RFC 2257
Agent Extensibility (AgentX) Protocol Version 1
Network Working Group M. Daniele Request for Comments: 2257 Digital Equipment Corporation Category: Standards Track B. Wijnen T.J. Watson Research Center, IBM Corp. D. Francisco, Ed. Cisco Systems, Inc. January 1998 Agent Extensibility (AgentX) Protocol Version 1 Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1998). All Rights Reserved. Table of Contents 1 Introduction......................................................4 2 The SNMP Framework................................................4 2.1 A Note on Terminology.........................................4 3 Extending the MIB.................................................5 3.1 Motivation for AgentX.........................................5 4 AgentX Framework..................................................6 4.1 AgentX Roles..................................................7 4.2 Applicability.................................................8 4.3 Design Features of AgentX.....................................9 4.4 Non-Goals....................................................10 5 AgentX Encodings.................................................10 5.1 Object Identifier............................................11 5.2 SearchRange..................................................13 5.3 Octet String.................................................14 5.4 Value Representation.........................................14 6 Protocol Definitions.............................................16 6.1 AgentX PDU Header............................................16
6.1.1 Context..................................................19
6.2 AgentX PDUs..................................................20
6.2.1 The agentx-Open-PDU......................................20
6.2.2 The agentx-Close-PDU.....................................21
6.2.3 The agentx-Register-PDU..................................22
6.2.4 The agentx-Unregister-PDU................................25
6.2.5 The agentx-Get-PDU.......................................27
6.2.6 The agentx-GetNext-PDU...................................29
6.2.7 The agentx-GetBulk-PDU...................................30
6.2.8 The agentx-TestSet-PDU...................................31
6.2.9 The agentx-CommitSet, -UndoSet, -CleanupSet
PDUs.....................................................33
6.2.10 The agentx-Notify-PDU...................................33
6.2.11 The agentx-Ping-PDU.....................................34
6.2.12 The agentx-IndexAllocate-PDU............................35
6.2.13 The agentx-IndexDeallocate-PDU..........................36
6.2.14 The agentx-AddAgentCaps-PDU.............................37
6.2.15 The agentx-RemoveAgentCaps-PDU..........................38
6.2.16 The agentx-Response-PDU.................................39
7 Elements of Procedure............................................41
7.1 Processing AgentX Administrative Messages....................42
7.1.1 Processing the agentx-Open-PDU...........................42
7.1.2 Processing the agentx-IndexAllocate-PDU..................43
7.1.3 Using the agentx-IndexAllocate-PDU.......................45
7.1.4 Processing the agentx-IndexDeallocate-PDU................47
7.1.5 Processing the agentx-Register-PDU.......................48
7.1.5.1 Handling Duplicate OID Ranges........................50
7.1.6 Processing the agentx-Unregister-PDU.....................51
7.1.7 Processing the agentx-AddAgentCaps-PDU...................51
7.1.8 Processing the agentx-RemoveAgentCaps-PDU................52
7.1.9 Processing the agentx-Close-PDU..........................52
7.1.10 Detecting Connection Loss...............................53
7.1.11 Processing the agentx-Notify-PDU........................53
7.1.12 Processing the agentx-Ping-PDU..........................54
7.2 Processing Received SNMP Protocol Messages...................54
7.2.1 Dispatching AgentX PDUs..................................55
7.2.1.1 agentx-Get-PDU.......................................57
7.2.1.2 agentx-GetNext-PDU...................................58
7.2.1.3 agentx-GetBulk-PDU...................................59
7.2.1.4 agentx-TestSet-PDU...................................60
7.2.1.5 Dispatch.............................................60
7.2.2 Subagent Processing of agentx-Get, GetNext,
GetBulk-PDUs.............................................61
7.2.2.1 Subagent Processing of the agentx-Get-PDU............61
7.2.2.2 Subagent Processing of the
agentx-GetNext-PDU...................................62
7.2.2.3 Subagent Processing of the
agentx-GetBulk-PDU...................................62
7.2.3 Subagent Processing of agentx-TestSet,
-CommitSet, -UndoSet, -CleanupSet-PDUs...................63
7.2.3.1 Subagent Processing of the
agentx-TestSet-PDU...................................64
7.2.3.2 Subagent Processing of the
agentx-CommitSet-PDU.................................65
7.2.3.3 Subagent Processing of the
agentx-UndoSet-PDU...................................65
7.2.3.4 Subagent Processing of the
agentx-CleanupSet-PDU................................65
7.2.4 Master Agent Processing of AgentX Responses..............66
7.2.4.1 Common Processing of All AgentX Response
PDUs.................................................66
7.2.4.2 Processing of Responses to agentx-Get-PDUs...........66
7.2.4.3 Processing of Responses to
agentx-GetNext-PDU and agentx-GetBulk-PDU............67
7.2.4.4 Processing of Responses to
