Queuing Mechanisms. Overview. Objectives

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1 Queuing Mechanisms Overview Objectives This module describes the queuing mechanisms that can be used on output interfaces. It includes the following topics: Queuing Overview FIFO Queuing Priority Queuing Custom Queuing Weighted Fair Queuing Distributed Weighted Fair Queuing Modified Deficit Round-robin IP RTP Prioritization Upon completion of this module, you will be able to perform the following tasks: Describe and configure FIFO Queuing (FQ) Describe and configure Priority Queuing (PQ) Describe and configure Custom Queuing (CQ) Describe and configure basic Weighted Fair Queuing (WFQ), distributed WFQ, ToS-based distributed WFQ and QoS-group-based distributed WFQ Describe and configure Modified Weighted Round-robin (MDRR) queuing Describe and configure IP RTP Prioritization

2 Queuing Overview Objectives Upon completion of this lesson, you will be able to perform the following tasks: Understand how queuing works on Cisco routers List the most used queuing mechanisms 3-2 Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

3 Queuing in Cisco IOS Cisco routers running Cisco IOS have a number of different queuing mechanisms This module focuses on the following: First In First Out (FIFO) Priority Queuing (PQ) Custom Queuing (CQ) Weighted Fair Queuing (WFQ) with the different distributed versions Modified Deficit Round Robin (MDRR) IP RTP Prioritization These mechnisms are implemented as software queues 2001, Cisco Systems, Inc. Queuing Mechanisms -5 The lesson discusses how output queuing mechanisms are implemented on Cisco routers running Cisco IOS. It discusses most of the queuing mechanisms in detail, except Class-based Weighted Fair Queuing and Class-based Low-latency Queuing, which are discussed in the IP QoS Modular QoS CLI (Chapter 2) module. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-3

4 Output Interface Queue Structure Forwarder Software Queuing System Hardware Queue (TxQ) Output Interface Any supported queuing mechanism Always FIFO Each Interface has its hardware and software queuing system The hardware queuing system always uses FIFO queuing (Transmission queue or TxQ) The software queuing system can be selected and configured depending on the platform and Cisco IOS version 2001, Cisco Systems, Inc. Queuing Mechanisms -6 Queuing on routers is necessary to accommodate bursts when the arrival rate of packets is greater than the departure rate due to one of the following two reasons: Input interface is faster than the output interface Output interface is receiving packets coming in from multiple other interfaces Initial implementations of queuing used a single FIFO (first-in first-out or first-come first-serve queuing) strategy. More complex queuing mechanisms were introduced when special requirements need routers to differentiate between packets of different importance. Queuing was split into two parts: The hardware queue that still uses FIFO strategy, which is necessary for the interface drivers to transmit packets one by one. The hardware queue is sometimes referred to as the transmit queue or TxQ. The software queue that schedules packets into the hardware queue based on the QoS requirements Listed on the previous graphic are some of the available software queuing strategies with their goals: FIFO: no differentiation of packets (true fairness but no guarantees) Priority Queuing (PQ): strict prioritizing of packets Custom Queuing (CQ): service (bandwidth) guaranteed to up to 16 classes Weighted Fair Queuing (WFQ) and Distributed WFQ: service (bandwidth) guarantee to individual flows Distributed ToS-based WFQ: service (bandwidth) guaranteed to up to 4 classes 3-4 Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

5 Distributed QoS-group-based WFQ: service (bandwidth) guaranteed to up to 100 classes Modified Deficit Round-robin (MDRR): service (bandwidth) guaranteed to up to 8 classes; low-delay guarantee if Strict or Alternate Priority is used IP RTP Prioritization: low-delay guarantee Most queuing mechanisms depend on the availability on different platforms and Cisco IOS versions. For example: MDRR is only available on Cisco series routers (GSR) Distributed ToS-based and QoS-group-based WFQ are only available on Cisco 7x00 series routers with Versatile Interface Processors (VIP) Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-5

6 Bypassing the Software Queue Software Queue Empty? Yes Hardware Queue Full? No Hardware Queue (TxQ) No Yes Software Queuing System When a packet is being forwarded the router will bypass the software queue if: the software queue is empty and the hardware queue is not full 2001, Cisco Systems, Inc. Queuing Mechanisms -7 The implementation of software queuing was optimized for periods when the interface is not congested. The software queuing system is bypassed whenever there is no packet in the software queue and there is room in the hardware queue. The software queue is, therefore, only used when data must wait to be placed into the hardware queue. 3-6 Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

7 Hardware Queue (TxQ) Size Routers determine the length of the hardware queue based on the configured bandwidth of the interface Long TxQ may result in poor performance of the software queue Short TxQ may result in a large number of interrupts which causes high CPU utilization and low link utilization 2001, Cisco Systems, Inc. Queuing Mechanisms -8 The double queuing strategy (software and hardware queue) has its impacts on the result of overall queuing: Software queue is used for a certain reason. If the hardware queue is too long it will contain a large number of packets scheduled in the FIFO fashion. This is probably against the QoS design that required a certain complex software queuing system (for example, Custom Queuing). So why use the hardware queue at all? Or why not just set its length to one? That would force all packets to go through the software queue and be scheduled one by one to the interface for transmission. This approach has the following drawbacks: Each time a packet is transmitted, the interface driver interrupts the CPU and requests more packets to be delivered into its hardware queue. Some queuing mechanisms have complex scheduling that takes time to deliver more packets. The interface does not send anything during that time (link utilization is decreased) if the hardware queue is empty because its maximum size is one. The CPU schedules packets one by one instead of many at the same time (in the same interrupt interval). This increases the CPU utilization. Choosing the appropriate length of the hardware queue is very important. The default TxQ size is determined by the IOS based on the bandwidth of the media and should be fine for most queuing implementations. Faster interfaces have longer hardware queues because they produce less delay. Slower interfaces have shorter hardware queues to prevent too much delay in the worst-case scenario where the entire hardware queue is full of MTU-sized packets. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-7

