< draft-irtf-nmrg-snmp-tcp-08.txt		draft-irtf-nmrg-snmp-tcp-09.txt >
			A new Request for Comments is now available in online RFC libraries.
	Network Working Group J. Schoenwaelder		RFC 3430
	Internet-Draft TU Braunschweig
	Expires: September 18, 2002 March 20, 2002
	SNMP over TCP Transport Mapping
	draft-irtf-nmrg-snmp-tcp-08.txt
	Status of this Memo
	This document is an Internet-Draft and is in full conformance with
	all provisions of Section 10 of RFC2026.
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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.
	Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. SNMP over TCP . . . . . . . . . . . . . . . . . . . . . . . . 3
	2.1 Serialization . . . . . . . . . . . . . . . . . . . . . . . . 3
	2.2 Well-Known Values . . . . . . . . . . . . . . . . . . . . . . 4
	2.3 Connection Management . . . . . . . . . . . . . . . . . . . . 4
	2.4 Reliable Transport versus Confirmed Operations . . . . . . . . 5
	3. Security Considerations . . . . . . . . . . . . . . . . . . . 6
	4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
	References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . 8
	A. Connection Establishment Alternatives . . . . . . . . . . . . 8
	Full Copyright Statement . . . . . . . . . . . . . . . . . . . 10
	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 RFC 1906 [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 RFC 1906 [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 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 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.
	2.1 Serialization
	Each instance of a message is serialized into a single BER-encoded
	message, using the algorithm specified in Section 8 of RFC 1906 [3].
	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
	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.
	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 xxxx [6]
	which cover TCP over IPv4 and IPv6.
	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.
	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.
	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 [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 signalled for confirmed
	requests 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.
	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
	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.
	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 RFC 2574 [10]
	and the View-based Access Control Model RFC 2575 [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.
	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
	to Version 3 of the Internet-standard Network Management
	Framework", RFC 2570, April 1999.
	[2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
	September 1981.
	[3] 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.
	[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", draft-ietf-ops-taddress-mib-02, March
	2002.
	[7] Kastenholz, F., "SNMP Communications Services", RFC 1270,
	October 1991.
	[8] Levi, D., Meyer, P. and B. Stewart, "SNMP Applications", RFC
	2573, April 1999.
	[9] Harrington, D., Presuhn, R. and B. Wijnen, "An Architecture for
	Describing SNMP Management Frameworks", RFC 2571, April 1999.
	[10] Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
	for version 3 of the Simple Network Management Protocol
	(SNMPv3)", RFC 2574, April 1999.
	[11] Wijnen, B., Presuhn, R. and K. McCloghrie, "View-based Access
	Control Model (VACM) for the Simple Network Management Protocol
	(SNMP)", RFC 2575, April 1999.
	[12] <http://www.irtf.org/>
	[13] <http://net-snmp.sourceforge.net/>
	[14] <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.
	The discussion below also assumes that the reader is familiar with
	the SNMPv3 notification forwarding model as defined in RFC 2573 [8].
	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		Title: Simple Network Management Protocol Over
	augmentation of the snmpTargetTable which lists alternate fallback		Transmission Control Protocol Transport Mapping
	transports endpoints of arbitrary transport domains. Multiple		Author(s): J. Schoenwaelder
	fallbacks may be possible by using a tag list approach. This		Status: Experimental
	provides a generic transport independent fallback mechanism which is		Date: December 2002
	independent of the TCP transport mapping defined in this memo.		Mailbox: schoenw@ibr.cs.tu-bs.de
			Pages: 10
			Characters: 21527
			Updates/Obsoletes/SeeAlso: None
	Another alternative is to make the notification originator		I-D Tag: draft-irtf-nmrg-snmp-tcp-09.txt
	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		URL: ftp://ftp.rfc-editor.org/in-notes/rfc3430.txt
	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
	(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.
	Full Copyright Statement		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.
	Copyright (C) The Internet Society (2002). All Rights Reserved.		This memo defines an Experimental Protocol for the Internet community.
			It does not specify an Internet standard of any kind. Discussion and
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