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rfc:rfc994

Network Working Group ANSI X3S3.3 86-80 Request for Comments: 994 ISO TC97/SC6/N 3998

                                                             March 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: Final Text of DIS 8473, Protocol for Providing the Connectionless-

      mode Network Service

Source: DIS 8473 Editor

ISO 8473 [Page 1] RFC 994 December 1986

Contents

1 Scope and Field of Application 6

2 References 7

SECTION ONE. GENERAL 9

3 Definitions 9

  3.1   Reference Model Definitions  . . . . . . . . . . . . . . . . .   9
  3.2   Service Conventions Definitions  . . . . . . . . . . . . . . .   9
  3.3   Network Layer Architecture Definitions . . . . . . . . . . . .   9
  3.4   Network Layer Addressing Definitions . . . . . . . . . . . . .  10
  3.5   Additional Definitions . . . . . . . . . . . . . . . . . . . .  10

4 Symbols and Abbreviations 11

  4.1   Data Units   . . . . . . . . . . . . . . . . . . . . . . . . .  11
  4.2   Protocol Data Units  . . . . . . . . . . . . . . . . . . . . .  11
  4.3   Protocol Data Unit Fields  . . . . . . . . . . . . . . . . . .  11
  4.4   Parameters . . . . . . . . . . . . . . . . . . . . . . . . . .  11
  4.5   Miscellaneous  . . . . . . . . . . . . . . . . . . . . . . . .  11

5 Overview of the Protocol 12

  5.1   Internal Organization of the Network Layer . . . . . . . . . .  12
  5.2   Subsets of the Protocol  . . . . . . . . . . . . . . . . . . .  12
  5.3   Addresses and Titles . . . . . . . . . . . . . . . . . . . . .  13
        5.3.1   Addresses  . . . . . . . . . . . . . . . . . . . . . .  13
        5.3.2   Network-entity Titles  . . . . . . . . . . . . . . . .  13
  5.4   Service Provided by the Network Layer  . . . . . . . . . . . .  14
  5.5   Underlying Service Assumed by the Protocol . . . . . . . . . .  14
        5.5.1   Subnetwork Points of Attachment  . . . . . . . . . . .  15
        5.5.2   Subnetwork Quality of Service  . . . . . . . . . . . .  15
        5.5.3   Subnetwork User Data   . . . . . . . . . . . . . . . .  16
        5.5.4   Subnetwork Dependent Convergence Functions . . . . . .  16
  5.6   Service Assumed from Local Environment . . . . . . . . . . . .  16

SECTION TWO. SPECIFICATION OF THE PROTOCOL 18

6 Protocol Functions 18

  6.1   PDU Composition Function . . . . . . . . . . . . . . . . . . .  18
  6.2   PDU Decomposition Function . . . . . . . . . . . . . . . . . .  19
  6.3   Header Format Analysis Function  . . . . . . . . . . . . . . .  19

ISO 8473 [Page 2] RFC 994 December 1986

  6.4   PDU Lifetime Control Function  . . . . . . . . . . . . . . . .  20
  6.5   Route PDU Function . . . . . . . . . . . . . . . . . . . . . .  20
  6.6   Forward PDU Function . . . . . . . . . . . . . . . . . . . . .  21
  6.7   Segmentation Function  . . . . . . . . . . . . . . . . . . . .  21
  6.8   Reassembly Function  . . . . . . . . . . . . . . . . . . . . .  22
  6.9   Discard PDU Function . . . . . . . . . . . . . . . . . . . . .  23
  6.10  Error Reporting Function . . . . . . . . . . . . . . . . . . .  24
        6.10.1  Overview . . . . . . . . . . . . . . . . . . . . . . .  24
        6.10.2  Requirements . . . . . . . . . . . . . . . . . . . . .  25
        6.10.3  Processing of Error Reports  . . . . . . . . . . . . .  25
        6.10.4  Relationship of Data PDU Options to Error Reports  . .  26
  6.11  PDU Header Error Detection . . . . . . . . . . . . . . . . . .  27
  6.12  Padding Function . . . . . . . . . . . . . . . . . . . . . . .  28
  6.13  Security . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
  6.14  Source Routing Function  . . . . . . . . . . . . . . . . . . .  28
  6.15  Record Route Function  . . . . . . . . . . . . . . . . . . . .  29
  6.16  Quality of Service Maintenance Function  . . . . . . . . . . .  30
  6.17  Priority Function  . . . . . . . . . . . . . . . . . . . . . .  31
  6.18  Congestion Notification Function . . . . . . . . . . . . . . .  31
  6.19  Classification of Functions  . . . . . . . . . . . . . . . . .  31

7 Structure and Encoding of PDUs 33

  7.1   Structure  . . . . . . . . . . . . . . . . . . . . . . . . . .  33
  7.2   Fixed Part . . . . . . . . . . . . . . . . . . . . . . . . . .  34
        7.2.1   General  . . . . . . . . . . . . . . . . . . . . . . .  34
        7.2.2   Network Layer Protocol Identifier  . . . . . . . . . .  34
        7.2.3   Length Indicator   . . . . . . . . . . . . . . . . . .  35
        7.2.4   Version/Protocol Identifier Extension  . . . . . . . .  35
        7.2.5   PDU Lifetime   . . . . . . . . . . . . . . . . . . . .  35
        7.2.6   Flags  . . . . . . . . . . . . . . . . . . . . . . . .  35
                7.2.6.1   Segmentation Permitted . . . . . . . . . . .  35
                7.2.6.2   More Segments  . . . . . . . . . . . . . . .  35
                7.2.6.3   Error Report   . . . . . . . . . . . . . . .  36
        7.2.7   Type Code  . . . . . . . . . . . . . . . . . . . . . .  36
        7.2.8   PDU Segment Length   . . . . . . . . . . . . . . . . .  36
        7.2.9   PDU Checksum   . . . . . . . . . . . . . . . . . . . .  36
  7.3   Address Part   . . . . . . . . . . . . . . . . . . . . . . . .  37
        7.3.1   General  . . . . . . . . . . . . . . . . . . . . . . .  37
                7.3.1.1   Destination and Source Addresses . . . . . .  37
  7.4   Segmentation Part  . . . . . . . . . . . . . . . . . . . . . .  38
        7.4.1   Data Unit Identifier . . . . . . . . . . . . . . . . .  38
        7.4.2   Segment Offset . . . . . . . . . . . . . . . . . . . .  38
        7.4.3   PDU Total Length . . . . . . . . . . . . . . . . . . .  39
  7.5   Options Part   . . . . . . . . . . . . . . . . . . . . . . . .  39
        7.5.1   General  . . . . . . . . . . . . . . . . . . . . . . .  39
        7.5.2   Padding  . . . . . . . . . . . . . . . . . . . . . . .  40
        7.5.3   Security . . . . . . . . . . . . . . . . . . . . . . .  40
                7.5.3.1   Source Address Specific  . . . . . . . . . .  41
                7.5.3.2   Destination Address Specific . . . . . . . .  41
                7.5.3.3   Globally Unique Security . . . . . . . . . .  41
        7.5.4   Source Routing   . . . . . . . . . . . . . . . . . . .  41

ISO 8473 [Page 3] RFC 994 December 1986

        7.5.5   Recording of Route . . . . . . . . . . . . . . . . . .  42
        7.5.6   Quality of Service Maintenance . . . . . . . . . . . .  43
                7.5.6.1   Source Address Specific  . . . . . . . . . .  43
                7.5.6.2   Destination Address Specific . . . . . . . .  43
                7.5.6.3   Globally Unique QoS  . . . . . . . . . . . .  43
        7.5.7   Priority   . . . . . . . . . . . . . . . . . . . . . .  44
  7.6   Data Part  . . . . . . . . . . . . . . . . . . . . . . . . . .  45
  7.7   Data (DT) PDU  . . . . . . . . . . . . . . . . . . . . . . . .  46
        7.7.1   Structure  . . . . . . . . . . . . . . . . . . . . . .  46
        7.7.1.1   Fixed Part . . . . . . . . . . . . . . . . . . . . .  47
        7.7.1.2   Addresses  . . . . . . . . . . . . . . . . . . . . .  47
        7.7.1.3   Segmentation . . . . . . . . . . . . . . . . . . . .  47
        7.7.1.4   Options  . . . . . . . . . . . . . . . . . . . . . .  47
        7.7.1.5   Data   . . . . . . . . . . . . . . . . . . . . . . .  47
  7.8   Inactive Network Layer Protocol  . . . . . . . . . . . . . . .  47
        7.8.1   Network Layer Protocol Id  . . . . . . . . . . . . . .  47
        7.8.2   Data Field   . . . . . . . . . . . . . . . . . . . . .  47
  7.9   Error Report PDU (ER)  . . . . . . . . . . . . . . . . . . . .  48
        7.9.1   Structure  . . . . . . . . . . . . . . . . . . . . . .  48
                7.9.1.1   Fixed Part . . . . . . . . . . . . . . . . .  49
                7.9.1.2   Addresses  . . . . . . . . . . . . . . . . .  49
                7.9.1.3   Options  . . . . . . . . . . . . . . . . . .  49
                7.9.1.4   Reason for Discard . . . . . . . . . . . . .  50
                7.9.1.5   Error Report Data Field  . . . . . . . . . .  51

8 Conformance 51

  8.1   Provision of Functions for Conformance . . . . . . . . . . . .  51

List of Tables

1 Service Primitives for Underlying Service . . . . . . . . . . . . 14 2 Service Primitives for Underlying Service . . . . . . . . . . . . 14 3 Timer Primitives . . . . . . . . . . . . . . . . . . . . . . . . 14 4 Categorization of Protocol Functions . . . . . . . . . . . . . . . 32 5 Valid PDU Types . . . . . . . . . . . . . . . . . . . . . . . . . 36 6 Encoding of Option Parameters . . . . . . . . . . . . . . . . . . 39 7 Reason for Discard . . . . . . . . . . . . . . . . . . . . . . . . 50 8 Categorization of Functions . . . . . . . . . . . . . . . . . . . 52

List of Figures

1 Interrelationship of Standards . . . . . . . . . . . . . . . . . 6 2 PDU Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3 PDU Header – Fixed Part . . . . . . . . . . . . . . . . . . . . . 34 4 PDU Header – Address Part . . . . . . . . . . . . . . . . . . . 37 5 Address Parameters . . . . . . . . . . . . . . . . . . . . . . . . 38 6 PDU Header – Segmentation Part . . . . . . . . . . . . . . . . . 38 7 PDU Header – Options Part . . . . . . . . . . . . . . . . . . . . 39 8 PDU Header – Data Field . . . . . . . . . . . . . . . . . . . . 45

ISO 8473 [Page 4] RFC 994 December 1986

9 DT PDU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 10 Inactive Network Layer Protocol . . . . . . . . . . . . . . . . . 47 11 Error Report PDU . . . . . . . . . . . . . . . . . . . . . . . . . 48

