OAM in Packet Transport NetworksLeon Bruckman

Corrigent Systems Inc., 101 Metro Drive, Ste. 680, San Jose, CA, 95110leonb@corrigent.com

E.Bert.BaschVerizon Laboratories, 40 Sylvan Road, Waltham MA 02451

Bert.basch@verizon.com

Abstract: Networks are transitioning from TDM to packet transport optimized architectures. Packetnetworks are based on technologies traditionally lacking OAM tools. We will present the OAM tools beingdeveloped and their application to the transport layers.© 2006 Optical Society of AmericaOCIS codes: (060.2330) Fiber optics communications; (060.4250) Networks

1. Packet networks environment

WAN and MAN networks are transitioning from TDM to packet transport optimized architectures; this change isdriven by the large increase in data traffic and the need to support this growth with networks that are cost effectiveand future proof. Networks optimized for packet transport take advantage of statistical multiplexing over widetransmission “pipes” and are not constrained to the rigid SONET containers.

Some of the technologies developed for packet transport include powerful OAM tools. The best example in thiscategory is ATM. ITU-T I.610 [1], defines a full set of OAM tools for ATM. Nevertheless ATM OAM has not beenwidely deployed (even though some of the defined OAM tools are common practice) and ATM seems to be loosingground to other technologies due to factors not related with its OAM capabilities.

The prevailing technologies on which packet networks are being build traditionally lack OAM tools. Some ofthem, like Ethernet, were developed for LAN environments where the added value of OAM tools is low; others, likeIP/MPLS, were developed primarily to “move” large amounts of data without SLA guarantees, mostly in a best-effort manner.

Small networks owned by a single organization who is also the only customer of the network services, can beoperated without automated and powerful OAM tools. LAN networks are a good example of these types of networks.But, large networks that may be owned by several organizations and provide services to several customers, presentnew challenges on the area of cost effective maintenance. This fact has been recognized by the main standard bodiesand efforts are being made to define OAM tools that will promote the acceptance of these technologies in the MANand WAN environment.

2. OAM definition

Operation, Administration, and Maintenance (OAM) is a group of management functions and tools that providenetwork fault indication, performance monitoring, diagnostic and testing functions, configuration and userprovisioning.

In this paper we will concentrate on two issues of OAM: fault management and performance monitoring.

3. Fault management

Fault management includes alarm surveillance, fault localization, fault correction and testing. Alarm surveillanceprovides the capability to monitor failures detected in the network elements (NEs). In support of alarm surveillance,the NEs should perform checks on hardware and software in order to detect failures, and generate alarms for suchfailures. Upon detecting a failure, in addition to generating and sending alarms to the management system, the NEsshould also send indications in the forward and backward directions in order to notify downstream/upstream NEsthat a failure has occurred (and some action may be required).

Fault localization determines the root cause of a failure. In addition to the initial failure information, it may usefailure information from other entities in order to correlate and localize the fault.

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Fault correction is responsible for the repair of a fault and for the control of procedures that use redundantresources to replace equipment or facilities that have failed.

Testing performs repair functions using some testing and diagnostic routines. Testing is characterized as theapplication of signals/messages and their measurement. A loopback is one example of a testing routine and can beactivated upon request.

4. Performance monitoring

Performance monitoring (PM) is the process of non-intrusive collection, analysis, and reporting of performance data.This data is used to assess and maintain the network as well as to document the quality of service to customers.Indications of service affecting degradation are sometimes used by fault management functions.

5. OAM in a layered network

Transport is built by protocols that are layered one on top of the other. Protocols on the same layer communicatebetween them, while the communication between layers is kept to a minimum with the goal of making layersagnostic of their upper and lower layers. From an OAM perspective each layer requires separate OAM tools, since itmay be handled by independent hardware and/or software, may expand over several sections of the lower layer, andmay be managed by a separate organization.

Lower layers must indicate OAM changes to upper layers in order for the upper layers to be able to take theright action as a consequence of the lower layer being down; usually upper layers don’t need to communicate OAMchanges to lower layers.

It should be noted that a protocol may also include a number of layers. Figure 1 shows an example of theSONET TDM protocol that includes three layers, and Figure 2 shows an example of the ATM protocol that includestwo layers.

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Fig. 1. SONET Layers

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6. What do we need OAM for?

There are two main reasons to require OAM: cost-effective network maintenance and compliance/verification of theService Level Agreement.

