Quidway S8500 Technical White Paper Series -- QoS ...€¦ · Quidway S8500 Technical White Paper Series ... 2 BASIC NETWORKING DIAGRAM ... configure according to the management requirements

Quidway S8500 Technical White Paper Series -- QoS Technical White Paper V1.00

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2005-10-24 Huawei-3Com confidential. No dispersion without permission. Page 1 of 17

Product name Confidentiality level

Quidway® S8500 Confidential Product version V100R002B02

Quidway® S8500 Technical White

Paper Series -- QoS Technical White Paper V1.00

Prepared by: Switch group Date: 2005-10-24

Revised by: Date:

Revised by: Date:

Authorized by: Date:

Huawei-3Com Technology Co., Ltd. All Rights Reserved


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Revision Record Date Version Description Prepared by

2005-5-9

V1.00 First draft finished S8500 R&D

2005-10-24

V1.00 Release International S&M-switch group


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TABLE OF CONTENTS

1 OVERVIEW ............................................................................................................4

2 BASIC NETWORKING DIAGRAM..........................................................................5

3 INTRODUCTION TO FEATURES...........................................................................5

3.1 SERVICE MODEL................................................................................................5

3.2 TRAFFIC CLASSIFICATION...................................................................................6

3.3 TRAFFIC MONITORING ........................................................................................6

3.4 PRIORITY TAGGING ............................................................................................7

3.5 QUEUE DISPATCHING .......................................................................................10

3.6 CONGESTION AVOIDANCE.................................................................................12

3.7 TRAFFIC SHAPING ...........................................................................................14

3.8 POLICY-BASED ROUTING (PBR) .......................................................................15

4 BASIC FLOW OF S8500 QOS..............................................................................16


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1 Overview

The traditional packet network does not distinguish all the packets and treats them in the

same way. Each router/switch adopts the First in First out (FIFO) policy for the processing

of all the packets, and tries its Best-Effort to send the packet to the destination. However,

they make no commitment or guarantee with regard to the transmission performance such

as delay, delay jitter, etc during packet transmission.

With fast development of the computer network, voices, images and important data, which

are sensitive to bandwidth, delay and jitter, are being increasingly transmitted over the

network. In this situation, the service resources over the network are enriched greatly on

one hand. On the other hand, higher requirements have been raised for the Quality of

Service (QoS).

The Ethernet technology is a network technology widely deployed today. At present,

Ethernet has become the dominant technology in various independent Local Area

Networks (LANs). Meanwhile, many Ethernet-style LANs have become part of the Internet.

In addition, with ceaseless development of the Ethernet technology, the Ethernet access

mode is becoming one of the leading access modes to Internet users. Therefore, to

realize the end-to-end full network QoS solution, it is inevitable that we need to consider

how to guarantee the QoS over Ethernet. This requires the Ethernet switching equipment

to adopt the Ethernet QoS technology, so as to provide different levels of QoS guarantees

for different types of service flows, especially to support the service flows that have higher

requirements for delay and jitter.


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2 Basic Networking Diagram

S8500

S8500

S8500

VPN1

VPN2

VPN3

VPN2

VPN3

QinQ

PE1 S8500

P S8500

PE2 S8500

MPLS DOMAIN

S8500

NAT

VPN1

Private address

Public address

S8500

S8500

S8500

VPN1

VPN2

VPN3

VPN2

VPN3

QinQ

PE1 S8500

P S8500

PE2 S8500

MPLS DOMAIN

S8500

NAT

VPN1

Private address

Public address

Figure 2-1 Basic networking diagram

3 Introduction to Features

3.1 Service Model

The service model refers to a group of end-to-end QoS functions. The simplest service

model is Best-Effort (FIFO). In this mode, the network equipment tries its best to forward

packets, but makes no commitment or guarantee for the transmission performance such

as delay, delay jitter, etc during packet transmission. To ensure the QoS during

transmission over the network, the concept of Diff-Serv is brought forward. Diff-Serv is a

multi-service model, usually used to provide the end-to-end QoS for some important

applications. It offers the special service according to the QoS specified by each packet,

thereby meeting different QoS requirements.