ISO 8473 [Page 5] RFC 994 December 1986

0 Introduction

 This Protocol Standard 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 interconnection.
 This Protocol Standard is positioned with respect to other related
 standards by the layers defined in the Reference Model for Open Sys-
 tems Interconnection (ISO 7498).  In particular, it is a protocol of
 the Network Layer. This Protocol may be used between network-entities
 in end systems or in Network Layer relay systems (or both).  It pro-
 vides the Connectionless-mode Network Service as defined in Addendum
 1 to the Network Service Definition Covering Connectionless-mode
 Transmission (ISO 8348/AD1).
 The interrelationship of these standards is illustrated in Figure 1
 below:
  1. ——————-+— ISO NETWORK SERVICE PROVIDER —–^—————–

| |

                     |                                      |
                     |                                      |
         PROTOCOL    |  REFERENCE TO AIMS  -----------------+
                     |
      SPECIFICATION  |  REFERENCE TO ASSUMPTIONS -----------+
                     |                                      |
                     |                                      |
                     |                                      |
 --------------------+---SUBNETWORK SERVICE DEFINITION(S)---v-----------------
               Figure 1: Interrelationship of Standards

1 Scope and Field of Application

 This International Standard specifies a protocol which is used to
 provide the Connectionless-mode Network Service as described in Ad-
 dendum 1 to the Network Service Definition Covering Connectionless-
 mode Transmission.  The protocol relies upon the provision of an
 underlying connectionless-mode service by real subnetworks and/or
 data links. The underlying connectionless-mode service assumed by the
 protocol may be obtained either directly, from a connectionless-mode
 real subnetwork, or indirectly, through the operation of an appropri-
 ate Subnetwork Dependent Convergence Function (SNDCF) or Protocol
 (SNDCP) over a connection-mode real subnetwork as described in ISO
 8648, Internal Organization of the Network Layer.

ISO 8473 [Page 6] RFC 994 December 1986

    This Standard specifies:
    a)  procedures for the connectionless transmission of data and
        control information from one network-entity to a peer
        network-entity;
    b)  the encoding of the protocol data units (PDUs) used for the
        transmission of data and control information, comprising a
        variable-length protocol header format;
    c)  procedures for the correct interpretation of protocol control
        information; and
    d)  the functional requirements for implementations claiming
        conformance to the Standard.
    The procedures are defined in terms of:
    a)  the interactions among peer network-entities through the
        exchange of protocol data units;
    b)  the interactions between a network-entity and a Network Service
        user through the exchange of Network Service primitives; and
    c)  the interactions between a network-entity and an underlying
        service provider through the exchange of service primitives.

2 References

 ISO 7498, Information Processing Systems --- Open Systems Intercon-
 nection --- Basic Reference Model
 DIS 7498/AD1, Information Processing Systems --- Open Systems In-
 terconnection --- 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 Net-
 work Service Definition Covering Connectionless-mode Transmission
 ISO 8348/AD2, Information Processing Systems --- Telecommunications
 and Information Exchange between Systems --- Addendum to the Net-
 work Service Definition Covering Network Layer Addressing*
 DIS 8648, Information Processing Systems --- Telecommunications and
 Information Exchange between Systems --- Internal Organization of the
 Network Layer

ISO 8473 [Page 7] RFC 994 December 1986

 ISO 8509, Technical Report --- OSI Service Conventions
 ISO 9074, A Formal Description Technique based on an Extended State
 Transition Model

  • At present, at the stage of Draft; publication anticipated in

due course.

ISO 8473 [Page 8] RFC 994 December 1986

                        SECTION  ONE.  GENERAL

3 Definitions

3.1 Reference Model Definitions

 This document makes use of the following concepts defined in ISO 7498:
   (a)  End system
   (b)  Network entity
   (c)  Network layer
   (d)  Network protocol
   (e)  Network protocol data unit
   (f)  Network relay
   (g)  Network service
   (h)  Network service access point
   (i)  Network service access point address
   (j)  Routing
   (k)  Service
   (l)  Service data unit

3.2 Service Conventions Definitions

 This Protocol Standard makes use of the following terms from the OSI
 Service Conventions Technical Report (ISO TR 8509):
   (a)  Service provider
   (b)  Service user

3.3 Network Layer Architecture Definitions

 This Protocol Standard makes use of the following terms from the
 Internal Organization of the Network Layer (ISO 8648):
   (a)  Intermediate system
   (b)  Relay system
   (c)  Subnetwork

ISO 8473 [Page 9] RFC 994 December 1986

3.4 Network Layer Addressing Definitions

 This Protocol Standard makes use of the following terms from ISO 8348/AD2,
 Addendum to the Network Service Definition Covering Network Layer
 addressing:
   (a)  Network addressing domain
   (b)  Network protocol address information
   (c)  Subnetwork point of attachment

3.5 Additional Definitions

 For the purposes of this Protocol Standard, the following definitions
 apply:
   (a)  derived PDU --- a protocol data unit whose fields are identical
        to those of an initial PDU, except that it carries only a segment
        of the user data from an N-UNITDATA request.
   (b)  initial PDU --- a protocol data unit carrying the whole of the
        userq data from an N-UNITDATA request.
   (c)  local matter --- a decision made by a system concerning its
        behavior in the Network Layer that is not prescribed or
        constrained by this Protocol Standard.
   (d)  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.
   (e)  reassembly --- the act of regenerating an initial PDU from two
        or more derived PDUs.
   (f)  segment --- a distinct unit of data consisting of part or all
        of the user data provided in the N-UNITDATA request and delivered
        in the N-UNITDATA indication.
   (g)  segmentation --- the act of generating two or more derived PDUs
        from an initial or derived PDU. The derived PDUs together carry
        the entire user data of the initial or derived PDU from which they
        were generated.
                                     Note:
        It is possible that such an initial PDU will never actually be
        generated for a particular N-UNITDATA request, owing to the
        immediate application of segmentation.

ISO 8473 [Page 10] RFC 994 December 1986

4 Symbols and Abbreviations

4.1 Data Units

   NSDU      Network Service Data Unit
   PDU       Protocol Data Unit
   SNSDU     Subnetwork Service Data Unit

4.2 Protocol Data Units

   DT PDU    Data Protocol Data Unit
   ER PDU    Error Report Protocol Data Unit

4.3 Protocol Data Unit Fields

   CS        Checksum
   DA        Destination Address
   DAL       Destination Address Length
   DUID      Data Unit Identifier
   E/R       Error Report Flag
   LI        Length Indicator
   LT        Lifetime
   MS        More Segments Flag
   NLPID     Network Layer Protocol Identifier
   SA        Source Address
   SAL       Source Address Length
   SL        Segment Length
   SO        Segment Offset
   SP        Segmentation Permitted Flag
   TL        Total Lengt
   TP        Type
   V/P       Version/Protocol Identifier Extension

4.4 Parameters

   DA      Destination Address
   QOS     Quality of Service
   SA      Source Address

4.5 Miscellaneous

   CLNP       Connectionless-mode Network Protocol
   NS         Network Service
   NPAI       Network Protocol Address Information
   NSAP       Network Service Access Point
   SDU        Service Data Uni
   SN         Subnetwork
   SNDCF      Subnetwork Dependent Convergence Function
   SNDCP      Subnetwork Dependent Convergence Protocol
   SNICP      Subnetwork Independent Convergence Protocol
   SNPA       Subnetwork Point of Attachment

ISO 8473 [Page 11] RFC 994 December 1986

5 Overview of the Protocol

5.1 Internal Organization of the Network Layer

 The architectural organization of the Network Layer is described in a
 separate document, Internal Organization of the Network Layer (ISO
 8648). ISO 8648 identifies and categorizes the way in which functions
 can be performed within the Network Layer by Network Layer protocols,
 thus providing a uniform framework for describing how protocols
 operating either individually or cooperatively in the Network Layer
 can be used to provide the OSI Network Service. This protocol is
 designed to be used in the context of the internetworking protocol
 approach to the provision of the Connectionless-mode Network Service
 defined in that Standard.
 This protocol is intended for use in the Subnetwork Independent Con-
 vergence Protocol (SNICP) role.  A protocol which fulfills the SNICP
 role operates to construct the OSI Network Service over a defined set
 of underlying services, performing functions which are necessary to
 support the uniform appearance of the OSI Connectionless-mode Network
 Service over a homogeneous or heterogeneous set of interconnected
 subnetworks.  This protocol is defined to accommodate variability
 where Subnetwork Dependent Convergence Protocols and/or Subnetwork
 Access Protocols do not provide all of the functions necessary to
 support the Connectionless-mode Network Service over all or part of
 the path from one NSAP to another.
 As described in ISO 8648, a protocol at the Network Layer may fulfill
 different roles in different configurations.  Although this protocol
 is designed particularly to be suitable for a SNICP role in the con-
 text of the internetworking protocol approach to the provision of the
 Connectionless-mode Network Service, it may also be used to fulfill
 other roles and may therefore be used in the context of other ap-
 proaches to subnetwork interconnection.
 The specification of this protocol begins with a definition of the
 underlying service which it assumes. This service is made available
 by the operation of other Network Layer protocols or through provi-
 sion of the Data Link Service. The underlying service assumed by this
 protocol is described in Clause 5.5.

5.2 Subsets of the Protocol

 Two proper subsets of the full protocol are defined which permit the
 use of known subnetwork characteristics and are therefore not subnet-
 work independent.
 The Inactive Network Layer protocol subset is a null-function subset
 which can be used when it is known that the source and destination
 end-systems are connected by a single subnetwork, and when none of
 the functions performed by the full protocol is required to provide

ISO 8473 [Page 12] RFC 994 December 1986

 the Connectionless-mode Network Service between any pair of end-
 systems.
 The Non-segmenting protocol subset permits simplification of the
 header where it is known that the source and destination end-systems
 are connected by subnetworks whose service data unit sizes are
 greater than or equal to a known bound which is large enough so that
 segmentation is not required. This subset is selected by setting the
 Segmentation Permitted flag to zero.

5.3 Addresses and Titles

 The following Clauses describe the addresses and titles used by this
 Protocol.

5.3.1 Addresses

 The Source Address and Destination Address parameters referred to in
 Clause 7.3 of this International Standard are OSI Network Service Ac-
 cess Point Addresses.  The syntax and semantics of an OSI Network
 Service Access Point Address are described in a separate document,
 ISO 8348/AD2, Addendum to the Network Service Definition Covering
 Network Layer Addressing.
 The encoding used by this protocol to convey NSAP Addresses shall be
 the preferred binary encoding specified in ISO 8348/AD2; the entire
 NSAP address, taken as a whole, is represented explicitly as a string
 of binary octets.  This string is conveyed in its entirety in the ad-
 dress fields described in Clause 7.3. The rules governing the genera-
 tion of the preferred binary encoding are described in ISO 8348/AD2.

