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rfc:rfc2741

Network Working Group M. Daniele Request for Comments: 2741 Compaq Computer Corporation Obsoletes: 2257 B. Wijnen Category: Standards Track T.J. Watson Research Center, IBM Corp.

                                                        M. Ellison, Ed.
                                      Ellison Software Consulting, Inc.
                                                      D. Francisco. Ed.
                                                    Cisco Systems, Inc.
                                                           January 2000
               Agent Extensibility (AgentX) Protocol
                             Version 1

Status of this Memo

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

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

 This memo defines a standardized framework for extensible SNMP
 agents.  It defines processing entities called master agents and
 subagents, a protocol (AgentX) used to communicate between them, and
 the elements of procedure by which the extensible agent processes
 SNMP protocol messages. This memo obsoletes RFC 2257.

Table of Contents

 1. Introduction.....................................................4
 2. The SNMP Management Framework....................................4
   2.1. A Note on Terminology........................................5
 3. Extending the MIB................................................5
   3.1. Motivation for AgentX........................................6
 4. AgentX Framework.................................................6
   4.1. AgentX Roles.................................................7
   4.2. Applicability................................................8
   4.3. Design Features of AgentX....................................9
   4.4. Non-Goals...................................................10

Daniele, et al. Standards Track [Page 1] RFC 2741 AgentX January 2000

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

Daniele, et al. Standards Track [Page 2] RFC 2741 AgentX January 2000

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

Daniele, et al. Standards Track [Page 3] RFC 2741 AgentX January 2000

1. Introduction

 This memo defines a standardized framework for extensible SNMP
 agents.  It defines processing entities called master agents and
 subagents, a protocol (AgentX) used to communicate between them, and
 the elements of procedure by which the extensible agent processes
 SNMP protocol messages.
 This memo obsoletes RFC 2257.  It is worth noting that most of the
 changes are for the purpose of clarification.  The only changes
 affecting AgentX protocol messages on the wire are:
  1. The agentx-Notify-PDU and agentx-Close-PDU now generate an

agentx-Response-PDU

  1. Three new error codes are available: parseFailed(266),

requestDenied(267), and processingError(268)

 Appendix A provides a detailed list of changes relative to RFC 2257.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [27].

2. The SNMP Management Framework

 The SNMP Management Framework presently consists of five major
 components:
 An overall architecture, described in RFC 2571 [1].
 Mechanisms for describing and naming objects and events for the
 purpose of management. The first version of this Structure of
 Management Information (SMI) is called SMIv1 and described in STD 16,
 RFC 1155 [2], STD 16, RFC 1212 [3] and RFC 1215 [4]. The second
 version, called SMIv2, is described in STD 58, RFC 2578 [5], STD 58,
 RFC 2579 [6] and STD 58, RFC 2580 [7].
 Message protocols for transferring management information. The first
 version of the SNMP message protocol is called SNMPv1 and described
 in STD 15, RFC 1157 [8]. A second version of the SNMP message
 protocol, which is not an Internet standards track protocol, is
 called SNMPv2c and described in RFC 1901 [9] and RFC 1906 [10]. The
 third version of the message protocol is called SNMPv3 and described
 in RFC 1906 [10], RFC 2572 [11] and RFC 2574 [12].
 Protocol operations for accessing management information. The first
 set of protocol operations and associated PDU formats is described in

Daniele, et al. Standards Track [Page 4] RFC 2741 AgentX January 2000

 STD 15, RFC 1157 [8]. A second set of protocol operations and
 associated PDU formats is described in RFC 1905 [13].
 A set of fundamental applications described in RFC 2573 [14] and the
 view-based access control mechanism described in RFC 2575 [15].
 A more detailed introduction to the current SNMP Management Framework
 can be found in RFC 2570 [16].
 Managed objects are accessed via a virtual information store, termed
 the Management Information Base or MIB.  Objects in the MIB are
 defined using the mechanisms defined in the SMI.

2.1. A Note on Terminology

 The term "variable" refers to an instance of a non-aggregate object
 type defined according to the conventions set forth in the SMIv2 (STD
 58, RFC 2578, [5]) or the textual conventions based on the SMIv2 (STD
 58, RFC 2579 [6]).  The term "variable binding" normally refers to
 the pairing of the name of a variable and its associated value.
 However, if certain kinds of exceptional conditions occur during
 processing of a retrieval request, a variable binding will pair a
 name and an indication of that exception.
 A variable-binding list is a simple list of variable bindings.
 The name of a variable is an OBJECT IDENTIFIER, which is the
 concatenation of the OBJECT IDENTIFIER of the corresponding object
 type together with an OBJECT IDENTIFIER fragment identifying the
 instance.  The OBJECT IDENTIFIER of the corresponding object-type is
 called the OBJECT IDENTIFIER prefix of the variable.

3. Extending the MIB

 New MIB modules that extend the Internet-standard MIB are
 continuously being defined by various IETF working groups.  It is
 also common for enterprises or individuals to create or extend
 enterprise-specific or experimental MIBs.
 As a result, managed devices are frequently complex collections of
 manageable components that have been independently installed on a
 managed node.  Each component provides instrumentation for the
 managed objects defined in the MIB module(s) it implements.
 The SNMP framework does not describe how the set of managed objects
 supported by a particular agent may be changed dynamically.

Daniele, et al. Standards Track [Page 5] RFC 2741 AgentX January 2000

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
  1. a "master" agent that is available on the standard transport

address and that accepts SNMP protocol messages

  1. a set of "subagents" that each contain management

instrumentation

  1. 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.
  1. a set of tools to aid subagent development, and a runtime (API)

environment that hides much of the protocol operation between a

       subagent and the master agent.
 The wide deployment of extensible SNMP agents, coupled with the lack
 of Internet standards in this area, makes it difficult to field
 SNMP-manageable applications.  A vendor may have to support several
 different subagent environments (APIs) in order to support different
 target platforms.
 It can also become quite cumbersome to configure subagents and
 (possibly multiple) master agents on a particular managed node.
 Specifying a standard protocol for agent extensibility (AgentX)
 provides the technical foundation required to solve both of these
 problems.  Independently developed AgentX-capable master agents and
 subagents will be able to interoperate at the protocol level.
 Vendors can continue to differentiate their products in all other
 respects.

4. AgentX Framework

 Within the SNMP framework, a managed node contains a processing
 entity, called an agent, which has access to management information.
 Within the AgentX framework, an agent is further defined to consist
 of:

Daniele, et al. Standards Track [Page 6] RFC 2741 AgentX January 2000

  1. a single processing entity called the master agent, which sends

and receives SNMP protocol messages in an agent role (as

       specified by the SNMP framework documents) but typically has
       little or no direct access to management information.
  1. zero or more processing entities called subagents, which are

"shielded" from the SNMP protocol messages processed by the

       master agent, but which have access to management information.
 The master and subagent entities communicate via AgentX protocol
 messages, as specified in this memo.  Other interfaces (if any) on
 these entities, and their associated protocols, are outside the scope
 of this document.  While some of the AgentX protocol messages appear
 similar in syntax and semantics to the SNMP, bear in mind that AgentX
 is not SNMP.
 The internal operations of AgentX are invisible to an SNMP entity
 operating in a manager role.  From a manager's point of view, an
 extensible agent behaves exactly as would a non-extensible
 (monolithic) agent that has access to the same management
 instrumentation.
 This transparency to managers is a fundamental requirement of AgentX,
 and is what differentiates AgentX subagents from SNMP proxy agents.

4.1. AgentX Roles

 An entity acting in a master agent role performs the following
 functions:
  1. Accepts AgentX session establishment requests from subagents.
  1. Accepts registration of MIB regions by subagents.
  1. Sends and accepts SNMP protocol messages on the agent's

specified transport addresses.

  1. 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.)
  1. Provides instrumentation for the MIB objects defined in RFC

1907 [17], and for any MIB objects relevant to any

       administrative framework it supports.

Daniele, et al. Standards Track [Page 7] RFC 2741 AgentX January 2000

  1. Sends and receives AgentX protocol messages to access

management information, based on the current registry of MIB

       regions.
  1. Forwards notifications on behalf of subagents.
 An entity acting in a subagent role performs the following functions:
  1. Initiates AgentX sessions with the master agent.
  1. Registers MIB regions with the master agent.
  1. Instantiates managed objects.
  1. Binds OIDs within its registered MIB regions to actual

variables.

  1. Performs management operations on variables.
  1. Initiates notifications.

4.2. Applicability

 It is intended that this memo specify the smallest amount of required
 behavior necessary to achieve the largest benefit, that is, to cover
 a very large number of possible MIB implementations and
 configurations with minimum complexity and low "cost of entry".
 This section discusses several typical usage scenarios.
 1) Subagents implement separate MIB modules -- for example, subagent
    `A' implements "mib-2", subagent `B' implements "host-resources".
    It is anticipated that this will be the most common subagent
    configuration.
 2) Subagents implement rows in a "simple table".  A simple table is
    one in which row creation is not specified, and for which the MIB
    does not define an object that counts entries in the table.
    Examples of simple tables are rdbmsDbTable, udpTable, and
    hrSWRunTable.
    This is the most commonly defined type of MIB table, and probably
    represents the next most typical configuration that AgentX would
    support.

Daniele, et al. Standards Track [Page 8] RFC 2741 AgentX January 2000

 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

Daniele, et al. Standards Track [Page 9] RFC 2741 AgentX January 2000

    carry them out; subagents are exclusively concerned with
    management instrumentation; and neither should intrude on the
    other's territory.
 2) A standard protocol and "rules of engagement" to enable
    interoperability between management instrumentation and extensible
    agents.
 3) Mechanisms for independently developed subagents to integrate into
    the extensible agent on a particular managed node in such a way
    that they need not be aware of any other existing subagents.
 4) A simple, deterministic registry and dispatching algorithm.  For a
    given extensible agent configuration, there is a single subagent
    who is "authoritative" for any particular region of the MIB (where
    "region" may extend from an entire MIB down to a single object-
    instance).
 5) Performance considerations.  It is likely that the master agent
    and all subagents will reside on the same host, and in such cases
    AgentX is more a form of inter-process communication than a
    traditional communications protocol.
    Some of the design decisions made with this in mind include:
  1. 32-bit alignment of data within PDUs
  1. Native byte-order encoding by subagents
  1. 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

Daniele, et al. Standards Track [Page 10] RFC 2741 AgentX January 2000

    typically required for independently developed subagents to
    coexist.  The set of supported extensible agent configurations is
    described above, in Section 4.2, "Applicability".
 Some possible future version of the AgentX protocol may provide
 coverage for one or more of these "non-goals" or for new goals that
 might be identified after greater deployment experience.

5. AgentX Encodings

 AgentX PDUs consist of a common header, followed by PDU-specific data
 of variable length.  Unlike SNMP PDUs, AgentX PDUs are not encoded
 using the BER (as specified in ISO 8824 [18]), but are transmitted as
 a contiguous byte stream.  The data within this stream is organized
 to provide natural alignment with respect to the start of the PDU,
 permitting direct (integer) access by the processing entities.
 The first four fields in the header are single-byte values.  A bit
 (NETWORK_BYTE_ORDER) in the third field (h.flags) is used to indicate
 the byte ordering of all multi-byte integer values in the PDU,
 including those which follow in the header itself.  This is described
 in more detail in Section 6.1, "AgentX PDU Header", below.
 PDUs are depicted in this memo using the following convention (where
 byte 1 is the first transmitted byte):
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  byte 1       |  byte 2       |  byte 3       |  byte 4       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  byte 5       |  byte 6       |  byte 7       |  byte 8       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields marked "<reserved>" are reserved for future use and must be
 zero-filled.

5.1. Object Identifier

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

Daniele, et al. Standards Track [Page 11] RFC 2741 AgentX January 2000

 Object Identifier
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  n_subid      |  prefix       |  include      |  <reserved>   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       sub-identifier #1                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       sub-identifier #n_subid                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Object Identifier header fields:
    n_subid
       The number (0-128) of sub-identifiers in the object identifier.
       An ordered list of "n_subid" 4-byte sub-identifiers follows the
       4-byte header.
    prefix
       An unsigned value used to reduce the length of object
       identifier encodings.  A non-zero value "x" is interpreted as
       the first sub-identifier after "internet" (1.3.6.1), and
       indicates an implicit prefix "internet.x" to the actual sub-
       identifiers encoded in the Object Identifier.  For example, a
       prefix field value 2 indicates an implicit prefix "1.3.6.1.2".
       A value of 0 in the prefix field indicates there is no prefix
       to the sub-identifiers.
    include
       Used only when the Object Identifier is the start of a
       SearchRange, as described in section 5.2, "SearchRange".
    sub-identifier 1, 2, ... n_subid
       A 4-byte unsigned integer, encoded according to the header's
       NETWORK_BYTE_ORDER bit.
 A null Object Identifier consists of the 4-byte header with all bytes
 set to 0.

