Operations and Management (OAM) Requirements

for Multi-Protocol Label Switched (MPLS) Networks

Status of This Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2006).

Abstract

This document specifies Operations and Management (OAM) requirements for Multi-Protocol Label Switching (MPLS), as well as for applications of MPLS, such as pseudo-wire voice and virtual private network services. These requirements have been gathered from network operators who have extensive experience deploying MPLS networks.

Table of Contents

   1. Introduction ....................................................2
   2. Document Conventions ............................................2
   3. Motivations .....................................................4
   4. Requirements ....................................................4
   5. Security Considerations ........................................11
   6. References .....................................................12
   7. Acknowledgements ...............................................13

1. Introduction

This document describes requirements for user and data plane Operations and Management (OAM) for Multi-Protocol Label Switching (MPLS). These requirements have been gathered from network operators who have extensive experience deploying MPLS networks. This document specifies OAM requirements for MPLS, as well as for applications of MPLS.

Currently, there are no specific mechanisms proposed to address these requirements. The goal of this document is to identify a commonly applicable set of requirements for MPLS OAM at this time. Specifically, a set of requirements that apply to the most common set of MPLS networks deployed by service provider organizations at the time this document was written. These requirements can then be used as a base for network management tool development and to guide the evolution of currently specified tools, as well as the specification of OAM functions that are intrinsic to protocols used in MPLS networks.

2. Document Conventions

2.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

Queuing/buffering Latency:

The delay caused by packet queuing (value is variable since it is dependent on the packet arrival rate, the packet length, and the link throughput).

   Probe-based-detection:     Active measurement tool that can measure
   
                              the consistency of an LSP [RFC4379].
   
   Defect:                    Any error condition that prevents a Label
                              Switched Path (LSP) from functioning
                              correctly.  For example, loss of an
                              Interior Gateway Protocol (IGP) path will
                              most likely result in an LSP not being
                              able to deliver traffic to its
                              destination.  Another example is the
                              interruption of the path for a TE tunnel.
                              These may be due to physical circuit
                              failures or failure of switching nodes to
                              operate as expected.

Multi-vendor/multi-provider network operation typically requires agreed upon definitions of defects (when it is broken and when it is not) such that both recovery procedures and service level specification impact can be specified.

   Head-end Label Switching
   Router (LSR):              The beginning of an LSP.  A head-end LSR
                              is also referred to as an ingress LSR.
   
   Tail-end Label Switching
   Router (LSR):              The end of an LSP.  A tail-end LSR is also
                              referred to as an egress LSR.
   
   Propagation Latency:       The delay added by the propagation of the
                              packet through the link (fixed value that
                              depends on the distance of the link and
                              the propagation speed).
   
   Transmission Latency:      The delay added by the transmission of the
                              packet over the link, i.e., the time it
                              takes to put the packet over the media
                              (value that depends on the link throughput
                              and packet length).
   
   Processing Latency:        The delay added by all the operations
                              related to the switching of labeled
                              packets (value is node implementation
                              specific and may be considered fixed and
                              constant for a given type of equipment).
   
   Node Latency:              The delay added by the network element
                              resulting from of the sum of the
                              transmission, processing, and
                              queuing/buffering latency.
   
   One-hop Delay:             The fixed delay experienced by a packet to
                              reach the next hop resulting from the of
                              the propagation latency, the transmission
                              latency, and the processing latency.
   
   Minimum Path Latency:      The sum of the one-hop delays experienced
                              by the packet when traveling from the
                              ingress to the egress LSR.
   
   Variable Path Latency:     The variation in the sum of the delays
                              experienced by packets transiting the
                              path, otherwise know as jitter.

2.2. Acronyms

ASBR:

Autonomous System Border Router

CE:

       Customer Edge

PE:

       Provider Edge

SP:

       Service Provider

ECMP:

         Equal-Cost Multi-path

LSP:

        Label Switched Path

LSP Ping: Label Switched Path Ping

LSR:

        Label Switching Router

OAM:

        Operations and Management

RSVP:

         Resource reSerVation Protocol

LDP:

        Label Distribution Protocol

DoS:

        Denial of Service

3. Motivations

This document was created to provide requirements that could be used to create consistent and useful OAM functionality that meets operational requirements of those service providers (SPs) who have deployed or are deploying MPLS.

