l Configuration of burst traffic feature in port rate limit and traffic policing. For details, see section Configuring Traffic Policing and section Configuring Line Rate.

l Configuration of the priority marking feature in VLAN. For details, see section Marking Packet Priority.

l Redirecting traffic to an aggregation group and removing outer VLAN tags when redirecting traffic to the specified port/aggregation group. For details, see section Traffic Redirecting.

l The burst function. For details, see section Burst.

Quality of Service (QoS) is a concept concerning service demand and supply. It reflects the ability to meet customer needs. Generally, QoS does not focus on grading services precisely, but on improving services under certain conditions.

In an internet, QoS refers to the ability of the network to forward packets. The evaluation on QoS of a network can be based on different aspects because the network may provide various services. Generally, QoS refers to the ability to provide improved service by solving the core issues such as delay, jitter, and packet loss ratio in the packet forwarding process.

1.1.2 Traditional Packet Forwarding Service

In traditional IP networks, packets are treated equally. That is, the FIFO (first in first out) policy is adopted for packet processing. Network resources required for packet forwarding is determined by the order in which packets arrive. All the packets share the resources of the network. Network resources available to the packets completely depend on the time they arrive. This service policy is known as Best-effort, which delivers the packets to their destination with the best effort, with no assurance and guarantee for delivery delay, jitter, packet loss ratio, reliability, and so on.

The traditional Best-Effort service policy is only suitable for applications insensitive to bandwidth and delay, such as WWW, E-mail and FTP.

1.1.3 New Applications and New Requirements

With the expansion of computer network, more and more networks become part of the Internet. The Internet gains rapid development in terms of scale, coverage and user quantities. More and more users use the Internet as a platform for their services and for data transmission.

Besides the traditional applications such as WWW, E-mail, and FTP, new services are developed on the Internet, such as tele-education, telemedicine, video telephone, videoconference and Video-on-Demand (VoD). Enterprise users expect to connect their regional branches together using VPN techniques for coping with daily business, for instance, accessing databases or manage remote equipments through Telnet.

All these new applications have one thing in common, that is, they have special requirements for bandwidth, delay, and jitter. For instance, bandwidth, delay, and jitter are critical for videoconference and VoD. As for other applications, such as transaction processing and Telnet, although bandwidth is not as critical, a too long delay may cause unexpected results. That is, they need to get serviced in time even if congestion occurs.

Newly emerging applications demand higher service performance from IP networks. In addition to simply delivering packets to their destinations, better network services are demanded, such as allocating dedicated bandwidth, reducing packet loss ratio, avoiding congestion, regulating network traffic, and setting priority of the packets. To meet those requirements, the network should be provided with better service capability.

1.1.4 Major Traffic Control Techniques

Figure 1-1 End-to-end QoS model

As shown in the figure above, traffic classification, traffic policing, traffic shaping, congestion management, and congestion avoidance are the foundations for a network to provide differentiated services. They are described as follow:

l Traffic classification identifies traffic based on certain matching rules. It is a prerequisite for differentiated services and is usually applied in the inbound direction of a port.

l Traffic policing confines traffic to a specific specification and is usually applied in the inbound direction of a port. You can configure restriction or penalty measures against the exceeding traffic to protect carrier benefits and network resources.

l Traffic shaping adapts output traffic rate usually to the input capability of the receiving device to avoid packet drop and port congestion. Traffic shaping is usually applied in the outbound direction of a port.

l Congestion management handles resource competition during network congestion. Generally, it adds packets to queues first, and then forwards the packets by using a scheduling algorithm. Congestion management is usually applied in the outbound direction of a port.

l Congestion avoidance monitors the use of network resources and drops packets actively when congestion reaches certain degree. It relieves network load by adjusting traffics. Congestion avoidance is usually applied in the outbound direction of a port.

Traffic classification is the basis of all the above-mentioned traffic management technologies. It identifies packets using certain rules and makes differentiated services possible. Traffic policing, traffic shaping, congestion management, and congestion avoidance are methods for implementing network traffic control and network resource management. They are occurrences of differentiated services.

1.2 QoS Supported by the S5600 Series Ethernet Switches

The S5600 series Ethernet switches support the QoS features listed in Table 1-1:

Table 1-1 QoS features supported by the S5600 series Ethernet switches

QoS Feature		Description	Refer to …
Traffic classification		Classify incoming traffic based on ACLs. The S5600 series support the following types of ACLs: l Basic ACLs l Advanced ACLs l Layer-2 ACLs l User-defined ACLs	l For information about ACLs, refer to the ACL Operation and ACL Command manuals. l For information about traffic classification, refer to Traffic Classification.
QoS action	The S5600 series support performing the following QoS actions for packets matching the specified ACL: l Priority marking l Traffic policing l Traffic redirecting l VLAN Mapping l Traffic accounting l Traffic mirroring		l For information about priority marking, refer to Priority Marking. l For information about traffic policing, refer to Traffic Policing. l For information about traffic redirecting, refer to Traffic Redirecting. l For information about VLAN Mapping, refer to VLAN Mapping. l For information about traffic accounting, refer to Flow-Based Traffic Accounting. l For information about traffic mirroring, refer to Traffic mirroring.
QoS action	You can configure the following QoS actions as required on the S5600 series: l Priority trust mode l Protocol packet priority l Line rate l Burst		l For information about priority trust mode, refer to Priority Trust Mode. l For information about specifying priority for protocol packets, refer to Protocol Priority. l For information about line rate, refer to Line Rate. l For information about the burst function, refer to Burst.
Congestion management		The S5600 series support SP and WRR queue scheduling algorithms and support the following three queue scheduling modes: l SP l WRR l SP+WRR	For information about SP and WRR, refer to Queue Scheduling.