Daniele, et al. Standards Track [Page 12] RFC 2741 AgentX January 2000

 Examples:
 sysDescr.0 (1.3.6.1.2.1.1.1.0)
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 4             | 2             | 0             | 0             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 0                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 1.2.3.4
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 4             | 0             | 0             | 0             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 2                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 3                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 4                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.2. SearchRange

 A SearchRange consists of two Object Identifiers.  In its
 communication with a subagent, the master agent uses a SearchRange to
 identify a requested variable binding, and, in GetNext and GetBulk
 operations, to set an upper bound on the names of managed object
 instances the subagent may send in reply.
 The first Object Identifier in a SearchRange (called the starting
 OID) indicates the beginning of the range.  It is frequently (but not
 necessarily) the name of a requested variable binding.
 The "include" field in this OID's header is a boolean value (0 or 1)
 indicating whether or not the starting OID is included in the range.
 The second object identifier (ending OID) indicates the non-inclusive
 end of the range, and its "include" field is always 0.  A null value
 for ending OID indicates an unbounded SearchRange.

Daniele, et al. Standards Track [Page 13] RFC 2741 AgentX January 2000

 Example:  To indicate a search range from 1.3.6.1.2.1.25.2
 (inclusive) to 1.3.6.1.2.1.25.2.1 (exclusive), the SearchRange would
 be:
 (start)
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 3             | 2             | 1             |       0       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 25                                                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 2                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 (end)
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 4             | 2             | 0             |       0       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 25                                                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 2                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 A SearchRangeList is a contiguous list of SearchRanges.

5.3. Octet String

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

Daniele, et al. Standards Track [Page 14] RFC 2741 AgentX January 2000

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Octet String Length (L)                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Octet L - 1  |  Octet L      |       Padding (as required)   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 A null Octet String consists of a 4-byte length field set to 0.

5.4. Value Representation

 Variable bindings may be encoded within the variable-length portion
 of some PDUs.  The representation of a variable binding (termed a
 VarBind) consists of a 2-byte type field, a name (Object Identifier),
 and the actual value data.
 VarBind
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          v.type               |          <reserved>           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 (v.name)
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  n_subid      |  prefix       |      0        |       0       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       sub-identifier #1                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       sub-identifier #n_subid                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 (v.data)
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       data                                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       data                                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 VarBind fields:
    v.type
 Indicates the variable binding's syntax, and must be one of the
 following values:

Daniele, et al. Standards Track [Page 15] RFC 2741 AgentX January 2000

            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:
  1. Integer, Counter32, Gauge32, and TimeTicks are encoded as 4

contiguous bytes, according to the header's

          NETWORK_BYTE_ORDER bit.
  1. Counter64 is encoded as 8 contiguous bytes, according to

the header's NETWORK_BYTE_ORDER bit.

  1. Object Identifiers are encoded as described in section 5.1,

Object Identifier.

  1. IpAddress, Opaque, and Octet String are all octet strings

and are encoded as described in section 5.3, "Octet

          String", Octet String.  Note that the octets used to
          represent IpAddress are always ordered most significant to
          least significant.
          Value data always follows v.name whenever v.type is one of
          the above types.  These data bytes are present even if they
          will not be used (as, for example, in certain types of
          index allocation).
  1. Null, noSuchObject, noSuchInstance, and endOfMibView do not

contain any encoded value. Value data never follows v.name

          in these cases.

Daniele, et al. Standards Track [Page 16] RFC 2741 AgentX January 2000

       Note that the VarBind itself does not contain the value size.
       That information is implied for the fixed-length types, and
       explicitly contained in the encodings of variable-length types
       Object Identifier and Octet String).
 A VarBindList is a contiguous list of VarBinds.  Within a
 VarBindList, a particular VarBind is identified by an index value.
 The first VarBind in a VarBindList has index value 1, the second has
 index value 2, and so on.

6. Protocol Definitions

6.1. AgentX PDU Header

 The AgentX PDU header is a fixed-format, 20-octet structure:
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   h.version   |    h.type     |    h.flags    |  <reserved>   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          h.sessionID                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        h.transactionID                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          h.packetID                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        h.payload_length                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 An AgentX PDU header contains the following fields:
    h.version
       The version of the AgentX protocol (1 for this memo).
    h.type
       The PDU type; one of the following values:
          agentx-Open-PDU             (1),
          agentx-Close-PDU            (2),
          agentx-Register-PDU         (3),
          agentx-Unregister-PDU       (4),
          agentx-Get-PDU              (5),
          agentx-GetNext-PDU          (6),
          agentx-GetBulk-PDU          (7),
          agentx-TestSet-PDU          (8),
          agentx-CommitSet-PDU        (9),
          agentx-UndoSet-PDU         (10),

Daniele, et al. Standards Track [Page 17] RFC 2741 AgentX January 2000

          agentx-CleanupSet-PDU      (11),
          agentx-Notify-PDU          (12),
          agentx-Ping-PDU            (13),
          agentx-IndexAllocate-PDU   (14),
          agentx-IndexDeallocate-PDU (15),
          agentx-AddAgentCaps-PDU    (16),
          agentx-RemoveAgentCaps-PDU (17),
          agentx-Response-PDU        (18)
          The set of PDU types for "administrative processing" are 1-4
          and 12-17.  The set of PDU types for "SNMP request
          processing" are 5-11.
    h.flags
          A bitmask, with bit 0 the least significant bit.  The bit
          definitions are as follows:
               Bit             Definition
               ---             ----------
               0               INSTANCE_REGISTRATION
               1               NEW_INDEX
               2               ANY_INDEX
               3               NON_DEFAULT_CONTEXT
               4               NETWORK_BYTE_ORDER
               5-7             (reserved)
          The NETWORK_BYTE_ORDER bit applies to all multi-byte integer
          values in the entire AgentX packet, including the remaining
          header fields.  If set, then network byte order (most
          significant byte first; "big endian") is used.  If not set,
          then least significant byte first ("little endian") is used.
          The NETWORK_BYTE_ORDER bit applies to all AgentX PDUs.
          The NON_DEFAULT_CONTEXT bit is used only in the AgentX PDUs
          described in section 6.1.1, "Context".
          The NEW_INDEX and ANY_INDEX bits are used only within the
          agentx-IndexAllocate-, and -IndexDeallocate-PDUs.
          The INSTANCE_REGISTRATION bit is used only within the
          agentx-Register-PDU.

Daniele, et al. Standards Track [Page 18] RFC 2741 AgentX January 2000

    h.sessionID
          The session ID uniquely identifies a session over which
          AgentX PDUs are exchanged between a subagent and the master
          agent.  The session ID has no significance and no defined
          value in the agentx-Open-PDU sent by a subagent to open a
          session with the master agent; in this case, the master
          agent will assign a unique session ID that it will pass back
          in the corresponding agentx-Response-PDU.  From that point
          on, that same session ID will appear in every AgentX PDU
          exchanged over that session between the master and the
          subagent.  A subagent may establish multiple AgentX sessions
          by sending multiple agentx-Open-PDUs to the master agent.
          In master agents that support multiple transport protocols,
          the sessionID should be globally unique rather than unique
          just to a particular transport.
    h.transactionID
          The transaction ID uniquely identifies, for a given session,
          the single SNMP management request (and single SNMP PDU)
          with which an AgentX PDU is associated.  If a single SNMP
          management request results in multiple AgentX PDUs being
          sent by the master agent with the same session ID, each of
          these AgentX PDUs must contain the same transaction ID;
          conversely, AgentX PDUs sent during a particular session,
          that result from distinct SNMP management requests, must
          have distinct transaction IDs within the limits of the 32-
          bit field).
          Note that the transaction ID is not the same as the SNMP
          PDU's request-id (as described in section 4.1 of RFC 1905
          [13], nor is it the same as the SNMP Message's msgID (as
          described in section 6.2 of RFC 2572 [11]), nor can it be,
          since a master agent might receive SNMP requests with the
          same request-ids or msgIDs from different managers.
          The transaction ID has no significance and no defined value
          in AgentX administrative PDUs, i.e., AgentX PDUs that are
          not associated with an SNMP management request.
    h.packetID
          A packet ID generated by the sender for all AgentX PDUs
          except the agentx-Response-PDU. In an agentx-Response-PDU,
          the packet ID must be the same as that in the received
          AgentX PDU to which it is a response.  A master agent might

Daniele, et al. Standards Track [Page 19] RFC 2741 AgentX January 2000

          use this field to associate subagent response PDUs with
          their corresponding request PDUs.  A subagent might use this
          field to correlate responses to multiple (batched)
          registrations.
    h.payload_length
          The size in octets of the PDU contents, excluding the 20-
          byte header.  As a result of the encoding schemes and PDU
          layouts, this value will always be either 0, or a multiple
          of 4.

6.1.1. Context

 In the SNMPv1 or SNMPv2c, the community string may be used as an
 index into a local repository of configuration information that may
 include community profiles or more complex context information. In
 SNMPv3 this notion of "context" is formalized (see section 3.3.1 in
 RFC 2571 [1].
 AgentX provides a mechanism for transmitting a context specification
 within relevant PDUs, but does not place any constraints on the
 content of that specification.
 An optional context field may be present in the agentx-Register-,
 UnRegister-, AddAgentCaps-, RemoveAgentCaps-, Get-, GetNext-,
 GetBulk-, IndexAllocate-, IndexDeallocate-, Notify-, TestSet-, and
 Ping- PDUs.
 If the NON_DEFAULT_CONTEXT bit in the AgentX header field h.flags is
 clear, then there is no context field in the PDU, and the operation
 refers to the default context.  (This does not mean there is a zero-
 length Octet String, it means there is no Octet String present.)  If
 the NON_DEFAULT_CONTEXT bit is set, then a context field immediately
 follows the AgentX header, and the operation refers to that specific
 context.  The context is represented as an Octet String.  There are
 no constraints on its length or contents.
 Thus, all of these AgentX PDUs (that is, those listed immediately
 above) refer to, or "indicate" a context, which is either the default
 context, or a non-default context explicitly named in the PDU.

6.2. AgentX PDUs

6.2.1. The agentx-Open-PDU

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

Daniele, et al. Standards Track [Page 20] RFC 2741 AgentX January 2000

 (AgentX header)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | h.version (1) |  h.type (1)   |    h.flags    |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          h.sessionID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.transactionID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           h.packetID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.payload_length                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  o.timeout    |                     <reserved>                |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (o.id)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  n_subid      |  prefix       |       0       |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             subidentifier #1                                  |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...                                                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             subidentifier #n_subid                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (o.descr)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Octet String Length (L)                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet L - 1  |  Octet L      |       Padding (as required)   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 An agentx-Open-PDU contains the following fields:
    o.timeout
          The length of time, in seconds, that a master agent should
          allow to elapse after dispatching a message on a session
          before it regards the subagent as not responding.  This is
          the default value for the session, and may be overridden by

Daniele, et al. Standards Track [Page 21] RFC 2741 AgentX January 2000

          values associated with specific registered MIB regions.  The
          default value of 0 indicates that there is no session-wide
          default value.
    o.id
          An Object Identifier that identifies the subagent.
          Subagents that do not support such an notion may send a null
          Object Identifier.
    o.descr
          An Octet String containing a DisplayString describing the
          subagent.

6.2.2. The agentx-Close-PDU

 An agentx-Close-PDU issued by either a subagent or the master agent
 terminates an AgentX session.
 (AgentX header)
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | h.version (1) |  h.type (2)   |    h.flags    |  <reserved>   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          h.sessionID                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        h.transactionID                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           h.packetID                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        h.payload_length                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  c.reason     |                     <reserved>                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 An agentx-Close-PDU contains the following field:
         c.reason
          An enumerated value that gives the reason that the master
          agent or subagent closed the AgentX session.  This field may
          take one of the following values:

Daniele, et al. Standards Track [Page 22] RFC 2741 AgentX January 2000

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

Daniele, et al. Standards Track [Page 23] RFC 2741 AgentX January 2000

  (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).

Daniele, et al. Standards Track [Page 24] RFC 2741 AgentX January 2000

    r.priority
          A value between 1 and 255, used to achieve a desired
          configuration when different sessions register identical or
          overlapping regions.  Subagents with no particular knowledge
          of priority should register with the default value of 127.
          In the master agent's dispatching algorithm, smaller values
          of r.priority take precedence over larger values, as
          described in section 7.1.4.1, "Handling Duplicate and
          Overlapping Subtrees".
    r.subtree
          An Object Identifier that names the basic subtree of a MIB
          region for which a subagent indicates its support. The term
          "subtree" is used generically here, it may represent a
          fully-qualified instance name, a partial instance name, a
          MIB table, an entire MIB, etc.
          The choice of what to register is implementation-specific;
          this memo does not specify permissible values.  Standard
          practice however is for a subagent to register at the
          highest level of the naming tree that makes sense.
          Registration of fully- qualified instances is typically done
          only when a subagent can perform management operations only
          on particular rows of a conceptual table.
          If r.subtree is in fact a fully qualified instance name, the
          INSTANCE_REGISTRATION bit in h.flags must be set, otherwise
          it must be cleared.  The master agent may save this
          information to optimize subsequent operational dispatching.
    r.range_subid
          Permits specifying a range in place of one of r.subtree's
          sub-identifiers.  If this value is 0, no range is being
          specified and there is no r.upper_bound field present in the
          PDU. In this case the MIB region being registered is the
          single subtree named by r.subtree.
          Otherwise the "r.range_subid"-th sub-identifier in r.subtree
          is a range lower bound, and the range upper bound sub-
          identifier (r.upper_bound) immediately follows r.subtree.
          In this case the MIB region being registered is the union of
          the subtrees formed by enumerating this range.

