Network Working Group Y. Weingarten
Internet-Draft Nokia Siemens Networks
Intended status: Informational S. Bryant
Expires: December 31, 2010 Cisco
N. Sprecher
Nokia Siemens Networks
D. Ceccarelli
D. Caviglia
F. Fondelli
Ericsson
M. Corsi
Altran
B. Wu
X. Dai
ZTE Corporation
June 29, 2010
MPLS-TP Ring Protection
draft-weingarten-mpls-tp-ring-protection-03.txt
Abstract
This document presents an applicability statement to address the
requirements for protection of ring topologies for Multi-Protocol
Label Switching Transport Profile (MPLS-TP) Label Switched Paths
(LSP) on multiple layers. The MPLS-TP Requirements document [TPReqs]
specifies specific criteria for justification of dedicated protection
mechanism for particular topologies, including optimizing the number
of OAM entities needed, minimizing the number of labels for
protection paths, minimzing the number of recovery elements in the
network, and minimizing the number of control and management
transactions necessary. The document proposes a methodology for ring
protection based on existing MPLS-TP survivability mechanisms,
without the need of specification of new constructs or protocols.
This document is a product of a joint Internet Engineering Task Force
(IETF) / International Telecommunications Union Telecommunications
Standardization Sector (ITU-T) effort to include an MPLS Transport
Profile within the IETF MPLS and PWE3 architectures to support the
capabilities and functionalities of a packet transport network as
defined by the ITU-T.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 31, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Problem statement . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology and Notation . . . . . . . . . . . . . . . . . 5
1.3. Contributing Authors . . . . . . . . . . . . . . . . . . . 6
2. P2P Ring Protection . . . . . . . . . . . . . . . . . . . . . 6
2.1. Wrapping . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Steering . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3. P2P ring protection with SPME . . . . . . . . . . . . . . 9
2.3.1. Path SPME for Steering . . . . . . . . . . . . . . . . 11
2.3.2. Wrapping with segment based SPME . . . . . . . . . . . 11
2.3.3. Wrapping node protection . . . . . . . . . . . . . . . 13
2.3.4. Wrapping for link and node protection . . . . . . . . 14
2.4. Analysis of p2p protection . . . . . . . . . . . . . . . . 15
3. P2MP protection . . . . . . . . . . . . . . . . . . . . . . . 16
3.1. Wrapping for p2mp LSP . . . . . . . . . . . . . . . . . . 16
3.1.1. Comparison of Wrapping and ROM-Wrapping . . . . . . . 18
3.1.2. Multiple Failures Comparison . . . . . . . . . . . . . 19
3.2. Steering for p2mp paths . . . . . . . . . . . . . . . . . 20
4. Coordination protocol . . . . . . . . . . . . . . . . . . . . 21
5. Interconnected rings . . . . . . . . . . . . . . . . . . . . . 22
6. Conclusions and Recommendations . . . . . . . . . . . . . . . 24
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
8. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
10. Informative References . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
Multi-Protocol Label Switching Transport Profile (MPLS-TP) is being
standardized as part of a joint effort between the Internet
Engineering Task Force (IETF) and the International Telecommunication
Union Standardization (ITU-T). These specifications are based on the
requirements that were generated from this joint effort.
The requirements for MPLS-TP [TPReqs] indicates a requirement to
support a network that may include sub-networks that constitute a
MPLS-TP ring as defined in the requirements. There were no
requirements specific to a ring topology indicated in the
requirements document. However, the requirements state that specific
protection mechanisms aimed at ring topologies may be developed if
these allow the network to optimize:
o Number of OAM entities needed to trigger the protection
o Number of recovery objects needed
o Number of labels required
o Number of control and management plane transactions during a
recovery operation
o Impact of signaling and routing information exchanged, in presence
of control plane
This document will propose a set of basic mechanisms that could be
used for the protection of the data flows that traverse a MPLS-TP
ring. The mechanism is based on existing MPLS and MPLS-TP protection
mechanisms. We show that this mechanism provides the ability to
protect all of the basic conditions within a reasonable time frame
and does optimize the criteria set out in [TPReqs] as summarized
above.
1.1. Problem statement
Ring topologies, as definied in [TPReqs], are used in transport
networks due to their ability to easily support both p2p and p2mp
transport paths. When designing a protection mechanism for a ring
topology, there is a need to address both -
1. Simple case of a transport path that enters a MPLS-TP capable
ring at one node, the ingress node, and exits the ring at a
single egress node possibly continuing beyond the ring.
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2. Case where the ring is being used as a branching point, i.e. the
transport path enters the MPLS-TP capable ring at the ingress
node and exits through a number of egress nodes, possibly
continuing beyond the ring.
