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RFC 2257 - Agent Extensibility (AgentX) Protocol Version 1


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Network Working Group                                         M. Daniele
Request for Comments: 2257                 Digital Equipment Corporation
Category: Standards Track                                      B. Wijnen
                                  T.J. Watson Research Center, IBM Corp.
                                                       D. Francisco, Ed.
                                                     Cisco Systems, Inc.
                                                            January 1998

                 Agent Extensibility (AgentX) Protocol
                               Version 1

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Table of Contents

   1 Introduction......................................................4

   2 The SNMP Framework................................................4
     2.1 A Note on Terminology.........................................4

   3 Extending the MIB.................................................5
     3.1 Motivation for AgentX.........................................5

   4 AgentX Framework..................................................6
     4.1 AgentX Roles..................................................7
     4.2 Applicability.................................................8
     4.3 Design Features of AgentX.....................................9
     4.4 Non-Goals....................................................10

   5 AgentX Encodings.................................................10
     5.1 Object Identifier............................................11
     5.2 SearchRange..................................................13
     5.3 Octet String.................................................14
     5.4 Value Representation.........................................14

   6 Protocol Definitions.............................................16
     6.1 AgentX PDU Header............................................16

       6.1.1 Context..................................................19
     6.2 AgentX PDUs..................................................20
       6.2.1 The agentx-Open-PDU......................................20
       6.2.2 The agentx-Close-PDU.....................................21
       6.2.3 The agentx-Register-PDU..................................22
       6.2.4 The agentx-Unregister-PDU................................25
       6.2.5 The agentx-Get-PDU.......................................27
       6.2.6 The agentx-GetNext-PDU...................................29
       6.2.7 The agentx-GetBulk-PDU...................................30
       6.2.8 The agentx-TestSet-PDU...................................31
       6.2.9 The agentx-CommitSet, -UndoSet, -CleanupSet
             PDUs.....................................................33
       6.2.10 The agentx-Notify-PDU...................................33
       6.2.11 The agentx-Ping-PDU.....................................34
       6.2.12 The agentx-IndexAllocate-PDU............................35
       6.2.13 The agentx-IndexDeallocate-PDU..........................36
       6.2.14 The agentx-AddAgentCaps-PDU.............................37
       6.2.15 The agentx-RemoveAgentCaps-PDU..........................38
       6.2.16 The agentx-Response-PDU.................................39

   7 Elements of Procedure............................................41
     7.1 Processing AgentX Administrative Messages....................42
       7.1.1 Processing the agentx-Open-PDU...........................42
       7.1.2 Processing the agentx-IndexAllocate-PDU..................43
       7.1.3 Using the agentx-IndexAllocate-PDU.......................45
       7.1.4 Processing the agentx-IndexDeallocate-PDU................47
       7.1.5 Processing the agentx-Register-PDU.......................48
         7.1.5.1 Handling Duplicate OID Ranges........................50
       7.1.6 Processing the agentx-Unregister-PDU.....................51
       7.1.7 Processing the agentx-AddAgentCaps-PDU...................51
       7.1.8 Processing the agentx-RemoveAgentCaps-PDU................52
       7.1.9 Processing the agentx-Close-PDU..........................52
       7.1.10 Detecting Connection Loss...............................53
       7.1.11 Processing the agentx-Notify-PDU........................53
       7.1.12 Processing the agentx-Ping-PDU..........................54
     7.2 Processing Received SNMP Protocol Messages...................54
       7.2.1 Dispatching AgentX PDUs..................................55
         7.2.1.1 agentx-Get-PDU.......................................57
         7.2.1.2 agentx-GetNext-PDU...................................58
         7.2.1.3 agentx-GetBulk-PDU...................................59
         7.2.1.4 agentx-TestSet-PDU...................................60
         7.2.1.5 Dispatch.............................................60
       7.2.2 Subagent Processing of agentx-Get, GetNext,
             GetBulk-PDUs.............................................61
         7.2.2.1 Subagent Processing of the agentx-Get-PDU............61
         7.2.2.2 Subagent Processing of the
                 agentx-GetNext-PDU...................................62

         7.2.2.3 Subagent Processing of the
                 agentx-GetBulk-PDU...................................62
       7.2.3 Subagent Processing of agentx-TestSet,
             -CommitSet, -UndoSet, -CleanupSet-PDUs...................63
         7.2.3.1 Subagent Processing of the
                 agentx-TestSet-PDU...................................64
         7.2.3.2 Subagent Processing of the
                 agentx-CommitSet-PDU.................................65
         7.2.3.3 Subagent Processing of the
                 agentx-UndoSet-PDU...................................65
         7.2.3.4 Subagent Processing of the
                 agentx-CleanupSet-PDU................................65
       7.2.4 Master Agent Processing of AgentX Responses..............66
         7.2.4.1 Common Processing of All AgentX Response
                 PDUs.................................................66
         7.2.4.2 Processing of Responses to agentx-Get-PDUs...........66
         7.2.4.3 Processing of Responses to
                 agentx-GetNext-PDU and agentx-GetBulk-PDU............67
         7.2.4.4 Processing of Responses to
                 agentx-TestSet-PDUs..................................68
         7.2.4.5 Processing of Responses to
                 agentx-CommitSet-PDUs................................68
         7.2.4.6 Processing of Responses to
                 agentx-UndoSet-PDUs..................................69
       7.2.5 Sending the SNMP Response-PDU............................69
       7.2.6 MIB Views................................................69
     7.3 State Transitions............................................70
       7.3.1 Set Transaction States...................................70
       7.3.2 Transport Connection States..............................71
       7.3.3 Session States...........................................73

   8 Transport Mappings...............................................74
     8.1 AgentX over TCP..............................................74
       8.1.1 Well-known Values........................................74
       8.1.2 Operation................................................74
     8.2 AgentX over UNIX-domain Sockets..............................75
       8.2.1 Well-known Values........................................75
       8.2.2 Operation................................................75

   9 Security Considerations..........................................76

   10 Acknowledgements................................................77

   11 Authors' and Editor's Addresses.................................77

   12 References......................................................78

   13 Full Copyright Statement........................................80

1.  Introduction

   This memo defines a standardized framework for extensible SNMP
   agents.  It defines processing entities called master agents and
   subagents, a protocol (AgentX) used to communicate between them, and
   the elements of procedure by which the extensible agent processes
   SNMP protocol messages.

2.  The SNMP Framework

   A management system contains:  several (potentially many) nodes, each
   with a processing entity, termed an agent, which has access to
   management instrumentation; at least one management station; and, a
   management protocol, used to convey management information between
   the agents and management stations.  Operations of the protocol are
   carried out under an administrative framework which defines
   authentication, authorization, access control, and privacy policies.

   Management stations execute management applications which monitor and
   control managed elements.  Managed elements are devices such as
   hosts, routers, terminal servers, etc., which are monitored and
   controlled via access to their management information.

   Management information is viewed as a collection of managed objects,
   residing in a virtual information store, termed the Management
   Information Base (MIB).  Collections of related objects are defined
   in MIB modules.  These modules are written using a subset of OSI's
   Abstract Syntax Notation One (ASN.1) [1], termed the Structure of
   Management Information (SMI) (see RFC 1902 [2]).

2.1.  A Note on Terminology

   The term "variable" refers to an instance of a non-aggregate object
   type defined according to the conventions set forth in the SMI (RFC
   1902, [2]) or the textual conventions based on the SMI (RFC 1903
   [3]).  The term "variable binding" normally refers to the pairing of
   the name of a variable and its associated value.  However, if certain
   kinds of exceptional conditions occur during processing of a
   retrieval request, a variable binding will pair a name and an
   indication of that exception.

   A variable-binding list is a simple list of variable bindings.

   The name of a variable is an OBJECT IDENTIFIER, which is the
   concatenation of the OBJECT IDENTIFIER of the corresponding object
   type together with an OBJECT IDENTIFIER fragment identifying the

   instance.  The OBJECT IDENTIFIER of the corresponding object-type is
   called the OBJECT IDENTIFIER prefix of the variable.  For the purpose
   of exposition, the original Internet-standard

   Network Management Framework, as described in RFCs 1155 (STD 16),
   1157 (STD 15), and 1212 (STD 16), is termed the SNMP version 1
   framework (SNMPv1).  The current framework, as described in RFCs
   1902-1908, is termed the SNMP version 2 framework (SNMPv2).

3.  Extending the MIB

   New MIB modules that extend the Internet-standard MIB are
   continuously being defined by various IETF working groups.  It is
   also common for enterprises or individuals to create or extend
   enterprise-specific or experimental MIBs.

   As a result, managed devices are frequently complex collections of
   manageable components that have been independently installed on a
   managed node.  Each component provides instrumentation for the
   managed objects defined in the MIB module(s) it implements.

   Neither the SNMP version 1 nor version 2 framework describes how the
   set of managed objects supported by a particular agent may be changed
   dynamically.

3.1.  Motivation for AgentX

   This very real need to dynamically extend the management objects
   within a node has given rise to a variety of "extensible agents",
   which typically comprise

      - a "master" agent that is available on the standard transport
        address and that accepts SNMP protocol messages

      - a set of "subagents" that each contain management
        instrumentation

      - a protocol that operates between the master agent and subagents,
        permitting subagents to "connect" to the master agent, and the
        master agent to multiplex received SNMP protocol messages
        amongst the subagents.

      - a set of tools to aid subagent development, and a runtime (API)
        environment that hides much of the protocol operation between a
        subagent and the master agent.

   The wide deployment of extensible SNMP agents, coupled with the lack
   of Internet standards in this area, makes it difficult to field
   SNMP-manageable applications.  A vendor may have to support several
   different subagent environments (APIs) in order to support different
   target platforms.

   It can also become quite cumbersome to configure subagents and
   (possibly multiple) master agents on a particular managed node.

   Specifying a standard protocol for agent extensibility (AgentX)
   provides the technical foundation required to solve both of these
   problems.  Independently developed AgentX-capable master agents and
   subagents will be able to interoperate at the protocol level.
   Vendors can continue to differentiate their products in all other
   respects.

4.  AgentX Framework

   Within the SNMP framework, a managed node contains a processing
   entity, called an agent, which has access to management information.

   Within the AgentX framework, an agent is further defined to consist
   of

      - a single processing entity called the master agent, which sends
        and receives SNMP protocol messages in an agent role (as
        specified by the SNMP version 1 and version 2 framework
        documents) but typically has little or no direct access to
        management information.

      - 0 or more processing entities called subagents, which are
        "shielded" from the SNMP protocol messages processed by the
        master agent, but which have access to management information.

   The master and subagent entities communicate via AgentX protocol
   messages, as specified in this memo.  Other interfaces (if any) on
   these entities, and their associated protocols, are outside the scope
   of this document.  While some of the AgentX protocol messages appear
   similar in syntax and semantics to the SNMP, bear in mind that AgentX
   is not SNMP.

   The internal operations of AgentX are invisible to an SNMP entity
   operating in a manager role.  From a manager's point of view, an
   extensible agent behaves exactly as would a non-extensible
   (monolithic) agent that has access to the same management
   instrumentation.

   This transparency to managers is a fundamental requirement of AgentX,
   and is what differentiates AgentX subagents from SNMP proxy agents.

4.1.  AgentX Roles

   An entity acting in a master agent role performs the following
   functions:

      - Accepts AgentX session establishment requests from subagents.

      - Accepts registration of MIB regions by subagents.

      - Sends and accepts SNMP protocol messages on the agent's
        specified transport addresses.

      - Implements the agent role Elements of Procedure specified
        for the administrative framework applicable to the SNMP protocol
        message, except where they specify performing management
        operations.  (The application of MIB views, and the access
        control policy for the managed node, are implemented by the
        master agent.)

      - Provides instrumentation for the MIB objects defined in RFC
        1907 [5], and for any MIB objects relevant to any administrative
        framework it supports.

      - Sends and receives AgentX protocol messages to access
        management information, based on the current registry of MIB
        regions.

      - Forwards notifications on behalf of subagents.

   An entity acting in a subagent role performs the following functions:

      - Initiates an AgentX session with the master agent.

      - Registers MIB regions with the master agent.

      - Instantiates managed objects.

      - Binds OIDs within its registered MIB regions to actual
        variables.

      - Performs management operations on variables.

      - Initiates notifications.

4.2  Applicability

   It is intended that this memo specify the smallest amount of required
   behavior necessary to achieve the largest benefit, that is, to cover
   a very large number of possible MIB implementations and
   configurations with minimum complexity and low "cost of entry".

   This section discusses several typical usage scenarios.

   1) Subagents implement separate MIB modules--for example,
      subagent A implements "mib-2", subagent b implements "host-
      resources".

      It is anticipated that this will be the most common subagent
      configuration.

   2) Subagents implement rows in a "simple table".  A simple table
      is one in which row creation is not specified, and for which the
      MIB does not define an object that counts entries in the table.
      Examples of simple tables are rdbmsDbTable, udpTable, and
      hrSWRunTable.

      This is the most commonly defined type of MIB table, and probably
      represents the next most typical configuration that AgentX would
      support.

   3) Subagents share MIBs along non-row partitions.  Subagents
      register "chunks" of the MIB that represent multiple rows, due to
      the nature of the MIB's index structure.  Examples include
      registering ipNetToMediaEntry.n, where n represents the ifIndex
      value for an interface implemented by the subagent, and
      tcpConnEntry.a.b.c.d, where a.b.c.d represents an IP address on an
      interface implemented by the subagent.

