draft-ietf-mpls-tp-ring-protection-04.txt   draft-ietf-mpls-tp-ring-protection-05.txt 
Network Working Group Y. Weingarten Network Working Group Y. Weingarten
Internet-Draft Internet-Draft
Intended status: Informational S. Bryant Intended status: Informational S. Bryant
Expires: August 29, 2013 Cisco Expires: October 6, 2013 Cisco
D. Ceccarelli D. Ceccarelli
D. Caviglia D. Caviglia
F. Fondelli F. Fondelli
Ericsson Ericsson
M. Corsi M. Corsi
Altran Altran
B. Wu B. Wu
X. Dai X. Dai
ZTE Corporation ZTE Corporation
February 25, 2013 April 4, 2013
Applicability of MPLS-TP Linear Protection for Ring Topologies Applicability of MPLS-TP Linear Protection for Ring Topologies
draft-ietf-mpls-tp-ring-protection-04.txt draft-ietf-mpls-tp-ring-protection-05.txt
Abstract Abstract
This document presents an applicability of existing MPLS protection This document presents an applicability of existing MPLS protection
mechanisms, both local and end-to-end, to Multi-Protocol Label mechanisms, both local and end-to-end, to Multi-Protocol Label
Switching Transport Profile (MPLS-TP) in ring topologies. Protection Switching Transport Profile (MPLS-TP) in ring topologies. This
on rings offers a number of opportunities for optimization as the document does not propose any new mechanisms or protocols.
protection choices are starkly limited (all traffic traveling one way Protection on rings offers a number of opportunities for optimization
around a ring can only be switched to travel the other way on the as the protection choices are starkly limited (all traffic traveling
ring), but also suffers from some complications caused by the one way around a ring can only be switched to travel the other way on
the ring), but also suffers from some complications caused by the
limitations of the topology. limitations of the topology.
Requirements for MPLS-TP protection especially for protection in ring Requirements for MPLS-TP protection especially for protection in ring
topologies are discussed in "Requirements of an MPLS Transport topologies are discussed in "Requirements of an MPLS Transport
Profile" (RFC 5654) and "MPLS Transport Profile (MPLS-TP) Profile" (RFC 5654) and "MPLS Transport Profile (MPLS-TP)
Survivability Framework" (RFC 6372). This document shows how MPLS-TP Survivability Framework" (RFC 6372). This document shows how MPLS-TP
linear protection as defined in RFC 6378 can be applied to single linear protection as defined in RFC 6378 can be applied to single
ring topologies, discusses how most of the requirements are met, and ring topologies, discusses how most of the requirements are met, and
describes scenarios in which the function provided by applying linear describes scenarios in which the function provided by applying linear
protection in a ring topology falls short of some of the protection in a ring topology falls short of some of the
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This Internet-Draft will expire on August 29, 2013. This Internet-Draft will expire on October 6, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Problem statement . . . . . . . . . . . . . . . . . . . . 4 1.1. Problem statement . . . . . . . . . . . . . . . . . . . . 4
1.2. Scope of the document . . . . . . . . . . . . . . . . . . 5 1.2. Scope of the document . . . . . . . . . . . . . . . . . . 5
1.3. Terminology and Notation . . . . . . . . . . . . . . . . . 5 1.3. Terminology and Notation . . . . . . . . . . . . . . . . . 6
1.4. Contributing Authors . . . . . . . . . . . . . . . . . . . 7 1.4. Contributing Authors . . . . . . . . . . . . . . . . . . . 7
2. P2P Ring Protection . . . . . . . . . . . . . . . . . . . . . 7 2. P2P Ring Protection . . . . . . . . . . . . . . . . . . . . . 7
2.1. Wrapping . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Wrapping . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Steering . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2. Steering . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3. SPME for p2p protection of a ring topology . . . . . . . . 10 2.3. SPME for p2p protection of a ring topology . . . . . . . . 10
2.3.1. Path SPME for Steering . . . . . . . . . . . . . . . . 11 2.3.1. Path SPME for Steering . . . . . . . . . . . . . . . . 12
2.3.2. Wrapping link protection with segment based SPME . . . 12 2.3.2. Wrapping link protection with segment based SPME . . . 13
2.3.3. Wrapping node protection . . . . . . . . . . . . . . . 14 2.3.3. Wrapping node protection . . . . . . . . . . . . . . . 14
2.3.4. Wrapping for link and node protection . . . . . . . . 14 2.3.4. Wrapping for link and node protection . . . . . . . . 15
2.4. Analysis of p2p protection . . . . . . . . . . . . . . . . 15 2.4. Analysis of p2p protection . . . . . . . . . . . . . . . . 16
2.4.1. Recommendations for protection of p2p paths 2.4.1. Recommendations for protection of p2p paths
traversing a ring . . . . . . . . . . . . . . . . . . 16 traversing a ring . . . . . . . . . . . . . . . . . . 16
3. P2MP protection . . . . . . . . . . . . . . . . . . . . . . . 16 3. P2MP protection . . . . . . . . . . . . . . . . . . . . . . . 17
3.1. Wrapping for p2mp LSP . . . . . . . . . . . . . . . . . . 16 3.1. Wrapping for p2mp LSP . . . . . . . . . . . . . . . . . . 17
3.1.1. Comparison of Wrapping and ROM-Wrapping . . . . . . . 18 3.1.1. Comparison of Wrapping and ROM-Wrapping . . . . . . . 19
3.1.2. Multiple Failures Comparison . . . . . . . . . . . . . 20 3.1.2. Multiple Failures Comparison . . . . . . . . . . . . . 20
3.2. Steering for p2mp paths . . . . . . . . . . . . . . . . . 20 3.2. Steering for p2mp paths . . . . . . . . . . . . . . . . . 21
3.2.1. Context labels . . . . . . . . . . . . . . . . . . . . 21 3.2.1. Context labels . . . . . . . . . . . . . . . . . . . . 21
3.2.2. Walkthrough using context labels . . . . . . . . . . . 23 3.2.2. Walkthrough using context labels . . . . . . . . . . . 23
4. Coordination protocol . . . . . . . . . . . . . . . . . . . . 24 4. Coordination protocol . . . . . . . . . . . . . . . . . . . . 25
5. Conclusions and Recommendations . . . . . . . . . . . . . . . 25 5. Conclusions and Recommendations . . . . . . . . . . . . . . . 26
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
7. Security Considerations . . . . . . . . . . . . . . . . . . . 26 7. Security Considerations . . . . . . . . . . . . . . . . . . . 27
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.1. Normative References . . . . . . . . . . . . . . . . . . . 26 9.1. Normative References . . . . . . . . . . . . . . . . . . . 27
9.2. Informative References . . . . . . . . . . . . . . . . . . 27 9.2. Informative References . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction 1. Introduction
Multi-Protocol Label Switching Transport Profile (MPLS-TP) has been Multi-Protocol Label Switching Transport Profile (MPLS-TP) has been
standardized as part of a joint effort between the Internet standardized as part of a joint effort between the Internet
Engineering Task Force (IETF) and the International Telecommunication Engineering Task Force (IETF) and the International Telecommunication
Union Standardization (ITU-T). These specifications are based on the Union Standardization (ITU-T). These specifications are based on the
requirements that were generated from this joint effort. requirements that were generated from this joint effort.