Within the ring segment of the transport path there is the need to
address the following different cases -
1. One of the ring links causes a fault condition. This could be
either a unidirectional or bidirectional fault, and should be
detected by the neighboring nodes.
2. One of the ring nodes causes a fault condition. This condition
is invariably a bidirectional fault (although in rare cases of
misconfiguration this could be detected as a unidirectional
fault) and should be detected by the two neighboring ring nodes.
3. An operator command is issued to a specific ring node. Examples
of these commands include Manual Switch, Forced Switch, or Clear
operations.
The protection domain addressed in this document is limited to the
traffic that is traversing the ring. Traffic on the transport path
prior to the ring ingress node or beyond the egress nodes may be
protected by some other mechanism.
1.2. Terminology and Notation
The terminology used in this document is based on the terminology
defined in the MPLS-TP framework documents:
o MPLS-TP Framework[TPFwk]
o MPLS-TP OAM Framework[OAMFwk]
o MPLS-TP Survivability Framework[SurvivFwk]
In addition, we describe the use of the label stack in connection
with the redirecting of data packets by the protection mechanism.
The following syntax will be used to describe the contents of the
label stack:
1. The label stack will be enclosed in square brackets ("[]")
2. Each level in the stack will be separated by the '|' character
3. The bottom of the stack will be denoted by the string "BOS"
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4. The label of an ingress LSP will be denoted by the string "LI"
and the label of the egress LSP will be denoted by the string
"LE"
5. The label for a SPME will be denoted by Px(y) where x and y are
LSR identifiers and the intention is to the label for LSR-x to
transmit to LSR-y over the SPME.
For example - the label stack [PB(G)|LI|BOS] denotes a stack whose
top-label is the SPME label for LSR-B to transmit the data packet to
LSR-G, the packet originated from a LSP that arrived at the ring with
label LI.
1.3. Contributing Authors
Akira Sakurai (NEC), Rolf Winter (NEC)
2. P2P Ring Protection
Classically there are two protection architecture mechanisms for ring
topologies, based on SDH specifications [G.841], that have been
proposed in various forums to perform recovery of a topological ring
network - "wrapping" and "steering". The following sub-sections will
examine these two mechanisms.
2.1. Wrapping
Wrapping is defined as a "local" protection architecture. This
mechanism is local to the LSRs that are neighbors to the detected
fault. When a fault is detected (either a link or node failure), the
neighboring LSR can identify that the fault would prevent forwarding
of the data along the data path. Therefore, in order to continue the
data along the path, there is need to "wrap" all data traffic around
the ring, on an alternate data path, until arriving at the LSR that
is on the opposite side of the fault. When this LSR also detects
that there is a fault condition on the LSP, it can identify that the
data traffic that is arriving on the alternate (protecting) data path
is intended for the "broken" LSP. Therefore, again taking a local
decision, can wrap the data back onto the normal working path until
the egress from the ring segment.
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____
=========>/ LSR\
* \__B_/ *
* @@@@@@@# *
* @ @# *
___* @ @# *___
/LSR\ @ @#/LSR\
\_C_/ @ #\_A_/
* @ # *
* @ XX
_*_ @ #*_
/LSR\@ /LSR\
\_D_/@ @\_F_/
* @ @#*
* @ @@#*
* @@____@##*
*/ LSR\*
\__E_/========>
===> connected LSP *** physical link
### working path @@@ wrapped data path
Figure 1: Wrapping protection for p2p path
In this figure we have a ring with a LSP that enters the ring at
LSR-B and exits at LSR-E. The normal working path follows through
B-A-F-E. If a signal fault is detected on the link 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 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 egress node. In this
protection scheme, the traffic will follow the path -
B-A-B-C-D-E-F-E.
This protection scheme is simple in the sense that there is no need
for coordination between the different LSR in the ring - only 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
the far-end). Coordination would only be needed to maintain co-
routed bidirectional traffic even in cases of a unidirectional fault
condition.
The following considerations should be taken into account when
considering use of wrapping protection:
o Detection of loss-of-continutiy or misconnectivity, should be
performed at the link level and/or per LSR when using node-level
protection. Configuration of the protection being performed (i.e.
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link protection or node protection) needs to be performed
a-priori, since the configuration of the proper protection path is
dependent upon this decision.
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
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)
alternate paths.
o When wrapping, the data is transmitted over some of the links
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
transmitted E-->F and F-->E. This means that there is additional
bandwith needed for this protection.
