MPLS-TP OAM Analysis
draft-sprecher-mpls-tp-oam-analysis-04.txt

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Abstract

The intention of this document is to analyze the set of requirements for Operations, Administration, and Maintenance (OAM) for the Transport Profile of MPLS(MPLS-TP) as defined in [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.), to evaluate whether existing OAM tools (either from the current MPLS toolset or from the ITU-T documents) can be applied to these requirements. Eventually, the purpose of the document is to recommend which of the existing tools should be extended and what new tools should be defined to support the set of OAM requirements for MPLS-TP.

Table of Contents

1.  Introduction
    1.1.  LSP Ping
    1.2.  MPLS BFD
    1.3.  PW VCCV
    1.4.  ITU Recommendation Y.1731
    1.5.  Acronyms
    1.6.  Organization of the document
2.  Architectural requirements and general principles of operation
    2.1.  Architectural and Principles of Operation – Recommendations and Guidelines
3.  MPLS-TP OAM Functions
    3.1.  Continuity Check and Connectivity Verification
        3.1.1.  Existing tools
        3.1.2.  Gap analysis
        3.1.3.  Recommendations and Guidelines
    3.2.  Alarm Notification
        3.2.1.  Existing tools
        3.2.2.  Recommendations and Guidelines
    3.3.  Diagnostic
        3.3.1.  Existing tools
        3.3.2.  Recommendations and Guidelines
    3.4.  Adjacency and Route Tracing
        3.4.1.  Existing tools
        3.4.2.  Recommendations and Guidelines
    3.5.  Lock
        3.5.1.  Existing tools
        3.5.2.  Recommendations and Guidelines
    3.6.  Remote Defect Indication
        3.6.1.  Existing tools
        3.6.2.  Recommendations and Guidelines
    3.7.  Client Fail Indication
        3.7.1.  Existing tools
        3.7.2.  Recommendations and Guidelines
    3.8.  Packet Loss
        3.8.1.  Existing tools
        3.8.2.  Recommendations and Guidelines
    3.9.  Delay Measurement
        3.9.1.  Existing tools
        3.9.2.  Recommendations and Guidelines
4.  Recommendations
5.  IANA Considerations
6.  Security Considerations
7.  Acknowledgements
8.  Informative References
§  Authors' Addresses

1.  Introduction

OAM (Operations, Administration, and Maintenance) plays a significant and fundamental role in carrier networks, providing methods for fault management and performance monitoring in both the transport and the service layers in order to improve their ability to support services with guaranteed and strict Service Level Agreements (SLAs) while reducing their operational costs.

[MPLS‑TP Reqs] (Nadeau, T. and C. Pignataro, “Requirements for the Trasport Profile of MPLS,” April 2009.) in general, and [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) in particular define a set of requirements for OAM functionality in MPLS-Transport Profile (MPLS-TP) for MPLS-TP Label Switched Paths (LSPs) (network infrastructure) and Pseudowires (PWs) (services).

The purpose of this document is to analyze the OAM requirements and evaluate whether existing OAM tools defined for MPLS can be used to meet the requirements, identify which tools need to be extended to comply with the requirements, and which new tools need to be defined. We also take the ITU-T OAM toolset, as defined in [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.), as a candidate to base these new tools upon. The existing tools that are evaluated include LSP Ping (defined in [LSP Ping] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.)), MPLS Bi-directional Forwarding Detection (BFD) (defined in [MPLS BFD] (Katz, D. and D. Ward, “BFD for Multipoint Networks,” December 2007.)) and Virtual Circuit Connectivity Verification (VCCV) (defined in [PW VCCV] (Nadeau, T. and C. Pignataro, “Pseudowire Virtual Circuit Connectivity Verification (VCCV): A Control Channel for Pseudowires,” December 2007.) and [VCCV BFD] (Nadeau, T. and C. Pignataro, “Bidirectional Forwarding Detection (BFD) for the Pseudowire Virtual Circuit Connectivity Verification (VCCV),” February 2008.)), and the ITU-T OAM toolset defined in [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.).