The MPLS-TP requirement document [RFC5654] includes a requirement to The MPLS-TP requirement document [RFC5654] includes a requirement to
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o Number of elements of recovery needed o Number of elements of recovery needed
o Number of labels required o Number of labels required
o Number of control and management plane transactions during a o Number of control and management plane transactions during a
maintenance operation maintenance operation
o Impact of signaling and routing information exchanged during o Impact of signaling and routing information exchanged during
protection, in presence of control plane protection, in presence of control plane
This document proposes a set of basic mechanisms that could be used This document describes how applying a set of basic MPLS-TP linear
for the protection of the data flows that traverse an MPLS-TP ring. protection mechanisms defined in [RFC6378] can be used to provide
These mechanisms are based on existing MPLS and MPLS-TP protection protection of the data flows that traverse an MPLS-TP ring. These
mechanisms. These mechanisms provide data flow protection due to any mechanisms provide data flow protection due to any switching trigger
switching trigger within a reasonable time frame and optimize the within a reasonable time frame and optimize the criteria set out in
criteria set out in [RFC5654], as summarized above. [RFC5654], as summarized above. This doxument does not define any
new protocol mechanisms or procedures.
A related topic in [RFC5654] addresses the required support for A related topic in [RFC5654] addresses the required support for
interconnected rings. This topic involves various scenarios that interconnected rings. This topic involves various scenarios that
require further study and will be addressed in a separate document, require further study and will be addressed in a separate document,
based on the principles outlined in this document. based on the principles outlined in this document.
1.1. Problem statement 1.1. Problem statement
Ring topologies, as defined in [RFC5654], are used in transport Ring topologies, as defined in [RFC5654], are used in transport
networks. When designing a protection mechanism for a single ring networks. When designing a protection mechanism for a single ring
topology, there is a need to address both - topology, there is a need to address both -
1. A point-to-point transport path that enters an MPLS-TP capable 1. A point-to-point transport path that either originates at or
ring at one node, the ingress node, and exits the ring at a enters an MPLS-TP capable ring at one node, the ingress node, and
single egress node possibly continuing beyond the ring. exits the ring at a single egress node possibly continuing beyond
the ring.
2. Where the ring is being used as a branching point for a point-to- 2. Where the ring is being used as a branching point for a point-to-
multipoint transport path, i.e. the transport path enters the multipoint transport path, i.e. the transport path originates at
MPLS-TP capable ring at the ingress node and exits through a or enters the MPLS-TP capable ring at the ingress node and exits
number of egress nodes, possibly continuing beyond the ring. through a number of egress nodes, possibly continuing beyond the
ring.
In either of these two situations, there is a need to address the In either of these two situations, there is a need to address the
following different cases - following different cases -
1. One of the ring links causes a fault condition. This could be 1. One of the ring links causes a fault condition. This could be
either a unidirectional or bidirectional fault, and should be either a unidirectional or bidirectional fault, and should be
detected by the neighboring nodes. detected by the neighboring nodes.
2. One of the ring nodes causes a fault condition. This condition 2. One of the ring nodes causes a fault condition. This condition
is invariably a bidirectional fault (although in rare cases of is invariably a bidirectional fault (although in rare cases of
misconfiguration this could be detected as a unidirectional misconfiguration this could be detected as a unidirectional
fault) and should be detected by the two neighboring ring nodes. fault) and should be detected by the two neighboring ring nodes.
3. An operator command is issued to a specific ring node. A 3. An operator command that changes the operational state of a node
description of the different operator commands is found in or a link, or specifically triggers a protection actionis issued
Section 4.13 of [RFC4427]. Examples of these commands include to a specific ring node. A description of the different operator
Manual Switch, Forced Switch, or Clear operations. commands is found in Section 4.13 of [RFC4427]. Examples of
these commands include Manual Switch, Forced Switch, or Clear
operations.
The protection domain addressed in this document is limited to the The protection domain addressed in this document is limited to the
traffic that traverses on the ring. Traffic on the transport path traffic that traverses on the ring. Traffic on the transport path
prior to the ring ingress node or beyond the egress nodes may be prior to the ring ingress node or beyond the egress nodes may be
protected by some other mechanism. protected by some other mechanism.
1.2. Scope of the document 1.2. Scope of the document
This document addresses the requirements that appear in Section This document addresses the requirements that appear in Section
2.5.6.1 of [RFC5654] on Ring Protection based on the application of 2.5.6.1 of [RFC5654] on Ring Protection based on the application of
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The authors would like to acknowledge the following individuals that The authors would like to acknowledge the following individuals that
contributed their insights and advice to this work: contributed their insights and advice to this work:
Nurit Sprecher (NSN) Nurit Sprecher (NSN)
Akira Sakurai (NEC) Akira Sakurai (NEC)
Rolf Winter (NEC) Rolf Winter (NEC)
Eric Osborne (Cisco)
2. P2P Ring Protection 2. P2P Ring Protection
There are two protection architecture mechanisms, that have There are two protection architecture mechanisms, that have
historically been applied to ring topologies, based on SDH historically been applied to ring topologies, based on SDH
specifications [G.841], and have been proposed in various forums to specifications [G.841], and have been proposed in various forums to
perform recovery of a topological ring network - "Wrapping" and perform recovery of a topological ring network - "Wrapping" and
"Steering". The following sub-sections examine these two mechanisms, "Steering". The following sub-sections examine these two mechanisms,
as applied to an MPLS transport network. as applied to an MPLS transport network.
