Internet DRAFT - draft-cheung-mpls-tp-mesh-protection
draft-cheung-mpls-tp-mesh-protection
MPLS Working Group T. Cheung
Internet-Draft J. Ryoo
Intended status: Standards Track ETRI
Expires: September 13, 2012 Y. Weingarten
N. Sprecher
Nokia Siemens Networks
D. King
Old Dog Consulting
March 12, 2012
MPLS-TP Shared Mesh Protection
draft-cheung-mpls-tp-mesh-protection-05.txt
Abstract
This document describes a mechanism to address the requirement to
support protection of Label Switched Paths (LSPs) in an MPLS
Transport Profile (MPLS-TP) mesh topology. The shared mesh
protection mechanism enables multiple protection paths within a
shared mesh protection domain to share protection resources for the
protection of working paths by coordinating protection switching
operations according to the priority assigned to each end-to-end
linear protection domain.
This document is a product of a joint Internet Engineering Task Force
(IETF) / International Telecommunications Union Telecommunications
Standardization Sector (ITU-T) effort to include an MPLS Transport
Profile within the IETF MPLS and PWE3 architectures to support the
capabilities and functionalities of a packet transport network as
defined by the ITU-T.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 13, 2012.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in this Document . . . . . . . . . . . . . . 5
2.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Definitions and Terminology . . . . . . . . . . . . . . . 5
3. Shared Mesh Protection Architecture . . . . . . . . . . . . . 6
3.1. Shared Mesh Protection Group . . . . . . . . . . . . . . . 7
3.2. Shared Start and End Nodes . . . . . . . . . . . . . . . . 8
3.3. SMP protocol communication channel . . . . . . . . . . . . 10
3.4. Network planning for SMP . . . . . . . . . . . . . . . . . 10
3.5. Preemption and race conditions . . . . . . . . . . . . . . 11
3.6. SMP Overview . . . . . . . . . . . . . . . . . . . . . . . 12
3.6.1. LP Protocol extensions for shared protection . . . . . 12
3.6.2. Notifying protection switching event . . . . . . . . . 13
3.6.3. Requesting lockout of protection . . . . . . . . . . . 13
3.6.4. Resolving race conditions . . . . . . . . . . . . . . 14
4. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. PDU Format . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2. Message Transmission . . . . . . . . . . . . . . . . . . . 17
5. Operation of Shared Mesh Protection . . . . . . . . . . . . . 17
6. Manageability Considerations . . . . . . . . . . . . . . . . . 20
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Normative References . . . . . . . . . . . . . . . . . . . 21
9.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
The MPLS Transport Profile (MPLS-TP) is a packet transport technology
based on a profile of the MPLS and Pseudowires (PW) as described in
[RFC3031], [RFC3985], and [RFC5085]. MPLS-TP is the application of
MPLS to the construction of packet-switched paths that are analogous
to traditional circuit-switched technologies. Requirements for
MPLS-TP are specified in [RFC5654].
An important feature of a transport network is its survivability
function and the ability to maintain or recover traffic following a
network failure or attack. According to Requirement 56 of [RFC5654],
MPLS-TP must provide protection and restoration mechanisms, and it
must also be possible to protect 100% of the traffic on the protected
path (Requirement 58).
1+1 and 1:1 linear protection meets these requirements by reserving
the equivalent amount of network resources for the protection paths
as is allocated to the normal traffic that is being protected. While
those dedicated protection mechanisms provide very good protection
capabilities, they are resource inefficient and will increase overall
network resource consumption. Deploying 1+1 and 1:1 protection
mechanisms for all services that require resiliency, dramatically
increases network costs.
[RFC5654] also establishes that MPLS-TP should support shared
protection (Requirement 68). 1:n end-to-end protection uses one
protection path to protect n working paths between the same two
endpoints. This improves overall network utilization, but the
resource (bandwidth) allocated to a protection path is typically not
sufficient to protect multiple simultaneous failures on different
working paths. If multiple working paths require concurrent
protection switching, the path with the highest priority should be
protected as described in [RFC6372].
In 1+1 and 1:1 protection, the end nodes of the working path must be
the same as those of the protection path. Similarly in 1:n
protection all pairs of end nodes of the n working paths are the
same, and the protection path must also have the same end nodes. In
the event that the MPLS-TP network scales up, the number of Label
Switched Paths (LSPs) having different end nodes will also increase.
