Network Working Group Jianhua Gao
Dan Li
Internet Draft Huawei
Snigdho Bardalai
Richard Rabbat
Fujitsu
Diego Caviglia
Dino Bramanti
Ericsson
Category: Standards Track
Expires: April 2007 October, 2006
Problem and Applicability Statements for the use of Generalized
Multi-Protocol Label Switching (GMPLS) to Support Multiplex Section
Shared Protection Ring (MS-SPRing)
draft-gao-ccamp-gmpls-msspring-01.txt
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Abstract
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In order to provide high availability for transport networks, link
protection technologies are adopted in the data plane. In present
Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network
(SONET) optical transport networks, one of these protection
technologies, shared ring protection technologies, such as the 2/4-
Fiber Bi-directional Multiplex Section Shared Protection Ring(MS-
SPRing), are widely used. The same technologies can also be applied
in Optical Transport Networks (OTNs). This document describes a set
of issues to be addressed when applying GMPLS) to support MS-SPRings,
and sets out how GMPLS can be applied.
Table of Contents
1. Introduction................................................3
2. Multiplex Section Shared Protection Ring Overview............3
2.1. 2-Fiber and 4-Fiber Bi-directional MS-SPRing............3
3. Use of MS-SPRings within GMPLS Networks......................3
4. Issues with MS-SPRing........................................3
4.1. Advertising a TE Link In an MS-SPRing...................4
4.2. Re-Advertising the TE Link in Failure States............5
4.2.1. Scenario for Two Fibers MS-SPRing..................6
4.2.2. Scenarios for Four Fibers MS-SPRing................7
4.3. LSP Re-Routing After MS-SPRing Failure..................8
4.4. Consistent Resource/Label Usage.........................9
4.5. SRLG Consideration......................................9
4.6. LSP End-to-end / Segment Recovery......................10
4.7. Ring Map configuration for squelching..................11
4.8. Data Plane and Control Plane misalignment..............11
5. Application of GMPLS to MS-SPRings..........................11
5.1. Routing...............................................12
5.1.1. TE Link Advertisement.............................12
5.1.2. Re-Advertising TE Links...........................13
5.1.3. SRLGs............................................13
5.2. Signaling.............................................14
5.3. LSP End-to-end / Segment Recovery......................14
6. Security Considerations.....................................14
7. Acknowledgments............................................15
8. References.................................................15
8.1. Normative References...................................15
8.2. Informative References.................................15
APPENDIX A: MS-SPRing Overview.................................16
a. Information needed by MS-SPRing..........................17
b. MS-SPRing Example........................................19
c. Time Slot Interchange (TSI)..............................23
d. Squelching..............................................24
9. Author's Addresses.........................................26
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10. Full Copyright Statement...................................27
11. Intellectual Property Statement............................27
1. Introduction
In order to provide high availability for transport networks, link
protection technologies are adopted in the data plane. In present
Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network
(SONET)optical transport networks, shared ring protection
technologies, such as the 2/4-Fiber Bi-directional Multiplex Section
Shared Protection Ring(MS-SPRing) [G.841], are widely used. The same
technologies can also be applied in Optical Transport Networks (OTNs).
This document describes a set of issues to be addressed when applying
Generalized Multiprotocol Label Switching (GMPLS) to support MS-
SPRings, and sets out how GMPLS can be applied.
2. Multiplex Section Shared Protection Ring Overview
2.1. 2-Fiber and 4-Fiber Bi-directional MS-SPRing
Please refer to Appendix for a quick MS-SPRing overview.
3. Use of MS-SPRings within GMPLS Networks
In this document, we only consider how to run GMPLS over a data plane
that includes network elements participating in MS-SPRing protection
rings. Controlling or emulating MS-SPRing using GMPLS are out of
scope.
This document will describe the GMPLS requirements and applicability
for setting up of LSPs in a network that has a MS-SPRing or multiple
MS-SPRings. In general the topology can be viewed as:
- A single ring in the network
- Multiple interconnected rings in the network
This document will also cover GMPLS requirements and applicability
for setting up of LSPs with end-to-end recovery or segment recovery
capability in a network as described above.
4. Issues with MS-SPRing
The following is a list summarizing the issues that need to be
addressed to enable an effective interworking and relationship
between GMPLS protocols policies and MS-SPRing inherent operating
rules.
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o Advertising TE Links belonging to MS-SPRing rings;
o Avoiding label change within a MS-SPRing due to No TimeSlot
Interchange constraint imposed by underlying data plane;
o Ring Map configuration for squelching;
Possible misalignment, in case of failure recovered by MS-SPRing,
between the actual data plane path of LSP and what is stored in the
RRO.
4.1. Advertising a TE Link In an MS-SPRing
When an MS-SPRing exists within a GMPLS network, it is able to
support multiple TE links between each pair of neighbor nodes in the
ring. Each TE link can use the features of the ring to provide
protection for the traffic it carries.
For a 2-Fiber bi-directional MS-SPRing, there are two fibers for each
span of the ring. Each fiber is equally divided into working and
protection channels, that is, each fiber provides protection for
traffic on the other fiber.