2.1. Wrapping 2.1. Wrapping
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\_C_/@@@@@@@@\_D_/@@@@@@@@\_E_/ \_C_/@@@@@@@@\_D_/@@@@@@@@\_E_/
===> connected LSP *** physical link ===> connected LSP *** physical link
### working path @@@ wrapped data path ### working path @@@ wrapped data path
Figure 1: Wrapping protection for p2p path Figure 1: Wrapping protection for p2p path
Consider the LSP that is shown in Figure 1 that enters the ring of Consider the LSP that is shown in Figure 1 that enters the ring of
LSRs at LSR-B and exits at LSR-E. The normal working path LSP LSRs at LSR-B and exits at LSR-E. The normal working path LSP
follows through LSRs B-A-F-E. If a fault is detected on the link follows through LSRs B-A-F-E. If a fault is detected on the link
A<-->F, then the wrapping mechanism decides that LSR-A would wrap the A<->F, then the wrapping mechanism decides that LSR-A would wrap the
traffic around the ring, on a wrapped data path A-B-C-D-E-F, to traffic around the ring, on a wrapped data path A-B-C-D-E-F, to
arrive at LSR-F (on the far side of the failed link). LSR-F would arrive at LSR-F (on the far side of the failed link). LSR-F would
then wrap the data packets back onto the working path F-->E to the then wrap the data packets back onto the working path F->E to the
egress node. In this protection scheme, the traffic will follow the egress node. In this protection scheme, the traffic will follow the
path - B-A-B-C-D-E-F-E. path - B-A-B-C-D-E-F-E.
This protection scheme is simple in the sense that there is no need This protection scheme is simple in the sense that there is no need
for coordination between the different LSR in the ring - only the for coordination between the different LSR in the ring - only the
LSRs that detect the fault must wrap the traffic, either onto the LSRs that detect the fault must wrap the traffic, either onto the
wrapped data path (at the near-end) or back to the working path (at wrapped data path (at the near-end) or back to the working path (at
the far-end). Coordination of the switchover to the protection path the far-end). Coordination of the switchover to the protection path
would be needed to maintain the traffic on a co-routed bidirectional would be needed to maintain the traffic on a co-routed bidirectional
LSP even in cases of a unidirectional fault condition. LSP even in cases of a unidirectional fault condition.
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dependent upon this decision. dependent upon this decision.
o There is a need to define a data-path that traverses the alternate o There is a need to define a data-path that traverses the alternate
path around the ring to connect between the two neighbors of the path around the ring to connect between the two neighbors of the
detected fault. If protecting both the links and the nodes of a detected fault. If protecting both the links and the nodes of a
LSP, then, for a ring with N nodes, there is a need for O(2N) LSP, then, for a ring with N nodes, there is a need for O(2N)
alternate paths. alternate paths.
o When wrapping, the data is transmitted over some of the links o When wrapping, the data is transmitted over some of the links
twice, once in each direction. For example, in the figure above twice, once in each direction. For example, in the figure above
the traffic is transmitted both B-->A and then A-->B, later it is the traffic is transmitted both B->A and then A->B, later it is
transmitted E-->F and F-->E. This means that there is additional transmitted E->F and F->E. This means that there is additional
bandwidth needed for this protection. bandwidth needed for this protection.
o If a double-fault situation occurs in the ring, then wrapping will o If a double-fault situation occurs in the ring, then wrapping will
not be able to deliver any packets except between the ingress and not be able to deliver any packets except between the ingress and
the first fault location encountered on the working path. This is the first fault location encountered on the working path. This is
based on the need for wrapping to connect between the neighbors of based on the need for wrapping to connect between the neighbors of
the fault location, and this is not possible in the segmented the fault location, and this is not possible in the segmented
ring. ring.
o The resource pre-allocation for all of the alternate-paths could o The resource pre-allocation for all of the alternate-paths could
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point (to the ring) to the egress point going in opposite directions point (to the ring) to the egress point going in opposite directions
around the ring. This is illustrated in Figure 2, where if we assume around the ring. This is illustrated in Figure 2, where if we assume
that the traffic needs to enter the ring from node B and exit through that the traffic needs to enter the ring from node B and exit through
node F, we could define a primary path through nodes B-A-F, and an node F, we could define a primary path through nodes B-A-F, and an
alternate path through the nodes B-C-D-E-F. In steering the alternate path through the nodes B-C-D-E-F. In steering the
switching is always performed by the ingress node (node B in switching is always performed by the ingress node (node B in
Figure 2). If a fault condition is detected anywhere on the working Figure 2). If a fault condition is detected anywhere on the working
path (B-A-F), then the traffic would be redirected by B to the path (B-A-F), then the traffic would be redirected by B to the
alternate path (i.e. B-C-D-E-F). alternate path (i.e. B-C-D-E-F).
___ ___ ___
======>/LSR\********/LSR\********/LSR\======>
\_B_/########\_A_/########\_F_/
*@ @*
*@ @*
*@ @*
_*@ ___ @*_
/LSR\********/LSR\********/LSR\
\_C_/@@@@@@@@\_D_/@@@@@@@@\_E_/
===> connected LSP *** physical link
### working path @@@ protection path
Figure 2: Steering protection in an MPLS-TP ring
This mechanism bears similarities to linear 1:1 protection [RFC6372]. This mechanism bears similarities to linear 1:1 protection [RFC6372].
The two paths around the ring act as the working and protection The two paths around the ring act as the working and protection
paths. There is need to communicate to the ingress node the need to paths. There is need to communicate to the ingress node the need to
switch over to the protection path and there is a need to coordinate switch over to the protection path and there is a need to coordinate
the switchover between the two end-points of the protected domain. the switchover between the two end-points of the protected domain.
