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Congestion Avoidance
Inner Mongolia University
Objectives
Upon completing this module, you will be able to: Describe random early detection (RED) Describe and configure weighted random early
detection (WRED)
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TCP Review
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Transmission Control Protocol - TCP
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IP Best-Effort Design Philosophy
Best-effort delivery Let everybody send Try to deliver what you can … and just drop the rest
source destination
IP network
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Congestion is Unavoidable
Two packets arrive at the same time The node can only transmit one … and either buffer or drop the other
If many packets arrive in short period of time The node cannot keep up with the arriving traffic … and the buffer may eventually overflow
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The Problem of Congestion
What is congestion? Load is higher than capacity
What do IP routers do? Drop the excess packets
Why is this bad? Wasted bandwidth for retransmissions
Load
Goodput“ congestioncollapse” Increase in load that
results in a decrease in useful work done.
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Ways to Deal With Congestion
Ignore the problem Many dropped (and retransmitted) packets Can cause congestion collapse
Reservations, like in circuit switching Pre-arrange bandwidth allocations Requires negotiation before sending packets
Pricing Don’t drop packets for the high-bidders Requires a payment model
Dynamic adjustment (TCP) Every sender infers the level of congestion And adapts its sending rate, for the greater good
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Many Important Questions
How does the sender know there is congestion? Explicit feedback from the network? Inference based on network performance?
How should the sender adapt? Explicit sending rate computed by the network? End host coordinates with other hosts? End host thinks globally but acts locally?
What is the performance objective? Maximizing goodput, even if some users suffer more? Fairness? (Whatever the heck that means!)
How fast should new TCP senders send?
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Inferring From Implicit Feedback
?
What does the end host see?What can the end host change?
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Where Congestion Happens: Links
Simple resource allocation: FIFO queue & drop-tail
Link bandwidth: first-in first-out queue Packets transmitted in the order they arrive
Buffer space: drop-tail queuing If the queue is full, drop the incoming packet
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How it Looks to the End Host
Packet delay Packet experiences high delay
Packet loss Packet gets dropped along the way
How does TCP sender learn this? Delay
• Round-trip time estimate Loss
• Timeout • Triple-duplicate acknowledgment
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What Can the End Host Do?
Upon detecting congestion Decrease the sending rate (e.g., divide in half) End host does its part to alleviate the congestion
But, what if conditions change? Suppose there is more bandwidth available Would be a shame to stay at a low sending rate
Upon not detecting congestion Increase the sending rate, a little at a time And see if the packets are successfully delivered
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TCP Congestion Window
Each TCP sender maintains a congestion window Maximum number of bytes to have in transit I.e., number of bytes still awaiting acknowledgments
Adapting the congestion window Decrease upon losing a packet: backing off Increase upon success: optimistically exploring Always struggling to find the right transfer rate
Both good and bad Pro: avoids having explicit feedback from network Con: under-shooting and over-shooting the rate
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Additive Increase, Multiplicative Decrease
How much to increase and decrease? Increase linearly, decrease multiplicatively A necessary condition for stability of TCP Consequences of over-sized window are much worse
than having an under-sized window• Over-sized window: packets dropped and retransmitted• Under-sized window: somewhat lower throughput
Multiplicative decrease On loss of packet, divide congestion window in half
Additive increase On success for last window of data, increase linearly
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Leads to the TCP “Sawtooth”
t
Window
halved
Loss
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Practical Details
Congestion window Represented in bytes, not in packets (Why?) Packets have MSS (Maximum Segment Size) bytes
Increasing the congestion window Increase by MSS on success for last window of data
Decreasing the congestion window Never drop congestion window below 1 MSS
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Receiver Window vs. Congestion Window
Flow control Keep a fast sender from overwhelming a slow
receiver
Congestion control Keep a set of senders from overloading the network
Different concepts, but similar mechanisms TCP flow control: receiver window TCP congestion control: congestion window TCP window: min{congestion window, receiver
window}
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How Should a New Flow Start
t
Window
But, could take a long time to get
started!
