Cnir Paper

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Algorithm for Topology Discovery in Resilient Packet Ring IEEE802.17

Shahid Hussain ABBASSI Department of Electrical Engineering, Air University,

E-9, Islamabad, Pakistan,[email protected],

http://www.au.edu.pk/dept_electrical_faculty.aspx

Abstract

Resilient Packet Ring (RPR) IEEE802.17 is ametropolitan area network protocol designed to combinecarrier class functions of SONET such as fast protectionand restoration with high bandwidth, granularity andutilization of Ethernet. RPR Topology discovery

mechanism is used to determine the direction on the ringwhich will provide the best path to the destination based

on hop count and ring direction. According to RFC 2892,the ‘Cisco SRP MAC Layer Protocol’ topology discovery packets (TDPs) are sent only on the outer ring andtopology map is updated if a TDP is found with the proper ring id. In this paper, we devise an algorithm inwhich ring id is neglected while accepting the TDP. The

algorithm also sends TDPs on both rings. This allows usto make a map of each ring and these maps can becompared to obtain lesser hop counts to the destination.This proposed algorithm works well even in the case of fiber breaks (ring wrap) and our method also leads toreduced travel times in the case of ring wraps.

Keywords: MAC, MAN, RPR, Topology Discovery.

1. Introduction

Figure 1. Existing Metro Technologies

Resilient Packet Ring (RPR) was designed to combinethe functions of SONET such as fast protection andrestoration with high bandwidth, granularity and

utilization of Ethernet [1]. Additionally, RPR technology provides fair bandwidth distribution lacking in other Ethernet solutions. RPR employs spatial reuse in order tomaximize the utilization of bandwidth, fairly distributes bandwidth, and provides fast traffic protection very likeAutomatic Switching in SONET. RPR provides full

bandwidth utilization and in the case of failure of nodesor fiber cut using a priority scheme protects the traffic.Furthermore, it provides all this functionality whilekeeping Ethernet’s advantages of low equipment costs,

high bandwidth granularity, and statistical multiplexingcapability.

Topology discovery mechanism in RPR is used to findthe best path to the destination node based on lesser hopcounts. It also provides wrap status of the ring around acertain node. Based on this information the data/real timetraffic packets are sent on the RPR ring. We introduce anew algorithm that sends the topology discovery packets

(TDPs) on both outer and inner rings. Each nodereceiving the packet appends its information to this packet except the source node. The source node onreceiving this packet builds/updates the topology map of the ring by comparing the information from packets from

both rings. The path with lesser hop counts is mentionedagainst the address of each node. In the case of ring wrapthe packet is returned on the ring other than the one onwhich it was sent. The packet is received and the map isupdated even in this case. In this way the packet travelsless as compared to the case in which it is sent on justone ring.

2. Existing Topology Discovery AlgorithmsTopology discovery is vital for Spatial Reuse Protocol(SRP) Ring selection and convenient management of thenetwork. Each node on the network, at predefined and

configurable intervals, sends the TDPs, after marking it

with its outgoing Ring ID inner or outer, appending itsown MAC address, ring id information, and setting its

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length field. This packet is then passed from node to node,with each node adding its MAC binding information tothe packet. If a wrap is found in the ring, the respective

wrapped node will indicate the wrap along with addingits MAC binding, and then the packet follows the wrap.While following a wrap, as packet has to traverse thesame node so the MAC binding information is notappended to the packet. This rule is followed until the packet leaves the wrap [2].

MAC Type field and 48 bit MAC address of the nodeconstitute the MAC binding. The first MAC binding isthat of the originator [2].Gradually, the node that generated the packet gets it back.It then checks if the packet has the same Incoming andoutgoing Ring IDs before accepting it. Determination of

weather incoming and outgoing Ring IDs of a packet aresame is to be done by checking the Ring ID found in theMAC Type field of the last MAC binding added to the packet. To prevent topology changes in transient states being incorporated into the topology map, the change in

the map occurs only after receiving two consecutivetopology packets indicating the similar pattern of topology [4].The topology information is not required for themechanism for protection. This can be used to calculatethe number of nodes in the ring and to calculate hopdistances to the nodes for determining the shortest path to

a node as there are two counter-rotating rings [4]. Table 1shows the format for MAC type. Figure 2 shows the RPR ring and wrap status in the event of fiber cut.