8 Queuing Components Forwarded Packets Software Queuing System Class 1? Add/Drop Queue 1 Class 2? Add/Drop Queue 2 Scheduler Hardware Queuing System Hardware Q Interface Class n? Add/Drop Queue n Each queuing mechanism has three main components that define it: Classification (selecting the class) Insertion policy (determining whether a packet can be enqueued) Service policy (scheduling packets to be put into the hardware queue) 2001, Cisco Systems, Inc. Queuing Mechanisms -9 The figure illustrates the actions that have to be taken before a packet can be transmitted: Most queuing mechanisms include classification of packets. Once a packet is classifie d, a router has to determine whether it can put the packet into the queue or it has to drop the packet. Most queuing mechanisms will drop a packet only if the corresponding queue is full (tail-drop). Some mechanisms use a more intelligent dropping scheme (Weighted Fair Queuing) or a random dropping scheme (Weighted Random Early Detection). If the packet is allowed to be enqueued it will be put into the FIFO queue for that particular class. Packets are then taken from the individual per-class queues and put into the hardware queue. Queuing systems differ in the following ways: Classification options: some mechanisms classify packets automatically (for example, WFQ), while other mechanisms require manual configuration of classification (for example, PQ or CQ). Insertion policy: most queuing mechanisms use the tail-dropping scheme. Scheduling policy: this is the most important part of every queuing mechanism because it determines the order in which the packets will leave the router. 3-8 Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

9 Summary After completing this lesson, you should be able to perform the following tasks: Understand how queuing works on Cisco routers List the most used queuing mechanisms Review Questions Answer the following questions: Which queuing mechanisms do Cisco routers support? When is a software queuing mechanisms not used? How does TxQ length affect the software queuing system? Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-9

10 FIFO Queuing Objectives Upon completion of this lesson, you will be able to perform the following tasks: Describe FIFO queuing Describe the drawbacks of FIFO queuing Configure FIFO queuing on Cisco routers Monitor and troubleshoot FIFO queuing 3-10 Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

11 FIFO Queuing Forwarded Packets FIFO Queuing System Hardware Queuing System All in one queue Tail-drop Queue 1 FIFO Scheduler Hardware Q Interface Routers serve packets in the first-come first-serve fashion FIFO uses one single queue All packets are classified into one class Newly arriving packets are dropped if the queue is full Software FIFO queue is basically an extension to the hardware FIFO queue 2001, Cisco Systems, Inc. Queuing Mechanisms-14 FIFO queuing has no classification because all packets belong to the same class. Packets are dropped when the output queue is full (tail-drop). The scheduler services packets in the order they arrived. Software FIFO queue is basically an extension of the hardware FIFO queue. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-11

12 Benefits and Drawbacks of FIFO Queuing + Benefits Simple and fast (one single queue with a simple scheduling mechanism) Supported on all platforms Supported in all switching paths Supported in all IOS versions Drawbacks Unfair allocation of bandwidth among multiple flows Causes starvation (aggressive flows can monopolize links) Causes jitter (bursts or packet trains temporarily fill the queue) 2001, Cisco Systems, Inc. Queuing Mechanisms-15 FIFO queuing might be regarded as the fairest queuing mechanism but it has a long list of drawbacks: FIFO does not fairly allocate bandwidth among multiple flows. Some flows receive more bandwidth because they use larger packets or send more packets. FIFO is extremely unfair when an aggressive flow is contesting with a fragile flow. Aggressive flows send a large number of packets, many of which are dropped. Fragile flows send a modest amount of packets and most of them are dropped because the queue is always full due to the aggressive flow. This type of behavior is called starvation. Short or long bursts cause a FIFO queue to fill. Packets entering an almost full queue have to wait a long time before they can be transmitted. Another time, the queue might be empty causing packets of the same flow to experience almost no delay. Variation in delay is called jitter. In spite of all the drawbacks FIFO is still the most used queuing mechanism because of the following benefits: It is simple and fast. Most high-end routers with fast interfaces are not really challenged by the drawbacks mentioned earlier. Furthermore, routers are not capable of complex classification and scheduling when they have to process a large number of packets per second. FIFO is, therefore, the most suitable queuing mechanisms on these platforms. It is supported on all platforms. It is supported in all IOS versions Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

13 Configuring FIFO Queuing Router(config-if)# no fair-queue FIFO queuing is enabled by default on all interfaces that have a default bandwidth of more than 2Mbsp Weighted Fair Queuing is enabled if the bandwidth is less than 2Mbps Disable WFQ to enable FIFO on interfaces that have less than 2Mbps of bandwidth 2001, Cisco Systems, Inc. Queuing Mechanisms-16 Cisco routers have two default queuing mechanisms: All interfaces with the default bandwidth above 2Mbps use FIFO queuing. No configuration is necessary on such interfaces. All interfaces with the default bandwidth below 2Mbps use Weighted Fair Queuing (WFQ). The no fair-queue command must be used to disable WFQ and enable FIFO. There is no special command that specifically enables FIFO. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-13

