Supporting Shared Mesh Protection in MPLS-TP Networks
draft-pan-shared-mesh-protection-03
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| Authors | Ping Pan , Rajan Rao , Biao Lu , Luyuan Fang , Andrew G. Malis , Fatai Zhang , Sam Aldrin , Fei Zhang , Mohana Saty Singamsetty | ||
| Last updated | 2011-10-31 (Latest revision 2011-07-11) | ||
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draft-pan-shared-mesh-protection-03
IETF Ping Pan, Ed.
Internet Draft Rajan Rao
Biao Lu
(Infinera)
Luyuan Fang
(Cisco)
Andrew G. Malis
(Verizon)
Fatai Zhang
Sam Aldrin
(Huawei)
Fei Zhang
(ZTE)
Mohana Singamsetty
(Tellabs)
Expires: January 31, 2012 October 31, 2011
Supporting Shared Mesh Protection in MPLS-TP Networks
draft-pan-shared-mesh-protection-03.txt
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Abstract
Shared mesh protection is a common protection and recovery mechanism
in transport networks, where multiple paths can share the same set
of network resources for protection purposes.
In the context of MPLS-TP, it has been explicitly requested as a
part of the overall solution (Req. 67, 68 and 69 in RFC5654 [1]).
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It's important to note that each MPLS-TP LSP may be associated with
transport network resources. In event of network failure, it may
require explicit activation on the protecting paths before switching
user traffic over.
In this memo, we define a lightweight signaling mechanism for
protecting path activation in shared mesh protection-enabled MPLS-TP
networks.
Table of Contents
1. Introduction...................................................3
2. Conventions used in this document..............................4
2.1. Acronyms..................................................4
2.2. Definitions and Terminology....Error! Bookmark not defined.
3. Solution Overview..............................................5
3.1. Protection Switching......................................7
3.2. Operation Overview........................................8
4. SMP Message and Action Definition..............................9
4.1. Protection Switching Control (PSC) Logic..................9
4.2. SMP Action Types.........................................11
4.3. PSC Signal to SMP Action Mapping.........................12
5. Protocol Definition...........................................13
5.1. Message Encapsulation....................................14
5.2. Reliable Messaging.......................................15
5.3. Message Scoping..........................................15
6. Processing Rules..............................................16
6.1. Enable a protecting path.................................16
6.2. Disable a protecting path................................17
6.3. Get protecting path status...............................17
6.4. Preemption...............................................17
7. Security Consideration........................................18
8. IANA Considerations...........................................18
9. Normative References..........................................18
10. Acknowledgments..............................................18
1. Introduction
Shared mesh protection (SMP) is a common traffic protection
mechanism in transport networks, where multiple paths can share the
same set of network resources for protection purposes.
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In the context of MPLS-TP, it has been explicitly requested as a
part of the overall solution (Req. 67, 68 and 69 in RFC5654 [1]).Its
operation has been further outlined in Section 4.7.6 of MPLS-TP
Survivability Framework [2].
It's important to note that each MPLS-TP LSP may be associated with
transport network resources. In event of network failure, it may
require explicit activation on the protecting paths before switching
user traffic over.
In this memo, we define a lightweight signaling mechanism for
protecting path activation in shared mesh protection-enabled MPLS-TP
networks.
Here are the key design goals:
1. Fast: The protocol is to activate the previously configured
protecting paths in a timely fashion, with minimal transport and
processing overhead. The goal is to support 50msec end-to-end
traffic switch-over in large transport networks.
2. Reliable message delivery: Activation and deactivation operation
have serious impact on user traffic. This requires the protocol to
adapt a low-overhead reliable messaging mechanism. The activation
messages may either traverse through a "trusted" transport
channel, or require some level of built-in reliability mechanism.
3. Modular: Depending on deployment scenarios, the signaling may need
to support functions such as preemption, resource re-allocation
and bi-directional activation in a modular fashion.
2. Conventions used in this document
Here are some of the conventions used in this document. The key
words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
2.1. Acronyms
This draft uses the following acronyms:
SMP Shared Mesh Protection
LO Lockout of protection
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DNR Do not revert
FS Forced Switch
SF Signal Fail
SD Signal Degrade
MS Manual Switch
NR No Request
WTR Wait-to-Restore
EXER Exercise
RR Reverse Request
ACK Acknowledgement
NACK Negative Acknowledgement
G-ACh Generic Associated Channel
MPLS-TP Transport Profile for MPLS
3. Solution Overview
In this section, we describe the SMP operation in the context of
MPLS-TP networks, and outline some of the relevant definitions.
