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RPR Technology White Paper

Hangzhou H3C Technologies Co., Ltd. /16

H3C SR8800 RPR Technology White Paper

Keywords: RPR, IP ring network

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

a highly efficient and reliable metropolitan area network (MAN) networking technology. RPR has

numerous advantages over earlier ring network technologies. This document describes its

implementation, characteristics, and basic applications, as well as the unique implementation on the

SR8800.

Acronyms:

Acronym Full spelling

FRR Fast ReRoute

FS Force Switch

LSP Label Switching Path

MPLS Multiprotocol Label Switching

MS Manual Switch

OAM Operations, Administration, Maintenance

RPR Resilient Packet Ring

TP Topology Protect

TC Topology Checksum



Table of Contents

Overview ······························································································································· 3

RPR Features ························································································································ 3 Concepts ························································································································· 3

Span ························································································································· 3 Edge ························································································································· 3 Wrapping ··················································································································· 3 Steering····················································································································· 3 Host ·························································································································· 4 Ringlet Selection Table ································································································ 4

Protocol Processing Mechanism ·························································································· 4 Data Operations on RPR Stations ················································································· 4 Efficient Bandwidth Usage ···························································································· 6 Automatic Topology Discovery ······················································································ 6 Topology Protection and Self-healing ············································································· 6 Fairness Algorithm ······································································································ 8 QoS Guarantee ·········································································································· 9 Miscellaneous ·········································································································· 11

RPR Data Frame Format ·································································································· 11

RPR Application Scenarios ·································································································· 12 Application in Small and Medium-Sized MANs and LANs ······················································ 13 Application in Large and Medium-Sized IP MANs ································································· 14

RPR Features on the SR8800 ································································································ 15 Powerful Service Switching Performance ············································································ 15 Complete QoS Capabilities ······························································································· 15 Abundant Ring Selection Mechanisms ················································································ 15 Ease of Configuration ······································································································ 15 High Reliability ················································································································ 15 Mate Port Smart Connection ····························································································· 16

References ·························································································································· 16



Overview Resilient packet ring (RPR) is a MAC layer technology standardized by the IEEE 802.17 workgroup. It is independent of the physical layer and can run on SONET/SDH, Ethernet, and dense wavelength division multiplexing (DWDM).

Integrating the high reliability of SDH self-healing and Ethernet advantages such as low cost, large bandwidth, flexibility, and scalability, the RPR technology provides bandwidth management with data optimization capability and high-performance multi-service transmission on a ring topology.

RPR Features

Concepts Figure 1 RPR ring topology

S0 S1 S2 S3 S4 S5 S253

Congestion domainS254

inner ringletouter ringlet

edge span links

Span

The portion of a ring bounded by two adjacent stations. A span comprises a pair of unidirectional links transmitting in opposite directions.

Edge

A span on which data frames are not allowed to pass is called an edge. An edge can result from fiber cut, signal degradation, manual switch, or any other error or protection action.

Wrapping

Wrapping is a protection mechanism of RPR. When an edge occurs (a span or station fails), this protection mechanism redirects traffic to the original destination by sending it in the opposite direction around the ring. The two ringlets thus form a closed single ring around the point of the failure. As wrapping allows quick switchover without ringlet selection update, data frame loss is minimized, but at the price of bandwidth.

Steering

Steering is another protection mechanism of RPR. In steering mode, the RPR stations on the ringlet update the ringlet selection upon detection of an edge. Based on the update result, the protected traffic is steered to the newly selected ringlet. The steering mechanism thus avoids the bandwidth



waste with the wrapping mechanism, but as it requires topology reconvergence, it can cause frame loss and service interruption.

Host

For the purpose of this document, the upper layer of the RPR MAC layer is referred to as the host. The host receives, processes, and transmits traffic destined for the local station.

Ringlet Selection Table

Each RPR station maintains a ringlet selection table, which stores information such as ringlets and hops to reach other stations on the RPR ring.

When the ring is closed, two paths are available for reaching a destination, of which the shortest one is selected by default.

Protocol Processing Mechanism RPR consists of two unidirectional counter-rotating ringlets identified as Ringlet 0 and Ringlet 1. All links on the ring operate at the same data rate. The two ringlets of RPR can transmit data frames and control frames at the same time.

