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RFC1067 - Simple Network Management Protocol

王朝other·作者佚名  2008-05-31
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Network Working Group J. Case

Request for Comments: 1067 University of Tennessee at Knoxville

M. Fedor

NYSERNet, Inc.

M. Schoffstall

Rensselaer Polytechnic Institute

J. Davin

Proteon, Inc.

August 1988

A Simple Network Management Protocol

Table of Contents

1. Status of this Memo ................................... 2

2. IntrodUCtion .......................................... 2

3. The SNMP Architecture ................................. 4

3.1 Goals of the Architecture ............................ 4

3.2 Elements of the Architecture ......................... 4

3.2.1 Scope of Management Information .................... 5

3.2.2 Representation of Management Information ........... 5

3.2.3 Operations Supported on Management Information ..... 6

3.2.4 Form and Meaning of Protocol Exchanges ............. 7

3.2.5 Definition of Administrative Relationships ......... 7

3.2.6 Form and Meaning of References to Managed Objects .. 11

3.2.6.1 Resolution of Ambiguous MIB References ........... 11

3.2.6.2 Resolution of References across MIB Versions...... 11

3.2.6.3 Identification of Object Instances ............... 11

3.2.6.3.1 ifTable Object Type Names ...................... 12

3.2.6.3.2 atTable Object Type Names ...................... 12

3.2.6.3.3 ipAddrTable Object Type Names .................. 13

3.2.6.3.4 ipRoutingTable Object Type Names ............... 13

3.2.6.3.5 tcpConnTable Object Type Names ................. 13

3.2.6.3.6 egpNeighTable Object Type Names ................ 14

4. Protocol Specification ................................ 15

4.1 Elements of Procedure ................................ 16

4.1.1 Common Constructs .................................. 18

4.1.2 The GetRequest-PDU ................................. 19

4.1.3 The GetNextRequest-PDU ............................. 20

4.1.3.1 Example of Table Traversal ....................... 22

4.1.4 The GetResponse-PDU ................................ 23

4.1.5 The SetRequest-PDU ................................. 24

4.1.6 The Trap-PDU ....................................... 26

4.1.6.1 The coldStart Trap ............................... 27

4.1.6.2 The warmStart Trap ............................... 27

4.1.6.3 The linkDown Trap ................................ 27

4.1.6.4 The linkUp Trap .................................. 27

4.1.6.5 The authenticationFailure Trap ................... 27

4.1.6.6 The egpNeighborLoss Trap ......................... 27

4.1.6.7 The enterpriseSpecific Trap ...................... 28

5. Definitions ........................................... 29

6. Acknowledgements ...................................... 32

7. References ............................................ 33

1. Status of this Memo

This memo defines a simple protocol by which management information

for a network element may be inspected or altered by logically remote

users. In particular, together with its companion memos which

describe the structure of management information along with the

initial management information base, these documents provide a

simple, workable architecture and system for managing TCP/IP-based

internets and in particular the Internet.

This memo specifies a draft standard for the Internet community.

TCP/IP implementations in the Internet which are network manageable

are eXPected to adopt and implement this specification.

Distribution of this memo is unlimited.

2. Introduction

As reported in RFC1052, IAB Recommendations for the Development of

Internet Network Management Standards [1], the Internet Activities

Board has directed the Internet Engineering Task Force (IETF) to

create two new working groups in the area of network management. One

group is charged with the further specification and definition of

elements to be included in the Management Information Base (MIB).

The other is charged with defining the modifications to the Simple

Network Management Protocol (SNMP) to accommodate the short-term

needs of the network vendor and operations communities, and to align

with the output of the MIB working group.

The MIB working group has produced two memos, one which defines a

Structure for Management Information (SMI) [2] for use by the managed

objects contained in the MIB. A second memo [3] defines the list of

managed objects.

The output of the SNMP Extensions working group is this memo, which

incorporates changes to the initial SNMP definition [4] required to

attain alignment with the output of the MIB working group. The

changes should be minimal in order to be consistent with the IAB's

directive that the working groups be "extremely sensitive to the need

to keep the SNMP simple." Although considerable care and debate has

gone into the changes to the SNMP which are reflected in this memo,

the resulting protocol is not backwardly-compatible with its

predecessor, the Simple Gateway Monitoring Protocol (SGMP) [5].

Although the syntax of the protocol has been altered, the original

philosophy, design decisions, and architecture remain intact. In

order to avoid confusion, new UDP ports have been allocated for use

by the protocol described in this memo.

3. The SNMP Architecture

Implicit in the SNMP architectural model is a collection of network

management stations and network elements. Network management

stations execute management applications which monitor and control

network elements. Network elements are devices such as hosts,

gateways, terminal servers, and the like, which have management

agents responsible for performing the network management functions

requested by the network management stations. The Simple Network

Management Protocol (SNMP) is used to communicate management

information between the network management stations and the agents in

the network elements.

3.1. Goals of the Architecture

The SNMP explicitly minimizes the number and complexity of management

functions realized by the management agent itself. This goal is

attractive in at least four respects:

(1) The development cost for management agent software

necessary to support the protocol is accordingly reduced.

(2) The degree of management function that is remotely

supported is accordingly increased, thereby admitting

fullest use of internet resources in the management task.

(3) The degree of management function that is remotely

supported is accordingly increased, thereby imposing the

fewest possible restrictions on the form and

sophistication of management tools.

(4) Simplified sets of management functions are easily

understood and used by developers of network management

tools.

A second goal of the protocol is that the functional paradigm for

monitoring and control be sufficiently extensible to accommodate

additional, possibly unanticipated ASPects of network operation and

management.

