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INFORMATIONAL
Network Working Group S. Yasukawa
Request for Comments: 4687 NTT Corporation
Category: Informational A. Farrel
Old Dog Consulting
D. King
Aria Networks Ltd.
T. Nadeau
Cisco Systems, Inc.
September 2006
Operations and Management (OAM) Requirements
for Point-to-Multipoint 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 (C) The Internet Society (2006).
Abstract
Multi-Protocol Label Switching (MPLS) has been extended to encompass
point-to-multipoint (P2MP) Label Switched Paths (LSPs). As with
point-to-point MPLS LSPs, the requirement to detect, handle, and
diagnose control and data plane defects is critical.
For operators deploying services based on P2MP MPLS LSPs, the
detection and specification of how to handle those defects are
important because such defects not only may affect the fundamentals
of an MPLS network, but also may impact service level specification
commitments for customers of their network.
This document describes requirements for data plane operations and
management for P2MP MPLS LSPs. These requirements apply to all forms
of P2MP MPLS LSPs, and include P2MP Traffic Engineered (TE) LSPs and
multicast LSPs.
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Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
2.1. Conventions Used in This Document ..........................3
2.2. Terminology ................................................3
2.3. Acronyms ...................................................3
3. Motivations .....................................................4
4. General Requirements ............................................4
4.1. Detection of Label Switch Path Defects .....................5
4.2. Diagnosis of a Broken Label Switch Path ....................6
4.3. Path Characterization ......................................6
4.4. Service Level Agreement Measurement ........................7
4.5. Frequency of OAM Execution .................................8
4.6. Alarm Suppression, Aggregation, and Layer Coordination .....8
4.7. Support for OAM Interworking for Fault Notification ........8
4.8. Error Detection and Recovery ...............................9
4.9. Standard Management Interfaces .............................9
4.10. Detection of Denial of Service Attacks ...................10
4.11. Per-LSP Accounting Requirements ..........................10
5. Security Considerations ........................................10
6. References .....................................................11
6.1. Normative References ......................................11
6.2. Informative References ....................................11
7. Acknowledgements ...............................................12
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1. Introduction
This document describes requirements for data plane operations and
management (OAM) for point-to-multipoint (P2MP) Multi-Protocol Label
Switching (MPLS). This document specifies OAM requirements for P2MP
MPLS, as well as for applications of P2MP MPLS.
These requirements apply to all forms of P2MP MPLS LSPs, and include
P2MP Traffic Engineered (TE) LSPs [RFC4461] and [P2MP-RSVP], as well
as multicast LDP LSPs [MCAST-LDP].
Note that the requirements for OAM for P2MP MPLS build heavily on the
requirements for OAM for point-to-point MPLS. These latter
requirements are described in [RFC4377] and are not repeated in this
document.
For a generic framework for OAM in MPLS networks, refer to [RFC4378].
2. Terminology
2.1. Conventions Used in This Document
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].
The requirements in this document apply to OAM mechanism and protocol
development, as opposed to the usual application of RFC 2119
requirements to an actual protocol, as this document does not specify
a protocol.
2.2. Terminology
Definitions of key terms for MPLS OAM are found in [RFC4377] and the
reader is assumed to be familiar with those definitions, which are
not repeated here.
[RFC4461] includes some important definitions and terms for use
within the context of P2MP MPLS. The reader should be familiar with
at least the terminology section of that document.
2.3. Acronyms
The following acronyms are used in this document.
CE: Customer Edge
DoS: Denial of service
ECMP: Equal Cost Multipath
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LDP: Label Distribution Protocol
LSP: Label Switched Path
LSR: Label Switching Router
OAM: Operations and Management
RSVP: Resource reSerVation Protocol
P2MP: Point-to-Multipoint
SP: Service Provider
TE: Traffic Engineering
3. Motivations
OAM for MPLS networks has been established as a fundamental
requirement both through operational experience and through its
documentation in numerous Internet Drafts. Many such documents (for
example, [RFC4379], [RFC3812], [RFC3813], [RFC3814], and [RFC3815])
developed specific solutions to individual issues or problems.
