



Network Working Group                                   ANSI X3S3.3 86-118
Request for Comments: 995                              ISO TC97/SC6/N 4053
                                                                April 1986





                                 I S O
             INTERNATIONAL ORGANIZATION FOR STANDARDIZATION
              ORGANISATION INTERNATIONALE DE NORMALISATION

     ______________________________________________________________________
    |                                                                     |
    |                            ISO/TC 97/SC 6                           |
    |                TELECOMMUNICATIONS AND INFORMATION                   |
    |                     EXCHANGE BETWEEN SYSTEMS                        |
    |                       Secretariat: USA (ANSI)                       |
    |                                                                     |
    |                                                                     |
    |_____________________________________________________________________|





  Title:    End System to Intermediate System Routing Exchange Protocol
            for use in conjunction with ISO 8473

  Source:   SC6/WG2
            Project 97.6.41




  ___________________________________________________________________________
 |This document is a progression of SC6/N3862, edited to incorporate member |
 |body comments and discussion at the Florence meeting of SC6/WG2. Pursuant |
 |to Recommendation 5 of that meeting, comments from member bodies on this  |
 |revision text are requested for discussion at the Tokyo meeting of SC6    |
 |and WGs.                                                                  |
 |__________________________________________________________________________|












ISO N4053                                                       [Page 1]

RFC 995                                                    December 1986


Contents

1   Introduction                                                           5

2   Scope and Field of Application                                         6

3   References                                                             7


SECTION ONE. GENERAL                                                       9

4   Definitions                                                            9
    4.1   Reference Model Definitions  . . . . . . . . . . . . . . . . .   9
    4.2   Network Layer Architecture Definitions   . . . . . . . . . . .   9
    4.3   Network Layer Addressing Definitions   . . . . . . . . . . . .   9
    4.4   Local Area Network Definitions   . . . . . . . . . . . . . . .  10
    4.5   Additional Definitions . . . . . . . . . . . . . . . . . . . .  10

5   Symbols and Abbreviations                                             10
    5.1   Data Units   . . . . . . . . . . . . . . . . . . . . . . . . .  10
    5.2   Protocol Data Units  . . . . . . . . . . . . . . . . . . . . .  10
    5.3   Protocol Data Unit Fields  . . . . . . . . . . . . . . . . . .  10
    5.4   Parameters . . . . . . . . . . . . . . . . . . . . . . . . . .  11
    5.5   Miscellaneous  . . . . . . . . . . . . . . . . . . . . . . . .  11

6   Overview of the Protocol                                              11
    6.1   Information Provided by the Protocol . . . . . . . . . . . . .  11
    6.2   Subsets of the Protocol. . . . . . . . . . . . . . . . . . . .  12
    6.3   Addressing   . . . . . . . . . . . . . . . . . . . . . . . . .  12
    6.4   Underlying Service Assumed by the Protocol   . . . . . . . . .  12
          6.4.1   Subnetwork Addresses . . . . . . . . . . . . . . . . .  12
          6.4.2   Subnetwork User Data . . . . . . . . . . . . . . . . .  13
    6.5   Service Assumed from Local Environment . . . . . . . . . . . .  13
    6.6   Subnetwork Types . . . . . . . . . . . . . . . . . . . . . . .  14
          6.6.1   Point-to-Point Subnetworks . . . . . . . . . . . . . .  15
          6.6.2   Broadcast Subnetworks  . . . . . . . . . . . . . . . .  15
          6.6.3   General Topology Subnetworks . . . . . . . . . . . . .  16


SECTION TWO. SPECIFICATION OF THE PROTOCOL                                18

7   Protocol Functions                                                    18
    7.1   Protocol Timers  . . . . . . . . . . . . . . . . . . . . . . .  18
          7.1.1   Configuration Timer  . . . . . . . . . . . . . . . . .  18
          7.1.2   Holding Timer  . . . . . . . . . . . . . . . . . . . .  18
    7.2   Report Configuration Function  . . . . . . . . . . . . . . . .  18
          7.2.1   Report Configuration by End Systems  . . . . . . . . .  19
          7.2.2   Report Configuration by Intermediate Systems . . . . .  19
    7.3   Record Configuration Function  . . . . . . . . . . . . . . . .  20
    7.4   Flush Old Configuration Function   . . . . . . . . . . . . . .  20
    7.5   Query Configuration Function . . . . . . . . . . . . . . . . .  20



ISO N4053                                                       [Page 2]

RFC 995                                                    December 1986


    7.6   Configuration Response Function  . . . . . . . . . . . . . . .  21
    7.7   Request Redirect Function. . . . . . . . . . . . . . . . . . .  22
    7.8   Record Redirect Function . . . . . . . . . . . . . . . . . . .  23
    7.9   Refresh Redirect Function  . . . . . . . . . . . . . . . . . .  23
    7.10  Flush Old Redirect Function  . . . . . . . . . . . . . . . . .  24
    7.11  PDU Header Error Detection   . . . . . . . . . . . . . . . . .  24
    7.12  Classification of Functions  . . . . . . . . . . . . . . . . .  25

8   Structure and Encoding of PDUs                                        25
    8.1   Structure  . . . . . . . . . . . . . . . . . . . . . . . . . .  26
    8.2   Fixed Part . . . . . . . . . . . . . . . . . . . . . . . . . .  26
          8.2.1   General  . . . . . . . . . . . . . . . . . . . . . . .  26
          8.2.2   Network Layer Protocol Identifier  . . . . . . . . . .  27
          8.2.3   Length Indicator . . . . . . . . . . . . . . . . . . .  27
          8.2.4   Version/Protocol Identifier Extension  . . . . . . . .  27
          8.2.5   Type Code  . . . . . . . . . . . . . . . . . . . . . .  28
          8.2.6   Holding Time . . . . . . . . . . . . . . . . . . . . .  28
          8.2.7   PDU Checksum . . . . . . . . . . . . . . . . . . . . .  28
    8.3   Network Address Part . . . . . . . . . . . . . . . . . . . . .  28
          8.3.1   General  . . . . . . . . . . . . . . . . . . . . . . .  28
          8.3.2   NPAI (Network Protocol Address Information) En-
                  coding . . . . . . . . . . . . . . . . . . . . . . . .  28
          8.3.3   Source Address Parameter for ESH PDU   . . . . . . . .  29
          8.3.4   Network Entity Title Parameter for ISH PDU . . . . . .  29
          8.3.5   Destination Address Parameter for RD PDU . . . . . . .  30
    8.4   Subnetwork Address Part  . . . . . . . . . . . . . . . . . . .  30
          8.4.1   Subnetwork Address Parameter for RD PDU  . . . . . . .  31
    8.5   Options Part . . . . . . . . . . . . . . . . . . . . . . . . .  31
          8.5.1   General  . . . . . . . . . . . . . . . . . . . . . . .  31
          8.5.2   Security . . . . . . . . . . . . . . . . . . . . . . .  32
          8.5.3   Quality of Service Maintenance . . . . . . . . . . . .  33
          8.5.4   Priority . . . . . . . . . . . . . . . . . . . . . . .  33
    8.6   End System Hello (ESH) PDU . . . . . . . . . . . . . . . . . .  34
          8.6.1   Structure  . . . . . . . . . . . . . . . . . . . . . .  34
    8.7   Intermediate System Hello (ISH) PDU  . . . . . . . . . . . . .  35
          8.7.1   Structure  . . . . . . . . . . . . . . . . . . . . . .  35
    8.8   Redirect (RD) PDU. . . . . . . . . . . . . . . . . . . . . . .  36
          8.8.1   Structure  . . . . . . . . . . . . . . . . . . . . . .  36