The network utilization benefit for sharing protection resources
among multiple protected domains for such LSPs will increase
accordingly.
Requirement 68 of [RFC5654] specifies that MPLS-TP should support 1:n
shared mesh recovery, and Requirement 69 states that MPLS-TP must
support sharing of protection resources. It may be possible that
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some working paths are sufficiently disjoint and would be unlikely to
be simultaneously affected by a single network failure. Typically,
such a scenario is hard to track in real network environments where
new services are often added and removed.
In mesh protection, network resources may be shared to provide
protection for working paths that do not share the same end nodes at
the edge of a protection domain. This type of protection can make
very efficient use of network resources, but requires coordination of
several segments in order to ensure that only a single traffic flow
is switched to the protection resources at any time.
[RFC4428] defines two shared mesh recovery schemes named (1:1)^n and
(M:N)^n. The (1:1)^n recovery scheme is a simple case of (M:N)^n
recovery scheme. In (1:1)^n protection, n working paths are
protected by n dedicated protection paths while sharing the same
protection bandwidth. The protection bandwidth can be optimized to
allow only one of the n working paths to be protected at any time.
In this case, it achieves network utilization similar to 1:n
protection.
It should be noted that the (1:1)^n protection scheme described in
[RFC4428] differs with that defined in [G.808.1] in that the former
allows each n pairs of working and protection paths to have different
end nodes while the latter applies to the case where all pairs have
the same end nodes.
This document defines a data-plane shared mesh protection mechanism
based on the concept of the (1:1)^n recovery scheme described in
[RFC4428] and a protocol for coordination of the shared protection
resources. The actual protection switching is controlled by end-to-
end linear protection, while the usage of the shared resources is
based on the protection switching priority assigned to each pair of
working and protection paths.
The shared mesh protection mechanism defined in this document
utilizes the existing MPLS-TP linear protection switching mechanism,
and assumes that the protection paths are established and ready to
forward data prior to a failure. Upon detection of a failure on a
working path, only the two end nodes of the failed working path
exchange their linear protection protocol messages to switch data
traffic. No explicit activation procedure to switch data traffic to
the protection path is needed in the intermediate nodes along the
protection path. However, the intermediate nodes that are part of
the shared segments need to coordinate the resource allocation on the
shared nodes and this coordination will be addressed by the protocol
proposed in this document.
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2. 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 [RFC2119].
2.1. Acronyms
This draft uses the following acronyms:
G-ACh Generic Associated Channel Header
LoP Lockout of Protection
LP Linear Protection
LSP Label Switched Path
MIP Maintenance Entity Group Intermediate Point
MPLS-TP Transport Profile for MPLS
P2P Point-to-point
P2MP Point-to-multipoint
PS Protection Switching
PW Pseudowire
RA Resource Allocation
SEN Shared End Node
SMP Shared Mesh Protection
SMPG Shared Mesh Protection Group
SPME Sub-Path Maintenance Entity
SRLG Shared Risk Link Group
SSN Shared Start Node
2.2. Definitions and Terminology
This document defines two protection domains as follows:
o End-to-end linear protection (LP) domain: A protection domain as
defined in [RFC6372] for protecting a P2P or P2MP LSP. It
consists of two or more endpoints at the boundary of the domain
and a working path and a protection path between the end nodes.
An end-to-end linear protection switching protocol runs within the
domain.
o Shared mesh protection (SMP) domain: A protection domain for
protecting a number of P2P or P2MP LSPs. It consists of a number
of end-to-end linear protection domains. Each end-to-end linear
protection domain shares protection resources with other domains.
The shared protection resource may be a node, link, transport path
segment or concatenated transport path segment. A shared mesh
protection switching protocol runs within the domain.
In addition, we define the following:
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o Protection segment: A protection segment is a portion of the end-
to-end protection path. An end-to-end protection path can be
broken down into separate protection segments. A protection
segment may either be a shared protection segment or a dedicated
protection segment. A shared protection segment is a segment that
has a set of resources, e.g. link bandwidth that is reserved as
shared amongst several end-to-end protection paths. A dedicated
protection segment is a segment of the end-to-end protection path
whose resources are reserved for use only by that protection path.
o Shared mesh protection group (SMPG): A protection group includes
the pairs of working and protection paths, whose working paths do
not belong to a single SRLG and whose protection paths share the
resource of a single shared protection segment. Note that an LSP
may belong to multiple SMPGs.
