tion (ECN) to IP' RFC 3168 [23]. These documents define that the six leftmost bits of the Traffic Class field indi- cate Differentiated Services Code Point (DSCP) and the two rightmost bits are for ECN. The DSCP bits of the Traffic Class field define Per-Hop Behavior (PHB) that a packet should receive at each node in its path through the Internet. The 6-bit pattern of 000000' is reserved for signaling default PHB. The IP packet forwarders aware of dif- ferentiated services, look at the 6-bit value and based on that can apply different treatment and different PHB for each packet. Hosts may be configured to mark selected packets they create with non-default DSCP values. However, the DSCP field values may be rewritten by forwarders in the Internet, as packets pass through different network boundaries where differentiated services are utilized. Due to intermediate nodes possibly modifying the DSCP field value, the field is not usable for end-to-end communications. A number of RFCs exist that define meanings for the DSCP bits. These include RFC 2474 that defines Class Selector Codepoints [22], RFC 2597 that defines DSCPs for assured forwarding [24] and expedited forwarding [25], and RFC 5865 for capacity-admitted traffic [26]. The two ECN bits of the Traffic Class field have been defined for four different meanings: '00' to indicate that ECN is not used, '10' and '01' indicate that endpoints are capable of understanding the ECN, and '11' to indicate that congestion is experienced in the network. The end points of a transport protocol that is capable of using ECN will set the ECN bits to either '10' or '01'. When congestion is experienced by a network node, but packets are not (yet) dropped, the node changes the ECN field bits to '11' in order to indicate the packet's receiver about network congestion taking place. The RFC 3168 does not take a stand on when nodes should set bits to '10' or '01'. When a node that learns about network congestion, the receiver of a packet that has ECN bits '11', has to take action at the 