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. 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 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. 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. 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. 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. RPR uses the SDH ring structure, and inherits a major feature, the powerful failure self-healing capability, which implements failure protection switching in 50ms. RPR supports automatic topology discovery. The protection information or topology information packets contain the topology information, which is broadcast on the ring network. 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).
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. RPR works on a concept of dual counter rotating rings called ringlets. These ringlets are set up by creating RPR stations at nodes where traffic is supposed to drop, per flow (a flow is the ingress and egress of data traffic). RPR uses Media Access Control protocol (MAC) messages to direct the traffic, which can use either ringlet of the ring. The nodes also negotiate for bandwidth among themselves using fairness algorithms, avoiding congestion and failed spans. The avoidance of failed spans is accomplished by using one of two techniques known as steering and wrapping. Under steering, if a node or span is broken, all nodes are notified of a topology change and they reroute their traffic. In wrapping, the traffic is looped back at the last node prior to the break and routed to the destination station.All traffic on the ring is assigned a Class of Service (CoS) and the standard specifies three classes. Class A (or High) traffic is a pure committed information rate (CIR) and is designed to support applications requiring low latency and jitter, such as voice and video. Class B (or Medium) traffic is a mix of both a CIR and an excess information rate (EIR; which is subject to fairness queuing). Class C (or Low) is best effort traffic, utilizing whatever bandwidth is available. This is primarily used to support Internet access traffic.

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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Fig (1):- 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. 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.

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Fig (2):- Traditional Packet Switching Structure.

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.

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Fig (3):-Processing 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.

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Fig (4):-The Destination Station Performs “Copy” And “Strip” Operations

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

 

  2.3. Frame Format:-

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. [9]

 

 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.

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Fig (5): – MAC Entity Structure

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 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. 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. 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. [4]

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. 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.
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:

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Fig (6): – Data Traffic Diagram

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. [3]

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.

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Fig (7):-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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Fig (8):-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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Fig (9):- 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. [4]

4. Advantages:-
An effort to bring SONET-like abilities to metro Ethernet networks, by adding support for a ring topology and fast recovery from fiber cuts and link failures at Layer 2. Being defined by the IEEE’s 802.17 working group.RPR uses Ethernet switching and a dual counter-rotating ring topology to provide SONET-like network resiliency and optimized bandwidth usage, while delivering multipoint Ethernet/IP services. RPR maintains its own protection scheme and uses physical-layer alarm information and Layer 2 protocol communications to detect node and/or link failures. When a failure is detected, the RPR switching mechanism restores networks in less than 50 millisecond. Because RPR is a Layer 2 MAC-based technology, it can operate over multiple physical layers, including SONET. Therefore, corporations can reap the benefits of RPR by having it ride over the SONET network to deliver the resilient, efficient, multipoint functionality and scalability of data applications such as VoIP, packet video, business continuance and distance learning. Or they can install multiservice provisioning platforms, which are optimized for TDM services but also can support advanced data applications via RPR over SONET. The advantage is that existing TDM services are maintained, while a smooth migration to packet-based services is enabled. Another major advantage of RPR’s dual-rotating ring design is that Ethernet traffic is sent in both directions on the ring to achieve the maximum bandwidth utilization. Unlike older ring-based data networks such as token ring or FDDI, RPR uses a spatial reuse mechanism. Rather than requiring traffic to traverse the entire ring even though a destination node is only a hop away, RPR sends it there directly, keeping the rest of the ring bandwidth available for use by other stations on the network. To further enhance the network efficiency and support multimedia applications, the IEEE has included a classification scheme and a fairness algorithm in the RPR specification. This guarantees that jitter- and delay-sensitive traffic is always given higher-priority access to the network. Meanwhile, best effort (Internet type) data traffic is ensured equal access and a “fair” share of the remaining ring bandwidth.
RPR also uses statistical multiplexing so that bandwidth can be oversubscribed, while establishing committed information rate (CIR) and peak-rate thresholds on a per-application basis. This guarantees each enterprise application a CIR and the ability to burst up to the peak rates when bandwidth is available. With such a mechanism, each department is charged only for using extra bandwidth rather than being billed for a larger, nailed-up circuit, regardless of use.
Widespread corporate adoption of RPR will help usher in the cost-effective transport of popular Ethernet and IP communications services. RPR transport will provide efficient bandwidth protection, accommodate bursty data traffic and provide the quality of service needed for these advanced packet applications. [2]