Need to start with a small CWND to avoid overloading the network.
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“Slow Start” Phase
Start with a small congestion window Initially, CWND is 1 Max Segment Size (MSS) So, initial sending rate is MSS/RTT
That could be pretty wasteful Might be much less than the actual bandwidth Linear increase takes a long time to accelerate
Slow-start phase (really “fast start”) Sender starts at a slow rate (hence the name) … but increases the rate exponentially … until the first loss event
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Slow Start in Action
Double CWND per round-trip time
D A D D A A D D
A A
D
A
Src
Dest
D
A
1 2 4 8
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Slow Start and the TCP Sawtooth
Loss
Exponential “slow start”
t
Window
Why is it called slow-start? Because TCP originally hadno congestion control mechanism. The source would just start by sending a whole receiver window’s worth of data.
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Two Kinds of Loss in TCP
Timeout Packet n is lost and detected via a timeout E.g., because all packets in flight were lost After the timeout, blasting away for the entire CWND … would trigger a very large burst in traffic So, better to start over with a low CWND
Triple duplicate ACK Packet n is lost, but packets n+1, n+2, etc. arrive Receiver sends duplicate acknowledgments … and the sender retransmits packet n quickly Do a multiplicative decrease and keep going
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Repeating Slow Start After Timeout
t
Window
Slow-start restart: Go back to CWND of 1, but take advantage of knowing the previous value of CWND.
Slow start in operation until it reaches half of
previous cwnd.
timeout
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Repeating Slow Start After Idle Period
Suppose a TCP connection goes idle for a while E.g., Telnet session where you don’t type for an hour
Eventually, the network conditions change Maybe many more flows are traversing the link E.g., maybe everybody has come back from lunch!
Dangerous to start transmitting at the old rate Previously-idle TCP sender might blast the network … causing excessive congestion and packet loss
So, some TCP implementations repeat slow start Slow-start restart after an idle period
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TCP Achieves Some Notion of Fairness
Effective utilization is not the only goal We also want to be fair to the various flows … but what the heck does that mean?
Simple definition: equal shares of the bandwidth N flows that each get 1/N of the bandwidth? But, what if the flows traverse different paths? E.g., bandwidth shared in proportion to the RTT
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What About Cheating?
Some folks are more fair than others Running multiple TCP connections in parallel Modifying the TCP implementation in the OS Use the User Datagram Protocol
What is the impact Good guys slow down to make room for you You get an unfair share of the bandwidth
Possible solutions? Routers detect cheating and drop excess packets? Peer pressure? ???
© 2001, Cisco Systems, Inc.
Random Early Detection
QOS v1.0—5-28
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Objectives
Upon completing this lesson, you will be able to: Explain the need for congestion avoidance
mechanisms Explain how RED works and how it can prevent
congestion Describe the benefits and drawbacks of RED
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Router Interface Congestion
Router interfaces congest when the output queue is full:
• Additional incoming packets are dropped.• Dropped packets may cause significant application
performance degradation.• By default, routers perform tail dropping.• Tail dropping has significant drawbacks.• WFQ, if configured, has a more intelligent dropping
scheme.
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Tail-Drop Flaws
Simple tail dropping has significant flaws:• TCP synchronization
• TCP starvation
• High delay and jitter
• No differentiated drop
• Poor feedback to TCP
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TCP Synchronization
Multiple TCP sessions start at different times. TCP window sizes are increased. Tail drops cause many packets of many sessions to be dropped at the
same time. TCP sessions restart at the same time (synchronization).
Flow A
Flow B
Flow C
Average link use
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TCP Starvation, Delay, and Jitter
Constant high buffer use (long queue) causes delay. More aggressive flows can cause other flows to starve. Variable buffer use causes jitter. There is no differentiated dropping.
