RFCs in HTML Format


RFC 1573

              Evolution of the Interfaces Group of MIB-II

Table of Contents

   1. Introduction .............................................    2
   2. The SNMPv2 Network Management Framework ..................    2
   2.1 Object Definitions ......................................    3
   3 Experience with the Interfaces Group ......................    3
   3.1 Areas of Clarification/Revision .........................    3
   3.1.1 Interface Numbering ...................................    4
   3.1.2 Interface Sub-Layers ..................................    4
   3.1.3 Virtual Circuits ......................................    5
   3.1.4 Bit, Character, and Fixed-Length Interfaces ...........    5
   3.1.5 Counter Size ..........................................    5
   3.1.6 Interface Speed .......................................    6
   3.1.7 Multicast/Broadcast Counters ..........................    6
   3.1.8 Addition of New ifType values .........................    6
   3.1.9 ifSpecific ............................................    6
   3.2 Clarifications/Revisions ................................    7
   3.2.1 Interface Numbering ...................................    7
   3.2.2 Interface Sub-Layers ..................................    8
   3.2.3 Guidance on Defining Sub-layers .......................   11
   3.2.4 Virtual Circuits ......................................   12
   3.2.5 Bit, Character, and Fixed-Length Interfaces ...........   12
   3.2.6 Counter Size ..........................................   14
   3.2.7 Interface Speed .......................................   16
   3.2.8 Multicast/Broadcast Counters ..........................   16
   3.2.9 Trap Enable ...........................................   17
   3.2.10 Addition of New ifType values ........................   17
   3.2.11 InterfaceIndex Textual Convention ....................   17
   3.2.12 IfAdminStatus and IfOperStatus .......................   18
   3.2.13 Traps ................................................   19
   3.2.14 ifSpecific ...........................................   20



McCloghrie & Kastenholz                                         [Page 1]

RFC 1573 Interfaces Group Evolution January 1994 3.3 Media-Specific MIB Applicability ........................ 20 4. Overview ................................................. 21 5. IANAifType Definition .................................... 22 6. Interfaces Group Definitions ............................. 24 7. Acknowledgements ......................................... 53 8. References ............................................... 53 9. Security Considerations .................................. 55 10. Authors' Addresses....................................... 55 1. Introduction This memo defines a portion of the Management Information Base (MIB) for use with network management protocols in the Internet community. In particular, it describes managed objects used for managing Network Interfaces. This memo discusses the 'interfaces' group of MIB-II, especially the experience gained from the definition of numerous media-specific MIB modules for use in conjunction with the 'interfaces' group for managing various sub-layers beneath the internetwork-layer. It proposes clarifications to, and extensions of, the architectural issues within the current model used for the 'interfaces' group. This memo also includes a MIB module. As well as including new MIB definitions to support the architectural extensions, this MIB module also re-specifies the 'interfaces' group of MIB-II in a manner which is both compliant to the SNMPv2 SMI and semantically-identical to the existing SNMPv1-based definitions. 2. The SNMPv2 Network Management Framework The SNMPv2 Network Management Framework consists of four major components. They are: o RFC 1442 which defines the SMI, the mechanisms used for describing and naming objects for the purpose of management. o STD 17, RFC 1213 defines MIB-II, the core set of managed objects for the Internet suite of protocols. o RFC 1445 which defines the administrative and other architectural aspects of the framework. o RFC 1448 which defines the protocol used for network access to managed objects. The Framework permits new objects to be defined for the purpose of experimentation and evaluation. McCloghrie & Kastenholz [Page 2]
RFC 1573 Interfaces Group Evolution January 1994 2.1. Object Definitions Managed objects are accessed via a virtual information store, termed the Management Information Base or MIB. Objects in the MIB are defined using the subset of Abstract Syntax Notation One (ASN.1) defined in the SMI. In particular, each object object type is named by an OBJECT IDENTIFIER, an administratively assigned name. The object type together with an object instance serves to uniquely identify a specific instantiation of the object. For human convenience, we often use a textual string, termed the descriptor, to refer to the object type. 3. Experience with the Interfaces Group One of the strengths of internetwork-layer protocols such as IP [6] is that they are designed to run over any network interface. In achieving this, IP considers any and all protocols it runs over as a single "network interface" layer. A similar view is taken by other internetwork-layer protocols. This concept is represented in MIB-II by the 'interfaces' group which defines a generic set of managed objects such that any network interface can be managed in an interface-independent manner through these managed objects. The 'interfaces' group provides the means for additional managed objects specific to particular types of network interface (e.g., a specific medium such as Ethernet) to be defined as extensions to the 'interfaces' group for media-specific management. Since the standardization of MIB-II, many such media-specific MIB modules have been defined. Experience in defining these media-specific MIB modules has shown that the model defined by MIB-II is too simplistic and/or static for some types of media-specific management. As a result, some of these media-specific MIB modules have assumed an evolution or loosening of the model. This memo is a proposal to document and standardize the evolution of the model and to fill in the gaps caused by that evolution. A previous effort to extend the interfaces group resulted in the publication of RFC 1229 [7]. As part of defining the evolution of the interfaces group, this memo applies that evolution to, and thereby incorporates, the RFC 1229 extensions. 3.1. Areas of Clarification/Revision There are several areas for which experience indicates that clarification, revision, or extension of the model would be helpful. The next sections discuss these. McCloghrie & Kastenholz [Page 3]
