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RFC 2741 - Agent Extensibility (AgentX) Protocol Version 1


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Network Working Group                                          M. Daniele
Request for Comments: 2741                    Compaq Computer Corporation
Obsoletes: 2257                                                 B. Wijnen
Category: Standards Track          T.J. Watson Research Center, IBM Corp.
                                                          M. Ellison, Ed.
                                        Ellison Software Consulting, Inc.
                                                        D. Francisco. Ed.
                                                      Cisco Systems, Inc.
                                                             January 2000

                 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 (2000).  All Rights Reserved.

Abstract

   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. This memo obsoletes RFC 2257.

Table of Contents

   1. Introduction.....................................................4
   2. The SNMP Management Framework....................................4
     2.1. A Note on Terminology........................................5
   3. Extending the MIB................................................5
     3.1. Motivation for AgentX........................................6
   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................................................11
     5.1. Object Identifier...........................................11
     5.2. SearchRange.................................................13
     5.3. Octet String................................................14
     5.4. Value Representation........................................15
   6. Protocol Definitions............................................17
     6.1. AgentX PDU Header...........................................17
       6.1.1. Context.................................................20
     6.2. AgentX PDUs.................................................20
       6.2.1. The agentx-Open-PDU.....................................20
       6.2.2. The agentx-Close-PDU....................................22
       6.2.3. The agentx-Register-PDU.................................23
       6.2.4. The agentx-Unregister-PDU...............................27
       6.2.5. The agentx-Get-PDU......................................29
       6.2.6. The agentx-GetNext-PDU..................................30
       6.2.7. The agentx-GetBulk-PDU..................................32
       6.2.8. The agentx-TestSet-PDU..................................34
       6.2.9. The agentx-CommitSet, -UndoSet, -CleanupSet PDUs........35
       6.2.10. The agentx-Notify-PDU..................................36
       6.2.11. The agentx-Ping-PDU....................................37
       6.2.12. The agentx-IndexAllocate-PDU...........................37
       6.2.13. The agentx-IndexDeallocate-PDU.........................38
       6.2.14. The agentx-AddAgentCaps-PDU............................39
       6.2.15. The agentx-RemoveAgentCaps-PDU.........................41
       6.2.16. The agentx-Response-PDU................................43
   7. Elements of Procedure...........................................45
     7.1. Processing AgentX Administrative Messages...................45
       7.1.1. Processing the agentx-Open-PDU..........................46
       7.1.2. Processing the agentx-IndexAllocate-PDU.................47
       7.1.3. Processing the agentx-IndexDeallocate-PDU...............49
       7.1.4. Processing the agentx-Register-PDU......................50
         7.1.4.1. Handling Duplicate and Overlapping Subtrees.........50
         7.1.4.2. Registering Stuff...................................51
           7.1.4.2.1. Registration Priority...........................51
           7.1.4.2.2. Index Allocation................................51
           7.1.4.2.3. Examples........................................53
       7.1.5. Processing the agentx-Unregister-PDU....................55
       7.1.6. Processing the agentx-AddAgentCaps-PDU..................55
       7.1.7. Processing the agentx-RemoveAgentCaps-PDU...............55
       7.1.8. Processing the agentx-Close-PDU.........................56
       7.1.9. Detecting Connection Loss...............................56
       7.1.10. Processing the agentx-Notify-PDU.......................56
       7.1.11. Processing the agentx-Ping-PDU.........................57
     7.2. Processing Received SNMP Protocol Messages..................58
       7.2.1. Dispatching AgentX PDUs.................................58
         7.2.1.1. agentx-Get-PDU......................................61
         7.2.1.2. agentx-GetNext-PDU..................................61
         7.2.1.3. agentx-GetBulk-PDU..................................62

         7.2.1.4. agentx-TestSet-PDU..................................63
         7.2.1.5. Dispatch............................................64
       7.2.2. Subagent Processing.....................................64
       7.2.3. Subagent Processing of agentx-Get, GetNext, GetBulk-PDUs65
         7.2.3.1. Subagent Processing of the agentx-Get-PDU...........65
         7.2.3.2. Subagent Processing of the agentx-GetNext-PDU.......66
         7.2.3.3. Subagent Processing of the agentx-GetBulk-PDU.......66
       7.2.4. Subagent Processing of agentx-TestSet, -CommitSet,
              -UndoSet, -CleanupSet-PDUs..............................67
         7.2.4.1. Subagent Processing of the agentx-TestSet-PDU.......68
         7.2.4.2. Subagent Processing of the agentx-CommitSet-PDU.....69
         7.2.4.3. Subagent Processing of the agentx-UndoSet-PDU.......69
         7.2.4.4. Subagent Processing of the agentx-CleanupSet-PDU....70
       7.2.5. Master Agent Processing of AgentX Responses.............70
         7.2.5.1. Common Processing of All AgentX Response PDUs.......70
         7.2.5.2. Processing of Responses to agentx-Get-PDUs..........70
         7.2.5.3. Processing of Responses to agentx-GetNext-PDU and
                  agentx-GetBulk-PDU..................................71
         7.2.5.4. Processing of Responses to agentx-TestSet-PDUs......72
         7.2.5.5. Processing of Responses to agentx-CommitSet-PDUs....73
         7.2.5.6. Processing of Responses to agentx-UndoSet-PDUs......74
       7.2.6. Sending the SNMP Response-PDU...........................74
       7.2.7. MIB Views...............................................74
     7.3. State Transitions...........................................75
       7.3.1. Set Transaction States..................................75
       7.3.2. Transport Connection States.............................77
       7.3.3. Session States..........................................78
   8. Transport Mappings..............................................79
     8.1. AgentX over TCP.............................................79
       8.1.1. Well-known Values.......................................79
       8.1.2. Operation...............................................79
     8.2. AgentX over UNIX-domain Sockets.............................80
       8.2.1. Well-known Values.......................................80
       8.2.2. Operation...............................................80
   9. Security Considerations.........................................81
   10. Acknowledgements...............................................82
   11. Authors' and Editor's Addresses................................83
   12. References.....................................................84
   13. Notices........................................................86
   Appendix A. Changes relative to RFC 2257 ..........................87
   Full Copyright Statement ..........................................91

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.

   This memo obsoletes RFC 2257.  It is worth noting that most of the
   changes are for the purpose of clarification.  The only changes
   affecting AgentX protocol messages on the wire are:

      -  The agentx-Notify-PDU and agentx-Close-PDU now generate an
         agentx-Response-PDU

      -  Three new error codes are available: parseFailed(266),
         requestDenied(267), and processingError(268)

   Appendix A provides a detailed list of changes relative to RFC 2257.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [27].

