IETF Draft Ken Owens
Multi-Protocol Label Switching Srinivas Makam
Expires: May 2001 Vishal Sharma
Ben Mack-Crane
Tellabs Operations, Inc.
Changcheng Huang
Carleton University
November 2000
A Path Protection/Restoration Mechanism for MPLS Networks
<draft-chang-mpls-path-protection-02.txt>
Status of this memo
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Abstract
It is expected that MPLS-based recovery could become a viable option
for obtaining faster restoration than layer 3 rerouting. To deliver
reliable service, however, multi-protocol label switching (MPLS)[1],
[2] requires a set of procedures to provide protection of the
traffic carried on the label switched paths (LSPs). This imposes
certain requirements on the path recovery process [3], and requires
procedures for the configuration of working and protection paths,
for the communication of fault information to appropriate switching
elements, and for the activation of appropriate switchover actions.
This document specifies a mechanism for path protection switching
and restoration in MPLS networks.
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Table of Contents Page
1. Introduction 2
2. Purpose and Motivation 3
3. Key Features of the Proposed Mechanism 4
4. Core MPLS Path Protection Components 6
4.1 Reverse Notification Tree (RNT) 7
4.2 Protection Domain 10
4.3 Multiple Faults 11
4.4 Timers and Thresholds 12
5.0 Configuration 13
5.1 Establishing a Protection Domain 13
5.1.1 Explicit Route Protection Information 14
5.1.2 Path Protection InformationInformation 15
5.2 Establishing a Recovery/Protection Path 16
5.3 Creating an RNT 16
5.4 Engineering a Protection Domain 17
5.5 Configuring Timers 18
6.0 Fault Detection 20
7.0 Fault Notification 21
8.0 Switch Over 22
9.0 Switchback or Restoration 22
10.0 Protocol Specific Extensions 23
11.0 Security Considerations 23
12.0 Acknowledgements 23
13.0 Intellectual Property Considerations 23
14.0 AuthorsÆ Addresses 23
15.0 References 24
1.0 Introduction
With the migration of real-time and high-priority traffic to IP
networks, and with the need for IP networks to increasingly carry
mission-critical business data, network survivability has become
critical for future IP networks. Current routing algorithms, despite
being robust and survivable, can take a substantial amount of time,
to recover from a failure, on the order of several seconds to
minutes, which can cause serious disruption of service in the
interim. This is unacceptable for many applications that require a
highly reliable service, and has motivated network providers to give
serious consideration to the issue of network survivability.
Path-oriented technologies, such as MPLS, can be used to support
advanced survivability requirements and enhance the reliability of
IP networks. Different from legacy IP networks, MPLS networks
establish label switched paths (LSPs), where packets with the same
label follow the same path. This potentially allows MPLS networks to
pre-establish protection LSPs for working LSPs, and achieve better
protection switching times than those in legacy IP networks. With
this objective in mind, the present contribution describes a
mechanism to protect paths (or path segments) in MPLS networks.
Before discussing the specifics of this contribution, we first
outline the major components of a path protection solution, and
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point out those that are within the scope of this document. A
complete solution for path protection requires the following
components:
(i) A method for selecting the working and protection paths.
(ii) A method for signaling the setup of the working and protection
paths.
(iii) A fault detection mechanism to detect faults along a path.
(iv) A fault notification mechanism, to convey information about the
occurrence of a fault to a network entity responsible for
reacting to the fault and taking appropriate corrective action.
(v) A switchover mechanism to move traffic over from the working
path to the protection path.
(vi) A repair detection mechanism, to detect that a fault along a
path has been repaired.
(vii) An (optional) switchback or restoration mechanism, for
switching traffic back to the original working path, once it
is discovered that the fault has corrected or has been
repaired.
Observe that component (i) consists of algorithms and techniques
that are used to select the working and protection paths based on
specific criteria, established via policy or other constraints, and
can be proprietary. It is therefore not subject to standardization,
and is outside the scope of this draft. Therefore, the protection
mechanism described later assumes that the working and protection
paths are known to the LSR responsible for path setup, and are
either communicated to it or are calculated by some intelligence at
that LSR. Component (ii), which involves establishing the working
and protection paths via signaling, is within the scope of the
draft, and is discussed in Section 3.1.
A detailed specification of fault detection mechanisms is outside
the scope of this draft, but the specification of how the path
protection mechanism works with different fault detection methods is
in scope, and is discussed in Section 5. In particular, we consider
how the mechanism works for two practical cases of interest: (a)
when only the end node of a path is responsible for detecting
faults, and (b) when all the nodes along the path are responsible
for detecting faults. The main focus of this draft is the
specification of an efficient fault notification mechanism that
takes LSP merging into account (irrespective of whether they are
physically or virtually merged). Switchover and switchback
mechanisms also are also within the scope of the draft, but
component (vi) is outside the scope of the draft, so the draft does
not specify the details of the mechanisms used to detect that a
fault has been repaired.
2.0 Motivation and Purpose
The framework document [3] lays out the various options for MPLS-
based restoration/recovery. However, candidate approaches
corresponding to various viable recovery options are still needed.
Our work on proposing a path protection mechanism for MPLS networks
is motivated by the belief that path protection (in conjunction with
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local repair) will be needed for truly reliable network operation.
The purpose of this contribution is to propose a path protection
mechanism that is:
(i) fast (compared to Layer 3, with the goal of being comparable to
SONET),
(ii) scalable,
(iii)bandwidth efficient,
(iv)allows for path merging (i.e., is merging compatible), and
(v) works at the MPLS layer (that is, only uses knowledge that is
commonly available to MPLS routing and signaling modules).
The major differences between this 02 version and the previous 01
version are:
-- Protection domain configuration details
-- Protection domain configuration information elements added
3.0 Key Features of the Proposed Mechanism
This contribution describes an MPLS-based path recovery mechanism
that can facilitate fast protection switching. The mechanism
currently supports 1:1 protection [3].