14 Configuring FIFO Queuing Router(config-if)# hold-queue <buffers> out FIFO queuing allows a maximum of 40 packets to be stored in the output queue This command can be used to increase or decrease the maximum number of buffered packets A large value can be set to support longer bursts (less drops, more buffer usage) A small value can be set to prevent bursts (more drops) 2001, Cisco Systems, Inc. Queuing Mechanisms-17 One of the considerations when using FIFO queuing is the maximum burst size. Routers allow (by default) up to 40 packets to be in the output queue. Shortening the queue causes more drops, especially for bursty sessions. Lengthening the queue allows more packets to be enqueued. A long queue should be used to allow bursts without drops. The hold-queue command is used to set the maximum number of packets in the output queue Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

15 FIFO Example interface Ethernet0/0 ip address ! interface Serial0/0 ip address no fair-queue hold-queue 50 out! The Ethernet interface has a default bandwidth of 10Mbps. FIFO is the default queuing and it does not need to be configured. The serial interface (A/S) has a default bandwidth of 128 kbps. WFQ is the default queuing and it has to be disabled to enable FIFO queuing. Up to 50 frames are allowed to be enqueued before the router will start tail-dropping newely arriving packets. 2001, Cisco Systems, Inc. Queuing Mechanisms-18 The example shows how FIFO can be enabled on an interface that uses WFQ by default. The serial interface in question has the default bandwidth of 128 kbps (below 2 Mbps). The ethernet interface has the default bandwidth of 10 Mbps (above 2 Mbps) and requires no configuration. The maximum output queue size was also slightly increased from the default 40 to 50. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-15

16 Monitoring and Troubleshooting FIFO Router# show interface [<interface>] The command displays information about the selected interface(s) Router#show interface Serial0/0 The queue is currently empty (0/50). Serial0/0 is up, line protocol is up There can be a maximum of 50 frames in the Hardware is PowerQUICC Serial queue (0/50). Internet address is /8 MTU 1500 bytes, BW 128 Kbit, DLY usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation HDLC, loopback not set Keepalive set (10 sec) FIFO queuing is enabled Last input 00:00:02, output 00:00:04, output hang neveron an interface with the Last clearing of "show interface" counters never default bandwidth of Queueing strategy: fifo 128kbps. Output queue 0/50, 0 drops; input queue 0/75, 0 drops 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 2001, Cisco Systems, Inc. Queuing Mechanisms-19 FIFO queuing is not supported by a large arsenal of show and debug commands. The show interface command can be used to determine the queuing strategy of an interface and to display the following statistics: The current queue size (buffer usage) The maximum queue size (default 40 or whatever is configured with the hold-queue out command) 3-16 Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

17 Summary FIFO queuing is the oldest queuing mechanism in the Cisco IOS. It is used on fast interfaces because of its simplicity and speed. FIFO produces undesirable behavior on congested (low-speed) interfaces that manifest itself as: Unfair allocation of bandwidth Starvation of less-aggressive flows Delay Jitter FIFO is the default queuing mechanism on all interfaces that have the default bandwidth of more than 2 Mbps. FIFO queuing can be enabled on interfaces with the default bandwidth of 2 Mbps or less by disabling WFQ. Review Questions Answer the following questions: Why is FIFO the fastest queuing mechanism? Describe the classification and scheduling of FIFO queuing. List the drawbacks of FIFO queuing. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-17

18 Priority Queuing Objectives Upon completion of this lesson, you will be able to perform the following tasks: Describe Priority Queuing Describe the benefits and drawbacks of Priority Queuing Configure Priority Queuing on Cisco routers Monitor and troubleshoot Priority Queuing 3-18 Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

19 Priority Queuing Forwarded Packets Priority Queuing System High? Tail-drop Queue 1 Medium? Tail-drop Queue 2 Pre-emptive Scheduler Hardware Queuing System Hardware Q Interface Normal? Tail-drop Queue 3 Low? Tail-drop Queue 4 Priority Queuing (PQ) uses four FIFO queues 2001, Cisco Systems, Inc. Queuing Mechanisms-24 Priority Queuing (PQ) is one of the first mechanisms that allowed classification of packets into multiple classes. Scheduling is done in strict priority. PQ can classify packets into one of the four queues: High queue Medium queue Normal queue (the default queue) Low queue Scheduling prefers packets in the same order. Each class uses one FIFO queue, where packets are dropped if a queue is full. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-19

20 Priority Queuing Classification Priority Queuing classification for IP supports the following options: Source interface IP access list (standard and extended) Packet size (greater or smaller than specified) Fragments TCP source or destination port numbers UDP source or destination port numbers 2001, Cisco Systems, Inc. Queuing Mechanisms-25 Priority Queuing can classify IP packets with the following tools: Direct matching on the source interface. Standard or extended IP Access list. Extended IP access lists support matching on the following parameters: Source IP address Destination IP address Source TCP or UDP port number or port range Destination TCP or UDP port number or port range IP precedence (high-order three bits of the ToS field) DSCP (high-order six bits of the ToS field) ToS value (bits one through four of the ToS field) Fragments TCP flags (ACK, SYN, RST, URG, PSH) Direct matching of TCP or UDP source and destination port numbers. Direct matching of fragments. Direct matching of packets based on their size Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

