Configuring QoS
Have you ever participated in a long-distance phone call that involved a satellite connection? The conversation might be interrupted with brief, but perceptible, gaps at odd intervals. Those gaps are the time, called the latency, between the arrival of packets being transmitted over the network. Some network traffic, such as voice and video, cannot tolerate long latency times. Quality of service (QoS) is a feature that lets you give priority to critical traffic, prevent bandwidth hogging, and manage network bottlenecks to prevent packet drops.
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Note For the ASASM, we suggest performing QoS on the switch instead of the ASASM. Switches have more capability in this area.
This chapter describes how to apply QoS policies and includes the following sections:
Information About QoS
You should consider that in an ever-changing network environment, QoS is not a one-time deployment, but an ongoing, essential part of network design.
This section describes the QoS features supported by the ASA and includes the following topics:
- Supported QoS Features
- What is a Token Bucket?
- Information About Policing
- Information About Priority Queuing
- Information About Traffic Shaping
- DSCP and DiffServ Preservation
Supported QoS Features
The ASA supports the following QoS features:
- Policing—To prevent individual flows from hogging the network bandwidth, you can limit the maximum bandwidth used per flow. See the “Information About Policing” section for more information.
- Priority queuing—For critical traffic that cannot tolerate latency, such as Voice over IP (VoIP), you can identify traffic for Low Latency Queuing (LLQ) so that it is always transmitted ahead of other traffic. See the “Information About Priority Queuing” section for more information.
- Traffic shaping—If you have a device that transmits packets at a high speed, such as a ASA with Fast Ethernet, and it is connected to a low speed device such as a cable modem, then the cable modem is a bottleneck at which packets are frequently dropped. To manage networks with differing line speeds, you can configure the ASA to transmit packets at a fixed slower rate. See the “Information About Traffic Shaping” section for more information.
What is a Token Bucket?
A token bucket is used to manage a device that regulates the data in a flow. For example, the regulator might be a traffic policer or a traffic shaper. A token bucket itself has no discard or priority policy. Rather, a token bucket discards tokens and leaves to the flow the problem of managing its transmission queue if the flow overdrives the regulator.
A token bucket is a formal definition of a rate of transfer. It has three components: a burst size, an average rate, and a time interval. Although the average rate is generally represented as bits per second, any two values may be derived from the third by the relation shown as follows:
average rate = burst size / time interval
Here are some definitions of these terms:
- Average rate—Also called the committed information rate (CIR), it specifies how much data can be sent or forwarded per unit time on average.
- Burst size—Also called the Committed Burst (Bc) size, it specifies in bits or bytes per burst how much traffic can be sent within a given unit of time to not create scheduling concerns. (For traffic shaping, it specifies bits per burst; for policing, it specifies bytes per burst.)
- Time interval—Also called the measurement interval, it specifies the time quantum in seconds per burst.
In the token bucket metaphor, tokens are put into the bucket at a certain rate. The bucket itself has a specified capacity. If the bucket fills to capacity, newly arriving tokens are discarded. Each token is permission for the source to send a certain number of bits into the network. To send a packet, the regulator must remove from the bucket a number of tokens equal in representation to the packet size.
If not enough tokens are in the bucket to send a packet, the packet either waits until the bucket has enough tokens (in the case of traffic shaping) or the packet is discarded or marked down (in the case of policing). If the bucket is already full of tokens, incoming tokens overflow and are not available to future packets. Thus, at any time, the largest burst a source can send into the network is roughly proportional to the size of the bucket.
Note that the token bucket mechanism used for traffic shaping has both a token bucket and a data buffer, or queue; if it did not have a data buffer, it would be a policer. For traffic shaping, packets that arrive that cannot be sent immediately are delayed in the data buffer.
