SNMPv2 Operations: Protocol Operations for Version 2 of the Simple Network Management Protocol (SNMPv2)

Network Working Group                               SNMPv2 Working Group
Request for Comments: 1905                                       J. Case
Obsoletes: 1448                                      SNMP Research, Inc.
Category: Standards Track                                  K. McCloghrie
                                                     Cisco Systems, Inc.
                                                                 M. Rose
                                            Dover Beach Consulting, Inc.
                                                           S. Waldbusser
                                          International Network Services
                                                            January 1996


                          Protocol Operations
                          for Version 2 of the
              Simple Network Management Protocol (SNMPv2)

Status of this Memo

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

1.  Introduction

   A management system contains:  several (potentially many) nodes, each
   with a processing entity, termed an agent, which has access to
   management instrumentation; at least one management station; and, a
   management protocol, used to convey management information between
   the agents and management stations.  Operations of the protocol are
   carried out under an administrative framework which defines
   authentication, authorization, access control, and privacy policies.

   Management stations execute management applications which monitor and
   control managed elements.  Managed elements are devices such as
   hosts, routers, terminal servers, etc., which are monitored and
   controlled via access to their management information.

   Management information is viewed as a collection of managed objects,
   residing in a virtual information store, termed the Management
   Information Base (MIB).  Collections of related objects are defined
   in MIB modules.  These modules are written using a subset of OSI's
   Abstract Syntax Notation One (ASN.1) [1], termed the Structure of
   Management Information (SMI) [2].

   The management protocol, version 2 of the Simple Network Management
   Protocol, provides for the exchange of messages which convey
   management information between the agents and the management
   stations.  The form of these messages is a message "wrapper" which
   encapsulates a Protocol Data Unit (PDU).  The form and meaning of the
   "wrapper" is determined by an administrative framework which defines
   both authentication and authorization policies.

   It is the purpose of this document, Protocol Operations for SNMPv2,
   to define the operations of the protocol with respect to the sending
   and receiving of the PDUs.

1.1.  A Note on Terminology

   For the purpose of exposition, the original Internet-standard Network
   Management Framework, as described in RFCs 1155 (STD 16), 1157 (STD
   15), and 1212 (STD 16), is termed the SNMP version 1 framework
   (SNMPv1).  The current framework is termed the SNMP version 2
   framework (SNMPv2).

2.  Overview

2.1.  Roles of Protocol Entities

   A SNMPv2 entity may operate in a manager role or an agent role.

   A SNMPv2 entity acts in an agent role when it performs SNMPv2
   management operations in response to received SNMPv2 protocol
   messages (other than an inform notification) or when it sends trap
   notifications.

   A SNMPv2 entity acts in a manager role when it initiates SNMPv2
   management operations by the generation of SNMPv2 protocol messages
   or when it performs SNMPv2 management operations in response to
   received trap or inform notifications.

   A SNMPv2 entity may support either or both roles, as dictated by its
   implementation and configuration.  Further, a SNMPv2 entity can also
   act in the role of a proxy agent, in which it appears to be acting in
   an agent role, but satisfies management requests by acting in a
   manager role with a remote entity.

2.2.  Management Information

   The term, variable, refers to an instance of a non-aggregate object
   type defined according to the conventions set forth in the SMI [2] or
   the textual conventions based on the SMI [3].  The term, variable
   binding, normally refers to the pairing of the name of a variable and
   its associated value.  However, if certain kinds of exceptional
   conditions occur during processing of a retrieval request, a variable
   binding will pair a name and an indication of that exception.

   A variable-binding list is a simple list of variable bindings.

   The name of a variable is an OBJECT IDENTIFIER which is the
   concatenation of the OBJECT IDENTIFIER of the corresponding object-
   type together with an OBJECT IDENTIFIER fragment identifying the
   instance.  The OBJECT IDENTIFIER of the corresponding object-type is
   called the OBJECT IDENTIFIER prefix of the variable.

2.3.  Access to Management Information

   Three types of access to management information are provided by the
   protocol.  One type is a request-response interaction, in which a
   SNMPv2 entity, acting in a manager role, sends a request to a SNMPv2
   entity, acting in an agent role, and the latter SNMPv2 entity then
   responds to the request.  This type is used to retrieve or modify
   management information associated with the managed device.

   A second type is also a request-response interaction, in which a
   SNMPv2 entity, acting in a manager role, sends a request to a SNMPv2
   entity, also acting in a manager role, and the latter SNMPv2 entity
   then responds to the request.  This type is used to notify a SNMPv2
   entity, acting in a manager role, of management information
   associated with another SNMPv2 entity, also acting in a manager role.

   The third type of access is an unconfirmed interaction, in which a
   SNMPv2 entity, acting in an agent role, sends a unsolicited message,
   termed a trap, to a SNMPv2 entity, acting in a manager role, and no
   response is returned.  This type is used to notify a SNMPv2 entity,
   acting in a manager role, of an exceptional situation, which has
   resulted in changes to management information associated with the
   managed device.

2.4.  Retransmission of Requests

   For all types of request in this protocol, the receiver is required
   under normal circumstances, to generate and transmit a response to
   the originator of the request.  Whether or not a request should be
   retransmitted if no corresponding response is received in an
   appropriate time interval, is at the discretion of the application
   originating the request.  This will normally depend on the urgency of
   the request.  However, such an application needs to act responsibly
   in respect to the frequency and duration of re-transmissions.

