3Com 7600 Traffic, Management Concepts, This congestion information, Generate backward RM-cells

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.
Traffic	This section contains some of the TM concepts in greater detail.
Management
Concepts

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,