5.3.2 Network-entity Titles

 A network-entity title is an identifier for a network-entity in an
 endsystem or intermediate-system. Network-entity titles are allocated
 from the same name space as NSAP addresses, and the determination of
 whether an address is an NSAP address or a network-entity title
 depends on the context in which the address is interpreted. The en-
 tries in the Source Routing and Recording of Route parameters defined
 in Clauses 7.5.4 and 7.5.5 are network-entity titles. The Source Ad-
 dress and Destination Address parameters in the Error Report PDU de-
 fined in Clause 7.9.1.2 are also network-entity titles.
 The encoding used by this protocol to convey network-entity titles
 shall also be the preferred binary encoding; again, the entire
 network-entity title, taken as a whole, is represented explicitly as
 a string of binary octets.  This string is conveyed in its entirety
 in the fields described in Clauses 7.5.4, 7.5.5, and 7.9.1.2.

ISO 8473 [Page 13] RFC 994 December 1986

5.4 Service Provided by the Network Layer

 The service provided by this protocol is the Connectionless-mode Net-
 work Service described in ISO 8348/AD1, Addendum to the Network Ser-
 vice Definition Covering Connectionless-mode Transmission.  The Net-
 work Service primitives provided are summarized in Table 1:
         _____________________________________________________________
        |             PRIMITIVES                    PARAMETERS        |
        |____________________________________________________________ |
        |  N_UNITDATA         .Request    |  N_Source_Address,        |
        |                     .Indication |  N_Destination_Address,   |
        |                                 |  N_Quality_of_Service,    |
        |                                 |  N_Userdata               |
        |_________________________________|___________________________|
             Table 1: Service Primitives for Underlying Service
 The Addendum to the Network Service Definition Covering
 Connectionless-mode Transmission (ISO 8348/AD1) states that the max-
 imum size of a connectionless-mode Network-service-data-unit (NSDU)
 is limited to 64512 octets.

5.5 Underlying Service Assumed by the Protocol

 The underlying service required to support this protocol is defined
 by the following primitives:
         _____________________________________________________________
        |             PRIMITIVES                    PARAMETERS        |
        |____________________________________________________________ |
        |  SN_UNITDATA        .Request    | SN_Source_Address,        |
        |                     .Indication | SN_Destination_Address,   |
        |                                 | SN_Quality_of_Service,    |
        |                                 | SN_Userdata               |
        |_________________________________|___________________________|
             Table 2: Service Primitives for Underlying Service
                                 Note:
 These service primitives are used to describe the abstract interface
 which exists between the ISO 8473 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.

ISO 8473 [Page 14] RFC 994 December 1986

5.5.1 Subnetwork Points of Attachment

 The source and destination addresses specify the points of attachment
 to a public or private subnetwork(s) involved in the transmission.
 Subnetwork Point of Attachment addresses (SNPAs) are defined by each
 individual subnetwork authority.
 The syntax and semantics of SNPAs are not defined in this Standard.

5.5.2 Subnetwork Quality of Service

 Subnetwork Quality of Service describes aspects of an underlying
 connectionless-mode service which are attributable solely to the
 underlying service.
 Associated with each connectionless-mode transmission, certain meas-
 ures of Quality of Service are requested when the primitive action is
 initiated.  These requested measures (or parameter values and op-
 tions) are based on a priori knowledge of the service(s) made avail-
 able to it by the subnetwork. Knowledge of the nature and type of
 service available is typically obtained prior to an invocation of the
 underlying connectionless-mode service.
 The Quality of Service parameters identified for the underlying
 connectionless-mode service may in some circumstances be directly
 derivable from or mappable onto those identified in the
 Connectionless-mode Network Service.  The following parameters as de-
 fined in ISO 8348/AD1, Addendum to the Network Service Definition
 Covering Connectionlessmode Transmission, may be employed:
   (a)  transit delay;
   (b)  protection against unauthorized access;
   (c)  cost determinants;
   (d)  priority; and
   (e)  residual error probability.
                                  Note:
      For those subnetworks which do not inherently provide Quality of
      Service as a parameter when the primitive action is initiated, it
      is a local matter as to how the semantics of the service requested
      might be preserved. In particular, there may be instances in which
      the Quality of Service requested cannot be maintained.  In such
      circumstances, an attempt shall be made to deliver the protocol
      data unit at whatever Quality of Service is available.

ISO 8473 [Page 15] RFC 994 December 1986

5.5.3 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 assumed by the CLNP is required to support a
 service data unit size of at least 512 octets.
 If the minimum service data unit sizes supported by all of the sub-
 networks involved in the transmission of a particular PDU are known
 to be large enough that segmentation is not required, then the Non-
 segmenting protocol subset may be used.

5.5.4 Subnetwork Dependent Convergence Functions

 Subnetwork Dependent Convergence Functions may be performed to pro-
 vide an underlying connectionless-mode service in the case where a
 real subnetwork does not inherently provide the connectionless-mode
 service assumed by the protocol.  If a subnetwork inherently provides
 a connection-mode service, a Subnetwork Dependent Convergence Func-
 tion provides a mapping into the required underlying service.  Sub-
 network Dependent Convergence Functions may also be required in those
 cases where functions assumed from the underlying service are not
 performed.  In some cases, this may require the operation of an ex-
 plicit protocol (i.e., a protocol involving explicit exchanges of
 protocol control information between peer network-entities) in the
 Subnetwork Dependent Convergence Protocol (SNDCP) role. However,
 there may also be cases where the functionality required to fulfill
 the SNDCP role consists simply of a set of rules for manipulating the
 underlying service.

5.6 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.

ISO 8473 [Page 16] RFC 994 December 1986

 The S--TIMER Cancel primitive is an indication to the local environ-
 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 3.
             __________________________________________________
            |        PRIMITIVES               PARAMETERS      |
            |_________________________________________________|
            |    S--TIMER     .Request  |  S-Time,            |
            |                           |  S-Name,            |
            |                           |  S-Subscript        |
            |                           |                     |
            |                 .Response |  S-Name,            |
            |                           |  S-Subscript        |
            |___________________________|_____________________|
                      Table 3: 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.

ISO 8473 [Page 17] RFC 994 December 1986

            SECTION  TWO.  SPECIFICATION OF THE PROTOCOL

6 Protocol Functions

 This Clause describes the functions performed as part of the Proto-
 col.
 Not all of the functions must be performed by every implementation.
 Clause 6.17 specifies which functions may be omitted, and the correct
 behavior when requested functions are not implemented.

6.1 PDU Composition Function

 This function is responsible for the construction of a protocol data
 unit according to the rules governing the encoding of PDUs given in
 Clause 7.  Protocol Control Information required for delivering the
 data unit to its destination is determined from current state and lo-
 cal information and from the parameters associated with the N-
 UNITDATA Request.
 Network Protocol Address Information (NPAI) for the Source Address
 and Destination Address fields of the PDU header is derived from the
 NS-Source-Address and NS-Destination-Address parameters. The NS-
 Destination-Address and NS-Quality-of-Service parameters, together
 with current state and local information, are used to determine which
 optional functions are to be selected. User data passed from the Net-
 work Service User (NS-Userdata) forms the Data field of the protocol
 data unit.
 During the composition of the protocol data unit, a Data Unit Iden-
 tifier is assigned to distinguish this request to transmit NS-
 Userdata to a particular destination NS User from other such re-
 quests. The originator of the PDU must choose the Data Unit Identif-
 ier so that it remains unique (for this Source and Destination ad-
 dress pair) for the maximum lifetime of the Initial PDU in the net-
 work; this rule applies for any PDUs derived from the Initial PDU as
 a result of the application of the Segmentation Function (see Clause
 6.7).  Derived PDUs are considered to correspond to the same Initial
 PDU, and hence the same N-UNITDATA Request, if they have the same
 Source Address, Destination Address, and Data Unit Identifier.
 The Data Unit Identifier is also available for ancillary functions
 such as error reporting (see Clause 6.10).
 The total length of the PDU in octets is determined by the originator
 and placed in the Total Length field of the PDU header. This field is
 not changed in any Derived PDU for the lifetime of the protocol data
 unit.

ISO 8473 [Page 18] RFC 994 December 1986

 When the Non-segmenting protocol subset is employed, neither the To-
 tal Length field nor the Data Unit Identifier field is present.  The
 rules governing the PDU composition function are modified in this
 case as follows. During the composition of the protocol data unit,
 the total length of the PDU in octets is determined by the originator
 and placed in the Segment Length field of the PDU header. This field
 is not changed for the lifetime of the PDU. No Data Unit Identifica-
 tion is provided.

6.2 PDU Decomposition Function

 This function is responsible for removing the Protocol Control Infor-
 mation from the protocol data unit.  During this process, information
 pertinent to the generation of the N-UNITDATA Indication is deter-
 mined as follows. The NS-Source-Address and NS-Destination-Address
 parameters of the N-UNITDATA Indication are recovered from the NPAI
 in the Source and Destination Address fields of the PDU header. The
 data field of the PDU received is reserved until all segments of the
 original service data unit have been received; collectively, these
 form the NS-Userdata parameter of the N-UNITDATA Indication.  Infor-
 mation relating to the Quality of Service provided during the
 transmission of the PDU is determined from the Quality of Service and
 other information contained in the Options Part of the PDU header.
 This information constitutes the NS-Quality-of-Service parameter of
 the N-UNITDATA Indication.

6.3 Header Format Analysis Function

 This function determines whether the full protocol described in this
 Standard is employed, or one of the defined proper subsets thereof.
 If the protocol data unit has a Network Layer Protocol Identifier in-
 dicating that this is a standard version of the Protocol, this func-
 tion determines whether a received PDU has reached its destination,
 using the Destination Address provided in the PDU. If the Destination
 Address provided in the PDU identifies an NSAP served by this
 network-entity, then the PDU has reached its destination; if not, it
 must be forwarded.
 If the protocol data unit has a Network Layer Protocol Identifier in-
 dicating that the Inactive Network Layer Protocol subset is in use,
 then no further analysis of the PDU header is required. The network-
 entity in this case determines that either the Subnetwork Point of
 Attachment address encoded as network protocol address information in
 the supporting subnetwork protocol corresponds directly to an NSAP
 address serviced by this network-entity or that an error has oc-
 curred. If the subnetwork protocol data unit has been delivered
 correctly, then the PDU may be decomposed according to the procedures
 described for that particular subnetwork protocol.

ISO 8473 [Page 19] RFC 994 December 1986

6.4 PDU Lifetime Control Function

 This function is used to enforce the maximum PDU lifetime. It is
 closely associated with the Header Format Analysis function.  This
 function determines whether a PDU received may be forwarded or wheth-
 er its assigned lifetime has expired, in which case it must be dis-
 carded.
 The operation of the PDU Lifetime Control function depends upon the
 Lifetime field in the PDU header.  This field contains, at any time,
 the remaining lifetime of the PDU (represented in units of 500 mil-
 liseconds). The Lifetime of the Initial PDU is determined by the ori-
 ginating network-entity, and placed in the Lifetime field of the PDU.
 When the Segmentation function is applied to a PDU, the value of the
 Lifetime field of the Initial PDU is copied into all of the Derived
 PDUs.
 The Lifetime of the PDU is decremented by every network-entity which
 processes the PDU. When a network-entity processes a PDU, it decre-
 ments the PDU Lifetime by at least one.  The value of the PDU Life-
 time field shall be decremented by more than one if the sum of:
    1.  the transit delay in the underlying service from which the PDU
        was received; and
    2.  the delay within the system processing the PDU
 exceeds or is estimated to exceed 500 milliseconds.  In this case,
 the lifetime field should be decremented by one for each additional
 500 milliseconds of delay. The determination of delay need not be
 precise, but where a precise value cannot be ascertained, the value
 used shall be an overestimate, not an underestimate.
 If the Lifetime field reaches a value of zero before the PDU is
 delivered to the destination, the PDU must be discarded.  The Error
 Reporting function shall be invoked as described in Clause 6.10, Er-
 ror Reporting Function, and may result in the generation of an Error
 Report PDU.  It is a local matter whether the destination network-
 entity performs the Lifetime Control function.