We refer to the figure below for illustration:
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----- B ------- C ----
/ \
/ \
A D
\ /
\ /
==== E === F === G ===
/ \
/ \
H K
\ /
\ /
----- I ------- J ----
Working paths: X = {A, B, C, D}, Y = {H, I, J, K}
Protecting paths: X' = {A, E, F, G, D}, Y' = {H, E, F, G, K}
The links between E, F and G are shared by both protecting paths.
All paths are established via MPLS-TP control plane prior to network
failure.
All paths are assumed to be bi-directional. An edge node is denoted
as a headend or tailend for a particular path in accordance to the
path setup direction.
Initially, the operators setup both working and protecting paths.
During setup, the operators specify the network resources for each
path.
The working path X and Y will configure the appropriate resources on
the intermediate nodes, however, the protecting paths, X' and Y',
will reserve the resources on the nodes, but won't occupy them.
Depending on network planning requirements (such as SRLG), X' and Y'
may share the same set of resources on node E, F and G. The resource
assignment is a part of the control-plane CAC operation taking place
on each node.
At some time, link B-C is cut. Node A will detect the outage, and
initiate activation messages to bring up the protecting path X'. The
intermediate nodes, E, F and G will program the switch fabric and
configure the appropriate resources. Upon the completion of the
activation, A will switch the user traffic to X'.
The operation may have extra caveat:
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1. Preemption: Protecting paths X' and Y' may share the same
resources on node E, F or G due to resource constraints. Y' has
higher priority than that of X'. In the previous example, X' is
up and running. When there is a link outage on I-J, H can
activate its protecting path Y'. On E, F or G, Y' can take over
the resources from X' for its own traffic. The behavior is
acceptable with the condition that A should be notified about
the preemption action.
2. Over-subscription (1:N): A unit of network resource may be
reserved by one or multiple protecting paths. In the example,
the network resources on E-F and F-G are shared by two
protecting paths, X' and Y'. In deployment, the over-
subscription ratio is an important factor on network resource
utilization.
3.1. Protection Switching
The entire activation and switch-over operation need to be within
the range of milliseconds to meet customer's expectation [1]. This
section illustrates how this may be achieved on MPLS-TP-enabled
transport switches. Note that this is for illustration of protection
switching operation, not mandating the implementation itself.
The diagram below illustrates the operation.
+---------------+
Control | MPLS-TP | Control
<=== Signaling ====| Control Plane |=== Signaling ===>
+---------------+
/ \
/ \ (MPLS label assignment)
/ \
/ \
+-------+ +------+ +-------+
Activation |Line | |Switch| |Line | Activation
<=== Messages ===|Module |===|Fabric|===|Module |=== Messages ===>
+-------+ +------+ +-------+
Typical MPLS-TP user flows (or, LSP's) are bi-directional, and setup
as co-routed or associated tunnels, with a MPLS label for each of
the upstream and downstream traffic. On this particular type of
transport switch, the control-plane can download the labels to the
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line modules. Subsequently, the line module will maintain a label
lookup table on all working and protecting paths.
Upon the detection of network failure, the headend nodes will
transmit activation messages along the MPLS LSP's. When receiving
the messages, the line modules can locate the associated protecting
path from the label lookup table, and perform activation procedure
by programming the switching fabric directly. Upon its success, the
line module will swap the label, and forward the activation messages
to the next hop.
In summary, the activation procedure involves efficient path lookup
and switch fabric re-programming.
To achieve the tight end-to-end switch-over budget, it's possible to
implement the entire activation procedure with hardware-assistance
(such as in FPGA or ASIC).
The activation messages are encapsulated with a MPLS-TP Generic
Associated Channel Header (GACH) [3]. Detailed message encoding is
explained in Section 6.
3.2. Operation Overview
To achieve high performance, the activation procedure is designed to
be simple and straightforward on the network nodes.
In this section, we describe the activation procedure using the same
figure shown before:
----- B ------- C ----
/ \
/ \
A D
\ /
\ /
==== E === F === G ===
/ \
/ \
H K
\ /
\ /
----- I ------- J ----
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Working paths: X = {A, B, C, D}, Y = {H, I, J, K}
Protecting paths: X' = {A, E, F, G, D}, Y' = {H, E, F, G, K}
Upon the detection of working path failure, the edge nodes, A, D, H
and K may trigger the activation messages to activate the protecting
paths, and redirect user traffic immediately after.
