Network Working Group SNMPv2 Working Group

Network Working Group SNMPv2 Working Group
Request for Comments: 1906 J. Case
Obsoletes: 1449 SNMP Research, Inc.
Category: Standards Track K. McCloghrie
Cisco Systems, Inc.
M. Rose
Dover Beach Consulting, Inc.
S. Waldbusser
International Network Services
January 1996

Transport Mappings for Version 2 of the
Simple Network Management Protocol (SNMPv2)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Table of Contents

1. Introduction ................................................ 2
1.1 A Note on Terminology ...................................... 2
2. Definitions ................................................. 3
3. SNMPv2 over UDP ............................................. 5
3.1 Serialization .............................................. 5
3.2 Well-known Values .......................................... 5
4. SNMPv2 over OSI ............................................. 6
4.1 Serialization .............................................. 6
4.2 Well-known Values .......................................... 6
5. SNMPv2 over DDP ............................................. 6
5.1 Serialization .............................................. 6
5.2 Well-known Values .......................................... 6
5.3 Discussion of AppleTalk Addressing ......................... 7
5.3.1 How to Acquire NBP names ................................. 8
5.3.2 When to Turn NBP names into DDP addresses ................ 8
5.3.3 How to Turn NBP names into DDP addresses ................. 8
5.3.4 What if NBP is broken .................................... 9
6. SNMPv2 over IPX ............................................. 9
6.1 Serialization .............................................. 9
6.2 Well-known Values .......................................... 9
7. Proxy to SNMPv1 ............................................. 10
8. Serialization using the Basic Encoding Rules ................ 10
8.1 Usage Example .............................................. 11

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9. Security Considerations ..................................... 11
10. Editor's Address ........................................... 12
11. Acknowledgements ........................................... 12
12. References ................................................. 13

1. Introduction

A management system contains: several (potentially many) nodes, each
with a processing entity, termed an agent, which has access to
management instrumentation; at least one management station; and, a
management protocol, used to convey management information between
the agents and management stations. Operations of the protocol are
carried out under an administrative framework which defines
authentication, authorization, access control, and privacy policies.

Management stations execute management applications which monitor and
control managed elements. Managed elements are devices such as
hosts, routers, terminal servers, etc., which are monitored and
controlled via access to their management information.

The management protocol, version 2 of the Simple Network Management
Protocol [1], may be used over a variety of protocol suites. It is
the purpose of this document to define how the SNMPv2 maps onto an
initial set of transport domains. Other mappings may be defined in
the future.

Although several mappings are defined, the mapping onto UDP is the
preferred mapping. As such, to provide for the greatest level of
interoperability, systems which choose to deploy other mappings
should also provide for proxy service to the UDP mapping.

1.1. A Note on Terminology

For the purpose of exposition, the original Internet-standard Network
Management Framework, as described in RFCs 1155 (STD 16), 1157 (STD
15), and 1212 (STD 16), is termed the SNMP version 1 framework
(SNMPv1). The current framework is termed the SNMP version 2
framework (SNMPv2).

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2. Definitions

SNMPv2-TM DEFINITIONS ::= BEGIN

IMPORTS
OBJECT-IDENTITY, snmpDomains, snmpProxys
FROM SNMPv2-SMI
TEXTUAL-CONVENTION
FROM SNMPv2-TC;

-- SNMPv2 over UDP over IPv4

snmpUDPDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMPv2 over UDP transport domain. The corresponding
transport address is of type SnmpUDPAddress."
::= { snmpDomains 1 }

SnmpUDPAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1d.1d.1d.1d/2d"
STATUS current
DESCRIPTION
"Represents a UDP address:

octets contents encoding
1-4 IP-address network-byte order
5-6 UDP-port network-byte order
"
SYNTAX OCTET STRING (SIZE (6))

-- SNMPv2 over OSI

snmpCLNSDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMPv2 over CLNS transport domain. The corresponding
transport address is of type SnmpOSIAddress."
::= { snmpDomains 2 }

snmpCONSDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMPv2 over CONS transport domain. The corresponding
transport address is of type SnmpOSIAddress."
::= { snmpDomains 3 }

