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Network Working Group                                   J. SchoenwaelderRequest for Comments: 3430                               TU BraunschweigCategory: Experimental                                     December 2002Simple Network Management Protocol (SNMP) overTransmission Control Protocol (TCP) Transport MappingStatus 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 Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .22.  SNMP over TCP  . . . . . . . . . . . . . . . . . . . . . . . .22.1 Serialization  . . . . . . . . . . . . . . . . . . . . . . . .22.2 Well-Known Values  . . . . . . . . . . . . . . . . . . . . . .32.3 Connection Management  . . . . . . . . . . . . . . . . . . . .32.4 Reliable Transport versus Confirmed Operations . . . . . . . .43.  Security Considerations  . . . . . . . . . . . . . . . . . . .54.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .6       References . . . . . . . . . . . . . . . . . . . . . . . . . .6A.  Connection Establishment Alternatives  . . . . . . . . . . . .8       Author's Address . . . . . . . . . . . . . . . . . . . . . . .9       Full Copyright Statement . . . . . . . . . . . . . . . . . . .10Schoenwaelder                 Experimental                      [Page 1]

RFC 3430            SNMP over TCP Transport Mapping        December 20021. 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 inBCP 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 inSection 8 of STD 62,RFC3417 [3].  The BER-encoded message is then sent over a TCPSchoenwaelder                 Experimental                      [Page 2]

RFC 3430            SNMP over TCP Transport Mapping        December 2002   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 byRFC 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.Schoenwaelder                 Experimental                      [Page 3]

RFC 3430            SNMP over TCP Transport Mapping        December 2002   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 inSection 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 inAppendix 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.Schoenwaelder                 Experimental                      [Page 4]

RFC 3430            SNMP over TCP Transport Mapping        December 2002   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,RFC3414 [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.Schoenwaelder                 Experimental                      [Page 5]

RFC 3430            SNMP over TCP Transport Mapping        December 20024. 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.Schoenwaelder                 Experimental                      [Page 6]

RFC 3430            SNMP over TCP Transport Mapping        December 2002   [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/>Schoenwaelder                 Experimental                      [Page 7]

RFC 3430            SNMP over TCP Transport Mapping        December 2002Appendix 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,RFC3413 [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.Schoenwaelder                 Experimental                      [Page 8]

RFC 3430            SNMP over TCP Transport Mapping        December 2002   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: schoenw@ibr.cs.tu-bs.deSchoenwaelder                 Experimental                      [Page 9]

RFC 3430            SNMP over TCP Transport Mapping        December 2002Full Copyright Statement   Copyright (C) The Internet Society (2002).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Schoenwaelder                 Experimental                     [Page 10]