3. Shared Mesh Protection Architecture
The shared mesh protection domain shown in Figure 1 has two end-to-
end linear protection domains. One consists of the two end nodes A
and E and includes one working path, ABCDE, and one dedicated
protection path APQRE. The second consists of end nodes V and Z and
one working path, VWXYZ, and the dedicated protection path, VPQRZ.
Those two LP domains include a shared protection segment PQR for
their protection paths. This illustrates a simple configuration of
shared mesh protection. Note that the two working paths, ABCDE and
VWXYZ, do not share endpoints so they cannot make use of 1:n
protection even though they also do not share any potential common
points of failure.
It is possible to apply linear protection to each of these working
paths individually. If there are no failures affecting either of the
two working paths, the shared protection segment PQR carries no
traffic (or only interruptible extra traffic). In the event of only
one failure on a working path, the segment PQR carries traffic from
the working path that detected the failure. Only in the event that
there are failures detected on both of the working paths is there a
conflict over the appropriate use of the shared protection segment
PQR. It is important to note that there are two distinct LSPs (i.e.
APQRE and VPQRZ) that are signaled over the shared protection segment
and that although we refer to the singular segment, the traffic is
actually being transported on separated transport paths.
Thus, it is possible for the network resources of segment PQR to be
shared by the two protection paths. In this way, shared mesh
protection can substantially reduce the amount of network resources
that need to be reserved to provide protection of the multiple paths
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within the same protection group.
A----B----C----D----E
\ /
\ /
\ /
P-----Q-----R
/ \
/ \
/ \
V----W----X----Y----Z
Figure 1: A Shared Mesh Protection Topology
3.1. Shared Mesh Protection Group
In Figure 1, two working paths, ABCDE and VWXYZ and their
corresponding protection paths, APQRE and VPQRZ, are considered a
Shared Mesh Protection Group (SMPG). As pointed out above, each
protection path belonging to a SMPG is an individual LSP, but it
shares the resources with others in a segment. The resources that
are being shared are the nodes, ports, links and bandwidth of the
segment.
The shared resources, for example bandwidth capacity, should be
reserved in partitions according to the different SMPGs at the
particular segment.
A------B-------C D------E
\ / / \
\ / / \
F---G----H-----J------K-----L
/ / \ \
/ M----------N \
/ \
V-------W-------X-------Y-------Z
Figure 2: Shared Mesh Protection Groups
To further clarify, consider the mesh network in Figure 2. In this
figure we have the following working paths and corresponding
protection paths:
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+----+--------------+-----------------+
| Wx | working path | protection path |
+----+--------------+-----------------+
| W1 | A-B-C | A-F-G-H-C |
| W2 | D-E | D-J-K-L-E |
| W3 | M-N | M-J-K-N |
| W4 | V-W-X-Y-Z | V-G-H-J-K-L-Z |
+----+--------------+-----------------+
In this network we would define three SMPG - characterized by the
three shared segments -
o S1 segment G-H - shared by W1 and W4
o S2 segment J-K - shared by W2, W3, and W4
o S3 segment K-L - shared by W2 and W4
The shared segment is always the smallest segment that is shared by
multiple protection paths. Therefore, even though segment J-K-L is
shared by W2 and W4, we split this into two shared segments - J-K and
K-L, since W3 also shares the resources of segment J-K.
In addition, this demonstrates that a single working path may be a
member of a number of SMPGs. Also a single SMPG may include more
than two working paths.
3.2. Shared Start and End Nodes
For the sake of the discussion of the SMP operation, we designate the
two end nodes of the shared protection segment as a Shared Start Node
(SSN) and Shared End Node (SEN). To simplify the discussion, this
designation is based on referencing the protection path as a pair of
unidirectional LSPs.
A SSN is the first node of a unidirectional shared protection
segment. For example, in Figure 1, node P is a SSN on unidirectional
protection paths A-P-Q-R-E and V-P-Q-R-Z. SSN may act as a
Maintenance Entity Group Intermediate Point (MIP) for each protection
path sharing the same protection resources.
Similarly, a SEN is defined as the last node of a unidirectional
shared protection segment (for example, node R on unidirectional
protection paths A-P-Q-R-E and V-P-Q-R-Z in Figure 1). A SEN acts as
a MIP on each protection path that shares the protection resource.