1.1.  LSP Ping

LSP Ping is a variation of ICMP Ping and traceroute [ICMP] (Postel, J., “Internet Control Message Protocol,” Sept 1981.)] adapted to the different needs of MPLS LSP. Forwarding, of the LSP Ping packets, is based upon the LSP Label and label stack, in order to guarantee that the echo messages are switched in-band (i.e. over the same data route) of the LSP. However, it should be noted that the messages are transmitted using IP/UDP encapsulation and IP addresses in the 127/8 (loopback) range. The use of the loopback range guarantees that the LSP Ping messages will be terminated, by a loss of connectivity or inability to continue on the path, without being transmitted beyond the LSP. The return message of the LSP Ping could be sent either on the return LSP of a corouted bidirectional LSP, or for associated bidirectional LSPs or unidirectional LSPs may be sent using IP forwarding to the IP address of the LSP ingress node.

LSP Ping extends the basic ICMP Ping operation (of data-plane connectivity and continuity check) with functionality to verify data-plane vs. control-plane consistency for a Forwarding Equivalence Class (FEC) and also Maximum Transmission Unit (MTU) problems. The traceroute functionality may be used to isolate and localize the MPLS faults, using the Time-to-live (TTL) indicator to incrementally identify the sub-path of the LSP that is succesfully traversed before the faulty link or node. LSP Ping is not dependent on the MPLS control-plane for its operation, i.e. even though the propagation of the LSP label may be performed over the control-plane via the Label Distribution Protocol (LDP).

LSP Ping can be activated both in on-demand and pro-active (asynchronous) modes, as defined in [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.).

[P2MP LSP Ping] (Nadeau, T. and A. Farrel, “Detecting Data Plane Failures in Point-to-Multipoint Multiprotocol Label Switching (MPLS) - Extensions to LSP Ping,” June 2008.) clarifies the applicability of LSP Ping to MPLS P2MP LSPs, and extends the techniques and mechanisms of LSP Ping to the MPLS P2MP environment.

[MPLS LSP Ping] (Bahadur, N. and K. Kompella, “Mechanism for performing LSP-Ping over MPLS tunnels,” June 2008.) extends LSP Ping to operate over MPLS tunnels or for a stitched LSP.

As pointed out above, TTL exhaust is the method used to terminate flows at intermediate LSRs, usually to locate a problem that was discovered previously.

Some of the drawbacks identified with LSP Ping include - LSP Ping is considered to be computational intensive as pointed out in [MPLS BFD] (Katz, D. and D. Ward, “BFD for Multipoint Networks,” December 2007.). Use of the loopback address range (to protect against leakage outside the LSP) assumes that all of the intermediate nodes support some IP functionality. When LSP bundling is employed in the network, there is no guarantee that the LSP Ping packets will follow the same physical path used by the data traffic.

1.2.  MPLS BFD

BFD (Bidirectional Forwarding Detection) is a mechanism that is defined for fast fault detection for point-to-point connections. BFD defines a simple packet that may be transmitted over any protocol, dependent on the application that is employing the mechanism. BFD is dependent upon creation of a session that is agreed upon by both ends of the link (which may be a single link, LSP, etc.) that is being checked. The session is assigned a separate identifier by each end of the path being monitored. This session identifier is by nature only unique within the context of node that assigned it. As part of the session creation, the end-points negotiate an agreed transmission rate for the BFD packets. BFD supports an echo function to check the continuity, and verify the reachability of the desired destination. BFD does not support neither a discovery mechanism nor a traceroute capability for fault localization, these must be provided by use of other mechanisms. The BFD packets support authentication between the routers being checked.

BFD can be used in pro-active (asynchronous) and on-demand modes, as defined in [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.), of operation.