4. Requirements

The following sections enumerate the OAM requirements gathered from service providers who have deployed MPLS and services based on MPLS networks. Each requirement is specified in detail to clarify its applicability. Although the requirements specified herein are defined by the IETF, they have been made consistent with requirements gathered by other standards bodies such as the ITU [Y1710].

4.1. Detection of Label Switched Path Defects

The ability to detect defects in a broken LSP MUST not require manual hop-by-hop troubleshooting of each LSR used to switch traffic for that LSP. For example, it is not desirable to manually visit each LSR along the data plane path transited by an LSP; instead, this function MUST be automated and able to be performed at some operator specified frequency from the origination point of that LSP. This implies solutions that are interoperable to allow for such automatic operation.

Furthermore, the automation of path liveliness is desired in cases where large numbers of LSPs might be tested. For example, automated ingress LSR to egress LSR testing functionality is desired for some LSPs. The goal is to detect LSP path defects before customers do, which requires detection and correction of LSP defects in a manner that is both predictable and within the constraints of the service level agreement under which the service is being offered. Simply put, the sum of the time it takes an OAM tool to detect a defect and the time needed for an operational support system to react to this defect, by possibly correcting it or notifying the customer, must fall within the bounds of the service level agreement in question.

Synchronization of detection time bounds by tools used to detect broken LSPs is required. Failure to specify defect detection time bounds may result in an ambiguity in test results. If the time to detect broken LSPs is known, then automated responses can be specified with respect and regard to resiliency and service level specification reporting. Further, if synchronization of detection time bounds is possible, an operational framework can be established to guide the design and specification of MPLS applications.

Although an ICMP-based ping [RFC792] can be sent through an LSP as an IP payload, the use of this tool to verify the defect-free operation of an LSP has the potential of returning erroneous results (both positive and negative) for a number of reasons. For example, in some cases, because the ICMP traffic is based on legally addressable IP addressing, it is possible for ICMP messages that are originally transmitted inside of an LSP to "fall out of the LSP" at some point along the path. In these cases, since ICMP packets are routable, a falsely positive response may be returned. In other cases, where the data plane of a specific LSP needs to be tested, it is difficult to guarantee that traffic based on an ICMP ping header is parsed and hashed to the same equal-cost multi-paths (ECMP) as the data traffic.

Any detection mechanisms that depend on receiving the status via a return path SHOULD provide multiple return options with the expectation that one of them will not be impacted by the original defect. An example of a case where a false negative might occur would be a mechanism that requires a functional MPLS return path. Since MPLS LSPs are unidirectional, it is possible that although the forward LSP, which is the LSP under test, might be functioning, the response from the destination LSR might be lost, thus giving the source LSR the false impression that the forward LSP is defective. However, if an alternate return path could be specified -- say IP for example -- then the source could specify this as the return path to the destination, and in this case, would receive a response indicating that the return LSP is defective.

The OAM packet MUST follow the customer data path exactly in order to reflect path liveliness used by customer data. Particular cases of interest are forwarding mechanisms, such as ECMP scenarios within the operator's network, whereby flows are load-shared across parallel paths (i.e., equal IGP cost). Where the customer traffic may be spread over multiple paths, the ability to detect failures on any of the path permutations is required. Where the spreading mechanism is payload specific, payloads need to have forwarding that is common with the traffic under test. Satisfying these requirements introduces complexity into ensuring that ECMP connectivity permutations are exercised and that defect detection occurs in a reasonable amount of time.