Both SEN and SSN are involved in coordinating the use of the
unidirectional shared protection segment during the shared mesh
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protection operation.
Table 1 summarizes the relationship between SSN and SEN of the shared
protection segment and protection paths sharing it as illustrated in
Figure 1.
Table 1: SSN/SEN in Figure 1
+----------------------+---------------------------+-----+-----+
| Protection paths | Shared protection segment | SSN | SEN |
+----------------------+---------------------------+-----+-----+
| A-P-Q-R-E, V-P-Q-R-Z | P-Q-R | P | R |
| E-R-Q-P-A, Z-R-Q-P-V | R-Q-P | R | P |
+----------------------+---------------------------+-----+-----+
Figure 3 shows a more complex example of the shared mesh protection
domain. Three working paths ABC, DEF, and GHJ are protected by the
protection paths APQC, DRSF, and GPQRSJ, respectively. Table 2
summarizes the relationship between SSN and SEN of the shared
protection segments and the related protection paths in the
protection domain illustrated in Figure 3.
A------B------C D------E------F
\ / \ /
\ / \ /
\ / \ /
P-----Q----------R-----S
/ \
/ \
/ \
G--------------H---------------J
Figure 3: A More Complex Mesh Protection Example
Table 2: SSN/SEN in Figure 3
+----------------------+---------------------------+-----+-----+
| Protection paths | Shared protection segment | SSN | SEN |
+----------------------+---------------------------+-----+-----+
| A-P-Q-C, G-P-Q-R-S-J | P-Q | P | Q |
| C-Q-P-A, J-S-R-Q-P-G | Q-P | Q | P |
| D-R-S-F, G-P-Q-R-S-J | R-S | R | S |
| F-S-R-D, J-S-R-Q-P-G | S-R | S | R |
+----------------------+---------------------------+-----+-----+
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3.3. SMP protocol communication channel
The MPLS-TP Framework [RFC5921] defines the concept of a Sub-Path
Maintenance Entity (SPME) and together with [RFC5586] defines the use
of the Generic Associated Channel (G-ACh) for communication of
MPLS-TP control protocols between the endpoints of a maintenance
entity. While the usual utility of a SPME is to allow tunneling of
transport traffic while monitoring the segment with in-band
connectivity verification messages, it is possible to use concept of
a SPME to describe a LSP that is dedicated to carry a control
protocol over the G-ACh between the endpoints of the shared
protection segment and the endpoints of the protection paths within
the SMPG.
For example, referring to the network in Figure 3, we would configure
the following SPME (without identifying the intermediate nodes):
A-P, G-P, P-Q, Q-C and Q-J (for SMPG 1 sharing P-Q), and
D-R, G-R, R-S, S-F and S-J (for SMPG 2 sharing R-S).
These SPMEs are bidirectional LSPs that are not used to carry any
data traffic, but only the SMP protocol traffic described in
Section 4.
The communication channel between the SSN and SEN of the shared
protection segment and between themselves and the endpoints of the
protection paths within the SMPG is to coordinate the allocation of
the shared segment to a single protection path during a protection
switching condition. This process is described more fully in
Section 3.6
3.4. Network planning for SMP
Shared mesh protection will typically be dependent upon careful
network planning. This includes:
o Preparing the working and protection paths for the different
services that require protection.
o Determining which working paths are disjoint and so will not be
subject to common failures. It should be clear that working paths
within the same SRLG should not be included in the same SMPG.
o Identifying which protection paths share network resources and can
constitute a SMPG. Signaling or configuring the proper path
information for the shared segment endpoints to allow for
communication between the corresponding endpoints of the shared
segment and the protection path.
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o Assigning Protection Switching Priority and a path identifier for
each working path within a shared mesh protection domain.
o Ensuring that working paths of high Protection Switching Priority
do not share resources on their protection paths in such a way
that would mean that one of them could be unprotected.
o Enabling the necessary shared mesh protection functions at the
endpoints of the shared protection segments. This includes
preparing the different SPME used for communication between the
corresponding endpoints of the shared protection segments and the
protection paths, as well as between the endpoints of the shared
protection segment.
Note that some control plane features of GMPLS may be used to
dynamically configure shared mesh protection. These features are out
of scope for this document which focuses on the operation of shared
mesh protection switching once it has been configured.
