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.\" smux.rf - SNMP MUX definitions
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.he 'draft'SMUX'May 1990'
.fo 'M.T. Rose''[Page %]'
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\fBSNMP MUX Protocol and MIB\fR
.sp 1
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Sun May 13 14:53:58 1990
.sp 2
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Marshall T. Rose
.sp 1
.ce 4
Performance Systems International, Inc.
11800 Sunrise Valley Drive
Suite 1100
Reston, VA  22091
.sp 1
.ce
[email protected]
.sp 5
.sh 1 "Status of this Memo"
.lp
This document defines a mechanism by which multiple UNIX daemons
may communicate with the local SNMP agent on a host.
This is a local mechanism.

.bp
.sh 1 "Introduction"
.lp
On kernel/user systems such as BSD UNIX,
an agent speaking the SNMP [1] is often implemented as a user-process,
which 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 the and other processes is problematic.
.lp
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.
.lp
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.
.lp
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.

.bp
.sh 1 "Architecture"
.lp
There are two approaches that can be taken when trying to integrate
arbitrary MIB modules with the SNMP agent: request-response and cache-ahead.
.lp
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.
.lp
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,
locally performs the operation,
and returns a response to the network management station.
The cache-ahead model is said to be peer-driven since,
after registration,
the SMUX peer initiates all transactions.
.lp
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.
.lp
Obviously,
the SMUX peer which employs the cache-ahead model acts as a \*(lqfirewall\*(rq
for those user-processes which actually implement the managed objects
in the given MIB module.
.lp
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.

.bp
.sh 1 "Protocol"
.lp
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.
.lp
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.
.lp
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.
.lp
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.
.lp
When either the SNMP agent or the SMUX peer wishes to release the SMUX
association,
the ClosePDU is issued and the connection is closed.

.sh 2 "Tricky Things"
.lp
Although straight-forward, there are a few nuances.

.sh 3 "Registration"
.lp
Associated with each registration is an integer priority,
from 0 to (2^31)-1.
The lower the value,
the higher the priority.
.lp
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.
.lp
When registering a subtree,
the special priority -1 may be used,
which selects the highest available priority.
.lp
Of course,
the SNMP agent may select an arbitrarily worse priority for a SMUX
peer,
based on local (configuration)  information.
.lp
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
\*(lqsubtree mounting effect\*(rq,
which hides the registrations by other SMUX peers of objects within
the subtree.
For example,
if a SMUX peer registered \*(lqsysDescr\*(rq,
and later another SMUX peer registered \*(lqsystem\*(rq
(which scopes \*(lqsysDescr\*(rq),
then all requests for \*(lqsysDescr\*(rq would be given to the latter
SMUX peer.
.lp
An SNMP agent should disallow any attempt to register at or within the SNMP
and SMUX subtrees of the MIB.
Other subtrees may be disallowed as an implementation-specific option.

.sh 3 "Removing Registration"
.lp
A SMUX peer may remove registrations for only those subtrees which it
has registered.
If the priority given in the RReq-PDU is -1,
then the registration of highest priority is selected for deletion.
Otherwise, only that registration with the precise registration
is selected.

.sh 3 "Variables in Requests"
.lp
When constructing an SNMP GetRequest-PDU, GetNextRequest-PDU, or
SetRequest-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.

.sh 3 "Request-ID"
.lp
When the SNMP agent constructs an SNMP GetRequest-PDU,
GetNextRequest-PDU, or SetRequest-PDU, for a SMUX peer,
the \fIrequest_id\fR field of the SNMP takes a special meaning.
Basically,
this field should take a different value for each SNMP PDU received by
the SNMP agent.
As such,
if an SNMP agent generates multiple PDUs for a SMUX peer,
upon receipt of a single PDU from the network management station,
then the \fIrequest_id\fR field of the PDUs sent to the SMUX peer
takes the same value
(which need bear no relationship to the value of the \fIrequest_id\fR
field of the PDU originally received by the SNMP agent.)

.sh 3 "The powerful get-next operator"
.lp
Each SMUX peer acts as though it contains the entire MIB when
processing the powerful get-next operator.
This means that the SNMP agent must check each variable named returned
in the SNMP GetResponse-PDU generated by a SMUX peer and see if it is in
the subtree of the managed object corresponding to original SNMP
GetNextRequest-PDU.
If not,
the SNMP agent will apply the get-next operator to the next managed object.
This may result in contacting a different SMUX peer,
depending on the registration topology.

