468 Chapter 6: Congestion Avoidance Through Drop Policies
Table 6-14 describes the overall logic of when RED discards packets, with the same ideas outlined in Figure 6-16.
Table 6-14 Three Categories of When RED Will Discard Packets, and How Many
Average Queue Depth Versus |
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Thresholds |
Action |
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Average < minimum threshold |
No packets dropped. |
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Minimum threshold < average depth < |
A percentage of packets dropped. Drop percentage increases |
maximum threshold |
from 0 to a maximum percent as the average depth moves |
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from the minimum threshold to the maximum. |
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Average depth > maximum threshold |
All new packets discarded similar to tail dropping. |
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Figure 6-16 RED Discarding Logic Using Average Depth, Minimum Threshold, and Maximum Threshold
Discard
Percentage
100%
Maximum
Discard
Percentage
Average Queue Depth
Minimum Threshold Maximum Threshold
Table 6-15 summarizes some of the key terms related to RED.
Table 6-15 RED Terminology
Term |
Meaning |
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Actual queue depth |
The actual number of packets in a queue at a particular point in time. |
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Average queue depth |
Calculated measurement based on the actual queue depth and the previous |
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average. Designed to adjust slowly to the rapid changes of the actual queue |
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depth. |
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Foundation Summary 469 |
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Table 6-15 RED Terminology (Continued) |
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Term |
Meaning |
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Minimum threshold |
Compares this setting to the average queue depth to decide whether |
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packets should be discarded. No packets are discarded if the average queue |
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depth falls below this minimum threshold. |
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Maximum threshold |
Compares this setting to the average queue depth to decide whether |
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packets should be discarded. All packets are discarded if the average queue |
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depth falls above this maximum threshold. |
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Mark probability |
Used to calculate the maximum percentage of packets discarded when the |
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denominator |
average queue depth falls between the minimum and maximum thresholds. |
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Exponential weighting |
Used to calculate the rate at which the average queue depth changes as |
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constant |
compared with the current queue depth. The larger the number, the slower |
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the change in the average queue depth. |
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Figure 6-17 shows the default WRED settings for precedence 0, with some nondefault settings for precedence 5 traffic.
Figure 6-17 Example WRED Settings for Precedences 0 and 5 for Thresholds and Discard Percent
Discard
Percentage 100% 
Precedence 0 Drop |
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Percentage (MPD = 10) |
10% |
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Precedence 3 Drop |
5% |
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Percentage (MPD = 20) |
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Average |
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Queue Depth |
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20 |
30 |
40 |
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Pecedence |
Pecedence |
Maximum |
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Precedence 3 |
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0 Minimum |
3 Minimum Threshold |
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Precedence 0 |
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Threshold |
Threshold |
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WRED measures the average queue depth of the FIFO queue on an interface, as shown in Figure 6-18.
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Foundation Summary 471 |
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Table 6-16 Command Reference for WRED |
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Command |
Mode and Function |
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random-detect [dscp-based | prec-based] |
Interface or class configuration mode; enables WRED, |
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specifying whether to react to precedence or DSCP. |
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random-detect [attach group-name] |
Interface configuration mode; enables per-VC WRED |
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on ATM interfaces by referring to a random-detect- |
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group. |
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random-detect-group group-name [dscp- |
Global configuration mode; creates a grouping of |
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based | prec-based] |
WRED parameters, which can be enabled on |
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individual ATM VCs using the random-detect attach |
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command. |
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random-detect precedence precedence min- |
Interface, class, or random-detect-group configuration |
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threshold max-threshold mark-prob- |
modes; overrides default settings for the specified |
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denominator |
precedence, for minimum and maximum WRED |
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thresholds, and for percentage of packets discarded. |
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random-detect dscp dscpvalue min- |
Interface, class, or random-detect-group configuration |
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threshold max-threshold [mark-probability- |
modes; overrides default settings for the specified |
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denominator] |
DSCP, for minimum and maximum WRED |
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thresholds, and for the percentage of packets |
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discarded. |
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random-detect exponential-weighting- |
Interface, class, or random-detect-group configuration |
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constant exponent |
modes; overrides default settings for exponential |
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weighting constant. Lower numbers make WRED |
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react quickly to changes in queue depth; higher |
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numbers make WRED react less quickly. |
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Table 6-17 Exec Command Reference for WRED |
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Command |
Function |
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show queue interface-name interface- |
Lists information about the packets that are waiting in |
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number [vc [vpi/] vci]] |
a queue on the interface |
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show queueing random-detect [interface |
Lists configuration and statistical information about |
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atm-subinterface [vc [[vpi/] vci]]] |
the queuing tool on an interface. |
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show interfaces |
Mentions whether WRED has been enabled on the |
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interface |
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show interface random-detect |
Lists information about WRED when distributed |
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WRED is running on a VIP interface |
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show policy-map [interface interface-name |
Lists WRED information when it is enabled inside an |
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interface-number] |
MQC policy map |
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472 Chapter 6: Congestion Avoidance Through Drop Policies
Fred classifies each flow into one of three FRED flow types, as listed in Table 6-18.
