Network Working Group W. Cheng
Internet-Draft L. Wang
Intended status: Standards Track H. Li
Expires: August 6, 2015 China Mobile
H. Helvoort
Hai Gaoming BV
K. Liu
J. Dong
J. He
Huawei Technologies
F. Li
China Academy of Telecommunication Research, MIIT., China
J. Yang
ZTE Corporation P.R.China
J. Wang
Fiberhome Telecommunication Technologies Co., LTD.
February 2, 2015
MPLS-TP Shared-Ring protection (MSRP) mechanism for ring topology
draft-cheng-mpls-tp-shared-ring-protection-04
Abstract
This document describes requirements, architecture and solutions for
MPLS-TP Shared Ring Protection (MSRP) in the ring topology for point-
to-point (P2P) services. The mechanism of MSRP is illustrated and
how it satisfies the requirements in RFC 5654 for optimized ring
protection is analyzed.
Requirements Language
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].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 6, 2015.
Copyright Notice
Copyright (c) 2015 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements for MPLS-TP Ring Protection . . . . . . . . . . 4
2.1. Recovery of Multiple Failures . . . . . . . . . . . . . . 4
2.2. Smooth Upgrade from Linear Protection to Ring Protection 4
2.3. Configuration Complexity . . . . . . . . . . . . . . . . 4
3. Terminology and Notation . . . . . . . . . . . . . . . . . . 5
4. Shared Ring Protection Architecture . . . . . . . . . . . . . 5
4.1. Ring Tunnel . . . . . . . . . . . . . . . . . . . . . . . 5
4.1.1. Establishment of Ring Tunnel . . . . . . . . . . . . 6
4.1.2. Label Assignment and Distribution . . . . . . . . . . 8
4.1.3. Forwarding Operation . . . . . . . . . . . . . . . . 8
4.2. Failure Detection . . . . . . . . . . . . . . . . . . . . 9
4.3. Ring Protection . . . . . . . . . . . . . . . . . . . . . 10
4.3.1. Wrapping . . . . . . . . . . . . . . . . . . . . . . 10
4.3.2. Short Wrapping . . . . . . . . . . . . . . . . . . . 12
4.3.3. Steering . . . . . . . . . . . . . . . . . . . . . . 13
4.4. Interconnected Ring Protection . . . . . . . . . . . . . 16
4.4.1. Interconnected Ring Topology . . . . . . . . . . . . 16
4.4.2. Interconnected Ring Protection Mechanisms . . . . . . 17
4.4.3. Ring Tunnels in Interconnected Rings . . . . . . . . 18
4.4.4. Interconnected Ring Switching Procedure . . . . . . . 20
4.4.5. Interconnected Ring Detection Mechanism . . . . . . . 21
5. Ring Protection Coordination Protocol . . . . . . . . . . . . 22
5.1. RPS Protocol . . . . . . . . . . . . . . . . . . . . . . 23
5.1.1. Transmission and Acceptance of RPS Requests . . . . . 25
5.1.2. RPS PDU Format . . . . . . . . . . . . . . . . . . . 25
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5.1.3. Ring Node RPS States . . . . . . . . . . . . . . . . 26
5.1.4. RPS State Transitions . . . . . . . . . . . . . . . . 27
5.2. RPS State Machine . . . . . . . . . . . . . . . . . . . . 30
5.2.1. Initial States . . . . . . . . . . . . . . . . . . . 30
5.2.2. State transitions When Local Request is Applied . . . 31
5.2.3. State Transitions When Remote Request is Applied . . 34
5.2.4. State Transitions When Request Addresses to Another
Node is Received . . . . . . . . . . . . . . . . . . 37
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40
7. Security Considerations . . . . . . . . . . . . . . . . . . . 40
8. Contributing Authors . . . . . . . . . . . . . . . . . . . . 40
9. Normative References . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction
As described in 2.5.6.1 of [RFC5654], Ring Protection of MPLS-TP
requirements , several service providers have expressed much interest
in operating MPLS-TP in ring topologies and require a high-level
survivability function in these topologies. In operational transport
network deployment, MPLS-TP networks are often constructed with ring
topologies. It calls for an efficient and optimized ring protection
mechanism to achieve simple operation and fast, sub 50 ms, recovery
performance.
The requirements for MPLS-TP [RFC5654] state that recovery mechanisms
which are optimized for ring topologies could be further developed if
it can provide the following features:
a. Minimize the number of OAM entities for protection
b. Minimize the number of elements of recovery
c. Minimize the required label number
d. Minimize the amount of control and management-plane transactions
during maintenance operation
e. Minimize the impact on information exchange during protection if
a control plane is supported
This document specifies MPLS-TP Shared-Ring Protection mechanisms
that can meet all those requirements on ring protection listed in
[RFC5654].
The basic concepts and architecture of Shared-Ring protection
mechanism are specified in this document. This document focuses on
the solutions for point-to-point transport paths. While the basic
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concepts may also apply to point-to-multipoint transport paths, the
solution for point-to-multipoint transport paths is under study and
will be presented in a separate document.
2. Requirements for MPLS-TP Ring Protection
The requirements for MPLS-TP ring protection are specified in
[RFC5654]. This document elaborates on the requirements in detail.
2.1. Recovery of Multiple Failures
MPLS-TP is expected to be used in carrier grade metro networks and
backbone transport networks to provide mobile backhaul, business
services etc., in which the network survivability is very important.
According to R106 B in [RFC5654], MPLS-TP recovery mechanisms in a
ring SHOULD protect against multiple failures. The following text
provides some more detailed illustration about "multiple failures".
In metro and backbone networks, a single risk factor often affects
multiple links or nodes. Some examples of risk factors are given as
follows:
o multiple links use fibers in one cable or pipeline
o Several nodes share one power supply system
o Weather sensitive micro-wave system
Once one of the above risk factors happens, multiple links or nodes
failures may occur simultaneously and those failed links or nodes may
be located on a single ring as well as on interconnected rings. Ring
protection against multiple failures should cover both multiple
failures on a single ring and multiple failures on interconnected
rings.
2.2. Smooth Upgrade from Linear Protection to Ring Protection
It is beneficial for service providers to upgrade the protection
scheme from linear protection to ring protection in their MPLS-TP
network without service interruption. In-service insertion and
removal of a node on the ring should also be supported. Therefore,
the MPLS-TP ring protection mechanism is supposed to be developed and
optimized for compliance with this smooth upgrading principle.
2.3. Configuration Complexity
Ring protection can reduce the dependency of configuration on the
quantity of services, thus will simplify the network protection
configuration and operation effort. This is because the ring
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protection makes use of the characteristics of ring topology and
mechanisms on the section layer. While in the application scenarios
of deploying linear protection in ring topology MPLS-TP network, the
configuration of protection has a close relationship with the
quantities of services carried. Especially in some large metro
networks with more than ten thousands of services in the access
nodes, the LSP linear protection capabilities of the metro core nodes
needs to be large enough to meet the network planning requirements,
which also leads to the complexity of network protection
configurations and operations.
3. Terminology and Notation
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.
It should be noted that the label stack may contain additional
layers. However, we only present the layers that are related to the
protection mechanism.
3. If the Label is assigned by Node X, the Node Name is enclosed in
bracket ("()")
4. Shared Ring Protection Architecture
4.1. Ring Tunnel
This document introduces a new logical layer of the ring for shared
ring protection in MPLS-TP networks. As shown in Figure 1, the new
logical layer consists of ring tunnels which provides a server layer
for the LSPs traverse the ring. Once a ring tunnel is established,
the configuration, management and protection of the ring are all
performed at the ring tunnel level. One port can carry multiple ring
tunnels, while one ring tunnel can carry multiple LSPs.