[MPLS BFD] (Katz, D. and D. Ward, “BFD for Multipoint Networks,” December 2007.) defines the use of BFD for P2P LSP end-points and is used to verify data-plane continuity. It uses a simple hello protocol which can be easily implemented in hardware. The end-points of the LSP exchange hello packets at negotiated regular intervals and an end-point is declared down when expected hello packets do not show up. Failures in each direction can be monitored independently using the same BFD session. The use of the BFD echo function and on-demand activation are outside the scope of the MPLS BFD specification.

The BFD session mechanism requires an additional (external) mechanism to bootstrap and bind the session to a particular LSP or FEC. LSP Ping is designated by [MPLS BFD] (Katz, D. and D. Ward, “BFD for Multipoint Networks,” December 2007.) as the bootstrap mechanism for the BFD session in an MPLS environment. The implication is that the session establishment BFD messages for MPLS are transmitted using a IP/UDP encapsulation.

In order to be able to identify certain extreme cases of mis-connectivity, it is necessary that each managed connection have its own unique identifiers. BFD uses Discriminator values to identify the connection being verified, at both ends of the path. These discriminator values are set by each end-node to be unique only in the context of that node. This limited scope of uniqueness would not identify a misconnection of crossing paths that could assign the same discriminators to the different sessions.

1.3.  PW VCCV

PW VCCV provides end-to-end fault detection and diagnostics for PWs (regardless of the underlying tunneling technology). The VCCV switching function provides a control channel associated with each PW (based on the PW Associated Channel Header (ACH) which is defined in [PW ACH] (Bryant, S., Swallow, G., Martini, L., and D. McPherson, “Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN,” February 2006.)), and allows sending OAM packets in-band with PW data (using CC Type 1: In-band VCCV)

VCCV currently supports the following OAM mechanisms: ICMP Ping, LSP Ping, and BFD. ICMP and LSP Ping are IP encapsulated before being sent over the PW ACH. BFD for VCCV supports two modes of encapsulation - either IP/UDP encapsulated (with IP/UDP header) or PW-ACH encapsulated (with no IP/UDP header) and provides support to signal the AC status. The use of the VCCV control channel provides the context, based on the MPLS-PW label, required to bind and bootstrap the BFD session to a particular pseudo wire (FEC), eliminating the need to exchange Discriminator values.

VCCV consists of two components: (1) signaled component to communicate VCCV capabilities as part of VC label, and (2) switching component to cause the PW payload to be treated as a control packet.

VCCV is not directly dependent upon the presence of a control plane. The VCCV capability negotiation may be performed as part of the PW signaling when LDP is used. In case of manual configuration of the PW, it is the responsibility of the operator to set consistent options at both ends.

1.4.  ITU Recommendation Y.1731

[Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) specifies a set of OAM procedures and related packet data unit (PDU) formats that meet the transport network requirements for OAM. These PDU and procedures address similar requirements to those outlined in [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.).

The PDU and procedures are described relative to an Ethernet environment, with the appropriate encapsulation for that environment. However, the actual PDU formats are technology agnostic and could be carried over different encapsulations, e.g. VCCV control channel. The OAM procedures, likewise, could be supported by MPLS-TP nodes just as they are supported by Ethernet nodes.

[Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) describes procedures to support the following OAM functions:

• Connectivity and Continuity Monitoring – for end-to-end checking
• Loopback functionality – to verify connectivity to intermediate nodes
• Link trace – provides information on the intermediate nodes of the path being monitored, may be used for fault localization.
• Alarm indication signaling – for alarm suppression in case of faults that are detected at the server layer.
• Remote defect indication &ndash as part of the Connectivity and Continuity Monitoring packets
• Performance monitoring – including measurement of packet delays both uni and bi-directional, measurement of the ratio of lost packets, and the effective bandwidth that is supported without packet loss.