Daniele, et al. Standards Track [Page 25] RFC 2741 AgentX January 2000

          Note that r.range_subid indicates the (1-based) index of
          this sub-identifier within the OID represented by r.subtree,
          regardless of whether or not r.subtree is encoded using a
          prefix. (See the example below.)
    r.upper_bound
          The upper bound of a sub-identifier's range.  This field is
          present only if r.range_subid is not 0.
          The use of r.range_subid and r.upper_bound provide a general
          shorthand mechanism for specifying a MIB region. For
          example, if r.subtree is the OID 1.3.6.1.2.1.2.2.1.1.7,
          r.range_subid is 10, and r.upper_bound is 22, the specified
          MIB region can be denoted 1.3.6.1.2.1.2.2.1.[1-22].7.
          Registering this region is equivalent to registering the
          union of subtrees
           1.3.6.1.2.1.2.2.1.1.7
           1.3.6.1.2.1.2.2.1.2.7
           1.3.6.1.2.1.2.2.1.3.7
           ...
           1.3.6.1.2.1.2.2.1.22.7
          One expected use of this mechanism is registering a
          conceptual row with a single PDU.  In the example above, the
          MIB region happens to be row 7 of the RFC 1573 ifTable.  The
          actual PDU would be:
 (AgentX header)
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | h.version (1) |  h.type (3)   |    h.flags    |  <reserved>   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          h.sessionID                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        h.transactionID                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           h.packetID                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        h.payload_length                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   r.timeout   |  r.priority   | 10            |  <reserved>   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Daniele, et al. Standards Track [Page 26] RFC 2741 AgentX January 2000

 (r.subtree)
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 6             |  2            |  0            |  <reserved>   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 2                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 2                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 7                                                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 (r.upper_bound)
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 22                                                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Note again that here r.range_subid is 10, even though n_subid in
 r.subtree is only 6.
 r.range_subid may be used at any level within a subtree, it need not
 represent row-level registration.  This mechanism may be used in any
 way that is convenient for a subagent to achieve its registrations.

6.2.4. The agentx-Unregister-PDU

 The agentx-Unregister-PDU is sent by a subagent to remove a MIB
 region that was previously registered on this session.
  (AgentX header)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | h.version (1) |  h.type (4)   |    h.flags    |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          h.sessionID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.transactionID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           h.packetID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.payload_length                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Daniele, et al. Standards Track [Page 27] RFC 2741 AgentX January 2000

  (u.context) OPTIONAL
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Octet String Length (L)                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet L - 1  |  Octet L      |       Padding (as required)   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |    <reserved> |  u.priority   | u.range_subid |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (u.subtree)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  n_subid      |  prefix       |      0        |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #1                                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #n_subid                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (u.upper_bound)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             optional upper-bound sub-identifier               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 An agentx-Unregister-PDU contains the following fields:
    u.context
          An optional non-default context.
    u.priority
          The priority at which this region was originally registered.
    u.subtree
          Indicates a previously-registered region of the MIB that a
          subagent no longer wishes to support.

Daniele, et al. Standards Track [Page 28] RFC 2741 AgentX January 2000

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

6.2.5. The agentx-Get-PDU

  (AgentX header)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | h.version (1) |  h.type (5)   |    h.flags    |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          h.sessionID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.transactionID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           h.packetID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.payload_length                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (g.context) OPTIONAL
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Octet String Length (L)                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet L - 1  |  Octet L      |       Padding (as required)   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (g.sr)
  (start 1)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  n_subid      |  prefix       |  include      |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #1                                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #n_subid                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Daniele, et al. Standards Track [Page 29] RFC 2741 AgentX January 2000

  (end 1)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | 0             | 0             | 0             |       0       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  (start n)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  n_subid      |  prefix       |  include      |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #1                                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #n_subid                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (end n)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | 0             | 0             | 0             |       0       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    An agentx-Get-PDU contains the following fields:
    g.context
          An optional non-default context.
    g.sr
          A SearchRangeList containing the requested variables for
          this session.  Within the agentx-Get-PDU, the Ending OIDs
          within SearchRanges are null-valued Object Identifiers.

6.2.6. The agentx-GetNext-PDU

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

Daniele, et al. Standards Track [Page 30] RFC 2741 AgentX January 2000

  (g.context) OPTIONAL
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Octet String Length (L)                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet L - 1  |  Octet L      |       Padding (as required)   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (g.sr)
  (start 1)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  n_subid      |  prefix       |  include      |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #1                                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #n_subid                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (end 1)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  n_subid      |  prefix       |      0        |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #1                                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #n_subid                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  (start n)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  n_subid      |  prefix       |  include      |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #1                                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #n_subid                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Daniele, et al. Standards Track [Page 31] RFC 2741 AgentX January 2000

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

Daniele, et al. Standards Track [Page 32] RFC 2741 AgentX January 2000

  (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.

Daniele, et al. Standards Track [Page 33] RFC 2741 AgentX January 2000

6.2.8. The agentx-TestSet-PDU

  (AgentX header)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | h.version (1) |  h.type (8)   |    h.flags    |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          h.sessionID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.transactionID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           h.packetID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.payload_length                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (t.context) OPTIONAL
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Octet String Length (L)                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet L - 1  |  Octet L      |       Padding (as required)   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (t.vb)
  (VarBind 1)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          v.type               |        <reserved>             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  n_subid      |  prefix       |      0        |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       sub-identifier #1                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       sub-identifier #n_subid                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       data                                    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       data                                    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...

Daniele, et al. Standards Track [Page 34] RFC 2741 AgentX January 2000

  (VarBind n)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          v.type               |        <reserved>             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  n_subid      |  prefix       |      0        |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       sub-identifier #1                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       sub-identifier #n_subid                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       data                                    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       data                                    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 An agentx-TestSet-PDU contains the following fields:
    t.context
          An optional non-default context.
    t.vb
          A VarBindList containing the requested VarBinds for this
          subagent.

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

 These PDUs consist of the AgentX header only.
 The agentx-CommitSet-, -UndoSet-, and -Cleanup-PDUs are used in
 processing an SNMP SetRequest operation.

Daniele, et al. Standards Track [Page 35] RFC 2741 AgentX January 2000

6.2.10. The agentx-Notify-PDU

 An agentx-Notify-PDU is sent by a subagent to cause the master agent
 to forward a notification.
  (AgentX header)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | h.version (1) |  h.type (12)  |    h.flags    |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          h.sessionID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.transactionID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           h.packetID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.payload_length                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (n.context) OPTIONAL
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Octet String Length (L)                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet L - 1  |  Octet L      |       Padding (as required)   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (n.vb)
  ...
 An agentx-Notify-PDU contains the following fields:
    n.context
          An optional non-default context.
    n.vb
          A VarBindList whose contents define the actual PDU to be
          sent.  This memo places the following restrictions on its
          contents:
  1. If the subagent supplies sysUpTime.0, it must be

present as the first varbind.

Daniele, et al. Standards Track [Page 36] RFC 2741 AgentX January 2000

  1. snmpTrapOID.0 must be present, as the second varbind

if sysUpTime.0 was supplied, as the first if it was

                not.

6.2.11. The agentx-Ping-PDU

 The agentx-Ping-PDU is sent by a subagent to the master agent to
 monitor the master agent's ability to receive and send AgentX PDUs
 over their AgentX session.
  (AgentX header)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | h.version (1) |  h.type (13)  |    h.flags    |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          h.sessionID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.transactionID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           h.packetID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.payload_length                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (p.context) OPTIONAL
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Octet String Length (L)                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet L - 1  |  Octet L      |       Padding (as required)   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 An agentx-Ping-PDU may contain the following field:
    p.context
          An optional non-default context.
 Using p.context a subagent can retrieve the sysUpTime value for a
 specific context, if required.

6.2.12. The agentx-IndexAllocate-PDU

 An agentx-IndexAllocate-PDU is sent by a subagent to request
 allocation of a value for specific index objects.  Refer to section
 7.1.4.2, "Registering Stuff", for suggested usage.

Daniele, et al. Standards Track [Page 37] RFC 2741 AgentX January 2000

  (AgentX header)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | h.version (1) |  h.type (14)  |    h.flags    |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          h.sessionID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.transactionID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           h.packetID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.payload_length                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (i.context) OPTIONAL
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Octet String Length (L)                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet L - 1  |  Octet L      |       Padding (as required)   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (i.vb)
  ...
 An agentx-IndexAllocate-PDU contains the following fields:
    i.context
          An optional non-default context.
    i.vb
          A VarBindList containing the index names and values
          requested for allocation.

6.2.13. The agentx-IndexDeallocate-PDU

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

Daniele, et al. Standards Track [Page 38] RFC 2741 AgentX January 2000

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | h.version (1) |  h.type (15)  |    h.flags    |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          h.sessionID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.transactionID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           h.packetID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.payload_length                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (i.context) OPTIONAL
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Octet String Length (L)                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet L - 1  |  Octet L      |       Padding (as required)   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (i.vb)
  ...
 An agentx-IndexDeallocate-PDU contains the following fields:
    i.context
          An optional non-default context.
    i.vb
          A VarBindList containing the index names and values to be
          released.

6.2.14. The agentx-AddAgentCaps-PDU

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

Daniele, et al. Standards Track [Page 39] RFC 2741 AgentX January 2000

  (AgentX header)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | h.version (1) |  h.type (16)  |    h.flags    |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          h.sessionID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.transactionID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           h.packetID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.payload_length                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (a.context) (OPTIONAL)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Octet String Length (L)                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet L - 1  |  Octet L      |       Optional Padding        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (a.id)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  n_subid      |  prefix       |      0        |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #1                                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             sub-identifier #n_subid                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  (a.descr)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Octet String Length (L)                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet 1      |  Octet 2      |   Octet 3     |   Octet 4     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Octet L - 1  |  Octet L      |       Optional Padding        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Daniele, et al. Standards Track [Page 40] RFC 2741 AgentX January 2000

 An agentx-AddAgentCaps-PDU contains the following fields:
    a.context
          An optional non-default context.
    a.id
          An Object Identifier containing the value of an invocation
          of the AGENT-CAPABILITIES macro, which the master agent
          exports as a value of sysORID for the indicated context.
          (Recall that the value of an invocation of an AGENT-
          CAPABILITIES macro is an object identifier that describes a
          precise level of support with respect to implemented MIB
          modules.  A more complete discussion of the AGENT-
          CAPABILITIES macro and related sysORID values can be found
          in section 6 of STD 58, RFC 2580 [7].)
    a.descr
          An Octet String containing a DisplayString to be used as the
          value of sysORDescr corresponding to the sysORID value
          above.

6.2.15. The agentx-RemoveAgentCaps-PDU

 An agentx-RemoveAgentCaps-PDU is generated by a subagent to request
 that the master agent stop exporting a particular value of sysORID.
 This value must have previously been advertised by the subagent in an
 agentx-AddAgentCaps-PDU for this session.
  (AgentX header)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | h.version (1) |  h.type (17)  |    h.flags    |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          h.sessionID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.transactionID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           h.packetID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.payload_length                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Daniele, et al. Standards Track [Page 41] RFC 2741 AgentX January 2000

  (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.

Daniele, et al. Standards Track [Page 42] RFC 2741 AgentX January 2000

6.2.16. The agentx-Response-PDU

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | h.version (1) |  h.type (18)  |    h.flags    |  <reserved>   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          h.sessionID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.transactionID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           h.packetID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        h.payload_length                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        res.sysUpTime                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             res.error         |     res.index                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ...
 An agentx-Response-PDU contains the following fields:
     h.sessionID
          If this is a response to an agentx-Open-PDU, then it
          contains the new and unique sessionID (as assigned by the
          master agent) for this session.
          Otherwise it must be identical to the h.sessionID value in
          the PDU to which this PDU is a response.
    h.transactionID
          Must be identical to the h.transactionID value in the PDU to
          which this PDU is a response.
          In an agentx response PDU from the master agent to the
          subagent, the value of h.transactionID has no significance
          and can be ignored by the subagent.
    h.packetID
          Must be identical to the h.packetID value in the PDU to
          which this PDU is a response.

Daniele, et al. Standards Track [Page 43] RFC 2741 AgentX January 2000

    res.sysUpTime
          This field contains the current value of sysUpTime for the
          context indicated within the PDU to which this PDU is a
          response.   It is relevant only in agentx response PDUs sent
          from the master  agent to a subagent in response to the set
          of administrative PDUs listed in section 6.1, "AgentX PDU
          Header".
          In an agentx response PDU from the subagent to the master
          agent, the value of res.sysUpTime has no significance and is
          ignored by the master agent.
    res.error
          Indicates error status.  Within responses to the set of
          "administrative" PDU types listed in section 6.1, "AgentX
          PDU Header", values are limited to the following:
             noAgentXError              (0),
             openFailed                 (256),
             notOpen                    (257),
             indexWrongType             (258),
             indexAlreadyAllocated      (259),
             indexNoneAvailable         (260),
             indexNotAllocated          (261),
             unsupportedContext         (262),
             duplicateRegistration      (263),
             unknownRegistration        (264),
             unknownAgentCaps           (265),
             parseError                 (266),
             requestDenied              (267),
             processingError            (268)
          Within responses to the set of "SNMP request processing" PDU
          types listed in section 6.1, "AgentX PDU Header", values may
          also include those defined for errors in the SNMPv2 PDU (RFC
          1905 [13]).
    res.index
          In error cases, this is the index of the failed variable
          binding within a received request PDU.  (Note: As explained
          in section 5.4, "Value Representation", the index values of
          variable bindings within a variable binding list are 1-
          based.)