Each RPR station is identified by a 48-bit MAC address. The MAC address of an RPR station on the RPR ring must be unique. The two physical optical interfaces of an RPR station correspond to one logical interface on the network layer.

Data Operations on RPR Stations

Stations on an RPR ring handle data frames by performing the following operations:

Insert: to place a frame received from outside of the RPR ring onto a ringlet. Transit: to pass a frame to the next station. As the frame is quickly forwarded at the RPR MAC

layer, the throughput of the RPR station is improved. For a multicast/unicast data frame, the RPR station also sends a copy to the upper layer.

Copy: to deliver an inbound frame from the ring to the upper layer. The copying of a frame does not imply its removal from the ring.

Strip, to remove a frame from a ringlet. A station strips a frame if the frame is destined for or sourced from the local station, or if the time to live (TTL) value of the frame expires.

The source station inserts the unicast frame into the data stream on Ringlet 0 or Ringlet 1, the transit stations transit the frame, and the destination station copies and strips the frame.

For a multicast or broadcast frame, the stations on the RPR ring copy and transit the frame. When the frame travels back to the source, the source station strips the frame from the ring.

Figure 2, Figure 3, and Figure 4 show how unicast, multicast, and broadcast frames are transmitted on an RPR ringlet respectively.



Figure 2 Unicast transmission on an RPR ring

Transit

Strip

Strip

Transit

Copy from ringlet 1

Copy from ringlet 0

Insert to ringlet 0

Insert to ringlet 1

TransitStrip

Figure 3 Broadcast transmission on an RPR ring

Copy from ringlet 0

Copy from ringlet 0

Copy from ringlet 0

Copy from ringlet 0

Strip

Strip

Insert to ringlet 0

Figure 4 Multicast transmission on an RPR ring

Insert to ringlet 0

Copy from ringlet 0

Transmit

Copy from ringlet 0

Strip

Transmit

TransmitStrip



Efficient Bandwidth Usage

RPR allows efficient bandwidth usage on a ring network:

Destination stripping: Traditional ring technologies such as SDH/SONET use source stripping, where a unicast frame is removed from the ring only after it returns to the source station, RPR adopts destination stripping where a unicast frame is removed from the ring as soon as it reaches the destination station.

Spatial reuse: The adding of individual frames is not synchronized between ringlets. On an RPR ring, the frame transmission event on any one link is independent of frame transmission events on other links. With the RPR ring topology, this allows per-link bandwidth to be utilized beyond that possible with other ring-based LAN technologies. By supporting concurrent per-ringlet transmissions, the bandwidth available to the stations on a ringlet exceeds the individual link capacity. On non-overlapping segments, concurrent transmissions of independent traffic are allowed. On overlapping segments, bandwidth allocated for traffic transmissions is assigned based on a bandwidth fairness algorithm.

Automatic bandwidth allocation: Different from the complex static bandwidth allocation with SDH, RPR supports bursty traffic, allowing fast service deployment.

No redundant bandwidth: Unlike SDH, RPR can transmit frames on both ringlets without having to reserve bandwidth for protection purpose. With RPR, the two ringlets back up each other to achieve self-healing.

Support for broadcast and multicast: For a broadcast or multicast frame, only one copy travels on the ring. This broadcast/multicast frame is copied and transited on each RPR station and stripped off from the ring when it travels back to the source station.

L2 rapid forwarding: As a station processes only frames destined for it, forwarding speed is improved.

Automatic Topology Discovery

Each station on an RPR ring uses topology and protection (TP) frames to broadcast its topology and protection status information. After receiving the information, other stations update their local topology databases. In this way, all stations on the ring eventually maintain a consistent topology database.

When detecting a protection state change, a station first sends eight TP frames at (short) intervals of 1 to 20 milliseconds (the default is 10 milliseconds). Then, it sends TP frames periodically at (long) intervals of 50 milliseconds to 10 seconds (the default is 100 milliseconds). This mechanism enables all stations on the ring to sense protection and topology changes timely and reliably, ensuring timely protection switchover in addition to topology synchronization.

The automatic topology discovery mechanism of RPR achieves “plug and play” for RPR stations, allowing the stations to obtain the ring topology information and be sensed by other stations automatically.