It should be noted that the PDU defined in [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) includes various information elements (fields) that may not be defined in [MPLS-TP OAM Framework]. These fields include information on the MEG-Level, OpCode, and version. Addressing of the PDU as defined in [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) is based on the MAC Address of the nodes, which would need to be adjusted to support other addressing schemes, length of additional information. The addressing information is carried in <Type, Length, Value> (TLV) fields that follow the actual PDU.

1.5.  Acronyms

This draft uses the following acronyms:

1.6.  Organization of the document

Section 2 of the document analyzes the requirements that are documented in [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) and provides basic principles of operation for the OAM functionality that is required.

Section 3 evaluates which existing tools can provide coverage for the different OAM functions that are required to support MPLS-TP.

Section 4 provides recommendations on what functionality could be covered by the existing toolset and what extensions or new tools would be needed in order to provide full coverage of the OAM functionality for MPLS-TP.

2.  Architectural requirements and general principles of operation

[MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) defines a set of requirements on OAM architecture and general principles of operations which are evaluated below:

• [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) requires that OAM mechanisms in MPLS-TP are independent of the transmission media and of the client service being emulated by the PW. The existing tools comply with this requirement.
• [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) requires that MPLS-TP OAM MUST be able to operate without IP functionality and without relying on control and/or management planes. It is required that OAM functionality MUST NOT be dependent on IP routing and forwarding capabilities. The existing tools do not rely on control and/or management plane, however the following should be observed regarding the reliance on IP functionality:
• LSP Ping, VCCV Ping, and MPLS BFD makes use of IP header (UDP/IP) and do not comply with the requirement. In the on-demand mode, LSP Ping also uses IP forwarding to reply back to the source router. This dependence on IP, has further implications concerning the use of LSP Ping as the bootstrap mechanism for BFD for MPLS.
• VCCV BFD supports the use of PW-ACH encapsulated BFD sessions for PWs and can comply with the requirement.
• Y.1731 PDU are technology agnostic and thereby not dependent on IP functionality. These PDU could be carried by a VCCV control channel.
• [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) requires that OAM tools for fault management do not rely on user traffic, and the existing MPLS OAM tools and Y.1731 already comply with this requirement.
• It is also required that OAM packets and the user traffic are congruent (i.e. OAM packets are transmitted in-band) and there is a need to differentiate OAM packets from user-plane ones.
• For PWs, VCCV provides a control channel that can be associated with each PW which allows sending OAM packets in band of PWs and allow the receiving end-point to intercept, interpret, and process them locally as OAM messages. VCCV defines different VCCV Connectivity Verification Types for MPLS (like ICMP Ping, LSP Ping and IP/UDP encapsulated BFD and PW-ACH encapsulated BFD).
• Currently there is no distinct OAM payload identifier in MPLS shim. BFD and LSP Ping packets for LSPs are carried over UDP/IP and are addressed to the loopback address range. The router at the end-point intercepts, interprets, and processes the packets.
• The Y.1731 PDU could be carried over a control channel defined along the data path and the processing of the PDU would occur at the destination indicated in the PDU.
• [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) requires that the MPLS-TP OAM mechanism allows the propagation of AC (Attachment Circuit) failures and their clearance across a MPLS-TP domain
• BFD for VCCV supports a mechanism for "Fault detection and AC/PW Fault status signaling." This can be used for both IP/UDP encapsulated or PW-ACH encapsulated BFD sessions, i.e. by setting the appropriate VCCV Connectivity Verification Type.This mechanism could support this requirement.
• [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) defines Maintenance Domain, Maintenance End Points (MEPs) and Maintenance Intermediate Points (MIPs). Means should be defined to provision these entities, both by static configuration (as it is required to operate OAM in the absence of any control plane or dynamic protocols) and by a control plane. Note that the Y.1731 functionality currently supports these entities.
• [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) requires a single OAM technology and consistent OAM capabilities for LSPs, PWs, MPLS-TP Links, and Tandem Connections. There is currently no mechanism in the IETF to support OAM for Tandem Connections. Also, the existing set of tools defines a different way of operating the OAM functions (e.g. LSP Ping to bootstrap MPLS BFD vs. VCCV). Currently, the Y.1731 functionality is defined for Ethernet paths, and the procedures would need to be redefined for the various MPLS-TP path concepts.
• [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) requires allowing OAM packets to be directed to an intermediate node (MIP) of a LSP/PW. Technically, this could be supported by the proper setting of the TTL value. However, the applicability of such a solution needs to be examined per OAM function. For details, see below.
• [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) suggests that OAM messages MAY be authenticated. BFD has a support for authentication. Other tools should support this capability as well. Y.1731 functionality uses the identification of the path for authentication.