For traffic shaping, a token bucket permits burstiness but bounds it. It guarantees that the burstiness is bounded so that the flow will never send faster than the token bucket capacity, divided by the time interval, plus the established rate at which tokens are placed in the token bucket. See the following formula:
(token bucket capacity in bits / time interval in seconds) + established rate in bps = maximum flow speed in bps
This method of bounding burstiness also guarantees that the long-term transmission rate will not exceed the established rate at which tokens are placed in the bucket.
Information About Policing
Policing is a way of ensuring that no traffic exceeds the maximum rate (in bits/second) that you configure, thus ensuring that no one traffic flow or class can take over the entire resource. When traffic exceeds the maximum rate, the ASA drops the excess traffic. Policing also sets the largest single burst of traffic allowed.
Information About Priority Queuing
LLQ priority queuing lets you prioritize certain traffic flows (such as latency-sensitive traffic like voice and video) ahead of other traffic.
The ASA supports two types of priority queuing:
- Standard priority queuing—Standard priority queuing uses an LLQ priority queue on an interface (see the “Configuring the Standard Priority Queue for an Interface” section), while all other traffic goes into the “best effort” queue. Because queues are not of infinite size, they can fill and overflow. When a queue is full, any additional packets cannot get into the queue and are dropped. This is called tail drop . To avoid having the queue fill up, you can increase the queue buffer size. You can also fine-tune the maximum number of packets allowed into the transmit queue. These options let you control the latency and robustness of the priority queuing. Packets in the LLQ queue are always transmitted before packets in the best effort queue.
- Hierarchical priority queuing—Hierarchical priority queuing is used on interfaces on which you enable a traffic shaping queue. A subset of the shaped traffic can be prioritized. The standard priority queue is not used. See the following guidelines about hierarchical priority queuing:
– Priority packets are always queued at the head of the shape queue so they are always transmitted ahead of other non-priority queued packets.
– Priority packets are never dropped from the shape queue unless the sustained rate of priority traffic exceeds the shape rate.
– For IPsec-encrypted packets, you can only match traffic based on the DSCP or precedence setting.
– IPsec-over-TCP is not supported for priority traffic classification.
Information About Traffic Shaping
Traffic shaping is used to match device and link speeds, thereby controlling packet loss, variable delay, and link saturation, which can cause jitter and delay.
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Note Traffic shaping is only supported on the ASA 5505, 5510, 5520, 5540, and 5550.
- Traffic shaping must be applied to all outgoing traffic on a physical interface or in the case of the ASA 5505, on a VLAN. You cannot configure traffic shaping for specific types of traffic.
- Traffic shaping is implemented when packets are ready to be transmitted on an interface, so the rate calculation is performed based on the actual size of a packet to be transmitted, including all the possible overhead such as the IPsec header and L2 header.
- The shaped traffic includes both through-the-box and from-the-box traffic.
- The shape rate calculation is based on the standard token bucket algorithm. The token bucket size is twice the Burst Size value. See the “What is a Token Bucket?” section.
- When bursty traffic exceeds the specified shape rate, packets are queued and transmitted later. Following are some characteristics regarding the shape queue (for information about hierarchical priority queuing, see the “Information About Priority Queuing” section):
– The queue size is calculated based on the shape rate. The queue can hold the equivalent of 200-milliseconds worth of shape rate traffic, assuming a 1500-byte packet. The minimum queue size is 64.
– When the queue limit is reached, packets are tail-dropped.
– Certain critical keep-alive packets such as OSPF Hello packets are never dropped.
– The time interval is derived by time_interval = burst_size / average_rate. The larger the time interval is, the burstier the shaped traffic might be, and the longer the link might be idle. The effect can be best understood using the following exaggerated example:
In the above example, the time interval is 1 second, which means, 1 Mbps of traffic can be bursted out within the first 10 milliseconds of the 1-second interval on a 100 Mbps FE link and leave the remaining 990 milliseconds idle without being able to send any packets until the next time interval. So if there is delay-sensitive traffic such as voice traffic, the Burst Size should be reduced compared to the average rate so the time interval is reduced.