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
the first fault location. This is based on the need for wrapping
to connect between the neighbors of the fault location, and this
is not possible in the segmented ring.
o The resource allocation for the alternate-paths could be
problematic, since most of these alternate paths will not be used
simultaneously. One possibility could be to allocate '0'
resources and depend on the NMS to allocate the proper resources
around the ring.
o Wrapping also involves greater latency in delivering the packets,
as a result of traversing the entire ring. This could be very
restrictive for large rings.
2.2. Steering
The second common scheme for ring protection redirects the traffic
from the ingress point to the alternate route around the ring to the
egress point. This is illustrated in Figure 2, where if a Signal
Fault is detected on the working path (B-A-F), then the traffic would
be redirected by B to the alternate path (i.e. B-C-D-E-F).
This mechanism is similar to linear 1:1 protection [SurvivFwk]. The
two paths around the ring act as the working and recovery paths.
There is need to communicate to the ingress node the need to switch
over to the recovery path and there is a need to coordinate the
switchover between the two end-points of the protected domain.
The following considerations must be taken into account regarding the
steering architecture:
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o It is necessary for the ingress node to be "informed" of the fault
condition in order to perform the protection switching.
o The process of "informing" the ingress node adds to the latency of
the protection switching process, after the detection of the fault
condition.
o While there is no need for double bandwidth for the data path,
there is the necessity for the ring to maintain enough capacity
for all of the data in both directions around the ring.
2.3. P2P ring protection with SPME
The MPLS-TP Framework document [TPFwk] defines a Sub-Path Maintenance
Entity (SPME) construct that can be defined between any two LSRs of a
MPLS-TP LSP. For MPLS-TP purposes, such a SPME may be configured as
a bidirectional co-routed path. A SPME may be used to aggregate all
LSP traffic that traverses the segment (from ingress LSR to egress
LSR) that is delineated by the SPME. This SPME may be monitored
using the OAM mechanisms as specified in the MPLS-TP OAM Framework
document [OAMFwk].
When defining a MPLS-TP ring as a protection domain, there is a need
to design a protection mechanism that protects all the LSPs that
cross the MPLS-TP ring. For this purpose, we associate a (working)
SPME with the segment of the transport path that traverses the ring.
In addition, we configure an alternate (protecting) SPME that
traverses the ring in the opposite direction around the ring. The
exact selection of the two SPMEs is dependent on the type of
transport path and protection that is being implemented and will be
detailed in the following sections.
Based on this architectural configuration for ring protection, it is
possible to restrict the number of alternate paths needed to protect
the data traversing the ring. Similarly, we can minimize the number
of OAM sessions needed to monitor the data traffic of the ring - by
monitoring the SPMEs, rather than monitoring each individual LSP.
The following figure shows a MPLS-TP ring that is part of a larger
MPLS-TP network. The ring could be used as a network segment that
may be traversed by numerous LSPs. In particular, the figure shows
that for all LSPs that connect to the ring at LSR-B and exit the ring
from LSR-F, we configure two SPME through the ring (the first SPME
traverses along B-A-F, and the second SPME traverses B-C-D-E-F).
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____
=========>/ LSR\
* \__B_/ *
* @ # *
* @ # *
__* @ # *___
/LSR\ @ #/LSR\
\_C_/ @ #\_A_/
* @ # *
* @ #*
_*_ @ #*_
/LSR\@ /LSR\========>
\_D_/@ \_F_/
* @ @*
* @ @*
* @@____@@*
*/ LSR\*
\__E_/
===> connected LSP *** physical link
### primary SPME @@@ secondary SPME
Figure 2: A MPLS-TP ring
In all of the following subsections, we use 1:1 linear protection
[SurvivFwk] [LinProtect] to perform protection switching and
coordination when a signal fault is detected. The actual
configuration of the SPMEs used may change dependent upon the choice
of methodology and this will be detailed in the following sections.
However, in all of these configurations the mechanism will be to
transmit the data traffic on the primary SPME, while using OAM
functionlity to detect signal fault conditions on either the primary
or the secondary SPME. If a signal fault is detected on the primary
SPME, then the mechanism described in [LinProtect] shall be used to
coordinate a switch-over of data traffic to the secondary SPME.
Assuming that the SPME is implemented as an hierarchial LSP, packets
that arrive at LSR-B with a label stack [L1|BOS] will have the SPME
label pushed at LSR-B (i.e. the packet will arrive at LSR-A with a
label stack of [PA(F)|L1|BOS], arrive at LSR-F with [PF(F)|L1|BOS]).
The SPME label will be popped by LSR-F and the LSP label will be
treated appropriately at LSR-F and forwarded along the LSP. This
scenario is true for all LSP that are aggregated by this primary
SPME.