2. The SNMP Management Framework

   The SNMP Management Framework presently consists of five major
   components:

   An overall architecture, described in RFC 2571 [1].

   Mechanisms for describing and naming objects and events for the
   purpose of management. The first version of this Structure of
   Management Information (SMI) is called SMIv1 and described in STD 16,
   RFC 1155 [2], STD 16, RFC 1212 [3] and RFC 1215 [4]. The second
   version, called SMIv2, is described in STD 58, RFC 2578 [5], STD 58,
   RFC 2579 [6] and STD 58, RFC 2580 [7].

   Message protocols for transferring management information. The first
   version of the SNMP message protocol is called SNMPv1 and described
   in STD 15, RFC 1157 [8]. A second version of the SNMP message
   protocol, which is not an Internet standards track protocol, is
   called SNMPv2c and described in RFC 1901 [9] and RFC 1906 [10]. The
   third version of the message protocol is called SNMPv3 and described
   in RFC 1906 [10], RFC 2572 [11] and RFC 2574 [12].

   Protocol operations for accessing management information. The first
   set of protocol operations and associated PDU formats is described in

   STD 15, RFC 1157 [8]. A second set of protocol operations and
   associated PDU formats is described in RFC 1905 [13].

   A set of fundamental applications described in RFC 2573 [14] and the
   view-based access control mechanism described in RFC 2575 [15].

   A more detailed introduction to the current SNMP Management Framework
   can be found in RFC 2570 [16].

   Managed objects are accessed via a virtual information store, termed
   the Management Information Base or MIB.  Objects in the MIB are
   defined using the mechanisms defined in the SMI.

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 SMIv2 (STD
   58, RFC 2578, [5]) or the textual conventions based on the SMIv2 (STD
   58, RFC 2579 [6]).  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.

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.

   The SNMP framework does not describe 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 framework documents) but typically has
         little or no direct access to management information.

      -  zero 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 [17], 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 AgentX sessions 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 tables that permit row creation, for
      example, the RMON historyControlTable.  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).

   5) Subagents implement rows in tables whose MIB also defines an
      object that counts entries in the table, for example the MIB-2
      ifTable (due to ifNumber).  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).

   Scenarios in these latter 2 categories 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, "Applicability".

   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 [18]), 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, "SearchRange".

      sub-identifier 1, 2, ... n_subid

         A 4-byte unsigned integer, encoded according to the header's
         NETWORK_BYTE_ORDER bit.

   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 (ending OID) indicates the non-inclusive
   end of the range, and its "include" field is always 0.  A null value
   for ending OID indicates an unbounded SearchRange.

   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 (encoded according to the header's
   NETWORK_BYTE_ORDER bit) 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, according to the header's
            NETWORK_BYTE_ORDER bit.

         -  Counter64 is encoded as 8 contiguous bytes, according to
            the header's NETWORK_BYTE_ORDER bit.

         -  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", Octet String.  Note that the octets used to
            represent IpAddress are always ordered most significant to
            least significant.

            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)

            The set of PDU types for "administrative processing" are 1-4
            and 12-17.  The set of PDU types for "SNMP request
            processing" are 5-11.

      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, "Context".

            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 session ID that it will pass back
            in the corresponding agentx-Response-PDU.  From that point
            on, that same session ID 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 session ID, 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
            [13], nor is it the same as the SNMP Message's msgID (as
            described in section 6.2 of RFC 2572 [11]), nor can it be,
            since a master agent might receive SNMP requests with the
            same request-ids or msgIDs 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 SNMPv2c, 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. In
   SNMPv3 this notion of "context" is formalized (see section 3.3.1 in
   RFC 2571 [1].

   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.  (This does not mean there is a zero-
   length Octet String, it means there is no Octet String present.)  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 on a session
            before it regards the subagent as not responding.  This is
            the default value for the session, and may be overridden by

            values associated with specific registered MIB regions.  The
            default value of 0 indicates that there is no session-wide
            default value.

      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.subtree)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  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 on a session
            before it regards the subagent as not responding.  r.timeout
            applies only to messages that concern MIB objects within
            r.subtree.  It overrides both the session's default 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 sessions register identical or
            overlapping regions.  Subagents with no particular knowledge
            of priority should register with the default value of 127.

            In the master agent's dispatching algorithm, smaller values
            of r.priority take precedence over larger values, as
            described in section 7.1.4.1, "Handling Duplicate and
            Overlapping Subtrees".

      r.subtree

            An Object Identifier that names the basic subtree of a MIB
            region for which a subagent indicates its support. The term
            "subtree" is used generically here, it may represent a
            fully-qualified instance name, a partial instance name, a
            MIB table, an entire MIB, etc.

            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.subtree 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.subtree's
            sub-identifiers.  If this value is 0, no range is being
            specified and there is no r.upper_bound field present in the
            PDU. In this case the MIB region being registered is the
            single subtree named by r.subtree.

            Otherwise the "r.range_subid"-th sub-identifier in r.subtree
            is a range lower bound, and the range upper bound sub-
            identifier (r.upper_bound) immediately follows r.subtree.
            In this case the MIB region being registered is the union of
            the subtrees formed by enumerating this range.

            Note that r.range_subid indicates the (1-based) index of
            this sub-identifier within the OID represented by r.subtree,
            regardless of whether or not r.subtree is encoded using a
            prefix. (See the example below.)

      r.upper_bound

            The upper bound of a sub-identifier's range.  This field is
            present only if r.range_subid is not 0.

            The use of r.range_subid and r.upper_bound provide a general
            shorthand mechanism for specifying a MIB region. For
            example, if r.subtree is the OID 1.3.6.1.2.1.2.2.1.1.7,
            r.range_subid is 10, and r.upper_bound is 22, the specified
            MIB region can be denoted 1.3.6.1.2.1.2.2.1.[1-22].7.
            Registering this region is equivalent to registering the
            union of subtrees

             1.3.6.1.2.1.2.2.1.1.7
             1.3.6.1.2.1.2.2.1.2.7
             1.3.6.1.2.1.2.2.1.3.7
             ...
             1.3.6.1.2.1.2.2.1.22.7

            One expected use of this mechanism is registering a
            conceptual row with a single PDU.  In the example above, the
            MIB region happens to be row 7 of the RFC 1573 ifTable.  The
            actual PDU would be:

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

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   r.timeout   |  r.priority   | 10            |  <reserved>   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

   Note again that here r.range_subid is 10, even though n_subid in
   r.subtree is only 6.

   r.range_subid may be used at any level within a subtree, it need not
   represent row-level registration.  This mechanism may be used in any
   way that is convenient for a subagent to achieve its registrations.

