rfc1270.txt

来自「RFC 的详细文档！」· 文本 代码 · 共 619 行 · 第 1/2 页

Network Working Group                              F. Kastenholz, Editor
Request for Comments: 1270               Clearpoint Research Corporation
                                                            October 1991


                      SNMP Communications Services

Status 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 Contents

   1. Abstract ..............................................    1
   2. Introduction ..........................................    1
   3. Standardization .......................................    3
   4. Interoperability ......................................    3
   5. To Transport or Not To Transport ......................    3
   6. Connection Oriented vs. Connectionless ................    6
   7. Which Protocol ........................................    8
   8. Security Considerations ...............................    9
   9. Appendix ..............................................    9
   10. References ...........................................   10
   11. Acknowledgements .....................................   11
   12. Author's Address .....................................   11

1.  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 in RFC 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 in
   RFC 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), and RFC 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 1991


3.  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 over



SNMP 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 the



SNMP 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,