9   Formal Description                                                    37

10  Conformance                                                           37

ANNEX A. SUPPORTING TECHNICAL MATERIAL                                    38
    A.1   Use of Timers  . . . . . . . . . . . . . . . . . . . . . . . .  38
          A.1.1   Example of Holding Time for Route Redirection  . . . .  38
          A.1.2   Example of Holding Timer for Configuration Informa-
                  tion . . . . . . . . . . . . . . . . . . . . . . . . .  39
    A.2   Refresh and timeout of Redirection information . . . . . . . .  39
    A.3   System Initialization Considerations . . . . . . . . . . . . .  40
    A.4   Optimizations for Flushing Redirects   . . . . . . . . . . . .  41



ISO N4053                                                       [Page 3]

RFC 995                                                    December 1986


List of Tables

    1     Service Primitives for Underlying Service  . . . . . . . . . .  12
    2     Timer Primitives . . . . . . . . . . . . . . . . . . . . . . .  14
    3     Categories of Protocol Functions . . . . . . . . . . . . . . .  25
    4     Valid PDU Types  . . . . . . . . . . . . . . . . . . . . . . .  28


List of Figures

    1     PDU Header -- Fixed Part . . . . . . . . . . . . . . . . . . .  27
    2      Address Parameters  . . . . . . . . . . . . . . . . . . . . .  29
    3     ESH PDU - Network Address Part   . . . . . . . . . . . . . . .  29
    4     ISH PDU - Network Address Part . . . . . . . . . . . . . . . .  30
    5     RD PDU - Network Address Part  . . . . . . . . . . . . . . . .  30
    6     ESH PDU - Address Part   . . . . . . . . . . . . . . . . . . .  31
    7     All PDUs - Options Part  . . . . . . . . . . . . . . . . . . .  31
    8     Encoding of Option Parameters  . . . . . . . . . . . . . . . .  32
    9     ESH PDU Format . . . . . . . . . . . . . . . . . . . . . . . .  34
    10    ISH PDU Format . . . . . . . . . . . . . . . . . . . . . . . .  35
    11    RD PDU Format when Redirect is to an IS  . . . . . . . . . . .  36
    12    RD PDU Format when Redirect is to an ES  . . . . . . . . . . .  37
































ISO N4053                                                       [Page 4]

RFC 995                                                    December 1986


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.



ISO N4053                                                       [Page 5]

RFC 995                                                    December 1986


      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.

2     Scope and Field of Application

   This International Standard specifies a protocol which is used by
   Network Layer entities operating ISO 8473 in End Systems and Inter-
   mediate Systems (referred to herein as ES and IS respectively) to
   maintain routing information.  The Protocol herein described relies
   upon the provision of a connectionless-mode underlying service.

     This Standard specifies:

     a)  procedures for the transmission of configuration and routing
         information between network entities residing in End Systems
         and network entities residing in Intermediate Systems;

     b)  the encoding of the protocol data units used for the transmission
         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.



ISO N4053                                                       [Page 6]

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






ISO N4053                                                       [Page 7]

RFC 995                                                    December 1986


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

   DIS 8802        Local Area Networks



















































ISO N4053                                                       [Page 8]

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



ISO N4053                                                       [Page 9]

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



ISO N4053                                                      [Page 10]

RFC 995                                                    December 1986


     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



                             Note:
      These service primitives are used to describe the abstract interface
      which exists between the protocol machine and an underlying real
      subnetwork or a Subnetwork Dependent Convergence Function which
      operates over a real subnetwork or real data link to provide the
      required underlying service.

6.4.1  Subnetwork Addresses

   The source and destination addresses specify the points of attachment
   to a public or private subnetwork(s) involved in the transmission
   (known as Subnetwork Points of Attachment, or SNPAs).Subnetwork ad-
   dresses are defined in the Service Definition of each individual sub-
   network.  This protocol is designed to take advantage of subnetworks
   which support broadcast, multicast, or other forms of multi-



ISO N4053                                                      [Page 12]

RFC 995                                                    December 1986


   destination addressing for n-way transmission. It is assumed that the
   SN_Destination_Address parameter may take on one of the following
   multi-destination addresses in addition to a normal single destina-
   tion address:

     All End System Network entities

     All Intermediate System Network entities

   Where a real subnetwork does not inherently support broadcast or oth-
   er forms of transmission to multi-destination addresses, a conver-
   gence function may be used to provide n-way transmission to these
   multi-destination addresses.

   When the SN_Destination_Address on the SN_UNITDATA.Request is a
   multi-destination address, the SN_Destination_Address parameter in
   the corresponding SN_UNITDATA.Indication shall be the same multi-
   destination address.

   The syntax and semantics of subnetwork addresses, except for the pro-
   perties described above, are not defined in this Protocol Standard.

6.4.2  Subnetwork User Data

   The SN_Userdata is an ordered multiple of octets, and is transferred
   transparently between the specified subnetwork points of attachment.

   The underlying service is required to support a service data unit
   size of at least that required to operate the Protocol for Providing
   the Connectionless Network Service (ISO 8473).

6.5   Service Assumed from Local Environment

   A timer service must be provided to allow the protocol entity to
   schedule events.

     There are three primitives associated with the S-TIMER service:

       1.  the S--TIMER Request,
       2.  the S--TIMER Response, and
       3.  the S--TIMER Cancel.

   The S--TIMER Request primitive indicates to the local environment
   that it should initiate a timer of the specified name and subscript
   and maintain it for the duration specified by the time parameter.

   The S--TIMER Response primitive is initiated by the local environment
   to indicate that the delay requested by the corresponding S-TIMER Re-
   quest primitive has elapsed.

   The S--TIMER Cancel primitive is an indication to the local environ-



ISO N4053                                                      [Page 13]

RFC 995                                                    December 1986


   ment that the specified timer(s) should be canceled.If the subscript
   parameter is not specified, then all timers with the specified name
   are canceled; otherwise, the timer of the given name and subscript is
   cancelled. If no timers correspond to the parameters specified, the
   local environment takes no action.

   The parameters of the S--TIMER service primitives are specified in
   Table 2.

          ___________________________________________
         |                          |               |
         | S--TIMER     .Request    | S-Time,       |
         |                          | S-Name,       |
         |                          | S-Subscript   |
         |                          |               |
         |              .Response   | S-Name,       |
         |                          | S-Subscript   |
         |__________________________|_______________|

                  Table 2: Timer Primitives

   The time parameter indicates the time duration of the specified ti-
   mer.  An identifiying label is associated with a timer by means of
   the name parameter.The subscript parameter specifies a value to dis-
   tinguish timers with the same name. The name and subscript taken to-
   gether constitute a unique reference to the timer.

   Timers used in association with a specific protocol funtion are de-
   fined under that protocol function.

                           Note:
     This International Standard does not define specific values for the
     timers.Any derivations described in this Standard are not mandatory.
     Timer values should be chosen so that the requested Quality of
     Service can be provided, given the known characteristics of the
     underlying service.

6.6   Subnetwork Types

   In order to evaluate the applicability of this protocol in particular
   configurations of End Systems, Intermediate Systems and subnetworks,
   three generic types of subnetwork are identified. These are:

     1.  the point-to-point subnetwork,

     2.  the broadcast subnetwork, and

     3.  the general topology subnetwork

    These subnetwork types are discussed in the following clauses.




ISO N4053                                                      [Page 14]

RFC 995                                                    December 1986


6.6.1  Point-to-Point Subnetworks

   A point-to-point subnetwork supports exactly two systems. The two
   systems may be either two End Systems, or an End System and a single
   Intermediate System. A single point-to-point data link connecting two
   Network Entities is an example of a point-to-point subnetwork.