3.5. Preemption and race conditions
In the normal operation of SMP, when a working path triggers a
protection switch, and requests allocation of the shared resources,
the SMP process should verify that the resources are available and
allocate them to the requesting protection path. There are some
cases where the determination of the availability is not simply
determined.
Within the SMP domain, there is a need to define a "Protection
Switching Priority" for each working path. This Protection Switching
Priority will be used to determine the use of the shared protection
resources in cases of possible preemption. When the shared resources
are in use protecting the traffic of a failed working path and a
second working path fails, the SMP process should compare the
Protection Switching Priority of the two working paths and if the
priority of the second path is higher than the priority of the
currently protected traffic, then this second path will preempt the
currently protected traffic. If the second path has a lower or equal
priority to the currently protected traffic, then the second path is
locked-out of the protection resources.
The Protection Switching Priority may be provisioned by the network
management system or configured by some other mechanism that is
outside the scope of this document.
There is an additional case where the SMP process needs to make a
determination of which working path should be allowed to be protected
using the shared resources. This is the case of multiple working
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paths triggering a protection switch virtually simultaneously. This
may result in a race condition where the two endpoints of the shared
protection segment ostensibly receive requests from two different
working paths. By default, working paths with equal priority result
in first-come first-served recovery. If multiple working paths
request protection switching simultaneously, a pre-defined identifier
assigned to each working path in the SMP domain MUST be used to
determine the priority among them. The definition of the identifier
is for further study.
3.6. SMP Overview
When a protection switching trigger is activated on any of the
working paths within a SMPG, then the local linear protection
mechanism (in 1:1 protection mode) should cause a protection switch.
If, as a result of the protection switch action, there is a need to
transmit working data on the protection path then the endpoint of LP
domain should inform the endpoint of the shared protection segment of
the allocation of the shared resources.
At this point, the shared segment endpoints should notify all of the
other protection paths in the SMPG that the resources have been
allocated, which could affect the linear protection actions relative
to future triggers.
3.6.1. LP Protocol extensions for shared protection
The shared mesh protection mechanism is designed to fully utilize the
existing end-to-end LP switching on the working paths. These LP
domains SHALL operate in revertive mode. The LP protocol should use
the normal procedures for LP without any changes except support for
the following additional functionalities:
o Function to generate a protection switching event message to the
SEN when a switching trigger occurs at the end-to-end LP domain.
Switching to the protection path or reverting to the working path
should be notified.
o Function to take a Lockout of Protection (LoP) request message
from the SEN, and incorporate it as the Lockout of Protection
(LoP) command assertion or clearance.
o Function to acknowledge the SEN when the LP domain completes the
LoP operation.
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3.6.2. Notifying protection switching event
If the endpoint of a working path detects a switching trigger, it
triggers the protection switching and exchanges LP switching protocol
messages with far endpoint. This operation is independent of the SMP
switching mechanism specified in this document.
At the same time, for the operation of SMP, the protection path
endpoint notifies its protection switching event to SENs by sending a
"Protection Switching (PS) Event" message.
The PS Event message MUST be transmitted immediately when an endpoint
of the end-to-end LP domain changes its selector position either from
working to protection or vice versa. The event message SHALL be
transmitted over the SPME that is configured between the protection
path endpoint and the SEN, using the G-ACh. When bidirectional
protection switching is being used by the working path, both
endpoints will transmit the event messages to their corresponding
SENs using the properly configured SPME. When unidirectional
protection switching is being used and a unidirectional failure is
detected, only the detecting endpoint will send the messages to its
corresponding SENs.
The endpoint of the protection path that is becoming active (or
released) sends the messages directly to each SEN. This requires
that N messages are sent, where N is the number of SMPG that the
working path is a member of. This, of course, implies that the
endpoints are pre-configured with knowledge of all SENs associated
with the SMPG.
3.6.3. Requesting lockout of protection
When a SEN receives the PS Event message notifying that protection
switching to the protection path has begun in an end-to-end LP domain
and that the shared resources are to be allocated, it compares the
Protection Switching Priority of the working path notifying the event
with those of other LP domains in the same SMPG.
