Network Layer

Overview

• Transport segment from sending to receiving host
• On sending side, encapsulate segments in to datagrams
• On receiving side, delivers segments to transport layer
• Network layer protocols in every host, router
• Router examines header fields in all IP datagrams passing through it

Two Key Functions

Forwarding

• Move packets from router’s input to appropriate router output

Routing

• Determine route taken by packets from source to destination

Data Plane

• Local, per-router function
• Determines how datagram arriving on router input port is forwarded to router output port
  • Forwarding function

Control Plane

• Network-wide logic
• Determines how datagram is routed among routers along end-end path from source host to destination host
• Two Approaches:
  • Traditional routing algorithms: implemented in routers
  • Software-defined networking: implemented in servers

Router Architecture

Input Port Functions

Decentralized Switching

• Using header field values, lookup output port using forwarding table in input port memory
  • Goal: Complete input port processing at “line speed” (Max capacity)
  • Queuing: if datagrams arrive faster than forwarding rate into switch fabric

Destination-based forwarding

Forward based only on destination IP address

Longest prefix matching

When looking for forwarding table entry for given destination address, use longest address prefix that matches destination address

• Often performed using ternary content addressable memories (TCAMs)
  • Content addressable: Present address to TCAM: retrieve address in one clock cycle, regardless of table size

Switching Fabrics

• Transfer packet from input buffer to appropriate output buffer
• Switching rate: Rate at which packets can be transferred from inputs to outputs
  Three types:
  

Switching via memory

First generation routers:

• Packet copied to system’s memory
• Limited by memory bandwidth

Switching via bus

• Datagram from input port memory to output port memory via a shared bus
• Bus contention: switching speed limited by bus bandwidth

Switching via interconnection network

• Overcome bus bandwidth limitations
• Fragmenting datagram into fixed length cells, switch cells through the fabric

Import port queuing

Fabric slower than input ports combined

• Head of the Line Blocking
  • Queued datagram at front of queue prevents others in queue from moving forward
    

Output port queuing

Buffering: Required when datagrams arrive from fabric faster than link transmission rate
Scheduling discipline: Chooses among queued datagrams for transmission

Packet Scheduling

Deciding which packet to send next on link

Internet Protocol (IP)

IPv4 and IPv6

Network Address Translation (NAT)

Generalized Forwarding

Control Plane

Routing: Determine route taken by packets from source to destination

• per-router control
• logically centralized control