RFC 1573 Interfaces Group Evolution January 1994 3.1.1. Interface Numbering MIB-II defines an object, ifNumber, whose value represents: "The number of network interfaces (regardless of their current state) present on this system." Each interface is identified by a unique value of the ifIndex object, and the description of ifIndex constrains its value as follows: "Its value ranges between 1 and the value of ifNumber. The value for each interface must remain constant at least from one re-initialization of the entity's network management system to the next re-initialization." This constancy requirement on the value of ifIndex for a particular interface is vital for efficient management. However, an increasing number of devices allow for the dynamic addition/removal of network interfaces. One example of this is a dynamic ability to configure the use of SLIP/PPP over a character-oriented port. For such dynamic additions/removals, the combination of the constancy requirement and the restriction that the value of ifIndex is less than ifNumber is problematic. 3.1.2. Interface Sub-Layers Experience in defining media-specific management information has shown the need to distinguish between the multiple sub-layers beneath the internetwork-layer. In addition, there is a need to manage these sub-layers in devices (e.g., MAC-layer bridges) which are unaware of which, if any, internetwork protocols run over these sub-layers. As such, a model of having a single conceptual row in the interfaces table (MIB-II's ifTable) represent a whole interface underneath the internetwork-layer, and having a single associated media-specific MIB module (referenced via the ifType object) is too simplistic. A further problem arises with the value of the ifType object which has enumerated values for each type of interface. Consider, for example, an interface with PPP running over an HDLC link which uses a RS232-like connector. Each of these sub-layers has its own media-specific MIB module. If all of this is represented by a single conceptual row in the ifTable, then an enumerated value for ifType is needed for that specific combination which maps to the specific combination of media-specific MIBs. Furthermore, there is still a lack of a method to describe the relationship of all the sub-layers of the MIB stack. An associated problem is that of upward and downward multiplexing of McCloghrie & Kastenholz [Page 4]
RFC 1573 Interfaces Group Evolution January 1994 the sub-layers. An example of upward multiplexing is MLP (Multi- Link-Procedure) which provides load-sharing over several serial lines by appearing as a single point-to-point link to the sub-layer(s) above. An example of downward multiplexing would be several instances of PPP, each framed within a separate X.25 virtual circuit, all of which run over one fractional T1 channel, concurrently with other uses of the T1 link. The current MIB structure does not allow for these sorts of relationships to be described. 3.1.3. Virtual Circuits Several of the sub-layers for which media-specific MIB modules have been defined are connection oriented (e.g., Frame Relay, X.25). Experience has shown that each effort to define such a MIB module revisits the question of whether separate conceptual rows in the ifTable are needed for each virtual circuit. Most, if not all, of these efforts to date have decided to have all virtual circuits reference a single conceptual row in the ifTable. 3.1.4. Bit, Character, and Fixed-Length Interfaces RS-232 is an example of a character-oriented sub-layer over which (e.g., through use of PPP) IP datagrams can be sent. Due to the packet-based nature of many of the objects in the ifTable, experience has shown that it is not appropriate to have a character-oriented sub-layer represented by a (whole) conceptual row in the ifTable. Experience has also shown that it is sometimes desirable to have some management information for bit-oriented interfaces, which are similarly difficult to represent by a (whole) conceptual row in the ifTable. For example, to manage the channels of a DS1 circuit, where only some of the channels are carrying packet-based data. A further complication is that some subnetwork technologies transmit data in fixed length transmission units. One example of such a technology is cell relay, and in particular Asynchronous Transfer Mode (ATM), which transmits data in fixed-length cells. Representing such a interface as a packet-based interface produces redundant objects if the relationship between the number of packets and the number of octets in either direction is fixed by the size of the transmission unit (e.g., the size of a cell). 3.1.5. Counter Size As the speed of network media increase, the minimum time in which a 32 bit counter will wrap decreases. For example, on an Ethernet, a stream of back-to-back, full-size packets will cause ifInOctets to wrap in just over 57 minutes. For a T3 line, the minimum wrap-time McCloghrie & Kastenholz [Page 5]
RFC 1573 Interfaces Group Evolution January 1994 is just over 12 minutes. For FDDI, it will wrap in 5.7 minutes. For a 1-gigabit medium, the counter might wrap in as little as 34 seconds. Requiring that interfaces be polled frequently enough not to miss a counter wrap will be increasingly problematic. 3.1.6. Interface Speed Network speeds are increasing. The range of ifSpeed is limited to reporting a maximum speed of (2**31)-1 bits/second, or approximately 2.2Gbs. SONET defines an OC-48 interface, which is defined at operating at 48 times 51 Mbs, which is a speed in excess of 2.4gbits. Thus, ifSpeed will be of diminishing utility over the next several years. 3.1.7. Multicast/Broadcast Counters The counters in the ifTable for packets addressed to a multicast or the broadcast address, are combined as counters of non-unicast packets. In contrast, the ifExtensions MIB [7] defines one set of counters for multicast, and a separate set for broadcast packets. With the separate counters, the original combined counters become redundant. 3.1.8. Addition of New ifType values Over time new ifType enumerated values have been needed for new interface types. With the syntax of ifType being defined in a MIB, this requires the new MIB to be re-issued in order to define the new values. In the past, re-issuing of the MIB has occurred only after several years. 3.1.9. ifSpecific The original definition of the OBJECT IDENTIFIER value of ifSpecific was not sufficently clear. As a result, different implementors have used it differently, and confusion has resulted. Some implementations have the value of ifSpecific be the OBJECT IDENTIFIER that defines the media-specific MIB, i.e., the "foo" of: foo OBJECT IDENTIFIER ::= { transmission xxx } while others have it be the OBJECT IDENTIFIER of the table or entry in the appropriate media-specific MIB (e.g. fooTable or fooEntry), while still others have it be the OBJECT IDENTIFIER of the index object of the table's row, including instance identifier (e.g., fooIfIndex.ifIndex). A definition based on the latter would not be sufficient unless it also allowed for media-specific MIBs which include several tables, where each table has its own, different, McCloghrie & Kastenholz [Page 6]
