The Expedited Forwarding PHB and DSCP Values
RFC 2598 defines the expedited forwarding per-hop behaviors. This RFC defines a very simple PHB (low latency, with a cap on bandwidth), and a single DSCP (EF) to represent it. Expedited forwarding simply states that a packet with the EF DSCP should minimize delay, jitter, and loss, up to a guaranteed bandwidth level for the class.
Like AF, the EF RFC suggests two QoS actions be performed to achieve the PHB. First, queuing must be used to minimize the time that EF packets spend in a queue. A queuing scheme that sends packets from this queue whenever a packet is waiting reduces delay. Anything that reduces delay reduces jitter. Always servicing the EF queue first greatly reduces the queue length, which in turn greatly reduces the chance of tail drop due to the queue being full. Therefore, EF's goal of reducing delay, jitter, and loss can be achieved with a queuing method such as Priority Queuing, with EF traffic in the most important queue.
The second component of the EF PHB is policing. If the input load of EF packets exceeds a configured rate, the excess packets are discarded. If 100 kbps is reserved for the EF BA, and 200 kbps enters the network, for example, supporting the extra traffic may be unreasonable. Why not just accept the extra traffic? The queuing method used to achieve low delay, low jitter, and low loss, would prevent other types of traffic from getting any bandwidth, because the queuing method always gives preference to EF traffic. Thus, EF protects the other traffic by capping the amount of bandwidth for the class. In other words, EF suggests a great level of service, but just up to the contracted amount of bandwidth.
The expedited forwarding PHB uses a DSCP name of EF, whose binary value is 101110, with a decimal value of 46.
Table 2-19 summarizes many of the key points about the various DiffServ PHBs.
|
PHB |
Key Components |
Names of DSCPs |
|
Best effort (BE) |
PHB for getting no specific QoS treatment |
DSCP BE (default) |
|
Class selector (CS) |
Uses 8 DSCPs, all with binary 0s for the last 3 bits. Used for backward compatibility with IP precedence. Uses "bigger-is-better" logic—the bigger the DSCP, the better the QoS treatment. |
CS1, CS2, CS3, CS4, CS5, CS6, CS7 |
|
Assured forwarding (AF) |
PHB consists of 2 components: queuing to provide a minimum bandwidth to each for 4 different queues, and 3 drop thresholds inside each queue. DSCPs do not always follow the "bigger-is-better" logic. |
AF11, AF12, AF13, AF21, AF22, AF23, AF31, AF32, AF33, AF41, AF42, AF43 |
|
Expedited forwarding (EF) |
PHB also has 2 components: queuing to provide low delay/jitter/loss plus a guaranteed amount of bandwidth, and policing to prevent EF from preventing other types of traffic from getting enough bandwidth. |
EF |
Continue reading here: The Integrated Services QoS Model
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