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+-------------
+-------------|
+-------------| |
=====PW1======| | |
| | Ring | Physical
=====PW2======| LSP | Tunnel | Port
| | |
=====PW3======| | |
+-------------| |
+-------------|
+-------------
Figure 1. The logical layers of the ring
The label stack used in MPLS-TP Shared Ring Protection mechanism is
shown as below:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ring tunnel Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSP Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. Label stack used in MPLS-TP Shared Ring Protection
4.1.1. Establishment of Ring Tunnel
The Ring tunnels are established based on the exit node. The exit
node is the node where traffic leaves the ring. LSPs which have the
same exit node on the ring share the same ring tunnels. In other
words, all the LSPs that traverse the ring and exit from the same
node share the same working ring tunnel and protection ring tunnel.
For each exit node, four ring tunnels are established:
o one clockwise working ring tunnel, which is protected by the
anticlockwise protection ring tunnel
o one anticlockwise protection ring tunnel
o one anticlockwise working ring tunnel, which is protected by the
clockwise protection ring tunnel
o one clockwise protection ring tunnel
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The structure of the protection tunnels are determined by the
selected protection mechanism. This will be detailed in subsequent
sections.
As shown in Figure 3, LSP 1, LSP 2 and LSP 3 enter the ring from Node
E, Node A and Node B, respectively, and all leave the ring at Node D.
To protect these LSPs that traverse the ring, a clockwise working
ring tunnel (RcW_D) via E->F->A->B->C->D, and its anticlockwise
protection ring tunnel (RaP_D) via D->C->B->A->F->E->D are
established, Also, an anti-clockwise working ring tunnel (RaW_D) via
C->B->A->F->E->D, and its clockwise protection ring tunnel (RcP_D)
via D->E->F->A->B->C->D are established. For simplicity Figure 3
only shows RcW_D and RaP_D. A similar provisioning should be applied
for any other node on the ring. In summary, for each node in
Figure 3 when acting as exit node, the ring tunnels are created as
follows:
o To Node A: RcW_A, RaW_A, RcP_A, RaP_A
o To Node B: RcW_B, RaW_B, RcP_B, RaP_B
o To Node C: RcW_C, RaW_C, RcP_C, RaP_C
o To Node D: RcW_D, RaW_D, RcP_D, RaP_D
o To Node E: RcW_E, RaW_E, RcP_E, RaP_E
o To Node F: RcW_F, RaW_F, RcP_F, RaP_F
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+---+#############+---+
| F |-------------| A | +-- LSP2
+---+*************+---+
#/* *\#
#/* *\#
#/* *\#
+---+ +---+
LSP1-+ | E | | B |+-- LSP3
+---+ +---+
#\ */#
#\ */#
#\ */#
+---+*************+---+
LSP1 +--| D |-------------| C |
LSP2 +---+#############+---+
LSP3
---- physical links
**** RcW_D
#### RaP_D
Figure 3. Ring tunnels in MSRP
Through these working and protection ring tunnels, LSPs which enter
the ring from any node can reach any exit nodes on the ring, and are
protected from failures on the ring.
4.1.2. Label Assignment and Distribution
The ring tunnel labels are downstream-assigned labels as defined in
[RFC3031]. The ring tunnel labels can be either configured
statically, provisioned by a controller, or distributed dynamically
via a control protocol.
4.1.3. Forwarding Operation
When an MPLS-TP transport path, such as an LSP, enters the ring, the
ingress node on the ring pushes the working ring tunnel label
according to the exit node and sends the traffic to the next hop.
The transit nodes on the working ring tunnel swap the ring tunnel
labels and forward the packets to the next hop. When the packet
arrives at the exit node, the exit node pops the ring tunnel label
and forwards the packets based on the inner LSP label and PW label.
Figure 4 shows the label operation in the MPLS-TP shared ring
protection mechanism. Assume that LSP1 enters the ring at Node A and
exits from Node D, and the following label operations are executed.
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1. Ingress node: Packets of LSP1 arrive at Node A with a label stack
[LSP1] and is supposed to be forwarded in the clockwise direction
of the ring. The clockwise working ring tunnel label RcW_D will
be pushed at Node A, the label stack for the forwarded packet at
Node A is changed to [RcW_D(B)|LSP1].
2. Transit nodes: In this case, Node B and Node C forward the
packets by swapping the working ring tunnel labels. For example,
the label [RcW_D(B)|LSP1] is swapped to [RcW_D(C)|LSP1] at Node
B.
3. Exit node: When the packet arrives at Node D (i.e. the exit node)
with label stack [RcW_D(D)|LSP1], Node D pops RcW_D(D), and
subsequently deals with the inner labels of LSP1.
4. All the LSPs that exit from the same node share the same set of
ring tunnel labels.
+---+#####[RaP_D(F)]######+---+
| F |---------------------| A | +-- LSP1
+---+*****[RcW_D(A)]******+---+
#/* *\#
[RaP_D(E)]#/*[RcW_D(F)] [RcW_D(B)]*\#[RaP_D(A)]
#/* *\#
+---+ +---+
| E | | B |
+---+ +---+
#\ */#
[RaP_D(D)]#\ [RxW_D(C)]*/#[RaP_D(B)]
#\ */#
+---+*****[RcW_D(D)]****+---+
LSP1 +-- | D |-------------------| C |
+---+#####[RaP_D(C)]####+---+
-----physical links ****** RcW_D ###### RaP_D
Figure 4. Label operation of MSRP
4.2. Failure Detection
The MPLS-TP section layer OAM is used to monitor the connectivity
between each two adjacent nodes on the ring using the mechanisms
defined in [RFC6371]. Protection switching is triggered by the
failure detected on the ring by the OAM mechanisms.
Two end ports of a link form a Maintenance Entity Group (MEG), and an
MEG end point (MEP) function is installed in each ring port. CC-V
OAM packets are periodically exchanged between each pair of MEPs to
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monitor the link health. Three or more consecutive CC-V packets
losses will be interpreted as a link failure.
A node failure is regarded as the failure of two links attached to
that node. The two nodes adjacent to the failed node detect the
failure in the links that are connected to the failed node.
4.3. Ring Protection
Taking the topology in Figure 4 as example, the LSP1 enters the ring
at Node A and leaves the ring at Node D. In normal state, LSP 1 is
carried by clockwise working ring tunnel (RcW_D) through the path
A->B->C->D, the label operation is:
[LSP1](original data traffic carried by LSP 1) ->
[RCW_D(B)|LSP1](NodeA) -> [RCW_D(C)|LSP1](NodeB) -> [RCW_D(D)|
LSP1](NodeC) -> [LSP1](data traffic carried by LSP 1). Then at node
D the packet will be forwarded based on label stack of LSP1.
The following sections describes the protection mechanisms used in
ring topology.
4.3.1. Wrapping
With the wrapping mechanism, the protection ring tunnel is a closed
ring identified by the exit node. As shown in Figure 4, the RaP_D is
the anticlockwise protection ring tunnel for the clockwise working
ring tunnel RcW_D. As specified in the following sections, the
closed ring protection tunnel can protect both the link failure and
the node failure.
4.3.1.1. Wrapping for Link Failure
When a link failure between Node B and Node C occurs, both Node B and
Node C detect the failure via OAM mechanism. Node B switches the
clockwise working ring tunnel (RcW_D) to the anticlockwise protection
ring tunnel (RaP_D) and Node C switches anticlockwise protection ring
tunnel(RaP_D) to the clockwise working ring tunnel(RcW_D). The data
traffic which enters the ring at Node A and exits at Node D follows
the path A->B->A->F->E->D->C->D. The label operation is:
[LSP1](Original data traffic) -> [RcW_D(B)|LSP1](Node A) ->
[RaP_D(A)|LSP1](Node B) -> [RaP_D(F)|LSP1](Node A) -> [RaP_D(E)|LSP1]
(Node F) -> [RaP_D(D)|LSP1] (Node E) -> [RaP_D(C)|LSP1] (Node D) ->
[RcW_D(D)|LSP1](Node C) -> [LSP1](data traffic exits the ring).
