1. Introduction

   This Protocol is one of a set of International Standards produced to
   facilitate the interconnection of open systems. The set of standards
   covers the services and protocols required to achieve such intercon-
   nection.

   This Protocol is positioned with respect to other related standards
   by the layers defined in the Reference Model for Open System Inter-
   connection (ISO 7498) and by the structure defined in the Internal
   Organization of the Network Layer (DIS 8648).  In particular, it is a
   protocol of the Network Layer.  This protocol permits End Systems and
   Intermediate Systems to exchange configuration and routing informa-
   tion to facilitate the operation of the routing and relaying func-
   tions of the Network Layer.

   The aspects of Network Layer routing that are concerned with communi-
   cation between end systems and intermediate systems on the same sub-
   network are to a great extent separable from the aspects that are
   concerned with communication among the intermediate systems that con-
   nect multiple subnetworks. This protocol addresses only the former
   aspects. It will be significantly enhanced by the cooperative opera-
   tion of an additional protocol that provides for the exchange of
   routing information among intermediate systems, but is useful whether
   or not such an additional protocol is available.

      This protocol provides solutions for the following practical problems:

      1.  How do end systems discover the existence and reachability of
          intermediate systems that can route NPDUs to destinations on
          subnetworks other than the one(s) to which the end system is
          directly connected?

      2.  How do end systems discover the existence and reachability of
          other end systems on the same subnetwork (when direct
          examination of the destination NSAP address does not provide
          information about the destination subnetwork)?

      3.  How do intermediate systems discover the existence and
          reachability of end systems on each of the subnetworks to
          which they are directly connected?

      4.  How do end systems decide which intermediate system to use
          to forward NPDUs to a particular destination when more than one
          intermediate system is accessible?

      The protocol assumes that:

      1.  Routing to a specified subnetwork point of attachment address
          (SNPA) on the same subnetwork is carried out satisfactorily by
          the subnetwork itself.

      2.  The subnetwork is not, however, capable of routing on a global
          basis using the NSAP address alone to achieve communication
          with a requested destination.

                                      Note:
          Consequently, it is not possible to use Application Layer
          communication to carry out the functions of this protocol.

      The protocol is connectionless, and is designed to:

      1.  minimize the amount of a priori state information needed by
          end systems before they can begin to communicate with other
          end systems;

      2.  minimize the amount of memory needed to store routing
          information in end systems; and

      3.  minimize the computational complexity of end system routing
          algorithms.

   The protocol is also designed to operate in close conjunction with
   the Protocol for the Provision of the Connectionless-mode Network
   Service (ISO 8473).  Since routing styles are usually closely related
   to communication styles, the information that this protocol provides
   to end systems and intermediate systems may or may not be appropriate
   information for supporting routing functions when a Network Layer
   protocol other than ISO 8473 is used.