RFC 926
This document is obsolete. Please refer to RFC 994.

Network Working Group                                                ISO
Request for Comments: 926                                  December 1984

    Protocol for Providing the Connectionless-Mode Network Services
                         (Informally - ISO IP)

                              ISO DIS 8473

Status of this Memo:

 This document is distributed as an RFC for information only.  It does
 not specify a standard for the ARPA-Internet.  Distribution of this
 memo is unlimited.


 This document has been prepared by retyping the text of ISO DIS 8473 of
 May 1984, which is currently undergoing voting within ISO as a Draft
 International Standard (DIS).  Although this RFC has been reviewed
 after typing, and is believed to be substantially correct, it is
 possible that typographic errors not present in the ISO document have
 been overlooked.

 Alex McKenzie

RFC 926                                                    December 1984

RFC 926                                                    December 1984

                           TABLE OF CONTENTS

 1   SCOPE AND FIELD OF APPLICATION........................ 2

 2   REFERENCES............................................ 3

 3   DEFINITIONS........................................... 4
 3.1   Reference Model Definitions......................... 4
 3.2   Service Conventions Definitions..................... 4
 3.3   Network Layer Architecture Definitions.............. 4
 3.4   Network Layer Addressing Definitions................ 5
 3.5   Additional Definitions.............................. 5

 4   SYMBOLS AND ABBREVIATIONS............................. 7
 4.1   Data Units.......................................... 7
 4.2   Protocol Data Units................................. 7
 4.3   Protocol Data Unit Fields........................... 7
 4.4   Parameters.......................................... 8
 4.5   Miscellaneous....................................... 8

 5   OVERVIEW OF THE PROTOCOL.............................. 9
 5.1   Internal Organization of the Network Layer.......... 9
 5.2   Subsets of the Protocol............................. 9
 5.3   Addressing......................................... 10
 5.4   Service Provided by the Network Layer.............. 10
 5.5   Service Assumed from the Subnetwork Service
    Provider.............................................. 11
 5.5.1   Subnetwork Addresses............................. 12
 5.5.2   Subnetwork Quality of Service.................... 12
 5.5.3   Subnetwork User Data............................. 13
 5.5.4   Subnetwork Dependent Convergence Functions....... 13
 5.6   Service Assumed from Local Evironment.............. 14

 6   PROTOCOL FUNCTIONS................................... 16
 6.1   PDU Composition Function........................... 16
 6.2   PDU Decomposition Function......................... 17
 6.3   Header Format Analysis Function.................... 17
 6.4   PDU Lifetime Control Function...................... 18
 6.5   Route PDU Function................................. 18
 6.6   Forward PDU Function............................... 19
 6.7   Segmentation Function.............................. 19
 6.8   Reassembly Function................................ 20
 6.9   Discard PDU Function............................... 21

ISO DIS 8473 (May 1984)                                         [Page i]

RFC 926                                                    December 1984

 6.10   Error Reporting Function.......................... 22
 6.10.1   Overview........................................ 22
 6.10.2   Requirements.................................... 23
 6.10.3   Processing of Error Reports..................... 24
 6.11   PDU Header Error Detection........................ 25
 6.12   Padding Function.................................. 26
 6.13   Security.......................................... 26
 6.14   Source Routing Function........................... 27
 6.15   Record Route Function............................. 28
 6.16   Quality of Service Maintenance Function........... 29
 6.17   Classification of Functions....................... 29

 7   STRUCTURE AND ENCODING OF PDUS....................... 32
 7.1   Structure.......................................... 32
 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............................................ 36   Segmentation Permitted and More Segments Flags. 36   Error Report Flag.............................. 37
 7.2.7   Type Code........................................ 37
 7.2.8   PDU Segment Length............................... 37
 7.2.9   PDUChecksum...................................... 38
 7.3   Address Part....................................... 38
 7.3.1   General.......................................... 38     Destination and Source Address Information... 39

 7.4   Segmentation Part.................................. 40
 7.4.1   Data Unit Identifier............................. 41
 7.4.2   Segment Offset................................... 41
 7.4.3   PDU Total Length................................. 41
 7.5   Options Part....................................... 41
 7.5.1   General.......................................... 41
 7.5.2   Padding.......................................... 43
 7.5.3   Security......................................... 43
 7.5.4   Source Routing................................... 44
 7.5.5   Recording of Route............................... 45
 7.5.6   Quality of Service Maintenance................... 46
 7.6   Priority........................................... 47

ISO DIS 8473 (May 1984)                                        [Page ii]

RFC 926                                                    December 1984

 7.7   Data Part.......................................... 47
 7.8   Data (DT) PDU...................................... 49
 7.8.1   Structure........................................ 49   Fixed Part..................................... 50   Addresses...................................... 50   Segmentation................................... 50   Options........................................ 50   Data........................................... 50
 7.9   Inactive Network Layer Protocol.................... 51
 7.9.1   Network Layer Protocol Id........................ 51
 7.9.2   Data Field....................................... 51
 7.10   Error Report PDU (ER)............................. 52
 7.10.1   Structure....................................... 52   Fixed Part.................................... 53   Addresses..................................... 53   Segmentation.................................. 53   Options....................................... 54   Reason for Discard............................ 54   Error Report Data Field....................... 55

 8   FORMAL DESCRIPTION................................... 56
 8.1   Values of the State Variable....................... 57
 8.2   Atomic Events...................................... 57
 8.2.1   N.UNITDATA_request and N.UNITDATA_indication..... 57
 8.2.2   SN.UNITDATA_request and SN.UNITDATA_indication... 58
 8.2.3   TIMER Atomic Events.............................. 59
 8.3   Operation of the Finite State Automation........... 59
 8.3.1   Type and Constant Definitions.................... 61
 8.3.2   Interface Definitions............................ 65
 8.3.3   Formal Machine Definition........................ 67

 9   CONFORMANCE.......................................... 84
 9.1   Provision of Functions for Conformance............. 84

ISO DIS 8473 (May 1984)                                       [Page iii]

RFC 926                                                    December 1984

ISO DIS 8473 (May 1984)                                        [Page iv]

RFC 926                                                    December 1984


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

 This Protocol Standard is positioned with respect to other related
 standards by the layers defined in the Reference Model for Open Systems
 Interconnection (ISO 7498). In particular, it is a protocol of the
 Network Layer. The Protocol herein described is a Subnetwork
 Independent Convergence Protocol combined with relay and routing
 functions as described in the Internal Organization of the Network
 Layer (ISO iiii). This Protocol provides the connectionless-mode
 Network Service as defined in ISO 8348/DAD1, Addendum to the Network
 Service Definition Covering Connectionless-mode Transmission, between
 Network Service users and/or Network Layer relay systems.

 The interrelationship of these standards is illustrated in Figure 0-1

      ______________OSI Network Service Definition______________  
                    |                             ^              
                    |                             |              
         Protocol     Reference to aims __________|              
      Specification | Reference to assumptions ___                
                    |                             |              
                    |                             |              
                    |                             v              
      ______________Subnetwork Service Definition(s) ___________  

              Figure 0-1.  Interrelationship of Standards

ISO DIS 8473 (May 1984)                                         [Page 1]

RFC 926                                                    December 1984


 This International Standard specifies a protocol which is used to
 provide the Connectionless-mode Network Service as described in ISO
 8348/DAD1, Addendum to the Network Service Definition Covering
 Connectionless-mode Transmission. The protocol herein described relies
 upon the provision of a connectionless-mode subnetwork service.

 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 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 a subnetwork service
      provider through the exchange of subnetwork service primitives.

ISO DIS 8473 (May 1984)                                         [Page 2]

RFC 926                                                    December 1984


 ISO 7498       Information Processing Systems - Open Systems
                Interconnection - Basic Reference Model

 DP 8524        Information Processing Systems - Open Systems
                Interconnection - Addendum to ISO 7498 Covering
                Connectionless-Mode Transmission

 DIS 8348       Information Processing Systems - Data Communications -
                Network Service Definition

 ISO 8348/DAD1  Information Processing Systems - Data Communications -
                Addendum to the Network Service Definition Covering
                Connectionless-Mode Transmission

 ISO 8348/DAD2  Information Processing Systems - Data Communications -
                Addendum to the Network Service Definition Covering
                Network Layer Addressing

 DP iiii        Information Processing Systems - Data Communications -
                Internal Organization of the Network Layer

 DP 8509        Information Processing Systems - Open Systems
                Interconnection - Service Conventions

 ISO TC97/SC16  A Formal Description Technique based on an N1825
                Extended State Transition Model

ISO DIS 8473 (May 1984)                                         [Page 3]

RFC 926                                                    December 1984



3.1  Reference Model Definitions

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

   a) Network layer
   b) Network service
   c) Network service access point
   d) network service access point address
   e) Network entity
   f) Routing
   f) Service
   h) Network protocol
   i) Network relay
   j) Network protocol data unit
   k) End system

3.2  Service Conventions Definitions

  This document makes use of the following concepts from the OSI Service
  Conventions (ISO 8509):

   l) Service user
   m) Service provider

3.3  Network Layer Architecture Definitions

  This document makes use of the following concepts from the Internal
  Organization of the Network Layer (ISO iiii):

   n) Subnetwork

ISO DIS 8473 (May 1984)                                         [Page 4]

RFC 926                                                    December 1984

   o) Relay system
   p) Intermediate system
   q) Subnetwork service

3.4  Network Layer Addressing Definitions

  This document makes use of the following concepts from DIS 8348/DAD2,
  Addendum to the Network Service Definition Covering Network layer

   r) Network entity title
   s) Network protocol address information
   t) Subnetwork address
   u) Domain

3.5  Additional Definitions

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

   a) automaton    -  a machine designed to follow automatically a
                      predetermined sequence of operations or to respond
                      to encoded instructions.

   b) local matter -  a decision made by a system concerning its
                      behavior in the Network Layer that is not subject
                      to the requirements of this Protocol.

   c) segment      -  part of the user data provided in the N_UNITDATA
                      request and delivered in the N_UNITDATA

   d) initial PDU  -  a protocol data unit carrying the whole of the
                      user data from an N_UNITDATA request.

   e) 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

ISO DIS 8473 (May 1984)                                         [Page 5]

RFC 926                                                    December 1984

   f) 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.]

   g) reassembly   -  the act of regenerating an initial PDU (in order
                      to issue an N_UNITDATA indication) from two or
                      more derived PDUs produced by segmentation.

ISO DIS 8473 (May 1984)                                         [Page 6]

RFC 926                                                    December 1984


4.1  Data Units

  PDU          Protocol Data Unit
  NSDU         Network Service 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

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

ISO DIS 8473 (May 1984)                                         [Page 7]

RFC 926                                                    December 1984

4.4  Parameters

  DA           Destination Address
  SA           Source Address
  QOS          Quality of Service

4.5  Miscellaneous

  SNICP        Subnetwork Independent Convergence Protocol
  SNDCP        Subnetwork Dependent Convergence Protocol
  SNAcP        Subnetwork Access Protocol
  SN           Subnetwork
  P            Protocol
  NSAP         Network Service Access Point
  SNSAP        Subnetwork Service Access Point
  NPAI         Network Protocol Address Information
  NS           Network Service

ISO DIS 8473 (May 1984)                                         [Page 8]

RFC 926                                                    December 1984


5.1  Internal Organization of the Network Layer

  The architecture of the Network Layer is described in a separate
  document, Internal Organization of the Network Layer (ISO iiii), in
  which an OSI Network Layer structure is defined, and a structure to
  classify protocols as an aid to the progression toward that structure
  is presented. This protocol is designed to be used in the context of
  the internetworking protocol approach defined in that document,
  between Network Service users and/or Network Layer relay systems. As
  described in the Internal Organization of the Network Layer, the
  protocol herein described is a Subnetwork Independent Convergence
  Protocol combined with relay and routing functions designed to allow
  the incorporation of existing network standards within the OSI

  A Subnetwork Independent Convergence Protocol is one which can be
  defined on a subnetwork independent basis and which is necessary to
  support the uniform appearance of the OSI Connectionless-mode Network
  Service between Network Service users and/or Network Layer relay
  systems over a set of interconnected homogeneous or heterogeneous
  subnetworks. This protocol is defined in just such a subnetwork
  independent way so as to minimize variability where subnetwork
  dependent and/or subnetwork access protocols do not provide the OSI
  Network Service.

  The subnetwork service required from the lower sublayers by the
  protocol described herein is identified in Section 5.5.

5.2  Subsets of the Protocol

  Two proper subsets of the full protocol are also defined which permit
  the use of known subnetwork characteristics, and are therefore not
  subnetwork independent.

  One protocol subset is for use where it is known that the source and
  destination end-systems are connected by a single subnetwork. This is
  known as the "Inactive Network Layer Protocol" subset. A second subset
  permits simplification of the header where it is known that the source
  and destination end-systems are connected by subnetworks whose
  subnetwork service data unit (SNSDU) sizes are greater than or equal
  to a known bound large enough for segmentation not to be required.
  This subset, selected by setting the "segmentation permitted" flag to
  zero, is known as the "non-segmenting" protocol subset.

