rfc926.txt

RFC 的详细文档！

 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
  addressing:

   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
                      indication.

   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
                      request.











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  SYMBOLS AND ABBREVIATIONS

 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  OVERVIEW OF THE PROTOCOL

 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
  framework.

  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]