The following considerations must be taken into account regarding the The following considerations must be taken into account regarding the
steering architecture: steering architecture:
o Steering relies on a failure detection method that is able to o Steering relies on a failure detection method that is able to
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*@ @* *@ @*
*@ @* *@ @*
*@ @* *@ @*
_*@ ___ @*_ _*@ ___ @*_
/LSR\********/LSR\********/LSR\ /LSR\********/LSR\********/LSR\
\_C_/@@@@@@@@\_D_/@@@@@@@@\_E_/ \_C_/@@@@@@@@\_D_/@@@@@@@@\_E_/
===> connected LSP *** physical link ===> connected LSP *** physical link
### primary SPME @@@ secondary SPME ### primary SPME @@@ secondary SPME
Figure 2: An MPLS-TP ring Figure 3: An MPLS-TP ring
In all of the following subsections, we use 1:1 linear protection In all of the following subsections, we use 1:1 linear protection
[RFC6372] [RFC6378] to perform protection switching and coordination [RFC6372] [RFC6378] to perform protection switching and coordination
when a signal fault is detected. The actual configuration of the when a signal fault is detected. The actual configuration of the
SPMEs used may change dependent upon the choice of methodology and SPMEs used may change dependent upon the choice of methodology and
this will be detailed in the following sections. However, in all of this will be detailed in the following sections. However, in all of
these configurations the mechanism will be to transmit the data these configurations the mechanism will be to transmit the data
traffic on the primary SPME, while applying OAM functionality over traffic on the primary SPME, while applying OAM functionality over
both the primary and the secondary SPME to detect signal fault both the primary and the secondary SPME to detect signal fault
conditions on either path. If a signal fault is detected on the conditions on either path. If a signal fault is detected on the
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all LSP data packets. all LSP data packets.
The protection SPME will continue to transmit the data packets until The protection SPME will continue to transmit the data packets until
the stable recovery of the fault condition. Upon recovery, i.e. the the stable recovery of the fault condition. Upon recovery, i.e. the
fault condition has cleared and the network is stabilized, the fault condition has cleared and the network is stabilized, the
ingress LSR could switch traffic back to the working SPME, if the ingress LSR could switch traffic back to the working SPME, if the
protection domain is configured for revertive behavior. protection domain is configured for revertive behavior.
The control of the protection switching, especially for cases of The control of the protection switching, especially for cases of
operator commands, would be covered by the protocol defined in operator commands, would be covered by the protocol defined in
[RFC6378]. [RFC6378].
2.3.2. Wrapping link protection with segment based SPME 2.3.2. Wrapping link protection with segment based SPME
It is possible to use the SPME mechanism to perform segment-based It is possible to use the SPME mechanism to perform segment-based
protection. For each link in the ring, we define two SPME - the protection. For each link in the ring, we define two SPME - the
first is a SPME between the two LSRs that are connected by the link, first is a SPME between the two LSRs that are connected by the link,
and the second SPME between these same two LSRs but traversing the and the second SPME between these same two LSRs but traversing the
entire ring (except the link that connects the LSRs). In Figure 3 we entire ring (except the link that connects the LSRs). In Figure 4 we
show the primary SPME that connects LSR-A & LSR-F over a segment show the primary SPME that connects LSR-A & LSR-F over a segment
connection, and the secondary SPME that connects these same LSRs by connection, and the secondary SPME that connects these same LSRs by
traversing the ring in the opposite direction. traversing the ring in the opposite direction.
___ ___ ___ ___ ___ ___
/LSR\********/LSR\********/LSR\ /LSR\********/LSR\********/LSR\
\_B_/@@@@@@@@\_A_/########\_F_/ \_B_/@@@@@@@@\_A_/########\_F_/
*@ *@ *@ *@
*@ *@ *@ *@
*@ *@ *@ *@
_*@ ___ _*@ _*@ ___ _*@
/LSR\********/LSR\********/LSR\ /LSR\********/LSR\********/LSR\
\_C_/@@@@@@@@\_D_/@@@@@@@@\_E_/ \_C_/@@@@@@@@\_D_/@@@@@@@@\_E_/
*** physical link *** physical link
### primary SPME @@@ secondary SPME ### primary SPME @@@ secondary SPME
Figure 3: Segment SPMEs Figure 4: Segment SPMEs
By applying OAM monitoring of these two SPME (at each LSR), it is By applying OAM monitoring of these two SPME (at each LSR), it is
possible to affect a wrapping protection mechanism for the LSP possible to affect a wrapping protection mechanism for the LSP
traffic that traverses the ring. The LSR on either side of the traffic that traverses the ring. The LSR on either side of the
segment would identify that there is a fault condition on the link segment would identify that there is a fault condition on the link
and redirect all LSP traffic to the secondary SPME. The traffic and redirect all LSP traffic to the secondary SPME. The traffic
would traverse the ring until arriving at the neighboring (relative would traverse the ring until arriving at the neighboring (relative
to the segment) LSR. At this point, the LSP traffic would be to the segment) LSR. At this point, the LSP traffic would be
redirected onto the original LSP, quite likely over the neighboring redirected onto the original LSP, quite likely over the neighboring
SPME. SPME.
skipping to change at page 14, line 13 skipping to change at page 14, line 26
possibly pushing a SPME label if the next segment is likewise possibly pushing a SPME label if the next segment is likewise
protected. protected.
2.3.3. Wrapping node protection 2.3.3. Wrapping node protection
Implementation of protection at the node level would be similar to Implementation of protection at the node level would be similar to
the mechanism described in the previous sub-section. The difference the mechanism described in the previous sub-section. The difference
would be in the SPMEs that are used. For node protection, the would be in the SPMEs that are used. For node protection, the
primary SPME would be configured between the two LSR that are primary SPME would be configured between the two LSR that are
connected to the node that is being protected (see SPME between LSR-A connected to the node that is being protected (see SPME between LSR-A
and LSR-E through LSR-F in Figure 4), and the secondary SPME would be and LSR-E through LSR-F in Figure 5), and the secondary SPME would be
configured between these same nodes, going around the ring (see configured between these same nodes, going around the ring (see
secondary SPME in Figure 4). secondary SPME in Figure 5).
___ ___ ___ ___ ___ ___
/LSR\********/LSR\********/LSR\ /LSR\********/LSR\********/LSR\
\_B_/@@@@@@@@\_A_/########\_F_/ \_B_/@@@@@@@@\_A_/########\_F_/
*@ *# *@ *#
*@ *# *@ *#
*@ *# *@ *#
_*@ ___ _*# _*@ ___ _*#
/LSR\********/LSR\********/LSR\ /LSR\********/LSR\********/LSR\
\_C_/@@@@@@@@\_D_/@@@@@@@@\_E_/ \_C_/@@@@@@@@\_D_/@@@@@@@@\_E_/
*** physical link *** physical link
### primary SPME @@@ secondary SPME ### primary SPME @@@ secondary SPME
Figure 4: Node-protection SPMEs Figure 5: Node-protection SPMEs
The protection mechanism would work similarly - based on 1:1 linear The protection mechanism would work similarly - based on 1:1 linear
protection [RFC6372], triggered by OAM functions on both SPMEs, and protection [RFC6372], triggered by OAM functions on both SPMEs, and
wrapping the data packets onto the secondary SPME at the ingress MEP wrapping the data packets onto the secondary SPME at the ingress MEP
(e.g. LSR-A in the figure) of the SPME and back onto the (e.g. LSR-A in the figure) of the SPME and back onto the
continuation of the LSP at the egress MEP (e.g. LSR-E in the figure) continuation of the LSP at the egress MEP (e.g. LSR-E in the figure)
of the SPME. of the SPME.