6.5 Route PDU Function

 This function determines the network-entity to which a protocol data
 unit should be forwarded and the underlying service that must be used
 to reach that network-entity, using the Destination Address and the
 total length of the PDU. Where segmentation is required, the Route
 PDU function further determines over which underlying service Derived
 PDUs/segments must be sent in order to reach that network-entity. The
 results of the Route PDU function are passed to the Forward PDU func-
 tion (along with the PDU itself) for further processing.  Selection
 of the underlying service that must be used to reach the "next" sys-

ISO 8473 [Page 20] RFC 994 December 1986

 tem in the route is initially influenced by the NS-Quality-of- Ser-
 vice parameter of the N-UNITDATA Request, which specifies the QoS re-
 quested by the sending NS User. Whether this QoS is to be provided
 directly by the CLNP, through the selection of the Quality of Service
 Maintenance parameter and other optional parameters, or through the
 QoS facilities offered by each of the underlying services is deter-
 mined prior to invocation of the Forward PDU function.  Route selec-
 tion by intermediate systems may subsequently be influenced by the
 values of the Quality of Service Maintenance parameter (if present),
 and other optional parameters (if present).

6.6 Forward PDU Function

 This function issues an SN-UNITDATA Request primitive (see Clause
 5.5), supplying the subnetwork or SNDCF identified by the Route PDU
 function with the protocol data unit as user data to be transmitted,
 the address information required by that subnetwork or SNDCF to iden-
 tify the "next" system within the subnetwork-specific addressing
 domain (this may be an intermediate-system or the destination end-
 system), and Quality of Service constraints (if any) to be considered
 in the processing of the user data.
 When the PDU to be forwarded is longer than the maximum service data
 user size provided by the underlying service, the Segmentation func-
 tion is applied (See Clause 6.7, which follows).

6.7 Segmentation Function

 Segmentation is performed when the size of the protocol data unit is
 greater than the maximum service data unit size supported by the
 underlying service to be used to transmit the PDU.
 Segmentation consists of composing two or more new PDUs (Derived
 PDUs) from the PDU received. The PDU received may be the Initial PDU,
 or it may be a Derived PDU. All of the header information from the
 PDU to be segmented, with the exception of the segment length and
 checksum fields of the fixed part, and the segment offset of the seg-
 mentation part, is duplicated in each Derived PDU, including all of
 the address part, the data unit identifier and total length of the
 segmentation part, and the options part (if present).
                                 Note:
     The rules for forwarding and segmentation guarantee that the
     header length is the same for all segments (Derived PDUs) of
     the Initial PDU, and is the same as the header length of the
     Initial PDU.  The size of a PDU header will not change due to
     operation of any protocol function.
 The user data encapsulated within the PDU received are divided such
 that the Derived PDUs satisfy the size requirements of the user data
 parameter field of the primitive used to access the underlying ser-

ISO 8473 [Page 21] RFC 994 December 1986

 vice.
 Derived PDUs are identified as being from the same Initial PDU by
 means of
   (a)  the source address,
   (b)  the destination address, and
   (c)  the data unit identifier.
 Segmentation shall not result in the generation of a Derived PDU con-
 taining less than eight (8) octets of user data.
 The following fields of the PDU header are used in conjunction with
 the Segmentation function:
    (a)  Segment Offset --- identifies, with respect to the start
         of the Initial PDU, the octet at which the segment begins;
    (b)  Segment Length --- specifies the number of octets in the
         Derived PDU, including both header and data;
    (c)  More Segments Flag --- is set to one if this Derived PDU
         does not contain, as its final octet of user data, the final
         octet of the Initial PDU; and
    (d)  Total Length --- specifies the entire length of the Initial
         PDU, including both header and data.
 Derived PDUs may be further segmented without constraining the rout-
 ing of the individual Derived PDUs.  The Segmentation Permitted flag
 is set to one to indicate that segmentation is permitted. If the Ini-
 tial PDU is not to be segmented at any point during its lifetime in
 the network, the flag is set to zero by the source network-entity.
 The setting of the Segmentation Permitted flag cannot be changed by
 any other network-entity for the lifetime of the Initial PDU and any
 Derived PDUs.

6.8 Reassembly Function

 The Reassembly function reconstructs the Initial PDU from the Derived
 PDUs generated by the operation of the Segmentation Function on the
 Initial PDU (and, recursively, on subsequent Derived PDUs).  A bound
 on the time during which segments (Derived PDUs) of an Initial PDU
 will be held at a reassembly point before being discarded is provid-
 ed, so that reassembly resources may be released when it is no longer
 expected that any outstanding segments of the Initial PDU will arrive
 at the reassembly point. Upon reception of a Derived PDU, a reassem-
 bly timer is initiated with a value which indicates the amount of

ISO 8473 [Page 22] RFC 994 December 1986

 time which must elapse before any outstanding segments of the Initial
 PDU shall be assumed to be lost.  When this timer expires, all seg-
 ments (Derived PDUs) of the Initial PDU held at the reassembly point
 are discarded, the resources allocated for those segments are freed,
 and if selected, an Error Report is generated (see Clause 6.10).
 While the exact relationship between reassembly lifetime and PDU
 lifetime is a local matter, the Reassembly Function must preserve the
 intent of the PDU lifetime. Consequently, the reassembly function
 must discard PDUs whose lifetime would otherwise have expired had
 they not been under the control of the reassembly function.
                                 Note:
       1. Methods of bounding reassembly lifetime are discussed in
          Annex B.
       2. The Segmentation and Reassembly functions are intended to
          be used in such a way that the fewest possible segments are
          generated at each segmentation point and reassembly takes
          place at the final destination of a PDU. However, other
          schemes which
           (a) interact with the routing algorithm to favor paths on
               which fewer segments are generated;
           (b) generate more segments than absolutely required in
               order to avoid additional segmentation at some subsequent
               point; or
           (c) allow partial or full reassembly at some intermediate
               point along the route
          are not precluded. The information necessary to enable the
          use of one of these alternative strategies may be made
          available through the operation of a Network Layer Management
          function or by other means.
       3. The originator of the Initial PDU determines the value of the
          Segmentation Permitted flag in the Initial PDU and all Derived
          PDUs (if any).  Partial or full reassembly in an intermediate
          system (Note 2 (c) above) cannot change this value in the
          Initial PDU or any PDU derived from it, and cannot therefore
          add or remove the segmentation part of the header.

6.9 Discard PDU Function

 This function performs all of the actions necessary to free the
 resources reserved by the network-entity when any of the following
 situations is encountered (Note: the list is not exhaustive):
   (a)  A violation of protocol procedure has occurred.

ISO 8473 [Page 23] RFC 994 December 1986

   (b)  A PDU is received whose checksum is inconsistent with its
        contents.
   (c)  A PDU is received, but due to local congestion, it cannot be
        processed.
   (d)  A PDU is received whose header cannot be analyzed.
   (e)  A PDU is received which cannot be segmented and cannot be
        forwarded because its length exceeds the maximum service data
        unit size supported by any underlying service available for
        transmission of the PDU to the next network-entity on the
        chosen route.
   (f)  A PDU is received whose destination address is unreachable or
        unknown.
   (g)  Incorrect or invalid source routing was specified. This may
        include a syntax error in the source routing field, an unknown
        or unreachable address in the source routing field, or a path
        which is not acceptable for other reasons.
   (h)  A PDU is received whose PDU lifetime has expired or whose
        lifetime expires during reassembly.
   (i)  A PDU is received which contains an unsupported option.

6.10 Error Reporting Function

6.10.1 Overview

 This function causes an attempt to return an Error Report PDU to the
 source network-entity when a protocol data unit is discarded in ac-
 cordance with Clause 6.9.
 The Error Report PDU identifies the discarded PDU, specifies the type
 of error detected, and identifies the location in the header of the
 discarded PDU at which the error was detected.  At least the entire
 header of the Discarded PDU (and, at the discretion of the originator
 of the Error Report PDU none, all, or part of the data field) is
 placed in the data field of the Error Report PDU.
 The originator of a Data PDU may control the generation of Error Re-
 port PDUs.  An Error Report flag in the original PDU is set by the
 source network-entity to indicate that an Error Report PDU is to be
 returned if the Initial PDU or any PDUs derived from it are discard-
 ed; if the flag is not set, Error Reports are to be suppressed.
                                  Note:
       1. The suppression of Error Report PDUs is controlled by the

ISO 8473 [Page 24] RFC 994 December 1986

          originating network-entity and not by the NS User.  Care
          should be exercised by the originator with regard to
          suppressing ER PDUs so that error reporting is not suppressed
          for every PDU generated.
       2. Non-receipt of an Error Report PDU does not imply correct
          delivery of a PDU issued by a source network-entity.

6.10.2 Requirements

 An Error Report PDU shall not be generated to report the discard of
 an Error Report PDU.
 An Error Report PDU shall not be generated to report the discard of a
 Data PDU unless that PDU has the Error Report flag set to allow Error
 Reports.
 If a Data PDU is discarded, and the Error Report flag has been set to
 allow Error Reports, an Error Report PDU shall be generated if the
 reason for discard is one of the reasons for discard enumerated in
 Clause 6.9, subject to the conditions described in Clause 6.10.4.
                                 Note:
     If a Data PDU with the E/R flag set to allow Error Reports is
     discarded for any other reason, an ER PDU may be generated (as
     an implementation option).