RFC 1573 Interfaces Group Evolution January 1994 indexing. 3.2. Clarifications/Revisions The following clarifications and/or revisions are proposed. 3.2.1. Interface Numbering One solution to the interface numbering problem would be to redefine ifNumber to be the largest value of ifIndex, but the utility of such an object is questionable, and such a re-definition would require ifNumber to be deprecated. Thus, an improvement would be to deprecate ifNumber and not replace it. However, the deprecation of ifNumber would require a change to that portion of ifIndex's definition which refers to ifNumber. So, since the definition of ifIndex must be changed anyway in order to solve the problem, changes to ifNumber do not benefit the solution. The solution adopted in this memo is to delete the requirement that the value of ifIndex must be less than the value of ifNumber, and to retain ifNumber with its current definition. It could be argued that this is a change in the semantics of ifIndex; however, all existing implementations conform to this new definition, and in the interests of not requiring changes in existing implementations and in the many existing media-specific MIBs, it is proposed that this change does not require ifIndex to be deprecated. This solution also results in the possibility of "holes" in the ifTable (i.e., the ifIndex values of conceptual rows in the ifTable are not necessarily contiguous), but SNMP's GetNext (and SNMPv2's GetBulk) operation easily deals with such holes. The value of ifNumber still represents the number of conceptual rows, which increases/decreases as new interfaces are dynamically added/removed. The vital constancy requirement is met by requiring that after an interface is dynamically removed, its ifIndex value is not re-used (by a different dynamically added interface) until after the following re-initialization of the network management system. This avoids the need for a priori assignment of ifIndex values for all possible interfaces which might be added dynamically. The exact meaning of a "different" interface is hard to define, and there will be gray areas. One important criterion is that a management station, not noticing that an interface has gone away and another come into existence, should not be confused when it calculates the difference between the counter values retrieved on successive polls for a particular ifIndex value. However, any firm definition in this document would likely to turn out to be inadequate. Instead, the following guidelines are offered to allow McCloghrie & Kastenholz [Page 7]
RFC 1573 Interfaces Group Evolution January 1994 implementors to choose what "different" means in their particular situation. A previously-unused value of ifIndex should be assigned to a dynamically added interface if: (1) the assignment of a previously-used ifIndex value to the interface could result in a discontinuity in the values of ifTable counters for that value of ifIndex; or, (2) an agent has no knowledge of whether the interface is the "same" or "different" from a previous interface incarnation. Because of the restriction of the value of ifIndex to be less than ifNumber, interfaces have been numbered with small integer values. This has led to the ability by humans to use the ifIndex values as (somewhat) user-friendly names for network interfaces (e.g., "interface number 3"). With the relaxation of the restriction on the value of ifIndex, there is now the possibility that ifIndex values could be assigned as very large numbers (e.g., memory addresses). Such numbers would be much less user-friendly. Therefore, this memo recommends that ifIndex values still be assigned as (relatively) small integer values starting at 1, even though the values in use at any one time are not necessarily contiguous. (Note that this makes remembering which values have been assigned easy for agents which dynamically add new interfaces.) This proposed change introduces a new problem of its own. Previously, there usually was a simple, direct, mapping of interfaces to the physical ports on systems. This mapping would be based on the ifIndex value. However, by removing the previous restrictions on the values allowed for ifIndex, along with the interface sub-layer concept (see the following section), mapping from interfaces to physical ports becomes increasingly problematic. To address this issue, a new object, ifName, is added to the MIB. This object contains the device's name for the interface of which the relevant entry in the ifTable is a component. For example, if a router has an interface named wan1, which is composed of PPP running over an RS-232 port, the ifName objects for the corresponding PPP and RS-232 entries in the ifTable will contain the string "wan1". 3.2.2. Interface Sub-Layers One possible but not recommended solution to the problem of representing multiple sub-layers would be to retain the concept of one conceptual row for all the sub-layers of an interface and have McCloghrie & Kastenholz [Page 8]
RFC 1573 Interfaces Group Evolution January 1994 each media-specific MIB module identify its "superior" and "subordinate" sub-layers through OBJECT IDENTIFIER "pointers". The drawbacks of this scheme are: 1) the superior/subordinate pointers are contained in the media-specific MIB modules, and thus, a manager could not learn the structure of an interface, without inspecting multiple pointers in different MIB modules; this is overly complex and only possible if the manager has knowledge of all the relevant media-specific MIB modules; 2) current MIB modules would all need to be retrofitted with these new "pointers"; 3) this scheme does not adequately address the problem of upward and downward multiplexing; and 4) enumerated values of ifType are needed for each combination of sub-layers. Another possible but not recommended scheme would be to retain the concept of one conceptual row for all the sub-layers of an interface and have a new separate MIB table to identify the "superior" and "subordinate" sub-layers which contain OBJECT IDENTIFIER "pointers" to media-specific MIB module(s) for each sub-layer. Effectively, one