2.3.4. Wrapping for link and node protection 2.3.4. Wrapping for link and node protection
In the different types of wrapping presented in Section 2.3.2 and In the different types of wrapping presented in Section 2.3.2 and
Section 2.3.3, there is a limitation that the protection mechanism Section 2.3.3, there is a limitation that the protection mechanism
must a priori decide whether it is protecting for link or node must a priori decide whether it is protecting for link or node
failure. In addition, the neighboring LSR, that detects the fault, failure. In addition, the neighboring LSR, that detects the fault,
cannot readily differentiate between a link failure or a node cannot readily differentiate between a link failure or a node
failure. failure.
It would be possible to configure extra SPME to protect both for link It would be possible to configure extra SPME to protect both for link
and node failures, arriving at a configuration of the ring that is and node failures, arriving at a configuration of the ring that is
shown in Figure 5. Choosing the SPME to use for the wrapping would, shown in Figure 6. Here there are three protection SPME configured:
however, then involve considerable effort and could result in the
protected traffic not sharing the same protection path in both o Secondary node#1 would be used to divert traffic as a result of an
directions. indication that LSR-F is not available, it redirects traffic to be
transmitted between LSR-A and LSR-E.
o Secondary node#2 would be used to divert traffic as a result of an
indication that LSR-A is not available, it redirects traffic to be
transmitted between LSR-F and LSR-B.
o Secondary segment would be used to divert traffic as a result of
an indication that the segment between LSR-A and LSR-F is not
available, it redirects traffic to be transmitted between LSR-A
and LSR-F on the long circuit of the ring.
Choosing the SPME to use for the wrapping would, however, then
involve considerable effort and could result in the protected traffic
not sharing the same protection path in both directions.
___ ++++++++ ___ ___ ___ ++++++++ ___ ___
/LSR\********/LSR\********/LSR\ /LSR\********/LSR\********/LSR\
\_B_/@@@@@@@@\_A_/########\_F_/ \_B_/@@@@@@@@\_A_/########\_F_/
$+*@ +*$ $+*@ +*$
$+*@ +*$ $+*@ +*$
$+*@ +*$ $+*@ +*$
$+*@ ++++++++ ___ ++++++++ +*$ $+*@ ++++++++ ___ ++++++++ +*$
/LSR\********/LSR\********/LSR\ /LSR\********/LSR\********/LSR\
\_C_/@@@@@@@@\_D_/@@@@@@@@\_E_/ \_C_/@@@@@@@@\_D_/@@@@@@@@\_E_/
$$$$$$$$ $$$$$$$$ $$$$$$$$ $$$$$$$$
*** physical link *** physical link
### primary SPME @@@ secondary node#1 SPME ### primary SPME @@@ secondary node#1 SPME
$$$ secondary node#2 SPME +++ secondary segment SPME $$$ secondary node#2 SPME +++ secondary segment SPME
Figure 5: Segment & Node protection SPMEs Figure 6: Segment & Node protection SPMEs
2.4. Analysis of p2p protection 2.4. Analysis of p2p protection
Analyzing the mechanisms described in the above subsections we can Analyzing the mechanisms described in the above subsections we can
point to the following observations (based on a ring with N nodes, point to the following observations (based on a ring with N nodes,
assumed to be not more than 16): assumed to be not more than 16):
o Number of SPME that need to be configured - for steering SPME o Number of SPME that need to be configured - for steering SPME
protection (Section 2.3.1) = O(2N^2) [two SPME from each ingress protection (Section 2.3.1) = O(2N^2) [two SPME from each ingress
LSR to each other node in the ring], for wrapping based on SPME LSR to each other node in the ring], for wrapping based on SPME
skipping to change at page 17, line 14 skipping to change at page 17, line 41
given, as the traffic has been wrapped back onto the normal working given, as the traffic has been wrapped back onto the normal working
path on the far-side of the detected fault and will continue to be path on the far-side of the detected fault and will continue to be
transported to all of the egress points. transported to all of the egress points.
It is possible to optimize the performance of the wrapping mechanism It is possible to optimize the performance of the wrapping mechanism
when applied to p2mp LSPs by exploiting the topology of ring when applied to p2mp LSPs by exploiting the topology of ring
networks. networks.
This improved mechanism, which we call Ring Optimized Multipoint This improved mechanism, which we call Ring Optimized Multipoint
Wrapping (ROM-Wrapping), behaves much the same as classical wrapping. Wrapping (ROM-Wrapping), behaves much the same as classical wrapping.
There is one difference - rather than configuring the protection LSP However, ROM-Wrapping configures protection p2mp LSP, relative to
between the end nodes of a failed link (link protection) or between each node that is considered a failure risk, from the upstream node
the upstream and downstream node of a failed node (node protection), and all egress nodes (for the particular LSP) downstream from the
the improved mechanism configures a protection p2mp LSP from the failure risk.
upstream (with respect to the failure) node and all egress nodes (for
the particular LSP) downstream from the failure.
Referring to Figure 6, it is possible to identify the protected Referring to Figure 7, it is possible to identify the protected
(working) LSP (A-B-[C]-[D]-E-[F]) and one possible backup (working) LSP (A-B-{C}-{D}-E-{F}) and one possible backup
(protection) LSP. This protection LSP will be used to wrap the data (protection) LSP (note:the egress nodes are indicated by the curly
back around the ring to protect against a failure on link B-C. This braces). This protection LSP will be used to wrap the data back
around the ring to protect against a failure on link B-C. This
protection LSP is also a p2mp LSP that is configured with egress protection LSP is also a p2mp LSP that is configured with egress
points (at nodes F, D, & C) complimentary to the broken working data points (at nodes F, D, & C) complimentary to the broken working data
path. path.
