Static is manually maintained; dynamic is updated via routing protocols like RIP, OSPF.

Unicast Routing Protocols (22.3)

  • Routing Protocols Purpose: Share knowledge regarding network paths.

  • Intra-domain vs. Inter-domain Routing:

    • Intra-domain: Routing within an AS (RIP, OSPF).

    • Inter-domain: Routing between ASs (BGP).

Distance Vector Routing

  • Each router maintains a table of minimum distances to all nodes it can reach, sharing this information with neighbors.

  • Convergence Issues: Slow convergence and potential instability due to routing loops.

Link State Routing

  • Nodes maintain a complete topology view to build optimal paths using algorithms.

  • Each node independently calculates shortest paths based on a shared state.

BGP (Border Gateway Protocol)

  • Operates as an inter-domain routing protocol, relying on path vector routing.

Multicast Routing Protocols (22.4)

  • Multicast vs. Unicast:

    • Multicast: one source to multiple destinations.

    • Unicast: one source to one destination.

Applications of Multicasting:

  • Distributed databases, teleconferencing, content delivery, etc.

Multicast Routing Protocols:

  1. MOSPF: Multicast OSPF, extends OSPF for multicast routing.

  2. DVMRP: Distance Vector Multicast Routing Protocol; uses metrics and shortest path for routing decisions.

  3. PIM: Protocol Independent Multicast (Dense Mode and Sparse Mode).

Core-Based Tree (CBT)

  • Utilizes a rendezvous router to manage group shared trees, supporting multicast efficiently across multiple paths.

Practice Questions Recap

  • Understanding concepts of delivery types, forwarding techniques, and the comparison of routing protocols.

  • Mastery of multicast protocols and their differences based on routing approaches and structural hierarchies in ISPs.

Key Terms

  • Autonomous System (AS), Unicast, Multicast, Distance Vector, Link State, BGP, RIP, OSPF, and Multicast Routing protocols.