Chapter 10. Quality of Service

This chapter discusses how to configure and use the Quality of Service (QoS) features described here:

• Comprehensive QoS Configuration

  • 10-1: Modular QoS Command-Line Interface (MQC) ” A " user -friendly" way to configure all QoS features within a single framework. This framework is based on the Diffserv model. Traffic is defined as class maps; class maps are used in policy maps; policy maps are applied to interfaces. You should consider using the modular QoS CLI rather than configuring a variety of individual QoS features, just for its intuitive and structured approach.

    Class maps perform versatile traffic classification. Policy maps define QoS policies by using traffic marking (reclassification), congestion management (WFQ and LLQ), congestion avoidance (WRED), and traffic policing and shaping.

• Packet Classification Techniques

  • 10-2: Network-Based Application Recognition (NBAR) ” Traffic can be intelligently and automatically recognized and classified based on a database of protocols. NBAR provides an easy-to-use classification of protocols ”even those that use dynamic TCP and UDP port assignments.

  • 10-3: Policy-Based Routing (PBR) ” Traffic can be classified with access lists, marked with an IP Precedence, and routed to specified interfaces.

  • 10-4: Quality of Service for VPNs ” Traffic can be classified before it is encrypted or encapsulated by a VPN mechanism.

  • 10-5: QoS Policy Propagation via BGP ” Traffic classification policies can be propagated throughout a large network through the use of BGP updates.

• Congestion Management Techniques

  • 10-6: Priority Queuing (PQ) ” Traffic is assigned to four strict priority queues for transmission on an interface.

  • 10-7: Custom Queuing (CQ) ” Traffic is assigned to a set of queues that each receives a proportional amount of the interface bandwidth.

  • 10-8: Weighted Fair Queuing (WFQ) ” Traffic is automatically classified into conversations or flows, each of which receives a weight according to the type of traffic. Low-volume traffic receives priority over high-volume traffic, providing timely delivery of low-volume interactive traffic.

• Congestion Avoidance Techniques

  • 10-9: Weighted Random Early Detection (WRED) ” Traffic is randomly dropped as congestion becomes apparent, causing the traffic source to reduce its transmission rate. WRED can drop packets based on IP Precedence or DSCP so that higher-priority traffic is delivered without being dropped.

• Traffic Policing and Shaping Techniques

  • 10-10: Committed Access Rate (CAR) ” Both inbound and outbound traffic can be rate-limited according to CAR policies. Policies can identify traffic according to interface, classification, addresses, application, and access lists. Each policy is configured with a bandwidth restriction and actions to take if the policy is met or exceeded.

  • 10-11: Generic Traffic Shaping (GTS) ” Outbound traffic can be shaped to a particular rate to prevent congestion.

  • 10-12: Frame Relay Traffic Shaping (FRTS) ” Outbound traffic on a Frame Relay interface can be shaped to conform to the CIR. Congestion notification methods can be used to adapt the shaping.

• QoS Signaling Techniques

  • 10-13: Use RSVP for QoS Signaling ” RSVP provides a means for an end node or router to request a resource reservation from the source to the destination network. End-to-end guaranteed QoS is the result.

• Improving QoS on Physical Links

  • 10-14: Link Efficiency Mechanisms ” At the interface level, some additional techniques are available to improve data transmission performance. Link Fragmentation and Interleaving (LFI) can adjust the transmitted packet size and order so that time-critical packets (voice, video, ERP, and so forth) can be inserted into the data stream at a guaranteed interpacket delay. Compressed Real-Time Protocol (CRTP) performs header compression to reduce the overhead involved in transporting time-critical packets.

QoS is also built around the concept of Differentiated Services (Diffserv), in which the QoS specification is carried within each packet. IP packets have a Type of Service (ToS) byte that is formatted according to the top row of Table 10-1. Bits P2, P1, and P0 form the IP Precedence value. Bits T3, T2, T1, and T0 form the ToS value.

For Differentiated Services, the same byte is called the Differentiated Services (DS) byte. It is formatted according to the bottom row of Table 10-1. Bits DS5 through DS0 form the Differentiated Services Code Point (DSCP). The DSCP is arranged to be backward-compatible with the IP Precedence bits, because the two quantities share the same byte in the IP header.T

Table 10-1. ToS and DSCP Byte Formats

ToS Byte	P2	P1	P0	T3	T2	T1	T0	Zero
DS Byte	DS5	DS4	DS3	DS2	DS1	DS0	Unused	Unused
	(Class selector)			(Drop precedence)

Bits DS5, DS4, and DS3 form the DSCP Class Selector. Classes 1 through 4 are the Assured Forwarding (AF) service levels. Each AF service level has three Drop Precedence categories: Low, Medium, and High. Traffic in the AF classes can be dropped. The greatest likelihood of dropping is in the Low category, and the least is in the High category.

Class 5 is also called the Expedited Forwarding (EF) class, offering premium service and the least likelihood of packet drops . The Default class selector (DSCP 000 000) offers only best-effort forwarding.

Table 10-2 shows how the IP Precedence names and bits are mapped to DSCP values. DSCP is broken down into Per-Hop Behavior (PHB), Class Selector, and Drop Precedence. Many times, DSCP values are referred to by their Codepoint names (such as AF23); these are also listed in the table. The DSCP bits are shown, along with their decimal equivalents. In many DSCP- related commands, you need to enter a decimal DSCP value, even though it is difficult to relate the decimal numbers to the corresponding DSCP service levels and PHBs. Use this table as a convenient cross-reference.

Table 10-2. Mapping of IP Precedence and DSCP Fields

IP Precedence (3 bits)			DSCP (6 bits)
Name	Value	Bits	Per-Hop Behavior	Class Selector	Drop Precedence	Code-point Name	DSCP Bits (Decimal)
Routine		000	Default			Default	000 000 (0)
Priority	1	001	AF	1	1: Low	AF11	001 010 (10)
					2: Medium	AF12	001 100 (12)
					3: High	AF13	001 110 (14)
Immediate	2	010	AF	2	1: Low	AF21	010 010 (18)
					2: Medium	AF22	010 100 (20)
					3: High	AF23	010 110 (22)
Flash	3	011	AF	3	1: Low	AF31	011 010 (26)
					2: Medium	AF32	011 100 (28)
					3: High	AF33	011 110 (30)
Flash Override	4	100	AF	4	1: Low	AF41	100 010 (34)
					2: Medium	AF42	100 100 (36)
					3: High	AF43	100 110 (38)
Critical	5	101	EF			EF	101 110 (46)
Internetwork Control	6	110	”
Network Control	7	111	”