Prec.0
Prec.0
Prec.0
Prec.0
Prec.0
Prec.0
Prec.0
Prec.0
Prec.3
Prec.3 Queue
Packets of Aggressive
Flows
Prec.3
Packets of Starving Flows
Delay
Packets experience long delay if the interface is constantly congested.
Prec.3
Prec.3
TCP does not react well if
multiple packets are
dropped.
Tail dropping does not look
at IP Precedence.
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Conclusion
Tail dropping should be avoided. Tail dropping can be avoided if congestion is
prevented. Congestion can be prevented if TCP sessions
(which still make up more than 80% of average Internet traffic) can be slowed down.
TCP sessions can be slowed down if some packets are occasionally dropped.
Therefore, packets should be dropped when an interface is nearing congestion.
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Random Early Detection
Random early detection (RED) is a mechanism that randomly drops packets even before a queue is full.
RED drops packets with increasing probability. RED result:
• TCP sessions slow down to the approximate rate of output-link bandwidth.
• Average queue size is small (much less than the maximum queue size).
IP Precedence can be used to drop lower-Precedence packets more aggressively than higher-Precedence packets.
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RED Profile
AverageQueueSize
DropProbability
10%
100%
20 40
MinimumThreshold
MaximumThreshold
MaximumDrop
Probability
No drop Random drop Full drop
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RED Modes
RED has three modes:• No drop—when the average queue size is between 0 and
the minimum threshold• Random drop—when the average queue size is between
the minimum and the maximum threshold• Full drop (tail drop)—when the average queue size is at
maximum threshold or above
Random drops should prevent congestion (prevent tail drops).
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Before RED
TCP synchronization prevents average link utilization close to the link bandwidth.
Tail drops cause TCP sessions to go into slow-start.
Flow A
Flow B
Flow C
Average link use
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After RED
Average link use is much closer to link bandwidth. Random drops cause TCP sessions to reduce
window sizes.
Average link use Flow A
Flow B
Flow C
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Summary
Upon completing this lesson, you should be able to: Explain the need for congestion avoidance
mechanisms Explain how RED works and how it can prevent
congestion Describe the benefits and drawbacks of RED
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Lesson Review
1.What are the main drawbacks of using tail dropping as a means of congestion control?
2.What does RED do to prevent TCP synchronization?
3.What are the three modes of RED?
Weighted Random Early Detection
© 2001, Cisco Systems, Inc. QOS v1.0—5-42
Inner Mongolia University
Objectives
Upon completing this lesson, you will be able to: Describe the weighted random early detection
(WRED) mechanism Configure WRED on Cisco routers Monitor and troubleshoot WRED on Cisco routers
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Weighted Random Early Detection
WRED uses a different RED profile for each weight. Each profile is identified by:
• Minimum threshold• Maximum threshold • Maximum drop probability
Weight can be:• IP Precedence (8 profiles)• DSCP (64 profiles)
WRED drops less important packets more aggressively than more important packets.
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WRED Profiles
WRED profiles can be manually set. WRED has 8 default value sets for IP Precedence–based WRED. WRED has 64 default value sets for DSCP–based WRED.
AverageQueueSize
DropProbability
10%
100%
20 4010
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IP Precedence and Class Selector Profiles
AverageQueueSize
DropProbability
10%
100%
20 40RSVP0 1 2 3 4 5 6 7
22 24 26 28 31 33 35 37
IP Precedence
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DSCP-Based WRED(Expedited Forwarding)
AverageQueueSize
DropProbability
10%
100%
20 40
EF
36
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DSCP-Based WRED(Assured Forwarding)
AverageQueueSize
DropProbability
10%
100%
20 40
Assured Forwarding High Drop
3224 28
Assured Forwarding Medium Drop
Assured Forwarding Low Drop
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WRED Building Blocks
IP PacketIP Packet WREDWRED
Calculate Average Queue Size
Calculate Average Queue Size
FIFO QueueFIFO Queue
Select WREDProfile
Select WREDProfile
CurrentQueueSize
IP PrecedenceorDSCP
Minimum Threshold Maximum Threshold Mark Probability Denominator
QueueFull?