   AgentX supports these three common configurations, and all
   permutations of them, completely.  The consensus is that they
   comprise a very large majority of current and likely future uses of
   multi-vendor extensible agent configurations.

   4) Subagents implement rows in "complex tables".  Complex tables
      here are defined as tables permitting row creation, or whose MIB
      also defines an object that counts entries in the table.  Examples
      include the MIB-2 ifTable (due to ifNumber), and the RMON
      historyControlTable.

   The subagent that implements such a counter object (like ifNumber)
   must go beyond AgentX to correctly implement it.  This is an
   implementation issue (and most new MIB designs no longer include such
   objects).

   To implement row creation in such tables, at least one AgentX
   subagent must register at a point "higher" in the OID tree than an
   individual row (per AgentX's dispatching procedure).  Again, this is
   an implementation issue.

   Scenarios in this category were thought to occur somewhat rarely in
   configurations where subagents are independently implemented by
   different vendors.  The focus of a standard protocol, however, must
   be in just those areas where multi- vendor interoperability must be
   assured.

   Note that it would be inefficient (due to AgentX registration
   overhead) to share a table among AgentX subagents if the table
   contains very dynamic instances, and each subagent registers fully
   qualified instances.  ipRouteTable could be an example of such a
   table in some environments.

4.3.  Design Features of AgentX

   The primary features of the design described in this memo are:

   1) A general architectural division of labor between master agent
      and subagent: The master agent is MIB ignorant and SNMP
      omniscient, while the subagent is SNMP ignorant and MIB omniscient
      (for the MIB variables it instantiates).  That is, master agents,
      exclusively, are concerned with SNMP protocol operations and the
      translations to and from AgentX protocol operations needed to
      carry them out; subagents are exclusively concerned with
      management instrumentation; and neither should intrude on the
      other's territory.

   2) A standard protocol and "rules of engagement" to enable
      interoperability between management instrumentation and extensible
      agents.

   3) Mechanisms for independently developed subagents to
      integrate into the extensible agent on a particular managed node
      in such a way that they need not be aware of any other existing
      subagents.

   4) A simple, deterministic registry and dispatching algorithm.
      For a given extensible agent configuration, there is a single
      subagent who is "authoritative" for any particular region of the
      MIB (where "region" may extend from an entire MIB down to a single
      object-instance).

   5) Performance considerations.  It is likely that the master
      agent and all subagents will reside on the same host, and in such
      cases AgentX is more a form of inter-process communication than a
      traditional communications protocol.

      Some of the design decisions made with this in mind include:

         - 32-bit alignment of data within PDUs

         - Native byte-order encoding by subagents

         - Large AgentX PDU payload sizes.

4.4  Non-Goals

   1) Subagent-to-subagent communication.  This is out of scope,
      due to the security ramifications and complexity involved.

   2) Subagent access (via the master agent) to MIB variables.
      This is not addressed, since various other mechanisms are
      available and it was not a fundamental requirement.

   3) The ability to accommodate every conceivable extensible
      agent configuration option. This was the most contentious aspect
      in the development of this protocol.  In essence, certain features
      currently available in some commercial extensible agent products
      are not included in AgentX.  Although useful or even vital in some
      implementation strategies, the rough consensus was that these
      features were not appropriate for an Internet Standard, or not
      typically required for independently developed subagents to
      coexist.  The set of supported extensible agent configurations is
      described above, in Section 4.2.

   Some possible future version of the AgentX protocol may provide
   coverage for one or more of these "non-goals" or for new goals that
   might be identified after greater deployment experience.

5.  AgentX Encodings

   AgentX PDUs consist of a common header, followed by PDU-specific data
   of variable length.  Unlike SNMP PDUs, AgentX PDUs are not encoded
   using the BER (as specified in ISO 8824 [1]), but are transmitted as

   a contiguous byte stream.  The data within this stream is organized
   to provide natural alignment with respect to the start of the PDU,
   permitting direct (integer) access by the processing entities.

   The first four fields in the header are single-byte values.  A bit
   (NETWORK_BYTE_ORDER) in the third field (h.flags) is used to indicate
   the byte ordering of all multi-byte integer values in the PDU,
   including those which follow in the header itself.  This is described
   in more detail in Section 6.1, "AgentX PDU Header", below.

   PDUs are depicted in this memo using the following convention (where
   byte 1 is the first transmitted byte):

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  byte 1       |  byte 2       |  byte 3       |  byte 4       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  byte 5       |  byte 6       |  byte 7       |  byte 8       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields marked "<reserved>" are reserved for future use and must be
   zero-filled.

5.1.  Object Identifier

   An object identifier is encoded as a 4-byte header, followed by a
   variable number of contiguous 4-byte fields representing sub-
   identifiers.  This representation (termed Object Identifier) is as
   follows:

   Object Identifier

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  n_subid      |  prefix       |  include      |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       sub-identifier #1                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       sub-identifier #n_subid                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Object Identifier header fields:

      n_subid

         The number (0-128) of sub-identifiers in the object identifier.
         An ordered list of "n_subid" 4-byte sub-identifiers follows the
         4-byte header.

      prefix

         An unsigned value used to reduce the length of object
         identifier encodings.  A non-zero value "x" is interpreted as
         the first sub-identifier after "internet" (1.3.6.1), and
         indicates an implicit prefix "internet.x" to the actual sub-
         identifiers encoded in the Object Identifier.  For example, a
         prefix field value 2 indicates an implicit prefix "1.3.6.1.2".
         A value of 0 in the prefix field indicates there is no prefix
         to the sub-identifiers.

      include

         Used only when the Object Identifier is the start of a
         SearchRange, as described in section 5.2.

   A null Object Identifier consists of the 4-byte header with all bytes
   set to 0.

   Examples:

   sysDescr.0 (1.3.6.1.2.1.1.1.0)

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 4             | 2             | 0             | 0             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 0                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   1.2.3.4

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 4             | 0             | 0             | 0             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 2                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 3                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 4                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.2.  SearchRange

   A SearchRange consists of two Object Identifiers.  In its
   communication with a subagent, the master agent uses a SearchRange to
   identify a requested variable binding, and, in GetNext and GetBulk
   operations, to set an upper bound on the names of managed object
   instances the subagent may send in reply.

   The first Object Identifier in a SearchRange (called the starting
   OID) indicates the beginning of the range.  It is frequently (but not
   necessarily) the name of a requested variable binding.

   The "include" field in this OID's header is a boolean value (0 or 1)
   indicating whether or not the starting OID is included in the range.

   The second object identifier indicates the non-inclusive end of the
   range, and its "include" field is always 0.

   Example:  To indicate a search range from 1.3.6.1.2.1.25.2
   (inclusive) to 1.3.6.1.2.1.25.2.1 (exclusive), the SearchRange would
   be

   (start)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 3             | 2             | 1             |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 25                                                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 2                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (end)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 4             | 2             | 0             |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 25                                                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 2                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A SearchRangeList is a contiguous list of SearchRanges.

5.3.  Octet String

   An octet string is represented by a contiguous series of bytes,
   beginning with a 4-byte integer whose value is the number of octets
   in the octet string, followed by the octets themselves.  This
   representation is termed an Octet String.  If the last octet does not
   end on a 4-byte offset from the start of the Octet String, padding
   bytes are appended to achieve alignment of following data.  This
   padding must be added even if the Octet String is the last item in
   the PDU.  Padding bytes must be zero filled.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Octet String Length (L)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Octet L - 1  |  Octet L      |       Padding (as required)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A null Octet String consists of a 4-byte length field set to 0.

5.4.  Value Representation

   Variable bindings may be encoded within the variable-length portion
   of some PDUs.  The representation of a variable binding (termed a
   VarBind) consists of a 2-byte type field, a name (Object Identifier),
   and the actual value data.

   VarBind

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          v.type               |          <reserved>           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (v.name)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  n_subid      |  prefix       |      0        |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       sub-identifier #1                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       sub-identifier #n_subid                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (v.data)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       data                                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       data                                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   VarBind fields:

   v.type

         Indicates the variable binding's syntax, and must be one of
         the following values:

                     Integer                  (2),
                     Octet String             (4),
                     Null                     (5),
                     Object Identifier        (6),
                     IpAddress               (64),
                     Counter32               (65),
                     Gauge32                 (66),
                     TimeTicks               (67),
                     Opaque                  (68),
                     Counter64               (70),
                     noSuchObject           (128),
                     noSuchInstance         (129),
                     endOfMibView           (130)

   v.name

         The Object Identifier which names the variable.

   v.data

         The actual value, encoded as follows:

          - Integer, Counter32, Gauge32, and TimeTicks are encoded as
            4 contiguous bytes.  If the NETWORK_BYTE_ORDER bit is set
            in h.flags, the bytes are ordered most significant to least
            significant, otherwise they are ordered least significant
            to most significant.

          - Counter64 is encoded as 8 contiguous bytes.  If the
            NETWORK_BYTE_ORDER bit is set in h.flags, the bytes are
            ordered most significant to least significant, otherwise
            they are ordered least significant to most significant.

          - Object Identifiers are encoded as described in section
            5.1, Object Identifier.

          - IpAddress, Opaque, and Octet String are all octet strings
            and are encoded as described in section 5.3, Octet String.

            Value data always follows v.name whenever v.type is one
            of the above types.  These data bytes are present even if
            they will not be used (as, for example, in certain types
            of index allocation).

          - Null, noSuchObject, noSuchInstance, and endOfMibView do not
            contain any encoded value.  Value data never follows
            v.name in these cases.

         Note that the VarBind itself does not contain the value size.
         That information is implied for the fixed-length types, and
         explicitly contained in the encodings of variable-length types
         (Object Identifier and Octet String).

   A VarBindList is a contiguous list of VarBinds.  Within a
   VarBindList, a particular VarBind is identified by an index value.
   The first VarBind in a VarBindList has index value 1, the second
   has index value 2, and so on.

6.  Protocol Definitions

6.1.  AgentX PDU Header

   The AgentX PDU header is a fixed-format, 20-octet structure:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   h.version   |    h.type     |    h.flags    |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          h.sessionID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.transactionID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          h.packetID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.payload_length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An AgentX PDU header contains the following fields:

      h.version

         The version of the AgentX protocol (1 for this memo).

      h.type

         The PDU type; one of the following values:

              agentx-Open-PDU             (1),
              agentx-Close-PDU            (2),
              agentx-Register-PDU         (3),
              agentx-Unregister-PDU       (4),
              agentx-Get-PDU              (5),
              agentx-GetNext-PDU          (6),
              agentx-GetBulk-PDU          (7),
              agentx-TestSet-PDU          (8),
              agentx-CommitSet-PDU        (9),
              agentx-UndoSet-PDU         (10),
              agentx-CleanupSet-PDU      (11),
              agentx-Notify-PDU          (12),
              agentx-Ping-PDU            (13),
              agentx-IndexAllocate-PDU   (14),
              agentx-IndexDeallocate-PDU (15),
              agentx-AddAgentCaps-PDU    (16),
              agentx-RemoveAgentCaps-PDU (17),
              agentx-Response-PDU        (18)

      h.flags

         A bitmask, with bit 0 the least significant bit.  The bit
         definitions are as follows:

                 Bit             Definition
                 ---             ----------
                 0               INSTANCE_REGISTRATION
                 1               NEW_INDEX
                 2               ANY_INDEX
                 3               NON_DEFAULT_CONTEXT
                 4               NETWORK_BYTE_ORDER
                 5-7             (reserved)

         The NETWORK_BYTE_ORDER bit applies to all multi-byte integer
         values in the entire AgentX packet, including the remaining
         header fields.  If set, then network byte order (most
         significant byte first; "big endian") is used.  If not set,
         then least significant byte first ("little endian") is used.

         The NETWORK_BYTE_ORDER bit applies to all AgentX PDUs.

         The NON_DEFAULT_CONTEXT bit is used only in the AgentX PDUs
         described in section 6.1.1.

         The NEW_INDEX and ANY_INDEX bits are used only within the
         agentx-IndexAllocate-, and -IndexDeallocate-PDUs.

         The INSTANCE_REGISTRATION bit is used only within the agentx-
         Register-PDU.

      h.sessionID

         The session ID uniquely identifies a session over which AgentX
         PDUs are exchanged between a subagent and the master agent.
         The session ID has no significance and no defined value in the
         agentx-Open-PDU sent by a subagent to open a session with the
         master agent; in this case, the master agent will assign a
         unique sessionID that it will pass back in the corresponding
         agentx-Response-PDU.  From that point on, that same sessionID
         will appear in every AgentX PDU exchanged over that session
         between the master and the subagent.  A subagent may establish
         multiple AgentX sessions by sending multiple agentx-Open-PDUs
         to the master agent.

         In master agents that support multiple transport protocols, the
         sessionID should be globally unique rather than unique just to
         a particular transport.

      h.transactionID

         The transaction ID uniquely identifies, for a given session,
         the single SNMP management request (and single SNMP PDU) with
         which an AgentX PDU is associated.  If a single SNMP management
         request results in multiple AgentX PDUs being sent by the
         master agent with the same sessionID, each of these AgentX PDUs
         must contain the same transaction ID; conversely, AgentX PDUs
         sent during a particular session, that result from distinct
         SNMP management requests, must have distinct transaction IDs
         within the limits of the 32-bit field).

         Note that the transaction ID is not the same as the SNMP PDU's
         request-id (as described in section 4.1 of RFC 1905 [4]), nor
         can it be, since a master agent might receive SNMP requests
         with the same request-ids from different managers.