Daniele, et al. Standards Track [Page 44] RFC 2741 AgentX January 2000

 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:

Daniele, et al. Standards Track [Page 45] RFC 2741 AgentX January 2000

  1. PDU length (h.payload) too short to contain current

construct (Object Identifier header indicates more sub-

             identifiers, VarBind v.type indicates data follows, etc)
  1. An unrecognized value is encountered for h.type, v.type,

etc.

    3) Otherwise, if h.sessionID does not correspond to a currently
       established session with this subagent, res.error is set to
       `notOpen'.
    4) Otherwise, if the NON_DEFAULT_CONTEXT bit is set and the master
       agent does not support the indicated context, res.error is set
       to `unsupportedContext'.  If the master agent does support the
       indicated context, the value of res.sysUpTime is set to the
       value of sysUpTime.0 for that context.
    Note: Non-default contexts might be added on the fly by the master
          agent, or the master agent might require such non-default
          contexts to be pre-configured.  The choice is
          implementation-specific.
    5) If resources cannot be allocated or some other condition
       prevents processing, res.error is set to `processingError'.
    6) At this point, if res.error is not `noAgentXError', the
       received PDU is not processed further.  If the received PDU's
       header was successfully parsed, the AgentX-Response-PDU is sent
       in reply.  If the received PDU contained a VarBindList which
       was successfully parsed, the AgentX-Response-PDU contains the
       identical VarBindList.  If the received PDU's header was not
       successfully parsed or for some other reason the master agent
       cannot send a reply, processing is complete.

7.1.1. Processing the agentx-Open-PDU

 When the master agent receives an agentx-Open-PDU, it processes it as
 follows:
 1) An agentx-Response-PDU is created, res.sysUpTime is set to the
    value of sysUpTime.0 for the default context, res.error is set to
    `noAgentXError', and res.index is set to 0.
 2) If the received PDU cannot be parsed, res.error is set to
    `parseError'.
 3) Otherwise, if the master agent is unable to open an AgentX session
    for any reason, res.error is set to `openFailed'.

Daniele, et al. Standards Track [Page 46] RFC 2741 AgentX January 2000

 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:
  1. 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.
  1. The o.timeout value is used in calculating response timeout

conditions for this session. This field is also referenced in

       the AgentX MIB (a work-in-progress) by the agentxSessionTimeout
       object.
  1. The o.id and o.descr fields are used for informational

purposes. These two fields are also referenced in the AgentX

       MIB (a work-in-progress) by the agentxSessionObjectID object,
       and by the agentxSessionDescr object.
 5) The agentx-Response-PDU is sent with the res.error field
    indicating the result of the session initiation.
 If processing was successful, an AgentX session is considered
 established between the master agent and the subagent.  An AgentX
 session is a distinct channel for the exchange of AgentX protocol
 messages between a master agent and one subagent, qualified by the
 session-specific attributes listed in 4) above.  AgentX session
 establishment is initiated by the subagent.  An AgentX session can be
 terminated by either the master agent or the subagent.

7.1.2. Processing the agentx-IndexAllocate-PDU

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

Daniele, et al. Standards Track [Page 47] RFC 2741 AgentX January 2000

 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:
  1. v.name is the OID prefix of the MIB OBJECT-TYPE for which a

value is to be allocated.

  1. v.type is the syntax of the MIB OBJECT-TYPE for which a value is

to be allocated.

  1. v.data indicates the specific index value requested. If the

NEW_INDEX or the ANY_INDEX bit is set, the actual value in

       v.data is ignored and an appropriate index value is generated.
    a) If there are no currently allocated index values for v.name in
       the indicated context, and v.type does not correspond to a
       valid index type value, the VarBind fails and res.error is set
       to `indexWrongType'.
    b) If there are currently allocated index values for v.name in the
       indicated context, but the syntax of those values does not
       match v.type, the VarBind fails and res.error is set to
       `indexWrongType'.
    c) Otherwise, if both the NEW_INDEX and ANY_INDEX bits are clear,
       allocation of a specific index value is being requested.  If
       the requested index is already allocated for v.name in the
       indicated context, the VarBind fails and res.error is set to
       `indexAlreadyAllocated'.
    d) Otherwise, if the NEW_INDEX bit is set, the master agent should
       generate the next available index value for v.name in the
       indicated context, with the constraint that this value must not
       have been allocated (even if subsequently released) to any
       subagent since the last re-initialization of the master agent.
       If no such value can be generated, the VarBind fails and
       res.error is set to `indexNoneAvailable'.
    e) Otherwise, if the ANY_INDEX bit is set, the master agent should
       generate an index value for v.name in the indicated context,
       with the constraint that this value is not currently allocated
       to any subagent.  If no such value can be generated, then the
       VarBind fails and res.error is set to `indexNoneAvailable'.

Daniele, et al. Standards Track [Page 48] RFC 2741 AgentX January 2000

 2) If all VarBinds are processed successfully, the agentx-Response-
    PDU is sent in reply with res.error set to `noAgentXError'.  A
    VarBindList is included that is identical to the one sent in the
    agentx-IndexAllocate-PDU, except that VarBinds requesting a
    NEW_INDEX or ANY_INDEX value are generated with an appropriate
    value.
    See section 7.1.4.2, "Registering Stuff" for more information on
    how subagents should perform index allocations.

7.1.3. Processing the agentx-IndexDeallocate-PDU

 When the master agent receives an agentx-IndexDeallocate-PDU, it
 performs the common processing described in section 7.1, "Processing
 AgentX Administrative Messages".  If as a result res.error is
 `noAgentXError', processing continues as follows:
 1) Each VarBind in the VarBindList is processed until either all are
    successful, or one fails.  If any VarBind fails, the agentx-
    Response-PDU is sent in reply, containing the original
    VarBindList, with res.index set to indicate the failed VarBind,
    and with res.error set as described subsequently.  All other
    VarBinds are ignored; no index values are released.
    VarBinds are processed as follows:
  1. v.name is the name of the index for which a value is to be

released

  1. v.type is the syntax of the index object
  1. v.data indicates the specific index value to be released. The

NEW_INDEX and ANY_INDEX bits are ignored.

    a) If the index value for the named index is not currently
       allocated to this session, the VarBind fails and res.error is
       set to `indexNotAllocated'.
 2) If all VarBinds are processed successfully, res.error is set to
    `noAgentXError' and the agentx-Response-PDU is sent.  A
    VarBindList is included which is identical to the one sent in the
    agentx-IndexDeallocate-PDU.
 All released index values are now available, and may be used in
 response to subsequent allocation requests for ANY_INDEX values and
 in response to subsequent allocation requests for the particular
 index value.

Daniele, et al. Standards Track [Page 49] RFC 2741 AgentX January 2000

7.1.4. Processing the agentx-Register-PDU

 When the master agent receives an agentx-Register-PDU, it performs
 the common processing described in section 7.1, "Processing AgentX
 Administrative Messages".  If as a result res.error is
 `noAgentXError', processing continues as follows:
 If any of the union of subtrees defined by this MIB region is exactly
 the same as any subtree defined by a MIB region currently registered
 within the indicated context, those subtrees are termed "duplicate
 subtrees".
 If any of the union of subtrees defined by this MIB region overlaps,
 or is itself overlapped by, any subtree defined by a MIB region
 currently registered within the indicated context, those subtrees are
 termed "overlapping subtrees".
 1) If this registration would result in duplicate subtrees registered
    with the same value of r.priority, the request fails and an
    agentx-Response-PDU is returned with res.error set to
    `duplicateRegistration'.
 2) Otherwise, if the master agent does not wish to permit this
    registration for implementation-specific reasons, the request
    fails and an agentx-Response-PDU is returned with res.error set to
    `requestDenied'.
 3) Otherwise, the agentx-Response-PDU is returned with res.error set
    to `noAgentXError'.
    The master agent adds this MIB region to its registration data
    store for the indicated context, to be considered during the
    dispatching phase for subsequently received SNMP protocol
    messages.

7.1.4.1. Handling Duplicate and Overlapping Subtrees

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

Daniele, et al. Standards Track [Page 50] RFC 2741 AgentX January 2000

    1) Choose the one whose original agentx-Register-PDU r.subtree
       contained the most subids, i.e., the most specific r.subtree.
       Note: The presence or absence of a range subid has no bearing
       on how "specific" one object identifier is compared to another.
    2) If still ambiguous, there were duplicate subtrees.  Choose the
       one whose original agentx-Register-PDU specified the smaller
       value of r.priority.

7.1.4.2. Registering Stuff

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

7.1.4.2.1. Registration Priority

 The r.priority field in the agentx-Register-PDU is intended to be
 manipulated by human administrators to achieve a desired subagent
 configuration.  Typically this would be needed where a legacy
 application registers a specific subtree, and a different
 (configurable) application may need to become authoritative for the
 identical subtree.
 The result of this configuration (the same subtree registered on
 different sessions with different priorities) is that the session
 using the better priority (see section 7.1.4.1, "Handling Duplicate
 and Overlapping Subtrees") will be authoritative.  The other session
 will simply never be dispatched to.
 This is useful in the case described above, but is NOT useful in
 other cases, particularly when subagents share tables indexed by
 arbitrary values (see below).  In general, subagents should register
 using the default priority (127).

7.1.4.2.2. Index Allocation

 Index allocation is a service provided by an AgentX master agent.  It
 provides generic support for sharing MIB conceptual tables among
 subagents who are assumed to have no knowledge of each other.
 The master agent maintains a database of index objects (OIDs), and,
 for each index, the values that have been allocated for it.  It is
 unaware of what MIB variables (if any) the index objects represent.

Daniele, et al. Standards Track [Page 51] RFC 2741 AgentX January 2000

 By convention, subagents use the MIB variable listed in the INDEX
 clause as the index object for which values must be allocated.  For
 tables indexed by multiple variables, values may be allocated for
 each index (although this is frequently unnecessary; see example 2
 below).  The subagent may request allocation of
        a) a specific index value
        b) an index value that is not currently allocated
        c) an index value that has never been allocated
 The last two alternatives reflect the uniqueness and constancy
 requirements present in many MIB specifications for arbitrary integer
 indexes (e.g., ifIndex in the IF-MIB (RFC 2233 [19]),
 snmpFddiSMTIndex in the FDDI MIB (RFC 1285 [20]), or
 sysApplInstallPkgIndex in the System Application MIB (RFC 2287
 [21])).  The need for subagents to share tables using such indexes is
 the main motivation for index allocation in AgentX.
 It is important to note that index allocation and MIB region
 registration are not coupled in the master agent. The current state
 of index allocations is not considered when processing registration
 requests, and the current registry is not considered when processing
 index allocation requests.  (This is mainly to accommodate non-AgentX
 subagents.)
 AgentX subagents should follow the model of "first request allocation
 of an index, then register the corresponding region".  Then a
 successful index allocation request gives a subagent a good hint (but
 no guarantee) of what it should be able to register.  The
 registration may fail (with `duplicateRegistration') because some
 other subagent session has already registered that row of the table.
 The recommended mechanism for subagents to register conceptual rows
 in a shared table is
 1) Successfully allocate an index value.
 2) Use that value to fully qualify the MIB region(s), and attempt to
    register using the default priority.
 3) If the registration fails with `duplicateRegistration' deallocate
    the previously allocated index value(s) for this row and go to
    step 1).

Daniele, et al. Standards Track [Page 52] RFC 2741 AgentX January 2000

 Note that index allocation is necessary only when the index in
 question is an arbitrary value, and hence the subagent has no other
 reasonable way to determine which index values to use.  When index
 values have intrinsic meaning it is not expected that subagents will
 allocate their index values.
 For example, RFC 1514's table of running software processes
 (hrSWRunTable) is indexed by the system's native process identifier
 (pid).  A subagent implementing the row of hrSWRunTable corresponding
 to its own process would simply register the region defining that
 row's object instances without allocating index values.

7.1.4.2.3. Examples

 Example 1:
    A subagent implements an interface, and wishes to register a
    single row of the RFC 2233 ifTable.  It requests an allocation for
    the index object "ifIndex", for a value that has never been
    allocated (since ifIndex values must be unique).  The master agent
    returns the value "7".
    The subagent now attempts to register row 7 of ifTable, by
    specifying a MIB region in the agentx-Register-PDU of
    1.3.6.1.2.1.2.2.1.[1-22].7.  If the registration succeeds, no
    further processing is required.  The master agent will dispatch to
    this subagent correctly.
    If the registration failed with `duplicateRegistration', the
    subagent should deallocate the failed index, request allocation of
    a new index i, and attempt to register ifTable.[1-22].i, until
    successful.
 Example 2:
    This same subagent wishes to register ipNetToMediaTable rows
    corresponding to its interface (ifIndex i).  Due to the structure
    of this table, no further index allocation need be done.  The
    subagent can register the MIB region ipNetToMediaTable.[1-4].i, It
    is claiming responsibility for all rows of the table whose value
    of ipNetToMediaIfIndex is i.

