Network Layer Overview

• Delivers data from the sending host to the receiving host.
• The sending side encapsulates segments in datagrams.
• The receiving side passes segments to the transport layer.
• Network-layer protocols are present in every host and router.
• Routers inspect header fields in every IP datagram that passes through them.

1. Forwarding
   : Moves a packet from a router's input-link interface to the appropriate output-link interface.
   • A data-plane function that operates within each router.
   • Usually implemented in hardware.
   • Uses a
     forwarding table
     to forward packets.
   • Packets may be lost or duplicated.
2. Routing
   : Determines the path from source to destination.
   • A control-plane function that operates across the network.
   • Routing algorithms determine which direction to forward packets and build the
     forwarding table
     .
   • Implemented in software.

• Determines how a datagram arriving at a router's input port is forwarded to an output port.
• Performs forwarding.
• Uses a forwarding table.

• Network-wide logic.
• Determines how datagrams are routed between routers along the path from the source host to the destination host.
• Configures forwarding tables.
• Two approaches to the control plane:
  • Traditional routing algorithms: Implemented in routers.
  • Software-defined networking (SDN): Implemented in a remote server or controller.

Individual routing-algorithm components in each router interact in the control plane.

An independent, usually remote controller interacts with local control agents (CAs).

• Services for individual datagrams:
  • Guaranteed delivery.
  • A delivery-time bound.
• Services for a flow of datagrams:
  • In-order delivery.
  • Guaranteed minimum bandwidth.
  • A bound on changes in packet spacing.