Coordination of the full OAM requirements for MPLS was achieved by
[RFC4377] in recognition of the fact that the previous piecemeal
approach could lead to inconsistent and inefficient applicability of
OAM techniques across the MPLS architecture, and might require
significant modifications to operational procedures and systems in
order to provide consistent and useful OAM functionality.
This document builds on these realizations and extends the statements
of MPLS OAM requirements to cover the new area of P2MP MPLS. That
is, this document captures the requirements for P2MP MPLS OAM in
advance of the development of specific solutions.
Nevertheless, at the time of writing, some effort had already been
expended to extend existing MPLS OAM solutions to cover P2MP MPLS
(for example, [P2MP-LSP-PING]). While this approach of extending
existing solutions may be reasonable, in order to ensure a consistent
OAM framework it is necessary to articulate the full set of
requirements in a single document. This will facilitate a uniform
set of MPLS OAM solutions spanning multiple MPLS deployments and
concurrent applications.
4. General Requirements
The general requirements described in this section are similar to
those described for point-to-point MPLS in [RFC4377]. The
subsections below do not repeat material from [RFC4377], but simply
give references to that document.
However, where the requirements for P2MP MPLS OAM differ from or are
more extensive than those expressed in [RFC4377], additional text is
supplied.
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In general, it should be noted that P2MP LSPs introduce a scalability
issue with respect to OAM that is not present in point-to-point MPLS.
That is, an individual P2MP LSP will have more than one egress and
the path to those egresses will very probably not be linear (for
example, it may have a tree structure). Since the number of egresses
for a single P2MP LSP is unknown and not bounded by any small number,
it follows that all mechanisms defined for OAM support MUST scale
well with the number of egresses and the complexity of the path of
the LSP. Mechanisms that are able to deal with individual egresses
will scale no worse than similar mechanisms for point-to-point LSPs,
but it is desirable to develop mechanisms that are able to leverage
the fact that multiple egresses are associated with a single LSP, and
so achieve better scaling.
4.1. Detection of Label Switch Path Defects
The ability to detect defects in a P2MP LSP SHOULD not require
manual, hop-by-hop troubleshooting of each LSR used to switch traffic
for that LSP, and SHOULD rely on proactive OAM procedures (such as
continuous path connectivity and Service Level Agreement (SLA)
measurement mechanisms). Any solutions SHOULD either extend or work
in close conjunction with existing solutions developed for point-to-
point MPLS, such as those specified in [RFC4379] where this
requirement is not contradicted by the other requirements in this
section. This will leverage existing software and hardware
deployments.
Note that P2MP LSPs may introduce additional scaling concerns for LSP
probing by tools such as [RFC4379]. As the number of leaves of a
P2MP LSP increases it potentially becomes more expensive to inspect
the LSP to detect defects. Any tool developed for this purpose MUST
be cognitive of this issue and MUST include techniques to reduce the
scaling impact of an increase in the number of leaves. Nevertheless,
it should also be noted that the introduction of additional leaves
may mean that the use of techniques such as [RFC4379] are less
appropriate for defect detection with P2MP LSPs, while the technique
may still remain useful for defect diagnosis as described in the next
section.
Due to the above scaling concerns, LSRs or other network resources
MUST NOT be overwhelmed by the operation of normal proactive OAM
procedures, and measures taken to protect LSRs and network resources
against being overwhelmed MUST NOT degrade the operational value or
responsiveness of proactive OAM procedures. Note that reactive OAM
may violate these limits (i.e., cause visible traffic degradation) if
it is necessary or useful to try to fix whatever has gone wrong.
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By "overwhelmed" we mean that it MUST NOT be possible for an LSR to
be so busy handling proactive OAM that it is unable to continue to
process control or data plane traffic at its advertised rate.
Similarly, a network resource (such as a data link) MUST NOT be
carrying so much proactive OAM traffic that it is unable to carry the
advertised data rate. At the same time, it is important to configure
proactive OAM, if it is in use, not to raise alarms caused by the
failure to receive an OAM message if the component responsible for
processing the messages is unable to process because other components
are consuming too many system resources -- such alarms might turn out
to be false.