A third goal is that the architecture be, as much as possible,

independent of the architecture and mechanisms of particular hosts or

particular gateways.

3.2. Elements of the Architecture

The SNMP architecture articulates a solution to the network

management problem in terms of:

(1) the scope of the management information communicated by

the protocol,

(2) the representation of the management information

communicated by the protocol,

(3) operations on management information supported by the

protocol,

(4) the form and meaning of exchanges among management

entities,

(5) the definition of administrative relationships among

management entities, and

(6) the form and meaning of references to management

information.

3.2.1. Scope of Management Information

The scope of the management information communicated by operation of

the SNMP is exactly that represented by instances of all non-

aggregate object types either defined in Internet-standard MIB or

defined elsewhere according to the conventions set forth in

Internet-standard SMI [2].

Support for aggregate object types in the MIB is neither required for

conformance with the SMI nor realized by the SNMP.

3.2.2. Representation of Management Information

Management information communicated by operation of the SNMP is

represented according to the subset of the ASN.1 language [6] that is

specified for the definition of non-aggregate types in the SMI.

The SGMP adopted the convention of using a well-defined subset of the

ASN.1 language [6]. The SNMP continues and extends this tradition by

utilizing a moderately more complex subset of ASN.1 for describing

managed objects and for describing the protocol data units used for

managing those objects. In addition, the desire to ease eventual

transition to OSI-based network management protocols led to the

definition in the ASN.1 language of an Internet-standard Structure of

Management Information (SMI) [2] and Management Information Base

(MIB) [3]. The use of the ASN.1 language, was, in part, encouraged

by the successful use of ASN.1 in earlier efforts, in particular, the

SGMP. The restrictions on the use of ASN.1 that are part of the SMI

contribute to the simplicity espoused and validated by experience

with the SGMP.

Also for the sake of simplicity, the SNMP uses only a subset of the

basic encoding rules of ASN.1 [7]. Namely, all encodings use the

definite-length form. Further, whenever permissible, non-constructor

encodings are used rather than constructor encodings. This

restriction applies to all aspects of ASN.1 encoding, both for the

top-level protocol data units and the data objects they contain.

3.2.3. Operations Supported on Management Information

The SNMP models all management agent functions as alterations or

inspections of variables. Thus, a protocol entity on a logically

remote host (possibly the network element itself) interacts with the

management agent resident on the network element in order to retrieve

(get) or alter (set) variables. This strategy has at least two

positive consequences:

(1) It has the effect of limiting the number of essential

management functions realized by the management agent to

two: one operation to assign a value to a specified

configuration or other parameter and another to retrieve

such a value.

(2) A second effect of this decision is to avoid introducing

into the protocol definition support for imperative

management commands: the number of such commands is in

practice ever-increasing, and the semantics of such

commands are in general arbitrarily complex.

The strategy implicit in the SNMP is that the monitoring of network

state at any significant level of detail is accomplished primarily by

polling for appropriate information on the part of the monitoring

center(s). A limited number of unsolicited messages (traps) guide

the timing and focus of the polling. Limiting the number of

unsolicited messages is consistent with the goal of simplicity and

minimizing the amount of traffic generated by the network management

function.

The exclusion of imperative commands from the set of explicitly

supported management functions is unlikely to preclude any desirable

management agent operation. Currently, most commands are requests

either to set the value of some parameter or to retrieve such a

value, and the function of the few imperative commands currently

supported is easily accommodated in an asynchronous mode by this

management model. In this scheme, an imperative command might be

realized as the setting of a parameter value that subsequently

triggers the desired action. For example, rather than implementing a

"reboot command," this action might be invoked by simply setting a

parameter indicating the number of seconds until system reboot.

3.2.4. Form and Meaning of Protocol Exchanges

The communication of management information among management entities

is realized in the SNMP through the exchange of protocol messages.

The form and meaning of those messages is defined below in Section 4.

Consistent with the goal of minimizing complexity of the management

agent, the exchange of SNMP messages requires only an unreliable

datagram service, and every message is entirely and independently

represented by a single transport datagram. While this document

specifies the exchange of messages via the UDP protocol [8], the

mechanisms of the SNMP are generally suitable for use with a wide

variety of transport services.

3.2.5. Definition of Administrative Relationships

The SNMP architecture admits a variety of administrative

relationships among entities that participate in the protocol. The

entities residing at management stations and network elements which

communicate with one another using the SNMP are termed SNMP

application entities. The peer processes which implement the SNMP,

and thus support the SNMP application entities, are termed protocol

entities.

A pairing of an SNMP agent with some arbitrary set of SNMP

application entities is called an SNMP community. Each SNMP

community is named by a string of octets, that is called the

community name for said community.

An SNMP message originated by an SNMP application entity that in fact

belongs to the SNMP community named by the community component of

said message is called an authentic SNMP message. The set of rules

by which an SNMP message is identified as an authentic SNMP message

for a particular SNMP community is called an authentication scheme.

An implementation of a function that identifies authentic SNMP

messages according to one or more authentication schemes is called an

authentication service.

Clearly, effective management of administrative relationships among

SNMP application entities requires authentication services that (by

the use of encryption or other techniques) are able to identify

authentic SNMP messages with a high degree of certainty. Some SNMP

implementations may wish to support only a trivial authentication

service that identifies all SNMP messages as authentic SNMP messages.

For any network element, a subset of objects in the MIB that pertain

to that element is called a SNMP MIB view. Note that the names of

the object types represented in a SNMP MIB view need not belong to a

single sub-tree of the object type name space.