2.5.  Message Sizes

   The maximum size of a SNMPv2 message is limited to the minimum of:

(1)  the maximum message size which the destination SNMPv2 entity can
     accept; and,

(2)  the maximum message size which the source SNMPv2 entity can
     generate.

   The former may be known on a per-recipient basis; and in the absence
   of such knowledge, is indicated by transport domain used when sending
   the message.  The latter is imposed by implementation-specific local
   constraints.

   Each transport mapping for the SNMPv2 indicates the minimum message
   size which a SNMPv2 implementation must be able to produce or
   consume.  Although implementations are encouraged to support larger
   values whenever possible, a conformant implementation must never
   generate messages larger than allowed by the receiving SNMPv2 entity.

   One of the aims of the GetBulkRequest-PDU, specified in this
   protocol, is to minimize the number of protocol exchanges required to
   retrieve a large amount of management information.  As such, this PDU
   type allows a SNMPv2 entity acting in a manager role to request that
   the response be as large as possible given the constraints on message
   sizes.  These constraints include the limits on the size of messages
   which the SNMPv2 entity acting in an agent role can generate, and the
   SNMPv2 entity acting in a manager role can receive.

   However, it is possible that such maximum sized messages may be
   larger than the Path MTU of the path across the network traversed by
   the messages.  In this situation, such messages are subject to
   fragmentation.  Fragmentation is generally considered to be harmful
   [4], since among other problems, it leads to a decrease in the
   reliability of the transfer of the messages.  Thus, a SNMPv2 entity
   which sends a GetBulkRequest-PDU must take care to set its parameters
   accordingly, so as to reduce the risk of fragmentation.  In
   particular, under conditions of network stress, only small values
   should be used for max-repetitions.

2.6.  Transport Mappings

   It is important to note that the exchange of SNMPv2 messages requires
   only an unreliable datagram service, with every message being
   entirely and independently contained in a single transport datagram.
   Specific transport mappings and encoding rules are specified
   elsewhere [5].  However, the preferred mapping is the use of the User
   Datagram Protocol [6].

3.  Definitions

     SNMPv2-PDU DEFINITIONS ::= BEGIN

     IMPORTS
         ObjectName, ObjectSyntax, Integer32
             FROM SNMPv2-SMI;


     -- protocol data units

     PDUs ::=
         CHOICE {
             get-request
                 GetRequest-PDU,

             get-next-request
                 GetNextRequest-PDU,

             get-bulk-request
                 GetBulkRequest-PDU,

             response
                 Response-PDU,

             set-request
                 SetRequest-PDU,

             inform-request
                 InformRequest-PDU,

             snmpV2-trap
                 SNMPv2-Trap-PDU,

             report
                 Report-PDU,
         }


     -- PDUs

     GetRequest-PDU ::=
         [0]
             IMPLICIT PDU

     GetNextRequest-PDU ::=

         [1]
             IMPLICIT PDU

     Response-PDU ::=
         [2]
             IMPLICIT PDU

     SetRequest-PDU ::=
         [3]
             IMPLICIT PDU

     -- [4] is obsolete

     GetBulkRequest-PDU ::=
         [5]
             IMPLICIT BulkPDU

     InformRequest-PDU ::=
         [6]
             IMPLICIT PDU

     SNMPv2-Trap-PDU ::=
         [7]
             IMPLICIT PDU

     --   Usage and precise semantics of Report-PDU are not presently
     --   defined.  Any SNMP administrative framework making use of
     --   this PDU must define its usage and semantics.
     Report-PDU ::=
         [8]
             IMPLICIT PDU

     max-bindings
         INTEGER ::= 2147483647

     PDU ::=
         SEQUENCE {
             request-id
                 Integer32,

             error-status            -- sometimes ignored
                 INTEGER {
                     noError(0),
                     tooBig(1),
                     noSuchName(2),   -- for proxy compatibility
                     badValue(3),     -- for proxy compatibility
                     readOnly(4),     -- for proxy compatibility
                     genErr(5),
                     noAccess(6),
                     wrongType(7),
                     wrongLength(8),
                     wrongEncoding(9),
                     wrongValue(10),
                     noCreation(11),
                     inconsistentValue(12),
                     resourceUnavailable(13),
                     commitFailed(14),
                     undoFailed(15),
                     authorizationError(16),
                     notWritable(17),
                     inconsistentName(18)
                 },

             error-index            -- sometimes ignored
                 INTEGER (0..max-bindings),

             variable-bindings   -- values are sometimes ignored
                 VarBindList
         }


     BulkPDU ::=                     -- MUST be identical in
         SEQUENCE {                  -- structure to PDU
             request-id
                 Integer32,

             non-repeaters
                 INTEGER (0..max-bindings),

             max-repetitions
                 INTEGER (0..max-bindings),

             variable-bindings       -- values are ignored
                 VarBindList
         }


     -- variable binding

     VarBind ::=
         SEQUENCE {
             name
                 ObjectName,

             CHOICE {
                 value

                     ObjectSyntax,

                 unSpecified         -- in retrieval requests
                         NULL,

                                     -- exceptions in responses
                 noSuchObject[0]
                         IMPLICIT NULL,

                 noSuchInstance[1]
                         IMPLICIT NULL,

                 endOfMibView[2]
                         IMPLICIT NULL
             }
         }


     -- variable-binding list

     VarBindList ::=
         SEQUENCE (SIZE (0..max-bindings)) OF
             VarBind


     END


4.  Protocol Specification

4.1.  Common Constructs

   The value of the request-id field in a Response-PDU takes the value
   of the request-id field in the request PDU to which it is a response.
   By use of the request-id value, a SNMPv2 application can distinguish
   the (potentially multiple) outstanding requests, and thereby
   correlate incoming responses with outstanding requests.  In cases
   where an unreliable datagram service is used, the request-id also
   provides a simple means of identifying messages duplicated by the
   network.  Use of the same request-id on a retransmission of a request
   allows the response to either the original transmission or the
   retransmission to satisfy the request.  However, in order to
   calculate the round trip time for transmission and processing of a
   request-response transaction, the SNMPv2 application needs to use a
   different request-id value on a retransmitted request.  The latter
   strategy is recommended for use in the majority of situations.