conceptual row in the ifTable would represent each combination of sub-layers between the internetwork-layer and the wire. While this scheme has fewer drawbacks, it does not support downward multiplexing, such as PPP over MLP; since MLP makes two (or more) serial lines appear to the layers above as a single physical interface, PPP over MLP should appear to the internetwork-layer as a single interface. However, this scheme would result in two (or more) conceptual rows in the ifTable and the internetwork-layer would run over both of them. This scheme also requires enumerated values of ifType for each combination of sub-layers. The solution adopted in this memo is to have an individual conceptual row in the ifTable to represent each sub-layer and have a new separate MIB table (the ifStackTable, see section 5 of this memo) to identify the "superior" and "subordinate" sub-layers through INTEGER "pointers" to the appropriate conceptual rows in the ifTable. This solution supports both upward and downward multiplexing. It also allows the IANAIfType to Media-Specific MIB mapping to identify the media-specific MIB module for each sub- layer. The new table (ifStackTable) need be referenced only to obtain information about layering. Enumerated values for ifType are required for each sub- layer only, not for combinations of them. However, this solution does require that the descriptions of some objects in the ifTable (specifically, ifType, ifPhysAddress, ifInUcastPkts, and ifOutUcastPkts) be generalized so as to apply to any sub-layer (rather than only to a sub-layer immediately beneath the network layer, as at present). It also requires that some objects (specifically, ifSpeed) need to have appropriate values identified for use when a generalized definition does not apply to a McCloghrie & Kastenholz [Page 9]
RFC 1573 Interfaces Group Evolution January 1994 particular sub-layer. In addition, this adopted solution makes no requirement that a device, in which a sub-layer is instrumented by a conceptual row of the ifTable, be aware of whether an internetwork protocol runs on top of (i.e., at some layer above) that sub-layer. In fact, the counters of packets received on an interface are defined as counting the number "delivered to a higher-layer protocol". This meaning of "higher-layer" includes: (1) Delivery to a forwarding module which accepts packets/frames/octets and forwards them on at the same protocol layer. For example, for the purposes of this definition, the forwarding module of a MAC-layer bridge is considered as a "higher-layer" to the MAC-layer of each port on the bridge. (2) Delivery to a higher sub-layer within a interface stack. For example, for the purposes of this definition, if a PPP module operated directly over a serial interface, the PPP module would be considered the higher sub-layer to the serial interface. (3) Delivery to a higher protocol layer which does not do packet forwarding for sub-layers that are "at the top of" the interface stack. For example, for the purposes of this definition, the local IP module would be considered the higher layer to a SLIP serial interface. Similarly, for output, the counters of packets transmitted out an interface are defined as counting the number "that higher-level protocols requested to be transmitted". This meaning of "higher- layer" includes: (1) A forwarding module, at the same protocol layer, which transmits packets/frames/octets that were received on an different interface. For example, for the purposes of this definition, the forwarding module of a MAC-layer bridge is considered as a "higher-layer" to the MAC-layer of each port on the bridge. (2) The next higher sub-layer within an interface stack. For example, for the purposes of this definition, if a PPP module operated directly over a serial interface, the PPP module would be a "higher layer" to the serial interface. McCloghrie & Kastenholz [Page 10]
RFC 1573 Interfaces Group Evolution January 1994 (3) For sub-layers that are "at the top of" the interface stack, a higher element in the network protocol stack. For example, for the purposes of this definition, the local IP module would be considered the higher layer to an Ethernet interface. 3.2.3. Guidance on Defining Sub-layers The designer of a media-specific MIB must decide whether to divide the interface into sub-layers, and if so, how to make the divisions. The following guidance is offered to assist the media-specific MIB designer in these decisions. In general, the number of entries in the ifTable should be kept to the minimum required for network management. In particular, a group of related interfaces should be treated as a single interface with one entry in the ifTable providing that: (1) None of the group of interfaces performs multiplexing for any other interface in the agent, (2) There is a meaningful and useful way for all of the ifTable's information (e.g., the counters, and the status variables), and all of the ifTable's capabilities (e.g., write access to ifAdminStatus), to apply to the group of interfaces as a whole. Under these circumstances, there should be one entry in the ifTable for such a group of interfaces, and any internal structure which needs to be represented to network management should be captured in a MIB module specific to the particular type of interface. Note that application of bullet 2 above to the ifTable's ifType object requires that there is a meaningful media-specific MIB and a meaningful ifType value which apply to the group of interfaces as a whole. For example, it is not appropriate to treat an HDLC sub-layer and an RS-232 sub-layer as a single ifTable entry when the media- specific MIBs and the ifType values for HDLC and RS-232 are separate (rather than combined). Note that the sub-layers of an interface on one device will sometimes be different to the sub-layers of the interconnected interface of another device. A simple example of this is a frame-relay DTE interface which connects to a frameRelayService interface, where the DTE interface has a different ifType value and media-specific MIB to the DCE interface. Also note that a media-specific MIB may mandate that a particular McCloghrie & Kastenholz [Page 11]