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2.3.1. Path SPME for Steering
A p2p SPME that traverses part of a ring has two Maintenance Entity
Group End Points (MEPs), each one acts as the ingress and egress in
one direction of the bidirectional SPME. Since the SPME is
traversing a ring we can take advantage of another characteristic of
a ring - there is always an alternative path between the two MEPs,
traversing the ring in the opposite direction. This alternative SPME
can be defined as the protection path for the working path that is
configured as part of the LSP and defined as a SPME.
For each pair of SPMEs that are defined in this way, it is possible
to verify the connectivity and continuity by applying the MPLS-TP OAM
functionality to both the working and recovery SPME. If a
discontinuity or mis-connectivity is detected then the MEPs will
become aware of this condition, and could perform a protection switch
of all LSPs to the alternate, recovery SPME.
This protection mechanism is identical to application of 1:1 linear
protection[SurvivFwk] [LinProtect] to the pair of SPMEs. Under
normal conditions, all LSP data traffic will be transmitted on the
working SPME. If the linear protection is triggered, by either the
OAM indication, an other fault indication trigger, or an operator
command, then the MEPs will select the recovery SPME to transmit all
LSP data packets.
The recovery SPME will continue to transmit the data packets until
the stable recovery of the fault condition. Upon recovery, the
ingress LSR could switch traffic back to the working SPME, if the
protection domain is configured for revertive behavior.
The control of the protection switching, especially for cases of
operator commands, would be covered by the protocol defined in
[LinProtect].
2.3.2. Wrapping with segment based SPME
It is possible to use the SPME mechanism to perform segment-based
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,
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
show the primary SPME that connects LSR-A & LSR-F over a segment
connection, and the secondary SPME that connects these same LSRs by
traversing the ring in the opposite direction.
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____
/ LSR\
* \__B_/ *
* @ @ *
* @ @ *
__* @ @ *___
/LSR\ @ @/LSR\
\_C_/ @ \_A_/
* @ #*
* @ #*
_*_ @ #*_
/LSR\@ /LSR\
\_D_/@ \_F_/
* @ @*
* @ @*
* @@____@@*
*/ LSR\*
\__E_/
*** physical link
### primary SPME @@@ secondary SPME
Figure 3: Segment SPMEs
By applying OAM monitoring of these two SPME (at each LSR), it is
possible to affect a wrapping protection mechanism for the LSP
traffic that traverses the ring. The LSR on either side of the
segment would identify that there is a fault condition on the link
and redirect all LSP traffic to the secondary SPME. The traffic
would traverse the ring until arriving at the neighboring (relative
to the segment) LSR. At this point, the LSP traffic would be
redirected onto the original LSP, quite likely over the neighboring
SPME.
Following the progression of the label stack through this switching
operation:
1. The data packet arrives at LSR-A with label stack [LI|BOS] (i.e.
top label from the LSP and bottom-of-stack indicator)
2. In the normal case (no switching), LSR-A forwards the packet with
label stack [PA1(F)|LE|BOS] (i.e. swap the label for the LSP, to
be acceptable to the SPME egress, and push the label for the
primary SPME from LSR-A to LSR-F).
3. When switching is in-effect, LSR-A forwards the packet with label
stack [PA2(F)|LE|BOS] (i.e. LSR-A pushed the label for the
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secondary SPME from LSR-A to LSR-F, after swapping the label of
the lower level LSP).
4. When the packet arrives at LSR-F, it will pop the SPME label,
process the LSP label, and forward the packet to the next point,
possibly pushing a SPME label if the next segment is likewise
protected.
2.3.3. Wrapping node protection
Implementation of protection at the node level would be similar to
the mechanism described in the previous sub-section. The difference
would be in the SPMEs that are used. For node protection, the
primary SPME would be configured between the two LSR that are
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
configured between these same nodes, going around the ring (see
secondary SPME in Figure 4).
____
/ LSR\
* \__B_/ *
* @ @ *
* @ @ *
__* @ @ *___
/LSR\ @ @/LSR\
\_C_/ @ \_A_/
* @ #*
* @ #*
_*_ @ #*_
/LSR\@ /LSR\
\_D_/@ \_F_/
* @ # *
* @ # *
* @@____ # *
*/ LSR\#*
\__E_/
*** physical link
### primary SPME @@@ secondary SPME
Figure 4: Node-protection SPMEs
The protection mechanism would work similarly - based on 1:1 linear
protection [SurvivFwk], triggered by OAM functions on both SPMEs, and
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
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continuation of the LSP at the egress MEP (e.g. LSR-E in the figure)
of the SPME.
2.3.4. Wrapping for link and node protection
In the different types of wrapping presented in sections 2.3.2 and
2.3.3, there is a limitation that the protection mechanism must a
priori decide whether it is protecting for link or node failure. In
addition, the neighboring LSR, that detects the fault, cannot readily
differentiate between a link failure or a node failure.