   A non-zero value of the error-status field in a Response-PDU is used
   to indicate that an exception occurred to prevent the processing of
   the request.  In these cases, a non-zero value of the Response-PDU's
   error-index field provides additional information by identifying
   which variable binding in the list caused the exception.  A variable
   binding is identified by its index value.  The first variable binding
   in a variable-binding list is index one, the second is index two,
   etc.

   SNMPv2 limits OBJECT IDENTIFIER values to a maximum of 128 sub-
   identifiers, where each sub-identifier has a maximum value of 2**32-
   1.

4.2.  PDU Processing

   It is mandatory that all SNMPv2 entities acting in an agent role be
   able to generate the following PDU types:  Response-PDU and SNMPv2-
   Trap-PDU; further, all such implementations must be able to receive
   the following PDU types:  GetRequest-PDU, GetNextRequest-PDU,
   GetBulkRequest-PDU, and SetRequest-PDU.

   It is mandatory that all SNMPv2 entities acting in a manager role be
   able to generate the following PDU types: GetRequest-PDU,
   GetNextRequest-PDU, GetBulkRequest-PDU, SetRequest-PDU,
   InformRequest-PDU, and Response-PDU; further, all such
   implementations must be able to receive the following PDU types:
   Response-PDU, SNMPv2-Trap-PDU,

   InformRequest-PDU;

   In the elements of procedure below, any field of a PDU which is not
   referenced by the relevant procedure is ignored by the receiving
   SNMPv2 entity.  However, all components of a PDU, including those
   whose values are ignored by the receiving SNMPv2 entity, must have
   valid ASN.1 syntax and encoding.  For example, some PDUs (e.g., the
   GetRequest-PDU) are concerned only with the name of a variable and
   not its value.  In this case, the value portion of the variable
   binding is ignored by the receiving SNMPv2 entity.  The unSpecified
   value is defined for use as the value portion of such bindings.

   On generating a management communication, the message "wrapper" to
   encapsulate the PDU is generated according to the "Elements of
   Procedure" of the administrative framework in use is followed.  While
   the definition of "max-bindings" does impose an upper-bound on the
   number of variable bindings, in practice, the size of a message is
   limited only by constraints on the maximum message size -- it is not
   limited by the number of variable bindings.

   On receiving a management communication, the "Elements of Procedure"
   of the administrative framework in use is followed, and if those
   procedures indicate that the operation contained within the message
   is to be performed locally, then those procedures also indicate the
   MIB view which is visible to the operation.

4.2.1.  The GetRequest-PDU

   A GetRequest-PDU is generated and transmitted at the request of a
   SNMPv2 application.

   Upon receipt of a GetRequest-PDU, the receiving SNMPv2 entity
   processes each variable binding in the variable-binding list to
   produce a Response-PDU.  All fields of the Response-PDU have the same
   values as the corresponding fields of the received request except as
   indicated below.  Each variable binding is processed as follows:

(1)  If the variable binding's name exactly matches the name of a
     variable accessible by this request, then the variable binding's
     value field is set to the value of the named variable.

(2)  Otherwise, if the variable binding's name does not have an OBJECT
     IDENTIFIER prefix which exactly matches the OBJECT IDENTIFIER
     prefix of any (potential) variable accessible by this request, then
     its value field is set to `noSuchObject'.

(3)  Otherwise, the variable binding's value field is set to
     `noSuchInstance'.

   If the processing of any variable binding fails for a reason other
   than listed above, then the Response-PDU is re-formatted with the
   same values in its request-id and variable-bindings fields as the
   received GetRequest-PDU, with the value of its error-status field set
   to `genErr', and the value of its error-index field is set to the
   index of the failed variable binding.

   Otherwise, the value of the Response-PDU's error-status field is set
   to `noError', and the value of its error-index field is zero.

   The generated Response-PDU is then encapsulated into a message.  If
   the size of the resultant message is less than or equal to both a
   local constraint and the maximum message size of the originator, it
   is transmitted to the originator of the GetRequest-PDU.

   Otherwise, an alternate Response-PDU is generated.  This alternate
   Response-PDU is formatted with the same value in its request-id field
   as the received GetRequest-PDU, with the value of its error-status
   field set to `tooBig', the value of its error-index field set to
   zero, and an empty variable-bindings field.  This alternate
   Response-PDU is then encapsulated into a message.  If the size of the
   resultant message is less than or equal to both a local constraint
   and the maximum message size of the originator, it is transmitted to
   the originator of the GetRequest-PDU.  Otherwise, the snmpSilentDrops
   [9] counter is incremented and the resultant message is discarded.

4.2.2.  The GetNextRequest-PDU

   A GetNextRequest-PDU is generated and transmitted at the request of a
   SNMPv2 application.