.sh 2 "Protocol Data Units"
.lp
The SMUX protocol data units are defined using Abstract Syntax
Notation One (ASN.1) [3]:
.sp
.in +.5i
.nf
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
    }


-- 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),
		    readOnly(1),
		    readWrite(2)
		}
	}

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

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

END
.fi
.in -.5i
.sp

.sh 2 "Mappings on Transport Service"
.lp
The SMUX protocol may be mapped onto any CO-mode transport service.
At present,
only one such mapping is defined.

.sh 3 "Mapping onto the TCP"
.lp
When using the TCP to provide the transport-backing for the SMUX
protocol,
the SNMP agent listens on TCP port 199.
.lp
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,
so no packetization protocol is required.

.bp
.sh 1 "MIB for the SMUX"
.lp
The MIB objects for the SMUX are implemented by the local SNMP agent:
.sp
.in +.5i
.nf
SMUX-MIB DEFINITIONS ::= BEGIN

IMPORTS
	enterprises, OBJECT-TYPE, ObjectName
		FROM RFC1155-SMI;


unix    OBJECT IDENTIFIER ::= { enterprises 4 }


smux	OBJECT IDENTIFIER ::= { unix 4 }

smuxPeerTable	OBJECT-TYPE
	SYNTAX	SEQUENCE OF SmuxPeerEntry
	ACCESS	not-accessible
	STATUS	mandatory
	::= { smux 1 }

smuxPeerEntry	OBJECT-TYPE
	SYNTAX	SmuxPeerEntry
	ACCESS	not-accessible
	STATUS	mandatory
--	INDEX	{ smuxPindex }
	::= { smuxPeerTable 1}

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

smuxPindex	OBJECT-TYPE
	SYNTAX	INTEGER
	ACCESS	read-write
	STATUS	mandatory
	::= { smuxPeerEntry 1 }

smuxPidentity	OBJECT-TYPE
	SYNTAX	OBJECT IDENTIFIER
	ACCESS	read-write
	STATUS	mandatory
	::= { smuxPeerEntry 2 }

smuxPdescription OBJECT-TYPE
	SYNTAX	DisplayString (SIZE (0..255))
	ACCESS	read-write
	STATUS	mandatory
	::= { smuxPeerEntry 3 }

smuxPstatus	OBJECT-TYPE
	SYNTAX	INTEGER { valid(1), invalid(2), connecting(3) }
	ACCESS	read-write
	STATUS	mandatory
	::= { smuxPeerEntry 4 }

smuxTreeTable	OBJECT-TYPE
	SYNTAX	SEQUENCE OF SmuxTreeEntry
	ACCESS	not-accessible
	STATUS	mandatory
	::= { smux 2 }

smuxTreeEntry	OBJECT-TYPE
	SYNTAX	SmuxTreeEntry
	ACCESS	not-accessible
	STATUS	mandatory
	::= { smuxTreeTable 1}

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

smuxTsubtree	OBJECT-TYPE
	SYNTAX	ObjectName
	ACCESS	read-write
	STATUS	mandatory
--	INDEX	{ smuxTsubtree, smuxTpriority }
	::= { smuxTreeEntry 1 }

smuxTpriority OBJECT-TYPE
	SYNTAX	INTEGER (0..2147483647)
	ACCESS	read-write
	STATUS	mandatory
	::= { smuxTreeEntry 2 }

smuxTindex OBJECT-TYPE
	SYNTAX	INTEGER (0..2147483647)
	ACCESS	read-write
	STATUS	mandatory
	::= { smuxTreeEntry 3 }

smuxTstatus	OBJECT-TYPE
	SYNTAX	INTEGER { valid(1), invalid(2) }
	ACCESS	read-write
	STATUS	mandatory
	::= { smuxTreeEntry 4 }