S8500 supports the QoS in the Diff-Serv mode.


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3.2 Traffic Classification

For Diff service, it is necessary to differentiate between different traffic on the network and

to specify different QoS parameters for the packets of different classes of service. Traffic

classification is to identify the packets that have the characteristics of a certain class

according to certain rules. Classification rules refer to the filtering rules the administrator

configure according to the management requirements. The classification rule can be very

simple, e.g. the ToS field in the IP header, according to which the traffic with different

priorities can be identified. Meanwhile, it can be very complex, e.g. the information on the

integrated link layer (layer 2), the network layer (layer 3), and the transport layer (layer 4)

like the MAC address, the IP protocol, the source IP address, the destination IP address

or the port number of the application, according to which the packets can be classified.

Generally, the classification rule is limited to information on the header of the

encapsulation packet, while the contents of the packet are rarely used as the classification

rule.

S8500 supports the ACL rules for traffic classification of the Layers 2, 3 and 4, including:

source MAC addresses, destination MAC addresses, VLAN IDs, source IP addresses,

destination IP addresses, source TCP/UDP port numbers, protocol types of the packets,

IP precedence levels, TOS priorities, DSCP priorities, whether the packet is fragmented,

etc.

3.3 Traffic Monitoring

To better serve the users with limited network resources, the QoS function can monitor

the traffic of a specified user on the input port, so as to make the traffic fit the network

resources allocated to it. Traffic monitoring uses the Token Bucket (TB) for traffic control.


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Figure 3-1 Schematic drawing for traffic monitoring

Figure 3-1 shows the process of traffic monitoring. First, the packet is classified. If the

packet belongs to a certain class which has specified the traffic characteristics, it will be

put into the token bucket for processing. If the token bucket contains enough tokens to

transmit the packet, the packet will pass. If the token bucket does not contain enough

tokens to transmit the packet, the packet will be discarded. In this way, the traffic

composed of a certain class of packets can be controlled.

The token bucket puts the tokens into the bucket at the rate set by the user. Meanwhile,

the volume of the bucket is set by the user as well. Therefore, when the quantity of the

tokens in the bucket reaches the volume of the bucket, new tokens will not be put into the

bucket. When the packet is processed by the token bucket, if the token bucket has

enough tokens to transmit the packet, the packet will be forwarded continuously.

Meanwhile, the quantity of the tokens in the bucket will decrease according to the length

of the packet. When the quantity of the tokens in the bucket becomes so small that the

packet cannot be transmitted any more, the packet will be discarded. In this way, the

traffic of packets can only be equal to or smaller than the rate at which the tokens are

generated, thereby reaching the goal of traffic monitoring.

S8500 supports traffic monitoring, with the granularity as 8Kbps.

3.4 Priority Tagging

By tagging the packet priority, the QoS of different packets can be distinguished. The

S8500 Ethernet switch can provide the service of tagging the priority for a specific packet.

The contents of the tag include TOS, DSCP, 802.1p, EXP, etc. These priority tags are fit

Packets to be sent on this

interface

Puts tokens into the bucket at the

specified rate

Token bucket

Discard packets

Continues transmission


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for different QoS models respectively, and defined in different models. The following

contents introduce the IP precedence, the TOS priority, the DSCP priority, the 802.1p

priority, and the EXP priority.

1. IP precedence, TOS priority and DSCP priority

Figure 3-2 IP precedence, TOS priority and DSCP priority

As shown in the figure, the TOS field of the IP header includes eight bits. Among them, the

former three bits represent the IP precedence, with the value ranging from 0 ~ 7. The four

bits -- bits No.3 ~ No.6, represent the TOS priority, with the value ranging from 0 ~ 15. In

RFC2474, the TOS field of the IP header has been defined anew, naming it the DS field.

The former six bits (bits 0 ~ 5) of this field represent the DSCP priority, with the value

ranging from 0 ~ 63, while the latter two bits (bits 6 ~ 7) are reserved.

2. 802.1p priority

The 802.1p priority is situated in the header of the layer-2 packet, and fit for the situation

where the layer-3 header needs no analysis while the QoS needs to be ensured at the

layer-2.

