Queuing Tools 265
WFQ places an absolute limit on the number of packets enqueued among all queues; this value is called the hold-queue limit. If a new packet arrives, and the hold-queue limit has been reached, the packet is discarded. That part of the decision is based not on a single queue, but on the whole WFQ queuing system for the interface.
The next decision is based on an individual queue. If a packet needs to be placed into a queue, and that queue’s congestive discard threshold (CDT) has been reached, the packet may be thrown away. CDT is a little like a maximum queue length for each flow’s queue, but WFQ puts a little twist on how the concept is used (hence the use of another term, instead of just calling it the maximum queue length). To appreciate how the CDT is used, examine Figure 4-17.
Figure 4-17 WFQ Modified Tail Drop and Congestive Discard Threshold
Hold-queue |
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CDT |
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Enqueue New Packet |
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Calculate SN |
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Limit |
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Exceeded? |
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This Queue? |
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No |
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Is There a Higher |
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Packet |
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Discarded |
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Another Queue? |
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SN Packet |
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The hold-queue size limits the total number of packets in all of the flow or conversation queues. However, CDT limits the number of packets in each individual queue. If CDT packets are already in the queue into which a packet should be placed, WFQ considers discarding the new packet. Normally, the new packet is discarded. If a packet with a larger SN has already been enqueued in a different queue, however, WFQ instead discards the packet with the larger SN! It’s like going to Disneyland, getting in line, and then being told that a bunch of VIPs showed up, so you cannot ride the ride, and you will have to come back later. (Hopefully Disney would not take you out of the line and send you to the bit bucket, though!) In short, WFQ can discard a packet in another flow when the queue for a different flow has exceeded CDT but still has lower sequence numbers. You can configure the CDT to a value between 1 and 4096, inclusive.
Finally, WFQ can be configured for a maximum of 4096 queues, but interestingly, the actual value can only be a power of 2 between 16 and 4096, inclusive. The IOS restricts the values because WFQ performs a hash algorithm to classify traffic, and the hash algorithm only works when the number of queues is one of these valid values.
268 Chapter 4: Congestion Management
Example 4-3 WFQ Configuration and show Commands (Continued)
Queueing strategy: weighted fair
Output queue: 126/1000/64/8249 (size/max total/threshold/drops)
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Conversations 6/7/256 (active/max active/max total) |
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Reserved Conversations 0/0 (allocated/max allocated) |
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Available Bandwidth 1158 |
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5 minute input rate |
52000 bits/sec, 97 packets/sec |
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5 minute output rate 58000 bits/sec, 78 |
packets/sec |
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36509 packets input, 2347716 bytes, 0 no buffer |
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Received 0 broadcasts, 0 |
runts, 0 giants, 0 throttles |
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1 input errors, 0 CRC, 1 |
frame, 0 overrun, 0 ignored, 0 abort |
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28212 packets output, 2623792 bytes, |
0 underruns |
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0 output errors, |
0 collisions, 5 interface resets |
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0 output buffer failures, 0 output buffers swapped out |
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10 carrier transitions |
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DCD=up DSR=up |
DTR=up RTS=up CTS=up |
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R3#show queueing fair |
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Current fair queue configuration: |
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Interface |
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threshold |
queues |
queues |
queues |
queues |
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Serial0/0 |
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Serial0/1 |
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R3#show queueing fair int s 0/0 |
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Current fair queue configuration: |
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Interface |
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threshold |
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R3# show queue s 0/0
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 11027 Queueing strategy: weighted fair
Output queue: 79/1000/64/11027 (size/max total/threshold/drops) Conversations 4/8/256 (active/max active/max total) Reserved Conversations 0/0 (allocated/max allocated) Available Bandwidth 1158 kilobits/sec
!Next stanza lists info about one of the VoIP calls
(depth/weight/total drops/no-buffer drops/interleaves) 37/5397/1359/0/0
Conversation 15, linktype: ip, length: 64
source: 192.168.3.254, destination: 192.168.2.251, id: 0x013B, ttl: 253, TOS: 184 prot: 17, source port 16772, destination port 19232
!Next stanza lists info about one of the VoIP calls
(depth/weight/total drops/no-buffer drops/interleaves) 37/5397/1359/0/0 Conversation 125, linktype: ip, length: 64
source: 192.168.3.254, destination: 192.168.2.251, id: 0x0134, ttl: 253, TOS: 184 prot: 17, source port 16638, destination port 19476
!Next stanza lists info about one of the HTTP TCP connections (depth/weight/total drops/no-buffer drops/interleaves) 1/10794/36/0/0 Conversation 33, linktype: ip, length: 1404
270 Chapter 4: Congestion Management
Example 4-3 WFQ Configuration and show Commands (Continued)
Queueing strategy: weighted fair
Output queue: 125/500/100/13567 (size/max total/threshold/drops)
Conversations 5/7/64 (active/max active/max total)
Reserved Conversations 0/0 (allocated/max allocated)
Available Bandwidth 1158 kilobits/sec
(depth/weight/total drops/no-buffer drops/interleaves) 61/5397/654/0/0 Conversation 61, linktype: ip, length: 64
source: 192.168.3.254, destination: 192.168.2.251, id: 0x0134, ttl: 253, TOS: 184 prot: 17, source port 16638, destination port 19476
(depth/weight/total drops/no-buffer drops/interleaves) 61/5397/653/0/0 Conversation 15, linktype: ip, length: 64
source: 192.168.3.254, destination: 192.168.2.251, id: 0x013B, ttl: 253, TOS: 184 prot: 17, source port 16772, destination port 19232
(depth/weight/total drops/no-buffer drops/interleaves) 1/10794/15/0/0 Conversation 34, linktype: ip, length: 1404
source: 192.168.3.100, destination: 192.168.1.100, id: 0x00A5, ttl: 127, TOS: 88 prot: 6, source port 80, destination port 1068
(depth/weight/total drops/no-buffer drops/interleaves) 1/10794/15/0/0 Conversation 33, linktype: ip, length: 1404
source: 192.168.3.100, destination: 192.168.1.100, id: 0x00A7, ttl: 127, TOS: 72 prot: 6, source port 80, destination port 1067
(depth/weight/total drops/no-buffer drops/interleaves) 1/32384/12/0/0 Conversation 29, linktype: ip, length: 1404
source: 192.168.3.100, destination: 192.168.1.100, id: 0x00A1, ttl: 127, TOS: 0 prot: 6, source port 1353, destination port 1065
To enable WFQ, the fair-queue interface subcommand is used. After enabling WFQ, the show interface command output shows the fact that WFQ is enabled. (See the highlighted portion of the first show interface command in Example 4-3.) Note that the hold queue default size of 1000 is shown in the show interface output as well. The show interface command and the show queueing fair commands both list the CDT (default 64), along with the maximum number of queues (256).
The most interesting show command for WFQ is the show queue command. Note that a summary section is listed first, followed by a stanza of output for each active flow. Each stanza lists statistics about the current size, number of drops, and so on. Each stanza also lists the details used for classification. For instance, each stanza of the first show queue command includes
a comment added by me. Knowing that two voice calls, one TCP connection for FTP, and two TCP connections for HTTP were being used, I could look at the source and destination addresses and ports numbers and decide which WFQ flows correlated to each of these user flows.