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Cisco IOS Quality of Service (QoS) software includes four types of traffic shaping:
All four traffic shaping methods are similar in implementation, though their command line interfaces (CLIs) differ somewhat and they use different types of queues to contain and shape traffic that is deferred. If a packet is deferred, GTS and Class-Based Shaping use a weighted fair queue to hold the delayed traffic. FRTS uses either a custom queue or a priority queue.
This section explains how traffic shaping works and describes the Cisco IOS QoS traffic shaping mechanisms. It also described traffic-shaping feature called Low Latency Traffic Shaping (LLQ).
This section contains the following topics:
Traffic shaping allows you to control outgoing traffic on an interface to match the traffic speed of the remote target interface and to ensure that the traffic conforms to specific policies. Traffic that adheres to a particular profile can be shaped to meet downstream requirements, thereby eliminating bottlenecks in topologies caused by data-rate mismatches.
The primary reasons to use traffic shaping are to control access to available bandwidth, to ensure that traffic conforms to specific policies, and to regulate the flow of traffic in order to avoid congestion. Some example reasons for using traffic shaping follow:
A similar, more complicated case would be a link-layer network giving indications of congestion with differing access rates on different attached data terminal equipment (DTE) devices. The network may be able to deliver more transit speed to a given DTE device at a specific time than at another time.
Traffic shaping prevents packet loss. Its use is especially important in Frame Relay networks because the switch cannot determine which packets take precedence or which packets should be dropped when congestion occurs.
Traffic shaping limits the rate of transmission of data. You can limit the data transfer to one of the following:
The rate of transfer depends on three components that constitute the token bucket: burst size, mean rate, and measurement (time) interval. The mean rate is equal to the burst size divided by the interval.
When traffic shaping is enabled, the bit rate of the interface does not exceed the mean rate over any integral multiple of the interval. During every interval, the burst size is usually the maximum number of bits that can be sent. Within the interval, however, the bit rate may be faster than the mean rate at any given time.
One additional variable applies to traffic shaping: Excess Burst Size (called the Be size). The Be Size corresponds to the number of noncommitted bits—those outside the committed information rate (CIR)—that are still accepted by the Frame Relay switch but are marked as discard eligible (DE).
The Be size allows more than the burst size to be sent during a time interval in certain situations. The switch allows the packets belonging to the Excess Burst to go through but it will mark them by setting the DE bit. Whether the packets are sent depends on how the switch is configured.
When the Be size equals 0, the interface sends no more than the burst size every interval, achieving an average rate no higher than the mean rate. However, when the Be size is greater than 0, the interface can send as many as Bc + Be bits in a burst, if the maximum amount was not sent in a previous time period. Whenever the number of bits sent during an interval is less than the burst size, the remaining number of bits can be sent in a later interval.
You can specify which Frame Relay packets have low priority or low time sensitivity. These packets are the first to be dropped when a Frame Relay switch is congested. The Discard Eligible (DE) bit allows a Frame Relay switch to identify such packets.
You can define DE lists that identify the characteristics of packets to be eligible for discarding, and you can also specify DE groups to identify the data-link connection identifier (DLCI) that is affected.
You can specify DE lists based on the protocol or the interface. You can also specify DE lists that are based on characteristics such as fragmentation of the packet, a specific TCP or User Datagram Protocol (UDP) port, an access list number, or a packet size.
GTS, Class-Based Shaping, DTS, and FRTS are similar in implementation, sharing the same code and data structures, but they differ in regard to their CLIs and the queue types they use.
Here are a few ways in which these mechanisms differ:
Table 10-1 summarizes these differences.
You can configure GTS to behave the same as FRTS by allocating one DLCI per subinterface and using GTS plus backward explicit congestion notification (BECN) support.
Traffic shaping smooths traffic by storing traffic above the configured rate in a queue.
When a packet arrives at the interface for transmission, the following sequence occurs:
1. If the queue is empty, the arriving packet is processed by the traffic shaper.
2. If the queue is not empty, the packet is placed in the queue.
When packets are in the queue, the traffic shaper removes the number of packets it can send from the queue at each time interval.
Generic Traffic Shaping (GTS) shapes traffic by reducing outbound traffic flow to avoid congestion. GTS constrains traffic to a particular bit rate using the token bucket mechanism. See the section "What is a Token Bucket" in the Cisco IOS Quality of Service Solutions Configuration Guide, Release 12.2:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos _c/fqcprt4/qcfpolsh.htm#1000909
GTS applies traffic shaping on a per-interface basis and can use access lists to select the traffic to shape. GTS works with a variety of Layer 2 technologies, including Frame Relay, ATM, Switched Multimegabit Data Service (SMDS), and Ethernet.
On a Frame Relay subinterface, GTS can be set up to adapt dynamically to available bandwidth by integrating backward explicit congestion notification (BECN) signals. GTS also can be shape traffic to a specified rate. GTS can be configured on an ATM/ATM Interface Processor (AIP) interface to respond to the Resource Reservation Protocol (RSVP) feature signalled over statically configured ATM permanent virtual circuits (PVCs).
