Token Bucket Mechanism 41
The show queueing output for af21, af31, and af41 indicates that the minimum threshold
for af11 would have been 33 by default. As shown, the minimum threshold is 20, based on
commands entered.
NOTE The afxx values shown in the show queueing output are related to the assured forwarding
per-hop behavior (AF PHB) discussed in Chapter 1. From the show queueing output, you
can see that afx3 packets will be dropped before afx2 packets, and afx2 packets will be
dropped before afx1 packets. This behavior was explained in detail in Chapter 1.
Incidentally, the term average queue depth has been used several times during this
discussion of WRED and, although a detailed explanation of this formula is beyond the
scope of this chapter, the formula used to calculate average queue depth is as follows:
Average = (old_average * (1 – 2
-n
)) + (current_queue_size * 2
-n
), where n is the
exponential weight factor, which is user configurable.
CAUTION If the value for n is set too high, WRED will not work properly and the net impact will be
roughly the same as if WRED were not in use at all.
At time of publication, the Catalyst 3550 Family and Catalyst 6500 Family of switches
support the WRED congestion management feature. Refer to the congestion management
section in each product chapter for the Catalyst 3550 Family and Catalyst 6500 Family of
switches for more details.
Token Bucket Mechanism
A token bucket is a formal definition of a rate of transfer. For this discussion, assume the
token bucket starts full. This implies the maximum amount of tokens is available to sustain
incoming traffic. Assume a bucket, which is being filled with tokens at a rate of x tokens per
refresh interval. Each token represents 1 bit of data. To successfully transmit a packet, there
must be a one-to-one match between bits and tokens. As a result, when a packet or frame
arrives at the port or interface, and enough tokens exist in the bucket to accommodate the
entire unit of data, the packet conforms to the contract, and therefore is forwarded. When
the packet is successfully transmitted, the number of tokens equal to the size of the trans-
mitted packet is removed from the bucket. Figure 2-1 illustrates the token bucket
mechanism.
42 Chapter 2: End-to-End QoS: Quality of Service at Layer 3 and Layer 2
Figure 2-1 Token Bucket Mechanism
If the actual ingress traffic rate exceeds the configured rate, and there are insufficient tokens
in the token bucket to accommodate the arriving traffic, the excess data is considered out-
of-profile and can be dealt with in one of two ways:
• Re-assign QoS values to appropriate header
• Drop packet
If the decision is to mark down the nonconforming traffic, the DSCP value is derived from
the mapping tables.
Excess
tokens are
discarded
Tokens
replenished
every refresh
interval
Number of
tokens equal to
size of packet
are removed
from bucket
Burst
Depth
Token Bucket Mechanism 43
The token bucket mechanism has three components: a burst size, a mean rate, and a time
interval (Tc). Although the mean rate is generally represented as bits per second, any two
values may be derived from the third by the relation shown as follows:
Mean rate = burst size / time interval
Here are some definitions of these terms:
• Mean rate—Also called the committed information rate (CIR), it specifies how much
data can be sent or forwarded per unit time on average.
• Burst size—Also called the committed burst (Bc) size, it specifies in bits (or bytes) per
burst how much traffic can be sent within a given unit of time to not create scheduling
concerns. (For a shaper, such as generic traffic shaping (GTS), it specifies bits per burst;
for a policer, such as committed access rate (CAR), it specifies bytes per burst.)
• Time interval—Also called the measurement interval, it specifies the time quantum
in seconds per burst.
By definition, over any integral multiple of the interval, the bit rate of the interface will not
exceed the mean rate. The bit rate, however, may be arbitrarily fast within the interval.
A token bucket is used to manage a device that regulates the data in a flow. For example,
the regulator might be a traffic policer, such as CAR, or a traffic shaper, such as Frame
Relay traffic shaping (FRTS) or GTS. A token bucket itself has no discard or priority policy.
Rather, a token bucket discards tokens and leaves to the flow the problem of managing its
transmission queue if the flow overdrives the regulator. (Neither CAR nor FRTS and GTS
implement either a true token bucket or true leaky bucket.)
In the token bucket metaphor, tokens are put into the bucket at a certain rate. The bucket
itself has a specified capacity. If the bucket fills to capacity, newly arriving tokens are
discarded. Each token is permission for the source to send a certain number of bits into the
network. To send a packet, the regulator must remove from the bucket a number of tokens
equal in representation to the packet size.
If not enough tokens are in the bucket to send a packet, the packet either waits until the
bucket has enough tokens (in the case of GTS) or the packet is discarded or marked down
(in the case of CAR). If the bucket is already full of tokens, incoming tokens overflow and
are not available to future packets. Thus, at any time, the largest burst a source can send into
the network is roughly proportional to the size of the bucket.
Note that the token bucket mechanism used for traffic shaping has both a token bucket and
a data buffer, or queue; if it did not have a data buffer, it would be a policer. For traffic
shaping, packets that arrive that cannot be sent immediately are delayed in the data buffer.
For traffic shaping, a token bucket permits burstiness but bounds it. It guarantees that the
burstiness is bounded so that the flow never sends faster than the capacity of the token
bucket plus the time interval multiplied by the established rate at which tokens are placed
in the bucket. It also guarantees that the long-term transmission rate does not exceed the
established rate at which tokens are placed in the bucket.
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