6.2.4. The agentx-Unregister-PDU

   The agentx-Unregister-PDU is sent by a subagent to remove a MIB
   region that was previously registered on this session.

    (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.subtree)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  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.subtree

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

      u.range_subid

            Indicates a sub-identifier in u.subtree is a range lower
            bound.

      u.upper_bound

            The upper bound of the range sub-identifier.  This field is
            present in the PDU only if u.range_subid is not 0.

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 session.  Within the agentx-Get-PDU, the Ending OIDs
            within SearchRanges are null-valued Object Identifiers.

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                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...

     An agentx-GetNext-PDU contains the following fields:

      g.context

            An optional non-default context.

      g.sr

            A SearchRangeList containing the requested variables for
            this session.

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)
    ...

   An agentx-GetBulk-PDU contains the following fields:

      g.context

            An optional non-default context.

      g.non_repeaters

            The number of variables in the SearchRangeList that are not
            repeaters.

      g.max_repetitions

            The maximum number of repetitions requested for repeating
            variables.

      g.sr

            A SearchRangeList containing the requested variables for
            this session.

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
   7.1.4.2, "Registering Stuff", 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 agent capabilities for the specified session.

    (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 STD 58, RFC 2580 [7].)

      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 for this session.

    (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 an 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
            context indicated within the PDU to which this PDU is a
            response.   It is relevant only in agentx response PDUs sent
            from the master  agent to a subagent in response to the set
            of administrative PDUs listed in section 6.1, "AgentX PDU
            Header".

            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.  Within responses to the set of
            "administrative" PDU types listed in section 6.1, "AgentX
            PDU Header", values are limited to the following:

               noAgentXError              (0),
               openFailed                 (256),
               notOpen                    (257),
               indexWrongType             (258),
               indexAlreadyAllocated      (259),
               indexNoneAvailable         (260),
               indexNotAllocated          (261),
               unsupportedContext         (262),
               duplicateRegistration      (263),
               unknownRegistration        (264),
               unknownAgentCaps           (265),
               parseError                 (266),
               requestDenied              (267),
               processingError            (268)

            Within responses to the set of "SNMP request processing" PDU
            types listed in section 6.1, "AgentX PDU Header", values may
            also include those defined for errors in the SNMPv2 PDU (RFC
            1905 [13]).

      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 res.index, 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.

   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., registration
        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.

   Processing is defined specifically for each PDU type in its own
   section.  For the master agent, many of these PDU types require the
   same initial processing steps.  This common processing is defined
   here, and referenced as needed in the PDU type-specific descriptions.

   Common Processing:

   The master agent initially processes a received AgentX PDU as
   follows:

      1) An agentx-Response-PDU is created, res.sysUpTime is set to the
         value of sysUpTime.0 for the default context, res.error is set
         to `noAgentXError', and res.index is set to 0.

      2) If the received PDU cannot be parsed, res.error is set to `
         parseError'.  Examples of a parse error are:

            - PDU length (h.payload) too short to contain current
               construct (Object Identifier header indicates more sub-
               identifiers, VarBind v.type indicates data follows, etc)

            - An unrecognized value is encountered for h.type, v.type,
               etc.

      3) Otherwise, if h.sessionID does not correspond to a currently
         established session with this subagent, res.error is set to
         `notOpen'.

      4) Otherwise, if the NON_DEFAULT_CONTEXT bit is set and the master
         agent does not support the indicated context, res.error is set
         to `unsupportedContext'.  If the master agent does support the
         indicated context, the value of res.sysUpTime is set to the
         value of sysUpTime.0 for that 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.

      5) If resources cannot be allocated or some other condition
         prevents processing, res.error is set to `processingError'.

      6) At this point, if res.error is not `noAgentXError', the
         received PDU is not processed further.  If the received PDU's
         header was successfully parsed, the AgentX-Response-PDU is sent
         in reply.  If the received PDU contained a VarBindList which
         was successfully parsed, the AgentX-Response-PDU contains the
         identical VarBindList.  If the received PDU's header was not
         successfully parsed or for some other reason the master agent
         cannot send a reply, processing is complete.

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, res.sysUpTime is set to the
      value of sysUpTime.0 for the default context, res.error is set to
      `noAgentXError', and res.index is set to 0.

   2) If the received PDU cannot be parsed, res.error is set to
      `parseError'.

   3) Otherwise, if the master agent is unable to open an AgentX session
      for any reason, res.error is set to `openFailed'.

   4) 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.

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

      -  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 session. This field is also referenced in
         the AgentX MIB (a work-in-progress) by the agentxSessionTimeout
         object.

      -  The o.id and o.descr fields are used for informational
         purposes.  These two fields are also referenced in the AgentX
         MIB (a work-in-progress) by the agentxSessionObjectID object,
         and by the agentxSessionDescr object.

   5) The agentx-Response-PDU is sent with the res.error field
      indicating the result of the session initiation.

   If processing was successful, 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
   performs the common processing described in section 7.1, "Processing
   AgentX Administrative Messages".  If as a result res.error is
   `noAgentXError', processing continues as follows:

   1) 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 OID prefix of the MIB OBJECT-TYPE for which a
         value is to be allocated.

      - v.type is the syntax of the MIB OBJECT-TYPE for which a value is
         to be allocated.

      -  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'.

   2) If all VarBinds are processed successfully, the agentx-Response-
      PDU is sent in reply with res.error set to `noAgentXError'.  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.

      See section 7.1.4.2, "Registering Stuff" for more information on
      how subagents should perform index allocations.

7.1.3. Processing the agentx-IndexDeallocate-PDU

   When the master agent receives an agentx-IndexDeallocate-PDU, it
   performs the common processing described in section 7.1, "Processing
   AgentX Administrative Messages".  If as a result res.error is
   `noAgentXError', processing continues as follows:

   1) 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 session, the VarBind fails and res.error is
         set to `indexNotAllocated'.

   2) If all VarBinds are processed successfully, res.error is set to
      `noAgentXError' 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 and
   in response to subsequent allocation requests for the particular
   index value.

7.1.4. Processing the agentx-Register-PDU

   When the master agent receives an agentx-Register-PDU, it performs
   the common processing described in section 7.1, "Processing AgentX
   Administrative Messages".  If as a result res.error is
   `noAgentXError', processing continues as follows:

   If any of the union of subtrees defined by this MIB region is exactly
   the same as any subtree defined by a MIB region currently registered
   within the indicated context, those subtrees are termed "duplicate
   subtrees".

   If any of the union of subtrees defined by this MIB region overlaps,
   or is itself overlapped by, any subtree defined by a MIB region
   currently registered within the indicated context, those subtrees are
   termed "overlapping subtrees".

