2.4.2.a.  Expedited Forwarding PHB

The EF PHB can be used for low delay and delay variation and assured bandwidth. The DS domains experience an end-to-end service while using EF. The delay and delay variation occur due to the congestion or basically traffic queues in the network. This increase in the traffic queue occurs when the departure rate is slower than the arrival rate in the same node. Hence a type of service that ensures no queues for some aggregate is equivalent to bounding rates such that, at every transit node, the aggregate's maximum arrival rate is less than it's minimum departure rate.

Now EF sets up the nodes in such a way that the aggregate has a minimum departure rate which is independent of the intensity of the other traffic at the node.Also the traffic conditioners at the network boundary execute the policing and shaping so that the arrival rate at any node is always less than the configured minimum departure rate.

EF PHB can be implemented in several ways. A simple priority queue will give the desired result, provided that there is no higher priority queue  that would make the packets be preempted for more than  a packet time at the configured rate. If a simple queue is maintained and is serviced by Weighted Round Robin scheduler in which  the bandwidth share assigned for the packets with EF is equal to the configured rate.

An important thing for the EF PHB  is the DSCP  for this behavior. The DSCP is 101100 which is recommended for EF PHB. If packets are marked for EF PHB then their DSCP can be marked again at a DS domain boundary only if the new DSCP satisfies the EF PHB. Also the packets that are marked with for EF PHBs should not be changed to any other type of PHB by a DS domain.
 
 
EF DSCP Value as standardized by the IETF 101100 ... which is shifted by two bits to read: 0x2e
In order to protect itself against denial of service attacks, the edge of a DS domain must police all EF marked packets to the negotiated rate with the upstream domain. This value should be less than the configured rate. Packets in excess of the negotiated value should be discarded.