5. Disadvantages of RPR:-
Due to its underlying ring topology and the applied fairness control algorithm, RPR suffers from the following limitations.
• Spatial reuse: In RPR, packets generally have to traverse multiple intermediate nodes in order to reach their destinations, and thus consume a considerable amount of ring bandwidth, resulting in limited spatial reuse.
• Oscillations under unbalanced traffic: Spatial reuse in RPR is further decreased due to severe and permanent oscillationsunder unbalanced and constant rate traffic inputs [2].Recently, novel fairness algorithms have been proposed that are able to mitigate the oscillations and achieve nearly complete spatial reuse [9. 10]. In particular, the so-called Distributed Virtual-Time Scheduling in Rings (DVSR) fairness control algorithm has attracted considerable attention [11].We examine an extended version of DVSR later when discussing fairness in RINGOSTAR.
• Single-failure resilience: RPR is able to recover only froma single link or node failure in a rather inefficient manner by wrapping and steering incoming traffic away from the failure on the opposite fiber ring. For instance, it was shown in [3] that in the event of a failure, the loss of traffic in a 63-nodeRPR network may be as high as 94 percent due to the increased length of the backup path. Furthermore, RPR’sresilience against a single failure is poorly suited to provide survivability in the presence of multiple failures, which is paramount in metro core networks [12].
• Hot-spot traffic pattern: Metro edge rings exhibit stronglyhubbed (hotspot) traffic where most traffic originating from a given access network is outbound toward metro core rings [4].RPR with its underlying ring topology supports such hotspot traffic inefficiently since outbound packets have to traverse many intermediate nodes along the fiber rings on their way to the hub due to missing alternate shorter paths.[2] 6. Conclusion:-

In this seminar, we presented of Resilient Packet Rings, a dynamic bandwidth allocation algorithm targeted to achieve high utilization, spatial reuse, and fairness in Resilient Packet Rings. We showed through analysis and simulations that DVSR overcomes imitations of current RPR draft algorithms and fully exploits spatial reuse, rapidly converges typically within two ring times, and closely approximates the Ring Ingress Aggregated with Spatial reuse (RIAS) fairness reference model.

7. References:-

[1] IEEE Research Paper for Improvement of Resilient Packet Ring Fairness (February 2005 Revised August 2005) “Fredrik DavikSimula Research Laboratory University of OsloEricsson” Research Norwa Simula Research Laboratory.

[2] IEEE Research Paper for High Performance Fair Bandwidth Allocation forResilient Packet Rings (V. Gambiroza, Y. Liu, P. Yuan, and E. KnightlyECE Department, Rice University
Houston, TX 77005, USA) January 2002,

[3] IEEE Research Paper for Ringostar: an evolutionary performance-enhancing wdm upgrade of IEEE 802.17 Resilient Packet Ring (Martin Herzog, Technical University Berlin
Martin Maier, Institut National de la Recherche Scientifique) February 2006
[4] IEEE802.17 Resilient packet ring (RPR) access method and physical layer specifications
[5] IEEE802.17Packet Transfer Delay Comparison of aStore-and-Forward and a Cut-Through Resilient Packet Ring (Dominic A. Schupke, Anton RiedlMunich University of Technology, Institute of Communication Networks, and Germany) Nov 2001.

[6] www. datacomm.huawei.com
[7] www. telephonyonline.com

[8] www. networkworld.com

[9] www.wikipedia.com

 

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