2.1.  Architectural and Principles of Operation – Recommendations and Guidelines

Based on the requirements analysis above, the following guidelines should be followed to create an OAM environment that could more fully support the requirements cited:

• Extend the PW Associate Channel Header (ACH) to provide a control channel at the path and section levels. This could then be associated with a MPLS-TP Link, LSP, or a Tandem Connection (TC). The ACH should then become a common mechanism for PW, LSP, MPLS-TP Link, and Tandem Connection.
• Create a VPCV (Virtual Path Connectivity Verification) definition that would apply the definitions and functionality of VCCV to the MPLS-TP environment for LSP or Tandem Connection. Need a generalized addressing scheme that can also support unique identification of the monitored paths (or connections).
• Create or extend the VCCV definition to define a mechanism that would apply the definitions and functionality of VCCV to PW Tandem Connections
• Apply BFD to these new mechanisms using the control channel encapsulation, as defined above – allowing use of BFD for MPLS-TP independent of IP functionality. This could be used to address the CC-V functionality
• The Y.1731 PDU set could be used as a basis for defining the information units to be transmitted over the VPCV. The actual procedures and addressing schemes would need to be adjusted for the MPLS-TP environment.
• Define a mechanism to create TCME and allow transmission of the traffic via the Tandem Connection using label stacking.
• Define a mechanism that could be used to address a MIP of a path in a unique way, to support the maintenance functions. This addressing should be flexible to allow support for different addressing schemes, and would supplement the TTL addressing of intermediate points.

Creating these extensions/mechanisms would fulfill the following architectural requirements, mentioned above:

• Independence of IP forwarding and routing.
• OAM packets should be transmitted in-band.
• Support a single OAM technology for LSP, PW, MPLS-TP Link, and TC.

In addition, the following additional requirements can be satisfied:

• Provide the ability to carry other types of communications (e.g., APS, Management Control Channel (MCC), Signalling Control Channel (SCC)), by defining new types of communication channels for PWs, MPLS-TP Links, and LSPs.
• The design of the OAM mechanisms for MPLS-TP MUST allow the ability to support vendor specific and experimental OAM functions.

3.  MPLS-TP OAM Functions

The following sections discuss the required OAM functions that were identified in [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.).

LSP Ping is not considered a candidate to fulfill the required functionality, due its failure to comply with the basic architectural requirement for independence from IP routing and forwarding, as documented in Section 2 of this document. However, usage of LSP Ping, in addition to the MPLS-TP OAM tools, or in MPLS-TP deployments with IP functionality is not precluded.

3.1.  Continuity Check and Connectivity Verification

Continuity Check and Connectivity Verification (CC-V) are OAM operations generally used in tandem, and compliment each other. Together they are used to detect loss of traffic continuity and misconnections between MEPs and are useful for applications like Fault Management, Performance Monitoring and Protection Switching, etc. To guarantee that CC-V can identify misconnections from cross-connections it is necessary that the tool use network-wide unique identifiers for the path that is being checked in the session.

3.1.1.  Existing tools

LSP Ping provides much of the functionality required for corouted bidirectional LSPs. As observed above, LSP Ping may be operated in both asynchronous and on-demand mode. Addressing is based on the LSP label and the basic functionality only requires support for the loopback address range in each node on the LSP path.