RFC 1573 Interfaces Group Evolution January 1994 ifTable counter does not apply and that its value must always be 0, signifying that the applicable event can not and does not occur for that type of interface; for example, ifInMulticastPkts and ifOutMulticastPkts on an interface type which has no multicast capability. In other circumstances, an agent must not always return 0 for any counter just because its implementation is incapable of detecting occurrences of the particular event; instead, it must return a noSuchName/noSuchObject error/exception when queried for the counter, even if this prevents the implementation from complying with the relevant MODULE-COMPLIANCE macro. These guidelines are just that - guidelines. The designer of a media-specific MIB is free to lay out the MIB in whatever SMI conformant manner is desired. However, in so doing, the media- specific MIB MUST completely specify the sub-layering model used for the MIB, and provide the assumptions, reasoning, and rationale used to develop that model. 3.2.4. Virtual Circuits This memo strongly recommends that connection-oriented sub-layers do not have a conceptual row in the ifTable for each virtual circuit. This avoids the proliferation of conceptual rows, especially those which have considerable redundant information. (Note, as a comparison, that connection-less sub-layers do not have conceptual rows for each remote address.) There may, however, be circumstances under which it is appropriate for a virtual circuit of a connection- oriented sub-layer to have its own conceptual row in the ifTable; an example of this might be PPP over an X.25 virtual circuit. The MIB in section 6 of this memo supports such circumstances. If a media-specific MIB wishes to assign an entry in the ifTable to each virtual circuit, the MIB designer must present the rationale for this decision in the media-specific MIB's specification. 3.2.5. Bit, Character, and Fixed-Length Interfaces About half the objects in the ifTable are applicable to every type of interface: packet-oriented, character-oriented, and bit-oriented. Of the other half, two are applicable to both character-oriented and packet-oriented interfaces, and the rest are applicable only to packet-oriented interfaces. Thus, while it is desirable for consistency to be able to represent any/all types of interfaces in the ifTable, it is not possible to implement the full ifTable for bit- and character-oriented sub-layers. One possible but not recommended solution to this problem would be to split the ifTable into two (or more) new MIB tables, one of which McCloghrie & Kastenholz [Page 12]
RFC 1573 Interfaces Group Evolution January 1994 would contain objects that are relevant only to packet-oriented interfaces (e.g., PPP), and another that may be used by all interfaces. This is highly undesirable since it would require changes in every agent implementing the ifTable (i.e., just about every existing SNMP agent). The solution adopted in this memo builds upon the fact that compliance statements in SNMPv2 (in contrast to SNMPv1) refer to object groups, where object groups are explicitly defined by listing the objects they contain. Thus, in SNMPv2, multiple compliance statements can be specified, one for all interfaces and additional ones for specific types of interfaces. The separate compliance statements can be based on separate object groups, where the object group for all interfaces can contain only those objects from the ifTable which are appropriate for every type of interfaces. Using this solution, every sub-layer can have its own conceptual row in the ifTable. Thus, section 6 of this memo contains definitions of the objects of the existing 'interfaces' group of MIB-II, in a manner which is both SNMPv2-compliant and semantically-equivalent to the existing MIB-II definitions. With equivalent semantics, and with the BER ("on the wire") encodings unchanged, these definitions retain the same OBJECT IDENTIFIER values as assigned by MIB-II. Thus, in general, no rewrite of existing agents which conform to MIB-II and the ifExtensions MIB is required. In addition, this memo defines several object groups for the purposes of defining which objects apply to which types of interface: (1) the ifGeneralGroup. This group contains those objects applicable to all types of network interfaces, including bit-oriented interfaces. (2) the ifPacketGroup. This group contains those objects applicable to packet-oriented network interfaces. (3) the ifFixedLengthGroup. This group contains the objects applicable not only to character-oriented interfaces, such as RS-232, but also to those subnetwork technologies, such as cell-relay/ATM, which transmit data in fixed length transmission units. As well as the octet counters, there are also a few other counters (e.g., the error counters) which are useful for this type of interface, but are currently defined as being packet-oriented. To accommodate this, the definitions of these counters are generalized to apply to character-oriented interfaces and fixed-length-transmission interfaces. McCloghrie & Kastenholz [Page 13]
RFC 1573 Interfaces Group Evolution January 1994 It should be noted that the octet counters in the ifTable aggregate octet counts for unicast and non-unicast packets into a single octet counter per direction (received/transmitted). Thus, with the above definition of fixed-length-transmission interfaces, where such interfaces which support non-unicast packets, separate counts of unicast and multicast/broadcast transmissions can only be maintained in a media-specific MIB module. 3.2.6. Counter Size Two approaches to addressing the shrinking minimum counter-wrap time problem were evaluated. Counters could be scaled, for example, ifInOctets could be changed to count received octets in, e.g., 1024 byte blocks. Alternatively, the size of the counter could be increased. Scaling the counters was rejected. While it provides acceptable performance at high count rates, at low rates it suffers. If there is little traffic on an interface, there might be a significant interval before enough counts occur to cause a counter to be incremented. Traffic would then appear to be very bursty, leading to incorrect conclusions of the network's performance. The alternative, which this memo adopts, is to provide expanded, 64 bit, counters. These counters are provided in new "high capacity" groups, The old, 32-bit, counters have not been deprecated. The 64-bit counters are to be used only when the 32-bit counters do not provide enough capacity; that is, the 32 bit counters could wrap too fast. For interfaces that operate at 20,000,000 (20 million) bits per second or less, 32-bit byte and packet counters MUST be used. For interfaces that operate faster than 20,000,000 bits/second, and slower than 650,000,000 bits/second, 32-bit packet counters MUST be used and 64-bit octet counters MUST be used. For interfaces that operate at 650,000,000 bits/second or faster, both 64-bit packet counters AND 64-bit octet counters MUST be used. These speed steps were chosen as reasonable compromises based on the following: (1) The cost of maintaining 64-bit counters is relatively high, so minimizing the number of agents which must support them is desirable. Common interfaces (such as Ethernet) should not require them. McCloghrie & Kastenholz [Page 14]