Bypass tunneling is for further study. First, because tunnel setup
itself is not adequately defined yet, and second, because even
assuming a tunnel could be setup, in the presence of tunnels (or
tunneled segments) the mechanism still requires the ability to setup
bi-directional tunnels, which is not defined yet. The mechanism has
several timers to enable it to inter-work with protection mechanisms
at other layers. Some of the key features of the protection
mechanism are:
-- Special tree structure to efficiently distribute fault and/or
recovery information.
Existing published proposals for MPLS recovery have not addressed
the issue of fault notification in detail. Specifically, none of
these proposals has discussed how to perform fault notification for
the label merging case. In this draft, we propose a new fault
notification structure called the reverse notification tree (RNT),
which makes fault notification efficient and scalable (we provide
details of the RNT in subsequent sections).
-- Lightweight notification mechanism.
The lack of MPLS OAM functionality requires the definition of a
lighweight stateless notification mechanism. Reliable transport
mechanisms, such as TCP, are typically state-oriented and therefore
difficult to scale. It is also very difficult to support point-to-
multipoint communications based on reliable transport mechanisms. In
our scheme, therefore, we use a stateless notification mechanism to
achieve scalability. The notification is based on the transmission
of packets that are sent periodically until the nodes responsible
for switchover learn of the fault. Since no acknowledgements or
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handshaking between adjacent nodes is needed, the mechanism works
only with timers and does not require the maintenance of state.
--Minimize delays of a recovery cycle.
An objective of the mechanism proposed in this draft is to minimize
the duration of the recovery cycle. Thus a stateless transport
mechanism together with high priority for control traffic minimizes
notification delay. Likewise, a simple label merging approach to
handle the traffic arriving on the working and protection paths
eliminates the need for synchronization (or handshaking) between the
LSRs at the two ends of a recovery path.
-- Work at the MPLS layer (that is, use information available to the
MPLS signaling and routing modules at the nodes)
The mechanism is designed to operate using only MPLS constructs and
the knowledge available to the MPLS modules at the nodes. Therefore,
the mechanism assumes, by default, that the working and protection
paths merge at a path merge LSR (PML) within the domain under
consideration. However, since the mechanism does not depend on the
path selection method, it also works in settings where a PML does
not exist, and a path selection algorithm (outside the scope of this
I-D) determines that the working and protection paths must terminate
at different egress LSRs. Note, however, that for the path selection
mechanism to be able to make this determination, it may need
knowledge beyond that which is commonly available to the MPLS
modules at a node. This is because determining whether a working
path can be protected by another path with a different egress LSR
requires Layer 3 knowledge to ascertain whether the LSR terminating
the recovery path is acceptable. In the remainder of this document,
we will focus on the PML case, with the understanding that the non-
PML case is also supported.
In addition to the key features outlined above, some other
characteristics of the mechanism are:
-- A liveness message to detect faults.
Although fault detection is outside the scope of this draft, we will
allow the existence of a generic æælivenessÆÆ message that can
complement any fault detection mechanism. This liveness message may,
for example, be provided as part of an user/control plane OAM
function, or by an existing Hello message (as the RSVP "Hello"
message) with an appropriately set timer value. We do not define
specific liveness mechanisms in this draft, deferring instead to
work on OAM in MPLS, which is where we expect such a liveness
message to be defined.
Our assumption is that faults fall into different classes, and that
different faults may be detected and corrected by different layers.
Some faults (for example, the loss of signal or transmitter faults)
may be detected and corrected by lower layer mechanisms (such as
SONET), while others (for example, failure of the reverse link) may
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be detected (but may not be corrected) by lower layers and may be
communicated to the MPLS layer. Still other faults (such as node
failures or faults on the reverse link) may not be detected by lower
layers, and will have to be detected and corrected at the MPLS
layer. Therefore, we adopt the liveness message as a complementary
fault detection mechanism.
We note that in this draft we confine our discussion of protection
to a single MPLS domain, and do not consider protection/recovery
across multiple MPLS domains or across multiple administrative
boundaries. We note, however, that protection mechanisms in
different domains may be concatenated, and that (at least initially)
these mechanisms may work autonomously, across the (possibly)
multiple points of attachment between two adjacent domains. However,
coordination of protection mechanisms across multiple domains or
across multiple transport technologies is currently out of the scope
of this document.
4.0 Core MPLS Path Protection Components
This document assumes the terminology given in[1], [2], [3] , and
introduces some additional terms. For the convenience of the reader,
we repeat here some of the terminology from earlier documents.
Working Path
The protected path that carries traffic before the occurrence of a
fault. The working path is the part of the LSP between the PSL and
the PML (if any) or, in the absence of a PML, between the PSL and an
egress LSR. A working path is denoted by the sequence of LSRs
through which it passes. For example, in Fig. 1, the working path
that starts at LSR 1 and terminates at LSR 7 is denoted by (1-2-3-4-
6-7).
Recovery Path
The path by which traffic is restored after the occurrence of a
fault. In other words, the path along which traffic is directed by
the recovery mechanism. The recovery path can either be an
equivalent recovery path and ensure no reduction in quality of
service or be a limited recovery path and thereby not guarantee the
same quality of service (or some other criteria of performance) as
the working path. A recovery path is also denoted by the sequence of
LSRs through which it passes. Again, in Fig. 1, the recovery path
that starts at LSR 1 and terminates at LSR 7 is denoted by (1-5-7).
Path Switch LSR (PSL)
An LSR that is the transmitter of both the working path traffic and
its corresponding recovery path traffic. The PSL is responsible for
switching of the traffic between the working path and the recovery
path. The PSL is the origin of the recovery traffic, but may or may
not be the origin of the working traffic (that is the working path
may be transiting the PSL).
Path Merge LSR (PML)
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An LSR that receives both working path traffic and its corresponding
recovery path traffic, and either merges their traffic into a single
outgoing path, or, it is itself the destination, passes the traffic
on to the higher layer protocols. The PML is the destination of the
recovery path but may or may not be the destination of the working
path.
Intermediate LSR
An LSR on a working or recovery path that is neither a PSL nor a PML
for that path.
FIS (Fault Indication Signal)
A signal that indicates that a fault along a path has occurred. It
is relayed by each intermediate LSR to its upstream or downstream
neighbor, until it reaches an LSR that is set up to perform MPLS
recovery.