Using the terminology of [RFC4202], the resources (i.e. timeslots or
lambdas) on the data links that make up the ring have protection
types "Enhanced", "Shared", "Dedicated 1:1"(for 4-fiber bi-MS-SPRing
when there is a link failure in the ring) or "Extra Traffic". It is
also desirable to support unprotected traffic across the data link
whose part of resource has been configured with MS-SPring, so the
resources may also have protection type "Unprotected".
A 1:1 shared protection type TE link, for example, can be supported
by two types link resources: one type of link resource is used to
carry the protected traffic, another type of link resource is used to
carry the extra traffic.
The question arises of how to advertise the TE link between two nodes
in the ring. Note that according to the definition in section 1.2 of
[RFC4203], the GMPLS routing protocols only support one protection
type per TE link. But should we advertise the shared, extra-traffic,
or unprotected traffic capabilities of the link between the two nodes?
For a 4-Fiber bi-directional MS-SPRing, there are two pairs of fibers
for each span of the ring. Each pair of fibers is defined as working
or protection channels. The same issue of choosing which protection
capabilities to advertise exists for the 4-Fiber bi-directional MS-
SPRing case.
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4.2. Re-Advertising the TE Link in Failure States
In 2/4-Fiber MS-SPRing, if a fiber is broken, all the protection
capability of TE links in MS-SPRing will be affected. For example, in
4-Fiber MS-SPRing, when a working fiber is broken, all the protected
traffic carried by that fiber is switched to the protection fiber.
The broken link that previously offered "Enhanced" protection can now
only provide the "Shared" protection for the protected traffic. The
link in the same span with the broken link that was previously
offered "Extra Traffic" can not carry the extra traffic any more. The
other links in the ring that previously offered "Enhanced" protection
can now only provides the " Dedicated 1:1" protection for the
protected traffic . the other links in the ring that was previously
offered "Extra Traffic" can still carry the extra traffic.
The link protection type would normally be configured and should not
be affected by the link failure: either the link is available, or it
isn't. But the available link resources change in the event of the
link failure.
The question is how to advertise (or re-advertise) the TE links in
the case of a failure on the ring. There may have two options with
respect to this situation.
Option 1: Advertising TE link is not available
We may simply advertise the TE link with configured protection type
(enhanced or shared) is not available, because the TE link could not
provide the protected service as it was promised before.
Option 2: Advertising TE link is degraded but available
Even when the TE link is not available with previous protection type,
it can still be used to carry the traffic. Especially in the case of
where the transport resource is limited, if we can use the degraded
TE link to establish the LSP, the transport resource utilization
could be improved. For example, if the configured protection type of
the TE link is "enhanced", in case of the link failure in the ring,
the TE link may be advertised with "shared" temporary protection type.
If the requested service is "shared" protection type, then this TE
link can be selected to establish the LSP with requested protection
type to carry the traffic. Once the link failure is recovered, the
protection type of the TE link should also be restored as "enhanced"
protection type.
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There are three typical fault scenarios, during the link failure, the
protection capability of TE links in MS-SPRing is described in the
following tables.
4.2.1. Scenario for Two Fibers MS-SPRing
+---------+ +---------+
| |------------->| |
| TNE1 | W1/P1 | TNE2 |
| |<-------------| |
+---------+ +---------+
^ | ^ |
| | | |
|W4/P4| |W2/P2|
| | | |
| v | v
+---------+ +---------+
| |------------->| |
| TNE4 | W3/P3 | TNE3 |
| |<-------------| |
+---------+ +---------+
Figure 1: MS-SPRing two fibers reference circuit
For two fibers MS-SPRing as shown in Figure 1, there are two TE links
in each pair of fibers. For example, W1 and P1 are in same pair of
fibers between TNE1 and TNE2. If the data link between TNE1 and TNE2
is broken, the protection capability which the TE links in the ring
can provide is as follows:
TE Link Initial advertised Able to provide protection
link protection type capability after the link
failure
-------------------------------------------------------------------
W1 Shared Unprotected
P1 Extra Traffic Unavailable
W2 Shared Unprotected
P2 Extra Traffic Unavailable
W3 Shared Unprotected
P3 Extra Traffic Unavailable
W4 Shared Unprotected
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P4 Extra Traffic Unavailable
4.2.2. Scenarios for Four Fibers MS-SPRing
+----------+ W1 +----------+
| |------------->| |
| |<-------------| |
| TNE1 | | TNE2 |
| |<-------------| |
| |------------->| |
+----------+ P1 +----------+
^ | ^ | | ^ | ^
| | | | | | | |
W4| | | |P4 P2| | | |W2
| | | | | | | |
| | | | | | | |
| v | v V | v |
+----------+ P3 +----------+
| |------------->| |
| |<-------------| |
| TNE4 | | TNE3 |
| |------------->| |
| |<-------------| |
+----------+ W3 +----------+
Figure 2: MS-SPRing four fibers reference circuit
There are two failure scenarios in four fibers MS-SPRing:
Scenario 1: If the TE link W1 is broken in Figure 2, the protection
capability which the TE links in the ring can provide is as follows:
TE Link Initial advertised Able to provide protection
link protection type capability after the link
failure
-------------------------------------------------------------------
W1 Enhanced Shared
P1 Extra Traffic Unavailable
W2 Enhanced Dedicated 1:1
P2 Extra Traffic Extra Traffic
W3 Enhanced Dedicated 1:1
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P3 Extra Traffic Extra Traffic
W4 Enhanced Dedicated 1:1
P4 Extra Traffic Extra Traffic
Scenario 2: If the TE link W1 and P1 are broken in Figure 2, the
protection capability which the TE links in the ring can provide is
as follows:
TE Link Initial advertised Able to provide protection
link protection type capability after the link
failure
-------------------------------------------------------------------
W1 Enhanced Extra Traffic
P1 Extra Traffic Unavailable
W2 Enhanced Dedicated 1:1
P2 Extra Traffic Unavailable
W3 Enhanced Dedicated 1:1
P3 Extra Traffic Unavailable
W4 Enhanced Dedicated 1:1
P4 Extra Traffic Unavailable
4.3. LSP Re-Routing After MS-SPRing Failure
When a failure occurs in an MS-SPRing, the affected TE links may be
re-advertised in the network in answer to the question in the
previous section. LSPs that used the TE links across the ring may
still be able to carry traffic (using the protection capabilities of
the ring), or may have been displaced (because they were extra
traffic, or because they were unprotected).
