Movatterモバイル変換

Network Working Group                              F. Kastenholz, EditorRequest for Comments: 1270               Clearpoint Research Corporation                                                            October 1991SNMP Communications ServicesStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard.  Distribution of this memo is   unlimited.Table of Contents1. Abstract ..............................................12. Introduction ..........................................13. Standardization .......................................34. Interoperability ......................................35. To Transport or Not To Transport ......................36. Connection Oriented vs. Connectionless ................67. Which Protocol ........................................88. Security Considerations ...............................99. Appendix ..............................................910. References ...........................................1011. Acknowledgements .....................................1112. Author's Address .....................................111.  Abstract   This memo is being distributed to members of the Internet community as   an Informational RFC.  The intent is to present a discussion on the   issues relating to the communications services for SNMP.  While the   issues discussed may not be directly relevant to the research problems   of the Internet, they may be interesting to a number of researchers   and implementors.2.  Introduction   This document discusses various issues to be considered when   determining the underlying communications services to be used by an   SNMP implementation.   As reported inRFC 1052, IAB Recommendations for the Development of   Internet Network Management Standards [8], a two-prong strategy for   network management of TCP/IP-based internets was undertaken.  In the   short-term, the Simple Network Management Protocol (SNMP), defined inRFC 1067, was to be used to manage nodes in the Internet community.SNMP Working Group                                              [Page 1]

RFC 1270              SNMP Communications Services          October 1991   In the long-term, the use of the OSI network management framework was   to be examined.  Two documents were produced to define the management   information:RFC 1065, which defined the Structure of Management   Information (SMI), andRFC 1066, which defined the Management   Information Base (MIB).  Both of these documents were designed so as   to be compatible with both the SNMP and the OSI network management   framework.   This strategy was quite successful in the short-term: Internet-based   network management technology was fielded, by both the research and   commercial communities, within a few months.  As a result of this,   portions of the Internet community became network manageable in a   timely fashion.   In May of 1990, the core documents were elevated to "Standard   Protocols" with "Recommended" status.  As such, the Internet-standard   network management framework consists of: Structure and Identification   of Management Information for TCP/IP-based internets,RFC 1155 [9],   which describes how managed objects contained in the MIB are defined;   Management Information Base for Network Management of TCP/IP-based   internets, which describes the managed objects contained in the MIB,RFC 1156 [10]; and, the Simple Network Management Protocol,RFC 1157   [1], which defines the protocol used to manage these objects.   In parallel with this activity, documents specifying how to transport   SNMP messages over protocols other than UDP/IP have been developed and   published: SNMP Over Ethernet [3], SNMP Over OSI [2], and SNMP Over   IPX [6] and it would be suprising if more were not developed.  These   memos have caused a degree of confusion in the community.  This   document is intended to disperse that confusion by discussing the   issues of relevance to an implementor when choosing how to encapsulate   SNMP packets.   None of these documents have been made full Internet Standards. SNMP   Over ISO and SNMP Over Ethernet are both Experimental protocols. SNMP   Over IPX [6] is an Internet Draft. Only the SNMP Specification [1] is   an Internet Standard.   No single transport scheme can be considered the absolute best   solution for all circumstances.  This note will argue that, except for   a very small set of special circumstances, operating SNMP over UDP/IP   is the optimal scheme.   This document does not present a standard or a protocol for the   Internet Community.  For production use in the Internet the SNMP and   its required communication services are specified in [1].SNMP Working Group                                              [Page 2]

RFC 1270              SNMP Communications Services          October 19913.  Standardization   Currently, the SNMP Specification [1] only specifies that the UDP   protocol be used to exchange SNMP messages.  While the IAB may   standardize other protocols for use in exchanging SNMP messages in the   future, only UDP is currently standardized for this purpose.   In order to claim full compliance with the SNMP Specification, an   implementation would have to use UDP for SNMP message exchange.4.  Interoperability   Interoperability is the degree to which the equipment produced by one   vendor can can operate with equipment produced by another vendor.   Related to Interoperability is compliance with a standard.  Everything   else being equal, a device that complies with some standard is more   likely to be interoperable than a device that does not.   For commercial product development, the pros and cons of developing an   interoperable product must be weighed and a choice made.  Both   engineering and marketing organizations participate in this process.   The Internet is the single largest market for SNMP systems.  A large   portion of SNMP systems will be developed with the Internet as a   target environment.  Therefore, it may be expected that the Internet's   needs and requirements will be the driving force for SNMP.  SNMP over   UDP/IP is specified as the "Internet Standard" protocol.  Therefore,   in order to operate in the Internet and be managed in that environment   on a production basis, a device must support SNMP over UDP/IP.  This   situation will lead to SNMP over UDP/IP being the most common method   of operating SNMP.  Therefore, the widest degree of interoperability   and widest acceptance of a commercial product will be attained by   operating SNMP over UDP/IP.   The preponderance of UDP/IP based network management stations also   strongly suggests that an agent should operate SNMP over UDP/IP.   The results of the interoperability decision drive a number of   technical decisions.  If interoperability is desired, then SNMP must be   operated over UDP/IP.5.  To Transport or Not To Transport   A major issue is whether SNMP should run on top of a transport-layer   protocol (such as UDP) or not.  Typically, the choice is to run over a   transport/network/data link protocol or just run over the datalink.   In fact, several protocols have been published for operating SNMP overSNMP Working Group                                              [Page 3]