Table 1. MAC Type Field

Bit Description

0 Reserved1 Ring ID

0 – Outer Ring1 – Inner Ring

2 Wrap Status0 – Node Unwrapped1 – Node Wrapped

3-7 Reserved

Figure 2. RPR Ring and Wrap Status

3. Introducing a New AlgorithmThe flow chart of this algorithm is given in figure 3. Thenew algorithm works as follows: It checks if thereceiving node is the source node of the packet and if the

ring id indicates the outer ring then it creates a temporaryaddress pool and copies all the addresses from the packet

into the pool. It also copies the wrap status of both sidesof the nodes and preferred ring id for receiving packetsinto temporary arrays. If the ring id indicates the inner ring then it performs the same actions as it did for theouter ring but using different pool variables specific tothe inner ring.

It then checks if one or more packets are received fromeach ring or more than one packet are received from anyone ring, then it picks one address and other informationof the node from the pool variables of outer ring andcopies these into the temporary variables. It then checksif the same address is found in the pool variables of inner

ring. If yes it checks where. If address is not found in

pool variables of inner ring it simply adds this address tothis pool. If address is found in the pool variables of inner ring at a distance more than that in the pool variables of outer ring, then it keeps the status of ring id and wrap of outer ring variables. It then copies the topology map

information from the pool variables of the inner ring tothe permanent pool variables of the node.If the receiving node is not the source node of the packetand time to live field value is greater than one then itchecks if the ring is same as that on which the packet wasoriginated.If yes then it copies address of the node, wrap status and

ring id to the packet. It also increments the topology

length by one and simultaneously decrements the time tolive field by one. It then calculates and adds the parity bitand checksum into the packet. Afterwards it adds the packet to the high priority transmit buffer of outer or inner ring of the node.

Problem in the previous algorithms is that each packetwill have to travel about twice the ring length in the caseof fiber break and ring wrap in the case if ring id is notneglected while accepting the packets.Advantages of above algorithm are: Exact map of thering nodes is obtained by circulating packets on both

rings and afterward comparing to get the best path for transmission of packets intended for that node. Secondly

Time for circulating of TDPs decreases in the case of ringwrap as packets does not need to travel whole of the ringand come back on wrapped ring as shown in figure 4, but packets on each ring travel for about half of the length of

the ring as shown in figure 5. In this way the packet senton the left, gets back with map of the ring on left side and packet sent on the right, gets back with the map of ringon the right side.

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Figure 3. Introducing Topology Discovery Flow Chart

Figure 4. TD Packet Path due to wrap proposed in Existing

Algorithms

Figure 5. TD Packet Path due to wrap proposed in new algorithm

4. ConclusionRPR being an important protocol for Metropolitan Area Networks should incorporate each and everyconsideration of different situations which can be createdin the actual data as well as real time traffic. Topology

Discovery is an important aspect of any algorithm. Thefuture prospects of the paper are: Circulation of onetopology discovery packet in the ring instead of everynode generating a new packet; Graphical representation

of ring map incorporating changes dynamically; trying toreduce the size of control packets.

5. AcknowledgementsI am thankful to Dr. Sarmad Abbasi for his kind guidancein writing the paper and to Mr. Hameer Abbasi in helpingto convert the program into algorithm.

6. References[1] Appian. Appian Communications Strategy for IEEE-

802.17 Resilient Packet Ring Networks. URL:

www.appiancom.com, 2001.[2] Cisco Systems, Spatial Reuse Protocol Technology.

URL:grouper.ieee.org/groups/802/17/documents/presentations/tutorial/srp_overview.pdf, 2000.

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[3] Corrigent Systems. Resilient Packet Ring TechnologyOverview.URL:www.corrigent.com/pdfs/RPR_Technology_010

3.pdf, 2001.[4] D. Tsiang, G.Suwala. The Cisco SRP MAC Layer

Protocol. RFC 2892. Cisco Systems. 2001.[5] RPR Alliance. An Introduction to Resilient Packet

Ring Technology. URL:www.rpralliance.org, 2001.[6] S.H.Abbassi, U.Farooq. Topology Discovery in

Resilient Packet Ring Technology. pp. 229-234. InProceedings of the International Network Conferencein Plymouth, UK. 2004.

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