RFC 2233
The Interfaces Group MIB using SMIv2
Network Working Group K. McCloghrie Request for Comments: 2233 Cisco Systems Obsoletes: 1573 F. Kastenholz Category: Standards Track FTP Software November 1997 The Interfaces Group MIB using SMIv2 Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1997). All Rights Reserved. Table of Contents 1 Introduction .............................................. 2 2 The SNMP Network Management Framework ..................... 2 2.1 Object Definitions ...................................... 3 3 Experience with the Interfaces Group ...................... 3 3.1 Clarifications/Revisions ................................ 3 3.1.1 Interface Sub-Layers .................................. 4 3.1.2 Guidance on Defining Sub-layers ....................... 6 3.1.3 Virtual Circuits ...................................... 8 3.1.4 Bit, Character, and Fixed-Length Interfaces ........... 8 3.1.5 Interface Numbering ................................... 10 3.1.6 Counter Size .......................................... 14 3.1.7 Interface Speed ....................................... 16 3.1.8 Multicast/Broadcast Counters .......................... 17 3.1.9 Trap Enable ........................................... 18 3.1.10 Addition of New ifType values ........................ 18 3.1.11 InterfaceIndex Textual Convention .................... 18 3.1.12 New states for IfOperStatus .......................... 19 3.1.13 IfAdminStatus and IfOperStatus ....................... 20 3.1.14 IfOperStatus in an Interface Stack ................... 21 3.1.15 Traps ................................................ 21 3.1.16 ifSpecific ........................................... 23 3.1.17 Creation/Deletion of Interfaces ...................... 24 3.1.18 All Values Must be Known ............................. 24 4 Media-Specific MIB Applicability .......................... 25
5 Overview .................................................. 26 6 Interfaces Group Definitions .............................. 26 7 Acknowledgements .......................................... 64 8 References ................................................ 64 9 Security Considerations ................................... 65 10 Authors' Addresses ....................................... 65 11 Full Copyright Statement ................................. 66 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 specifies clarifications to, and extensions of, the architectural issues within the previous 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 that is both compliant to the SNMPv2 SMI and semantically- identical to the existing SNMPv1-based definitions. The key words "MUST" and "MUST NOT" in this document are to be interpreted as described in RFC 2119 [10]. 2. The SNMP Network Management Framework The SNMP Network Management Framework presently consists of three major components. They are: o RFC 1902 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 STD 15, RFC 1157 and RFC 1905 which define two versions of the protocol used for network access to managed objects.
The Framework permits new objects to be defined for the purpose of experimentation and evaluation. 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 assume an evolution or loosening of the model. This memo documents and standardizes that evolution of the model and fills in the gaps caused by that evolution. This memo also incorporates the interfaces group extensions documented in RFC 1229 [7]. 3.1. Clarifications/Revisions There are several areas for which experience has indicated that clarification, revision, or extension of the model would be helpful. The following sections discuss the changes in the interfaces group adopted by this memo in each of these areas.
In some sections, one or more paragraphs contain discussion of rejected alternatives to the model adopted in this memo. Readers not familiar with the MIB-II model and not interested in the rationale behind the new model may want to skip these paragraphs. 3.1.1. 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, such a model still lacks 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 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 MIB structure must allow these sorts of relationships to be described. Several solutions for representing multiple sub-layers were rejected. One was to retain the concept of one conceptual row for all the sub-layers of an interface and have each media-specific MIB module identify its "superior" and "subordinate" sub-layers through OBJECT IDENTIFIER "pointers". This scheme would have several drawbacks: the superior/subordinate pointers would be contained in the media-specific MIB modules; thus, a manager could not learn the structure of an interface without inspecting multiple pointers in different MIB modules; this would be overly
complex and only possible if the manager had knowledge of all the relevant media-specific MIB modules; MIB modules would all need to be retrofitted with these new "pointers"; this scheme would not adequately address the problem of upward and downward multiplexing; and finally, enumerated values of ifType would be needed for each combination of sub-layers. Another rejected solution also retained the concept of one conceptual row for all the sub-layers of an interface but had a new separate MIB table to identify the "superior" and "subordinate" sub-layers and to contain OBJECT IDENTIFIER "pointers" to the media-specific MIB module 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 still would not support downward multiplexing, such as PPP over MLP: observe that MLP makes two (or more) serial lines appear to the layers above as a single physical interface, and thus PPP over MLP should appear to the internetwork-layer as a single interface; in contrast, this scheme would result in two (or more) conceptual rows in the ifTable, both of which the internetwork-layer would run over. This scheme would also require enumerated values of ifType for each combination of sub-layers. The solution adopted by 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 6 below) 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, allows the IANAifType to Media-Specific MIB mapping to identify the media-specific MIB module for that sub-layer, such that the new table need only be referenced to obtain information about layering, and it only requires enumerated values of ifType for each sub-layer, not for combinations of them. However, it 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 previously), plus some (specifically, ifSpeed) which need to have appropriate values identified for use when a generalized definition does not apply to a 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.