GTS is supported on most media and encapsulation types on the router. GTS can be applied to a specific access list on an interface.
Figure 10-1 shows how GTS works.
For information on how to configure GTS, see the chapter "Configuring Generic Traffic Shaping" in the Cisco IOS Quality of Service Solutions Configuration Guide, Release 12.2:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos _c/fqcprt4/qcfgts.htm#80560
For information on traffic shaping commands, see the Cisco IOS Quality of Service Solutions Command Reference, Release 12.2 manual:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_r/index.htm
Class-Based Traffic Shaping can be enabled on any interface that supports GTS.
Using Class-Based Traffic Shaping, you can perform the following tasks:
Flow-based WFQ applies weights to traffic to classify the traffic into conversations and determine how much bandwidth each conversation is allowed. These weights and traffic classifications are dependent on and limited to the seven IP Precedence levels.
CBWFQ allows you to define what constitutes a class based on criteria that exceed the confines of flow. CBWFQ allows you to use ACLs and protocols or input interface names to define how traffic will be classified, thereby providing coarser granularity. You need not maintain traffic classification on a flow basis. Moreover, you can configure up to 64 discrete classes in a service policy.
For information on how to configure Class-Based Shaping, see the chapter "Configuring Class-Based Shaping" in the Cisco IOS Quality of Service Solutions Configuration Guide, Release 12.2:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos _c/fqcprt4/qcfcbshp.htm#80464
For information on traffic shaping commands, see the Cisco IOS Quality of Service Solutions Command Reference, Release 12.2 manual:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_r/index.htm
Peak and average traffic shaping is configured on a per-interface or per-class basis, and cannot be used in conjunction with commands used to configure GTS from previous versions of Cisco IOS. These commands include the following:
Adaptive traffic shaping for Frame Relay networks is not supported using the Class-Based Shaping feature. To configure adaptive GTS for Frame Relay networks, you must use the commands from releases prior to Release 12.1(2) of Cisco IOS software.
Cisco has long provided support for FECN for DECnet and OSI, and BECN for Systems Network Architecture (SNA) traffic using Logical Link Control, type 2 (LLC2) encapsulation via RFC 1490 and DE bit support. FRTS builds upon this existing Frame Relay support with additional capabilities that improve the scalability and performance of a Frame Relay network, increasing the density of VCs and improving response time.
As is also true of GTS, FRTS can eliminate bottlenecks in Frame Relay networks that have high-speed connections at the central site and low-speed connections at branch sites. You can configure rate enforcement—a peak rate configured to limit outbound traffic—to limit the rate at which data is sent on the VC at the central site.
Using FRTS, you can configure rate enforcement to either the CIR or some other defined value such as the excess information rate on a per-VC basis. The ability to allow the transmission speed used by the router to be controlled by criteria other than line speed provides a mechanism for sharing media by multiple VCs. You can allocate bandwidth to each VC, creating a virtual time-division multiplexing (TDM) network.
You can also define PQ, CQ, and WFQ at the VC or subinterface level. Using these queueing methods allows for finer granularity in the prioritization and queueing of traffic, providing more control over the traffic flow on an individual VC. If you combine CQ with the per-VC queueing and rate enforcement capabilities, you enable Frame Relay VCs to carry multiple traffic types such as IP, SNA, and Internetwork Packet Exchange (IPX) with bandwidth guaranteed for each traffic type.
Using information contained in the BECN-tagged packets received from the network, FRTS can also dynamically throttle traffic. With BECN-based throttling, packets are held in the buffers of the router to reduce the data flow from the router into the Frame Relay network. The throttling is done on a per-VC basis and the transmission rate is adjusted based on the number of BECN-tagged packets received.
With the Cisco FRTS feature, you can integrate ATM ForeSight closed-loop congestion control to actively adapt to downstream congestion conditions.
In Frame Relay networks, BECNs and FECNs indicate congestion. BECN and FECN are specified by bits within a Frame Relay frame.
FECNs are generated when data is sent out a congested interface; they indicate to a DTE device that congestion was encountered. Traffic is marked with BECN if the queue for the opposite direction is deep enough to trigger FECNs at the current time.
BECNs notify the sender to decrease the transmission rate. If the traffic is one-way only (such as multicast traffic), there is no reverse traffic with BECNs to notify the sender to slow down. Thus, when a DTE device receives an FECN, it first determines if it is sending any data in return. If it is sending return data, this data will get marked with a BECN on its way to the other DTE device. However, if the DTE device is not sending any data, the DTE device can send a Q.922 TEST RESPONSE message with the BECN bit set.
When an interface configured with traffic shaping receives a BECN, it immediately decreases its maximum rate by a large amount. If, after several intervals, the interface has not received another BECN and traffic is waiting in the queue, the maximum rate increases slightly. The dynamically adjusted maximum rate is called the derived rate.
The derived rate will always be between the upper bound and the lower bound configured on the interface.