END
.fi
.in -.5i
.sp

.sh 2 "Identification of OBJECT instances for use with the SNMP"
.lp
The names for all object types in the MIB are defined explicitly either
in the Internet-standard MIB or in other documents which conform to
the naming conventions of the SMI[5]. The SMI requires
that conformant management protocols define mechanisms
for identifying individual instances of those object types
for a particular network element.
.lp
Each instance of any object type defined in the MIB is identified
in SNMP operations by a unique name called its "variable name."
In general, the name of an SNMP variable is an OBJECT IDENTIFIER
of the form x.y, where x is the name of a non-aggregate object type 
defined in the MIB and y is an OBJECT IDENTIFIER fragment
that, in a way specific to the named object type, identifies the
desired instance.
.lp
This naming strategy admits the fullest exploitation of the
semantics of the powerful SNMP get-next operator,
because it assigns names for related variables so as to be contiguous
in the lexicographical ordering of all variable names
known in the MIB.
.lp
The type-specific naming of object instances is defined below
for a number of classes of object types.
Instances of
an object type to which
none of the following naming conventions are applicable are
named by OBJECT IDENTIFIERs of the form
x.0, where x is the name of said object type in the MIB definition.
.lp
For example,
suppose one wanted to identify an instance of the variable sysDescr.
The object class for sysDescr is:
.sp
.in +0.25
.nf
iso org dod internet mgmt mib system sysDescr
 1   3   6     1      2    1    1       1
.fi
.in -0.25
.sp
Hence,
the object type, x, would be 1.3.6.1.2.1.1.1 to which
is appended an instance sub-identifier of 0.
That is, 1.3.6.1.2.1.1.1.0
identifies the one and only instance of sysDescr.

.sh 3 "smuxPeerTable Object Type Names"
.lp
The name of a SMUX peer relationship, s, is the OBJECT IDENTIFIER
value of the form i, where i has the value of that
instance of the smuxPindex object type associated with s.
.lp
For each object type, t, for which the defined name, n, has
a prefix of smuxPeerEntry, an instance, i, of t is named by an OBJECT
IDENTIFIER of the form n.s, where s is the name of the SMUX peer relationship
about which i represents information.
.lp
For example,
suppose one wanted to identify the instance of the variable smuxPidentity
associated with peer relationship number 2.
Accordingly, smuxPidentity.2 would identify the desired instance.

.sh 3 "smuxTreeTable Object Type Names"
.lp
The name of a SMUX subtree relationship, s, is the OBJECT IDENTIFIER
value of the form i.j, where i has the value of that
instance of the smuxTsubtree object type,
and j has the value of that instance of the smuxTpriority object type,
associated with s.
.lp
For each object type, t, for which the defined name, n, has
a prefix of smuxTreeEntry, an instance, i, of t is named by an OBJECT
IDENTIFIER of the form n.s.t.u,
where s is the number of sub-identifiers in the corresponding instance
of smuxTsubtree,
t is the value of the instance of smuxTsubtree corresponding to this
entry,
and u is the value of the instance of smuxTpriority corresponding to
this entry.
.lp
For example,
suppose one wanted to identify the instance of the variable smuxTindex
associated with the subtree 1.3.6.1.5 for priority 3.
Accordingly, smuxTindex.5.1.3.6.1.5.3 would identify the desired instance.

.bp
.sh 1 "Acknowledgements"
.lp
SMUX was designed one afternoon by these people:
.sp
.in +.5i
.nf
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
.fi
.in -.5i
.sp

.bp
.sh 1 "References"
.ip [1]
J.D.\0Case, M.S.\0Fedor, M.L.\0Schoffstall, and J.R.\0Davin,
\fISimple Network Management Protocol\fR,
Internet Working Group Request for Comments 1157.
Network Information Center, SRI International, Menlo Park, California,
(May, 1990).
.ip [2]
K.\0McCloghrie and M.T.\0Rose,
\fIManagement Information Base for Network Management of TCP/IP-based
internets\fR,
Internet Working Group Request for Comments 1156.
Network Information Center, SRI International, Menlo Park, California,
(May, 1990).
.ip [3]
\fIInformation processing systems \(em Open Systems Interconnection \(em
Specification of Abstract Syntax Notation One (ASN.1)\fP, International
Organization for Standardization.  International Standard 8824,
(December, 1987).
.ip [4]
\fIInformation processing systems \(em Open Systems Interconnection \(em
Specification of Basic Encoding Rules for Abstract Notation
One (ASN.1)\fP, International Organization for Standardization.
International Standard 8825,
(December, 1987).
.ip [5]
M.T.\0Rose and K.\0McCloghrie,
\fIStructure and Identification of Management Information for
TCP/IP\-based internets\fR,
Internet Working Group Request for Comments 1155.
Network Information Center, SRI International, Menlo Park, California,
(May, 1990).

.bp
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\fBTable of Contents\fR
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