RFC 1573 Interfaces Group Evolution January 1994 (2) 64-bit counters are a new feature, introduced in SNMPv2. It is reasonable to expect that support for them will be spotty for the immediate future. Thus, we wish to limit them to as few systems as possible. This, in effect, means that 64-bit counters should be limited to higher speed interfaces. Ethernet (10,000,000 bps) and Token Ring (16,000,000 bps) are fairly wide-spread so it seems reasonable to not require 64- bit counters for these interfaces. (3) The 32-bit octet counters will wrap in the following times, for the following interfaces (when transmitting maximum-sized packets back-to-back): - Ethernet: 57 minutes, - 16 megabit Token Ring: 36 minutes, - A US T3 line (45 megabits): 12 minutes, - FDDI: 5.7 minutes (4) The 32-bit packet counters wraps in about 57 minutes when 64-byte packets are transmitted back-to-back on a 650,000,000 bit/second link. As an aside, a 1-terabit (1,000 gigabits) link will cause a 64 bit octet counter to wrap in just under 5 years. Conversely, an 81,000,000 terabit/second link is required to cause a 64-bit counter to wrap in 30 minutes. We believe that, while technology rapidly marches forward, this link speed will not be achieved for at least several years, leaving sufficient time to evaluate the introduction of 96 bit counters. When 64-bit counters are in use, the 32-bit counters MUST still be available. They will report the low 32-bits of the associated 64-bit count (e.g., ifInOctets will report the least significant 32 bits of ifHCInOctets). This enhances inter-operability with existing implementations at a very minimal cost to agents. The new "high capacity" groups are: (1) the ifHCFixedLengthGroup for character-oriented/fixed-length interfaces, and the ifHCPacketGroup for packet-based interfaces; both of these groups include 64 bit counters for octets, and McCloghrie & Kastenholz [Page 15]
RFC 1573 Interfaces Group Evolution January 1994 (2) the ifVHCPacketGroup for packet-based interfaces; this group includes 64 bit counters for octets and packets. 3.2.7. Interface Speed In order to deal with increasing interface speeds, we have added an ifHighSpeed object. This object reports the speed of the interface in 1,000,000 (1 million) bits/second units. Thus, the true speed of the interface will be the value reported by this object, plus or minus 500,000 bits/second. Other alternatives considered were: (1) Making the interface speed a 64-bit gauge. This was rejected since the current SMI does not allow such a syntax. Furthermore, even if 64-bit gauges were available, their use would require additional complexity in agents due to an increased requirement for 64-bit operations. (2) We also considered making "high-32 bit" and "low-32-bit" objects which, when combined, would be a 64-bit value. This simply seemed overly complex for what we are trying to do. Furthermore, a full 64-bits of precision does not seem necessary. The value of ifHighSpeed will be the only report of interface speed for interfaces that are faster than 4,294,967,295 bits per second. At this speed, the granularity of ifHighSpeed will be 1,000,000 bits per second, thus the error will be 1/4294, or about 0.02%. This seems reasonable. (3) Adding a "scale" object, which would define the units which ifSpeed's value is. This would require two additional objects; one for the scaling object, and one to replace the current ifSpeed. This later object is required since the semantics of ifSpeed would be significantly altered, and manager stations which do not understand the new semantics would be confused. 3.2.8. Multicast/Broadcast Counters To avoid the redundancy of counting all non-unicast packets as well as having individual multicast and broadcast packet counters, we deprecate the use of the non-unicast counters, which can be derived McCloghrie & Kastenholz [Page 16]
RFC 1573 Interfaces Group Evolution January 1994 from the values of the others. For the output broadcast and multicast counters defined in RFC 1229, their definitions varied slightly from the packet counters in the ifTable, in that they did not count errors/discarded packets. To align the definitions better, the old counters are deprecated and replaced by new definitions. Counters with 64 bits of range are also needed, as explained above. 3.2.9. Trap Enable In the multi-layer interface model, each sub-layer for which there is an entry in the ifTable can generate linkUp/Down Traps. Since interface state changes would tend to propagate through the interface (from top to bottom, or bottom to top), it is likely that several traps would be generated for each linkUp/Down occurrence. It is desirable to provide a mechanism for manager stations to control the generation of these traps. To this end, the ifLinkUpDownTrapEnable object has been added. This object allows managers to limit generation of traps to just the sub-layers of interest. The default setting should limit the number of traps generated to one per interface per linkUp/Down event. Furthermore, it seems that the conditions that cause these state changes that are of most interest to network managers occur at the lowest level of an interface stack. Therefore we specify that by default, only the lowest sub-layer of the interface generate traps. 3.2.10. Addition of New ifType values The syntax of ifType is changed to be a textual convention, such that the enumerated integer values are now defined in the textual convention, IANAifType, which can be re-specified (with additional values) without issuing a new version of this document. The Internet Assigned Number Authority (IANA) is responsible for the assignment of all Internet numbers, including various SNMP-related numbers, and specifically, new ifType values. Thus, this document defines two MIB modules: one to define the MIB for the 'interfaces' group, and a second to define the first version of the IANAifType textual convention. The latter will be periodically re-issued by the IANA. 3.2.11. InterfaceIndex Textual Convention A new textual convention, InterfaceIndex, has been defined. This textual convention "contains" all of the semantics of the ifIndex object. This allows other mib modules to easily import the semantics McCloghrie & Kastenholz [Page 17]