An element of the set { READ-ONLY, READ-WRITE } is called an SNMP

Access mode.

A pairing of a SNMP access mode with a SNMP MIB view is called an

SNMP community profile. A SNMP community profile represents

specified access privileges to variables in a specified MIB view. For

every variable in the MIB view in a given SNMP community profile,

access to that variable is represented by the profile according to

the following conventions:

(1) if said variable is defined in the MIB with "Access:" of

"none," it is unavailable as an operand for any operator;

(2) if said variable is defined in the MIB with "Access:" of

"read-write" or "write-only" and the access mode of the

given profile is READ-WRITE, that variable is available

as an operand for the get, set, and trap operations;

(3) otherwise, the variable is available as an operand for

the get and trap operations.

(4) In those cases where a "write-only" variable is an

operand used for the get or trap operations, the value

given for the variable is implementation-specific.

A pairing of a SNMP community with a SNMP community profile is called

a SNMP access policy. An access policy represents a specified

community profile afforded by the SNMP agent of a specified SNMP

community to other members of that community. All administrative

relationships among SNMP application entities are architecturally

defined in terms of SNMP access policies.

For every SNMP access policy, if the network element on which the

SNMP agent for the specified SNMP community resides is not that to

which the MIB view for the specified profile pertains, then that

policy is called a SNMP proxy access policy. The SNMP agent

associated with a proxy access policy is called a SNMP proxy agent.

While careless definition of proxy access policies can result in

management loops, prudent definition of proxy policies is useful in

at least two ways:

(1) It permits the monitoring and control of network elements

which are otherwise not addressable using the management

protocol and the transport protocol. That is, a proxy

agent may provide a protocol conversion function allowing

a management station to apply a consistent management

framework to all network elements, including devices such

as modems, multiplexors, and other devices which support

different management frameworks.

(2) It potentially shields network elements from elaborate

access control policies. For example, a proxy agent may

implement sophisticated access control whereby diverse

subsets of variables within the MIB are made accessible

to different management stations without increasing the

complexity of the network element.

By way of example, Figure 1 illustrates the relationship between

management stations, proxy agents, and management agents. In this

example, the proxy agent is envisioned to be a normal Internet

Network Operations Center (INOC) of some administrative domain which

has a standard managerial relationship with a set of management

agents.

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

Region #1 INOC Region #2 INOC PC in Region #3

Domain=Region #1 Domain=Region #2 Domain=Region #3

CPU=super-mini-1 CPU=super-mini-1 CPU=Clone-1

PCommunity=pub PCommunity=pub PCommunity=slate

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

/\ /\ /

\/

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

+--------------> Region #3 INOC <-------------+

Domain=Region #3

CPU=super-mini-2

PCommunity=pub,

slate

DCommunity=secret

+--------------> <-------------+

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

/\

\/ \/ \/

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

Domain=Region#3 Domain=Region#3 Domain=Region#3

CPU=router-1 CPU=mainframe-1 CPU=modem-1

DCommunity=secret DCommunity=secret DCommunity=secret

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

Domain: the administrative domain of the element

PCommunity: the name of a community utilizing a proxy agent

DCommunity: the name of a direct community

Figure 1

Example Network Management Configuration

3.2.6. Form and Meaning of References to Managed Objects

The SMI requires that the definition of a conformant management

protocol address:

(1) the resolution of ambiguous MIB references,

(2) the resolution of MIB references in the presence multiple

MIB versions, and

(3) the identification of particular instances of object

types defined in the MIB.

3.2.6.1. Resolution of Ambiguous MIB References

Because the scope of any SNMP operation is conceptually confined to

objects relevant to a single network element, and because all SNMP

references to MIB objects are (implicitly or explicitly) by unique

variable names, there is no possibility that any SNMP reference to

any object type defined in the MIB could resolve to multiple

instances of that type.

3.2.6.2. Resolution of References across MIB Versions

The object instance referred to by any SNMP operation is exactly that

specified as part of the operation request or (in the case of a get-

next operation) its immediate successor in the MIB as a whole. In

particular, a reference to an object as part of some version of the

Internet-standard MIB does not resolve to any object that is not part

of said version of the Internet-standard MIB, except in the case that

the requested operation is get-next and the specified object name is

lexicographically last among the names of all objects presented as

part of said version of the Internet-Standard MIB.

3.2.6.3. Identification of Object Instances

The names for all object types in the MIB are defined explicitly

either in the Internet-standard MIB or in other documents which

conform to the naming conventions of the SMI. The SMI requires that

conformant management protocols define mechanisms for identifying

individual instances of those object types for a particular network

element.

Each instance of any object type defined in the MIB is identified in

SNMP operations by a unique name called its "variable name." In

general, the name of an SNMP variable is an OBJECT IDENTIFIER of the

form x.y, where x is the name of a non-aggregate object type defined

in the MIB and y is an OBJECT IDENTIFIER fragment that, in a way

specific to the named object type, identifies the desired instance.

This naming strategy admits the fullest exploitation of the semantics

of the GetNextRequest-PDU (see Section 4), because it assigns names

for related variables so as to be contiguous in the lexicographical

ordering of all variable names known in the MIB.

The type-specific naming of object instances is defined below for a

number of classes of object types. Instances of an object type to

which none of the following naming conventions are applicable are

named by OBJECT IDENTIFIERs of the form x.0, where x is the name of

said object type in the MIB definition.

