RFC1227

Network Working Group M. Rose Request for Comments: 1227 Performance Systems International, Inc.

                                                               May 1991

                      SNMP MUX Protocol and MIB

Status of this Memo

  This memo suggests a mechanism by which a user process may associate
  itself with the local SNMP agent on a host, in order to implement
  portions of the MIB.  This mechanism would be local to the host.

  This is an Experimental Protocol for the Internet community.
  Discussion and suggestions for improvement are requested.  Please
  refer to the current edition of the "IAB Official Protocol Standards"
  for the standardization state and status of this protocol.
  Distribution of this memo is unlimited.

Table of Contents

  1. Introduction ..........................................    1
  2. Architecture ..........................................    2
  3. Protocol ..............................................    3
  3.1 Tricky Things ........................................    3
  3.1.1 Registration .......................................    4
  3.1.2 Removing Registration ..............................    4
  3.1.3 Atomic Sets ........................................    4
  3.1.4 Variables in Requests ..............................    5
  3.1.5 Request-ID .........................................    5
  3.1.6 The powerful get-next operator .....................    5
  3.2 Protocol Data Units ..................................    6
  3.3 Mappings on Transport Service ........................    8
  3.3.1 Mapping onto the TCP ...............................    8
  4. MIB for the SMUX ......................................    9
  5. Acknowledgements ......................................   12
  6. References ............................................   12
  7. Security Considerations................................   13
  8. Author's Address.......................................   13

1. Introduction

  On typical kernel/user systems, an agent speaking the SNMP [1] is
  often implemented as a user-process, that reads kernel variables in
  order to realize the Internet-standard MIB [2].  This approach works
  fine as long as all of the information needed by the SNMP agent
  resides in either the kernel or in stable storage (i.e., files).
  However, when other user-processes are employed to implement other

RFC 1227 SMUX May 1991

  network services, such as routing protocols, communication between
  the SNMP agent and other processes is problematic.

  In order to solve this problem, a new protocol, the SNMP multiplexing
  (SMUX) protocol is introduced.  When a user-process, termed a SMUX
  peer, wishes to export a MIB module, it initiates a SMUX association
  to the local SNMP agent, registers itself, and (later) fields
  management operations for objects in the MIB module.

  Carrying this approach to its fullest, it is possible to generalize
  the SNMP agent so that it knows about only the SNMP group of the
  Internet-standard MIB.  All other portions of the Internet-standard
  MIB can be implemented by another process.  This is quite useful, for
  example, when a computer manufacturer wishes to provide SNMP access
  for its operating system in binary form.

  In addition to defining the SMUX protocol, this document defines a
  MIB for the SMUX.  Obviously, this MIB module must also be
  implemented in the local SNMP agent.

2. Architecture

  There are two approaches that can be taken when trying to integrate
  arbitrary MIB modules with the SNMP agent: request-response and
  cache-ahead.

  The request-response model simply propagates the SNMP requests
  received by the SNMP agent to the user process which exported the MIB
  module.  The SMUX peer then performs the operation and returns a
  response.  In turn, the SNMP agent propagates this response back to
  the network management station.  The request-response model is said
  to be agent-driven since, after registration, the SNMP agent
  initiates all transactions.

  The cache-ahead model requires that the SMUX peer, after
  registration, periodically updates the SNMP agent with the subtree
  for the MIB module which has been registered.  The SNMP agent, upon
  receiving an SNMP request for information retrieval, locally performs
  the operation, and returns a response to the network management
  station.  (SNMP set requests are given immediately to the SMUX peer.)
  The cache-ahead model is said to be peer-driven since, after
  registration, the SMUX peer initiates all transactions.

  There are advantages and disadvantages to both approaches.  As such,
  the architecture envisioned supports both models in the following
  fashion: the protocol between the SNMP agent and the SMUX peer is
  based on the request-response model.  However, the SMUX peer, may
  itself be a user-process which employs the cache-ahead model with

RFC 1227 SMUX May 1991

  other user-processes.

  Obviously, the SMUX peer which employs the cache-ahead model acts as
  a "firewall" for those user-processes which actually implement the
  managed objects in the given MIB module.