QueueFull?
No
Yes
Tail DropRandom Drop
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Configuring WRED and DWRED
random-detectrandom-detect
Router(config-if)#
• Enables IP Precedence–based WRED• Default service profile is used• Nondistributed WRED cannot be combined
with fancy queuing—FIFO queuing has to be used
• WRED can run distributed on VIP-based interfaces (DWRED)
• DWRED can be combined with DWFQ
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Changing the WRED Profile
random-detect precedence precedence min-threshold max-threshold mark-prob-denominatorrandom-detect precedence precedence min-threshold max-threshold mark-prob-denominator
Router(config-if)#
• Changes RED profile for specified IP Precedence value
• Packet drop probability at maximum threshold is 1 / mark-prob-denominator
• Nonweighted RED is achieved by using the same RED profile for all precedence values
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nt
navgavg QtQtQ 2)21()()1(
Changing WRED Sensitivity to Bursts
random-detect exponential-weighting-constant nrandom-detect exponential-weighting-constant n
Router(config-if)#
• WRED takes the average queue size to determine the current WRED mode (no drop, random drop, full drop).
• High values of n allow short bursts.
• Low values of n make WRED more burst-sensitive.
• Default value (9) should be used in most scenarios.
• Average output queue size with n =9 is
averaget+1 = averaget * 0.998 + queue_sizet * 0.002
Current Queue Size
Previous Average Queue Size
New Average Queue size
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Configuring DSCP-Based WRED
random-detect {prec-based | dscp-based}random-detect {prec-based | dscp-based}
Router(config-if)#
• Selects WRED mode• Precedence-based WRED is the default
mode• DSCP-based command uses 64 profiles
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Changing the WRED Profile
random-detect dscp dscp min-threshold max-threshold mark-prob-denominatorrandom-detect dscp dscp min-threshold max-threshold mark-prob-denominator
Router(config-if)#
• Changes RED profile for specified DSCP value
• Packet drop probability at maximum threshold is 1 / mark-prob-denominator
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WRED Case Study
WRED is applied to a core link in a network with these IP Precedence definitions:
IP IP PPrec.rec. MeaningMeaning
00 High-dropHigh-drop,, best best--effort trafficeffort traffic
Low-dropLow-drop,, best-effort traffic best-effort traffic11
33 Premium traffic in the contractPremium traffic in the contract
22 Premium traffic outside of the contractPremium traffic outside of the contract
44 UnusedUnused
55 VoiceVoice overover IPIP
66 Routing protocol trafficRouting protocol traffic
77 Routing protocol trafficRouting protocol traffic
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WRED Case StudyGuidelines
Best-effort traffic should be dropped before premium traffic. Out-of-contract or high-drop, best-effort traffic should be dropped
very aggressively. Voice traffic should be dropped only under extreme congestion. Routing protocol traffic should be less drop resistant than VoIP
(depends on the routing protocol and control over amount of VoIP traffic).
Configure WRED with default values on an interface first and tune the per-precedence parameters based on default values.
