What Does Network Layer Do?
                  •      End-to-end deliver packet from 
                                                                                                               application
                         sending to receiving hosts, “hop-                                                     transport
                                                                                                                network
                         by-hop” thru network                                                                   data link                                     network
                                                                                                                physical                 network              data link              network
                            – A network-wide concern!                                                                                   data link             physical               data link
                            – Involves every router, host in the                                                                         physical                                    physical
                                                                                                                                                              network
                                  network                                                                                                                     data link
                                                                                                                                                              physical           network
                  •                                                                                                                                                             data link
                         Compare:                                                                                                                                                physical
                            – Transport layer                                                                                              network                    network
                                                                                                                                          data link                  data link
                                      • between two end hosts                                                                              physical                   physical
                            – Data link layer                                                                                                              network
                                                                                                                                                           data link            application
                                      • over a physical link directly                                                                                       physical             transport
                                                                                                                                                                                  network
                                           connecting two (or more) physically                                                                                                   data link
                                            hosts                                                                                                                                 physical
                              CSci4211:           Network Layer: Data Plane Part 1                                                                                                       2
                                            What Does Network Layer Do?
        •      transport segment from sending 
               to receiving host 
        •                                                                                                   application
               on sending side encapsulates                                                                  transport
                                                                                                              network
               segments into datagrams                                                                       data link                                      network
                                                                                                              physical                network               data link              network
        •      on rcving side, delivers segments                                                                                      data link             physical              data link
                                                                                                                                      physical                                     physical
               to transport layer                                                                                                                           network
                                                                                                                                                           data link
                                                                                                                                                            physical          network
        •      network layer protocols in every                                                                                                                               data link
               host, router                                                                                                                                                   physical
                                                                                                                                        network                    network
                                                                                                                                        data link                  data link
        •      Router examines header fields in                                                                                         physical                   physical
                                                                                                                                                         network
               all IP datagrams passing                                                                                                                  data link            application
                                                                                                                                                         physical              transport
               through it                                                                                                                                                      network
                                                                                                                                                                               data link
                                                                                                                                                                               physical
                              CSci4211:           Network Layer: Data Plane Part 1                                                                                                       3
                     Network Layer Functions
       •  Addressing
           – Globally unique address for each routable device
               • Logical address, unlike MAC address (as you’ll see later)
           – Assigned by network operator
               • Need to map to MAC address (as you’ll see later)
       •  Routing: building a “map” of network
           – Which path to use to forward packets from src to dest
       •  Forwarding: delivery of packets hop by hop
           – From input port to appropriate output port in a router
            Routing and forwarding depend on network service models: datagram 
             vs. virtual circuit
            CSci4211:           Network Layer: Data Plane Part 1       4
               Two Key Network-Layer Functions
    • forwarding: move packets 
       from router’s input to            analogy:
       appropriate router output         • routing: process of planning 
    • routing: determine route              trip from source to dest
       taken by packets from source      • forwarding: process of 
       to dest.                             getting through single 
        – routing algorithms                interchange
            CSci4211:           Network Layer: Data Plane Part 1      5
           Network Layer: Data Plane, Control Plane
     Data plane                            Control plane
     •                                     • network-wide logic
      local, per-router function
     •                                     • determines how datagram is routed 
      determines how datagram arriving       among routers along end-end path from 
     on router input port is forwarded to    source host to destination host
     router output port                    • two control-plane approaches:
     •
      forwarding function                      • traditional routing algorithms: 
                                                 implemented in routers
                                               • software-defined networking (SDN): 
       values in arriving                        implemented in (remote) servers
       packet header
               0111           1
                             3 2
             CSci4211:           Network Layer: Data Plane Part 1               6