21 Priority Queuing Classification Priority Queuing also supports classification of other protocols with the following options: Protocol-specific access list (if available for the specified protocol) Packet size (greater or smaller than specified) Some of the supported protocols are: IPX CLNS DECNET AppleTalk VINES DLSw 2001, Cisco Systems, Inc. Queuing Mechanisms-26 Priority Queuing is a multi-protocol QoS mechanism because it supports classification tools for other (non-ip) protocols. The figure lists the match options for some of the supported Layer-3 protocols as well as other higher-layer protocols. Although other protocols are supported, the classification options are not as powerful as with IP. Most protocols can use their corresponding access lists to classify packets. Matching on packet size is also available with all supported protocols. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-21

22 Priority Queuing Insertion Policy Each queue has a maximum number of packets that it can hold (queue size). After a packet is classified to one of the following queues the router will enqueue the packet if the queue limit has not been reached (tail-drop within each class). 2001, Cisco Systems, Inc. Queuing Mechanisms-27 Priority Queuing is basically a collection of four parallel FIFO queues. Each queue suffers from all FIFO problems isolated to the class (unfair, starvation, delay, jitter). Each queue uses the tail-drop scheme when the queue is full. Each of the four queues can be configured with the maximum number of packets that it can hold Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

23 Priority Queuing Scheduling Packet in HIGH queue? Yes No Packet in MEDIUM queue? No Yes Packet in NORMAL queue? No Yes Packet in LOW queue? No Yes Dispatch Packet And start checking the HIGH queue again Hardware Q 2001, Cisco Systems, Inc. Queuing Mechanisms-28 Priority Queuing uses strict priority scheduling. As long as there are packets in the high queue no other queue will be served. If the high queue is empty the router starts serving the medium queue. Congestion in any of the queues, except the low queue, causes a different type of starvation. A congested higher-priority queue causes all lower-priority queues to starve (class starvation). Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-23

24 + Benefits Benefits and Drawbacks of Priority Queuing Provides low-delay propagation to high-priority packets Supported on most platforms Supported in all IOS versions (above 10.0) Drawbacks All drawbacks of FIFO queuing within a single class Starvation of lower-priority classes when higherpriority classes are congested Manual configuration of classification on every hop 2001, Cisco Systems, Inc. Queuing Mechanisms-29 As mentioned previously, Priority Queuing suffers from the same drawbacks as FIFO queuing, except it is localized to four classes. Each class can experience starvation, delay and jitter if one or more flows in the class cause congestion. Furthermore, one higher-priority queue can cause all other queues to starve if it is congested. Priority Queuing requires manual configuration of classification. The main benefit of PQ is that it enables the user to create a class that is used for applications that require low delay (high queue) Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

25 Configuring Priority Queuing Configure priority lists Configure classification Select a queue Set maximum queue size Apply the priority list to outbound traffic on an interface 2001, Cisco Systems, Inc. Queuing Mechanisms-30 The configuration of Priority Queuing can be split into the following four steps: 1. Classify data into four classes 2. Assign a queue to each class 3. Set the maximum queue size (if the default is not appropriate) 4. Apply the priority queuing system to one or more interfaces Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-25

26 Priority Queuing Classification Router(config)# priority-list list-number protocol protocol-name {high medium normal low} queue-keyword keyword-value Selects the queue based on layer-3 protocol Additional classification (queue-keyword): fragment (IP packets with non-zero fragment offset) gt/lt <size>: based on packet size (including L2 frame) list <acl>: ACL classification tcp/udp <port>: TCP or UDP port number System and link-level messages are classified in high by default 2001, Cisco Systems, Inc. Queuing Mechanisms -31 The first three configuration steps are achieved using the priority-list command. A Priority Queuing system is identified with a common number (list-number). Priority Queuing supports the following direct classification options of IP packets: 1. Match fragments 2. Match packets based on their size 3. Match packets based on their source or destination TCP/UDP port number A far more powerful classification tool is an access list (standard or extended) Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

27 Priority Queuing Classification Router(config)# priority-list list-number interface intf {high medium normal low} Classifies the packet based on incoming interface Router(config)# priority-list list-number default {high medium normal low} Classifies all unclassified packets in a default queue 2001, Cisco Systems, Inc. Queuing Mechanisms-32 Additionally, PQ supports classification based on source interface. By default, all traffic not specifically classified goes into the normal queue. This behavior can be changed by using the priority-list default command. Note The priority-list commands are evaluated in the order they were entered. This is especially important when overlapping classification is configured for separate queues. For example: Line 1: all IP traffic goes into the high priority queue Line 2: all TCP traffic goes into the medium queue The medium queue in this example would never get any packets because it appears second in the configuration and it is a subset of the first line. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-27

28 Priority Queuing Scheduling and Dropping Parameters Router(config)# priority-list list-number queue-limit high medium normal low Specifies the maximum queue sizes of individual priority queues Router(config-if)# priority-group list Assigns Priority Queuing definition to an interface 2001, Cisco Systems, Inc. Queuing Mechanisms-33 Priority Queuing uses the following default maximum queue sizes for the four queues: High queue has a default queue limit of 20 Medium queue has a default queue limit of 40 Normal queue has a default queue limit of 60 Low queue has a default queue limit of 80 The last configuration step is to apply a priority-list to an interface. Use the priority-group command with a corresponding priority-list number to enable Priority Queuing on an interface Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