4.2. Diagnosis of a Broken Label Switched Path

The ability to diagnose a broken LSP and to isolate the failed component (i.e., link or node) in the path is required. For example, note that specifying recovery actions for mis-branching defects in an LDP network is a particularly difficult case. Diagnosis of defects and isolation of the failed component is best accomplished via a path trace function that can return the entire list of LSRs and links used by a certain LSP (or at least the set of LSRs/links up to the location of the defect). The tracing capability SHOULD include the ability to trace recursive paths, such as when nested LSPs are used. This path trace function MUST also be capable of diagnosing LSP mis- merging by permitting comparison of expected vs. actual forwarding behavior at any LSR in the path. The path trace capability SHOULD be capable of being executed from the head-end Label Switching Router (LSR) and may permit downstream path components to be traced from an intermediate mid-point LSR. Additionally, the path trace function MUST have the ability to support ECMP scenarios described in Section 4.1.

4.3. Path Characterization

The path characterization function is the ability to reveal details of LSR forwarding operations. These details can then be compared during subsequent testing relevant to OAM functionality. This includes but is not limited to:

• consistent use of pipe or uniform time to live (TTL) models by an LSR [RFC3443].

• sufficient details that allow the test origin to exercise all path permutations related to load spreading (e.g., ECMP).

• stack operations performed by the LSR, such as pushes, pops, and TTL propagation at penultimate hop LSRs.

4.4. Service Level Agreement Measurement

Mechanisms are required to measure the diverse aspects of Service Level Agreements, which include:

• latency - amount of time required for traffic to transit the network

• packet loss

• jitter - measurement of latency variation

• defect free forwarding - the service is considered to be available, or the service is unavailable and other aspects of performance measurement do not have meaning.

Such measurements can be made independently of the user traffic or via a hybrid of user traffic measurement and OAM probing.

At least one mechanism is required to measure the number of OAM packets. In addition, the ability to measure the quantitative aspects of LSPs, such as jitter, delay, latency, and loss, MUST be available in order to determine whether the traffic for a specific LSP is traveling within the operator-specified tolerances.

Any method considered SHOULD be capable of measuring the latency of an LSP with minimal impact on network resources. See Section 2.1 for definitions of the various quantitative aspects of LSPs.

4.5. Frequency of OAM Execution

The operator MUST have the flexibility to configure OAM parameters to meet their specific operational requirements.

This includes the frequency of the execution of any OAM functions. The ability to synchronize OAM operations is required to permit a consistent measurement of service level agreements. To elaborate, there are defect conditions, such as mis-branching or misdirection of traffic, for which probe-based detection mechanisms that incur significant mismatches in their detection frequency may result in flapping. This can be addressed either by synchronizing the rate or having the probes self-identify their probe rate. For example, when the probing mechanisms are bootstrapping, they might negotiate and ultimately agree on a probing rate, therefore providing a consistent probing frequency and avoiding the aforementioned problems.

One observation would be that wide-spread deployment of MPLS, common implementation of monitoring tools, and the need for inter-carrier synchronization of defect and service level specification handling will drive specification of OAM parameters to commonly agreed on values. Such values will have to be harmonized with the surrounding technologies (e.g., SONET/SDH, ATM) to be useful. This will become particularly important as networks scale and mis-configuration can result in churn, alarm flapping, etc.

4.6. Alarm Suppression, Aggregation, and Layer Coordination

Network elements MUST provide alarm suppression functionality that prevents the generation of a superfluous generation of alarms by simply discarding them (or not generating them in the first place), or by aggregating them together, thereby greatly reducing the number of notifications emitted. When viewed in conjunction with the requirement in Section 4.7 below, this typically requires fault notification to the LSP egress that may have specific time constraints if the application using the LSP independently implements path continuity testing (for example, ATM I.610 Continuity check (CC)[I610]). These constraints apply to LSPs that are monitored. The nature of MPLS applications allows for the possibility of having multiple MPLS applications attempt to respond to defects simultaneously, e.g., layer-3 MPLS VPNs that utilize Traffic Engineered tunnels where a failure occurs on the LSP carrying the Traffic Engineered tunnel. This failure would affect the VPN traffic that uses the tunnel's LSP. Mechanisms are required to coordinate network responses to defects.