(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.
3.1.2. Guidance on Defining Sub-layers
The designer of a media-specific MIB must decide whether to divide
the interface into sub-layers or not, 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).
Subject to the above, it is appropriate to assign an ifIndex value
to any interface that can occur in an interface stack (in the
ifStackTable) where the bottom of the stack is a physical
interface (ifConnectorPresent has the value 'true') and there is a
layer-3 or other application that "points down" to the top of this
stack. An example of an application that points down to the top
of the stack is the Character MIB [9].
Note that the sub-layers of an interface on one device will
sometimes be different from the sub-layers of the interconnected
interface of another device; for example, for a frame-relay DTE
interface connected a frameRelayService interface, the inter-
connected DTE and DCE interfaces have different ifType values and
media-specific MIBs.
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 doing so, 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.
3.1.3. 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. 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. 3.1.4. 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). 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. A rejected solution to this problem would be to split the ifTable into two (or more) new MIB tables, one of which 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 ifGeneralInformationGroup. 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.
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.
3.1.5. 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. Redefining ifNumber to be the largest value of ifIndex was rejected since it would not help. Such a re-definition would require ifNumber to be deprecated and the utility of the redefined object would be questionable. Alternatively, ifNumber could be deprecated and not replaced. 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 just 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. This is a minor change in the semantics of ifIndex; however, all existing agent implementations conform to this new definition, and in the interests of not requiring changes to existing agent implementations and to the many existing media-specific MIBs, this memo assumes that this change does not require ifIndex to be deprecated. Experience indicates that this assumption does "break" a few management applications, but this is considered preferable to breaking all agent implementations. 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 requirement for constancy (between re-initializations) of an interface's ifIndex value 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 assignment (in advance) of ifIndex values for all possible interfaces that might be added dynamically. The exact meaning of a "different" interface is hard to define, and there will be gray areas. Any firm definition in this document would likely to turn out to be inadequate. Instead, implementors must choose what it means in their particular situation, subject to the following rules:
(1) a previously-unused value of ifIndex must be assigned to a
dynamically added interface if an agent has no knowledge of
whether the interface is the "same" or "different" to a
previously incarnated interface.
(2) a management station, not noticing that an interface has gone
away and another has come into existence, must not be
confused when calculating the difference between the counter
values retrieved on successive polls for a particular ifIndex
value.
When the new interface is the same as an old interface, but a
discontinuity in the value of the interface's counters cannot be
avoided, the ifTable has (until now) required that a new ifIndex
value be assigned to the returning interface. That is, either all
counter values have had to be retained during the absence of an
interface in order to use the same ifIndex value on that
interface's return, or else a new ifIndex value has had to be
assigned to the returning interface. Both alternatives have
proved to be burdensome to some implementations:
(1) maintaining the counter values may not be possible (e.g., if
they are maintained on removable hardware),
(2) using a new ifIndex value presents extra work for management
applications. While the potential need for such extra work
is unavoidable on agent re-initializations, it is desirable
to avoid it between re-initializations.