It is possible to combine the link protection mechanism presented in
section 2.3.2 with the protection mechanism of section 2.3.3 to give
more complete coverage. For each segment, we configure both a
secondary link protection SPME as well as a two secondary node
protection SPME, i.e. one for each direction of the bidirectional
segment SPME (see Figure 5). When a protection switch is triggered,
the ingress LSR of the segment will examine the packet ring
destination. Only if this destination is for the LSR connected to
the "secondary link" SPME, then the packets will be wrapped onto this
secondary SPME. For all other cases, the data packets will be
wrapped onto the "secondary node" SPME. In all cases the egress LSR
for the secondary SPME will wrap the data traffic back onto the
working LSP/SPME.
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____
/ LSR\
* \__B_/ *
* @+$ +@ *
* @+$ +@ *
__* @+$ +@ *___
/LSR\ @+$ +@/LSR\
\_C_/ @+$ \_A_/
* @+$ #*
* @+$ #*
_*_ @+$ #*_
/LSR\@+$ /LSR\
\_D_/@+$ \_F_/
* @+$ $+*
* @+$ $+*
* @++$+$+$+*
*/ LSR\*
\__E_/
*** physical link
### primary SPME @@@ secondary node#1 SPME
$$$ secondary node#2 SPME +++ secondary segment SPME
Figure 5: Segment & Node protection SPMEs
2.4. Analysis of p2p protection
Analyzing the mechanisms described in the above subsections we can
point to the following observations (based on a ring with N nodes):
o Number of SPME that need to be configured - for path SPME (sec
2.3.1) = O(2N^2) [two SPME from each ingress LSR to each other
node in the ring], for segment SPME (sec 2.3.4) = O(4N) [four SPME
for each link in the ring]
o Number of OAM sessions at each node - for path SPME = O(2N), for
segment SPME = 4
o Bandwidth requirements - for path SPME: single bandwidth at each
link, for segment SPME: double bandwidth at links that are between
ingress and wrapping node and between second wrapping node and
egress.
o Special considerations - for path SPME: latency of OAM detection
of fault condition by ingress MEP [using Alarm-reporting could
optimize over using CC-V only], for segment SPME: need to examine
data packet ring destination before selecting bypass SPME.
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3. P2MP protection
[TPReqs] requires that ring protection must provide protection for
unidirectional point-to-multipoint paths through the ring. Ring
topologies provide a ready platform for supporting such data paths.
A p2mp LSP in an MPLS-TP ring would be characterized by a single
ingress LSR and multiple egress LSRs. The following sub-sections
will present methods to address the protection of the ring-based
sections of these LSP.
3.1. Wrapping for p2mp LSP
When protecting a p2mp ring data path using the wrapping
architecture, the basic operation is similar to the description
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
transported to all of the egress points.
It is possible to optimize the performance of the wrapping mechanism
when applied to p2mp LSPs by exploiting the topology of ring
networks.
This improved mechanism, which we call Ring Optimized Multicast
Wrapping (ROM-Wrapping), behaves much the same as classical wrapping.
There is one difference - rather than configuring the recovery LSP
between the end nodes of a failed link (link protection) or between
the upstream and downstream node of a failed node (node protection),
the improved mechanism configures a recovery p2mp LSP from the
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
(working) LSP (A-B-[C]-[D]-E-[F]) and one possible backup
(protection) LSP. 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
points (at nodes F, D, & C) complimentary to the broken working data
path.
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|
|
V Ingress
___ _V_ ___
/LSR\ /LSR\**************/LSR\
<@@\_F_/@@@@@@@@@@@@@\_A_/@@@@@@@@@@@@@@\_B_/
@ * *
@ * *
@ * XXXX Failure
@ * *
@_* ___ __*
/LSR\*************/LSR\**************/LSR\
\_E_/@@@@@@@@@@@@@\_D_/@@@@@@@@@@@@@@\_C_/
@ @
@ @
V V
*** working LSP @@@ protection LSP
Figure 6: P2MP ROM Wrapping
Using this mechanism, there is a need to configure a particular
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
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.
Protected LSP: A->B->[C]->[D]->E->[F]
---- LINK/NODE PROTECTION----
A's Backup: A->[F]->E->[D]->[C]
B's Backup: B->A->[F]->E->[D]->[C]
C's Backup: C->B->A->[F]->E->[D]
D's Backup: 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
mechanism, as opposed to the SPME based protection mechanisms that
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
applications that are based on p2mp distribution schemes. While ROM-
Wrapping can be applied to any network topology, it is particularly
efficient for interconnected ring topologies.