agentx-TestSet-PDUs..................................68
7.2.4.5 Processing of Responses to
agentx-CommitSet-PDUs................................68
7.2.4.6 Processing of Responses to
agentx-UndoSet-PDUs..................................69
7.2.5 Sending the SNMP Response-PDU............................69
7.2.6 MIB Views................................................69
7.3 State Transitions............................................70
7.3.1 Set Transaction States...................................70
7.3.2 Transport Connection States..............................71
7.3.3 Session States...........................................73
8 Transport Mappings...............................................74
8.1 AgentX over TCP..............................................74
8.1.1 Well-known Values........................................74
8.1.2 Operation................................................74
8.2 AgentX over UNIX-domain Sockets..............................75
8.2.1 Well-known Values........................................75
8.2.2 Operation................................................75
9 Security Considerations..........................................76
10 Acknowledgements................................................77
11 Authors' and Editor's Addresses.................................77
12 References......................................................78
13 Full Copyright Statement........................................80
1. Introduction This memo defines a standardized framework for extensible SNMP agents. It defines processing entities called master agents and subagents, a protocol (AgentX) used to communicate between them, and the elements of procedure by which the extensible agent processes SNMP protocol messages. 2. The SNMP Framework A management system contains: several (potentially many) nodes, each with a processing entity, termed an agent, which has access to management instrumentation; at least one management station; and, a management protocol, used to convey management information between the agents and management stations. Operations of the protocol are carried out under an administrative framework which defines authentication, authorization, access control, and privacy policies. Management stations execute management applications which monitor and control managed elements. Managed elements are devices such as hosts, routers, terminal servers, etc., which are monitored and controlled via access to their management information. Management information is viewed as a collection of managed objects, residing in a virtual information store, termed the Management Information Base (MIB). Collections of related objects are defined in MIB modules. These modules are written using a subset of OSI's Abstract Syntax Notation One (ASN.1) [1], termed the Structure of Management Information (SMI) (see RFC 1902 [2]). 2.1. A Note on Terminology The term "variable" refers to an instance of a non-aggregate object type defined according to the conventions set forth in the SMI (RFC 1902, [2]) or the textual conventions based on the SMI (RFC 1903 [3]). The term "variable binding" normally refers to the pairing of the name of a variable and its associated value. However, if certain kinds of exceptional conditions occur during processing of a retrieval request, a variable binding will pair a name and an indication of that exception. A variable-binding list is a simple list of variable bindings. The name of a variable is an OBJECT IDENTIFIER, which is the concatenation of the OBJECT IDENTIFIER of the corresponding object type together with an OBJECT IDENTIFIER fragment identifying the
instance. The OBJECT IDENTIFIER of the corresponding object-type is called the OBJECT IDENTIFIER prefix of the variable. For the purpose of exposition, the original Internet-standard Network Management Framework, as described in RFCs 1155 (STD 16), 1157 (STD 15), and 1212 (STD 16), is termed the SNMP version 1 framework (SNMPv1). The current framework, as described in RFCs 1902-1908, is termed the SNMP version 2 framework (SNMPv2). 3. Extending the MIB New MIB modules that extend the Internet-standard MIB are continuously being defined by various IETF working groups. It is also common for enterprises or individuals to create or extend enterprise-specific or experimental MIBs. As a result, managed devices are frequently complex collections of manageable components that have been independently installed on a managed node. Each component provides instrumentation for the managed objects defined in the MIB module(s) it implements. Neither the SNMP version 1 nor version 2 framework describes how the set of managed objects supported by a particular agent may be changed dynamically. 3.1. Motivation for AgentX This very real need to dynamically extend the management objects within a node has given rise to a variety of "extensible agents", which typically comprise - a "master" agent that is available on the standard transport address and that accepts SNMP protocol messages - a set of "subagents" that each contain management instrumentation - a protocol that operates between the master agent and subagents, permitting subagents to "connect" to the master agent, and the master agent to multiplex received SNMP protocol messages amongst the subagents. - a set of tools to aid subagent development, and a runtime (API) environment that hides much of the protocol operation between a subagent and the master agent.