To address this, a new object, ifCounterDiscontinuityTime, has
been defined to record the time of the last discontinuity in an
interface's counters. By monitoring the value of this new object,
a management application can now detect counter discontinuities
without the ifIndex value of the interface being changed. Thus,
an agent which implements this new object should, when a new
interface is the same as an old interface, retain that interface's
ifIndex value and update if necessary the interface's value of
ifCounterDiscontinuityTime. With this new object, a management
application must, when calculating differences between counter
values retrieved on successive polls, discard any calculated
difference for which the value of ifCounterDiscontinuityTime is
different for the two polls. (Note that this test must be
performed in addition to the normal checking of sysUpTime to
detect an agent re-initialization.) Since such discards are a
waste of network management processing and bandwidth, an agent
should not update the value of ifCounterDiscontinuityTime unless
absolutely necessary.
While defining this new object is a change in the semantics of the
ifTable counter objects, it is impractical to deprecate and
redefine all these counters because of their wide deployment and
importance. Also, a survey of implementations indicates that many
agents and management applications do not correctly implement this
aspect of the current semantics (because of the burdensome issues
mentioned above), such that the practical implications of such a
change is small. Thus, this breach of the SMI's rules is
considered to be acceptable.
Note, however, that the addition of ifCounterDiscontinuityTime
does not change the fact that:
It is necessary at certain times for the assignment of ifIndex
values to change on a reinitialization of the agent (such as a
reboot).
The possibility of ifIndex value re-assignment must be
accommodated by a management application whenever the value of
sysUpTime is reset to zero.
Note also that some agents support multiple "naming scopes", e.g.,
for an SNMPv1 agent, multiple values of the SNMPv1 community
string. For such an agent (e.g., a CNM agent which supports a
different subset of interfaces for different customers), there is
no required relationship between the ifIndex values which identify
interfaces in one naming scope and those which identify interfaces
in another naming scope. It is the agent's choice as to whether
the same or different ifIndex values identify the same or
different interfaces in different naming scopes.
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).
A new problem is introduced by representing each sub-layer as an ifTable entry. 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 having an ifTable entry for each interface sub-layer, 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 local name (e.g., the name used at the device's local console) for the interface of which the relevant entry in the ifTable is a component. For example, consider a router having an interface composed of PPP running over an RS-232 port. If the router uses the name "wan1" for the (combined) interface, then the ifName objects for the corresponding PPP and RS-232 entries in the ifTable would both have the value "wan1". On the other hand, if the router uses the name "wan1.1" for the PPP interface and "wan1.2" for the RS-232 port, then the ifName objects for the corresponding PPP and RS-232 entries in the ifTable would have the values "wan1.1" and "wan1.2", respectively. As an another example, consider an agent which responds to SNMP queries concerning an interface on some other (proxied) device: if such a proxied device associates a particular identifier with an interface, then it is appropriate to use this identifier as the value of the interface's ifName, since the local console in this case is that of the proxied device. In contrast, the existing ifDescr object is intended to contain a description of an interface, whereas another new object, ifAlias, provides a location in which a network management application can store a non-volatile interface-naming value of its own choice. The ifAlias object allows a network manager to give one or more interfaces their own unique names, irrespective of any interface- stack relationship. Further, the ifAlias name is non-volatile, and thus an interface must retain its assigned ifAlias value across reboots, even if an agent chooses a new ifIndex value for the interface. 3.1.6. Counter Size As the speed of network media increase, the minimum time in which a 32 bit counter will wrap decreases. For example, a 10Mbs stream of back-to-back, full-size packets causes ifInOctets to wrap in just over 57 minutes; at 100Mbs, the minimum wrap time is 5.7 minutes, and at 1Gbs, the minimum is 34 seconds. Requiring that interfaces be polled frequently enough not to miss a counter wrap is increasingly problematic.
A rejected solution to this problem was to scale the counters; for
example, ifInOctets could be changed to count received octets in,
say, 1024 byte blocks. While it would provide acceptable
functionality at high rates of the counted-events, at low rates it
suffers. If there is little traffic on an interface, there might
be a significant interval before enough of the counted-events
occur to cause the scaled counter to be incremented. Traffic
would then appear to be very bursty, leading to incorrect
conclusions of the network's performance.
Instead, this memo adopts 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, when 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, 64-bit packet
counters AND 64-bit octet counters MUST be used.