29 Sample PQ Configuration E0 WAN core E1 Core interface serial0 Branch priority-group 1 office priority-list 1 protocol ip high list 101 priority-list 1 interface ethernet 0 medium priority-list 1 default normal priority-list 1 queue-limit access-list 101 permit tcp any any eq , Cisco Systems, Inc. Queuing Mechanisms-34 The figure illustrates a simple example where outbound traffic is classified into the following three classes: 1. All outbound telnet sessions (access list 101) are using the high priority queue 2. All traffic coming into the router via interface Ethernet 0 is forwarded through the medium queue 3. All other traffic is using the default normal queue Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-29

30 Monitoring Priority Queuing Router# show interface interface Displays information and statistics about queuing on interface Router# show queueing [priority custom fair random-detect] interface Displays queuing parameters on interfaces Router# show queue interface Displays queue contents 2001, Cisco Systems, Inc. Queuing Mechanisms-35 The show interface command can be used to determine the queuing strategy of an interface. If the queuing strategy is PQ some statistics are also displayed. The show queueing priority command can be used to display all non-default parameters of priority lists. Note To use the show queueing command, you must enter at least the first six characters to differentiate the command (show queuei vs. show queue) Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

31 Show Interface Router#show interface serial 1/0 Serial1/0 is up, line protocol is up Hardware is M4T Internet address is /8 MTU 1500 bytes, BW 19 Kbit, DLY usec, rely 255/255, load 93/255 Encapsulation HDLC, crc 16, loopback not set Keepalive set (10 sec) Last input 00:00:00, output 00:00:00, output hang never Last clearing of "show interface" counters never Input queue: 0/75/0 (size/max/drops); Total output drops: 0 Queueing strategy: priority-list 1 Output queue (queue priority: size/max/drops): high: 0/20/0, medium: 0/40/0, normal: 0/60/0, low: 0/80/0 5 minute input rate bits/sec, 8 packets/sec 5 minute output rate 7000 bits/sec, 8 packets/sec rest ignored , Cisco Systems, Inc. Queuing Mechanisms-36 The show interface command displays the parameters and the statistics of all four priority queues. The first parameter is the current size of the queue, the second is the maximum allowed size of the queue and the third parameter is the number of drops since the last clearing of counters. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-31

32 Show Queuing Priority Router#show queueing priority Current priority queue configuration: List Queue Args 1 high protocol ip list medium interface Ethernet6/0 The show queueing priority command displays only the non-default parameters 2001, Cisco Systems, Inc. Queuing Mechanisms-37 The show queueing priority command lists all non-default parameters. The figure shows the two parameters: All packets permitted by access list 101 go into the high queue All packets coming from interface Ethernet6/0 go into the medium queue The commands that set default parameters are not displayed, either in the running configuration or by using this command Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

33 Summary Priority Queuing uses four FIFO queues. Strict priority queuing is used. Starvation within a single class or starvation of lower-priority classes is possible when one flow congests a higher-priority queue. Priority queuing can be used to guarantee all the bandwidth and low-delay propagation. Review Questions Answer the following questions: When would you use priority queuing? What are the benefits and drawbacks of priority queuing? How many classes does priority queuing support? How does priority queuing schedule packets? Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-33

34 Custom Queuing Objectives Upon completion of this lesson, you will be able to perform the following tasks: Describe Custom Queuing Describe the benefits and drawbacks of Custom Queuing Configure Custom Queuing on Cisco routers Monitor and troubleshoot Custom Queuing 3-34 Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

35 Custom Queuing Forwarded Packets Custom Queuing System Class 1? Tail-drop Queue 1 Class 2? Tail-drop Queue 2 Round Robin Scheduler Hardware Queuing System Hardware Q Interface Class 16? Tail-drop Queue 16 Custom Queuing (CQ) uses 16 FIFO queues for user defined traffic classes 2001, Cisco Systems, Inc. Queuing Mechanisms-42 Custom Queuing (CQ) is similar to Priority Queuing in the way it is configured and in the supported classification options. The scheduling, however, is completely different. CQ uses up to 16 queues that can be used for user-defined classes. The classification options are identical to those of Priority Queuing. The scheduling mechanism uses the round-robin service where each queue is allowed to forward a certain number of bytes (not packets). Tail-drop is still used within each individual queue. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-35

36 Custom Queuing Classification Custom Queuing classification for IP supports the following options: Source interface IP access list (standard and extended) Packet size (greater or smaller than specified) Fragments TCP source or destination port numbers UDP source or destination port numbers Custom Queuing classification is identical to that of Priority Queuing 2001, Cisco Systems, Inc. Queuing Mechanisms-43 Custom Queuing (similar to Priority Queuing) can classify IP packets with the following tools: Direct matching on the source interface. Standard or extended IP Access list. Extended IP access lists support matching on the following parameters: Source IP address Destination IP address Source TCP or UDP port number or port range Destination TCP or UDP port number or port range IP precedence (high-order three bits of the ToS field) DSCP (high-order six bits of the ToS field) ToS value (bits one through four of the ToS field) Fragments TCP flags (ACK, SYN, RST, URG, PSH) Direct matching of TCP or UDP source and destination port numbers. Direct matching of fragments. Direct matching of packets based on their size Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

