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This feature module describes the Low Latency Queueing feature for the Versatile Interface Processor (VIP). It includes information on the benefits of the new feature, supported platforms, related documents, and so forth.
This document includes the following sections:
Without Low Latency Queueing, CBWFQ provides weighted fair queueing based on defined classes with no priority queue available for real-time traffic. CBWFQ allows you to define traffic classes and then assign characteristics to that class. For example, you can designate the minimum bandwidth delivered to the class during congestion.
For CBWFQ, the weight for a packet belonging to a specific class is derived from the bandwidth you assigned to the class when you configured it. Therefore, the bandwidth assigned to the packets of a class determines the order in which packets are sent. All packets are serviced fairly based on weight; no class of packets may be granted priority. This approach can pose problems for voice traffic on low bandwidth links that are largely intolerant of delay, especially variation in delay. For voice traffic, variations in delay might introduce irregularities of transmission manifesting as jitter in the heard conversation.
The Low Latency Queueing feature provides priority queueing for CBWFQ, reducing jitter in voice conversations. Configured by the priority command, Low Latency Queueing enables use of a single priority queue within CBWFQ at the class level, allowing you to direct traffic belonging to a class to the CBWFQ priority queue. To enqueue class traffic to the priority queue, you configure the priority command for the class after you specify the named class within a policy map. (Classes to which the priority command is applied are considered priority classes.) Within a policy map, you can give one or more classes priority status. When multiple classes within a single policy map are configured as priority classes, all traffic from these classes is enqueued to the same, single, priority queue.
When you specify the priority command for a class, it takes a bandwidth argument that gives maximum bandwidth in kilobits per second (kbps). You use this parameter to specify the maximum amount of bandwidth allocated for packets belonging to the class configured with the priority command. The bandwidth parameter both guarantees bandwidth to the priority class and restrains the flow of packets from the priority class.
In the event of congestion, when the bandwidth is exceeded, policing is used to drop packets. Voice traffic enqueued to the priority queue is UDP-based and therefore not adaptive to the early packet drop characteristic of Weighted Random Early Detection (WRED). Because WRED is ineffective, you cannot use the WRED random-detect command with the priority command. In addition, because policing is used to drop packets and queue limit is not imposed, the queue-limit command cannot be used with the priority command.
When congestion occurs, traffic destined for the priority queue is metered to ensure that the bandwidth allocation configured for the class to which the traffic belongs is not exceeded.
Priority traffic metering has the following qualities:
With metering, the classes are policed and rate-limited individually. That is, although a single policy map might contain four priority classes, all of which are enqueued in a single priority queue, they are each treated as separate flows with separate bandwidth allocations and constraints.
It is important to note that because bandwidth for the priority class is specified as a parameter to the priority command, you cannot also configure the bandwidth command for a priority class. To do so is a configuration violation that would only introduce confusion in relation to the amount of bandwidth to allocate.
The bandwidth allocated for a priority queue always includes the Layer 2 encapsulation header. However, it does not include other headers, such as ATM cell tax overheads. When you calculate the amount of bandwidth to allocate for a given priority class, you must account for the fact the Layer 2 headers are included. When ATM is used, you must account for the fact that ATM cell tax overhead is not included. You must also allow bandwidth for the possibility of jitter introduced by routers in the voice path.
Consider this case that uses ATM. Suppose a voice stream of 60 bytes emitting 50 packets per second is encoded using G.729. Prior to converting the voice stream to cells, the meter for the priority queue used for the voice stream assesses the length of the packet after the Layer 2 LLC headers have been added.
Given the 8-byte Layer 2 LLC header, the meter will take into account a 68-byte packet. Because ATM cells are a standard 53 bytes long, before the 68-kbps packet is emitted on the line, it is divided into two 53-byte ATM cells. Thus, the bandwidth consumed by this flow is 106 bytes per packet.
For this case, then, you must configure the bandwidth to be at least 27.2 kbps (68*50*8 = 27.2 kbps). However, recall that you must also allow for the cell tax overhead, which is not accounted for by the configured bandwidth. In other words, the sum of the bandwidths for all classes must be less than the interface bandwidth by at least (106-68)*50*8 = 15.2 kbps. You should also remember to allow bandwidth for router-introduced jitter.
The priority queueing scheme allows delay-sensitive data such as voice to be dequeued and sent first---that is, before packets in other queues are dequeued. Delay-sensitive data is given preferential treatment over other traffic. This feature provides priority queueing on ATM virtual circuits (VCs);
Because you can configure the priority status for a class within CBWFQ, you are no longer limited to UDP port numbers to stipulate priority flows. Instead, all of the valid match criteria used to specify traffic for a class now applies to priority traffic.
By configuring the maximum amount of bandwidth allocated for packets belonging to a class, you can avoid starving non-priority traffic.
The Low Latency Queueing feature is related to the following features:
None
This feature supports no new MIBs.
For descriptions of supported MIBs and how to use MIBs, see the Cisco MIB web site on CCO at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.
None
To use this feature, you should be familiar with the following:
To give priority to a class within a policy map, use the following commands in the policy-map class configuration mode:
| Command | Purpose |
|---|---|
Router# show interfaces [interface-type interface-number] fair-queue | Displays information and statistics about weighted fair queuing for a VIP-based interface. |
Router# show policy-map policy map name | Displays the contents of a policy map, including the priority setting in a specific policy map. |
The priority queue is the queue whose conversation ID is equal to the number of dynamic queues plus 8. The packets in the priority queue have a weight of 0.