  Note that this document describes only the SMUX protocol, for the
  request-response model.  Each SMUX peer is free to define a cache-
  ahead protocol specific for the application at hand.

3. Protocol

  The SMUX protocol is simple: the SNMP agent listens for incoming
  connections.  Upon establishing a connection, the SMUX peer issues an
  OpenPDU to initialize the SMUX association.  If the SNMP agent
  declines the association, it issues a closePDU and closes the
  connection.  If the SNMP agent accepts the association, no response
  is issued by the SNMP agent.

  For each subtree defined in a MIB module that the SMUX peer wishes to
  register (or unregister), the SMUX peer issues a RReqPDU.  When the
  SNMP agent receives such a PDU, it issues a corresponding RRspPDU.
  The SNMP agent returns RRspPDUs in the same order as the RReqPDUs
  were received.

  When the SMUX peer wishes to issue a trap, it issues an SNMP Trap-
  PDU.  When the SNMP agent receives such a PDU, it propagates this to
  the network management stations that it is configured to send traps
  to.

  When the SNMP agent receives an SNMP GetRequest-PDU, GetNextRequest-
  PDU, or SetRequest-PDU which includes one or more variable-bindings
  within a subtree registered by a SMUX peer, the SNMP agent sends an
  equivalent SNMP PDU containing only those variables within the
  subtree registered by that SMUX peer.  When the SMUX peer receives
  such a PDU, it applies the indicated operation and issues a
  corresponding SNMP GetResponse-PDU.  The SNMP agent then correlates
  this result and propagates the resulting GetResponse-PDU to the
  network management station.

  When either the SNMP agent or the SMUX peer wishes to release the
  SMUX association, the ClosePDU is issued, the connection is closed,
  and all subtree registrations for that association are released.

3.1. Tricky Things

  Although straight-forward, there are a few nuances.

RFC 1227 SMUX May 1991

3.1.1. Registration

  Associated with each registration is an integer priority, from 0 to
  (2^31)-1.  The lower the value, the higher the priority.

  Multiple SMUX peers may register the same subtree.  However, they
  must do so at different priorities (i.e., a subtree and a priority
  uniquely identifies a SMUX peer).  As such, if a SMUX peer wishes to
  register a subtree at a priority which is already taken, the SNMP
  agent repeatedly increments the integer value until an unused
  priority is found.

  When registering a subtree, the special priority -1 may be used,
  which selects the highest available priority.

  Of course, the SNMP agent may select an arbitrarily worse priority
  for a SMUX peer, based on local (configuration) information.

  Note that when a subtree is registered, the SMUX peer with the
  highest registration priority is exclusively consulted for all
  operations on that subtree.  Further note that SNMP agents must
  enforce the "subtree mounting effect", which hides the registrations
  by other SMUX peers of objects within the subtree.  For example, if a
  SMUX peer registered "sysDescr", and later another SMUX peer
  registered "system" (which scopes "sysDescr"), then all requests for
  "sysDescr" would be given to the latter SMUX peer.

  An SNMP agent should disallow any attempt to register above, at, or
  below, the SNMP and SMUX subtrees of the MIB.  Other subtrees may be
  disallowed as an implementation-specific option.

3.1.2. Removing Registration

  A SMUX peer may remove registrations for only those subtrees which it
  has registered.  If the priority given in the RReqPDU is -1, then the
  registration of highest priority is selected for deletion.
  Otherwise, only that registration with the precise priority is
  selected.

3.1.3. Atomic Sets

  A simple two-phase commit protocol is used between the SNMP agent and
  the SMUX peers.  When an SNMP SetRequest-PDU is received, the SNMP
  agent determines which SMUX peers will participate in the
  transaction.  For each of these peers, at least one SNMP SetRequest-
  PDU is sent, with only those variables of interest to that peer.

  Each SMUX peer must either accept or refuse the set operation,

RFC 1227 SMUX May 1991

  without actually performing the operation.  If the peer opts to
  accept, it sends back an SNMP GetResponse-PDU with an error-status of
  noError(0); otherwise, if the peer refuses to perform the operation,
  then an SNMP GetResponse-PDU is returned with a non-zero error-
  status.