Daniele, et al. Standards Track [Page 53] RFC 2741 AgentX January 2000

 Example 3:
    A network device consists of a set of processors, each of which
    accepts network connections for a unique set of IP addresses.
    Further, each processor contains a subagent that implements
    tcpConnTable.  In order to represent tcpConnTable for the entire
    managed device, the subagents need to share tcpConnTable.
    In this case, no index allocation need be done at all.  Each
    subagent can register a MIB region of tcpConnTable.[1-5].a.b.c.d,
    where a.b.c.d represents an unique IP address of the individual
    processor.
    Each subagent is claiming responsibility for the region of
    tcpConnTable where the value of tcpConnLocalAddress is a.b.c.d.
 Example 4:
    The Application Management MIB (RFC 2564 [22]) uses two objects to
    index several tables.  A partial description of them is:
    applSrvIndex     OBJECT-TYPE
           SYNTAX      Unsigned32 (1..'ffffffff'h)
           MAX-ACCESS  read-only
           STATUS      current
           DESCRIPTION
              "An applSrvIndex is the system-unique identifier
              of an instance of a service.  The value is unique
              not only across all instances of a given service,
              but also across all services in a system."
    applSrvName     OBJECT-TYPE
           SYNTAX     SnmpAdminString
           MAX-ACCESS read-only
           STATUS     current
           DESCRIPTION
              "The human-readable name of a service.  Where
              appropriate, as in the case where a service can
              be identified in terms of a single protocol, the
              strings should be established names such as those
              assigned by IANA and found in STD 2 [23], or
              defined by some other authority.  In some cases
              private conventions apply and the string should
              in these cases be consistent with these
              non-standard conventions. An applicability
              statement may specify the service name(s) to be
              used."

Daniele, et al. Standards Track [Page 54] RFC 2741 AgentX January 2000

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

7.1.5. Processing the agentx-Unregister-PDU

 When the master agent receives an agentx-Unregister-PDU, it performs
 the common processing described in section 7.1, "Processing AgentX
 Administrative Messages".  If as a result res.error is `
 noAgentXError', processing continues as follows:
 1) If u.subtree, u.priority, u.range_subid (and if u.range_subid is
    not 0, u.upper_bound), and the indicated context do not match an
    existing registration made during this session, the agentx-
    Response-PDU is returned with res.error set to `
    unknownRegistration'.
 2) Otherwise, the agentx-Response-PDU is sent in reply with res.error
    set to `noAgentXError', and the previous registration is removed
    from the registration data store.

7.1.6. Processing the agentx-AddAgentCaps-PDU

 When the master agent receives an agentx-AddAgentCaps-PDU, it
 performs the common processing described in section 7.1, "Processing
 AgentX Administrative Messages".  If as a result res.error is `
 noAgentXError', processing continues as follows:
 1) The master agent adds this agent capabilities information to the
    sysORTable for the indicated context.  An agentx-Response-PDU is
    sent in reply with res.error set to `noAgentXError'.

7.1.7. Processing the agentx-RemoveAgentCaps-PDU

 When the master agent receives an agentx-RemoveAgentCaps-PDU, it
 performs the common processing described in section 7.1, "Processing
 AgentX Administrative Messages".  If as a result res.error is
 `noAgentXError', processing continues as follows:
 1) If the combination of a.id and the optional a.context does not
    represent a sysORTable entry that was added by this subagent
    during this session, the agentx-Response-PDU is returned with
    res.error set to `unknownAgentCaps'.

Daniele, et al. Standards Track [Page 55] RFC 2741 AgentX January 2000

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

7.1.8. Processing the agentx-Close-PDU

 When the master agent receives an agentx-Close-PDU, it performs the
 common processing described in section 7.1, "Processing AgentX
 Administrative Messages", with the exception that step 4) is not
 performed since the agentx-Close-PDU does may not contain a context
 field. If as a result res.error is `noAgentXError', processing
 continues as follows:
 1) The master agent closes the AgentX session as described below, and
    sends in reply the agentx-Response-PDU with res.error set to
    `noAgentXError':
  1. All MIB regions that have been registered during this session

are unregistered, as described in section 7.1.5, "Processing

       the agentx-Unregister-PDU".
  1. All index values allocated during this session are freed, as

described in section 7.1.3, "Processing the agentx-

       IndexDeallocate-PDU".
  1. All sysORID values that were registered during this session are

removed, as described in section 7.1.7, "Processing the

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

7.1.9. Detecting Connection Loss

 If a master agent is able to detect (from the underlying transport)
 that a subagent cannot receive AgentX PDUs, it should close all
 affected AgentX sessions as described in section 7.1.8, "Processing
 the agentx-Close-PDU", step 1).

7.1.10. Processing the agentx-Notify-PDU

 A subagent sending SNMPv1 trap information must map this into
 (minimally) a value of snmpTrapOID.0, as described in 3.1.2 of RFC
 1908 [24].

Daniele, et al. Standards Track [Page 56] RFC 2741 AgentX January 2000

 When the master agent receives an agentx-Notify-PDU, it performs the
 common processing described in section 7.1, "Processing AgentX
 Administrative Messages".  If as a result res.error is
 `noAgentXError',  processing continues as follows:
 1) If the first VarBind is sysUpTime.0;
    (a)  if the second VarBind is not snmpTrapOID.0, res.error is set
         to `processingError' and res.index to 2
    (b)  otherwise these two VarBinds are used as the first two
         VarBinds within the generated notification.
 2) If the first VarBind is not sysUpTime.0;
    (a)  if the first VarBind is not snmpTrapOID.0, res.error is set
         to `processingError' and res.index to 1
    (b)  otherwise this VarBind is used for snmpTrapOID.0 within the
         generated notification, and the master agent uses the current
         value of sysUpTime.0 for the indicated context as sysUpTime.0
         within the notification.
 3) An agentx-Response-PDU is sent containing the original
    VarBindList, and with res.error and res.index set as described
    above.  If res.error is `noAgentXError', notifications are sent
    according to the implementation-specific configuration of the
    master agent.  If SNMPv1 Trap PDUs are generated, the recommended
    mapping is as described in RFC 2089 [25].  If res.error indicates
    an error in processing, no notifications are generated.
    Note that the master agent's successful response indicates the
    agentx-Notify-PDU was received and validated.  It does not
    indicate that any particular notifications were actually generated
    or received by notification targets.

7.1.11. Processing the agentx-Ping-PDU

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

Daniele, et al. Standards Track [Page 57] RFC 2741 AgentX January 2000

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

7.2. Processing Received SNMP Protocol Messages

 When an SNMP GetRequest, GetNextRequest, GetBulkRequest, or
 SetRequest protocol message is received by the master agent, the
 master agent applies its access control policy.
 In particular, for SNMPv1 or SNMPv2c protocol messages, the master
 agent applies the Elements of Procedure defined in section 4.1 of STD
 15, RFC 1157 [8] that apply to receiving entities.  For SNMPv3, the
 master agent applies an Access Control Model, possibly the View-based
 Access Control Model (see RFC 2575 [15]), as described in section
 3.1.2 and section 4.3 of RFC 2571 [1].
 For SNMPv1 and SNMPv2c, the master agent uses the community string as
 an index into a local repository of configuration information that
 may include community profiles or more complex context information.
 For SNMPv3, the master agent uses the SNMP Context (see section 3.3.1
 of RFC 2571 [1]) for these purposes.
 If application of the access control policy results in a valid SNMP
 request PDU, then an SNMP Response-PDU is constructed from
 information gathered in the exchange of AgentX PDUs between the
 master agent and one or more subagents.  Upon receipt and initial
 validation of an SNMP request PDU, a master agent uses the procedures
 described below to dispatch AgentX PDUs to the proper subagents,
 marshal the subagent responses, and construct an SNMP response PDU.

7.2.1. Dispatching AgentX PDUs

 Upon receipt and initial validation of an SNMP request PDU, a master
 agent uses the procedures described below to dispatch AgentX PDUs to
 the proper subagents.
 General Rules of Procedure
 While processing a particular SNMP request, the master agent may send
 one or more AgentX PDUs on one or more subagent sessions.  The
 following rules of procedure apply in general to the AgentX master
 agent.  PDU-specific rules are listed in the applicable sections.

Daniele, et al. Standards Track [Page 58] RFC 2741 AgentX January 2000

 1) Honoring the registry
    Because AgentX supports registration of duplicate and overlapping
    regions, it is possible for the master agent to obtain a value for
    a requested varbind from within multiple registered MIB regions.
    The master agent must ensure that the value (or exception)
    actually returned in the SNMP response PDU is taken from the
    authoritative region (as defined in section 7.1.4.1, "Handling
    Duplicate and Overlapping Subtrees").
 2) GetNext and GetBulk Processing
    The master agent may choose to send agentx-Get-PDUs while
    servicing an SNMP GetNextRequest-PDU.  The master agent may choose
    to send agentx-Get-PDUs or agentx-GetNext-PDUs while servicing an
    SNMP GetBulkRequest-PDU.  One possible reason for this would be if
    the current iteration has targeted instance-level registrations.
    The master agent may choose to "scope" the possible instances
    returned by a subagent by specifying an ending OID in the
    SearchRange.  If such scoping is used, typically the ending OID
    would be the first lexicographical successor to the target region
    that was registered on a session other than the target session.
    Regardless of this choice, rule (1) must be obeyed.
    The master agent may require multiple request-response iterations
    on the same subagent session, to determine the final value of all
    requested variables.
    All AgentX PDUs sent on the session while processing a given SNMP
    request must contain identical values of transactionID.  Each
    different SNMP request processed by the master agent must present
    a unique value of transactionID (within the limits of the 32-bit
    field) to the session.
 3) Number and order of variables sent per AgentX PDU
    For Get/GetNext/GetBulk operations, at any stage of the possibly
    iterative process, the master agent may need to dispatch several
    SearchRanges to a particular subagent session.  The master agent
    may send one, some, or all of the SearchRanges in a single AgentX
    PDU.
    The master agent must ensure that the correct contents and
    ordering of the VarBindList in the SNMP Response-PDU are
    maintained.

Daniele, et al. Standards Track [Page 59] RFC 2741 AgentX January 2000

    The following rules govern the number of VarBinds in a given
    AgentX PDU:
       a) The subagent must support processing of AgentX PDUs with
          multiple VarBinds.
       b) When processing an SNMP Set request, the master agent must
          send all of the VarBinds applicable to a particular subagent
          session in a single agentx-TestSet-PDU.
       c) When processing an SNMP Get, GetNext, or GetBulk request,
          the master agent may send a single AgentX PDU on the session
          with all applicable VarBinds, or multiple PDUs with single
          VarBinds, or something in between those extremes. The
          determination of which method to use in a particular case is
          implementation-specific.
 4) Timeout Values
    The master agent chooses a timeout value for each MIB region being
    queried, which is
       a) the value specified during registration of the MIB region,
          if it was non-zero
       b) otherwise, the value specified during establishment of the
          session in which this region was subsequently registered, if
          that value was non-zero
       c) otherwise, or, if the specified value is not practical, the
          master agent's implementaton-specific default value
    When an AgentX PDU that references multiple MIB regions is
    dispatched, the timeout value used for the PDU is the maximum
    value of the timeouts so determined for each of the referenced MIB
    regions.
 5) Context
    If the master agent has determined that a specific non-default
    context is associated with the SNMP request PDU, that context is
    encoded into the AgentX PDU's context field and the
    NON_DEFAULT_CONTEXT bit is set in h.flags.
    Otherwise, no context Octet String is added to the PDU, and the
    NON_DEFAULT_CONTEXT bit is cleared.

Daniele, et al. Standards Track [Page 60] RFC 2741 AgentX January 2000

7.2.1.1. agentx-Get-PDU

 Each variable binding in the SNMP request PDU is processed as
 follows:
 (1)  Identify the target MIB region.
      Within a lexicographically ordered set of registered MIB
      regions, valid for the indicated context, locate the
      authoritative region (according to section 7.1.4.1, "Handling
      Duplicate and Overlapping Subtrees") that contains the binding's
      name.
 (2)  If no such region exists, the variable binding is not processed
      further, and its value is set to `noSuchObject'.
 (3)  Identify the subagent session in which this region was
      registered, termed the target session.
 (4)  If this is the first variable binding to be dispatched over the
      target session in a request-response exchange entailed in the
      processing of this management request:
  1. Create an agentx-Get-PDU for this session, with the header

fields initialized as described above (see section 6.1,

          "AgentX PDU Header").
 (5)  Add a SearchRange to the end of the target session's PDU for
      this variable binding.
  1. The variable binding's name is encoded into the starting OID.
  1. The ending OID is encoded as null.

7.2.1.2. agentx-GetNext-PDU

 Each variable binding in the SNMP request PDU is processed as
 follows:
 (1)  Identify the target MIB region.
      Within a lexicographically ordered set of registered MIB
      regions, valid for the indicated context, locate the
      authoritative region (according to section 7.1.4.1, "Handling
      Duplicate and Overlapping Subtrees") that
      a) contains the variable binding's name and is not a fully
         qualified instance, or

Daniele, et al. Standards Track [Page 61] RFC 2741 AgentX January 2000

      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:
  1. Create an agentx-GetNext-PDU for the session, with the header

fields initialized as described above (see section 6.1,

         "AgentX PDU Header").
 (5)  Add a SearchRange to the end of the target session's agentx-
      GetNext-PDU for this variable binding.
  1. if (1a) applies, the variable binding's name is encoded into

the starting OID, and the OID's "include" field is set to 0.

  1. if (1b) applies, the target region's r.subtree is encoded

into the starting OID, and its "include" field is set to 1.

         (This is the recommended method.  An implementation may
         choose to use a Starting OID value that precedes r.subtree,
         in which case the include bit must be 0.  A starting OID
         value that succeeds r.subtree is not permitted.)
  1. the Ending OID for the SearchRange is encoded to be either

NULL, or a value that lexicographically succeeds the Starting

         OID.  This is an implementation-specific choice depending on
         how the master agent wishes to "scope" the possible returned
         instances.