1.3 Introduction to QoS Functions

1.3.1 Traffic Classification

Traffic here refers to service traffic; that is, all the packets passing the switch.

Traffic classification means identifying packets that conform to certain characteristics according to certain rules. It is the foundation for providing differentiated services.

In traffic classification, the priority bit in the type of service (ToS) field in IP packet header can be used to identify packets of different priorities. The network administrator can also define traffic classification policies to identify packets by the combination of source address, destination address, MAC address, IP protocol or the port number of an application. Normally, traffic classification is done by checking the information carried in packet header. Packet payload is rarely adopted for traffic classification. The identifying rule is unlimited in range. It can be a quintuplet consisting of source address, source port number, protocol number, destination address, and destination port number. It can also be simply a network segment.

1.3.2 Priority Trust Mode

I. Introduction to precedence types

1) IP precedence, ToS precedence, and DSCP precedence

Figure 1-2 DS field and ToS byte

The ToS field in an IP header contains eight bits numbered 0 through 7, among which,

l The first three bits indicate IP precedence in the range 0 to 7.

l Bit 3 to bit 6 indicate ToS precedence in the range of 0 to 15.

l In RFC2474, the ToS field in IP packet header is also known as DS field. The first six bits (bit 0 through bit 5) of the DS field indicate differentiated service codepoint (DSCP) in the range of 0 to 63, and the last two bits (bit 6 and bit 7) are reserved.

Table 1-2 Description on IP Precedence

IP Precedence (decimal)	IP Precedence (binary)	Description
0	000	Routine
1	001	priority
2	010	immediate
3	011	flash
4	100	flash-override
5	101	critical
6	110	internet
7	111	network

In a network providing differentiated services, traffics are grouped into the following four classes, and packets are processed according to their DSCP values.

l Expedited Forwarding (EF) class: In this class, packets can be forwarded regardless of link share of other traffic. The class is suitable for preferential services with low delay, low packet loss ratio, low jitter, and assured bandwidth (such as virtual leased line);

l Assured forwarding (AF) class: This class is further divided into four subclasses (AF1/2/3/4) and a subclass is further divided into three drop priorities, so the AF service level can be segmented. The QoS rank of the AF class is lower than that of the EF class;

l Class selector (CS) class: This class comes from the IP ToS field and includes eight subclasses;

l Best Effort (BE) class: This class is a special class without any assurance in the CS class. The AF class can be degraded to the BE class if it exceeds the limit. Current IP network traffic belongs to this class by default.

Table 1-3 Description on DSCP precedence values

DSCP value (decimal)	DSCP value (binary)	Description
46	101110	ef
10	001010	af11
12	001100	af12
14	001110	af13
18	010010	af21
20	010100	af22
22	010110	af23
26	011010	af31
28	011100	af32
30	011110	af33
34	100010	af41
36	100100	af42
38	100110	af43
8	001000	cs1
16	010000	cs2
24	011000	cs3
32	100000	cs4
40	101000	cs5
48	110000	cs6
56	111000	cs7
0	000000	be (default)

2) 802.1p priority

802.1p priority lies in Layer 2 packet headers and is applicable to occasions where the Layer 3 packet header does not need analysis but QoS must be assured at Layer 2.

Figure 1-3 An Ethernet frame with an 802.1Q tag header

As shown in the figure above, the 4-byte 802.1Q tag header consists of the tag protocol identifier (TPID, two bytes in length), whose value is 0x8100, and the tag control information (TCI, two bytes in length). Figure 1-4 describes the detailed contents of an 802.1Q tag header.

Figure 1-4 802.1Q tag headers

In the figure above, the priority field (three bits in length) in TCI is 802.1p priority (also known as CoS precedence), which ranges from 0 to 7.

Table 1-4 Description on 802.1p priority

802.1p priority (decimal)	802.1p priority (binary)	Description
0	000	best-effort
1	001	background
2	010	spare
3	011	excellent-effort
4	100	controlled-load
5	101	video
6	110	voice
7	111	network-management

The precedence is called 802.1p priority because the related applications of this precedence are defined in detail in the 802.1p specifications.

3) Local precedence

Local precedence is a locally significant precedence that the device assigns to a packet. A local precedence value corresponds to one of the eight hardware output queues. Packets with the highest local precedence are processed preferentially. As local precedence is used only for internal queuing, a packet does not carry it after leaving the queue.

