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466 Chapter 6: Congestion Avoidance Through Drop Policies

# Foundation Summary

The “Foundation Summary” is a collection of tables and ﬁgures that provide a convenient review of many key concepts in this chapter. For those of you already comfortable with the topics in this chapter, this summary could help you recall a few details. For those of you who just read this chapter, this review should help solidify some key facts. For any of you doing your ﬁnal prep before the exam, these tables and ﬁgures are a convenient way to review the day before the exam.

Figure 6-13 outlines the process of packet loss, halving CWND, and slow start.

Figure 6-13 Slamming the CWND Shut and the Slow Start Process After No Acknowledgment Received

Timeout!

Note: CWND units Are Segment Size in This Example

CWND = 1

CWND = 2

CWND = 3 CWND = 4

 SN = 1 ACK = 2 SN = 2 SN = 3 ACK = 3 = 4 ACK

Figure 6-14 shows a graph of CWND after packet loss just using slow start, and another with slow start plus congestion avoidance.

Foundation Summary 467

Figure 6-14 Graphs of CWND with Slow Start and Congestion Avoidance

 CWND CWND Before Lost ACK Congestion
 1 Segment Size Time Time Slow Start Only CWND>SSthresh Slow Start and Congestion Avoidance

The key information about TCP and UDP operation when packets are dropped is summarized in the following list:

UDP senders do not reduce or increase sending rates as a result of lost packets.

TCP senders do reduce their sending rates as a result of lost packets.

TCP senders decide to use either the receiver window or the CWND, based on whichever is lower at the time.

TCP slow start and congestion avoidance dictate how fast the CWND rises after the window was lowered due to packet loss.

The graph in Figure 6-15 shows the proven behavior in the Internet with many TCP connections.

Figure 6-15 Graph of Global Synchronization

 Line Rate Actual Bit Rate

Average

Rate

Time

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 Thresholds Action Average < minimum threshold No packets dropped. 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 from the minimum threshold to the maximum. Average depth > maximum threshold All new packets discarded similar to tail dropping.

Figure 6-16 RED Discarding Logic Using Average Depth, Minimum Threshold, and Maximum Threshold

Percentage

100%

Maximum

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 Actual queue depth The actual number of packets in a queue at a particular point in time. Average queue depth Calculated measurement based on the actual queue depth and the previous average. Designed to adjust slowly to the rapid changes of the actual queue depth.
 Foundation Summary 469 Table 6-15 RED Terminology (Continued) Term Meaning Minimum threshold Compares this setting to the average queue depth to decide whether packets should be discarded. No packets are discarded if the average queue depth falls below this minimum threshold. Maximum threshold Compares this setting to the average queue depth to decide whether packets should be discarded. All packets are discarded if the average queue depth falls above this maximum threshold. Mark probability Used to calculate the maximum percentage of packets discarded when the denominator average queue depth falls between the minimum and maximum thresholds. Exponential weighting Used to calculate the rate at which the average queue depth changes as constant compared with the current queue depth. The larger the number, the slower the change in the average queue depth.

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

Percentage 100%

 Precedence 0 Drop Percentage (MPD = 10) 10% Precedence 3 Drop 5% Percentage (MPD = 20) Average Queue Depth 20 30 40 Pecedence Pecedence Maximum Precedence 3 0 Minimum 3 Minimum Threshold Precedence 0 Threshold Threshold

WRED measures the average queue depth of the FIFO queue on an interface, as shown in Figure 6-18.

470 Chapter 6: Congestion Avoidance Through Drop Policies

Figure 6-18 FIFO Output Queue and WRED Interaction

Output Interface on a Router

Average Queue Depth Based on

Class Queue Actual Depth

 WRED FIFO Interface New Packet TX Ring Decision Output Queue

Packet

WRED can be applied to each class queue with CBWFQ, as shown in Figure 6-19.

Figure 6-19 WRED with CBWFQ

Average Queue Depth Based on

Class Queue Actual Depth

 WRED Class 1 Output Decision Queue Average Queue Depth Based on Class Queue Actual Depth Classification WRED Class 2 Output TX Ring Decision Queue

.

.

Average Queue Depth Based on

Class Queue Actual Depth

 WRED Class N Output Decision Queue

Tables 6-16 and 6-17 list the WRED conﬁguration and show commands, respectively.