Figure 3-3 Frame structure of the 802.1Q protocol

As shown in the figure, each host, which supports the 802.1Q protocol, adds a four-byte

802.1Q tag header to the end of the source address of the original Ethernet frame header

when sending the packet. This four-byte 802.1Q tag header includes a two-byte Tag

Protocol Identifier (TPID, with the value as 0x8100) and two-byte Tag Control Information

(TCI). The TPID is a new type defined by IEEE, used to indicate the packet with the


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802.1Q tag added to. The figure below shows the detailed contents of the 802.1Q tag

header.

Figure 3-4 Schematic drawing for the 802.1p priority

In the above figure, the Priority field in the TCI byte represents the 802.1p priority. It

consists of three bits, the value of which range from 0 ~ 7. The three bits indicate the

priority of the frame. Eight types of priorities are available, mainly used to determine which

packet is to be transmitted first when the switch is congested. The reason why this priority

is named the 802.1p priority is that the applications related to this priority are defined in

detail in the 802.1p standard.

In the Vlan vpn (QinQ) field, Differentiated service is applied. That is, the packets are

classified by using the inner-layer VLAN or 802.1p priority information, thus determining

the priority of packet queue dispatching and packet discard.

Figure 3-5 Schematic drawing for the application of 802.1p mirroring

S8500

VPN2

VPN2

VPN3

QinQ

VPN3

The 802.1p priority identifies the inner-layer tag

and determines the dispatch and discard level of

the packet on the interface QinQ

QinQ

S8500

VPN2

VPN2

VPN3

QinQ

VPN3

QinQ

QinQ


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3. EXP priority

Figure 3-6 Schematic drawing for the MPLS tag

In the Ethernet MPLS packet, as there is a shim between the link layer (layer-2) and the

network layer (layer-3), we can expand the unused EXP field in the shim. This field

contains three bits, which determine the priority for dispatching and discarding the packet.

Figure 3-7 Schematic drawing for the application of the EXP priority tag

3.5 Queue Dispatching

Upon network congestion, the method of queue dispatching is usually adopted to solve

the problem that multiple packets contend for the resources at the same time. S8500

provides the following two queue dispatching algorithms: the Strict-Priority (SP) queue

dispatching algorithm and the Weighted Round Robin (WRR) dispatching algorithm.

1. SP dispatching algorithm

PE1 S8500

P S8500

PE2 S8500

MPLS DOMAIN

PE1 S8500

P S8500

PE2 S8500

MPLS DOMAIN

Maps EXP according to DSCP of the IP packet;

determines the dispatching and discard level of the

packet on the outbound interface

Classifies the traffic according to EXP of the MPLS

packet; determines the dispatching and discard level

of the packet on the outbound interface


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Packets sent via thisinterface

high priority

Low priority

Classify

Packets sent

Sending queueDequeue

queue 7

queue 6

queue 5

queue 4

queue 3

queue 2

queue 1

queue 0

Figure 3-8 Schematic drawing for SP queue dispatching

The SP queue dispatching algorithm is designed for the application of key services. Key

services share an important feature, i.e. requiring obtaining the service first upon

congestion so as to reduce the delay of response. Taking the port with eight output

queues for example, the priority queue divides the eight output queues of the port into

eight classes, i.e. queues 7 ~ 0, whose priorities decrease in turn.

Upon queue dispatching, the SP algorithm first sends the packets in the queue with a

higher priority by strictly following the preferential order. When the queue with a higher

priority is empty, the SP algorithm will send the packets in the queue with a lower priority.

Therefore, the packets of a key service will be put into the queue with a higher priority

while the packets of a non-key service (e.g. E-Mail) will be put into the queue with a lower

priority. In this way, the packets of a key service are sent first while the packets of a

non-key service are sent during the idle intervals when the key service is being

processed.

The disadvantage of the SP algorithm is: upon congestion, if the queue with a higher

priority contains packets for a long time, the packets in the queue with a lower priority will

starve to death as they are not served.

