Huawei Technologies Co., Ltd. - 123seminarsonly.com · Huawei Technologies Co., ... The data operation for “transit” is similar to that of the SDH ADM equipment,

Technical White Paper for Resilient Packet Ring (RPR)

Huawei Technologies Co., Ltd.


Copyright ©2007 Huawei Technologies Co., Ltd. All Rights Reserved ihttp://datacomm.huawei.com

Table of Contents

1 Introduction......................................................................................................................... 1

2 Technical Overview ............................................................................................................ 1

2.1 Structure Overview.......................................................................................................... 1

2.2 Data Operation ................................................................................................................ 3

2.3 Frame Format.................................................................................................................. 5

2.4 MAC Entity Structure....................................................................................................... 6

2.5 Queuing Technique ......................................................................................................... 9

2.6 Fair Algorithm ................................................................................................................ 12

2.7 Failure Self-healing........................................................................................................ 13

2.8 Topology Discovery....................................................................................................... 14

2.9 Management Protection ................................................................................................ 15

3 Typical Applications.......................................................................................................... 15

3.1 IP MAN Application........................................................................................................ 15

3.2 LAN Application ............................................................................................................. 17

Appendix A References ........................................................................................................... 18

Appendix B Acronyms and Abbreviations................................................................................ 18


Copyright ©2007 Huawei Technologies Co., Ltd. All Rights Reserved 1http://datacomm.huawei.com


Abstract: Resilient Packet Ring (RPR) is an international standard for establishing IP ring networks,

offering a highly efficient and reliable MAN networking technology. Compared with the old

ring network technology, it features numerous unique advantages. This document

describes its implementation, characteristics and basic applications.

Keywords: RPR, MAN, Ring

1 Introduction

Integrating the intelligent features of IP network, economical feature of Ethernet, and high

bandwidth utilization and availability of optical fiber ring network, RPR (Resilient Packet

Ring) is an ideal networking solution for IP MAN. RPR makes it possible for a carrier to

provide carrier-class services in a MAN at a low cost, offering network reliability of SDH

level but at a much lower transmission cost. RPR is different from traditional MAC with its

most appealing feature of carrier-class reliability. This feature allows it to address

data-oriented service transmission requirements and to form an integrated transmission

solution capable of multi-service processing.

2 Technical Overview

2.1 Structure Overview

Similar to the SDH topology, RPR is a reciprocal dual-ring topology, with each optical span

working at the same rate. The difference is that both the two rings of RPR can transmit data.

These two rings are referred to as Ringlet0 and Ringlet1 respectively.



Station (station)

Ringlet0 (ringlet0)

Link (link)

Ringlet1(ringlet1)

Span (span)

Domain (domain)

Data are transmitted clockwise on Ringlet0 while anti-clockwise on Ringlet1.

Each RPR station uses a 48-bit MAC address used in Ethernet as its address ID. From the

perspective of the link layer of the RPR station, these two pairs of physical optical ports of

transmission/reception are only one link layer interface. From the perspective of the

network layer, only one IP address needs to be allocated.

The link between two adjacent RPR stations is refereed to as a span, and multiple

continuous spans and the stations on them constitute a domain.

From the perspective of a station, its packet switching structure has changed immensely in

comparison with the traditional packet switching structure.

Traffic TrafficTraffic

Bandwidth management

This structure is similar to the ring road of a city, where the stations on the ring are directly

connected, with barely any traffic lights needed, and hence higher efficiency. One RPR

station has one MAC entity and two physical layer entities. The physical layer entities are



associated with the links. Referred to as the access point, the MAC entity includes one

MAC control entity and two MAC service link entities. Each access point is associated with

a loop. By direction, physical layer entities are divided into east physical layer and west

physical layer. The east and west are based on the assumption that the station is to the

north of RPR. The “Tx interface” of the east physical layer and the “Rx interface” of the west

physical layer are connected via the MAC entity into the Ringlet0 of RPR. Similarly, the “Rx

interface” of the east physical layer and the “Tx interface” of the west physical layer are

connected into the Ringlet1 of RPR.

