Subject: Draft of updated RFC 2257

From: Mike Daniele <[email-masked]>
Date: Thu, 28 Jan 99 16:40:47 -0500

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

                 Agent Extensibility (AgentX) Protocol
                               Version 1

Status of this Memo

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

Copyright Notice

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

Table of Contents

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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RFC 2257                        AgentX                      January 1998

1.  Introduction

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

2.  The SNMP Framework

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

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

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

2.1.  A Note on Terminology

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

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

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

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

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

3.  Extending the MIB

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

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

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

3.1.  Motivation for AgentX

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

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

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

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

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

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

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

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

4.  AgentX Framework

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

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

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

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

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

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

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

4.1.  AgentX Roles

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

      - Accepts AgentX session establishment requests from subagents.

      - Accepts registration of MIB regions by subagents.

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

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

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

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

      - Forwards notifications on behalf of subagents.

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

      - Initiates 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.

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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.

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RFC 2257                        AgentX                      January 1998

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

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

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

         - 32-bit alignment of data within PDUs

         - Native byte-order encoding by subagents

         - Large AgentX PDU payload sizes.

4.4  Non-Goals

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

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

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

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

5.  AgentX Encodings

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

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   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.

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      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.

      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                                                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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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.

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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                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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   (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.

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          - 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.
	    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).

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      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.

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

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

      h.sessionID

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

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

      h.transactionID

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

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

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

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      h.packetID

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

      h.payload_length

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

6.1.1.  Context

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

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

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

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

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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:

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      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:

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      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                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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    (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).

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      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 7.1.5.1.

      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:

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   (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. 

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   (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:

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      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                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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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                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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    (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)

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    (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.

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      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:

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      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:

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      p.context

         An optional non-default context.

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

6.2.12.  The agentx-IndexAllocate-PDU

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

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

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

    (i.vb)
    ...

   An agentx-IndexAllocate-PDU contains the following fields:

      i.context

         An optional non-default context.

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      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.

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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     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   An agentx-AddAgentCaps-PDU contains the following fields:

      a.context

         An optional non-default context.

      a.id

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

      a.descr

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

6.2.15.  The agentx-RemoveAgentCaps-PDU

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

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

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   (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                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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RFC 2257                        AgentX                      January 1998

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        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 6.1.

         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 6.1, 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),
                parseFailed                (266),
                requestDenied		   (267),
                processingError            (268)

         Within responses to the set of "SNMP request processing" PDU 
         types listed in 6.1, values may also include those defined for 
         errors in the SNMPv2 PDUs (RFC 1905 [4]).

      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

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

           o 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 identicial 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'.  Examples of a parse error are

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

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

   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.

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

	 << FIX ME: Should we refer to mib spec now? >>

   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 7.1.
   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'.

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RFC 2257                        AgentX                      January 1998

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

   5) If all VarBinds are processed successfully, the
      agentx-Response-PDU is sent in reply with res.error set to
      `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 "Registering Stuff" for more information on how subagents should
      perform index allocations.

7.1.4   Processing the agentx-IndexDeallocate-PDU

   When the master agent receives an agentx-IndexDeallocate-PDU, it
   performs the common processing described in 7.1.
   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'.

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RFC 2257                        AgentX                      January 1998

   5) 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
      for the particular index.

7.1.5.  Processing the agentx-Register-PDU

   When the master agent receives an agentx-Register-PDU, it
   performs the common processing described in 7.1.
   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.

7.1.5.1.  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 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.

7.1.5.2 Registering Stuff

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

7.1.5.2.1 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 authoratative 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 (7.1.5.1) will be
   authoratative.  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).

7.1.5.2.2 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 1573 [11]),
   snmpFddiSMTIndex in the FDDI MIB (RFC 1285 [12]), or
   sysApplInstallPkgIndex in the System Application MIB [13]).  The need
   for subagents to share tables using such indexes is the main
   motivation for index allocation in AgentX.

   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 accomodate
   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.

7.1.5.2.3 Examples

   Example 1:

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

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

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

   Example 2:

      This same subagent wishes to register ipNetToMediaTable rows
      corresponding to its interface (ifIndex i).  Due to 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.

      << FIX ME: examples from applmib refer to an i-d. >>

   Example 4:

      The Application Management MIB [APPL-MIB] 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 [13], 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.

7.1.6.  Processing the agentx-Unregister-PDU

   When the master agent receives an agentx-Unregister-PDU, it
   performs the common processing described in 7.1.
   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.

7.1.7.  Processing the agentx-AddAgentCaps-PDU

   When the master agent receives an agentx-AddagentCaps-PDU, it
   performs the common processing described in 7.1.
   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'.

7.1.8.  Processing the agentx-RemoveAgentCaps-PDU

   When the master agent receives an agentx-RemoveAgentCaps-PDU, it
   performs the common processing described in 7.1.
   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'.