7. Network maintenance

By using OAM tools it is possible to detect failures within a very short period of their occurrence, find the rootcause of the failure, and take the appropriate actions to fix the problem, usually to send the technician to the rightlocation with the right tools and replacement parts.

Furthermore, OAM tools can detect soft failures and provide indications for routine maintenance actions beforea hard failure occurs.

8. Service Level Agreement (SLA)

More and more customers (mostly business, but not only) are able to monitor the network performance and reclaimreimbursements if the service is not according to the agreed SLA. To avoid not meeting the SLA and to be able toevaluate the customer’s complaints, the service provider needs OAM tools that can measure all the SLA parameters.

In some cases service providers use general “thumb rules” based on long term network measurement toguarantee the SLA, but in dynamic and fast developing packet networks this scheme does not provide enoughaccuracy leading to the sub-optimal use of available resources.

9. OAM challenges in packet networks

Due to the special characteristics of packet networks, numerous challenges need to be dealt with, some of which willbe presented in the following sections.

10. Large amount of OAM entities

In legacy TDM networks the amount of OAM entities is either small when handling high rate interfaces (e.g. STS-n),or at least bounded when handling lower rate interfaces (e.g. DS-1). In packet networks the amount of flows that areactive in a transport network may be very large, and providing OAM for each of them requires large amounts ofhigh-speed hardware and storage space.

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11. Statistical multiplexing impact

One of the main advantages of packet networks is their ability to provide statistical multiplexing gain by combiningthe flows of several customers into a shared media and taking advantage of the timing differences in the arrival ofbursts from the different sources. Statistical multiplexing is strongly coupled with traffic management that assureshigh-priority traffic delivery and fair sharing of the remaining available bandwidth during congestion events. Sincethe resources are overbooked, buffers are used to cope with momentarily congestion events; in case that these eventsare not resolved within a short period the buffers may become full and new packets arriving to the port are discarded.

The above method adds complexity to OAM operation. For high priority flows the OAM packet can be markedwith the same quality of service as the flow to which it is attached to, and since it has high priority the probability ofthe OAM packet being discarded is very low. On the other hand, if the flow under test is a low priority flow then theuser is faced with two options: either transmit the OAM packet with higher priority than the flow under test in orderto avoid OAM packet discard, or use the same priority of service for the OAM packets as the user data packets. Inthe former case there is a high probability that the OAM packet will flow through a different hardware path than theone used by the flow under test, and in the later case OAM packets may be discarded, leading to wrong conclusionsregarding the actual status of the flow (note that discard of low priority packets during congestion is an allowedoperation).

12. Connection characteristics

Packet protocols may be connection oriented (e.g. ATM, MPLS) or connectionless (e.g. Ethernet). Some of theconnection-oriented protocols require bi-directional paths only, even for unidirectional services (e.g. ATM), butothers define only unidirectional paths (e.g. MPLS). In the later case there is no straight forward return path forsimple OAM operations such as loopbacks, and other methods must be defined [2].

For connectionless protocols the route taken by a flow may change due to changes in the network status, andcare should be exercised to verify that the OAM packets follow the right route, and that the actual preferred routecan be discovered.

13. Control and data planes

Connection-oriented packet protocols include a control plane and a data forwarding plane. The control plane(sometimes called "signaling") is responsible for setting up the connections so that the data plane can forward thepackets to their desired destination. The control plane is based on special packets that may or may not follow thesame path as the normal data packets. In the latter case, the control path may be operating normally even though thedata path is completely down, or the problem may affect only the data plane. OAM tools must allow forunambiguous indications regarding which is the failed plane.

14. Network topologies

Legacy networks support different types of topologies, but OAM is usually implemented point to point since themain driver for OAM is the maintenance of the physical layer interfaces; in packet networks different topologiesrequire special treatment, even when using the same technology. For example, the maintenance requirements of apoint-to-point Ethernet link are different from the maintenance requirements of a shared-media Ethernet link.

The most common packet network topologies are: point-to-point, hub-and-spoke, full mesh and rings. OAMshould be able to provide common tools to maintain all these topologies.