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+---+#####[RaP_D(F)]######+---+
| F |---------------------| A | +-- LSP1
+---+*****[RcW_D(A)]******+---+
#/* *\#
[RaP_D(E)]#/*[RcW_D(F)] [RcW_D(B)]*\#RaP_D(A)
#/* *\#
+---+ +---+
| E | | B |
+---+ +---+
#\ *x#
[RaP_D(D)]#\ [RcW_D(C)]*x#RaP_D(B)
#\ *x#
+---+*****[RcW_D(D)]****+---+
LSP1 +-- | D |-------------------| C |
+---+#####[RaP_D(C)]####+---+
-----physical links xxxx Failure Link
****** RcW_D ###### RaP_D
Figure 5.Wrapping for link failure
4.3.1.2. Wrapping for Node Failure
When Node B fails, Node A detects the failure between A and B and
switches the clockwise work ring tunnel (RcW_D) to the anticlockwise
protection ring tunnel(RaP_D), Node C detects the failure between C
and B and switches the anticlockwise protection ring tunnel(RaP_D) to
the clockwise working ring tunnel(RcW_D). The data traffic which
enters the ring at Node A and exits at Node D follows the path
A->F->E->D->C->D. The label operation is:
[LSP1](original data traffic carried by LSP 1) ->
[RaP_D(F)|LSP1](NodeA) -> [RaP_D(E)|LSP1](NodeF) ->
[RaP_D(D)|LSP1](NodeE) -> [RaP_D(C)|LSP1] (NodeD) -> [RcW_D(D)|LSP1]
(NodeC) -> [LSP1](data traffic carried by LSP 1).
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+---+#####[RaP_D(F)]######+---+
| F |---------------------| A | +-- LSP1
+---+*****[RcW_D(A)]******+---+
#/* *\#
[RaP_D(E)]#/*[RcW_D(F)] [RcW_D(B)]*\#RaP_D(A)
#/* *\#
+---+ xxxxx
| E | x B x
+---+ xxxxx
#\ */#
[RaP_D(D)]#\ [RcW_D(C)]*/#RaP_D(B)
#\ */#
+---+*****[RcW_D(D)]****+---+
LSP1 +-- | D |-------------------| C |
+---+#####[RaP_D(C)]####+---+
-----physical links xxxxxx Failure Node
*****RcW_D ###### RaP_D
Figure 6. Wrapping for node failure
4.3.2. Short Wrapping
With the traditional wrapping protection scheme, Protection switching
is executed at both nodes detecting the failure, consequently the
traffic will be wrapped twice. This mechanism will cause additional
latency and bandwidth consumption when traffic is switched to the
protection path.
With short wrapping protection, data traffic switching is executed
only at the upstream node detecting the link failure, and exits the
ring in the protection ring tunnel at the exit node. This scheme can
reduce the additional latency and bandwidth consumption when traffic
is switched to the protection path.
In the traditional wrapping solution, the protection ring tunnel is a
closed ring in normal state, while in the short wrapping solution,
the protection ring tunnel is ended at the exit node, which is
similar to the working ring tunnel. Short wrapping is easy to
implement in shared ring protection because both the working and
protection ring tunnels are terminated on the exit nodes. Figure 7
shows the clockwise working ring tunnel and the anticlockwise
protection ring tunnel with node D as the exit node.
As shown in Figure 7, in normal state, LSP 1 is carried by the
clockwise working ring tunnel (RcW_D) through the path A->B->C->D.
When a link failure between Node B and Node C occurs, Node B switches
The working ring tunnel RcW_D to the protection ring tunnel RaP_D in
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the opposite direction. The difference occurs in the protection ring
tunnel at exit node. In short wrapping protection, Rap_D ends in
Node D and then traffic will be forwarded based on the LSP labels.
Thus with short wrapping mechanism, LSP1 will follow the path
A->B->A->F->E->D when link failure between Node B and Node C happens.
For node failure, the protection with short wrapping is similar to
the mechanism with link failure.
+---+#####[RaP_D(F)]######+---+
| F |---------------------| A | +-- LSP1
+---+*****[RcW_D(A)]******+---+
#/* *\#
[RaP_D(E)]#/*[RcW_D(F)] [RcW_D(B)]*\#RaP_D(A)
#/* *\#
+---+ +---+
| E | | B |
+---+ +---+
#\ *x#
[RaP_D(D)]#\ [RcW_D(C)]*x#RaP_D(B)
#\ *x#
+---+*****[RcW_D(D)]****+---+
LSP1 +-- | D |-------------------| C |
+---+ +---+
----- physical links xxxxx Failure Link
****** RcW_D ###### RaP_D
Figure 7. Short wrapping for link failure
4.3.3. Steering
In ring topology, each working ring tunnel is associated with a
protection ring tunnel in the opposite direction, and every node can
obtain the ring topology either by configuration or via some topology
discovery mechanism. When a failure occurs in the ring, the nodes
that detect the failure will transmit the failure information in the
opposite direction of the failure hop by hop on the ring. When a
node receives the message that identifies a failure, it can quickly
determine the location of the fault by using the topology information
that is maintained by the node, then it can determine whether the
LSPs entering the ring locally need to switchover or not. For LSPs
that needs to switchover, it will switch the LSPs from the working
ring tunnels to its corresponding protection ring tunnels.
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+--LSP l
+-+-+-+-+-+-+-+ +---+ ###[RaP_D(F)]### +---/ +-+-+-+-+-+-+-+
|F|A|B|C|D|E|F| | F | ---------------- | A | |A|B|C|D|E|F|A|
+-+-+-+-+-+-+-+ +---+ ***[RcW_D(A)]*** +---+ +-+-+-+-+-+-+-+
|I|I|I|S|I|I| |I|I|S|I|I|I|
+-+-+-+-+-+-+ #/* *\# +-+-+-+-+-+-+
[RaP_D(E)] #/* [RcW_D(B)] *\# [RaP_D(A)]
#/* [RcW_D(F)] *\#
+-+-+-+-+-+-+-+ #/* *\#
|E|F|A|B|C|D|E| +---+ +---+ +-- LSP 2
+-+-+-+-+-+-+-+ | E | | B | +-+-+-+-+-+-+-+
|I|I|I|I|S|I| +---+ +---+ |B|C|D|E|F|A|B|
+-+-+-+-+-+-+ #\* */# +-+-+-+-+-+-+-+
#\* [RcW_D(E)] [RcW_D(C)] */# |I|S|I|I|I|I|
[RaP_D(D)] #\* */# +-+-+-+-+-+-+
#\* */# [RaP_D(B)]
+-+-+-+-+-+-+-+ +---+ [RcW_D(D)] +---+ +-+-+-+-+-+-+-+
|D|E|F|A|B|C|D| +-- | D | xxxxxxxxxxxxxxxxx | C | |C|D|E|F|A|B|C|
+-+-+-+-+-+-+-+ LSP 1 +---+ [RaP_D(C)] +---+ +-+-+-+-+-+-+-+
|I|I|I|I|I|S| LSP 2 |S|I|I|I|I|I|
+-+-+-+-+-+-+ +-+-+-+-+-+-+
----- physical links ***** RcW_D ##### RaP_D
Figure 8. Steering operation and protection switching
As shown in Figure 8, LSP1 enters the ring from Node A while LSP2
enters the ring from Node B, and both of them have the same
destination node D.
In the normal state, LSP 1 is carried by the clockwise working ring
tunnel (RcW_D) through the path A->B->C->D, the label operation is:
[LSP1] -> [RcW_D(B)|LSP1](NodeA) -> [RcW_D(C)| LSP1](NodeB) ->
[RcW_D(D)|LSP1](NodeC) -> [LSP1] (data traffic carried by LSP 1) .
LSP2 is carried by the clockwise working ring tunnel (RcW_D) throught
the path B->C->D, the label operation is: [LSP2] ->
[RcW_D(C)|LSP2](NodeB) -> [RcW_D(D)|LSP2](NodeC) -> [LSP2] (data
traffic carried by LSP 1) .