How QoS Features Interact
You can configure each of the QoS features alone if desired for the ASA. Often, though, you configure multiple QoS features on the ASA so you can prioritize some traffic, for example, and prevent other traffic from causing bandwidth problems.
See the following supported feature combinations per interface:
You cannot configure priority queuing and policing for the same set of traffic.
- Traffic shaping (for all traffic on an interface) + Hierarchical priority queuing (for a subset of traffic).
You cannot configure traffic shaping and standard priority queuing for the same interface; only hierarchical priority queuing is allowed. For example, if you configure standard priority queuing for the global policy, and then configure traffic shaping for a specific interface, the feature you configured last is rejected because the global policy overlaps the interface policy.
Typically, if you enable traffic shaping, you do not also enable policing for the same traffic, although the ASA does not restrict you from configuring this.
DSCP and DiffServ Preservation
- DSCP markings are preserved on all traffic passing through the ASA.
- The ASA does not locally mark/remark any classified traffic, but it honors the Expedited Forwarding (EF) DSCP bits of every packet to determine if it requires “priority” handling and will direct those packets to the LLQ.
- DiffServ marking is preserved on packets when they traverse the service provider backbone so that QoS can be applied in transit (QoS tunnel pre-classification).
Licensing Requirements for QoS
The following table shows the licensing requirements for this feature:
Guidelines and Limitations
This section includes the guidelines and limitations for this feature.
Supported in single context mode only. Does not support multiple context mode.
Supported in routed firewall mode only. Does not support transparent firewall mode.
- Traffic shaping is only supported on the ASA 5505, 5510, 5520, 5540, and 5550. Multi-core models (such as the ASA 5500-X) do not support shaping.
- (ASA 5580) You cannot create a standard priority queue for a Ten Gigabit Ethernet interface. Note : For the ASA 5585-X, standard priority queuing is supported on a Ten Gigabit Interface.
- (ASA 5512-X through ASA 5555-X) Priority queuing is not supported on the Management 0/0 interface.
- (ASASM) Only policing is supported.
Additional Guidelines and Limitations
- QoS is applied unidirectionally; only traffic that enters (or exits, depending on the QoS feature) the interface to which you apply the policy map is affected. See the “Feature Directionality” section for more information.
- For traffic shaping, you can only use the class-default class map, which is automatically created by the ASA, and which matches all traffic.
- For priority traffic, you cannot use the class-default class map.
- For hierarchical priority queuing, for encrypted VPN traffic, you can only match traffic based on the DSCP or precedence setting; you cannot match a tunnel group.
- For hierarchical priority queuing, IPsec-over-TCP traffic is not supported.
- You cannot configure traffic shaping and standard priority queuing for the same interface; only hierarchical priority queuing is allowed.
- For standard priority queuing, the queue must be configured for a physical interface or, for the ASA 5505 or ASASM, a VLAN.
- For policing, to-the-box traffic is not supported.
- For policing, traffic to and from a VPN tunnel bypass interface is not supported.
- For policing, when you match a tunnel group class map, only outbound policing is supported.
Configuring QoS
This section includes the following topics:
- Determining the Queue and TX Ring Limits for a Standard Priority Queue
- Configuring the Standard Priority Queue for an Interface
- Configuring a Service Rule for Standard Priority Queuing and Policing
- Configuring a Service Rule for Traffic Shaping and Hierarchical Priority Queuing
Determining the Queue and TX Ring Limits for a Standard Priority Queue
To determine the priority queue and TX ring limits, use the worksheets below.
Delay (ms)3
Delay (ms)6
Configuring the Standard Priority Queue for an Interface
If you enable standard priority queuing for traffic on a physical interface, then you need to also create the priority queue on each interface. Each physical interface uses two queues: one for priority traffic, and the other for all other traffic. For the other traffic, you can optionally configure policing.
