RFC 1227

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

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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

   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

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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

   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.

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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

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,

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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-

   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

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

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.

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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]:


           DisplayString, ObjectName
                   FROM RFC1155-SMI

                   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

           registerRequest -- SMUX peer uses

           registerResponse -- SNMP agent uses

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

           commitOrRollback -- SNMP agent uses

   -- open PDU
   -- currently only simple authentication

   OpenPDU ::=
       CHOICE {

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   SimpleOpen ::=
           SEQUENCE {
               version     -- of SMUX protocol
                   INTEGER {

               identity    -- of SMUX peer, authoritative
                   OBJECT IDENTIFIER,

               description -- of SMUX peer, implementation-specific

               password    -- zero length indicates no authentication
                   OCTET STRING

   -- close PDU

   ClosePDU ::=
           INTEGER {

   -- insert PDU

   RReqPDU ::=
           SEQUENCE {

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


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

   RRspPDU ::=
           INTEGER {

              -- on success the non-negative priority is returned

   SOutPDU ::=
           INTEGER {


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.

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RFC 1227                          SMUX                          May 1991

4.  MIB for the SMUX

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


                          FROM RFC1155-SMI
                          FROM RFC1212;

          unix    OBJECT IDENTIFIER ::= { enterprises 4 }

          smux    OBJECT IDENTIFIER ::= { unix 4 }

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

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

          SmuxPeerEntry ::=
              SEQUENCE {
                      OBJECT IDENTIFIER,

          smuxPindex      OBJECT-TYPE
                  SYNTAX  INTEGER
                  ACCESS  read-only

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

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

          smuxPdescription OBJECT-TYPE
                  SYNTAX  DisplayString (SIZE (0..255))
                  ACCESS  read-only
                  STATUS  mandatory
                      "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
                      "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
                      "The SMUX tree table."
                  ::= { smux 2 }

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RFC 1227                          SMUX                          May 1991

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

          SmuxTreeEntry ::=
              SEQUENCE {
                      OBJECT IDENTIFIER,

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

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

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

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

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RFC 1227                          SMUX                          May 1991

                  ACCESS  read-write
                  STATUS  mandatory
                      "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 }


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

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

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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

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: mrose@psi.com
   X.500:  rose, psi, us

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