The paths of the LSPs can be re-computed based on the current TE link
capabilities, and the re-routing process may be invoked if necessary.
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The question to be answered here is which LSPs should be
recomputed/re-routed first: those that have degraded protection
(below the original service request level), but where the traffic is
still being delivered; or those where the traffic has been completely
disrupted?
4.4. Consistent Resource/Label Usage
If there is a connection across the MS-SPRing through the two non-
adjacent nodes then, according to the switching principle of MS-
SPRing, when two link failures occur along the connection path in the
MS-SPRing and there is still physical connectivity between the two
non-adjacent nodes in the other direction around the MS-SPRing, the
connection can be protected.
Thus, if the label (time slot or wavelength) is changed at a transit
node along the LSP's path around the MS-SPRing, it may not be
possible to protect the LSP, and mis-connection may occur.
The question arises of how to ensure that the LSP is allocated the
same label (time slot or wavelength) along its path in the MS-SPRing
to ensure that the LSP gains full maximal protection. This means Time
Slot Interchange (TSI) is not allowed when the LSP switches from the
incoming span to the outgoing span part of the same MS-SPRing. TSI
can be allowed in case the incoming span and outgoing span belongs to
different MS-SPRings in the same node.
Regarding such constraint about avoidance of the TimeSlot Interchange
within MS-SPRing, you can find a brief description in section c of
the Appendix.
4.5. SRLG Consideration
The nature of the MS-SPRing technology is that any failure in a ring
may affect all of the TE links which cross the MS-SPRing.
When computing paths across the network, it is desirable for the
computation to be aware of TE links that appear to be disjoint but
actually use the same underlying resources because these TE links are
subject to the same failure conditions. This is usually achieved by
defining a Shared Risk Link Group (SRLG) and having each TE link
advertise its membership of the same group.
The question is how to indicate that two TE links that cross an MS-
SPRing that may not have the same entry or exit point in common are
members of the same SRLG, and how to coordinate the allocation of the
SRLG ID for advertisement by the end points of the TE links.
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4.6. LSP End-to-end / Segment Recovery
Setting up LSPs with recovery in a network with MS-SPRing requires
that every non-MS-SPRing link in the LSP has a backup LSP in order to
provide end-to-end protection.
MS-SPRing links do not require any backup LSPs because there is a
backup route available for enhanced traffic that goes around the ring.
There is in fact a restriction in branching out from a MS-SPRing
entry point or merging at a MS-SPRing exit point. Whereas it is
allowed to merge at the MS-SPRing entry point and branch at the MS-
SPRing exit point.
+-------------------+
| |
+-----+ +-----+
| | | | | |
|--/ | | \--|
|--\ |A B| /--|
| | | | | |
+-----+ +-----+
| |
+-------------------+
In the figure above if nodes A and B are MS-SPRing nodes then the
branch point at node A i.e. the ring entry point is not allowed.
Neither the merge point at node B i.e. the ring exit point is not
allowed.
+-------------------+
| |
+-----+ +-----+ +-----+ +-----+
| | | | | | | | | |
<->|-----|------|--/ | | \--|-------|-- |
| \ |A | | |B C| | | D| \ |
+-----+ +-----+ +-----+ +-----+
| | | |
| | MS-SPRing | |
+-----+ +-----+ +-----+ +-----+
| \ | | | | | | | | \ |
| --|------|--/ | | \--|-------|-----|<->
| |E | |F G| | H| |
+-----+ +-----+ +-----+ +-----+
| |
+-------------------+
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In the figure above nodes B, C, F and G are part of the MS-SPRing.
Nodes A, E, D and H are non-MS-SPRing nodes. This example shows that
merging is allowed at the ring entry point and branching is allowed
at the ring exit point.
4.7. Ring Map configuration for squelching
Please refer to Section d of the Appendix for an overview of the
squelching issue introduced by MS-SPRing operation.
4.8. Data Plane and Control Plane misalignment
When a failure affects an LSP that traverses an MS-SPRing protected
ring the data plane scenario is the same as in Figure A-3 in the
apppendix.