RFC 1270              SNMP Communications Services          October 1991   several different datalink and transport protocols.   Operation of SNMP over a Transport and Network protocol stack   is preferred.  These protocols provide at least five functions   that are of vital importance to the movement of SNMP packets   through a network:          o Routing               The network layer provides routing functions, which               improves the overall utility of network management.  The               network has the ability to re-route packets around failed               areas.  This allows network management to continue               operating during localized losses of service (It should               be noted that these losses of service occur not only               because of failures, but also for non-failure reasons               such as preventive maintenance).          o Media Independence               The network layer provides a high degree of media               independence.  By using this capability, many different               types of network elements may be managed.  Tying SNMP to               a particular data link protocol limits the management               scope of those SNMP entities to just those devices that               use that datalink protocol.          o End-to-End Checksum               The end-to-end checksum provided by transport protocols               improves the reliability of the data transfer.          o Multiplexing/Demultiplexing               Transport protocols provide multiplexing and               demultiplexing services.  These services facilitate the               many-to-many management relationships possible with SNMP.          o Fragmentation and Reassembly               This is related to media independence.  IP allows SNMP               packets to transit media with differing MTU sizes.  This               capability is not available for datalink specific               transmission schemes.               Fragmentation and Reassembly does reduce the overall               robustness of network management since, if any single               fragment is lost along the way, the operation will fail.               The worse the network operates, the higher the               probability that a fragment will get lost or delayed.               For monitoring and data gathering while the network is               operating normally, Fragmentation and Reassembly is not a               problem. When the network is operating poorly (and theSNMP Working Group                                              [Page 4]

RFC 1270              SNMP Communications Services          October 1991               network operators are typically trying to diagnose and               repair a failure), small packets should to be used,               preventing the packet from being fragmented.   There are other services and functions that are provided by a   connection oriented transport.  These services and functions are not   desired for SNMP.  A later section will explore this issue in more   detail.   The main drawbacks that are cited with respect to using Transport and   Network layers in the managed object are: a) Increased development   time and b) Increased resource requirements.  These arguments are   less than compelling.   There are several excellent public domain or freely redistributable   UDP/IP stacks that provide enough support for SNMP.  The effort   required to port the essential components of one of these stacks is   small compared to the overall effort of installing the SNMP software.   The additional resources required in the managed object to support   UDP/IP are minimal.  CPU resources are required only when actually   transmitting or receiving a packet.  The largest single resource   requirement of a UDP/IP is calculating the UDP checksum, which is   very small compared to the cost of doing the ASN.1 encoding/decoding,   Object Identifier lookup, and so on.   The author has personal knowledge of a UDP/IP stack that was   developed expressly for the purpose of supporting SNMP.  This stack   requires less than 4Kb of code space.  It is a minimalist   implementation of UDP/IP in that it is "just enough" so support SNMP.   This stack supports UDP, IP, ARP, and handles ICMP redirect and echo   request messages.  Furthermore, this stack was developed by a single   person in approximately two months.  Obviously, neither the   development effort nor the memory requirements are large.   The network overhead of using UDP/IP is relatively small.  A UDP/IP   header requires 28 octets (assuming no IP options).  Since the UDP is   connectionless, it will generate no overhead traffic of its own (such   as TCP SYNs, FINs, and ACKs).   The growing popularity of internetworking outside of The Internet   mandates that SNMP operate over, at least, a network layer protocol.   These internetworks consist of a number of networks all connected   together with routers.  In order to traverse a router, a packet must   be one of the network layer protocols that the router understands.   Therefore, for SNMP management to be deployed in an internetwork, the   SNMP entities in that internetwork must use a network layer protocol.   SNMP over a datalink can not traverse a router.SNMP Working Group                                              [Page 5]