Topology Protection and Self-healing

RPR can provide protection against sudden failures, allowing services to recover within 50 milliseconds. RPR provides two protection mechanisms: steering and wrapping.

In steering mode, a station, upon detection of a fault on a ringlet, broadcasts the protection state change with TP frames on the ring. This also triggers ringlet selection. When other stations receive



the TP frames, they transit to the corresponding protection state, recalculate the reachability of the stations on the ring, and update their ringlet selection tables to select the ringlet that retains connectivity to the destination stations.

Unlike the steering mode, the wrapping mode does not involve ringlet selection update on the entire ring. Instead, the stations at both sides of a point of failure transit to the wrapping mode upon detection of the failure while other stations transmit traffic along the original path. When the protected traffic arrives at the station on one side of the point of failure, it is directed to the opposing healthy ringlet to reach the station on the other side of the point of failure. Then the protected traffic travels along the original ringlet to reach the destination.

Compared with the steering mode, the wrapping mode provides quicker protection resulting in less frame loss but requires more bandwidth.

Figure 5 Path from Station A to Station B before a fault occurs

Path before switchover

Station A

Station B

Figure 6 Path steering upon detection of a fault (A -> B)

Steered path

Station A

Station B



Figure 7 Path wrapping upon detection of a fault (A -> B)

Wrapped path

Station B

Station A

To benefit from both, the RPR implementation of the SR8800 adopts the wrap-then-steer mode. In this mode, RPR starts the wrapping mode once a link fails to ensure continuity of the ongoing service and reduce short-term frame loss, and switches to the steering mode after the topology converges to improve long-term ringlet utilization efficiencies.

Fairness Algorithm

Resources on a ring network are shared among the stations. RPR provides a global fairness algorithm on the entire ring network to guarantee the fairness of the sharing and improve bandwidth usage efficiency. The fairness algorithm of RPR can regulate traffic dynamically to minimize the possibility of congestion and to handle bursty large volume traffic effectively, thus ensuring normal operation of the network.

To achieve bandwidth allocation fairness, each RPR station monitors the use of its bandwidth and provides an explicit backpressure mechanism between stations. With this mechanism, the station notifies a source station of the current available capability, having the source station regulate traffic transmission. Thus, bandwidth allocation fairness is achieved on the ring.

The fairness algorithm of RPR involves the following three aspects:

Determining the congestion threshold on a station Determining the broadcast rate to the upstream station Determining the traffic insertion rate on a station

When congestion occurs on a station, the station sends an advertisement on the ringlet opposite to the data transmission direction to advertise a fair rate. Receiving the advertisement, the upstream station then decreases the frame insertion rate down to the advertised fair rate. If congestion also occurs on the current station, it does the same as its downstream station did.

To guarantee high-priority services, bandwidth management regulates low-priority data frames, but not high-priority data frames or control frames. The bandwidth management capability of RPR allows for bandwidth allocation efficiency and fairness, which cannot be provided by Ethernet or other ring network technologies where bandwidth management is not available.

The following figure illustrates how the fairness algorithm works on an RPR ring comprising stations A, B, C, D, E, and F. Suppose the bandwidth of each link is 10 Gbps and traffic travels Ringlet 0.



Figure 8 Bandwidth fairness algorithm

D

10G RPR

C

F

A

B

E

4 Gbps3.3 Gbps

4 Gbps2 Gbps3 Gbps3.3 Gbps

4 Gbps3 Gbps3.3 Gbps

Send

control

framesSend control frames

Congested

The following is what occurs on the RPR ring:

1) Both stations C and B send 4 Gbps traffic to station D. They share bandwidth on span C–D and represent 8-Gbps bandwidth in total. As the link bandwidth is 10 Gbps, no congestion is present.

2) Station A also sends 4 Gbps traffic to station D. As a result, the total traffic on span C–D reaches 12 Gbps, exceeding the maximum link bandwidth (10 Gbps). Congestion thus occurs on span C–D.

3) With the fairness algorithm, station C performs calculation immediately after detecting the congestion to decrease the rate of putting traffic onto the ring to 2 Gbps. In the mean time, it sends control frames to station B reversely along the inner ringlet to transmit congestion and fairness algorithm information.