         The transaction ID has no significance and no defined value in
         AgentX administrative PDUs, i.e., AgentX PDUs that are not
         associated with an SNMP management request.

      h.packetID

         A packet ID generated by the sender for all AgentX PDUs except
         the agentx-Response-PDU. In an agentx-Response-PDU, the packet
         ID must be the same as that in the received AgentX PDU to which
         it is a response.  A master agent might use this field to
         associate subagent response PDUs with their corresponding
         request PDUs.  A subagent might use this field to correlate
         responses to multiple (batched) registrations.

      h.payload_length

         The size in octets of the PDU contents, excluding the 20-byte
         header.  As a result of the encoding schemes and PDU layouts,
         this value will always be either 0, or a multiple of 4.

6.1.1.  Context

   In the SNMPv1 or v2c frameworks, the community string may be used as
   an index into a local repository of configuration information that
   may include community profiles or more complex context information.
   Future versions of the SNMP will likely formalize this notion of
   "context".

   AgentX provides a mechanism for transmitting a context specification
   within relevant PDUs, but does not place any constraints on the
   content of that specification.

   An optional context field may be present in the agentx-Register-,
   UnRegister-, AddAgentCaps-, RemoveAgentCaps-, Get-, GetNext-,
   GetBulk-, IndexAllocate-, IndexDeallocate-, Notify-, TestSet-, and
   Ping- PDUs.

   If the NON_DEFAULT_CONTEXT bit in the AgentX header field h.flags is
   clear, then there is no context field in the PDU, and the operation
   refers to the default context.

   If the NON_DEFAULT_CONTEXT bit is set, then a context field
   immediately follows the AgentX header, and the operation refers to
   that specific context.  The context is represented as an Octet
   String.  There are no constraints on its length or contents.

   Thus, all of these AgentX PDUs (that is, those listed immediately
   above) refer to, or "indicate" a context, which is either the default
   context, or a non-default context explicitly named in the PDU.

6.2.  AgentX PDUs

6.2.1.  The agentx-Open-PDU

   An agentx-Open-PDU is generated by a subagent to request
   establishment of an AgentX session with the master agent.

   (AgentX header)

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | h.version (1) |  h.type (1)   |    h.flags    |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          h.sessionID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.transactionID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           h.packetID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.payload_length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  o.timeout    |                     <reserved>                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (o.id)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  n_subid      |  prefix       |       0       |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             subidentifier #1                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ...                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             subidentifier #n_subid                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (o.descr)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Octet String Length (L)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Octet L - 1  |  Octet L      |       Padding (as required)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An agentx-Open-PDU contains the following fields:

      o.timeout

         The length of time, in seconds, that a master agent should
         allow to elapse after dispatching a message to a subagent
         before it regards the subagent as not responding.  This is a
         subagent-wide default value that may be overridden by values
         associated with specific registered MIB regions.  The default
         value of 0 indicates that no subagent-wide value is requested.

      o.id

         An Object Identifier that identifies the subagent.  Subagents
         that do not support such an notion may send a null Object
         Identifier.

      o.descr

         An Octet String containing a DisplayString describing the
         subagent.

6.2.2.  The agentx-Close-PDU

   An agentx-Close-PDU issued by either a subagent or the master agent
   terminates an AgentX session.

   (AgentX header)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | h.version (1) |  h.type (2)   |    h.flags    |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          h.sessionID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.transactionID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           h.packetID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.payload_length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  c.reason     |                     <reserved>                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An agentx-Close-PDU contains the following field:

      c.reason

         An enumerated value that gives the reason that the master agent
         or subagent closed the AgentX session.  This field may take one
         of the following values:

            reasonOther(1)
               None of the following reasons

            reasonParseError(2)
               Too many AgentX parse errors from peer

            reasonProtocolError(3)
               Too many AgentX protocol errors from peer

            reasonTimeouts(4)
               Too many timeouts waiting for peer

            reasonShutdown(5)
               Sending entity is shutting down

            reasonByManager(6)
               Due to Set operation; this reason code can be used only
               by the master agent, in response to an SNMP management
               request.

6.2.3.  The agentx-Register-PDU

   An agentx-Register-PDU is generated by a subagent for each region of
   the MIB variable naming tree (within one or more contexts) that it
   wishes to support.

    (AgentX header)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | h.version (1) |  h.type (3)   |    h.flags    |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          h.sessionID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.transactionID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           h.packetID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.payload_length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (r.context) (OPTIONAL)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Octet String Length (L)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet L - 1  |  Octet L      |       Padding (as required)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  r.timeout    |  r.priority   | r.range_subid |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (r.region)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  n_subid      |  prefix       |      0        |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #1                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #n_subid                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (r.upper_bound)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             optional upper-bound sub-identifier               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An agentx-Register-PDU contains the following fields:

      r.context

         An optional non-default context.

      r.timeout

         The length of time, in seconds, that a master agent should
         allow to elapse after dispatching a message to a subagent
         before it regards the subagent as not responding.  r.timeout
         applies only to messages that concern MIB objects within
         r.region.  It overrides both the subagent-wide value (if any)
         indicated when the AgentX session with the master agent was
         established, and the master agent's default timeout.  The
         default value for r.timeout is 0 (no override).

      r.priority

         A value between 1 and 255, used to achieve a desired
         configuration when different subagents register identical or
         overlapping regions.  Subagents with no particular knowledge of
         priority should register with the default value of 255 (lowest
         priority).

         In the master agent's dispatching algorithm, smaller values of
         r.priority take precedence over larger values, as described in
         section 7.1.5.1.

      r.region

         An Object Identifier that, in conjunction with r.range_subid,
         indicates a region of the MIB that a subagent wishes to
         support.  It may be a fully-qualified instance name, a partial
         instance name, a MIB table, an entire MIB, or ranges of any of
         these.

         The choice of what to register is implementation-specific; this
         memo does not specify permissible values.  Standard practice
         however is for a subagent to register at the highest level of
         the naming tree that makes sense.  Registration of fully-
         qualified instances is typically done only when a subagent can
         perform management operations only on particular rows of a
         conceptual table.

         If r.region is in fact a fully qualified instance name, the
         INSTANCE_REGISTRATION bit in h.flags must be set, otherwise it
         must be cleared.  The master agent may save this information to
         optimize subsequent operational dispatching.

      r.range_subid

         Permits specifying a range in place of one of r.region's sub-
         identifiers.  If this value is 0, no range is specified.
         Otherwise the "r.range_subid"-th sub-identifier in r.region is
         a range lower bound, and the range upper bound sub-identifier
         (r.upper_bound) immediately follows r.region.

         This permits registering a conceptual row with a single PDU.
         For example, the following PDU would register row 7 of the RFC
         1573 ifTable (1.3.6.1.2.1.2.2.1.1-22.7):

   (AgentX header)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | h.version (1) |  h.type (3)   |    h.flags    |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          h.sessionID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.transactionID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           h.packetID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.payload_length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   r.timeout   |  r.priority   | 5             |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (r.region)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 6             |  2            |  0            |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 2                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 2                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 7                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (r.upper_bound)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 22                                                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6.2.4.  The agentx-Unregister-PDU

   The agentx-Unregister-PDU is sent by a subagent to remove a
   previously registered MIB region from the master agent's OID space.

   (AgentX header)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | h.version (1) |  h.type (4)   |    h.flags    |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          h.sessionID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.transactionID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           h.packetID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.payload_length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (u.context) OPTIONAL
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Octet String Length (L)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Octet L - 1  |  Octet L      |       Padding (as required)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    <reserved> |  u.priority   | u.range_subid |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (u.region)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  n_subid      |  prefix       |      0        |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             sub-identifier #1                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             sub-identifier #n_subid                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (u.upper_bound)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             optional upper-bound sub-identifier               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An agentx-Unregister-PDU contains the following fields:

      u.context

         An optional non-default context.

      u.priority

         The priority at which this region was originally registered.

      u.region

         Indicates a previously-registered region of the MIB that a
         subagent no longer wishes to support.

6.2.5.  The agentx-Get-PDU

    (AgentX header)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | h.version (1) |  h.type (5)   |    h.flags    |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          h.sessionID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.transactionID                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           h.packetID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.payload_length                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (g.context) OPTIONAL
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Octet String Length (L)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet L - 1  |  Octet L      |       Padding (as required)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (g.sr)

    (start 1)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  n_subid      |  prefix       |  include      |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #1                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #n_subid                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (end 1)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | 0             | 0             | 0             |       0       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    (start n)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  n_subid      |  prefix       |  include      |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #1                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #n_subid                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (end n)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | 0             | 0             | 0             |       0       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An agentx-Get-PDU contains the following fields:

      g.context

         An optional non-default context.

      g.sr

         A SearchRangeList containing the requested variables for this
         subagent.

6.2.6.  The agentx-GetNext-PDU

    (AgentX header)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | h.version (1) |  h.type (6)   |    h.flags    |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          h.sessionID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.transactionID                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           h.packetID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.payload_length                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (g.context) OPTIONAL
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Octet String Length (L)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet L - 1  |  Octet L      |       Padding (as required)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (g.sr)

    (start 1)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  n_subid      |  prefix       |  include      |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #1                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #n_subid                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (end 1)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  n_subid      |  prefix       |      0        |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #1                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #n_subid                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...

    (start n)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  n_subid      |  prefix       |  include      |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #1                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #n_subid                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (end n)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  n_subid      |  prefix       |      0        |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #1                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #n_subid                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...

6.2.7.  The agentx-GetBulk-PDU

    (AgentX header)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | h.version (1) |  h.type (7)   |    h.flags    |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          h.sessionID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.transactionID                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           h.packetID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.payload_length                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (g.context) OPTIONAL
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Octet String Length (L)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet L - 1  |  Octet L      |       Padding (as required)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             g.non_repeaters   |     g.max_repetitions         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (g.sr) as in agentx-GetNext-PDU above
    ...

6.2.8.  The agentx-TestSet-PDU

    (AgentX header)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | h.version (1) |  h.type (8)   |    h.flags    |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          h.sessionID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.transactionID                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           h.packetID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.payload_length                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (t.context) OPTIONAL
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Octet String Length (L)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet L - 1  |  Octet L      |       Padding (as required)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (t.vb)

    (VarBind 1)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          v.type               |        <reserved>             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  n_subid      |  prefix       |      0        |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       sub-identifier #1                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       sub-identifier #n_subid                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       data                                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       data                                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...

    (VarBind n)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          v.type               |        <reserved>             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  n_subid      |  prefix       |      0        |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       sub-identifier #1                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       sub-identifier #n_subid                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       data                                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       data                                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An agentx-TestSet-PDU contains the following fields:

      t.context

         An optional non-default context.

      t.vb

         A VarBindList containing the requested VarBinds for this
         subagent.

6.2.9.  The agentx-CommitSet, -UndoSet, -CleanupSet PDUs

   These PDUs consist of the AgentX header only.

   The agentx-CommitSet-, -UndoSet-, and -Cleanup-PDUs are used in
   processing an SNMP SetRequest operation.

6.2.10.  The agentx-Notify-PDU

   An agentx-Notify-PDU is sent by a subagent to cause the master agent
   to forward a notification.

    (AgentX header)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | h.version (1) |  h.type (12)  |    h.flags    |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          h.sessionID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.transactionID                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           h.packetID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.payload_length                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (n.context) OPTIONAL
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Octet String Length (L)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet L - 1  |  Octet L      |       Padding (as required)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (n.vb)
    ...

   An agentx-Notify-PDU contains the following fields:

      n.context

         An optional non-default context.

      n.vb

         A VarBindList whose contents define the actual PDU to be sent.
         This memo places the following restrictions on its contents:

             - If the subagent supplies sysUpTime.0, it must be
               present as the first varbind.

             - snmpTrapOID.0 must be present, as the second
               varbind if sysUpTime.0 was supplied, as the first if it
               was not.

6.2.11  The agentx-Ping-PDU

   The agentx-Ping-PDU is sent by a subagent to the master agent to
   monitor the master agent's ability to receive and send AgentX PDUs
   over their AgentX session.

    (AgentX header)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | h.version (1) |  h.type (13)  |    h.flags    |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          h.sessionID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.transactionID                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           h.packetID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.payload_length                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (p.context) OPTIONAL
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Octet String Length (L)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet L - 1  |  Octet L      |       Padding (as required)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An agentx-Ping-PDU may contain the following field:

      p.context

         An optional non-default context.

   Using p.context a subagent can retrieve the sysUpTime value for a
   specific context, if required.

6.2.12.  The agentx-IndexAllocate-PDU

   An agentx-IndexAllocate-PDU is sent by a subagent to request
   allocation of a value for specific index objects.  Refer to section
   7.1.3 (Using the agentx-IndexAllocate-PDU) for suggested usage.

    (AgentX header)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | h.version (1) |  h.type (14)  |    h.flags    |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          h.sessionID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.transactionID                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           h.packetID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.payload_length                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (i.context) OPTIONAL
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Octet String Length (L)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet L - 1  |  Octet L      |       Padding (as required)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (i.vb)
    ...

   An agentx-IndexAllocate-PDU contains the following fields:

      i.context

         An optional non-default context.

      i.vb

         A VarBindList containing the index names and values requested
         for allocation.

6.2.13.  The agentx-IndexDeallocate-PDU

   An agentx-IndexDeallocate-PDU is sent by a subagent to release
   previously allocated index values.

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | h.version (1) |  h.type (15)  |    h.flags    |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          h.sessionID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.transactionID                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           h.packetID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.payload_length                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (i.context) OPTIONAL
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Octet String Length (L)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet L - 1  |  Octet L      |       Padding (as required)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (i.vb)
    ...