7.2.1.3. agentx-GetBulk-PDU

 (Note: The outline of the following procedure is based closely on
 section 4.2.3, "The GetBulkRequest-PDU" of RFC 1905 [13].  Please
 refer to it for details on the format of the SNMP GetBulkRequest-PDU
 itself.)

Daniele, et al. Standards Track [Page 62] RFC 2741 AgentX January 2000

 Each variable binding in the request PDU is processed as follows:
 (1)  Identify the authoritative target region and target session,
      exactly as described for the agentx-GetNext-PDU (see section
      7.2.1.2, "agentx-GetNext-PDU").
 (2)  If this is the first variable binding to be dispatched over the
      target session in a request-response exchange entailed in the
      processing of this management request:
  1. Create an agentx-GetBulk-PDU for the session, with the header

fields initialized as described above (see section 6.1,

         "AgentX PDU Header").
 (3)  Add a SearchRange to the end of the target session's agentx-
      GetBulk-PDU for this variable binding, as described for the
      agentx-GetNext-PDU.  If the variable binding was a non-repeater
      in the original request PDU, it must be a non-repeater in the
      agentx-GetBulk-PDU.
 The value of g.max_repetitions in the agentx-GetBulk-PDU may be less
 than (but not greater than) the value in the original request PDU.
 The master agent may make such alterations due to simple sanity
 checking, optimizations for the current iteration based on the
 registry, the maximum possible size of a potential Response-PDU,
 known constraints of the AgentX transport, or any other
 implementation-specific constraint.

7.2.1.4. agentx-TestSet-PDU

 AgentX employs test-commit-undo-cleanup phases to achieve "as if
 simultaneous" semantics of the SNMP SetRequest-PDU within the
 extensible agent.  The initial phase involves the agentx-TestSet-PDU.
 Each variable binding in the SNMP request PDU is processed in order,
 as follows:
 (1)  Identify the target MIB region and target session exactly as
      described in section 7.2.1.1, "agentx-Get-PDU", step 1).
      Within a lexicographically ordered set of OID ranges, valid for
      the indicated context, locate the authoritative range that
      contains the variable binding's name.
 (2)  If no such target region exists, this variable binding fails
      with an error of `notWritable'.  Processing is complete for this
      request.

Daniele, et al. Standards Track [Page 63] RFC 2741 AgentX January 2000

 (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:
  1. create an agentx-TestSet-PDU for the session, with the header

fields initialized as described above (see section 6.1,

         "AgentX PDU Header").
 (4)  Add a VarBind to the end of the target session's PDU for this
      variable binding, as described in section 5.4, "Value
      Representation".
 Note that all VarBinds applicable to a given session must be sent in
 a single agentx-TestSet-PDU.

7.2.1.5. Dispatch

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

7.2.2. Subagent Processing

 A subagent initially processes a received AgentX PDU as follows:
  1. 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.
  1. If the received PDU is any other type:
 1) an agentx-Response-PDU is created whose header fields are
    identical to the received request PDU except that h.type is set to
    Response, res.error to `noError', res.index to 0, and the
    VarBindList to null.
 2) If the received PDU cannot be parsed, res.error is set to
    `parseError'.

Daniele, et al. Standards Track [Page 64] RFC 2741 AgentX January 2000

 3) Otherwise, if h.sessionID does not correspond to a currently
    established session, res.error is set to `notOpen'.
 4) At this point, if res.error is not `noError', the received PDU is
    not processed further.  If the received PDU's header was
    successfully parsed, the AgentX-Response-PDU is sent in reply.  If
    the received PDU's header was not successfully parsed or for some
    other reason the subagent cannot send a reply, processing is
    complete.

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

 A conformant AgentX subagent must support the agentx-Get, -GetNext,
 and -GetBulk PDUs, and must support multiple variables being supplied
 in each PDU.
 When a subagent receives an agentx-Get-, GetNext-, or GetBulk-PDU, it
 performs the indicated management operations and returns an agentx-
 Response-PDU.
 Each SearchRange in the request PDU's SearchRangeList is processed as
 described below, and a VarBind is added in the corresponding location
 of the agentx-Response-PDU's  VarbindList.  If processing should fail
 for any reason not described below, res.error is set to `genErr',
 res.index to the index of the failed SearchRange, the VarBindList is
 reset to null, and this agentx-Response-PDU is returned to the master
 agent.

7.2.3.1. Subagent Processing of the agentx-Get-PDU

 Upon the subagent's receipt of an agentx-Get-PDU, each SearchRange in
 the request is processed as follows:
 (1)  The starting OID is copied to v.name.
 (2)  If the starting OID exactly matches the name of a variable
      instantiated by this subagent within the indicated context and
      session, v.type and v.data are encoded to represent the
      variable's syntax and value, as described in section 5.4, "Value
      Representation".
 (3)  Otherwise, if the starting OID does not match the object
      identifier prefix of any variable instantiated within the
      indicated context and session, the VarBind is set to
      `noSuchObject', in the manner described in section 5.4, "Value
      Representation".

Daniele, et al. Standards Track [Page 65] RFC 2741 AgentX January 2000

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

7.2.3.2. Subagent Processing of the agentx-GetNext-PDU

 Upon the subagent's receipt of an agentx-GetNext-PDU, each
 SearchRange in the request is processed as follows:
 (1)  The subagent searches for a variable within the
      lexicographically ordered list of variable names for all
      variables it instantiates (without regard to registration of
      regions) within the indicated context and session, as follows:
  1. if the "include" field of the starting OID is 0, the

variable's name is the closest lexicographical successor to

         the starting OID.
  1. 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.
  1. 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

Daniele, et al. Standards Track [Page 66] RFC 2741 AgentX January 2000

 to M, and for each repeated SearchRange s, such that s is greater
 than zero and less than or equal to R, the (N+((i-1)*R)+s)-th VarBind
 is added to the agentx-Response-PDU as follows:
    1) The subagent searches for a variable within the
       lexicographically ordered list of variable names for all
       variables it instantiates (without regard to registration of
       regions) within the indicated context and session, for which
       the following are all true:
  1. 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.)
  1. 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
  1. For any iteration i, all s values of v.type are `

endOfMibView'.

  1. An AgentX transport constraint or other implementation-

specific constraint is reached.

7.2.4. Subagent Processing of agentx-TestSet, -CommitSet, -UndoSet,

  1. 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.

Daniele, et al. Standards Track [Page 67] RFC 2741 AgentX January 2000

 These four PDUs are used to collectively perform the indicated
 management operation.  An agentx-Response-PDU is sent in reply to
 each of the PDUs (except -CleanupSet), to inform the master agent of
 the state of the operation.
 The master agent must serialize Set transactions for each session.
 That is, a session need not handle multiple concurrent Set
 transactions.
 These Response-PDUs do not contain a VarBindList.

7.2.4.1. Subagent Processing of the agentx-TestSet-PDU

 Upon the subagent's receipt of an agentx-TestSet-PDU, each VarBind in
 the PDU is validated until they are all successful, or until one
 fails, as described in section 4.2.5 of RFC 1905 [13]. The subagent
 validates variables with respect to the context and session indicated
 in the testSet-PDU.
 If each VarBind is successful, the subagent has a further
 responsibility to ensure the availability of all resources (memory,
 write access, etc.) required for successfully carrying out a
 subsequent agentx-CommitSet operation.  If this cannot be guaranteed,
 the subagent should set res.error to `resourceUnavailable'.  As a
 result of this validation step, an agentx-Response-PDU is sent in
 reply whose res.error field is set to one of the following SNMPv2 PDU
 error-status values (see section 3, "Definitions", in RFC 1905 [13]):
          noError                    (0),
          genErr                     (5),
          noAccess                   (6),
          wrongType                  (7),
          wrongLength                (8),
          wrongEncoding              (9),
          wrongValue                (10),
          noCreation                (11),
          inconsistentValue         (12),
          resourceUnavailable       (13),
          notWritable               (17),
          inconsistentName          (18)
 If this value is not `noError', the res.index field must be set to
 the index of the VarBind for which validation failed.

Daniele, et al. Standards Track [Page 68] RFC 2741 AgentX January 2000

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

7.2.4.2. Subagent Processing of the agentx-CommitSet-PDU

 The agentx-CommitSet-PDU indicates that the subagent should actually
 perform (as described in the post-validation sections of 4.2.5 of RFC
 1905 [13]) the management operation indicated by the previous
 TestSet-PDU.  After carrying out the management operation, the
 subagent sends in reply an agentx-Response-PDU whose res.error field
 is set to one of the following SNMPv2 PDU error-status values (see
 section 3, "Definitions", in RFC 1905 [13]):
          noError                    (0),
          commitFailed              (14)
 If this value is `commitFailed', the res.index field must be set to
 the index of the VarBind (as it occurred in the agentx-TestSet-PDU)
 for which the operation failed.  Otherwise res.index is set to 0.

7.2.4.3. Subagent Processing of the agentx-UndoSet-PDU

 The agentx-UndoSet-PDU indicates that the subagent should undo the
 management operation requested in a preceding CommitSet-PDU.  The
 undo process is as described in section 4.2.5 of RFC 1905 [13].
 After carrying out the undo process, the subagent sends in reply an
 agentx-Response-PDU whose res.error field is set to one of the
 following SNMPv2 PDU error-status values (see section 3,
 "Definitions", in RFC 1905 [13]):
          noError                    (0),
          undoFailed                (15)
 If this value is `undoFailed', the res.index field must be set to the
 index of the VarBind (as it occurred in the agentx-TestSet-PDU) for
 which the operation failed.  Otherwise res.index is set to 0.
 This PDU also signals the end of processing of the management
 operation initiated by the previous TestSet-PDU.  The subagent should
 release resources, etc. as described in section 7.2.4.4, "Subagent
 Processing of the agentx-CleanupSet-PDU".

Daniele, et al. Standards Track [Page 69] RFC 2741 AgentX January 2000

7.2.4.4. Subagent Processing of the agentx-CleanupSet-PDU

 The agentx-CleanupSet-PDU signals the end of processing of the
 management operation requested in the previous TestSet-PDU.  This is
 an indication to the subagent that it may now release any resources
 it may have reserved in order to carry out the management request.
 No response is sent by the subagent.

7.2.5. Master Agent Processing of AgentX Responses

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

7.2.5.1. Common Processing of All AgentX Response PDUs

 1) If a response is not received on a session within the timeout
    interval for this dispatch, it is treated as if the subagent had
    returned `genErr' and processed as described below.
    A timeout may be due to a variety of reasons, and does not
    necessarily denote a failed or malfunctioning subagent.  As such,
    the master agent's response to a subagent timeout is
    implementation-specific, but with the following constraint:
    A session that times out on three consecutive AgentX requests is
    considered unable to respond, and the master agent must close the
    AgentX session as described in section 7.1.8, "Processing the
    agentx-Close-PDU", step (2).
 2) Otherwise, the h.packetID, h.sessionID, and h.transactionID fields
    of the AgentX response PDU are used to correlate subagent
    responses.  If the response does not pertain to this SNMP
    operation, it is ignored.
 3) Otherwise, the responses are processed jointly to form the SNMP
    response PDU.

7.2.5.2. Processing of Responses to agentx-Get-PDUs

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

Daniele, et al. Standards Track [Page 70] RFC 2741 AgentX January 2000

 1) For any received AgentX response PDU, if res.error is not
    `noError', the SNMP response PDU's error code is set to this
    value.  If res.error contains an AgentX specific value (e.g.
    `parseError'), the SNMP response PDU's error code is set to a
    value of genErr instead.  Also, the SNMP response PDU's error
    index is set to the index of the variable binding corresponding to
    the failed VarBind in the subagent's AgentX response PDU.
    All other AgentX response PDUs received due to processing this
    SNMP request are ignored.  Processing is complete; the SNMP
    Response PDU is ready to be sent (see section 7.2.6, "Sending the
    SNMP Response-PDU").
 2) Otherwise, the content of each VarBind in the AgentX response PDU
    is used to update the corresponding variable binding in the SNMP
    Response-PDU.