4) Upon receiving the fairness control frames, station B immediately decreases its traffic rate and sends fairness control frames to station A. According to the fairness algorithm, both station C and station B adjust traffic rate to 3 Gbps.

5) After receiving the control frames, station A acts likewise.

By repeating this process, stations A, B and C adjust their traffic rates to 3.3 Gbps, sharing bandwidth fairly.

In this example, absolute fairness is maintained. RPR, however, allows exclusive bandwidth allocation and weighted bandwidth allocation. Thus, traffic rate can be different at each station depending on its fairness weight.

QoS Guarantee

With the capability of 50 milliseconds self-healing, efficient bandwidth use, and advanced RPR-Fa algorithm, RPR can provide QoS guarantee for services, achieving high reliability, large throughput, low delay, and low loss rate.

RPR services fall into three classes: class A, class B and class C, with decreasing priorities.

Class A: Provides an allocated, guaranteed data rate, a low end-to-end delay, and low jitter to support time division multiplexing (TDM) services. It is subdivided into subclasses A0 and A1. For the A0 service, bandwidth is reserved on the entire ring and the unused reserved bandwidth



cannot be used for lower priority services. For the subclass A1 and class B services, bandwidth is reclaimable and the unused bandwidth can be used for lower priority services.

Class B: Provides an allocated, guaranteed data rate, and low delay and jitter to transmit data by the priority order. Class B can be divided into two subclasses: committed information rate (CIR) and excess information rate (EIR), that is, B-CIR and B-EIR.

Class C: Provides a best-effort service with no allocated or guaranteed data rate and no bounds on delay or jitter for traditional IP traffic.

RPR uses the Sc field to indicate the priority of an RPR frame.

For traffic to be forwarded, the RPR MAC layer adopts either a single transit queue or dual transit queues. On a single-queue station, all traffic is transmitted in a first in first out (FIFO) queue regardless of their priorities. On a dual-queue station, traffic is transmitted either in the primary transit queue or in the secondary transit queue as follows:

Class A traffic: transmitted in the primary transit queue. Class B traffic: transmitted in the secondary transit queue. The subclass B-CIR traffic has

precedence over the class C traffic and is not regulated by the fairness algorithm. The subclass B-EIR traffic has the same priority as the class C traffic and is regulated by the fairness algorithm.

Class C traffic: transmitted in the secondary transit queue.

A CIR is allocated for the class B services. Traffic conforming to the CIR has precedence over the nonconforming traffic (B-EIR traffic).

The RPR MAC layer controls the traffic transmission order, which differs with the queue model.

On a dual-queue station

The RPR MAC layer assigns traffic sent by the host to the host queue and traffic to be forwarded for other stations to the transit queues. The RPR MAC layer dequeues frames from the transit queues in the following order:

1) Frames in the primary transit queue. 2) Class A frames so long as the secondary transit queue is not full. In case the length of the

secondary transit queue exceeds a specified threshold, the frames in the queue are sent. 3) B-CIR frames. 4) B-EIR and Class C frames, if the fairness algorithm is obeyed. 5) Frames in the secondary transit queue, if no higher priority frames are waiting for transmission. On a single-queue station

Frames in the transit queue are transmitted first, regardless of their priorities. The class C and subclass B-EIR frames are regulated by the fairness algorithm.

RPR supports bandwidth reservation, providing perfect QoS guarantee for reserved bandwidth. Thus, the traditional voice traffic can be transmitted.

Because the fairness algorithm of RPR does not regulate the high-priority traffic, all high-priority traffic will always be sent prior to low-priority traffic. To prevent excessive high-priority traffic from affecting low-priority traffic, you are recommended to set a threshold for high-priority traffic.

RPR provides multiple static traffic shaping methods such as rate limiting (using a rate limiter) for high- and low-priority data frames. For low-priority data frames, RPR also provides dynamic traffic shaping.



The QoS guarantee measures of RPR ensure that an excellent QoS guarantee can be provided on an RPR ring even if the hosts do not provide QoS guarantee.

Miscellaneous

As a physical layer-independent MAC layer protocol, RPR provides physical layer interfaces to support Ethernet, DWDM and SONET/SDH.

