130	CLASS OF SERVICE AND QUALITY OF SERVICE

Diffserv provides a schema for signalling and enforcing QoS from end to end. Diffserv introduces two important concepts. With IP forwarding, the DSCP is evaluated at each hop along the path. At each hop, each class of service is known as a per-hop behaviour (PHB). A behaviour aggregate (BA) represents the sum of all traffic traversing an interface to which the same PHB applies.

9.3.3 MPLS EXP

As with the various devices at Layer 2 and the ToS byte at Layer 3, the EXP bits in the MPLS header are used to identify the queue into which MPLS packets are placed on egress. Also, in common with other marking mechanisms, they provide a persistent mechanism of marking each packet with an identifier that can be used to apply a consistent behaviour from the ingress of the network to the egress of the network.

9.4 INTEGRATING QoS AT LAYER 2, IN IP AND IN MPLS

It is somewhat convenient that there are three bits assigned for QoS in Ethernet frames, IP and MPLS because this makes it possible to create an integrated policy that maps all three QoS mechanisms to each other. As packets pass from one format to another, a consistent behaviour can be achieved.

This can be achieved by the creation of rewrite policies that write the bits in one of the headers based upon the queue into which the packets have been placed. This may not provide a perfect mapping between one set of headers and another. For example, if a device supports four queues, then it might be necessary to place packets marked with two different classes into a single queue, for example, see Table 9.1.

In this case, a rewrite policy cannot differentiate between packets marked 000 and packets marked 001. Since all such marked packets are put into queue 0, they can only

Table 9.1 Mappings from IP ToS bits settings to output queuers using four output queues

Class of Service	QoS marking	Queue
	(assumes 3 bits)
Best Effort	000	0
Best Effort—High Priority	001	0
Assured Delivery	010	1
Assured Delivery—High Priority	011	1
Low Latency	100	2
Low Latency—Low Jitter	101	2
Network Management	110	3
Network Control	111	3

9.4 INTEGRATING QoS AT LAYER 2, IN IP AND IN MPLS

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be rewritten with one value. So, if a rewrite of the 802.1p bits is required, it would be necessary to select either 000 or 001. While the 802.1p bits can only be one of four values, it is not necessary to rewrite the bits in the higher layer protocols that will be maintained across the network.

9.4.1DIFFSERV INTEGRATION WITH MPLS

In order to integrate DiffServ with MPLS, two types of traffic engineered LSPs have been developed. The first type uses the EXP bits to carry the class of service information and is known as an E-LSP. Since DiffServ uses 6 bits and there are only 3 EXP bits, it is clear that each of the EXP bits has to represent an average of 8 DSCPs. It is only necessary to create a single LSP between the ingress and egress LSRs and simply identify the class of service by the EXP bits. Since it is possible to simply configure a mapping from EXP to Diffserv and back again on every LSR, there is no signalling required at the time the LSP is established. This means that the amount of MPLS state required is kept to a minimum. However, this reduction in state is at the cost of a reduction in granularity.

The second type uses a different label to represent each of the different classes of service. They are known as L-LSPs. One LSP is created between the ingress and egress LSRs for each class of service and traffic for each class is associated with the relevant FEC. In order to achieve this, it is necessary to signal the class of service at the time the LSP is established. This requires an additional RSVP object (DIFFSERV) or an additional LDP TLV (DiffServ). This makes it possible to exactly map each DSCP to a unique LSP. However, this increased granularity comes at the cost of significantly increased MPLS state.