   Upon receipt of a GetNextRequest-PDU, the receiving SNMPv2 entity
   processes each variable binding in the variable-binding list to
   produce a Response-PDU.  All fields of the Response-PDU have the same
   values as the corresponding fields of the received request except as
   indicated below.  Each variable binding is processed as follows:

(1)  The variable is located which is in the lexicographically ordered
     list of the names of all variables which are accessible by this
     request and whose name is the first lexicographic successor of the
     variable binding's name in the incoming GetNextRequest-PDU.  The
     corresponding variable binding's name and value fields in the
     Response-PDU are set to the name and value of the located variable.

(2)  If the requested variable binding's name does not lexicographically
     precede the name of any variable accessible by this request, i.e.,
     there is no lexicographic successor, then the corresponding
     variable binding produced in the Response-PDU has its value field
     set to `endOfMibView', and its name field set to the variable
     binding's name in the request.

   If the processing of any variable binding fails for a reason other
   than listed above, then the Response-PDU is re-formatted with the
   same values in its request-id and variable-bindings fields as the
   received GetNextRequest-PDU, with the value of its error-status field
   set to `genErr', and the value of its error-index field is set to the
   index of the failed variable binding.

   Otherwise, the value of the Response-PDU's error-status field is set
   to `noError', and the value of its error-index field is zero.

   The generated Response-PDU is then encapsulated into a message.  If
   the size of the resultant message is less than or equal to both a
   local constraint and the maximum message size of the originator, it
   is transmitted to the originator of the GetNextRequest-PDU.

   Otherwise, an alternate Response-PDU is generated.  This alternate
   Response-PDU is formatted with the same values in its request-id
   field as the received GetNextRequest-PDU, with the value of its
   error-status field set to `tooBig', the value of its error-index
   field set to zero, and an empty variable-bindings field.  This
   alternate Response-PDU is then encapsulated into a message.  If the
   size of the resultant message is less than or equal to both a local
   constraint and the maximum message size of the originator, it is
   transmitted to the originator of the GetNextRequest-PDU.  Otherwise,
   the snmpSilentDrops [9] counter is incremented and the resultant
   message is discarded.

4.2.2.1.  Example of Table Traversal

   An important use of the GetNextRequest-PDU is the traversal of
   conceptual tables of information within a MIB.  The semantics of this
   type of request, together with the method of identifying individual
   instances of objects in the MIB, provides access to related objects
   in the MIB as if they enjoyed a tabular organization.

   In the protocol exchange sketched below, a SNMPv2 application
   retrieves the media-dependent physical address and the address-
   mapping type for each entry in the IP net-to-media Address
   Translation Table [7] of a particular network element.  It also
   retrieves the value of sysUpTime [9], at which the mappings existed.
   Suppose that the agent's IP net-to-media table has three entries:

  Interface-Number  Network-Address  Physical-Address  Type

         1            10.0.0.51     00:00:10:01:23:45  static
         1             9.2.3.4      00:00:10:54:32:10  dynamic
         2            10.0.0.15     00:00:10:98:76:54  dynamic

   The SNMPv2 entity acting in a manager role begins by sending a
   GetNextRequest-PDU containing the indicated OBJECT IDENTIFIER values
   as the requested variable names:

    GetNextRequest ( sysUpTime,
                     ipNetToMediaPhysAddress,
                     ipNetToMediaType )

   The SNMPv2 entity acting in an agent role responds with a Response-
   PDU:

    Response (( sysUpTime.0 =  "123456" ),
              ( ipNetToMediaPhysAddress.1.9.2.3.4 =
                                         "000010543210" ),
              ( ipNetToMediaType.1.9.2.3.4 =  "dynamic" ))

   The SNMPv2 entity acting in a manager role continues with:

    GetNextRequest ( sysUpTime,
                     ipNetToMediaPhysAddress.1.9.2.3.4,
                     ipNetToMediaType.1.9.2.3.4 )

   The SNMPv2 entity acting in an agent role responds with:

    Response (( sysUpTime.0 =  "123461" ),
              ( ipNetToMediaPhysAddress.1.10.0.0.51 =
                                          "000010012345" ),
              ( ipNetToMediaType.1.10.0.0.51 =  "static" ))

   The SNMPv2 entity acting in a manager role continues with:

    GetNextRequest ( sysUpTime,
                     ipNetToMediaPhysAddress.1.10.0.0.51,
                     ipNetToMediaType.1.10.0.0.51 )

   The SNMPv2 entity acting in an agent role responds with:

    Response (( sysUpTime.0 =  "123466" ),
              ( ipNetToMediaPhysAddress.2.10.0.0.15 =
                                           "000010987654" ),
              ( ipNetToMediaType.2.10.0.0.15 =  "dynamic" ))

   The SNMPv2 entity acting in a manager role continues with:

    GetNextRequest ( sysUpTime,
                     ipNetToMediaPhysAddress.2.10.0.0.15,
                     ipNetToMediaType.2.10.0.0.15 )

   As there are no further entries in the table, the SNMPv2 entity
   acting in an agent role responds with the variables that are next in
   the lexicographical ordering of the accessible object names, for
   example:

    Response (( sysUpTime.0 =  "123471" ),
              ( ipNetToMediaNetAddress.1.9.2.3.4 =
                                               "9.2.3.4" ),
              ( ipRoutingDiscards.0 =  "2" ))

   This response signals the end of the table to the SNMPv2 entity
   acting in a manager role.

4.2.3.  The GetBulkRequest-PDU

   A GetBulkRequest-PDU is generated and transmitted at the request of a
   SNMPv2 application.  The purpose of the GetBulkRequest-PDU is to
   request the transfer of a potentially large amount of data,
   including, but not limited to, the efficient and rapid retrieval of
   large tables.