Referring to that scheme, data Traffic is flowing through:
Node-A<-->Node-B<-->Node-A<-->Node-F<-->Node-E<-->Node-D<-->Node-C<--
>Node-D
while from a control plane perspective traffic is still flowing
through Node-A<-->Node-B<-->Node-C<-->Node-D.
It may be noted that the loop Node-A<-->Node-B<-->Node-A could be
bridged, releasing the protection channels for extra traffic use thus
increasing the availability of extra traffic resources. It has to be
noted however that doing this requires an update to the Ring map.
The afore mentioned misalignment between control and data plane
arises because the control plane is un-aware of the failure.
In such case a mechanism targeted at realigning the control plane
view with the actual scenario present in data plane may be required.
In other words, when data plane inherent protection scheme imposes
autonomously (i.e. not as a result of a control plane
command)changes to relevant LSP characteristic, a communication
mechanism that re-aligns control plane to data plane state may be
required if an exact map of data plane resources held by a LSP is
needed within control plane .
5. Application of GMPLS to MS-SPRings
With reference to the issues detailed in previous sections, possible
solutions, allowing an effective and harmless overlaying of Control
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Plane protocols over a data plane enhanced with MS-SPRing inherent
protection mechanism, are introduced.
Part of the mentioned issues may be addressed by exploiting routing
related methods, while mechanisms based on signalling may used for
others.
It has to be noted that structural modification to routing or
signalling protocols have been avoided in proposed solutions. However,
issues such as Squelching management (see section 4.7 in this
document and details in appendix) or modification of actual data
plane LSP segments within the MS-SPRing ring transparently to control
plane(see 4.8) are not covered at present in following paragraphs. A
solution to such problems seem to imply a major impact (i.e.
enhancement) on GMPLS protocols mechanisms or operation
5.1. Routing
5.1.1. TE Link Advertisement
Each node in the MS-SPRing is responsible for advertising the TE
links between the adjacent nodes. There are two options:
o Three distinct TE links may be advertised for each pair of
adjacent nodes in the MS-SPRing, i.e. an MS-SPRing component link
or span. Such advertisements would reflect the split of ring
resources into working, protection/extra-traffic, and unprotected
resources. That is, one TE link could be advertised as "Dedicated
1:1", one TE link as "Extra Traffic", and one TE link as
"unprotected". An entity computing the path of an LSP through the
network could then be aware of what resources existed at each
capability, and could select the appropriate TE link for the level
of service required.
o A single TE link may be advertised with protection type "Enhanced".
Path computation could select this link to support any protected
traffic, extra traffic, or unprotected traffic. In this mode of
operation, it is the responsibility of the entry node to allocate
resources on the ring commensurate with service level requested
for the LSP in the signaling message.
A hybrid mechanism has been proposed where a single TE link could be
advertised with multiple protection types. This is contrary to the
specification in [RFC4202] and does not appear to gain anything over
the second option stated above. Therefore, this document does not
propose any extensions to the routing protocols. Should an operator
wish to distinguish the different ring resources (or even ring
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directions) while still using the second option listed above, they
can define a link bundle (see [RFC4201]).
5.1.2. Re-Advertising TE Links
When a failure occurs in an MS-SPRing, the protection capabilities of
some TE links may be decreased. Some TE links may become incapable of
providing link level protection, and some may be completely broken.
Lastly, when the ring is repaired, the protection capabilities of the
TE links that cross the ring is restored.
The new protection capabilities of a link can be advertised in a
routing protocol update advertisement. As with advertisements for
changes in available bandwidth, care should be taken by
implementations not to generate updates too frequently. Depending on
administrative network policy it may be better to favor updates that
remove capabilities over updates that restore capabilities.
Note that the coordination of the interaction between distinct TE
links that cross the ring is the responsibility of the management
system that control the MS-SPRing.
[RFC4203] and [RFC4205] define how the SRLGs that apply to a TE link
may be advertised by routing protocols. Management plane coordination
can be used so that all nodes on an MS-SPRing know the SRLG ID for
the ring and can advertise it as part of the TE link advertisements
that they originate for TE links that cross the ring.
It is possible that it will be advantageous for a path computing
entity to know the type of association represented by an SRLG. For
example, it may be beneficial to be able to distinguish an SRLG that
means that two TE links share a duct, from and SRLG that means that
two TE links cross the same road bridge. Similarly, it may be
advantageous to know that the SRLG advertised indicates that the TE
links share the same ring because this will indicate that the shared
risk is graded through a loss of protection capabilities before a
complete loss of connectivity.
At the moment there are no classifications of SRLG IDs standardized.
An operator is free to sub-divide the space of SRLG IDs within their
Administrative Domain and place meaning on the sub-divisions.
Standardization of the classification of SRLGs is for future study.
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5.2. Signaling
If label continuity is the responsibility of the management plane
then there is no additional requirement on GMPLS to control the
allocation of labels within the ring, and the entry and exit nodes
are responsible for selecting suitable labels for use on the TE link.
In this case, if an inappropriate label is provided in the Explicit
Route object (ERO) signaled to the entry point node, it may be
necessary for the node to reject the signaling message.
In order to support the consistent label allocation when the ring is
controlled using GMPLS, the Label Set object can be used with a
single label member for the LSPs signaled within the ring. If the
label in label set object is not available on a certain node in the
MS-SPRing, an Acceptable Label Set object can be returned to
indicate which labels would have been consistently available.