| |
| |
V Ingress V Ingress
___ _V_ ___ ___ _V_ ___
/LSR\ /LSR\**************/LSR\ /LSR\ /LSR\**************/LSR\
<@@\_F_/@@@@@@@@@@@@@\_A_/@@@@@@@@@@@@@@\_B_/ <@@\_F_/@@@@@@@@@@@@@\_A_/@@@@@@@@@@@@@@\_B_/
skipping to change at page 17, line 48 skipping to change at page 18, line 26
@ * * @ * *
@_* ___ __* @_* ___ __*
/LSR\*************/LSR\**************/LSR\ /LSR\*************/LSR\**************/LSR\
\_E_/@@@@@@@@@@@@@\_D_/@@@@@@@@@@@@@@\_C_/ \_E_/@@@@@@@@@@@@@\_D_/@@@@@@@@@@@@@@\_C_/
@ @ @ @
@ @ @ @
V V V V
*** working LSP @@@ protection LSP *** working LSP @@@ protection LSP
Figure 6: P2MP ROM Wrapping Figure 7: P2MP ROM Wrapping
Using this mechanism, there is a need to configure a particular Using this mechanism, there is a need to configure a particular
protection LSP for each node on the working LSP. In the table below, protection LSP for each node on the working LSP. In the table below,
"X's Backup" is the backup path activated by node X as a consequence "X's Backup" is the backup path activated by node X as a consequence
of a failure affecting node Y (downstream node with respect to X) or of a failure affecting node Y (downstream node with respect to X) or
link X-Y, and square brackets, in the path,indicate egress nodes. link X-Y, and square brackets, in the path,indicate egress nodes.
Protected LSP: A->B->[C]->[D]->E->[F] Protected LSP: A->B->{C}->{D}->E->{F}
---- LINK/NODE PROTECTION---- -- LINK/NODE PROTECTION --
A's Backup: A->[F]->E->[D]->[C] A's Backup: A->{F}->E->{D}->{C}
B's Backup: B->A->[F]->E->[D]->[C] B's Backup: B->A->{F}->E->{D}->{C}
C's Backup: C->B->A->[F]->E->[D] C's Backup: C->B->A->{F}->E->{D}
D's Backup: D->C->B->A->[F] D's Backup: D->C->B->A->{F}
E's Backup: E->D->C->B->A->[F] E's Backup: E->D->C->B->A->{F}
It should be noted that ROM-Wrapping is an LSP based protection It should be noted that ROM-Wrapping is an LSP based protection
mechanism, as opposed to the SPME based protection mechanisms that mechanism, as opposed to the SPME based protection mechanisms that
are presented in other sections of this draft. While this may seem are presented in other sections of this draft. While this may seem
to be limited in scope, the mechanism may be very efficient for many to be limited in scope, the mechanism may be very efficient for many
applications that are based on p2mp distribution schemes. While ROM- applications that are based on p2mp distribution schemes. While ROM-
Wrapping can be applied to any network topology, it is particularly Wrapping can be applied to any network topology, it is particularly
efficient for interconnected ring topologies. efficient for interconnected ring topologies.
3.1.1. Comparison of Wrapping and ROM-Wrapping 3.1.1. Comparison of Wrapping and ROM-Wrapping
skipping to change at page 18, line 49 skipping to change at page 19, line 27
ROM-Wrapping however does not have this limitation, because there is ROM-Wrapping however does not have this limitation, because there is
no distinction between node and link protection. Whether link B-C or no distinction between node and link protection. Whether link B-C or
node C fails, in either case the rerouting will attempt to reach C. node C fails, in either case the rerouting will attempt to reach C.
If the failure is on the link, the traffic will be delivered to C, If the failure is on the link, the traffic will be delivered to C,
while if the failure is at node C, the traffic will be rerouted while if the failure is at node C, the traffic will be rerouted
correctly until node D, and will be blocked at this point. However, correctly until node D, and will be blocked at this point. However,
all egress nodes up-to the failure will be able to deliver the all egress nodes up-to the failure will be able to deliver the
traffic properly. traffic properly.
A second aspect is the number of hops needed to properly deliver the A second aspect is the number of hops needed to properly deliver the
traffic. Referring to the example shown in Figure 6, where a failure traffic. Referring to the example shown in Figure 7, where a failure
is detected on link B-C, the following table lists the set of nodes is detected on link B-C, the following table lists the set of nodes
traversed by the data in the protection: traversed by the data in the protection:
Basic Wrapping: Basic Wrapping:
A-B B-A-F-E-D-C [C]-[D]-E-[F] A-B B-A-F-E-D-C {C}-{D}-E-{F}
"Upstream" segment backup path "Downstream" segment "Upstream" segment backup path "Downstream" segment
with respect to the with respect to the with respect to the with respect to the
failure failure failure failure
ROM Wrapping: ROM Wrapping:
A-B B-A-[F]-E-[D]-[C] .. A-B B-A-{F}-E-{D}-{C} ..
"Upstream" segment backup path "Upstream" segment backup path
with respect to the with respect to the
failure failure
Comparing the two lists of nodes, it is possible to see that in this Comparing the two lists of nodes, it is possible to see that in this
particular case the number of hops crossed using the simple Wrapping particular case the number of hops crossed using the simple Wrapping
is significantly higher than the number of hops crossed by the is significantly higher than the number of hops crossed by the
traffic when ROM-Wrapping is used. Generally, the number of hops for traffic when ROM-Wrapping is used. Generally, the number of hops for
basic Wrapping is always higher or at least equal compared to ROM- basic Wrapping is always higher or at least equal compared to ROM-
Wrapping. This implies a certain waste of bandwidth on all links Wrapping. This implies a certain waste of bandwidth on all links
skipping to change at page 19, line 48 skipping to change at page 20, line 26
does not require support of such extra traffic. does not require support of such extra traffic.
The two recovery mechanism require different protection bandwidths. The two recovery mechanism require different protection bandwidths.
In the case of Wrapping, the bandwidth used is M in both directions In the case of Wrapping, the bandwidth used is M in both directions
of many of the links. While in case of ROM-Wrapping, only the links of many of the links. While in case of ROM-Wrapping, only the links
from the ingress node to the node performing the actual wrapping from the ingress node to the node performing the actual wrapping
utilize M bandwidth in both directions, while all other links utilize utilize M bandwidth in both directions, while all other links utilize
M bandwidth only in the counterclockwise direction. M bandwidth only in the counterclockwise direction.
Consider the case of a failure detected on link B-C as shown in Consider the case of a failure detected on link B-C as shown in
Figure 6. The following table lists the bandwidth utilization on Figure 7. The following table lists the bandwidth utilization on
each link (in units equal to M), for each recovery mechanism and for each link (in units equal to M), for each recovery mechanism and for
each direction (CW=clockwise, CCW=counterclockwise). each direction (CW=clockwise, CCW=counterclockwise).
