RFC 2863 The Interfaces Group MIB June 2000
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
McCloghrie & Kastenholz Standards Track [Page 5]