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Note The standard priority queue is not required for hierarchical priority queuing with traffic shaping; see the “Information About Priority Queuing” section for more information.
Restrictions
- (ASASM) The ASASM does not support priority queuing.
- (ASA 5580) You cannot create a standard priority queue for a Ten Gigabit Ethernet interface. Note : For the ASA 5585-X, standard priority queuing is supported on a Ten Gigabit Interface.
- (ASA 5512-X through ASA 5555-X) Priority queuing is not supported on the Management 0/0 interface.
Detailed Steps
Examples
The following example establishes a priority queue on interface “outside” (the GigabitEthernet0/1 interface), with the default queue-limit and tx-ring-limit:
The following example establishes a priority queue on the interface “outside” (the GigabitEthernet0/1 interface), sets the queue-limit to 260 packets, and sets the tx-ring-limit to 3:
Configuring a Service Rule for Standard Priority Queuing and Policing
You can configure standard priority queuing and policing for different class maps within the same policy map. See the “How QoS Features Interact” section for information about valid QoS configurations.
Restrictions
- You cannot use the class-default class map for priority traffic.
- You cannot configure traffic shaping and standard priority queuing for the same interface; only hierarchical priority queuing is allowed.
- (ASASM) The ASASM only supports policing.
- For policing, to-the-box traffic is not supported.
- For policing, traffic to and from a VPN tunnel bypass interface is not supported.
- For policing, when you match a tunnel group class map, only outbound policing is supported.
Detailed Steps
For priority traffic, creates a class map to identify the traffic for which you want to perform priority queuing.
Specifies the traffic in the class map. See the “Identifying Traffic (Layer 3/4 Class Maps)” section for more information.
For policing traffic, creates a class map to identify the traffic for which you want to perform policing.
Specifies the traffic in the class map. See the “Identifying Traffic (Layer 3/4 Class Maps)” section for more information.
Identifies the class map you created for prioritized traffic in Step 1.
Identifies the class map you created for policed traffic in Step 3.
police {output | input} conform-rate [conform-burst] [conform-action [drop | transmit]] [exceed-action [drop | transmit]]
Configures policing for the class. See the followingoptions:
- conform-burst argument—Specifies the maximum number of instantaneous bytes allowed in a sustained burst before throttling to the conforming rate value, between 1000 and 512000000 bytes.
- conform-action —Sets the action to take when the rate is less than the conform_burst value.
- conform-rate— Sets the rate limit for this traffic flow; between 8000 and 2000000000 bits per second.]
- drop —Drops the packet.
- exceed-action —Sets the action to take when the rate is between the conform-rate value and the conform-burst value.
- input —Enables policing of traffic flowing in the input direction.
- output —Enables policing of traffic flowing in the output direction.
- transmit —Transmits the packet.
service-policy policymap_name {global | interface interface_name}
ciscoasa(config)# service-policy QoS_policy interface inside
Activates the policy map on one or more interfaces. global applies the policy map to all interfaces, and interface applies the policy to one interface. Only one global policy is allowed. You can override the global policy on an interface by applying a service policy to that interface. You can only apply one policy map to each interface.
Examples
Example 23-1 Class Map Examples for VPN Traffic
In the following example, the class-map command classifies both tunneled and non-tunneled traffic according to the traffic type:
ciscoasa(config)#
access-list tunneled extended permit ip 10.10.34.0 255.255.255.0 192.168.10.0 255.255.255.0ciscoasa(config-cmap)#
description"This class-map matches all best-effort traffic for tunnel-grp1."
The following example shows a way of policing a flow within a tunnel, provided the classed traffic is not specified as a tunnel, but does go through the tunnel. In this example, 192.168.10.10 is the address of the host machine on the private side of the remote tunnel, and the ACL is named “host-over-l2l”. By creating a class-map (named “host-specific”), you can then police the “host-specific” class before the LAN-to-LAN connection polices the tunnel. In this example, the “host-specific” traffic is rate-limited before the tunnel, then the tunnel is rate-limited:
The following example builds on the configuration developed in the previous section. As in the previous example, there are two named class-maps: tcp_traffic and TG1-voice.