   1) If this registration would result in duplicate subtrees registered
      with the same value of r.priority, the request fails and an
      agentx-Response-PDU is returned with res.error set to
      `duplicateRegistration'.

   2) Otherwise, if the master agent does not wish to permit this
      registration for implementation-specific reasons, the request
      fails and an agentx-Response-PDU is returned with res.error set to
      `requestDenied'.

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

      The master agent adds this MIB region to its registration data
      store for the indicated context, to be considered during the
      dispatching phase for subsequently received SNMP protocol
      messages.

7.1.4.1.  Handling Duplicate and Overlapping Subtrees

   As a result of this registration algorithm there are likely to be
   duplicate and/or overlapping subtrees within the registration data
   store of the master agent.  Whenever the master agent's dispatching
   algorithm (see section 7.2.1, "Dispatching AgentX PDUs") determines
   that there are multiple subtrees that could potentially contain the
   same MIB object instances, the master agent selects one to use,
   termed the 'authoritative region', as follows:

      1) Choose the one whose original agentx-Register-PDU r.subtree
         contained the most subids, i.e., the most specific r.subtree.
         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 subtrees.  Choose the
         one whose original agentx-Register-PDU specified the smaller
         value of r.priority.

7.1.4.2.  Registering Stuff

   This section describes more fully how AgentX subagents use the
   agentx-IndexAllocate-PDU and agentx-Register-PDU to achieve desired
   configurations.

7.1.4.2.1.     Registration Priority

   The r.priority field in the agentx-Register-PDU is intended to be
   manipulated by human administrators to achieve a desired subagent
   configuration.  Typically this would be needed where a legacy
   application registers a specific subtree, and a different
   (configurable) application may need to become authoritative for the
   identical subtree.

   The result of this configuration (the same subtree registered on
   different sessions with different priorities) is that the session
   using the better priority (see section 7.1.4.1, "Handling Duplicate
   and Overlapping Subtrees") will be authoritative.  The other session
   will simply never be dispatched to.

   This is useful in the case described above, but is NOT useful in
   other cases, particularly when subagents share tables indexed by
   arbitrary values (see below).  In general, subagents should register
   using the default priority (127).

7.1.4.2.2.     Index Allocation

   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.

   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) a specific index value
          b) an index value that is not currently allocated
          c) 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 2233 [19]),
   snmpFddiSMTIndex in the FDDI MIB (RFC 1285 [20]), or
   sysApplInstallPkgIndex in the System Application MIB (RFC 2287
   [21])).  The need for subagents to share tables using such indexes is
   the main motivation for index allocation in AgentX.

   It is important to note that index allocation and MIB region
   registration are not coupled in the master agent. 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.  (This is mainly to accommodate non-AgentX
   subagents.)

   AgentX subagents should 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.  The
   registration may fail (with `duplicateRegistration') because some
   other subagent session has already registered that row of the table.

   The recommended mechanism for subagents to register conceptual rows
   in a shared table is

   1) Successfully allocate an index value.

   2) Use that value to fully qualify the MIB region(s), and attempt to
      register using the default priority.

   3) If the registration fails with `duplicateRegistration' deallocate
      the previously allocated index value(s) for this row and go to
      step 1).

   Note that index allocation is necessary only when the index in
   question is an arbitrary value, and hence the subagent has no other
   reasonable way to determine which index values to use.  When index
   values have intrinsic meaning it is not expected that subagents will
   allocate their index values.

   For example, RFC 1514's table of running software processes
   (hrSWRunTable) is indexed by the system's native process identifier
   (pid).  A subagent implementing the row of hrSWRunTable corresponding
   to its own process would simply register the region defining that
   row's object instances without allocating index values.

7.1.4.2.3.     Examples

   Example 1:

      A subagent implements an interface, and wishes to register a
      single row of the RFC 2233 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.

      If the registration failed with `duplicateRegistration', the
      subagent should deallocate the failed index, 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 the 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.

   Example 4:

      The Application Management MIB (RFC 2564 [22]) uses two objects to
      index several tables.  A partial description of them is:

      applSrvIndex     OBJECT-TYPE
             SYNTAX      Unsigned32 (1..'ffffffff'h)
             MAX-ACCESS  read-only
             STATUS      current
             DESCRIPTION
                "An applSrvIndex is the system-unique identifier
                of an instance of a service.  The value is unique
                not only across all instances of a given service,
                but also across all services in a system."

      applSrvName     OBJECT-TYPE
             SYNTAX     SnmpAdminString
             MAX-ACCESS read-only
             STATUS     current
             DESCRIPTION
                "The human-readable name of a service.  Where
                appropriate, as in the case where a service can
                be identified in terms of a single protocol, the
                strings should be established names such as those
                assigned by IANA and found in STD 2 [23], or
                defined by some other authority.  In some cases
                private conventions apply and the string should
                in these cases be consistent with these
                non-standard conventions. An applicability
                statement may specify the service name(s) to be
                used."

      Since applSrvIndex is an arbitrary value, it would be reasonable
      for subagents to allocate values for this index.  applSrvName
      however has intrinsic meaning and any values a subagent would use
      should be known a priori, hence it is not reasonable for subagents
      to allocate values of this index.

7.1.5. Processing the agentx-Unregister-PDU

   When the master agent receives an agentx-Unregister-PDU, it performs
   the common processing described in section 7.1, "Processing AgentX
   Administrative Messages".  If as a result res.error is `
   noAgentXError', processing continues as follows:

   1) If u.subtree, u.priority, u.range_subid (and if u.range_subid is
      not 0, u.upper_bound), 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'.

   2) Otherwise, the agentx-Response-PDU is sent in reply with res.error
      set to `noAgentXError', and the previous registration is removed
      from the registration data store.

7.1.6. Processing the agentx-AddAgentCaps-PDU

   When the master agent receives an agentx-AddAgentCaps-PDU, it
   performs the common processing described in section 7.1, "Processing
   AgentX Administrative Messages".  If as a result res.error is `
   noAgentXError', processing continues as follows:

   1) The master agent adds this agent capabilities information to the
      sysORTable for the indicated context.  An agentx-Response-PDU is
      sent in reply with res.error set to `noAgentXError'.

7.1.7. Processing the agentx-RemoveAgentCaps-PDU

   When the master agent receives an agentx-RemoveAgentCaps-PDU, it
   performs the common processing described in section 7.1, "Processing
   AgentX Administrative Messages".  If as a result res.error is
   `noAgentXError', processing continues as follows:

   1) 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'.