II. Priority trust mode

After a packet enters a switch, the switch sets the 802.1p priority and local precedence for the packet according to its own capability and the corresponding rules.

1) For a packet carrying no 802.1q tag

When a packet carrying no 802.1q tag reaches the port of a switch, the switch replaces the 802.1p priority of the received packet with the port priority, searches for the local precedence corresponding to the port priority of the receiving port in the 802.1p-to-local precedence mapping table, and assigns the local precedence to the packet.

2) For an 802.1q tagged packet

When an 802.1q tagged packet reaches the port of a switch, you can use the priority trust on the receiving port to configure the port to trust packet priority or use the priority command on the receiving port to configure the port to trust port priority. By default, port priority is trusted and the priority of a port is 0.

l Trusting port priority

In this mode, the switch replaces the 802.1p priority of the received packet with the port priority, searches for the local precedence corresponding to the port priority of the receiving port in the 802.1p-to-local precedence mapping table, and assigns the local precedence to the packet.

l Trusting packet priority

In this mode, the switch searches for the local precedence corresponding to the 802.1p priority of the packet in the 802.1p-to-local precedence mapping table and assigns the local precedence to the packet.

Table 1-5 shows the default 802.1p priority-to-local precedence mapping table. You can modify the default mapping tables at the CLI. For detailed configuration procedure, refer to Configuring the Mapping between 802.1p Priority and Local Precedence.

Table 1-5 802.1p priority-to-local precedence mapping table

802.1p priority	Local precedence
0	2
1	0
2	1
3	3
4	4
5	5
6	6
7	7

1.3.3 Protocol Priority

Protocol packets generated by a switch carry their own priority. You can set a new IP precedence or DSCP precedence for the specific type of protocol packets to implement QoS.

1.3.4 Priority Marking

The priority marking function is to reassign priority for the traffic matching an ACL referenced for traffic classification.

l If 802.1p priority marking is configured, the traffic will be mapped to the local precedence corresponding to the re-marked 802.1p priority and assigned to the output queue corresponding to the local precedence.

l If local precedence marking is configured, the traffic will be assigned to the output queue corresponding to the re-marked local precedence.

l If IP precedence or DSCP marking is configured, the traffic will be marked with new IP precedence or DSCP precedence.

1.3.5 Traffic Policing

The network will be made more congested by plenty of continuous burst packets if the traffic of each user is not limited. The traffic of each user must be limited in order to make better use of the limited network resources and provide better service for more users. For example, traffic can be limited to get only its committed resources during a time period to avoid network congestion caused by excessive bursts.

Traffic policing is a kind of traffic control policy used to limit the traffic and the resource occupied by supervising the traffic. The regulation policy is implemented according to the evaluation result on the premise of knowing whether the traffic exceeds the specification when traffic policing is performed. Normally, token bucket is used for traffic evaluation.

I. Token bucket

The token bucket can be considered as a container with a certain capacity to hold tokens. The system puts tokens into the bucket at the set rate. When the token bucket is full, the extra tokens will overflow and the number of tokens in the bucket stops increasing.

Figure 1-5 Evaluate the traffic with the token bucket

II. Evaluating the traffic with the token bucket

When token bucket is used for traffic evaluation, the number of the tokens in the token bucket determines the amount of the packets that can be forwarded. If the number of tokens in the bucket is enough to forward the packets, the traffic is conforming to the specification; otherwise, the traffic is nonconforming or excess.

Parameters concerning token bucket include:

l Average rate: The rate at which tokens are put into the bucket, namely, the permitted average rate of the traffic. It is generally set to committed information rate (CIR).

l Burst size: The capacity of the token bucket, namely, the maximum traffic size that is permitted in each burst. It is generally set to committed burst size (CBS). The set burst size must be greater than the maximum packet length.

One evaluation is performed on each arriving packet. In each evaluation, if the number of tokens in the bucket is enough, the traffic is conforming to the specification and you must take away some tokens whose number is corresponding to the packet forwarding authority; if the number of tokens in the bucket is not enough, it means that too many tokens have been used and the traffic is excess.

III. Traffic policing

The typical application of traffic policing is to supervise specific traffic into the network and limit it to a reasonable range, or to "discipline" the extra traffic. In this way, the network resources and the interests of the operators are protected. For example, you can limit HTTP packets to be within 50% of the network bandwidth. If the traffic of a certain connection is excess, traffic policing can choose to drop the packets or to reset the priority of the packets.

Traffic policing is widely used in policing the traffic into the network of internet service providers (ISPs). Traffic policing can identify the policed traffic and perform pre-defined policing actions based on different evaluation results. These actions include:

l Drop. Drop the packet whose evaluation result is “nonconforming”.

l Modify the DSCP precedence and forward. Modify the DSCP precedence of the packets whose evaluation result is “nonconforming” and then forward them.

Table of Contents

Strategy Partner :

Chapter 1 QoS Configuration

1.1 Overview

1.1.1 Introduction to QoS