   An agentx-IndexDeallocate-PDU contains the following fields:

      i.context

         An optional non-default context.

      i.vb

         A VarBindList containing the index names and values to be
         released.

6.2.14.  The agentx-AddAgentCaps-PDU

   An agentx-AddAgentCaps-PDU is generated by a subagent to inform the
   master agent of its agent capabilities.

    (AgentX header)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | h.version (1) |  h.type (16)  |    h.flags    |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          h.sessionID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.transactionID                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           h.packetID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        h.payload_length                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (a.context) (OPTIONAL)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Octet String Length (L)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet L - 1  |  Octet L      |       Optional Padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (a.id)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  n_subid      |  prefix       |      0        |  <reserved>   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #1                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             sub-identifier #n_subid                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (a.descr)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Octet String Length (L)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Octet L - 1  |  Octet L      |       Optional Padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An agentx-AddAgentCaps-PDU contains the following fields:

      a.context

         An optional non-default context.

      a.id

         An Object Identifier containing the value of an invocation of
         the AGENT-CAPABILITIES macro, which the master agent exports as
         a value of sysORID for the indicated context.  (Recall that the
         value of an invocation of an AGENT-CAPABILITIES macro is an
         object identifier that describes a precise level of support
         with respect to implemented MIB modules.  A more complete
         discussion of the AGENT-CAPABILITIES macro and related sysORID
         values can be found in section 6 of RFC 1904 [10].)

      a.descr

         An Octet String containing a DisplayString to be used as the
         value of sysORDescr corresponding to the sysORID value above.

6.2.15.  The agentx-RemoveAgentCaps-PDU

   An agentx-RemoveAgentCaps-PDU is generated by a subagent to request
   that the master agent stop exporting a particular value of sysORID.
   This value must have previously been advertised by the subagent in an
   agentx-AddAgentCaps-PDU.

   (AgentX header)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | h.version (1) |  h.type (17)  |    h.flags    |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          h.sessionID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.transactionID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           h.packetID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.payload_length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (a.context) (OPTIONAL)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Octet String Length (L)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Octet L - 1  |  Octet L      |       Optional Padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (a.id)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  n_subid      |  prefix       |       0       |   <reserved>  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             sub-identifier #1                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             sub-identifier #n_subid                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An agentx-RemoveAgentCaps-PDU contains the following fields:

      a.context

         An optional non-default context.

      a.id

         An ObjectIdentifier containing the value of sysORID that should
         no longer be exported.

6.2.16.  The agentx-Response-PDU

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | h.version (1) |  h.type (18)  |    h.flags    |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          h.sessionID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.transactionID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           h.packetID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        h.payload_length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        res.sysUpTime                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             res.error         |     res.index                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ...

   An agentx-Response-PDU contains the following fields:

      h.sessionID

         If this is a response to a agentx-Open-PDU, then it contains
         the new and unique sessionID (as assigned by the master agent)
         for this session.

         Otherwise it must be identical to the h.sessionID value in the
         PDU to which this PDU is a response.

      h.transactionID

         Must be identical to the h.transactionID value in the PDU to
         which this PDU is a response.

         In an agentx response PDU from the master agent to the
         subagent, the value of h.transactionID has no significance and
         can be ignored by the subagent.

      h.packetID

         Must be identical to the h.packetID value in the PDU to which
         this PDU is a response.

      res.sysUpTime

         This field contains the current value of sysUpTime for the
         indicated context.  It is relevant only in agentx response PDUs
         sent from the master agent to a subagent in response to the
         following agentx PDUs:

               agentx-Open-PDU               (1),
               agentx-Close-PDU              (2),
               agentx-Register-PDU           (3),
               agentx-Unregister-PDU         (4),
               agentx-Ping-PDU              (13),
               agentx-IndexAllocate-PDU     (14),
               agentx-IndexDeallocate-PDU   (15),
               agentx-AddAgentCaps-PDU      (16),
               agentx-RemoveAgentCaps-PDU   (17)

         In an agentx response PDU from the subagent to the master
         agent, the value of res.sysUpTime has no significance and is
         ignored by the master agent.

      res.error

         Indicates error status (including `noError').  Values are
         limited to those defined for errors in the SNMPv2 SMI (RFC 1905
         [4]), and the following AgentX-specific values:

                openFailed                 (256),
                notOpen                    (257),
                indexWrongType             (258),
                indexAlreadyAllocated      (259),
                indexNoneAvailable         (260),
                indexNotAllocated          (261),
                unsupportedContext         (262),
                duplicateRegistration      (263),
                unknownRegistration        (264),
                unknownAgentCaps           (265)

      res.index

         In error cases, this is the index of the failed variable
         binding within a received request PDU.  (Note: As explained in
         section 5.4, Value Representation, the index values of variable
         bindings within a variable binding list are 1-based.)

   A VarBindList may follow these latter two fields, depending on which
   AgentX PDU is being responded to.  These data are specified in the
   subsequent elements of procedure.

7.  Elements of Procedure

   This section describes the actions of protocol entities (master
   agents and subagents) implementing the AgentX protocol.  Note,
   however, that it is not intended to constrain the internal
   architecture of any conformant implementation.

   Specific error conditions and associated actions are described in
   various places.  Other error conditions not specifically mentioned
   fall into one of two categories, "parse" errors and "protocol"
   errors.

   A parse error occurs when a receiving entity cannot decode the PDU.
   For instance, a VarBind contains an unknown type, or a PDU contains a
   malformed Object Identifier.

   A protocol error occurs when a receiving entity can parse a PDU, but
   the resulting data is unspecified.  For instance, an agentx-
   Response-PDU is successfully parsed, but contains an unknown
   res.error value.

   An implementation may choose either to ignore such messages, or to
   close the session on which they are received, using the appropriate
   reason code as defined in the agentx-Close-PDU.

   The actions of AgentX protocol entities can be broadly categorized
   under two headings, each of which is described separately:

      (1) processing AgentX administrative messages (e.g., connection
          requests from a subagent to a master agent); and

      (2) processing SNMP messages (the coordinated actions of a
          master agent and one or more subagents in processing, for
          example, a received SNMP GetRequest-PDU).

7.1.  Processing AgentX Administrative Messages

   This subsection describes the actions of AgentX protocol entities in
   processing AgentX administrative messages.  Such messages include
   those involved in establishing and terminating an AgentX session
   between a subagent and a master agent, those by which a subagent
   requests allocation of instance index values, and those by which a
   subagent communicates to a master agent which MIB regions it
   supports.

7.1.1.  Processing the agentx-Open-PDU

   When the master agent receives an agentx-Open-PDU, it processes it as
   follows:

   1) An agentx-Response-PDU is created and res.sysUpTime is set to
      the value of sysUpTime.0 for the indicated context.

   2) If the master agent is unable to open an AgentX session for
      any reason, it may refuse the session establishment request,
      sending in reply the agentx-Response-PDU, with res.error field set
      to `openFailed'.

   3) Otherwise:  The master agent assigns a sessionID to the new
      session and puts the value in the h.sessionID field of the
      agentx-Response-PDU.  This value must be unique among all existing
      open sessions.

   4) The master agent retains session-specific information
      from the PDU for this subagent:

       - The NETWORK_BYTE_ORDER value in h.flags is retained.
         All subsequent AgentX protocol operations initiated by the
         master agent for this session must use this byte ordering and
         set this bit accordingly.

         The subagent typically sets this bit to correspond to its
         native byte ordering, and typically does not vary byte ordering
         for an initiated session.  The master agent must be able to
         decode each PDU according to the h.flag NETWORK_BYTE_ORDER bit
         in the PDU, but does not need to toggle its retained value for
         the session if the subagent varies its byte ordering.

       - The o.timeout value is used in calculating response
         timeout conditions for this subagent.

       - The o.id and o.descr fields are used for informational
         purposes.  (Such purposes are implementation-specific for now,
         and may be used in a possible future standard AgentX MIB.)

   5) The agentx-Response-PDU is sent with the res.error field
      set to `noError'.

   At this point, an AgentX session is considered established between
   the master agent and the subagent.  An AgentX session is a distinct
   channel for the exchange of AgentX protocol messages between a master
   agent and one subagent, qualified by the session-specific attributes
   listed in 4) above.  AgentX session establishment is initiated by the
   subagent.  An AgentX session can be terminated by either the master
   agent or the subagent.

7.1.2.  Processing the agentx-IndexAllocate-PDU

   When the master agent receives an agentx-IndexAllocate-PDU, it
   processes it as follows:

   1) An agentx-Response-PDU is created and res.sysUpTime is set to
      the value of sysUpTime.0 for the default context.

   2) If h.sessionID does not correspond to a currently established
      session with this subagent, the agentx-Response-PDU is sent in
      reply with res.error set to `notOpen'.

   3) If the NON_DEFAULT_CONTEXT bit is set, and the master agent
      supports only a default context, the agentx-Response-PDU is
      returned with res.error set to `unsupportedContext', and the
      requested allocation fails.  Otherwise: The value of res.sysUpTime
      is set to the value of sysUpTime.0 for the indicated context.

   4) Each VarBind in the VarBindList is processed until either all
      are successful, or one fails.  If any VarBind fails, the agentx-
      Response-PDU is sent in reply containing the original VarBindList,
      with res.index set to indicate the failed VarBind, and with
      res.error set as described subsequently.  All other VarBinds are
      ignored; no index values are allocated.

      VarBinds are processed as follows:

      - v.name is the name of the index for which a value is to be
        allocated.

      - v.type is the syntax of the index object.

      - v.data indicates the specific index value requested.
        If the NEW_INDEX or the ANY_INDEX bit is set, the actual value
        in v.data is ignored and an appropriate index value is
        generated.

      a) If there are no currently allocated index values for v.name
         in the indicated context, and v.type does not correspond to a
         valid index type value, the VarBind fails and res.error is set
         to `indexWrongType'.

      b) If there are currently allocated index values for v.name
         in the indicated context, but the syntax of those values does
         not match v.type, the VarBind fails and res.error is set to
         `indexWrongType'.

      c) Otherwise, if both the NEW_INDEX and ANY_INDEX bits are
         clear, allocation of a specific index value is being requested.
         If the requested index is already allocated for v.name in the
         indicated context, the VarBind fails and res.error is set to
         `indexAlreadyAllocated'.

      d) Otherwise, if the NEW_INDEX bit is set, the master agent
         should generate the next available index value for v.name in
         the indicated context, with the constraint that this value must
         not have been allocated (even if subsequently released) to any
         subagent since the last re-initialization of the master agent.
         If no such value can be generated, the VarBind fails and
         res.error is set to `indexNoneAvailable'.

      e) Otherwise, if the ANY_INDEX bit is set, the master agent
         should generate an index value for v.name in the indicated
         context, with the constraint that this value is not currently
         allocated to any subagent.  If no such value can be generated,
         then the VarBind fails and res.error is set to
         `indexNoneAvailable'.

   5) If all VarBinds are processed successfully, the
      agentx-Response-PDU is sent in reply with res.error set to
      `noError'.  A VarBindList is included that is identical to the one
      sent in the agentx-IndexAllocate-PDU, except that VarBinds
      requesting a NEW_INDEX or ANY_INDEX value are generated with an
      appropriate value.

7.1.3.  Using the agentx-IndexAllocate-PDU

   Index allocation is a service provided by an AgentX master agent.  It
   provides generic support for sharing MIB conceptual tables among
   subagents who are assumed to have no knowledge of each other.

   Each subagent sharing a table should first request allocation of
   index values, then use those index values to qualify MIB regions in
   its subsequent registrations.

   The master agent maintains a database of index objects (OIDs), and,
   for each index, the values that have been allocated for it.  It is
   unaware of what MIB variables (if any) the index objects represent.

   By convention, subagents use the MIB variable listed in the INDEX
   clause as the index object for which values must be allocated.  For
   tables indexed by multiple variables, values may be allocated for
   each index (although this is frequently unnecessary; see example 2
   below).  The subagent may request allocation of

        - a specific index value - an index value that is not currently
        allocated - an index value that has never been allocated

   The last two alternatives reflect the uniqueness and constancy
   requirements present in many MIB specifications for arbitrary integer
   indexes (e.g., ifIndex in the IF MIB (RFC 1573 [11]),
   snmpFddiSMTIndex in the FDDI MIB (RFC 1285 [12]), or
   sysApplInstallPkgIndex in the System Application MIB [13]).  The need
   for subagents to share tables using such indexes is the main
   motivation for index allocation in AgentX.

   Example 1:

      A subagent implements an interface, and wishes to register a
      single row of the RFC 1573 ifTable.  It requests an allocation for
      the index object "ifIndex", for a value that has never been
      allocated (since ifIndex values must be unique).  The master agent
      returns the value "7".

      The subagent now attempts to register row 7 of ifTable, by
      specifying a MIB region in the agentx-Register-PDU of
      1.3.6.1.2.1.2.2.1.[1-22].7.  If the registration succeeds, no
      further processing is required.  The master agent will dispatch to
      this subagent correctly.

      But the registration may fail.  Index allocation and MIB region
      registration are not coupled in the master agent.  Some other
      subagent may have already registered ifTable row 7 without first
      having requested allocation of the index.  The current state of
      index allocations is not considered when processing registration
      requests, and the current registry is not considered when
      processing index allocation requests.  If subagents follow the
      model of "first request allocation of an index, then register the
      corresponding region", then a successful index allocation request
      gives a subagent a good hint (but no guarantee) of what it should
      be able to register.