   Configuration Information on a point-to-point Subnetwork.On a point-
   to-point subnetwork the Configuration Information of this protocol
   informs the communicating Network entities of the following:

     1.  Whether the topology consists only of two End Systems, or

     2.  One of the two systems is a Intermediate System.

                           Note:
     On a point-to-point subnetwork, if both systems are Intermediate Systems,
     then this protocol is inapplicable to the situation, since a IS-to-IS
     protocol should be employed instead. However, there is no reason why
     the configuration information could not be employed in a IS-to-IS
     environment to ascertain the topology and initiate operation of a
     IS-to-IS protocol.

   The Intermediate System is informed of the NSAP address(es) supported
   by the Network entity in the End System. This permits reachability
   information and routing metrics concerning these NSAPs to be dissem-
   inated to other Intermediate Systems for the purpose of calculating
   routes to/from this End System.

   Route Redirection Information on a point-to-point Subnetwork.  Route
   Redirection Information is not employed on point-to-point subnetworks
   because there are never any alternate routes.

6.6.2  Broadcast Subnetworks

   A Broadcast subnetwork supports an arbitrary number of End Systems
   and Intermediate Systems, and additionally is capable of transmitting
   a single SNPDU to all or a subset of these systems in response to a
   single SN_UNITDATA.Request.An example of a broadcast subnetwork is a
   LAN (local area network) conforming to DIS8802/2, type 1 operation.


   Configuration Information on a broadcast Subnetwork.On a broadcast
   subnetwork the Configuration Information of this protocol is employed
   to inform the communicating Network entities of the following:

     1.  End Systems are informed of the reachability, Network entity Title,
         and SNPA address(es) of each active Intermediate System on the
         subnetwork.




ISO N4053                                                      [Page 15]

RFC 995                                                    December 1986


     2.  Intermediate Systems are informed of the NSAP addresses supported
         by each End System and the Subnetwork address of the ES. Once the
         Intermediate System obtains this information, reachability
         information and routing metrics concerning these NSAPs may be
         disseminated to other ISs for the purpose of calculating routes
         to/from each ES on the subnetwork.

     3.  In the absence of an available Intermediate System, End Systems may
         query over a broadcast subnetwork to discover whether a particular
         NSAP is reachable on the subnetwork, and if so, what SNPA address
         to use to reach that NSAP.

   Route Redirection Information on broadcast Subnetworks.Route Redirec-
   tion Information may be employed on broadcast subnetworks to permit
   Intermediate Systems to inform End Systems of superior routes to a
   destination NSAP. The superior route might be another IS on the same
   subnetwork as the ES, or it might be the destination ES itself, if it
   is directly reachable on the same subnetwork as the source ES.

6.6.3  General Topology Subnetworks

   A general topology subnetwork supports an arbitrary number of End
   Systems and Intermediate Systems, but does not support a convenient
   multidestination connectionless transmission facility as does a
   broadcast subnetwork.An example of a general topology subnetwork is a
   subnetwork employing X.25 or ISO 8208.

                                 Note:
     The crucial distinguishing characteristic between the broadcast
     subnetwork and the general topology subnetwork is the "cost" of an
     n-way transmission to a potentially large subset of the systems on
     the subnetwork. On a general topology subnetwork, the cost is assumed
     to be close to the cost of sending an individual PDU to each SNPA on
     the subnetwork.  Conversely, on a broadcast subnetwork the cost is
     assumed to be close to the cost of sending a single PDU to one SNPA
     on the subnetwork.  Intermediate situations between these extremes
     are of course possible. In such cases it would be possible to treat the
     subnetwork as either in the broadcast or general topology categories.

   Configuration Information on a general topology Subnetwork.  On a
   general topology subnetwork the Configuration Information is general-
   ly not employed because this protocol can be very costly in the util-
   ization (and charging for) subnetwork resources.


   Route Redirection Information on a general topology Subnetwork.
   Route Redirection Information may be employed on general topology
   subnetworks to permit Intermediate Systems to inform End Systems of
   superior routes to a destination NSAP. The superior route might be
   another IS on the same subnetwork as the ES, or it might be the des-
   tination ES itself, if it is directly reachable on the same subnet-



ISO N4053                                                      [Page 16]

RFC 995                                                    December 1986


   work as the source ES.





















































ISO N4053                                                      [Page 17]

RFC 995                                                    December 1986


                SECTION  TWO. SPECIFICATION OF THE PROTOCOL

7     Protocol Functions

   This section describes the functions performed as part of the Proto-
   col.  Not all of the functions must be performed by every implementa-
   tion.  Clause 7.12 specifies which functions may be omitted and the
   correct behavior where requested functions are not implemented.

7.1     Protocol Timers

   Many of the protocol functions are timer based. This means that they
   are executed upon expiration of a timer rather than upon receipt of a
   PDU or invocation of a service primitive. The two major types of ti-
   mers employed by the protocol are the Configuration Timer (CT) and
   the Holding Timer (HT).

7.1.1    Configuration Timer

   The Configuration Timer is a local timer (i.e. maintained indepen-
   dently by each system) which performs the Report Configuration func-
   tion (see section 7.2).  The timer determines how often a system re-
   ports its availability to the other systems on the same subnetwork.
   The shorter the Configuration Timer, the more quickly other systems
   on the subnetwork will become aware when the reporting system becomes
   available or unavailable. The increased responsiveness must be traded
   off against increased use of resources in the subnetwork and in the
   recipient systems.

7.1.2    Holding Timer

   The Holding Timer applies to both Configuration Information and Route
   Redirection Information. The value of the Holding Timer is set by the
   source of the information and transmitted in the appropriate PDU. The
   recipient of the information is expected to retain the information no
   longer than the Holding Timer. Old Configuration or Route Redirection
   information must be discarded after the Holding Timer expires to en-
   sure the correct operation of the protocol.

   Further discussion of the rationale for these timers and guidelines
   for their use may be found in annex 10.

7.2   Report Configuration Function

   The Report Configuration Function is used by End Systems and Inter-
   mediate Systems to inform each other of their reachability and
   current subnetwork address. This function is invoked every time the
   local Configuration Timer (CT) expires in an ES or IS. It is also in-
   voked upon receipt of a Query Configuration PDU from another End Sys-
   tem.




ISO N4053                                                      [Page 18]

RFC 995                                                    December 1986


   7.2.1  Report Configuration by End Systems

   An End System constructs and transmits one ESH PDU (ESH stands for
   "End System Hello") for each NSAP it serves, and issues one
   SN_UNITDATA.- Request with the ESH PDU as the SNSDU on each subnet-
   work to which it is attached.

                                   Note:
     The necessity to transmit a separate ESH PDU for each NSAP served by
     the Network entity arises from the lack of a formalized relationship
     between Network Entity Titles and NSAP addresses. If this relationship
     could be constrained to require that all NSAP addresses be assigned as
     leaf subdomains of a domain represented by the local Network entity's
     Network entity Title, then a single ESH PDU could be transmitted
     containing the ESs Network entity Title.The Network entity Title
     would then imply which NSAPs might be present at that End system.

   The Holding Timer (HT) field is set to approximately twice the ESs
   Configuration Timer (CT) parameter. This variable is set to a value
   large enough so that even if every other ESH PDU is discarded (due to
   lack of resources), or otherwise lost in the subnetwork, the confi-
   guration information will still be maintained. The value must be set
   small enough so that Intermediate Systems can respond in a timely
   fashion to End Systems becoming available or unavailable.

   The SN_Destination_Address parameter is set to the group address that
   indicates "All Intermediate System Network Entities". This ensures
   that a single transmission on a broadcast subnetwork will reach all
   of the active Intermediate Systems.