FRS (Fault Recovery Signal)
A signal that indicates that a fault along a path has been repaired.
Like the FIS, it is relayed by each intermediate LSR to its upstream
or downstream neighbor, until it reaches an LSR that performs a
switchback to the path for which the FIS was received.
Liveness Message
A generic name for any message exchanged periodically between two
adjacent LSRs that serves as a link probing mechanism. It provides
an integrity check of the forward and backward directions of the
link between the two LSRs as well as a check of neighbor liveness.
Path Continuity Test
A test that verifies the integrity and continuity of a path or a
path segment. The details of such a test are beyond the scope of
this draft. (This could be accomplished, for example, by sending a
control message along the same links and nodes as those traversed by
the data traffic.)
4.1 Reverse Notification Tree
Since LSPs are unidirectional entities and recovery requires the
notification of faults to the LSR(s) responsible for switchover to
the recovery path, a mechanism must be provided for the fault
indication and the fault repair notification to travel from the
point of occurrence of the fault back to the PSL(s). The situation
is complicated in the following two cases:
(i) Physically merged LSPs: With label merging multiple working
paths may converge to form a multipoint-to-point tree, with the
PSLs as the leaves. In this case, therefore, the fault
indication and -repair notification should be able to travel
along a reverse path of the working path to all the PSLs
affected by the fault. For example, in Fig. 1, for a fault along
link 34 the affected PSLs are 1 and 9, where as for a fault
along link 23, the only affected PSL is 1.
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(ii) Virtually merged LSPs: When several LSPs originating at
different LSRs share a common segment beyond some node, and
share a common identifier (such as the SESSION ID in RSVP-TE),
we call such LSPs virtually merged. In this case also, savings
in notification can be realized by sending a single
notification towards the affected PSLs along segments shared by
the LSPs emanating from these PSLs, and allowing the
notification to branch out at the merge node(s). For example,
in Fig. 1, for a failure along link 67 a single notification
could be sent for working paths 1-2-3-4-6-7 and 8-9-3-4-6-7
along their common segment 7-6-4-3. The notification would
branch out at node 3, which is the node where the LSP from node
1 to node 7 and the LSP from node 8 to node 7 merge.
In both the cases above, an appropriate notification path can be
provided by the reverse notification tree (RNT which is a point-to-
multipoint tree that is an exact mirror image of the converged
working paths, along which the FIS and the FRS travel. (see Fig.
1). There are several advantages to using an RNT:
-- The RNT can be established in association with the working
path(s), simply by making each LSR along a working path remember
its upstream neighbor (or the collection of upstream neighbors
whose working paths converge at the LSR and exit as one). Thus,
no multicast routing is required. We elaborate more on the RNT in
Section 3.
-- Only one RNT is required for all the working paths that merge
(either physically or virtually) to form the multipoint-to-point
forward path. The RNT is rooted at an appropriately chosen LSR
along the common segment of the merged working LSPs and is
terminated at the PSLs. All intermediate LSRs on the converged
working paths share the same RNT.
Therefore, the RNT enables a reduction in the signaling overhead
associated with recovery. Unlike schemes that treat each LSP
independently, and require signaling between a PSL and the PML
individually for each LSP, the RNT allows for only one (or a small
number of) signaling messages on the shared segments of the LSPs.
-- The RNT can be implemented either at Layer 3 or at Layer 2. In
either case, the delay along the RNT needs to be carefully
controlled. This may be ensured by giving the highest priority to
the fault and repair notification packets, which travel along the
RNT.
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PML
+----+ L[11,13] +----+ +----+
| 11 |------+ +======| 14 |=========================| 15 |
| | | || | | P[14,15] | |
+----+ | || +----+ +----+
| || | :
+----+ ||P[13,14] | |
| 13 |======+ | :
PSL | |-------+ | |
+----+<-..-: | | :
| | | | |
L[12,13]| : |L[13,5] | :
+----+ | +----+ L[5,15] | |
| 12 |------+ | |-----------------------------------+ :
| | +===| 5 |<-.-..-..-..-..-..-..-..-..-..-..-..-+
+----+ || | |======================================+
P[1,5] || +----+ P[5,7] ||
+============+ ||
|| ||
|| ||
+----+ +----+ L[2,3] L[4,6] +----+ L[6,7] +----+
| 1 |----| 2 |--------+ +-------| 6 |----------| 7 |
| |<.-.| |<-..-+ | | +-..-<| |<-..-..-..| |
+----+ +----+ N32 : |I23 | : +----+ +----+
PSL | | | | PML ||
: | | : ||
| | | | ||
: | L[3,4] | : ||
+----+ I34 +----+ ||
| 3 |--------| 4 | P[10,7]||
| |<-..-..-| | ||
+----+ N43 +----+ ||
I93 | | ||
| : ||
| |N39 ||
| : ||
+----+ +----+ | | +----+ ||
| 8 |-----| 9 |----+ : | 10 |=============+
| | | |<-..-.+ P[9,10] | |
+----+ +----+==========================+----+
PSL
Legend:
--- = Working path
=== = Protection path
-..- = Reverse Notification Tree
---- = Working path
L[x,y] = Working path link between nodes x and y.
P[x,y] = Protection path link between nodes x and y.
Lxy = Label used by the LSP traversing link L[x,y] from x to y.
Nxy = Label used for RNT traffic sent from node x to node y.
Ixy = Interface between nodes x and y.
Figure 1: Illustration of MPLS protection configuration
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4.2 Protection Domain
A protection domain is defined as the set of LSRs over which a
working path and its corresponding recovery path are routed. Thus,
a protection domain is bounded by the LSRs that provide the
switching and (if needed) the merging functions for MPLS protection,
namely, the PSL and the PML (if present), respectively.