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Sample WRED ProfileP
ack
et
Dis
card
Pro
ba
bili
ty
AverageQueue Size
0.1
RSVP
15
10
20
25
30
35
37
Precedence 2
Precedence 0
Precedence 3
Precedence 1
VoIP
Routing
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WRED Configuration
interface Serial 0/1/0 ip address 200.200.14.250 255.255.255.252 random-detect random-detect precedence 0 10 25 10 random-detect precedence 1 20 35 10 random-detect precedence 2 15 25 10 random-detect precedence 3 25 35 10 random-detect precedence 4 1 2 1 random-detect precedence 5 35 40 10 random-detect precedence 6 30 40 10 random-detect precedence 7 30 40 10
interface Serial 0/1/0 ip address 200.200.14.250 255.255.255.252 random-detect random-detect precedence 0 10 25 10 random-detect precedence 1 20 35 10 random-detect precedence 2 15 25 10 random-detect precedence 3 25 35 10 random-detect precedence 4 1 2 1 random-detect precedence 5 35 40 10 random-detect precedence 6 30 40 10 random-detect precedence 7 30 40 10
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Monitoring WRED
show interface• Displays the queuing/dropping mechanism in use
• Displays WRED parameters (VIP only) show queueing
• Displays the RED profile for each interface show queue
• Displays the interfaces output queue show interface random-detect
• Displays RED statistics (VIP only)
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Interface Parameters
Router#show interface serial 1/0Serial1/0 is up, line protocol is up Hardware is CD2430 in sync mode Internet address is 192.168.1.2/30 MTU 1500 bytes, BW 128 Kbit, DLY 200 usec, rely 255/255 ... Encapsulation HDLC, loopback not set, keepalive set (10 sec) Last input 00:00:07, output 00:00:07, output hang never Last clearing of "show interface" counters never Input queue: 2/75/0 (size/max/drops); Total output drops: 0 Queueing strategy: random early detection (WRED) 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 337102 packets input, 27357987 bytes, 0 no buffer Received 265169 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort ... rest deleted ...
Router#show interface serial 1/0Serial1/0 is up, line protocol is up Hardware is CD2430 in sync mode Internet address is 192.168.1.2/30 MTU 1500 bytes, BW 128 Kbit, DLY 200 usec, rely 255/255 ... Encapsulation HDLC, loopback not set, keepalive set (10 sec) Last input 00:00:07, output 00:00:07, output hang never Last clearing of "show interface" counters never Input queue: 2/75/0 (size/max/drops); Total output drops: 0 Queueing strategy: random early detection (WRED) 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 337102 packets input, 27357987 bytes, 0 no buffer Received 265169 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort ... rest deleted ...
show interface intfshow interface intf
Router#
• Displays interface parameters
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WRED Parameters and Statistics
Router#show queueing random-detectCurrent random-detect configuration: Serial1/0 Queueing strategy: random early detection (WRED) Exp-weight-constant: 9 (1/512) Mean queue depth: 38
Class Random Tail Minimum Maximum Mark drop drop threshold threshold probability 0 174 34 20 40 1/10 1 0 0 22 40 1/10 2 0 0 24 40 1/10 3 0 0 26 40 1/10 4 0 0 28 40 1/10 5 0 0 31 40 1/10 6 6 3 33 40 1/10 7 0 0 35 40 1/10 rsvp 0 0 37 40 1/10
Router#show queueing random-detectCurrent random-detect configuration: Serial1/0 Queueing strategy: random early detection (WRED) Exp-weight-constant: 9 (1/512) Mean queue depth: 38
Class Random Tail Minimum Maximum Mark drop drop threshold threshold probability 0 174 34 20 40 1/10 1 0 0 22 40 1/10 2 0 0 24 40 1/10 3 0 0 26 40 1/10 4 0 0 28 40 1/10 5 0 0 31 40 1/10 6 6 3 33 40 1/10 7 0 0 35 40 1/10 rsvp 0 0 37 40 1/10
show queueing random-detectshow queueing random-detect
Router#
• Displays per-interface parameters WRED statistics
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DWRED Parameters and Statistics