The wide deployment of extensible SNMP agents, coupled with the lack of Internet standards in this area, makes it difficult to field SNMP-manageable applications. A vendor may have to support several different subagent environments (APIs) in order to support different target platforms. It can also become quite cumbersome to configure subagents and (possibly multiple) master agents on a particular managed node. Specifying a standard protocol for agent extensibility (AgentX) provides the technical foundation required to solve both of these problems. Independently developed AgentX-capable master agents and subagents will be able to interoperate at the protocol level. Vendors can continue to differentiate their products in all other respects. 4. AgentX Framework Within the SNMP framework, a managed node contains a processing entity, called an agent, which has access to management information. Within the AgentX framework, an agent is further defined to consist of - a single processing entity called the master agent, which sends and receives SNMP protocol messages in an agent role (as specified by the SNMP version 1 and version 2 framework documents) but typically has little or no direct access to management information. - 0 or more processing entities called subagents, which are "shielded" from the SNMP protocol messages processed by the master agent, but which have access to management information. The master and subagent entities communicate via AgentX protocol messages, as specified in this memo. Other interfaces (if any) on these entities, and their associated protocols, are outside the scope of this document. While some of the AgentX protocol messages appear similar in syntax and semantics to the SNMP, bear in mind that AgentX is not SNMP. The internal operations of AgentX are invisible to an SNMP entity operating in a manager role. From a manager's point of view, an extensible agent behaves exactly as would a non-extensible (monolithic) agent that has access to the same management instrumentation.
This transparency to managers is a fundamental requirement of AgentX, and is what differentiates AgentX subagents from SNMP proxy agents. 4.1. AgentX Roles An entity acting in a master agent role performs the following functions: - Accepts AgentX session establishment requests from subagents. - Accepts registration of MIB regions by subagents. - Sends and accepts SNMP protocol messages on the agent's specified transport addresses. - Implements the agent role Elements of Procedure specified for the administrative framework applicable to the SNMP protocol message, except where they specify performing management operations. (The application of MIB views, and the access control policy for the managed node, are implemented by the master agent.) - Provides instrumentation for the MIB objects defined in RFC 1907 [5], and for any MIB objects relevant to any administrative framework it supports. - Sends and receives AgentX protocol messages to access management information, based on the current registry of MIB regions. - Forwards notifications on behalf of subagents. An entity acting in a subagent role performs the following functions: - Initiates an AgentX session with the master agent. - Registers MIB regions with the master agent. - Instantiates managed objects. - Binds OIDs within its registered MIB regions to actual variables. - Performs management operations on variables. - Initiates notifications.
4.2 Applicability It is intended that this memo specify the smallest amount of required behavior necessary to achieve the largest benefit, that is, to cover a very large number of possible MIB implementations and configurations with minimum complexity and low "cost of entry". This section discusses several typical usage scenarios. 1) Subagents implement separate MIB modules--for example, subagent A implements "mib-2", subagent b implements "host- resources". It is anticipated that this will be the most common subagent configuration. 2) Subagents implement rows in a "simple table". A simple table is one in which row creation is not specified, and for which the MIB does not define an object that counts entries in the table. Examples of simple tables are rdbmsDbTable, udpTable, and hrSWRunTable. This is the most commonly defined type of MIB table, and probably represents the next most typical configuration that AgentX would support. 3) Subagents share MIBs along non-row partitions. Subagents register "chunks" of the MIB that represent multiple rows, due to the nature of the MIB's index structure. Examples include registering ipNetToMediaEntry.n, where n represents the ifIndex value for an interface implemented by the subagent, and tcpConnEntry.a.b.c.d, where a.b.c.d represents an IP address on an interface implemented by the subagent. AgentX supports these three common configurations, and all permutations of them, completely. The consensus is that they comprise a very large majority of current and likely future uses of multi-vendor extensible agent configurations. 4) Subagents implement rows in "complex tables". Complex tables here are defined as tables permitting row creation, or whose MIB also defines an object that counts entries in the table. Examples include the MIB-2 ifTable (due to ifNumber), and the RMON historyControlTable.