The SEN determines which of the LP domains (within the SMPG) have a
lower or equal priority to that of the notifying LP domain. The SEN
then sends a "Lockout of Protection (LoP) Request" message to the
endpoints of these protection paths that is equivalent to a "Lockout
of Protection" operator command. This prevents any protection
switching actions in those LP domains. For those LP domains having
higher priorities no request is transmitted and those LP domains may
continue to perform protection switching actions which they require.
When a protection path endpoint receives the LoP Request message from
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an SEN, it SHOULD react as if a LoP command was received, according
to the actions dictated by the LP protocol. Since the LoP command
has the highest priority in the LP switching protocol, it will
inhibit any further protection switching in the LP domain.
If the LP domain that received the LoP Request message is currently
transmitting traffic on the protection path, it SHALL immediately
stop transmitting the traffic on the protection path and release the
allocated resources.
When the protection path endpoint completes LoP operation, it SHOULD
immediately reply with a "LoP Acknowledgement" message to inform the
completion of the LoP operation to the SEN.
To minimize potential congestion that may occur when a protection
path having higher priority pre-empts other protection paths having
lower priorities, the SEN SHOULD block forwarding traffic from the
protection paths having lower priorities until it receives the LoP
Ack message from the endpoints of those protection paths.
When a SEN receives a PS Event message indicating that the shared
protection resources are being released, i.e. the LP domain is
reverting to normal state, it sends a LoP Request message to the
endpoints of all the protection paths in the SMPG that were
previously locked (i.e. those with equal or lower priority) to clear
the LoP command. The endpoint of the protection path that receives
this message SHALL react as if a Clear command was received.
3.6.4. Resolving race conditions
As was pointed out in Section 3.5 there are some cases, in particular
in unidirectional protection switching triggers, of simultaneous
protection switching that could cause race conditions. In these use-
cases there is a need for the two end nodes of the shared protection
segment, i.e. the SEN and the SSN, to coordinate the selection of the
LP domain that will be allocated the shared protection resources.
For this purpose, additional messages are defined that are
transmitted on the SPME that is defined between the end nodes of the
shared protection segment. When a SEN receives a PS Event message
from a LP domain indicating that protection switching to the
protection path has begun, it SHALL send a "Resource Allocation (RA)
Notification" message to the SSN that the resources have been
allocated, with an indication of the working path identifier. This
allocation needs to be confirmed for cases where both end nodes
report allocation to different working path identifiers.
The race condition can occur only when more than one protection paths
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are configured to have the same Protection Switching Priority within
a SMP domain.
When the SSN receives the RA Notification message from the SEN, it
first checks whether it has received a PS Event message from an
endpoint of the other LP domain having the same Protection Switching
Priority that corresponds to the LP domain sending the RA
notification message.
If both have the same priority, the SSN compares the working path
identifier and sends an RA Ack message to the SEN only when it is
determined that the working path identifier contained in the RA
Notification message to have been allocated the shared protection
resources. Each working path or LP domain has a unique identifier
within a SMP domain and rules for deciding the priority will be
defined in later.
The SEN does not perform the LoP request procedure described in
Section 3.6.3 until it receives an RA Ack from the SSN. This results
in the overall protection switching time to be increased. To avoid
this, it is RECOMMENDED to configure none of the working paths
sharing the protection segment in a SMP domain to have the same
Protection Switching Priority.
4. Protocol
4.1. PDU Format
The shared mesh protection protocol messages MUST be sent over a
G-ACh as defined in [RFC5586].
The shared mesh protection protocol messages are as follows:
o Protection Switching (PS) Event message [sent from protection path
endpoint to SEN]
o Lockout of Protection (LoP) Request message [sent from SEN to
protection path endpoint]
o Lockout of Protection (LoP) Acknowledgement message [sent from
protection path endpoint to SEN]
o Resource Allocation (RA) Notification message [sent from SEN to
SSN]
o Resource Allocation (RA) Acknowledgement message [sent from SSN to
SEN]
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The channel type in ACH is used to indicate that the message is a SMP
protocol message. The protocol message MUST follow the ACH.
