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RFC 2741

Agent Extensibility (AgentX) Protocol Version 1

Pages: 91
Draft Standard
Obsoletes:  2257
Part 1 of 4 – Pages 1 to 17
None   None   Next

Top   ToC   RFC2741 - Page 1
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
Top   ToC   RFC2741 - Page 2
   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
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         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
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1. Introduction

This memo defines a standardized framework for extensible SNMP agents. It defines processing entities called master agents and subagents, a protocol (AgentX) used to communicate between them, and the elements of procedure by which the extensible agent processes SNMP protocol messages. This memo obsoletes RFC 2257. It is worth noting that most of the changes are for the purpose of clarification. The only changes affecting AgentX protocol messages on the wire are: - The agentx-Notify-PDU and agentx-Close-PDU now generate an agentx-Response-PDU - Three new error codes are available: parseFailed(266), requestDenied(267), and processingError(268) Appendix A provides a detailed list of changes relative to RFC 2257. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [27].

2. The SNMP Management Framework

The SNMP Management Framework presently consists of five major components: An overall architecture, described in RFC 2571 [1]. Mechanisms for describing and naming objects and events for the purpose of management. The first version of this Structure of Management Information (SMI) is called SMIv1 and described in STD 16, RFC 1155 [2], STD 16, RFC 1212 [3] and RFC 1215 [4]. The second version, called SMIv2, is described in STD 58, RFC 2578 [5], STD 58, RFC 2579 [6] and STD 58, RFC 2580 [7]. Message protocols for transferring management information. The first version of the SNMP message protocol is called SNMPv1 and described in STD 15, RFC 1157 [8]. A second version of the SNMP message protocol, which is not an Internet standards track protocol, is called SNMPv2c and described in RFC 1901 [9] and RFC 1906 [10]. The third version of the message protocol is called SNMPv3 and described in RFC 1906 [10], RFC 2572 [11] and RFC 2574 [12]. Protocol operations for accessing management information. The first set of protocol operations and associated PDU formats is described in
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   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.
Top   ToC   RFC2741 - Page 6

3.1. Motivation for AgentX

This very real need to dynamically extend the management objects within a node has given rise to a variety of "extensible agents", which typically comprise - a "master" agent that is available on the standard transport address and that accepts SNMP protocol messages - a set of "subagents" that each contain management instrumentation - a protocol that operates between the master agent and subagents, permitting subagents to "connect" to the master agent, and the master agent to multiplex received SNMP protocol messages amongst the subagents. - a set of tools to aid subagent development, and a runtime (API) environment that hides much of the protocol operation between a subagent and the master agent. The wide deployment of extensible SNMP agents, coupled with the lack of Internet standards in this area, makes it difficult to field SNMP-manageable applications. A vendor may have to support several different subagent environments (APIs) in order to support different target platforms. It can also become quite cumbersome to configure subagents and (possibly multiple) master agents on a particular managed node. Specifying a standard protocol for agent extensibility (AgentX) provides the technical foundation required to solve both of these problems. Independently developed AgentX-capable master agents and subagents will be able to interoperate at the protocol level. Vendors can continue to differentiate their products in all other respects.

4. AgentX Framework

Within the SNMP framework, a managed node contains a processing entity, called an agent, which has access to management information. Within the AgentX framework, an agent is further defined to consist of:
Top   ToC   RFC2741 - Page 7
      -  a single processing entity called the master agent, which sends
         and receives SNMP protocol messages in an agent role (as
         specified by the SNMP framework documents) but typically has
         little or no direct access to management information.

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

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

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

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

4.1. AgentX Roles

An entity acting in a master agent role performs the following functions: - Accepts AgentX session establishment requests from subagents. - Accepts registration of MIB regions by subagents. - Sends and accepts SNMP protocol messages on the agent's specified transport addresses. - Implements the agent role Elements of Procedure specified for the administrative framework applicable to the SNMP protocol message, except where they specify performing management operations. (The application of MIB views, and the access control policy for the managed node, are implemented by the master agent.) - Provides instrumentation for the MIB objects defined in RFC 1907 [17], and for any MIB objects relevant to any administrative framework it supports.
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      -  Sends and receives AgentX protocol messages to access
         management information, based on the current registry of MIB
         regions.

      -  Forwards notifications on behalf of subagents.

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

      -  Initiates AgentX sessions with the master agent.

      -  Registers MIB regions with the master agent.

      -  Instantiates managed objects.

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

      -  Performs management operations on variables.

      -  Initiates notifications.

4.2. Applicability

It is intended that this memo specify the smallest amount of required behavior necessary to achieve the largest benefit, that is, to cover a very large number of possible MIB implementations and configurations with minimum complexity and low "cost of entry". This section discusses several typical usage scenarios. 1) Subagents implement separate MIB modules -- for example, subagent `A' implements "mib-2", subagent `B' implements "host-resources". It is anticipated that this will be the most common subagent configuration. 2) Subagents implement rows in a "simple table". A simple table is one in which row creation is not specified, and for which the MIB does not define an object that counts entries in the table. Examples of simple tables are rdbmsDbTable, udpTable, and hrSWRunTable. This is the most commonly defined type of MIB table, and probably represents the next most typical configuration that AgentX would support.
Top   ToC   RFC2741 - Page 9
   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
Top   ToC   RFC2741 - Page 10
      carry them out; subagents are exclusively concerned with
      management instrumentation; and neither should intrude on the
      other's territory.

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

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

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

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

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

         - 32-bit alignment of data within PDUs

         - Native byte-order encoding by subagents

         - Large AgentX PDU payload sizes.

4.4. Non-Goals

1) Subagent-to-subagent communication. This is out of scope, due to the security ramifications and complexity involved. 2) Subagent access (via the master agent) to MIB variables. This is not addressed, since various other mechanisms are available and it was not a fundamental requirement. 3) The ability to accommodate every conceivable extensible agent configuration option. This was the most contentious aspect in the development of this protocol. In essence, certain features currently available in some commercial extensible agent products are not included in AgentX. Although useful or even vital in some implementation strategies, the rough consensus was that these features were not appropriate for an Internet Standard, or not
Top   ToC   RFC2741 - Page 11
      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:
Top   ToC   RFC2741 - Page 12
   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.
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   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.
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   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.
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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     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:
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              Integer                  (2),
              Octet String             (4),
              Null                     (5),
              Object Identifier        (6),
              IpAddress               (64),
              Counter32               (65),
              Gauge32                 (66),
              TimeTicks               (67),
              Opaque                  (68),
              Counter64               (70),
              noSuchObject           (128),
              noSuchInstance         (129),
              endOfMibView           (130)

      v.name

         The Object Identifier which names the variable.

      v.data

         The actual value, encoded as follows:

         -  Integer, Counter32, Gauge32, and TimeTicks are encoded as 4
            contiguous bytes, according to the header's
            NETWORK_BYTE_ORDER bit.

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

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

         -  IpAddress, Opaque, and Octet String are all octet strings
            and are encoded as described in section 5.3, "Octet
            String", Octet String.  Note that the octets used to
            represent IpAddress are always ordered most significant to
            least significant.

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

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



(page 17 continued on part 2)

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