                                   Note:
     The actual value of the SN_Destination_Address used to mean "All
     Intermediate System Network Entities" is subnetwork dependent and will
     most likely vary from subnetwork to subnetwork. It would of course be
     desirable that on widely-used subnetwork types (such as those based
     on DIS 8802) that this value and the value of the "All End System
     Network Entities" group address, be standardized.

7.2.2  Report Configuration by Intermediate Systems

   An Intermediate System constructs a single ISH PDU (ISH stands for
   "Intermediate System Hello") containing the ISs Network Entity Title
   and issues one SN_UNITDATA.Request with the ISH PDU as the SNSDU on
   each subnetwork to which it is attached.

   The Holding Timer (HT) field is set to approximately twice the Inter-
   mediate System's Configuration Timer (CT) parameter. This variable is
   set to a value large enough so that even if every other ISH PDU is
   discarded (due to lack of resources), or otherwise lost in the sub-
   network, the configuration information will still be maintained.The
   value must be set small enough so that End Systems will quickly cease



ISO N4053                                                      [Page 19]

RFC 995                                                    December 1986


   to use ISs that have failed, thus preventing "black holes" in the
   Network.

   The SN_Destination_Address parameter is set to the group address that
   indicates "All End System Network Entities".This ensures that a sin-
   gle transmission on a broadcast subnetwork will reach all of the ac-
   tive End Systems.

7.3   Record Configuration Function

   The Record Configuration function receives ESH or ISH PDUs, extracts
   the configuration information, and adds or replaces the corresponding
   configuration information in the local Network entity's Routing In-
   formation base.  If insufficient space is available to store new con-
   figuration information, the PDU is discarded. No Error Report is gen-
   erated.

                                     Note:
     The protocol is described such that End Systems receive and record
     only ISH PDUs and Intermediate Systems receive and process only
     ESH PDUs. If an ES so desires however, it may decide to process ESH
     PDUs as well (on a broadcast network this is easily done by enabling
     the appropriate group address). There is potentially some performance
     improvement to be gained by doing this, at the expense of extra memory,
     and possibly extra processing cycles in the End System.The
     ES, by recording other ESs' Configuration information, may be able
     to route NPDUs directly to ESs on the local subnetwork without first
     being redirected by a Intermediate System.

     Similarly, Intermediate Systems may choose to receive the ISH PDUs
     of other ISs, allowing this protocol to be used as the initialization and
     topology maintenance portion of a full IS-to-IS routing protocol.
     Both of these possibilities are for further study.

7.4   Flush Old Configuration Function

   The Flush Old Configuration Function is executed to remove Configura-
   tion entries in the routing information base whose Holding Timer has
   expired.  When the Holding Time for an ES or IS expires, this func-
   tion removes the corresponding entry from the routing information
   base of the local Network Entity.

7.5   Query Configuration Function

   The Query Configuration Function is performed under the following
   circumstances:

     1. The End System is attached to a broadcast subnetwork,

     2. There is no Intermediate System currently reachable on the
        subnetwork (i.e. no ISH PDUs have been received since the last



ISO N4053                                                      [Page 20]

RFC 995                                                    December 1986


        information was flushed by the Flush Old Configuration Function),

     3. The Network Layer's Route PDU Function needs to obtain the SNPA
        address to which to forward a PDU destined for a certain NSAP, and

     4. The SNPA address cannot be obtained either by a local transformation
        or a local table lookup.

                                     Note:
     Despite appearances, this is actually a quite common case, since it
     is likely that there will be numerous isolated Local Area Networks
     without Intermediate Systems to rely upon for obtaining routing
     information (e.g.via the Request Redirect Function of this protocol).
     Further, if the Intermediate System(s) are temporarily unavailable,
     without this capability communication on the local subnetwork would
     suffer unless manually-entered tables were present in each End System
     or all NSAPs of the subnetwork had the subnetwork SNPA address
     embedded in them.

   The End System, when needing to route an NPDU to a destination NSAP
   whose SNPA is unknown issues an SN_UNITDATA.Request with the NPDU as
   the SN_Userdata.The SN_Destination_Address parameter is set to the
   group address that indicates "All End System Network Entities".

   Subsequently an ESH PDU may be received containing the NSAP address
   along with the corresponding SNPA address (see clause 7.6). In such a
   case the End System executes the Record Configuration function for
   the NSAP, and therefore will be able to route subsequent PDUs to that
   destination using the specified SNPA. If no ESH PDU is received, the
   End System may declare the destination NSAP is not reachable. The
   length of time to wait for a response before indicating a failure or
   the possibility of repeating the process some number of times before
   returning a failure are local matters and are not specified in this
   standard.

7.6   Configuration Response Function

   The Configuration Response function is performed when an End System
   attached to a broadcast subnetwork receives an NPDU addressed to one
   of its NSAPs, with the SN_Destination_Address from the
   SN_UNITDATA.Indication set to the group address "All End System
   Netowrk Entities". This occurs as a result of another ES having per-
   formed the Query Configuration function described in clause 7.5.

   The End System constructs an ESH PDU identical in content to the ESH
   PDU constructed by the Report Configuration function (see clause
   7.2.1) for the NSAP to which the received NPDU was addressed.It then
   transmits the ESH PDU to the source of the original NPDU by issuing
   an SN_UNITDATA.Request with the SN_Destination_Address set to the
   value of the SN_Source_Address received in the SN_UNITDATA.Indication
   with the original NPDU.



ISO N4053                                                      [Page 21]

RFC 995                                                    December 1986


7.7     Request Redirect Function

   The Request Redirect Function is present only in Intermediate Systems
   and is closely coupled with the Routing and Relaying Functions of In-
   termediate Systems. The Request Redirect Function is coupled with the
   "Route PDU Function" described in clause 6.5 of ISO 8473. The Request
   Redirect Function is performed after the Route PDU function has cal-
   culated the next hop of the Data PDU's path.

   When an NPDU is to be forwarded by a Intermediate System, the Request
   Redirect Function first examines the SN_Source_Address associated
   with the SN_UNITDATA.Indication which received the SNSDU (containing
   this NPDU). If the SN_Source_Address is not from an End System on the
   local subnetwork (determined by examining the Configuration informa-
   tion obtained through the Record Configuration Function), then this
   function does no further processing of the NPDU.

   If the NPDU was received directly from an ES the output of the ISs
   Routing and Relaying function for this NPDU is examined. This output
   will contain, among other things, the following pieces of informa-
   tion:

     1. a local identifier for the subnetwork over which to forward the NPDU,
        plus either

     2. the Network entity title and subnetwork address of the IS to which to
        forward the NPDU, or

     3. the subnetwork address of the destination End System.

   The Request Redirect function must now determine if the source ES
   could have sent the NPDU directly to the Network entity the Inter-
   mediate System is about to forward the PDU to. If any of the follow-
   ing conditions hold, the source ESshould be informed of the "better"
   path (by sending an RD PDU to the originating ES):

     1. The next hop is to the destination system, and the destination is
        directly reachable (at subnetwork address BSNPA) on the source ESs
        subnetwork, or

     2. The next hop is to a Intermediate System which is connected to the
        same subnetwork as the ES.

   If the better path exists, the IS first completes normal processing
   of the received NPDU and forwards it.It then constructs a Redirect
   PDU (RD PDU) containing the Destination Address of the original NPDU,
   the subnetwork address of the better next hop (BSNPA), the Network
   Entity Title of the IS to which the ES is being redirected (unless
   the redirect is to the destination ES), a Holding Time (HT), QoS
   Maintenance, Priority, and Security options that were present in the
   Data NPDU (these are simply copied from the Data PDU). The HT is set



ISO N4053                                                      [Page 22]

RFC 995                                                    December 1986


   to the value of the local Redirect Timer (RT). See Annex A for a dis-
   cussion of how to choose the value of RT.  If there are insufficient
   resources to both forward the original NPDU and to generate and send
   an RD PDU, the original NPDU must be given preference.  The Inter-
   mediate System (assuming it has sufficient resources) then sends the
   RD PDU to the source End System using the SN_Source_Address of the
   received NPDU as the SN_Destination_Address for the SN_UNITDATA.-
   Reqeust.