In practice, of course, the requirements in the previous paragraph
may be met by careful specification of the anticipated data
throughput of LSRs or data links. However, it should be recalled
that proactive OAM procedures may be scaled linearly with the number
of LSPs, and the number of LSPs is not necessarily a function of the
available bandwidth in an LSR or on a data link.
4.2. Diagnosis of a Broken Label Switch Path
The ability to diagnose a broken P2MP LSP and to isolate the failed
component (i.e., link or node) in the path is REQUIRED. These
functions include a path connectivity test that can test all branches
and leaves of a P2MP LSP for reachability, as well as a path tracing
function. Note that this requirement is distinct from the
requirement to detect errors or failures described in the previous
section. In practice, Detection and Diagnosis/Isolation MAY be
performed by separate or the same mechanisms according to the way in
which the other requirements are met.
It MUST be possible for the operator (or an automated process) to
stipulate a timeout after which the failure to see a response shall
be flagged as an error.
Any mechanism developed to perform these functions is subject to the
scalability concerns expressed in section 4.
4.3. Path Characterization
The path characterization function [RFC4377] is the ability to reveal
details of LSR forwarding operations for P2MP LSPs. These details
can then be compared later during subsequent testing relevant to OAM
functionality. Therefore, LSRs supporting P2MP LSPs MUST provide
mechanisms that allow operators to interrogate and characterize P2MP
paths.
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Since P2MP paths are more complex than the paths of point-to-point
LSPs, the scaling concerns expressed in section 4 apply.
Note that path characterization SHOULD lead to the operator being
able to determine the full tree for a P2MP LSP. That is, it is not
sufficient to know the list of LSRs in the tree, but it is important
to know their relative order and where the LSP branches.
Since, in some cases, the control plane state and data paths may
branch at different points from the control plane and data plane
topologies (for example, Figure 1), it is not sufficient to present
the order of LSRs, but it is important that the branching points on
that tree are clearly identified.
E
/
A---B---C===D
\
F
Figure 1. An example P2MP tree where the data path and control
plane state branch at C, but the topology branches at D.
A diagnostic tool that meets the path characterization requirements
SHOULD collect information that is easy to process to determine the
P2MP tree for a P2MP LSP, rather than provide information that must
be post-processed with some complexity.
4.4. Service Level Agreement Measurement
Mechanisms are required to measure the diverse aspects of Service
Level Agreements for services that utilize P2MP LSPs. The aspects
are listed in [RFC4377].
Service Level Agreements are often measured in terms of the quality
and rate of data delivery. In the context of P2MP MPLS, data is
delivered to multiple egress nodes. The mechanisms MUST, therefore,
be capable of measuring the aspects of Service Level Agreements as
they apply to each of the egress points to a P2MP LSP. At the same
time, in order to diagnose issues with meeting Service Level
Agreements, mechanisms SHOULD be provided to measure the aspects of
the agreements at key points within the network such as at branch
nodes on the P2MP tree.
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4.5. Frequency of OAM Execution
As stipulated in [RFC4377], the operator MUST have the flexibility to
configure OAM parameters to meet their specific operational
requirements. This requirement is potentially more important in P2MP
deployments where the effects of the execution of OAM functions can
be potentially much greater than in a non-P2MP configuration. For
example, a mechanism that causes each egress of a P2MP LSP to respond
could result in a large burst of responses to a single OAM request.
Therefore, solutions produced SHOULD NOT impose any fixed limitations
on the frequency of the execution of any OAM functions.
4.6. Alarm Suppression, Aggregation, and Layer Coordination
As described in [RFC4377], network elements MUST provide alarm
suppression and aggregation mechanisms to prevent the generation of
superfluous alarms within or across network layers. The same time
constraint issues identified in [RFC4377] also apply to P2MP LSPs.
A P2MP LSP also brings the possibility of a single fault causing a
larger number of alarms than for a point-to-point LSP. This can
happen because there are a larger number of downstream LSRs (for
example, a larger number of egresses). The resultant multiplier in
the number of alarms could cause swamping of the alarm management
systems to which the alarms are reported, and serves as a multiplier
to the number of potentially duplicate alarms raised by the network.
