Network Working Group J. Schoenwaelder
Internet-Draft TU Braunschweig
Expires: August 26, 2002 February 25, 2002
SNMP over TCP Transport Mapping
draft-irtf-nmrg-snmp-tcp-07.txt
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This memo defines a transport mapping for using the Simple Network
Management Protocol (SNMP) over TCP. The transport mapping can be
used with any version of SNMP. This document extends the transport
mappings defined in RFC 1906.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. SNMP over TCP . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Serialization . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 Well-Known Values . . . . . . . . . . . . . . . . . . . . . . 6
3.3 Connection Management . . . . . . . . . . . . . . . . . . . . 6
3.4 Reliable Transport versus Confirmed Operations . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
A. Connection Establishment Alternatives . . . . . . . . . . . . 10
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The SNMP Management Framework presently consists of five major
components:
o An overall architecture, described in RFC 2571 [1].
o Mechanisms for describing and naming objects and events for the
purpose of management. The first version of this Structure of
Management Information (SMI) is called SMIv1 and described in STD
16, RFC 1155 [2], STD 16, RFC 1212 [3] and RFC 1215 [4]. The
second version, called SMIv2, is described in STD 58, RFC 2578
[5], STD 58, RFC 2579 [6] and STD 58, RFC 2580 [7].
o Message protocols for transferring management information. The
first version of the SNMP message protocol is called SNMPv1 and
described in STD 15, RFC 1157 [8]. A second version of the SNMP
message protocol, which is not an Internet standards track
protocol, is called SNMPv2c and described in RFC 1901 [9] and RFC
1906 [10]. The third version of the message protocol is called
SNMPv3 and described in RFC 1906 [10], RFC 2572 [11] and RFC 2574
[12].
o Protocol operations for accessing management information. The
first set of protocol operations and associated PDU formats is
described in STD 15, RFC 1157 [8]. A second set of protocol
operations and associated PDU formats is described in RFC 1905
[13].
o A set of fundamental applications described in RFC 2573 [14] and
the view-based access control mechanism described in RFC 2575
[15].
A more detailed introduction to the current SNMP Management Framework
can be found in RFC 2570 [16].
Managed objects are accessed via a virtual information store, termed
the Management Information Base or MIB. Objects in the MIB are
defined using the mechanisms defined in the SMI.
This memo defines a transport mapping for using the Simple Network
Management Protocol (SNMP) over TCP. The transport mapping can be
used with any version of SNMP. This document extends the transport
mappings defined in RFC 1906 [10].
The SNMP over TCP transport mapping is an optional transport mapping.
SNMP protocol engines that implement the SNMP over TCP transport
mapping MUST also implement the SNMP over UDP transport mapping as
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defined in RFC 1906 [10].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [17].
2. Definitions
IRTF-NMRG-SNMP-TCP-TM DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-IDENTITY, experimental
FROM SNMPv2-SMI
TEXTUAL-CONVENTION
FROM SNMPv2-TC;
nmrgSnmpDomains MODULE-IDENTITY
LAST-UPDATED "200202250000Z"
ORGANIZATION "IRTF Network Management Research Group"
CONTACT-INFO
"Juergen Schoenwaelder
TU Braunschweig
Bueltenweg 74/75
38106 Braunschweig
Germany
Phone: +49 531 391-3283
Email: schoenw@ibr.cs.tu-bs.de"
DESCRIPTION
"This MIB module defines the SNMP over TCP transport mapping."
REVISION "200202250000Z"
DESCRIPTION
"Initial version, published as RFC XXXX."
::= { experimental nmrg(91) 1 }
-- SNMP over TCP over IPv4
snmpTCPDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMP over TCP over IPv4 transport domain. The
corresponding transport address is of type SnmpTCPAddress."
::= { nmrgSnmpDomains 1 }
SnmpTCPAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1d.1d.1d.1d/2d"
STATUS current
DESCRIPTION
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"Represents a TCP/IPv4 address:
octets contents encoding
1-4 IP-address network-byte order
5-6 TCP-port network-byte order
"
SYNTAX OCTET STRING (SIZE (6))
END
3. SNMP over TCP
SNMP over TCP is an experimental optional transport mapping. It is
primarily defined to support more efficient bulk transfer mechanisms
within the SNMP framework [20].
