1、 International Telecommunication Union ITU-T Y.1720TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (12/2006) SERIES Y: GLOBAL INFORMATION INFRASTRUCTURE, INTERNET PROTOCOL ASPECTS AND NEXT-GENERATION NETWORKS Internet protocol aspects Operation, administration and maintenance Protection switching fo
2、r MPLS networks ITU-T Recommendation Y.1720 ITU-T Y-SERIES RECOMMENDATIONS GLOBAL INFORMATION INFRASTRUCTURE, INTERNET PROTOCOL ASPECTS AND NEXT-GENERATION NETWORKS GLOBAL INFORMATION INFRASTRUCTURE General Y.100Y.199 Services, applications and middleware Y.200Y.299 Network aspects Y.300Y.399 Interf
3、aces and protocols Y.400Y.499 Numbering, addressing and naming Y.500Y.599 Operation, administration and maintenance Y.600Y.699 Security Y.700Y.799 Performances Y.800Y.899 INTERNET PROTOCOL ASPECTS General Y.1000Y.1099 Services and applications Y.1100Y.1199 Architecture, access, network capabilities
4、and resource management Y.1200Y.1299 Transport Y.1300Y.1399 Interworking Y.1400Y.1499 Quality of service and network performance Y.1500Y.1599 Signalling Y.1600Y.1699 Operation, administration and maintenance Y.1700Y.1799 Charging Y.1800Y.1899 NEXT GENERATION NETWORKS Frameworks and functional archit
5、ecture models Y.2000Y.2099 Quality of Service and performance Y.2100Y.2199 Service aspects: Service capabilities and service architecture Y.2200Y.2249 Service aspects: Interoperability of services and networks in NGN Y.2250Y.2299 Numbering, naming and addressing Y.2300Y.2399 Network management Y.240
6、0Y.2499 Network control architectures and protocols Y.2500Y.2599 Security Y.2700Y.2799 Generalized mobility Y.2800Y.2899 For further details, please refer to the list of ITU-T Recommendations. ITU-T Rec. Y.1720 (12/2006) i ITU-T Recommendation Y.1720 Protection switching for MPLS networks Summary IT
7、U-T Recommendation Y.1720 provides requirements and mechanisms for 1+1, 1:1, shared mesh, and packet 1+1 protection switching functionality for the user-plane in MPLS layer networks. The mechanism defined herein is designed to support end-to-end point-to-point LSPs. Protection switching functionalit
8、y for multipoint-to-point and point-to-multipoint LSP are for further study. m:n protection switching is for further study. Hitless protection switching is outside the scope of this version of the Recommendation. Source ITU-T Recommendation Y.1720 was approved on 14 December 2006 by ITU-T Study Grou
9、p 15 (2005-2008) under the ITU-T Recommendation A.8 procedure. Keywords LSP, MPLS, PML, protection switching, PSL, re-routing. ii ITU-T Rec. Y.1720 (12/2006) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU
10、Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardizat
11、ion Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information techno
12、logy which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. Complianc
13、e with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure e.g. interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. The words “shall“ or some other obligato
14、ry language such as “must“ and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation o
15、f this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the
16、 date of approval of this Recommendation, ITU had received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementers are cautioned that this may not represent the latest information and are therefore strongly urged to consult
17、 the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2007 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. Y.1720 (12/2006) iii CONTENTS Page 1 Scope 1 2 References. 1 3 Definitions 2 3.1 Def
18、initions defined elsewhere 2 3.2 Terms defined in this Recommendation. 3 4 Abbreviations and acronyms 3 5 Requirements 4 6 Principles 5 7 Mechanisms 6 7.1 Unidirectional protection switching . 6 7.2 Mechanisms of bidirectional protection switching. 15 8 Security aspects 15 Appendix I Example of prot
19、ection capacity sharing for shared mesh protection switching. 16 Appendix II Packet 1+1 example realization 18 II.1 Dual-feed and select mechanism 19 II.2 Analysis of the packet 1+1 scheme 19 Bibliography. 27 ITU-T Rec. Y.1720 (12/2006) 1 ITU-T Recommendation Y.1720 Protection switching for MPLS net