Table 6-18 FRED Flow Types
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Transport |
FRED |
Flow Type |
Description |
Protocol |
Discard Policy |
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Robust |
Adapts to lost packets by slowing down |
TCP |
Moderate discard rates |
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the rate of sending packets |
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Fragile |
Does not adapt to lost packets by |
UDP |
Low discard rates |
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slowing down, but the number of packets |
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sent is not excessive |
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Nonadaptive |
Does not adapt to lost packets by |
UDP |
High discard rates |
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slowing down, and the number of |
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packets sent is excessive |
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The terms used by FRED to describe the processes covered so far are listed in Table 6-19.
Table 6-19 FRED Terminology
Term |
Definition |
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Average per-flow queue size |
A calculated value, based on the formula maximum queue size/ |
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number of active flows. |
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Active flow |
A flow that currently has packets in the queue. |
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Maximum per-flow queue size |
A calculated value, based on the formula (average queue size * |
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scaling factor). (This same formula is used in the previous example |
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that results in an answer of 16.) This value is used to determine |
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which flows are fragile, and which are nonadaptive. |
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Scaling factor |
Number used in the calculation of maximum per-flow queue size, |
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which may be changed using configuration. |
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Average depth factor |
Another name for scaling factor. |
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FRED configuration and show commands are listed in Tables 6-20 and 6-21.
Table 6-20 Command Reference for FRED
Command |
Mode and Function |
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random-detect flow |
Interface configuration mode; enables FRED on the |
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interface. |
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random-detect flow average-depth-factor |
Interface configuration mode; changes scaling factor. |
scaling-factor |
The lower the number, the more aggressively FRED |
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discards packets for flows using more than their fair |
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share of the available queue space. |
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Foundation Summary 473 |
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Table 6-20 Command Reference for FRED (Continued) |
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Command |
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Mode and Function |
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random-detect flow count number |
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Interface configuration mode; overrides default |
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setting for the maximum number of concurrent flows |
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tracked by FRED. (The default is 256.) |
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random-detect precedence precedence |
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Interface, class, or random-detect-group configura- |
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min-threshold max-threshold mark-prob- |
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tion modes; overrides default settings for the speci- |
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denominator |
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fied precedence, for minimum and maximum WRED |
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thresholds, and for percentage of packets discarded. |
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random-detect dscp dscpvalue min-threshold |
Interface, class, or random-detect-group configura- |
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max-threshold [mark-probability- |
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tion modes; overrides default settings for the speci- |
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denominator] |
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fied DSCP, for minimum and maximum WRED |
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thresholds, and for percentage of packets discarded. |
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random-detect exponential-weighting- |
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Interface, class, or random-detect-group configura- |
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constant exponent |
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tion modes; overrides default settings for exponential |
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weighting constant. Lower numbers make WRED |
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react quickly to changes in queue depth; higher num- |
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bers make WRED react less quickly. |
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Table 6-21 Exec Command Reference for FRED |
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Command |
Function |
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show queue interface-name interface- |
Lists information about the packets that are waiting in a |
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number [vc [vpi/] vci]] |
queue on the interface |
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show queueing random-detect [interface |
Lists configuration and statistical information about the |
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atm-subinterface [vc [[vpi/] vci]]] |
queuing tool on an interface |
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show interfaces |
Mentions whether WRED has been enabled on the |
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interface |
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show interface random-detect |
Lists information about WRED when distributed WRED |
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is running on a VIP interface |
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Table 6-22 summarizes many of the key concepts when comparing WRED and FRED.
474 Chapter 6: Congestion Avoidance Through Drop Policies
Table 6-22 WRED Versus FRED
Feature |
WRED |
FRED |
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Discards packets to avoid congestion |
Yes |
Yes |
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Can be enabled on the physical interface concurrently with a |
No |
No |
queuing tool |
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Can be combined with CBWFQ or LLQ policy map |
Yes |
No |
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Bases drop decision, at least in part, on different thresholds per |
Yes |
Yes |
precedence or DSCP value |
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Bases drop decision, at least in part, on per-flow queue depth |
No |
Yes |
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