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15. Error ratio calculation

One of the main network performance monitoring parameters is the error ratio. Degradation of network performancecan be detected at early stages when the error performance of the network deteriorates, and corrective actions can beimplemented before any hard failure occurs. In TDM networks error calculation is simple since the signals areconstant and repetitive regardless of their payload; all that is required is to know how many errors occurred during apredefined period and the Bit Error Ratio (BER) can be evaluated. Packets usually include an error monitoring fieldin their header or trailer, but the amount of bits transmitted during any period is not fixed, and it depends on thepayload activity. Furthermore, the length of the packet is not always constant (except for cell technologies such asATM) and an error detected in a large packet or in a short packet should be accounted with a different weight.

Some technologies such as RPR [3], include a packet that is transmitted at constant intervals and can be used forBER calculations.

16. Synchronization of end-to-end parameter computation

In packet networks it is very common to provide the packet loss ratio as a parameter for the SLA. Packet loss ratio isrelated to the amount of packets sent by the source and the amount of packets received by the destination. Toimplement transmit and receive packet counters is trivial, but in order to be able to verify if packets have been lost itis not enough to subtract the values of these counters since packets may be in transit (already transmitted, but not yetreceived). OAM should provide a method to signal to the destination how many packets have been sent during aninterval so that the destination can evaluate the packet loss ratio. The same issue applies if the misrouted packetsratio has to be calculated (misrouted packets are the packets sent to a wrong destination due to undetected networkerrors).

17. Special OAM requirements

Due to the characteristics of packet networks, some new OAM mechanisms have to be defined. Simple failuremonitoring tasks such as total path failure are not trivial in packet networks; loss of activity in a packet flow mayindicate a loss failure, but it may also be the result of no packets being transmitted by the source, a normal behaviorfor bursty services.

In TDM networks signal delay is almost constant from the point of view of the service, and it is mostlyinfluenced by the timing signal jitter and wander. Delay in packet networks is heavily influenced by the networkcongestion status. Services that are adversely influenced by delay and delay variation, such as voice and video,include the delay and delay variation as an SLA parameter, but measuring the delay and delay variation requiresaccurate timing sources and the ability to pass time stamps. Software handling of the time-stamped packets may addlarge amounts of inaccuracy to the delay measurements.

Another requirement is to find the route taken by a flow in a packet network when connectionless protocols areused. Usually there is more than a single path from source to destination, and without a trace route mechanism it isimpossible to verify which is the actual route selected for a specific packet flow.

18. Standardization

For OAM to be an effective tool, comprehensive standards have to be developed and accepted by equipmentmanufacturers and service providers, and implemented in equipment throughout the network. Standards allowinterworking between equipment from different vendors and limit the amount of training required to maintain andoperate the network, but if most of the installed equipment does not support the new OAM standards the value ofthese tools is questionable at best.

The situation today for some technologies, especially Ethernet networks, is that a large amount of legacyequipment that does not support the new OAM tools exists in the network. Standards are defined in a way that OAMtools can be added gradually allowing legacy equipment to pass the OAM information transparently. Still, in orderto have end-to-end OAM, at least the edges of the network must be upgraded with the OAM tools.

Standard bodies are making efforts to create the required standards; some of them work in parallel with differentdegrees of synchronization between them. As an example, MPLS OAM is being defined by IETF [4], and EthernetOAM by IEEE [5], and ITU-T [6].

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19. OAM in real-life networks

Figure 3 shows a view of a network that uses several technologies and the domains of the OAM tools.

Fig. 3. Network View and OAM Domains

In the following sections we will describe the capabilities of advanced packet OAM with references to EthernetOAM as being defined by ITU-T [6]. The reason to choose Ethernet OAM is that Ethernet services are becoming akey offering in the service provider’s networks. It should be noted that most of the principles apply to any otherOAM packet or cell technology.

As noted previously, there are two standard bodies defining Ethernet OAM: IEEE and ITU-T. We will focus ourexamples on the ITU-T definitions since it includes mechanisms beyond the ones defined by IEEE (e.g. AIS/RDIand delay measurements) that provide additional OAM tools for large MAN/WAN transport networks.

Munefumi Tsurusawa and Hideaki Tanaka [7], demonstrated the value of Ethernet OAM to detect and localizefailures in packet networks and to evaluate performance of these networks.

20. OAM frames

OAM frames are special frames that carry OAM information related to the service status (e.g. alarms, errors, lostframes, delays) or to maintenance actions (e.g. continuity check, loopbacks). OAM frames are identified by a specialmark (e.g. an Ethertype in Ethernet) and they are copied and/or stripped by maintenance entities, while other entitiesin the network subject them to the same treatment as normal (non-OAM) frames.