2.2 Data Operation

In agreement with the ring, the stations are designed with ADM data switching for various

data operations. Common basic data operations are:

Insert: It is the process that the station equipment inserts the packets forwarded from other

interfaces into the data stream of the RPR ring;

Copy: It is the process that the station equipment receives data from the data stream of the

RPR ring and gives them to the upper layer for processing;

Transit: It is the process that the data stream passing a station is forwarded to the next

station;

Strip: It is the process that the data passing a station is stopped from further forwarding.

The data operation for “transit” is similar to that of the SDH ADM equipment, in that the

“transit” data streams are not processed by the upper-layer equipment, which greatly

enhances the processing performance of the equipment. Such ADM switching of packets

can easily support various high-speed link interfaces.

The stations use one or any combination of these basic data operations to implement

unicast, multicast and broadcast traffic.

Below please find the schematic diagram for unicast:



Insert to outer ring (insert)

Insert to inner ring (insert)

Transit (transit)

Copy from outer ring (copy)

Strip (strip)

Copy from inner ring (copy)

Transit (transit)

At the source station, the “insert” operation is performed to load the data to Ringlet0 or

Ringlet1. The destination station performs “copy” and “strip” operations. The stations in

between only perform the “transit” operation.

It is worth noting that RPR performs “strip” at the destination station for unicast traffic,

which is different from the traditional ring network technology, where “strip” is performed at

the source station. That the destination station performs the “strip” operation can effectively

enhance bandwidth utilization, so that the space reuse of bandwidth becomes more

effective.

For multicast and broadcast traffic, there are multiple destination stations, so a data

transmission mechanism different from that of unicast should be used. Below please find

an implementation solution of broadcast traffic:



Strip

Download

Download

Download

Download

This solution is Ringlet0 broadcast. Other solutions are Ringlet0 broadcast and dual-ring

broadcast.

2.3 Frame Format

The format of a RPR frame is shown as below:

扩展环控制 (1 字节)

基本环控制 (1 字节)

FCS (4 bytes)

Data (N bytes)

Protocol type (2 bytes)

HEX (2 bytes)

Expanded ring control (1 byte)

TTL base (1 byte)

Source MAC address (6 bytes)

Destination MAC address (6 bytes)

Basic ring control (1 byte)

TTL (1byte)

paritywescftferi paritywescftferi

ressopsffef ressopsffef

Sub-ring ID Frame type Back-track option

Fair option Service class Reserved bit

Expanded frame Reserved bit

Flooding form Strict order

Pass source

Except the ring control byte that reflects the RPR feature, other fields are very similar to

those of the Ethernet frame format. Usually, the Maximum Transmission Unit (MTU) of a



RPR frame is 1616 bytes, and that of an oversized frame is 9216.

The ring control byte contains many control contents, for example, ring selection

information, fair bandwidth allocation option, frame type, service class, fault switching

method, broadcast flag, etc. It provides various functions including active performance

monitoring and fault monitoring, to ensure rich, flexible and efficient ring operations that

can meet the high requirements of the networks for ring network technology.

2.4 MAC Entity Structure

For a RPR station, the MAC entity is the most important part. The MAC entity must

exchange data and control with the upper layer, while working well with various physical

interfaces. Undoubtedly, it needs a flexible and efficient layered model.

Below is the layered reference model of MAC. Generally, there are the service layer, MAC

layer and physical layer. Between the service layer and MAC layer is the MAC service

interface, and between the MAC layer and physical layer is the physical service interface.

In addition, all these three layers have management interfaces for coordination with the

MAC management layer.

MAC layer management

MAC layer

Service access

MAC service layer

MAC control layer

Fair control

Topology discovery

OMAP processing

Protection switching control

Ring routing

Data transmission channel

Packet header processing, FCS check

MAC Layer

Physical layer service accessPhysical layer service access Adaptation sub-layer

Physical layer

The MAC entity contains one MAC control sub-layer and two MAC data channel sub-layers.

These two MAC data channels are for the data exchange of Ringlet 1 and Ringlet 0



respectively. The MAC control entity receives/sends data frames over these two data

channels, and interacts with the MAC client for control and data via the MAC service

interface. This structure is shown in the figure below:

MAC control

MAC

MAC service interface

Physical service interface

Outer ring data channel

Inner ring data channel

Receive ReceiveReceive Send

Send framesReceive frames

Send frames Receive frames

MAC control request

MAC control indication

MAC data request MAC data indication

West physical layer interface East physical layer interface

With this structure, multiple stations can be connected to form a complete end-to-end MAC

service processing flow. Here, the easiest example is given: There are three RPR stations.