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

7.1.8. Processing the agentx-Close-PDU

   When the master agent receives an agentx-Close-PDU, it performs the
   common processing described in section 7.1, "Processing AgentX
   Administrative Messages", with the exception that step 4) is not
   performed since the agentx-Close-PDU does may not contain a context
   field. If as a result res.error is `noAgentXError', processing
   continues as follows:

   1) The master agent closes the AgentX session as described below, and
      sends in reply the agentx-Response-PDU with res.error set to
      `noAgentXError':

      -  All MIB regions that have been registered during this session
         are unregistered, as described in section 7.1.5, "Processing
         the agentx-Unregister-PDU".

      -  All index values allocated during this session are freed, as
         described in section 7.1.3, "Processing the agentx-
         IndexDeallocate-PDU".

      -  All sysORID values that were registered during this session are
         removed, as described in section 7.1.7, "Processing the
         agentx-RemoveAgentCaps-PDU".

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

7.1.9. 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 section 7.1.8, "Processing
   the agentx-Close-PDU", step 1).

7.1.10. 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 [24].

   When the master agent receives an agentx-Notify-PDU, it performs the
   common processing described in section 7.1, "Processing AgentX
   Administrative Messages".  If as a result res.error is
   `noAgentXError',  processing continues as follows:

   1) If the first VarBind is sysUpTime.0;

      (a)  if the second VarBind is not snmpTrapOID.0, res.error is set
           to `processingError' and res.index to 2

      (b)  otherwise these two VarBinds are used as the first two
           VarBinds within the generated notification.

   2) If the first VarBind is not sysUpTime.0;

      (a)  if the first VarBind is not snmpTrapOID.0, res.error is set
           to `processingError' and res.index to 1

      (b)  otherwise this VarBind is used for snmpTrapOID.0 within the
           generated notification, and the master agent uses the current
           value of sysUpTime.0 for the indicated context as sysUpTime.0
           within the notification.

   3) An agentx-Response-PDU is sent containing the original
      VarBindList, and with res.error and res.index set as described
      above.  If res.error is `noAgentXError', 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 [25].  If res.error indicates
      an error in processing, no notifications are generated.

      Note that the master agent's successful response indicates the
      agentx-Notify-PDU was received and validated.  It does not
      indicate that any particular notifications were actually generated
      or received by notification targets.

7.1.11. Processing the agentx-Ping-PDU

   When the master agent receives an agentx-Ping-PDU, it performs the
   common processing described in section 7.1, "Processing AgentX
   Administrative Messages".     If as a result res.error is `
   noAgentXError', processing continues as follows:

      1) An agentx-Response-PDU is sent in reply.

   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 MIB 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 protocol messages, the master
   agent applies the Elements of Procedure defined in section 4.1 of STD
   15, RFC 1157 [8] that apply to receiving entities.  For SNMPv3, the
   master agent applies an Access Control Model, possibly the View-based
   Access Control Model (see RFC 2575 [15]), as described in section
   3.1.2 and section 4.3 of RFC 2571 [1].

   For SNMPv1 and SNMPv2c, 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.
   For SNMPv3, the master agent uses the SNMP Context (see section 3.3.1
   of RFC 2571 [1]) for these purposes.

   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.

   General Rules of Procedure

   While processing a particular SNMP request, the master agent may send
   one or more AgentX PDUs on one or more subagent sessions.  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 registration of duplicate and overlapping
      regions, 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 7.1.4.1, "Handling
      Duplicate and Overlapping Subtrees").

   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 region
      that was registered on a session other than the target session.
      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 agentx-TestSet-PDU.

         c) When processing an SNMP Get, GetNext, or GetBulk request,
            the master agent may send a single AgentX PDU on the session
            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, or, if the specified value is not practical, the
            master agent's implementaton-specific 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 MIB region.

        Within a lexicographically ordered set of registered MIB
        regions, valid for the indicated context, locate the
        authoritative region (according to section 7.1.4.1, "Handling
        Duplicate and Overlapping Subtrees") that contains the binding's
        name.

   (2)  If no such region 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 section 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 MIB region.

        Within a lexicographically ordered set of registered MIB
        regions, valid for the indicated context, locate the
        authoritative region (according to section 7.1.4.1, "Handling
        Duplicate and Overlapping Subtrees") that

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

        b) is the first lexicographical successor to the variable
           binding's name.

   (2)  If no such region 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 section 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 region's r.subtree is encoded
           into the starting OID, and its "include" field is set to 1.
           (This is the recommended method.  An implementation may
           choose to use a Starting OID value that precedes r.subtree,
           in which case the include bit must be 0.  A starting OID
           value that succeeds r.subtree is not permitted.)

        -  the Ending OID for the SearchRange is encoded to be either
           NULL, or a value that lexicographically succeeds the Starting
           OID.  This is an implementation-specific choice depending on
           how the master agent wishes to "scope" the possible returned
           instances.

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 [13].  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 region and target session,
        exactly as described for the agentx-GetNext-PDU (see section
        7.2.1.2, "agentx-GetNext-PDU").

   (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 section 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 MIB region and target session exactly as
        described in section 7.2.1.1, "agentx-Get-PDU", step 1).

        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 target region exists, this variable binding fails
        with an error of `notWritable'.  Processing is complete for this
        request.

   (3)  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 section 6.1,
           "AgentX PDU Header").

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

   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 section 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

   A subagent initially processes a received AgentX PDU as follows:

   -  If the received PDU is an agentx-Response-PDU:

   1) If there are any errors parsing or interpreting the PDU, it is
      silently dropped.

   2) Otherwise the response is matched to the original request via
      h.packetID, and handled in an implementation-specific manner.  For
      example, if this response indicates an error attempting to
      register a MIB region, the subagent may wish to register a
      different region, or log an error and halt, etc.

   -  If the received PDU is any other type:

   1) an agentx-Response-PDU is created whose header fields are
      identical to the received request PDU except that h.type is set to
      Response, res.error to `noError', res.index to 0, and the
      VarBindList to null.

   2) If the received PDU cannot be parsed, res.error is set to
      `parseError'.

   3) Otherwise, if h.sessionID does not correspond to a currently
      established session, res.error is set to `notOpen'.

   4) At this point, if res.error is not `noError', the received PDU is
      not processed further.  If the received PDU's header was
      successfully parsed, the AgentX-Response-PDU is sent in reply.  If
      the received PDU's header was not successfully parsed or for some
      other reason the subagent cannot send a reply, processing is
      complete.

7.2.3. 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.

   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.

7.2.3.1.  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".

7.2.3.2.  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, as follows:

        -  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 a variable is successfully located, v.name is set to that
        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 the subagent cannot locate an appropriate variable, 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".

7.2.3.3.  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.

7.2.4. 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 the agentx-TestSet-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 (except -CleanupSet), to inform the master agent of
   the state of the operation.

   The master agent must serialize Set transactions for each session.
   That is, a session need not handle multiple concurrent Set
   transactions.