OAM maintenance actions may be on-demand or proactive. On-demand actions generate OAM frames onlyafter a management request has been issued due to some condition that needs validations or fault localization, whileproactive actions generate frames constantly in a way that loss of these frames indicates a fault condition.

The term "frame" is used in the remaining of this paper instead of the term "packet", in accordance with thereferred standards.

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21. Maintenance entities

A maintenance entity is an entity that requires management, and is bounded by maintenance end points.Maintenance entities are organized in group (MEGs) so that all maintenance entities in a MEG belong to the sameconnectivity and administrative level. This arrangement allows having different levels of maintenance for customersand providers, and for different providers.

There are two types of maintenance entities: a MEG end point (MEP) and a MEG intermediate point (MIP). SeeFigure 4 for an example of these entities.

A MEP is capable of initiating and terminating OAM frames. It does not terminate or modify the non-OAMtransit frames, but it is able to observe the frames for monitoring information such as number of transmitted/receivedframes. The MEP generates and terminates OAM frames belonging to an administrative domain, and is transparentto OAM frames belonging to higher administrative levels; as such it protects the administrative domain from OAMframes belonging to the same or lower levels by blocking them from entering the maintenance domain, and itprevents OAM frames from within the administrative level to leak outside the level.

A MIP is able to react to some OAM frames, but it does not generate any OAM frames. It does not perform anyaction on non-OAM frames.

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22. OAM functions

OAM functions enabled by the OAM frames are: Fault Management and Performance Monitoring. Following aresome examples of OAM frames and the defect conditions that can be detected by using them.

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23. Continuity check

When configured for continuity check (CC) a MEP transmits CC frames periodically, and expects to receive CCframes from the MEP at the other end of the maintenance entity. Failure to receive CC frames within a configuredperiod (normally 3.5 times the CC generation rate), or reception of CC frames from unexpected sources (e.g. lowerMEG levels, different maintenance entities) triggers an alarm indication.

Continuity check is used to detect loss of continuity between any pair of MEPs, and to detect unintendedconnectivity or mismerges. Note however that some mismerge conditions can not be detected by the CC frames.Figures 5a and 5b show examples of such cases.

In the example of Figure 5a the bidirectional mismatch will not be detected by MEPs A1 and A2 since they aretransparent to higher MEG levels; MEPs B1 and B2 will detect the mismerge. In the example of Figure 5b themismerge will remain undetected since B1 stations are not aware of the problem. Note that if in the example ofFigure 5b the connections would be completely misrouted (and not duplicated) then B2 MEP would detect a loss ofcontinuity.

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Fig. 5. Mismerge Detection Limitations: (a) Bidirectional mismerge; (b) Unidirectional mismerge

Note however that this mismerge conditions can be detected by the performance monitoring function since thenumber of frames received by A2 will be larger than the number of frames transmitted by A1. Still the exactmisconfiguration or fault that led to the mismerge will not be identified and other tools have to be used to localizethe fault.

24. Loopbacks

A loopback (LB) is an on-demand function that can be used to verify connectivity of a MEP with a MIP or a peerMEP. A MEP can be instructed to transmit a LB frame, or a series of LB frames, to a specific MEP or MIP; it canalso transmit a multicast LB frame to all MEPs in its domain (according to [6], MIPs do not respond to multicast LBframes).

LB tests can either be in-service (without disturbing the non-OAM data flows) or out-of-service, in which caseonly LB frames will be transmitted to perform deep diagnostics operations. A MEP or MIP receiving an LB framewill return an LB response frame (except for the case of an LB multicast in which case a MIP will not respond).

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When a multicast LB is used all MEPs in the domain that are connected to the MEP generating the LB multicastframe will respond with an LB response frame. This operation may cause a storm of LB responses if many MEPshave connectivity to the LB-generating MEP. To avoid this storm, each MEP should generate the LB response aftera random period.

25. Alarm Indication Signal and Remote Defect Indication

Alarm Indication Signal (AIS) is used to suppress alarms at the entities downstream of the location of a detectedfault at the same or lower layer, and Remote Defect Indication (RDI) is used to indicate an alarm condition upstreamof the location of the detected fault. In Ethernet OAM, as defined by ITU-T [6], the RDI is indicated in the payloadof the CC frame.

Only MEPs are allowed to generate and monitor AIS and RDI. Once a condition requiring generation of AIS orRDI has been detected, the indication is transmitted periodically until the defect condition has been repaired.