If the link between nodes C and D fails, according to the fault
detection and distribution mechanisms, Node D will find out that
there is a failure in the link between C and D, and it will update
the link state of its ring topology, changing the link between C and
D from normal to fault. In the direction that opposite to the
failure position, Node D will send the state report message to Node
E, informing Node E of the fault between C and D, and E will update
the link state of its ring topology accordingly, changing the link
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between C and D from normal to fault. In this way, the state report
message is sent hop by hop in the clockwise direction. Similar to
Node D, Node C will send the failure information in the anti-
clockwise direction.
When Node A receives the failure report message and updates the link
state of its ring topology, it is aware that there is a fault on the
clockwise working ring tunnel to node D (RcW_D), and LSP 1 enters the
ring locally and is carried by this ring tunnel, thus Node A will
decide to switch the LSP1 onto the anticlockwise protection ring
tunnel to node D (RaP_D). After the switchover, LSP1 will follow the
path A->F->E->D, the label operation is: [LSP1] -> [RaP_D(F)|
LSP1](NodeA) -> [RaP_D(E)|LSP1](NodeF) -> [RaP_D(D)|LSP1](NodeE) ->
[LSP1] (data traffic carried by LSP 1).
The same also apply to the operation of LSP2. When Node B updates
the link state of its ring topology, and finds out that the working
ring tunnel RcW_D has failed, it will switch the LSP2 to the
anticlockwise protection tunnel RaP_D. After the switchover, LSP2
goes through the path B->A->F->E->D, and the label operation is:
[LSP2] -> [RaP_D(A)|LSP2](NodeB) -> [RaP_D(F)|LSP2](NodeA) ->
[RaP_D(E)|LSP2](NodeF) -> [RaP_D(D)|LSP2](NodeE) -> [LSP2](data
traffic carried by LSP 2).
Then assume the link between nodes A and B breaks down, as shown in
Figure 9. Similar to the above failure case, Node B will detect a
fault in the link between A and B, and it will update the link state
of its ring topology, changing the link state between A and B from
normal to fault. The state report message is sent hop by hop in the
clockwise direction, notifying every node that there is a fault
between node A and B, and every node updates the link state of its
ring topology. As a result, Node A will detect a fault in the
working ring tunnel to node D, and switch LSP1 to the protection ring
tunnel, while Node B determine that the working ring tunnel for LSP2
still works fine, and will not perform the switchover.
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/-- LSP l
+-+-+-+-+-+-+-+ +---+ ###[RaP_D(F)]#### +---/ +-+-+-+-+-+-+-+
|F|A|B|C|D|E|F| | F | ----------------- | A | |A|B|C|D|E|F|A|
+-+-+-+-+-+-+-+ +---+ ***[RcW_D(A)]**** +---+ +-+-+-+-+-+-+-+
|I|S|I|I|I|I| #/* x |S|I|I|I|I|I|
+-+-+-+-+-+-+ #/* x +-+-+-+-+-+-+
[RaP_D(E)] #/*[RcW_D(F)] [RcW_D(B)]x [RaP_D(A)]
#/* x +-- LSP 2
+-+-+-+-+-+-+-+ +---+ +---++-+-+-+-+-+-+-+
|E|F|A|B|C|D|E| | E | | B ||B|C|D|E|F|A|B|
+-+-+-+-+-+-+-+ +---+ +---++-+-+-+-+-+-+-+
|I|I|S|I|I|I| #\* */# |I|I|I|I|I|S|
+-+-+-+-+-+-+ #\*[RcW_D(E)] [RcW_D(C)] */# +-+-+-+-+-+-+
[RaP_D(D)] #\* */# [RaP_D(B)]
+-+-+-+-+-+-+-+ #\* */# +-+-+-+-+-+-+-+
|D|E|F|A|B|C|D| +---+ ***[RcW_D(D)]*** +---+ |C|D|E|F|A|B|C|
+-+-+-+-+-+-+-+ +-- | D | ---------------- | C | +-+-+-+-+-+-+-+
|I|I|I|S|I|I| LSP1 +---+ ###[RaP_D(C)]### +---+ |I|I|I|I|S|I|
+-+-+-+-+-+-+ LSP2 +-+-+-+-+-+-+
----- physical links ***** RcW_D ##### RaP_D
Figure 9. Steering operation and protection switching (2)
4.4. Interconnected Ring Protection
4.4.1. Interconnected Ring Topology
Interconnected ring topology is often used in MPLS-TP networks. This
document will discuss two typical interconnected ring topologies:
1. Single-node interconnected rings
In single-node interconnected rings, the connection between
the two rings is through a single node. Because the
interconnection node is in fact a single point of failure,
this topology should be avoided in real transport networks.
Figure 10 shows the topology of single-node interconnected
rings. Node C is the interconnection node between Ring1 and
Ring2.
2. Dual-node interconnected rings
In dual-node interconnected rings, the connection between the
two rings is through two nodes. The two interconnection nodes
belong to both interconnected rings. This topology can
recover from one interconnection node failure.
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Figure 10 shows the topology of single-node interconnected rings.
Node C is the interconnection node between Ring1 and Ring2.
+---+ +---+ +---+ +---+
| A |------| B |----- -----| G |------| H |
+---+ +---+ \ / +---+ +---+
| \ / |
| \ +---+ / |
| Ring1 | C | Ring2 |
| / +---+ \ |
| / \ |
+---+ +---+ / \ +---+ +---+
| F |------| E |----- -----| J |------| I |
+---+ +---+ +---+ +---+
Figure 10. Single-node interconnected rings
Figure 11 shows the topology of dual-node interconnected rings.
Nodes C and Node D are the interconnection nodes between Ring1 and
Ring2.
+---+ +---+ +---+ +---+ +---+
| A |------| B |------| C |------| G |------| H |
+---+ +---+ +---+ +---+ +---+
| | | |
| | | |
| Ring1 | | Ring2 |
| | | |
| | | |
+---+ +---+ +---+ +---+ +---+
| F |------| E |------| D |------| J |------| I |
+---+ +---+ +---+ +---+ +---+
Figure 11. Dual-node interconnected rings
4.4.2. Interconnected Ring Protection Mechanisms
Interconnected rings can be regarded as two independent rings. Ring
protection switching protocol operates on each ring independently.
Failure in one ring only triggers protection switching on the ring
itself and does not affect the other ring. Protection switching in a
single ring is same as the one described in section 4.3.
The service LSPs that traverse the interconnected rings via the
interconnection nodes MUST use different ring tunnels in different
rings. On the interconnection node, the ring tunnel label used in
the source ring will be popped, and the ring tunnel label of
destination ring will be pushed
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For the protected interconnection node in dual-node interconnected
ring, the service LSPs in the interconnection nodes should use the
same LSP label. So any interconnection node can terminate a source
ring runnel and push a destination ring tunnel according to the
service LSP label.
Two interconnection nodes can be managed as a virtual interconnection
node group. Each ring should assign ring tunnels to the virtual
interconnection node group. The interconnection nodes in the group
should terminate the working ring tunnel in each ring. The
protection ring tunnel is an open ring to switch with the working
ring tunnel at the nodes that detect the fault and ends at the egress
node.
When the service traffic passes through the interconnection node, the
direction of the working ring tunnels in each ring for this service
traffic should be the same. For example, if the working ring tunnel
follows the clockwise direction in Ring1, the working ring tunnel for
the same service traffic in Ring2 also follows the clockwise
direction when the service leaves Ring1 and enters Ring2.
4.4.3. Ring Tunnels in Interconnected Rings
The same ring tunnels as described in section 4.1 are used in each
ring of the interconnected rings. Note that ring tunnels to the
virtual interconnection node group will be established by each ring
of the interconnected rings, i.e.:
o one clockwise working ring tunnel to the virtual interconnection
node group
o one anticlockwise protection ring tunnel to the virtual
interconnection node group
o one anticlockwise working ring tunnel to the virtual
interconnection node group
o one clockwise protection ring tunnel to the virtual
interconnection node group
These ring tunnels will terminated at all nodes in the virtual
interconnection node group.