The subagent that implements such a counter object (like ifNumber) must go beyond AgentX to correctly implement it. This is an implementation issue (and most new MIB designs no longer include such objects). To implement row creation in such tables, at least one AgentX subagent must register at a point "higher" in the OID tree than an individual row (per AgentX's dispatching procedure). Again, this is an implementation issue. Scenarios in this category were thought to occur somewhat rarely in configurations where subagents are independently implemented by different vendors. The focus of a standard protocol, however, must be in just those areas where multi- vendor interoperability must be assured. Note that it would be inefficient (due to AgentX registration overhead) to share a table among AgentX subagents if the table contains very dynamic instances, and each subagent registers fully qualified instances. ipRouteTable could be an example of such a table in some environments. 4.3. Design Features of AgentX The primary features of the design described in this memo are: 1) A general architectural division of labor between master agent and subagent: The master agent is MIB ignorant and SNMP omniscient, while the subagent is SNMP ignorant and MIB omniscient (for the MIB variables it instantiates). That is, master agents, exclusively, are concerned with SNMP protocol operations and the translations to and from AgentX protocol operations needed to carry them out; subagents are exclusively concerned with management instrumentation; and neither should intrude on the other's territory. 2) A standard protocol and "rules of engagement" to enable interoperability between management instrumentation and extensible agents. 3) Mechanisms for independently developed subagents to integrate into the extensible agent on a particular managed node in such a way that they need not be aware of any other existing subagents.
4) A simple, deterministic registry and dispatching algorithm.
For a given extensible agent configuration, there is a single
subagent who is "authoritative" for any particular region of the
MIB (where "region" may extend from an entire MIB down to a single
object-instance).
5) Performance considerations. It is likely that the master
agent and all subagents will reside on the same host, and in such
cases AgentX is more a form of inter-process communication than a
traditional communications protocol.
Some of the design decisions made with this in mind include:
- 32-bit alignment of data within PDUs
- Native byte-order encoding by subagents
- Large AgentX PDU payload sizes.
4.4 Non-Goals
1) Subagent-to-subagent communication. This is out of scope,
due to the security ramifications and complexity involved.
2) Subagent access (via the master agent) to MIB variables.
This is not addressed, since various other mechanisms are
available and it was not a fundamental requirement.
3) The ability to accommodate every conceivable extensible
agent configuration option. This was the most contentious aspect
in the development of this protocol. In essence, certain features
currently available in some commercial extensible agent products
are not included in AgentX. Although useful or even vital in some
implementation strategies, the rough consensus was that these
features were not appropriate for an Internet Standard, or not
typically required for independently developed subagents to
coexist. The set of supported extensible agent configurations is
described above, in Section 4.2.
Some possible future version of the AgentX protocol may provide
coverage for one or more of these "non-goals" or for new goals that
might be identified after greater deployment experience.
5. AgentX Encodings
AgentX PDUs consist of a common header, followed by PDU-specific data
of variable length. Unlike SNMP PDUs, AgentX PDUs are not encoded
using the BER (as specified in ISO 8824 [1]), but are transmitted as
a contiguous byte stream. The data within this stream is organized to provide natural alignment with respect to the start of the PDU, permitting direct (integer) access by the processing entities. The first four fields in the header are single-byte values. A bit (NETWORK_BYTE_ORDER) in the third field (h.flags) is used to indicate the byte ordering of all multi-byte integer values in the PDU, including those which follow in the header itself. This is described in more detail in Section 6.1, "AgentX PDU Header", below. PDUs are depicted in this memo using the following convention (where byte 1 is the first transmitted byte): +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | byte 1 | byte 2 | byte 3 | byte 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | byte 5 | byte 6 | byte 7 | byte 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Fields marked "<reserved>" are reserved for future use and must be zero-filled. 5.1. Object Identifier An object identifier is encoded as a 4-byte header, followed by a variable number of contiguous 4-byte fields representing sub- identifiers. This representation (termed Object Identifier) is as follows: Object Identifier +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | n_subid | prefix | include | <reserved> | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sub-identifier #1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sub-identifier #n_subid | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Object Identifier header fields: n_subid The number (0-128) of sub-identifiers in the object identifier. An ordered list of "n_subid" 4-byte sub-identifiers follows the 4-byte header.
prefix
An unsigned value used to reduce the length of object
identifier encodings. A non-zero value "x" is interpreted as
the first sub-identifier after "internet" (1.3.6.1), and
indicates an implicit prefix "internet.x" to the actual sub-
identifiers encoded in the Object Identifier. For example, a
prefix field value 2 indicates an implicit prefix "1.3.6.1.2".
A value of 0 in the prefix field indicates there is no prefix
to the sub-identifiers.
include
Used only when the Object Identifier is the start of a
SearchRange, as described in section 5.2.
A null Object Identifier consists of the 4-byte header with all bytes
set to 0.