Suppose that one data stream is originated from station 1 (S1), passes through S2, and

terminates at S3. The whole data stream flow is shown in the following diagram. As can be

seen in this diagram, the MAC control entity works only when it needs to interact with the

MAC client, while the MAC control entity barely deals with the intermediate stations. For

unicast, this means that only the source station and destination station need to use the

MAC control entity to process the data. In normal cases, for a data stream, various stations

use the same data channel for connection, either Ringlet0 data channel or Ringlet1 data

channel, for better service continuity.




MAC client

MAC data request

MAC control

MAC



PHY

West interface

S1

West interface

S2

West interface

S3

MAC data indication

MAC client

MAC control MAC control





East interface East interface East interface

The MAC control entity contains the functions of the data and control layers, including such

important functions as fair control, protection, topology discovery, sub-ring selection,

running management and maintenance and data encapsulation/encapsulation, as shown

in the following diagram:

MAC

Control

MAC


Physical service interface



ReceiveReceive

Send Send

Send frames Receive framesSend frames Receive frames

MAC control request MAC control indication MAC data request MAC data indication

West physical layer interface

Frame reception/sending

Data interface processing


Control interface processing

Fair control, protection, topology database, path calculation, OAM

Encapsulation/Decapsulation, sub-ring selection, frame collection

East physical layer interface

The MAC data channel is directly associated with the data transmission of each respective

sub-ring. It performs the following four functions: 1. Traffic shaping (for ordered entry into



the shared ring media); 2. Data frame staging at the source station, and data frame

queuing at the transit stations; 3. Selecting data frames for transfer to the local client or

control sub-layer; 4. Selecting data frames to be stripped from the ring.

MAC controlMAC


Receive Send

Send frames Receive frames

MAC control requestMAC control indication MAC data request MAC data indication

Staging/queuing



Shaping

Send framesReceive frames

Select the frames to transfer Select the frames to transfer

Shaping

Select the frames to stripWest sending/east reception

East sending/west reception


ReceivePhysical service interface

West PHY East PHY


2.5 Queuing Technique

When RPR processes transit traffic, there are two queuing and forwarding methods:

Store-and-forward and direct-through. The storage-and-forward method is easy to

implement, while the direct-through method offers higher efficiency. The store-and-forward

mode is the basis that must be supported. Even when the direct-through method is used,

the store-and-forward method may still be used, for example, when the direct-through

queue is temporarily blocked.

According to the ADM switching method of the RPR service, the RPR MAC has the “insert”

buffering queue and “transit” buffering queue.

One RPR station has three “insert” buffer queues, Queue A, Queue B and Queue C, which

correspond to data service classes A, B, and C, for which different scheduling priorities are

provided. RPR divides the traffic to insert into these three classes: Class A, Class B and

Class C. Class A is for low-delay/strict jitter traffic of high priority, with lowest end-to-end

delay and jitter provided and Committed Information Rate (CIR). Class B is for Committed



Information Rate (CIR) and Excess Information Rate (EIR) traffic of medium priority, where

certain bandwidth and end-to-end delay and jitter must be ensured for CIR, but no need for

EIR. Class C is for best-effort common traffic of low priority, with no bandwidth definition.

In the following two diagrams, each service channel (on each ring) of the MAC of RPR ring

can have one or two transit queues – PTQ (Primary Transit Queue) and STQ (Secondary

Transit Queue). Transit traffic of Class A passes the PTQ, and transit traffic of Class B and

Class C passes the STQ.

Of the decision mechanism of RPR MAC, the priorities for “insert” and “transit” traffic are in

the following sequence: Transit PTQ, insert Class A, transit STQ, insert Class B, and insert

Class C.

Check

Shape

Stage

Shape

MAC control sub-layer

Control

PTQ

DataSe

rvice

clas

s B in

serte

d

Servi

ce cl

ass A

inse

rted

Servi

ce cl

ass C

inse

rted

Station client

Data channel sub-layer

To other data channels

Class A/B/C

Send services A/B/C

Fair control mechanism

The above is the schematic diagram for a single transit queue, where all types of transit

traffic are scheduled in a single queue.