The originator of a request/response transaction chooses the
transport protocol for the entire transaction. The transport
protocol MUST NOT change during a transaction.
In general, originators of request/response transactions are free to
use the transport they assume is the best in a given situation.
However, since TCP has a larger footprint on resource usage than UDP,
engines using SNMP over TCP may choose to switch back to UDP by
refusing new TCP connections whenever necessary (e.g. too many open
TCP connections).
When selecting the transport, it is useful to consider how SNMP
interacts with TCP acknowledgements and timers. In particular,
infrequent SNMP interactions over TCP may lead to additional IP
packets carrying acknowledgements for SNMP responses if there is no
chance to piggyback them. Furthermore, it is recommended to
configure SNMP timers to fire later when using SNMP over TCP to avoid
application specific timeouts before the TCP timers have expired.
3.1 Serialization
Each instance of a message is serialized into a single BER-encoded
message, using the algorithm specified in Section 8 of RFC 1906 [10].
The BER-encoded message is then sent over a TCP connection. An SNMP
engine MUST NOT interleave SNMP messages within the TCP byte stream.
All the bytes of one SNMP message must be sent before any bytes of a
different SNMP message.
It is possible to exchange multiple SNMP request/response pairs over
a single (persistent) TCP connection. TCP connections are per
default full-duplex and data can travel in both directions at
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different speeds. It is therefore possible to send multiple SNMP
messages to a remote SNMP engine before receiving responses from the
same SNMP engine. Note that an SNMP engine is not required to return
responses in the same order as it received the requests.
It is possible that the underlying TCP implementation delivers byte
sequences that do not coincide with SNMP message boundaries. A
receiving SNMP engine MUST therefore use the length field in the BER-
encoded SNMP message to separate multiple requests sent over a single
TCP connection.
3.2 Well-Known Values
It is RECOMMENDED that administrators configure their SNMP entities
containing command responders to listen on TCP port 161 for incoming
connections. It is also RECOMMENDED that SNMP entities containing
notification receivers be configured to listen on TCP port 162 for
connection requests.
When an SNMP entity uses the TCP transport mapping, it MUST be
capable of accepting messages that are at least 8192 octets in size.
Implementation of larger values is encouraged whenever possible.
3.3 Connection Management
The use of TCP connections introduces costs [18]. Connection
establishment and teardown cause additional network traffic.
Furthermore, maintaining open connections binds resources in the
network layer of the underlying operating system.
SNMP over TCP is intended to be used when the size of the transferred
data is large since TCP offers flow control and efficient
segmentation. The transport of large amounts of management data via
SNMP over UDP requires many request/response interactions with small-
sized SNMP over UDP messages, which causes latency to increase
excessively.
TCP connections are established on behalf of the SNMP applications
which initiate a transaction. In particular, command generator
applications are responsible for opening TCP connections to command
responder applications and notification originator applications are
responsible to initiate TCP connections to notification receiver
applications, which are selected as described in Section 3 of RFC
2573 [14]. If the TCP connection cannot be established, then the
transaction is aborted and reported to the application as a timeout
error condition. Alternative connection establishment procedures are
discussed in Appendix A but are not part of this specification.
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All SNMP entities (whether in an agent role or manager role) can
close TCP connections at any point in time. This ensures that SNMP
entities can control their resource usage and shut down TCP
connections that are not used. Note that SNMP engines are not
required to process SNMP messages if the incoming half of the TCP
connection is closed while the outgoing half remains open.
The processing of any outstanding SNMP requests when both sides of
the TCP connection have been closed is implementation dependent. The
sending SNMP entity SHOULD therefore not make assumptions about the
processing of outstanding SNMP requests once a TCP connection is
closed. A timeout error condition SHOULD be signalled for confirmed
requests if the TCP connection is closed before a response has been
received.
3.4 Reliable Transport versus Confirmed Operations
The transport of SNMP messages over TCP results in a reliable
exchange of SNMP messages between SNMP engines. In particular, TCP
guarantees (in the absence of security attacks) that the delivered
data is not damaged, lost, duplicated, or delivered out of order
[19].
The SNMP protocol has been designed to support confirmed as well as
unconfirmed operations [1]. The inform-request protocol operation is
an example for a confirmed operation while the snmpV2-trap operation
is an example for an unconfirmed operation.
