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An example of how these two policing schemes are used is shown in Figure 7.18. The top diagram refers to the excess token bucket, and the bottom one to the committed token bucket. The rules described in the above paragraph for marking and dropping apply. All four packets arrive at rates higher than CDR. As we can see, packets 1 and 3 go through. Packet 2 arrives at a time when the committed token bucket does not have enough tokens, but there are enough tokens in the excess token bucket. As a result, the token count TC is left unchanged, the token count TE is reduced by the size of packet 2, and packet 2 is marked and let into the network. Packets 4 and 5 are dropped, since they arrive at a time when neither token buckets have enough tokens. In both cases the token counts TC and TE are unchanged.
The five traffic parameters - PDR, PBS, CDR, CBS, and EBS - can be set to different values so as to create different classes of service, such as a delay sensitive service and a
THE CONSTRAINED-BASED ROUTING LABEL DISTRIBUTION PROTOCOL
(marked) 3 (dropped) (dropped)
Figure 7.18 An example of the two policing schemes.
best effort service. They can also be set up to provide different ATM service categories. Examples of how these five parameters can be set so that to provide different classes of services are given below in Section 7.2.5.
As mentioned above, the output of the committed token bucket is the traffic that will enter the network. A bandwidth allocation algorithm can be used at each LSR to decide whether the new CR-LSP will be accepted or not. As in ATM, different schemes can be used to calculate how much bandwidth should be allocated to the CR-LSP. The simplest scheme is to allocate a bandwidth equal to CDR. This is equivalent to the peak rate allocation scheme in ATM.
The flags, frequency, and weight fields
The traffic parameters TLV can be included in the label mapping message. This permits an LSR to replace the proposed value for one or more traffic parameter by a lower value. The flags field defines which of the traffic parameters are negotiable; that is, they can be replaced by an LSR with a lower value. It consists of one 2-bit reserved subfield and six 1-bit flags. Five of these flags are associated with the five traffic parameters, and the sixth flag is associated with the weight field. Specifically, flag F1 corresponds to PDR, flag F2 corresponds to PBS, flag F3 corresponds to CDR, flag F4 corresponds to CBS, flag F5 corresponds to EBS, and flag F6 corresponds to the weight field. Each flag indicates whether its associated traffic parameter is negotiable or not. Flag F6 indicates whether the weight is negotiable or not. If a flag is set to 0, then the associated traffic parameter is not negotiable. Otherwise, it is negotiable.
As mentioned above, the CDR can be used to allocate bandwidth to a CR-LSP. The exact allocated bandwidth can vary over time, but the average bandwidth calculated during this time should be at least equal to CDR. The 8-bit frequency field is used to specify this period. The following frequency codes have been defined:
Unspecified (value 0).
Frequent (value 1): That is, the available rate should average at least the CDR when measured over any time interval equal to or longer than a small number of shortest packet times transmitted at the CDR.
LABEL DISTRIBUTION PROTOCOLS
VeryFrequent (value 2): That is, the available rate should average at least the CDR when measured over any time interval equal to or longer than the shortest packet time transmitted at the CDR.
Reserved (values 3 to 255).
Finally, the 8-bit weight field is used to indicate the CR-LSPs relative share of the excess bandwidth. Weight values range from 1 to 255. The value 0 means that the weight is not applicable.
7.2.5 Classes of Service
Class services can be constructed by appropriately manipulating the traffic parameters, and the rules regarding passing, marking, and dropping a packet. In Table 7.1, we give the traffic parameters and rules for marking and dropping packets for three classes of service: delay sensitive (DS) service, throughput sensitive (TS) service, and best effort (BE) service. In the delay sensitive service, the network commits with high probability to deliver packets at a rate of PDR with minimum delay. Packets in excess of PDR will be discarded. In the throughput sensitive service, the network commits to deliver with high
Table 7.1 Traffic parameters - DS, TS, and BE service classes.
Traffic Delay Throughput Best effort
Parameters sensitive sensitive
PDR User-specific User-specific Infinite
PBS User-specific User-specific Infinite
CDR PDR User-specific Infinite
CBS PBS User-specific Infinite
EBS 0 0 0