7.8   Record Redirect Function

   The Record Redirect Function is present only in End Systems. This
   function is invoked whenever an RD PDU is received. It extracts the
   redirect information and adds or replaces the corresponding redirec-
   tion information in the local Network entity's Routing Information
   base. The essential information is the redirection mapping from a
   Destination Address to a subnetwork address, along with the Priority,
   Security, and QoS Maintenance options and the Holding Time for which
   this mapping is to be considered valid. If the Redirect was to anoth-
   er Intermediate System, the Network Entity Title of the IS is record-
   ed as well.

                                    Note:
     If insufficient memory is available to store new redirection information,
     the RD PDU may be safely discarded since the original Intermediate
     System will continue to forward PDUs on behalf of this Network entity
     anyway.

7.9   Refresh Redirect Function

   The Refresh Redirect Function is present only in End Systems. This
   function is invoked whenever an NPDU is received by a destination ES.
   It is closely coupled with the function that processes received NPDUs
   at a destination Network Entity.This is the "PDU Decomposition" func-
   tion in ISO 8473.  The purpose of this function is to increase the
   longevity of a redirection without allowing an incorrect route to
   persist indefinitely.  The Source Address (SA), Priority, Security,
   and QoS options are extracted and compared to any Destination Address
   and QoS parameters being maintained in the Routing Information base
   (such information would have been stored by the Record Redirect Func-
   tion). If a corresponding entry is found, the previous hop of the PDU
   is obtained from the SN_Source_Address parameter of the
   SN_Unitdata.Indication primitive by which it was received.  If this
   address matches the next hop address stored with the redirection in-
   formation, the remaining holding time for the redirection is reset to
   the original holding timer that was obtained from the RD PDU.

                                       Note:
     The purpose of this function is to avoid timing out redirection entries
     when the Network entity is receiving return traffic from the destination
     via the same path over which it is currently sending traffic.This is



ISO N4053                                                      [Page 23]

RFC 995                                                    December 1986


     particularly useful when the destination system is on the same subnetwork
     as the source, since after one redirect no IS need be involved in
     the ES-to-ES traffic.

     This function must operate in a very conservative fashion however,
     to prevent the formation of black holes. The remaining holding time
     should be refreshed only under the exact conditions specified above.
     For a discussion of the issues surrounding the refresh of redirection
     information, see Annex 10.

7.10   Flush Old Redirect Function

   The Flush Old Redirect Function is executed to remove Configuration
   entries in the routing information base whose Holding Timer has ex-
   pired.  When the Holding Time for an ES or IS expires, this function
   removes the corresponding entry from the routing information base of
   the local Network Entity.

7.11   PDU Header Error Detection

   The PDU Header Error Detection function protects against failure of
   Intermediate or End System Network entities due to the processing of
   erroneous information in the PDU header.The function is realized by a
   checksum computed on the entire PDU header. The checksum is verified
   at each point at which the PDU is processed. If the checksum calcula-
   tion fails, the PDU must be discarded.

   The use of the Header Error Detection function is optional and is
   selected by the originating Network Entity. If the function is not
   used, the checksum field of the PDU header is set to zero.

   If the function is selected by the originating Network Entity, the
   value of the checksum field causes the following formulf to be satis-
   fied:

        (The Sum from i=1 to L of a(i)) (mod   255) = 0

        (The Sum from i=1 to L of  (L - i + 1) * a(i))  (mod   255) = 0


   where L = the number of octets in the PDU header, and a(i) = the value of
   the octet at position i. The first octet in the PDU header is considered to
   occupy position i = 0.

   When the function is in use, neither octet of the checksum field may be
   set to zero.








ISO N4053                                                      [Page 24]

RFC 995                                                    December 1986


7.12   Classification of Functions

   Implementations do not have to support all of the functions described
   in clause 7. Functions are divided into four categories:

   Type A:   These functions must be supported in all cases.

   Type B:   These functions must be supported by Systems which implement
             the Configuration Information.

   Type C:   These functions must be supported by Systems which implement
             the Redirect Information.

   Type D:   These functions are optional.

   If a PDU is received which invokes an optional function that is not
   implemented, that PDU is discarded.

   Table 3 shows how the functions are divided into these four
   categories, and to which type of system (ES, IS, or both) they apply.

    ______________________________________________________________
    | Function                      |   Category |   System Type |
    |_______________________________|____________|_______________|
    | Report Configuration          |      B     |      ES,IS    |
    | Record Configuration          |      B     |      ES,IS    |
    | Configuration Response        |      A     |       ES      |
    | Flush Old Configuration       |      B     |      ES,IS    |
    | Request Redirect              |      C     |       IS      |
    | Query Configuration           |      B     |       ES      |
    | Record Redirect               |      C     |       ES      |
    | Refresh Redirect              |      D     |       ES      |
    | Flush Old Redirect            |      C     |       ES      |
    | PDU Header Error Detection    |      A     |      ES,IS    |
    |_______________________________|____________|_______________|

            Table 3: Categories of Protocol Functions

8   Structure and Encoding of PDUs

                              Note:
     The encoding of the PDUs for this protocol is compatible with that
     used in ISO 8473.

                       Temporary Note:
     The method employed for describing the encoding of PDUs is provisional.
     Member bodies are requested to comment on whether another
     method (such as ASN.1 with an appropriate concrete syntax) would
     be preferable.





ISO N4053                                                      [Page 25]

RFC 995                                                    December 1986


8.1   Structure

   All Protocol Data Units shall contain an integral number of
   octets.The octets in a PDU are numbered starting from one (1) and in-
   creasing in the order in which they are put into an SNSDU. The bits
   in an octet are numbered from one (1) to eight (8), where bit one (1)
   is the low-order bit.  When consecutive octets are used to represent
   a binary number, the lower octet number has the most significant
   value.

   Any subnetwork supporting this protocol is required to state in its
   specification the way octets are transferred, using the terms "most
   significant bit" and "least significant bit". The PDUs of this proto-
   col are defined using the terms "most significant bit" and "least
   significant bit".

                             Note:
     When the encoding of a PDU is represented using a diagram in this
     section, the following representation is used:

     a) octets are shown with the lowest numbered octet to the left,
        higher number octets being further to the right;
     b) within an octet, bits are shown with bit eight (8) to the left and
        bit one (1) to the right.

    PDUs shall contain, in the following order:

     1.  the fixed part;

     2.  the Network address part;

     3.  the Subnetwork address part, if present; and

     4.  the Options part, if present.

8.2   Fixed Part

8.2.1  General

   The fixed part contains frequently occurring parameters including the
   type code (ESH, ISH, or RD) of the protocol data unit.The length and
   the structure of the fixed part are defined by the PDU code.












ISO N4053                                                      [Page 26]

RFC 995                                                    December 1986


   The fixed part has the following format:

                                               Octet
      ________________________________________
      |    Network Layer Protocol Identifier |    1
      |______________________________________|
      |           Length Indicator           |    2
      |______________________________________|
      |      Version/Protocol Id Extension   |    3
      |______________________________________|
      |        reserved (must be zero)       |    4
      |______________________________________|
      | 0 |0 |0 |           Type             |    5
      |___|__|__|____________________________|
      |           Holding Time               |   6,7
      |______________________________________|
      |             Checksum                 |   8,9
      |______________________________________|

         Figure 1: PDU Header -- Fixed Part


8.2.2  Network Layer Protocol Identifier

   The value of this field shall be 1000 0010.