There is an important difference between an unconfirmed protocol
operation sent over a reliable transport and a confirmed protocol
operation. A reliable transport such as TCP only guarantees that
delivered data is not damaged, lost, duplicated, or delivered out of
order. It does not guarantee that the delivered data was actually
processed in any way by the application process. Furthermore, even a
reliable transport such as TCP cannot guarantee that data sent to a
remote system is eventually delivered on the remote system. Even a
graceful close of the TCP connection does not guarantee that the
receiving TCP engine has actually delivered all the data to an
application process.
With a confirmed SNMP operation, the receiving SNMP engine
acknowledges that the data was actually received. Depending on the
SNMP protocol operation, a confirmation may indicate that further
processing was done. For example, the response to an inform-request
protocol operation also indicates to the notification originator that
the notification passed the security model and that it was delivered
to the notification receiver application. Similarily, the response
to a set-request indicates that the data passed the transport, the
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authentication mechanism and that the write request was actually
processed by the command responder.
A reliable transport is thus only a poor approximation for confirmed
operations. Applications that need confirmation of delivery or
processing are encouraged to use the confirmed operations, such as
the inform-request, rather than using unconfirmed operations, such as
snmpV2-trap, over a reliable transport.
4. Security Considerations
It is recommended that implementors consider the security features as
provided by the SNMPv3 framework in order to provide SNMP security.
Specifically, the use of the User-based Security Model RFC 2574 [12]
and the View-based Access Control Model RFC 2575 [15] is recommended.
It is then a customer/user responsibility to ensure that the SNMP
entity giving access to a MIB is properly configured to give access
to the objects only to those principals (users) that have legitimate
rights to indeed GET or SET (change) them.
The SNMP over TCP transport mapping does not have any impact on the
security mechanisms provided by SNMPv3. However, SNMP over TCP may
introduce new vulnerabilities to denial of service attacks (such as
TCP syn flooding) that do not exist in this form in other transport
mappings.
5. Acknowledgments
This document is the result of discussions within the Network
Management Research Group (NMRG) of the Internet Research Task
Force[21] (IRTF). Special thanks to Luca Deri, Jean-Philippe Martin-
Flatin, Aiko Pras, Ron Sprenkels, and Bert Wijnen for their comments
and suggestions.
Additional useful comments have been made by Mike Ayers, Jeff Case,
Mike Daniele, David Harrington, Lauren Heintz, Keith McCloghrie,
Olivier Miakinen, and Dave Shield.
Luca Deri, Wes Hardaker, Bert Helthuis, and Erik Schoenfelder helped
to create prototype implementations. The SNMP over TCP transport
mapping is currently supported by the NET-SNMP package[22] and the
Linux CMU SNMP package[23].
References
[1] Harrington, D., Presuhn, R. and B. Wijnen, "An Architecture for
Describing SNMP Management Frameworks", RFC 2571, April 1999.
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[2] Rose, M. and K. McCloghrie, "Structure and Identification of
Management Information for TCP/IP-based Internets", STD 16, RFC
1155, May 1990.
[3] Rose, M. and K. McCloghrie, "Concise MIB Definitions", STD 16,
RFC 1212, March 1991.
[4] Rose, M., "A Convention for Defining Traps for use with the
SNMP", RFC 1215, March 1991.
[5] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
M. and S. Waldbusser, "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[6] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
M. and S. Waldbusser, "Textual Conventions for SMIv2", STD 58,
RFC 2579, April 1999.
[7] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
M. and S. Waldbusser, "Conformance Statements for SMIv2", STD
58, RFC 2580, April 1999.
[8] Case, J., Fedor, M., Schoffstall, M. and J. Davin, "A Simple
Network Management Protocol (SNMP)", STD 15, RFC 1157, May
1990.
[9] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
"Introduction to Community-based SNMPv2", RFC 1901, January
1996.
[10] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
"Transport Mappings for Version 2 of the Simple Network
Management Protocol (SNMPv2)", RFC 1906, January 1996.
[11] Case, J., Harrington, D., Presuhn, R. and B. Wijnen, "Message
Processing and Dispatching for the Simple Network Management
Protocol (SNMP)", RFC 2572, April 1999.
[12] Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
for version 3 of the Simple Network Management Protocol
(SNMPv3)", RFC 2574, April 1999.