+----------+----------+--------------+ +----------+----------+--------------+
| | Wrapping | ROM-Wrapping | | | Wrapping | ROM-Wrapping |
+----------+----------+--------------+ +----------+----------+--------------+
| Link A-B | CW+CCW | CW+CCW | | Link A-B | CW+CCW | CW+CCW |
| Link A-F | CCW | CCW | | Link A-F | CCW | CCW |
| Link F-E | CW+CCW | CCW | | Link F-E | CW+CCW | CCW |
| Link E-D | CW+CCW | CCW | | Link E-D | CW+CCW | CCW |
skipping to change at page 20, line 24 skipping to change at page 20, line 49
3.1.2. Multiple Failures Comparison 3.1.2. Multiple Failures Comparison
A further comparison between Wrapping and ROM-Wrapping can be done A further comparison between Wrapping and ROM-Wrapping can be done
with respect to their ability to react to multiple failures. The with respect to their ability to react to multiple failures. The
wrapping recovery mechanism does not have the ability to recover from wrapping recovery mechanism does not have the ability to recover from
multiple failures on a ring network, while ROM-Wrapping is able to multiple failures on a ring network, while ROM-Wrapping is able to
recover, from some multiple failures. recover, from some multiple failures.
Consider, for example, a double link failure affecting links B-C and Consider, for example, a double link failure affecting links B-C and
C-D shown in Figure 6. The Wrapping mechanism is not able to recover C-D shown in Figure 7. The Wrapping mechanism is not able to recover
from the failure because B, upon detecting the failure, has no from the failure because B, upon detecting the failure, has no
alternative paths to reach C. The whole P2MP traffic is lost. The alternative paths to reach C. The whole P2MP traffic is lost. The
ROM-Wrapping mechanism is able to partially recover from the failure, ROM-Wrapping mechanism is able to partially recover from the failure,
because the backup P2MP LSP to node F and node D is correctly set up because the backup P2MP LSP to node F and node D is correctly set up
and continues delivering traffic. and continues delivering traffic.
3.2. Steering for p2mp paths 3.2. Steering for p2mp paths
When protecting p2mp traffic that uses an MPLS-TP ring as its When protecting p2mp traffic that uses an MPLS-TP ring as its
branching point, i.e. it enters the ring at a head-end node and exits branching point, i.e. it enters the ring at a head-end node and exits
skipping to change at page 21, line 10 skipping to change at page 21, line 35
select the traffic from whichever of the two SPME whose traffic select the traffic from whichever of the two SPME whose traffic
arrives at that node, i.e. since one of the two (presumably the arrives at that node, i.e. since one of the two (presumably the
working SPME) will be blocked by the failure. In this way, all working SPME) will be blocked by the failure. In this way, all
egress nodes are able to receive the data traffic. While each node egress nodes are able to receive the data traffic. While each node
detects that there is connectivity from the ingress point, it detects that there is connectivity from the ingress point, it
continues to select the data that is coming from the working SPME. continues to select the data that is coming from the working SPME.
If a particular node stops receiving the connectivity messages from If a particular node stops receiving the connectivity messages from
the working SPME, it identifies that it must select to read the data the working SPME, it identifies that it must select to read the data
packets from the protection SPME. packets from the protection SPME.
3.2.1. Context labels
Figure 8 shows the two unidirectional p2mp SPME that are configured
from LSR-A with egress points at all of the nodes on the ring. The
clockwise SPME (i.e. A-B-C-D-E-F) is configured as the working SPME,
that will aggregate all traffic for p2mp LSPs that enter the ring at
LSR-A and must be sent out of the ring at any subset of the ring
nodes. The counter-clockwise SPME (i.e. A-F-E-D-C-B) is configured
as the protection SPME.
^ ^ ^ ^ ^ ^
_|_ _|_ _|_ _|_ _|_ _|_
----->/LSR\********/LSR\********/LSR\ ----->/LSR\********/LSR\********/LSR\
\_A_/========\_B_/========\_C_/ \_A_/========\_B_/========\_C_/
+* <+++++++++*|| +* <+++++++++*||
+* +*|| +* +*||
+* +*|| +* +*||
+* +*|| +* +*||
+*_ ++++++++ ___ +++++++++*|| +*_ ++++++++ ___ +++++++++*||
/LSR\********/LSR\********/LSR\ /LSR\********/LSR\********/LSR\
\_F_/<=======\_E_/========\_D_/ \_F_/<=======\_E_/========\_D_/
| | | | | |
V V V V V V
---> connected LSP *** physical link ---> connected LSP *** physical link
=== working SPME +++ protection SPME === working SPME +++ protection SPME
Figure 7: P2MP SPMEs Figure 8: P2MP SPMEs
3.2.1. Context labels
Figure 7 shows the two unidirectional p2mp SPME that are configured
from LSR-A with egress points at all of the nodes on the ring. The
clockwise SPME (i.e. A-B-C-D-E-F) is configured as the working SPME,
that will aggregate all traffic for p2mp LSPs that enter the ring at
LSR-A and must be sent out of the ring at any subset of the ring
nodes. The counter-clockwise SPME (i.e. A-F-E-D-C-B) is configured
as the protection SPME.
[RFC5331] defines the concept of context labels. A context- [RFC5331] defines the concept of context labels. A context-
identifying label defines a context label space that is used to identifying label defines a context label space that is used to
interpret the context-specific labels (found directly below the interpret the context-specific labels (found directly below the
context- identifying label) for a specific tunnel. The SPME label is context- identifying label) for a specific tunnel. The SPME label is
a context- identifying label. This means that at each hop the node a context- identifying label. This means that at each hop the node
that receives the SPME label uses it to point not directly to a that receives the SPME label uses it to point not directly to a
forwarding table, but to a LIB. As a node receives an SPME label it forwarding table, but to a Label Information Base (LIB). As a node
examines it, discovers that it is a context label, pops off the SPME receives an SPME label it examines it, discovers that it is a context
label, and looks up the next label down in the stack in the LIB label, pops off the SPME label, and looks up the next label down in
indicated by the context label. the stack in the LIB indicated by the context label.
