Chapter 4: Network Layer

4.1: Introduction

• Network layer = IP layer
• Transport segments from sending to receiving host
• On the sending side, segments are encapsulated into datagrams
• On the receiving side, segments are delivered to transport layer
• Network layer protocols in every host and router
• Router examines header fields in all IP datagrams passing through it

Key Network-Layer functions

1. Routing: determine the path taken by packets from source to dest
   1. Build a minimum-cost table at each router
   2. Table has next-hop neighbor for each possible destination
   3. Goal: send packet along the least-expensive path
2. Forwarding: move packets from a router’s input port to the router’s output port (physical interface (NIC) inside router, not a TCP/UDP port!)
   1. Table lookup
   2. Port-to-port transfer
   3. Goal: efficiency
   4. Note this is within a single router!

1. Connection setup in certain network architectures, e.g. ATM (Asynchronous Transfer Mode):
   1. Before datagrams flow in such networks, two hosts and intermediate routers establish virtual circuit (VC). Routers get involved to set up a path

4.2: Virtual Circuit and Datagram Networks

Virtual Circuits

• VCs may create a path that behaves much like a telephone circuit (no congestion, low delay, no loss)
• Call setup for each connection before data can flow
• Each packet carries a VC tag instead of the 5-tuple <src addr, dest addr, src port, dest port, proto>
• Every router on source-dest path maintains “state” for each passing connection
  • Mapping from tags to next-hop router
• Fraction of router resources (bandwidth, buffers) are allocated to each VC

If this was a forwarding table for a datagram network, then the 2 columns would be destination address and outgoing interface.

Datagram Networks

• No call setup at network layer