Traffic Management Concepts
13-3
Q
Directly insert feedback control information into RM-cells when they
pass in the forward or backward direction
Q
Indirectly inform the source about congestion by setting the EFCI bit in
the data cell header of the cells of the forward information flow. In
this case, the destination will update the backward RM-cells based on
this congestion information.
Q
Generate backward RM-cells.
Traff ic
Management
Concepts
This section contains some of the TM concepts in greater detail.
Explicit Forward
Congestion Indication
The
Explicit Forward Congestion Indication
(EFCI) is a congestion
notification mechanism that is set in the cell header by a network
element in an pre-congested or congested state. A network element in an
impending congested state or a congested state may set an EFCI in the
cell header so that this indication may be examined by the destination
end-system. For example, the end-system may use this indication to
implement a protocol that adaptively lowers the cell rate of the
connection during congestion or impending congestion. A network
element that is not in a congested state or an impending congested state
will not modify the value of this indication. An impending congested
state is the state when a network element is operating around its
engineered capacity level.
Cell Loss Priority
For some service categories the end system may generate traffic flows of
cells with
Cell Loss Priority
(CLP) marking. A congested network element
may selectively discard cells explicitly identified as belonging to a
non-compliant ATM connection and/or those cells with their CLP bit set.
This is to protect cells without their CLP bit set from being discarded for
as long as possible.
Implicit Rate Control
In implicit rate control (binary mode) the EFCI bit in the cell header is
checked. If there has been congestion on the forward path (recognized at
the destination end station by the EFCI bits of the incoming data cells),
the
Congestion Indication
(CI) field within the backward RM cell is
marked (set to one). The source end station receives the backward RM
cell and acts upon it. If the CI field is set or if RM cells are not returned,
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