  The SNMP agent examines all of the responses.  If at least one SMUX
  peer refused the operation, then a SMUX SOutPDU is sent to each SMUX
  peer, with value rollback, telling the SMUX peer to discard any
  knowledge of the requested operation.

  Otherwise if all SMUX peers accepted the operation, then a SMUX
  SOutPDU is sent to each SMUX peer, with value commit, telling the
  SMUX peer to perform the operation.

  In either case, the SMUX peer does not generate a response to the
  SMUX SOutPDU.

3.1.4. Variables in Requests

  When constructing an SNMP GetRequest-PDU or GetNextRequest-PDU for a
  SMUX peer, the SNMP agent may send one, or more than one variable in
  a single request.  In all cases, the SNMP agent should process each
  variable sequentially, and block accordingly when a SMUX peer is
  contacted.

3.1.5. Request-ID

  When the SNMP agent constructs an SNMP GetRequest-PDU,
  GetNextRequest-PDU, or SetRequest-PDU, for a SMUX peer, the
  request_id field of the SNMP takes a special meaning: if an SNMP
  agent generates multiple PDUs for a SMUX peer, upon receipt of a
  single PDU from the network management station, then the request_id
  field of the PDUs sent to the SMUX peer must take the same value
  (which need bear no relationship to the value of the request_id field
  of the PDU originally received by the SNMP agent.)

3.1.6. The powerful get-next operator

  Each SMUX peer acts as though it contains the entire MIB when
  processing a SNMP GetNext-PDU from the SNMP agent.  This means that
  the SNMP agent must check each variable returned in the SNMP
  GetResponse-PDU generated by the SMUX peer to ensure that each
  variable is still within the same registered subtree as caused the
  SNMP GetNext-PDU to be sent to that peer.  For each variable which is
  not, the SNMP agent must include it in a SNMP GetNext-PDU to the peer
  for the succeeding registered subtree, until responses are available
  for all variables within their expected registered subtree.

RFC 1227 SMUX May 1991

3.2. Protocol Data Units

  The SMUX protocol data units are defined using Abstract Syntax
  Notation One (ASN.1) [3]:

  SMUX DEFINITIONS ::= BEGIN

  IMPORTS
          DisplayString, ObjectName
                  FROM RFC1155-SMI

          PDUs
                  FROM RFC1157-SNMP;

  -- tags for SMUX-specific PDUs are application-wide to
  -- avoid conflict with tags for current (and future)
  -- SNMP-generic PDUs

  SMUX-PDUs ::=
      CHOICE {
          open            -- SMUX peer uses
              OpenPDU,    -- immediately after TCP open

          close           -- either uses immediately before TCP close
              ClosePDU,

          registerRequest -- SMUX peer uses
              RReqPDU,

          registerResponse -- SNMP agent uses
              RRspPDU,

              PDUs,       -- note that roles are reversed:
                          --   SNMP agent does get/get-next/set
                          --   SMUX peer does get-response/trap

          commitOrRollback -- SNMP agent uses
              SOutPDU
     }

  -- open PDU
  -- currently only simple authentication

  OpenPDU ::=
      CHOICE {
         simple

RFC 1227 SMUX May 1991

             SimpleOpen
      }

  SimpleOpen ::=
      [APPLICATION 0] IMPLICIT
          SEQUENCE {
              version     -- of SMUX protocol
                  INTEGER {
                      version-1(0)
                  },

              identity    -- of SMUX peer, authoritative
                  OBJECT IDENTIFIER,

              description -- of SMUX peer, implementation-specific
                  DisplayString,

              password    -- zero length indicates no authentication
                  OCTET STRING
          }

  -- close PDU

  ClosePDU ::=
      [APPLICATION 1] IMPLICIT
          INTEGER {
              goingDown(0),
              unsupportedVersion(1),
              packetFormat(2),
              protocolError(3),
              internalError(4),
              authenticationFailure(5)
          }