These speed thresholds 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 10Mbs Ethernet) should
not require them.
(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):
- 10Mbs Ethernet: 57 minutes,
- 16Mbs Token Ring: 36 minutes,
- a US T3 line (45 megabits): 12 minutes,
- FDDI: 5.7 minutes
(4) The 32-bit packet counters wrap 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/second (1,000 Gbs) 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
(2) the ifVHCPacketGroup for packet-based interfaces; this group
includes 64 bit counters for octets and packets.
3.1.7. 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 2.4Gbs.
Thus, ifSpeed is insufficient for the future, and this memo defines
an additional object: ifHighSpeed.
The ifHighSpeed 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 (but rejected) 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.
3.1.8. Multicast/Broadcast Counters
In MIB-II, the ifTable counters for multicast and broadcast packets
are combined as counters of non-unicast packets. In contrast, the
ifExtensions MIB [7] defined one set of counters for multicast, and a
separate set for broadcast packets. With the separate counters, the
original combined counters become redundant. To avoid this
redundancy, the non-unicast counters are deprecated.
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. Thus,
this memo defines new objects with better aligned definitions.
Counters with 64 bits of range are also needed, as explained above.
3.1.9. 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 state changes 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. 3.1.10. Addition of New ifType values Over time, there is the need to add new ifType enumerated values for new interface types. If the syntax of ifType were defined in the MIB in section 6, then a new version of this MIB would have to be re- issued in order to define new values. In the past, re- issuing of a MIB has occurred only after several years. Therefore, 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, defined in a different document. This allows additional values to be documented without having to re-issue 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. 3.1.11. 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 of ifIndex.
3.1.12. New states for IfOperStatus Three new states have been added to ifOperStatus: 'dormant', 'notPresent', and 'lowerLayerDown'. The dormant state indicates that the relevant interface is not actually in a condition to pass packets (i.e., it is not "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 notPresent state is a refinement on the down state which indicates that the relevant interface is down specifically because some component (typically, a hardware component) is not present in the managed system. Examples of use of the notPresent state are: (1) to allow an interface's conceptual row including its counter values to be retained across a "hot swap" of a card/module, and/or (2) to allow an interface's conceptual row to be created, and thereby enable interfaces to be pre-configured prior to installation of the hardware needed to make the interface operational. Agents are not required to support interfaces in the notPresent state. However, from a conceptual viewpoint, when a row in the ifTable is created, it first enters the notPresent state and then subsequently transitions into the down state; similarly, when a row in the ifTable is deleted, it first enters the notPresent state and then subsequently the object instances are deleted. For an agent with no support for notPresent, both of these transitions (from the notPresent state to the down state, and from the notPresent state to the instances being removed) are immediate, i.e., the transition does not last long enough to be recorded by ifOperStatus. Even for those agents which do support interfaces in the notPresent state, the length of time and conditions under which an interface stays in the notPresent state is implementation-specific.
The lowerLayerDown state is also a refinement on the down state. This new state indicates that this interface runs "on top of" one or more other interfaces (see ifStackTable) and that this interface is down specifically because one or more of these lower-layer interfaces are down. 3.1.13. IfAdminStatus and IfOperStatus The down state of ifOperStatus 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 (or notPresent) 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 be down or notPresent. 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: (1) Change to the up state if and only if the interface is able to send and receive packets. (2) Change to the lowerLayerDown state if and only if the interface is prevented from entering the up state because of the state of one or more of the interfaces beneath it in the interface stack.
(3) Change to the dormant state if and only if the interface is
found to be operable, but the interface is waiting for other,
external, events to occur before it can transmit or receive
packets. Presumably when the expected events occur, the
interface will then change to the up state.
(4) Remain in the down state if an error or other fault condition
is detected on the interface.
(5) Change to the unknown state if, for some reason, the state of
the interface can not be ascertained.
(6) Change to the testing state if some test(s) must be performed
on the interface. Presumably after completion of the test, the
interface's state will change to up, dormant, or down, as
appropriate.
(7) Remain in the notPresent state if interface components are
missing.