4.7. Support for OAM Inter-working for Fault Notification

An LSR supporting the inter-working of one or more networking technologies over MPLS MUST be able to translate an MPLS defect into the native technology's error condition. For example, errors occurring over an MPLS transport LSP that supports an emulated ATM VC MUST translate errors into native ATM OAM Alarm Indication Signal (AIS) cells at the termination points of the LSP. The mechanism SHOULD consider possible bounded detection time parameters, e.g., a "hold off" function before reacting to synchronize with the OAM functions.

One goal would be alarm suppression by the upper layer using the LSP. As observed in Section 4.5, this requires that MPLS perform detection in a bounded timeframe in order to initiate alarm suppression prior to the upper layer independently detecting the defect.

4.8. Error Detection and Recovery

Recovery from a fault by a network element can be facilitated by MPLS OAM procedures. These procedures will detect a broader range of defects than that of simple link and node failures. Since MPLS LSPs may span multiple routing areas and service provider domains, fault recovery and error detection should be possible in these configurations as well as in the more simplified single-area/domain configurations.

Recovery from faults SHOULD be automatic. It is a requirement that faults SHOULD be detected (and possibly corrected) by the network operator prior to customers of the service in question detecting them.

4.9. Standard Management Interfaces

The wide-spread deployment of MPLS requires common information modeling of management and control of OAM functionality. Evidence of this is reflected in the standard IETF MPLS-related MIB modules (e.g., [RFC3813][RFC3812][RFC3814]) for fault, statistics, and configuration management. These standard interfaces provide operators with common programmatic interface access to Operations and Management functions and their statuses. However, gaps in coverage of MIB modules to OAM and other features exist; therefore, MIB modules corresponding to new protocol functions or network tools are required.

4.10. Detection of Denial of Service Attacks

The ability to detect denial of service (DoS) attacks against the data or control planes MUST be part of any security management related to MPLS OAM tools or techniques.

4.11. Per-LSP Accounting Requirements

In an MPLS network, service providers can measure traffic from an LSR to the egress of the network using some MPLS related MIBs, for example. This means that it is reasonable to know how much traffic is traveling from location to location (i.e., a traffic matrix) by analyzing the flow of traffic. Therefore, traffic accounting in an MPLS network can be summarized as the following three items:

(1) Collecting information to design network

For the purpose of optimized network design, a service

provider may offer the traffic information. Optimizing

network design needs this information.

(2) Providing a Service Level Specification

Providers and their customers MAY need to verify high-level service level specifications, either to continuously optimize their networks, or to offer guaranteed bandwidth services. Therefore, traffic accounting to monitor MPLS applications is required.

      (3) Inter-AS environment

Service providers that offer inter-AS services require accounting of those services.

These three motivations need to satisfy the following:

          -  In (1) and (2), collection of information on a per-LSP
          
             basis is a minimum level of granularity for collecting
             accounting information at both of ingress and egress of an
             LSP.

• In (3), SP's ASBR carry out interconnection functions as an intermediate LSR. Therefore, identifying a pair of ingress and egress LSRs using each LSP is needed to determine the cost of the service that a customer is using.

4.11.1. Requirements

Accounting on a per-LSP basis encompasses the following set of functions:

(1) At an ingress LSR, accounting of traffic through LSPs that

begin at each egress in question.

(2) At an intermediate LSR, accounting of traffic through LSPs for

each pair of ingress to egress.

(3) At egress LSR, accounting of traffic through LSPs for each

ingress.

(4) All LSRs containing LSPs that are being measured need to have

a common identifier to distinguish each LSP. The identifier MUST be unique to each LSP, and its mapping to LSP SHOULD be provided whether from manual or automatic configuration.

In the case of non-merged LSPs, this can be achieved by simply reading traffic counters for the label stack associated with the LSP at any LSR along its path. However, in order to measure merged LSPs, an LSR MUST have a means to distinguish the source of each flow so as to disambiguate the statistics.