RFC 1270              SNMP Communications Services          October 1991   There are some circumstances where running SNMP over some datalink is   appropriate.   There are schemes are under development to provide Out-Of-Band (OOB)   management access to network devices.  This OOB access is typically   provided over point-to-point or dial-up connections.  Since these   connections are dedicated to OOB network management and go directly   from the network management station to the managed device, a   Transport/Network protocol may not be necessary.   Using a Transport/Network protocol on these links may be easier from   a development point of view though.  It is probably a simple   configuration operation to have the management station's IP use a   serial port rather than the "normal" (e.g., Ethernet) port for   traffic destined for a particular node.   If the Out-Of-Band link is also used as a "primary" route to some   nodes, then the functions of a network-layer are required.  These   functions are readily supplied by using UDP/IP.   For a datalink interface and driver (e.g., a PC Ethernet interface   card) that must be manageable independent of the higher level   protocol suite (which might NOT be manageable), operating SNMP   directly over the datalink is reasonable.  It is not known, a priori,   what higher-level protocol services may be available, so those   services can not be used.  If an arbitrary choice is made for   example, to put in an elementary UDP/IP stack, then there may be two   independent UDP/IPs in the system (which is undesireable as this   would require two IP addresses per managed node), or a new protocol   stack will be introduced into the environment.6.  Connection Oriented vs. Connectionless   While this section primarily addresses itself to transport layer   issues, its basic discussion of connection oriented vs connectionless   applies to any layer which provides communication services for SNMP.   For SNMP, connectionless transport service (UDP) is specified in the   Protocol Specification [1].  This choice was made after careful study   and consideration by the original SNMP developers.   The prime motivation of this choice is that SNMP must continue to   operate (if at all possible) when the network is operating at its   worst.  For other applications, such as Telnet or FTP, the user can   always "try again later" if the network is operating poorly.  On the   other hand, the major purpose of a network management protocol is to   fix the network when it is operating poorly so the "try again later"   strategy is useless.SNMP Working Group                                              [Page 6]

RFC 1270              SNMP Communications Services          October 1991   By using a connectionless transport protocol, SNMP takes on the   responsibility of reliable data transmission (A SNMP application may   time out outstanding requests and either retransmit them or abort   them as appropriate).  However, the SNMP requires these functions   only of the sender of a Request PDU (get, getNext, or Set), which   typically is a network management station.  Since the Agent only   generates responses, it need not perform any of these functions.   This vastly reduces the resource and functional requirements on the   Agent.   If a connection oriented transport is used, then a fundamental design   choice must be made with respect to connection maintenance:          (1)  Keep a connection open to each managed object on the               network,          (2)  Establish and tear down connections on a per-operation               basis, or          (3)  Keep a fixed number of connections open and, when another               connection is needed, use some algorithm (e.g., LRU) to               select one for closing and opening to the new agent.   All of these alternatives pose severe problems, and because of them,   each is undesirable.   The first option reduces the amount of resources required to perform   a single operation in that the connection establishment and   termination cost is "amortized" over many operations.  On the other   hand, keeping a connection open implies that the management station   needs to maintain a large number of connection records (in the   hundreds or even thousands).  Furthermore, if either side of the   connection engages in "keep-alives" (even though such behavior is   frowned upon), a large amount of traffic will be generated, consuming   a large amount of network resources, all for no gain.   The second option reduces the amount of idle resources such as   connection records, but vastly increases the amount of resources   required to perform an operation.  A connection must be established,   the request Message sent and the response returned, and then the   connection closed for each operation.  For a TCP, this would   typically require 10 separate packet transfers plus the TCP Time-Wait   (see the Appendix for details).   In the face of pathological network problems, a connection oriented   transport protocol may simply cease to operate because the   probability of getting all of these packets through reduces to a very   small number.SNMP Working Group                                              [Page 7]

RFC 1270              SNMP Communications Services          October 1991   The third option requires that the management station maintain   connection usage information in order to implement the LRU algorithm.   This excessively complicates the management station.  Furthermore,   this option tends to reduce to the second option when doing health   check polling for a number of agents that is large compared to the   number of supported connections.   A connection oriented transport protocol may provide services that   are undesirable or unneeded by SNMP.   For example, one application of network management is to poll nodes   to determine if they are up or not.  When a node is up, it makes   little difference whether SNMP operates over TCP or UDP.  However, if   the node goes down then TCP will eventually close the connection.   Every poll request must then be translated into a TCP Open request   while the managed node is down.  Once the node comes up, the send   must then be done.   For connection oriented transports, the transport ACK does not   necessarily indicate delivery of data to the destination application   process (for TCP, see section 2.6 of [4]).  The SNMP would still need   its own timeout/retry procedure to ensure that the SNMP software   actually got the packet.   A connection oriented transport such as TCP provides flow control for   the data stream.  Because of the lock-step nature of SNMP protocol   exchanges, this is not a service that SNMP requires.   Architectural purists may argue that an "Application" layer entity   (SNMP) must not perform operations that are properly the realm of the   Transport layer (timeouts, retransmissions, and so on).  While   architecturally pure, this line of reasoning is not relevant.  The   network management applications and protocols are monitoring the   "health" of the network and, as a result, have the best information   and are in the best position to adapt their own behavior to the state   of the network, and thereby, continuing operations in the face of   network adversity.7.  Which Protocol   The final point of discussion is the actual choice of a protocol to   support SNMP.   If a device is destined for use in the Internet then it must operate   SNMP over UDP/IP.   If the device is operating in some other protocol environment, then   SNMP ought to use the transport services that are native to thatSNMP Working Group                                              [Page 8]