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3.1.1. Comparison of Wrapping and ROM-Wrapping
It is possible to compare the Wrapping and the ROM-Wrapping
mechanisms in different aspects, and show some improvements offered
by ROM-Wrapping.
When configuring the protection LSP for Wrapping it is necessary to
configure for a specific failure: link protection or node protection.
If the protection method is configured to protect node failures but
the actual failure affects a link, this could result in failing to
deliver traffic to the node, when it should be possible to.
ROM-Wrapping however does not have this limitation, because there is
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.
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
correctly until node D, and will be blocked at this point. However,
all egress nodes upto the failure will be able to deliver the traffic
properly.
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
is detected on link B-C, the following table lists the set of nodes
traversed by the data in the protection:
Basic Wrapping:
A-B B-A-F-E-D-C [C]-[D]-E-[F]
"Upstream" segment backup path "Downstream" segment
with respect to the with respect to the
failure failure
ROM Wrapping:
A-B B-A-[F]-E-[D]-[C] ..
"Upstream" segment backup path
with respect to the
failure
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
is significantly higher than the number of hops crossed by the
traffic when ROM-Wrapping is used. Generally, the number of hops for
basic Wrapping is always higher or at least equal compared to ROM-
Wrapping. This implies a certain waste of bandwidth on all links
that are crossed in both directions.
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Considering the ring network previously seen, it is possible to do
some bandwidth utilization considerations. The protected LSP is set
up from A to F clockwise and an M Mbps bandwidth is reserved along
the path. All the protection LSPs are preprovisioned
counterclockwise, each of them may also have reserved bandwidth M.
These LSPs share the same bandwidth in a SE (Shared Explicit) [RSVP]
style.
The bandwidth reserved counterclockwise is not used when the
protected LSP is properly working and could, in theory, be used for
extra traffic [RFC4427]. However, it should be noted that [TPReqs]
does not require support of such extra traffic.
The two recovery mechanism require different protection bandwidths.
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
from the ingress node to the node performing the actual wrapping
utilize M bandwidth in both directions, while all other links utilize
M bandwidth only in the counterclockwise direction.
Consider the case of a failure detected on link B-C as shown in
Figure 6. The following table lists the bandwidth utilization on
each link (in units equal to M), for each recovery mechanism and for
each direction (CW=clockwise, CCW=counterclockwise).
+----------+----------+--------------+
| | Wrapping | ROM-Wrapping |
+----------+----------+--------------+
| Link A-B | CW+CCW | CW+CCW |
| Link A-F | CCW | CCW |
| Link F-E | CW+CCW | CCW |
| Link E-D | CW+CCW | CCW |
| Link D-C | CW+CCW | CCW |
+----------+----------+--------------+
3.1.2. Multiple Failures Comparison
A further comparison between Wrapping and ROM-Wrapping can be done
with respect to their ability to react to multiple failures. The
wrapping recovery mechanism does not have the ability to recover from
multiple failures on a ring network, while ROM-Wrapping is able to
recover, from some multiple failures.
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
from the failure because B, upon detecting the failure, has no
alternative paths to reach C. The whole P2MP traffic is lost. The
ROM-Wrapping mechanism is able to partially recover from the failure,
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because the backup P2MP LSP to node F and node D is correctly set up
and continues delivering traffic.
3.2. Steering for p2mp paths
To take advantage of the ring topology in protecting the data traffic
over p2mp LSPs, we can configure two p2mp unidirectional SPME from
each node on the ring that traverse the ring in both directions.
These SPME will be configured with an egress at each ring node. For
every LSP that enters the ring at a given node the traffic will be
sent through one of these SPME (the working SPME) - pushing the SPME
label onto the packet label stack. Each LSR on the ring will forward
a copy of the packet along the SPME, but check the packet by popping
the SPME label and examining the underlying LSP label. If this LSR
is an egress point for the LSP it will treat the data packet
appropriately. If the LSR is not an egress point for the LSP, the
packet will be silently dropped.
^ ^ ^
_|_ _|_ _|_
----->/LSR\********/LSR\********/LSR\
\_A_/========\_B_/========\_C_/
+* <+++++++++*||
+* +*||
+* +*||
+* +*||
+*_ ++++++++ +++++++++*||
/LSR\********/LSR\********/LSR\
\_F_/<=======\_E_/========\_D_/
| | |
V V V
---> connected LSP *** physical link
=== primary p2mp SPME +++ secondary p2mp SPME
Figure 7: P2MP SPMEs
Using this SPME architecture, we define a 1+1 linear protection
mechanism [SurvivFwk] for all protected data traffic traversing the
MPLS-TP ring. The data for a particular p2mp LSP is transmitted on
both the working and recovery SPME, using a permanent bridge. While
each node detects that there is connectivity from the ingress point,
it continues to select the data that is coming from the working path.