Router#show interfaces random-detect FastEthernet1/0/0 queue size 0 packets output 29692, drops 0 WRED: queue average 0 weight 1/512 Precedence 0: 109 min threshold, 218 max threshold, 1/10 mark weight 1 packets output, drops: 0 random, 0 threshold Precedence 1: 122 min threshold, 218 max threshold, 1/10 mark weight (no traffic) Precedence 2: 135 min threshold, 218 max threshold, 1/10 mark weight 14845 packets output, drops: 0 random, 0 threshold Precedence 3: 148 min threshold, 218 max threshold, 1/10 mark weight (no traffic) Precedence 4: 161 min threshold, 218 max threshold, 1/10 mark weight (no traffic) Precedence 5: 174 min threshold, 218 max threshold, 1/10 mark weight (no traffic) Precedence 6: 187 min threshold, 218 max threshold, 1/10 mark weight 14846 packets output, drops: 0 random, 0 threshold Precedence 7: 200 min threshold, 218 max threshold, 1/10 mark weight (no traffic)
Router#show interfaces random-detect FastEthernet1/0/0 queue size 0 packets output 29692, drops 0 WRED: queue average 0 weight 1/512 Precedence 0: 109 min threshold, 218 max threshold, 1/10 mark weight 1 packets output, drops: 0 random, 0 threshold Precedence 1: 122 min threshold, 218 max threshold, 1/10 mark weight (no traffic) Precedence 2: 135 min threshold, 218 max threshold, 1/10 mark weight 14845 packets output, drops: 0 random, 0 threshold Precedence 3: 148 min threshold, 218 max threshold, 1/10 mark weight (no traffic) Precedence 4: 161 min threshold, 218 max threshold, 1/10 mark weight (no traffic) Precedence 5: 174 min threshold, 218 max threshold, 1/10 mark weight (no traffic) Precedence 6: 187 min threshold, 218 max threshold, 1/10 mark weight 14846 packets output, drops: 0 random, 0 threshold Precedence 7: 200 min threshold, 218 max threshold, 1/10 mark weight (no traffic)
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Queue Details
Router#show queue serial 1/0Output queue for Serial1/0 is 65/0
Packet 1, linktype: ip, length: 1504, flags: 0x48 source: 192.168.1.2, destination: 192.168.1.2, id: 0x001A, ttl: 255, prot: 1 data: 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD
Packet 2, linktype: ip, length: 1504, flags: 0x48 source: 192.168.1.2, destination: 192.168.1.2, id: 0x001A, ttl: 255, prot: 1 data: 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD
Packet 3, linktype: ip, length: 1504, flags: 0x48 source: 192.168.1.2, destination: 192.168.1.2, id: 0x001A, ttl: 255, prot: 1 data: 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD... rest deleted ...
Router#show queue serial 1/0Output queue for Serial1/0 is 65/0
Packet 1, linktype: ip, length: 1504, flags: 0x48 source: 192.168.1.2, destination: 192.168.1.2, id: 0x001A, ttl: 255, prot: 1 data: 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD
Packet 2, linktype: ip, length: 1504, flags: 0x48 source: 192.168.1.2, destination: 192.168.1.2, id: 0x001A, ttl: 255, prot: 1 data: 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD
Packet 3, linktype: ip, length: 1504, flags: 0x48 source: 192.168.1.2, destination: 192.168.1.2, id: 0x001A, ttl: 255, prot: 1 data: 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD 0xABCD... rest deleted ...
show queue intfshow queue intf
Router#
• Displays queue contents
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WRED Caveats and Restrictions
Because the same policy is applied to all flows, a single nonadaptive flow can monopolize the buffer resources at an interface:
• WRED is suitable when TCP represents at least 80% of the traffic .
• Non-TCP traffic should be rate limited. Non distributed WRED implementation is mutually
exclusive with PQ, CQ, and WFQ.
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Summary
Upon completing this lesson, you should be able to: Describe the weighted random early detection
(WRED) mechanism Configure WRED on Cisco routers Monitor and troubleshoot WRED on Cisco routers
Inner Mongolia University
Module Summary
Upon completing this module, you should be able to: Describe random early detection (RED) Describe and configure weighted random early
detection (WRED)
Inner Mongolia University© 2001, Cisco Systems, Inc. IP QoS Traffic Shaping and Policing-67