ISO DIS 8473 (May 1984)                                         [Page 9]

RFC 926                                                    December 1984

5.3  Addressing

  The Source Address and Destination Address parameters referred to in
  Section 7.3 of this International Standard are OSI Network Service
  Access Point Addresses. The syntax and semantics of an OSI Network
  Service Access Point Address, the syntax and encoding of the Network
  Protocol Address Information employed by this Protocol, and the
  relationship between the NSAP and the NPAI is described in a separate
  document, ISO 8348/DAD2, Addendum to the Network Service Definition
  covering Network Layer Addressing.

  The syntax and semantics of the titles and addresses used for relaying
  and routing are also described in ISO 8348/DAD2.

5.4  Service Provided by the Network Layer

  The service provided by the protocol herein described is a
  connectionless-mode Network Service. The connectionless-mode Network
  Service is described in document ISO 8348/DAD1, Addendum to the
  Network Service Definition Covering Connectionless-mode Transmission.
  The Network Service primitives provided are summarized below:

ISO DIS 8473 (May 1984)                                        [Page 10]

RFC 926                                                    December 1984

                 Primitives                Parameters          
      |                           |                            |
      | N_UNITDATA Request        | NS_Destination_Address,    |
      |            Indication     | NS_Source_Address,         |
      |                           | NS_Quality_of_Service,     |
      |                           | NS_Userdata                |

                 Table 5-1.  Network Service Primitives

  The Addendum to the Network Service Definition Covering
  Connectionless-mode Transmission (ISO 8348/DAD1) states that the
  maximum size of a connectionless-mode Network-service-data-unit is
  limited to 64512 octets.

5.5  Service Assumed from the Subnetwork Service provider

  The subnetwork service required to support this protocol is defined as
  comprising the following primitives:

                Primitives                  Parameters          
      |                           |                            |
      | SN_UNITDATA Request       | SN_Destination_Address,    |
      |             Indication    | SN_Source_Address,         |
      |                           | SN_Quality_of_Service,     |
      |                           | SN_Userdata                |

               Table 5-2.  Subnetwork Service Primitives

ISO DIS 8473 (May 1984)                                        [Page 11]

RFC 926                                                    December 1984

  5.5.1  Subnetwork Addresses

   The source and destination addresses specify the points of attachment
   to a public or private subnetwork(s) involved in the transmission.
   Subnetwork addresses are defined in the Service Definition of each
   individual subnetwork.

   The syntax and semantics of subnetwork addresses are not defined in
   this Protocol Standard.

  5.5.2  Subnetwork Quality of Service

   Subnetwork Quality of Service describes aspects of a subnetwork
   connectionless-mode service which are attributable solely to the
   subnetwork service provider.

   Associated with each subnetwork connectionless-mode transmission,
   certain measures of quality of service are requested when the
   primitive action is initiated. These requested measures (or parameter
   values and options) are based on a priori knowledge by the Network
   Service provider of the service(s) made available to it by the
   subnetwork. Knowledge of the nature and type of service available is
   typically obtained prior to an invocation of the subnetwork
   connectionless-mode service.


     The quality of service parameters identified for the subnetwork
     connectionless-mode service may in some circumstances be directly
     derivable from or mappable onto those identified in the
     connectionless-mode Network Service; e.g., the parameters

      a)  transit delay;
      b)  protection against unauthorized access;
      c)  cost determinants;
      d)  priority; and
      e)  residual error probability

     as defined in ISO 8348/DAD1, Addendum to the Network Service
     Definition Covering Connectionless-mode Transmission, may be

ISO DIS 8473 (May 1984)                                        [Page 12]

RFC 926                                                    December 1984

     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, the subnetwork service provider shall attempt to
     deliver the protocol data unit at whatever Quality of Service is

  5.5.3  Subnetwork User Data

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

   The subnetwork service is required to support a subnetwork service
   data unit size of at least the maximum size of the Data PDU header
   plus one octet of NS-Userdata. This requires a minimum subnetwork
   service data unit size of 256 octets.

   Where the subnetwork service can support a subnetwork service data
   unit (SNSDU) size greater than the size of the Data PDU header plus
   one octet of NS_Userdata, the protocol may take advantage of this. In
   particular, if all SNSDU sizes of the subnetworks involved 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
   provide a connectionless-mode subnetwork service in the case where
   subnetworks also provide a connection-oriented subnetwork service. If
   a subnetwork provides a connection-oriented service, some subnetwork
   dependent function is assumed to provide a mapping into the required
   subnetwork service described in the preceding text.

   A Subnetwork Dependent Convergence Protocol may also be employed in
   those cases where functions assumed from the subnetwork service
   provider are not performed.

ISO DIS 8473 (May 1984)                                        [Page 13]

RFC 926                                                    December 1984

5.6  Service Assumed from Local Evironment

  A timer service is provided to allow the protocol entity to schedule

  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
  request primitive has elapsed.

  The S_TIMER cancel primitive is an indication to the local environment
  that the specified timer(s) should be cancelled. If the subscript
  parameter is not specified, then all timers with the specified name
  are cancelled; 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:

ISO DIS 8473 (May 1984)                                        [Page 14]

RFC 926                                                    December 1984

            Primitives                  Parameters            
      |                           |                            |
      | S_TIMER Request           | S_Time                     |
      |                           | S_Name                     |
      |                           | S_Subscript                |
      |                           |                            |
      | S_TIMER Response          | S_Name                     |
      |         Cancel            | S_Subscript                |

                      Table 5-3.  Timer Primitives

  The time parameter indicates the time duration of the specified timer.
  An identifying label is associated with a timer by means of the name
  parameter. The subscript parameter specifies a value to distinguish
  timers with the same name. The name and subscript taken together
  constitute a unique reference to the timer.

ISO DIS 8473 (May 1984)                                        [Page 15]

RFC 926                                                    December 1984



 This section describes the functions performed as part of the Protocol.

 Not all of the functions must be performed by every implementation.
 Section 6.17 specifies which functions may be omitted and the correct
 behavior where 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 of protocol given in Section 7. Protocol
  Control Information required for delivering the data unit to its
  destination is determined from current state information and from the
  parameters provided with the N_UNITDATA Request; e.g., source and
  destination addresses, QOS, etc. User data passed from the Network
  Service user in the N_UNITDATA Request forms the Data field of the
  protocol data unit.

  During the composition of the protocol data unit, a Data Unit
  Identifier is assigned to identify uniquely all segments of the
  corresponding NS_Userdata. The "Reassemble PDU" function considers
  PDUs to correspond to the same Initial PDU, and hence N_UNITDATA
  request, if they have the same Source and Destination Addresses and
  Data Unit Identifier.

  The Data Unit Identifier is available for ancillary functions such as
  error reporting. The originator of the PDU must choose the Data Unit
  Identifier so that it remains unique (for this Source and Destination
  Address pair) for the maximum lifetime of the PDU (or any Derived
  PDUs) in the network.

ISO DIS 8473 (May 1984)                                        [Page 16]

RFC 926                                                    December 1984

  During the composition of the PDU, a value of the total length of the
  PDU 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.

  Where the non-segmenting subset is employed, neither the Total Length
  field nor the Data Unit Identifier field is present. During the
  composition of the protocol data unit, a value of the total length of
  the PDU 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.

6.2  PDU Decomposition Function

  This function is responsible for removing the Protocol Control
  Information from the protocol data unit. During this process,
  information pertinent to the generation of the N_UNITDATA Indication
  is retained. The data field of the PDU received is reserved until all
  segments of the original service data unit have been received; this is
  the NS_Userdata 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
  indicating that this is a standard version of the Protocol, this
  function determines whether a PDU received has reached its destination
  using the destination address provided in the PDU is the same as the
  one which addresses 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
  indicating that the Inactive Network Layer Protocol subset is in use,
  then no further analysis of the PDU header is required. The

ISO DIS 8473 (May 1984)                                        [Page 17]

RFC 926                                                    December 1984

  network-entity in this case determines that either the network address
  encoded in the network protocol address information of a supporting
  subnetwork protocol corresponds to a network Service Access Point
  address served by this network-entity, or that an error has occurred.
  If the subnetwork PDU has been delivered correctly, then the protocol
  data unit may be decomposed according to the procedure described for
  that particular subnetwork protocol.

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 whether
  its assigned lifetime has expired, in which case it must be discarded.

  The operation of the 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
  Milliseconds). The Lifetime of the Initial PDU is determined by the
  originating network-entity, and placed in the Lifetime field of the

6.5  Route PDU Function

  This function determines the network-entity to which a protocol data
  unit should be forwarded, using the destination NSAP address
  parameters, Quality of Service parameter, and/or other parameters. It
  determines the subnetwork which must be transited to reach that
  network-entity. Where segmentation occurs, it further determines which
  subnetwork(s) the segments may transit to reach that network-entity.

ISO DIS 8473 (May 1984)                                        [Page 18]

RFC 926                                                    December 1984

6.6  Forward PDU Function

  This function issues a subnetwork service primitive (see Section 5.5)
  supplying the subnetwork identified by the "Route PDU" function with
  the protocol data unit as an SNSDU, and the address information
  required by that subnetwork to identify the "next" intermediate-system
  within the subnetwork-specific address domain.

  When an Error Report PDU is to be forwarded, and is longer than the
  maximum user data acceptable by the subnetwork, it shall be truncated
  to the maximum acceptable length ad forwarded with no other change.
  When a Data PDU is to be forwarded ad is longer than the maximum user
  data acceptable by the subnetwork, the Segmentation function is
  applied (See Section 6.7, which follows).

6.7  Segmentation Function

  Segmentation is performed when the size of the protocol data unit is
  greater than the maximum size of the user data parameter field of the
  subnetwork service primitive.

  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. The Protocol Control Information required to
  identify, route, and forward a PDU is duplicated in each PDU derived
  from the Initial PDU. The user data encapsulated within the PDU
  received is divided such that the Derived PDUs satisfy the size
  requirements of the user data parameter field of the subnetwork
  service primitive.

  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.

ISO DIS 8473 (May 1984)                                        [Page 19]

RFC 926                                                    December 1984

  The following fields of the PDU header are used in conjunction with
  the Segmentation function:

   a)  Segment Offset - identifies at which octet in the data field of
       the Initial PDU the segment begins;

   b)  Segment Length - specifies the number of octets in the Derived
       PDU, including both header and data;

   c)  More Segments Flag - 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 routing
  of the individual Derived PDUs.

  A Segmentation Permitted flag is set to one to indicate that
  segmentation is permitted. If the Initial PDU is not to be segmented
  at any point during its lifetime in the network, the flag is set to

  When the "Segmentation Permitted" flag is set to zero, the non-
  segmenting protocol subset is in use.

6.8  Reassembly Function

  The Reassembly Function reconstructs the Initial PDU transmitted to
  the destination network-entity from the Derived PDUs generated during
  the lifetime of the Initial PDU.

  A bound on the time during which segments (Derived PDUs) of an Initial
  PDU will be held at a reassembly point is provided so that resources
  may be released when it is no longer expected that any outstanding
  segments of the Initial PDU will arrive at the reassembly point. When
  such an event occurs, segments (Derived PDUs) of the Initial PDU held
  at the reassembly point are discarded, the resources allocated for
  those segments are freed,

ISO DIS 8473 (May 1984)                                        [Page 20]

RFC 926                                                    December 1984

  and if selected, an Error Report is generated.


    The design of the Segmentation and Reassembly functions is intended
    principally to be used such that reassembly takes place at the
    destination. 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/full reassembly at some point along the route
         where it is known that the subnetwork with the smallest PDU
         size has been transited

    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.

    While the exact relationship between reassembly lifetime and PDU
    lifetime is a local matter, the reassembly algorithm 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

6.9  Discard PDU Function

  This function performs all of the actions necessary to free the
  resources reserved by the network-entity in any of the following
  situations (Note: the list is not exhaustive):

   a)  A violation of protocol procedure has occurred.

   b)  A PDU is received whose checksum is inconsistent with its

ISO DIS 8473 (May 1984)                                        [Page 21]

RFC 926                                                    December 1984

   c)  A PDU is received, but due to 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 subnetwork
       service data unit size.

   f)  A PDU is received whose destination address is unreachable or

   g)  Incorrect or invalid source routing was specified. This may
       include a syntax error in the source routing field, and 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 the 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 the return of an Error Report PDU to the source
   network-entity when a protocol data unit is discarded. An "error
   report flag" in the original PDU is set by the source network-entity
   to indicate whether or not Error Report PDUs are to be returned.

   The Error Report PDU identifies the discarded PDU, specifies the type
   of error detected, and identifies the location at which the error was
   detected. Part or all of the discarded PDU is included in the data
   field of the Error Report PDU.

   The address of the originator of the Data Protocol Data Unit is

ISO DIS 8473 (May 1984)                                        [Page 22]

RFC 926                                                    December 1984

   conveyed as both the destination address of the Error Report PDU as
   well as the source address of the original Data PDU; the latter is
   contained in the Data field of the Error Report PDU. The address of
   the originator of the Error Report PDU is contained in the source
   address field of the header of the Error Report PDU.