In other words, a protection domain in bounded by the PSL at one
end, and by the LSRs that form the end of the working or protection
path at the other. In general, if the working and protection paths
do not merge within the MPLS domain, the LSRs at the end of the
working and protection paths may be egress LSRs. The PSL and the PML
(alternatively, the end points of the working and protection paths
within the MPLS domain under consideration) are identified during
the setting up of an LSP, either via an offline algorithm or by an
algorithm that runs at the head-end of an LSP to decide the specific
nodes that the LSP must pass through. (Note that segments of the LSP
between the PSL and the PML may be loosely routed, as long as the
PSL and PML are known). Recovery should ideally be performed between
the source and destination (end-to-end), but in some cases segment
recovery may be desired (for example, when certain segments are more
unreliable than others) or may be the only option (due to the
topology of the network, see Fig. 1). For example, in Fig. 1, the
working path 8-9-3-4-6-7, can only have protection on the segment 9-
3-4-6-7.
Note that when multiple LSPs merge into a single LSP or when
multiple LSPs that share a common identifier follow the same path
beyond some node, the working paths corresponding to these LSPs also
converge. As explained in Section 4.4, an RNT can be used in this
case for propagating the failure and repair notification back to the
concerned PSL(s). We can therefore have a situation where different
protection domains share a common RNT. A protection domain is
denoted by specifying the working path and the recovery path. For
example, in Fig. 1, the protection domain bounded by LSR 1 and LSR
7, is denoted by (1-2-3-4-6-7, 1-5-7).
4.2.1 Relationship between protection domains with different RNTs
When protection domains have different RNTs, two cases may arise,
depending on whether or not any portions of the two domains overlap,
that is, have nodes or links in common. If the protection domains do
not overlap, the protection domains are independent (note that by
virtue of the RNTs in the two domains being different, neither the
working paths nor the RNTs in the two domains can overlap). In other
words, failures in one domain do not interact with failures in the
other domain. For example, the protection domain defined by (9-3-4-
6-7, 9-10-7) is completely independent of the domain defined by (11-
13-5-15, 11-13-14-15). As a result, as long as faults occur in
independent domains, the network shown in Fig. 1 can tolerate
multiple -faults (for example, simultaneous failures on the working
path in each domain).
If protection domains with disjoint RNTs overlap, it implies that
the protection path of one intersects the working path of the other.
Therefore, although failures on the working paths of the two domains
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do not affect one another, failures on the protection path of one
may affect the working path of the other and visa versa. For
example, the protection domain defined by (1-2-3-4-6-7, 1-5-7) is
not independent of the domain defined by (11-13-5-15, 11-13-14-15)
since LSR 5 lies on the protection path in the former domain and on
the working path in the latter domain.
4.2.2 Relationship between protection domains with the same RNT
When protection domains have the same RNT, different failures along
the working paths may affect both paths differently. As shown in
Fig. 1, for example, working paths 1-2-3-4-5-7 and 9-3-4-6-7 share
the same RNT. As a result, for a failure on some segments of the
working path, both domains will be affected, resulting in a
protection switch in both (for example, the segment 3-4-6-7 in Fig.
1). Likewise, for failures on other segments of the working path,
only one domain may be affected (for example, failure on segment 2-3
affects only the first working path 1-2-3-4-6-7, where as failure on
the segment 9-3 affects only the second working path 9-3-4-6-7).
4.3 Multiple Faults
We note that transferring the working traffic to the recovery path
is enough to take care of multiple faults on the working path.
However, if multiple faults happen such that there is at least one
failure on both the working and recovery paths, MPLS layer recovery
may no longer suffice. In this case, the network will either have to
allow for Layer 3 rerouting or have the PSL inform the administrator
via an alarm, thus enabling the manual reconfiguration of a
different working and backup path. (An OAM functionality could
greatly simplify such communication.) Note that for a PSL to be able
to generate an alarm, it must also have a mechanism for detecting
faults on the recovery path, such as a RNT for the recovery path (to
allow for the fault notification on the recovery path to be
propagated to the PSL).
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4.4 Timers and Thresholds
For its proper operation, the protection mechanism described in this
contribution uses the following timers and thresholds:
Timer or Sym Function
Threshold bol
Inter FIS t1 Interval at which successive FIS packets are
packet timer transmitted by a LSR to its upstream
neighbor.
Max. FIS t2 Max. time for which FIS packets are
duration timer transmitted by an LSR to its upstream peer.
Inter FRS t1Æ Interval at which successive FRS packets are
packet timer sent by a LSR to its upstream neighbor.
Max. FRS t2Æ Max. time for which the FRS packets are sent
duration timer by an LSR to its upstream neighbor.
Liveness msg. t4 Interval at which successive liveness
sender timer messages are sent by an LSR to peer LSRs that
have a working path (and RNT) through this
LSR.
Liveness msg. t4' A timer set to count down the interval at the
receiver end of which a liveness message should be
timeout timer received.
Hold-off Timer t5 Interval between the detection of a failure
[3] at an LSR, and the generation of the first
FIS message, to allow time for lower layer
protection to take effect.
Wait-to-Restore T6 Interval between the detection of a
Timer [3] recovery/failure at an LSR, and the
generation of the first FRS message, to allow
time for the stability of restoration.
Lost liveness K No. of liveness messages that can be lost
message before an LSR will declare a fault and
threshold generate the first FIS.
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5.0 Configuration
In the following sections, we describe the operation of the path
protection mechanism, and explain the various steps involved with
reference to Fig. 1.
Protection configuration consists of two aspects: establishing the
protection domain and creating the reverse notification tree. The
protection domain configuration involves either configuring the
working and protection path pair or the protection path of an
established working path. These aspects are discussed in this
section.
5.1 Establishing a Protection Domain
The label distribution protocol encompasses negotiations in which
two label distribution peers engage in order to learn of each
other's MPLS capabilities. The label distribution protocol is used
to establish a protection domain via signaling. The protection
domain consists of a working path and a recovery/protection path
pair. MPLS defines two methods for label distribution, Label
Distribution Protocol (LDP/CR-LDP) and ReSerVation Protocol (RSVP).
Our mechanism is designed to work with either of these label
distribution protocols.
LDP/CR-LDP and RSVP allow the path to be setup loosely (each node
determines itÆs next hop) or explicitly (each node has been given
itÆs next hop). We assume that protection paths will be setup
explicitly, however there is no requirement for this. These
protocols are being extended to provide a mechanism by which
protection establishment can be signaled and created. The
funtionality being introduced is:
-- Explicit Route Protection information to identify the PSL and
PML, and thus the protection domain.