We assume that there is a consistent definition of priority levels
among the paths throughout the network. At activation time, each
node may rely on the priority levels to potentially preempt other
paths.
When the nodes detect path preemption on a particular node, they
should inform all relevant nodes to free the resources by sending
out notification messages. Upon the reception of notification
messages, the relevant nodes will send out de-activation messages.
To optimize traffic protection and resource management, each headend
may periodically poll the protecting paths about resource
availability. The intermediate nodes have the option to inform the
current resource utilization.
Note that, upon the detection of a working path failure, both
headend and tailend may initiate the activation simultaneously
(known as bi-directional activation). This may expedite the
activation time. However, both headend and tailend nodes need to
coordinate the order of protecting paths for activation, since there
may be multiple protecting paths for each working path (i.e., 1:N
protection). For clarity, we will describe the operation from
headend in the memo. The tailend operation will be available in the
subsequent revisions.
4. SMP Message and Action Definition
4.1. Protection Switching Control (PSC) Logic
Protection switching processes the local triggers described in
requirements 74-79 of [1] together with inputs received from the
tailend node. Based on these inputs the headend will take SMP
actions, and transmit different protocol messages.
Here, we reuse the switching control logic described in MPLS Linear
Protection [6], with the following logical decomposition at headend
node:
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Server Indication Control Plane Indication
-----------------+ +-------------
Operator Command | | OAM Indication
----------------+ | | +---------------
| | | |
V V V V
+---------------+ +-------+
| Local Request |<--------| WTR |
| logic |WTR Exps | Timer |
+---------------+ +-------+
| ^
Highest local|request |
V | Start/Stop
+-----------------+ |
Remote PSC | PSC Control |------------+
------------>| logic |
Request +-----------------+
|
| Action +------------+
+---------------->| Message |
| Generator |
+------------+
|
Output PSC | Message
V
The Local Request logic unit accepts the triggers from the OAM,
external operator commands, from the local control plane (when
present), and the Wait-to-Restore timer. By considering all of these
local request sources it determines the highest priority local
request. This high-priority request is passed to the PSC Control
logic, that will cross-check this local request with the information
received from the tailend node. The PSC Control logic uses this
input to determine what actions need to be taken, e.g. local actions
at the headend, or what message should be sent to the tailend node.
Specifically, the signals could be the following:
o Clear - if the operator cancels an active local administrative
command, i.e. LO/FS/MS.
o Lockout of Protection (LO) - if the operator requested to prevent
switching data traffic to the protection path, for any purpose.
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o Signal Fail (SF) - if any of the Server Layer, Control plane, or
OAM indications signaled a failure condition on either the
protection path or one of the working paths.
o Signal Degrade (SD) - if any of the Server Layer, Control plane,
or OAM indications signaled a degraded transmission condition on
either the protection path or one of the working paths.
o Forced Switch (FS) - if the operator requested that traffic be
switched from one of the working paths to the protection path,
o Manual Switch (MS) - if the operator requested that traffic is
switched from the working path to the protection path. This is
only relevant if there is no currently active fault condition or
Operator command.
o WTR Expires - generated by the WTR timer completing its period.
If none of the input sources have generated any input then the
request logic should generate a No Request (NR) request.
In addition to the local requests, the PSC Control Logic SHALL
accept PSC messages from the tailend node of the transport path.
Remote messages indicate the status of the transport path from the
viewpoint of the tailend nodes. The remote requests may include
remote LO, SF, SD, FS, MS, WTR and NR.
Much of the signal definition is further described in ITU G.709 and
G.873.1.
4.2. SMP Action Types
As shown in the previous section, SMP requires four actions types
throughout the operation:
o ACTIVATION: This action is triggered by the head-end (or tail-
end) to activate a protecting connection. The intermediate nodes
need to propagate this towards the other end of the protecting
connection.
o DE-ACTIVATION: This action is used to de-activate a particular
protecting connection. This can be originated by one end of a
protecting connection (i.e. head-end, or tail-end). The
intermediate nodes need to propagate this towards the other end
of the protecting connection.
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o QUERY: This action is used when an operator decides to query a
particular protecting connection.
o NOTIFICATION: SMP operation requires the coordination between
nodes. The coordination takes places in two occasions:
(1) The activation/de-activation is initiated at the headend
(tailend) nodes. To avoid potential mis-connection, the user
traffic cannot be switched on to the protecting connection until
the reception of an acknowledgement from the tailend (headend)
nodes.
(2) If an intermediate node cannot process the activation
requests, due to lack of resources or preemption levels, it needs
to report the failure to the request originators.