If a particular node stops receiving the connectivity messages from
the working path SPME, it identifies that it must switch its selector
to read the data packets from the recovery SPME.
Figure 7 shows the two unidirectional p2mp SPME that are configured
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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 recovery SPME. Applying 1+1 linear protection to these two
SPME - a packet that arrives at LSR-A with a label stack [LI|BOS]
will be forwarded on the clockwise SPME with a label stack [PA1(F)|
LE|BOS] and concurrently on the counter-clockwise SPME with a label
stack [PA2(B)|LE|BOS].
Assume that the LSP "LI" has egress points at LSR-C & LSR-E. When
the packet arrives at LSR-B, LSR-D, and LSR-F, the LSR will forward
the data packet to continue along the SPME, e.g. at LSR-D the packet
will be forwarded with label stack [PD1(F)|LE|BOS]. But in addition
LSR-D will retain a copy of the packet, pop the SPME label and
examine the underlying LSP label. Since LSR-D is not an egress point
for LSP-LI, the packet will be dropped. At LSR-C the data packet
will be forwarded along the SPME (with label stack [PC1(F)|LE|BOS],
but when the retained copy is examined (after popping the SPME label)
it will determine that the packet is intended for this LSR as an
egress point and switch the LSP label forwarding this packet with the
label stack [LE|BOS].
If a fault condition is detected, then some of the nodes will cease
to receive the packets from the clockwise (working) SPME. These LSR
should then begin to switch their selector bridge to accept the data
packets from the counter-clockwise SPME (i.e. A-F-E-D-C-B), using a
similar logic to that presented in the previous paragraph.
This architecture has the added advantages that there is no need for
the ingress node to identify the existence of the misconnectivity,
and there is no need for a return path from the egress points to the
ingress.
4. Coordination protocol
The Survivability Framework [SurvivFwk] indicates that there is a
need to coordinate protection switching between the end-points of a
protected bidirectional domain. The coordination is necessary for
particular cases, in order to maintain the co-routed nature of the
bidirectional transport path. The particular cases where this
becomes necessary include cases of unidirectional fault detection and
use of operator commands.
By using the same mechanisms defined in [LinProtect], for linear
protection, to apply for ring protection we are able to gain a
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consistent solution for this coordination between the end-points of
the protection domain. The Protection State Coordination Protocol
that is specified in [LinProtect] provides coverage for all the
coordination cases, including support for operator commands, e.g.
Forced-Switch.
5. Interconnected rings
The Requirements document [TPReqs] states that the ring protection
must support a single ring that may be interconnected to other rings.
In addition, traffic that traverses a number of rings within a
network of interconnected rings must be protected even if the
interconnection nodes and links fail.
When interconnecting rings in a network there are two common
interconnection schemes:
o Dual-node interconnect - when the interconnected rings are
interconnected by two nodes from each ring (see Figure 8)
o Single-node interconnect - when the connection between the
interconnected rings are through a single node (see Figure 9)
The protection schemes presented in Section 2 are capable of
protecting each interconnected ring as a separate entity independent
of the other rings in the network. This protects the traffic that
traverses the entire network, as each ring will continue to transfer
the traffic to the interconnection points, and from there to the next
ring.
When the interconnection nodes or links fail, there is the need to
protect these connection points. Therefore, it should be noted that
in the case of single-node interconnect the interconnection node
(LSR-A in Figure 9) is a single-point of failure and such an
interconnection scheme should be avoided. In the case of the dual-
node interconnect scheme in Figure 8, the connection point over LSR-
A<-->LSR-6 and LSR-F<-->LSR-5 could use basic linear protection as
defined in [SurvivFwk] and [LinProtect] .
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____ ___
/ LSR\ /LSR\
* \__B_/ * *\_1_/*
* * * *
* * * *
___* *___ _*_ * ___
/LSR\ /LSR\****/LSR\ /LSR\
\_C_/ \_A_/ \_6_/ \_2_/
* Ring #1 * * Ring #2 *
* * * *
_*_ _*_ _*_ _*_
/LSR\ /LSR\ /LSR\ /LSR\
\_D_/ \_F_/****\_5_/ \_3_/
* * * *
* * * *
* ____ * * ____ *
*/ LSR\* */LSR \*
\__E_/ \__4_/
*** physical link
Figure 8: Dual-node interconnected rings
____ ___
/ LSR\ /LSR\
* \__B_/ * *\_1_/*
* * * *
* * * *
___* *___ * * ___
/LSR\ /LSR\* /LSR\
\_C_/ \_A_/ \_2_/
* Ring #1 * * Ring #2 *
* * * *
_*_ _*_ * ___ _*_
/LSR\ /LSR\ */LSR\ /LSR\
\_D_/ \_F_/ \_5_/ \_3_/
* * * *
* * * *
* ____ * *___ *
*/ LSR\* /LSR\*
\__E_/ \_4_/
*** physical link
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Figure 9: Single-node interconnected rings
6. Conclusions and Recommendations
Based on the use of the Path Segment Tunnel construct, defined in
[TPFwk] and [OAMFwk], it is possible to define a protection mechanism
for MPLS-TP rings that is based on linear protection [SurvivFwk].