3.1.14. IfOperStatus in an Interface Stack
When an interface is a part of an interface-stack, but is not the
lowest interface in the stack, then:
(1) ifOperStatus has the value 'up' if it is able to pass packets
due to one or more interfaces below it in the stack being 'up',
irrespective of whether other interfaces below it are 'down',
'dormant', 'notPresent', 'lowerLayerDown', 'unknown' or
'testing'.
(2) ifOperStatus may have the value 'up' or 'dormant' if one or
more interfaces below it in the stack are 'dormant', and all
others below it are either 'down', 'dormant', 'notPresent',
'lowerLayerDown', 'unknown' or 'testing'.
(3) ifOperStatus has the value 'lowerLayerDown' while all
interfaces below it in the stack are either 'down',
'notPresent', 'lowerLayerDown', or 'testing'.
3.1.15. Traps
The exact definition of when linkUp and linkDown traps are generated
has been changed to reflect the changes to ifAdminStatus and
ifOperStatus.
Operational experience indicates that management stations are most
concerned with an interface being in the down state and the fact that
this state may indicate a failure. Thus, it is most useful to
instrument transitions into/out of either the up state or the down
state.
Instrumenting transitions into or out of the up state was rejected
since it would have the drawback that a demand interface might have
many transitions between up and dormant, leading to many linkUp traps
and no linkDown traps. Furthermore, if a node's only interface is
the demand interface, then a transition to dormant would entail
generation of a linkDown trap, necessitating bringing the link to the
up state (and a linkUp trap)!!
On the other hand, instrumenting transitions into or out of the down
state (to/from all other states except notPresent) has the
advantages:
(1) A transition into the down state (from a state other than
notPresent) will occur when an error is detected on an
interface. Error conditions are presumably of great interest to
network managers.
(2) Departing the down state (to a state other than the
notPresent state) generally indicates that the interface is
going to either up or dormant, both of which are considered
"healthy" states.
Furthermore, it is believed that generating traps on transitions into
or out of the down state (except to/from the notPresent state) is
generally consistent with current usage and interpretation of these
traps by manager stations.
Transitions to/from the notPresent state are concerned with the
insertion and removal of hardware, and are outside the scope of these
traps.
Therefore, this memo defines that LinkUp and linkDown traps are
generated on just after ifOperStatus leaves, or just before it
enters, the down state, respectively; except that LinkUp and linkDown
traps never generated on transitions to/from the notPresent state.
Note that this definition allows a node with only one interface to
transmit a linkDown trap before that interface goes down. (Of
course, when the interface is going down because of a failure
condition, the linkDown trap probably cannot be successfully
transmitted anyway.)
Some interfaces perform a link "training" function when trying to bring the interface up. In the event that such an interface were defective, then the training function would fail and the interface would remain down, and the training function might be repeated at appropriate intervals. If the interface, while performing this training function, were considered to the in the testing state, then linkUp and linkDown traps would be generated for each start and end of the training function. This is not the intent of the linkUp and linkDown traps, and therefore, while performing such a training function, the interface's state should be represented as down. An exception to the above generation of linkUp/linkDown traps on changes in ifOperStatus, occurs when an interface is "flapping", i.e., when it is rapidly oscillating between the up and down states. If traps were generated for each such oscillation, the network and the network management system would be flooded with unnecessary traps. In such a situation, the agent should rate- limit its generation of traps. 3.1.16. ifSpecific The original definition of the OBJECT IDENTIFIER value of ifSpecific was not sufficiently clear. As a result, different implementors used it differently, and confusion resulted. Some implementations set the value of ifSpecific to the OBJECT IDENTIFIER that defines the media- specific MIB, i.e., the "foo" of: foo OBJECT IDENTIFIER ::= { transmission xxx } while others set it to be OBJECT IDENTIFIER of the specific table or entry in the appropriate media-specific MIB (i.e., fooTable or fooEntry), while still others set it be the OBJECT IDENTIFIER of the index object of the table's row, including instance identifier, (i.e., 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) indexing. The only definition that can both be made explicit and can cover all the useful situations is to have ifSpecific be the most general value for the media-specific MIB module (the first example given above). This effectively makes it redundant because it contains no more information than is provided by ifType. Thus, ifSpecific has been deprecated.