37 Custom Queuing Insertion Policy Each queue has a maximum number of packets that it can hold (queue size). After a packet is classified to one of the following queues the router will enqueue the packet if the queue limit has not been reached (tail-drop within each class). 2001, Cisco Systems, Inc. Queuing Mechanisms-44 Once the packet is classified a router has to determine if the packet can be enqueued. The packet is dropped if the queue is full. Each queue, unless configured otherwise, can buffer up to 20 packets before the first packet is dropped. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-37

38 Custom Queuing Scheduling No Packet in Queue N? No Next Queue (increase N) Yes Is Queue N over the threshold? Yes Dispatch Packet Hardware Q Custom Queuing uses round-robin service policy Each queue is allowed to forward a configurable amount of bytes (threshold) in one round 2001, Cisco Systems, Inc. Queuing Mechanisms-45 Custom Queuing uses round-robin scheduling, where each queue gets some service (bandwidth). Each queue is configured with the number of bytes (byte-count) it can send in one round. The last packet is always sent, even if the total amount of bytes sent in one round is above the limit (byte-count). The router then starts processing the next queue Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

39 Custom Queuing Scheduling Parameters Threshold (byte-count) = 3000 Up to 4499 bytes can be forwarded in one round in the worst case The threshold (byte count) parameter specifies the lower boundary on how many bytes the system allows to be delivered from a given queue during a particular cycle The router is allowed to send the entire packet even if the sum of all bytes is more than the threshold 2001, Cisco Systems, Inc. Queuing Mechanisms-46 The figure illustrates the worst case scenario where the following parameters were used to implement Custom Queuing on an interface: MTU of the interface is 1500 bytes Byte-count is 3000 (twice the MTU) The example shows how the router first sent two packets with a total size of 2999 bytes. Since this is still within the limit (3000) the router can send the next packet (MTU-sized). The result was that the queue received almost 50% more bandwidth in this round than it should. This is one of the drawbacks of Custom Queuing it does not allocate bandwidth accurately. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-39

40 CQ Design Guideline Configure the amount to remove from a queue in each round to configure the proportional weight of the queue Amounts to remove should approximate a small multiple of the interface s MTU Ratio between largest and smallest queue should be a small positive integer, not more than 10:1 2001, Cisco Systems, Inc. Queuing Mechanisms-47 The limit or weight of the queue is configured in bytes. The accuracy of Custom Queuing depends on the weight (byte-count) and the MTU. If the ratio between the byte-count and the MTU is too small CQ will not allocate bandwidth accurately. If the ratio between the byte-count and the MTU is too large CQ will cause long delays. This problem is discussed in detail on the next two pages Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

41 Delay vs. Bandwidth Allocation Queue Queue 2 Round Robin Scheduler 64 kbps MTU=1500 Queue BW (Queue 1) = bc1/(bc1+bc2+bc3) = 4500/9000 = 50% Delay (Queue 1) = (bc2+bc3)/bandwidth = 562ms Worst-case Delay (Queue 1) = ((bc2+1499) +(bc3+1499))/bandwidth = 937ms 2001, Cisco Systems, Inc. Queuing Mechanisms-48 The figure illustrates sample calculations of bandwidth guarantees and the maximum delay. The time it takes to complete a round depends on the bandwidth of the interface, the MTU size and the sum of all queue byte-counts. The case study parameters are: The first queue uses a byte-count of 4500 (three times the MTU) 5999 bytes can be sent in the worst case The second queue uses a byte-count of 3000 (two times the MTU) 4499 bytes can be sent in the worst case The third queue uses byte-count 1500 (MTU) 2999 bytes can be sent in the worst case The first calculation shows that the first queue should receive approximately 50% of the bandwidth. The second calculation shows the round-robin delay of 562ms for Queue 1 when all classes are congested. The third calculation shows the round-robin delay of 937ms for Queue 1 when all classes are congested and manage to send the maximum number of bytes (byte-count + MTU - 1) in one round. Although this worst case is very unlikely it is also unlikely that classes will use the exact configured maximum. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-41

42 Worst-case Delay MTU=1500, byte-count (4500, 3000, 1500) Max(delay)=( )*8/64000=1312 ms MTU=1000, byte-count (4500, 3000, 1500) Max(delay)=( )*8/64000=1187 ms MTU=250, byte-count (450, 300, 150) Max(delay)=( )*8/64000=156 ms Expected delay=( )*8/64000=125 ms Custom queuing is not appropriate for lowdelay environment. Changing MTU and bytecounts might be a workaround. 2001, Cisco Systems, Inc. Queuing Mechanisms-49 The figure shows several calculations where the worst-case maximum delay was reduced by reducing both the MTU and the byte-counts. Note The calculation merely shows the impact the MTU and the byte-count have on the delay. Lowering the MTU is not a recommended solution because it potentially increases the CPU utilization of the router due to fragmentation of packets Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

43 + Benefits Benefits and Drawbacks of Custom Queuing Guarantees throughput to traffic classes (prevents starvation between traffic classes) Supported on most platforms Supported in all IOS versions (above 10.0) Drawbacks All drawbacks of FIFO queuing within a single class Manual configuration of classification on every hop Not accurate bandwidth allocation High jitter due to implementation of scheduling 2001, Cisco Systems, Inc. Queuing Mechanisms-50 In addition to all the benefits and drawbacks of Priority Queuing, Custom Queuing can also guarantee bandwidth to up to 16 classes. Custom Queuing can cause all queues to experience delay due to the implementation of scheduling (one round can take a long time). Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-43