The label below this context-identifying label should be used by the The label below this context-identifying label should be used by the
forwarding function of the node to decide the actions taken for this forwarding function of the node to decide the actions taken for this
packet. In MPLS-TP protection of ring topologies there are two packet. In MPLS-TP protection of ring topologies there are two
context LIBs. One is the context LIB for the working SPME and the context LIBs. One is the context LIB for the working SPME and the
other is the context LIB for the P-SPME. All context LIBs have a other is the context LIB for the P-SPME. All context LIBs have a
behavior defined for the e2e LSP label but the behavior at each node behavior defined for the e2e LSP label but the behavior at each node
may be different in the context of each SPME. may be different in the context of each SPME.
For example, using the ring that is shown in Figure 7, if the working For example, using the ring that is shown in Figure 8, if the working
SPME is configured to have a context-identifying label of CW at each SPME is configured to have a context-identifying label of CW at each
node on the ring and the protection SPME is configured to have a node on the ring and the protection SPME is configured to have a
context-identifying label of CP at each node. For the specific LSP context-identifying label of CP at each node. For the specific LSP
we will designate the context-specific label used on the working SPME we will designate the context-specific label used on the working SPME
as WL(x-y) to be the label used as node-x to forward the packet to as WL(x-y) to be the label used as node-x to forward the packet to
node-y. Similarly, for the context-specific labels on the protection node-y. Similarly, for the context-specific labels on the protection
SPME would be designated PL(x-y). An explicit example of label SPME would be designated PL(x-y). An explicit example of label
values appears in the next sub-section. values appears in the next sub-section.
Applying 1+1 linear protection, as outlined above, for a p2mp LSP Applying 1+1 linear protection, as outlined above, for a p2mp LSP
that enters the ring at LSR-A and has egress points from the ring at that enters the ring at LSR-A and has egress points from the ring at
LSR-C and LSR-E using the two SPME shown in Figure 7 then a packet LSR-C and LSR-E using the two SPME shown in Figure 8 then a packet
that arrives at LSR-A with a label stack [LI+S] will be forwarded on that arrives at LSR-A with a label stack [LI+S] will be forwarded on
the working SPME with a label stack [CW | WL(A-B)]. The packet the working SPME with a label stack [CW | WL(A-B)]. The packet
should then be forwarded to LSR-C arriving with a label [CW | should then be forwarded to LSR-C arriving with a label [CW |
WL(B-C)], where WL(B-C) should instruct the forwarding function to WL(B-C)], where WL(B-C) should instruct the forwarding function to
egress the packet with [LE(C)] and forward a copy to LSR-D with label egress the packet with [LE(C)] and forward a copy to LSR-D with label
stack [CW | WL(C-D)]. stack [CW | WL(C-D)].
If a fault condition is detected, for example on the link C-D, then If a fault condition is detected, for example on the link C-D, then
the nodes that are beyond the fault point, in this example nodes the nodes that are beyond the fault point, in this example nodes
LSR-D, LSR-E, and LSR-F, will cease to receive the data packets from LSR-D, LSR-E, and LSR-F, will cease to receive the data packets from
skipping to change at page 23, line 9 skipping to change at page 23, line 42
This architecture has the added advantages that there is no need for This architecture has the added advantages that there is no need for
the ingress node to identify the existence of the mis-connectivity, the ingress node to identify the existence of the mis-connectivity,
and there is no need for a return path from the egress points to the and there is no need for a return path from the egress points to the
ingress. ingress.
3.2.2. Walkthrough using context labels 3.2.2. Walkthrough using context labels
In order to better demonstrate the use of the context labels we In order to better demonstrate the use of the context labels we
present a walkthrough of an example application of the p2mp present a walkthrough of an example application of the p2mp
protection presented in this section. Referring to Figure 8, there protection presented in this section. Referring to Figure 9, there
is a p2mp LSP that traverses the ring, entering the ring at LSR-B and is a p2mp LSP that traverses the ring, entering the ring at LSR-B and
branching off at LSR-D, LSR-E, and LSR-H and does not continue beyond branching off at LSR-D, LSR-E, and LSR-H and does not continue beyond
LSR-H. For purposes of protection two p2mp unidirectional SPME are LSR-H. For purposes of protection two p2mp unidirectional SPME are
configured on the ring starting from LSR-B. One of the SPME, the configured on the ring starting from LSR-B. One of the SPME, the
working SPME, is configured with egress points at each of the LSR - working SPME, is configured with egress points at each of the LSR -
C, D, E, F, G, H, J, K, A. The second SPME, the protection SPME, is C, D, E, F, G, H, J, K, A. The second SPME, the protection SPME, is
configured with egress points at each of the LSR - A, K, J, H, G, F, configured with egress points at each of the LSR - A, K, J, H, G, F,
E, D, C. E, D, C.
^ ^ ^ ^ ^ ^ ^ ^
skipping to change at page 23, line 39 skipping to change at page 24, line 25
/LSR\********/LSR\********/LSR\*******/LSR\*******/LSR\ /LSR\********/LSR\********/LSR\*******/LSR\*******/LSR\
\_A_/<=======\_K_/========\_J_/=======\_H_/=======\_G_/ \_A_/<=======\_K_/========\_J_/=======\_H_/=======\_G_/
+ + + +Xxxxxxxxxx + + + + +Xxxxxxxxxx +
v v v v v v v v v v
v v v v v v v v v v
xxx p2mp LSP (X LSP egress) *** physical link xxx p2mp LSP (X LSP egress) *** physical link
=== working SPME +++ protection SPME === working SPME +++ protection SPME
+>> protection SPME egress +>> protection SPME egress
Figure 8: P2MP SPMEs Figure 9: P2MP SPMEs
For this example we suppose that the LSP traffic enters the ring at For this example we suppose that the LSP traffic enters the ring at
LSR-B with the label stack [99], leaves the ring at LSR-D with stack LSR-B with the label stack [99], leaves the ring at LSR-D with stack
[199], at LSR-E with stack [299], and LSR-H with stack [399]. [199], at LSR-E with stack [299], and LSR-H with stack [399].
While it is possible for the context-identifying label for the SPME While it is possible for the context-identifying label for the SPME
be configured as a different value at each LSR, for the sake of this be configured as a different value at each LSR, for the sake of this
example we will suppose a configuration of 200 as the context- example we will suppose a configuration of 200 as the context-
identifying label for the working SPME at each of the LSR in the identifying label for the working SPME at each of the LSR in the
ring, and 400 as the context-identifying label for the protection ring, and 400 as the context-identifying label for the protection
 End of changes. 48 change blocks. 
99 lines changed or deleted 136 lines changed or added

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