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
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
Rose [Page 1]
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.
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
Rose [Page 2]
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.
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.
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.
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.
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,
Rose [Page 4]
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.
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.
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.)
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.
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.
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 }
Rose [Page 10]
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}
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 }
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 }
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
[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.
Rose [Page 12]
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.