                     Temporary Note:
     The value 1000 0010 is provisional, pending resolution of the NLPID
     issue in SC6.

   This field identifies this Network Layer Protocol as ISO SC6/N4053,
   End System to Intermediate System Routing Exchange Protocol for use in
   conjunction with ISO 8473.

8.2.3  Length Indicator

   The length is indicated by a binary number, with a maximum value of
   254 (1111 1110).The length indicated is the length of the entire PDU
   (which consists entirely of header, since this protocol does not car-
   ry user data) in octets, as described in clause 8.1. The value 255
   (1111 1111) is reserved for possible future extensions.

8.2.4  Version/Protocol Identifier Extension

   The value of this field is binary 0000 0001. This identifies a stan-
   dard version of ISO xxxx, End System to Intermediate System Routing
   Exchange Protocol for use in conjunction with ISO 8473.







ISO N4053                                                      [Page 27]

RFC 995                                                    December 1986


8.2.5  Type Code

   The Type code field identifies the type of the protocol data unit.
   Allowed values are given in table 4.

   _____________________________________________________
   |            | Bits               5   4   3   2   1 |
   |____________|______________________________________|
   |____________|______________________________________|
   |ESH PDU     |                    0   0   0   1   0 |
   |____________|______________________________________|
   |ISH PDU     |                    0   0   1   0   0 |
   |____________|______________________________________|
   |RD PDU      |                    0   0   1   1   0 |
   |____________|______________________________________|

                   Table 4: Valid PDU Types

   All other PDU type values are reserved.

8.2.6  Holding Time

   The Holding Time field specifies for how long the receiving Network
   entity should retain the configuration/routing information contained
   in this PDU.  The receiving Network entity should discard any infor-
   mation obtained from this PDU from its internal state when the hold-
   ing time expires.  The Holding time field is encoded as an integral
   number of micro-fortnights.

8.2.7  PDU Checksum

   The checksum is computed on the entire PDU header. A checksum value
   of zero is reserved to indicate that the checksum is to be ignored.
   The operation of the PDU Header Error Detection function (Clause
   7.11) ensures that the value zero does not represent a valid check-
   sum. A non-zero value indicates that the checksum must be processed.
   If the checksum calculation fails, the PDU must be discarded.

8.3   Network Address Part

8.3.1  General

   Address parameters are distinguished by their location. The different
   PDU types carry different address parameters however.The ESH PDU car-
   ries a Source NSAP address (SA); the ISH PDU carries a Intermediate
   System Network entity Title (NET); and the RD PDU carries a Destina-
   tion NSAP address (DA), and possibly a Network Entity Title (NET).

8.3.2  NPAI (Network Protocol Address Information) Encoding

   The Destination and Source Addresses are Network Service Access Point



ISO N4053                                                      [Page 28]

RFC 995                                                    December 1986


   addresses as defined in ISO 8348/AD2, Addendum to the Network Service
   Definition Covering Network Layer addressing.The Network Entity Title
   address parameter is defined in clause 4.5. The Destination Address,
   Source Address, and Network Entity Title are encoded as NPAI using
   the binary syntax defined in clause 8.3.1 of ISO 8348/AD2.

   The address information is of variable length. Each address parameter
   is encoded as follows:

         _______________________________________________
         | Octet  | Address parameter Length Indicator |
         |   n    |                (e.g., 'm')         |
         |________|____________________________________|
         | Octets |                                    |
         | n + 1  |       Address Parameter Value      |
         |  thru  |                                    |
         | n + m  |                                    |
         |________|____________________________________|

                       Figure 2:  Address Parameters
8.3.3  Source Address Parameter for ESH PDU

   The Source Address is the NSAP address of an NSAP served by the Net-
   work entity sending the ESH PDU. It is encoded in the ESH PDU as fol-
   lows:


                                                   Octet
         ________________________________________
         |Source Address Length Indicator (SAL) |   10
         |______________________________________|
         |                                      |   11
         :           Source Address (SA)        :
         |                                      |  m - 1
         |______________________________________|

          Figure 3: ESH PDU - Network Address Part

8.3.4  Network Entity Title Parameter for ISH PDU

   The Network entity Title parameter is the Network Entity Title of the
   Intermediate System sending the ISH PDU. It is encoded in the ISH PDU
   as follows:











ISO N4053                                                      [Page 29]

RFC 995                                                    December 1986


                                                   Octet
   _______________________________________________
   |Network Entity Title Length Indicator (NETL) |  10
   |_____________________________________________|
   |                                             |  11
   :           Network Entity Title (NET)        :
   |                                             | m - 1
   |_____________________________________________|

      Figure 4: ISH PDU - Network Address Part

8.3.5  Destination Address Parameter for RD PDU

   The Destination Address is the NSAP address of a destination associ-
   ated with some NPDU being forwarded by the Intermediate System send-
   ing the RD PDU. It is encoded in the RD PDU as follows:

                                                  Octet
    _____________________________________________
    |Destination Address Length Indicator (DAL) |  10
    |___________________________________________|
    |                                           |  11
    :           Destination Address (DA)        :
    |                                           | m - 1
    |___________________________________________|

       Figure 5: RD PDU - Network Address Part


8.4   Subnetwork Address Part

   The Subnetwork Address Part is present only in RD PDUs.It is used to
   indicate the subnetwork address of another Network entity on the same
   subnetwork as the End System (and Intermediate System) which may be a
   better path to the destination specified in the Network Address Part.
   The Subnetwork Address parameter is encoded in the same manner as the
   Network Address parameters.

















ISO N4053                                                      [Page 30]

RFC 995                                                    December 1986


8.4.1    Subnetwork Address Parameter for RD PDU

    The Subnetwork Address Parameter is encoded in the RD PDU as fol-
   lows:

                                                   Octet
    _______________________________________________
    |Subnetwork Address Length Indicator (BSNPAL) |   m
    |_____________________________________________|
    |                                             | m + 1
    :         Subnetwork Address (BSNPA)          :
    |                                             | n - 1
    |_____________________________________________|

           Figure 6: ESH PDU - Address Part

 8.5     Options Part

 8.5.1    General

    The options part is used to convey optional parameters. The options
   part
    of the PDU header is illustrated below:

                                                        Octet
    ___________________________________________________
    |                                                  |   p
    :                    Options                       :
    |                                                  |   q
    |__________________________________________________|

               Figure 7: All PDUs - Options Part

   If the options part is present, it may contain one or more parame-
   ters.  The number of parameters that may be contained in the options
   part is constrained by the length of the options part, which is
   determined by the following formula:

      PDU Header Length - (length of fixed part + length of address
      part + length of segmentation part),

   and by the length of the individual optional parameters.

   Parameters defined in the options part may appear in any order.  Du-
   plication of options is not permitted.Receipt of a PDU with an option
   duplicated must be treated as a protocol error.








ISO N4053                                                      [Page 31]

RFC 995                                                    December 1986


   The encoding of parameters contained within the options part of the
   PDU header is illustrated below in figure 8.

          Octets
          _________________________________
          |     n      |  Parameter Code  |
          |____________|__________________|
          |   n + 1    | Parameter Length |
          |____________|__________________|
          |   n + 2    |                  |
          |     to     | Parameter Value  |
          | n + m + 1  |                  |
          |____________|__________________|

      Figure 8:  Encoding of Option Parameters

   The parameter code field is coded in binary and, without extensions,
   provides a maximum of 255 different parameters. No parameter codes
   use bits 8 and 7 with the value 00, so the actual maximum number of
   parameters is lower. A parameter code of 255 (binary 1111 1111) is
   reserved for possible future extensions.