To tune your RTP bandwidth or decrease RTP traffic if the priority queue is experiencing drops, use one or more of the following commands:
This section provides the following configuration example:
The priority command can be enabled on an ATM subinterface, and that subinterface must have only one enabled ATM PVC. This configuration provides a sufficient amount of ATM PVC support.
In the following example, a priority queue (with a guaranteed allowed bandwidth of 50 kbps) is reserved for traffic that is sent from the source address (10.10.10.10) to the destination address (10.10.10.20), in the range of ports 16384 through 20000 and 53000 through 56000.
First, the following commands configure access list 102 to match the desired voice traffic:
router(config)# access-list 102 permit udp host 10.10.10.10 host 10.10.10.20 range 16384 20000 router(config)# access-list 102 permit udp host 10.10.10.10 host 10.10.10.20 range 53000 56000
Next, the class map voice is defined, and the policy map policy1 is created; a priority queue for the class voice is reserved, a bandwidth of 20 kbps is configured for the class bar, and the default class is configured for flow-based fair-queuing. The service-policy command then attaches the policy map to subinterface atm1/0.1:
router(config)# class-map voice router(config-cmap)# match access-group 102 router(config)# policy-map policy1 router(config-pmap)# class voice router(config-pmap-c)# priority 50 router(config-pmap)# class bar router(config-pmap-c)# bandwidth 20 router(config-pmap)# class class-default router(config-pmap-c)# fair-queue router(config)# interface atm1/0.1 router(config-subif)# service-policy output policy1
This section documents the new priority command that configures the Low Latency Queueing feature. All other commands used with this feature are documented in the Cisco IOS Release 12.0 command reference publications or Cisco IOS Release 12.0 T feature module publications.
bandwidth | Guaranteed allowed bandwidth (in kbps) for the priority traffic. Beyond the guaranteed bandwidth, the priority traffic will be dropped in the event of congestion to ensure that the nonpriority traffic is not starved. |
No default behavior or values.
Policy-map class configuration
| Release | Modification |
|---|---|
12.0(5)XE5 | This command was introduced for the Versatile Interface Processor. |
This command provides priority queueing for CBWFQ. Priority queueing allows delay-sensitive data such as voice to be dequeued and sent first (before packets in other queues are dequeued), giving delay-sensitive data preferential treatment over other traffic.
The priority command allows you to set up classes based on a variety of criteria (not just UDP ports) and assign priority to them, and is available for use on serial interfaces and ATM PVCs.
The bandwidth argument is used to specify the maximum amount of bandwidth allocated for packets belonging to a class configured with the priority command. The bandwidth parameter both guarantees bandwidth to the priority class and restrains the flow of packets from the priority class.
When the device is not congested, the priority class traffic is allowed to exceed its allocated bandwidth. When the device is congested, the priority class traffic above the allocated bandwidth is discarded.
Keep the following guidelines in mind when using the priority command:
policy-map policy1class voice1priority 24class voice2priority 48class databandwidth 20
The following example configures priority queueing with a guaranteed bandwidth of 50 kbps for the policy map "policy1":
router(config)# policy-map policy1 router(config-pmap)# class voice router(config-pmap-c)# priority 50
| Command | Description |
|---|---|
Router# show interfaces [interface-type interface-number] fair-queue | Displays information and statistics about weighted fair queuing for a VIP-based interface. |
show policy-map policy map name | Displays the contents of a policy map, including the priority setting of a specific policy map. |
CBWFQ---Class-Based Weighted Fair Queueing. Extends the standard WFQ functionality to provide support for user-defined traffic classes.
Class-Based Weighted Fair Queueing---See CBWFQ.
LLC---logical link control. Higher of the two data link layer sublayers defined by the IEEE. The LLC sublayer handles error control, flow control, framing, and MAC-sublayer addressing. The most prevalent LLC protocol is IEEE 802.2, which includes both connectionless and connection-oriented variants.
logical link control---See LLC.
PPP---Point-to-Point Protocol. Successor to SLIP that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. Whereas SLIP was designed to work with IP, PPP was designed to work with several network layer protocols, such as IP, IPX, and ARA. PPP also has built-in security mechanisms, such as CHAP and PAP. PPP relies on two protocols: LCP and NCP.
Random Early Detection---See RED.
RED---Random Early Detection. A congestion avoidance mechanism that takes advantage of TCP's congestion control mechanism. By randomly dropping packets prior to periods of high congestion, RED tells the packet source to decrease its transmission rate. Assuming the packet source is using TCP, it will decrease its transmission rate until all the packets reach their destination, indicating that the congestion is cleared.
RTP---Real-Time Transport Protocol. One of the IPv6 protocols. RTP is designed to provide end-to-end network transport functions for applications sending real-time data such as audio, video, or simulation data over multicast or unicast network services. RTP provides services such as payload type identification, sequence numbering, time-stamping, and delivery monitoring to real-time applications.
SNA---Systems Network Architecture. Large, complex, feature-rich network architecture developed in the 1970s by IBM. Similar in some respects to the OSI reference model, but with a number of differences.
UDP---User Datagram Protocol. Connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols. UDP is defined in RFC 768.
weighted fair queueing---See WFQ.
WFQ---weighted fair queueing. Congestion management algorithm that identifies conversations (in the form of traffic streams), separates packets that belong to each conversation, and ensures that capacity is shared fairly between these individual conversations. WFQ is an automatic way of stabilizing network behavior during congestion and results in increased performance and reduced retransmission.
Posted: Fri Dec 17 17:00:53 PST 1999
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