6.10.3 Processing of Error Reports

 An Error Report PDU is composed from information contained in the
 header of the discarded Data PDU to which the Error Report refers.
 The contents of the Source Address field of the discarded Data PDU
 are used as the Destination Address of the Error Report PDU. This
 value, which in the context of the Data PDU was used as an NSAP Ad-
 dress, is used in the context of the Error Report PDU as the
 network-entity title of the network-entity that originated the Data
 PDU. The network- entity title of the originator of the Error Report
 PDU is conveyed in the Source Address field of the header of the Er-
 ror Report PDU. The value of the Lifetime field is determined in ac-
 cordance with Clause 6.4. Optional parameters are selected in accor-
 dance with Clause 6.10.4.
 Segmentation of Error Report PDUs is not permitted; hence, no Segmen-
 tation Part is present.  The total length of the ER PDU in octets is
 placed in the Segment Length field of the ER PDU header. This field
 is not changed during the lifetime of the ER PDU. If the originator
 of the ER PDU determines that the size of the ER PDU exceeds the max-
 imum service data unit size of the underlying service, the ER PDU
 shall be truncated to the maximum service data unit size (see Clause
 5.5.3) and forwarded with no other change. Error Report PDUs are
 routed and forwarded by intermediate-system network-entities in the

ISO 8473 [Page 25] RFC 994 December 1986

 same way as Data PDUs.
                                  Note:
     The requirement that the underlying service assumed by the CLNP
     must be capable of supporting a service data unit size of at least
     512 octets guarantees that the entire header of the discarded Data
     PDU can be conveyed in the data field of any ER PDU.
 When an ER PDU is decomposed upon reaching its destination, informa-
 tion that may be used to interpret and act upon the Error Report is
 obtained as follows. The network-entity title recovered from the NPAI
 in the Source Address field of the ER PDU header is used to identify
 the network-entity which generated the Error Report.  The reason for
 generating the Error Report is extracted from the Options Part of the
 PDU header. The entire header of the discarded Data PDU (and part or
 all of the original user data) is extracted from the data field of
 the ER PDU to assist in determining the nature of the error.

6.10.4 Relationship of Data PDU Options to Error Reports

 The generation of an Error Report is affected by options that are
 present in the corresponding Data PDU. The presence of options in the
 original Data PDU that are not supported by the system which has dis-
 carded that PDU may cause the suppression of an Error Report even if
 the original Data PDU indicated that an Error Report should be gen-
 erated in the event of a discard.
 The processing of an Error Report is also affected by options which
 are present in the corresponding Data PDU. In particular, options
 selected for the original Data PDU affect which options are included
 in the corresponding Error Report PDU. The selection of options for
 an Error Report PDU is governed by the following requirements:
   (a)  If the Priority Option or the QoS Maintenance Option is selected
        in the original Data PDU, and the system generating the Error
        Report PDU supports the option, then the Error Report PDU shall
        specify the option.
   (b)  If the Security Option is selected in the Data PDU, and the system
        generating the Error Report supports this option, then the Error
        Report PDU shall specify the option using the value that was
        specified in the original Data PDU. If the system does not support
        the Security Option, an Error Report must not be generated for
        a Data PDU that selects the Security Option.
   (c)  If the Complete Source Route Option is selected in the original
        Data PDU, and the system generating the Error Report PDU supports
        this option, then the error Report shall specify the Complete Source
        Route option.  The Source Route parameter value is obtained by
        extracting from the original Data PDU that portion of the complete
        source route that has already been traversed, and reversing the

ISO 8473 [Page 26] RFC 994 December 1986

        order of network-entity titles which comprise the list.
        If the system does not support the Complete Source Route Option,
        an Error Report must not be generated for a Data PDU that selects
        the Complete Source Route option.
   (d)  The Padding, Partial Source Routing, and Record Route Options,
        if supported, may be specified in the Error Report PDU.
                                  Note:
          The values of the optional parameters in (d) above may be
          derived as a local matter, or they may be based upon the
          corresponding values in the original Data PDU.

6.11 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 veri-
 fied at each point at which the PDU header is processed.  If the
 checksum calculation fails, the PDU must be discarded.  If PDU header
 fields are modified (for example, due to operation of the lifetime
 function), then the checksum is modified so that the checksum remains
 valid.
 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 formulae to be sa-
 tisfied:
      (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.
                                 Note:
     1. To ensure that inadvertent modification of a header while a
        PDU is being processed by an intermediate system (for
        example, due to a memory fault) may still be detected by the
        PDU Header Error function, an intermediate system network-

ISO 8473 [Page 27] RFC 994 December 1986

        entity must not recompute the checksum for the entire header,
        even if fields are modified.
     2. Annex C contains descriptions of algorithms which may be
        used to calculate the correct value of the checksum field
        when the PDU is created, and to update the value of the
        checksum field when the header is modified.

6.12 Padding Function

 The padding function is provided to allow space to be reserved in the
 PDU header which is not used to support any other function.  Octet
 alignment must be maintained.
                                 Note:
     An example of the use of this function is to cause the data field
     of a PDU to begin on a convenient boundary for the originating
     network-entity, such as a computer word boundary.

6.13 Security

 The provision of protection services (e.g., data origin authentica-
 tion, data confidentiality, and data integrity of a single
 connectionless-mode NSDU) is performed by the Security Function.
 The Security Function is related to the Protection from Unauthorized
 Access Quality of Service parameter described in ISO 8348/AD1, Adden-
 dum to the Network Service Definition Covering Connectionless-mode
 Transmission. The function is realized through selection of the secu-
 rity parameter in the options part of the PDU header.
 This Standard does not specify the way in which protection services
 are to be provided; it only provides for the encoding of security in-
 formation in the PDU header. To facilitate interoperation between
 end-systems and network relay-systems by avoiding different interpre-
 tations of the same encoding, a means to distinguish user-defined
 security encodings from standardized security encodings is described
 in Clause 7.5.3.
                                 Note:
     As an implementation consideration, data origin authentication
     may be provided through the use of a cryptographically generated
     or enciphered checksum (unique from the PDU Header Error Detection
     mechanism); data confidentiality and data integrity may be
     provided via route control mechanisms.

6.14 Source Routing Function

 The Source Routing function allows the originator to specify the path
 a generated PDU must take. Source routing may only be selected by the

ISO 8473 [Page 28] RFC 994 December 1986

 originator of a PDU. Source Routing is accomplished using a list of
 network-entity titles held in a parameter within the options part of
 the PDU header.  The length of this parameter is determined by the
 originating network-entity, and does not change as the PDU traverses
 the network.
 The Source Route parameter includes information used by the originat-
 ing end-system when determining the initial route of the PDU. Only
 the titles of intermediate system network-entities are included in
 the list; the network-entity title of the destination of the PDU is
 not included in the list.
 Associated with the list of network-entity titles is an indicator
 which identifies the next entry in the list to be used; this indica-
 tor is advanced by the receiver of the PDU when the next title in the
 list matches its own. The indicator is updated as the PDU is forward-
 ed so as to identify the appropriate entry at each stage of relaying.
 Two forms of the Source Routing function are provided.  The first
 form, referred to as Complete Source Routing, requires that the
 specified path must be taken; that is, only those systems identified
 in the list may be visited by the PDU while en route to the destina-
 tion, and each system must be visited in the order specified. If the
 specified path cannot be taken, the PDU must be discarded. Clause
 6.10 describes the circumstances in which an attempt shall be made to
 inform the originator of the discard using the Error Reporting func-
 tion.
 The second form is referred to as Partial Source Routing. Again, each
 system identified in the list must be visited in the order specified
 while en route to the destination.  However, with this form of source
 routing the PDU may take any path necessary to arrive at the next in-
 termediate system in the list, which may include visiting intermedi-
 ate systems that are not identified in the list. The PDU will not be
 discarded (for source routing related reasons) unless one of the sys-
 tems specified cannot be reached by any available route.

6.15 Record Route Function

 The Record Route function records the path(s) taken by a PDU as it
 traverses a series of intermediate systems. A recorded route consists
 of a list of network-entity titles held in a parameter within the op-
 tions part of the PDU header. The length of this parameter is deter-
 mined by the originating network-entity, and does not change as the
 PDU traverses the network.
 The list is constructed as the PDU is forwarded along a path towards
 its destination.  Only the titles of intermediate system network-
 entities are included in the recorded route. The network-entity title
 of the originator of the PDU is not recorded in the list.

ISO 8473 [Page 29] RFC 994 December 1986

 When an intermediate system network-entity processes a PDU containing
 the Record Route parameter, the system adds its own networkentity ti-
 tle at the end of the list of recorded network-entity titles.  An in-
 dicator is maintained to identify the next available octet to be used
 for recording of route. This indicator is updated as entries are ad-
 ded to the list as follows. The length of the entry to be added to
 the list is added to the value of the next available octet indicator,
 and this sum is compared with the length of the Record Route parame-
 ter.  If the addition of the entry to the list would exceed the size
 of the parameter, the next available octet indicator is set to indi-
 cate that route recording has been terminated. The network-entity ti-
 tle is not added to the list. The PDU may still be forwarded to its
 final destination, without further addition of network-entity titles.
 If the addition of the entry would not exceed the size of the Record
 Route parameter, the next available octet indicator is updated with
 the new value, and the network-entity title is added to the head of
 the list after the other entries have been moved.
 Two forms of the Record Route function are provided.  The first form
 is referred to as Complete Route Recording.  It requires that the
 list of network-entity titles be a complete and accurate record of
 all intermediate systems visited by a PDU (including Derived PDUs),
 except when a shortage of space in the record route option field
 causes termination of recording of route, as described above. When
 Complete Route Recording is selected, PDU reassembly at intermediate
 systems is performed only when the Derived PDUs that are reassembled
 all took the same route; otherwise, the PDU is discarded, and if
 selected, an Error Report is generated (see Clause 6.10).
 The second form is referred to as Partial Route Recording. It also
 requires a record of intermediate systems visited by a PDU. When Par-
 tial Route Recording is selected, PDU reassembly at intermediate sys-
 tems is always permitted.  When reassembly is performed at an inter-
 mediate system, the route recorded in any of the Derived PDUs may be
 placed in the PDU resulting from the reassembly.
                                 Note:
     The Record Route function is intended to be used in the diagnosis
     of subnetwork problems and/or to provide a return path that could
     be used as a source route in a subsequent PDU.

6.16 Quality of Service Maintenance Function

 The Quality of Service Maintenance function provides information to
 network-entities in intermediate systems which may be used to make
 routing decisions where such decisions affect the overall QoS provid-
 ed to NS users. This information is conveyed to intermediate system
 network- entities in a parameter in the options part of the PDU
 header.

ISO 8473 [Page 30] RFC 994 December 1986

 In those instances where the QoS requested cannot be maintained, in-
 termediate system network-entities shall attempt to deliver the PDU
 at a QoS different from the QoS requested. Intermediate system
 network-entities do not necessarily provide a notification of failure
 to meet the requested Quality of Service.

6.17 Priority Function

 The Priority function allows a PDU with a numerically higher priority
 value to be processed preferentially with respect to other PDUs with
 numerically lower priority values. The function is realized through
 selection of a parameter in the options part of the PDU header.
 The lowest priority value is zero; a source network-entity that does
 not support the Priority function must set the Priority value to
 zero.  The Priority function provides a means whereby the resources
 of end and intermediate system network-entities, such as outgoing
 transmission queues and buffers, can be used preferentially to pro-
 cess higher-priority PDUs ahead of lower-priority PDUs. The specific
 action taken by an individual network-entity to support the Priority
 function is a local matter.