-- Path Protection information to configure the nodes in the
protection domain.
The establishment of the protection domain requires the
identification of the working path and the protection path. There
are two separate cases to consider: non-merged (point-to-point) and
merged (multipoint-to-point). The working and protection paths for
RSVP/CR-LDP are identified as follows:
Non-merged:
-- RSVP: Same Sender Template (IP tunnel sender IP address,
LSPID)
-- Cr-LDP: Same LSPID TLV (Ingress LSR Router ID and Local CR-LSP
ID)
Merged:
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-- RSVP: Same session object (IP tunnel end point address and
Tunnel ID)
-- Cr-LDP: Same FEC TLV (Host Address and Prefix)
5.1.1 Explicit Route Protection Information
In order to identify the PSL, PML, and the nodes between the PSL and
PML that make up a protection domain, anExplicit Route Protection
fieldhas been added to the Explicit Route Field of CR-LDP and RSVP-
TE [8][9]. The Explicit Route Protection field will first appear
when the configuration message reaches the PSL. This denotes the
start of a protection domain. When the PSL processes the Explicit
Route Protection field, it will modify the configuration message
with a Path Protection Field that is directly derived from the
Explicit Route Protection Field and then forwards the configuration
message.
The configuration message will continue along the path until the
second Explicit Route Protection Field is evaluated at the PML. This
denotes the end of the protection domain. When the PML processes the
Explicit Route Protection Field, it will remove the Path Protection
Field from the configuration message and then forward the message.
This same process would be perfomed for each protection domain along
the configuration message path. For path protection it is critical
to identify the PSL,PML, and nodes within the protection domain. The
following attributes are specified in this field.
1. The Router ID of the PSL or PML;
2. Whether the node processing the Explicit Route Protection field
at the current hop is a PSL or PML;
3. What the protection type is 1+1, 1:1, etc.;
4. Whether this is the configuration message for the working or
protection path;
5. If the protection path resources can be used for extra traffic
becides the protected traffic;
6. Whether the RNT is implemented as a Hop-by-hop (Layer 3) LSP,
as an MPLS (Layer 2) LSP, or over SONET K1/K2 bytes;
7. What to configure the hold-off and wait-to-restore timers; and
8. If the protection switching action is revertive.
For example, the Explicit Route Field of the configuration message
might look like the following:
Ipv4 Address A
Ipv4 Address B
Explicit Route Protection (PSL, Router ID = current hop Ipv4
Router ID B)
Ipv4 Address C
Ipv4 Address D
Ipv4 Address E
Ipv4 Address F
Explicit Route Protection (PML, Router ID = current Hop Ipv4
Router ID F)
Ipv4 Address G
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Denotes the Explicit Route path of two Ipv4 hops (A and B) with the
second Ipv4 (B) hop identified as the PSL by the presence of the
Explicit Route Protection field. The PSL signifies the beginning of
the protection domain and as such creates the Path Protection Field
in the configuration message and forwards the message to the next
hop.
The configuration message continues for four more hops with the
nodes processing the Path Protection Field. The fourth IPv4 (F)hop
is identified as the PML by the presence of the Explicit Route
Protection field. The PML signifies the end of the protection domain
and as such removes the Path Protection Field from the configuration
message prior to forwarding the message to the last hop. This
process could continue if other protection domains are present after
the PML.
5.1.2 Path Protection Information
The Path Protection specifies whether path protection is activated,
identifies whether the path is the working path or protection path,
and configures each node with in the protection domain[8][9]. The
PSL node learns during a working/protection path configuration
process, which working and protection paths are coupled together.
The PML node learns during a working/protection path configuration
process, which working and protection paths are merged to the
outgoing network switch element. The PSL/PML pair constitute a
protection domain.
The attributes required to establish the Protection Domain are
defined in the framework[3]:
1 RNT Type: Specifies whether the RNT is implemented as a Hop-by-
hop (Layer 3) LSP, as an MPLS (Layer 2) LSP, or over SONET
K1/K2 bytes.
2 Timer Options: Specifies the hold-off and wait-to-restore time
requirements.
3 Revertive Option: Specifies whether the recovery action is
revertive.
5.2 Establishing a Protection/Recovery Path
The establishment of the recovery path requires the identification
of the working path. There are two separate cases to consider: non-
merged (point-to-point) and merged (point-to-multipoint). For path
protection mechanisms, the working and protection paths for are
identified as follows:
Non-merged:
-- RSVP: Same Sender Template (IP tunnel sender IP address,
LSPID)
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-- Cr-LDP: Same LSPID TLV (Ingress LSR Router ID and Local CR-LSP
ID)
Merged:
-- RSVP: Same session object (IP tunnel end point address and
Tunnel ID)
-- Cr-LDP: Same FEC TLV (Host Address and Prefix)
The Explicit Route Protection Field would only carry the protection
path configuration information. The configuration of the protection
path would be identical to the description provided in 5.1 for the
protection path.
In most cases, the working path and its corresponding recovery path
would be specified during LSP setup, either via a path selection
algorithm (running at a centralized location or at an ingress LSR)
or via administrative configuration. Observe that the specification
of the path, does not, strictly speaking, require the entire path to
be explicitly specified. Rather, it requires only that the PSL and
PML (or in the absence of a PML, the path egress points out of the
MPLS domain) be specified, with the segments between them being
loosely routed, if required. In other words, the path would be
established between the two nodes at the boundaries of the
protection domain via (possibly loose) explicit (or source) routing
using LDP [4], [5] /RSVP [6], [7] signaling (alternatively, via
constraint-based routing, or using manual configuration).
5.3 Creating the RNT
The RNT is used for propagating the FIS and the FRS, and can be
created by a simple extension to the LSP setup process. Note: An
MPLS OAM notification is preferable and could make use of the RNT.