For example, suppose one wanted to identify an instance of the

variable sysDescr The object class for sysDescr is:

iso org dod internet mgmt mib system sysDescr

1 3 6 1 2 1 1 1

Hence, the object type, x, would be 1.3.6.1.2.1.1.1 to which is

appended an instance sub-identifier of 0. That is, 1.3.6.1.2.1.1.1.0

identifies the one and only instance of sysDescr.

3.2.6.3.1. ifTable Object Type Names

The name of a subnet interface, s, is the OBJECT IDENTIFIER value of

the form i, where i has the value of that instance of the ifIndex

object type associated with s.

For each object type, t, for which the defined name, n, has a prefix

of ifEntry, an instance, i, of t is named by an OBJECT IDENTIFIER of

the form n.s, where s is the name of the subnet interface about which

i represents information.

For example, suppose one wanted to identify the instance of the

variable ifType associated with interface 2. Accordingly, ifType.2

would identify the desired instance.

3.2.6.3.2. atTable Object Type Names

The name of an AT-cached network address, x, is an OBJECT IDENTIFIER

of the form 1.a.b.c.d, where a.b.c.d is the value (in the familiar

"dot" notation) of the atNetAddress object type associated with x.

The name of an address translation equivalence e is an OBJECT

IDENTIFIER value of the form s.w, such that s is the value of that

instance of the atIndex object type associated with e and such that w

is the name of the AT-cached network address associated with e.

For each object type, t, for which the defined name, n, has a prefix

of atEntry, an instance, i, of t is named by an OBJECT IDENTIFIER of

the form n.y, where y is the name of the address translation

equivalence about which i represents information.

For example, suppose one wanted to find the physical address of an

entry in the address translation table (ARP cache) associated with an

IP address of 89.1.1.42 and interface 3. Accordingly,

atPhysAddress.3.1.89.1.1.42 would identify the desired instance.

3.2.6.3.3. ipAddrTable Object Type Names

The name of an IP-addressable network element, x, is the OBJECT

IDENTIFIER of the form a.b.c.d such that a.b.c.d is the value (in the

familiar "dot" notation) of that instance of the ipAdEntAddr object

type associated with x.

For each object type, t, for which the defined name, n, has a prefix

of ipAddrEntry, an instance, i, of t is named by an OBJECT IDENTIFIER

of the form n.y, where y is the name of the IP-addressable network

element about which i represents information.

For example, suppose one wanted to find the network mask of an entry

in the IP interface table associated with an IP address of 89.1.1.42.

Accordingly, ipAdEntNetMask.89.1.1.42 would identify the desired

instance.

3.2.6.3.4. ipRoutingTable Object Type Names

The name of an IP route, x, is the OBJECT IDENTIFIER of the form

a.b.c.d such that a.b.c.d is the value (in the familiar "dot"

notation) of that instance of the ipRouteDest object type associated

with x.

For each object type, t, for which the defined name, n, has a prefix

of ipRoutingEntry, an instance, i, of t is named by an OBJECT

IDENTIFIER of the form n.y, where y is the name of the IP route about

which i represents information.

For example, suppose one wanted to find the next hop of an entry in

the IP routing table associated with the destination of 89.1.1.42.

Accordingly, ipRouteNextHop.89.1.1.42 would identify the desired

instance.

3.2.6.3.5. tcpConnTable Object Type Names

The name of a TCP connection, x, is the OBJECT IDENTIFIER of the form

a.b.c.d.e.f.g.h.i.j such that a.b.c.d is the value (in the familiar

"dot" notation) of that instance of the tcpConnLocalAddress object

type associated with x and such that f.g.h.i is the value (in the

familiar "dot" notation) of that instance of the tcpConnRemoteAddress

object type associated with x and such that e is the value of that

instance of the tcpConnLocalPort object type associated with x and

such that j is the value of that instance of the tcpConnRemotePort

object type associated with x.

For each object type, t, for which the defined name, n, has a prefix

of tcpConnEntry, an instance, i, of t is named by an OBJECT

IDENTIFIER of the form n.y, where y is the name of the TCP connection

about which i represents information.

For example, suppose one wanted to find the state of a TCP connection

between the local address of 89.1.1.42 on TCP port 21 and the remote

address of 10.0.0.51 on TCP port 2059. Accordingly,

tcpConnState.89.1.1.42.21.10.0.0.51.2059 would identify the desired

instance.

3.2.6.3.6. egpNeighTable Object Type Names

The name of an EGP neighbor, x, is the OBJECT IDENTIFIER of the form

a.b.c.d such that a.b.c.d is the value (in the familiar "dot"

notation) of that instance of the egpNeighAddr object type associated

with x.

For each object type, t, for which the defined name, n, has a prefix

of egpNeighEntry, an instance, i, of t is named by an OBJECT

IDENTIFIER of the form n.y, where y is the name of the EGP neighbor

about which i represents information.

For example, suppose one wanted to find the neighbor state for the IP

address of 89.1.1.42. Accordingly, egpNeighState.89.1.1.42 would

identify the desired instance.

4. Protocol Specification

The network management protocol is an application protocol by which

the variables of an agent's MIB may be inspected or altered.

Communication among protocol entities is accomplished by the exchange

of messages, each of which is entirely and independently represented

within a single UDP datagram using the basic encoding rules of ASN.1

(as discussed in Section 3.2.2). A message consists of a version

identifier, an SNMP community name, and a protocol data unit (PDU).

A protocol entity receives messages at UDP port 161 on the host with

which it is associated for all messages except for those which report

traps (i.e., all messages except those which contain the Trap-PDU).