When failure occurs in a MSPRing, the data plane connection may be
switched to a different protection path transparently to control
plane. The control plane will not reroute the affected end-to-end
LSPs to be aligned with the actual data plane connection path because
the LSPs are protected successfully according to MS-SPRing way of
operation. The control plane doesn't actually need to perceive the
failure in the TE link around MS-SPRing, still being present a
misalignment (limited within the MS-SPRing ring) between the LSP path
records within control plane and actual data plane path.
5.3. LSP End-to-end / Segment Recovery
In order to ensure that the MS-SPRing branching and merging
restrictions are met it is necessary to create appropriate segment
LSPs in order to provide the backup paths for the non-MS-SPRing
segments.
In order to compute paths for the segment LSPs it would be required
to identify the MS-SPRing links. In case of intra-domain LSPs there
is no issue because the protection-type information is advertised as
part of the TE link. In case of inter-domain LSPs some extensions may
be required to be made to the RRO / XRO objects [EXCLUDE].
6. Security Considerations
End-to-end security within the GMPLS network is not adversely
impacted by the use of MS-SPRings within the network. Indeed, the
protection properties of the ring enhance the resilience of the GMPLS
LSP to physical attacks on the network infrastructure.
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The use of a GMPLS control plane to operate the MS-SPRing itself
raises the same security benefits and concerns as exist when an
entire network is migrated from the management plane (or from an pre-
existing control plane) to GMPLS.
Further notes on GMPLS security can be found in [RFC3945].
7. Acknowledgments
We would like to thank Adrian Farrel for his useful comments.
8. References
8.1. Normative References
[RFC3945] Mannie, E., Ed., "Generalized Multiprotocol Label Switching
(GMPLS) Architecture", RFC 3945, October 2004.
[RFC4202] Kompella, K., Ed., and Y. Rekhter, Ed., "Routing Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4202, October 2005.
[RFC4203] Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, October 2005.
[RFC4205] Kompella, K. Ed. and Y. Rekhter, Ed., "Intermediate System
to Intermediate System (IS-IS) Extensions in Support of
Generalized Multi-Protocol Label Switching (GMPLS)", RFC
4205, October 2005.
[G.841] ITU-T "Types and characteristics of SDH network protection
architectures", October 1998.
8.2. Informative References
[RFC4201] Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling in
MPLS Traffic Engineering (TE)", RFC 4201, October 2005.
[EXCLUDE] C-Y. Lee, A. Farrel and S De Cnodder, "Exclude Routes -
Extension to RSVP-TE", draft-ietf-ccamp-rsvp-te-exclude-
route, work in progress.
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APPENDIX A: MS-SPRing Overview
The main reference for this Section is ITU-T G.841. G.841 defines
two different kinds of MS-SPRing namely two fibers and four fibers.
Two-fiber MS switched rings require only two fibers for each span of
the ring. Each fiber carries both working channels and protection
channels. On each fiber, up to half the channels are defined as
working channels and up to half are defined as protection channels.
It is possible that some channels are not protected at all, being
defined as Non-pre-emptible Unprotected Traffic (NUT) channels.
The traffic carried on working channels inside one fiber is protected
by channels going in the opposite direction around the ring. This
allows for a bi-directional transport of normal traffic and makes
possible a sharing of the protection resources when needed.
The following picture illustrates the two fibers case, no NUT in this
example.
WPx links are 50% for worker traffic and 50% for protection traffic,
e.g. and STM-16 links have 8 AU-4 timeslot for worker traffic and 8
AU-4 timeslot for protection.
TNE A TNE B TNE C
+----------+ +----------+ +----------+
| | | | | |
| |----------->| |----------->| |
| | WP1 | | WP2 | |
| |<-----------| |<-----------| |
| | | | | |
+----------+ +----------+ +----------+
^ | ^ |
| | | |
| WP3 | WP Links Resources are | WP4 |
| | 50% Worker | |
| | 50% Protection | |
| v | v
+----------+ +----------+ +----------+
| | | | | |
| |----------->| |----------->| |
| | WP5 | | WP6 | |
| |<-----------| |<-----------| |
| | | | | |
+----------+ +----------+ +----------+
TNE F TNE E TNE D
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Figure A-1: MS-SPRing two fibers reference circuit
Four-fiber MS shared protection rings require four fibers for each
span of the ring. As illustrated in Figure 1-2, working and
protection channels are carried over different fibers: two multiplex
sections transmitting in opposite directions carry the working
channels while two multiplex sections, also transmitting in opposite
directions, carry the protection channels. This enables the bi-
directional transport of normal traffic, sharing as well the
protection capability.
The following picture illustrates the reference circuit (four fiber
MS-SPRing) used in this Section.