   These Response-PDUs do not contain a VarBindList.

7.2.4.1.  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 [13]. 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 PDU
   error-status values (see section 3, "Definitions", in RFC 1905 [13]):

            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.

7.2.4.2.  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 [13]) 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 PDU error-status values (see
   section 3, "Definitions", in RFC 1905 [13]):

            noError                    (0),
            commitFailed              (14)

   If this value is `commitFailed', the res.index field must be set to
   the index of the VarBind (as it occurred in the agentx-TestSet-PDU)
   for which the operation failed.  Otherwise res.index is set to 0.

7.2.4.3.  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 [13].

   After carrying out the undo process, the subagent sends in reply an
   agentx-Response-PDU whose res.error field is set to one of the
   following SNMPv2 PDU error-status values (see section 3,
   "Definitions", in RFC 1905 [13]):

            noError                    (0),
            undoFailed                (15)

   If this value is `undoFailed', the res.index field must be set to the
   index of the VarBind (as it occurred in the agentx-TestSet-PDU) 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 7.2.4.4, "Subagent
   Processing of the agentx-CleanupSet-PDU".

7.2.4.4.  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.

7.2.5. 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.

7.2.5.1.  Common Processing of All AgentX Response PDUs

   1) If a response is not received on a session 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 session that times out on three consecutive AgentX requests is
      considered unable to respond, and the master agent must close the
      AgentX session as described in section 7.1.8, "Processing the
      agentx-Close-PDU", 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.

7.2.5.2.  Processing of Responses to agentx-Get-PDUs

   After common processing of the subagent's response to an agentx-Get-
   PDU (see section 7.2.5.1, "Common Processing of All AgentX Response
   PDUs", 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.  If res.error contains an AgentX specific value (e.g.
      `parseError'), the SNMP response PDU's error code is set to a
      value of genErr instead.  Also, the SNMP response PDU's error
      index is set 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 7.2.6, "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.

7.2.5.3.  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 section 7.2.5.1, "Common
   Processing of All AgentX Response PDUs", 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.  If res.error contains an AgentX specific value (e.g.
      `parseError'), the SNMP response PDU's error code is set to a
      value of genErr instead.  Also, the SNMP response PDU's error
      index is set 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 7.2.6, "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 7.2.1.2, "agentx-GetNext-PDU"), in which only
      those VarBinds whose v.type is `endOfMibView' are processed.

   4) For each such VarBind, an authoritative target MIB region is
      identified in which the master agent expects to find suitable MIB
      variables.  The target session is the one on which this new target
      region was registered.

      The starting OID in each SearchRange is set to the value of v.name
      for the corresponding VarBind, and its "include" field is set to
      0.

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

   6) The AgentX PDUs are sent on the target session(s), and the
      responses are received and processed according to the steps
      described in section 7.2.5, "Master Agent Processing of AgentX
      Responses".

   7) This process continues iteratively until a complete SNMP
      Response-PDU has been built, or until there remain no
      authoritative MIB regions to query.

   Note that r.subtree for the new target region identified in step 4)
   may not lexicographically succeed r.subtree for the region that has
   returned `endOfMibView'.  For example, consider the following
   registry:

        session A   `mib-2' (1.3.6.1.2.1)
        session B   `ip'    (1.3.6.1.2.1.4)
        session C   `tcp'   (1.3.6.1.2.1.6)

   If while processing a GetNext-Request-PDU session B returns
   `endOfMibView' for a variable name within 1.3.6.1.2.1.4, the target
   MIB region identified in step 4) would be 1.3.6.1.2.1 (since it may
   contain variables whose names precede 1.3.6.1.2.1.6).

   Note also that if session A returned variables from within
   1.3.6.1.2.1.6, they must be discarded since session A is NOT
   authoritative for that region.

7.2.5.4.  Processing of Responses to agentx-TestSet-PDUs

   After common processing of the subagent's response to an agentx-
   TestSet-PDU (see section 7.2.5.1, "Common Processing of All AgentX
    Response PDUs", 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,
   "State Transitions".

   The set of all sessions who have been sent an agentx-TestSet-PDU for
   this particular transaction are referred to below as "involved
   sessions".

   1) If any target session'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 all involved sessions.
      Processing is complete; the SNMP response PDU is constructed as
      described below in 7.2.6, "Sending the SNMP Response-PDU".

   2) Otherwise an agentx-CommitSet-PDU is sent to all involved
      sessions.

7.2.5.5.  Processing of Responses to agentx-CommitSet-PDUs

   After common processing of the subagent's response to an agentx-
   CommitSet-PDU (see section 7.2.5.1, "Common Processing of All AgentX
   Response PDUs", 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.  If res.error contains an AgentX
      specific value (e.g. `parseError'), the SNMP response PDU's error
      code is set to a value of genErr instead.  Also, the SNMP response
      PDU's error index is set to the index of the VarBind corresponding
      to the failed VarBind in the agentx-TestSet-PDU.

      An agentx-UndoSet-PDU is sent to each target session that has been
      sent an agentx-CommitSet-PDU.  An agentx-CleanupSet-PDU is sent to
      the remainder of the involved sessions.

   2) Otherwise an agentx-CleanupSet-PDU is sent to all involved
      sessions.  Processing is complete; the SNMP response PDU is
      constructed as described below in section 7.2.6, "Sending the SNMP
      Response-PDU".

7.2.5.6.  Processing of Responses to agentx-UndoSet-PDUs

   After common processing of the subagent's response to an agentx-
   UndoSet-PDU (see section 7.2.5.1, "Common Processing of All AgentX
   Response PDUs", above), processing continues with the following
   steps:

   1) If any response is `undoFailed' the SNMP response PDU's error code
      is set to this value.  Also, the SNMP response PDU's error index
      is set to 0.

   2) Otherwise, if any response is not `noError' the SNMP response
      PDU's error code is set to this value.  Also, the SNMP response
      PDU's error index is set to the index of the VarBind corresponding
      to the failed VarBind in the agentx-TestSet-PDU. If res.error is
      an AgentX specific value (e.g. `parseError'), the SNMP response
      PDU's error code is set to a value of genErr instead.

   3) Otherwise the SNMP response PDU's error code and error index were
      set in section 7.2.5.5 step 1)

7.2.6. Sending the SNMP Response-PDU

   Once the processing described in section 7.2.5, "Master Agent
   Processing of AgentX Responses" 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 SNMPv1
   PDU.  In such cases the required PDU mapping is that defined in RFC
   2089 [25].  (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.)

