The network layer runs on all internet devices.

• Includes switches, routers, hosts

The network layer takes a segment from the sending host to the receiving host:

• Sender - encapsulates segment into datagram, pass to link layer
• Receiver - deliver segment to transport layer protocol

Function of a router:

• Examine header fields in all IP datagrams passing through it
• Move datagram from input port to output port along end-to-end path

There are 2 key functions of the network layer:

• Routing
  • Determine route taken by packets from source to destination
    • Involves routing algorithms to determine best path
  • Part of control plane
• Forwarding
  • Move packet from input to appropriate output link depending on IP header, forwarding table
  • Part of data plane

Planes for network layer:

• Data plane

  • Implemented on switches

  • Has local, per-router functionality

  • Determines how datagram on router is forwarded to router output port

    

• Control plane

  • Involves network-wide logic

  • Determines how datagram is routed among routers along end-end path from source to destination host

  • Involves routing algorithms - find best path according to network

  • Traditional routing algorithm - implement in routers

    • Need to commit to a single router (i.e. vendor lock-in)
    • Per-router control plane ⇒ individual routing algorithm components in every router interact in control plane
      • Information is collected via communication with other routers

    

  • Software-defined networking (SDN) - implement in remote servers

    • Can be deployed remotely on proprietary hardware
    • Remote controller computes and installs forwarding tables in routers
    • Control plane pushes output to data plane
    • Data plane uses information to forward packets
    • Monitoring information is pushed by routers to control plane

Network Service Model