Messages which report traps should be received on UDP port 162 for

further processing. An implementation of this protocol need not

accept messages whose length exceeds 484 octets. However, it is

recommended that implementations support larger datagrams whenever

feasible.

It is mandatory that all implementations of the SNMP support the five

PDUs: GetRequest-PDU, GetNextRequest-PDU, GetResponse-PDU,

SetRequest-PDU, and Trap-PDU.

RFC1067-SNMP DEFINITIONS ::= BEGIN

IMPORTS

ObjectName, ObjectSyntax, NetworkAddress, IpAddress, TimeTicks

FROM RFC1065-SMI;

-- top-level message

Message ::=

SEQUENCE {

version -- version-1 for this RFC

INTEGER {

version-1(0)

},

community -- community name

OCTET STRING,

data -- e.g., PDUs if trivial

ANY -- authentication is being used

}

-- protocol data units

PDUs ::=

CHOICE {

get-request

GetRequest-PDU,

get-next-request

GetNextRequest-PDU,

get-response

GetResponse-PDU,

set-request

SetRequest-PDU,

trap

Trap-PDU

}

-- the individual PDUs and commonly used

-- data types will be defined later

END

4.1. Elements of Procedure

This section describes the actions of a protocol entity implementing

the SNMP. Note, however, that it is not intended to constrain the

internal architecture of any conformant implementation.

In the text that follows, the term transport address is used. In the

case of the UDP, a transport address consists of an IP address along

with a UDP port. Other transport services may be used to support the

SNMP. In these cases, the definition of a transport address should

be made accordingly.

The top-level actions of a protocol entity which generates a message

are as follows:

(1) It first constructs the appropriate PDU, e.g., the

GetRequest-PDU, as an ASN.1 object.

(2) It then passes this ASN.1 object along with a community

name its source transport address and the destination

transport address, to the service which implements the

desired authentication scheme. This authentication

service returns another ASN.1 object.

(3) The protocol entity then constructs an ASN.1 Message

object, using the community name and the resulting ASN.1

object.

(4) This new ASN.1 object is then serialized, using the basic

encoding rules of ASN.1, and then sent using a transport

service to the peer protocol entity.

Similarly, the top-level actions of a protocol entity which receives

a message are as follows:

(1) It performs a rudimentary parse of the incoming datagram

to build an ASN.1 object corresponding to an ASN.1

Message object. If the parse fails, it discards the

datagram and performs no further actions.

(2) It then verifies the version number of the SNMP message.

If there is a mismatch, it discards the datagram and

performs no further actions.

(3) The protocol entity then passes the community name and

user data found in the ASN.1 Message object, along with

the datagram's source and destination transport addresses

to the service which implements the desired

authentication scheme. This entity returns another ASN.1

object, or signals an authentication failure. In the

latter case, the protocol entity notes this failure,

(possibly) generates a trap, and discards the datagram

and performs no further actions.

(4) The protocol entity then performs a rudimentary parse on

the ASN.1 object returned from the authentication service

to build an ASN.1 object corresponding to an ASN.1 PDUs

object. If the parse fails, it discards the datagram and

performs no further actions. Otherwise, using the named

SNMP community, the appropriate profile is selected, and

the PDU is processed accordingly. If, as a result of

this processing, a message is returned then the source

transport address that the response message is sent from

shall be identical to the destination transport address

that the original request message was sent to.

4.1.1. Common Constructs

Before introducing the six PDU types of the protocol, it is

appropriate to consider some of the ASN.1 constructs used frequently:

-- request/response information

RequestID ::=

INTEGER

ErrorStatus ::=

INTEGER {

noError(0),

tooBig(1),

noSuchName(2),

badValue(3),

readOnly(4)

genErr(5)

}

ErrorIndex ::=

INTEGER

-- variable bindings

VarBind ::=

SEQUENCE {

name

ObjectName,

value

ObjectSyntax

}

VarBindList ::=

SEQUENCE OF

VarBind

RequestIDs are used to distinguish among outstanding requests. By

use of the RequestID, an SNMP application entity can correlate

incoming responses with outstanding requests. In cases where an

unreliable datagram service is being used, the RequestID also

provides a simple means of identifying messages duplicated by the

network.

A non-zero instance of ErrorStatus is used to indicate that an

exception occurred while processing a request. In these cases,

ErrorIndex may provide additional information by indicating which

variable in a list caused the exception.

The term variable refers to an instance of a managed object. A

variable binding, or VarBind, refers to the pairing of the name of a

variable to the variable's value. A VarBindList is a simple list of

variable names and corresponding values. Some PDUs are concerned

only with the name of a variable and not its value (e.g., the

GetRequest-PDU). In this case, the value portion of the binding is

ignored by the protocol entity. However, the value portion must

still have valid ASN.1 syntax and encoding. It is recommended that

the ASN.1 value NULL be used for the value portion of such bindings.

4.1.2. The GetRequest-PDU

The form of the GetRequest-PDU is:

GetRequest-PDU ::=

[0]

IMPLICIT SEQUENCE {

request-id

RequestID,

error-status -- always 0

ErrorStatus,

error-index -- always 0

ErrorIndex,

variable-bindings

VarBindList

}

The GetRequest-PDU is generated by a protocol entity only at the

request of its SNMP application entity.

Upon receipt of the GetRequest-PDU, the receiving protocol entity

responds according to any applicable rule in the list below:

(1) If, for any object named in the variable-bindings field,

the object's name does not exactly match the name of some

object available for get operations in the relevant MIB

view, then the receiving entity sends to the originator

of the received message the GetResponse-PDU of identical

form, except that the value of the error-status field is

noSuchName, and the value of the error-index field is the

index of said object name component in the received

message.