For example, all the ring tunnels on Ring1 of Figure 12 are
established as follows:
o To Node A: R1cW_A, R1aW_A, R1cP_A, R1aP_A
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o To Node B: R1cW_B, R1aW_B, R1cP_B, R1aP_B
o To Node C: R1cW_C, R1aW_C, R1cP_C, R1aP_C
o To Node D: R1cW_D, R1aW_D, R1cP_D, R1aP_D
o To Node E: R1cW_E, R1aW_E, R1cP_E, R1aP_E
o To Node F: R1cW_F, R1aW_F, R1cP_F, R1aP_F
o To the virtual interconnection node group (including Node F and
Node A): R1cW_F&A, R1aW_F&A, R1cP_F&A, R1aP_F&A;
All the ring tunnels established in Ring2 in Figure 12 are
provisioned as follows:
o To Node A: R2cW_A, R2aW_A, R2cP_A, R2aP_A
o To Node F: R2cW_F, R2aW_F, R2cP_F, R2aP_F
o To Node G: R2cW_G, R2aW_G, R2cP_G, R2aP_G
o To Node H: R2cW_H, R2aW_H, R2cP_H, R2aP_H
o To Node I: R2cW_I, R2aW_I, R2cP_I, R2aP_I
o To Node J: R2cW_J, R2aW_J, R2cP_J, R2aP_J
o To the virtual interconnection node group(including Node F and
Node A): R2cW_FandA, R2aW_FandA, R2cP_FandA, R2aP_FandA
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+---+cccccccccccc +---+
| H |-------------| I |--->LSP1
+---+ +---+
c/a a\
c/a a\
c/a a\
+---+ +---+
| G | Ring2 | J |
+---+ +---+
c\a a/c
c\a a/c
c\a aaaaaaaaaaaaa a/c
+---+ccccccccccccc+---+
| F |-------------| A |
+---+ccccccccccccc+---+
c/aaaaaaaaaaaaaaaaaaa a\
c/ a\
c/ a\
+---+ +---+
| E | Ring1 | B |
+---+ +---+
c\a a/c
c\a a/c
c\a a/c
+---+aaaaaaaaaaaa +---+
LSP1--->| D |-------------| C |
+---+ccccccccccccc+---+
ccccccccccc R1cW_F&A
aaaaaaaaaaa R1aP_F&A
ccccccccccc R2cW_I
aaaaaaaaaaa R2aP_I
Figure 12. Ring tunnels for the interconnected rings
4.4.4. Interconnected Ring Switching Procedure
As shown in Figure 12, for the service traffic LSP1 which enters
Ring1 at Node D and exits Ring1 at Node F and continues to enter
Ring2 at Node F and exits Ring2 at Node I, the protection scheme is
described below.
In normal state, LSP1 follows R1cW_F&A in Ring1 and R2CW_I in Ring2.
The label used for the working ring tunnel R1cW_F&A in Ring1 is
popped and the label used for the working ring tunnel R2cW_I will be
pushed based the inner label lookup at the interconnection node F.
The working path that the service traffic LSP1 follows is:
LSP1->R1cW_F&A (D->E->F)->R2cW_I(F->G->H->I)->LSP1.
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In case of link failure, for example, when a failure occurs on the
link between Node F and Node E, Nodes F and E will detect the failure
and execute protection switching as described in 2.2.1.1. The path
that the service traffic LSP1 follows after switching change to
LSP1->R1cW_F&A(D->E)->R1aP_F&A(E->D->C->B->A->F)->R1cW_F(F)
->R2cW_I(F->G->H->I)->LSP1.
In case of a non interconnection node failure, for example, when the
failure occurs at Node E in Ring1, Nodes F and E will detect the
failure and execute protection switching as described in 2.2.1.2.
The path that the service traffic LSP1 follows after switching
becomes: LSP1->R1cW_F&A(D)->R1aP_F&A(D->C->B->A->F)->
R1cW_F(F)->R2cW_I(F->G->H->I).
In case of an interconnection node failure, for example, when the
failure occurs at the interconnection Node F. Nodes E and A in Ring1
will detect the failure, and execute protection switching as
described in 2.2.1.2. Nodes G and A in Ring2 will also detects the
failure, and execute protection switching. The path that the service
traffic LSP1 follows after switching is:
LSP1->R1cW_F&A(D->E)->R1aP_F&A(E->D->C->B->A)->R1cW_A(A)
->R2aP_I(A->J->I)->LSP1.
4.4.5. Interconnected Ring Detection Mechanism
As show in Figure 13, the service traffic LSP1 traverses A->B-C in
Ring1 and C->G->H->I in Ring2. Node C and Node D are the
interconnection nodes. When both the link between Node C and Node G
and the link between Node C and Node D fail, the ring tunnel from
Node C to Node I in Ring 2 becomes unreachable. However, Node D is
still available, and LSP1 can still reach Node I.
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+---+ *********+---+**********+---+ +---+**********+---+
LSP1->| A |----------| B |----------| C |XXXXXXXXXX| G |----------| H |
+---+##########+---+##########+---+ +---+##########+---+
|# X #|*
|# X #|*
|# Ring1 X Ring2 #|*
|# X #|*
|# X #|*
+---+##########+---+##########+---+######### +---+##########+---+
| F |----------| E |----------| D |----------| J |----------| I | ->LSP1
+---+ +---+ +---+ +---+ +---+
*********** R1cW_C&D
########### R1aP_C&D
*********** R2cW_I
########### R2aP_I
Figure 13. Interconnected ring
In order to achieve this, the interconnection nodes need to know the
ring topology of each ring so that they can judge whether a node is
reachable. This judgment is based on the knowledge of each ring
topology and the fault location as described in section 3.4. The
ring topology can be obtained from the NMS or topology discovery
mechanisms. The fault location can be obtained by transmitting the
fault information around the ring. The nodes that detect the failure
will transmit the fault information in the opposite direction node by
node in the ring. When the interconnection node receives the message
that informs the failure, it will quickly calculate the location of
the fault by the topology information that is maintained by itself
and determines whether the LSPs entering the ring at itself can reach
the destination. If the destination node is reachable, the LSP will
exit the source ring and enter the destination ring. If the
destination node is not reachable, the LSP will switch to the
anticlockwise protection ring tunnel.
In Figure 13, Node C determines that the ring tunnel to Node I is
unreachable, the service traffic LSP1 for which the destination node
on the ring tunnel is Node I should switch to the protection LSP
(R1aP_C&D) and consequently the service traffic LSP1 traverses the
interconnected rings at Node D. Node D will remove the ring tunnel
label of Ring1 and add the ring tunnel label of Ring2.
5. Ring Protection Coordination Protocol
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5.1. RPS Protocol
The MSRP protection operation MUST be controlled with the help of the
Ring Protection Switch Protocol (RPS). The RPS processes in the each
of the individual ring nodes that form the ring SHOULD communicate
using the G-ACh channel.
The RPS protocol MUST carry the ring status information and RPS
requests, i.e., automatically initiated and externally initiated,
between the ring nodes.
Each node on the ring MUST be uniquely identified by assigning it a
node ID. The maximum number of nodes on the ring supported by the
RPS protocol is 127. The node ID SHOULD be independent of the order
in which the nodes appear on the ring. The node ID is used to
identity the source and destination nodes of each RPS request.
Each node SHOULD have a ring map containing information about the
sequence of the nodes around the ring. The method of configuring the
nodes with the ring maps is TBD.
When no protection switches are active on the ring, each node MUST
dispatch periodically RPS requests to the two adjacent nodes,
indicating No Request (NR). When a node determines that a protection
switching is required, it MUST send the appropriate RPS request in
both directions.
+---+ A->B(NR) +---+ B->C(NR) +---+ C->D(NR)
-------| A |-------------| B |-------------| C |-------
(NR)F<-A +---+ (NR)A<-B +---+ (NR)B<-C +---+
Figure 14. RPS communication between the ring nodes in case of
no failures in the ring
A destination node is a node that is adjacent to a node that
identified a failed span. When a node that is not the destination
node receives an RPS request and it has no higher priority local
request, it MUST transfer in the same direction the RPS request as
received. In this way, the switching nodes can maintain direct RPS
protocol communication in the ring.
