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.