      If the registration failed, the subagent should request allocation
      of a new index i, and attempt to register ifTable.[1-22].i, until
      successful.

   Example 2:

      This same subagent wishes to register ipNetToMediaTable rows
      corresponding to its interface (ifIndex i).  Due to structure of
      this table, no further index allocation need be done.  The
      subagent can register the MIB region ipNetToMediaTable.[1-4].i, It
      is claiming responsibility for all rows of the table whose value
      of ipNetToMediaIfIndex is i.

   Example 3:

      A network device consists of a set of processors, each of which
      accepts network connections for a unique set of IP addresses.

      Further, each processor contains a subagent that implements
      tcpConnTable.  In order to represent tcpConnTable for the entire
      managed device, the subagents need to share tcpConnTable.

      In this case, no index allocation need be done at all.  Each
      subagent can register a MIB region of tcpConnTable.[1-5].a.b.c.d,
      where a.b.c.d represents an unique IP address of the individual
      processor.

      Each subagent is claiming responsibility for the region of
      tcpConnTable where the value of tcpConnLocalAddress is a.b.c.d.

7.1.4   Processing the agentx-IndexDeallocate-PDU

   When the master agent receives an agentx-IndexDeallocate-PDU, it
   processes it as follows:

   1) An agentx-Response-PDU is created and res.sysUpTime is set to
      the value of sysUpTime.0 for the default context.

   2) If h.sessionID does not correspond to a currently
      established session with this subagent, the agentx-Response-PDU is
      sent in reply with res.error set to `notOpen'.

   3) If the NON_DEFAULT_CONTEXT bit is set, and the master agent
      supports only a default context, the agentx-Response-PDU is
      returned with res.error set to `unsupportedContext', and the
      requested deallocation fails.  Otherwise: The value of
      res.sysUpTime is set to the value of sysUpTime.0 for the indicated
      context.

   4) Each VarBind in the VarBindList is processed until either all
      are successful, or one fails.  If any VarBind fails, the agentx-
      Response-PDU is sent in reply, containing the original
      VarBindList, with res.index set to indicate the failed VarBind,
      and with res.error set as described subsequently.  All other
      VarBinds are ignored; no index values are released.

      VarBinds are processed as follows:

      - v.name is the name of the index for which a value is to be
        released

      - v.type is the syntax of the index object

      - v.data indicates the specific index value to be released.
        The NEW_INDEX and ANY_INDEX bits are ignored.

      a) If the index value for the named index is not currently
         allocated to this subagent, the VarBind fails and res.error is
         set to `indexNotAllocated'.

   5) If all VarBinds are processed successfully, res.error is
      set to `noError' and the agentx-Response-PDU is sent.  A
      VarBindList is included which is identical to the one sent in the
      agentx-IndexDeallocate-PDU.

      All released index values are now available, and may be used in
      response to subsequent allocation requests for ANY_INDEX values
      for the particular index.

7.1.5.  Processing the agentx-Register-PDU

   When the master agent receives an agentx-Register-PDU, it processes
   it as follows:

   1) An agentx-Response-PDU is created and res.sysUpTime is set to
      the value of sysUpTime.0 for the default context.

   2) If h.sessionID does not correspond to a currently
      established session with this subagent, the agentx-Response-PDU is
      sent in reply with res.error set to `notOpen'.

   3) If the NON_DEFAULT_CONTEXT bit is set, and the master agent
      supports only a default context, the agentx-Response-PDU is
      returned with res.error set to `unsupportedContext', and the
      requested registration fails.  Otherwise:  The value of
      res.sysUpTime is set to the value of sysUpTime.0 for the indicated
      context.

      Note: Non-default contexts might be added on the fly by
            the master agent, or the master agent might require such
            non-default contexts to be pre-configured.  The choice is
            implementation-specific.

   4) Characterize the request.

      If r.region (or any of its set of Object Identifiers, if r.range
      is non-zero) is exactly the same as any currently registered value
      of r.region (or any of its set of Object Identifiers), this
      registration is termed a duplicate region.

      If r.region (or any of its set of Object Identifiers, if r.range
      is non-zero) is a subtree of, or contains, any currently
      registered value of r.region (or any of its set of Object
      Identifiers), this registration is termed an overlapping region.

      If the NON_DEFAULT_CONTEXT bit is set, this region is to be
      logically registered within the context indicated by r.context.

      Otherwise this region is to be logically registered within the
      default context.

      A registration that would result in a duplicate region with the
      same priority and within the same context as that of a current
      registration is termed a duplicate registration.

   5) Otherwise, if this is a duplicate registration, the
      agentx-Response-PDU is returned with res.error set to
      `duplicateRegistration', and the requested registration fails.

   6) Otherwise, the agentx-Response-PDU is returned with res.error
      set to `noError'.

      The master agent adds this region to its registered OID space for
      the indicated context, to be considered during the dispatching
      phase for subsequently received SNMP protocol messages.

      Note: The following algorithm describes maintaining a set of OID
      ranges derived from "splitting" registered regions.  The algorithm
      for operational dispatching is also stated in terms of these OID
      ranges.

      These OID ranges are a useful explanatory device, but are not
      required for a correct implementation.

       - If r.region (R1) is a subtree of a currently registered
         region (R2), split R2 into 3 new regions (R2a, R2b, and R2c)
         such that R2b is an exact duplicate of R1.  Now remove R2 and
         add R1, R2a, R2b, and R2c to the master agent's
         lexicographically ordered set of ranges (the registered OID
         space).  Note: Though newly-added ranges R1 and R2b are
         identical in terms of the MIB objects they contain, they are
         registered by different subagents, possibly at different
         priorities.

         For instance, if subagent S2 registered "ip" (R2 is
         1.3.6.1.2.1.4) and subagent S1 subsequently registered
         "ipNetToMediaTable" (R1 is 1.3.6.1.2.1.4.22), the resulting set
         of registered regions would be:

   1.3.6.1.2.1.4    up to but not including 1.3.6.1.2.1.4.22     (by S2)
   1.3.6.1.2.1.4.22 up to but not including 1.3.6.1.2.1.4.23     (by S2)
   1.3.6.1.2.1.4.22 up to but not including 1.3.6.1.2.1.4.23     (by S1)
   1.3.6.1.2.1.4.23 up to but not including 1.3.6.1.2.1.5        (by S2)

       - If r.region (R1) overlaps one or more currently registered
         regions, then for each overlapped region (R2) split R1 into 3
         new ranges (R1a, R1b, R1c) such that R1b is an exact
         duplicate of R2.  Add R1b and R2 into the lexicographically
         ordered set of regions.  Apply (5) above iteratively to R1a and
         R1c (since they may overlap, or be subtrees of, other regions).

         For instance, given the currently registered regions in the
         example above, if subagent S3 now registers mib-2 (R1 is
         1.3.6.1.2.1) the resulting set of regions would be:

   1.3.6.1.2.1      up to but not including 1.3.6.1.2.1.4        (by S3)
   1.3.6.1.2.1.4    up to but not including 1.3.6.1.2.1.4.22     (by S2)
   1.3.6.1.2.1.4    up to but not including 1.3.6.1.2.1.4.22     (by S3)
   1.3.6.1.2.1.4.22 up to but not including 1.3.6.1.2.1.4.23     (by S2)
   1.3.6.1.2.1.4.22 up to but not including 1.3.6.1.2.1.4.23     (by S1)
   1.3.6.1.2.1.4.22 up to but not including 1.3.6.1.2.1.4.23     (by S3)
   1.3.6.1.2.1.4.23 up to but not including 1.3.6.1.2.1.5        (by S2)
   1.3.6.1.2.1.4.23 up to but not including 1.3.6.1.2.1.5        (by S3)
   1.3.6.1.2.1.5    up to but not including 1.3.6.1.2.2          (by S3)

   Note that at registration time a region may be split into multiple
   OID ranges due to pre-existing registrations, or as a result of any
   subsequent registration.  This region splitting is transparent to
   subagents.  Hence the master agent must always be able to associate
   any OID range with the information contained in its original agentx-
   Register-PDU.

7.1.5.1.  Handling Duplicate OID Ranges

   As a result of this registration algorithm there are likely to be
   duplicate OID ranges (regions of identical MIB objects registered to
   different subagents) in the master agent's registered OID space.
   Whenever the master agent's dispatching algorithm (see 7.2.1,
   Dispatching AgentX PDUs) results in  a duplicate OID range, the
   master agent selects one to use, termed the 'authoritative region',
   as follows:

      1) Choose the one whose original agentx-Register-PDU
         r.region contained the most subids, i.e., the most specific
         r.region.  Note: The presence or absence of a range subid has
         no bearing on how "specific" one object identifier is compared
         to another.

      2) If still ambiguous, there were duplicate regions.  Choose the
         one whose original agentx-Register-PDU specified the smaller
         value of r.priority.

7.1.6.  Processing the agentx-Unregister-PDU

   1) An agentx-Response-PDU is created and res.sysUpTime is set to
      the value of sysUpTime.0 for the default context.

   2) If h.sessionID does not correspond to a currently
      established session with this subagent, the agentx-Response-PDU is
      sent in reply with res.error set to `notOpen'.

   3) If the NON_DEFAULT_CONTEXT bit is set, and the master agent
      supports only a default context, the agentx-Response-PDU is
      returned with res.error set to `unsupportedContext', and the
      requested unregistration fails.  Otherwise:  The value of
      res.sysUpTime is set to the value of sysUpTime.0 for the indicated
      context.

   4) If u.region, u.priority, and the indicated context do not match
      an existing registration made during this session, the agentx-
      Response-PDU is returned with res.error set to
      `unknownRegistration'.

   5) Otherwise, the agentx-Response-PDU is sent in reply with res.error
      set to `noError', and the previous registration is removed:

      - The master agent removes u.region from its registered OID space
        within the indicated context.  If the original region had been
        split, all such related regions are removed.

        For instance, given the example registry above, if subagent S2
        unregisters "ip", the resulting registry would be:

   1.3.6.1.2.1      up to but not including 1.3.6.1.2.1.4        (by S3)
   1.3.6.1.2.1.4    up to but not including 1.3.6.1.2.1.4.22     (by S3)
   1.3.6.1.2.1.4.22 up to but not including 1.3.6.1.2.1.4.23     (by S1)
   1.3.6.1.2.1.4.22 up to but not including 1.3.6.1.2.1.4.23     (by S3)
   1.3.6.1.2.1.4.23 up to but not including 1.3.6.1.2.1.5        (by S3)
   1.3.6.1.2.1.5    up to but not including 1.3.6.1.2.2          (by S3)

7.1.7.  Processing the agentx-AddAgentCaps-PDU

   When the master agent receives an agentx-AddAgentCaps-PDU, it
   processes it as follows:

   1) An agentx-Response-PDU is created and res.sysUpTime is set to
      the value of sysUpTime.0 for the default context.

   2) If h.sessionID does not correspond to a currently
      established session with this subagent, the agentx-Response-PDU is
      sent in reply with res.error set to `notOpen'.

   3) If the NON_DEFAULT_CONTEXT bit is set, and the master agent
      supports only a default context, the agentx-Response-PDU is
      returned with res.error set to `unsupportedContext', and the
      requested operation fails.  Otherwise:  The value of res.sysUpTime
      is set to the value of sysUpTime.0 for the indicated context.

   4) Otherwise, the master agent adds the subagent's capabilities
      information to the sysORTable for the indicated context.  An
      agentx-Response-PDU is sent in reply with res.error set to
      `noError'.

7.1.8.  Processing the agentx-RemoveAgentCaps-PDU

   1) An agentx-Response-PDU is created and res.sysUpTime is set to
      the value of sysUpTime.0 for the default context.

   2) If h.sessionID does not correspond to a currently
      established session with this subagent, the agentx-Response-PDU is
      sent in reply with res.error set to `notOpen'.

   3) If the NON_DEFAULT_CONTEXT bit is set, and the master agent
      supports only a default context, the agentx-Response-PDU is
      returned with res.error set to `unsupportedContext', and the
      requested operation fails.  Otherwise:  The value of res.sysUpTime
      is set to the value of sysUpTime.0 for the indicated context.

   4) If the combination of a.id and the optional a.context does not
      represent a sysORTable entry that was added by this subagent,
      during this session, the agentx-Response-PDU is returned with
      res.error set to `unknownAgentCaps'.

   5) Otherwise the master agent deletes the corresponding sysORTable
      entry and sends in reply the agentx-Response-PDU, with res.error
      set to `noError'.

7.1.9.  Processing the agentx-Close-PDU

   When the master agent receives an agentx-Close-PDU, it processes it
   as follows:

   1) An agentx-Response-PDU is created and res.sysUpTime is set to
      the value of sysUpTime.0 for the default context.

   2) If h.sessionID does not correspond to a currently
      established session with this subagent, the agentx-Response-PDU is
      sent in reply with res.error set to `notOpen'.

   3) Otherwise, the master agent closes the AgentX session
      as described below.  No agentx-Response-PDU is sent.

      - All MIB regions that have been registered during this session
        are unregistered, as described in 7.1.6.

      - All index values allocated during this session are freed, as
        described in section 7.1.4.

      - All sysORID values that were registered during this session
        are removed, as described in section 7.1.8.

   The master agent does not maintain state for closed sessions.  If a
   subagent wishes to re-establish a session after receiving an agentx-
   Close-PDU, it needs to re-register MIB regions, agent capabilities,
   etc.

7.1.10.  Detecting Connection Loss

   If a master agent is able to detect (from the underlying transport)
   that a subagent cannot receive AgentX PDUs, it should close all
   affected AgentX sessions as described in 7.1.9, step 3).