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SnmpOSIAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "*1x:/1x:"
STATUS current
DESCRIPTION
"Represents an OSI transport-address:

octets contents encoding
1 length of NSAP 'n' as an unsigned-integer
(either 0 or from 3 to 20)
2..(n+1) NSAP concrete binary representation
(n+2)..m TSEL string of (up to 64) octets
"
SYNTAX OCTET STRING (SIZE (1 | 4..85))

-- SNMPv2 over DDP

snmpDDPDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMPv2 over DDP transport domain. The corresponding
transport address is of type SnmpNBPAddress."
::= { snmpDomains 4 }

SnmpNBPAddress ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Represents an NBP name:

octets contents encoding
1 length of object 'n' as an unsigned integer
2..(n+1) object string of (up to 32) octets
n+2 length of type 'p' as an unsigned integer
(n+3)..(n+2+p) type string of (up to 32) octets
n+3+p length of zone 'q' as an unsigned integer
(n+4+p)..(n+3+p+q) zone string of (up to 32) octets

For comparison purposes, strings are case-insensitive All
strings may contain any octet other than 255 (hex ff)."
SYNTAX OCTET STRING (SIZE (3..99))

-- SNMPv2 over IPX

snmpIPXDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMPv2 over IPX transport domain. The corresponding

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transport address is of type SnmpIPXAddress."
::= { snmpDomains 5 }

SnmpIPXAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "4x.1x:1x:1x:1x:1x:1x.2d"
STATUS current
DESCRIPTION
"Represents an IPX address:

octets contents encoding
1-4 network-number network-byte order
5-10 physical-address network-byte order
11-12 socket-number network-byte order
"
SYNTAX OCTET STRING (SIZE (12))

-- for proxy to SNMPv1 (RFC 1157)

rfc1157Proxy OBJECT IDENTIFIER ::= { snmpProxys 1 }

rfc1157Domain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The transport domain for SNMPv1 over UDP. The
corresponding transport address is of type SnmpUDPAddress."
::= { rfc1157Proxy 1 }

-- ::= { rfc1157Proxy 2 } this OID is obsolete

END

3. SNMPv2 over UDP

This is the preferred transport mapping.

3.1. Serialization

Each instance of a message is serialized (i.e., encoded according to
the convention of [1]) onto a single UDP[2] datagram, using the
algorithm specified in Section 8.

3.2. Well-known Values

It is suggested that administrators configure their SNMPv2 entities
acting in an agent role to listen on UDP port 161. Further, it is
suggested that notification sinks be configured to listen on UDP port

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162.

When an SNMPv2 entity uses this transport mapping, it must be capable
of accepting messages that are at least 484 octets in size.
Implementation of larger values is encouraged whenever possible.

4. SNMPv2 over OSI

This is an optional transport mapping.

4.1. Serialization

Each instance of a message is serialized onto a single TSDU [3,4] for
the OSI Connectionless-mode Transport Service (CLTS), using the
algorithm specified in Section 8.

4.2. Well-known Values

It is suggested that administrators configure their SNMPv2 entities
acting in an agent role to listen on transport selector "snmp-l"
(which consists of six ASCII characters), when using a CL-mode
network service to realize the CLTS. Further, it is suggested that
notification sinks be configured to listen on transport selector
"snmpt-l" (which consists of seven ASCII characters, six letters and
a hyphen) when using a CL-mode network service to realize the CLTS.
Similarly, when using a CO-mode network service to realize the CLTS,
the suggested transport selectors are "snmp-o" and "snmpt-o", for
agent and notification sink, respectively.

When an SNMPv2 entity uses this transport mapping, it must be capable
of accepting messages that are at least 484 octets in size.
Implementation of larger values is encouraged whenever possible.

5. SNMPv2 over DDP

This is an optional transport mapping.

5.1. Serialization

Each instance of a message is serialized onto a single DDP datagram
[5], using the algorithm specified in Section 8.

5.2. Well-known Values

SNMPv2 messages are sent using DDP protocol type 8. SNMPv2 entities
acting in an agent role listens on DDP socket number 8, whilst
notification sinks listen on DDP socket number 9.