2. WRR dispatching algorithm


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The port of the switch supports eight output queues. The WRR dispatching algorithm

dispatches the queues in turn, so as to ensure that each queue can be served for a

certain period of time. Taking the port with eight priority queues for example, the WRR

algorithm configures a weight for each of them, i.e. w7, w6, w5, w4, w3, w2, w1, and w0

respectively. A weight represents the proportion of resources obtained. For a 100M port,

the weights of the WRR queues can be set to 25, 15, 5, 5, 25, 15, 5, and 5 (corresponding

to w7, w6, w5, w4, w3, w2, w1, and w0 respectively). In this way, it can be ensured that

the queue with the lowest priority can obtain at least 5Mbit/s bandwidth, thereby avoiding

the disadvantage of the SP algorithm that the packets in the queue with a lower priority

are not served for a long time.

Another advantage of the WRR algorithm is: although the queues are dispatched in turn,

they are not assigned the fixed time segment. That is, if a queue is empty, the next queue

will be dispatched at once, thereby taking full use of the bandwidth resources.

3.6 Congestion Avoidance

Upon network congestion, common network equipment adopts the tail drop method to

avoid congestion. That is, all the incoming packets will be discarded when the queue

length has reached the set value. However, for the packets of the TCP type, etc, TCP

timeout will be caused as a large number of packets are discarded. This can trigger the

slow start-up and congestion avoidance mechanism of the TCP, which will reduce the

number of packets sent by the TCP. When a queue discards the packets of multiple TCP

connections at the same time, the multiple TCP connections will simultaneously enter the

slow start-up and congestion avoidance state, which is called TCP global synchronization.

In this case, the packets sent to the queues from multiple TCP connections will decrease

simultaneously, resulting in that the number of the packets sent to the queues cannot

match the rate at which the packets are sent over the line and utilization of the line

bandwidth decreases. Furthermore, the traffic of packets sent to the queues fluctuates up

and down, causing the traffic over the line to change between the two extremes


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frequently.

To avoid the above cases, S8500 supports the discard policy of Weighted Random Early

Detection (WRED). Users can set the maximum and minimum thresholds for the length of

the queue. When the length of the queue is smaller than the minimum threshold, no

packet will be discarded. When the length of the queue is between the maximum and

minimum thresholds, WRED begins to discard packets. The longer the queue length is,

the higher the probability of discarding packets will be. When the length of the queue is

greater than the maximum threshold, all the incoming packets will be discarded.

As WRED discards packets randomly, global synchronization of the TCP can be avoided.

When the packets of a TCP connection are discarded and the transmission speed is

slowing down, other TCP connections still keep relatively high transmission speeds. In

this way, there are always TCP connections that transmit packets at high speeds and

therefore utilization of the line bandwidth is improved.

Directly using the maximum and minimum thresholds as well as the length of the queue to

make comparison and discard packets (this is to set the absolute length of the queue

threshold), will result in unfair treatment to the bursting data flow, which is unfavorable for

the transmission of data flows. Therefore, when making comparison with the maximum

and minimum thresholds and discarding packets, the average length of the queue is

adopted (this is to set the relative value of the queue limit compared with the average

length). The average length of the queue is the result of low-pass filtering the queue

length. It reflects the variation trend of the queue, but is not sensitive to the bursting

changes in queue length, thereby avoiding unfair treatment to the bursting data flow.

For the packets with different priorities of each queue, S8500 can set different queue

length filtering factors, minimum thresholds, maximum thresholds, and discard

probabilities. Meanwhile, it can offer different discard features for the packets with

different priorities of different queues.

In case of congestion, S8500 will drop the packets as soon as possible to release the

queue resources and try not to put the packets into the queue with a higher delay, thus

removing congestion.


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When receiving packets, S8500 assigns the discard levels for them (also called packet

coloring). The discard levels are numbered from 0 ~ 2. “2” stands for red, “1” stands for

yellow, and “0” stands for green. Upon congestion, red packets will be discarded first while

the green ones are discarded last.

The parameters and discard thresholds for congestion avoidance can be configured

respectively by queue and by discard level.