   Upon receipt of a GetBulkRequest-PDU, the receiving SNMPv2 entity
   processes each variable binding in the variable-binding list to
   produce a Response-PDU with its request-id field having the same
   value as in the request.  Processing begins by examining the values
   in the non-repeaters and max-repetitions fields.  If the value in the
   non-repeaters field is less than zero, then the value of the field is
   set to zero.  Similarly, if the value in the max-repetitions field is
   less than zero, then the value of the field is set to zero.

   For the GetBulkRequest-PDU type, the successful processing of each
   variable binding in the request generates zero or more variable
   bindings in the Response-PDU.  That is, the one-to-one mapping
   between the variable bindings of the GetRequest-PDU, GetNextRequest-
   PDU, and SetRequest-PDU types and the resultant Response-PDUs does
   not apply for the mapping between the variable bindings of a
   GetBulkRequest-PDU and the resultant Response-PDU.

   The values of the non-repeaters and max-repetitions fields in the
   request specify the processing requested.  One variable binding in
   the Response-PDU is requested for the first N variable bindings in
   the request and M variable bindings are requested for each of the R
   remaining variable bindings in the request.  Consequently, the total
   number of requested variable bindings communicated by the request is
   given by N + (M * R), where N is the minimum of:  a) the value of the
   non-repeaters field in the request, and b) the number of variable
   bindings in the request; M is the value of the max-repetitions field
   in the request; and R is the maximum of:  a) number of variable
   bindings in the request - N, and b)  zero.

   The receiving SNMPv2 entity produces a Response-PDU with up to the
   total number of requested variable bindings communicated by the
   request.  The request-id shall have the same value as the received
   GetBulkRequest-PDU.

   If N is greater than zero, the first through the (N)-th variable
   bindings of the Response-PDU are each produced as follows:

(1)  The variable is located which is in the lexicographically ordered
     list of the names of all variables which are accessible by this
     request and whose name is the first lexicographic successor of the
     variable binding's name in the incoming GetBulkRequest-PDU.  The
     corresponding variable binding's name and value fields in the
     Response-PDU are set to the name and value of the located variable.

(2)  If the requested variable binding's name does not lexicographically
     precede the name of any variable accessible by this request, i.e.,
     there is no lexicographic successor, then the corresponding
     variable binding produced in the Response-PDU has its value field
     set to `endOfMibView', and its name field set to the variable
     binding's name in the request.

   If M and R are non-zero, the (N + 1)-th and subsequent variable
   bindings of the Response-PDU are each produced in a similar manner.
   For each iteration i, such that i is greater than zero and less than
   or equal to M, and for each repeated variable, r, such that r is
   greater than zero and less than or equal to R, the (N + ( (i-1) * R )
   + r)-th variable binding of the Response-PDU is produced as follows:

(1)  The variable which is in the lexicographically ordered list of the
     names of all variables which are accessible by this request and
     whose name is the (i)-th lexicographic successor of the (N + r)-th
     variable binding's name in the incoming GetBulkRequest-PDU is
     located and the variable binding's name and value fields are set to
     the name and value of the located variable.

(2)  If there is no (i)-th lexicographic successor, then the
     corresponding variable binding produced in the Response-PDU has its
     value field set to `endOfMibView', and its name field set to either
     the last lexicographic successor, or if there are no lexicographic
     successors, to the (N + r)-th variable binding's name in the
     request.

   While the maximum number of variable bindings in the Response-PDU is
   bounded by N + (M * R), the response may be generated with a lesser
   number of variable bindings (possibly zero) for either of three
   reasons.

(1)  If the size of the message encapsulating the Response-PDU
     containing the requested number of variable bindings would be
     greater than either a local constraint or the maximum message size
     of the originator, then the response is generated with a lesser
     number of variable bindings.  This lesser number is the ordered set
     of variable bindings with some of the variable bindings at the end
     of the set removed, such that the size of the message encapsulating
     the Response-PDU is approximately equal to but no greater than
     either a local constraint or the maximum message size of the
     originator.  Note that the number of variable bindings removed has
     no relationship to the values of N, M, or R.

(2)  The response may also be generated with a lesser number of variable
     bindings if for some value of iteration i, such that i is greater
     than zero and less than or equal to M, that all of the generated
     variable bindings have the value field set to the `endOfMibView'.
     In this case, the variable bindings may be truncated after the (N +
     (i * R))-th variable binding.

(3)  In the event that the processing of a request with many repetitions
     requires a significantly greater amount of processing time than a
     normal request, then an agent may terminate the request with less
     than the full number of repetitions, providing at least one
     repetition is completed.

   If the processing of any variable binding fails for a reason other
   than listed above, then the Response-PDU is re-formatted with the
   same values in its request-id and variable-bindings fields as the
   received GetBulkRequest-PDU, with the value of its error-status field
   set to `genErr', and the value of its error-index field is set to the
   index of the variable binding in the original request which
   corresponds to the failed variable binding.

   Otherwise, the value of the Response-PDU's error-status field is set
   to `noError', and the value of its error-index field to zero.

   The generated Response-PDU (possibly with an empty variable-bindings
   field) is then encapsulated into a message.  If the size of the
   resultant message is less than or equal to both a local constraint
   and the maximum message size of the originator, it is transmitted to
   the originator of the GetBulkRequest-PDU.  Otherwise, the
   snmpSilentDrops [9] counter is incremented and the resultant message
   is discarded.