RPR allows great bandwidth scalability. For example, your can scale RPR ring bandwidth from 155 Mbps to 10 Gbps, and even to 40 Gbps.

A very important feature of RPR is that it avoids the N2 issue successfully to achieve full connectivity at the MAC layer for N stations with only N links. Compared with SDH, POS and Ethernet, RPR has lower link cost.

RPR is an optimized Ethernet technology. It supports all Ethernet protocols and services.

RPR supports equipment interoperability at the ring level. For example, you can connect ATM devices, routers, and TDM devices to the same RPR ring. These networks share the physical links and total bandwidth of the ring while being transparent to each other.

RPR provides complete MIB features, which allow RPR to provide an extraordinary operations and maintenance platform, achieving operability and manageability.

RPR Data Frame Format Figure 9 RPR extended frame format

MAC C

MAC D

MAC A

MAC B

FCS (4 octets)

Data (N octets)

Protocol type (2 octets)

HEX (2 octets)

Extended control (1 octet)

Basic control (1 octet)

TTL base number (1 octet)

TTL (1 octet)



Figure 10 RPR basic frame format

TTL (1 octet)

Basic control (1 octet)

MAC B

MAC A

TTL base number (1 octet)

Extended control (1 octet)

HEX (2 octets)

Protocol type (2 octets)

Data (N octets)

FCS (4 octets)

There are two types of RPR data frames: basic data frames and extended data frames, as shown in the figures above.

Basic data frames are forwarded at Layer 3 while extended data frames are forwarded at Layer 2. Judging by the frame structures, basic data frames use less bandwidth. For this reason, unicast and broadcast frames on the SR8800 are all forwarded as basic data frames.

RPR Application Scenarios This section presents various RPR application scenarios. Interconnected RPR stations form RPR rings, which are similar to SDH rings. RPR rings may intersect or touch. Devices connected to RPR stations can insert/copy traffic onto/from RPR rings through related cards (other than the one connected to the RPR ring) of the RPR stations.

RPR supports various routing protocols. Protection switch due to fiber cut for example does not result in route reconvergence or MPLS LSP re-establishment. This is because on an RPR ring, there are two paths to each destination station. When one fails, traffic can travel over the other to reach the destination. The protection switchover takes less than 50 milliseconds, far less than the hello interval of routing protocol neighbors. Moreover, the DOWN event of a physical port will not be reported to the upper layers, unless both physical ports of an RPR station are down. Therefore, using RPR rings to connect distribution and access services can provide high reliability and stability, whereas with STP or other ring network technologies, path switchover can result in update of service protocols involving routing, MPLS LSP, ARP, MAC, and so on. MPLS over RPR rings delivers unique advantages, providing sub-50 millisecond convergence for LSP services without FRR or a backup LSP.



Application in Small and Medium-Sized MANs and LANs Figure 11 RPR solution for small and medium-sized MANs

IP backbone network

SR88 SR88

SR88 SR88

MAN core/distribution layer

RPR ringMAN access layer

E1/E3 GE/FE GE/FE

Ethernet access/distribution

High-speed leased line access

VPN access

For a small or medium-sized city, an RPR ring can be built in the MAN. One or two of the stations can be used as the core and egress uplinked to the backbone network. Other stations are deployed at important offices in the city to provide the following services:

The access/distribution of Ethernet traffic in those areas The access of various high- and low-rate leased line users: Various interfaces such as E1, E3,

POS, and ATM can be provided on these stations, allowing them to serve as leased line access routers.

The access of various VPN users: MPLS links can be established on the RPR ring, allowing routers to serve as MPLS VPN PE devices to access various VPN users.

RPR can operate at the core layer of local area networks (LANs) with geographically-dispersed agencies or branches, such as government networks, enterprise networks, and campus networks, connect offices, data centers, and the Internet, and logically optimize the existing FDDI ring networks, reserving the features as being a self-healing ring. Such application is shown in the figure below.