7.1.11.  Processing the agentx-Notify-PDU

   A subagent sending SNMPv1 trap information must map this into
   (minimally) a value of snmpTrapOID.0, as described in 3.1.2 of RFC
   1908 [8].

   The master agent processes the agentx-Notify-PDU as follows:

   1) If h.sessionID does not correspond to a currently
         established session with this subagent, an agentx-Response-PDU
         is sent in reply with res.error set to `notOpen', and
         res.sysUpTime set to the value of sysUpTime.0 for the indicated
         context.

   2) The VarBindList is parsed.  If it does not contain a value for
         sysUpTime.0, the master agent supplies the current value of
         sysUpTime.0 for the indicated context.  If the next VarBind
         (either the first or second VarBind; see section 6.2.10.1) is
         not snmpTrapOID.0, the master agent ceases further processing
         of the notification.

   3) Notifications are sent according to the implementation-specific
         configuration of the master agent.

         If SNMPv1 Trap PDUs are generated, the recommended mapping is
         as described in RFC 2089 [9].

         Except in the case of a `notOpen' error as described in (1)
         above, no agentx-Response-PDU is sent to the subagent when the
         master agent finishes processing the notification.

7.1.12.  Processing the agentx-Ping-PDU

   When the master agent receives an agentx-Ping-PDU, it processes it as
   follows:

   1) An agentx-Response-PDU is created and res.sysUpTime is set to
      the value of sysUpTime.0 for the default context.

   2) If h.sessionID does not correspond to a currently
      established session with this subagent, the agentx-Response-PDU is
      sent in reply with res.error set to `notOpen'.

   3) If the NON_DEFAULT_CONTEXT bit is set, and the master agent
      supports only a default context, the agentx-Response-PDU is
      returned with res.error set to `unsupportedContext'.  Otherwise:
      The value of res.sysUpTime is set to the value of sysUpTime.0 for
      the indicated context.

   4) The agentx-Response-PDU is sent, with res.error set to
      `noError'.

   If a subagent does not receive a response to its pings, or if it is
   able to detect (from the underlying transport) that the master agent
   is not able to receive AgentX messages, then it eventually must
   initiate a new AgentX session, re-register its regions, etc.

7.2.  Processing Received SNMP Protocol Messages

   When an SNMP GetRequest, GetNextRequest, GetBulkRequest, or
   SetRequest protocol message is received by the master agent, the
   master agent applies its access control policy.

   In particular, for SNMPv1 or SNMPv2c PDUs, the master agent applies
   the Elements of Procedure defined in section 4.1 of RFC 1157 [6] that
   apply to receiving entities.  (For other versions of SNMP, the master
   agent applies the access control policy defined in the Elements of
   Procedure for those versions.)

   In the SNMPv1 or v2c frameworks, the master agent uses the community
   string as an index into a local repository of configuration
   information that may include community profiles or more complex
   context information.

   If application of the access control policy results in a valid SNMP
   request PDU, then an SNMP Response-PDU is constructed from
   information gathered in the exchange of AgentX PDUs between the
   master agent and one or more subagents.  Upon receipt and initial
   validation of an SNMP request PDU, a master agent uses the procedures
   described below to dispatch AgentX PDUs to the proper subagents,
   marshal the subagent responses, and construct an SNMP response PDU.

7.2.1.  Dispatching AgentX PDUs

   Upon receipt and initial validation of an SNMP request PDU, a master
   agent uses the procedures described below to dispatch AgentX PDUs to
   the proper subagents.

   Note: In the following procedures, an object identifier is said to be
   "contained" within an OID range when both of the following are true:

       - The object identifier does not lexicographically precede
         the range.

       - The object identifier lexicographically precedes the end
         of the range.

   General Rules of Procedure

   While processing a particular SNMP request, the master agent may send
   one or more AgentX PDUs to one or more subagents.  The following
   rules of procedure apply in general to the AgentX master agent. PDU-
   specific rules are listed in the applicable sections.

   1) Honoring the registry

      Because AgentX supports overlapping registrations, it is possible
      for the master agent to obtain a value for a requested varbind
      from within multiple registered MIB regions.

      The master agent must ensure that the value (or exception)
      actually returned in the SNMP response PDU is taken from the
      authoritative region (as defined in section 7.1.5.1).

   2) GetNext and GetBulk Processing

      The master agent may choose to send agentx-Get-PDUs while
      servicing an SNMP GetNextRequest-PDU.  The master agent may choose
      to send agentx-Get-PDUs or agentx-GetNext-PDUs while servicing an
      SNMP GetBulkRequest-PDU.  One possible reason for this would be if
      the current iteration has targeted instance-level registrations.

      The master agent may choose to "scope" the possible instances
      returned by a subagent by specifying an ending OID in the
      SearchRange.  If such scoping is used, typically the ending OID
      would be the first lexicographical successor to the target OID
      range that was registered by a subagent other than the target
      subagent.  Regardless of this choice, rule (1) must be obeyed.

      The master agent may require multiple request-response iterations
      on the same subagent session, to determine the final value of all
      requested variables.

      All AgentX PDUs sent on the session while processing a given SNMP
      request must contain identical values of transactionID.  Each
      different SNMP request processed by the master agent must present
      a unique value of transactionID (within the limits of the 32-bit
      field) to the session.

   3) Number and order of variables sent per AgentX PDU

      For Get/GetNext/GetBulk operations, at any stage of the possibly
      iterative process, the master agent may need to dispatch several
      SearchRanges to a particular subagent session.  The master agent
      may send one, some, or all of the SearchRanges in a single AgentX
      PDU.

      The master agent must ensure that the correct contents and
      ordering of the VarBindList in the SNMP Response-PDU are
      maintained.

      The following rules govern the number of VarBinds in a given
      AgentX PDU:

         a) The subagent must support processing of AgentX PDUs
            with multiple VarBinds.

         b) When processing an SNMP Set request, the master agent
            must send all of the VarBinds applicable to a particular
            subagent session in a single Test/Set transaction.

         c) When processing an SNMP Get, GetNext, or GetBulk request,
            the master agent may send a single AgentX PDU to the
            subagent with all applicable VarBinds, or multiple PDUs with
            single VarBinds, or something in between those extremes. The
            determination of which method to use in a particular case is
            implementation-specific.

   4) Timeout Values

      The master agent chooses a timeout value for each MIB region being
      queried, which is

         a) the value specified during registration of the MIB region,
            if it was non-zero

         b) otherwise, the value specified during establishment of
            the session in which this region was subsequently
            registered, if that value was non-zero.

         c) otherwise, the master agent's default value

      When an AgentX PDU that references multiple MIB regions is
      dispatched, the timeout value used for the PDU is the maximum
      value of the timeouts so determined for each of the referenced MIB
      regions.

   5) Context

      If the master agent has determined that a specific non-default
      context is associated with the SNMP request PDU, that context is
      encoded into the AgentX PDU's context field and the
      NON_DEFAULT_CONTEXT bit is set in h.flags.

      Otherwise, no context Octet String is added to the PDU, and the
      NON_DEFAULT_CONTEXT bit is cleared.

7.2.1.1.  agentx-Get-PDU

   Each variable binding in the SNMP request PDU is processed as
   follows:

   (1) Identify the target OID range.

       Within a lexicographically ordered set of OID ranges, valid for
       the indicated context, locate the authoritative region that
       contains the binding's name.

   (2) If no such OID range exists, the variable binding is not
       processed further, and its value is set to `noSuchObject'.

   (3) Identify the subagent session in which this region was
       registered, termed the target session.

   (4) If this is the first variable binding to be dispatched over
       the target session in a request-response exchange entailed in the
       processing of this management request:

       - Create an agentx-Get-PDU for this session, with the header
         fields initialized as described above (see 6.1 AgentX PDU
         Header).

   (5) Add a SearchRange to the end of the target session's PDU
       for this variable binding.

       - The variable binding's name is encoded into the starting OID.

       - The ending OID is encoded as null.

7.2.1.2.  agentx-GetNext-PDU

   Each variable binding in the SNMP request PDU is processed as
   follows:

   (1) Identify the target OID range.

       Within a lexicographically ordered set of OID ranges, valid for
       the indicated context, locate

        a) the authoritative OID range that contains the variable
           binding's name and is not a fully qualified instance, or

        b) the authoritative OID range that is the first
           lexicographical successor to the variable binding's name.

   (2) If no such OID range exists, the variable binding is not
       processed further, and its value is set to `endOfMibView'.

   (3) Identify the subagent session in which this region was
       registered, termed the target session.

   (4) If this is the first variable binding to be dispatched over the
       target session in a request-response exchange entailed in the
       processing of this management request:

       - Create an agentx-GetNext-PDU for the session, with
         the header fields initialized as described above (see 6.1
         AgentX PDU Header).

   (5) Add a SearchRange to the end of the target session's
       agentx-GetNext-PDU for this variable binding.

        - if (1a) applies, the variable binding's name is encoded
          into the starting OID, and the OID's "include" field is set to
          0.

        - if (1b) applies, the target OID is encoded into the starting
          OID, and its "include" field is set to 1.

7.2.1.3.  agentx-GetBulk-PDU

   (Note: The outline of the following procedure is based closely on
   section 4.2.3, "The GetBulkRequest-PDU" of RFC 1905 [4].  Please
   refer to it for details on the format of the SNMP GetBulkRequest-PDU
   itself.)

   Each variable binding in the request PDU is processed as follows:

   (1) Identify the authoritative target OID range and target session,
       exactly as described for the agentx-GetNext-PDU (see 7.2.1.2).

   (2) If this is the first variable binding to be dispatched over the
       target session in a request-response exchange entailed in the
       processing of this management request:

       - Create an agentx-GetBulk-PDU for the session, with
         the header fields initialized as described above (see 6.1
         AgentX PDU Header).

   (3) Add a SearchRange to the end of the target session's
       agentx-GetBulk-PDU for this variable binding, as described for
       the agentx-GetNext-PDU.  If the variable binding was a non-
       repeater in the original request PDU, it must be a non-repeater
       in the agentx-GetBulk-PDU.

   The value of g.max_repetitions in the agentx-GetBulk-PDU may be less
   than (but not greater than) the value in the original request PDU.

   The master agent may make such alterations due to simple sanity
   checking, optimizations for the current iteration based on the
   registry, the maximum possible size of a potential Response-PDU,
   known constraints of the AgentX transport, or any other
   implementation-specific constraint.

7.2.1.4.  agentx-TestSet-PDU

   AgentX employs test-commit-undo-cleanup phases to achieve "as if
   simultaneous" semantics of the SNMP SetRequest-PDU within the
   extensible agent.  The initial phase involves the agentx-TestSet-PDU.

   Each variable binding in the SNMP request PDU is processed in order,
   as follows:

   (1) Identify the target OID range.

       Within a lexicographically ordered set of OID ranges, valid for
       the indicated context, locate the authoritative range that
       contains the variable binding's name.

   (2) If no such OID range exists, this variable binding fails with an
       error of `notWritable'.  Processing is complete for this request.

   (3) Identify the single subagent responsible for this OID range,
       termed the target subagent, and the applicable session, termed
       the target session.

   (4) If this is the first variable binding to be dispatched over
       the target session in a request-response exchange entailed in the
       processing of this management request:

       - create an agentx-TestSet-PDU for the session, with the
         header fields initialized as described above (see 6.1 AgentX
         PDU Header).

   (5) Add a VarBind to the end of the target session's PDU
       for this variable binding, as described in section 5.4.

    Note that all VarBinds applicable to a given session must be sent in
    a single agentx-TestSet-PDU.

7.2.1.5.  Dispatch

   A timeout value is calculated for each PDU to be sent, which is the
   maximum value of the timeouts determined for each of the PDU's
   SearchRanges (as described above in 7.2.1 Dispatching AgentX PDUs,
   item 4). Each pending PDU is mapped (via its h.sessionID value) to a
   particular transport domain/endpoint, as described in section 8
   (Transport Mappings).

7.2.2.  Subagent Processing of agentx-Get, GetNext, GetBulk-PDUs

   A conformant AgentX subagent must support the agentx-Get, -GetNext,
   and -GetBulk PDUs, and must support multiple variables being supplied
   in each PDU.

   When a subagent receives an agentx-Get-, GetNext-, or GetBulk-PDU, it
   performs the indicated management operations and returns an agentx-
   Response-PDU.

   The agentx-Response-PDU header fields are identical to the received
   request PDU except that, at the start of processing, the subagent
   initializes h.type to Response, res.error to `noError', res.index to
   0, and the VarBindList to null.

   Each SearchRange in the request PDU's SearchRangeList is processed as
   described below, and a VarBind is added in the corresponding location
   of the agentx-Response-PDU's  VarbindList.  If processing should fail
   for any reason not described below, res.error is set to `genErr',
   res.index to the index of the failed SearchRange, the VarBindList is
   reset to null, and this agentx-Response-PDU is returned to the master
   agent.

7.2.2.1.  Subagent Processing of the agentx-Get-PDU

   Upon the subagent's receipt of an agentx-Get-PDU, each SearchRange in
   the request is processed as follows:

   (1) The starting OID is copied to v.name.

   (2) If the starting OID exactly matches the name of a
       variable instantiated by this subagent within the indicated
       context and session, v.type and v.data are encoded to represent
       the variable's syntax and value, as described in section 5.4,
       Value Representation.

   (3) Otherwise, if the starting OID does not match the object
       identifier prefix of any variable instantiated within the
       indicated context and session, the VarBind is set to
       `noSuchObject', in the manner described in section 5.4, Value
       Representation.