Check

Shape

Stage

ShapeControl

PTQ

STQ

Data

Class A

Class B/C

Servi

ce cl

ass A

inse

rted

Servi

ce cl

ass B

inse

rted

Servi

ce cl

ass C

inse

rted

Station client Send services A/B/C

MAC control sub-layer

Data channel sub-layer

Fair control mechanism

To other data channels

This diagram shows the double transit queues. Traffic of Class A is in the PTQ, and traffic

of Class B and C is in the STQ. In other words, for double-transit-queue RPR, the RPR loop

uses separate buffering queues for traffic of high and low priority, and uses strict priority

queue for switching. In other words, the decision mechanism of MAC of the RPR ring will

first process the traffic of high priority in whatever circumstance, and the traffic of low

priority will not affect the real-time switching of that of high priority.

Transit queue is similar to the lane on a ring road in a city: A single queue is equivalent to a

single lane, where all the vehicles run; double queues are equivalent to two lanes, where

cars run on the fast lane and trunks on the slow lane. Obviously, double queues are

superior to single queue technically. Queue scheduling of class A traffic is not affected that

of classes B and C, so the traffic of high priority with low delay is ensured. However, RPR

still takes the single-queue mode as an option, out of consideration of reduced cost. In the

single queue mode, traffic of classes A, B, and C are not divided for queuing, so the

hardware is much easier to implement, with much lower cost. The single queue mode can

be used for networks where only simple data services are provided and performance is not

so important, to reduce cost. However, for the IP MAN and backbone networks, which bear

multiple services, including high-quality services, the double-transit-ring mode must be

used. For large education networks and enterprise networks which usually also bear IP

voice and video services requiring high performance, the double-transit-ring mode is also

recommended.



2.6 Fair Algorithm

RPR allows the stations to share the bandwidth resources available. When the data traffic

is low, RPR can meet the needs of all the stations for traffic loading. When the traffic

becomes heavy, link overload or traffic congestion may occur, as the needs of the traffic for

bandwidth not fully satisfied. In such a circumstance, some stations occupy excessive

bandwidth, by relying on their advantages in position (near) or time (first), while affecting

other stations. To ensure that all the stations can share the bandwidth fairly in the event of

congestion or overload, RPR presents a special fair algorithm for fair bandwidth sharing

and allocation.

The fair algorithm of RPR is a distributed fair algorithm, where the stations transfer the

information required via control messages, including rate allowed, rate recommended, and

strategy indication. Fair algorithm includes traffic measurement and strategy processing

and the multiple stages during the processing, for ultimate achievement of fair allocation.

Bandwidth fairness and congestion control mechanism are functions of the MAC control

sub-layer of the data link layer of RPR. The RPR fair algorithm is applicable to services

where contention for bandwidth is required, that is, EIR services and best-effort services.

The fair algorithm protocol implemented in the fair control unit has the following functions:

Detects and eliminates congestion;

Transmits and receives the fair control messages between the RPR stations;

Provides access control for ring bandwidth based on the service classes, and uses the

even or weighted fair algorithm to control the utilization of the entire ring bandwidth;

Provides separate bandwidth fair operations for Ringlet 0 and Ringlet 1, and allocates

all the bandwidth between any two stations on the ring to the users as global

resources;

Each station can control the rate at which to forward packets to the ring based on the

service class and utilization of the bandwidth on the ring, to ensure every station has

the fair ring bandwidth allocated;

Flows on the different sub-rings in the opposite direction based on the bandwidth fair

control frame and the associated data stream



RPR supports monopolized and weighted fairness arrangement, where the traffic inserted

at each node is not necessarily equal. To avoid Head-of-line blocking, RPR supports the

multi-choke algorithm, but the fair algorithm is more reliable. The advertise rate mechanism

is recommended for smoother value adjustment, so that no large fluctuation of traffic

occurs.

2.7 Failure Self-healing

RPR uses the SDH ring structure, and inherits a major feature, the powerful failure

self-healing capability, which implements failure protection switching in 50ms. The

following diagram illustrates the protection in the event of a failure on the link. Inside the

stations at both ends of the failed link, Ringlet0 and Ringlet 1 are connected to form a new

ring network.

