Network Working Group S. Bryant, Ed. Internet-Draft Cisco Intended status: Informational Y. Weingarten, Ed. Expires: January 8, 2010 N. Sprecher, Ed. Nokia Siemens Networks July 7, 2009 MPLS-TP Ring Protection draft-weingarten-mpls-tp-ring-protection-00.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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. This Internet-Draft will expire on January 8, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This document describes mechanisms to address the requirements for Bryant, et al. Expires January 8, 2010 [Page 1] Internet-Draft MPLS-TP LP July 2009 protection of ring topologies for Multi-Protocol Label Switching Transport Profile (MPLS-TP) Label Switched Paths (LSP) and Pseudowires (PW) on multiple layers. Ring topologies offer the possibility of reducing the OAM overhead while providing a simplified protection mechanism. The document analyzes two basic ring protection schemes and explains how ring protection can be viewed as an application of linear protection. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Contributing Authors . . . . . . . . . . . . . . . . . . . 3 2. Ring Topologies . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Ring protection schemes . . . . . . . . . . . . . . . . . . . . 4 3.1. Wrapping . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Steering . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Conclusions and Recommendations . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 8 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 8. Informative References . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8 Bryant, et al. Expires January 8, 2010 [Page 2] Internet-Draft MPLS-TP LP July 2009 1. Introduction Multi-Protocol Label Switching Transport Profile (MPLS-TP) is being standardized as part of a joint effort between the Internet Engineering Task Force (IETF) and the International Telecommunication Union Standardization (ITU-T). The specifications are based on the requirements that were generated from this joint effort. The requirements for MPLS-TP [MPLS-TP Reqs] inidcates that there is requirement to support a network that may include sections that constitute a MPLS-TP ring (either logical or physical). The support for ring topologies as stated in the requirements is based on the ability to demonstrate that this topology allows the network to optimize either the protection or the number of Operations, Administration & Maintenance (OAM) entities needed to maintain the network. This document will examine different proposed mechanisms for protection of a ring in the context of MPLS-TP and try and determine how they may optimize the protection and the OAM procedures for a ring topology. Finally, we plan to show how the generic protection mechanisms can be used to address the requirements in an optimized manner. 1.1. Contributing Authors 2. Ring Topologies The MPLS-TP Requirements [MPLS-TP Reqs] defines a ring as a topology in which each LSR is connected to exactly two neighboring LSRs, each via a single point-to-point birectional MPLS-TP capable link. A ring provides certain advantages in transport networks, including: o Configuration of point-to-multipoint paths around a ring are easily accomplished. o There are always two paths between any two LSRs on a ring that can be easily identified and associated. o It is believed that the number of OAM entities needed, in order to detect faults and perform recovery actions, may be minimized in a ring topology. The following figure shows a MPLS-TP ring that is a segment that may be traversed by numerous LSPs or PWs. In particular, the figure shows that for all LSP that connect to the ring through LSR-B and exit the ring from LSR-F we can define two paths through the ring Bryant, et al. Expires January 8, 2010 [Page 3] Internet-Draft MPLS-TP LP July 2009 (the first path along B-A-F, and the second B-C-D-E-F). ____ =========>/ LSR\ * \__B_/ * * @ # * * @ # * __* @ # *___ /LSR\ @ #/LSR\ \_C_/ @ #\_A_/ * @ # * * @ #* _*_ @ #*_ /LSR\@ /LSR\========> \_D_/@ \_F_/ * @ @* * @ @* * @@____@@* */ LSR\* \__E_/ ===> connected LSP *** physical link ### logical path @@@ secondary logical path Figure 1: A MPLS-TP ring 3. Ring protection schemes There are two classic mechanisms that have been proposed in various forums to perform recovery of a topological ring network - "wrapping" and "steering". The following sub-sections will examine these two mechanisms. 3.1. Wrapping The "easier" recovery architecture is "wrapping". This mechanism is local to the LSRs that are neighbors to the detected fault. When a fault is detected, the neighboring LSR "wrap" all data traffic around the ring until arriving at the LSR that is on the opposite side of the fault, at which point the traffic continues on the normal working path until the egress from the ring segment. Bryant, et al. Expires January 8, 2010 [Page 4] Internet-Draft MPLS-TP LP July 2009 ____ =========>/ LSR\ * \__B_/ * * @@@@@@@# * * @ @# * ___* @ @# *___ /LSR\ @ @#/LSR\ \_C_/ @ #\_A_/ * @ # * * @ XX _*_ @ #*_ /LSR\@ /LSR\ \_D_/@ @\_F_/ * @ @#* * @ @@#* * @@____@##* */ LSR\* \__E_/========> ===> connected LSP *** physical link ### logical path @@@ Bypass tunnel Figure 2: Wrapping protection In this figure we have a ring with a LSP that enters the ring at LSR-B and exits at LSR-E. The normal working path follows through B-A-F-E. If a signal fault is detected on the link A<-->F, then there is the need for configuring a bypass tunnel [FRR] between A & F. The traffic will be transmitted over this bypass tunnel from A to F, and then will continue on the normal working path from F->E. Essentially, in this protection scheme, the traffic will follow the path - B-A-B-C-D-E-F-E. This protection scheme is simple in the sense that there is no need for coordination between the different LSR in the ring - only the LSR that detect fault must wrap the traffic, either via the bypass tunnel (at the near-end) or back to the normal path (at the far-end). When applying this scheme to a MPLS-TP ring topology segment there are the following considerations: o The OAM should be performed at either the