Network Working Group
Request for Comments: 1227
M. Rose
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 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 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.

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, 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.

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
              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
           
                   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.

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
                  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 }
          
          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) }
                  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.
   
   [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: mrose@psi.com
   X.500:  rose, psi, us