The Label Request Message
164 LABEL DISTRIBUTION PROTOCOLS
Committed data rate CDR, committed burst size CBS, and excess burst size The traffic that is sent to the network, which is the output of the token bucket P, is policed
using the token bucket C, referred to as the committed token bucket. The maximum size of token bucket C is set equal to the committed burst size CBS, expressed in bytes, and
the token bucket is replenished at the committed data rate CDR, expressed in bytessec. The output of this token bucket is referred to as the committed rate which is the amount
of bandwidth the network should allocate for the CR-LSP.
In addition to C, it is possible to use a second policing token bucket E, referred to as the excess token bucket. The maximum size of this token bucket is equal to the excess
burst size EBS , expressed in bytes, and the token bucket is replenished at the committed
data rate CDR , expressed in bytessec. As will be seen below, this token bucket can be
used to decide whether a violating packet should be marked and let into the network, or it should be dropped.
The operation of the committed and excess token buckets is as follows: •
Initially, the token count in the committed token bucket T
C
= CBS
, and the token count in the excess token bucket T
E
= EBS
. •
Thereafter, T
C
and T
E
are updated every second as follows: ◦ If T
C
CBS , then T
C
is incremented by M bytes it should not exceed CBS . ◦ If T
E
EBS , then T
E
is incremented by M bytes it should not exceed EBS where CDR = M bytessec.
• The following action is taken when a packet of size B arrives:
◦ If T
C
− B ≥
0, then there are enough tokens in the committed token bucket for the packet, and T
C
= T
C
− B
. ◦ If T
C
− B
0 and T
E
− B ≥
0, then there not enough tokens in the committed token bucket, but there are enough tokens in the excess token bucket, and T
E
= T
E
− B
. ◦ If T
C
− B
0 and T
E
− B
0, then there not enough tokens in the committed token bucket or in the excess token bucket, and T
C
or T
E
are not decremented. Note that if CDR is positive infinity, then an arriving packet will never be in excess of
either token bucket counts. The action taken when the size of a packet exceeds the token count in the token bucket
either committed or excess is implementation dependent. For instance, if the packet size is larger than the token count in the committed token bucket, but less than the token count
in the excess token bucket, then we might chose to mark it and let into the network. If the packet size is larger than both of the token counts, then we might choose to drop it.
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 T
C
is left unchanged, the token count T
E
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 T
C
and T
E
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 165
Packet 2 marked
T
C
CBS
Packet 1 Packet
3 Time
T
E
EBS
Packet 4 dropped
Packet 5 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.
166 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-LSP’s relative share of the excess bandwidth. Weight values range from 1 to 255. The value 0 means that the weight
is not applicable.