Adding a third class map provides a basis for defining a tunneled and non-tunneled QoS policy, as follows, which creates a simple QoS policy for tunneled and non-tunneled traffic, assigning packets of the class TG1-voice to the low latency queue and setting rate limits on the tcp_traffic and TG1-best-effort traffic flows.
(Optional) Configuring the Hierarchical Priority Queuing Policy
You can optionally configure priority queuing for a subset of latency-sensitive traffic.
Guidelines
- One side-effect of priority queuing is packet re-ordering. For IPsec packets, out-of-order packets that are not within the anti-replay window generate warning syslog messages. These warnings are false alarms in the case of priority queuing. You can configure the IPsec anti-replay window size to avoid possible false alarms. See the crypto ipsec security-association replay command in the command reference.
- For hierarchical priority queuing, you do not need to create a priority queue on an interface.
Detailed Steps
For hierarchical priority queuing, creates a class map to identify the traffic for which you want to perform priority queuing.
Specifies the traffic in the class map. See the “Identifying Traffic (Layer 3/4 Class Maps)” section for more information. For encrypted VPN traffic, you can only match traffic based on the DSCP or precedence setting; you cannot match a tunnel group.
Specifies the class map you created in Step 1.
Applies the priority queuing action to a class map.
Note This policy has not yet been activated. You must activate it as part of the shaping policy. See the “Configuring the Service Rule” section.Configuring the Service Rule
Adds or edits a policy map. This policy map must be different from the hierarchical priority-queuing map.
Identifies all traffic for traffic shaping; you can only use the class-default class map, which is defined as match any, because the ASA requires all traffic to be matched for traffic shaping.
Enables traffic shaping, where the average rate argument sets the average rate of traffic in bits per second over a given fixed time period, between 64000 and 154400000. Specify a value that is a multiple of 8000. See the “Information About Traffic Shaping” section for more information about how the time period is calculated.
The burst_size argument sets the average burst size in bits that can be transmitted over a given fixed time period, between 2048 and 154400000. Specify a value that is a multiple of 128. If you do not specify the burst_size, the default value is equivalent to 4-milliseconds of traffic at the specified average rate. For example, if the average rate is 1000000 bits per second, 4 ms worth = 1000000 * 4/1000 = 4000.
Configures hierarchical priority queuing, where the priority_policy_map_name is the policy map you created for prioritized traffic in the “(Optional) Configuring the Hierarchical Priority Queuing Policy” section.
service-policy policymap_name interface interface_name
ciscoasa(config)# service-policy shape-policy interface inside
Monitoring QoS
This section includes the following topics:
- Viewing QoS Police Statistics
- Viewing QoS Standard Priority Statistics
- Viewing QoS Shaping Statistics
- Viewing QoS Standard Priority Queue Statistics
Viewing QoS Police Statistics
To view the QoS statistics for traffic policing, use the show service-policy command with the police keyword:
The following is sample output for the show service-policy police command:
Viewing QoS Standard Priority Statistics
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Note “Aggregate drop” denotes the aggregated drop in this interface; “aggregate transmit” denotes the aggregated number of transmitted packets in this interface.
Viewing QoS Shaping Statistics
The following is sample output of the show service policy shape command, which includes service policies that include the shape command and the service-policy command that calls the hierarchical priority policy and the related statistics:
Viewing QoS Standard Priority Queue Statistics
- “Packets Dropped” denotes the overall number of packets that have been dropped in this queue.
- “Packets Transmit” denotes the overall number of packets that have been transmitted in this queue.
- “Packets Enqueued” denotes the overall number of packets that have been queued in this queue.
- “Current Q Length” denotes the current depth of this queue.
- “Max Q Length” denotes the maximum depth that ever occurred in this queue.