BFD defines functionality that can be used to support the pro-active OAM CC-V function when operated in the asynchronous mode. However, the current definition of basic BFD is dependent on use of LSP Ping to bootstrap the BFD session. Regarding the connectivity functional aspects, basic BFD has a limitation that it uses only locally unique (to each node) session identifiers.

VCCV can be used to carry BFD packets that are not IP/UDP encapsulated for CC-V on a PW and use the PW label to identify the path.

Y.1731 provides functionality for all aspects of CC-V for an Ethernet environment, this could be translated for the MPLS-TP environment. The CCM PDU defined in [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) includes the ability to set the frequency of the messages that are transmitted, and provides for attaching the address of the path (in the Ethernet case – the MEG Level) and a sequencing number to verify that CCM messages were not dropped.

3.1.2.  Gap analysis

There is currently no single MPLS tool that gives coverage for all aspects of CC-V functionality.

LSP Ping could be used to cover the cases of corouted bidirectional LSPs. However, there is a certain amount of computational overhead involved with use of LSP Ping (as was observed in sec 1.1), the verification of the control-plane, and the need to support the loopback functionality at each intermediate node.

BFD could be extended to fill the gaps indicated above. The extension would include:

• A mechanism should be defined to carry BFD packets over LSP without reliance on IP functionality.
• A mechanism should be defined to bootstrap BFD sessions for MPLS that is not dependent on UDP.
• BFD needs to be used in conjunction with "globally" unique identifiers for the path or ME being checked to allow connectivity verfication support. There are two possibilities, to allow BFD to support this new type of identifier –
  • Change the semantics of the two Discriminator fields that exist in BFD and have each node select the ME unique identifier. This may have backward compatibility implications.
  • Create a new optional field in the packet carrying the BFD that would identify the path being checked, in addition to the existing session identifiers.
• Extensions to BFD would be needed to cover P2MP connections.

Use of the Y.1731 functionality is another option that should be considered. The basic PDU for CCM includes (in the flags field) an indication of the frequency of the packets [eliminating the need to "negotiate" the frequency between the end-points], and also a flag used for RDI. The procedure itself would need adaptation to comply with the MPLS environment.

An additional option would be to create a new tool that would give coverage for both aspects of CC-V according to the requirements and the principles of operation (see section 2.1). This option is less preferable.

3.1.3.  Recommendations and Guidelines

Extend BFD to resolve the gaps, using a new optional field for the unique path identifier. And optionally support the PDU format defined in [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) with appropriate adjustments to support the MPLS-TP architecture.

Note that [MPLS BFD] (Katz, D. and D. Ward, “BFD for Multipoint Networks,” December 2007.) defines a method for using BFD to provide verification of multipoint or multicast connectivity.

3.2.  Alarm Notification

Alarm Notification is a function that is used by a server layer MEP to notify a failure condition to its client layer MEP(s) in order to suppress alarms that may be generated by maintenance domains of the client layer as a result of the failure condition in the server layer. This function should also have the capability to differentiate an administrative lock from a failure condition at a different execution level.

3.2.1.  Existing tools

There is no mechanism defined in the IETF to support this function. Y.1731 does define a PDU and procedure for this functionality.

3.2.2.  Recommendations and Guidelines

Define a tool to support Alarm Notification. This tool could be designed around the PDU proposed by [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) that includes support for an indication of the frequency at which these messages are transmitted after the alarm is raised until it is cleared.

3.3.  Diagnostic

A diagnostic test is a function that is used between MEPs to verify bandwidth throughput, packet loss, bit errors, etc. This is usually performed by sending packets of varying sizes at increasing rates (until the limits of the service level) to measure the actual utilization.

3.3.1.  Existing tools

There is no mechanism defined in the IETF to support this function. [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) describes a function that is dependent on sending a series of TST packets (this is a PDU whose size can be varied) at differing frequencies.

3.3.2.  Recommendations and Guidelines

Define a tool to support Diagnostic that could be based on the Y.1731 function.