7.2.5.3. Processing of Responses to agentx-GetNext-PDU and

              agentx-GetBulk-PDU
 After common processing of the subagent's response to an agentx-
 GetNext-PDU or agentx-GetBulk-PDU (see section 7.2.5.1, "Common
 Processing of All AgentX Response PDUs", above), processing continues
 with the following steps:
 1) For any received AgentX response PDU, if res.error is not
    `noError', the SNMP response PDU's error code is set to this
    value.  If res.error contains an AgentX specific value (e.g.
    `parseError'), the SNMP response PDU's error code is set to a
    value of genErr instead.  Also, the SNMP response PDU's error
    index is set to the index of the variable binding corresponding to
    the failed VarBind in the subagent's AgentX response PDU.
    All other AgentX response PDUs received due to processing this
    SNMP request are ignored.  Processing is complete; the SNMP
    response PDU is ready to be sent (see section 7.2.6, "Sending the
    SNMP Response-PDU").
 2) Otherwise, the content of each VarBind in the AgentX response PDU
    is used to update the corresponding VarBind in the SNMP response
    PDU.
 After all expected AgentX response PDUs have been processed, if any
 VarBinds still contain the value `endOfMibView' in their v.type
 fields, processing must continue:

Daniele, et al. Standards Track [Page 71] RFC 2741 AgentX January 2000

 3) A new iteration of AgentX request dispatching is initiated (as
    described in section 7.2.1.2, "agentx-GetNext-PDU"), in which only
    those VarBinds whose v.type is `endOfMibView' are processed.
 4) For each such VarBind, an authoritative target MIB region is
    identified in which the master agent expects to find suitable MIB
    variables.  The target session is the one on which this new target
    region was registered.
    The starting OID in each SearchRange is set to the value of v.name
    for the corresponding VarBind, and its "include" field is set to
    0.
 5) The value of transactionID must be identical to the value used
    during the previous iteration.
 6) The AgentX PDUs are sent on the target session(s), and the
    responses are received and processed according to the steps
    described in section 7.2.5, "Master Agent Processing of AgentX
    Responses".
 7) This process continues iteratively until a complete SNMP
    Response-PDU has been built, or until there remain no
    authoritative MIB regions to query.
 Note that r.subtree for the new target region identified in step 4)
 may not lexicographically succeed r.subtree for the region that has
 returned `endOfMibView'.  For example, consider the following
 registry:
      session A   `mib-2' (1.3.6.1.2.1)
      session B   `ip'    (1.3.6.1.2.1.4)
      session C   `tcp'   (1.3.6.1.2.1.6)
 If while processing a GetNext-Request-PDU session B returns
 `endOfMibView' for a variable name within 1.3.6.1.2.1.4, the target
 MIB region identified in step 4) would be 1.3.6.1.2.1 (since it may
 contain variables whose names precede 1.3.6.1.2.1.6).
 Note also that if session A returned variables from within
 1.3.6.1.2.1.6, they must be discarded since session A is NOT
 authoritative for that region.

7.2.5.4. Processing of Responses to agentx-TestSet-PDUs

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

Daniele, et al. Standards Track [Page 72] RFC 2741 AgentX January 2000

 exchange of AgentX PDUs.  The value of h.transactionID in the
 agentx-CommitSet, -UndoSet, and -CleanupSet-PDUs must be identical to
 the value sent in the testSet-PDU.
 The state transitions and PDU sequences are depicted in section 7.3,
 "State Transitions".
 The set of all sessions who have been sent an agentx-TestSet-PDU for
 this particular transaction are referred to below as "involved
 sessions".
 1) If any target session's response is not `noError', all other
    agentx-Response-PDUs received due to processing this SNMP request
    are ignored.
    An agentx-CleanupSet-PDU is sent to all involved sessions.
    Processing is complete; the SNMP response PDU is constructed as
    described below in 7.2.6, "Sending the SNMP Response-PDU".
 2) Otherwise an agentx-CommitSet-PDU is sent to all involved
    sessions.

7.2.5.5. Processing of Responses to agentx-CommitSet-PDUs

 After common processing of the subagent's response to an agentx-
 CommitSet-PDU (see section 7.2.5.1, "Common Processing of All AgentX
 Response PDUs", above), processing continues with the following
 steps:
 1) If any response is not `noError', the SNMP response PDU's error
    code is set to this value.  If res.error contains an AgentX
    specific value (e.g. `parseError'), the SNMP response PDU's error
    code is set to a value of genErr instead.  Also, the SNMP response
    PDU's error index is set to the index of the VarBind corresponding
    to the failed VarBind in the agentx-TestSet-PDU.
    An agentx-UndoSet-PDU is sent to each target session that has been
    sent an agentx-CommitSet-PDU.  An agentx-CleanupSet-PDU is sent to
    the remainder of the involved sessions.
 2) Otherwise an agentx-CleanupSet-PDU is sent to all involved
    sessions.  Processing is complete; the SNMP response PDU is
    constructed as described below in section 7.2.6, "Sending the SNMP
    Response-PDU".

Daniele, et al. Standards Track [Page 73] RFC 2741 AgentX January 2000

7.2.5.6. Processing of Responses to agentx-UndoSet-PDUs

 After common processing of the subagent's response to an agentx-
 UndoSet-PDU (see section 7.2.5.1, "Common Processing of All AgentX
 Response PDUs", above), processing continues with the following
 steps:
 1) If any response is `undoFailed' the SNMP response PDU's error code
    is set to this value.  Also, the SNMP response PDU's error index
    is set to 0.
 2) Otherwise, if any response is not `noError' the SNMP response
    PDU's error code is set to this value.  Also, the SNMP response
    PDU's error index is set to the index of the VarBind corresponding
    to the failed VarBind in the agentx-TestSet-PDU. If res.error is
    an AgentX specific value (e.g. `parseError'), the SNMP response
    PDU's error code is set to a value of genErr instead.
 3) Otherwise the SNMP response PDU's error code and error index were
    set in section 7.2.5.5 step 1)

7.2.6. Sending the SNMP Response-PDU

 Once the processing described in section 7.2.5, "Master Agent
 Processing of AgentX Responses" is complete, there is an SNMP
 response PDU available.  The master agent now implements the Elements
 of Procedure for the applicable version of the SNMP protocol in order
 to encapsulate the PDU into a message, and transmit it to the
 originator of the SNMP management request.  Note that this may
 involve altering the PDU contents (for instance, to replace the
 original VarBinds if an error condition is to be returned).
 The response PDU may also be altered in order to support the SNMPv1
 PDU.  In such cases the required PDU mapping is that defined in RFC
 2089 [25].  (Note in particular that the rules for handling Counter64
 syntax may require re-sending AgentX GetBulk or GetNext PDUs until a
 VarBind of suitable syntax is returned.)

7.2.7. MIB Views

 AgentX subagents are not aware of MIB views, since view information
 is not contained in AgentX PDUs.
 As stated above, the descriptions of procedures in section 7,
 "Elements of Procedure", of this memo are not intended to constrain
 the internal architecture of any conformant implementation.  In
 particular, the master agent procedures described in section 7.2.1,

Daniele, et al. Standards Track [Page 74] RFC 2741 AgentX January 2000

 "Dispatching AgentX PDUs" and in section 7.2.5, "Master Agent
 Processing of AgentX Responses" may be altered so as to optimize
 AgentX exchanges when implementing MIB views.
 Such optimizations are beyond the scope of this memo.  But note that
 section 7.2.3, "Subagent Processing of agentx-Get, GetNext, GetBulk-
 PDUs",  defines subagent behavior in such a way that alteration of
 SearchRanges may be used in such optimizations.

7.3. State Transitions

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

7.3.1. Set Transaction States

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

Daniele, et al. Standards Track [Page 75] RFC 2741 AgentX January 2000

                                     STATE
          +---------------+--------------+---------+--------+--------
          |       A       |      B       |   C     |   D    |   E
          |   (Initial    |    TestOK    | Commit  | Test   | Commit
          |     State)    |              |  OK     | Fail   |  Fail
          |               |              |         |        |
  EVENT   |               |              |         |        |
 ---------+---------------+--------------+---------+--------+--------
          | 7.2.4.1       |              |         |        |
 Receive  | All varbinds  |              |         |        |
 TestSet  | OK?           |      X       |    X    |   X    |    X
 PDU      |   Yes ->B     |              |         |        |
          |   No  ->D     |              |         |        |
 ---------+---------------+--------------+---------+--------+--------
          |               |  7.2.4.2     |         |        |
 Receive  |               |  NoError?    |         |        |
 Commit-  |       X       |   Yes ->C    |    X    |   X    |    X
 Set PDU  |               |   No  ->E    |         |        |
 ---------+---------------+--------------+---------+--------+--------
 Receive  |               |              | 7.2.4.3 |        |7.2.4.3
 UndoSet  |       X       |       X      | ->done  |   X    | ->done
 PDU      |               |              |         |        |
 ---------+---------------+--------------+---------+--------+--------
 Receive  |               |  7.2.4.4     | 7.2.4.4 |7.2.4.4 |
 Cleanup- |       X       |   ->done     | ->done  | ->done |   X
 Set PDU  |               |              |         |        |
 ---------+---------------+--------------+---------+--------+--------
 Session  |               | rollback     | undo    |        |
 Loss     |  ->done       |  ->done      |  ->done | ->done | ->done
 ---------+---------------+--------------+---------+--------+--------
 There are three possible sequences that a subagent may follow for a
 particular set transaction:
    1) TestSet CommitSet CleanupSet
    2) TestSet CommitSet UndoSet
    3) TestSet           CleanupSet
 Note that a single PDU sequence may result in multiple paths through
 the finite state machine (FSM).  For example, the sequence
    TestSet CommitSet UndoSet
 may walk through either of these two state sequences:
    (initial) TestOK CommitOK   (done)
    (initial) TestOK CommitFail (done)

Daniele, et al. Standards Track [Page 76] RFC 2741 AgentX January 2000

7.3.2. Transport Connection States

 The following table presents, from the master agent's perspective,
 the state transitions involved in transport connection setup and
 teardown:
                  STATE
                 +--------------+--------------
                 |      A       |      B
                 | No transport |  Transport
                 |              |  connected
                 |              |
 EVENT           |              |
 ----------------+--------------+--------------
 Transport       |              |
 connect         |     ->B      |      X
 indication      |              |
 ----------------+--------------+--------------
 Receive         |              | if no resources
 Open-PDU        |              | available
                 |              | reject, else
                 |      X       | establish
                 |              | session
                 |              |
                 |              |     ->B
 ----------------+--------------+--------------
 Receive         |              | if matching
 Response-PDU    |              | session id,
                 |              | feed to that
                 |      X       | session's FSM
                 |              | else ignore
                 |              |
                 |              |     ->B
 ----------------+--------------+--------------
 Receive other   |              | if matching
 PDUs            |              | session id,
                 |              | feed to that
                 |      X       | session's FSM
                 |              | else reject
                 |              |
                 |              |     ->B
 ----------------+--------------+--------------
 Transport       |              |notify all
 disconnect      |              |sessions on
 indication      |      X       |this transport
                 |              |
                 |              |     ->A
 ----------------+--------------+--------------

Daniele, et al. Standards Track [Page 77] RFC 2741 AgentX January 2000

7.3.3. Session States

 The following table presents, from the master agent's perspective,
 the state transitions involved in session setup and teardown:
                            STATE
                +-------------+----------------
                |     A       |      B
                |  No session |  Session
                |             |  established
 EVENT          |             |
 ---------------+-------------+----------------
                |  7.1.1      |
 Receive        |             |      X
 Open PDU       |    ->B      |
 ---------------+-------------+----------------
                |             |  7.1.8
 Receive        |      X      |
 Close PDU      |             |    ->A
 ---------------+-------------+----------------
 Receive        |             |  7.1.4
 Register PDU   |      X      |
                |             |    ->B
 ---------------+-------------+----------------
 Receive        |             |  7.1.5
 Unregister     |      X      |
 PDU            |             |    ->B
 ---------------+-------------+----------------
 Receive        |             |
 Get PDU        |             |
 GetNext PDU    |             |
 GetBulk PDU    |      X      |       X
 TestSet PDU    |             |
 CommitSet PDU  |             |
 UndoSet PDU    |             |
 CleanupSet PDU |             |
 ---------------+-------------+----------------
 Receive        |             |  7.1.10
 Notify PDU     |      X      |
                |             |    ->B
 ---------------+-------------+----------------
 Receive Ping   |             |  7.1.11
 PDU            |      X      |
                |             |    ->B
 ---------------+-------------+----------------
 (continued next page)

Daniele, et al. Standards Track [Page 78] RFC 2741 AgentX January 2000

  1. ————–+————-+—————-

Receive | | 7.1.2

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

8. Transport Mappings

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

8.1. AgentX over TCP

8.1.1. Well-known Values

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

8.1.2. Operation

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

Daniele, et al. Standards Track [Page 79] RFC 2741 AgentX January 2000

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

8.2. AgentX over UNIX-domain Sockets

 Many (BSD-derived) implementations of the UNIX operating system
 support the UNIX pathname address family (AF_UNIX) for socket
 communications.  This provides a convenient method of sending and
 receiving data between processes on the same host.
 Mapping AgentX to this transport is useful for environments that
  1. wish to guarantee subagents are running on the same managed

node as the master agent, and where

  1. 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.

Daniele, et al. Standards Track [Page 80] RFC 2741 AgentX January 2000

9. Security Considerations

 This memo defines a protocol between two processing entities, one of
 which (the master agent) is assumed to perform authentication of
 received SNMP requests and to control access to management
 information.  The master agent performs these security operations
 independently of the other processing entity (the subagent).
 Security considerations require three questions to be answered:
    1. Is a particular subagent allowed to initiate a session with a
       particular master agent?
    2. During an AgentX session, is any SNMP security-related
       information (for example, community names) passed from the
       master agent to the subagent?
    3. During an AgentX session, what part of the MIB tree is this
       subagent allowed to register?
 The answer to the third question is: A subagent can register any
 subtree (subject to AgentX elements of procedure, section 7.1.4,
 "Processing the agentx-Register-PDU").  Currently there is no access
 control mechanism defined in AgentX. A concern here is that a
 malicious subagent that registers an unauthorized "sensitive"
 subtree, could see modification requests to those objects, or by
 giving its own clever answer to NMS queries, could cause the NMS to
 do something that leads to information disclosure or other damage.
 The answer to the second question is: No.
 Now we can answer the first question.  AgentX does not contain a
 mechanism for authorizing/refusing session initiations.  Thus,
 controlling subagent access to the master agent may only be done at a
 lower layer (e.g., transport).
 An AgentX subagent can connect to a master agent using either a
 network transport mechanism (e.g., TCP), or a "local" mechanism
 (e.g., shared memory, named pipes).
 In the case where a local transport mechanism is used and both
 subagent and master agent are running on the same host, connection
 authorization can be delegated to the operating system features.  The
 answer to the first security question then becomes: "If and only if
 the subagent has sufficient privileges, then the operating system
 will allow the connection".

