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 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract Li Expires April 2007 [Page 1] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 2] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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. Li Expires April 2007 [Page 3] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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. Li Expires April 2007 [Page 4] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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. Li Expires April 2007 [Page 5] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 6] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 7] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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. Li Expires April 2007 [Page 8] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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. Li Expires April 2007 [Page 9] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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| | +-----+ +-----+ +-----+ +-----+ | | +-------------------+ Li Expires April 2007 [Page 10] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 11] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 12] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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. 5.1.3. SRLGs [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. Li Expires April 2007 [Page 13] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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. Li Expires April 2007 [Page 14] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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. Li Expires April 2007 [Page 15] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 16] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 17] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 18] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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. Li Expires April 2007 [Page 19] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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: Li Expires April 2007 [Page 20] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 21] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 22] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 23] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 24] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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. Li Expires April 2007 [Page 25] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 Li Expires April 2007 [Page 26] Internet-Draft draft-gao-ccamp-msspring-00.txt October 2006 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 10. 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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". Li Expires April 2007 [Page 28]