20、works 1 Scope This Recommendation provides requirements and mechanisms for 1+1, 1:1, shared mesh, and packet 1+1 protection switching functionality for the user-plane in MPLS networks. The mechanism defined herein is designed to support end-to-end point-to-point LSPs. Protection switching functional
21、ity for multipoint-to-point and point-to-multipoint LSP are for further study. m:n protection switching is for further study. Hitless protection switching is outside the scope of this version of the Recommendation. 2 References The following ITU-T Recommendations and other references contain provisi
22、ons which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibili
23、ty of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendat
24、ion. ITU-T G.805 ITU-T Recommendation G.805 (2000), Generic functional architecture of transport networks. NOTE There is a limitation of the applicability of the architecture specified by ITU-T G.805. It is not applicable to LDP based multipoint-to-point LSP and the case where PHP is in effect with
25、the egress not supporting MPLS data plane. ITU-T G.841 ITU-T Recommendation G.841 (1998), Types and characteristics of SDH network protection architectures. ITU-T I.630 ITU-T Recommendation I.630 (1999), ATM protection switching. ITU-T M.20 ITU-T Recommendation M.20 (1992), Maintenance philosophy fo
26、r telecommunication networks. ITU-T M.495 ITU-T Recommendation M.495 (1988), Transmission restoration and transmission route diversity: Terminology and general principles. ITU-T Y.1710 ITU-T Recommendation Y.1710 (2002), Requirements for Operation 2) SF is declared on the connected LSP (i.e., workin
27、g LSP or protection LSP) and is not declared on the other LSP and the hold-off timer has expired; or 3) the wait to restore timer expires (revertive mode) and SF is not declared on the working LSP. 7.1.2.1 Manual control Manual control of the protection switching function may be transferred from the
28、 operation system. 7.1.2.2 Signal fail declaration conditions 7.1.2.2.1 1+1 architecture For 1+1 architecture, signal fail (SF) is declared when the sink point of the protection domain enters the LSP Trail sink Near-End Defect State by entering the dServer, dLOCV, dTTSI_Mismatch, dTTSI_Mismerge, dEx
29、cess, or dUnknown condition. In order to achieve fast protection (the requirement for fast protection is under study), SF can be declared when an FDI packet is received by the sink of the protection domain before it enters other defect conditions (e.g., dLOCV). It allows fast protection against the
30、defects sourced from layers below the MPLS layer (and this requires that the incoming FDI have the DT codepoint 0x0101). In addition, the FDD function can be used to achieve a faster declaration of the signal fail condition. NOTE It is only to be used if the lower layer is not protected. If the lowe
31、r layer is also protected, it may lead to unnecessary protection switching by declaring SF on reception of FDI packets. In the case where the CV or FFD function is not activated, SF is declared when an FDI packet is received by the sink of the protection domain. It only applies to the defects source
32、d from layers below the MPLS layer (and this requires that the incoming FDI have the DT codepoint 0x0101). 7.1.2.2.2 1:1 architecture For 1:1 architecture, signal fail (SF) is declared when: the source of the protection domain enters the Trail sink Far-End Defect State by receiving a BDI packet (fro
33、m the return LSP or out of band). ITU-T Rec. Y.1720 (12/2006) 13 7.1.2.2.3 Shared mesh architecture Shared mesh architecture is an extension of 1:1 architecture. Signal fail (SF) is declared as in 1:1 architecture. NOTE Protection against a bidirectional LSP defect is for further study. 7.1.3 Compli
34、ance with network objectives The following network objectives apply: 1) Operating modes Revertive and non-revertive switching are provided. 2) Manual control Operator control via lockout of protection, forced switch and manual switch commands are supported. 3) Other switch initiation criteria Signal
35、 fail, wait to restore, and no request are supported in addition to the manual control commands listed above, as criteria for initiating (or preventing) a protection switch. 7.1.4 Switch initiation criteria The following switch initiation criteria exist: 1) an externally initiated command (clear, lo