7.2.7. 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,
   "Elements of Procedure", of this memo are not intended to constrain
   the internal architecture of any conformant implementation.  In
   particular, the master agent procedures described in section 7.2.1,

   "Dispatching AgentX PDUs" and in section 7.2.5, "Master Agent
   Processing of AgentX Responses" 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, "Subagent Processing of agentx-Get, GetNext, GetBulk-
   PDUs",  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   |               |              |         |        |
   ---------+---------------+--------------+---------+--------+--------
            | 7.2.4.1       |              |         |        |
   Receive  | All varbinds  |              |         |        |
   TestSet  | OK?           |      X       |    X    |   X    |    X
   PDU      |   Yes ->B     |              |         |        |
            |   No  ->D     |              |         |        |
   ---------+---------------+--------------+---------+--------+--------
            |               |  7.2.4.2     |         |        |
   Receive  |               |  NoError?    |         |        |
   Commit-  |       X       |   Yes ->C    |    X    |   X    |    X
   Set PDU  |               |   No  ->E    |         |        |
   ---------+---------------+--------------+---------+--------+--------
   Receive  |               |              | 7.2.4.3 |        |7.2.4.3
   UndoSet  |       X       |       X      | ->done  |   X    | ->done
   PDU      |               |              |         |        |
   ---------+---------------+--------------+---------+--------+--------
   Receive  |               |  7.2.4.4     | 7.2.4.4 |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 no resources
   Open-PDU        |              | available
                   |              | 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      |
   ---------------+-------------+----------------
                  |             |  7.1.8
   Receive        |      X      |
   Close PDU      |             |    ->A
   ---------------+-------------+----------------
   Receive        |             |  7.1.4
   Register PDU   |      X      |
                  |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |  7.1.5
   Unregister     |      X      |
   PDU            |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |
   Get PDU        |             |
   GetNext PDU    |             |
   GetBulk PDU    |      X      |       X
   TestSet PDU    |             |
   CommitSet PDU  |             |
   UndoSet PDU    |             |
   CleanupSet PDU |             |
   ---------------+-------------+----------------
   Receive        |             |  7.1.10
   Notify PDU     |      X      |
                  |             |    ->B
   ---------------+-------------+----------------
   Receive Ping   |             |  7.1.11
   PDU            |      X      |
                  |             |    ->B
   ---------------+-------------+----------------
   (continued next page)

   ---------------+-------------+----------------
   Receive        |             |  7.1.2
   IndexAllocate  |      X      |
   PDU            |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |  7.1.3
   IndexDeallocate|      X      |
   PDU            |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |  7.1.6
   AddAgentxCaps  |      X      |
   PDU            |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |  7.1.7
   RemoveAgentxCap|      X      |
   PDU            |             |    ->B
   ---------------+-------------+----------------
   Receive        |             |  7.2.5
   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.

   The AgentX entity must not "interleave" AgentX PDUs within the TCP
   byte stream.  All the bytes of one PDU must be sent before any bytes
   of a different PDU.  The receiving entity must be prepared for TCP to
   deliver byte sequences that do not coincide with AgentX PDU
   boundaries.

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.

   The AgentX entity must not "interleave" AgentX PDUs within the socket
   byte stream.  All the bytes of one PDU must be sent before any bytes
   of a different PDU.  The receiving entity must be prepared for the
   socket to deliver byte sequences that do not coincide with AgentX PDU
   boundaries.

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 7.1.4,
   "Processing the agentx-Register-PDU").  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, SSL, or IPsec SHOULD be used to
   control and protect subagent connections in this mode of operation.

   However, we RECOMMEND 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 [26].

   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, Lauren
      Heintz, Dave Keeney, Harmen van der Linde, Bob Natale, Aleksey
      Romanov, Don Ryan, and Juergen Schoenwaelder.

   An honorable mention is extended to Randy Presuhn in recognition for
   his numerous technical contributions to this specification; for his
   many answers provided on (and hosting of) the AgentX e-mail list and
   ftp site, and, for the valued support and guidance Randy provided to
   the Working Group chair.

   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
   Compaq Computer Corporation
   110 Spit Brook Rd
   Nashua, NH 03062

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

   Bert Wijnen
   IBM T.J.Watson Research
   Schagen 33
   3461 GL Linschoten
   Netherlands

   Phone: +31-348-432-794
   EMail: wijnen@vnet.ibm.com

   Mark Ellison (WG editor)
   Ellison Software Consulting, Inc.
   38 Salem Road
   Atkinson, NH  03811

   Phone: +1-603-362-9270
   EMail: ellison@world.std.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]   Harrington, D., Presuhn, R. and B. Wijnen, "An Architecture for
         Describing SNMP Management Frameworks", RFC 2571, April 1999.

   [2]   Rose, M. and K. McCloghrie, "Structure and Identification of
         Management Information for TCP/IP-based Internets", STD 16, RFC
         1155, May 1990.

   [3]   Rose, M. and K. McCloghrie, "Concise MIB Definitions", STD 16,
         RFC 1212, March 1991.

   [4]   Rose, M., "A Convention for Defining Traps for use with the
         SNMP", RFC 1215, March 1991.

   [5]   McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
         M. and S. Waldbusser, "Structure of Management Information
         Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.

   [6]   McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
         M. and S. Waldbusser, "Textual Conventions for SMIv2", STD 58,
         RFC 2579, April 1999.

   [7]   McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
         M. and S. Waldbusser, "Conformance Statements for SMIv2", STD
         58, RFC 2580, April 1999.

   [8]   Case, J., Fedor, M., Schoffstall, M. and J. Davin, "Simple
         Network Management Protocol", STD 15, RFC 1157, May 1990.

   [9]   Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
         "Introduction to Community-based SNMPv2", RFC 1901, January
         1996.

   [10]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
         "Transport Mappings for Version 2 of the Simple Network
         Management Protocol (SNMPv2)", RFC 1906, January 1996.

   [11]  Case, J., Harrington D., Presuhn R. and B. Wijnen, "Message
         Processing and Dispatching for the Simple Network Management
         Protocol (SNMP)", RFC 2572, April 1999.

   [12]  Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
         for version 3 of the Simple Network Management Protocol
         (SNMPv3)", RFC 2574, April 1999.

   [13]  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.

   [14]  Levi, D., Meyer, P. and B. Stewart, "SNMPv3 Applications", RFC
         2573, April 1999.

   [15]  Wijnen, B., Presuhn, R. and K. McCloghrie, "View-based Access
         Control Model (VACM) for the Simple Network Management Protocol
         (SNMP)", RFC 2575, April 1999.

   [16]  Case, J., Mundy, R., Partain, D. and B. Stewart, "Introduction
         to Version 3 of the Internet-standard Network Management
         Framework", RFC 2570, April 1999.

   [17]  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.