link level (by defining a TCME between each adjacent pair of LSR) and/or per LSR (by defining a TCME between the LSR that are neighbors of the protected LSR) when using node-level protection. o For each protected TCME there is a need to define a bypass tunnel that traverses the alternate path around the ring to connect Bryant, et al. Expires January 8, 2010 [Page 5] Internet-Draft MPLS-TP LP July 2009 between the two ends of the TCME. If protecting both the links and the nodes, then, for a ring with N nodes, there is a need for O(2N) bypass tunnels. o Protection of point-to-multipoint paths is similar to the simple protection since the data continues along the original path after wrapping around the ring. The one exception is the case where the failed node was one of the egress points for the data. o When wrapping the data is transmitted over some of the links twice, once in each direction. For example, in the figure above the traffic is transmitted both B->A and then A->B, later it is transmitted E->F and F->E. This means that there is additional bandwith needed for this protection. o The wrapping also involves greater latency in delivering the packets, as a result of traversing the entire ring. o The resource allocation for the bypass tunnels could be problematic, since most of the tunnels will not be used simultaneously. One possibility could be to allocate '0' resources and depend on the NMS to allocate the proper resources around the ring. 3.2. Steering The second common scheme for ring protection redirects the traffic from the ingress point to the alternate route around the ring to the egress point. This is illustrated in Figure 1 above, where if a Signal Fault is detected on the working path (B-A-F), then the traffic is redirected by B to the secondary path (i.e. B-C-D-E-F). When considering this mechanism it is almost identical to linear 1:1 protection. The two paths around the ring act as the working and recovery paths. There is need to communicate to the ingress node the need to switch over to the protection path and there is a need to coordinate the switchover between the two end-points of the protected path. There is one aspect that this diverts from the basic linear protection scheme - in the number of OAM sessions that would be neccesary to detect faults in the protected domain. Whereas, using generic linear protection would neccesitate a separate OAM session per LSP that traverses the ring, when using ring protection there is apossiblity of taking proper advantage of the realization that we are dealing with a ring and reduce the number of OAM sessions. This is done by defining a OAM session on the basis of a Path Segment Tunnel (PST), i.e. between any two nodes of the ring. This would lead to Bryant, et al. Expires January 8, 2010 [Page 6] Internet-Draft MPLS-TP LP July 2009 the number of OAM sessions for a ring with N nodes to be O(N*N/2), which could be very large. However, taking into consideration that the required support of rings, is for rings with up-to 16 nodes - this implies that the number of OAM sessions should be on the order of (16*16/2) or 128. This form of OAM would allow the ingress LSR to directly detect any faulty situations and redirect traffic to the secondary path without the need for any additional communication to the LSR. The following observations can be drawn from using this protection mechanism for MPLS-TP ring topologies: o Steering can be based on linear protection for the protection of a single ring. For cases of interconnected rings further study is necessary. o The number of OAM sessions can be greatly minimized, relative to using plain linear protection, by running the OAM sessions and the protection mechanisms on the ring segments for all LSPs that traverse the ring over that specific segment, rather than running a OAM session per LSP. This fulfills the objective presented in [MPLS-TP Reqs]. o Point-to-multipoint paths through the ring would need further consideration. 4. Conclusions and Recommendations In order to fulfill the requirements for protection of ring topologies for MPLS-TP networks, according to the conditions stated in [MPLS-TP Reqs], the protection should be based on MPLS-TP 1:1 linear protection. This mechanism will cover the cases of a single fault in a single ring topology. When defining the OAM behavior of the ring nodes, they should define a segment of all the LSPs that traverse a path within the ring. The OAM should be executed for each ring path, i.e. PST, to detect faults and trigger the protection switching within the ring. 5. IANA Considerations This document makes no request of IANA. Note to RFC Editor: this section may be removed on publication as an RFC. Bryant, et al. Expires January 8, 2010 [Page 7] Internet-Draft MPLS-TP LP July 2009 6. Security Considerations This document does not by itself raise any particular security considerations. 7. Acknowledgements The authors would like to thank all members of the teams (the Joint Working Team, the MPLS Interoperability Design Team in IETF and the T-MPLS Ad Hoc Group in ITU-T) involved in the definition and specification of MPLS Transport Profile. 8. Informative References [FRR] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute Exensions to RSVP-TE for LSP Tunnels", RFC 4090, May 2005. [MPLS-TP Reqs] Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N., and S. Ueno, "Requirements for the Trasport Profile of MPLS", ID draft-ietf-mpls-tp-requirements-09, June 2009. [MPLS-TP Surv Fwk] Sprecher, N. and A. Farrel, "MPLS-TP Survivability Framework", ID draft-ietf-mpls-tp-requirements-09, April 2009. Authors' Addresses Stewart Bryant (editor) Cisco United Kingdom Email: stbryant@cisco.com Yaacov Weingarten (editor) Nokia Siemens Networks 3 Hanagar St. Neve Ne'eman B Hod Hasharon, 45241 Israel Phone: +972-9-775 1827 Email: yaacov.weingarten@nsn.com Bryant, et al. Expires January 8, 2010 [Page 8] Internet-Draft MPLS-TP LP July 2009 Nurit Sprecher (editor) Nokia Siemens Networks 3 Hanagar St. Neve Ne'eman B Hod Hasharon, 45241 Israel Phone: +972-9-775 1229 Email: nurit.sprecher@nsn.com Bryant, et al. Expires January 8, 2010 [Page 9]