It is conceivable that this message can be used to report the
location of the fault, with respect to a protecting connection so
that the head-end may use this information as one of the criteria
for restoring the working transport entity. The fault location
could be used by the head-end node to select among a list of
possible protecting connections associated with the working
connection (i.e. avoid the faulty location), or to determine that
none of the provisioned protecting connections is usable at the
time the failure is reported and then fallback to restoring the
working connection.
4.3. PSC Signal to SMP Action Mapping
In SMP operation, there is the action-signal mapping:
Activation action: FS, SF, SD, MS,
De-activation action: NR
Query action: EXER
Notification action: ACK, NACK (see next section)
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5. Protocol Definition
Each SMP message has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|Request|Rsv|R| Reserved | Status | Seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Version: 1
o Request:
o 1111b: Lockout of Protection (LO)
o 1110b: Forced Switch (FS). This triggers activation
o 1100b: Signal Fail (SF). This triggers activation
o 1011b: Acknowledgement (ACK). This is to acknowledge a
successful activation/de-activation request
o 1010b: Signal Degrade (SD). This triggers activation
o 1001b: Negative Acknowledgement (NACK). This is to report
failure in activation/de-activation process.
o 1000b: Manual Switch (MS). This triggers activation
o 0110b: Wait-to-Restore (WTR). Used for revertive switching
o 0100b: Exercise (EXER). Triggers SMP query
o 0001b: Do Not Revert (DNR). Used for revertive switching
o 0000b: No Request (NR). This triggers de-activation
o R: Revertive field
o 0: non-revertive mode
o 1: revertive mode
o Rsv/Reserved: This field is reserved for future use
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o Status: this informs the status of the AMP activation. This field
is only relevant with ACK and NACK requests. Specifically, the
Status Code has the following encoding value and definition:
o 1: end-to-end ack
o 2: hop-to-hop ack
o 3: no such path
o 4: no more resource for the path
o 5: preempted by another path
o 6: system failure
o 7: shared resource has been taken by other paths
o Seq: This uniquely identifies a particular message. This field is
defined to support reliable message delivery
Note that the message format and naming convention are very similar
to that of MPLS linear protection [6] and ITU G.873.1.
5.1. Message Encapsulation
SMP messages use MPLS labels to identify the paths. Further, the
messages are encapsulated in GAL/GACH:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPLS Label stack |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GAL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Version| Reserved | Activation Channel Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Activation Message Payload |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o GAL is described in [3]
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o Activation Channel Type is the GACH channel number assigned to
the protocol. This uniquely identifies the activation messages.
Specifically, the messages have the following message format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | Exp |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label (13) | Exp |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Version| Reserved | Activation Channel Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|Request|Rsv|R| Reserved | Status | Seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.2. Reliable Messaging
The activation procedure adapts a simple two-way handshake reliable
messaging.
Each node maintains a sequence number generator. Each new sending
message will have a new sequence number. After sending a message,
the node will wait for a response with the same sequence number.
Specifically, upon the generation of activation, de-activation,
query and notification messages, the message sender expects to
receive acknowledgement in reply with same sequence number.
If a sender is not getting the reply within a time interval, it will
retransmit the same message with a new sequence number, and starts
to wait again. After multiple retries (by default, 3), the sender
will declare activation failure, and alarm the operators for further
service.
5.3. Message Scoping
Activation signaling uses MPLS label TTL to control how far the
message would traverse. Here are the processing rules on each
intermediate node:
o On receive, if the message has label TTL = 0, the node must drop
the packet without further processing
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o The receiving node must always decrement the label TTL value by
one. If TTL = 0 after the decrement, the node must process the
message. Otherwise, the node must forward the message without
further processing (unless, of course, the node is headend or
tailend)
o On transmission, the node will adjust the TTL value. For hop-by-
hop messages, TTL = 1. Otherwise, TTL = 0xFF, by default.
6. Processing Rules
6.1. Enable a protecting path
Upon the detection of network failure (SF/SD/FS) on a working path,
the headend node identifies the corresponding MPLS-TP label and
initiates the protection switching by sending an activation message.
The activation messages always use MPLS label TTL = 1 to force hop-
by-hop process. Upon reception, a next-hop node will locate the
corresponding path and activate the path.
If the activation message is received on an intermediate node, due
to label TTL expiry, the message is processed and then propagated to
the next hop of the MPLS TP LSP, by setting the MPLS TP label TTL =
1.
The headend node will declare the success of the activation only
when it gets a positive reply from the tailend node. This requires
that the tailend nodes must reply the messages with ACK to the
headend nodes in all cases.