   (4) Otherwise, the VarBind is set to `noSuchInstance'
       in the manner described in section 5.4, Value Representation.

7.2.2.2.  Subagent Processing of the agentx-GetNext-PDU

   Upon the subagent's receipt of an agentx-GetNext-PDU, each
   SearchRange in the request is processed as follows:

   (1) The subagent searches for a variable within the
       lexicographically ordered list of variable names for all
       variables it instantiates (without regard to registration of
       regions) within the indicated context and session, for which the
       following are all true:

       - if the "include" field of the starting OID is 0, the
         variable's name is the closest lexicographical successor to the
         starting OID.

       - if the "include" field of the starting OID is 1, the
         variable's name is either equal to, or the closest
         lexicographical successor to, the starting OID.

       - If the ending OID is not null, the variable's name
         lexicographically precedes the ending OID.

       If all of these conditions are met, v.name is set to the located
       variable's name.  v.type and v.data are encoded to represent the
       variable's syntax and value, as described in section 5.4, Value
       Representation.

   (2) If no such variable exists, v.name is set to the starting OID,
       and the VarBind is set to `endOfMibView', in the manner described
       in section 5.4, Value Representation.

7.2.2.3.  Subagent Processing of the agentx-GetBulk-PDU

   A maximum of N + (M * R) VarBinds are returned, where

      N equals g.non_repeaters,
      M equals g.max_repetitions, and
      R is (number of SearchRanges in the GetBulk request) - N.

   The first N SearchRanges are processed exactly as for the agentx-
   GetNext-PDU.

   If M and R are both non-zero, the remaining R SearchRanges are
   processed iteratively to produce potentially many VarBinds.  For each
   iteration i, such that i is greater than zero and less than or equal
   to M, and for each repeated SearchRange s, such that s is greater
   than zero and less than or equal to R, the (N+((i-1)*R)+s)-th VarBind
   is added to the agentx-Response-PDU as follows:

      1) The subagent searches for a variable within the
         lexicographically ordered list of variable names for all
         variables it instantiates (without regard to registration of
         regions) within the indicated context and session, for which
         the following are all true:

          - The variable's name is the (i)-th lexicographical successor
            to the (N+s)-th requested OID.

            (Note that if i is 0 and the "include" field is 1, the
            variable's name may be equivalent to, or the first
            lexicographical successor to, the (N+s)-th requested OID.)

          - If the ending OID is not null, the variable's name
            lexicographically precedes the ending OID.

         If all of these conditions are met, v.name is set to the
         located variable's name.  v.type and v.data are encoded to
         represent the variable's syntax and value, as described in
         section 5.4, Value Representation.

      2) If no such variable exists, the VarBind is set to
         `endOfMibView' as described in section 5.4, Value
         Representation.  v.name is set to v.name of the (N+((i-
         2)*R)+s)-th VarBind unless i is currently 1, in which case it
         is set to the value of the starting OID in the (N+s)-th
         SearchRange.

   Note that further iterative processing should stop if

        - For any iteration i, all s values of v.type are
          `endOfMibView'.

        - An AgentX transport constraint or other
          implementation-specific constraint is reached.

7.2.3.  Subagent Processing of agentx-TestSet, -CommitSet, -UndoSet,
                  -CleanupSet-PDUs

   A conformant AgentX subagent must support the agentx-TestSet,
   -CommitSet, -UndoSet, and -CleanupSet PDUs, and must support multiple
   variables being supplied in each PDU.

   These four PDUs are used to collectively perform the indicated
   management operation.  An agentx-Response-PDU is sent in reply to
   each of the PDUs, to inform the master agent of the state of the
   operation.

   The agentx-Response-PDU header fields are identical to the received
   request PDU except that, at the start of processing, the subagent
   initializes h.type to Response, res.error to `noError', and res.index
   to 0.

   These Response-PDUs do not contain a VarBindList.

7.2.3.1.  Subagent Processing of the agentx-TestSet-PDU

   Upon the subagent's receipt of an agentx-TestSet-PDU, each VarBind in
   the PDU is validated until they are all successful, or until one
   fails, as described in section 4.2.5 of RFC 1905 [4]. The subagent
   validates variables with respect to the context and session indicated
   in the testSet-PDU.

   If each VarBind is successful, the subagent has a further
   responsibility to ensure the availability of all resources (memory,
   write access, etc.) required for successfully carrying out a
   subsequent agentx-CommitSet operation.  If this cannot be guaranteed,
   the subagent should set res.error to `resourceUnavailable'.

   As a result of this validation step, an agentx-Response-PDU is sent
   in reply whose res.error field is set to one of the following (SNMPv2
   SMI) values:

            noError                    (0),
            genErr                     (5),
            noAccess                   (6),
            wrongType                  (7),
            wrongLength                (8),
            wrongEncoding              (9),
            wrongValue                (10),
            noCreation                (11),
            inconsistentValue         (12),
            resourceUnavailable       (13),
            notWritable               (17),
            inconsistentName          (18)

   If this value is not `noError', the res.index field must be set to
   the index of the VarBind for which validation failed.

   Implementation of rigorous validation code may be one of the most
   demanding aspects of subagent development.  Implementors are strongly
   encouraged to do this right, so as to avoid if at all possible the
   extensible agent's having to return `commitFailed' or `undoFailed'
   during subsequent processing.

7.2.3.2.  Subagent Processing of the agentx-CommitSet-PDU

   The agentx-CommitSet-PDU indicates that the subagent should actually
   perform (as described in the post-validation sections of 4.2.5 of RFC
   1905 [4]) the management operation indicated by the previous
   TestSet-PDU.  After carrying out the management operation, the
   subagent sends in reply an agentx-Response-PDU whose res.error field
   is set to one of the following (SNMPv2 SMI) values:

            noError                    (0),
            commitFailed              (14)

   If this value is `commitFailed', the res.index field must be set to
   the index of the VarBind for which the operation failed.  Otherwise
   res.index is set to 0.

7.2.3.3.  Subagent Processing of the agentx-UndoSet-PDU

   The agentx-UndoSet-PDU indicates that the subagent should undo the
   management operation requested in a preceding CommitSet-PDU.  The
   undo process is as described in section 4.2.5 of RFC 1905 [4].

   After carrying out the undo process, the subagent sends in reply an
   agentx-Response-PDU whose res.index field is set to 0, and whose
   res.error field is set to one of the following (SNMPv2 SMI) values:

            noError                    (0),
            undoFailed                (15)

   If this value is `undoFailed', the res.index field must be set to the
   index of the VarBind for which the operation failed.  Otherwise
   res.index is set to 0.

   This PDU also signals the end of processing of the management
   operation initiated by the previous TestSet-PDU.  The subagent should
   release resources, etc. as described in section 7.2.3.4.

7.2.3.4.  Subagent Processing of the agentx-CleanupSet-PDU

   The agentx-CleanupSet-PDU signals the end of processing of the
   management operation requested in the previous TestSet-PDU.  This is
   an indication to the subagent that it may now release any resources
   it may have reserved in order to carry out the management request.

   No response is sent by the subagent.

7.2.4.  Master Agent Processing of AgentX Responses

   The master agent now marshals all subagent AgentX response PDUs and
   builds an SNMP response PDU.  In the next several subsections, the
   initial processing of all subagent AgentX response PDUs is described,
   followed by descriptions of subsequent processing for each specific
   subagent Response.

7.2.4.1.  Common Processing of All AgentX Response PDUs

   1) If a subagent does not respond within the timeout interval for
      this dispatch, it is treated as if the subagent had returned
      `genErr' and processed as described below.

      A timeout may be due to a variety of reasons, and does not
      necessarily denote a failed or malfunctioning subagent.  As such,
      the master agent's response to a subagent timeout is
      implementation-specific, but with the following constraint:

         A subagent that times out on three consecutive requests is
         considered unable to respond, and the master agent must close
         the AgentX session as described in 7.1.9, step (2).

   2) Otherwise, the h.packetID, h.sessionID, and h.transactionID
      fields of the AgentX response PDU are used to correlate subagent
      responses.  If the response does not pertain to this SNMP
      operation, it is ignored.

   3) Otherwise, the responses are processed jointly to form the SNMP
      response PDU.

7.2.4.2.  Processing of Responses to agentx-Get-PDUs

   After common processing of the subagent's response to an agentx-Get-
   PDU (see 7.2.4.1 above), processing continues with the following
   steps:

   1)  For any received AgentX response PDU, if res.error is not
       `noError', the SNMP response PDU's error code is set to this
       value, and its error index to the index of the variable binding
       corresponding to the failed VarBind in the subagent's AgentX
       response PDU.

       All other AgentX response PDUs received due to processing this
       SNMP request are ignored.  Processing is complete; the SNMP
       Response PDU is ready to be sent (see section 7.2.5, Sending the
       SNMP Response-PDU).

   2)  Otherwise, the content of each VarBind in the AgentX response PDU
       is used to update the corresponding variable binding in the SNMP
       Response-PDU.

7.2.4.3.  Processing of Responses to agentx-GetNext-PDU and
                 agentx-GetBulk-PDU

   After common processing of the subagent's response to an agentx-
   GetNext-PDU or agentx-GetBulk-PDU (see 7.2.4.1 above), processing
   continues with the following steps:

   1)  For any received AgentX response PDU, if res.error is not
       `noError', the SNMP response PDU's error code is set to this
       value, and its error index to the index of the VarBind
       corresponding to the failed VarBind in the subagent's AgentX
       response PDU.

       All other AgentX response PDUs received due to processing this
       SNMP request are ignored.  Processing is complete; the SNMP
       response PDU is ready to be sent (see section 7.2.5, Sending the
       SNMP Response PDU).

   2)  Otherwise, the content of each VarBind in the AgentX response
       PDU is used to update the corresponding VarBind in the SNMP
       response PDU.

   After all expected AgentX response PDUs have been processed, if any
   VarBinds still contain the value `endOfMibView' in their v.type
   fields, processing must continue:

   3)  A new iteration of AgentX request dispatching is initiated
       (as described in section 7.2.1.1), in which only those VarBinds
       whose v.type is `endOfMibView' are processed.

   4)  For each such VarBind, a target OID range is identified
       which is the lexicographical successor to the target OID range
       for this VarBind on the last iteration.  The target subagent is
       the one that registered the target OID range.  The target session
       is the one in which the target OID range was registered.

       If an agentx-GetNext- or GetBulk-PDU is being dispatched, the
       starting OID in the SearchRanges is set to the target OID range,
       and its "include" field is set to 1.

   5)  The value of transactionID must be identical to the value
       used during the previous iteration.

   6)  The AgentX PDUs are sent to the subagent(s), and the responses
       are received and processed according to the steps described in
       section 7.2.4.

   7)  This process continues iteratively until a complete SNMP
       Response-PDU has been built, or until there remain no target OID
       range lexicographical successors.

7.2.4.4.  Processing of Responses to agentx-TestSet-PDUs

   After common processing of the subagent's response to an agentx-
   TestSet-PDU (see 7.2.4.1 above), processing continues with the
   further exchange of AgentX PDUs.  The value of h.transactionID in the
   agentx-CommitSet, -UndoSet, and -CleanupSet-PDUs must be identical to
   the value sent in the testSet-PDU.

   The state transitions and PDU sequences are depicted in section 7.3.

   1)  If any target subagent's response is not `noError', all other
       agentx-Response-PDUs received due to processing this SNMP request
       are ignored.

       An agentx-CleanupSet-PDU is sent to each target subagent that has
       been sent a agentx-TestSet-PDU.

       Processing is complete; the SNMP response PDU is constructed as
       described below in 7.2.4.6.

   2)  Otherwise an agentx-CommitSet-PDU is sent to each target
       subagent.

7.2.4.5.  Processing of Responses to agentx-CommitSet-PDUs

   After common processing of the subagent's response to an agentx-
   CommitSet-PDU (see 7.2.4.1 above), processing continues with the
   following steps:

   1)  If any response is not `noError', all other
       agentx-Response-PDUs received due to processing this SNMP request
       are ignored.

       An agentx-UndoSet-PDU is sent to each target subagent that has
       been sent a agentx-CommitSet-PDU.  All other subagents are sent a
       agentx-CleanupSet-PDU.

   2)  Otherwise an agentx-CleanupSet-PDU is sent to each target
       subagent.  Processing is complete; the SNMP response PDU is
       constructed as described below in 7.2.4.6.

7.2.4.6.  Processing of Responses to agentx-UndoSet-PDUs

   After common processing of the subagent's response to an agentx-
   UndoSet-PDU (see 7.2.4.1 above), processing continues with the
   following steps:

   1)  If any response is not `noError' the SNMP response
       PDU's error code is set to this value, and its error index to the
       index of the VarBind corresponding to the failed VarBind in the
       agentx-TestSet-PDU.

       Otherwise the SNMP response PDU's error code is set to `noError'
       and its error index to 0.

7.2.5.  Sending the SNMP Response-PDU

   Once the processing described in sections 7.2.1 - 7.2.4 is complete,
   there is an SNMP response PDU available.  The master agent now
   implements the Elements of Procedure for the applicable version of
   the SNMP protocol in order to encapsulate the PDU into a message, and
   transmit it to the originator of the SNMP management request.  Note
   that this may involve altering the PDU contents (for instance, to
   replace the original VarBinds if an error condition is to be
   returned).