3.1.17. Creation/Deletion of Interfaces While some interfaces, for example, most physical interfaces, cannot be created via network management, other interfaces such as logical interfaces sometimes can be. The ifTable contains only generic information about an interface. Almost all 'create-able' interfaces have other, media-specific, information through which configuration parameters may be supplied prior to creating such an interface. Thus, the ifTable does not itself support the creation or deletion of an interface (specifically, it has no RowStatus [2] column). Rather, if a particular interface type supports the dynamic creation and/or deletion of an interface of that type, then that media-specific MIB should include an appropriate RowStatus object (see the ATM LAN- Emulation Client MIB [8] for an example of a MIB which does this). Typically, when such a RowStatus object is created/deleted, then the conceptual row in the ifTable appears/disappears as a by-product, and an ifIndex value (chosen by the agent) is stored in an appropriate object in the media-specific MIB. 3.1.18. All Values Must be Known There are a number of situations where an agent does not know the value of one or more objects for a particular interface. In all such circumstances, an agent MUST NOT instantiate an object with an incorrect value; rather, it MUST respond with the appropriate error/exception condition (e.g., noSuchInstance for SNMPv2). One example is where an agent is unable to count the occurrences defined by one (or more) of the ifTable counters. In this circumstance, the agent MUST NOT instantiate the particular counter with a value of, say, zero. To do so would be to provide mis- information to a network management application reading the zero value, and thereby assuming that there have been no occurrences of the event (e.g., no input errors because ifInErrors is always zero). Sometimes the lack of knowledge of an object's value is temporary. For example, when the MTU of an interface is a configured value and a device dynamically learns the configured value through (after) exchanging messages over the interface (e.g., ATM LAN- Emulation [8]). In such a case, the value is not known until after the ifTable entry has already been created. In such a case, the ifTable entry should be created without an instance of the object whose value is unknown; later, when the value becomes known, the missing object can then be instantiated (e.g., the instance of ifMtu is only instantiated once the interface's MTU becomes known).
As a result of this "known values" rule, management applications MUST be able to cope with the responses to retrieving the object instances within a conceptual row of the ifTable revealing that some of the row's columnar objects are missing/not available. 4. Media-Specific MIB Applicability The exact use and semantics of many objects in this MIB are open to some interpretation. This is a result of the generic nature of this MIB. It is not always possible to come up with specific, unambiguous, text that covers all cases and yet preserves the generic nature of the MIB. Therefore, it is incumbent upon a media-specific MIB designer to, wherever necessary, clarify the use of the objects in this MIB with respect to the media-specific MIB. Specific areas of clarification include Layering Model The media-specific MIB designer MUST completely and unambiguously specify the layering model used. Each individual sub-layer must be identified, as must the ifStackTable's portrayal of the relationship(s) between the sub-layers. Virtual Circuits The media-specific MIB designer MUST specify whether virtual circuits are assigned entries in the ifTable or not. If they are, compelling rationale must be presented. ifRcvAddressTable The media-specific MIB designer MUST specify the applicability of the ifRcvAddressTable. ifType For each of the ifType values to which the media-specific MIB applies, it must specify the mapping of ifType values to media- specific MIB module(s) and instances of MIB objects within those modules. However, wherever this interface MIB is specific in the semantics, DESCRIPTION, or applicability of objects, the media-specific MIB designer MUST NOT change said semantics, DESCRIPTION, or applicability.
5. Overview This MIB consists of 4 tables: ifTable This table is the ifTable from MIB-II. ifXTable This table contains objects that have been added to the Interface MIB as a result of the Interface Evolution effort, or replacements for objects of the original (MIB-II) ifTable that were deprecated because the semantics of said objects have significantly changed. This table also contains objects that were previously in the ifExtnsTable. ifStackTable This table contains objects that define the relationships among the sub-layers of an interface. ifRcvAddressTable This table contains objects that are used to define the media- level addresses which this interface will receive. This table is a generic table. The designers of media- specific MIBs must define exactly how this table applies to their specific MIB.
(page 26 continued on part 2)