This mechanism would be based on 1:1 linear protection for
bidirectional or unidirectional p2p data paths, and 1+1 linear
protection for unidirectional p2mp paths. It has been shown that
this protection architecture and mechanism fulfills the criteria
defined in [TPReqs] for justification of designing a specific
protection mechanism for a ring topology.
It has also been shown that basing the ring protection on the
existing linear protection mechanisms defined in [SurvivFwk] and
[LinProtect], the operator has a choice of using either the wrapping
or steering methodology for protection of both p2p and p2mp data
traffic. In addition, there is no need to define any new
coordination protocol to complete this protection, instead depending
upon the OAM functionality [outlined in [OAMFwk] and specified in
various documents] and the coordination protocol specified for linear
protection in [LinProtect].
7. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
8. Security Considerations
To be added in future version.
9. Acknowledgements
The authors would like to thank all members of the teams (the Joint
Working Team, the MPLS Interoperability Design Team in IETF and the
T-MPLS Ad Hoc Group in ITU-T) involved in the definition and
specification of MPLS Transport Profile.
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10. Informative References
[FRR] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Exensions to RSVP-TE for LSP Tunnels", RFC 4090, May 2005.
[TPReqs] Niven-Jenkins, B., Nadeau, T., and C. Pignataro,
"Requirements for the Transport Profile of MPLS",
RFC 5654, April 2009.
[TPFwk] Bocci, M., Bryant, S., Frost, D., and L. Levrau, "MPLS-TP
Framework", ID draft-ietf-mpls-tp-framework-12, May 2010.
[OAMFwk] Niven-Jenkins, B., Allan, D., and I. Busi, "MPLS-TP OAM
Framework", ID draft-ietf-mpls-tp-oam-framework-06,
May 2010.
[SurvivFwk]
Sprecher, N. and A. Farrel, "MPLS-TP Survivability
Framework", ID draft-ietf-mpls-tp-survive-fwk-06,
June 2010.
[LinProtect]
Sprecher, N., Bryant, S., van Helvoort, H., Fulignoli, A.,
and Y. Weingarten, "MPLS-TP Linear Protection",
ID draft-ietf-mpls-tp-linear-protection-02, October 2009.
[RSVP] Braden, R., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) - Functional
Specifications", RFC 2205, September 1997.
[RFC4427] Mannie, E. and D. Papadimitriou, "Recovery (Protection and
Restoration) Terminology for GMPLS", RFC 4427, March 2006.
[G.841] "Types and characteristics of SDH network protection
architectures", ITU-T G.841, October 1998.
Authors' Addresses
Yaacov Weingarten
Nokia Siemens Networks
3 Hanagar St. Neve Ne'eman B
Hod Hasharon, 45241
Israel
Phone: +972-9-775 1827
Email: yaacov.weingarten@nsn.com
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Stewart Bryant
Cisco
United Kingdom
Email: stbryant@cisco.com
Nurit Sprecher
Nokia Siemens Networks
3 Hanagar St. Neve Ne'eman B
Hod Hasharon, 45241
Israel
Email: nurit.sprecher@nsn.com
Danielle Ceccarelli
Ericsson
Via A. Negrone 1/A
Genova, Sestri Ponente
Italy
Email: daniele.ceccarelli@ericsson.com
Diego Caviglia
Ericsson
Via A. Negrone 1/A
Genova, Sestri Ponente
Italy
Email: diego.caviglia@ericsson.com
Francesco Fondelli
Ericsson
Via A. Negrone 1/A
Genova, Sestri Ponente
Italy
Email: francesco.fondelli@ericsson.com
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Marco Corsi
Altran
Via A. Negrone 1/A
Genova, Sestri Ponente
Italy
Email: marco.corsi@altran.it
Bo Wu
ZTE Corporation
4F,RD Building 2,Zijinghua Road
Nanjing, Yuhuatai District
P.R.China
Email: wu.bo@zte.com.cn
Xuehui Dai
ZTE Corporation
4F,RD Building 2,Zijinghua Road
Nanjing, Yuhuatai District
P.R.China
Email: dai.xuehui@zte.com.cn
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