36、ckout of protection, forced switch, manual switch); 2) an automatically initiated command (signal fail) associated with a protection domain; or 3) a state (wait to restore, no request) of the protection switching function. All requests are local (i.e., protection sink for 1+1 architecture and protec
37、tion source for 1:1 architecture). The priority of local requests is given in Table 1. Table 1 Priority of local requests Local request (i.e., automatically initiated command,state, or externally initiated command) Order of priority Clear Highest Lockout of protection | Forced switch | Signal fail |
38、 Manual switch | Wait to restore | No request Lowest NOTE 1 A forced switch for the working LSP should not be overridden by a signal fail on the protection LSP. Since unidirectional protection switching is being performed, and no APS protocol is supported over the protection LSP, signal fail on the
39、protection LSP does not interfere with the ability to perform a forced switch for the working LSP. NOTE 2 A forced switch for the protection LSP is not defined because this function may be achieved via a lockout of the protection command. 14 ITU-T Rec. Y.1720 (12/2006) 7.1.4.1 Externally initiated c
40、ommands Externally initiated commands are listed below in descending order of priority. The functionality of each is described below. clear: This command clears all of the externally initiated switch commands listed below. lockout of protection (LoP): Fix the selector position on the working LSP. Pr
41、events the selector from switching to the protection LSP when it is selecting the working LSP. Switches the selector from the protection to the working LSP when it is selecting the protection LSP. forced switch (FS) for working LSP: Switches the selector from the working LSP to the protection LSP (u
42、nless a higher priority switch request (i.e., LoP) is in effect). manual switch (MS) for working LSP: Switches the selector from the working LSP to the protection LSP (unless an equal or higher priority switch request (i.e., LoP, FS, SF or MS) is in effect). manual switch (MS) for protection LSP: Sw
43、itches the selector from the protection LSP to the working LSP (unless an equal or higher priority switch request (i.e., LoP, FS, SF or MS) is in effect). 7.1.4.2 FDI triggered protection switch In the case of FDI triggered protection switching, if the LSP with SF never enters a near end defect stat
44、e, there may be a need to prevent frequent transitions. If so, some time may be defined that must pass before taking another protection switching action. This is for further study. 7.1.4.3 States Wait to restore is only applicable for the revertive mode and applies to a working LSP. This state is en
45、tered by the local protection switching function in conditions where working traffic is being received via the protection LSP when the working LSP is restored, if local protection switching requests have been previously active and now become inactive. It prevents reversion back to select the working
46、 LSP until the wait to restore timer has expired. The wait to restore time may be configured by the operator in 1 minute steps between 1 and 30 minutes; the default value is 12 minutes. No request is the state entered by the local protection switching function under all conditions where no local pro
47、tection switching requests (including wait to restore) are active. 7.1.5 Protection switching protocol In the unidirectional 1+1, 1:1 and shared mesh protection switching architecture, there is no need for APS protocol. 7.1.6 Unidirectional protection switching algorithm operation 7.1.6.1 Control of
48、 the selector In the 1+1, 1:1 and shared mesh architecture in the unidirectional protection switching operation, the selector is controlled by the highest priority local (i.e., sink of the protection domain for 1+1 architecture; source of the protection domain for 1:1 architecture) request (automati
49、cally initiated command, state, or externally initiated command). Therefore, each end operates independently of the other. If a condition of equal priority (e.g., SF) exists on both LSPs, switching shall not be performed. In packet 1+1, the packets are selected based on a packet level selector which uses identifiers (sequence numbers) carried within transmitted packets. ITU-T Rec. Y.1720 (12/2006) 15 7.1.6.2 Revertive mode In revertive mode of operation, under conditions where working traffic is being transmitted via the protectio
