rfc994.txt

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   time which must elapse before any outstanding segments of the Initial
   PDU shall be assumed to be lost.  When this timer expires, all seg-
   ments (Derived PDUs) of the Initial PDU held at the reassembly point
   are discarded, the resources allocated for those segments are freed,
   and if selected, an Error Report is generated (see Clause 6.10).
   While the exact relationship between reassembly lifetime and PDU
   lifetime is a local matter, the Reassembly Function must preserve the
   intent of the PDU lifetime. Consequently, the reassembly function
   must discard PDUs whose lifetime would otherwise have expired had
   they not been under the control of the reassembly function.

                                   Note:

         1. Methods of bounding reassembly lifetime are discussed in
            Annex B.

         2. The Segmentation and Reassembly functions are intended to
            be used in such a way that the fewest possible segments are
            generated at each segmentation point and reassembly takes
            place at the final destination of a PDU. However, other
            schemes which

             (a) interact with the routing algorithm to favor paths on
                 which fewer segments are generated;

             (b) generate more segments than absolutely required in
                 order to avoid additional segmentation at some subsequent
                 point; or

             (c) allow partial or full reassembly at some intermediate
                 point along the route

            are not precluded. The information necessary to enable the
            use of one of these alternative strategies may be made
            available through the operation of a Network Layer Management
            function or by other means.

         3. The originator of the Initial PDU determines the value of the
            Segmentation Permitted flag in the Initial PDU and all Derived
            PDUs (if any).  Partial or full reassembly in an intermediate
            system (Note 2 (c) above) cannot change this value in the
            Initial PDU or any PDU derived from it, and cannot therefore
            add or remove the segmentation part of the header.

6.9     Discard PDU Function

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

     (a)  A violation of protocol procedure has occurred.



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     (b)  A PDU is received whose checksum is inconsistent with its
          contents.

     (c)  A PDU is received, but due to local congestion, it cannot be
          processed.

     (d)  A PDU is received whose header cannot be analyzed.

     (e)  A PDU is received which cannot be segmented and cannot be
          forwarded because its length exceeds the maximum service data
          unit size supported by any underlying service available for
          transmission of the PDU to the next network-entity on the
          chosen route.

     (f)  A PDU is received whose destination address is unreachable or
          unknown.

     (g)  Incorrect or invalid source routing was specified. This may
          include a syntax error in the source routing field, an unknown
          or unreachable address in the source routing field, or a path
          which is not acceptable for other reasons.

     (h)  A PDU is received whose PDU lifetime has expired or whose
          lifetime expires during reassembly.

     (i)  A PDU is received which contains an unsupported option.

6.10     Error Reporting Function

6.10.1     Overview

   This function causes an attempt to return an Error Report PDU to the
   source network-entity when a protocol data unit is discarded in ac-
   cordance with Clause 6.9.

   The Error Report PDU identifies the discarded PDU, specifies the type
   of error detected, and identifies the location in the header of the
   discarded PDU at which the error was detected.  At least the entire
   header of the Discarded PDU (and, at the discretion of the originator
   of the Error Report PDU none, all, or part of the data field) is
   placed in the data field of the Error Report PDU.

   The originator of a Data PDU may control the generation of Error Re-
   port PDUs.  An Error Report flag in the original PDU is set by the
   source network-entity to indicate that an Error Report PDU is to be
   returned if the Initial PDU or any PDUs derived from it are discard-
   ed; if the flag is not set, Error Reports are to be suppressed.

                                    Note:

         1. The suppression of Error Report PDUs is controlled by the



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            originating network-entity and not by the NS User.  Care
            should be exercised by the originator with regard to
            suppressing ER PDUs so that error reporting is not suppressed
            for every PDU generated.

         2. Non-receipt of an Error Report PDU does not imply correct
            delivery of a PDU issued by a source network-entity.

6.10.2     Requirements

   An Error Report PDU shall not be generated to report the discard of
   an Error Report PDU.

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

   If a Data PDU is discarded, and the Error Report flag has been set to
   allow Error Reports, an Error Report PDU shall be generated if the
   reason for discard is one of the reasons for discard enumerated in
   Clause 6.9, subject to the conditions described in Clause 6.10.4.

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

6.10.3     Processing of Error Reports

   An Error Report PDU is composed from information contained in the
   header of the discarded Data PDU to which the Error Report refers.
   The contents of the Source Address field of the discarded Data PDU
   are used as the Destination Address of the Error Report PDU. This
   value, which in the context of the Data PDU was used as an NSAP Ad-
   dress, is used in the context of the Error Report PDU as the
   network-entity title of the network-entity that originated the Data
   PDU. The network- entity title of the originator of the Error Report
   PDU is conveyed in the Source Address field of the header of the Er-
   ror Report PDU. The value of the Lifetime field is determined in ac-
   cordance with Clause 6.4. Optional parameters are selected in accor-
   dance with Clause 6.10.4.