   The parameter length field indicates the length, in octets, of the
   parameter value field.The length is indicated by a positive binary
   number, m, with a theoretical maximum value of 254. the practical
   maximum value of m is lower. For example, in the case of a single
   parameter contained within the options part, two octets are required
   for the parameter code and the parameter length indicators. Thus, the
   value of m is limited to:

       m = 252-(length of fixed part +length of address part
       +length of segmentation part   )

   For each succeeding parameter the maximum value of m decreases.  The
   parameter value field contains the value of the parameter identified
   in the parameter code field.

   The following parameters are permitted in the options part.

8.5.2    Security

   The Security parameter conveys information about the security re-
   quested in the Data PDU that caused the containing RD PDU to be gen-
   erated.  This parameter has the same encoding and semantics as the
   Security parameter in ISO 8473.

       Parameter Code:        1100 0101

       Parameter Length:        variable

       Parameter Value:        See Section 7.5.3 of ISO 8473



ISO N4053                                                      [Page 32]

RFC 995                                                    December 1986


8.5.3    Quality of Service Maintenance

   The Quality of Service parameter conveys information about the quali-
   ty of service requested in the Data PDU that caused the containing RD
   PDU to be generated.

   This parameter has the same encoding and semantics as the QoS Mainte-
   nance parameter in ISO 8473.

      Parameter Code:        1100 0011

      Parameter Length:        variable

      Parameter Value:        See Section 7.5.6 of ISO 8473

8.5.4    Priority

   The Priority parameter conveys information about the priority re-
   quested in the Data PDU that caused the containing RD PDU to be gen-
   erated.

   This parameter has the same encoding and semantics as the Priority
   parameter in ISO 8473.

      Parameter Code:        1100 1101

      Parameter Length:        one octet

      Parameter Value:        See Section 7.5.7 of ISO 8473

























ISO N4053                                                      [Page 33]

RFC 995                                                    December 1986


8.6   End System Hello (ESH) PDU

8.6.1  Structure

   The ESH PDU has the following format:

                                                    Octet
       ____________________________________________
       |    Network Layer Protocol Identifier     |   1
       |__________________________________________|
       |             Length Indicator             |   2
       |__________________________________________|
       |       Version/Protocol Id Extension      |   3
       |__________________________________________|
       |         reserved (must be zero)          |   4
       |__________________________________________|
       |0 |0 |0 |              Type               |   5
       |__|__|__|_________________________________|
       |               Holding Time               |  6,7
       |__________________________________________|
       |                 Checksum                 |  8,9
       |__________________________________________|
       |    Source Address Length Indicator (SAL) |  10
       |__________________________________________|
       |                                          |  11
       :           Source Address (SA)            :
       |                                          | m - 1
       |__________________________________________|
       |                                          |   m
       :                  Options                 :
       |                                          | p - 1
       |__________________________________________|

                 Figure 9: ESH PDU Format




















ISO N4053                                                      [Page 34]

RFC 995                                                    December 1986


8.7   Intermediate System Hello (ISH) PDU

8.7.1  Structure

   The ISH PDU has the following format:


                                                   Octet
   _______________________________________________
   |        Network Layer Protocol Identifier    |   1
   |_____________________________________________|
   |                 Length Indicator            |   2
   |_____________________________________________|
   |          Version/Protocol Id Extension      |   3
   |_____________________________________________|
   |             reserved (must be zero)         |   4
   |_____________________________________________|
   |0 |0 |0 |                  Type              |   5
   |__|__|__|____________________________________|
   |                   Holding Time              |  6,7
   |_____________________________________________|
   |                     Checksum                |  8,9
   |_____________________________________________|
   |Network Entity Title Length Indicator (NETL) |  10
   |_____________________________________________|
   |                                             |  11
   :           Network Entity Title (NET)        :
   |                                             | m - 1
   |_____________________________________________|
   |                                             |  m
   :                      Options                :
   |                                             | p - 1
   |_____________________________________________|

              Figure 10: ISH PDU Format



















ISO N4053                                                      [Page 35]

RFC 995                                                    December 1986


8.8   Redirect (RD) PDU

8.8.1  Structure

   The RD PDU has the following format:

                                                    Octet
   ______________________________________________
   |         Network Layer Protocol Identifier   |    1
   |_____________________________________________|
   |                  Length Indicator           |    2
   |_____________________________________________|
   |           Version/Protocol Id Extension     |    3
   |_____________________________________________|
   |               reserved (must be zero)       |    4
   |_____________________________________________|
   |0 |0 |0 |                   Type             |    5
   |__|__|__|____________________________________|
   |                    Holding Time             |   6,7
   |_____________________________________________|
   |                      Checksum               |   8,9
   |_____________________________________________|
   |   Destination Address Length Indicator (DAL)|   10
   |_____________________________________________|
   |                                             |   11
   :              Destination Address (DA)       :
   |                                             | m - 1
   |_____________________________________________|
   |Subnetwork Address Length Indicator (BSNPAL) |   m
   |_____________________________________________|
   |                                             | m + 1
   :          Subnetwork Address (DBSNPA)        :
   |                                             | n - 1
   |_____________________________________________|
   |Network Entity Title Length Indicator (NETL) |   n
   |_____________________________________________|
   |                                             | n + 1
   :            Network Entity Title (NET)       :
   |                                             | p - 1
   |_____________________________________________|
   |                                             |   p
   :                        Options              :
   |                                             | q - 1
   |_____________________________________________|

   Figure 11: RD PDU Format when Redirect is to an IS








ISO N4053                                                      [Page 36]

RFC 995                                                    December 1986


                                                    Octet
   ______________________________________________
   |         Network Layer Protocol Identifier   |    1
   |_____________________________________________|
   |                  Length Indicator           |    2
   |_____________________________________________|
   |           Version/Protocol Id Extension     |    3
   |_____________________________________________|
   |               reserved (must be zero)       |    4
   |_____________________________________________|
   |0 |0 |0 |                   Type             |    5
   |__|__|__|____________________________________|
   |                    Holding Time             |   6,7
   |_____________________________________________|
   |                      Checksum               |   8,9
   |_____________________________________________|
   |   Destination Address Length Indicator (DAL)|   10
   |_____________________________________________|
   |                                             |   11
   :              Destination Address (DA)       :
   |                                             | m - 1
   |_____________________________________________|
   |Subnetwork Address Length Indicator (BSNPAL) |   m
   |_____________________________________________|
   |                                             | m + 1
   :          Subnetwork Address (DBSNPA)        :
   |                                             | n - 1
   |_____________________________________________|
   |                   NETL = 0                  |   n
   |_____________________________________________|
   |                                             | n + 1
   :                   Options                   :
   |                                             | p - 1
   |_____________________________________________|
   |              Quality of Service             | n + 1
   |_____________________________________________|

   Figure 12: RD PDU Format when Redirect is to an ES


9   Formal Description

   {Maybe next pass...}

10    Conformance

   See Clause 6.2.







ISO N4053                                                      [Page 37]

RFC 995                                                    December 1986


                  ANNEX A.  SUPPORTING TECHNICAL MATERIAL


A.1   Use of Timers

   This protocol makes extensive use of timers to ensure the timeliness
   and accuracy of information disseminated using the Configuration and
   Route Redirection functions.This section discusses the rationale for
   using these timers and provides some background for how they operate.