Examples:
sysDescr.0 (1.3.6.1.2.1.1.1.0)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | 2 | 0 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1.2.3.4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | 0 | 0 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.2. SearchRange A SearchRange consists of two Object Identifiers. In its communication with a subagent, the master agent uses a SearchRange to identify a requested variable binding, and, in GetNext and GetBulk operations, to set an upper bound on the names of managed object instances the subagent may send in reply. The first Object Identifier in a SearchRange (called the starting OID) indicates the beginning of the range. It is frequently (but not necessarily) the name of a requested variable binding. The "include" field in this OID's header is a boolean value (0 or 1) indicating whether or not the starting OID is included in the range. The second object identifier indicates the non-inclusive end of the range, and its "include" field is always 0. Example: To indicate a search range from 1.3.6.1.2.1.25.2 (inclusive) to 1.3.6.1.2.1.25.2.1 (exclusive), the SearchRange would be (start) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 3 | 2 | 1 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 25 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ (end) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4 | 2 | 0 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 25 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A SearchRangeList is a contiguous list of SearchRanges.
5.3. Octet String An octet string is represented by a contiguous series of bytes, beginning with a 4-byte integer whose value is the number of octets in the octet string, followed by the octets themselves. This representation is termed an Octet String. If the last octet does not end on a 4-byte offset from the start of the Octet String, padding bytes are appended to achieve alignment of following data. This padding must be added even if the Octet String is the last item in the PDU. Padding bytes must be zero filled. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Octet String Length (L) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Octet 1 | Octet 2 | Octet 3 | Octet 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Octet L - 1 | Octet L | Padding (as required) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A null Octet String consists of a 4-byte length field set to 0. 5.4. Value Representation Variable bindings may be encoded within the variable-length portion of some PDUs. The representation of a variable binding (termed a VarBind) consists of a 2-byte type field, a name (Object Identifier), and the actual value data. VarBind +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | v.type | <reserved> | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ (v.name) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | n_subid | prefix | 0 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sub-identifier #1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sub-identifier #n_subid | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(v.data)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
VarBind fields:
v.type
Indicates the variable binding's syntax, and must be one of
the following values:
Integer (2),
Octet String (4),
Null (5),
Object Identifier (6),
IpAddress (64),
Counter32 (65),
Gauge32 (66),
TimeTicks (67),
Opaque (68),
Counter64 (70),
noSuchObject (128),
noSuchInstance (129),
endOfMibView (130)
v.name
The Object Identifier which names the variable.
v.data
The actual value, encoded as follows:
- Integer, Counter32, Gauge32, and TimeTicks are encoded as
4 contiguous bytes. If the NETWORK_BYTE_ORDER bit is set
in h.flags, the bytes are ordered most significant to least
significant, otherwise they are ordered least significant
to most significant.
- Counter64 is encoded as 8 contiguous bytes. If the
NETWORK_BYTE_ORDER bit is set in h.flags, the bytes are
ordered most significant to least significant, otherwise
they are ordered least significant to most significant.
- Object Identifiers are encoded as described in section
5.1, Object Identifier.
- IpAddress, Opaque, and Octet String are all octet strings
and are encoded as described in section 5.3, Octet String.
Value data always follows v.name whenever v.type is one
of the above types. These data bytes are present even if
they will not be used (as, for example, in certain types
of index allocation).
- Null, noSuchObject, noSuchInstance, and endOfMibView do not
contain any encoded value. Value data never follows
v.name in these cases.
Note that the VarBind itself does not contain the value size.
That information is implied for the fixed-length types, and
explicitly contained in the encodings of variable-length types
(Object Identifier and Octet String).
A VarBindList is a contiguous list of VarBinds. Within a
VarBindList, a particular VarBind is identified by an index value.
The first VarBind in a VarBindList has index value 1, the second
has index value 2, and so on.
6. Protocol Definitions
6.1. AgentX PDU Header
The AgentX PDU header is a fixed-format, 20-octet structure:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| h.version | h.type | h.flags | <reserved> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| h.sessionID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| h.transactionID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| h.packetID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| h.payload_length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An AgentX PDU header contains the following fields:
h.version
The version of the AgentX protocol (1 for this memo).