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

   [19]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB
         using SMIv2", RFC 2233, November 1997.

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

   [21]  Krupczak, C. and J. Saperia, "Definitions of System-Level
         Managed Objects for Applications", RFC 2287, April 1997.

   [22]  Kalbfleisch, C., Krupczak, C., Presuhn, R. and J. Saperia,
         "Application Management MIB", RFC 2564, May 1999.

   [23]  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
         1700, October 1994.

   [24]  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.

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

   [26]  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.

   [27]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

13. Notices

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

A. Changes relative to RFC 2257

   Changes on the wire:

      -  The agentx-Notify-PDU and agentx-Close-PDU now generate an
         agentx-Response-PDU.

      -  The res.error field may contain three new error codes:
         parseFailed(266), requestDenied(267), and processingError(268).

   Clarifications to the text of the memo:

      -  Modified the text of step (4) in section 4.2, "Applicability"
         to separate the two concerns of row creation, and counters that
         count rows.

      -  The use of the r.range_subid field is more clearly defined in
         section 6.2.3, "The agentx-Register-PDU".

      -  Default priority (127) for registration added to the
         description of r.priority in section 6.2.3, "The agentx-
         Register-PDU".

      -  Made the distinction of "administrative processing" PDUs and
         "SNMP request processing" PDUs in section 6.1, "AgentX PDU
         Header" description of h.type.  This distinction is used in the
         Elements of Procedure relative to the res.sysuptime and
         res.error fields.

      -  Rewrote portions of text in section 6.2.3, "The agentx-
         Register-PDU" to be more explicit about the following points:

            -  There is a default registration priority of 127.
            -  Improved the description of r.range_subid, independent of
               the prefix in r.region.
            -  Improved description and examples of how to use the
               registration mechanism.
            -  Added a description for r.upper_bound.
            -  changed r.region to r.subtree (because the text used the
               terms "region", "range", and "OID range" in too loose a
               fashion.  r.subtree can not represent anything more by
               itself than a simple subtree.  In conjunction with
               r.range_subid and r.upper_bound, it can represent a
               "region", that is, a union of subtrees)

   -  Modified the text in section 6.2.4, "The agentx-Unregister-PDU" to
      include a description of u.range_subid and u.upper_bound

   -  Added use of the `requestDenied' error code in section 7.1.4,
      "Processing the agentx-Register-PDU".

   -  Removed text in section 7, "Elements of Procedure" on parse errors
      and protocol errors.

   -  Added a new section, 7.1, "Processing AgentX Administrative
      Messages" which defines common processing and how to use the
      `parseError' and `processingError' instead of closing a session,
      and how to handle context.

   -  Removed the common processing text from the other administrative
      processing Elements of Procedure sections, and added a reference
      to section 7.1, "Processing AgentX Administrative Messages".  The
      affected sections are:

            -  7.1.2,  "Processing the agentx-IndexAllocate-PDU"
            -  7.1.3,  "Processing the agentx-IndexDeallocate-PDU"
            -  7.1.4,  "Processing the agentx-Register-PDU"
            -  7.1.5,  "Processing the agentx-Unregister-PDU"
            -  7.1.6,  "Processing the agentx-AddAgentCaps-PDU"
            -  7.1.7,  "Processing the agentx-RemoveAgentCaps-PDU"
            -  7.1.8,  "Processing the agentx-Close-PDU"
            -  7.1.10, "Processing the agentx-Notify-PDU"
            -  7.1.11, "Processing the agentx-Ping-PDU"

   -  Reworked the text in section 7.1.1, "Processing the
      agentx-Open-PDU" to include new error codes, and, to eliminate
      reference to an indicated context.

   -  Modified the text in Section 7.1.10, "Processing the
      agentx-Notify-PDU" to state that context checking is performed.

   -  Substantially modified the text in section 7.1.4.1, "Handling
      Duplicate and Overlapping Subtrees".

   -  Removed the section on "Using the agentx-IndexAllocate-PDU" and
      added section 7.1.4.2, "Registering Stuff".  This change is
      intended to provide a more concise and a more cohesive
      description of how things are supposed to work.

   -  Modified the test in section 7.1.5, "Processing the
      agentx-Unregister-PDU" to require a match on u.range_subid and
      on u.upper_bound when these fields were applicable in the
      corresponding agentx-Register-PDU.

   -  Removed all references to "splitting", and all uses of the term
      "OID range".  The text now refers to regions or subtrees
      directly, and relies on rule (1), "Honoring the Registry", in
      section 7.2.1, "Dispatching AgentX PDUs".

   -  Modified text in clause 4(c) of section 7.2.1, "Dispatching
      AgentX PDUs", clarifying that the master agent can use its
      implementation-specific default timeout value when the timeout
      value registered by the subagent is impractical.

   -  Added text in section 7.2.2, "Subagent Processing" describing
      common processing.

   -  Added an example to the text in section 7.2.5.3, "Processing of
      Responses to agentx-GetNext-PDU and       agentx-GetBulk-PDU",
      and, removed the definition of "contains" from this section.

   -  Modified text in step (1) of section 7.2.5.5, "Processing of
      Responses to agentx-CommitSet-PDUs", eliminating directive for
      master agent to ignore additional responses to
      agentx-CommitSet-PDUs after the first error response.

   -  Modified text in section 7.2.5.6, "Processing of Responses to
      agentx-UndoSet-PDUs", cleaning up commit/undo elements of
      procedure per feedback received on the AgentX email list.

   -  Modified the text in section 8.1.2, "Operation" to explicitly
      prohibit interleaved sends, and, added a caution about
      exchanging AgentX messages via TCP.

   -  Modified text to be more explicit that the OID in the
      agentx-Allocate-PDU is an OBJECT-TYPE and does not contain any
      instance sub-identifiers.

   -  Replaced the term "subagent" with the term "session" in many
      places throughout the text.

   -  Modified the text relative to master agent processing of the
      agentx-TestSet-PDU, agentx-CommitSet-PDU, and the
      agentx-UndoSet-PDU to explicitly state that only "involved"
      sessions receive an agentx-CommitSet-PDU, and possibly, an
      agentx-UndoSet-PDU.

   -  Modified the text to use the term "transaction", instead of
      "packet" (and others), where appropriate.  This helps
      distinguish the overall transaction from a particular sequence
      of packets or PDUs.

   -  Modified the text to explicitly state that a session is not
      required to support concurrent sets.

   -  Added section 13, "Notices".

   -  Added text to section 1, Introduction, relative to BCP 14 key
      words.

   -  Modified text to section 9, Security Considerations, to include
      use of BCP 14 key words.

   -  Modified text to section 9, Security Considerations, to include
      IPSEC as a suggested Transport Layer Security.

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