4.11.2. Location of Accounting

It is not realistic for LSRs to perform the described operations on all LSPs that exist in a network. At a minimum, per-LSP based accounting SHOULD be performed on the edges of the network -- at the edges of both LSPs and the MPLS domain.

5. Security Considerations

Provisions to any of the network mechanisms designed to satisfy the requirements described herein are required to prevent their unauthorized use. Likewise, these network mechanisms MUST provide a means by which an operator can prevent denial of service attacks if those network mechanisms are used in such an attack.

LSP mis-merging has security implications beyond that of simply being a network defect. LSP mis-merging can happen due to a number of potential sources of failure, some of which (due to MPLS label stacking) are new to MPLS.

The performance of diagnostic functions and path characterization involve extracting a significant amount of information about network construction that the network operator MAY consider private.

6. References

6.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

6.2. Informative References

   [RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
             Label Switched (MPLS) Data Plane Failures", RFC 4379,
             February 2006.
   
   [RFC3812] Srinivasan, C., Viswanathan, A., and T. Nadeau,
             "Multiprotocol Label Switching (MPLS) Traffic Engineering
             (TE) Management Information Base (MIB)", RFC 3812, June
             2004.
   
   [RFC3813] Srinivasan, C., Viswanathan, A., and T. Nadeau,
             "Multiprotocol Label Switching (MPLS) Label Switching
             Router (LSR) Management Information Base (MIB)", RFC 3813,
             June 2004.
   
   [RFC3814] Nadeau, T., Srinivasan, C., and A. Viswanathan,
             "Multiprotocol Label Switching (MPLS) Forwarding
             Equivalence Class To Next Hop Label Forwarding Entry
             (FEC-To-NHLFE) Management Information Base (MIB)", RFC
             3814, June 2004.
   
   [Y1710]   ITU-T Recommendation Y.1710, "Requirements for OAM
             Functionality In MPLS Networks"
   
   [I610]    ITU-T Recommendation I.610, "B-ISDN operations and
             maintenance principles and functions", February 1999
   
   [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 2434,
             October 1998.
   
   [RFC792]  Postel, J., "Internet Control Message Protocol", STD 5, RFC
             792, September 1981.
   
   [RFC3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing in
             Multi-Protocol Label Switching (MPLS) Networks", RFC 3443,
             January 2003.

7. Acknowledgements

   The authors wish to acknowledge and thank the following individuals
   for their valuable comments to this document:  Adrian Smith, British
   Telecom; Chou Lan Pok, SBC; Mr. Ikejiri, NTT Communications; and Mr.
   Kumaki, KDDI.  Hari Rakotoranto, Miya Kohno, Cisco Systems; Luyuan
   Fang, AT&T; Danny McPherson, TCB; Dr. Ken Nagami, Ikuo Nakagawa,
   Intec Netcore, and David Meyer.

Authors' Addresses

Comments should be made directly to the MPLS mailing list at mpls@lists.ietf.org.

Thomas D. Nadeau
Cisco Systems, Inc.
300 Beaver Brook Road
Boxboro, MA 01719

   Phone: +1-978-936-1470
   EMail: tnadeau@cisco.com
   
   Monique Jeanne Morrow
   Cisco Systems, Inc.
   Glatt-Com, 2nd Floor
   CH-8301
   Switzerland
   
   Phone:  (0)1 878-9412
   EMail: mmorrow@cisco.com

George Swallow
Cisco Systems, Inc.
300 Beaver Brook Road
Boxboro, MA 01719

   Phone: +1-978-936-1398
   EMail: swallow@cisco.com

David Allan
Nortel Networks
3500 Carling Ave.
Ottawa, Ontario, CANADA

   Phone: 1-613-763-6362
   EMail: dallan@nortel.com
   
   Satoru Matsushima
   Japan Telecom
   1-9-1, Higashi-Shinbashi, Minato-ku
   Tokyo, 105-7316 Japan
   
   Phone: +81-3-6889-1092
   EMail: satoru@ft.solteria.net

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