   Segmentation of Error Report PDUs is not permitted; hence, no Segmen-
   tation Part is present.  The total length of the ER PDU in octets is
   placed in the Segment Length field of the ER PDU header. This field
   is not changed during the lifetime of the ER PDU. If the originator
   of the ER PDU determines that the size of the ER PDU exceeds the max-
   imum service data unit size of the underlying service, the ER PDU
   shall be truncated to the maximum service data unit size (see Clause
   5.5.3) and forwarded with no other change. Error Report PDUs are
   routed and forwarded by intermediate-system network-entities in the



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   same way as Data PDUs.

                                    Note:
       The requirement that the underlying service assumed by the CLNP
       must be capable of supporting a service data unit size of at least
       512 octets guarantees that the entire header of the discarded Data
       PDU can be conveyed in the data field of any ER PDU.

   When an ER PDU is decomposed upon reaching its destination, informa-
   tion that may be used to interpret and act upon the Error Report is
   obtained as follows. The network-entity title recovered from the NPAI
   in the Source Address field of the ER PDU header is used to identify
   the network-entity which generated the Error Report.  The reason for
   generating the Error Report is extracted from the Options Part of the
   PDU header. The entire header of the discarded Data PDU (and part or
   all of the original user data) is extracted from the data field of
   the ER PDU to assist in determining the nature of the error.

6.10.4     Relationship of Data PDU Options to Error Reports

   The generation of an Error Report is affected by options that are
   present in the corresponding Data PDU. The presence of options in the
   original Data PDU that are not supported by the system which has dis-
   carded that PDU may cause the suppression of an Error Report even if
   the original Data PDU indicated that an Error Report should be gen-
   erated in the event of a discard.

   The processing of an Error Report is also affected by options which
   are present in the corresponding Data PDU. In particular, options
   selected for the original Data PDU affect which options are included
   in the corresponding Error Report PDU. The selection of options for
   an Error Report PDU is governed by the following requirements:

     (a)  If the Priority Option or the QoS Maintenance Option is selected
          in the original Data PDU, and the system generating the Error
          Report PDU supports the option, then the Error Report PDU shall
          specify the option.

     (b)  If the Security Option is selected in the Data PDU, and the system
          generating the Error Report supports this option, then the Error
          Report PDU shall specify the option using the value that was
          specified in the original Data PDU. If the system does not support
          the Security Option, an Error Report must not be generated for
          a Data PDU that selects the Security Option.

     (c)  If the Complete Source Route Option is selected in the original
          Data PDU, and the system generating the Error Report PDU supports
          this option, then the error Report shall specify the Complete Source
          Route option.  The Source Route parameter value is obtained by
          extracting from the original Data PDU that portion of the complete
          source route that has already been traversed, and reversing the



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          order of network-entity titles which comprise the list.
          If the system does not support the Complete Source Route Option,
          an Error Report must not be generated for a Data PDU that selects
          the Complete Source Route option.

     (d)  The Padding, Partial Source Routing, and Record Route Options,
          if supported, may be specified in the Error Report PDU.

                                    Note:
            The values of the optional parameters in (d) above may be
            derived as a local matter, or they may be based upon the
            corresponding values in the original Data PDU.

6.11     PDU Header Error Detection

   The PDU Header Error Detection function protects against failure of
   intermediate or end-system network-entities due to the processing of
   erroneous information in the PDU header.  The function is realized by
   a checksum computed on the entire PDU header. The checksum is veri-
   fied at each point at which the PDU header is processed.  If the
   checksum calculation fails, the PDU must be discarded.  If PDU header
   fields are modified (for example, due to operation of the lifetime
   function), then the checksum is modified so that the checksum remains
   valid.

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

   If the function is selected by the originating network-entity, the
   value of the checksum field causes the following formulae to be sa-
   tisfied:

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

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


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

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

                                   Note:

       1. To ensure that inadvertent modification of a header while a
          PDU is being processed by an intermediate system (for
          example, due to a memory fault) may still be detected by the
          PDU Header Error function, an intermediate system network-



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          entity must not recompute the checksum for the entire header,
          even if fields are modified.

       2. Annex C contains descriptions of algorithms which may be
          used to calculate the correct value of the checksum field
          when the PDU is created, and to update the value of the
          checksum field when the header is modified.

6.12     Padding Function

   The padding function is provided to allow space to be reserved in the
   PDU header which is not used to support any other function.  Octet
   alignment must be maintained.

                                   Note:
       An example of the use of this function is to cause the data field
       of a PDU to begin on a convenient boundary for the originating
       network-entity, such as a computer word boundary.

6.13     Security

   The provision of protection services (e.g., data origin authentica-
   tion, data confidentiality, and data integrity of a single
   connectionless-mode NSDU) is performed by the Security Function.

   The Security Function is related to the Protection from Unauthorized
   Access Quality of Service parameter described in ISO 8348/AD1, Adden-
   dum to the Network Service Definition Covering Connectionles