Network Working Group J. Schoenwaelder

Network Working Group J. Schoenwaelder
Request for Comments: 3430 TU Braunschweig
Category: Experimental December 2002

Simple Network Management Protocol (SNMP) over
Transmission Control Protocol (TCP) Transport Mapping

Status of this Memo

This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

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 STD 62, RFC 3417.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. SNMP over TCP . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1 Serialization . . . . . . . . . . . . . . . . . . . . . . . . 2
2.2 Well-Known Values . . . . . . . . . . . . . . . . . . . . . . 3
2.3 Connection Management . . . . . . . . . . . . . . . . . . . . 3
2.4 Reliable Transport versus Confirmed Operations . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
A. Connection Establishment Alternatives . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 10

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1. Introduction

This memo defines a transport mapping for using the Simple Network
Management Protocol (SNMP) [1] over TCP [2]. The transport mapping
can be used with any version of SNMP. This document extends the
transport mappings defined in STD 62, RFC 3417 [3].

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
defined in STD 62, RFC 3417 [3].

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 BCP 14, RFC 2119 [4].

2. SNMP over TCP

SNMP over TCP is an optional transport mapping. It is primarily
defined to support more efficient bulk transfer mechanisms within the
SNMP framework [5].

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 acknowledgments and timers. In particular,
infrequent SNMP interactions over TCP may lead to additional IP
packets carrying acknowledgments for SNMP responses if there is no
chance to piggyback them. Furthermore, it is recommended to
configure SNMP retransmission timers to fire later when using SNMP
over TCP to avoid application specific timeouts before the TCP timers
have expired.

2.1 Serialization

Each instance of a message is serialized into a single BER-encoded
message, using the algorithm specified in Section 8 of STD 62, RFC
3417 [3]. The BER-encoded message is then sent over a TCP

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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 by default
full-duplex and data can travel in both directions at 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 align 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 (framing). An SNMP engine which looses framing
(for example due to ASN.1 parse errors) SHOULD close the TCP
connection. The connection initiator will then be responsible for
establishing a new TCP connection.

2.2 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.

SNMP over TCP transport addresses are identified by using the generic
TCP transport domain and address definitions provided by RFC 3419
[6], which cover TCP over IPv4 and IPv6.

When an SNMP entity uses the TCP transport mapping, it MUST be
capable of accepting and generating messages that are at least 8192
octets in size. Implementation of larger values is encouraged
whenever possible.

2.3 Connection Management

The use of TCP connections introduces costs [7]. Connection
establishment and teardown cause additional network traffic.
Furthermore, maintaining open connections binds resources in the
network layer of the underlying operating system.

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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 for initiating TCP connections to notification receiver
applications, which are selected as described in Section 3 of STD 62,
RFC 3413 [8]. 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.

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 signaled for confirmed
operations if the TCP connection is closed before a response has been
received.

2.4 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 [2].

The SNMP protocol has been designed to support confirmed as well as
unconfirmed operations [9]. The inform-request protocol operation is
an example for a confirmed operation while the snmpV2-trap operation
is an example for an unconfirmed operation.

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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 indicates to the notification originator that the
notification passed the transport, the security model and that it was
queued for delivery to the notification receiver application.
Similarly, the response to a set-request indicates that the data
passed the transport, the security model 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.

3. 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 STD 62, RFC
3414 [10] and the View-based Access Control Model STD 62, RFC 3415
[11] 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.

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4. Acknowledgments

This document is the result of discussions within the Network
Management Research Group (NMRG) of the Internet Research Task
Force[12] (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[13] and the
Linux CMU SNMP package[14].

References

[1] Case, J., Mundy, R., Partain, D. and B. Stewart, "Introduction
and Applicability Statements for Internet-Standard Management
Framework", RFC 3410, December 2002.

[2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.

[3] Presuhn, R., Ed., "Transport Mappings for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3417, December 2002.

[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[5] Sprenkels, R. and J. Martin-Flatin, "Bulk Transfers of MIB
Data", Simple Times 7(1), March 1999.

[6] Daniele, M. and J. Schoenwaelder, "Textual Conventions for
Transport Addresses", RFC 3419, December 2002.

[7] Kastenholz, F., "SNMP Communications Services", RFC 1270,
October 1991.

[8] Levi, D., Meyer, P. and B. Stewart, "Simple Network Management
Protocol (SNMP) Applications", STD 62, RFC 3413, December 2002.

[9] Harrington, D., Presuhn, R. and B. Wijnen, "An Architecture for
Describing Simple Network Management Protocol (SNMP) Management
Frameworks", STD 62, RFC 3411, December 2002.

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[10] Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
for version 3 of the Simple Network Management Protocol
(SNMPv3)", STD 62, RFC 3414, December 2002.

[11] Wijnen, B., Presuhn, R. and K. McCloghrie, "View-based Access
Control Model (VACM) for the Simple Network Management Protocol
(SNMP)", STD 62, RFC 3415, December 2002.

[12] <http://www.irtf.org/>

[13] <http://net-snmp.sourceforge.net/>

[14] <http://www.gaertner.de/snmp/>

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RFC 3430 SNMP over TCP Transport Mapping December 2002

Appendix A. Connection Establishment Alternatives

This memo defines a simple connection establishment scheme where the
notification originator or command generator application is
responsible for establishing 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.
The discussion below also assumes that the reader is familiar with
the SNMPv3 notification forwarding model as defined in STD 62, RFC
3413 [8].

The variations that have been discussed are basically driven by the
idea of providing 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 augmenting 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
case 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
transport 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.

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Another alternative is to make the notification originator
responsible for retrying 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 for establishing 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 (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 applications 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.

Author's Address

Juergen Schoenwaelder
TU Braunschweig
Bueltenweg 74/75
38106 Braunschweig
Germany
Phone: +49 531 391-3283
EMail: [email protected]

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Full Copyright Statement

Copyright (C) The Internet Society (2002). All Rights Reserved.

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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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