6.18 Congestion Notification Function

 To allow NS Users to take appropriate action when congestion is ex-
 perienced within the NS provider, intermediate systems may inform the
 destination network-entity of congestion through the use of a flag in
 the QoS Maintenance parameter in the options part of the PDU header.
 The value of this flag is initially set to zero (0) by the originator
 of the PDU and may be set to one (1) by any intermediate system which
 processes the PDU to indicate that it is experiencing congestion. The
 criteria for determining when this action is to be taken are a local
 matter.
                                  Note:
   Congestion typically corresponds to inavailability of buffer space
   to maintain output queues. An appropriate policy for indicating
   congestion may be based upon the depth of the output queue selected
   for a PDU (according to its destination address or other routing
   information). When the depth of a particular output queue exceeds
   a certain proportion of the depth of that queue, an intermediate
   system will start to discard PDUs. The intermediate system will set
   the Congestion Experienced flag in the next PDU to be forwarded
   and may continue to do so until the condition is alleviated.

6.19 Classification of Functions

 Implementations are not required to support all of the functions
 described in Clauses 6.1 through 6.18. Functions are divided into
 three categories:

ISO 8473 [Page 31] RFC 994 December 1986

 Type 1: These functions must be supported.
 Type 2: These functions may or may not be supported.
         If an implementation does not support a Type 2 function, and the
         function is selected in a PDU, then that PDU must be discarded,
         and an Error Report PDU must be generated and forwarded to the
         originating network-entity, providing that the Error Report flag is
         set and the conditions of Clause 6.10.4 are satisfied.
 Type 3: These functions may or may not be supported.
         If an implementation does not support a Type 3 function, and the
         function is selected in a PDU, then the function is not performed,
         and the PDU is processed exactly as though the function had not
         been selected.  The protocol data unit shall not be discarded for
         this reason.
 Table 4 shows how the functions are divided into these three categories:

_

FULL NON INACTIVE
FUNCTION PROTOCOL SEGMENTING SUBSET
SUBSET
__
PDU Composition 1 1 1
PDU Composition 1 1 1
Header Format Analysis 1 1 1
PDU Lifetime Control 1 1 N/A
Route PDU 1 1 N/A
Forward PDU 1 1 N/A
Segment PDU 1 N/A N/A
Reassemble PDU 1 N/A N/A
Discard PDU 1 1 N/A
Error Reporting (Note 1) 1 1 N/A
Header Error Detection (Note 1) 1 1 N/A
Security 1 2 N/A
Complete Source Routing 1 2 N/A
Complete Route Recording 2 2 N/A
Partial Source Routing 3 3 N/A
Partial Route Recording 3 3 N/A
Priority 3 3 N/A
QoS Maintenance 3 3 N/A
Congestion Notification 3 3 N/A
Padding 3 3 N/A
__
         Table 4: Categorization of Protocol Functions

ISO 8473 [Page 32] RFC 994 December 1986

                              Note:
   1. While the Error Reporting and Header Error Detection functions
      must be provided, they are provided only when selected
      by the sending Network Service user.
   2. The rationale for the inclusion of type 3 functions is that in
      the case of some functions it is more important to forward
      the PDUs between intermediate systems or deliver them to
      an end-system than it is to support the functions.  Type 3
      functions should be used in those cases where they are of an
      advisory nature; they cannot cause a PDU to be discarded
      when they are not supported.

7 Structure and Encoding of PDUs

7.1 Structure

 All Protocol Data Units shall contain an integral number of octets.
 The octets in a PDU are numbered starting from one (1) and increasing
 in the order in which they are submitted to the underlying service.
 The bits in an octet are numbered from one (1) to eight (8), where
 bit one (1) is the low-order (least significant) bit.
 When consecutive octets are used to represent a binary number, the
 lower octet number has the most significant value.
 Any implementation supporting this protocol is required to state in
 its specification the way in which octets are transferred, using the
 terms "most significant bit" and "least significant bit". The PDUs of
 this protocol 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
     Clause 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 address part;
     3.  the segmentation part, if present;
     4.  the Options part, if present;
 and the data field, if present. This structure is illustrated in Figure 2:

ISO 8473 [Page 33] RFC 994 December 1986

7.2 Fixed Part

7.2.1 General

 The fixed part of the PDU header contains frequently occurring param-
 eters including the type code (DT or ER) of the protocol data unit.
 The length and the structure of the fixed part are defined by the PDU
 code.
 The fixed part has the following format:
                         Part                      Described in
           ___________________________________
           |          Fixed Part             |      Section 7.2
           |_________________________________|
           |          Address Part           |      Section 7.3
           |_________________________________|
           |       Segmentation Part         |      Section 7.4
           |_________________________________|
           |          Options Part           |      Section 7.5
           |_________________________________|
           |              Data               |      Section 7.6
           |_________________________________|
                 Figure 2: PDU Structure
                                                      Octet
           ________________________________________
           |   Network Layer Protocol Identifier  |    1
           |______________________________________|
           |          Length Indicator            |    2
           |______________________________________|
           |     Version/Protocol Id Extension    |    3
           |______________________________________|
           |              Lifetime                |    4
           |______________________________________|
           |    SP  vline M S vline e/R | Type    |    5
           |______________________________________|
           |            Segment Length            |   6,7
           |______________________________________|
           |               Checksum               |   8,9
           |______________________________________|
               Figure 3: PDU Header -- Fixed Part

7.2.2 Network Layer Protocol Identifier

 The value of this field is set to binary 1000 0001 to identify this
 Network Layer protocol as ISO 8473, Protocol for Providing the
 Connectionless- mode Network Service. The value of this field is set

ISO 8473 [Page 34] RFC 994 December 1986

 to binary 0000 0000 to identify the Inactive Network Layer protocol
 subset.

7.2.3 Length Indicator

 The length is indicated by a binary number, with a maximum value of
 254 (1111 1110).  The length indicated is the length in octets of the
 header, as described in Clause 7.1. The value 255 (1111 1111) is
 reserved for possible future extensions.
                                 Note:
   The rules for forwarding and segmentation guarantee that the header
   length is the same for all segments (Derived PDUs) of the Initial
   PDU, and is the same as the header length of the Initial PDU.
   The size of a PDU header will not change due to operation of any
   protocol function.

7.2.4 Version/Protocol Identifier Extension

 The value of this field is binary 0000 0001, which identifies the
 standard Version 1 of ISO 8473, Protocol for Providing the
 Connectionless-mode Network Service.

7.2.5 PDU Lifetime

 The PDU Lifetime field is encoded as a binary number representing the
 remaining lifetime of the PDU, in units of 500 milliseconds.

7.2.6 Flags

7.2.6.1 Segmentation Permitted

 The Segmentation Permitted flag indicates whether segmentation is
 permitted. Its value is determined by the originator of the PDU and
 cannot be changed by any other network-entity for the lifetime of the
 Initial PDU and any Derived PDUs.
 A value of one (1) indicates that segmentation is permitted. A value
 of zero (0) indicates that the non-segmenting protocol subset is em-
 ployed.  When the value of zero is selected, the segmentation part of
 the PDU header is not present, and the Segment Length field serves as
 the Total Length field (see Clause 7.2.8).

7.2.6.2 More Segments

 The More Segments flag indicates whether the data segment in this PDU
 contains (as its last octet) the last octet of the User Data in the
 NSDU.  When the More Segments flag is set to one (1), segmentation
 has taken place and the last octet of the NSDU is not contained in
 this PDU. The More Segments flag cannot be set to one (1) if the Seg-
 mentation Permitted flag is not set to one (1).

ISO 8473 [Page 35] RFC 994 December 1986

 When the More Segments flag is set to zero (0), the last octet of the
 Data Part of the PDU is the last octet of the NSDU.

7.2.6.3 Error Report

 When the Error Report flag is set to one, the rules in Clause 6.10
 are used to determine whether to generate an Error Report PDU if it
 is necessary to discard this Data PDU.
 When the Error Report flag is set to zero, discard of the Data PDU
 will not cause the generation of an Error Report PDU.

7.2.7 Type Code

 The Type code field identifies the type of the protocol data unit.
 Allowed values are given in Table 5:
            __________________________________________________
            |         | Bits               5   4   3   2   1 |
            |_________|______________________________________|
            | DT PDU  |                    1   1   1   0   0 |
            |_________|______________________________________|
            | ER PDU  |                    0   0   0   0   1 |
            |_________|______________________________________|
                       Table 5: Valid PDU Types

7.2.8 PDU Segment Length

 The Segment Length field specifies the entire length, in octets, of
 the Derived PDU, including both header and data (if present).  When
 the full protocol is employed and a PDU is not segmented, the value
 of this field is identical to the value of the Total Length field lo-
 cated in the Segmentation Part of the header.
 When the non-segmenting protocol subset is employed, no segmentation
 part is present in the header. In this subset, the Segment Length
 field specifies the entire length of the Initial PDU, including both
 header and data (if present). The Segment Length field is not changed
 for the lifetime of the PDU.

7.2.9 PDU Checksum

 The checksum is computed on the entire PDU header.  For the Data PDU,
 this includes the segmentation and options parts (if present). For
 the Error Report PDU, this includes the reason for discard field as
 well.
 A checksum value of zero is reserved to indicate that the checksum is
 to be ignored.  The operation of the PDU Header Error Detection func-
 tion (Clause 6.11) ensures that the value zero does not represent a

ISO 8473 [Page 36] RFC 994 December 1986

 valid checksum. A non-zero value indicates that the checksum must be
 processed. If the checksum calculation fails, the PDU must be dis-
 carded.

7.3 Address Part

7.3.1 General

 Address parameters are distinguished by their location, immediately
 following the fixed part of the PDU header. The address part is il-
 lustrated Figure 4:
                                                            Octet
              ____________________________________________
             |    Destination Address Length Indicator   |   10
             |___________________________________________|
             |                                           |   11
             :              Destination Address          :
             |                                           |  m - 1
             |___________________________________________|
             |     Source Address Length Indicator       |    m
             |___________________________________________|
             |                                           |  m + 1
             :               Source Address              :
             |                                           |  n - 1
             |___________________________________________|
               Figure 4: PDU Header -- Address Part

7.3.1.1 Destination and Source Addresses

 The Destination and Source addresses used by this protocol are Net-
 work Service Access Point addresses as defined in ISO 8348/AD2, Ad-
 dendum to the Network Service Definition Covering Network Layer Ad-
 dressing.
 The Destination and Source Addresses are variable length. The Desti-
 nation and Source Address fields are encoded as Network Protocol Ad-
 dress Information using the Preferred Binary Encoding defined in
 Clause 8.3.1 of ISO 8348/AD2.
 The Destination Address Length Indicator field specifies the length
 of the Destination Address in octets. The Destination Address field
 follows the Destination Address Length Indicator field.
 The Source Address Length Indicator field specifies the length of the
 Source Address in octets.  The Source Address Length Indicator field
 follows the Destination Address field. The Source Address field fol-
 lows the Source Address Length Indicator field.