During the establishment of the working path, the signaling message
carries with it the identity (address) of the upstream node that
sent it (for example, via the path attribute in RSVP). Each LSR
along the path simply remembers the identity of its immediately
prior upstream neighbor on each incoming link. Through the neighbor
discovery mechanism of the routing protocol, each LSR finds the
interface connecting it to the upstream LSRs. (It is assumed in this
draft that there is a bi-directional connection between two
neighboring LSRs, such as a bi-directional SONET link, a bi-
directional lower layer network link (e.g., an ATM VP), or a pair of
bi-directional tunnels over an IP subnetwork.) The node then creates
an ææinverseÆÆ cross-connect table that for each protected outgoing
LSP maintains a list of the incoming LSPs that merge into that
outgoing LSP, together with the identity of the upstream node and
incoming interface that each incoming LSP comes through. Upon
receiving an FIS, an LSR extracts the labels contained in it (which
are the labels of the protected LSPs that use the outgoing link that
the FIS was received on) and checks whether the current LSR is the
PSL for that LSP. If it is it terminates the FIS. Otherwise, it
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consults its inverse cross-connect table to determine the identity
of the upstream nodes that the protected LSPs come from, and creates
and transmits an FIS to each of them.
Therefore, based on whether the RNT is implemented at Layer 3 or
Layer 2, two cases arise:
If the RNT is implemented by a point-to-multipoint LSP, then the
working path can be bound to the ingress label and interface of the
RNT LSP at a LSR. Note that the RNT only be a point-to-multipoint
LSP in the case of mergeing, otherwise the RNT is implemented as a
point-to-point LSP. The ingress label and interface can then be used
as an index into the "inverse" cross-connect table to find the
egress labels and interfaces of the RNT LSP as shown in Table 2.
Upon receiving an FIS, an LSR extracts the labels and checks whether
it is the PSL for that LSP. If it is, it terminates the FIS.
Otherwise, it consults its inverse cross-connect table to determine
the outgoing labels and interfaces, performs a label swap and
forwards the FIS to the appropriate upstream node(s). For example,
consider Figure 1, and assume that a Layer 2 point-to-multipoint
RNT, rooted at LSR 7 and extending to LSRs 1 and 9, is bound to the
multipoint-to-point forward paths starting at LSRs 1 and 8 and
terminating at LSR 7. Now in case of a fault on link L[4,6], LSR 3
receives an FIS on the RNT in a labeled packet with label N43. It
uses this label as an index into its inverse cross-connect table,
and learns that there are two previous nodes (namely those reachable
via interfaces I23 and I93 respectively) that the FIS needs to be
forwarded to. It encapsulates the received FIS into a labeled
packets with labels N32 and N39, and dispatches them along
interfaces I23 and I93 respectively.
Ingress Ingress Egress Egress Egress Egress
Label of Interface Label of Interface Label of Interface
RNT of RNT RNT of RNT RNT of RNT
N43 I34 N32 I23 N39 I93
Table 2. An example inverse cross-connect table for LSR 3 using MPLS
(Layer 2) RNT
If the RNT is implemented by a hop-by-hop Layer 3 mechanism, using,
for example, UDP packets (with a specific port number to identify
notification message type), then the egress label and interface of
the working path can be used as an index into the inverse cross-
connect table to obtain the IP addresses of the previous hop(s) and
the associated outgoing interface(s), as illustrated in Table 3. On
each hop, the FIS carried in the UDP packet carries the label and
interface of the working path for that hop. Thus, if the receiving
node is not a PSL, the label and interface in the FIS can be
extracted and can be used to access the inverse cross-connect table.
The label and interface used by the working LSP on the hop(s) to the
upstream node(s) are then inserted into FIS packet(s), and the FIS
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packet(s) transmitted to the appropriate upstream node(s) along the
interface specified the inverse cross-connect table. Therefore, in
the hop-by-hop mechanism the FIS packets are not forwarded by a node
to its previous hops using its default layer 3 forwarding table.
Rather, these packets are forwarded via the outgoing interface
extracted from the nodeÆs inverse cross-connect table. As in the
example above, in case of a fault on link L[4,6], LSR 3 receives an
FIS from LSR 4 that contains the outgoing label L34 and the outgoing
interface I34 of the LSP affected by the fault. LSR3 uses these to
index its inverse cross-connect table (see Table 3), and learns, as
before, that there are two previous nodes (those reachable via
interfaces I23 and I93, respectively) that must receive an FIS. It
then creates two FIS packets as follows. The first, for transmission
along interface I23, contains the label L23 used by LSR 2 to
transmit data to LSR 3 along the working LSP. The second, for
transmission along interface I93, contains the label L93 used by LSR
9 to transmit data to LSR 3 along the working LSP.
Egress Egress Next Hop Egress Ingress
Label of Interface IP Interface Label of
Working of Address of RNT working
Path Working of RNT path
Path
L34 I34 I9 I93 L93
I2 I23 L23
Table 3. An example inverse cross-connect table for LSR 3 using a
hop-by-hop (Layer 3) RNT
The roles of the various core protection/recovery components are:
PSL: The PSL must be able to correlate the RNT with the working and
recovery paths. To this end, it maintains a table with a list of
working LSPs protected by an RNT, and the identity of the recovery
LSPs that each working path is to be switched to in the event of a
failure on the working path. It need not maintain an inverse cross-
connect table (for those LSPs and working paths for which it is the
PSL).
PML: The PML, in general, has to remember all of its upstream
neighbors and associate them with the appropriate working paths and
RNTs. If the PML is also the root of the RNT, it has to associate
each of its upstream nodes with a working path and RNT, but it need
not maintain an inverse cross-connect table (for those LSPs and
working paths for which it is a PML).
Intermediate LSR: An intermediate LSR has to only remember all of
its upstream neighbors and associate them with the appropriate
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working paths and RNTs, and has to maintain an "inverse" cross-
connect table.
5.4 Engineering a Protection Domain
For 1:1 protection, the bandwidth (if any) reserved for a
protection/recovery path should be the same as the bandwidth
reserved for its corresponding working path. This guarantees the
same bandwidth for the protected traffic after protection switching.