Figure 12 RPR solution for LANs

Center

Branch

Branch

RPR ring

Branch

Subsidiary company

Subsidiary body

Internet

Application in Large and Medium-Sized IP MANs Figure 13 RPR solution for large and medium-sized IP MANs

SR88 SR88

10G RPR

SR88Ethernet access/distribution

High-speed leased line access VPN access

MAN distribution layer

IP backbone network

PP

Y

SR88

E1/E3 GE/FE

MAN access layer

PE

PE

SR88

SR88

MAN core layer

2.5G RPR

2.5G RPR

GE/FE

SR88

SR88

A large or medium-sized IP MAN contains a large number of core and distribution nodes. Hence, it usually adopts the typical three-layer architecture, namely, the core layer, distribution layer, and access layer. In such case, multiple RPR rings are often used for networking. On the core layer, a



core 10G RPR ring is built, and on the distribution layer, multiple 2.5G edge RPR rings are built. The core ring and edge rings can intersect or touch. When two rings intersect, they have two connection points and thus provide higher reliability. Therefore, you are recommended to deploy intersected rings wherever possible.

RPR Features on the SR8800

Powerful Service Switching Performance According to the RPR protocol, fault detection should be done within 10 milliseconds and service switching within 50 milliseconds. The RPR implementation on the SR8800 can complete service switching as fast as within 20 milliseconds, fully satisfying the carrier-class requirement and employing the strength of RPR.

Complete QoS Capabilities The RPR implementation on the SR8800 provides complete QoS capabilities for RPR traffic. It supports the access control list (ACL), rate limiting, traffic shaping, queuing, and almost all QoS features available on Ethernet. It supports mappings from 802.1p, MPLS and IP priorities to RPR priorities. Depending on customer needs, it supports services of Class A, B and C, providing bandwidth guarantee and other service differentiation capabilities. In addition, the RPR implementation on the SR8800 uses a fairness algorithm to ensure fair access of stations to ring bandwidth, allowing for bandwidth usage efficiency, congestion avoidance, and congestion alarming.

Abundant Ring Selection Mechanisms The RPR implementation on the SR8800 supports multiple ring selection modes.

By default, dynamic ringlet selection (shortest path selection) is adopted on an RPR ring. Dynamic ringlet selection results in a ringlet selection table that contains the shortest paths to other RPR stations on the ring upon topology convergence. This ringlet selection table does not change when the topology is stable.

In addition, you can configure static ringlet selection entries, which have higher priority over dynamic entries.

Ease of Configuration On the SR8800 working as an RPR station, RPR provides a logical RPR interface for you to make related configuration as if on a common Ethernet port. The two RPR physical ports bound with the logical RPR interface are transparent to the upper layers. All services are configured on the logical RPR interface rather than on the two physical ports respectively.

High Reliability An RPR station is identical to an RPR logical interface bound with two physical ports. The two physical ports are used to transmit data to or receive data from the ring and the logical interface is provided for you to make configuration. The SR8800 supports distributed RPR, where the two



physical ports are located on different interface cards. Comparatively, centralized RPR means that the two physical ports are located on the same interface card. In centralized RPR, when the interface card where the two physical ports are located goes down, the RPR station fails. In distributed RPR, the down event of one interface card can cause the physical port on the card to go down and thus edge to the RPR station. The RPR station can still work in protection mode to restore services in less than 50 milliseconds. The distributed RPR feature provides enhanced redundancy and greatly improves the reliability of RPR application in metropolitan area ring networks.

Mate Port Smart Connection An RPR logical interface is bound with two physical ports. Each physical port is accompanied by a mate port. Before you bind two physical ports with the same logical interface, you must connect their mate ports with optical fibers first for the RPR station to forward traffic.

The 2.5G RPR subcards of the SR8800 support both centralized RPR and distributed RPR. When the two RPR physical ports are on the same 2.5G subcard, you can enable the RPR mate port smart connection function. With the function enabled, RPR automatically connects the two mate ports internally without you having to connect them manually, thus optimizing the centralized RPR implementation.

References 1. IEEE Draft P802.17/D3.0

2. Hangzhou H3C Technologies Co., Ltd. RPR Technical Specifications

Copyright ©2009 Hangzhou H3C Technologies Co., Ltd. All rights reserved.

No part of this manual may be reproduced or transmitted in any form or by any means without prior written consent of Hangzhou H3C

Technologies Co., Ltd.

The information in this document is subject to change without notice.

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