(2) If, for any object named in the variable-bindings field,

the object is an aggregate type (as defined in the SMI),

then the receiving entity sends to the originator of the

received message the GetResponse-PDU of identical form,

except that the value of the error-status field is

noSuchName, and the value of the error-index field is the

index of said object name component in the received

message.

(3) If the size of the GetResponse-PDU generated as described

below would exceed a local limitation, then the receiving

entity sends to the originator of the received message

the GetResponse-PDU of identical form, except that the

value of the error-status field is tooBig, and the value

of the error-index field is zero.

(4) If, for any object named in the variable-bindings field,

the value of the object cannot be retrieved for reasons

not covered by any of the foregoing rules, then the

receiving entity sends to the originator of the received

message the GetResponse-PDU of identical form, except

that the value of the error-status field is genErr and

the value of the error-index field is the index of said

object name component in the received message.

If none of the foregoing rules apply, then the receiving protocol

entity sends to the originator of the received message the

GetResponse-PDU such that, for each object named in the variable-

bindings field of the received message, the corresponding component

of the GetResponse-PDU represents the name and value of that

variable. The value of the error- status field of the GetResponse-

PDU is noError and the value of the error-index field is zero. The

value of the request-id field of the GetResponse-PDU is that of the

received message.

4.1.3. The GetNextRequest-PDU

The form of the GetNextRequest-PDU is identical to that of the

GetRequest-PDU except for the indication of the PDU type. In the

ASN.1 language:

GetNextRequest-PDU ::=

[1]

IMPLICIT SEQUENCE {

request-id

RequestID,

error-status -- always 0

ErrorStatus,

error-index -- always 0

ErrorIndex,

variable-bindings

VarBindList

}

The GetNextRequest-PDU is generated by a protocol entity only at the

request of its SNMP application entity.

Upon receipt of the GetNextRequest-PDU, the receiving protocol entity

responds according to any applicable rule in the list below:

(1) If, for any object name in the variable-bindings field,

that name does not lexicographically precede the name of

some object available for get operations in the relevant

MIB view, then the receiving entity sends to the

originator of the received message the GetResponse-PDU of

identical form, except that the value of the error-status

field is noSuchName, and the value of the error-index

field is the index of said object name component in the

received message.

(2) If the size of the GetResponse-PDU generated as described

below would exceed a local limitation, then the receiving

entity sends to the originator of the received message

the GetResponse-PDU of identical form, except that the

value of the error-status field is tooBig, and the value

of the error-index field is zero.

(3) If, for any object named in the variable-bindings field,

the value of the lexicographical successor to the named

object cannot be retrieved for reasons not covered by any

of the foregoing rules, then the receiving entity sends

to the originator of the received message the

GetResponse-PDU of identical form, except that the value

of the error-status field is genErr and the value of the

error-index field is the index of said object name

component in the received message.

If none of the foregoing rules apply, then the receiving protocol

entity sends to the originator of the received message the

GetResponse-PDU such that, for each name in the variable-bindings

field of the received message, the corresponding component of the

GetResponse-PDU represents the name and value of that object whose

name is, in the lexicographical ordering of the names of all objects

available for get operations in the relevant MIB view, together with

the value of the name field of the given component, the immediate

successor to that value. The value of the error-status field of the

GetResponse-PDU is noError and the value of the errorindex field is

zero. The value of the request-id field of the GetResponse-PDU is

that of the received message.

4.1.3.1. Example of Table Traversal

One important use of the GetNextRequest-PDU is the traversal of

conceptual tables of information within the MIB. The semantics of

this type of SNMP message, together with the protocol-specific

mechanisms for identifying individual instances of object types in

the MIB, affords access to related objects in the MIB as if they

enjoyed a tabular organization.

By the SNMP exchange sketched below, an SNMP application entity might

extract the destination address and next hop gateway for each entry

in the routing table of a particular network element. Suppose that

this routing table has three entries:

Destination NextHop Metric

10.0.0.99 89.1.1.42 5

9.1.2.3 99.0.0.3 3

10.0.0.51 89.1.1.42 5

The management station sends to the SNMP agent a GetNextRequest-PDU

containing the indicated OBJECT IDENTIFIER values as the requested

variable names:

GetNextRequest ( ipRouteDest, ipRouteNextHop, ipRouteMetric1 )

The SNMP agent responds with a GetResponse-PDU:

GetResponse (( ipRouteDest.9.1.2.3 = "9.1.2.3" ),

( ipRouteNextHop.9.1.2.3 = "99.0.0.3" ),

( ipRouteMetric1.9.1.2.3 = 3 ))

The management station continues with:

GetNextRequest ( ipRouteDest.9.1.2.3,

ipRouteNextHop.9.1.2.3,

ipRouteMetric1.9.1.2.3 )

The SNMP agent responds:

GetResponse (( ipRouteDest.10.0.0.51 = "10.0.0.51" ),

( ipRouteNextHop.10.0.0.51 = "89.1.1.42" ),

( ipRouteMetric1.10.0.0.51 = 5 ))

The management station continues with:

GetNextRequest ( ipRouteDest.10.0.0.51,

ipRouteNextHop.10.0.0.51,

ipRouteMetric1.10.0.0.51 )

The SNMP agent responds:

GetResponse (( ipRouteDest.10.0.0.99 = "10.0.0.99" ),

( ipRouteNextHop.10.0.0.99 = "89.1.1.42" ),

( ipRouteMetric1.10.0.0.99 = 5 ))

The management station continues with:

GetNextRequest ( ipRouteDest.10.0.0.99,

ipRouteNextHop.10.0.0.99,

ipRouteMetric1.10.0.0.99 )

As there are no further entries in the table, the SNMP agent returns

those objects that are next in the lexicographical ordering of the

known object names. This response signals the end of the routing

table to the management station.