+---+ C->B(SF) +---+ B->C(SF) +---+ C->B(SF)
-------| A |-------------| B |----- X -----| C |-------
(SF)C<-B +---+ (SF)C<-B +---+ (SF)B<-C +---+
Figure 15. RPS communication between the ring nodes in case of
failure between nodes B and C
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Note that in the case of a bidirectional failure such as a cable cut,
the two adjacent nodes detect the failure and send each other an RPS
request in opposite directions.
o In rings utilizing the wrapping protection. When the destination
node receives the RPS request it MUST perform the switch from/to
the working ring tunnels to/from the protection ring tunnels if it
has no higher priority active RPS request.
o In rings utilizing the steering protection. When a ring switch is
required, any node MUST perform the switches if its added/dropped
traffic is affected by the failure. Determination of the affected
traffic SHOULD be performed by examining the RPS requests
(indicating the nodes adjacent to the failure or failures) and the
stored ring maps (indicating the relative position of the failure
and the added traffic destined towards that failure).
When the failure has cleared and the Wait-to-Restore (WTR) timer has
expired, the nodes sourcing RPS requests MUST drop their respective
switches (tail end) and MUST source an RPS request carrying the NR
code. The node receiving from both directions such RPS request (head
end) MUST drop its protection switches.
A protection switch MUST be initiated by one of the criteria
specified in Section 3.2. A failure of the RPS protocol or
controller MUST NOT trigger a protection switch.
Ring switches MUST be preempted by higher priority RPS requests. For
example, consider a protection switch that is active due to a manual
switch request on the given span, and another protection switch is
required due to a failure on another span. Then an RPS request MUST
be generated, the former protection switch MUST be dropped, and the
latter protection switch established.
MSRP mechanism SHOULD support multiple protection switches in the
ring, resulting the ring being segmented into two or more separate
segments. This may happen when several RPS requests of the same
priority exist in the ring due to multiple failures or external
switch commands.
Proper operation of the MSRP mechanism relies on all nodes having
knowledge of the state of the ring (nodes and spans) so that nodes do
not preempt existing RPS request unless they have a higher-priority
RPS request. In order to accommodate ring state knowledge, during a
protection switch the RPS requests MUST be sent in both directions.
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5.1.1. Transmission and Acceptance of RPS Requests
A new RPS request MUST be transmitted immediately when a change in
the transmitted status occurs.
The first three RPS protocol messages carrying new RPS request SHOULD
be transmitted as fast as possible. For fast protection switching
within 50 ms, the interval of the first three RPS protocol messages
SHOULD be 3.3 ms. The successive RPS requests SHOULD be transmitted
with the interval of 5 seconds.
5.1.2. RPS PDU Format
Figure 16 depicts the format of an RPS packet that is sent on the
G-ACh. The Channel Type field is set to indicate that the message is
an RPS message. The ACH MUST NOT include the ACH TLV Header
[RFC5586] meaning that no ACH TLVs can be included in the message.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|0 0 0 0|0 0 0 0 0 0 0 0| RPS Channel Type (TBD) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dest Node ID | Src Node ID | Request | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16. G-ACh RPS Packet Format
The following fields MUST be provided:
o Destination Node ID: The destination node ID MUST always be set to
value of the node ID of the adjacent node. Valid destination node
ID values are 1-127.
o Source node ID: The source node ID MUST always be set to the value
of the node ID generating the RPS request. Valid source node ID
values are 1-127.
o RPS request code: A code consisting of four bits as specified
below:
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+-------------+-----------------------------+----------+
| Bits 4-1 | Condition, State | Priority |
| (MSB - LSB) | or external Request | |
+-------------------------------------------+----------+
| 1 1 1 1 | Lockout of Protection (LP) | highest |
| 1 1 0 1 | Forced Switch (FS) | |
| 1 0 1 1 | Signal Fail (SF) | |
| 0 1 1 0 | Manual Switch (MS) | |
| 0 1 0 1 | Wait-To-Restore (WTR) | |
| 0 0 1 1 | Exercise (EXER) | |
| 0 0 0 1 | Reverse Request (RR) | |
| 0 0 0 0 | No Request (NR) | lowest |
+-------------+-----------------------------+----------+
5.1.3. Ring Node RPS States
Idle state: A node is in the idle state when it has no RPS request
and is sourcing and receiving NR code to/from both directions.
Switching state: A node not in the idle or pass-through states is in
the switching state.
Pass-through state: A node is in the pass-through state when its
highest priority RPS request is a request not destined to it or
sourced by it. The pass-through is bidirectional.
5.1.3.1. Idle State
A node in the idle state MUST source the NR request in both
directions.
A node in the idle state MUST terminate RPS requests flow in both
directions.
A node in the idle state MUST block the traffic flow on protection
LSPs/tunnels in both directions.
5.1.3.2. Switching State
A node in the switching state MUST source RPS request to adjacent
node with its highest RPS request code in both directions when it
detects a failure or receives an external command.
A node in the switching state MUST terminate RPS requests flow in
both directions.
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As soon as it receives an RPS request from the short path, the node
to which it is addressed MUST acknowledge the RPS request by replying
with the RR code on the short path, and with the received RPS request
code on the long path.
This rule refers to the unidirectional failure detection: the RR
SHOULD be issued only when the node does not detect the failure
condition (i.e., the node is a head end), that is, it is not
applicable when a bidirectional failure is detected, because, in this
case, both nodes adjacent to the failure will send an RPS request for
the failure on both paths (short and long).
The following switches MUST be allowed to coexist:
o LP and LP
o FS and FS
o SF and SF
o FS and SF
When multiple MS RPS requests over different spans exist at the same
time, no switch SHOULD be executed and existing switches MUST be
dropped. The nodes MUST signal, anyway, the MS RPS request code.
Multiple EXER requests MUST be allowed to coexist in the ring.
A node in a ring switching state that receives the external command
LP for the affected span MUST drop its switch and MUST signal NR for
the locked span if there is no other RPS request on another span.
Node still SHOULD signal relevant RPS request for another span.
5.1.3.3. Pass-through State
When a node is in a pass-through state, it MUST transfer the received
RPS Request in the same direction.
When a node is in a pass-through state, it MUST enable the traffic
flow on protection ring tunnels in both directions.
5.1.4. RPS State Transitions
All state transitions are triggered by an incoming RPS request
change, a WTR expiration, an externally initiated command, or locally
detected MPLS-TP section failure conditions.
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RPS requests due to a locally detected failure, an externally
initiated command, or received RPS request shall pre-empt existing
RPS requests in the prioritized order given in Section 3.1.2, unless
the requests are allowed to coexist.
5.1.4.1. Transitions Between Idle and Pass-through States
The transition from the idle state to pass-through state MUST be
triggered by a valid RPS request change, in any direction, from the
NR code to any other code, as long as the new request is not destined
to the node itself. Both directions move then into a pass-through
state, so that, traffic entering the node through the protection Ring
tunnels are transferred transparently through the node.
A node MUST revert from pass-through state to the idle state when it
detects NR codes incoming from both directions. Both directions
revert simultaneously from the pass-through state to the idle state.
5.1.4.2. Transitions Between Idle and Switching States
Transition of a node from the idle state to the switching state MUST
be triggered by one of the following conditions:
o A valid RPS request change from the NR code to any code received
on either the long or the short path and destined to this node
o An externally initiated command for this node
o The detection of an MPLS-TP section layer failure at this node
Actions taken at a node in the idle state upon transition to
switching state are:
o For all protection switch requests, except EXER and LP, the node
MUST execute the switch
o For EXER, and LP, the node MUST signal appropriate request but not
execute the switch
A node MUST revert from the switching state to the idle state when it
detects NR codes received from both directions.
o At the tail end: When a WTR time expires or an externally
initiated command is cleared at a node, the node MUST drop its
switch, transit to the Idle State and signal the NR code in both
directions.