S8500 supports two discard algorithms:

l Tail drop: when discarding packets, this algorithm determines whether to discard

packets according to the discard thresholds of the red, yellow and green queues

which are divided based on the discard levels. When the red, yellow and green

packets respectively exceed the maximum thresholds of the corresponding queues,

the system begins to discard the packets that come in after the maximum threshold of

the queue is reached.

l WRED: this algorithm takes the discard level into account when discarding the

packets according to different queues. When the red, yellow and green packets

respectively exceed the minimum thresholds of the corresponding queues, the

system begins to discard packets that are between the minimum and maximum

thresholds of the queue with a certain slope. When the red, yellow and green packets

exceed the maximum thresholds of the corresponding queues, the system begins to

discard the packets that come in after the maximum threshold of the queue is

reached.

3.7 Traffic Shaping

Traffic shaping is to control the rate at which the packets are transmitted, so as to send

them at a steady rate. Traffic shaping is usually intended to make the packet rate match

the downstream equipment, thus avoiding unnecessary packet discard and congestion.

The major difference between traffic shaping and traffic monitoring is: the former is to

cache the packets that exceed the rate limit so as to send them at a steady rate while the

latter is to discard the packets that exceed the rate limit. But traffic shaping can increase


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the delay while traffic monitoring does not result in additional delay. S8500 supports traffic

shaping intended for the port, i.e. to carry out traffic shaping for all the traffic on the port.

Meanwhile, it supports traffic shaping intended solely for a specific output queue on the

port.

3.8 Policy-Based Routing (PBR)

S 8 5 00

N A T

Public address

Private address

S 8 5 0 0

S 8 5 00

N A T

Public address

Private address

S 8 5 0 0

Figure 3-9 Schematic drawing for the application of PBR

S8500 realizes the PBR function by classifying the packets and then configuring

redirection for the packets of a certain class. As shown in Figure 3-9, S8500 identifies the

packet which accesses the public network and whose source IP is the private network

address by classifying the source and destination IP addresses of the packet. Then, it

redirects the packet trough PBR to the NAT equipment for NAT, thus enabling it to access

Internet.


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PQ

WRR

PQ+WRR

Classification

A

Drop

CAR

GTS

Queue0

Queue1

Queue2

Queue7

RED

WRED

Drop Congest

DSCP COS CAR

S-D D-S S-Pt D-pt Protocol

TOS

AC

L

Ingress

packets

4 Basic Flow of S8500 QoS

Figure 3-10 Flow chart of S8500 QoS

S8500 distinguishes different traffic by using different policies through traffic classification.

The policies include the source MAC, the destination MAC, the Ethernet type, VLAN,

802.1p priority, ip-protocol, the source IP, the destination IP, the application port number,

the icmp packet type, ip-precedence, TOS, DSCP, EXP, as well as the vlan and 802.1p

priority in the inner-layer TAG of the QINQ packet.

After classifying the traffic in detail, in addition to simple passage permission and discard

control, the Policy Control List (PCL) of S8500 can implement abundant actions for the

traffic such as traffic monitoring, traffic statistics, QoS parameters (the 802.1p priority,

DSCP, EXP, the discard priority, etc), retagging, packet mirroring, packet redirection,

determining the outgoing queue, etc.

After marking the discard level of the traffic through mapping the packet priority, the

congestion avoidance module adopts the packet drop policy according to the discard

mode set by the user and according to the maximum queue limit and the minimum queue

limit of the red, yellow and green packets. In the tail drop mode, when the red, yellow and

green packets respectively exceed the minimum thresholds of the corresponding queues,

the system begins to discard the packets that are between the minimum and maximum

queue limits with a certain slope. When the red, yellow and green packets respectively

exceed the maximum thresholds of the corresponding queues, the system begins to

Egress

Queue


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discard the packets that come in after the maximum threshold of the queue is reached.

The packets, which pass congestion avoidance and are permitted to be forwarded, will

enter the relevant queues. The queue management module uses the SP or WRR

algorithm to dispatch the packets. During the forwarding of the packets, traffic shaping is

carried out for the outgoing rate of the packets according to the size of the token bucket.

Documents

Quidway S8500 Technical White Paper Series -- QoS ...€¦ · Quidway S8500 Technical White Paper Series ... 2 BASIC NETWORKING DIAGRAM ... configure according to the management requirements