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Technical White Paper for
Resilient Packet Ring (RPR)
Huawei Technologies Co., Ltd.
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Technical White Paper for Resilient Packet Ring (RPR)
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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
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Technical White Paper for Resilient Packet Ring (RPR)
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Technical White Paper for Resilient
Packet Ring (RPR)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.
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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
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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:
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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 effectivelyenhance 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:
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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.
MA C l a y er m an a g em en t
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
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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 frames
Receive
frames
Send frames Receive frames
MAC control
requestMAC control
indication
MAC datarequest
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.
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MAC service interface
MAC client
MAC data request
MAC control
MAC
Inner ring data
channel
Outer ring data
channel
PHY
West interface
S1
West interface
S2
West interface
S3
MAC data indication
MAC client
MAC control MAC control
Outer ring datachannel
Outer ring data
channel
Inner ring data
channel
Inner ring data
channel
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
MAC service interface
Physical service interface
Outer ring data channel
Inner ring data channel
ReceiveReceive
Send Send
Send frames Receive frames
Send frames Receive frames
MAC control request MAC control indicationMAC data
requestMAC data indication
West physical layer interface
Frame reception/sending
Data interface processing
Frame reception/sending
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
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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
MAC service interface
Receive Send
Send frames Receive frames
MAC control request
MAC control indication MAC data request MAC data indication
Staging/queuing
Inner ring data channel
Frame reception/sending
Shaping
Send framesReceive frames
Select the frames to transfer Select the frames to transfer
Shaping
Select the frames to strip
West sending/east reception
East sending/west reception
Outer ring data channel
ReceivePhysical service interface
West PHYEast PHY
Frame reception/sending
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
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Check
Shape
Stage
ShapeControl
PTQ
STQ
Data
Class A
Class B/C
S e r v i c e c l a s s A i n s e r t e d
S e r v i c e c l a s s
B i n s e r t e d
S e r v i c e c l a s s C i n s e r t e d
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 willfirst 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.
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2.6 Fair Algorithm
RPR allows the stations to share the bandwidth resources available. When the data trafficis 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
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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:
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Normal data transmission Protection self-healing at linkfailure
Protection self-healing atlink 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
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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.
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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.
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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:
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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