[13] 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.
[14] Levi, D., Meyer, P. and B. Stewart, "SNMP Applications", RFC
2573, April 1999.
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[15] Wijnen, B., Presuhn, R. and K. McCloghrie, "View-based Access
Control Model (VACM) for the Simple Network Management Protocol
(SNMP)", RFC 2575, April 1999.
[16] Case, J., Mundy, R., Partain, D. and B. Stewart, "Introduction
to Version 3 of the Internet-standard Network Management
Framework", RFC 2570, April 1999.
[17] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[18] Kastenholz, F., "SNMP Communications Services", RFC 1270,
October 1991.
[19] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[20] Sprenkels, R. and J. Martin-Flatin, "Bulk Transfers of MIB
Data", Simple Times 7(1), March 1999.
[21] <http://www.irtf.org/>
[22] <http://net-snmp.sourceforge.net/>
[23] <http://www.gaertner.de/snmp/>
Author's Address
Juergen Schoenwaelder
TU Braunschweig
Bueltenweg 74/75
38106 Braunschweig
Germany
Phone: +49 531 391-3283
EMail: schoenw@ibr.cs.tu-bs.de
Appendix A. Connection Establishment Alternatives
This memo defines a simple connection establishment scheme where the
notification originator or command generator application is
responsible to establish TCP connections to notification receiver or
command responder applications. The purpose of this section is to
document variations or alternatives of this scheme which have been
discussed during the development of this specification. The
discussion below focuses on notification originator applications
since this is case where people seem to have diverging viewpoints.
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The discussion below also assumes that the reader is familiar with
the SNMPv3 notification forwarding model as defined in RFC 2573 [14].
The variations that have been discussed are basically driven by the
idea to provide fallback mechanisms in cases where TCP connection
establishment from the notification originator to the notification
receiver fails. The approach specified in this memo simply drops
notifications if the TCP connection cannot be established. This
implies that notification originators which need reliable
notification delivery must implement a local notification log in
order to keep a history of notifications that could not be delivered.
Another option is to deliver notifications via UDP in case TCP
connection establishment fails. This might require to augment the
snmpTargetTable with columns that provide information about the
alternate UDP transport domain and address. In general, this
approach only helps to deliver notifications in cases where the
notification receiver is unable to accept more TCP connections. In
other fault scenarios (e.g. routing problems in the network), the
UDP packet would have no or only marginally better chances to reach
the notification receiver. This implies that notification
originators which need reliable notification delivery still need to
implement a local notification log in order to keep a history of
notifications in cases the UDP packets do not reach the destination.
A generalization of this approach leads to the idea of a sparse
augmentation of the snmpTargetTable which lists alternate fallback
transports endpoints of arbitrary transport domains. Multiple
fallbacks may be possible by using a tag list approach. This
provides a generic transport independent fallback mechanism which is
independent of the TCP transport mapping defined in this memo.
Another alternative is to make the notification originator
responsible to retry connection establishment. This could be
accomplished by augmenting the snmpTargetTable with additional
columns that specify retry counts and timeouts or by adapting the
existing snmpTargetAddrTimeout and snmpTargetAddrRetryCount columns
in the snmpTargetTable. But even this approach requires a local
notification log in order to handle situations where all retries have
failed.
A fundamentally different approach is to make the notification
receiver responsible to establish the TCP connection to the
notification originator. This approach has the advantage that the
notification originator does not necessarily need a list of pre-
configured notification receiver transport addresses. The current
notification forwarding model however relies on the snmpTargetTable
to identify notification targets. So the question comes up whether
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(a) new entries are added to the snmpTargetTable when a connection is
established or whether (b) connections are only accepted if they
match pre-configured snmpTargetTable entries. Note that the target
selection logic relies on a tag list which can not be reasonably
populated when a connection is accepted. So only option (b) seems to
be compliant with the current notification forwarding logic. Another
issue to consider is the vulnerability to denial of service attacks.
A notification originator can be easily attacked by syn-flooding
attacks if it listens for incoming TCP connections. Finally, in
order to let notification originator and notification receiver
appplications coexist easily on a single system, it would be
necessary to assign new default port numbers on which notification
originators listen for incoming TCP connections.
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Copyright (C) The Internet Society (2002). All Rights Reserved.
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