44 Custom Queuing Classification Router(config)# queue-list list-number protocol protocol-name queue-number queue-keyword keyword-value Classifies the packet into a custom queue based on protocol and other protocol-specific criteria Selection criteria identical to priority queuing Router(config)# queue-list list-number interface incoming-intf queue-number Classifies the packet into a custom queue based on incoming interface 2001, Cisco Systems, Inc. Queuing Mechanisms-51 Custom queuing uses the same classification options as Priority Queuing. Instead of using names queues are numbered (1 to 16) Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

45 Custom Queuing Classification Router(config)# queue-list list-number default queue-number Classifies all unclassified packets into a default queue Router(config-if)# custom-queue list-number Starts custom queuing on an interface and assigns specified CQ definition to the interface 2001, Cisco Systems, Inc. Queuing Mechanisms-52 All traffic that is not specifically classified is put into Queue 1. Use the queue -list default command to change the default queue. Use the custom-queue interface command to apply a queue-list to an interface. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-45

46 Custom Queuing Scheduling Parameters Router(config)# queue-list list queue queue-number byte-count bc Specifies the lower boundary on how many bytes the system allows to be delivered from a given queue during one round-robin cycle Default: 1500 bytes Router(config)# queue-list list queue queue-number limit limit Specifies the maximum number of packets in a queue Incoming packets are tail-dropped if the limit is exceeded 2001, Cisco Systems, Inc. Queuing Mechanisms-53 Use the byte-count option to change the default weight of a queue (default equals MTU size) Use the limit option to change the number of packets that a queue can hold (default is 20) 3-46 Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

47 Custom Queuing with Preemptive Queues Forwarded Packets Custom Queuing System Class 0? Tail-drop Queue 0 Custom Queuing has queue 0 for system and link-level messages which use pre-emptive scheduling Class 1? Tail-drop Queue 1 Hardware Queuing System Class 2? Tail-drop Queue 2 Round Robin Scheduler Pre -emptive Scheduler Hardware Q Intf Class 16? Tail-drop Queue 16 Queue 1 is the lowest custom queue that is serviced by the round robin scheduler 2001, Cisco Systems, Inc. Queuing Mechanisms-54 Custom queuing has another queue Queue 0. This queue is used for system packets (routing protocols, link-level messages). This queue is not served by the round-robin scheduler. Instead, a strict priority scheduler is used to prioritize packets from this queue. Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-47

48 Custom Queuing with Preemptive Queues Forwarded Packets Custom Queuing System Class 0? Tail-drop Queue 0 Custom queues can be configured to use the pre-emptive scheduler Class 1? Tail-drop Queue 1 Hardware Queuing System Class 2? Tail-drop Queue 2 Pre -emptive Scheduler Hardware Q Intf Round Robin Scheduler Queue 2 is now the lowest custom queue that is serviced by the round robin scheduler Class 16? Tail-drop Queue , Cisco Systems, Inc. Queuing Mechanisms-55 The strict priority scheduler can be extended to other queues that are normally served by the round-robin scheduler. The figure illustrates how Queue 1 was moved into the priority-scheduled part of the Custom Queuing system. The delimiter can be set to any queue by specifying the lowest custom queue (Queue 2 in this example). In fact, Custom Queuing can be turned into Priority Queuing with 17 queues if Queue 16 is selected as the lowest custom queue Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

49 Custom Queuing Scheduling Parameters Router(config)# queue-list list-number lowest-custom queue-number Set the lowest queue to be treated as custom queue Queues below the specified queue are pre-emptive priority queues (Q1 having highest priority) Queue 0 is always treated as pre-emptive System and link-level messages are classified in Q0 by default 2001, Cisco Systems, Inc. Queuing Mechanisms-56 Use the lowest-custom option to achieve the following: All queues from Queue 0 to the queue before the one specified in the command use priority queuing (Queue 0 has the highest priority) All queues from the one specified in the command to Queue 16 use the round-robin scheduler Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-49

50 Custom Queuing Example E0 WAN core E1 Branch office interface serial 1/0 custom-queue-list 5 Core! queue-list 5 protocol ip 1 list 101 queue-list 5 queue 1 limit 40 queue-list 5 lowest-custom 2 queue-list 5 interface ethernet 0/0 2 queue-list 5 queue 2 byte-count 3000 queue-list 5 protocol ip 3 queue-list 5 queue 3 byte-count 5000 queue-list 5 default 4! access-list 101 permit ip any any precedence , Cisco Systems, Inc. Queuing Mechanisms-57 The figure shows a sample configuration where four queues are used: Queue 1 is used for delay-sensitive applications (marked with IP precedence 5). It uses the strict priority scheduler. Queue 2 is used for all packets coming from interface Ethernet0/0. Queue 3 is used for all IP packets that do not end in one of the first two queues. Queue 4 is used for all other traffic Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

51 Custom Queuing - Show Interface Router#show interface serial 1/0 Serial1/0 is up, line protocol is up Hardware is M4T Internet address is /8 MTU 1500 bytes, BW 19 Kbit, DLY usec, rely 255/255, load 107/255 Encapsulation HDLC, crc 16, loopback not set Keepalive set (10 sec) Last input 00:00:00, output 00:00:00, output hang never Last clearing of "show interface" counters never Input queue: 0/75/0 (size/max/drops); Total output drops: 0 Queueing strategy: custom-list 5 Output queues: (queue #: size/max/drops) 0: 0/20/0 1: 0/40/0 2: 0/20/0 3: 0/20/0 4: 0/20/0 5: 0/20/0 6: 0/20/0 7: 0/20/0 8: 0/20/0 9: 0/20/0 10: 0/20/0 11: 0/20/0 12: 0/20/0 13: 0/20/0 14: 0/20/0 15: 0/20/0 16: 0/20/0 rest ignored , Cisco Systems, Inc. Queuing Mechanisms-58 The show interface command is used to determine the queuing strategy of an interface. If custom queuing is used on an interface the following information is also displayed: The number of the queue-list Statistics for each of the 17 queues: Current size Maximum size Total number of drops since the last clearing of counters Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-51