ISO 8473 [Page 37] RFC 994 December 1986

 Each address parameter is encoded as illustrated in Table 5:
              ______________________________________________
              | Octet  | Address parameter Length Indicator |
              |   n    |                (e.g., 'm')         |
              |________|____________________________________|
              | Octets |                                    |
              |  n + 1 |       Address Parameter Value      |
              |  thru  |                                    |
              |  n + m |                                    |
              |________|____________________________________|
                        Figure 5:  Address Parameters

7.4 Segmentation Part

 If the Segmentation Permitted Flag in the Fixed Part of the PDU
 Header (Octet 4, Bit 8) is set to one, the segmentation part of the
 header, illustrated in Figure 6, must be present:
 If the Segmentation Permitted flag is set to zero, the non-segmenting
 protocol subset is in use.
                                                Octet
                  ________________________
                  | Data Unit Identifier |       n, n + 1
                  |______________________|
                  |    Segment Offset    |   n + 2, n + 3
                  |______________________|
                  |     Total Length     |   n + 4, n + 5
                  |______________________|
           Figure 6: PDU Header -- Segmentation Part

7.4.1 Data Unit Identifier

 The Data Unit Identifier identifies an Initial PDU (and hence, its
 Derived PDUs) so that a segmented data unit may be correctly reassem-
 bled. The Data Unit Identifier size is two octets.

7.4.2 Segment Offset

 For each Derived PDU, the Segment Offset field specifies the relative
 position of the segment contained in the data field of the Derived
 PDU with respect to the start of the data field of the Initial PDU.
 The offset is measured in units of octets. The offset of the first
 segment (and hence, the Initial PDU) is zero; an unsegmented (Initial
 PDU) has a segment offset value of zero (0). The value of this field
 shall be a multiple of eight 8).

ISO 8473 [Page 38] RFC 994 December 1986

7.4.3 PDU Total Length

 The Total Length field specifies the entire length of the Initial
 PDU, including both the header and data.  This field is not changed
 for the lifetime of the Initial PDU (and hence, its Derived PDUs).

7.5 Options Part

7.5.1 General

 The options part is used to convey optional parameters.  The options
 part of the PDU header is illustrated below:
                                                           Octet
      ___________________________________________________
      |                                                  | n + 6
      :                    Options                       :
      |                                                  |   p
      |__________________________________________________|
                Figure 7: PDU Header -- 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 Protocol Data
 Unit with an option duplicated should be treated as a protocol error.
 The rules governing the treatment of protocol errors are described in
 Clause 6.10, Error Reporting Function.
 The encoding of parameters contained within the options part of the
 PDU header is illustrated in Table 6:
              Octets
              ___________________________________________
              |     n      |       Parameter Code       |
              |____________|____________________________|
              |   n + 1    |  Parameter Length (e.g.m)  |
              |____________|____________________________|
              |   n + 2    |                            |
              |     to     |     Parameter Value        |
              | n + m + 1  |                            |
              |____________|____________________________|

ISO 8473 [Page 39] RFC 994 December 1986

                  Table 6: Encoding of 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 seg-
 mentation 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.

7.5.2 Padding

 The padding parameter is used to lengthen the PDU header to a con-
 venient size (See Clause 6.12).
 Parameter Code:        1100 1100
 Parameter Length:      variable
 Parameter Value:       any value is allowed

7.5.3 Security

 This parameter allows a unique and unambiguous security level to be
 assigned to a protocol data unit.
 Parameter Code:        1100 0101
 Parameter Length:      variable
 Parameter Value:       The high order two bits of the first octet
                        specify the Security Format Code, where:
          Security      Type of Security Field:
         Format Code

ISO 8473 [Page 40] RFC 994 December 1986

             00         Reserved
             01         Source Address Specific
             10         Destination Address Specific
             11         Globally Unique
        The rest of the first octet is reserved and must be zero.  The
        remainder of the Parameter Value field specifies the security
        level as described in the following Clauses.

7.5.3.1 Source Address Specific

 The Security Format Code value of binary "01" indicates that the
 remaining octets of the parameter value field specify a security lev-
 el which is unique and unambiguous in the context of the security
 classification system employed by the authority responsible for as-
 signing the source NSAP Address.

7.5.3.2 Destination Address Specific

 The Security Format Code value of binary "10" indicates that the
 remaining octets of the parameter value field specify a security lev-
 el which is unique and unambiguous in the context of the security
 classification system employed by the authority responsible for as-
 signing the destination NSAP Address.

7.5.3.3 Globally Unique Security

 The Security Format Code value of binary "11" indicates that the
 remaining octets of the parameter value field specify a globally
 unique and unambiguous security level.  This security classification
 system is not specified in this Standard.

7.5.4 Source Routing

 The source routing parameter specifies, either completely or partial-
 ly, the route to be taken from Source Network Address to Destination
 Network Address.
 Parameter Code:        1100 0101
 Parameter Length:      variable
 Parameter Value:       2 octets of control information succeeded by a
        concatenation of ordered network-entity title entries (ordered
        from source to destination)
 The first octet of the parameter value is the type code, and has the
 following significance:
        0000 0000    partial source routing
        0000 0001    complete source routing
                     <all other values reserved>

ISO 8473 [Page 41] RFC 994 December 1986

 The second octet indicates the octet offset of the next network-
 entity title entry to be processed in the list.  It is relative to
 the start of the parameter, such that a value of three (3) indicates
 that the next network-entity title entry begins immediately after
 this control octet. Successive octets are indicated by corresponding-
 ly larger values of this indicator.
 The third octet begins the network-entity title list. The list con-
 sists of variable length network-entity title entries.  The first oc-
 tet of entry identifies the length of the network-entity title which
 comprises the re- mainder of the entry.

7.5.5 Recording of Route

 The recording of route parameter identifies the route of intermediate
 systems traversed by the PDU.
 Parameter Code:        1100 1011
 Parameter Length:      variable
 Parameter Value:       2 octets of control information succeeded by a
        con catenation of ordered network-entity title entries (ordered
        from destination to source)
 The first octet of the parameter value is the type code, and has the
 following significance:
       0000 0000    Partial Recording of Route in progress
       0000 0001    Complete Recording of Route in progress
                    <all other values reserved>
 The second octet identifies the first octet not currently used for a
 recorded network-entity title, and therefore also the end of the
 list. It is encoded relative to the start of the parameter value,
 such that a value of three (3) indicates that no network-entity ti-
 tles have yet been recorded.  A value of all ones is used to indicate
 that route recording has been terminated.
 The third octet begins the network-entity title list. The list con-
 sists of variable length network-entity title entries.  The first oc-
 tet of each entry specifies the length of the network-entity title
 comprising the remainder of the entry.  Network-entity title entries
 are always added to the beginning of the list; that is, the most re-
 cently added entry will begin in the third octet of the parameter
 value.
                                   Note:
      The length of the Record Route parameter is determined by the
      originator of the PDU and is not changed during the lifetime of
      the PDU; hence, the operation of the Record Route function does

ISO 8473 [Page 42] RFC 994 December 1986

      not affect the length of the header.

7.5.6 Quality of Service Maintenance

 The Quality of Service parameter conveys information about the quali-
 ty of service requested by the originating Network Service user.
 Network-entities in intermediate systems may (but are not required
 to) make use of this information as an aid in selecting a route when
 more than one route satisfying other routing criteria is available
 and the available routes are known to differ with respect to Quality
 of Service see Clause 6.16).
   Parameter Code:        1100 0011
   Parameter Length:      variable
   Parameter Value:       The high order two bits of the first octet
         specify the  QoS Format Code, where:
      QoS Format      Type of QoS
          Code        Field
           00         Reserved
           01         Source Address Specific
           10         Destination Address Specific
           11         Globally Unique
 The rest of the first octet is reserved and must be zero. The
 remainder of the Parameter Value field specifies the QoS as described
 in the following Clauses.

7.5.6.1 Source Address Specific

 The QoS Format Code value of binary "01" indicates that the remaining
 octets of the parameter value field specify a QoS which is unique and
 unambiguous in the context of the QoS Maintenance system employed by
 the authority responsible for assigning the source NSAP Address.

7.5.6.2 Destination Address Specific

 The QoS Format Code value of binary "10" indicates that the remaining
 octets of the parameter value field specify a QoS which is unique and
 unambiguous in the context of the QoS Maintenance system employed by
 the authority responsible for assigning the destination NSAP Address.

7.5.6.3 Globally Unique QoS

 The QoS Format Code value of binary "11" indicates that the remainder
 of the parameter value field specifies a globally unique QoS Mainte-
 nance field. When the globally unique QoS Maintenance function is em-
 ployed, the parameter value field must have a total length of one oc-
 tet, which is assigned the following values:
       Bits 8 and 7:   QoS Format Code of binary "11"

ISO 8473 [Page 43] RFC 994 December 1986

       Bit 6:          Reserved
       Bit 5:          sequencing vs. transit delay
       Bit 4:          congestion experienced
       Bit 3:          transit delay vs. cost
       Bit 2:          residual error probability vs. transit delay
       Bit 1:          residual error probability vs. cost
 Bit 5 is set to one to indicate that, where possible, routing deci-
 sions should favor sending all PDUs to the specified destination NSAP
 address over a single path (in order to maintain sequence) over
 minimizing transit delay. A value of zero (0) indicates that, where
 possible, routing decisions should favor low transit delay over se-
 quence preservation.
 Bit 4 is set to zero by the network-entity which originates the pro-
 tocol data unit. It is set to one by an intermiediate system to indi-
 cate that this PDU has visited a congested intermediate system, and
 appropriate action should be taken by the destination network-entity.
 Once the congestion experienced bit is set by an intermediate system,
 it may not be reset by any intermediate system traversed by the PDU
 further along the path towards the destination.
 Bit 3 is set to one to indicate that, where possible, routing deci-
 sions should favor low transit delay over low cost. A value of 0 in-
 dicates that routing decisions should favor low cost over low transit
 delay.
 Bit 2 set to one to indicate that, where possible, routing decisions
 should favor low residual error probability over low transit delay.
 A value of zero indicates that routing decisions should favor low
 transit delay over low residual error probability.
 Bit 1 is set to one to indicate that, where possible, routing deci-
 sions should favor low residual error probability over low cost.  A
 value of 0 indicates that routing decisions should favor low cost
 over low residual error probability.

7.5.7 Priority

 The value of the Priority parameter indicates the relative priority
 of the protocol data unit.  Intermediate systems that support this
 option shall make use of this information in routing and in ordering
 PDUs for transmission.
 Parameter Code:        1100 1101
 Parameter Length:      one octet
 Parameter Value:       0000 0000 - Normal (Default) through
        0000 1110 - Highest
        <all other values reserved>

ISO 8473 [Page 44] RFC 994 December 1986

 The values 0000 0001 through 0000 1110 are to be used for higher
 priority protocol data units. If an intermediate system does not sup-
 port this option, all PDUs shall be treated as if the field had the
 value 0000 0000.