TNE A TNE B TNE C
+------------+ +------------+ +------------+
| |----------->| |----------->| |
| | W1 | | W2 | |
| |<-----------| |<-----------| |
| | | | | |
| |===========>| |===========>| |
| | P1 | | P2 | |
| |<===========| |<===========| |
+------------+ +------------+ +------------+
^ | ^ l ^ l ^ |
| | l l l l | |
| | l l l l | |
|W3| lP3l ------------ Working Link lP4l |W4|
| | l l llll and === Protection Link l l | |
| | l l l l | |
| v l v l v | v
+------------+ +------------+ +------------+
| |===========>| |===========>| |
| | P5 | | P6 | |
| |<===========| |<===========| |
| | | | | |
| |----------->| |----------->| |
| | W5 | | W6 | |
| |<-----------| |<-----------| |
+------------+ +------------+ +------------+
TNE F TNE E TNE D
Figure A-2: Reference circuit for MS-SPRing, four fibers variant
a. Information needed by MS-SPRing
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The MS-SPRing protection mechanism is implemented via a SDH
signalling protocol known as Automatic Protection Switching (APS).
This protocol makes use of SDH overhead bytes (K1 and K2, MS overhead
bytes) as a means to transport its own information. APS is not
detailed here as it is outside the scope of this document.
Each node on the ring shall be assigned an ID that is a number from 0
to 15, allowing a maximum of 16 nodes on the ring. Such ID value is
not related to the position of corresponding node in the ring, i.e.
the order of the nodes is not tied to nodes ID assignment.
Each node has a ring topology map that associates a node's ID with
its address.
With respect to the Figures 1-1/2 the ring topology map is:
TNE-ID TNE-Address
1 B
2 F
3 A
4 E
5 C
6 D
Table A-1 Ring Topology map
The following tables represent the traffic matrix of the ring and the
squelching (for definition of squelching please refer to Section 1.4)
tables of the TNEs.
+------------------------------------------------------------------+
| AU | <---- West Nodes East ---->|
| Number | A B C D E F A|
+--------+---------------------------------------------------------+
| 1 | <--------> <------------------> |
| 2 | <-----------------------------> |
| 3 | <--------------------><-------------------------> |
+------------------------------------------------------------------+
Table A-2 Traffic Matrix
+---------------------------------+---------------------------------
TNE-A | TNE-B
AU West East | AU West East
Src Dst Src Dst | Src Dst Src Dst
1 A B | 1 B A B D
2 A D | 2 D A A D
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3 A C | 3 C A C F
----------------------------------+----------------------------------
----------------------------------+---------------------------------
TNE-C | TNE-D
AU West East | AU West East
Src Dst Src Dst | Src Dst Src Dst
1 D B B D | 1 D B B D
2 D A A D | 2 D A A D
3 C A C F | 3 F C C F
----------------------------------+---------------------------------
----------------------------------+---------------------------------
TNE-E | TNE-F
AU West East | AU West East
Src Dst Src Dst | Src Dst Src Dst
1 | 1
2 | 2
3 F C C F | 3 F C C F
----------------------------------+---------------------------------
Table A-3 Squelching Table
When a node determines that a protection switch is required, it
sources the appropriate bridge request using the APS protocol to the
node at the far end of the affected MS (for more details on how APS
carries that information please refer to G.841 [G.841]).
We'll call Ring Map the sum of the information contained in all the
above Tables.
b. MS-SPRing Example
The worker circuit follows this path (4_fibers/2_fibers):
TNE-A <-Link W1/WP1-> TNE-B <-Link W2/WP2-> TNE-C <-Link W4/WP4->
TNE-D : AU Timeslot 1
In four fibers scenario failure of Links W2 and P2 triggers the MS-
SPRing protection.
Traffic is protected using this path:
TNE-A <-Link W1-> TNE-B [Internal Bridge] <-Link P1-> TNE-A <-Link
P3-> TNE F <-Link P5-> TNE-E <-Link P6-> TNE-D <-Link P4-> TNE-C
[Internal Bridge] <- Link W4 -> TNE-D: AU Timeslot 1.
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The following picture illustrates the state of the network after the
recovery, by means of MS-SPRing, of the failure.
1
TNE A v TNE B TNE C
+----------|-+ +------------+ +------------+
| +------------->----->----+ |XXXXXXXXXXXX| +--------+ |
| | W1 | | | W2 | | | |
| |<-----------| | |XXXXXXXXXXXX| | | |
| | | v | | ^ v |
| |===========>| | |XXXXXXXXXXXX| | | |
| | P1 | | | P2 | | | |
| +--<===========-----<----+ |XXXXXXXXXXXX| | | |
+----------|-+ +------------+ +-|--------|-+
^ | ^ l ^ l ^ |
| | l l l l | |
| | l l l l | |
|W3| lP3l ------------ Working Link lP4l |W4|
| | l l llll and === Protection Link l l | |
| | l l l l | |
| v l v l v | v
+----------|-+ +------------+ +-|--------|-+
| +--===========>------->------===========>--+ | |
| | P5 | | P6 | | |
| |<===========| |<===========| | |
| | | | | v |
| |----------->| |----------->| | |
| | W5 | | W6 | | |
| |<-----------| |<-----------| | |
+------------+ +------------+ +----------|-+
TNE F TNE E TNE D v
1
Figure A-3: MS-SPRing four fibers ring-switching
The extra traffic in spans P1, P3, P4, P5 and P6 is affected.
Note that in this case working traffic passes the same section two
times, i.e. link W1 and link P1, link P4 and link W4.
If these sections are very long, e.g. in trans-oceanic applications,
the propagation delay is affected considerably and will result in a
degradation of performance.