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o At the head end: Upon reception of the NR code, from both
directions, the head-end node MUST drop its switch, transition to
Idle State and signal the NR code in both directions.
5.1.4.3. Transitions Between Switching States
When a node that is currently executing any protection switch
receives a higher priority RPS request (due to a locally detected
failure, an externally initiated command, or a ring protection switch
request destined to it) for the same span, it MUST update the
priority of the switch it is executing to the priority of the
received RPS request.
When a failure condition clears at a node, the node MUST enter WTR
condition and remain in it for the appropriate time-out interval,
unless:
o A different RPS request with a higher priority than WTR is
received
o Another failure is detected
o An externally initiated command becomes active
The node MUST send out a WTR code on both the long and short paths.
When a node that is executing a switch in response to incoming SF RPS
request (not due to a locally detected failure) receives a WTR code
(unidirectional failure case), it MUST send out RR code on the short
path and the WTR on the long path.
5.1.4.4. Transitions Between Switching and Pass-through States
When a node that is currently executing a switch receives an RPS
request for a non-adjacent span of higher priority than the switch it
is executing, it MUST drop its switch immediately and enter the pass-
through state.
The transition of a node from pass-through to switching state MUST be
triggered by:
o An equal priority, a higher priority, or an allowed coexisting
externally initiated command
o The detection of an equal priority, a higher priority, or an
allowed coexisting automatic initiated command
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o The receipt of an equal, a higher priority, or an allowed
coexisting RPS request destined to this node
5.2. RPS State Machine
5.2.1. Initial States
+-----------------------------------+----------------+
| State | Signaled RPS |
+-----------------------------------+----------------+
| A | Idle | NR |
| | Working: no switch | |
| | Protection: no switch | |
+-----+-----------------------------+----------------+
| B | Pass-trough | N/A |
| | Working: no switch | |
| | Protection: pass through | |
+-----+-----------------------------+----------------+
| C | Switching - LP | LP |
| | Working: no switch | |
| | Protection: no switch | |
+-----+-----------------------------+----------------+
| D | Idle - LW | NR |
| | Working: no switch | |
| | Protection: no switch | |
+-----+-----------------------------+----------------+
| E | Switching - FS | FS |
| | Working: switched | |
| | Protection: switched | |
+-----+-----------------------------+----------------+
| F | Switching - SF | SF |
| | Working: switched | |
| | Protection: switched | |
+-----+-----------------------------+----------------+
| G | Switching - MS | MS |
| | Working: switched | |
| | Protection: switched | |
+-----+-----------------------------+----------------+
| H | Switching - WTR | WTR |
| | Working: switched | |
| | Protection: switched | |
+-----+-----------------------------+----------------+
| I | Switching - EXER | EXER |
| | Working: no switch | |
| | Protection: no switch | |
+-----+-----------------------------+----------------+
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5.2.2. State transitions When Local Request is Applied
In the state description below 'O' means that new local request will
be rejected because of exiting request.
=====================================================================
Initial state New request New state
------------- ----------- ---------
A (Idle) LP C (Switching - LP)
LW D (Idle - LW)
FS E (Switching - FS)
SF F (Switching - SF)
Recover from SF N/A
MS G (Switching - MS)
Clear N/A
WTR expires N/A
EXER I (Switching - EXER)
=====================================================================
Initial state New request New state
------------- ----------- ---------
B (Pass-trough) LP C (Switching - LP)
LW B (Pass-trough)
FS O - if current state is due to
LP sent by another node
E (Switching - FS) - otherwise
SF O - if current state is due to
LP sent by another node
F (Switching - SF) - otherwise
Recover from SF N/A
MS O - if current state is due to
LP, SF or FS sent by
another node
G (Switching - MS) - otherwise
Clear N/A
WTR expires N/A
EXER O
=====================================================================
Initial state New request New state
------------- ----------- ---------
C (Switching - LP) LP N/A
LW O
FS O
SF O
Recover from SF N/A
MS O
Clear A (Idle) - if there is no
failure in the ring
F (Switching - SF) - if there
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is a failure at this node
B (Pass-trough) - if there is
a failure at another node
WTR expires N/A
EXER O
=====================================================================
Initial state New request New state
------------- ----------- ---------
D (Idle - LW) LP C (Switching - LP)
LW N/A - if on the same span
D (Idle - LW) - if on another
span
FS O - if on the same span
E (Switching - FS) - if on
another span
SF O - if on the addressed span
F (Switching - SF) - if on
another span
Recover from SF N/A
MS O - if on the same span
G (Switching - MS) - if on
another span
Clear A (Idle) - if there is no
failure on addressed span
F (Switching - SF) - if there
is a failure on this span
WTR expires N/A
EXER O
=====================================================================
Initial state New request New state
------------- ----------- ---------
E (Switching - FS) LP C (Switching - LP)
LW O - if on another span
D (Idle - LW) - if on the same
span
FS N/A - if on the same span
E (Switching - FS) - if on
another span
SF O - if on the addressed span
E (Switching - FS) - if on
another span
Recover from SF N/A
MS O
Clear A (Idle) - if there is no
failure in the ring
F (Switching - SF) - if there
is a failure at this node
B (Pass-trough) - if there is
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a failure at another node
WTR expires N/A
EXER O
=====================================================================
Initial state New request New state
------------- ----------- ---------
F (Switching - SF) LP C (Switching - LP)
LW O - if on another span
D (Idle - LW) - if on the same
span
FS E (Switching - FS)
SF N/A - if on the same span
F (Switching - SF) - if on
another span
Recover from SF H (Switching - WTR)
MS O
Clear N/A
WTR expires N/A
EXER O
=====================================================================
Initial state New request New state
------------- ----------- ---------
G (Switching - MS) LP C (Switching - LP)
LW O - if on another span
D (Idle - LW) - if on the same
span
FS E (Switching - FS)
SF F (Switching - SF)
Recover from SF N/A
MS N/A - if on the same span
G (Switching - MS) - if on
another span release the
switches but signal MS
Clear A
WTR expires N/A
EXER O
=====================================================================
Initial state New request New state
------------- ----------- ---------
H (Switching - WTR) LP C (Switching - LP)
LW D (Idle - W)
FS E (Switching - FS)
SF F (Switching - SF)
Recover from SF N/A
MS G (Switching - MS)
Clear A
WTR expires A
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EXER O
=====================================================================
Initial state New request New state
------------- ----------- ---------
I (Switching - EXER) LP C (Switching - LP)
LW D (idle - W)
FS E (Switching - FS)
SF F (Switching - SF)
Recover from SF N/A
MS G (Switching - MS)
Clear A
WTR expires N/A
EXER N/A - if on the same span
I (Switching - EXER)
=====================================================================
5.2.3. State Transitions When Remote Request is Applied
The priority of a remote request does not depend on the side from
which the request is received.