   The response PDU may also be altered in order to support the SNMP
   version 1 framework.  In such cases the required mapping is that
   defined in RFC 2089 [9].  (Note in particular that the rules for
   handling Counter64 syntax may require re-sending AgentX GetBulk or
   GetNext PDUs until a VarBind of suitable syntax is returned.)

7.2.6.  MIB Views

   AgentX subagents are not aware of MIB views, since view information
   is not contained in AgentX PDUs.

   As stated above, the descriptions of procedures in section 7 of this
   memo are not intended to constrain the internal architecture of any
   conformant implementation.  In particular, the master agent
   procedures described in sections 7.2.1 and 7.2.4 may be altered so as
   to optimize AgentX exchanges when implementing MIB views.

   Such optimizations are beyond the scope of this memo.  But note that
   section 7.2.3 defines subagent behavior in such a way that alteration
   of SearchRanges may be used in such optimizations.

7.3.  State Transitions

   State diagrams are presented from the master agent's perspective for
   transport connection and session establishment, and from the
   subagent's perspective for Set transaction processing.

7.3.1.  Set Transaction States

   The following table presents, from the subagent's perspective, the
   state transitions involved in Set transaction processing:

                                       STATE
            +----------------+--------------+---------+--------+--------
            |       A        |      B       |   C     |   D    |   E
            |   (Initial     |    TestOK    | Commit  | Test   | Commit
            |     State)     |              |  OK     | Fail   |  Fail
            |                |              |         |        |
    EVENT   |                |              |         |        |
   ---------+----------------+--------------+---------+--------+--------
            | 7.2.3.1        |              |         |        |
   Receive  | All varbinds   |              |         |        |
   TestSet  | OK?            |      X       |    X    |   X    |    X
   PDU      |   Yes ->B      |              |         |        |
            |   No  ->D      |              |         |        |
   ---------+----------------+--------------+---------+--------+--------
            |                |  7.2.3.2     |         |        |
   Receive  |                |  NoError?    |         |        |
   Commit-  |       X        |   Yes ->C    |    X    |   X    |    X
   Set PDU  |                |   No  ->E    |         |        |
   ---------+----------------+--------------+---------+--------+--------
   Receive  |                |              | 7.2.3.3 |        |7.2.4.5
   UndoSet  |       X        |       X      | ->done  |   X    | ->done
   PDU      |                |              |         |        |
   ---------+----------------+--------------+---------+--------+--------
   Receive  |                |  7.2.4.4     | 7.2.3.4 |7.2.4.4 |
   Cleanup- |       X        |   ->done     | ->done  | ->done |   X
   Set PDU  |                |              |         |        |
   ---------+----------------+--------------+---------+--------+--------
   Session  |                | rollback     | undo    |        |
   Loss     |  ->done        |  ->done      |  ->done | ->done | ->done
   ---------+----------------+--------------+---------+--------+--------

   There are three possible sequences that a subagent may follow for a
   particular set transaction:

      1) TestSet CommitSet CleanupSet
      2) TestSet CommitSet UndoSet
      3) TestSet           CleanupSet

   Note that a single PDU sequence may result in multiple paths through
   the finite state machine (FSM).  For example, the sequence

      TestSet CommitSet UndoSet

   may walk through either of these two state sequences:

      (initial) TestOK CommitOK   (done)
      (initial) TestOK CommitFail (done)

7.3.2  Transport Connection States

   The following table presents, from the master agent's perspective,
   the state transitions involved in transport connection setup and
   teardown:

                                STATE
                   +--------------+--------------
                   |      A       |      B
                   | No transport |  Transport
                   |              |  connected
                   |              |
   EVENT           |              |
   ----------------+--------------+--------------
   Transport       |              |
   connect         |     ->B      |      X
   indication      |              |
   ----------------+--------------+--------------
   Receive         |              | if duplicate
   Open-PDU        |              | session id,
                   |              | reject, else
                   |      X       | establish
                   |              | session
                   |              |
                   |              |     ->B
   ----------------+--------------+--------------
   Receive         |              | if matching
   Response-PDU    |              | session id,
                   |              | feed to that
                   |      X       | session's FSM
                   |              | else ignore
                   |              |
                   |              |     ->B
   ----------------+--------------+--------------
   Receive other   |              | if matching
   PDUs            |              | session id,
                   |              | feed to that
                   |      X       | session's FSM
                   |              | else reject
                   |              |
                   |              |     ->B
   ----------------+--------------+--------------
   Transport       |              |notify all
   disconnect      |              |sessions on
   indication      |      X       |this transport
                   |              |
                   |              |     ->A
   ----------------+--------------+--------------

7.3.3  Session States

   The following table presents, from the master agent's perspective,
   the state transitions involved in session setup and teardown:

                              STATE
                  +-------------+----------------
                  |     A       |      B
                  |  No session |  Session
                  |             |  established
   EVENT          |             |
   ---------------+-------------+----------------
                  |  7.1.1      |
   Receive        |             |      X
   Open PDU       |    ->B      |
   ---------------+-------------+----------------
                  |             |  7.1.9
   Receive        |      X      |
   Close PDU      |             |    ->A
   ---------------+-------------+----------------
   Receive        |             |  7.1.5
   Register PDU   |      X      |
                  |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |  7.1.6
   Unregister     |      X      |
   PDU            |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |
   Get PDU        |             |
   GetNext PDU    |             |
   GetBulk PDU    |      X      |       X
   TestSet PDU    |             |
   CommitSet PDU  |             |
   UndoSet PDU    |             |
   CleanupSet PDU |             |
   ---------------+-------------+----------------
   Receive        |             |  7.1.11
   Notify PDU     |      X      |
                  |             |    ->B
   ---------------+-------------+----------------
   Receive Ping   |             |  7.1.12
   PDU            |      X      |
                  |             |    ->B
   ---------------+-------------+----------------
   (continued next page)

   ---------------+-------------+----------------
   Receive        |             |  7.1.2
   IndexAllocate  |      X      |
   PDU            |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |  7.1.4
   IndexDeallocate|      X      |
   PDU            |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |  7.1.7
   AddAgentxCaps  |      X      |
   PDU            |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |  7.1.8
   RemoveAgentxCap|      X      |
   PDU            |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |  7.2.4
   Response PDU   |      X      |
                  |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |
   Other PDU      |      X      |       X
   ---------------+-------------+----------------

8.  Transport Mappings

   The same AgentX PDU formats, encodings, and elements of procedure are
   used regardless of the underlying transport.

8.1.  AgentX over TCP

8.1.1.  Well-known Values

   The master agent accepts TCP connection requests for the well-known
   port 705.  Subagents connect to the master agent using this port
   number.

8.1.2.  Operation

   Once a TCP connection has been established, the AgentX peers use this
   connection to carry all AgentX PDUs. Multiple AgentX sessions may be
   established using the same TCP connection.  AgentX PDUs are sent
   within an AgentX session.  AgentX peers are responsible for mapping
   the h.sessionID to a particular TCP connection.

   All AgentX PDUs are presented individually to the TCP, to be sent as
   the data portion of a TCP PDU.

8.2.  AgentX over UNIX-domain Sockets

   Many (BSD-derived) implementations of the UNIX operating system
   support the UNIX pathname address family (AF_UNIX) for socket
   communications.  This provides a convenient method of sending and
   receiving data between processes on the same host.

   Mapping AgentX to this transport is useful for environments that

       - wish to guarantee subagents are running on the same
         managed node as the master agent, and where

       - sockets provide better performance than TCP or UDP,
         especially in the presence of heavy network I/O

8.2.1.  Well-known Values

   The master agent creates a well-known UNIX-domain socket endpoint
   called "/var/agentx/master".  (It may create other, implementation-
   specific endpoints.)

   This endpoint name uses the character set encoding native to the
   managed node, and represents a UNIX-domain stream (SOCK_STREAM)
   socket.

8.2.2.  Operation

   Once a connection has been established, the AgentX peers use this
   connection to carry all AgentX PDUs.

   Multiple AgentX sessions may be established using the same
   connection.  AgentX PDUs are sent within an AgentX session.  AgentX
   peers are responsible for mapping the h.sessionID to a particular
   connection.

   All AgentX PDUs are presented individually to the socket layer, to be
   sent in the data stream.

9.  Security Considerations

   This memo defines a protocol between two processing entities, one of
   which (the master agent) is assumed to perform authentication of
   received SNMP requests and to control access to management
   information.  The master agent performs these security operations
   independently of the other processing entity (the subagent).

   Security considerations require three questions to be answered:

      1. Is a particular subagent allowed to initiate a session with a
         particular master agent?

      2. During an AgentX session, is any SNMP security-related
         information (for example, community names) passed from the
         master agent to the subagent?

      3. During an AgentX session, what part of the MIB tree is this
         subagent allowed to register?

   The answer to the third question is: A subagent can register any
   subtree (subject to AgentX elements of procedure, section 7.1.5).
   Currently there is no access control mechanism defined in AgentX. A
   concern here is that a malicious subagent that registers an
   unauthorized "sensitive" subtree, could see modification requests to
   those objects, or by giving its own clever answer to NMS queries,
   could cause the NMS to do something that leads to information
   disclosure or other damage.

   The answer to the second question is: No.

   Now we can answer the first question.  AgentX does not contain a
   mechanism for authorizing/refusing session initiations.  Thus,
   controlling subagent access to the master agent may only be done at a
   lower layer (e.g., transport).

   An AgentX subagent can connect to a master agent using either a
   network transport mechanism (e.g., TCP), or a "local" mechanism
   (e.g., shared memory, named pipes).

   In the case where a local transport mechanism is used and both
   subagent and master agent are running on the same host, connection
   authorization can be delegated to the operating system features.  The
   answer to the first security question then becomes: "If and only if
   the subagent has sufficient privileges, then the operating system
   will allow the connection".

   If a network transport is used, currently there is no inherent
   security.  Transport Layer Security or SSL could be used to control
   subagent connections, but that is beyond the scope of this document.

   Thus it is recommended that subagents always run on the same host as
   the master agent and that operating system features be used to ensure
   that only properly authorized subagents can establish connections to
   the master agent.

10.  Acknowledgements

   The initial development of this memo was heavily influenced by the
   DPI 2.0 specification RFC 1592 [7].

   This document was produced by the IETF Agent Extensibility (AgentX)
   Working Group, and benefited especially from the contributions of the
   following working group members:

      David Battle, Uri Blumenthal, Jeff Case, Maria Greene, Dave
      Keeney, Harmen van der Linde, Bob Natale, Randy Presuhn, Aleksey
      Romanov, Don Ryan, and Juergen Schoenwaelder.

   The AgentX Working Group is chaired by:

   Bob Natale
   ACE*COMM Corporation
   704 Quince Orchard Road
   Gaithersburg MD  20878

   Phone: +1-301-721-3000
   Fax:   +1-301-721-3001
   EMail: bnatale@acecomm.com

11.  Authors' and Editor's Addresses

   Mike Daniele
   Digital Equipment Corporation
   110 Spit Brook Rd
   Nashua, NH 03062

   Phone: +1-603-881-1423
   EMail: daniele@zk3.dec.com

   Bert Wijnen
   IBM Professional Services
   Watsonweg 2
   1423 ND Uithoorn
   The Netherlands

   Phone: +31-79-322-8316
   EMail: wijnen@vnet.ibm.com

   Dale Francisco (editor)
   Cisco Systems
   150 Castilian Dr
   Goleta CA 93117

   Phone: +1-805-961-3642
   Fax:   +1-805-961-3600
   EMail: dfrancis@cisco.com

12.  References

[1]  Information processing systems - Open Systems Interconnection -
     Specification of Abstract Syntax Notation One (ASN.1),
     International Organization for Standardization.  International
     Standard 8824, (December, 1987).

[2]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
     "Structure of Management Information for Version 2 of the Simple
     Network Management Protocol (SNMPv2)", RFC 1902, January 1996.

[3]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
     "Textual Conventions for Version 2 of the Simple Network Management
     Protocol (SNMPv2)", RFC 1903, January 1996.

[4]  Case, J., McCloghrie, K., Rose, M., and S. Waldbusser,
     "Protocol Operations for Version 2 of the Simple Network Management
     Protocol (SNMPv2)", RFC 1905, January 1996.

[5]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
     "Management Information Base for Version 2 of the Simple Network
     Management Protocol (SNMPv2)", RFC 1907, January 1996.

[6]  Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple Network
     Management Protocol", STD 15, RFC 1157, SNMP Research, Performance
     Systems International, MIT Laboratory for Computer Science, May
     1990.

[7]  Wijnen, B., Carpenter, G., Curran, K., Sehgal, A. and G. Waters,
     "Simple Network Management Protocol: Distributed Protocol
     Interface, Version 2.0", RFC 1592, March 1994.

[8]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
     "Coexistence between Version 1 and Version 2 of the Internet-
     standard Network Management Framework", RFC 1908, January 1996.

[9]  Wijnen, B. and D. Levi, "V2ToV1: Mapping SNMPv2 onto SNMPv1
     Within a Bilingual SNMP Agent", RFC 2089, January 1997.

[10] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
     "Conformance Statements for Version 2 of the Simple Network
     Management Protocol (SNMPv2)", RFC 1904, January 1996.

[11] McCloghrie, K. and F. Kastenholz, "Evolution of the
     Interfaces Group of MIB-II", RFC 1573, January 1994.

[12] Case, J., "FDDI Management Information Base", RFC 1285,
     January 1992.

[13] Application MIB Working Group, Krupczak, C., and J. Saperia,
     "Definitions of System-Level Managed Objects for Applications",
     draft-ietf-applmib-sysapplmib-08.txt, 15 Apr 1997.

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