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Administrators must configure their SNMPv2 entities acting in an
agent role to use NBP type "SNMP Agent" (which consists of ten ASCII
characters), whilst notification sinks must be configured to use NBP
type "SNMP Trap Handler" (which consists of seventeen ASCII
characters).

The NBP name for agents and notification sinks should be stable - NBP
names should not change any more often than the IP address of a
typical TCP/IP node. It is suggested that the NBP name be stored in
some form of stable storage.

When an SNMPv2 entity uses this transport mapping, it must be capable
of accepting messages that are at least 484 octets in size.
Implementation of larger values is encouraged whenever possible.

5.3. Discussion of AppleTalk Addressing

The AppleTalk protocol suite has certain features not manifest in the
TCP/IP suite. AppleTalk's naming strategy and the dynamic nature of
address assignment can cause problems for SNMPv2 entities that wish
to manage AppleTalk networks. TCP/IP nodes have an associated IP
address which distinguishes each from the other. In contrast,
AppleTalk nodes generally have no such characteristic. The network-
level address, while often relatively stable, can change at every
reboot (or more frequently).

Thus, when SNMPv2 is mapped over DDP, nodes are identified by a
"name", rather than by an "address". Hence, all AppleTalk nodes that
implement this mapping are required to respond to NBP lookups and
confirms (e.g., implement the NBP protocol stub), which guarantees
that a mapping from NBP name to DDP address will be possible.

In determining the SNMP identity to register for an SNMPv2 entity, it
is suggested that the SNMP identity be a name which is associated
with other network services offered by the machine.

NBP lookups, which are used to map NBP names into DDP addresses, can
cause large amounts of network traffic as well as consume CPU
resources. It is also the case that the ability to perform an NBP
lookup is sensitive to certain network disruptions (such as zone
table inconsistencies) which would not prevent direct AppleTalk
communications between two SNMPv2 entities.

Thus, it is recommended that NBP lookups be used infrequently,
primarily to create a cache of name-to-address mappings. These
cached mappings should then be used for any further SNMP traffic. It
is recommended that SNMPv2 entities acting in a manager role should
maintain this cache between reboots. This caching can help minimize

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network traffic, reduce CPU load on the network, and allow for (some
amount of) network trouble shooting when the basic name-to-address
translation mechanism is broken.

5.3.1. How to Acquire NBP names

An SNMPv2 entity acting in a manager role may have a pre-configured
list of names of "known" SNMPv2 entities acting in an agent role.
Similarly, an SNMPv2 entity acting in a manager role might interact
with an operator. Finally, an SNMPv2 entity acting in a manager role
might communicate with all SNMPv2 entities acting in an agent role in
a set of zones or networks.

5.3.2. When to Turn NBP names into DDP addresses

When an SNMPv2 entity uses a cache entry to address an SNMP packet,
it should attempt to confirm the validity mapping, if the mapping
hasn't been confirmed within the last T1 seconds. This cache entry
lifetime, T1, has a minimum, default value of 60 seconds, and should
be configurable.

An SNMPv2 entity acting in a manager role may decide to prime its
cache of names prior to actually communicating with another SNMPv2
entity. In general, it is expected that such an entity may want to
keep certain mappings "more current" than other mappings, e.g., those
nodes which represent the network infrastructure (e.g., routers) may
be deemed "more important".

Note that an SNMPv2 entity acting in a manager role should not prime
its entire cache upon initialization - rather, it should attempt
resolutions over an extended period of time (perhaps in some pre-
determined or configured priority order). Each of these resolutions
might, in fact, be a wildcard lookup in a given zone.

An SNMPv2 entity acting in an agent role must never prime its cache.
Such an entity should do NBP lookups (or confirms) only when it needs
to send an SNMP trap. When generating a response, such an entity
does not need to confirm a cache entry.

5.3.3. How to Turn NBP names into DDP addresses

If the only piece of information available is the NBP name, then an
NBP lookup should be performed to turn that name into a DDP address.
However, if there is a piece of stale information, it can be used as
a hint to perform an NBP confirm (which sends a unicast to the
network address which is presumed to be the target of the name
lookup) to see if the stale information is, in fact, still valid.