7.6 Data Part

 The Data part of the PDU is structured as an ordered multiple of oc-
 tets, which is identical to the same ordered multiple of octets
 specified for the NS-Userdata parameter of the N-UNITDATA Request and
 Indication primitives. The data field is illustrated in Figure 8:
                                                               Octet
          ___________________________________________________
          |                                                  | p + 1
          :                      Data                        :
          |                                                  |   z
          |__________________________________________________|
                     Figure 8: PDU Header -- Data Field

ISO 8473 [Page 45] RFC 994 December 1986

7.7 Data (DT) PDU

7.7.1 Structure

 The DT PDU has the following format:
           __________________________________________
           |   Network Layer Protocol Identifier    |       1
           |________________________________________|
           |            Length Indicator            |       2
           |________________________________________|
           |      Version/Protocol Id Extension     |       3
           |________________________________________|
           |                Lifetime                |       4
           |________________________________________|
           |  S P  vline  M S vline e/R |  Type     |       5
           |____________________________|___________|
           |             Segment Length             |      6,7
           |________________________________________|
           |                Checksum                |      8,9
           |________________________________________|
           |  Destination Address Length Indicator  |      10
           |________________________________________|
           |                                        |      11
           :          Destination Address           :
           |________________________________________|     m - 1
           |    Source Address Length Indicator     |       m
           |________________________________________|
           |                                        |     m + 1
           :             Source Address             :
           |                                        |     n - 1
           |________________________________________|
           |          Data Unit Identifier          |    n, n + 1
           |________________________________________|
           |             Segment Offset             |  n + 2, n + 3
           |________________________________________|
           |              Total Length              |  n + 4, n + 5
           |________________________________________|
           |                                        |    n + 6
           |                 Options                |
           |                                        |      p
           |________________________________________|
           |                                        |    p + 1
           |                  Data                  |
           |                                        |      z
           |________________________________________|
                       Figure 9: DT PDU

ISO 8473 [Page 46] RFC 994 December 1986

7.7.1.1 Fixed Part

 1)   Network Layer Protocol Identifier      See Clause 7.2.2
 2)   Length Indicator                       See Clause 7.2.3
 3)   Version/Protocol Id Extension          See Clause 7.2.4
 4)   Lifetime                               See Clause 7.2.5
 5)   SP, MS, E/R                            See Clause 7.2.6
 6)   Type Code                              See Clause 7.2.7
 7)   Segment Length                         See Clause 7.2.8
 8)   Checksum                               See Clause 7.2.9

7.7.1.2 Addresses

 See Clause 7.3.

7.7.1.3 Segmentation

 See Clause 7.4.

7.7.1.4 Options

 See Clause 7.5.

7.7.1.5 Data

 See Clause 7.7.

7.8 Inactive Network Layer Protocol

                                                    Octet
                ____________________________________
                |Network Layer Protocol Identifier |  1
                |__________________________________|
                |                                  |  2
                |                Data              |
                |                                  |  2 - n
                |__________________________________|
              Figure 10: Inactive Network Layer Protocol

7.8.1 Network Layer Protocol Id

 The value of the Network Layer Protocol Identifier field is binary
 zero (0000 0000).

7.8.2 Data Field

 The length of the NS-Userdata parameter is constrained to be less
 than or equal to the value of the length of the SN-Userdata parameter
 minus one (see Clause 7.7).

ISO 8473 [Page 47] RFC 994 December 1986

7.9 Error Report PDU (ER)

7.9.1 Structure

 The ER PDU has the following format:
                                                           Octet
            ______________________________________________
            |     Network Layer Protocol Identifier      |   1
            |____________________________________________|
            |              Length Indicator              |   2
            |____________________________________________|
            |        Version/Protocol Id Extension       |   3
            |____________________________________________|
            |                   Lifetime                 |   4
            |____________________________________________|
            |   SP= 0  vline MS= 0 vline Reserved | Type |   5
            |_____________________________________|______|
            |               Segment Length               |  6,7
            |____________________________________________|
            |                 Checksum                   |  8,9
            |____________________________________________|
            |    Destination Address Length Indicator    |  10
            |____________________________________________|
            |                                            |  11
            :            Destination Address             :
            |                                            | m - 1
            |____________________________________________|
            |     Source Address Length Indicator        |   m
            |____________________________________________|
            |                                            | m + 1
            :               Source Address               :
            |                                            | n - 1
            |____________________________________________|
            |                                            |   n
            |                   Options                  |
            |                                            | p - 1
            |____________________________________________|
            |                                            |   p
            |             Reason for Discard             |
            |                                            | q - 1
            |____________________________________________|
            |                                            |   q
            |          Error Report Data Field           |
            |                                            |   z
            |____________________________________________|
                     Figure 11: Error Report PDU

ISO 8473 [Page 48] RFC 994 December 1986

7.9.1.1 Fixed Part

 The fixed part of the Error Report Protocol Data Unit is composed in
 the same way as a new (Initial) Data PDU. References are provided to
 previous Clauses describing the encoding of the fields comprising the
 fixed part:
       1)   Network Layer Protocol Identifier      See Clause 7.2.2
       2)   Length Indicator                       See Clause 7.2.3
       3)   Version/Protocol Id Extension          See Clause 7.2.4
       4)   Lifetime                               See Clause 7.2.5
       5)   SP, MS, E/R                            Always set to zero,
                                                   (See Clause 6.10)
       6)   Type Code                              See Clause 7.2.7
       7)   Segment Length                         See Clause 7.2.8
       8)   Checksum                               See Clause 7.2.9

7.9.1.2 Addresses

 See Clause 7.3.
 The Destination Address specifies the network-entity title of the origi-
 nator of the discarded PDU. The Source Address specifies the title of the
 intermediate-system or end-system network-entity initiating the Error
 Report PDU.

7.9.1.3 Options

 See Clause 7.5.

ISO 8473 [Page 49] RFC 994 December 1986

7.9.1.4 Reason for Discard

 This parameter is valid only for the Error Report PDU.
 Parameter Code:      1100 0001
 Parameter Length:    two octets
 Parameter Value:     type of error encoded in binary.  Values are listed
                      in Table 7:

_ | Parameter Value | Class of | Meaning | | Octet 1 Octet 2| Error | | ||_|_|

0000 0000 Reason not specified
0001 Protocol Procedure Error
0010 Incorrect Checksum
0011 General PDU Discarded due to Congestion
0100 Header Syntax Error (cannot be parsed)
0101 Segmentation needed but not permitted
0110 Incomplete PDU Received
0111 Duplicate Option
|_|_| | 1000 0000 | Address | Destination Address Unreachable | | 0001 | | Destination Address Unknown | |__
1001 0000 Unspecified Source Routing Error
0001 Source Syntax Error in Source Routing Field
0010 Routing Unknown Address in Source Routing Field
0011 Path not Acceptable
|_|_| | 1010 0000 | Lifetime | Lifetime Expired while Data Unit in Transit | | 0001 | | Lifetime Expired during Reassembly | |__
1011 0000 Unsupported Option not Specified
0001 PDU Unsupported Protocol Version
0010 Discarded Unsupported Security Option
0011 Unsupported Source Routing Option
0100 Unsupported Recording of Route Option
|_|_| | 1100 0000 | Reassembly | Reassembly interference | |__
                    Table 7: Reasons for Discard
 The first octet of the parameter value contains an error type code.
 If the error in the discarded Data PDU can be localized to a particu-
 lar field, the number of the first octet of that field is stored in
 the second octet of the reason for discard parameter field. If the
 error cannot be localized to a particular field, or if the error is a
 checksum error, then the value zero is stored in the second octet of
 the reason for discard parameter field.

ISO 8473 [Page 50] RFC 994 December 1986

7.9.1.5 Error Report Data Field

 This field contains the entire header of the discarded Data PDU, and
 may contain some or all of the data field of the discarded PDU.

8 Conformance

 For conformance to this International Standard, the ability to ori-
 ginate, manipulate, and receive PDUs in accordance with the full pro-
 tocol (as opposed to the non-segmenting or Inactive Network Layer
 Protocol subsets) is required.
 Additionally, conformance to the Standard requires provision of the
 protocol functions described in Clause 6. Provision of the optional
 functions described in Clause 6.18 and enumerated in Table 9-1 must
 meet the requirements described therein. Exceptions to this require-
 ment are described in Clause 8.1 below.
 Additionally, conformance to the Standard requires adherence to the
 structure and encoding of PDUs of Clause 7.
 If and only if the above requirements are met is there conformance to
 this International Standard.

8.1 Provision of Functions for Conformance

 The following table categorizes the functions in Clause 6 with
 respect to the type of system providing the function:
                                  Note:
     1. The support of the PDU Composition and Forward PDU functions
        is necessary for the generation of Error Report PDUs.
     2. The Segment PDU function is in general mandatory for an
        intermediate system. However, a system which is to be
        connected only to subnetworks all offering the same maximum
        SDU size (such as identical Local Area Networks) will not
        need to perform this function and therefore does not need to
        implement it.
        If this function is not implemented, this shall be stated
        as part of the specification of the implementation.
     3. The correct treatment of the padding function requires no
        processing. A conforming implementation shall support the
        function, to the extent of ignoring this parameter wherever
        it may appear.
     4. This function may or may not be supported.  If an
        implementation does not support this function, and the

ISO 8473 [Page 51] RFC 994 December 1986

        function is selected in a PDU, then the PDU shall be discarded,
        and an ER PDU shall be generated and forwarded to the
        originating network-entity if the Error Report flag is set
        and the conditions of Clause 6.10.4 are satisfied.
     5. This function may or may not be supported.  If an implementation
        does not support this function, and the function is selected
        in a PDU, then the function is not performed and the PDU is
        processed exactly as though the function had not been
        selected. The PDU shall not be discarded for this reason.
  ___________________________________________________________________
  | Function                   |     Send   |   Forward |   Receive |
  |____________________________|____________|___________|___________|
  | PDU Composition            |      M     |  (Note 1) |  (Note 1) |
  | PDU Decomposition          |      M     |     -     |     M     |
  | Header Format Analysis     |      -     |     M     |     M     |
  | PDU Lifetime Control       |            |     M     |     I     |
  | Route PDU                  |      -     |     M     |     -     |
  | Forward PDU                |      M     |     M     |  (Note 1) |
  | Segment PDU                |      M     |  (Note 2) |     -     |
  | Reassemble PDU             |      -     |     I     |     M     |
  | Discard PDU                |      -     |     M     |     M     |
  | Error Reporting            |      M     |     M     |     M     |
  | Header Error Detection     |   (Note 3) |     M     |     M     |
  |                            |            |           |           |
  | Security                   |      -     |  (Note 3) | (Note 4)  |
  | Complete Source Routing    |      -     |  (Note 4) |     -     |
  | Complete Route Recording   |      -     |  (Note 4) |     -     |
  | Partial Source Routing     |      -     |  (Note 5) |     -     |
  | Partial Route Recording    |      -     |  (Note 5) |     -     |
  | Priority                   |      -     |  (Note 5) |     -     |
  | QoS Maintenance            |      -     |  (Note 5) |     -     |
  | Congestion Notification    |      -     |  (Note 5) |     -     |
  | Padding                    |      -     |  (Note 5) | (Note 3)  |
  |____________________________|____________|___________|___________|
                  Table 8: Categorization of Functions
                                   Key:
 M: Mandatory Function; this function must be implemented.
  1. : Not applicable.
 I: Implementation option, as described in the text.
 NOTE:  See notes above

ISO 8473 [Page 52]

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