Alarm aggregation or limitation techniques MUST be applied within any
solution, or be available within an implementation, so that this
scaling issue can be reduced. Note that this requirement introduces
a second dimension to the concept of alarm aggregation. Where
previously it applied to the correlation and suppression of alarms
generated by different network layers, it now also applies to similar
techniques applied to alarms generated by multiple downstream LSRs.
4.7. Support for OAM Interworking for Fault Notification
[RFC4377] specifies that an LSR supporting the interworking of one or
more networking technologies over MPLS MUST be able to translate an
MPLS defect into the native technology's error condition. This also
applies to any LSR supporting P2MP LSPs. However, careful attention
to the requirements for alarm suppression stipulated therein and in
section 4.6 SHOULD be observed.
Note that the time constraints for fault notification and alarm
propagation affect the solutions that might be applied to the
scalability problem inherent in certain OAM techniques applied to
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P2MP LSPs. For example, a solution to the issue of a large number of
egresses all responding to some form of probe request at the same
time might be to make the probes less frequent -- but this might
affect the ability to detect and/or report faults.
Where fault notification to the egress is required, there is the
possibility that a single fault will give rise to multiple
notifications, one to each egress node of the P2MP that is downstream
of the fault. Any mechanisms MUST manage this scaling issue while
still continuing to deliver fault notifications in a timely manner.
Where fault notification to the ingress is required, the mechanisms
MUST ensure that the notification identifies the egress nodes of the
P2MP LSP that are impacted (that is, those downstream of the fault)
and does not falsely imply that all egress nodes are impacted.
4.8. Error Detection and Recovery
Recovery from a fault by a network element can be facilitated by MPLS
OAM procedures. As described in [RFC4377], these procedures will
detect a broad range of defects, and SHOULD be operable where MPLS
P2MP LSPs span multiple routing areas or multiple Service Provider
domains.
The same requirements as those expressed in [RFC4377] with respect to
automatic repair and operator intervention ahead of customer
detection of faults apply to P2MP LSPs.
It should be observed that faults in P2MP LSPs MAY be recovered
through techniques described in [P2MP-RSVP].
4.9. Standard Management Interfaces
The widespread deployment of MPLS requires common information
modeling of management and control of OAM functionality. This is
reflected in the integration of standard MPLS-related MIBs [RFC3812],
[RFC3813], [RFC3814], [RFC3815] for fault, statistics, and
configuration management. These standard interfaces provide
operators with common programmatic interface access to operations and
management functions and their status.
The standard MPLS-related MIB modules [RFC3812], [RFC3813],
[RFC3814], and [RFC3815] SHOULD be extended wherever possible, to
support P2MP LSPs, the associated OAM functions on these LSPs, and
the applications that utilize P2MP LSPs. Extending them will
facilitate the reuse of existing management software both in LSRs and
in management systems. In cases where the existing MIB modules
cannot be extended, then new MIB modules MUST be created.
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4.10. Detection of Denial of Service Attacks
The ability to detect denial of service (DoS) attacks against the
data or control planes that signal P2MP LSPs MUST be part of any
security management related to MPLS OAM tools or techniques.
4.11. Per-LSP Accounting Requirements
In an MPLS network where P2MP LSPs are in use, Service Providers can
measure traffic from an LSR to the egress of the network using some
MPLS-related MIB modules (see section 4.9), for example. Other
interfaces MAY exist as well and enable the creation of traffic
matrices so that it is possible to know how much traffic is traveling
from where to where within the network.
Analysis of traffic flows to produce a traffic matrix is more
complicated where P2MP LSPs are deployed because there is no simple
pairing relationship between an ingress and a single egress.
Fundamental to understanding traffic flows within a network that
supports P2MP LSPs will be the knowledge of where the traffic is
branched for each LSP within the network, that is, where within the
network the branch nodes for the LSPs are located and what their
relationship is to links and other LSRs. Traffic flow and accounting
tools MUST take this fact into account.