  -- insert PDU

  RReqPDU ::=
      [APPLICATION 2] IMPLICIT
          SEQUENCE {
              subtree
                  ObjectName,

              priority    -- the lower the better, "-1" means default
                  INTEGER (-1..2147483647),

              operation

RFC 1227 SMUX May 1991

                  INTEGER {
                      delete(0),    -- remove registration
                      readOnly(1),  -- add registration, objects are RO
                      readWrite(2)  --   .., objects are RW
                  }
          }

  RRspPDU ::=
      [APPLICATION 3] IMPLICIT
          INTEGER {
              failure(-1)

             -- on success the non-negative priority is returned
          }

  SOutPDU ::=
      [APPLICATION 4] IMPLICIT
          INTEGER {
              commit(0),
              rollback(1)
          }

  END

3.3. Mappings on Transport Service

  The SMUX protocol may be mapped onto any CO-mode transport service.
  At present, only one such mapping is defined.

3.3.1. Mapping onto the TCP

  When using the TCP to provide the transport-backing for the SMUX
  protocol, the SNMP agent listens on TCP port 199.

  Each SMUX PDU is serialized using the Basic Encoding Rules [4] and
  sent on the TCP.  As ASN.1 objects are self-delimiting when encoding
  using the BER, no packetization protocol is required.

RFC 1227 SMUX May 1991

4. MIB for the SMUX

  The MIB objects for the SMUX are implemented by the local SNMP agent:

         SMUX-MIB DEFINITIONS ::= BEGIN

         IMPORTS
                 enterprises
                         FROM RFC1155-SMI
                 OBJECT-TYPE
                         FROM RFC1212;

         unix    OBJECT IDENTIFIER ::= { enterprises 4 }

         smux    OBJECT IDENTIFIER ::= { unix 4 }

         smuxPeerTable   OBJECT-TYPE
                 SYNTAX  SEQUENCE OF SmuxPeerEntry
                 ACCESS  not-accessible
                 STATUS  mandatory
                 DESCRIPTION
                     "The SMUX peer table."
                 ::= { smux 1 }

         smuxPeerEntry   OBJECT-TYPE
                 SYNTAX  SmuxPeerEntry
                 ACCESS  not-accessible
                 STATUS  mandatory
                 DESCRIPTION
                     "An entry in the SMUX peer table."
                 INDEX   { smuxPindex }
                 ::= { smuxPeerTable 1}

         SmuxPeerEntry ::=
             SEQUENCE {
                 smuxPindex
                     INTEGER,
                 smuxPidentity
                     OBJECT IDENTIFIER,
                 smuxPdescription
                     DisplayString,
                 smuxPstatus
                     INTEGER
             }

         smuxPindex      OBJECT-TYPE
                 SYNTAX  INTEGER
                 ACCESS  read-only

RFC 1227 SMUX May 1991

                 STATUS  mandatory
                 DESCRIPTION
                     "An index which uniquely identifies a SMUX peer."
                 ::= { smuxPeerEntry 1 }

         smuxPidentity   OBJECT-TYPE
                 SYNTAX  OBJECT IDENTIFIER
                 ACCESS  read-only
                 STATUS  mandatory
                 DESCRIPTION
                     "The authoritative designation for a SMUX peer."
                 ::= { smuxPeerEntry 2 }

         smuxPdescription OBJECT-TYPE
                 SYNTAX  DisplayString (SIZE (0..255))
                 ACCESS  read-only
                 STATUS  mandatory
                 DESCRIPTION
                     "A human-readable description of a SMUX peer."
                 ::= { smuxPeerEntry 3 }

         smuxPstatus     OBJECT-TYPE
                 SYNTAX  INTEGER { valid(1), invalid(2), connecting(3) }
                 ACCESS  read-write
                 STATUS  mandatory
                 DESCRIPTION
                     "The type of SMUX peer.