     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 discarding
   of a PDU that itself contains an Error Report.

   An Error Report PDU shall not be generated upon discarding of a PDU
   unless that PDU has the Error Report flag set to allow Error Reports.

   If a Data PDU is discarded, and has the Error Report flag set to
   allow Error Reports, an Error Report PDU shall be generated if the
   reason for discard (See Section 6.9)  is

    a)  destination address unreachable,

    b)  source routing failure,

    c)  unsupported options, or

    d)  protocol violation.

ISO DIS 8473 (May 1984)                                        [Page 23]

RFC 926                                                    December 1984


     It is intended that this list shall include all nontransient
     reasons for discard; the list may therefore need to be amended or
     extended in the light of any changes made in the definitions of
     such reasons.

   If a Data PDU with the Error Report flag set to allow Error Reports
   is discarded for any other reason, an Error Report PDU may be
   generated (as an implementation option).

  6.10.3  Processing of Error Reports

   Error Report PDUs are forwarded by intermediate network-entities in
   the same way as Data PDUs. It is possible that an Error Report PDU
   may be longer than the maximum user data size of a subnetwork that
   must be traversed to reach the origin of the discarded PDU. In this
   case, the Forward PDU function shall truncate the PDU to the maximum
   size acceptable.

   The entire header of the discarded data unit shall be included in the
   data field of the Error Report PDU. Some or all of the data field of
   the discarded data unit may also be included.


     Since the suppression of Error Report PDUs is controlled by the
     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.

ISO DIS 8473 (May 1984)                                        [Page 24]

RFC 926                                                    December 1984

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 PDU header. The checksum is verified at each
  point at which the PDU header is processed. If PDU header fields are
  modified (for example, due to lifetime function), then the checksum is
  modified so that the checksum remains valid.

  An intermediate system network-entity must not recompute the checksum
  for the entire header, even if fields are modified.


    This is 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

  The use of this 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

   (SUM)     a   = 0  (modulo 255)

   (SUM)     (L-i+1) a   = 0 (modulo 255)

    Where L = the number of octets in the PDU header, and
          a = value of octet at position i.

ISO DIS 8473 (May 1984)                                        [Page 25]

RFC 926                                                    December 1984

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

  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 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.


    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

  An issue related to the quality of the network service is the
  protection of information flowing between transport-entities. A system
  may wish to control the distribution of secure data by assigning
  levels of security to PDUs. As a local consideration, the Network
  Service user could be authenticated to ascertain whether the user has
  permission to engage in communication at a particular security level
  before sending the PDU. While no protocol exchange is required in the
  authentication process, the optional security parameter in the options
  part of the PDU header may be employed to convey the particular
  security level between peer network-entities.

  The syntax and semantics of the security parameter are not specified
  by this Standard. The security parameter is related to the "protection
  from unauthorized access" Quality of service parameter described in
  ISO 8348/DAD1, Addendum to the Network Service Definition Covering
  Connectionless-mode Transmission. However, to facilitate
  interoperation between end-systems and relay-systems by avoiding
  different interpretations of the same encoding, a mechanism is
  provided to distinguish user-defined security encoding from
  standardized security encoding.

ISO DIS 8473 (May 1984)                                        [Page 26]

RFC 926                                                    December 1984

6.14  Source Routing Function

  The Source Routing function allows the originator to specify the path
  a generated PDU must take. Source routing can only be selected by the
  originator of a PDU. Source Routing is accomplished using a list of
  intermediate system addresses (or titles, see Section 5.3 and 5.5.1)
  held in a parameter within the options part of the PDU Header. The
  size of the option field is determined by the originating
  network-entity. The length of this option does not change as the PDU
  traverses the network. Associated with this list is an indicator which
  identifies the next entry in the list to be used; this indicator is
  advanced by the receiver of the PDU when the next address matches its
  own address. The indicator is updated as the PDU is forwarded so as to
  identify the appropriate entry at each stage of relaying.

  Two forms of the source routing option are provided. The first form,
  referred to as complete source routing, requires that the specified
  path must be taken; if the specified path cannot be taken, the PDU
  must be discarded. The source may be informed of the discard using the
  Error Reporting function described in Section 6.10.

  The second form is referred to as partial source routing. Again, each
  address in the list must be visited in the order specified while on
  route to the destination. However, with this form of source routing
  the PDU may take any path necessary to arrive at the next address in
  the list. The PDU will not be discarded (for source routing related
  causes) unless one of the addresses specified cannot be reached by any
  available route.

ISO DIS 8473 (May 1984)                                        [Page 27]

RFC 926                                                    December 1984

6.15  Record Route Function

  The Record Route function permits the exact recording of the paths
  taken by a PDU as it traverses a series of interconnected subnetworks.
  A recorded route is composed of a list of intermediate system
  addresses held in a parameter within the options part of the PDU
  header. The size of the option field is determined by the originating
  network-entity. The length of this option does not change as the PDU
  traverses the network.

  The list is constructed as the PDU traverses a set of interconnected
  subnetworks. Only intermediate system addresses are included in the
  recorded route. The address of the originator of the PDU is not
  recorded in the list. When an intermediate system network-entity
  processes a PDU containing the record route parameter, the system
  inserts its own address (or titles, see Sections 5.3 or 5.5.1) into
  the list of recorded addresses.

  The record route option contains an indicator which identifies the
  next available octet to be used for recording of route. This
  identifier is updated as entries are added to the list. If the
  addition of the current address to the list would exceed the size of
  the option field, the indicator is set to show that recording of route
  has terminated. The PDU may still be forwarded to its final
  destination, without further addition of intermediate system


    The Record Route function is principally intended to be used in the
    diagnosis of network problems. Its mechanism has been designed on
    this basis, and may provide a return path.

ISO DIS 8473 (May 1984)                                        [Page 28]

RFC 926                                                    December 1984

6.16  Quality of Service Maintenance Function

  In order to support the Quality of Service requested by Network
  Service users, the Protocol may need to make QOS information available
  at intermediate systems. This information may be used by network
  entities in intermediate systems to make routing decisions where such
  decisions affect the overall QOS provided to NS users.

  In those instances where the QOS indicated cannot be maintained, the
  NS provider will attempt to deliver the PDU at a QOS less than that
  indicated. The NS provider will not necessarily provide a notification
  of failure to meet the indicated quality of service.

6.17  Classification of Functions

  Implementations do not have to support all of the functions described
  in Section 6. Functions are divided into three categories:

   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 by a PDU, then the PDU shall be
            discarded, and an Error Report PDU shall be generated and
            forwarded to the originating network-entity, providing that
            the Error Report flag is set.

   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 by a PDU, then the function is not
            performed and the PDU is processed exactly as though the
            function was not selected. The protocol data unit shall not
            be discarded.

  Table 6-1 shows how the functions are divided into these three

ISO DIS 8473 (May 1984)                                        [Page 29]

RFC 926                                                    December 1984

         | Function                       |  Type            |
         |                                |                  |
         | PDU Composition                |  1               |
         | PDU Decomposition              |  1               |
         | Header Format Analysis         |  1               |
         | PDU Lifetime Control           |  1               |
         | Route PDU                      |  1               |
         | Forward PDU                    |  1               |
         | Segment PDU                    |  1               |
         | Reassemble PDU                 |  1               |
         | Discard PDU                    |  1               |
         | Error Reporting                |  1 (note 1)      |
         | PDU Header Error Detection     |  1 (note 1)      |
         | Padding                        |  1 (notes 1   2) |
         | Security                       |  2               |
         | Complete Source Routing        |  2               |
         | Partial Source Routing         |  3               |
         | Priority                       |  3               |
         | Record Route                   |  3               |
         | Quality of Service Maintenance |  3               |

            Table 6-1.  Categorization of Protocol Functions

ISO DIS 8473 (May 1984)                                        [Page 30]

RFC 926                                                    December 1984


   1)  While the Padding, Error Reporting, and Header Error Detection
       functions must be provided, they are provided only when selected
       by the sending Network Service user.

   2)  The correct treatment of the Padding function involves no
       processing. Therefore, this could equally be described as a Type
       3 function.

   3)  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 and should not be the cause of the discarding of a PDU
       when not supported.

ISO DIS 8473 (May 1984)                                        [Page 31]

RFC 926                                                    December 1984


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 put into an SNSDU. The bits in an octet
  are numbered from one (1) to eight (8), where bit one (1) is the
  low-order bit.

  When consecutive octets are used to represent a binary number, the
  lower octet number has the most significant value.

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


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

     a)  octets are shown with the lowest numbered octet to the left,
         higher number octets being further to the right;

     b)  within an octet, bits are shown with bit eight (8) to the left
         and bit one (1) to the right.

  PDUs shall contain, in the following order:

   1)  the header, comprising:

    a)  the fixed part;

    b)  the address part;

    c)  the segmentation part, if present;

    d)  the options part, if present


ISO DIS 8473 (May 1984)                                        [Page 32]

RFC 926                                                    December 1984

   2)  the data field, if present.

  This structure is illustrated below:

                       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 7-1.  PDU Structure

ISO DIS 8473 (May 1984)                                        [Page 33]

RFC 926                                                    December 1984

7.2 Fixed Part

  7.2.1 General

   The fixed part contains frequently occuring parameters 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:

            | Network Layer Protocol Identifier  |     1    
            |         Length Indicator           |     2    
            |   Version/Protocol Id Extension    |     3    
            |            Lifetime                |     4    
            |S |M |E/R|         Type             |     5    
            | P| S|   |                          |          
            |          Segment Length            |    6,7  
            |             Checksum               |    8,9  

                  Figure 7-2.  PDU Header--Fixed Part

  7.2.2 Network Layer Protocol Identifier

   The value of this field shall be binary 1000 0001. This field
   identifies this Network Layer Protocol as ISO 8473, Protocol for
   Providing the Connectionless-mode Network Service.

ISO DIS 8473 (May 1984)                                        [Page 34]

RFC 926                                                    December 1984

  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 Section 7.1, Structure. The value 255 (1111
   1111) is reserved for possible future extensions.


     The rules for forwarding and segmentation ensure 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.

  7.2.4 Version/Protocol Identifier Extension

   The value of this field is binary 0000 0001. This Identifies a
   standard version of ISO 8473, Protocol for Providing the
   Connectionless-mode Network Service.

  7.2.5 PDU Lifetime

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

   The Lifetime field is set by the originating network-entity, and is
   decremented by every network-entity which processes the PDU. The PDU
   shall be discarded if the value of the field reaches zero.

   When a network-entity processes a PDU, it decrements the Lifetime by
   at least one. The Lifetime shall be decremented by more than one if
   the sum of:

    1)  the transit delay in the subnetwork from which the PDU was
        received; and

ISO DIS 8473 (May 1984)                                        [Page 35]

RFC 926                                                    December 1984

    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 error exists the value used shall be an
   overestimate, not an underestimate.

   If the Lifetime reaches a value of zero before the PDU is delivered
   to the destination, the PDU shall be discarded. The Error Reporting
   function shall be invoked, as described in Section 6.10, Error
   Reporting Function, and may result in the generation of an ER PDU. It
   is a local matter whether the destination network-entity performs the
   Lifetime Control function.

   When the Segmentation function is applied to a PDU, the Lifetime
   field is copied into all of the Derived PDUs.

  7.2.6 Flags Segmentation Permitted and More Segments Flags

    The Segmentation Permitted flag determines whether segmentation is
    permitted. A value of one indicates that segmentation is permitted.

    A value of zero indicates that the non-segmenting protocol subset is
    employed. Where this is the case, the segmentation part of the PDU
    header is not present, and the Segment Length field serves as the
    Total Length field.

    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) then
    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 Segmentation Permitted flag is not set to one (1).

ISO DIS 8473 (May 1984)                                        [Page 36]

RFC 926                                                    December 1984

    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. Error Report Flag

    When the Error Report flag is set to one, the rules in Section 6.10
    are used to determine whether to generate an Error Report PDU upon
    discard of the PDU.

    When the Error Report flag is set to zero, discard of the 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 7-1:

                                Bits    5 4 3 2 1  
                    |  DT PDU  |        1 1 1 0 0 |
                    |  ER PDU  |        0 0 0 0 1 |

                      Table 7-1.  Valid PDU Types

  7.2.8 PDU Segment Length

   The Segment Length field specifies the entire length of the PDU
   segment including both header and data, if present. When the full
   protocol is employed and a PDU is not segmented, then the value of
   this field is identical to the value of the Total Length field
   located in the Segmentation Part of the header.