5. Security Considerations
This document introduces no new security issues compared with
[RFC4377]. It is worth highlighting, however, that any tool designed
to satisfy the requirements described in this document MUST include
provisions to prevent its unauthorized use. Likewise, these tools
MUST provide a means by which an operator can prevent denial of
service attacks if those tools are used in such an attack. LSP mis-
merging is described in [RFC4377] where it is pointed out that it has
security implications beyond simply being a network defect. It needs
to be stressed that it is in the nature of P2MP traffic flows that
any erroneous delivery (such as caused by LSP mis-merging) is likely
to have more far-reaching consequences since the traffic will be
mis-delivered to multiple receivers.
As with the OAM functions described in [RFC4377], the performance of
diagnostic functions and path characterization may involve the
extraction of a significant amount of information about network
construction. The network operator MAY consider this information
private and wish to take steps to secure it, but further, the volume
of this information may be considered as a threat to the integrity of
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the network if it is extracted in bulk. This issue may be greater in
P2MP MPLS because of the potential for a large number of receivers on
a single LSP and the consequent extensive path of the LSP.
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.
[RFC4377] Nadeau, T., Morrow, M., Swallow, G., Allan, D., and
S. Matsushima, "Operations and Management (OAM)
Requirements for Multi-Protocol Label Switched
(MPLS) Networks", RFC 4377, February 2006.
6.2. Informative References
[MCAST-LDP] Minei, I., Ed., Kompella, K., Wijnands, I., Ed., and
B. Thomas, "Label Distribution Protocol Extensions
for Point-to-Multipoint and Multipoint-to-Multipoint
Label Switched Paths", Work in Progress, June 2006.
[P2MP-LSP-PING] Yasukawa, S., Farrel, A., Ali, Z., and B. Fenner,
"Detecting Data Plane Failures in Point-to-
Multipoint MPLS Traffic Engineering - Extensions to
LSP Ping", Work in Progress, April 2006.
[P2MP-RSVP] Aggarwal, R., Papadimitriou, D., and S. Yasukawa,
"Extensions to RSVP-TE for Point to Multipoint TE
LSPs", Work in Progress, July 2006.
[RFC3812] Srinivasan, C., Viswanathan, A. and T. Nadeau,
"MPLS Traffic Engineering Management Information
Base Using SMIv2", RFC3812, June 2004.
[RFC3813] Srinivasan, C., Viswanathan, A. and T. Nadeau,
"MPLS Label Switch Router Management Information
Base Using SMIv2", RFC3813, June 2004.
[RFC3814] Nadeau, T., Srinivasan, C., and A. Viswanathan,
"Multiprotocol Label Switching (MPLS) FEC-To-NHLFE
(FTN) Management Information Base", RFC3814, June
2004.
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[RFC3815] Cucchiara, J., Sjostrand, H., and Luciani, J.,
"Definitions of Managed Objects for the
Multiprotocol Label Switching (MPLS), Label
Distribution Protocol (LDP)", RFC 3815, June 2004.
[RFC4378] Allan, D. and T. Nadeau, "A Framework for Multi-
Protocol Label Switching (MPLS) Operations and
Management (OAM)", RFC 4378, February 2006.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-
Protocol Label Switched (MPLS) Data Plane Failures",
RFC 4379, February 2006.
[RFC4461] Yasukawa, S., Ed., "Signaling Requirements for
Point-to-Multipoint Traffic-Engineered MPLS Label
Switched Paths (LSPs)", RFC 4461, April 2006.
7. Acknowledgements
The authors wish to acknowledge and thank the following individuals
for their valuable comments on this document: Rahul Aggarwal, Neil
Harrison, Ben Niven-Jenkins, and Dimitri Papadimitriou.
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Authors' Addresses
Seisho Yasukawa
NTT Corporation
(R&D Strategy Department)
3-1, Otemachi 2-Chome Chiyodaku,
Tokyo 100-8116 Japan
Phone: +81 3 5205 5341
EMail: s.yasukawa@hco.ntt.co.jp
Adrian Farrel
Old Dog Consulting
Phone: +44 (0) 1978 860944
EMail: adrian@olddog.co.uk
Daniel King
Aria Networks Ltd.
Phone: +44 (0)1249 665923
EMail: daniel.king@aria-networks.com
Thomas D. Nadeau
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
EMail: tnadeau@cisco.com
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