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|Version| Reserved | Channel Type = Shared Mesh P. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Shared Mesh Protection Protocol Message |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Shared mesh protection protocol message header
The SMP protocol message format is defined as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Mode | Type | ST| Reserved | ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Shared mesh protection protocol message format
Each protocol message includes the following fields:
o Version - Version indicator
0x0 - reserved
0x1 - This version
0x2~0xF - reserved
o Mode - SMP switching and operation mode
0x0 - Bidirectional
0x1 - Unidirectional
0x2~0xF - reserved
o Type - SMP protocol message type indicator
0x0 - reserved
0x1 - PS Event
0x2 - LoP Request
0x3 - LoP Acknowledgement
0x4 - RA Notification
0x5 - RA Acknowledgement
0x6~0xF - reserved
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o ST - SMP protocol message sub-type indicator
0x0 - Switch to working (for PS Event),
Clear LoP (for LoP Request)
0x1 - Switch to protection (for PS Event),
Assert LoP (for LoP Request)
0x2~0x3 - reserved
o ID - This is either the identifier of the LP domain that is
sending the message or the working path that was allocated
to the resources (dependent upon the message).
0x0 - reserved
0x1~0xFE - ID value
0xFF - reserved
o Reserved - reserved fields
All reserved bits SHOULD be zero upon transmission, and MUST be
ignored on reception.
4.2. Message Transmission
A new message must be transmitted immediately. The first three
messages should be transmitted as fast as possible so that fast
protection switching is possible even if one or two messages are lost
or corrupted. The interval of the first three messages should be
less than 3.3ms. Messages after the first three should be
transmitted with the interval of 5 seconds.
If no valid message is received, the last valid received information
remains applicable.
5. Operation of Shared Mesh Protection
This section illustrates the operation of the SMP protocol based on
the example illustrated in Figure 3 and the following assumptions:
o The SMP domain consists of the following end-to-end LP domains
(LPDs):
* LPD1: Working path ABC (W1) / Protection path APQC (P1)
* LPD2: Working path GHJ (W2) / Protection path GPQRSJ (P2)
* LPD3: Working path DEF (W3) / Protection path DRSF (P3)
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o The SMP domain includes the following SMPG:
* S1: LPD1 & LPD2 (Shared protection segment - PQ)
* S2: LPD3 & LPD2 (Shared protection segment - RS)
o Protection Switching Priority is LPD1 > LPD2 > LPD3 (i.e. LPD1
has the highest priority.)
o All working paths are protected by 1:1 bidirectional protection
switching.
If a unidirectional failure occurs on W2 in the direction from node H
to node G as shown in Figure 6, SMP will perform the following:
a. Node G detects the failure, and initiates linear protection
switching for the failed W2.
b. At the same time, node G transmits the PS Event message notifying
the SENs of the shared protection segments for S1 & S2, i.e. P
and R, that a protection switching event occurred to node G.
c. SEN P compares the Protection Switching Priority of LPD2 with
those of other members of S1, i.e. LPD1. In this example, since
the priority of LPD1 is higher than LPD2, SEN P does not send any
message to node A.
d. SEN R compares the Protection Switching Priority of LPD2 with
those of other members of S2, i.e. LPD3. In this example, as
the priority of LPD3 is lower than LPD2, SEN R sends the RA
Notification message to the SSN S, blocks forwarding of P3 and
sends the LoP Request message requesting the assertion of LoP to
node D.
e. SSN S does not process the RA Notification message. (Since in
this example, all the LP domains are configured to have different
Protection Switching Priorities.)
f. Node D takes the LoP Request message as input to the LP
switching, and follows the LP procedure to process the end-to-end
LoP command. After completion of the LoP operation, node D sends
the LoP Ack message to SEN R.
g. SEN R unblocks forwarding of P3 upon receiving the LoP Ack
message from node D.
h. Since LPD2 operates in 1:1 bidirectional protection switching
mode, node J performs the switching operations (i.e. switches its
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bridge and selector state) to synchronize with node G, and also
transmits the PS Event message to node S and Q, which are SENs
for G->H->J. Using a parallel procedure to that described in
steps c & d, SEN S sends the LoP Request message to node F while
the SEN Q does not take an action to node C.
W1 W3
==A======B======C== ==D======E======F==
\ / \ /
\ LPD1 / \ LPD3 /
\ / \ / == : Normal traffic
P=====Q================R=====S
// \\
// LPD2 \\
// \\
==G------xx---------H------------------J==
SF on G<-H W2
Figure 6: Shared Mesh Protection Example 1
Figure 7 shows a progression from Figure 6. While LPD2 is in
protecting state with its traffic transported on protection path P2,
another unidirectional failure occurs on W1 in the direction from
node B to node A.