RFC 1573 Interfaces Group Evolution January 1994 of ifIndex. 3.2.12. IfAdminStatus and IfOperStatus A new state has been added to ifOperStatus: dormant. This state indicates that the relevant interface is not actually in a condition to pass packets (i.e., up) but is in a "pending" state, waiting for some external event. For "on-demand" interfaces, this new state identifies the situation where the interface is waiting for events to place it in the up state. Examples of such events might be: (1) having packets to transmit before establishing a connection to a remote system. (2) having a remote system establish a connection to the interface (e.g., dialing up to a slip-server). The down state now has two meanings, depending on the value of ifAdminStatus. (1) If ifAdminStatus is not down and ifOperStatus is down, then a fault condition is presumed to exist on the interface. (2) If ifAdminStatus is down, then ifOperStatus will normally also be down, i.e., there is not (necessarily) a fault condition on the interface. Note that when ifAdminStatus transitions to down, ifOperStatus will normally also transition to down. In this situation, it is possible that ifOperStatus's transition will not occur immediately, but rather after a small time lag to complete certain operations before going "down"; for example, it might need to finish transmitting a packet. If a manager station finds that ifAdminStatus is down and ifOperStatus is not down for a particular interface, the manager station should wait a short while and check again. If the condition still exists only then should it raise an error indication. Naturally, it should also ensure that ifLastChange has not changed during this interval. Whenever an interface table entry is created (usually as a result of system initialization), the relevant instance of ifAdminStatus is set to down, and presumably ifOperStatus will also be down. An interface may be enabled in two ways: either as a result of explicit management action (e.g., setting ifAdminStatus to up) or as a result of the managed system's initialization process. When ifAdminStatus changes to the up state, the related ifOperStatus should do one of the following: McCloghrie & Kastenholz [Page 18]
RFC 1573 Interfaces Group Evolution January 1994
RFC 1573 Interfaces Group Evolution January 1994 -- difference is that this table makes use of the RowStatus -- textual convention, while ifExtnsRcvAddr did not. ifRcvAddressTable OBJECT-TYPE SYNTAX SEQUENCE OF IfRcvAddressEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "This table contains an entry for each address (broadcast, multicast, or uni-cast) for which the system will receive packets/frames on a particular interface, except as follows: - for an interface operating in promiscuous mode, entries are only required for those addresses for which the system would receive frames were it not operating in promiscuous mode. - for 802.5 functional addresses, only one entry is required, for the address which has the functional address bit ANDed with the bit mask of all functional addresses for which the interface will accept frames." ::= { ifMIBObjects 4 } ifRcvAddressEntry OBJECT-TYPE SYNTAX IfRcvAddressEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "A list of objects identifying an address for which the system will accept packets/frames on the particular interface identified by the index value ifIndex." INDEX { ifIndex, ifRcvAddressAddress } ::= { ifRcvAddressTable 1 } IfRcvAddressEntry ::= SEQUENCE { ifRcvAddressAddress PhysAddress, ifRcvAddressStatus RowStatus, ifRcvAddressType INTEGER } ifRcvAddressAddress OBJECT-TYPE SYNTAX PhysAddress MAX-ACCESS read-create STATUS current DESCRIPTION McCloghrie & Kastenholz [Page 47]
RFC 1573 Interfaces Group Evolution January 1994 "An address for which the system will accept packets/frames on this entry's interface." ::= { ifRcvAddressEntry 1 } ifRcvAddressStatus OBJECT-TYPE SYNTAX RowStatus MAX-ACCESS read-write STATUS current DESCRIPTION "This object is used to create and delete rows in the ifRcvAddressTable." ::= { ifRcvAddressEntry 2 } ifRcvAddressType OBJECT-TYPE SYNTAX INTEGER { other(1), volatile(2), nonVolatile(3) } MAX-ACCESS read-create STATUS current DESCRIPTION "This object has the value nonVolatile(3) for those entries in the table which are valid and will not be deleted by the next restart of the managed system. Entries having the value volatile(2) are valid and exist, but have not been saved, so that will not exist after the next restart of the managed system. Entries having the value other(1) are valid and exist but are not classified as to whether they will continue to exist after the next restart." DEFVAL { volatile } ::= { ifRcvAddressEntry 3 } -- definition of interface-related traps. linkDown NOTIFICATION-TYPE OBJECTS { ifIndex, ifAdminStatus, ifOperStatus } STATUS current DESCRIPTION "A linkDown trap signifies that the SNMPv2 entity, acting in an agent role, has detected that the ifOperStatus object for one of its communication links McCloghrie & Kastenholz [Page 48]
RFC 1573 Interfaces Group Evolution January 1994 is about to transition into the down state." ::= { snmpTraps 3 } linkUp NOTIFICATION-TYPE OBJECTS { ifIndex, ifAdminStatus, ifOperStatus } STATUS current DESCRIPTION "A linkUp trap signifies that the SNMPv2 entity, acting in an agent role, has detected that the ifOperStatus object for one of its communication links has transitioned out of the down state." ::= { snmpTraps 4 } -- conformance information ifConformance OBJECT IDENTIFIER ::= { ifMIB 2 } ifGroups OBJECT IDENTIFIER ::= { ifConformance 1 } ifCompliances OBJECT IDENTIFIER ::= { ifConformance 2 } -- compliance statements ifCompliance MODULE-COMPLIANCE STATUS current DESCRIPTION "The compliance statement for SNMPv2 entities which have network interfaces." MODULE -- this module MANDATORY-GROUPS { ifGeneralGroup, ifStackGroup } GROUP ifFixedLengthGroup DESCRIPTION "This group is mandatory for all network interfaces which are character-oriented or transmit data in fixed-length transmission units." GROUP ifHCFixedLengthGroup DESCRIPTION "This group is mandatory only for those network interfaces which are character-oriented or transmit data in fixed-length transmission units, and for which the value of the corresponding instance of ifSpeed is greater than 20,000,000 bits/second." GROUP ifPacketGroup McCloghrie & Kastenholz [Page 49]