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An NBP name to DDP address mapping can also be confirmed implicitly
using only SNMP transactions. For example, an SNMPv2 entity acting
in a manager role issuing a retrieval operation could also retrieve
the relevant objects from the NBP group [6] for the SNMPv2 entity
acting in an agent role. This information can then be correlated
with the source DDP address of the response.

5.3.4. What if NBP is broken

Under some circumstances, there may be connectivity between two
SNMPv2 entities, but the NBP mapping machinery may be broken, e.g.,

o the NBP FwdReq (forward NBP lookup onto local attached network)
mechanism might be broken at a router on the other entity's
network; or,

o the NBP BrRq (NBP broadcast request) mechanism might be broken
at a router on the entity's own network; or,

o NBP might be broken on the other entity's node.

An SNMPv2 entity acting in a manager role which is dedicated to
AppleTalk management might choose to alleviate some of these failures
by directly implementing the router portion of NBP. For example,
such an entity might already know all the zones on the AppleTalk
internet and the networks on which each zone appears. Given an NBP
lookup which fails, the entity could send an NBP FwdReq to the
network in which the agent was last located. If that failed, the
station could then send an NBP LkUp (NBP lookup packet) as a directed
(DDP) multicast to each network number on that network. Of the above
(single) failures, this combined approach will solve the case where
either the local router's BrRq-to-FwdReq mechanism is broken or the
remote router's FwdReq-to-LkUp mechanism is broken.

6. SNMPv2 over IPX

This is an optional transport mapping.

6.1. Serialization

Each instance of a message is serialized onto a single IPX datagram
[7], using the algorithm specified in Section 8.

6.2. Well-known Values

SNMPv2 messages are sent using IPX packet type 4 (i.e., Packet
Exchange Protocol).

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It is suggested that administrators configure their SNMPv2 entities
acting in an agent role to listen on IPX socket 36879 (900f
hexadecimal). Further, it is suggested that notification sinks be
configured to listen on IPX socket 36880 (9010 hexadecimal)

When an SNMPv2 entity uses this transport mapping, it must be capable
of accepting messages that are at least 546 octets in size.
Implementation of larger values is encouraged whenever possible.

7. Proxy to SNMPv1

In order to provide proxy to SNMPv1 [8], it may be useful to define a
transport domain, rfc1157Domain, which indicates the transport
mapping for SNMP messages as defined in RFC 1157. Section 3.1 of [9]
specifies the behavior of the proxy agent.

8. Serialization using the Basic Encoding Rules

When the Basic Encoding Rules [10] are used for serialization:

(1) When encoding the length field, only the definite form is used; use
of the indefinite form encoding is prohibited. Note that when
using the definite-long form, it is permissible to use more than
the minimum number of length octets necessary to encode the length
field.

(2) When encoding the value field, the primitive form shall be used for
all simple types, i.e., INTEGER, OCTET STRING, and OBJECT
IDENTIFIER (either IMPLICIT or explicit). The constructed form of
encoding shall be used only for structured types, i.e., a SEQUENCE
or an IMPLICIT SEQUENCE.

(3) When encoding an object whose syntax is described using the BITS
construct, the value is encoded as an OCTET STRING, in which all
the named bits in (the definition of) the bitstring, commencing
with the first bit and proceeding to the last bit, are placed in
bits 8 to 1 of the first octet, followed by bits 8 to 1 of each
subsequent octet in turn, followed by as many bits as are needed of
the final subsequent octet, commencing with bit 8. Remaining bits,
if any, of the final octet are set to zero on generation and
ignored on receipt.

These restrictions apply to all aspects of ASN.1 encoding, including
the message wrappers, protocol data units, and the data objects they
contain.