=====================================================================
Initial state New request New state
------------- ----------- ---------
A (Idle) LP C (Switching - LP)
FS E (Switching - FS)
SF F (Switching - SF)
MS G (Switching - MS)
WTR N/A
EXER I (Switching - EXER)
RR N/A
NR A (Idle)
=====================================================================
Initial state New request New state
------------- ----------- ---------
B (Pass-trough) LP C (Switching - LP)
FS N/A - cannot happen when there
is LP request in the ring
E (Switching - FS) - otherwise
SF N/A - cannot happen when there
is LP request in the ring
F (Switching - SF) - otherwise
MS N/A - cannot happen when there
is LP, FS or SF request
in the ring
G (Switching - MS) - otherwise
WTR N/A - cannot happen when there
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is LP, FS, SF or MS
request in the ring
EXER N/A - cannot happen when there
is LP, FS, SF, MS or WTR
request in the ring
I (Switching - EXER) -
otherwise
RR N/A
NR A (Idle) - if received from
both sides
=====================================================================
Initial state New request New state
------------- ----------- ---------
C (Switching - LP) LP C (Switching - LP)
FS N/A - cannot happen when there
is LP request in the ring
SF N/A - cannot happen when there
is LP request in the ring
MS N/A - cannot happen when there
is LP request in the ring
WTR N/A
EXER N/A - cannot happen when there
is LP request in the ring
RR C (Switching - LP)
NR N/A
=====================================================================
Initial state New request New state
------------- ----------- ---------
D (Idle - LW) LP C (Switching - LP)
FS E (Switching - FS)
SF F (Switching - SF)
MS G (Switching - MS)
WTR N/A
EXER I (Switching - EXER)
RR N/A
NR D (Idle - LW)
=====================================================================
Initial state New request New state
------------- ----------- ---------
E (Switching - FS) LP C (Switching - LP)
FS E (Switching - FS)
SF E (Switching - FS)
MS N/A - cannot happen when there
is FS request in the ring
WTR N/A
EXER N/A - cannot happen when there
is FS request in the ring
RR E (Switching - FS)
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NR N/A
=====================================================================
Initial state New request New state
------------- ----------- ---------
F (Switching - SF) LP C (Switching - LP)
FS F (Switching - SF)
SF F (Switching - SF)
MS N/A - cannot happen when there
is SF request in the ring
WTR N/A
EXER N/A - cannot happen when there
is SF request in the ring
RR F (Switching - SF)
NR N/A
=====================================================================
Initial state New request New state
------------- ----------- ---------
G (Switching - MS) LP C (Switching - LP)
FS E (Switching - FS)
SF F (Switching - SF)
MS G (Switching - MS) - release
the switches but signal MS
WTR N/A
EXER N/A - cannot happen when there
is MS request in the ring
RR G (Switching - MS)
NR N/A
=====================================================================
Initial state New request New state
------------- ----------- ---------
H (Switching - WTR) LP C (Switching - LP)
FS E (Switching - FS)
SF F (Switching - SF)
MS G (Switching - MS)
WTR H (Switching - WTR)
EXER N/A - cannot happen when there
is WTR request in the ring
RR H (Switching - WTR)
NR N/A
=====================================================================
Initial state New request New state
------------- ----------- ---------
I (Switching - EXER) LP C (Switching - LP)
FS E (Switching - FS)
SF F (Switching - SF)
MS G (Switching - MS)
WTR N/A
EXER I (Switching - EXER)
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RR I (Switching - EXER)
NR N/A
=====================================================================
5.2.4. State Transitions When Request Addresses to Another Node is
Received
The priority of a remote request does not depend on the side from
which the request is received.
=====================================================================
Initial state New request New state
------------- ----------- ---------
A (Idle) LP B (Pass-trough)
FS B (Pass-trough)
SF B (Pass-trough)
MS B (Pass-trough)
WTR B (Pass-trough)
EXER B (Pass-trough)
RR N/A
NR N/A
=====================================================================
Initial state New request New state
------------- ----------- ---------
B (Pass-trough) LP B (Pass-trough)
FS N/A - cannot happen when there
is LP request in the ring
B (Pass-trough) - otherwise
SF N/A - cannot happen when there
is LP request in the ring
B (Pass-trough) - otherwise
MS N/A - cannot happen when there
is LP, FS or SF request
in the ring
B (Pass-trough) - otherwise
WTR N/A - cannot happen when there
is LP, FS, SF or MS
request in the ring
B (Pass-trough) - otherwise
EXER N/A - cannot happen when there
is LP, FS, SF, MS or WTR
request in the ring
B (Pass-trough) - otherwise
RR N/A
NR B (Pass-trough)
=====================================================================
Initial state New request New state
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------------- ----------- ---------
C (Switching - LP) LP C (Switching - LP)
FS N/A - cannot happen when there
is LP request in the ring
SF N/A - cannot happen when there
is LP request in the ring
MS N/A - cannot happen when there
is LP request in the ring
WTR N/A - cannot happen when there
is LP in the ring
EXER N/A - cannot happen when there
is LP request in the ring
RR N/A
NR N/A
=====================================================================
Initial state New request New state
------------- ----------- ---------
D (Idle - LW) LP B (Pass-trough)
FS B (Pass-trough)
SF B (Pass-trough)
MS B (Pass-trough)
WTR B (Pass-trough)
EXER B (Pass-trough)
RR N/A
NR N/A
=====================================================================
Initial state New request New state
------------- ----------- ---------
E (Switching - FS) LP B (Pass-trough)
FS E (Switching - FS)
SF E (Switching - FS)
MS N/A - cannot happen when there
is FS request in the ring
WTR N/A - cannot happen when there
is FS request in the ring
EXER N/A - cannot happen when there
is FS request in the ring
RR N/A
NR N/A
=====================================================================
Initial state New request New state
------------- ----------- ---------
F (Switching - SF) LP B (Pass-trough)
FS F (Switching - SF)
SF F (Switching - SF)
MS N/A - cannot happen when there
is SF request in the ring
WTR N/A - cannot happen when there
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is SF request in the ring
EXER N/A - cannot happen when there
is SF request in the ring
RR N/A
NR N/A
=====================================================================
Initial state New request New state
------------- ----------- ---------
G (Switching - MS) LP B (Pass-trough)
FS B (Pass-trough)
SF B (Pass-trough)
MS G (Switching - MS) - release
the switches but signal MS
WTR N/A - cannot happen when there
is MS request in the ring
EXER N/A - cannot happen when there
is MS request in the ring
RR N/A
NR N/A
=====================================================================
Initial state New request New state
------------- ----------- ---------
H (Switching - WTR) LP B (Pass-trough)
FS B (Pass-trough)
SF B (Pass-trough)
MS B (Pass-trough)
WTR N/A
EXER N/A - cannot happen when there
is WTR request in the ring
RR N/A
NR N/A
=====================================================================
Initial state New request New state
I (Switching - EXER) LP B (Pass-trough)
FS B (Pass-trough)
SF B (Pass-trough)
MS B (Pass-trough)
WTR N/A
EXER I (Switching - EXER)
RR N/A
NR N/A
=====================================================================
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6. IANA Considerations
The Channel Types for the Generic Associated Channel are allocated
from the IANA PW Associated Channel Type registry defined in
[RFC4446] and updated by [RFC5586].
IANA is requested to allocate a further Channel Type as follows:
o TBA Ring Protection Switching (RPS)
Note to RFC Editor: this section may be removed on publication as an
RFC.
7. Security Considerations
This document does not by itself raise any particular security
considerations.
8. Contributing Authors
Wen Ye, Minxue Wang, Sheng Liu (China Mobile)
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
[RFC5586] Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
Associated Channel", RFC 5586, June 2009.
[RFC5654] Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N.,
and S. Ueno, "Requirements of an MPLS Transport Profile",
RFC 5654, September 2009.
[RFC6371] Busi, I. and D. Allan, "Operations, Administration, and
Maintenance Framework for MPLS-Based Transport Networks",
RFC 6371, September 2011.
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Authors' Addresses
Weiqiang Cheng
China Mobile
Email: chengweiqiang@chinamobile.com
Lei Wang
China Mobile
Email: wangleiyj@chinamobile.com
Han Li
China Mobile
Email: lihan@chinamobile.com
Huub van Helvoort
Hai Gaoming BV
Email: huubatwork@gmail.com
Kai Liu
Huawei Technologies
Email: alex.liukai@huawei.com
Jie Dong
Huawei Technologies
Email: jie.dong@huawei.com
Jia He
Huawei Technologies
Email: hejia@huawei.com
Fang Li
China Academy of Telecommunication Research, MIIT., China
Email: lifang@ritt.cn
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Jian Yang
ZTE Corporation P.R.China
Email: yang.jian90@zte.com.cn
Junfang Wang
Fiberhome Telecommunication Technologies Co., LTD.
Email: wjf@fiberhome.com.cn
Cheng, et al. Expires August 6, 2015 [Page 42]