4.2.3.1.  Another Example of Table Traversal

   This example demonstrates how the GetBulkRequest-PDU can be used as
   an alternative to the GetNextRequest-PDU.  The same traversal of the
   IP net-to-media table as shown in Section 4.2.2.1 is achieved with
   fewer exchanges.

   The SNMPv2 entity acting in a manager role begins by sending a
   GetBulkRequest-PDU with the modest max-repetitions value of 2, and
   containing the indicated OBJECT IDENTIFIER values as the requested
   variable names:

    GetBulkRequest [ non-repeaters = 1, max-repetitions = 2 ]
                    ( sysUpTime,
                      ipNetToMediaPhysAddress,
                      ipNetToMediaType )

   The SNMPv2 entity acting in an agent role responds with a Response-PDU:

    Response (( sysUpTime.0 =  "123456" ),
              ( ipNetToMediaPhysAddress.1.9.2.3.4 =
                                         "000010543210" ),
              ( ipNetToMediaType.1.9.2.3.4 =  "dynamic" ),
              ( ipNetToMediaPhysAddress.1.10.0.0.51 =
                                          "000010012345" ),
              ( ipNetToMediaType.1.10.0.0.51 =  "static" ))

   The SNMPv2 entity acting in a manager role continues with:

       GetBulkRequest [ non-repeaters = 1, max-repetitions = 2 ]
                       ( sysUpTime,
                         ipNetToMediaPhysAddress.1.10.0.0.51,
                         ipNetToMediaType.1.10.0.0.51 )

   The SNMPv2 entity acting in an agent role responds with:

    Response (( sysUpTime.0 =  "123466" ),
              ( ipNetToMediaPhysAddress.2.10.0.0.15 =
                                         "000010987654" ),
              ( ipNetToMediaType.2.10.0.0.15 =
                                              "dynamic" ),
              ( ipNetToMediaNetAddress.1.9.2.3.4 =
                                              "9.2.3.4" ),
              ( ipRoutingDiscards.0 =  "2" ))

   This response signals the end of the table to the SNMPv2 entity
   acting in a manager role.

4.2.4.  The Response-PDU

   The Response-PDU is generated by a SNMPv2 entity only upon receipt of
   a GetRequest-PDU, GetNextRequest-PDU, GetBulkRequest-PDU,
   SetRequest-PDU, or InformRequest-PDU, as described elsewhere in this
   document.

   If the error-status field of the Response-PDU is non-zero, the value
   fields of the variable bindings in the variable binding list are
   ignored.

   If both the error-status field and the error-index field of the
   Response-PDU are non-zero, then the value of the error-index field is
   the index of the variable binding (in the variable-binding list of
   the corresponding request) for which the request failed.  The first
   variable binding in a request's variable-binding list is index one,
   the second is index two, etc.

   A compliant SNMPv2 entity acting in a manager role must be able to
   properly receive and handle a Response-PDU with an error-status field
   equal to `noSuchName', `badValue', or `readOnly'.  (See Section 3.1.2
   of [8].)

   Upon receipt of a Response-PDU, the receiving SNMPv2 entity presents
   its contents to the SNMPv2 application which generated the request
   with the same request-id value.

4.2.5.  The SetRequest-PDU

   A SetRequest-PDU is generated and transmitted at the request of a
   SNMPv2 application.

   Upon receipt of a SetRequest-PDU, the receiving SNMPv2 entity
   determines the size of a message encapsulating a Response-PDU having
   the same values in its request-id and variable-bindings fields as the
   received SetRequest-PDU, and the largest possible sizes of the
   error-status and error-index fields.  If the determined message size
   is greater than either a local constraint or the maximum message size
   of the originator, then an alternate Response-PDU is generated,
   transmitted to the originator of the SetRequest-PDU, and processing
   of the SetRequest-PDU terminates immediately thereafter.  This
   alternate Response-PDU is formatted with the same values in its
   request-id field as the received SetRequest-PDU, with the value of
   its error-status field set to `tooBig', the value of its error-index
   field set to zero, and an empty variable-bindings field.  This
   alternate Response-PDU is then encapsulated into a message.  If the
   size of the resultant message is less than or equal to both a local
   constraint and the maximum message size of the originator, it is
   transmitted to the originator of the SetRequest-PDU.  Otherwise, the
   snmpSilentDrops [9] counter is incremented and the resultant message
   is discarded.  Regardless, processing of the SetRequest-PDU
   terminates.

   Otherwise, the receiving SNMPv2 entity processes each variable
   binding in the variable-binding list to produce a Response-PDU.  All
   fields of the Response-PDU have the same values as the corresponding
   fields of the received request except as indicated below.

   The variable bindings are conceptually processed as a two phase
   operation.  In the first phase, each variable binding is validated;
   if all validations are successful, then each variable is altered in
   the second phase.  Of course, implementors are at liberty to
   implement either the first, or second, or both, of these conceptual
   phases as multiple implementation phases.  Indeed, such multiple
   implementation phases may be necessary in some cases to ensure
   consistency.

   The following validations are performed in the first phase on each
   variable binding until they are all successful, or until one fails:

(1)  If the variable binding's name specifies an existing or non-
     existent variable to which this request is/would be denied access
     because it is/would not be in the appropriate MIB view, then the
     value of the Response-PDU's error-status field is set to
     `noAccess', and the value of its error-index field is set to the
     index of the failed variable binding.