3.4.  Adjacency and Route Tracing

Functinality of route determination is used to determine the route of a connection across the MPLS transport network. [MPLS‑TP OAM Reqs] (Vigoureux, M., Betts, M., and D. Ward, “Requirements for OAM in MPLS Transport Networks,” April 2009.) defines two closely related operations – one, Adjacency, for discovery of neighboring nodes and the other, Route Tracing, for determination of the path that is being traversed and location of a fault identified by e.g. the CC-V tool.

3.4.1.  Existing tools

LSP Ping supports a trace route function that could be used for co-routed bidirectional paths. This could support the second type of fnctionality.

However, the discovery aspect that is described by the Adjacency function does not have any available tools, neither in the IETF toolset nor in the ITU recommendations.

3.4.2.  Recommendations and Guidelines

Define a new tool to support the Adjacency functionality.

For the Route Trace functionality, either extend the LSP Ping functionality to support other options, i.e. PW, associated bidirectional LSP, or define a new tool.

3.5.  Lock

The Lock function allows the system to block off transmission of data along a LSP. When a path end-point receives a command, e.g. from the management system, that the path is blocked, the end-point informs the far-end that the path has been locked and that no data should be transmitted. This function is used on-demand.

3.5.1.  Existing tools

There is no mechanism defined in the IETF to support this function. Y.1731 does define a PDU and procedure for this functionality.

3.5.2.  Recommendations and Guidelines

Define a tool to support Lock. This tool could be designed around the procedure proposed by [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) that includes support for an indication of the frequency at which these messages are transmitted until the lock situation is cleared.

3.6.  Remote Defect Indication

Remote Defect Indication (RDI) is used by a MEP to notify its peer MEP that a defect, usually a unidirectional defect, is detected on a bi-directional connection between them.

This function should be supported in pro-active mode.

3.6.1.  Existing tools

There is no mechanism defined in the IETF to fully support this functionality, however BFD supports a mechanism of informing the far-end that the session has gone down, and the Diagnostic field indicates the reason. Similarly, when LSP Ping is used for a corouted bidirectional LSP the far-end LER could notify that there was a misconnectivity.

In [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) this functionality is defined as part of the CC-V function as a flag in the PDU.

3.6.2.  Recommendations and Guidelines

Either create a dedicated mechanism for this functionality or extend the BFD session functionality to support the functionality without disrupting the CC or CV functionality. Such an extension could be similar to that suggested by the ITU recommendation

3.7.  Client Fail Indication

Client Fail Indication (CFI) function is used to propagate an indication of a failure to the far-end sink when alarm suppression in the client layer is not supported.

3.7.1.  Existing tools

There is a possibility of using the BFD over VCCV mechanism for "Fault detection and AC/PW Fault status signalling". However, there is a need to differentiate between faults on the AC and the PW.

3.7.2.  Recommendations and Guidelines

Either extend the BFD tool or define a tool to support Client Fail Indication propagation.

3.8.  Packet Loss

Packet Loss is a function that is used to verify the quality of the service. This function indicates the ratio of packets that are not delivered out of all packets that are transmitted by the path source.

There are two possible ways of determining this measurement –

• Using OAM packets, it is possible to compute the statistics based on a series of OAM packets. This, however, has the disadvantage of being artificial, and may not be representative since part of the packet loss may be dependent upon packet sizes.
• Sending delimiting messages for the start and end of a measurement period during which the source and sink of the path count the packets transmitted and received. After the end delimiter, the ratio would be calculated by the path OAM entity.

3.8.1.  Existing tools

There is no mechanism defined in the IETF to support this function. [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) describes a function that is based on sending the CCM packets [used for CC-V support (see sec 3.1)] for proactive support and specialized loss-measurement packets for on-demand measurement. These packets include information (in the additional TLV fields) of packet counters that are maintained by each of the end-points of a path. These counters maintain a count of packets transmitted by the ingress end-point and the count of packets received from the far-end of the path by the egress end-point.