                     Setting this object to the value invalid(2) has
                     the effect of invaliding the corresponding entry
                     in the smuxPeerTable.  It is an implementation-
                     specific matter as to whether the agent removes an
                     invalidated entry from the table.  Accordingly,
                     management stations must be prepared to receive
                     tabular information from agents that correspond to
                     entries not currently in use.  Proper
                     interpretation of such entries requires
                     examination of the relative smuxPstatus object."
                 ::= { smuxPeerEntry 4 }

         smuxTreeTable   OBJECT-TYPE
                 SYNTAX  SEQUENCE OF SmuxTreeEntry
                 ACCESS  not-accessible
                 STATUS  mandatory
                 DESCRIPTION
                     "The SMUX tree table."
                 ::= { smux 2 }

RFC 1227 SMUX May 1991

         smuxTreeEntry   OBJECT-TYPE
                 SYNTAX  SmuxTreeEntry
                 ACCESS  not-accessible
                 STATUS  mandatory
                 DESCRIPTION
                     "An entry in the SMUX tree table."
                 INDEX   { smuxTsubtree, smuxTpriority }
                 ::= { smuxTreeTable 1}

         SmuxTreeEntry ::=
             SEQUENCE {
                 smuxTsubtree
                     OBJECT IDENTIFIER,
                 smuxTpriority
                     INTEGER,
                 smuxTindex
                     INTEGER,
                 smuxTstatus
                     INTEGER
             }

         smuxTsubtree    OBJECT-TYPE
                 SYNTAX  OBJECT IDENTIFIER
                 ACCESS  read-only
                 STATUS  mandatory
                 DESCRIPTION
                     "The MIB subtree being exported by a SMUX peer."
                 ::= { smuxTreeEntry 1 }

         smuxTpriority OBJECT-TYPE
                 SYNTAX  INTEGER (0..'07fffffff'h)
                 ACCESS  read-only
                 STATUS  mandatory
                 DESCRIPTION
                     "The SMUX peer's priority when exporting the MIB
                     subtree."
                 ::= { smuxTreeEntry 2 }

         smuxTindex OBJECT-TYPE
                 SYNTAX  INTEGER
                 ACCESS  read-only
                 STATUS  mandatory
                 DESCRIPTION
                     "The SMUX peer's identity."
                 ::= { smuxTreeEntry 3 }

         smuxTstatus     OBJECT-TYPE
                 SYNTAX  INTEGER { valid(1), invalid(2) }

RFC 1227 SMUX May 1991

                 ACCESS  read-write
                 STATUS  mandatory
                 DESCRIPTION
                     "The type of SMUX tree.

                     Setting this object to the value invalid(2) has
                     the effect of invaliding the corresponding entry
                     in the smuxTreeTable.  It is an implementation-
                     specific matter as to whether the agent removes an
                     invalidated entry from the table.  Accordingly,
                     management stations must be prepared to receive
                     tabular information from agents that correspond to
                     entries not currently in use.  Proper
                     interpretation of such entries requires
                     examination of the relative smuxTstatus object."
                 ::= { smuxTreeEntry 4 }

         END

5. Acknowledgements

  SMUX was designed one afternoon by these people:

              Jeffrey S. Case, UTK-CS
              James R. Davin, MIT-LCS
              Mark S. Fedor, PSI
              Jeffrey C. Honig, Cornell
              Louie A. Mamakos, UMD
              Michael G. Petry, UMD
              Yakov Rekhter, IBM
              Marshall T. Rose, PSI

6. References

  [1] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple
      Network Management Protocol", RFC 1157, SNMP Research,
      Performance Systems International, Performance Systems
      International, MIT Laboratory for Computer Science, May 1990.

  [2] McCloghrie K., and M. Rose, "Management Information Base for
      Network Management of TCP/IP-based Internets", RFC 1156,
      Performance Systems International and Hughes LAN Systems, May
      1990.

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

RFC 1227 SMUX May 1991

  [4] 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.

  [5] Rose, M., and K. McCloghrie, "Structure and Identification of
      Management Information for TCP/IP-based Internets", RFC 1155,
      Performance Systems International and Hughes LAN Systems, May
      1990.

7. Security Considerations

  Security issues are not discussed in this memo.

8. Author's Address

  Marshall T. Rose
  Performance Systems International, Inc.
  5201 Great America Parkway
  Suite 3106
  Santa Clara, CA  95054

  Phone: +1 408 562 6222

  EMail: [email protected]
  X.500:  rose, psi, us