(2)  Otherwise, if there are no variables which share the same OBJECT
     IDENTIFIER prefix as the variable binding's name, and which are
     able to be created or modified no matter what new value is
     specified, then the value of the Response-PDU's error-status field
     is set to `notWritable', and the value of its error-index field is
     set to the index of the failed variable binding.

(3)  Otherwise, if the variable binding's value field specifies,
     according to the ASN.1 language, a type which is inconsistent with
     that required for all variables which share the same OBJECT
     IDENTIFIER prefix as the variable binding's name, then the value of
     the Response-PDU's error-status field is set to `wrongType', and
     the value of its error-index field is set to the index of the
     failed variable binding.

(4)  Otherwise, if the variable binding's value field specifies,
     according to the ASN.1 language, a length which is inconsistent
     with that required for all variables which share the same OBJECT
     IDENTIFIER prefix as the variable binding's name, then the value of
     the Response-PDU's error-status field is set to `wrongLength', and
     the value of its error-index field is set to the index of the
     failed variable binding.

(5)  Otherwise, if the variable binding's value field contains an ASN.1
     encoding which is inconsistent with that field's ASN.1 tag, then
     the value of the Response-PDU's error-status field is set to
     `wrongEncoding', and the value of its error-index field is set to
     the index of the failed variable binding.  (Note that not all
     implementation strategies will generate this error.)

(6)  Otherwise, if the variable binding's value field specifies a value
     which could under no circumstances be assigned to the variable,
     then the value of the Response-PDU's error-status field is set to
     `wrongValue', and the value of its error-index field is set to the
     index of the failed variable binding.

(7)  Otherwise, if the variable binding's name specifies a variable
     which does not exist and could not ever be created (even though
     some variables sharing the same OBJECT IDENTIFIER prefix might
     under some circumstances be able to be created), then the value of
     the Response-PDU's error-status field is set to `noCreation', and
     the value of its error-index field is set to the index of the
     failed variable binding.

(8)  Otherwise, if the variable binding's name specifies a variable
     which does not exist but can not be created under the present
     circumstances (even though it could be created under other
     circumstances), then the value of the Response-PDU's error-status
     field is set to `inconsistentName', and the value of its error-
     index field is set to the index of the failed variable binding.

(9)  Otherwise, if the variable binding's name specifies a variable
     which exists but can not be modified no matter what new value is
     specified, then the value of the Response-PDU's error-status field
     is set to `notWritable', and the value of its error-index field is
     set to the index of the failed variable binding.

(10) Otherwise, if the variable binding's value field specifies a value
     that could under other circumstances be held by the variable, but
     is presently inconsistent or otherwise unable to be assigned to the
     variable, then the value of the Response-PDU's error-status field
     is set to `inconsistentValue', and the value of its error-index
     field is set to the index of the failed variable binding.

(11) When, during the above steps, the assignment of the value specified
     by the variable binding's value field to the specified variable
     requires the allocation of a resource which is presently
     unavailable, then the value of the Response-PDU's error-status
     field is set to `resourceUnavailable', and the value of its error-
     index field is set to the index of the failed variable binding.

(12) If the processing of the variable binding fails for a reason other
     than listed above, then the value of the Response-PDU's error-
     status field is set to `genErr', and the value of its error-index
     field is set to the index of the failed variable binding.

(13) Otherwise, the validation of the variable binding succeeds.

   At the end of the first phase, if the validation of all variable
   bindings succeeded, then the value of the Response-PDU's error-status
   field is set to `noError' and the value of its error-index field is
   zero, and processing continues as follows.

   For each variable binding in the request, the named variable is
   created if necessary, and the specified value is assigned to it.
   Each of these variable assignments occurs as if simultaneously with
   respect to all other assignments specified in the same request.
   However, if the same variable is named more than once in a single
   request, with different associated values, then the actual assignment
   made to that variable is implementation-specific.

   If any of these assignments fail (even after all the previous
   validations), then all other assignments are undone, and the
   Response-PDU is modified to have the value of its error-status field
   set to `commitFailed', and the value of its error-index field set to
   the index of the failed variable binding.

   If and only if it is not possible to undo all the assignments, then
   the Response-PDU is modified to have the value of its error-status
   field set to `undoFailed', and the value of its error-index field is
   set to zero.  Note that implementations are strongly encouraged to
   take all possible measures to avoid use of either `commitFailed' or
   `undoFailed' - these two error-status codes are not to be taken as
   license to take the easy way out in an implementation.

   Finally, the generated Response-PDU is encapsulated into a message,
   and transmitted to the originator of the SetRequest-PDU.

4.2.6.  The SNMPv2-Trap-PDU

   A SNMPv2-Trap-PDU is generated and transmitted by a SNMPv2 entity
   acting in an agent role when an exceptional situation occurs.

   The destination(s) to which a SNMPv2-Trap-PDU is sent is determined
   in an implementation-dependent fashion by the SNMPv2 entity.  The
   first two variable bindings in the variable binding list of an
   SNMPv2-Trap-PDU are sysUpTime.0 [9] and snmpTrapOID.0 [9]
   respectively.  If the OBJECTS clause is present in the invocation of
   the corresponding NOTIFICATION-TYPE macro, then each corresponding
   variable, as instantiated by this notification, is copied, in order,
   to the variable-bindings field.  If any additional variables are
   being included (at the option of the generating SNMPv2 entity), then
   each is copied to the variable-bindings field.

4.2.7.  The InformRequest-PDU

   An InformRequest-PDU is generated and transmitted at the request of
   an application in a SNMPv2 entity acting in a manager role, that
   wishes to notify another application (in a SNMPv2 entity also acting
   in a manager role) of information in a MIB view which is remote to
   the receiving application.