26. Link Trace

Link trace (LT) can be used to learn the sequence of MIPs and MEPs adjacent to a MEP; it can also detect faultssince if faults or loops are present in the network the resulting sequence of MIPs and MEPs will be different fromthe expected one (provided the expected one is known by the user).

Figure 6 shows an example of the operation of the LT message frame as defined in [6]. The LT message frameis transmitted by the MEP with a multicast destination address (MC) and its source address (A1). The target MEPaddress (A3 in the figure) is carried to ensure that the MEPs and MIPs can verify the LT message final destination,and the original LT message generating MEP (A1 in the figure) is also carried so that the MIPs and MEP know thedestination address for the LT reply message. As can be seen in the figure, each MIP forwards the LT message withits own address as the source address.

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Fig. 6. LT Monitor Frame Transmission

Figure 7 shows the LT reply message. As can be seen from the figure, the only MEP that responds is A3 since itrecognized its address as the target address in the LT message, and each MIP in the path also responds with an LTreply since they have A3 as an entry in their MAC learning table. LT replies should spaced by a random delay toavoid OAM frame storms.

As defined in [6], only MIPs that have the target address in their MAC learning table respond with an LT reply.As a result, for the first LT message, A1 MEP will only receive the LT reply from A3, since MIPs a1 and a2 don’thave A3 in their MAC learning table (no frame with A3 as its source address passed through them), but forsubsequent LT messages the MIPs will be able to respond with the LT reply since they can learn the A3 addressfrom the first LT reply. This behavior will repeat itself every time an LT message is sent if the time between linktrace actions is larger than the MAC learning table aging time, unless A3 generates other frames with its MACaddress as the source address.

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27. Loss measurement

Loss measurement frames carry the number of transmitted frames from a MEP to its peer. Frame loss can becalculated by comparing the amount of frames received with the amount of frames transmitted. One of thechallenges is to be able to insert the loss measurement frame in the right position of the frame flow; inaccurateinsertions may lead to inaccurate measurement of frame losses. Another challenge is for the receiver to be able tosynchronize its received frames counter with the arrival of the loss measurement frame.

28. Delay measurements

There are two models of frame delay measurement: one-way delay measurement and two-way delay measurement.In one-way delay measurement, a frame with a time stamp indicating the transmission time is sent to the

destination. The destination receives the frame and compares the local time with the time of transmission in order todetermine the delay; by periodically measuring the delay it can also evaluate the delay variation. The main problemwith one-way delay measurement is that it requires a high degree of clock synchronization between the end points ofthe connection, and this is not always practical.

In two-way delay measurement, a frame with a time stamp indicating the transmission time is sent to thedestination. The destination receives the frame and loops it back to the source. The source receives the looped-backframe and compares the local time with the time of transmission to determine the delay; by periodically measuringthe delay it can also evaluate the delay variation. One issue with two-way delay measurements is that it onlyprovides an estimation of the end-to-end performance, but it does not indicate the ratio between the delay of thereceive and of the transmit paths. Another issue is that accuracy is impaired with the time it takes for the peer MEPto loop back the frame. This latter problem can be alleviated by adding time stamps that indicate the latency of theloopback operation and subtracting this value from the measured delay.

29. Conclusion

Packet networks present new challenges to equipment manufacturers and service providers, these challenges must beresolved before large packet networks will be deployed. Standard bodies are in the advanced steps of defining therequired tools to provide carrier-class OAM.

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30. References

[1] ITU-T I.610, "B-ISDN operation and maintenance principles and functions".

[2] IETF RFC 4379, "Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures".

[3] IEEE 802.17-2004, "Resilient packet ring (RPR) access method and physical layer specifications".

[4] IETF OAM draft-ietf-mpls-p2mp-oam-reqs-01.txt, "Requirements for Point-to-Multipoint MPLS Networks".

[5] IEEE 802.1ag, "Virtual Bridged Local Area Networks - Amendment 5: Connectivity Fault Management".

[6] ITU-T Recommendation Y.1731, "OAM functions and mechanics for Ethernet based networks".

[7] Munefumi Tsurusawa and Hideaki Tanaka, "Management Capability of Wide-Area Ethernet Network Using Ethernet OAM FunctionalityBased-on a Resilient Packet Ring Equipment", IEEE Globecom 2006 ATPON.

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