 Foundation Summary 471 Table 6-16 Command Reference for WRED Command Mode and Function random-detect [dscp-based | prec-based] Interface or class conﬁguration mode; enables WRED, specifying whether to react to precedence or DSCP. random-detect [attach group-name] Interface conﬁguration mode; enables per-VC WRED on ATM interfaces by referring to a random-detect- group. random-detect-group group-name [dscp- Global conﬁguration mode; creates a grouping of based | prec-based] WRED parameters, which can be enabled on individual ATM VCs using the random-detect attach command. random-detect precedence precedence min- Interface, class, or random-detect-group conﬁguration threshold max-threshold mark-prob- modes; overrides default settings for the speciﬁed denominator precedence, for minimum and maximum WRED thresholds, and for percentage of packets discarded. random-detect dscp dscpvalue min- Interface, class, or random-detect-group conﬁguration threshold max-threshold [mark-probability- modes; overrides default settings for the speciﬁed denominator] DSCP, for minimum and maximum WRED thresholds, and for the percentage of packets discarded. random-detect exponential-weighting- Interface, class, or random-detect-group conﬁguration constant exponent modes; overrides default settings for exponential weighting constant. Lower numbers make WRED react quickly to changes in queue depth; higher numbers make WRED react less quickly. Table 6-17 Exec Command Reference for WRED Command Function show queue interface-name interface- Lists information about the packets that are waiting in number [vc [vpi/] vci]] a queue on the interface show queueing random-detect [interface Lists conﬁguration and statistical information about atm-subinterface [vc [[vpi/] vci]]] the queuing tool on an interface. show interfaces Mentions whether WRED has been enabled on the interface show interface random-detect Lists information about WRED when distributed WRED is running on a VIP interface show policy-map [interface interface-name Lists WRED information when it is enabled inside an interface-number] MQC policy map

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

 Transport FRED Flow Type Description Protocol Discard Policy Robust Adapts to lost packets by slowing down TCP Moderate discard rates the rate of sending packets Fragile Does not adapt to lost packets by UDP Low discard rates slowing down, but the number of packets sent is not excessive Nonadaptive Does not adapt to lost packets by UDP High discard rates slowing down, and the number of packets sent is excessive

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 Average per-ﬂow queue size A calculated value, based on the formula maximum queue size/ number of active ﬂows. Active ﬂow A ﬂow that currently has packets in the queue. Maximum per-ﬂow queue size A calculated value, based on the formula (average queue size * scaling factor). (This same formula is used in the previous example that results in an answer of 16.) This value is used to determine which ﬂows are fragile, and which are nonadaptive. Scaling factor Number used in the calculation of maximum per-ﬂow queue size, which may be changed using conﬁguration. Average depth factor Another name for scaling factor.

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 random-detect ﬂow Interface conﬁguration mode; enables FRED on the interface. random-detect ﬂow average-depth-factor Interface conﬁguration mode; changes scaling factor. scaling-factor The lower the number, the more aggressively FRED discards packets for ﬂows using more than their fair share of the available queue space.
 Foundation Summary 473 Table 6-20 Command Reference for FRED (Continued) Command Mode and Function random-detect ﬂow count number Interface conﬁguration mode; overrides default setting for the maximum number of concurrent ﬂows tracked by FRED. (The default is 256.) random-detect precedence precedence Interface, class, or random-detect-group conﬁgura- min-threshold max-threshold mark-prob- tion modes; overrides default settings for the speci- denominator ﬁed precedence, for minimum and maximum WRED thresholds, and for percentage of packets discarded. random-detect dscp dscpvalue min-threshold Interface, class, or random-detect-group conﬁgura- max-threshold [mark-probability- tion modes; overrides default settings for the speci- denominator] ﬁed DSCP, for minimum and maximum WRED thresholds, and for percentage of packets discarded. random-detect exponential-weighting- Interface, class, or random-detect-group conﬁgura- constant exponent tion modes; overrides default settings for exponential weighting constant. Lower numbers make WRED react quickly to changes in queue depth; higher num- bers make WRED react less quickly. Table 6-21 Exec Command Reference for FRED Command Function show queue interface-name interface- Lists information about the packets that are waiting in a number [vc [vpi/] vci]] queue on the interface show queueing random-detect [interface Lists conﬁguration and statistical information about the atm-subinterface [vc [[vpi/] vci]]] queuing tool on an interface show interfaces Mentions whether WRED has been enabled on the interface show interface random-detect Lists information about WRED when distributed WRED is running on a VIP interface

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 Discards packets to avoid congestion Yes Yes Can be enabled on the physical interface concurrently with a No No queuing tool Can be combined with CBWFQ or LLQ policy map Yes No Bases drop decision, at least in part, on different thresholds per Yes Yes precedence or DSCP value Bases drop decision, at least in part, on per-ﬂow queue depth No Yes