Figure 1.

  Internet protocol interfaces on one side to the higher level
  host-to-host protocols and on the other side to the local network
  protocol.  In this context a "local network" may be a small network in
  a building or a large network such as the ARPANET.

2.2.  Model of Operation

  The  model of operation for transmitting a datagram from one
  application program to another is illustrated by the following
  scenario:

    We suppose that this transmission will involve one intermediate
    gateway.

    The sending application program prepares its data and calls on its
    local internet module to send that data as a datagram and passes the
    destination address and other parameters as arguments of the call.

    The internet module prepares a datagram header and attaches the data
    to it.  The internet module determines a local network address for
    this internet address, in this case it is the address of a gateway.



 

                                                          September 1981
Internet Protocol
Overview



    It sends this datagram and the local network address to the local
    network interface.

    The local network interface creates a local network header, and
    attaches the datagram to it, then sends the result via the local
    network.

    The datagram arrives at a gateway host wrapped in the local network
    header, the local network interface strips off this header, and
    turns the datagram over to the internet module.  The internet module
    determines from the internet address that the datagram is to be
    forwarded to another host in a second network.  The internet module
    determines a local net address for the destination host.  It calls
    on the local network interface for that network to send the
    datagram.

    This local network interface creates a local network header and
    attaches the datagram sending the result to the destination host.

    At this destination host the datagram is stripped of the local net
    header by the local network interface and handed to the internet
    module.

    The internet module determines that the datagram is for an
    application program in this host.  It passes the data to the
    application program in response to a system call, passing the source
    address and other parameters as results of the call.

                                    
   Application                                           Application
   Program                                                   Program
         \                                                   /      
       Internet Module      Internet Module      Internet Module    
             \                 /       \                /           
             LNI-1          LNI-1      LNI-2         LNI-2          
                \           /             \          /              
               Local Network 1           Local Network 2            



                            Transmission Path

                                Figure 2








 

September 1981                                                          
                                                       Internet Protocol
                                                                Overview



2.3.  Function Description

  The function or purpose of Internet Protocol is to move datagrams
  through an interconnected set of networks.  This is done by passing
  the datagrams from one internet module to another until the
  destination is reached.  The internet modules reside in hosts and
  gateways in the internet system.  The datagrams are routed from one