In this case, the shared mesh protection will operate as follows:
a. Node A detects the failure, and initiates the linear protection
switching for the failed W1.
b. At the same time, node A transmits the PS Event message notifying
SEN for S1, i.e. node P, that a protection switching event
occurred.
c. SEN P compares the Protection Switching Priority of LPD1 with
those of the other members in S1, in this case LPD2. In this
example, since the priority of LPD2 is lower than LPD1, SEN P
sends the RA Notification message requesting the assertion of LoP
to node G.
d. SSN Q does not process the RA Notification message. (Since in
this example, all the LPDs are configured to have different
Protection Switching Priorities.)
e. Node G accepts the LoP Request message as input to linear
protection switching, and follows LP procedure to process the LoP
command. When LPD2 is forced to lockout its protection path P2,
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it may try to find another available path. m:n protection or
other recovery mechanism may be used for this, but this
discussion is out of scope for this document. After completion
of the LoP operation, node G sends the LoP Ack message to SEN P.
f. SEN P unblocks forwarding of P2 upon receiving the LoP Ack
message from node G.
g. As node G changes its bridge and selector states from protection
to working, it will transmit the PS Event message to the SENs of
S1 & S2, i.e. P & R, notifying that the shared protection
resources should be released.
h. SEN P compares the Protection Switching Priority of LPD2 with the
other members of S1, i.e. LPD1, and does not transmit any
message to node A, but SEN R sends the LoP Request message to
request the clear of LoP to node D, after comparing the
Protection Switching Priorities of the members of S2.
i. Node D accepts the message as input to the linear protection
switching, and follows the LP procedures to clear the LoP
command.
SF on
A<-B W1 W3
==A-xx---B------C== ==D======E======F==
\\ // \ /
\\ LPD1 // \ LPD3 /
\\ // \ / == : Normal traffic
P=====Q---------------R-----S
/ \
/ LPD2 \
/ \
==G------xx---------H-----------------J==
SF on G<-H W2
Figure 7: Shared Mesh Protection Example 2
NOTE: Examples for race condition to be provided in the next version.
6. Manageability Considerations
To be added in future version.
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7. IANA Considerations
To be added in future version.
8. Security Considerations
To be added in future version.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
[RFC5654] Niven-Jenkins, B., Nadeau, T., and C. Pignataro,
"Requirements for the Transport Profile of MPLS",
RFC 5654, April 2009.
9.2. Informative References
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC5921] Bocci, M., Bryant, S., Frost, D., and L. Levrau, "MPLS-TP
Framework", RFC 5921, July 2010.
[RFC6372] Sprecher, N. and A. Farrel, "MPLS-TP Survivability
Framework", RFC 6372, Sept 2011.
[RFC5085] Nadeau, T. and C. Pignataro, "Pseudo Wire (PW) Virtual
Circuit Connectivity Verification ((VCCV): A Control
Channel for Pseudowires", RFC 5085, Dec 2007.
[RFC5586] Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
Associated Channel", RFC 5586, June 2009.
[RFC4428] Papadimitriou, D. and E. Mannie, "Analysis of Generalized
Multi-Protocol Label Switching (GMPLS) based Recovery
Mechanisms (including Protection and Restoration) Recovery
(Protection and Restoration)", RFC 4428, March 2006.
[G.808.1] SG15, "Generic Protection Switching - Linear trail and
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subnetwork protection", ITU-T G.808.1, Feb 2010.
Authors' Addresses
Tae-sik Cheung
ETRI
161 Gajeong
Yuseong, Daejeon 305-700
South Korea
Phone: +82 42 860 5646
Email: cts@etri.re.kr
Jeong-dong Ryoo
ETRI
161 Gajeong
Yuseong, Daejeon 305-700
South Korea
Phone: +82 42 860 5384
Email: ryoo@etri.re.kr
Yaacov Weingarten
Nokia Siemens Networks
3 Hanagar St. Neve Ne'eman B
Hod Hasharon, 45241
Israel
Phone: +972-54-220 0977
Email: wyaacov@gmail.com
Nurit Sprecher
Nokia Siemens Networks
3 Hanagar St. Neve Ne'eman B
Hod Hasharon, 45241
Israel
Email: nurit.sprecher@nsn.com
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Daniel King
Old Dog Consulting
United Kingdom
Email: daniel@olddog.co.uk
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