RFC 1573 Interfaces Group Evolution January 1994 DESCRIPTION "This group is mandatory for all network interfaces which are packet-oriented." GROUP ifHCPacketGroup DESCRIPTION "This group is mandatory only for those network interfaces which are packet-oriented and for which the value of the corresponding instance of ifSpeed is greater than 650,000,000 bits/second." GROUP ifTestGroup DESCRIPTION "This group is optional. Media-specific MIBs which require interface tests are strongly encouraged to use this group for invoking tests and reporting results. A medium specific MIB which has mandatory tests may make implementation of this group mandatory." GROUP ifRcvAddressGroup DESCRIPTION "The applicability of this group MUST be defined by the media-specific MIBs. Media-specific MIBs must define the exact meaning, use, and semantics of the addresses in this group." OBJECT ifLinkUpDownTrapEnable MIN-ACCESS read-only DESCRIPTION "Write access is not required." OBJECT ifPromiscuousMode MIN-ACCESS read-only DESCRIPTION "Write access is not required." OBJECT ifStackStatus SYNTAX INTEGER { active(1) } -- subset of RowStatus MIN-ACCESS read-only DESCRIPTION "Write access is not required, and only one of the six enumerated values for the RowStatus textual convention need be supported, specifically: active(1)." OBJECT ifAdminStatus SYNTAX INTEGER { up(1), down(2) } MIN-ACCESS read-only DESCRIPTION "Write access is not required, nor is support for the McCloghrie & Kastenholz [Page 50]
RFC 1573 Interfaces Group Evolution January 1994 value testing(3)." ::= { ifCompliances 1 } -- units of conformance ifGeneralGroup OBJECT-GROUP OBJECTS { ifDescr, ifType, ifSpeed, ifPhysAddress, ifAdminStatus, ifOperStatus, ifLastChange, ifLinkUpDownTrapEnable, ifConnectorPresent, ifHighSpeed, ifName } STATUS current DESCRIPTION "A collection of objects providing information applicable to all network interfaces." ::= { ifGroups 1 } -- the following five groups are mutually exclusive; at most -- one of these groups is implemented for any interface ifFixedLengthGroup OBJECT-GROUP OBJECTS { ifInOctets, ifOutOctets, ifInUnknownProtos, ifInErrors, ifOutErrors } STATUS current DESCRIPTION "A collection of objects providing information specific to non-high speed, character-oriented or fixed-length-transmission network interfaces. (Non- high speed interfaces transmit and receive at speeds less than or equal to 20,000,000 bits/second.)" ::= { ifGroups 2 } ifHCFixedLengthGroup OBJECT-GROUP OBJECTS { ifHCInOctets, ifHCOutOctets, ifInOctets, ifOutOctets, ifInUnknownProtos, ifInErrors, ifOutErrors } STATUS current DESCRIPTION "A collection of objects providing information specific to high speed (greater than 20,000,000 bits/second) character-oriented or fixed-length- transmission network interfaces." ::= { ifGroups 3 } ifPacketGroup OBJECT-GROUP OBJECTS { ifInOctets, ifOutOctets, ifInUnknownProtos, ifInErrors, ifOutErrors, ifMtu, ifInUcastPkts, ifInMulticastPkts, McCloghrie & Kastenholz [Page 51]
RFC 1573 Interfaces Group Evolution January 1994 ifInBroadcastPkts, ifInDiscards, ifOutUcastPkts, ifOutMulticastPkts, ifOutBroadcastPkts, ifOutDiscards, ifPromiscuousMode } STATUS current DESCRIPTION "A collection of objects providing information specific to non-high speed, packet-oriented network interfaces. (Non-high speed interfaces transmit and receive at speeds less than or equal to 20,000,000 bits/second.)" ::= { ifGroups 4 } ifHCPacketGroup OBJECT-GROUP OBJECTS { ifHCInOctets, ifHCOutOctets, ifInOctets, ifOutOctets, ifInUnknownProtos, ifInErrors, ifOutErrors, ifMtu, ifInUcastPkts, ifInMulticastPkts, ifInBroadcastPkts, ifInDiscards, ifOutUcastPkts, ifOutMulticastPkts, ifOutBroadcastPkts, ifOutDiscards, ifPromiscuousMode } STATUS current DESCRIPTION "A collection of objects providing information specific to high speed (greater than 20,000,000 bits/second but less than or equal to 650,000,000 bits/second) packet-oriented network interfaces." ::= { ifGroups 5 } ifVHCPacketGroup OBJECT-GROUP OBJECTS { ifHCInUcastPkts, ifHCInMulticastPkts, ifHCInBroadcastPkts, ifHCOutUcastPkts, ifHCOutMulticastPkts, ifHCOutBroadcastPkts, ifHCInOctets, ifHCOutOctets, ifInOctets, ifOutOctets, ifInUnknownProtos, ifInErrors, ifOutErrors, ifMtu, ifInUcastPkts, ifInMulticastPkts, ifInBroadcastPkts, ifInDiscards, ifOutUcastPkts, ifOutMulticastPkts, ifOutBroadcastPkts, ifOutDiscards, ifPromiscuousMode } STATUS current DESCRIPTION "A collection of objects providing information specific to higher speed (greater than 650,000,000 bits/second) packet-oriented network interfaces." ::= { ifGroups 6 } McCloghrie & Kastenholz [Page 52]
RFC 1573 Interfaces Group Evolution January 1994 ifRcvAddressGroup OBJECT-GROUP OBJECTS { ifRcvAddressStatus, ifRcvAddressType } STATUS current DESCRIPTION "A collection of objects providing information on the multiple addresses which an interface receives." ::= { ifGroups 7 } ifTestGroup OBJECT-GROUP OBJECTS { ifTestId, ifTestStatus, ifTestType, ifTestResult, ifTestCode, ifTestOwner } STATUS current DESCRIPTION "A collection of objects providing the ability to invoke tests on an interface." ::= { ifGroups 8 } ifStackGroup OBJECT-GROUP OBJECTS { ifStackStatus } STATUS current DESCRIPTION "A collection of objects providing information on the layering of MIB-II interfaces." ::= { ifGroups 9 } END 7. Acknowledgements This memo has been produced by the IETF's Interfaces MIB Working Group. The initial proposal to the working group was the result of conversations and discussions with many people, including at least the following: Fred Baker, Ted Brunner, Chuck Davin, Jeremy Greene, Marshall Rose, Kaj Tesink, and Dean Throop. 8. References [1] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Structure of Management Information for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1442, SNMP Research, Inc., Hughes LAN Systems, Dover Beach Consulting, Inc., Carnegie Mellon University, April 1993. [2] Galvin, J., and K. McCloghrie, "Administrative Model for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1445, Trusted Information Systems, Hughes LAN Systems, April 1993. McCloghrie & Kastenholz [Page 53]
RFC 1573 Interfaces Group Evolution January 1994 [3] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Protocol Operations for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1448, SNMP Research, Inc., Hughes LAN Systems, Dover Beach Consulting, Inc., Carnegie Mellon University, April 1993. [4] McCloghrie, K., and M. Rose, "Management Information Base for Network Management of TCP/IP-based internets - MIB-II", STD 17, RFC 1213, Hughes LAN Systems, Performance Systems International, March 1991. [5] 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. [6] Postel, J., "Internet Protocol", STD 5, RFC 791, USC/Information Sciences Institute, September 1981. [7] McCloghrie, K., "Extensions to the Generic-Interface MIB", RFC 1229, Hughes LAN Systems, May 1991. [8] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Textual Conventions for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1443, SNMP Research, Inc., Hughes LAN Systems, Dover Beach Consulting, Inc., Carnegie Mellon University, April 1993. McCloghrie & Kastenholz [Page 54]
RFC 1573 Interfaces Group Evolution January 1994



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