If the LSRs on the protection path support excess mode [3], the
bandwidth reserved on the protection path for protecting high
priority traffic can be used by other lower priority traffic
streams. That is, lower priority traffic that has a segment in
common with the recovery path, use the bandwidth of the recovery
path, as long as the recovery path is not called into use. When the
recovery path is pressed into service, the low priority traffic will
be discarded to allow for the actual working traffic to take its
place. Also, if delay, jitter or other QoS parameters are to be
satisfied, the protection path in 1:1 protection should be chosen
such that these requirements are satisfied.
Since the volume of signaling traffic (e.g., FIS/FRS messages, or
liveness messages) is small, in general bandwidth need not be
reserved for the signaling traffic provided that there exist other
mechanisms that can ensure that the delay requirements of signaling
messages are met (by using, for example, the highest priority for
signaling messages).
For bypass tunneling protection, multiple working LSPs may share the
same protection bandwidth by tunneling protection LSPs over a common
path. This requires that the working paths of these LSPs be
disjoint, except at the PSL and PML, so that they can be assumed to
not all fail at the same time. In this case, the bandwidth reserved
on the tunnel will be the maximum of all individual paths.
Otherwise, a bypass tunnel could be created to carry all the backup
paths, with the bandwidth reserved for the tunnel being the maximum
bandwidth required over all failure scenarios on the working LSPs.
5.5 Configuring Timers
The purpose of timers t1/t1' is to control the tradeoff between
notification delay of the FIS/FRS and the resources consumed when
sending the FIS/FRS. If t1/t1' is large, it may take a relatively
long time for the node that initiated the FIS/FRS transmission to
send the second the FIS/FRS if the first FIS/FRS message is lost,
thereby increasing notification delay. On the other hand, if t1/t1'
is small, the repetitive sending of FIS/FRS messages may waste
bandwidth and processing power because the first message may already
have reached the PSL(s).
It is assumed that after t2/t2' it is not necessary to do protection
at MPLS layer, either because it is no longer useful or because by
that time an upper layer protection mechanism will have been
triggered.
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The timers t4/t4' are used to control the frequency of liveness
messages sent between neighboring LSRs, so their purpose is the same
as those of timers t1/t1Æ. While frequent exchanges of liveness
messages can unnecessarily consume network resources, too few
exchanges may delay the discovery of faults. To accommodate delay
jitter, t4' may be set at a slightly different value from t4.
The timers t5/t6 are used to allow lower layer protection to take
effect before initiating MPLS layer recovery mechanisms (for
example, an automatic protection switching between fibers that
comprise a link between two LSRs). Following the detection of a
fault/fault repair S/FRS packet, respectively. This allows for the
lower layer protection to take effect and for the LSR to learn this
through one of several ways: via an indication from a lower layer,
or by the resumption of the reception of a liveness message, or by
the lack of LF, LD, PF or PD conditions (see definitions in [3]).
The threshold K helps to minimize false alarms due to the occasional
loss of a liveness message, which may occur, for example, either due
to a temporary impairment in a link or a peer LSR or due to a buffer
overflow.
6.0 Fault Detection
Each LSR must be able to detect certain types of faults, such as PF,
PD, LF, and LD [3] and propagate an FIS message towards the PSL.
Here we consider unidirectional link faults, bi-directional (or
complete) link faults, and node faults.
Essentially, the node upstream of the fault must be able to
detect/learn about the fault. This motivates the need for a
"liveness"message, which allows a node upstream of the fault to
detect the fault either directly or implicitly. Also, the fault
detection mechanism must provide the trigger for generating the FIS.
Broadly, the types of mechanisms that could be triggers for the FIS
are:
i) Lower layer mechanisms
ii) MPLS-based detection mechanisms, which may be used to detect
link faults, via a liveness message for example.
iii) User-plane OAM mechanisms, such as a path continuity test,
which may be used to detect other faults, such as mis-
connections (arising from incorrect entries in the label
forwarding table, for example).
The fault types that need to be detected are:
-- Unidirectional Link Fault: A uni-directional fault implies that
only one direction of a bi-directional link has experienced a
fault
-- Downlink Fault: A fault on a link in the downstream direction
will be detected by the node downstream of the faulty link,
either via the PF or PD condition being detected at the MPLS
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layer, or via LF or LD signals being propagated to the MPLS
layer by the lower layer or via the absence of liveness
messages.
-- Uplink Fault: A fault on a link in the upstream direction will
be detected by a node upstream of the faulty link, either via a
LF or LD being detected at the lower layer and propagated to
the MPLS layer (if there was traffic on this reverse link), or
via the PD or PF condition being detected at the MPLS layer, or
via absence of liveness messages.
-- Bi-directional link fault or node fault: When both directions
of the link have a fault (as in the case of a fiber cut), nodes
at both ends of the link will detect the fault either due to
the LF or PF signal or due to the absence of liveness messages.
7.0 Fault Notification
The rapid notification of a fault is effected by the propagation of
the FIS message along the RNT. Due to the timers built into the
FIS/FRS propagation mechanism, the transportation of FIS/FRS
messages does not require a reliable mechanism like TCP. Any LSR
may generate an FIS.
For instance, in Fig. 1 if link L23 fails, LSR 3 will detect it and
transmit a FIS to LSR 2 (after waiting for time T2), its upstream
neighbor along link L23. The FIS will contain the incoming labels
(at node 3) of those LSPs on link L23 that have protection enabled.
Upon receiving the FIS message, LSR 2 will consult its inverse-cross
connect table and generate an FIS message for LSR 1, which on
receiving the first FIS packet will wait for time t3 before
performing a protection switch. The node which initiates the FIS
will continue to send FIS messages at an interval of t1 until timer
t2 expires. After t2 expires it is assumed that either upper layer
protection will be triggered or enough number of FIS messages will
have been sent to reach the desired reliability in conveying fault
information to the PSL(s).
The roles of the various core protection switching components are:
PSL: The PSL does not generate a FIS message, but must be able to
detect FIS packets.
PML: The PML must be able to generate the FIS packets in response to
detecting failure, and should transmit them over the RNT. The PML
begins FIS transmission after continuously detecting a fault for T2
time units, and does so every t1 time units for a maximum of t2 time
units.