For trans-oceanic applications intermediate nodes, not adjacent to an
affected section, will switch bridges as well.
Traffic is now protected using this path:
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TNE-A [Internal Bridge] TNE-A <-Link P3-> TNE F <-Link P5-> TNE-E <-
Link P6-> TNE-D [Internal Bridge] TNE-D: AU Timeslot 1.
The following picture illustrates the state of the network after the
recovery, by means of trans-oceanic MS-SPRing, of the failure.
1
TNE A v TNE B TNE C
+----------|-+ +------------+ +------------+
| | |----------->| |XXXXXXXXXXXX| |
| | | W1 | | W2 | |
| | |<-----------| |XXXXXXXXXXXX| |
| v | | | | |
| | |===========>| |XXXXXXXXXXXX| |
| | | P1 | | P2 | |
| | |<===========| |XXXXXXXXXXXX| |
+----------|-+ +------------+ +------------+
^ | ^ l ^ l ^ |
| | l l l l | |
| | l l l l | |
|W3| lP3l ------------ Working Link lP4l |W4|
| | l l llll and === Protection Link l l | |
| | l l l l | |
| v l v l v | v
+----------|-+ +------------+ +------------+
| +--===========>------->------===========>------>----+ |
| | P5 | | P6 | | |
| |<===========| |<===========| | |
| | | | | v |
| |----------->| |----------->| | |
| | W5 | | W6 | | |
| |<-----------| |<-----------| | |
+------------+ +------------+ +----------|-+
TNE F TNE E TNE D v
1
Figure A-4: MS-SPRing four fibers trans-oceanic ring-switching
Only the extra traffic in spans P3, P5 and P6 is affected.
Instead of bridging all working traffic to the protections channels
in the nodes adjacent to the failure in trans-oceanic ring-switching
the individual AU tributaries are switched in their ingress and
egress nodes using ring maps and APS information. Due to the transfer
and evaluation of the information more time is required for the
protection switch to complete, the objective is 300 ms or less.
Because tributaries are switched in their ingress and egress nodes no
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squelching is required and protection channels not required for
protection may carry pre-empted extra traffic.
A mechanism is required to auto-provision the ring maps and maintain
their consistency.
In two fibers scenario failure of Links W2 and P2 triggers the MS-
SPRing protection.
Traffic is protected using this path:
TNE-A <-Link WP1-> TNE-B [Internal Bridge] <-Link WP1-> TNE-A <-Link
WP3-> TNE F <-Link WP5-> TNE-E <-Link WP6-> TNE-D <-Link WP4-> TNE-C
[Internal Bridge] <- Link WP4 -> TNE-D: AU Timeslot 1 is used on all
the links.
The following picture illustrates the state of the network after the
recovery from the failure done by means of MS-SPRing mechanism.
|
TNE A v TNE B TNE C
+------------+ +------------+ +------------+
| | | | | | |
| | | | | | |
| +-|----------->|------+ |xxxxxxxxxxx>| +-----+ |
| | WP1 | | | WP2 | | | |
| +----|<-----------|------+ |<-----------| ^ v |
| | | | | | | | |
| | | | | | | | |
+------------+ +------------+ +------------+
^ | ^ |
| | WP Links Resources are | |
| WP3 | 50% Worker | WP4 |
| | 50% Protection | |
| v | v
+------------+ +------------+ +------------+
| | | | | | | | |
| | | | | | | | |
| +----|----------->|------------|----------->|---+ | |
| | WP5 | | WP6 | | |
| |<-----------| |<-----------| | |
| | | | | | |
| | | | | | |
+------------+ +------------+ +------------+
TNE F TNE E TNE D v
Figure A-5: MS-SPRing two fibers ring-switching
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Figure 1-3 and 1-5 illustrates the so-called "ring-switching"
protection.
In four-fiber MS-SPRing there exists also "span-switching" and in
this case only the working fibers are cut while the protection fibers
remain intact. See figure 1-6. The protection switch affects the
extra traffic in span P2.
1
TNE A v TNE B TNE C
+----------|-+ +------------+ +------------+
| +------------->----->----+ |XXXXXXXXXXXX| +--------+ |
| | W1 | | | W2 | | | |
| |<-----------| v |XXXXXXXXXXXX| ^ | |
| | | | | | | | |
| |===========>| +---===========>--+ v |
| | P1 | | P2 | | |
| |<===========| |<===========| | |
+------------+ +------------+ +----------|-+
^ | ^ l ^ l ^ |
| | l l l l | |
| | l l l l | |
|W3| lP3l ------------ Working Link lP4l |W4|
| | l l llll and === Protection Link l l | |
| | l l l l | |
| v l v l v | v
+------------+ +------------+ +----------|-+
| |===========>| |===========>| | |
| | P5 | | P6 | | |
| |<===========| |<===========| | |
| | | | | v |
| |----------->| |----------->| | |
| | W5 | | W6 | | |
| |<-----------| |<-----------| | |
+------------+ +------------+ +----------|-+
TNE F TNE E TNE D v
1
Figure A-6: MS-SPRing span-switch
c. Time Slot Interchange (TSI)
TSI is the connection function capability of changing the time slot
position of through-connected traffic (i.e. traffic that is not added
or dropped from the node). At present there is no TSI capability
specified in nodes belonging to a MS-SPRing sub network. Channels at
MS-SPRing node's egress are nailed to the same timeslot used by the
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same channels at node's ingress. This is a currently required
condition to ensure MS-SPRing correct operation.
d. Squelching
Squelching is defined as the process of inserting AU-AIS in order to
prevent misconnections. The squelching process application over
traffic results in an "all 1's" signal.