If the headend node is not receiving the acknowledgement within a
time internal, it will retransmit another activation message with a
different Seq number.
If the headend node is not receiving a positive reply within a
longer time interval, it will declare activation failure.
If an intermediate node cannot activate a protecting path, it will
reply a message with NACK to report failure. When the headend node
receives the message for failure, it must initiate the de-activation
messages to clean up networks resources on all the relevant nodes on
the path.
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6.2. Disable a protecting path
The headend removes the network resources on a path by sending the
de-activation messages.
In the message, the MPLS label represents the path to be de-
activated. The MPLS TTL is one to force hop-by-hop processing.
Upon reception, a node will de-activate the path, by freeing the
resources from the data-plane.
As a part of the clean-up procedure, each de-activation message must
traverse through and be processed on all the nodes of the
corresponding path. When the de-activation message reaches to the
tailend node, the tailend is required to reply with an
acknowledgement message to the headend.
The de-activation process is complete when the headend receives the
corresponding acknowledgement message from the tailend.
6.3. Get protecting path status
The operators have the option to trigger the query messages from the
headend to check on the protecting path periodically or on-demand.
The process procedure on each node is very similar to that of the
activation messages on the intermediate nodes, except the query
messages should not trigger any network resource re-programming.
Upon reception, the node will check the availability of resources.
If the resource is no longer available, the node will reply an NACK
with error conditions.
6.4. Preemption
The preemption operation typically takes place when processing an
activation message.
If the activating network resources have been used by another path
and carrying user traffic, the node needs to compare the priority
levels.
If the existing path has higher priority, the node needs to reject
the activation request by sending an ACK to the corresponding
headend to inform the unavailability of network resources.
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If the new path has higher priority, the node will reallocate the
resource to the new path, and send an ACK to old path's headend node
to inform about the preemption.
7. Security Consideration
The protection activation takes place in a controlled networking
environment. Nevertheless, it is expected that the edge nodes will
encapsulate and transport external traffic into separated tunnels,
and the intermediate nodes will never have to process them.
8. IANA Considerations
Activation messages are encapsulated in MPLS-TP with a specific GACH
channel type that needs to be assigned by IANA.
9. Normative References
[1] RFC 5654: Requirements of an MPLS Transport Profile, B. Niven-
Jenkins, D. Brungard, M. Betts, N. Sprecher, S. Ueno,
September 2009
[2] IETF draft, Multiprotocol Label Switching Transport Profile
Survivability Framework (draft-ietf-mpls-tp-survive-fwk-
06.txt), N. Sprecher, A. Farrel, June 2010
[3] RFC5586 - Vigoureux,, M., Bocci, M., Swallow, G., Aggarwal,
R., and D. Ward, "MPLS Generic Associated Channel", May 2009.
[4] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[5] Crocker, D. and Overell, P.(Editors), "Augmented BNF for
Syntax Specifications: ABNF", RFC 2234, Internet Mail
Consortium and Demon Internet Ltd., November 1997.
[6] IETF draft, MPLS-TP Linear Protection, (draft-ietf-mpls-tp-
linear-protection-09.txt), Bryant, et al.
10. Acknowledgments
Authors like to thank Eric Osborne, Lou Berger, Nabil Bitar and
Deborah Brungard for detailed feedback on the earlier work, and the
guidance and recommendation for this proposal.
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Internet-Draft Shared Mesh Protection in MPLS-TP October 2011
We also thank Maneesh Jain, Mohit Misra, Yalin Wang, Ted Sprague,
Ann Gui and Tony Jorgenson for discussion on network operation,
feasibility and implementation methodology.
During ITU-T SG15 Interim meeting in May 2011, we have had long
discussion with the G.SMP contributors, in particular Fatai Zhang,
Bin Lu, Maarten Vissers and Jeong-dong Ryoo. We thank their feedback
and corrections.
Authors' Addresses
Ping Pan
Email: ppan@infinera.com
Rajan Rao
Email: rrao@infinera.com
Biao Lu
Email: blu@infinera.com
Luyuan Fang
Email: lufang@cisco.com
Andrew G. Malis
Email: andrew.g.malis@verizon.com
Fatai Zhang
Email: zhangfatai@huawei.com
Sam Aldrin
Email: sam.aldrin@huawei.com
Fei Zhang
Email: zhang.fei3@zte.com.cn
Sri Mohana Satya Srinivas Singamsetty
Email: SriMohanS@Tellabs.com
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