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RFC 1906 Transport Mappings for SNMPv2 January 1996

8.1. Usage Example

As an example of applying the Basic Encoding Rules, suppose one
wanted to encode an instance of the GetBulkRequest-PDU [1]:

[5] IMPLICIT SEQUENCE {
request-id 1414684022,
non-repeaters 1,
max-repetitions 2,
variable-bindings {
{ name sysUpTime,
value { unspecified NULL } },
{ name ipNetToMediaPhysAddress,
value { unspecified NULL } },
{ name ipNetToMediaType,
value { unspecified NULL } }
}
}

Applying the BER, this would be encoded (in hexadecimal) as:

[5] IMPLICIT SEQUENCE a5 82 00 39
INTEGER 02 04 52 54 5d 76
INTEGER 02 01 01
INTEGER 02 01 02
SEQUENCE 30 2b
SEQUENCE 30 0b
OBJECT IDENTIFIER 06 07 2b 06 01 02 01 01 03
NULL 05 00
SEQUENCE 30 0d
OBJECT IDENTIFIER 06 09 2b 06 01 02 01 04 16 01 02
NULL 05 00
SEQUENCE 30 0d
OBJECT IDENTIFIER 06 09 2b 06 01 02 01 04 16 01 04
NULL 05 00

Note that the initial SEQUENCE is not encoded using the minimum
number of length octets. (The first octet of the length, 82,
indicates that the length of the content is encoded in the next two
octets.)

9. Security Considerations

Security issues are not discussed in this memo.

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10. Editor's Address

Keith McCloghrie
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
US

Phone: +1 408 526 5260
EMail: [email protected]

11. Acknowledgements

This document is the result of significant work by the four major
contributors:

Jeffrey D. Case (SNMP Research, [email protected])
Keith McCloghrie (Cisco Systems, [email protected])
Marshall T. Rose (Dover Beach Consulting, [email protected])
Steven Waldbusser (International Network Services, [email protected])

In addition, the contributions of the SNMPv2 Working Group are
acknowledged. In particular, a special thanks is extended for the
contributions of:

Alexander I. Alten (Novell)
Dave Arneson (Cabletron)
Uri Blumenthal (IBM)
Doug Book (Chipcom)
Kim Curran (Bell-Northern Research)
Jim Galvin (Trusted Information Systems)
Maria Greene (Ascom Timeplex)
Iain Hanson (Digital)
Dave Harrington (Cabletron)
Nguyen Hien (IBM)
Jeff Johnson (Cisco Systems)
Michael Kornegay (Object Quest)
Deirdre Kostick (AT&T Bell Labs)
David Levi (SNMP Research)
Daniel Mahoney (Cabletron)
Bob Natale (ACE*COMM)
Brian O'Keefe (Hewlett Packard)
Andrew Pearson (SNMP Research)
Dave Perkins (Peer Networks)
Randy Presuhn (Peer Networks)
Aleksey Romanov (Quality Quorum)
Shawn Routhier (Epilogue)
Jon Saperia (BGS Systems)

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RFC 1906 Transport Mappings for SNMPv2 January 1996

Bob Stewart (Cisco Systems, [email protected]), chair
Kaj Tesink (Bellcore)
Glenn Waters (Bell-Northern Research)
Bert Wijnen (IBM)

12. References

[1] SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
S. Waldbusser, "Protocol Operations for Version 2 of the Simple
Network Management Protocol (SNMPv2)", RFC 1905, January 1996.

[2] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
USC/Information Sciences Institute, August 1980.

[3] Information processing systems - Open Systems Interconnection -
Transport Service Definition, International Organization for
Standardization. International Standard 8072, (June, 1986).

[4] Information processing systems - Open Systems Interconnection -
Transport Service Definition - Addendum 1: Connectionless-mode
Transmission, International Organization for Standardization.
International Standard 8072/AD 1, (December, 1986).

[5] G. Sidhu, R. Andrews, A. Oppenheimer, Inside AppleTalk (second
edition). Addison-Wesley, 1990.

[6] Waldbusser, S., "AppleTalk Management Information Base", RFC 1243,
Carnegie Mellon University, July 1991.

[7] Network System Technical Interface Overview. Novell, Inc, (June,
1989).

[8] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple Network
Management Protocol", STD 15, RFC 1157, SNMP Research, Performance
Systems International, MIT Laboratory for Computer Science, May
1990.

[9] SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
S. Waldbusser, "Coexistence between Version 1 and Version 2 of the
Internet-standard Network Management Framework", RFC 1908,
January 1996.

[10] Information processing systems - Open Systems Interconnection -
Specification of Basic Encoding Rules for Abstract Syntax Notation
One (ASN.1), International Organization for Standardization.
International Standard 8825, December 1987.

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