RFC 985 (rfc985) - Page 3 of 23

Requirements for Internet gateways - draft

RFC 985                                                         May 1986
Requirements for Internet Gateways -- DRAFT


      architecturally layered upon it.  ICMP provides error reporting,
      flow control and first-hop gateway redirection.  Reliable data
      delivery is provided in the protocol suite by TCP, which provides
      end-end retransmission, resequencing and connection control.
      Connectionless service is provided by the User Datagram Protocol
      (UDP).

      The Internet community presently includes several thousand hosts
      connected to over 400 networks with about 120 gateways.  There are
      now well over 2400 hosts registered in the ARPA domain alone and
      an unknown number registered in other domains, with the total
      increasing at about ten percent each month.  Many of the hosts,
      gateways and networks in the Internet community are administered
      by civil organizations, including universities, research
      laboratories and equipment manufacturers.  Most of the remainder
      are administered by the US DoD and considered part of the DDN
      Internet, which presently consists of three sets of networks: the
      experimental segment, or ARPANET, the unclassified segment, or
      MILNET, and the classified segment, which does not yet have a
      collective name.

      The Internet model includes constituent networks, called local
      networks to distinguish them from the Internet system as a whole,
      which are required only to provide datagram (connectionless)
      transport.  This requires only best-effort delivery of individual
      packets, or datagrams.  Each datagram carries 32-bit source and
      destination addresses, which are encoded in three formats
      providing a two-part address, one of which is the local-network
      number and the other the host number on that local net.  According
      to the Internet service specification, datagrams can be delivered
      out of order, be lost or duplicated and/or contain errors.  In
      those networks providing connection-oriented service the extra
      reliability provided by virtual circuits enhances the end-end
      robustness of the system, but is not strictly necessary.

      Local networks are connected together in the Internet model by
      means of Internet gateways.  These gateways provide datagram
      transport only and normally seek to minimize the state information
      necessary to sustain this service in the interest of routing
      flexibility and robustness.  In the conventional model the gateway
      has a physical interface and address on each of the local nets
      between which it provides forwarding services.  The gateway also
      participates in one or more distributed routing or reachability
      algorithm such as the Gateway-Gateway Protocol (GGP) or Exterior
      Gateway Protocol (EGP) in order to maintain its routing tables.




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