Explicit congestion notification

From Hill2dot0

In frame relay, the term explicit congestion notification (ECN) refers to the passing of explicit information from the network to the FRAD to alert the CPE of a congestion event related to a particular virtual circuit (VC). There are two bit defined in the frame relay header that provide this function. They are known as the Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification (BECN) bits. These bits are always transmitted as 0 by the FRAD. They can be found in immediately following the DLCI in the second octet of the address field in the frame relay header. Specifically, they are the 5th and 6th bits in the second octet, and they are followed by the discard eligibility (DE) bit.

FECN & BECN Operation

If a frame passes through a point of congestion on its way from one user-to-network interface (UNI) to another, the network (typically at the ingress or egress to the network) sets (changes to 1) the FECN bit in the frame. Upon receiving a frame with the FECN bit set, the receiving FRAD knows there is inbound congestion on that particular VC.

The network, however, does not simply set the bit in the frame that is passing through the point of congestion. It also sets the BECN bit in any frame traveling in the reverse direction on the VC. When a FRAD receives a frame on a particular VC with a BECN bit set, it knows that the frames it is sending are experiencing congestion on their way through the network.

CPE Response to BECN & FECN

Note that there is little that can be done about the receipt of a frame with the FECN bit set, since the FRAD is on the receiving end of the transmission and the frame has already passed through the point of congestion. Most devices simply note it as an "interesting event" and can report information such as the frequency with which that event occurs per VC.

There is something that can be done about receipt of a frame with the BECN bit set. This circuit is experiencing congestion in the forward direction and the CPE could respond by reducing its transmission rate, perhaps to its negotiated CIR. Is this a wise course of action? If the congestion is occurring inside the network, this is not a wise course of action because there is no assurance that the other customers of the network are being equally well behaved. If, however, congestion typically occurs at the edge of the network (i.e., typically at the egress switch), then the customer is actually competing with themselves and they should adjust traffic rates to make maximal use of their network resources.

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| FECN & BECN