h.type
The PDU type; one of the following values:
agentx-Open-PDU (1),
agentx-Close-PDU (2),
agentx-Register-PDU (3),
agentx-Unregister-PDU (4),
agentx-Get-PDU (5),
agentx-GetNext-PDU (6),
agentx-GetBulk-PDU (7),
agentx-TestSet-PDU (8),
agentx-CommitSet-PDU (9),
agentx-UndoSet-PDU (10),
agentx-CleanupSet-PDU (11),
agentx-Notify-PDU (12),
agentx-Ping-PDU (13),
agentx-IndexAllocate-PDU (14),
agentx-IndexDeallocate-PDU (15),
agentx-AddAgentCaps-PDU (16),
agentx-RemoveAgentCaps-PDU (17),
agentx-Response-PDU (18)
h.flags
A bitmask, with bit 0 the least significant bit. The bit
definitions are as follows:
Bit Definition
--- ----------
0 INSTANCE_REGISTRATION
1 NEW_INDEX
2 ANY_INDEX
3 NON_DEFAULT_CONTEXT
4 NETWORK_BYTE_ORDER
5-7 (reserved)
The NETWORK_BYTE_ORDER bit applies to all multi-byte integer
values in the entire AgentX packet, including the remaining
header fields. If set, then network byte order (most
significant byte first; "big endian") is used. If not set,
then least significant byte first ("little endian") is used.
The NETWORK_BYTE_ORDER bit applies to all AgentX PDUs.
The NON_DEFAULT_CONTEXT bit is used only in the AgentX PDUs
described in section 6.1.1.
The NEW_INDEX and ANY_INDEX bits are used only within the
agentx-IndexAllocate-, and -IndexDeallocate-PDUs.
The INSTANCE_REGISTRATION bit is used only within the agentx-
Register-PDU.
h.sessionID
The session ID uniquely identifies a session over which AgentX
PDUs are exchanged between a subagent and the master agent.
The session ID has no significance and no defined value in the
agentx-Open-PDU sent by a subagent to open a session with the
master agent; in this case, the master agent will assign a
unique sessionID that it will pass back in the corresponding
agentx-Response-PDU. From that point on, that same sessionID
will appear in every AgentX PDU exchanged over that session
between the master and the subagent. A subagent may establish
multiple AgentX sessions by sending multiple agentx-Open-PDUs
to the master agent.
In master agents that support multiple transport protocols, the
sessionID should be globally unique rather than unique just to
a particular transport.
h.transactionID
The transaction ID uniquely identifies, for a given session,
the single SNMP management request (and single SNMP PDU) with
which an AgentX PDU is associated. If a single SNMP management
request results in multiple AgentX PDUs being sent by the
master agent with the same sessionID, each of these AgentX PDUs
must contain the same transaction ID; conversely, AgentX PDUs
sent during a particular session, that result from distinct
SNMP management requests, must have distinct transaction IDs
within the limits of the 32-bit field).
Note that the transaction ID is not the same as the SNMP PDU's
request-id (as described in section 4.1 of RFC 1905 [4]), nor
can it be, since a master agent might receive SNMP requests
with the same request-ids from different managers.
The transaction ID has no significance and no defined value in
AgentX administrative PDUs, i.e., AgentX PDUs that are not
associated with an SNMP management request.
h.packetID
A packet ID generated by the sender for all AgentX PDUs except
the agentx-Response-PDU. In an agentx-Response-PDU, the packet
ID must be the same as that in the received AgentX PDU to which
it is a response. A master agent might use this field to
associate subagent response PDUs with their corresponding
request PDUs. A subagent might use this field to correlate
responses to multiple (batched) registrations.
h.payload_length
The size in octets of the PDU contents, excluding the 20-byte
header. As a result of the encoding schemes and PDU layouts,
this value will always be either 0, or a multiple of 4.
6.1.1. Context
In the SNMPv1 or v2c frameworks, the community string may be used as
an index into a local repository of configuration information that
may include community profiles or more complex context information.
Future versions of the SNMP will likely formalize this notion of
"context".
AgentX provides a mechanism for transmitting a context specification
within relevant PDUs, but does not place any constraints on the
content of that specification.
An optional context field may be present in the agentx-Register-,
UnRegister-, AddAgentCaps-, RemoveAgentCaps-, Get-, GetNext-,
GetBulk-, IndexAllocate-, IndexDeallocate-, Notify-, TestSet-, and
Ping- PDUs.
If the NON_DEFAULT_CONTEXT bit in the AgentX header field h.flags is
clear, then there is no context field in the PDU, and the operation
refers to the default context.
If the NON_DEFAULT_CONTEXT bit is set, then a context field
immediately follows the AgentX header, and the operation refers to
that specific context. The context is represented as an Octet
String. There are no constraints on its length or contents.
Thus, all of these AgentX PDUs (that is, those listed immediately
above) refer to, or "indicate" a context, which is either the default
context, or a non-default context explicitly named in the PDU.
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