Daniele, et al. Standards Track [Page 81] RFC 2741 AgentX January 2000

 If a network transport is used, currently there is no inherent
 security.  Transport Layer Security, SSL, or IPsec SHOULD be used to
 control and protect subagent connections in this mode of operation.
 However, we RECOMMEND that subagents always run on the same host as
 the master agent and that operating system features be used to ensure
 that only properly authorized subagents can establish connections to
 the master agent.

10. Acknowledgements

 The initial development of this memo was heavily influenced by the
 DPI 2.0 specification RFC 1592 [26].
 This document was produced by the IETF Agent Extensibility (AgentX)
 Working Group, and benefited especially from the contributions of the
 following working group members:
    David Battle, Uri Blumenthal, Jeff Case, Maria Greene, Lauren
    Heintz, Dave Keeney, Harmen van der Linde, Bob Natale, Aleksey
    Romanov, Don Ryan, and Juergen Schoenwaelder.
 An honorable mention is extended to Randy Presuhn in recognition for
 his numerous technical contributions to this specification; for his
 many answers provided on (and hosting of) the AgentX e-mail list and
 ftp site, and, for the valued support and guidance Randy provided to
 the Working Group chair.
 The AgentX Working Group is chaired by:
 Bob Natale
 ACE*COMM Corporation
 704 Quince Orchard Road
 Gaithersburg, MD  20878
 Phone: +1-301-721-3000
 Fax:   +1-301-721-3001
 EMail: bnatale@acecomm.com

Daniele, et al. Standards Track [Page 82] RFC 2741 AgentX January 2000

11. Authors' and Editor's Addresses

 Mike Daniele
 Compaq Computer Corporation
 110 Spit Brook Rd
 Nashua, NH 03062
 Phone: +1-603-881-1423
 EMail: daniele@zk3.dec.com
 Bert Wijnen
 IBM T.J.Watson Research
 Schagen 33
 3461 GL Linschoten
 Netherlands
 Phone: +31-348-432-794
 EMail: wijnen@vnet.ibm.com
 Mark Ellison (WG editor)
 Ellison Software Consulting, Inc.
 38 Salem Road
 Atkinson, NH  03811
 Phone: +1-603-362-9270
 EMail: ellison@world.std.com
 Dale Francisco (editor)
 Cisco Systems
 150 Castilian Dr
 Goleta CA 93117
 Phone: +1-805-961-3642
 Fax:   +1-805-961-3600
 EMail: dfrancis@cisco.com

Daniele, et al. Standards Track [Page 83] RFC 2741 AgentX January 2000

12. References

 [1]   Harrington, D., Presuhn, R. and B. Wijnen, "An Architecture for
       Describing SNMP Management Frameworks", RFC 2571, April 1999.
 [2]   Rose, M. and K. McCloghrie, "Structure and Identification of
       Management Information for TCP/IP-based Internets", STD 16, RFC
       1155, May 1990.
 [3]   Rose, M. and K. McCloghrie, "Concise MIB Definitions", STD 16,
       RFC 1212, March 1991.
 [4]   Rose, M., "A Convention for Defining Traps for use with the
       SNMP", RFC 1215, March 1991.
 [5]   McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
       M. and S. Waldbusser, "Structure of Management Information
       Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
 [6]   McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
       M. and S. Waldbusser, "Textual Conventions for SMIv2", STD 58,
       RFC 2579, April 1999.
 [7]   McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
       M. and S. Waldbusser, "Conformance Statements for SMIv2", STD
       58, RFC 2580, April 1999.
 [8]   Case, J., Fedor, M., Schoffstall, M. and J. Davin, "Simple
       Network Management Protocol", STD 15, RFC 1157, May 1990.
 [9]   Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
       "Introduction to Community-based SNMPv2", RFC 1901, January
       1996.
 [10]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
       "Transport Mappings for Version 2 of the Simple Network
       Management Protocol (SNMPv2)", RFC 1906, January 1996.
 [11]  Case, J., Harrington D., Presuhn R. and B. Wijnen, "Message
       Processing and Dispatching for the Simple Network Management
       Protocol (SNMP)", RFC 2572, April 1999.
 [12]  Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
       for version 3 of the Simple Network Management Protocol
       (SNMPv3)", RFC 2574, April 1999.

Daniele, et al. Standards Track [Page 84] RFC 2741 AgentX January 2000

 [13]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Protocol
       Operations for Version 2 of the Simple Network Management
       Protocol (SNMPv2)", RFC 1905, January 1996.
 [14]  Levi, D., Meyer, P. and B. Stewart, "SNMPv3 Applications", RFC
       2573, April 1999.
 [15]  Wijnen, B., Presuhn, R. and K. McCloghrie, "View-based Access
       Control Model (VACM) for the Simple Network Management Protocol
       (SNMP)", RFC 2575, April 1999.
 [16]  Case, J., Mundy, R., Partain, D. and B. Stewart, "Introduction
       to Version 3 of the Internet-standard Network Management
       Framework", RFC 2570, April 1999.
 [17]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
       "Management Information Base for Version 2 of the Simple
       Network Management Protocol (SNMPv2)", RFC 1907, January 1996.
 [18]  Information processing systems - Open Systems Interconnection -
       Specification of Abstract Syntax Notation One (ASN.1),
       International Organization for Standardization.  International
       Standard 8824, (December, 1987).
 [19]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB
       using SMIv2", RFC 2233, November 1997.
 [20]  Case, J., "FDDI Management Information Base", RFC 1285, January
       1992.
 [21]  Krupczak, C. and J. Saperia, "Definitions of System-Level
       Managed Objects for Applications", RFC 2287, April 1997.
 [22]  Kalbfleisch, C., Krupczak, C., Presuhn, R. and J. Saperia,
       "Application Management MIB", RFC 2564, May 1999.
 [23]  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
       1700, October 1994.
 [24]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
       "Coexistence between Version 1 and Version 2 of the Internet-
       standard Network Management Framework", RFC 1908, January 1996.
 [25]  Wijnen, B. and D. Levi, "V2ToV1: Mapping SNMPv2 onto SNMPv1
       Within a Bilingual SNMP Agent", RFC 2089, January 1997.

Daniele, et al. Standards Track [Page 85] RFC 2741 AgentX January 2000

 [26]  Wijnen, B., Carpenter, G., Curran, K., Sehgal, A. and G.
       Waters, "Simple Network Management Protocol: Distributed
       Protocol Interface, Version 2.0", RFC 1592, March 1994.
 [27]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.

13. Notices

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

Daniele, et al. Standards Track [Page 86] RFC 2741 AgentX January 2000

A. Changes relative to RFC 2257

 Changes on the wire:
  1. The agentx-Notify-PDU and agentx-Close-PDU now generate an

agentx-Response-PDU.

  1. The res.error field may contain three new error codes:

parseFailed(266), requestDenied(267), and processingError(268).

 Clarifications to the text of the memo:
  1. Modified the text of step (4) in section 4.2, "Applicability"

to separate the two concerns of row creation, and counters that

       count rows.
  1. The use of the r.range_subid field is more clearly defined in

section 6.2.3, "The agentx-Register-PDU".

  1. Default priority (127) for registration added to the

description of r.priority in section 6.2.3, "The agentx-

       Register-PDU".
  1. Made the distinction of "administrative processing" PDUs and

"SNMP request processing" PDUs in section 6.1, "AgentX PDU

       Header" description of h.type.  This distinction is used in the
       Elements of Procedure relative to the res.sysuptime and
       res.error fields.
  1. Rewrote portions of text in section 6.2.3, "The agentx-

Register-PDU" to be more explicit about the following points:

  1. There is a default registration priority of 127.
  2. Improved the description of r.range_subid, independent of

the prefix in r.region.

  1. Improved description and examples of how to use the

registration mechanism.

  1. Added a description for r.upper_bound.
  2. changed r.region to r.subtree (because the text used the

terms "region", "range", and "OID range" in too loose a

             fashion.  r.subtree can not represent anything more by
             itself than a simple subtree.  In conjunction with
             r.range_subid and r.upper_bound, it can represent a
             "region", that is, a union of subtrees)
  1. Modified the text in section 6.2.4, "The agentx-Unregister-PDU" to

include a description of u.range_subid and u.upper_bound

Daniele, et al. Standards Track [Page 87] RFC 2741 AgentX January 2000

  1. Added use of the `requestDenied' error code in section 7.1.4,

"Processing the agentx-Register-PDU".

  1. Removed text in section 7, "Elements of Procedure" on parse errors

and protocol errors.

  1. Added a new section, 7.1, "Processing AgentX Administrative

Messages" which defines common processing and how to use the

    `parseError' and `processingError' instead of closing a session,
    and how to handle context.
  1. Removed the common processing text from the other administrative

processing Elements of Procedure sections, and added a reference

    to section 7.1, "Processing AgentX Administrative Messages".  The
    affected sections are:
  1. 7.1.2, "Processing the agentx-IndexAllocate-PDU"
  2. 7.1.3, "Processing the agentx-IndexDeallocate-PDU"
  3. 7.1.4, "Processing the agentx-Register-PDU"
  4. 7.1.5, "Processing the agentx-Unregister-PDU"
  5. 7.1.6, "Processing the agentx-AddAgentCaps-PDU"
  6. 7.1.7, "Processing the agentx-RemoveAgentCaps-PDU"
  7. 7.1.8, "Processing the agentx-Close-PDU"
  8. 7.1.10, "Processing the agentx-Notify-PDU"
  9. 7.1.11, "Processing the agentx-Ping-PDU"
  1. Reworked the text in section 7.1.1, "Processing the

agentx-Open-PDU" to include new error codes, and, to eliminate

    reference to an indicated context.
  1. Modified the text in Section 7.1.10, "Processing the

agentx-Notify-PDU" to state that context checking is performed.

  1. Substantially modified the text in section 7.1.4.1, "Handling

Duplicate and Overlapping Subtrees".

  1. Removed the section on "Using the agentx-IndexAllocate-PDU" and

added section 7.1.4.2, "Registering Stuff". This change is

    intended to provide a more concise and a more cohesive
    description of how things are supposed to work.
  1. Modified the test in section 7.1.5, "Processing the

agentx-Unregister-PDU" to require a match on u.range_subid and

    on u.upper_bound when these fields were applicable in the
    corresponding agentx-Register-PDU.

Daniele, et al. Standards Track [Page 88] RFC 2741 AgentX January 2000

  1. Removed all references to "splitting", and all uses of the term

"OID range". The text now refers to regions or subtrees

    directly, and relies on rule (1), "Honoring the Registry", in
    section 7.2.1, "Dispatching AgentX PDUs".
  1. Modified text in clause 4© of section 7.2.1, "Dispatching

AgentX PDUs", clarifying that the master agent can use its

    implementation-specific default timeout value when the timeout
    value registered by the subagent is impractical.
  1. Added text in section 7.2.2, "Subagent Processing" describing

common processing.

  1. Added an example to the text in section 7.2.5.3, "Processing of

Responses to agentx-GetNext-PDU and agentx-GetBulk-PDU",

    and, removed the definition of "contains" from this section.
  1. Modified text in step (1) of section 7.2.5.5, "Processing of

Responses to agentx-CommitSet-PDUs", eliminating directive for

    master agent to ignore additional responses to
    agentx-CommitSet-PDUs after the first error response.
  1. Modified text in section 7.2.5.6, "Processing of Responses to

agentx-UndoSet-PDUs", cleaning up commit/undo elements of

    procedure per feedback received on the AgentX email list.
  1. Modified the text in section 8.1.2, "Operation" to explicitly

prohibit interleaved sends, and, added a caution about

    exchanging AgentX messages via TCP.
  1. Modified text to be more explicit that the OID in the

agentx-Allocate-PDU is an OBJECT-TYPE and does not contain any

    instance sub-identifiers.
  1. Replaced the term "subagent" with the term "session" in many

places throughout the text.

  1. Modified the text relative to master agent processing of the

agentx-TestSet-PDU, agentx-CommitSet-PDU, and the

    agentx-UndoSet-PDU to explicitly state that only "involved"
    sessions receive an agentx-CommitSet-PDU, and possibly, an
    agentx-UndoSet-PDU.
  1. Modified the text to use the term "transaction", instead of

"packet" (and others), where appropriate. This helps

    distinguish the overall transaction from a particular sequence
    of packets or PDUs.

Daniele, et al. Standards Track [Page 89] RFC 2741 AgentX January 2000

  1. Modified the text to explicitly state that a session is not

required to support concurrent sets.

  1. Added section 13, "Notices".
  1. Added text to section 1, Introduction, relative to BCP 14 key

words.

  1. Modified text to section 9, Security Considerations, to include

use of BCP 14 key words.

  1. Modified text to section 9, Security Considerations, to include

IPSEC as a suggested Transport Layer Security.

Daniele, et al. Standards Track [Page 90] RFC 2741 AgentX January 2000

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Daniele, et al. Standards Track [Page 91]

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