   Systems using this protocol learn about other systems exclusively by
   receiving PDUs sent by those systems. In a connectionless environ-
   ment, a system must periodically receive updated information to en-
   sure that the information it previously received is still correct.
   For example, if a system on a subnetwork becomes unavailable (either
   it has ceased operating, or its SNPA becomes inoperative) the only
   way another system can detect this fact is by the absence of
   transmissions from that system. If information were retained in the
   absence of new PDUs being received, configuration and/or routing in-
   formation would inevitably become incorrect. The Holding Timers
   specified by this protocol guarantee that old information will not be
   retained indefinitely.

   A useful way of thinking of the configuration and route redirection
   information is as a cache maintained by each system. The cache is
   periodically flushed to ensure that only up-to-date information is
   stored.Unlike most caches, however, the time to retain information is
   not a purely local matter.  Rather, information is held for a period
   of time specified by the source of the information. Some examples
   will help clarify this operation.

A.1.1   Example of Holding Time for Route Redirection

   Route Redirection Information is obtained by an End System through
   the Request Redirect function (see clause 7.7).It is quite possible
   that a Intermediate System might redirect an End System to another IS
   which has recently become unavailable (this might happen if the IS-
   to-IS routing algorithm is still converging following a configuration
   change). If the Holding Timer were not present, or was set very long
   by the sending IS, an End System would have been redirected into a
   Black Hole from which none of its Data PDUs would ever emerge. The
   length of the Holding Timer on Redirects specifies, in essence, the
   length of time black holes are permitted to exist.

   On the other hand, setting the Holding Timer on Route Redirects very
   short to minimize the effect of black holes has other undesirable
   consequences.First, for each PDU that causes a redirect, an addition-
   al PDU beside the original Data PDU must be composed and transmitted;
   this increases overhead. Second, each time a "working" redirect's
   Holding Timer expires, the redirected End System will revert to a
   poorer route for at least one PDU.



ISO N4053                                                      [Page 38]

RFC 995                                                    December 1986


A.1.2   Example of Holding Timer for Configuration Information

   A similar type of problem can occur with respect to Configuration in-
   formation. If the Holding Time of a ISH PDU (see clause 7.2.2) is set
   very long, and the only Intermediate System (which has been sending
   this Configuration Information) on the subnetwork becomes unavail-
   able, a subnetwork-wide black hole can form. During this time, End
   Systems on the subnetwork may not be able to communicate with each
   other because they presume that a Intermediate System is operating
   which will forward their Data PDUs to destination ESs on the local
   subnetwork and return RD PDUs.Once the Holding Time expires, the ESs
   will realize that no IS is available and will take their only
   recourse, which is to send their traffic directly on the local sub-
   network.

   Given the types of problems that can occur, it is important that
   responsibility for incorrect information can be unambiguously as-
   signed to the source of the information. For this reason all Holding
   Timers are calculated by the source of the Configuration or Route
   Redirection information and communicated explicitly to each recipient
   in the appropriate PDU.

A.2   Refresh and timeout of Redirection information

   The protocol allows End Systems to refresh redirection information
   without first allowing the holding time to expire and being redirect-
   ed by a Intermediate System for a second (or subsequent) time. Such
   schemes are prevalent in connectionless subnetworks and are often
   called "reverse path information", "previous hop cache" or something
   similar.

   Refreshing the redirection information has obvious performance bene-
   fits, but can be dangerous if not handled in a very conservative
   fashion. In order for a redirection to be safely refreshed, all of
   the following conditions must hold:

     1. The source address of the received PDU must be exactly the same
        as the destination address specified in a prior RD PDU (this
        defines a "match" on the redirection information). Making
        assumptions about the equivalence of abbreviated addresses,
        group addresses, or similar "special" addresses is dangerous
        since routing for these addresses cannot be assumed to be
        the same.

     2. The Quality of Service parameters of the received PDU must be
        exactly the same as the QoS parameters specified in the matching
        (by destination address) redirection entry.Again, there is no
        guarantee that PDUs with different QoS parameters will be routed
        the same way. It is quite possible that the redirected path is
        even a black hole for certain values of the QoS parameters (the
        security field is a good example).



ISO N4053                                                      [Page 39]

RFC 995                                                    December 1986


     3. The "previous hop" of the received Data PDU must match the "next
        hop" stored in the redirection information. Specifically, the
        SN_Source_Address of the SN_UNITDATA.Indication which received the
        PDU must match exactly the SN_Destination_Address specified in the
        redirect to be used for sending traffic via the SN_UNITDATA.Request
        primitive. This comparison ensures that redirects are refreshed only
        when the reverse traffic is being received from the same IS (or
        destination ES) as the forward traffic is being sent through (or
        to). This check make certain that redirects are not refreshed for
        just on the basis of traffic being received from the destination.
        It is quite possible that the traffic is simply indicating that the
        forward path in use is not working!

   Note that these conditions still allow refresh in the most useful and
   common cases where either the destination is another ES on the same
   subnetwork as the source ES, or the redirection is to a IS which is
   passing traffic to/from the destination in both directions (i.e. the
   path is symmetric).

A.3   System Initialization Considerations

   This protocol is designed to make the exchange of information as free
   as possible from dependencies between the two types of systems.
   therefore, it is not possible for an End System to request all Inter-
   mediate Systems on a subnetwork to report their configuration, nor is
   it possible for an Intermediate System to request all End Systems on
   a subnetwork to report their configuration.

   In certain operating environments a constraint may be imposed than an
   ES, upon becoming operational, must discover the existence of an IS
   as soon as possible.The converse relationship also holds if it is
   necessary for an IS to discover the existence of End Systems as soon
   as possible. In both cases the availability of this information is
   normally determined by the Configuration Timer of the system for
   which the knowledge is desired. there is therefore a tradeoff between
   the overhead associated with performing the Report and Record Confi-
   guration functions and the timely availability of the configuration
   information. Decreasing the Configuration Timer increases the availa-
   bility at the expense of an increase in overhead.

   The following solution is recommended for addressing the constraint
   described above. When the Record Configuration function is invoked in
   either an End System or an Intermediate System, the function will
   determine if the received configuration information was previously
   unknown.If this is the case, then the Report Configuration function
   may be invoked before the expiration of the system's Configuration
   Timer. The Hello PDU generated by the Report Configuration function
   is then sent only to the Network Entity whose configuration was pre-
   viously unknown. Thus when an ES or IS first becomes operational it
   immediately reports its configuration. As soon as systems of the oth-
   er type discover the new network entity, they will make their own



ISO N4053                                                      [Page 40]

RFC 995                                                    December 1986


   configuration known to this entity.

   The additional overhead incurred by this solution is minimal. Also,
   since the discovery of new configurations is made timely by this ap-
   proach the Configuration Timer period can be increased in order to
   decrease the overhead of the configuration functions, provided that
   other factors not discussed here are accounted for by the longer time
   period.One caveat is that the first Hello PDU generated by a system
   may be lost during transmission. To solve this problem one or more
   additional PDUs may be transmitted at short time intervals during
   this initialization period.

   Note that this solution may be implemented in ISs only, in ESs only,
   or in both Intermediate and End Systems.This decision is purely a lo-
   cal matter and may be alterable through System Management.

A.4     Optimizations for Flushing Redirects

   An ES will attempt to forward NPDUs through an IS to which it has
   been redirected until the Holding Timer specified in the RD PDU has
   expired, even if that IS is no longer reachable.  Under certain cir-
   cumstances, it is possible to do better and recognize the existence
   of a black hole sooner. In particular, if the ES expects to hear ISH
   PDUs from the IS to which it has been redirected, and the Holding Ti-
   mer for that IS expires, all knowledge of the IS may be forgotten by
   the ES. This includes any redirects, which may be flushed (see the
   Flush Old Redirect function) even though their timeouts have not ex-
   pired.


























ISO N4053                                                      [Page 41]