1. Squelching to avoid misconnected traffic
To perform a ring switch, the protection channels are essentially
shared among each span of the ring. Also, extra traffic may reside in
the protection channels when the protection channels are not
currently being used to restore normal traffic transported on the
working channels. Thus, each protection channel time slot is subject
to use by multiple services (services from the same time slot but on
different spans, and service from extra traffic). With no extra
traffic on the ring, under certain multiple point failures, such as
those that cause node(s) isolation, services (from the same time slot
but on different spans) may contend for access to the same protection
channel time slot. This yields a potential for misconnected traffic.
With extra traffic on the ring, even under single point failures,
normal traffic on the working channels may contend for access to the
same protection channel time slot that carries the extra traffic.
This also yields a potential for misconnected traffic.
Without a mechanism to prevent misconnection, the following failure
scenario would yield misconnections.
Referring to Figure 1-1, two circuits traverse the MS-SPRing namely
circuit Q and R the path that they traverse is:
Circuit R: TNE-A <-Link WP3 AU 1-> TNE-F
Circuit Q: TNE-A <-Link WP1 AU 1-> TNE-B <-Link WP2 AU 1-> TNE-C
Suppose a cut in both the spans between nodes A and F and between
nodes A and B (isolating node A, that is the same as a TNE A failure)
causes circuits Q and R to attempt to access time slot #1P on the
protection channels. The mechanism for the MS-SPRing protection is
as depicted in previous sub-section.
A potential misconnection is determined by identifying the nodes that
will act as the switching nodes for a bridge request, and by
examining the traffic that will be affected by the switch. The
switching nodes can be determined from the node addresses in the K1
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and K2 bytes. The switching nodes determine the traffic affected by
the protection switch from the information contained in their ring
maps and from the identifications of the switching nodes. Potential
misconnections shall be squelched by inserting the appropriate AU-AIS
in those time slots where misconnected traffic could occur.
Specifically, the traffic that is sourced or dropped at the node(s)
isolated from the ring by the failure shall be squelched. For rings
operating at an AU-4 level, this squelching occurs at the switching
nodes. AU level squelching occurs for the normal or extra traffic
into or out of the protection channels (i.e. normal traffic into or
out of working channels is never squelched).
For example, consider a segment of a ring consisting of three nodes,
A, B, and C where B has failed. In a typical scenario, both A and C
will send bridge requests destined for B. When A sees the bridge
request from C, and sees that B is between A and C (from the node
map), it can deduce that B is isolated from the ring. A and C will
use their respective maps to find out which channels are added or
dropped by B. A and C will squelch these channels before the ring
switch is performed by inserting AU-AIS. Thus, any node on the ring
that was connected to B will now receive AIS on those channels.
Each of the ring maps, then, shall contain at minimum:
1. a ring map that contains information regarding the order in which
the nodes appear on the ring;
2. a cross-connect map that contains the AU-4 time-slot assignments
for traffic that is both terminated at that node and passed-
through that node;
3. a squelch table that contains, for each of these AU-4 time slots,
the node addresses at which the traffic enters and exits the ring;
and
4. an optional indication of whether the AU is being accessed at the
lower order VC level somewhere on the ring (not covered by this
Document)
An example of such ring maps and squelching table is given in this
Appendix Section a.
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9. Author's Addresses
Jianhua Gao
Huawei Technologies Co., Ltd.
F3-5-B R&D Center, Huawei Base,
Bantian, Longgang District
Shenzhen 518129
P.R.China
Phone: +86 755 2897 2902
Email: gjhhit@huawei.com
Dan Li
Huawei Technologies Co., Ltd.
F3-5-B R&D Center, Huawei Base,
Bantian, Longgang District
Shenzhen 518129
P.R.China
Phone: +86 755 2897 2910
Email: danli@huawei.com
Huub van Helvoort
Huawei Technologies, Ltd.
Kolkgriend 38, 1356 BC Almere
The Netherlands
Phone: +31 36 5315076
Email: hhelvoort@huawei.com
Snigdho C. Bardalai
Fujitsu Network Communications, Inc.
2801 Telecom Parkway,
Richardson, Texas 75082
United States of America
Phone: +1 972 479 2951
Email: snigdho.bardalai@us.fujitsu.com
Richard Rabbat
Fujitsu
1240 East Arques Ave, MS 345
Sunnyvale, CA 94085
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United States of America
Phone: +1 408-530-4537
Email: rabbat@alum.mit.edu
Diego Caviglia
Ericsson
Via A. Negrone 1/A 16153
Genoa Italy
Phone: +39 010 600 3736
Email: diego.caviglia@(marconi.com, ericsson.com)
Dino Bramanti
Ericsson
Via Moruzzi 1
C/O Area Ricerca CNR
Pisa, Italy
Email: dino.bramanti@marconi.com
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