Abstract—A method to monitor the quality of a MPLS LSP at the user, the service provider and the network provider’s aspect is suggested. The CV packet of the MPLS OAM packets is modified, and one bit for checking the incoming error is defined, the BIP-8 byte is added to evaluate the signal quality at the service provider and the network provider level respectively. By performing the tandem connection monitoring like the above the possible conflict between network providers can be avoided as the error location will be clearly identified. Index terms—TCM, OAM, MPLS

A. INTRODUCTION

In view of operation and maintenance for MPLS network, ITU-T Y.1711 published in 2004 details the related OAM (Operation, Administration and Maintenance) packet frame structures and their operation mechanism. According to the Y.1711, it is seen that MPLS network should have at least four types of surveillance mechanisms depending on the monitoring purposes. For the lower layer failure indication of the MPLS network the maintenance signals such as FDI (Forward Defect Indication) and BDI (Backward Defect Indication) signals are required. To periodically check the signal continuity at the egress node, the CV (Continuity Verification) and FFD (Fast Failure Detection) signals should be offered. For the purpose of connection check between two ends of the LSP (Label Switched Path), TTSI (Trail Termination Source Indication) signal is respectively defined in the CV and the FFD frame. If a network operator wishes to see the health of the return LSP path, the loop-back function should be supported. The above four surveillance mechanisms are key features in terms of the MPLS OAM roles, and depending on the raised defects how to deal with the OAM packet are in detail specified in Y.1711 as well. On the other hand, the currently defined MPLS OAM has not been reflected yet for the case of the error in the previous network. In other word, even though the error is being detected, but there is no way to clarify the error location. Consequently, no information is given to the user, the service provider and even the network

1 This work was supported by the IT R&D program of MIC/IITA. [2006-S-064-02, BcN Network Engineering]

providers can not share the network error information. In order to resolve such difficulties the tandem connection monitoring functionality is suggested for the MPLS network.

B. TCM FOR AN MPLS NETWORK

It is believed that the MPLS network should be

monitored at the level of the network providers, the service providers and the user because it might travel across multiple network provider domains as shown in figure 1. In order to build the tandem connection monitoring network, the CV of the MPLS OAM packet is modified as a TCM packet since it has one second periodic transmission nature and the reserved bytes unused in the current version.

MPLS LSP

NP A

SP TCM

CE CE

NP TCM A NP TCM B NP TCM C

TCM point TCM point TCM point

User TCM

NP B NP C

PE PE

TCM point

Figure 1 MPLS TCM boundaries

The CV packet shown in figure 2 contains three reserved byte and eighteen padding byte which carries null data just to form 44 byte MPLS OAM packet. The TCM information may be delivered using the three unused byte in CV packet to satisfy compatibility with the current version without extending the length of the OAM packets.

Function Type (01Hex)

Reserve(all 00Hex)

Padding(all 00Hex)LSP TTSI BIP16

1 octet 3 octet 20 octet 18 octet 2 octet

Figure 2 CV packet structure

The first octet “function type” within CV packet in figure 1 is a code-point used to differentiate OAM packets. The currently assigned code-point values are of 01Hex to 07Hex. All other code-points are reserved for future standardization in ITU-T. We will assign 08 Hex for the TCM of a MPLS network.

Kyung Gyu Chun, Soon Suk Lee, and Young Sun Kim Tandem Connection Monitoring in the MPLS Network

Electronics and Telecommunication Research Institute161 Gajeong-dong, Yuseong-gu, Daejeon, Korea

{jkq, sslee, sunkim}@etri.re.kr

Proceedings of Asia-Pacific Conference on Communications 2007

1-4244-1374-5/07/$25.00 ©2007 IEEE 345

As shown in figure 3, the TCM bytes consist of two kinds of the BIP-8 byte and the incoming error (IE) bit for the service provider (SP) or the network provider (NP). The BIP-8 is used for the TCM packet error check and the incoming error bit for the previous network’s error check, respectively.

Function Type (08Hex) TCM paddingLSP TTSI BIP-16

1 octet 3 octet 20 octet 18 octet 2 octet

1 octet

SP BIP-8x x

1 2 3 4 5 6 7 8

NPIE NP BIP-8

1 octet

SPIE

1 bit 1 bit Figure 3 TCM packet structure

Figure 4 shows the TCM operation for the client of

MPLS network. In view of the users, sometimes, they may wish to evaluate the signal quality being serviced. To satisfy such the user’s needs, as the customer equipment should be capable of the LSP generation and termination, we assume that the customer equipment has MPLS functionalities in this paper.

At the user level, the BIP-16 bytes in figure 3 are calculated so that the entire TCM packet except for the SP and the NP BIP-8 is summed by Exclusive OR logic. The calculated BIP-16 is saved and transmitted by the TCM packet at the ingress CE node. During the TCM packet transmission, the intermediate provider equipments (PE) must not modify the BIP-16 byte, because it should be monitored and calculated only at the customer equipment. At the egress CE node the received TCM packet except for the BIP-16 byte is recalculated by the BIP-16 check algorithm and compared it with the BIP-16 byte of the TCM packet. The BIP-16 calculation is done by summing every two byte as the following. BIP-16 = Function type plus IE (2byte including the null bits) ⊕ TTSI (2x10 byte) ⊕ Padding (2x9 byte), where “⊕” denotes the exclusive OR function.

