rfc2863.txt

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         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."



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   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 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 turn out to
   be inadequate.  Instead, implementors must choose what it means in
   their particular situation, subject to the following rules:



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   (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



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   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 re-initialization 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.



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   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.



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   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 supported.
   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
   supported and 64-bit octet counters MUST be supported.  For