   The destination(s) to which an InformRequest-PDU is sent is specified
   by the requesting application.  The first two variable bindings in
   the variable binding list of an InformRequest-PDU are sysUpTime.0 [9]
   and snmpTrapOID.0 [9] respectively.  If the OBJECTS clause is present
   in the invocation of the corresponding NOTIFICATION-TYPE macro, then
   each corresponding variable, as instantiated by this notification, is
   copied, in order, to the variable-bindings field.

   Upon receipt of an InformRequest-PDU, the receiving SNMPv2 entity
   determines the size of a message encapsulating a Response-PDU with
   the same values in its request-id, error-status, error-index and
   variable-bindings fields as the received InformRequest-PDU.  If the
   determined message size is greater than either a local constraint or
   the maximum message size of the originator, then an alternate
   Response-PDU is generated, transmitted to the originator of the
   InformRequest-PDU, and processing of the InformRequest-PDU terminates
   immediately thereafter.  This alternate Response-PDU is formatted
   with the same values in its request-id field as the received
   InformRequest-PDU, with the value of its error-status field set to
   `tooBig', the value of its error-index field set to zero, and an
   empty variable-bindings field.  This alternate Response-PDU is then
   encapsulated into a message.  If the size of the resultant message is
   less than or equal to both a local constraint and the maximum message
   size of the originator, it is transmitted to the originator of the
   InformRequest-PDU.  Otherwise, the snmpSilentDrops [9] counter is
   incremented and the resultant message is discarded.  Regardless,
   processing of the InformRequest-PDU terminates.

   Otherwise, the receiving SNMPv2 entity:

(1)  presents its contents to the appropriate SNMPv2 application;

(2)  generates a Response-PDU with the same values in its request-id and
     variable-bindings fields as the received InformRequest-PDU, with
     the value of its error-status field is set to `noError' and the
     value of its error-index field is zero; and

(3)  transmits the generated Response-PDU to the originator of the
     InformRequest-PDU.

5.  Security Considerations

   Security issues are not discussed in this memo.

6.  Editor's Address

   Keith McCloghrie
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134-1706
   US

   Phone: +1 408 526 5260
   EMail: kzm@cisco.com

7.  Acknowledgements

   This document is the result of significant work by the four major
   contributors:

   Jeffrey D. Case (SNMP Research, case@snmp.com)
   Keith McCloghrie (Cisco Systems, kzm@cisco.com)
   Marshall T. Rose (Dover Beach Consulting, mrose@dbc.mtview.ca.us)
   Steven Waldbusser (International Network Services, stevew@uni.ins.com)

   In addition, the contributions of the SNMPv2 Working Group are
   acknowledged.  In particular, a special thanks is extended for the
   contributions of:

     Alexander I. Alten (Novell)
     Dave Arneson (Cabletron)
     Uri Blumenthal (IBM)
     Doug Book (Chipcom)
     Kim Curran (Bell-Northern Research)
     Jim Galvin (Trusted Information Systems)
     Maria Greene (Ascom Timeplex)
     Iain Hanson (Digital)
     Dave Harrington (Cabletron)
     Nguyen Hien (IBM)
     Jeff Johnson (Cisco Systems)
     Michael Kornegay (Object Quest)
     Deirdre Kostick (AT&T Bell Labs)
     David Levi (SNMP Research)
     Daniel Mahoney (Cabletron)
     Bob Natale (ACE*COMM)
     Brian O'Keefe (Hewlett Packard)
     Andrew Pearson (SNMP Research)
     Dave Perkins (Peer Networks)
     Randy Presuhn (Peer Networks)
     Aleksey Romanov (Quality Quorum)
     Shawn Routhier (Epilogue)
     Jon Saperia (BGS Systems)
     Bob Stewart (Cisco Systems, bstewart@cisco.com), chair
     Kaj Tesink (Bellcore)
     Glenn Waters (Bell-Northern Research)
     Bert Wijnen (IBM)

8.  References

[1]  Information processing systems - Open Systems Interconnection -
     Specification of Abstract Syntax Notation One (ASN.1),
     International Organization for Standardization.  International
     Standard 8824, (December, 1987).

[2]  SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     S. Waldbusser, "Structure of Management Information for Version 2
     of the Simple Network Management Protocol (SNMPv2)", RFC 1902,
     January 1996.

[3]  SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     S. Waldbusser, "Textual Conventions for Version 2 of the Simple
     Network Management Protocol (SNMPv2)", RFC 1903, January 1996.

[4]  Kent, C., and J. Mogul, Fragmentation Considered Harmful,
     Proceedings, ACM SIGCOMM '87, Stowe, VT, (August 1987).

[5]  SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     S. Waldbusser, "Transport Mappings for Version 2 of the Simple
     Network Management Protocol (SNMPv2)", RFC 1906, January 1996.

[6]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
     USC/Information Sciences Institute, August 1980.

[7]  McCloghrie, K., and M. Rose, Editors, "Management Information
     Base for Network Management of TCP/IP-based internets:
     MIB-II", STD 17, RFC 1213, March 1991.

[8]  SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     S. Waldbusser, "Coexistence between Version 1 and Version 2
     of the Internet-standard Network Management Framework", RFC 1908,
     January 1996.

[9]  SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     S. Waldbusser, "Management Information Base for Version 2 of the
     Simple Network Management Protocol (SNMPv2)", RFC 1907,
     January 1996.