rfc995.txt

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         of the configuration and routing information;

     c)  procedures for the correct interpretation of protocol control
         information; and

     d)  the functional requirements for implementations claiming
         conformance to this Standard.



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RFC 995                                                    December 1986


     The procedures are defined in terms of:

     a)  the interactions between End System and Intermediate System
         network entities through the exchange of protocol data units;
         and

     b)  the interactions between a network entity and an underlying
         service provider through the exchange of subnetwork service
         primitives.

   This protocol does not specify any protocol elements or algorithms for
   facilitating routing and relaying among Intermediate Systems. Such
   functions are intentionally beyond the scope of this protocol.

3     References


   ISO 7498        Information Processing Systems --- Open Systems Intercon-
                   nection - Basic Reference Model

   DIS 7498/DAD1   Information Processing Systems --- Open Systems Intercon-
                   nection - Addendum to ISO 7498 Covering Connectionless-
                   mode Transmission

   ISO 8348        Information Processing Systems --- Telecommunications and
                   Information Exchange between Systems - Network Service
                   Definition

   ISO 8348/AD1    Information Processing Systems --- Telecommunications and
                   Information Exchange between Systems - Addendum to the
                   Network Service Definition Covering Connectionless-mode
                   Transmission


   ISO 8348/AD2    Information Processing Systems --- Telecommunications and
                   Information Exchange between Systems - Addendum to the
                   Network Service Definition Covering Network Layer Address-
                   ing


   ISO 8473        Information Processing Systems --- Telecommunications and
                   Information Exchange between Systems - Protocol for Pro-
                   viding the Connectionless Network Service


   DIS 8648        Information Processing Systems --- Telecommunications and
                   Information Exchange between Systems - Internal Organiza-
                   tion of the Network Layer






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RFC 995                                                    December 1986


   SC21/N965       OSI Management Framework --- Seventh Working Draft

   DIS 8802        Local Area Networks



















































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RFC 995                                                    December 1986


                          SECTION  ONE.  GENERAL

4     Definitions

4.1     Reference Model Definitions

   This document makes use of the following concepts defined in ISO 7498:

     (a)  Network layer

     (b)  Network service access point

     (c)  Network service access point address

     (d)  Network entity

     (e)  Routing

     (f)  Network protocol

     (g)  Network relay

     (h)  Network protocol data unit

4.2     Network Layer Architecture Definitions

   This document makes use of the following concepts from DIS 8648, Internal
   Organization of the Network Layer:

     (a)  Subnetwork

     (b)  End System

     (c)  Intermediate System

     (d)  Subnetwork Service

     (e)  Subnetwork Access Protocol

     (f)  Subnetwork Independent Convergence Protocol

4.3     Network Layer Addressing Definitions

   This document makes use of the following concepts from DIS 8348/DAD2,
   Addendum to the Network Service Definition Covering Network Layer Ad-
   dressing:


    (a)  Subnetwork address

    (b)  Subnetwork point of attachment



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RFC 995                                                    December 1986


4.4     Local Area Network Definitions

   This document makes use of the following concepts from DIS 8802, Local
   Area Networks:

    (a)  multicast address

    (b)  broadcast medium

4.5     Additional Definitions

   For the purposes of this document, the following definitions apply:

   Configuration:   The collection of End and Intermediate Systems
         attached to a single subnetwork, defined in terms of the
         system types, NSAP addresses present, Network Entities
         present, and the correspondence between systems and SNPA
         addresses.

   Network Entity Title:   An identifier for a network entity which
         has the same abstract syntax as an NSAP address, and which
         can be used to unambiguously identify a network entity in
         an End or Intermediate System.

5     Symbols and Abbreviations

5.1     Data Units
    PDU        Protocol Data Unit
    SNSDU      Subnetwork Service Data Unit

5.2     Protocol Data Units

    ESH PDU      End System Hello Protocol Data Unit
    ISH PDU      Intermediate System Hello Protocol Data Unit
    RD PDU       Redirect Protocol Data Unit

5.3     Protocol Data Unit Fields

     NPID        Network Layer Protocol Identifier
     LI          Length Indicator
     V/P         Version/Protocol Identifier Extension
     TP          Type
     CS          Checksum
     NETL        Network entity Title Length
     NET         Network entity Title
     DAL         Destination Address Length
     DA          Destination Address
     SAL         Source Address Length
     SA          Source Address
     BSNPAL      SN Address Length of better route to destination
     BSNPA       SN Address of better route to destination



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     HT          Holding timer

5.4     Parameters
     CT    Configuration Timer
     RT    Redirect Timer

5.5     Miscellaneous

     ES        End System
     IS        Intermediate System
     SN        Subnetwork
     SNACP     Subnetwork Access Protocol
     SNICP     Subnetwork Independent Convergence Protocol

6     Overview of the Protocol

6.1     Information Provided by the Protocol

   This Protocol provides two types of information to Network entities
   which support its operation:

     a) Configuration Information, and

     b) Route Redirection Information

   Configuration Information permits End Systems to discover the ex-
   istence and reachability of Intermediate Systems and permits Inter-
   mediate Systems to discover the existence and reachability of End
   Systems.  This information allows ESs and ISs attached to the same
   subnetwork to dynamically discover each other's existence and availa-
   bility, thus eliminating the need for manual intervention at ESs and
   ISs to establish the identity of Network entities that can be used to
   route NPDUs.

   Configuration Information also permits End Systems to obtain informa-
   tion about each other in the absence of an available Intermediate
   System.

                                    Note:
     The term "configuration information" is not intended in the broad
     sense of configuration as used in the context of OSI system
     management.  Rather, only the functions specifically defined herein
     are intended.

   Route Redirection Information allows Intermediate Systems to inform
   End Systems of (potentially) better paths to use when forwarding
   NPDUs to a particular destination. A better path could either be
   another IS on the same subnetwork as the ES, or the destination ES
   itself, if it on the same subnetwork as the source ES. Allowing the
   ISs to inform the ESs of routes minimizes the complexity of routing
   decisions in End Systems and improves performance because the ESs may



ISO N4053                                                      [Page 11]

RFC 995                                                    December 1986


   make use of the better IS or local subnetwork access for subsequent
   transmissions.

6.2     Subsets of the Protocol

   A Network Entity may choose to support either the Configuration In-
   formation, the Route Redirection Information, neither, or both. If
   the Configuration Information is supported, it is not required that
   it be employed over all subnetworks to which the Network entity is
   attached.

 6.3   Addressing

   The Source Address and Destination Address parameters referred to in
   this International Standard are OSI Network Service Access Point Ad-
   dresses. The syntax and semantics of an OSI Network Service Access
   Point Address are described in a separate document, ISO 8348/DAD2,
   Addendum to the Network Service Definition covering Network Layer Ad-
   dressing.

6.4   Underlying Service Assumed by the Protocol

   The underlying service required to support this protocol is defined
   by the primitives in Table 1.

     _________________________________________________________________
     |    SN_UNITDATA          .Request    | SN_Destination_Address, |
     |                         .Indication | SN_Source_Address,      |
     |                                     | SN_Quality_of_Service,  |
     |                                     | SN_Userdata             |
     |_____________________________________|_________________________|

            Table 1: Service Primitives for Underlying Service