As the IE bit is used for indication of the incoming error, if an error takes place within the network provider domain, the IE bit whether it’s SP-IE or NP-IE will be set to “1”. As such, by using the IE bit the at user level the network error will be detected since the calculated BIP-16 at the egress CE node will differ from the carried BIP-16 due to the IE bit set to “1”.

CECE PE

NP X NP Y NP ZClient A Client B

User TCM

BIP-16 insertion BIP-16 calculation

PE PE PE PE PE

Figure 4 Parity check operation for user level TCM

The figure 5 illustrates the TCM operation at the

service provider level. The service providers shall be responsible to safely deliver the user traffic through their own leased network regardless of the network providers. In the aspect of service providers they will wish to distinguish whether it is the caused error by their user or their network provider.

Service Provider TCM

BIP-8 insertionCheck Incoming Error

BIP-8 calculationCheck Incoming Error bit

CECE PE

NP X NP Y NP ZClient A Client B

PE PE PE PE PE

Figure 5 Parity check operation for SP level TCM To investigate the incoming error from the user side,

i.e. the CE A shown in figure 5, the first node of the service provider checks the BIP-16 and if occurred the error the SP-IE bit is set to “1”, which indicates the original signal is in error and is referred to the last service provider node, i.e. NP Z. The BIP-8 is calculated with the entire TCM packet except three TCM bytes whether the incoming error is present or not, which is used for the signal quality check of the service provider’s MPLS domain. The BIP-8 calculation is done by summing every byte as the following. SP BIP-8 = Function type ⊕ TTSI ⊕ Padding ⊕ BIP-16, where ⊕ denotes the exclusive OR function. If no error occurs in the network provider domains, the

calculated SP BIP-8 at the service provider’s egress node will have the same value as the delivered one from the ingress node, but at one of the network provider domain when the error occurs, the calculated BIP-8 will be different from the carried one or the NP-IE set to “1”, then it will result in the SP BIP-8 error.

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NP X TCM NP Y TCM

Insert NP BIP-8Check Error & Set IE bit

Insert NP BIP-8Check Error & Set IE bit

Calculate NP BIP-8 Calculate NP BIP-8

CECE PE

NP X NP Y NP ZClient A Client B

PE PE PE PE PE

NP Z TCM

Insert NP BIP-8Check Error & Set IE bit

Calculate NP BIP-8

Figure 6 Parity check operation for NP level TCM

In case where a LSP is deployed across multiple

network provider domains, and furthermore the LSP is in an error it will be desirable to clarify its error cause. As the error is caused by the previous network or the network inside fault, the NP BIP-8 is used. The operation method of the NP BIP-8 and the NP IE are the same as in that of the SP BIP-8 byte and SP IE bit, but the BIP-8 and the IE at the NP domain are updated because every NP domain should be independently monitored. On the other hand, at the first network provider node, the ingress PE of the NP X shown in figure 6, the BIP-16 from the client A instead of the BIP-8 is used to check the incoming error.

C. TCM FINITE STATE MACHINE

Without reference to the surveillance domain the TCM

packet is processed with the same manner at the first and the last node of each network. The TCM packet processing algorithms are shown in figure 7 and figure 8, respectively. At the ingress node as illustrated in figure 7, the BIP-8 for the received TCM packet is recalculated and compared to the value of BIP-8 byte contained in the TCM packet. If two values are different, and then the incoming error bit is set. Regardless of the incoming error a new BIP-8 is calculated for each domain monitoring and it is sent to the egress node through the MPLS network.

At the egress node as shown in figure 8, the IE bit is firstly checked, and the incoming error is reported to the network provider. If the calculated BIP-8 differs from the carried one, the network inside error is reported to the network provider. By using the BIP-N (N=8, 16) and the IE, the network inside and outside error can be identified. By investigating the reported error each network provider will be able to make decision of the network error cause.

Start

Calculate BIP-8,Compare the received to

the calculated BIP-8

BIP-8 error?

Insert a new BIP-8

End

No

Yes

Send TCM packet

Set “IE” bit

Receive incoming TCM packet

Figure 7 TCM packets processing at the ingress node

Receive TCM packet

Calculate BIP-8

BIP-8 error ?

End

No

Yes

IE bit = “1” ?

No

Yes

Start

Report Incoming Error

Report packet Error

Send TCM packet to the next

,

Figure 8 TCM packets processing at the egress node

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D. SUMMARY AND CONSIDERATIONS

In the MPLS network going through multiple network providers, the tandem connection monitoring functionality is presented, which make it possible to evaluate the path quality at the user, the service provider and the network provider level.

The TCM packet is designed modifying the CV packet, in which the BIP-8 byte and the IE bit are newly added. The IE bit allows each domain owner to detect the external error, and the BIP-N (N=8, 16) is used for the internal error check. Each network provider individually operates the IE bit and the BIP-8 byte, which enables each domain to be independently monitored of the network provider. It is assumed that the TCM prevents conflict between network providers, and the service provider is able to ensure the network quality to the user since it has a capability of clarifying whether it is the internal or the external error. Finally, it is believed that the TCM packet can be used for the continuity check instead of the CV packet due to its periodic transmission characteristic.

References [1] ITU-T Recommendation Y.1711 (02/2004),

Operation & Maintenance mechanism for MPLS networks.

[2] ITU-T Recommendation G.709/Y.1331 (03/2003), Interface for the Optical Transport Network (OTN)

[3] ITU-T Recommendation G.798 (01/2002), Characteristic of optical transport network hierarchy equipment functional blocks

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