For the traffic being transmitted on the ring, there are two protection modes: Wrap and

Steering (also known as the source route). The following diagram illustrates these two

protection modes:



Normal data transmission Protection self-healing at link failure

Protection self-healing at link failure

Wrap f(wrap) Steering (steering)

The left diagram shows the normal data traffic before the failure, where traffic goes from

station A to Station D over the Ringlet 0, covering the path A-B-C-D;

The middle diagram shows the wrap protection in the event of a failure. When the failure

occurs, optical loop-back is made at the stations on both ends of the failed link and so is the

data path. The overall path is A-B-A-F-E-D-C-D;

The right diagram shows the steering protection mode in the event of a failure, where the

data traffic from station A to station D goes the shortcut path, over the other ring (Ringlet1),

to the destination. The path is A-F-E-D.

The advantage of the wrap mode is that the failure switching is completed in a very short

time (within 50ms), with very few packets lost and hence no traffic interruption. However,

the problem is that much bandwidth is occupied.

The steering mode avoids waste of bandwidth, but it takes a long time to recover due to the

re-convergence, which may cause the interruption of some services.

2.8 Topology Discovery

RPR supports automatic topology discovery. The protection information or topology



information packets contain the topology information, which is broadcast on the ring

network. The possible topology structures are all loop-back structure and chain structure

(when some links fail).

Automatic discovery is helpful for the protection in the event of link failure, and it also

provides good support for network expansion, in enabling station level plug-n-play. In other

words, a station can be added or deleted to or from the ring network without manual

configuration of data.

2.9 Management Protection

As mentioned above, the RPR frame structure contains many option parameters for

performance management, fault management and configuration management, which laid

a good foundation for RPR’s Maintenance, Administration and Maintenance (OAM). RPR

implements fault monitoring, location and isolation on the RPR layer through the special

control frames.

3 Typical Applications

3.1 IP MAN Application

For small and medium-sized cities, a RPR ring can be built on the MAN. One or two of the

nodes can be used as the core and egress, which are connected upward to the backbone

network. Other nodes are distributed at the important offices in the city, for the

access/convergence of Ethernet traffic in those areas. Or, various interfaces such as E1,

E3, POS, and ATM can be provided. Therefore, it can also serve as the leased line access

router, to access various low-rate leased line subscribers. Or, the RPR ring can be

established on the MPLS, and the router is used as the MPLS VPN PE equipment, to

access various VPN subscribers.



RPR solution for small and medium-sized IP MAN

For medium or large-sized IP MAN, the more core and convergence nodes, the larger the

network. Usually, the typical three-layer architecture (core layer, convergence layer, and

access layer) is used, so multiple RPRs are often used for networking. On the core layer, a

core 2.5G/10G RPR ring is built, and on the convergence layer, multiple 2.5G edge RPR

rings are built. The core ring and the edge rings can be connected in intersection or

tangency. Intersection has two connection points, and provides higher reliability. Therefore,

it is recommended that intersection should be used wherever possible.



RPR solution for large and medium sized IP MAN

3.2 LAN Application

RPR can provide the core layer for the LANs with distributed agencies or branches, such

as government networks, enterprise networks and campus enterprise, provides office user

connections, data center connections, and Internet connections, offers logical optimization

to the existing FDDI ring network, and reserves the features of a self-healing ring. The

application is shown in the following diagram:



Application of RPR on LAN

With the RPR ring network, IP can be born on bare optical fibers. This way, even without

special transmission equipment, the bearer network platform can be provided for multiple

IP-based services including data, voice and video.

Appendix A References

IEEE802.17 Resilient packet ring (RPR) access method and physical layer specifications

Appendix B Acronyms and Abbreviations

Acronym/Abbreviation Full Spelling

ADM Add/Drop Multiplexer

CIR Committed Information Rate

EIR Excess Information Rate



Acronym/Abbreviation Full Spelling

FCS Frame Check Sequence

HEX Header Error Check

IP Internet Protocol

MAC Medium Access Control

MPLS Multiprotocol Label Switching

MTU Maximum Transfer Unit

PE Provider Edge

PTQ Primary Transit Queue

RPR Resilient Packet Ring

SDH Synchronous Digital Hierarchy

STQ Secondary Transit Queue

TTL Time To Live

VPN Virtual Private Network

Documents

Huawei Technologies Co., Ltd. - 123seminarsonly.com · Huawei Technologies Co., ... The data operation for “transit” is similar to that of the SDH ADM equipment,