7.1.9.  Processing the agentx-Close-PDU

   When the master agent receives an agentx-Close-PDU, it
   performs the common processing described in 7.1, 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 7.1.6.

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

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

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

7.1.10.  Detecting Connection Loss

   If a master agent is able to detect (from the underlying transport)
   that a subagent cannot receive AgentX PDUs, it should close all
   affected AgentX sessions as described in 7.1.9, step 1).

7.1.11.  Processing the agentx-Notify-PDU

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

   When the master agent receives an agentx-Notify-PDU, it
   performs the common processing described in 7.1.
   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.

   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 VarBinds 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 [9].  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.

      << FIX ME: Is 2089 correct reference now? >>

7.1.12.  Processing the agentx-Ping-PDU

   When the master agent receives an agentx-Ping-PDU, it
   performs the common processing described in 7.1.

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

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RFC 2257                        AgentX                      January 1998

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

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

   << FIX ME: Do we want to mention SNMPv3? >>

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 7.1.5.1).

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RFC 2257                        AgentX                      January 1998

   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.

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RFC 2257                        AgentX                      January 1998

         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, 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 7.1.5.1) that contains 
       the binding's name.

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RFC 2257                        AgentX                      January 1998

   (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 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 7.1.5.1) 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:

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RFC 2257                        AgentX                      January 1998

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

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

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

        - if (1b) applies, the target 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 preceeds 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 [4].  Please
   refer to it for details on the format of the SNMP GetBulkRequest-PDU
   itself.)

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

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

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

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

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

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

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

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RFC 2257                        AgentX                      January 1998

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 7.2.1.1 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 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.

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

7.2.1.5.  Dispatch

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

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RFC 2257                        AgentX                      January 1998

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:

   3) 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.

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

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

   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'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.

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RFC 2257                        AgentX                      January 1998

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:

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RFC 2257                        AgentX                      January 1998

      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.

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 [4]. The subagent
   validates variables with respect to the context and session indicated
   in the testSet-PDU.

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

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

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

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

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

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RFC 2257                        AgentX                      January 1998

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 [4]) the management operation indicated by the previous
   TestSet-PDU.  After carrying out the management operation, the
   subagent sends in reply an agentx-Response-PDU whose res.error field
   is set to one of the following (SNMPv2 SMI) values:

            noError                    (0),
            commitFailed              (14)

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

7.2.4.3.  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 [4].

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

            noError                    (0),
            undoFailed                (15)

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

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

7.2.4.4.  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.

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RFC 2257                        AgentX                      January 1998

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 7.1.9, step (2).

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

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

7.2.5.2.  Processing of Responses to agentx-Get-PDUs

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

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

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

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RFC 2257                        AgentX                      January 1998

   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 7.2.4.1 above), processing
   continues with the following steps:

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

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

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

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

   3)  A new iteration of AgentX request dispatching is initiated
       (as described in section 7.2.1.2), in which only those VarBinds
       whose v.type is `endOfMibView' are processed.

   4)  For each such VarBind, an authoratative 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.

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RFC 2257                        AgentX                      January 1998

   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 7.2.4.

   7)  This process continues iteratively until a complete SNMP
       Response-PDU has been built, or until there remain no 
       authoratative 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 preceed 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
   authoratative 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 7.2.4.1 above), processing continues with the
   further exchange of AgentX PDUs.  The value of h.transactionID in the
   agentx-CommitSet, -UndoSet, and -CleanupSet-PDUs must be identical to
   the value sent in the testSet-PDU.

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

   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 7.2.4.6.

   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 7.2.4.1 above), processing continues with the
   following steps:

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

       An agentx-UndoSet-PDU is sent to each target 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 7.2.4.6.

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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 7.2.4.1 above), processing continues with the
   following steps:

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

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

7.2.6.  Sending the SNMP Response-PDU

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

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

   << FIX ME: 2089 still correct?>>

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

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

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7.3.  State Transitions

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

7.3.1.  Set Transaction States

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

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

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

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

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   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:

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                                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
   ----------------+--------------+--------------

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7.3.3  Session States

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

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

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

8.  Transport Mappings

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

8.1.  AgentX over TCP

8.1.1.  Well-known Values

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

8.1.2.  Operation

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

   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.

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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).

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   Security considerations require three questions to be answered:

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

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

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

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

   The answer to the second question is: No.

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

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

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

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

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

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10.  Acknowledgements

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

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

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

   The AgentX Working Group is chaired by:

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

   Phone: +1-301-721-3000
   Fax:   +1-301-721-3001
   EMail: [email-masked]

11.  Authors' and Editor's Addresses

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

   Phone: +1-603-881-1423
   EMail: [email-masked]

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

   Phone: +31-79-322-8316
   EMail: [email-masked]

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

   Phone: +1-805-961-3642
   Fax:   +1-805-961-3600
   EMail: [email-masked]

12.  References

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

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

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

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

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

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

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

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

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

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

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

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

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

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13.  Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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