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Keywords: [SNMP-MUX|h]
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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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
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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.
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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,
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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.
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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]:
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
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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
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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.
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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
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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 }
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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) }
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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.
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[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
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