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04. Network Layer

$\gdef \N{\mathbb{N}} \gdef \Z{\mathbb{Z}} \gdef \Q{\mathbb{Q}} \gdef \R{\mathbb{R}} \gdef \C{\mathbb{C}} \gdef \setcomp#1{\overline{#1}} \gdef \sseq{\subseteq} \gdef \pset#1{\mathcal{P}(#1)} \gdef \covariant{\operatorname{Cov}} \gdef \of{\circ} \gdef \p{^{\prime}} \gdef \pp{^{\prime\prime}} \gdef \ppp{^{\prime\prime\prime}} \gdef \pn#1{^{\prime\times{#1}}} $

Services and Protocols
  • Transport segment from sending host to receiving host
    • Sender: Encapsulates segments from the Transport Layer into datagrams, and passes them on to the Link Layer
    • Receiver: Delivers segments to the Transport Layer protocol.

Routers examine header fields in all IP datagrams passing through it, and move datagrams from input ports to output ports to transfer datagrams along their end to end path.

Two key network layer functions
  • Forwarding: Move packets from a router’s input link to the appropriate router output link
  • Routing: Determine the route taken by packets from source to destination
Data Plane and Control Plane
Data Plane
  • Local, per router function
  • Determines how datagram arriving on router input port is forwarded to the router output port
Control Plane
  • Network-wide logic
  • Determines how datagram is routed among routers along end-to-end path from the source to the destination host.
  • There are two Control Plane approaches:
    • Traditional routing algorithms, implemented in routers
    • Software Defined Networking (SDN), implemented in remote servers
Network service model
IP Datagram Format

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Protocol Field

Has a dedicated value for the protocol being used.

  • Transport Layer Protocols
    • TCP
    • UDP
  • Network Layer Protocols
    • ICMP
    • IGMP
    • OSPF
Fragmentation Field

Can either be set to Do not Fragment, or More Fragments