52 Show Queueing Custom Router#show queueing custom Current custom queue configuration: List Queue Args 5 3 default 5 1 protocol ip list interface Ethernet0/0 5 1 byte-count 3000 limit byte-count 5000 The show queueing custom command displays only the non-default parameters 2001, Cisco Systems, Inc. Queuing Mechanisms-59 The show queueing custom command can be used to display all non-default parameters of Custom Queuing Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

53 Summary Custom Queuing introduces a scheduler that can guarantee bandwidth to 16 classes. In addition to the round-robin scheduling between 16 classes, a number of classes can be switched to priority scheduling. Review Questions Answer the following questions: When would you use custom queuing? What are the benefits and drawbacks of custom queuing? How many classes does custom queuing support? How does custom queuing schedule packets? Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-53

54 Weighted Fair Queuing Objectives Upon completion of this lesson, you will be able to perform the following tasks: Describe WFQ Describe the benefits and drawbacks of WFQ Configure WFQ on Cisco routers Monitor and troubleshoot WFQ 3-54 Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

55 Weighted Fair Queuing Queuing algorithm should fairly share the bandwidth among flows by: reducing response time for interactive flows by scheduling them to the front of the queue preventing high volume conversations from monopolizing an interface Implementation: Messages are sorted into conversations (flows) and transmitted by the order of the last bit crossing its channel Unfairness is reinstated by introducing weight (IP precedence) to give proportionately more bandwidth to flows with higher weight 2001, Cisco Systems, Inc. Queuing Mechanisms-64 Weighted Fair Queuing (WFQ) was introduced as a solution to the problems of the following queuing mechanisms: FIFO queuing causes starvation, delay and jitter PQ causes starvation of other lower-priority classes and suffers from all FIFO problems within each of the four queues CQ causes long delays and also suffers from all FIFO problems within each of the 16 queues The idea of WFQ is to: Have a dedicated queue for each flow (no starvation, delay or jitter within the queue) Fairly and accurately allocate bandwidth among all flows (minimum scheduling delay, guaranteed service) Use IP precedence as weight when allocating bandwidth Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-55

56 Weighted Fair Queuing Forwarded Packets Weighted Fair Queuing System Flow 1? WFQ-drop Queue 1 Flow 2? WFQ-drop Queue 2 WFQ Scheduler Hardware Queuing System Hardware Q Interface Flow N? WFQ-drop Queue N WFQ uses per-flow FIFO queues 2001, Cisco Systems, Inc. Queuing Mechanisms-65 WFQ uses automatic classification. Manually defined classes are not supported. WFQ dropping is not a simple tail-drop. WFQ drops packets of the most aggressive flows. WFQ scheduler is a simulation of a TDM system (time-division multiplexer). The bandwidth is equally distributed to all active flows Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

57 Weighted Fair Queuing Implementations Implementation parameters Queuing platform: central CPU or VIP Classification mechanism Weighted fairness Modified Tail-Drop within each queue 2001, Cisco Systems, Inc. Queuing Mechanisms-66 WFQ is supported on most Cisco routers as well as Versatile Interface Processors (VIP). The implementation on the VIP slightly differs from the one discussed in this lesson. Classification identifies a flow and assigns a queue to the flow Weight is used for scheduling to give proportionately more bandwidth to flows with a higher IP precedence Tail-dropping scheme is improved to drop packets of the most aggressive flows Copyright 2001, Cisco Systems, Inc. Queuing Mechanisms 3-57

58 WFQ Classification Src. Addr. IP TCP Payload Dst. Proto. ToS Src. Addr. Port Hash Algorithm Dst. Port WFQ Classification uses the following parameters: source IP address destination IP address source TCP or UDP port destination TCP or UDP port transport protocol type of service (ToS) field A hash algorithm is used to produce the index of the queue where the packet is enqueued #queue (Index of the queue) Packets of the same flow end up in the same queue ToS field is the only parameter that might change causing packets of the same flow to end up in different queues 2001, Cisco Systems, Inc. Queuing Mechanisms-67 WFQ classification has to identify individual flows (the term conversation is also used to signify flows). A flow is identified based on the following information taken from the IP header and the TCP or UDP headers: Source IP address Destination IP address Protocol number (identifying TCP or UDP) Type of Service Field Source TCP/UDP port number Destination TCP/UDP port number All these parameters are usually fixed for a single flow, although there are some exceptions: A QoS design could mark packets with different IP precedence values even if they belong to the same flow. This kind of behavior should be avoided when using WFQ. Some applications change port numbers (for example, TFTP). If packets of the same flow do not have the same parameters (for example, a different ToS field) the packets can end up in different queues and reordering can occur. The parameters are used as input for a hash algorithm that produces a fixed-length number that is used as the index of the queue Queuing Mechanisms Copyright 2001, Cisco Systems, Inc.

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