Intermediate LSR: An intermediate LSR must be able to
generate/forward FIS packets, either as a result of continuously
detecting a fault for T2 time units or in response to a received FIS
packet. It must transmit these to all its affected upstream
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neighbors as per its inverse cross-connect table. Again, it does so
every t1 time units for a maximum of t2 time units.
8.0 Switch Over
The switch over is the actual switching of the working traffic from
the working path to the recovery path. This is performed by a PSL,
t3 time units after the reception of the first FIS packet.
For example, in Fig. 1, consider protection domain (1-2-3-4-6-7, 1-
5-7). When link L34 fails, the PSL LSR 1 on learning of the failure
will perform a protection switch of the protected traffic from the
working path 1-2-3-4-6-7 to the backup path 1-5-7. Notice that LSR 7
acts as a protection merge LSR, merging traffic from the working and
backup paths. Since buffered packets from LSR 4 may continue to
arrive at LSR 7 even after the protection switch (the dampening
timer t43 at the PSL tends to mitigate this), a short-term
misordering of packets may happen at LSR 7, until the buffers on the
working path drain out.
The role of the core protection components is as follows:
PSL: Performs the protection switch upon receipt of the FIS message,
but after waiting for time t3 following the first FIS message.
PML: The PML automatically merges protection traffic with working
traffic. For a short period of time this may cause misordering of
packets, since packets buffered at LSRs downstream of the fault may
continue to arrive at the PML along the working path.
Intermediate LSR: The intermediate LSR has no special action.
9.0 Switch Back
Switch back or restoration is the transfer of working traffic from
the recovery path to the working path, once the working path is
repaired. This may be because the recovery path may be a limited
recovery path [3], or because the working path is deemed to be
preferred [3] in some respect. Restoration may be automatic or it
may be performed by manual intervention (or not performed at all).
In the revertive mode, restoration is performed upon the receipt of
the FRS message. A path continuity test may be performed to ensure
the integrity of the entire path before switching. I n the non-
revertive mode it may be performed by operator intervention.
The role of the core protection components is similar here to what
it is for protection switching. The PML does not need to do
anything, unless it was the node that detected the failure, in which
case it transmits a FRS upstream t6 time units after continuously
detecting recover signal from lower layer or after detecting
liveness messages from its peers. The intermediate LSR generates the
FRS message if it was the node that detected the recovery or
generates a FRS to relay the restoration status received from a
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downstream node. The PSL performs the restoration switch t3Æ seconds
after receiving the first FIS message.
10.0 Protocol Specific Extensions
The signaling protocol specific extensions needed to implement the
mechanism outlined in this draft are discussed in separate documents
[8],[9].
11.0 Security Considerations
The MPLS protection that is specified herein does not raise any
security issues that are not already present in the MPLS
architecture.
12.0 Intellectual Property Considerations
In accordance with the intellectual property rights procedures of
the IETF standards process, to the extent that Tellabs has patents,
pending applications and/or other intellectual property rights that
are essential to implementation of any subject matter submitted by
Tellabs that is included in a standard, Tellabs is prepared to
grant, on the basis of reciprocity (grantback), a license on such
subject matter under terms and conditions that are reasonable and
non-discriminatory.
13.0 Acknowledgements
We would like to thank our colleague Ben Mack-Crane, and members of
the MPLS WG list, in particular Dave Allan, Bora Akyol, Neil
Harrisson, Ping Pan, and J. Noel Chiappa, for suggestions, feedback,
and corrections to the first version of this draft.
14.0 AuthorsÆ Addresses
Changcheng Huang Vishal Sharma
Department of Systems and Tellabs Research Center
Computer Engineering
Carleton University One Kendall Square
1125 Colonel By Drive Bldg. 100, Ste. 121
Ottawa, Ontario K1S 5B6 Cambridge, MA 02139-1562
Phone: (613) 520-2600 ext. 2477 Phone: 617-577-8760
Changcheng.huang@sce.carleton.ca Vishal.Sharma@tellabs.com
Srinivas Makam Ken Owens
Tellabs Operations, Inc. Tellabs Operations, Inc.
4951 Indiana Avenue 1106 Fourth Street
Lisle, IL 60532 St. Louis, MO 63126
Phone: 630-512-7217 Phone: 314-918-1579
Srinivas.Makam@tellabs.com Ken.Owens@tellabs.com
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15.0 References
[1] Rosen, E., Viswanathan, A., and Callon, R., "Multiprotocol Label
Switching Architecture", Work in Progress, Internet Draft <draft-
ietf-mpls-arch-07.txt>, July 2000.
[2] Callon, R., Doolan, P., Feldman, N., Fredette, A., Swallow, G.,
Viswanathan, A., "A Framework for Multiprotocol Label Switching",
Work in Progress, Internet Draft <draft-ietf-mpls-framework-
05.txt>, September 1999.
[3] Makam, V., Sharma, V., Huang, C., Owens, K., Mack-Crane, B., et
al, "A Framework for MPLS-based Recovery", Work in Progress,
Internet Draft <draft-ietf-mpls-recovery-frmwrk-00.txt>,
September 2000.
[4] Andersson, L., Doolan, P., Feldman, N., Fredette, A., Thomas,
B., "LDP Specification", Work in Progress, Internet Draft <draft-
ietf-mpls-ldp-11.txt>, August 2000.
[5] Jamoussi, B. "Constraint-Based LSP Setup using LDP", Work in
Progress, Internet Draft <draft-ietf-mpls-cr-ldp-04.txt>, July
2000.
[6] Braden, R., Zhang, L., Berson, S., Herzog, S., "Resource
ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
[7] Awduche, D. et al "Extensions to RSVP for LSP Tunnels", Work in
Progress, Internet Draft <draft-ietf-mpls-rsvp-lsp-tunnel-07.txt,
August 2000.
[8] Huang, C., Sharma, V., Makam. V, and Owens, K., "Extensions to
RSVP-TE for MPLS Path Protection", Internet Draft, <draft-chang-
rsvpte-path-protection-ext-01.txt>, November 2000.
[9] Owens, K., Sharma, V., Makam. V, and Huang, C., "Extensions to
CR-LDP for MPLS Path Protection", Internet Draft, <draft-owens-
crldp-path-protection-ext-00.txt>, November, 2000.
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