ISO DIS 8473 (May 1984)                                        [Page 37]

RFC 926                                                    December 1984

   When the Non-segmenting protocol subset is employed, no segmentation
   part is present in the header. In this subset, the Segment Length
   field serves as the Total Length field of the header (see Section

  7.2.9 PDU Checksum

   The checksum is computed on the entire PDU header. This includes the
   segmentation and options parts, if present. A checksum value of zero
   is reserved to indicate that the checksum is to be ignored. The
   operation of the PDU Header Error Detection function ensures that the
   value zero does not represent a valid checksum. A non-zero value
   indicates that the checksum must be processed or the PDU must be

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
   illustrated below:

ISO DIS 8473 (May 1984)                                        [Page 38]

RFC 926                                                    December 1984

          |                                      |          
          | Destination Address Length Indicator |      10  
          |                                      |          
          |                                      |      11  
          |         Destination Address          |          
          |                                      |      m-1  
          |                                      |          
          |   Source Address Length Indicator    |       m  
          |                                      |          
          |                                      |      m+1  
          |           Source Address             |          
          |                                      |      n-1  

                 Figure 7-3.  PDU header--Address Part Destination and Source Address Information

    The Destination and Source addresses are Network Service Access
    Point addresses as defined in ISO 8348/DAD2, Addendum to the Network
    Service Definition Covering Network Layer Addressing.

    The Destination and Source Address information is of variable

    The Destination Address Length Indicator field specifies the length
    of the Destination Address in number of 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 number of octets. The Source Address
    Length Indicator field follows the Destination Address field. The
    Source Address field follows the Source Address Length Indicator

ISO DIS 8473 (May 1984)                                        [Page 39]

RFC 926                                                    December 1984

    Each address parameter is encoded as follows:

                      Bits   8   7   6   5   4   3   2   1  
            | Octet  | Address parameter Length Indicator |
            |   n    |           (e.g., 'm')              |
            | Octets |                                    |
            |  n+1   |     Address Parameter Value        |
            | thru   |                                    |
            |  n+m   |                                    |

                     Table 7-2.  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 below, must be present:

                |  Data Unit Identifier  |     n,n+1  
                |     Segment Offset     |    n+2,n+3  
                |      Total Length      |    n+4,n+5  

               Figure 7-4.  PDU Header--Segmentation Part

  Where the "Segmentation Permitted" flag is set to zero, the
  nonsegmenting protocol subset is in use.

ISO DIS 8473 (May 1984)                                        [Page 40]

RFC 926                                                    December 1984

  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
   reassembled by the destination network-entity. The Data Unit
   Identifier size is two octets.

  7.4.2 Segment Offset

   For each segment the Segment Offset field specifies the relative
   position of the segment in the data part of the Initial PDU with
   respect to the start of the data field. The offset is measured in
   units of octets. The offset of the first segment is zero.

  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 in
   any segment (Derived PDU) for the lifetime of the PDU.

7.5 Options Part

  7.5.1 General

   The options part is used to convey optional parameters. If the
   options part is present, it contains one or more parameters. The
   number of parameters that may be contained in the options part is
   indicated by the length of the options part which is:

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

ISO DIS 8473 (May 1984)                                        [Page 41]

RFC 926                                                    December 1984

   The options part of the PDU header is illustrated below:

                   |                    |       n+6  
                   |      Options       |            
                   |                    |       p    

                 Figure 7-5.  PDU Header--Options Part

   Each parameter contained within the options part of the PDU header is
   encoded as follows:

                          BITS    8  7  6  5  4  3  2  1  
             |  Octets  |                               |
             |    n     |  Parameter Code               |
             |   n+1    |  Parameter Length (e.g., 'm') |
             |   n+2    |  Parameter Value              |
             |  n+m+1   |                               |

                   Table 7-3.  Encoding of Parameters

   The parameter code field is coded in binary and, without extensions,
   provides a maximum number of 255 different parameters. However, as
   noted below, bits 8 and 7 cannot take every possible value, so the
   practical maximum number of different parameters is less. A parameter
   code of 255 (binary 1111 1111) is reserved for possible extensions of
   the parameter code.

   The parameter length field indicates the length, in octets, of the
   parameter value field. The length is indicated by a binary number,
   'm', with a theoretical maximum value of 255. 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 indication itself. Thus, the
   value of 'm' is limited to:

ISO DIS 8473 (May 1984)                                        [Page 42]

RFC 926                                                    December 1984

     253 - (length of fixed part +
     length of address part +
     length of segmentation part).

   For each succeeding parameter the maximum value of 'm' decreases.

   The parameter value field contains the value of the parameter
   identified in the parameter code field.

   No parameter codes use bits 8 and 7 with the value 00.

   Implementations shall accept the parameters defined in the options
   part in any order. Duplication of options (where detected) is not
   permitted. Receipt of a PDU with an option duplicated should be
   treated as a protocol error. The rules governing the treatment of
   protocol errors are described in Section 6.10, Error Reporting

   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
   convenient size (See Section 6.12).

    Parameter Code:       1100 1100
    Parameter Length:     variable
    Parameter Value:      any value is allowed

  7.5.3 Security

   This parameter is user defined.

    Parameter Code:       1100 0101
    Parameter Length:     variable
    Parameter Value:

     High order bit of first octet is Security Domain bit, S, to be
     interpreted as follows:

ISO DIS 8473 (May 1984)                                        [Page 43]

RFC 926                                                    December 1984


       | S | User Defined        ----


       | S | CODE | ORGANIZATION ----


       CODE = This field contains a geographic or non-geographic code to
              which the option applies.

       ORGANIZATION = This is a further subdivision of the CODE field
                      and is determined by an administrator of the
                      geographic or non-geographic domain identified by
                      the value of CODE.

  7.5.4 Source Routing

   The source routing parameter specifies, either completely or
   partially, the route to be taken from Source Network Address to
   Destination Network Address.

    Parameter Code:      1100 1000
    Parameter Length:    variable
    Parameter Value:     2 octet control information
                         succeeded by a concatenation
                         of ordered address fields
                         (ordered from source to destination)

ISO DIS 8473 (May 1984)                                        [Page 44]

RFC 926                                                    December 1984

   The first octet of the parameter value is the type code. This has the
   following significance.

    0000 0001     complete source routing
    0000 0000     partial source routing

    <all other values reserved>

   The second octet indicates the octet offset of the next address to be
   processed in the list. A value of three (3) indicates that the next
   address begins immediately after this control octet. Successive
   octets are indicated by correspondingly larger values of this

   The third octet begins the intermediate-system address list. The
   address list consists of variable length address fields. The first
   octet of each address field identifies the length of the address
   which comprises the remainder of the address field.

  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:      two octets control information
                          succeeded  by a concatenation of
                          ordered addresses

   The first octet is used to indicate that the recording of route has
   been terminated owing to lack of space in the option. It has the
   following significance:

    0000 0000     Recording of Route still in progress
    1111 1111     Recording of Route terminated

    <all other values reserved>

ISO DIS 8473 (May 1984)                                        [Page 45]

RFC 926                                                    December 1984

   The second octet identifies the next octet which may be used to
   record an address. It is encoded relative to the start of the
   parameter, such that a value of three (3) indicates that the octet
   after this one is the next to be used.

   The third octet begins the address list. The address list consists of
   variable length address fields. The first octet of each address field
   identifies the length of the address which comprises the remainder of
   the field. Address fields are always added to the beginning of the
   address list; i.e., the most recently added field will begin in the
   third octet of the parameter value.

  7.5.6 Quality of Service Maintenance

   The Quality of Service parameter conveys information about the
   quality of service requested by the originating Network Service user.
   At intermediate systems, Network Layer relay entities 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 Section 6.16).

    Parameter Code:       1100 0011
    Parameter Length:     one octet
    Parameter Value:      Bit 8:  transit delay vs. cost
                          Bit 7:  residual error probability vs.
                                  transit delay
                          Bit 6:  residual error probability vs.
                          Bits 5 thru 0 are not specified.

   Bit 8 is set to one indicates that where possible, routing decision
   should favor low transit delay over low cost. A value of 0 indicates
   that routing decisions should favor low cost over low transit delay.

ISO DIS 8473 (May 1984)                                        [Page 46]

RFC 926                                                    December 1984

   Bit 7 set to one indicates 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 6 is set to one indicates that where possible, routing decisions
   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.

7.6 Priority

  The priority parameter carries the relative priority of the protocol
  data unit. Intermediate systems that support this option should make
  use of this information in routing and in ordering PDUs for

   Parameter Code:       1100 1100
   Parameter Length:     one octet
   Parameter Value:      0000 0000 - Normal (Default)
                         0000 1111 - Highest

  The values 0000 0001 through 0000 1111 are to be used for higher
  priority protocol data units. If an intermediate system does not
  support this option then all PDUs shall be treated as if the field had
  the value 0000 0000.

7.7 Data Part

  The Data part of the PDU is structured as an ordered multiple of
  octets, 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 below:

ISO DIS 8473 (May 1984)                                        [Page 47]

RFC 926                                                    December 1984

                  |                  |      p+1      
                  |       Data       |                
                  |                  |       z        

                  Figure 7-6.  PDU header--Data Field

ISO DIS 8473 (May 1984)                                        [Page 48]

RFC 926                                                    December 1984

7.8 Data (DT) PDU

  7.8.1 Structure

   The DT PDU has the following format:

     |  Network Layer Protocol Identifier   |      1              
     |           Length Indicator           |      2              
     |   Version/Protocol Id Extension      |      3              
     |              Lifetime                |      4              
     |SP|MS|E/R|      Type                  |      5              
     |           Segment Length             |     6,7            
     |              Checksum                |     8,9            
     | Destination Address Length Indicator |     10              
     |         Destination Address          |     11 through m-1  
     |    Source Address Length Indicator   |      m              
     |            Source Address            |     m+1 through n-1
     |         Data Unit Identifier         |     n,n+1          
     |            Segment Offset            |     n+2,n+3        
     |             Total Length             |     n+4,n+5        
     |                Options               |     n+6 through p  
     |                 Data                 |     p+1 through z  

                     Figure 7-7.  PDU Header Format

ISO DIS 8473 (May 1984)                                        [Page 49]

RFC 926                                                    December 1984 Fixed Part

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

    See Section 7.3. Segmentation

    See Section 7.4. Options

    See Section 7.5. Data

    See Section 7.7.

ISO DIS 8473 (May 1984)                                        [Page 50]

RFC 926                                                    December 1984

7.9 Inactive Network Layer Protocol

            |  Network Layer Protocol Id  |     1          
            |           Data              |     2 through n

              Figure 7-9.  Inactive Network Layer Protocol

  7.9.1 Network Layer Protocol Id

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

  7.9.2 Data Field

   See Section 7.7.

   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.

ISO DIS 8473 (May 1984)                                        [Page 51]

RFC 926                                                    December 1984

7.10 Error Report PDU (ER)

  7.10.1 Structure

     |   Network Layer Protocol Identifier  |       1            
     |           Length Indicator           |       2            
     |     Version/Protocol Id Extension    |       3            
     |               Lifetime               |       4            
     |SP|MS|E/R|       Type                 |       5            
     |             Segment Length           |      6,7            
     |                Checksum              |      8,9            
     | Destination Address Length Indicator |      10            
     |         Destination Address          |     10 through m-1  
     |     Source Address Length Indicator  |       m            
     |             Source Address           |     m+1 through n-1
     |          Data Unit Identifier        |     n,n+1          
     |             Segment Offset           |     n+2,n+3        
     |              Total Length            |     n+4,n+5        
     |                Options               |     n+6 through p-1
     |           Reason for Discard         |     p through q-1  
     |       Error Report Data Field        |       z            

                     Figure 7-10.  Error Report PDU

ISO DIS 8473 (May 1984)                                        [Page 52]

RFC 926                                                    December 1984 Fixed Part

    The fixed part of the Error Report Protocol Data Unit is set as
    though this is a new (Initial) PDU. Thus, references are provided to
    precious sections describing the composition of the fields
    comprising the fixed part:

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

    See Section 7.3.

    The Destination Address specifies the original source of the
    discarded PDU. The Source Address specifies the intermediate system
    or end system network-entity initiating the Error Report PDU. Segmentation

    See Section 7.4.

ISO DIS 8473 (May 1984)                                        [Page 53]

RFC 926                                                    December 1984 Options

    See Section 7.5. Reason for Discard

    This parameter is only valid for the Error Report PDU. It provides a
    report on the discarded protocol data unit.

    Parameter Code:

     1100 0001

    Parameter Length:

     two octets
     type of error encoded in binary:

      0000 0000:  Reason not specified.
      0000 0001:  Protocol Procedure Error.
                  other than below:
      0000 0010:  Incorrect checksum.
      0000 0011:  PDU discarded due to congestion.
      0000 0100:  Header syntax error (header cannot
                  be parsed).
      0000 0101:  Segmentation is needed but is not

      1000 xxxx:  Addressing Error:
                  0000 0000:  Destination Address
                  1000 0001:  Destination Address

      1001 xxxx:  Source Routing Error:
                  1001 0000:  Unspecified Source
                              Routing error.
                  1001 0001:  Syntax error in Source
                              Routing field.
                  1001 0010:  Unknown Address in
                              Source Routing field.
                  1001 0011:  Path not acceptable.

ISO DIS 8473 (May 1984)                                        [Page 54]

RFC 926                                                    December 1984

      1010 xxxx:  Lifetime Expiration:
                  1010 0000:  Lifetime expired while
                              data unit in transit.
                  1010 0001:  Lifetime expired
                              during reassembly.