RFC 1270              SNMP Communications Services          October 1991   environment.  It may make very little sense to introduce a new   protocol stack into a network in order to provide just one service.   For example, it could require that the network operations staff   understand and be able to administer and operate two protocol stacks,   that hosts and routers understand both protocols, and so on.  It may   also be bureaucratically impossible to introduce UDP/IP into the   environment (the "We are only a FOONET shop - if it doesn't speak   FOONET, we don't want it" argument).   References [2] and [6] are experimental standards for operating SNMP   over IPX and OSI respectively.  In these environments, those   standards ought to be adhered to.8.  Security Considerations   Security issues are not discussed in this memo.9.  Appendix   This appendix details the TCP packet transfers required to perform a   single SNMP operation assuming that the connection is established   only for that operation and that a single SNMP operation (e.g., get   request) is performed.  We also assume that all operations are   "normal" i.e., that there are no lost packets, no simultaneous opens,   no half opens, and no simultaneous closes.  We also ignore the   possibility of TCP segmentation and IP fragmentation.   The nomenclature used to illustrate the packet transactions is the   same as that used in the TCP Specification [4].SNMP Working Group                                              [Page 9]

RFC 1270              SNMP Communications Services          October 1991              Packet  Management                         Managed              Number  Station                            Object                               Connection Open...               1         >--<CTL=SYN>----------------------->               2         <--<CTL=SYN,ACK>-------------------<               3         >--<CTL=ACK>----------------------->                           Connection now open,                           SNMP Request is sent.               4         >--<DATA=SNMP Request>------------->                           Response comes back               5         <--<DATA=SNMP Response, CTL=ACK>---<               6         >--<CTL=ACK>----------------------->                           Operation is complete,                           Management station initiates the                           close.               7         >--<CTL=FIN,ACK>------------------->               8         <--<CTL=ACK>-----------------------<               9         <--<CTL=FIN,ACK>-------------------<              10         >--<CTL=ACK>----------------------->                          Wait 2 MSL                          Connection now closed.   Some optimizations are possible IF the TCP has knowledge of the type   of operation.  However, a general purpose TCP would not be tuned to   SNMP operations so those optimizations would not be done.10.  References   [1] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple       Network Management Protocol",RFC 1157, SNMP Research,       Performance Systems International, Performance Systems       International, MIT Laboratory for Computer Science, May 1990.   [2] Rose, M., Editor, "SNMP over OSI",RFC 1161, Performance Systems       International, Inc., June 1990.   [3] Schoffstall, M., Davin, C., Fedor, M., and J. Case, "SNMP over       Ethernet",RFC 1089, Rensselaer Polytechnic Institute, MIT       Laboratory for Computer Science, NYSERNet, Inc., University of       Tennessee at Knoxville, February 1989.   [4] Postel, J., "Transmission Control Protocol - DARPA Internet       Program Protocol Specification",RFC 793, DARPA, September 1981.   [5] Postel, J., "User Datagram Protocol",RFC 768, USC/Information       Sciences Institute, August 1980.   [6] Wormley, R., "SNMP Over IPX", draft in process, August 1990.SNMP Working Group                                             [Page 10]

RFC 1270              SNMP Communications Services          October 1991   [7] Postel, J., Editor, "IAB Official Protocol Standards",RFC 1250,       IAB, August 1991.   [8] Cerf, V., "IAB Recommendations for the Development of Internet       Network Management Standards",RFC 1052, NRI, April 1988.   [9] Rose M., and K. McCloghrie, "Structure and Identification of       Management Information for TCP/IP-based internets",RFC 1155,       Performance Systems International, Hughes LAN Systems, May 1990.  [10] McCloghrie K., and M. Rose, "Management Information Base for       Network Management of TCP/IP-based internets",RFC 1156, Hughes       LAN Systems, Performance Systems International, May 1990.11.  Acknowledgements   The author wishes to thank Jeff Case, Chuck Davin and Keith   McCloghrie for their technical and editorial contributions to this   document.12.  Author's Address   Frank Kastenholz   Clearpoint Research Corporation   35 Parkwood Drive   Hopkinton, Mass. 01748   Phone: (508) 435-2000   Email: kasten@europa.clearpoint.comSNMP Working Group                                             [Page 11]