3.8.2.  Recommendations and Guidelines

One possibility is to define a mechanism to support Packet Loss Measurement, based on the delimiting messages. This would include a way for delimiting the periods for monitoring the packet transmissions to measure the loss ratios, and computation of the ratio between received and transmitted packets.

A second possibility would be to define a functionality based on the description of the loss-measurement function defined in [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) that is dependent on the counters maintained, by the MPLS LSR (as described in [RFC3813] (Srinivasan, C., Viswanathan, A., and T. Nadeau, “Multiprotocol Label Switching (MPLS) Label Switching Router (LSR) Management Information Base (MIB),” June 2004.), of received and transmitted octets.

3.9.  Delay Measurement

Delay Measurement is a function that is used to measure one-way or two-way delay of a packet transmission between a pair of MEPs. Where:

• One-way packet delay is the time elapsed from the start of transmission of the first bit of the packet by a source node until the reception of the first bit of that packet by the destination node.
• Two-way packet delay is the time elapsed from the start of transmission of the first bit of the packet by a source node until the reception of the last bit of the loop-backed packet by the same source node, when the loopback is performed at the packet's destination node.

Similarly to the packet loss measurement this could be performed in one of two ways –

• Using OAM packets – checking delay (either one-way or two-way) in transmission of OAM packets. May not fully reflect delay of larger packets, however, gives feedback on general service level.
• Using delimited periods of transmission – may be too intrusive on the client traffic.

3.9.1.  Existing tools

There is no mechanism defined in the IETF toolset that fulfills all of the MPLS-TP OAM requirements.

[Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) describes a function in which specific OAM packets are sent with a transmission time-stamp from one end of the managed path to the other end (these are transparent to the intermediate nodes). The delay measurement is supported for both unidirectional and bidirectional measurement of the delay.

3.9.2.  Recommendations and Guidelines

Define a mechanism that would allow to support Delay Measurement. The mechanism should be based on measurement of the delay in transmission and reception of OAM packets, transmitted in-band with normal traffic. This tool could be based on the tool defined in [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.).

4.  Recommendations

• Define a maintenance entity that could be applied both to LSPs and PWs that would support management of a sub-path. This entity should allow for transmission of traffic by means of label stacking and proper TTL setting.
• Extend the control and the management planes to support the configuration of the OAM maintenance entities and the set of functions to be supported by these entities.
• Extend the ACH to provide a control channel for MPLS-TP Links, LSPs, and Tandem Connections.
• Define a mechanism that would allow the unique addressing of the elements that need to be monitored, e.g., the connections, MEPs, and MIPs of a path. This mechanism needs to be flexible enough to support different addressing schemes, e.g. IP addresses, NSAP, connection names.
• Define a VPCV mechanism for LSP and Tandem Connection. This mechanism should reuse, as much as possible, the same principles of operation as VCCV. The ACH should be extended to support CV types for each of the tools that are defined below, in a way that is consistent for PW, LSP and Tandem Connection.
• The appropriate assignment of network-wide unique identifiers needed to support connectivity verification should be considered.
• Tools should be defined to support the following functions. The tools could be based on the procedures and PDU format defined by [Y.1731] (International Telecommunications Union - Standardization, “OAM functions and mechanisms for Ethernet based networks,” May 2006.) or extensions to existing MPLS tools:
• On-demand connectivity verification
• Alarm suppression
• Packet loss measurement
• Diagnostic test
• Route determination
• Delay measurement
• Remote defect indication
• Client fail indication
• The tools may have the capability to authenticate the messages.

5.  IANA Considerations

This document makes no request of IANA.

Note to RFC Editor: this section may be removed on publication as an RFC.

6.  Security Considerations

This document does not by itself raise any particular security considerations.

7.  Acknowledgements

The authors wish to thank xxxxxxx for his review and proposed enhancements to the text.

8. Informative References

Authors' Addresses