      1011 xxxx:  PDU discarded due to unsupported
                  1011 0000:  unsupported option not
                  1011 0001:  unsupported padding
                  1011 0010:  unsupported security
                  1011 0011:  unsupported source
                              routing option.
                  1011 0100:  unsupported recording
                              of route option.
                  1011 0101:  unsupported QoS
                              Maintenance option.

     The second octet contains a pointer to the field in the associated
     discarded PDU which caused the error. If no one particular field
     can be associated with the error, then this field contains the
     value of zero. Error Report Data Field

    This field provides all or a portion of the discarded PDU. The
    octets comprising this field contain the rejected or discarded PDU
    up to and including the octet which caused the rejection/discard.

ISO DIS 8473 (May 1984)                                        [Page 55]

RFC 926                                                    December 1984


 The operation of the protocol is modelled as a finite state automaton
 governed by a state variable with three values. The behavior of the
 automaton is defined with respect to individual independent Protocol
 Data Units. A transition of the automaton is prompted by the occurrence
 of an atomic event at one of three interfaces:

  1) an interface to the Transport Layer, defined by the service
      primitives of the Addendum to the Network Service Definition
      Covering Connectionless-mode Transmission;

  2) an interface to the subnetwork service provider, defined by the
      SN_UNITDATA primitive of Section 5.5 of this Standard;

  3) an interface to an implementation-dependent timer function defined
      by the TIMER primitives described in Section 5.6 of this Standard.

 In addition, a transition of the automaton may be prompted by the
 occurrence of a condition of the automaton.

 The atomic events are defined in Section 8.2. The occurrence of an
 atomic event is not in itself sufficient to cause a transition to take
 place; other conditions, called "enabling conditions" may also have to
 be met before a particular transition can take place. Enabling
 conditions are boolean expressions that depend on the values of
 parameters associated with the corresponding atomic event (that is, the
 parameters of some primitive), and on the values of locally maintained

 More than one enabling condition -- and therefore, more than one
 possible transition -- may be associated with a single atomic event. In
 every such case, the enabling conditions are mutually exclusive, so
 that for any given combination of atomic event and parameter values,
 only one state transition can take place.

 Associated with each transition is an action, or "output." Actions
 consist of changes to the values of local variables and the sequential
 performance of zero or more functions. The operation of the finite
 state automaton is completely specified in Section 8.3 by defining the
 action associated with every possible transition.

ISO DIS 8473 (May 1984)                                        [Page 56]

RFC 926                                                    December 1984

8.1  Values of the State Variable

  The protocol state variable has three values:

  1)  INITIAL       The automaton is created in the INITIAL state.  No
                    transition may carry the automaton into the INITIAL

  2)  REASSEMBLING  The automaton is in the REASSEMBLING state for the
                    period in which it is assembling PDU segments into a
                    complete PDU.

  3)  CLOSED        The final state of the automaton is the  CLOSED
                    state.  When the automaton enters the CLOSED state
                    it ceases to exist.

8.2  Atomic Events

  An atomic event is the transfer of a unit of information across an
  interface.  The description of an atomic event specifies a primitive
  (such as an N_UNITDATA.Request), and the service boundary at which it
  is invoked (such as the Network Service boundary). The direction of
  information flow across the boundary is implied by the definition of
  each of the primitives.

  8.2.1  N.UNITDATA_request and N.UNITDATA_indication

   The N.UNITDATA_request and N.UNITDATA_indication atomic events occur
   at the Network Service boundary. They are defined by the Addendum to
   the Network Service Definition Covering Connectionless Data
   Transmission (ISO 8348/DAD1).

ISO DIS 8473 (May 1984)                                        [Page 57]

RFC 926                                                    December 1984

   N.UNITDATA_request    (NS Source_Address,

   N.UNITDATA_indication (NS_Source_Address,
                          NS_Quality_of_Service, NS_Userdata)

   The     parameters     of     the     N.UNITDATA_request      and
   N.UNITDATA_indication  are  collectively  referred  to as Network
   Service Data Unit (NSDUs).

  8.2.2  SN.UNITDATA_request and SN.UNITDATA_indication

   The SN.UNITDATA_request and SN.UNITDATA_indication atomic events
   occur at the interface between the Protocol described herein and a
   subnetwork service provider. They are defined in Section 5.5 of this

   SN.UNITDATA_request    (SN_Source_Address,

   SN.UNITDATA_indication (SN_Source_Address,

   The parameters of the SN_UNITDATA request and SN_UNITDATA Indication
   are collectively referred to as Subnetwork Service Data Units

   The value of the SN_Userdata parameter may represent an Initial PDU
   or a Derived PDU.

ISO DIS 8473 (May 1984)                                        [Page 58]

RFC 926                                                    December 1984

  8.2.3  TIMER Atomic Events

   The TIMER atomic events occur at the interface between the Protocol
   described herein and its local environment. They are defined in
   Section 5.6 of this Standard.

    S.TIMER_request  (Time,

    S.TIMER_cancel   (Name

    S.TIMER_response (Name,

8.3  Operation of the Finite State Automation

  The operation of the automaton is defined by use of the formal
  description technique and notation specified in ISO/TC97/SC16 N1347.
  This technique is based on an extended finite state transition model
  and the Pascal programming language. The technique makes use of strong
  variable typing to reduce ambiguity in interpretation of the

  This specification formally specifies an abstract machine which
  provides a single instance of the Connectionless-Mode Network Service
  by use of the Protocol For Providing the Connectionless-Mode Network
  Service. It should be emphasized that this formal specification does
  not in any way constrain the internal operation or design of any
  actual implementation. For example, it is not required that the
  program segments contained in the state transitions will actually
  appear as part of an actual implementation. A formal protocol
  specification is useful in that it goes as far as possible to
  eliminate any degree of ambiguity or vagueness in the specification of
  a protocol standard.

  The formal specification contained here specifies the behavior of a
  single finite-state machine, which provides the protocol

ISO DIS 8473 (May 1984)                                        [Page 59]

RFC 926                                                    December 1984

  behavior corresponding to a single independent service request. It is
  expected that any actual implementation will be able to handle
  behavior corresponding to many simultaneous finite state machines.

ISO DIS 8473 (May 1984)                                        [Page 60]

RFC 926                                                    December 1984

  8.3.1  Type and Constant Definitions


    ZERO  = 0;
    max_user_data = 64512;


    NSAP_addr_type  = ...;

     { NSAP_addr_type defines the data type for NSAP addresses, as
     passed across the Network Service Boundary. }

    NPAI_addr_type  = ...;

     { NPAI_addr_type defines the data type for the addresses carried in
     PDUs. }

    SN_addr_type    = ...;

     { SN_addr_type defines the data type for addresses in the
     underlying service used by this protocol. }

    quality_of_service_type = ...;

     { Quality_of_service_type defines the data type for the QOS
     parameter passed across the Network Service boundary. }

    SN_QOS_type     = ...;

     { SN_QOS_type defines the data type for the QOS parameter, if any,
     passed to the underlying service used by this protocol. }

    data_type       = ...;

     { Data_type defines the data type for user data. Conceptually this
     is equivalent to a variable length binary string. }

    buffer_type     = ...;

     { Buffer_type defines the data type for the memory resources used
     in sending and receiving of user data.  This provides capabilities
     required for segmentation and reassembly. }

ISO DIS 8473 (May 1984)                                        [Page 61]

RFC 926                                                    December 1984

    timer_name_type = (lifetime_timer);
    timer_data_type = ...;

    network_layer_protocol_id_type = (ISO_8473_protocol_id);
    version_id_type  = (version1);
    pdu_tp_type      = (DT, ER);

    options_type    = ...;

     { Options_type defines the data type used to store the options part
     of the PDU header. }

    subnet_id_type  = ...;

     { The subnet_id_type defines the data type used to locally identify
     a particular underlying service used by this protocol.  In general
     there may be multiple underlying subnetwork (or data link)
     services. }

    error_type      = (NO_ERROR,
                       INCORRECT_CHECKSUM, CONGESTION,

ISO DIS 8473 (May 1984)                                        [Page 62]

RFC 926                                                    December 1984

   nsdu_type = record
                   da   : NSAP_addr_type;
                   sa   : NSAP_addr_type;
                   qos  : quality_of_service_type;
                   data : data_type;

   pdu_type = record
                   nlp_id   : network_layer_protocol_id_type;
                   hli      : integer;
                   vp_id    : version_id_type; lifetime : integer;
                   sp       : boolean;
                   ms       : boolean;
                   er_flag  : boolean;
                   pdu_tp   : pdu_tp_type;
                   seg_len  : integer;
                   checksum : integer;
                   da_len   : integer;
                   da       : NPAI_addr_type;
                   sa_len   : integer;
                   sa       : NPAI_addr_type;
                   du_id    : optional integer;
                   so       : optional integer;
                   tot_len  : optional integer;
                      { du_id, so, and tot_len are present
                       only if sp has the value TRUE. }
                   options  : options_type;
                   data     : data_type;

ISO DIS 8473 (May 1984)                                        [Page 63]

RFC 926                                                    December 1984

   route_result_type =

            subnet_id    : subnet_id_type;
            sn_da        : SN_addr_type;
            sn_sa        : SN_addr_type;
            segment_size : integer;

ISO DIS 8473 (May 1984)                                        [Page 64]

RFC 926                                                    December 1984

  8.3.2  Interface Definitions

   channel Network_access_point (User, Provider);

    by User:
           (NS_Destination_address : NSAP_addr_type;
            NS_Source_address      : NSAP_addr_type;
            NS_Quality_of_Service  : quality_of_service_type;
            NS_Userdata            : data_type);

    by Provider:
           (NS_Destination_address : NSAP_addr_type;
            NS_Source_address      : NSAP_addr_type;
            NS_Quality_of_Service  : quality_of_service_type;
            NS_Userdata            : data_type);

   channel Subnetwork_access_point (User, Provider);

    by User:
           (SN_Destination_address : SN_addr_type;
            SN_Source_address      : SN_addr_type;
            SN_Quality_of_Service  : SN_QOS_type;
            SN_Userdata            : pdu_type);

    by Provider:
           (SN_Destination_address : SN_addr_type;
            SN_Source_address      : SN_addr_type;
            SN_Quality_of_Service  : SN_QOS_type;
            SN_Userdata            : pdu_type);

   channel System_access_point (User, Provider);

    by User:
           (Time      : integer;
            Name      : timer_name_type;
            Subscript : integer);

ISO DIS 8473 (May 1984)                                        [Page 65]

RFC 926                                                    December 1984


             (Name      : timer_name_type;
              Subscript : integer);

    by Provider:
           (Name      : timer_name_type;
            Subscript : integer);

ISO DIS 8473 (May 1984)                                        [Page 66]

RFC 926                                                    December 1984

  8.3.3  Formal Machine Definition

   module Connectionless_Network_Protocol_Machine
        (N:  Network_access_point (Provider) common queue;
         SN: array [subnet_id_type] of Subnetwork_access_point
                                          (User) common queue;
         S:  System_access_point (User) individual queue );

       nsdu    : nsdu_type;
       pdu     : pdu_type;
       rcv_buf : buffer_type;


ISO DIS 8473 (May 1984)                                        [Page 67]

RFC 926                                                    December 1984

   procedure send_error_report (error : error_type;
                                pdu   : pdu_type);

        er_pdu : pdu_type;

     if (pdu.er_flag) then
       er_pdu.nlp_id   := ISO_8473_protocol_id;
       er_pdu.vp_id    := version1;
       er_pdu.lifetime := get_er_lifetime(pdu.sa);
       er_pdu.sp       := get_er_seg_per(pdu);
       er_pdu.ms       := FALSE;
       er_pdu.er_flag  := FALSE;
       er_pdu.pdu_tp   := ER;
       er_pdu.da_len   := pdu.sa_len;
       er_pdu.da       := pdu.sa;
       er_pdu.sa_len   := get_local_NPAI_addr_len;
       er_pdu.sa       := get_local_NPAI_addr;
       er_pdu.options  := get_er_options
       er_pdu.hli      := get_header_length
                          (er_pdu.da_len, er_pdu.sa_len,
       er_pdu.data     := get_er_data_field(error, pdu);
       if (er_pdu.sp) then
                           er_pdu.du_id   :=
                           er_pdu.so      := ZERO;
                           er_pdu.tot_len := er_pdu.hli +

ISO DIS 8473 (May 1984)                                        [Page 68]

RFC 926                                                    December 1984

       if (NPAI_addr_local(er_pdu.da))

ISO DIS 8473 (May 1984)                                        [Page 69]

RFC 926                                                    December 1984

   procedure send_pdu (pdu : pdu_type);


     rte_result   : route_result_type;
     error_code   : error_type;
     send_buf     : buffer_type;
     data_maxsize : integer;
     more_seg     : boolean;
     sn_qos       : SN_QOS_type;


     send_buf := make_buffer(pdu.data);
     more_seg := pdu.ms;



       error_code := check_parameters

       if (error_code = NO_ERROR) then


                           rte_result := route(pdu.hli,

                           data_maxsize := rte_result.segment_size -
                           pdu.data     := extract(send_buf,
                           pdu.seg_len  := pdu.hli + size(pdu.data);

