Explicit Congestion Notification

Explicit Congestion Notification ( ECN ) is an extension of the network protocol TCP / IP for congestion control and is described in RFC 3168 (2001). Using ECN, a router can indicate an impending overload by simply marking a bit in the IP header . ECN is defined on the IP level as well as on the TCP level of the network layer model and requires two bits each from the former TOS byte in the IP and TCP header. Two bits from a 6-bit wide block are used in the TCP header and are reserved for future use.

If all routers on the way between the source and the sink are ECN-capable, this procedure can be used to reduce the number of packets discarded due to overload, in that the sink reports back to the source that the router indicates a risk of overload, which then reduces the data rate.

Those who use ECN should be aware that some administrators have not yet implemented the changed semantics of the TOS byte through RFC 3168 in September 2001. Routers and firewalls even from well-known companies sometimes handle the ECN bits unpredictably. There is therefore a risk that a connection with activated ECN will not be established.

However, ECN can only be used if both hosts declare their willingness to do so. The method uses an ECN bit to (explicitly) indicate the traffic jam, instead of just discarding the packets from the network scheduler with the RED algorithm (implicit notification choke packet ). The sending station can then deliberately throttle its data flow. The dedicated information (ECN bit) does not artificially deteriorate the line quality, as is the case with RED / WRED, but not all network participants can use the ECN process and therefore discard packets that have set the ECN bit. Since the correct behavior in the case of non-ECN-capable components would be to ignore the ECN bit, these devices are to be regarded as faulty.

A similar method is the DEC bit .