For more information on configuring Frame Relay, refer to the chapter "Configuring Frame Relay" in the Cisco IOS Wide-Area Networking Configuration Guide, Release 12.2 manual:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fwan _c/wcffrely.htm
For information on configuring Frame Relay as it relates to voice traffic, refer to the chapter "Configuring Voice Over Frame Relay" in the Cisco IOS Voice, Video, and Fax Configuration Guide, Release 12.2 manual:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fvvf ax_c/vvfvofr.htm
For information on Frame Relay commands, see the section "Frame Relay Commands" in the Cisco IOS Wide-Area Networking Command Reference, Release 12.2 manual:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fwan _r/frcmds/index.htm
For information on Frame Relay ATM commands, see the section "Frame Relay—ATM Internetworking Commands" in the Cisco IOS Wide-Area Networking Command Reference, Release 12.2 manual:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fwan _r/frcmds/index.htm
FRTS applies only to Frame Relay PVCs and switched virtual circuits (SVCs).
DTS provides a method of managing the bandwidth of an interface to avoid congestion, to meet remote site requirements, and to conform to a service rate that is provided on that interface.
DTS uses queues to buffer traffic surges that can congest a network and send the data to the network at a regulated rate. This ensures that traffic will behave to the configured descriptor, as defined by the CIR, Bc, and Be. With the defined average bit rate and burst size that is acceptable on that shaped entity, you can derive a time interval value.
Distributed Cisco Express Forwarding (dCEF) must be enabled on the interface before DTS can be enabled.
A policy map and class maps must be created before DTS is enabled.
The Be size allows more than the Bc size to be sent during a time interval under certain conditions. Therefore, DTS provides two types of shape commands: average and peak. When shape average is configured, the interface sends no more than the Bc size for each interval, achieving an average rate no higher than the CIR. When the shape peak command is configured, the interface sends Bc plus Be bits in each interval.
In a link layer network such as Frame Relay, the network sends messages with the forward explicit congestion notification (FECN) or BECN if there is congestion. With the DTS feature, the traffic shaping adaptive mode takes advantage of these signals and adjusts the traffic descriptors, thereby regulating the amount of traffic entering or leaving the interface accordingly.
DTS provides the following key benefits:
This feature runs on Cisco 7500 series routers with VIP2-40, VIP2-50, or greater.
For information on how to configure DTS, see the chapter "Configuring Distributed Traffic Shaping" in the Cisco IOS Quality of Service Solutions Command Reference, Release 12.2 manual:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos _c/fqcprt4/qcfdts.htm
DTS does not support the following:
The LLQ feature brings strict PQ to CBWFQ. Strict PQ allows delay-sensitive data such as voice to be dequeued and sent before packets in other queues are dequeued.
Without LLQ, CBWFQ provides WFQ based on defined classes with no strict priority queue available for real-time traffic. CBWFQ allows you to define traffic classes and then assign characteristics to these classes. 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 at configuration. 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 strict priority. This scheme poses problems for voice traffic that is largely intolerant of delay, especially variation in delay. For voice traffic, variations in delay introduce irregularities of transmission, such as jitter in the heard conversation.
LLQ provides strict priority queueing for CBWFQ, reducing jitter in voice conversations. Configured by the priority command, LLQ enables use of a single, strict priority queue within CBWFQ at the class level. This allows you to direct traffic belonging to a class to the CBWFQ strict priority queue. To enqueue class traffic to the strict priority queue, specify the named class within a policy map and then configure the priority command for the class. (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, strict priority queue.
One of the ways in which the strict PQ used within CBWFQ differs from its use outside CBWFQ is in the parameters it takes. Outside CBWFQ, you can use the ip rtp priority command to specify the range of UDP ports whose voice traffic flows are given priority service.
Using the priority command, you are no longer limited to a UDP port number to stipulate priority flows because you can configure the priority status for a class within CBWFQ. Instead, all of the valid match criteria used to specify traffic for a class now apply to priority traffic. These methods of specifying traffic for a class include matching on access lists, protocols, and input interfaces. Moreover, within an access, list you can specify that traffic matches are allowed based on the IP differentiated services code point (DSCP) value. This value is set using the first six bits of the ToS byte in the IP header.
Although it is possible to enqueue various types of real-time traffic to the strict priority queue, Cisco strongly recommends that you direct only voice traffic to this queue. The reason is that voice traffic is well-behaved, whereas other types of real-time traffic are not well-behaved. Moreover, voice traffic requires nonvariable delays to avoid jitter.
Real-time traffic such as video could introduce variation in delay, thereby thwarting the steadiness of delay required for successful voice traffic transmission.
For more conceptual information about LLQ, see the section "Weighted Fair Queueing" in the Cisco IOS Quality of Service Solutions Configuration Guide, Release 12.2:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos _c/fqcprt2/qcfconmg.htm#xtocid46014
For information on how to configure LLQ, see the chapter "Configuring Weighted Fair Queueing" in the Cisco IOS Quality of Service Solutions Configuration Guide, Release 12.2:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos _c/fqcprt2/qcfwfq.htm
Posted: Sat Apr 5 03:59:42 PST 2003
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