                           if (size(send_buf) = ZERO) then
                               pdu.ms   := more_seg
                               pdu.ms   := TRUE;

ISO DIS 8473 (May 1984)                                        [Page 70]

RFC 926                                                    December 1984

                           pdu.checksum := get_checksum(pdu);
                           sn_qos       := get_sn_qos

                           out SN[rte_result.subnet_id].UNITDATA_request

                           pdu.so := pdu.so + data_maxsize;


       else if (error_code = CONGESTION) then


                           if (send_er_on_congestion (pdu)) then
                               send_error_report(CONGESTION, pdu);



                        send_error_report(error_code, pdu);


     until (size_buf(data_buf) = ZERO) or
           (error_code <> NO_ERROR);


ISO DIS 8473 (May 1984)                                        [Page 71]

RFC 926                                                    December 1984

   procedure allocate_reassembly_resources
            (pdu_tot_len : integer);

    { This procedure allocates resources required for reassembly of a
    PDU of the specified total length.  If this requires discarding of a
    PDU in which the ER flag is set, then an error report is returned to
    the source of the discarded data unit. }

   function check_parameters
        (hli     : integer;
         sp      : boolean;
         da      : NPAI_addr_type;
         options : options_type;
         datalen : integer) : error_type;

    { This function examines various parameters associated with a PDU,
    to determine whether forwarding of the PDU can continue.  If a
    result of NO_ERROR is returned, then the primitive route can be
    called to specify the route and segment size.  Otherwise this
    function specifies the reason that an error has occurred. }

   function data_unit_complete
        (buf : buffer_type) : boolean;

    { This function returns a boolean value specifying whether the PDU
    stored in the specified buffer has been completely received. }

ISO DIS 8473 (May 1984)                                        [Page 72]

RFC 926                                                    December 1984

   function elapsed_time : integer;

    { This function returns an estimate of the time elapsed, in 500
    microsecond increments, since the PDU was transmitted by the
    previous peer network entity.  This estimate includes both time
    spent in transit, and any time to be spent in buffers within the
    local system.  Although this estimate need not be precise,
    overestimates are preferable to underestimates, as underestimating
    the time elapsed may defeat the intent of the lifetime function. }

   procedure empty_buffer
        (buf : buffer_type);

    { This procedure empties the specified buffer. }

   function extract
        (buf    : buffer_type;
         amount : integer) : data_type;

    { This function removes the specified amount of data from
    the specified buffer, and returns this data as the function
    value. }

   procedure free_reassembly_resources;

    { This procedure releases the resources that had been previously
    allocated by the procedure allocate_reassembly_resources. }

   function get_checksum
        (pdu : pdu_type) : integer;

    { This function returns the 16 bit integer value to be placed in the
    checksum field of the PDU.  If the checksum facility is not being
    used, then this function returns the value zero.  The algorithm for
    producing a correct checksum value is specified in Annex A. }

   function get_data_unit_id
        (da : NPAI_addr_type) : integer;

    { This function returns a data unit identifier which is unique for
    the specified destination address. }

ISO DIS 8473 (May 1984)                                        [Page 73]

RFC 926                                                    December 1984

   function get_er_data_field
        (error : error_type;
         pdu   : pdu_type) : data_type;

    { This function returns the correct data field for an error report,
    based on the information that the specified PDU is being discarded
    due to the specified error.  The data field of an error report must
    include the header of the discarded PDU, and may optionally contain
    additional user data. }

   function get_er_flag
        (nsdu : nsdu_type) : boolean;

    { This function returns a boolean value to be used as the error
    report flag in a PDU which transmits the specified nsdu.  If the PDU
    must be discarded at some future time, an error report can be
    returned only if this value is set to TRUE. }

   function get_er_lifetime
        (da : NPAI_addr_type) : integer;

    { This function returns the lifetime value to be used for an error
    report being sent to the specified destination address. }

   function get_er_options
        (error   : error_type;
         da      : NPAI_addr_type;
         options : options_type) : options_type;

    { This function returns the options field of an error report, based
    on the reason for discard, and the destination address and options
    field of the discarded PDU.  The options field contains the reason
    for discard option, and may contain other optional fields. }

ISO DIS 8473 (May 1984)                                        [Page 74]

RFC 926                                                    December 1984

   function get_er_seg_per

        (pdu     : pdu_type) : boolean;

    { This function returns the boolean value which will be used for the
    segmentation permitted flag of an error report. }

   function get_header_len
        (da_len  : integer;
         sa_len  : integer;
         sp      : boolean;
         options : options_type) : integer;

    { This function returns the header length, in octets.  This depends
    upon the lengths of the source and destination addresses, whether
    the segmentation part of the header is present, and the length of
    the options part. }

   function get_lifetime
        (da  : NSAP_addr_type;
         qos : quality_of_service_type) : lifetime_type;

    { This function returns the lifetime value to be used for a PDU,
    based upon the destination address and requested quality of service.

   function get_local_NPAI_addr : NPAI_addr_type;

    { This functions returns the local address as used in the protocol
    header. }

   function get_local_NPAI_addr_len : integer;

    { This functions returns the length of the local address as used in
    the protocol header. }

ISO DIS 8473 (May 1984)                                        [Page 75]

RFC 926                                                    December 1984

   function get_NPAI
        (addr : NSAP_addr_type) : NPAI_addr_type;

    { This function returns the network address as used in the protocol
    header, or "Network Protocol Addressing Information", corresponding
    to the specified NSAP address. }

   function get_NPAI_len
        (addr : NSAP_addr_type) : integer;

    { This function returns the length of the network address
    corresponding to a specified NSAP address. }

   function get_NSAP_addr
        (addr : NPAI_addr_type;
         len  : integer) : NSAP_addr_type;

    { This function returns the NSAP address corresponding to the
    network protocol addressing information (as it appears in the
    protocol header) of the specified length. }

   function get_options
        (da  : NSAP_addr_type;
         qos : quality_of_service_type) : options_type;

    { This function returns the options field for a PDU, based on the
    requested destination address and quality of service. }

   function get_seg_permitted
        (da : NSAP_addr_type;
         qos : quality_of_service_type) : boolean;

    { This function returns the boolean value to be used in the
    segmentation permitted field of a PDU.  This value may depend upon
    the destination address, requested quality of service, and the
    length of the user data. }

ISO DIS 8473 (May 1984)                                        [Page 76]

RFC 926                                                    December 1984

   function get_sn_qos
        (subnet_id : subnet_id_type;

          options   : options_type) : SN_QOS_type;

    { This function returns the quality of service to be used on the
    specified subnetwork, in order to obtain the quality of service (if
    any) and other parameters requested in the options part of the PDU.

   function get_qos
        (options : options_type) : quality_of_service_type;

    { This function determines, to the extent possible, the quality of
    service that was obtained for a particular PDU, based upon the
    quality of service and other information contained in the options
    part of the PDU header. }

   function make_buffer
        (data : data_type) : buffer_type;

    { This function places the specified data in a newly created buffer.
    The precise manner of handling buffers is implementation specific.
    This newly created buffer is returned as the function value. }

   procedure merge_seg
        (buf   : buffer_type;
         so    : integer;
         data  : data_type);

    { This procedure merges the specified data into the specified
    buffer, based on the specified segment offset of the data. }

   function NPAI_addr_local
        (addr : NPAI_addr_type) : boolean;

    { This function returns the boolean value TRUE only if the specified
    network protocol addressing information specifies a local address. }

ISO DIS 8473 (May 1984)                                        [Page 77]

RFC 926                                                    December 1984

   function NSAP_addr_local
        (addr : NSAP_addr_type) : boolean;

    { This function returns the boolean value TRUE only if the specified
    NSAP address specifies a local address. }

   procedure post_error_report
        (er_pdu : pdu_type);

    { This procedure posts the specified error report (ER) type PDU to
    the appropriate local entity that handles error reports. }

   function route
        (hli     : integer;
         sp      : boolean;
         da      : NPAI_addr_type;
         options : options_type;
         datalen : integer) : route_result_type;

    { This function determines the route to be followed by a PDU
    segment, as well as the segment size.  Note that in general, the
    segment size and route may be mutually dependent.  This
    determination is made on the basis of the header length, the
    segmentation permitted flag, the destination address, several
    parameters (such as source routing) contained in the options part of
    the PDU header, and the length of data.  This function returns a
    structure that specifies the subnetwork on which the segment should
    be transmitted, the source and destination addresses to be used on
    the subnetwork, and the segment size.  This routine may only be
    called if the primitive function check_parameters has already
    determined that an error will not occur. }

ISO DIS 8473 (May 1984)                                        [Page 78]

RFC 926                                                    December 1984

   function send_er_on_congestion
       (pdu : pdu_type) : boolean;

    { This function returns the boolean value true if an error report
    should be sent when the indicated data unit is discarded due to
    congestion.  Note that if the value true is returned, then the
    er_flag field of the discarded data unit must still be checked
    before an error report can be sent. }

   function size
       (data : data_type) : integer;

    { This function returns the length, in octets, of the specified
    data. }

   function size_buf
       (buf : buffer_type) : integer;

    { This function returns the length, in octets, of the data contained
    in the specified buffer. }


        state to INITIAL;

ISO DIS 8473 (May 1984)                                        [Page 79]

RFC 926                                                    December 1984

   trans  (* begin transitions *)

   from INITIAL  to  CLOSED
   when      N.UNITDATA_request
   provided  not NSAP_addr_local(NS_Destination_Address)

     nsdu.da   := NS_Destination_Address;
     nsdu.sa   := NS_Source_Address;
     nsdu.qos  := NS_Quality_o  _Service;
     nsdu.data := NS_Userdata;

     pdu.nlp_id   := ISO_8473_protocol_id;
     pdu.vp_id    := version1;
     pdu.lifetime := get_lifetime(nsdu.da, nsdu.qos);
     pdu.sp       := get_seg_permitted(nsdu.da, nsdu.qos);
     pdu.ms       := FALSE;
     pdu.er_flag  := get_er_flag(nsdu);
     pdu.pdu_tp   := DT;
     pdu.da_len   := get_NPAI_len(nsdu.da);
     pdu.da       := get_NPAI(nsdu.da);
     pdu.sa_len   := get_NPAI_len(nsdu.sa);
     pdu.sa       := get_NPAI(nsdu.sa);
     pdu.options  := get_options(nsdu.da, nsdu.qos);
     pdu.data     := nsdu.data;

     pdu.hli      := get_header_len(pdu.da_len,

     if (pdu.sp) then
             pdu.du_id    := get_data_unit_id(pdu.da);
             pdu.so       := ZERO;
             pdu.tot_len  := pdu.hli  +  size(pdu.data);

     if (size(pdu.data) > max_user_data) then
           send_error_report(TOO_MUCH_USER_DATA, pdu)

ISO DIS 8473 (May 1984)                                        [Page 80]

RFC 926                                                    December 1984

   from INITIAL  to  CLOSED
   when      N.UNITDATA_request
   provided  NSAP_addr_local(NS_Destination_Address)

     nsdu.da   := NS_Destination_Address;
     nsdu.sa   := NS_Source_Address;
     nsdu.qos  := NS_Quality_of_Service;
     nsdu.data := NS_Userdata;

     out N.UNITDATA_indication
         (nsdu.da, nsdu.sa, nsdu.qos, nsdu.data);


   from INITIAL  to  CLOSED
   when      SN[subnet_id].UNITDATA_indication
   provided  NPAI_addr_local(SN_Userdata.da)  and
             SN_Userdata.so       =  ZERO     and
             not  SN_Userdata.ms

     pdu := SN_Userdata;

     if (pdu.pdu_tp = DT) then
         out N.UNITDATA_indication
            (get_NSAP_addr(pdu.da_len, pdu.da),
             get_NSAP_addr(pdu.sa_len, pdu.sa),



ISO DIS 8473 (May 1984)                                        [Page 81]

RFC 926                                                    December 1984

   when      SN[subnet_id].UNITDATA_indication
   provided  NPAI_addr_local(SN_Userdata.da)    and
             ((SN_Userdata.so > ZERO) or (SN_Userdata.ms))

     pdu := SN_Userdata;


     out S.TIMER_request


   from INITIAL  to  CLOSED
   when      SN[subnet_id].UNITDATA_indication
   provided  not NPAI_addr_local(SN_Userdata.da)

     pdu := SN_Userdata;

     if (pdu.lifetime > elapsed_time) then
         pdu.lifetime := pdu.lifetime - elapsed_time;
       send_error_report(LIFETIME_EXPIRED, pdu);


ISO DIS 8473 (May 1984)                                        [Page 82]

RFC 926                                                    December 1984

   when      SN[subnet_id].UNITDATA_indication
   provided  (SN_Userdata.du_id   = pdu.du_id)   and
             (SN_Userdata.da_len  = pdu.da_len)  and
             (SN_Userdata.da      = pdu.da)      and
             (SN_Userdata.sa_len  = pdu.sa_len)  and
             (SN_Userdata.sa      = pdu.sa)



   provided  data_unit_complete(rcv_buf)
   no delay

     if (pdu.pdu_tp = DT) then
         out N.UNITDATA_indication
            (get_NSAP_addr(pdu.da_len, pdu.da),
             get_NSAP_addr(pdu.sa_len, pdu.sa),
             extract (rcv_buf, size_buf(rcv_buf)))
    out S.TIMER_cancel(lifetime_timer,ZERO);


   when      S.TIMER_indication

     send_error_report(LIFETIME_EXPIRED, pdu);


ISO DIS 8473 (May 1984)                                        [Page 83]

RFC 926                                                    December 1984


 For conformance to this International Standard, the ability to
 originate, manipulate, and receive PDUs in accordance with the full
 protocol (as opposed to the "non-segmenting" or "Inactive Network Layer
 Protocol" subsets) is required.