4.1.4. The GetResponse-PDU

The form of the GetResponse-PDU is identical to that of the

GetRequest-PDU except for the indication of the PDU type. In the

ASN.1 language:

GetResponse-PDU ::=

[2]

IMPLICIT SEQUENCE {

request-id

RequestID,

error-status

ErrorStatus,

error-index

ErrorIndex,

variable-bindings

VarBindList

}

The GetResponse-PDU is generated by a protocol entity only upon

receipt of the GetRequest-PDU, GetNextRequest-PDU, or SetRequest-PDU,

as described elsewhere in this document.

Upon receipt of the GetResponse-PDU, the receiving protocol entity

presents its contents to its SNMP application entity.

4.1.5. The SetRequest-PDU

The form of the SetRequest-PDU is identical to that of the

GetRequest-PDU except for the indication of the PDU type. In the

ASN.1 language:

SetRequest-PDU ::=

[3]

IMPLICIT SEQUENCE {

request-id

RequestID,

error-status -- always 0

ErrorStatus,

error-index -- always 0

ErrorIndex,

variable-bindings

VarBindList

}

The SetRequest-PDU is generated by a protocol entity only at the

request of its SNMP application entity.

Upon receipt of the SetRequest-PDU, the receiving entity responds

according to any applicable rule in the list below:

(1) If, for any object named in the variable-bindings field,

the object is not available for set operations in the

relevant MIB view, then the receiving entity sends to the

originator of the received message the GetResponse-PDU of

identical form, except that the value of the error-status

field is noSuchName, and the value of the error-index

field is the index of said object name component in the

received message.

(2) If, for any object named in the variable-bindings field,

the contents of the value field does not, according to

the ASN.1 language, manifest a type, length, and value

that is consistent with that required for the variable,

then the receiving entity sends to the originator of the

received message the GetResponse-PDU of identical form,

except that the value of the error-status field is

badValue, and the value of the error-index field is the

index of said object name in the received message.

(3) If the size of the Get Response type message generated as

described below would exceed a local limitation, then the

receiving entity sends to the originator of the received

message the GetResponse-PDU of identical form, except

that the value of the error-status field is tooBig, and

the value of the error-index field is zero.

(4) If, for any object named in the variable-bindings field,

the value of the named object cannot be altered for

reasons not covered by any of the foregoing rules, then

the receiving entity sends to the originator of the

received message the GetResponse-PDU of identical form,

except that the value of the error-status field is genErr

and the value of the error-index field is the index of

said object name component in the received message.

If none of the foregoing rules apply, then for each object named in

the variable-bindings field of the received message, the

corresponding value is assigned to the variable. Each variable

assignment specified by the SetRequest-PDU should be effected as if

simultaneously set with respect to all other assignments specified in

the same message.

The receiving entity then sends to the originator of the received

message the GetResponse-PDU of identical form except that the value

of the error-status field of the generated message is noError and the

value of the error-index field is zero.

4.1.6. The Trap-PDU

The form of the Trap-PDU is:

Trap-PDU ::=

[4]

IMPLICIT SEQUENCE {

enterprise -- type of object generating

-- trap, see sysObjectID in [2]

OBJECT IDENTIFIER,

agent-addr -- address of object generating

NetworkAddress, -- trap

generic-trap -- generic trap type

INTEGER {

coldStart(0),

warmStart(1),

linkDown(2),

linkUp(3),

authenticationFailure(4),

egpNeighborLoss(5),

enterpriseSpecific(6)

},

specific-trap -- specific code, present even

INTEGER, -- if generic-trap is not

-- enterpriseSpecific

time-stamp -- time elapsed between the last

TimeTicks, -- (re)initialization of the network

-- entity and the generation of the

trap

variable-bindings -- "interesting" information

VarBindList

}

The Trap-PDU is generated by a protocol entity only at the request of

the SNMP application entity. The means by which an SNMP application

entity selects the destination addresses of the SNMP application

entities is implementation-specific.

Upon receipt of the Trap-PDU, the receiving protocol entity presents

its contents to its SNMP application entity.

The significance of the variable-bindings component of the Trap-PDU

is implementation-specific.

Interpretations of the value of the generic-trap field are:

4.1.6.1. The coldStart Trap

A coldStart(0) trap signifies that the sending protocol entity is

reinitializing itself such that the agent's configuration or the

protocol entity implementation may be altered.

4.1.6.2. The warmStart Trap

A warmStart(1) trap signifies that the sending protocol entity is

reinitializing itself such that neither the agent configuration nor

the protocol entity implementation is altered.

4.1.6.3. The linkDown Trap

A linkDown(2) trap signifies that the sending protocol entity

recognizes a failure in one of the communication links represented in

the agent's configuration.

The Trap-PDU of type linkDown contains as the first element of its

variable-bindings, the name and value of the ifIndex instance for the

affected interface.

4.1.6.4. The linkUp Trap

A linkUp(3) trap signifies that the sending protocol entity

recognizes that one of the communication links represented in the

agent's configuration has come up.

The Trap-PDU of type linkUp contains as the first element of its

variable-bindings, the name and value of the ifIndex instance for the

affected interface.