 Additionally, the provision of the optional functions described in
 Section 6.17 and enumerated in Table 9-1 must meet the requirements
 described therein.

 Additionally, conformance to the Standard requires adherence to the
 formal description of Section 8 and to the structure and encoding of
 PDUs of Section 7.

 If and only if the above requirements are met is there conformance to
 this International Standard.

9.1  Provision of Functions for Conformance

  The following table categorizes the functions in Section 6 with
  respect to the type of system providing the function:

ISO DIS 8473 (May 1984)                                        [Page 84]

RFC 926                                                    December 1984

  | Function                   |  Send  | Forward | Receive |
  | PDU Composition            |   M    |    -    |    -    |
  | PDU Decomposition          |   M    |    -    |    M    |
  | Header Format Analysis     |   -    |    M    |    M    |
  | PDU Lifetime Control       |   -    |    M    |    I    |
  | Route PDU                  |   -    |    M    |    -    |
  | Forward PDU                |   M    |    M    |    -    |
  | Segment PDU                |   M    | (note 1)|    -    |
  | Reassemble PDU             |   -    |    I    |    M    |
  | Discard PDU                |   -    |    M    |    M    |
  | Error Reporting            |   -    |    M    |    M    |
  | PDU Header Error Detection |   M    |    M    |    M    |
  | Padding                    |(note 2)| (note 2)| (note 2)|
  | Security                   |   -    | (note 3)| (note 3)|
  | Complete Source Routing    |   -    | (note 3)|    -    |
  | Partial Source Routing     |   -    | (note 4)|    -    |
  | Record Route               |   -    | (note 4)|    -    |
  | QoS Maintenance            |   -    | (note 4)|    -    |

                Table 9-1.  Categorization of Functions

  | KEY:                                                    |
  |       M : Mandatory Function; must be implemented       |
  |       - : Not applicable                                |
  |       I : Implementation option, as described in text   |


   1)  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 SNSDU 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.

ISO DIS 8473 (May 1984)                                        [Page 85]

RFC 926                                                    December 1984

   2)  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.

   3)  This function may or may not be supported. If an implementation
       does not support this function, and the function is selected by 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.

   4)  This function may or may not be supported. If an implementation
       does not support this function, and the function is selected by a
       PDU, then the function is not provided and the PDU is processed
       exactly as though the function was not selected. The PDU shall
       not be discarded.

ISO DIS 8473 (May 1984)                                        [Page 86]

RFC 926                                                    December 1984


 (These annexes are provided for information for implementors and are
 not an integral part of the body of the Standard.)


A.1  Data Unit Lifetime

  There are two primary purposes of providing a PDU lifetime capability
  in the ISO 8473 Protocol. One purpose is to ensure against unlimited
  looping of protocol data units. Although the routing algorithm should
  ensure that it will be very rare for data to loop, the PDU lifetime
  field provides additional assurance that loops will be limited in

  The other important purpose of the lifetime capability is to provide
  for a means by which the originating network entity can limit the
  Maximum NSDU lifetime. ISO Transport Protocol Class 4 assumes that
  there is a particular Maximum NSDU Lifetime in order to protect
  against certain error states in the connection establishment and
  termination phases. If a TPDU does not arrive within this time, then
  there is no chance that it will ever arrive. It is necessary to make
  this assumption, even if the Network Layer does not guarantee any
  particular upper bound on NSDU lifetime. It is much easier for
  Transport Protocol Class 4 to deal with occasional lost TPDUs than to
  deal with occasional very late TPDUs. For this reason, it is
  preferable to discard very late TPDUs than to deliver them. Note that
  NSDU lifetime is not directly associated with the retransmission of
  lost TPDUs, but relates to the problem of distinguishing old
  (duplicate) TPDUs from new TPDUs.

  Maximum NSDU Lifetime must be provided to transport protocol entity in
  units of time; a transport entity cannot count "hops". Thus NSDU
  lifetime must be calculated in units of time in order to be useful in
  determining Transport timer values.

  In the absence of any guaranteed bound, it is common to simply guess
  some value which seems like a reasonable compromise. In essence one is
  simply assuming that "surely no TPDU would ever take more than 'x'
  seconds to traverse the network." This value is probably chosen by
  observation of past performance, and may

ISO DIS 8473 (May 1984)                                        [Page 87]

RFC 926                                                    December 1984

  vary with source and destination.

  Three possible ways to deal with the requirement for a limit on the
  maximum NSDU lifetime are: (1) specify lifetime in units of time,
  thereby requiring intermediate systems to decrement the lifetime field
  by a value which is an upper bound on the time spent since the
  previous intermediate system, and have the Network Layer discard
  protocol data units whose lifetime has expired; (2) provide a
  mechanism in the Transport Layer to recognize and discard old TPDUs;
  or (3) ignore the problem, anticipating that the resulting
  difficulties will be rare. Which solution should be followed depends
  in part upon how difficult it is to implement solutions (1) and (2),
  and how strong the transport requirement for a bounded time to live
  really is.

  There is a problem with solution (2) above, in that transport entities
  are inherently transient. In case of a computer system outage or other
  error, or in the case where one of the two endpoints of a connection
  closes without waiting for a sufficient period of time (approximately
  twice Maximum NSDU Lifetime), it is possible for the Transport Layer
  to have no way to know whether a particular TPDU is old unless
  globally synchronized clocks are used (which is unlikely). On the
  other hand, it is expected that intermediate systems will be
  comparatively stable. In addition, even if intermediate systems do
  fail and resume processing without memory of the recent past, it will
  still be possible (in most instances) for the intermediate system to
  easily comply with lifetime in units of time, as discussed below.

  It is not necessary for each intermediate system to subtract a precise
  measure of the time that has passed since an NPDU (containing the TPDU
  or a segment thereof) has left the previous intermediate system. It is
  sufficient to subtract an upper bound on the time taken. In most
  cases, an intermediate system may simply subtract a constant value
  which depends upon the typical near-maximum delays that are
  encountered in a specific subnetwork. It is only necessary to make an
  accurate estimate on a per NPDU basis for those subnetworks which have
  both a relatively large maximum delay, and a relatively large
  variation in delay.

  As an example, assume that a particular local area network has short
  average delays, with overall delays generally in the 1 to 5

ISO DIS 8473 (May 1984)                                        [Page 88]

RFC 926                                                    December 1984

  millisecond range and with occasional delays up to 20 milliseconds. In
  this case, although the relative range in delays might be large (a
  factor of 20), it would still not be necessary to measure the delay
  for actual NPDUs. A constant value of 20 milliseconds (or more) can be
  subtracted for all delays ranging from .5 seconds to .6 seconds (.5
  seconds for the propagation delay, 0 to .1 seconds for queueing delay)
  then the constant value .6 seconds could be used.

  If a third subnetwork had normal delays ranging from .1 to 1 second,
  but occasionally delivered an NPDU after a delay of 15 seconds, the
  intermediate system attached to this subnetwork might be required to
  determine how long it has actually take the PDU to transit the
  subnetwork. In this last example, it is likely to be more useful to
  have the intermediate systems determine when the delays are extreme ad
  discard very old NPDUs, as occasional large delays are precisely what
  causes the Transport Protocol the most trouble.

  In addition to the time delay within each subnetwork, it is important
  to consider the time delay within intermediate systems. It should be
  relatively simple for those gateways which expect to hold on to some
  data-units for significant periods of time to decrement the lifetime

  Having observed that (i) the Transport Protocol requires Maximum NSDU
  to be calculated in units of time; (ii) in the great majority of
  cases, it is not difficult for intermediate systems to determine a
  valid upper bound on subnetwork transit time; and (iii) those few
  cases where the gateways must actually measure the time take by a NPDU
  are precisely the cases where such measurement truly needs to be made,
  it can be concluded that NSDU lifetime should in fact be measured in
  units of time, and that intermediate systems should required to
  decrement the lifetime field of the ISO 8473 Protocol by a value which
  represents an upper bound on the time actually taken since the
  lifetime field was last decremented.

A.2  Reassembly Lifetime Control

  In order to ensure a bound on the lifetime of NSDUs, and to
  effectively manage reassembly buffers in the Network Layer, the
  Reassembly Function described in Section 6 must control the

ISO DIS 8473 (May 1984)                                        [Page 89]

RFC 926                                                    December 1984

  lifetime of segments representing partially assembled PDUs. This annex
  discusses methods of bounding reassembly lifetime and suggests some
  implementation guidelines for the reassembly function.

  When segments of a PDU arrive at a destination network-entity, they
  are buffered until an entire PDU is received, assembled, and passed to
  the PDU Decomposition Function. The connectionless Internetwork
  Protocol does not guarantee the delivery of PDUs; hence, it is
  possible for some segments of a PDU to be lost or delayed such that
  the entire PDU cannot be assembled in a reasonable length of time. In
  the case of loss of a PDU "segment", for example, this could be
  forever. There are a number of possible schemes to prevent this:

   a)  Per-PDU reassembly timers,

   b)  Extension of the PDU Lifetime control function, and

   c)  Coupling of the Transport Retransmission timers.

  Each of these methods is discussed in the subsections which follow.

  A.2.1  Method (a)

   assigns a "reassembly lifetime" to each PDU received and identified
   by its Data-unit Identifier. This is a local, real time which is
   assigned by the reassembly function and decremented while some, but
   not all segments of the PDU are being buffered by the destination
   network-entity. If the timer expires, all segments of the PDU are
   discarded, thus freeing the reassembly buffers and preventing a "very
   old" PDU from being confused with a newer one bearing the same
   Data-unit Identifier. For this scheme to function properly, the
   timers must be assigned in such a fashion as to prevent the
   phenomenon of Reassembly Interference (discussed below). In
   particular, the following guidelines should be followed:

    1)  The Reassembly Lifetime must be much less than the maximum PDU
        lifetime of the network (to prevent the confusion of old and new

ISO DIS 8473 (May 1984)                                        [Page 90]

RFC 926                                                    December 1984

    2)  The lifetime should be less than the Transport protocol's
        retransmission timers minus the average transit time of the
        network. If this is not done, extra buffers are tied up holding
        data which has already been retransmitted by the Transport
        Protocol. (Note that an assumption has been made that such
        timers are integral to the Transport Protocol, which in some
        sense, dictates that retransmission functions must exist in the
        Transport Protocol employed).

  A.2.2  Method (b)

   is feasible if the PDU lifetime control function operates based on
   real or virtual time rather than hop-count. In this scheme, the
   lifetime field of all PDU segments of a Data-unit continues to be
   decremented by the reassembly function of the destination
   network-entity as if the PdU were still in transit (in a sense, it
   still is). When the lifetime of any segment of a partially
   reassembled PDU expires, all segments of that PDU are discarded. This
   scheme is attractive since the delivery behavior of the ISO 8473
   Protocol would be identical for segmented and unsegmented PDUs.

  A.2.3  Method (c)

   couples the reassembly lifetime directly to the Transport Protocol's
   retransmission timers, and requires that Transport Layer management
   make known to Network Layer Management (and hence, the Reassembly
   Function) the values of its retransmission timers for each source
   from which it expects to be receiving traffic. When a PDU segment is
   received from a source, the retransmission time minus the anticipated
   transit time becomes the reassembly lifetime of that PDU. If this
   timer expires before the entire PDU has been reassembled, all
   segments of the PDU are discarded. This scheme is attractive since it
   has a low probability of holding PDU segments that have already been
   retransmitted by the source Transport-entity; it has, however, the
   disadvantage of depending on reliable operation of the Transport
   Protocol to work effectively. If the retransmission timers are not
   set correctly, it is possible that all PDUs would be discarded too
   soon, and the Transport Protocol would make no progress.

A.3  The Power of the Header Error Detection Function

ISO DIS 8473 (May 1984)                                        [Page 91]

RFC 926                                                    December 1984

  A.3.1  General

   The form of the checksum used for PDU header error detection is such
   that it is easily calculated in software or firmware using only two
   additions per octet of header, yet it has an error detection power
   approaching (but not quite equalling) that of techniques (such as
   cyclic polynomial checks) which involve calculations that are much
   more time- or space-consuming. This annex discusses the power of this
   error detection function.

   The checksum consists of two octets, either of which can assume any
   value except zero. That is, 255 distinct values for each octet are
   possible. The calculation of the two octets is such that the value of
   either is independent of the value of the other, so the checksum has
   a total of 255 x 255 = 65025 values. If one considers all ways in
   which the PDU header might be corrupted as equally likely, then there
   is only one chance in 65025 that the checksum will have the correct
   value for any particular corruption. This corresponds to 0.0015  of
   all possible errors.

   The remainder of this annex considers particular classes of errors
   that are likely to be encountered. The hope is that the error
   detection function will be found to be more powerful, or at least no
   less powerful, against these classes as compared to errors in

  A.3.2  Bit Alteration Errors

   First considered are classes of errors in which bits are altered, but
   no bits are inserted nor deleted. This section does not consider the
   case where the checksum itself is erroneously set to be all zero;
   this case is discussed in section A.3.4.

   A burst error of length b is a corruption of the header in which all
   of the altered bits (no more than b in number) are within a single
   span of consecutively transmitted bits that is b bits long. Checksums
   are usually expected to do well against burst errors of a length not
   exceeding the number of bits in the header error detection parameter
   (16 for the PDU header). The PDU header error detection parameter in
   fact fails to detect only 0.000019  of all such errors, each distinct
   burst error of length 16 or less being considered to be equally
   likely. In particular,

ISO DIS 8473 (May 1984)                                        [Page 92]

RFC 926                                                    December 1984

   it cannot detect an 8-bit burst in which an octet of zero is altered
   to an octet of 255 (all bits = 1) or vice versa. Similarly, it fails
   to detect the swapping of two adjacent octets only if one is zero and
   the other is 255.

   The PDU header error detection, as should be expected, detects all
   errors involving only a single altered bit.

   Undetected errors involving only two altered bits should occur only
   if the two bits are widely separated (and even then only rarely). The
   PDU header error detection detects all double bit errors for which
   the spacing between the two altered bits is less than 2040 bits = 255
   octets. Since this separation exceeds the maximum header length, all
   double bit errors are detected.

   The power to detect double bit errors is an advantage of the checksum
   algorithm used for the protocol, versus a simple modulo 65536
   summation of the header split into 16 bit fields. This simple
   summation would not catch all such double bit errors. In fact, double
   bit errors with a spacing as little as 16 bits apart could go

  A.3.3  Bit Insertion/Deletion Errors

   Although errors involving the insertion or deletion of bits are in
   general neither more nor less likely to go undetected than are all
   other kinds of general errors, at least one class of such errors is
   of special concern. If octets, all equal to either zero or 255, are
   inserted at a point such that the simple sum CO in the running
   calculation (described in Annex C) happens to equal zero, then the
   error will go undetected. This is of concern primarily because there
   are two points in the calculation for which this value for the sum is
   not a rare happenstance, but is expected; namely, at the beginning
   and the end. That is, if the header is preceded or followed by
   inserted octets all equal to zero or 255 then no error is detected.
   Both cases are examined separately.

   Insertion of erroneous octets at the beginning of the header
   completely misaligns the header fields, causing them to be
   misinterpreted. In particular, the first inserted octet is
   interpreted as the network layer protocol identifier, probably
   eliminating any knowledge that the data unit is related to the

ISO DIS 8473 (May 1984)                                        [Page 93]

RFC 926                                                    December 1984

   ISO 8473 Protocol, and thereby eliminating any attempt to perform the
   checksum calculation or invoking a different form of checksum
   calculation. An initial octet of zero is reserved for the Inactive
   Network Layer Protocol. This is indeed a problem but not one which
   can be ascribed to the form of checksum being used. Therefore, it is
   not discussed further here.

   Insertion of erroneous octets at the end of the header, in the
   absence of other errors, is impossible because the length field
   unequivocally defines where the header ends. Insertion or deletion of
   octets at the end of the header requires an alteration in the value
   of the octet defining the header length. Such an alteration implies
   that the value of the calculated sum at the end of the header would
   not be expected to have the dangerous value of zero and consequently
   that the error is just as likely to be detected as is any error in

   Insertion of an erroneous octet in the middle of the header is
   primarily of concern if the inserted octet has either the value zero
   or 255, and if the variable CO happens to have the value zero at this
   point. In most cases, this error will completely destroy the parsing
   of the header, which will cause the data unit to e discarded. In
   addition, in the absence of any other error, the last octet of the
   header will be thought to be data. This in turn will cause the header
   to end in the wrong place. In the case where the header otherwise can
   parse correctly, the last field will be found to be missing. Even in
   the case where necessary, the length field is the padding option, and
   therefore not necessary, the length field for the padding function
   will be inconsistent with the header length field, and therefore the
   error can be detected.

  A.3.4  Checksum Non-calculation Errors

   Use of the header error detection function is optional. The choice of
   not using it is indicated by a checksum parameter value of zero. This
   creates the possibility that the two octets of the checksum parameter
   (neither of which is generated as being zero) could both be altered
   to zero. This would in effect be an error not detected by the
   checksum since the check would not be made. One of three
   possibilities exists:

    1)  A burst error of length sixteen (16) which sets the entire

ISO DIS 8473 (May 1984)                                        [Page 94]

RFC 926                                                    December 1984

    checksum to zero. Such an error could not be detected; however, it
        requires a particular positioning of the burst within the
        header. [A calculation of its effect on overall detectability of
        burst errors depends upon the length of the header.]

    2)  All single bit errors are detected. Since both octets of the
        checksum field must be non-zero when the checksum is being used,
        no single bit error can set the checksum to zero.

    3)  Where each of the two octets of the checksum parameter has a
        value that is a power of two, such that only one bit in each
        equals one (1), then a zeroing of the checksum parameter could
        result in an undetected double bit error. Furthermore, the two
        altered bits have a separation of less than sixteen (16), and
        could be consecutive. This is clearly a decline from the
        complete detectability previously described.

   Where a particular administration is highly concerned about the
   possibility of accidental zeroing of the checksum among data units
   within its domain, then the administration may impose the restriction
   that all data units whose source or destination lie within its domain
   must make use of the header error detection function. Any data units
   which do not could be discarded, nor would they be allowed outside
   the domain. This protects against errors that occur within the
   domain, and would protect all data units whose source or destination
   lies within the domain, even where the data path between all such
   pairs crosses other domains (errors outside the protected domain

ISO DIS 8473 (May 1984)                                        [Page 95]

RFC 926                                                    December 1984

                      ANNEX B.  NETWORK MANAGEMENT

 The following topics are considered to be major components of Network
 Layer management:

  A.  Routing

   Considered by many to be the most crucial element of Network Layer
   management, since management of the Routing algorithms for networking
   seem to be an absolutely necessary prerequisite to a practical
   networking scheme.

   Routing management consists of three parts; forwarding, decision, and
   update. Management of forwarding is the process of interpreting the
   Network Layer address to properly forward NSDUs on its next network
   hop on a route through the network. Management of decision is the
   process of choosing routes for either connections or NSDUs, depending
   on whether the network is operating a connection-oriented or
   connectionless protocol. The decision component will be driven by a
   number of considerations, not the least of which are those associated
   with Quality of Service. Management of update is the management
   protocol(s) used to exchange information among
   intermediate-systems/network- entities which is used in the decision
   component to determine routes.

   To what extent is it desirable and/or practical to pursue a single
   OSI network routing algorithm and associated Management protocol(s)?
   It is generally understood that it is impractical to expect ISO to
   adopt a single global routing algorithm. On the other hand, it is
   recognized that having no standard at all upon which to make routing
   decisions effectively prevents an internetwork protocol from working
   at all. One possible compromise would be to define the principles for
   the behavior of an internetwork routing algorithm. A possible next
   step would be to specify the types of information that must be
   propagated among the intermediate-systems/network-entities via their
   update procedures. The details of the updating protocol might then be
   left to bilateral agreements among the cooperating administrations.

ISO DIS 8473 (May 1984)                                        [Page 96]

RFC 926                                                    December 1984

  B.  Statistical Analysis

   These management functions relate to the gathering and reporting of
   information about the real-time behavior of the global network. They
   consist of Data counts such as number of PDUs forwarded, entering
   traffic, etc., and Event Counts such as topology changes, quality of
   service changes, etc.

  C.  Network Control

   These management functions are those related to the control of the
   global network, and possibly could be performed by a Network Control
   Center(s). The control functions needed are not al all clear. Neither
   are the issues relating to what organization(s) is/are responsible
   for the management of the environment. Should there be a Network
   Control Center distinct from those provided by the subnetwork
   administrations? What subnetwork management information is needed by
   the network management components to perform their functions?

  D.  Directory Mapping Functions

   Does the Network layer contain a Directory function as defined in the
   Reference Model? Current opinion is that the Network Layer restricts
   itself to the function of mapping NSAP addresses to routes.

  E.  Congestion Control

   Does this come under the umbrella of Network Layer management? How?

  F.  Configuration Control

   This is tightly associated with the concepts of Resource Management,
   and is generally considered to be somehow concerned with the control
   of the resources used in the management of the global network. The
   resources which have to be managed are Bandwidth (use of subnetwork
   resources), Processor (CPU), and Memory (buffers). Where is the
   responsibility for resources assigned, and are they appropriate for
   standardization? It appears that these

ISO DIS 8473 (May 1984)                                        [Page 97]

RFC 926                                                    December 1984

   functions are tightly related to how one signals changes in Quality
   of Service.

  G.  Accounting

   What entities, administrations, etc., are responsible for network
   accounting? How does this happen? What accounting information, if
   any, is required from the subnetworks in order to charge for network
   resources? Who is charged? To what degree is this to be standardized?

ISO DIS 8473 (May 1984)                                        [Page 98]

RFC 926                                                    December 1984


 This Annex describes algorithm which may be used to computer, check and
 update the checksum field of the PDU Header in order to provide the PDU
 Header Error Detection function described in Section 6.11.

C.1  Symbols used in algorithms

  CO,C1  variables used in the algorithms
  i      number (i.e., position) of an octet within the header
  n      number (i.e., position) of the first octet of the checksum
         parameter (n=8)
  L      length of the PDU header in octets
  X      value of octet one of the checksum parameter
  Y      value of octet two of the checksum parameter
  a      octet occupying position i of the PDU header

C.2  Arithmetic Conventions

  Addition is performed in one of the two following modes:

   a)  modulo 255 arithmetic;

   b)  eight-bit one's complement arithmetic in which, if any of the
       variables has the value minus zero (i.e., 255) it shall be
       regarded as though it was plus zero (i.e., 0).

C.3  Algorithm for Generating Checksum Parameters

  A:  Construct the complete PDU header with the value of the checksum
      parameter field set to zero;

  B:  Initialize C0 and C1 to zero;

  C:  Process each octet of the PDU header sequentially from i = 1 to L

   a)  adding the value of the octet to C0; then

   b)  adding the value of C0 to C1;

  D:  Calculate X = (L-8)C0 - C1 (modulo 255) and Y = (L-7) (-C0) + C1
      (modulo 255)

ISO DIS 8473 (May 1984)                                        [Page 99]

RFC 926                                                    December 1984

  E:  If X = 0, set X = 255;

  F:  If Y = 0, set Y = 255;

  G:  Place the values X and Y in octets 8 and 9 respectively.

C.4  Algorithm for Checking Checksum Parameters

  A:  If octets 8 and 9 of PDU header both contain 0 (all bits off),
      then the checksum calculation has succeeded; otherwise initialize
      C1 = 0, C0 - 0 and proceed;

  B:  process each octet of the PDU header sequentially from i = 1 to L

   a)  adding the value of the octet to C0; then

   b)  adding the value of C0 to C1;

  C:  If, when all the octets have been processed, C0 = C1 = 0 (modulo
      255) then the checksum calculation has succeeded; otherwise, the
      checksum calculation has failed.

C.5  Algorithm to adjust checksum parameter when an octet is altered

  This algorithm adjusts the checksum when an octet (such as the
  lifetime field) is altered. Suppose the value in octet k is changed by
  Z = new_value - old_value.

  If X and Y denote the checksum values held in octets n and n+1,
  respectively, then adjust X and Y as follows:

   If X = 0 and Y = 0 do nothing, else;
        X := (k-n-1)Z + X (modulo 255) and
        Y := (n-k)Z + Y   (modulo 255).
   If X is equal to zero, then set it to 255; and
   similarly for Y.

  For this Protocol, n = 8. If the octet being altered is the lifetime
  field, k = 4. For the case where the lifetime is decreased by 1 unit
  (Z = -1), the results simplify to

ISO DIS 8473 (May 1984)                                       [Page 100]

RFC 926                                                    December 1984

   X := X + 5 (modulo 255) and
   Y := Y - 4 (modulo 255).


    To derive this result, assume that when octet k has the value Z
    added to it then X and Y have values ZX and ZY added to them. For
    the checksum parameters to satisfy the conditions of Section 6.11
    both before and after the values are added, the following is

     Z + ZX + ZY = 0 (modulo 255) and
     (L-k+1)Z + (L-n+1)ZX + (L-n)ZY = 0 (modulo 255).

  Solving these equations simultaneously yields ZX = (k-n-1)Z and ZY +

ISO DIS 8473 (May 1984)                                       [Page 101]