4.1.6.5. The authenticationFailure Trap

An authenticationFailure(4) trap signifies that the sending protocol

entity is the addressee of a protocol message that is not properly

authenticated. While implementations of the SNMP must be capable of

generating this trap, they must also be capable of suppressing the

emission of such traps via an implementation-specific mechanism.

4.1.6.6. The egpNeighborLoss Trap

An egpNeighborLoss(5) trap signifies that an EGP neighbor for whom

the sending protocol entity was an EGP peer has been marked down and

the peer relationship no longer oBTains.

The Trap-PDU of type egpNeighborLoss contains as the first element of

its variable-bindings, the name and value of the egpNeighAddr

instance for the affected neighbor.

4.1.6.7. The enterpriseSpecific Trap

A enterpriseSpecific(6) trap signifies that the sending protocol

entity recognizes that some enterprise-specific event has occurred.

The specific-trap field identifies the particular trap which

occurred.

5. Definitions

RFC1067-SNMP DEFINITIONS ::= BEGIN

IMPORTS

ObjectName, ObjectSyntax, NetworkAddress, IpAddress, TimeTicks

FROM RFC1065-SMI;

-- top-level message

Message ::=

SEQUENCE {

version -- version-1 for this RFC

INTEGER {

version-1(0)

},

community -- community name

OCTET STRING,

data -- e.g., PDUs if trivial

ANY -- authentication is being used

}

-- protocol data units

PDUs ::=

CHOICE {

get-request

GetRequest-PDU,

get-next-request

GetNextRequest-PDU,

get-response

GetResponse-PDU,

set-request

SetRequest-PDU,

trap

Trap-PDU

}

-- PDUs

GetRequest-PDU ::=

[0]

IMPLICIT PDU

GetNextRequest-PDU ::=

[1]

IMPLICIT PDU

GetResponse-PDU ::=

[2]

IMPLICIT PDU

SetRequest-PDU ::=

[3]

IMPLICIT PDU

PDU ::=

SEQUENCE {

request-id

INTEGER,

error-status -- sometimes ignored

INTEGER {

noError(0),

tooBig(1),

noSuchName(2),

badValue(3),

readOnly(4),

genErr(5)

},

error-index -- sometimes ignored

INTEGER,

variable-bindings -- values are sometimes ignored

VarBindList

}

Trap-PDU ::=

[4]

IMPLICIT SEQUENCE {

enterprise -- type of object generating

-- trap, see sysObjectID in [2]

OBJECT IDENTIFIER,

agent-addr -- address of object generating

NetworkAddress, -- trap

generic-trap -- generic trap type

INTEGER {

coldStart(0),

warmStart(1),

linkDown(2),

linkUp(3),

authenticationFailure(4),

egpNeighborLoss(5),

enterpriseSpecific(6)

},

specific-trap -- specific code, present even

INTEGER, -- if generic-trap is not

-- enterpriseSpecific

time-stamp -- time elapsed between the last

TimeTicks, -- (re)initialization of the

network

-- entity and the generation of the

trap

variable-bindings -- "interesting" information

VarBindList

}

-- variable bindings

VarBind ::=

SEQUENCE {

name

ObjectName,

value

ObjectSyntax

}

VarBindList ::=

SEQUENCE OF

VarBind

END

6. Acknowledgements

This memo was influenced by the IETF SNMP Extensions working

group:

Karl Auerbach, Epilogue Technology

K. Ramesh Babu, Excelan

Amatzia Ben-Artzi, 3Com/Bridge

Lawrence Besaw, Hewlett-Packard

Jeffrey D. Case, University of Tennessee at Knoxville

Anthony Chung, Sytek

James Davidson, The Wollongong Group

James R. Davin, Proteon

Mark S. Fedor, NYSERNet

Phill Gross, The MITRE Corporation

Satish Joshi, ACC

Dan Lynch, Advanced Computing Environments

Keith McCloghrie, The Wollongong Group

Marshall T. Rose, The Wollongong Group (chair)

Greg Satz, cisco

Martin Lee Schoffstall, Rensselaer Polytechnic Institute

Wengyik Yeong, NYSERNet

7. References

[1] Cerf, V., "IAB Recommendations for the Development of

Internet Network Management Standards", RFC1052, IAB,

April 1988.

[2] Rose, M., and K. McCloghrie, "Structure and Identification

of Management Information for TCP/IP-based internets",

RFC1065, TWG, August 1988.

[3] McCloghrie, K., and M. Rose, "Management Information Base

for Network Management of TCP/IP-based internets",

RFC1066, TWG, August 1988.

[4] Case, J., M. Fedor, M. Schoffstall, and J. Davin,

"A Simple Network Management Protocol", Internet

Engineering Task Force working note, Network Information

Center, SRI International, Menlo Park, California,

March 1988.

[5] Davin, J., J. Case, M. Fedor, and M. Schoffstall,

"A Simple Gateway Monitoring Protocol", RFC1028,

Proteon, University of Tennessee at Knoxville,

Cornell University, and Rensselaer Polytechnic

Institute, November 1987.

[6] Information processing systems - Open Systems

Interconnection, "Specification of Abstract Syntax

Notation One (ASN.1)", International Organization for

Standardization, International Standard 8824,

December 1987.

[7] Information processing systems - Open Systems

Interconnection, "Specification of Basic Encoding Rules

for Abstract Notation One (ASN.1)", International

Organization for Standardization, International Standard

8825, December 1987.

[8] Postel, J., "User Datagram Protocol", RFC768,

USC/Information Sciences Institute, November 1980.

 
 
 
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