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rfc3063 - Network Working Group Y Ohba Request for Comments...

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Network Working Group Y. Ohba Request for Comments: 3063 Y. Katsube Category: Experimental Toshiba E. Rosen Cisco Systems P. Doolan Ennovate Networks February 2001 MPLS Loop Prevention Mechanism Status of this Memo This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract This paper presents a simple mechanism, based on "threads", which can be used to prevent Multiprotocol Label Switching (MPLS) from setting up label switched path (LSPs) which have loops. The mechanism is compatible with, but does not require, VC merge. The mechanism can be used with either the ordered downstream-on-demand allocation or ordered downstream allocation. The amount of information that must be passed in a protocol message is tightly bounded (i.e., no path- vector is used). When a node needs to change its next hop, a distributed procedure is executed, but only nodes which are downstream of the change are involved. Ohba, et al. Experimental [Page 1]
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RFC 3063 MPLS Loop Prevention Mechanism February 2001 Table of Contents 1 Introduction .......................................... 2 2 Basic definitions ..................................... 3 3 Thread basics ......................................... 5 3.1 Thread attributes ..................................... 5 3.2 Thread loop ........................................... 7 3.3 Primitive thread actions .............................. 7 3.4 Examples of primitive thread actions ................. 10 4 Thread algorithm ...................................... 14 5 Applicability of the algorithm ........................ 14 5.1 LSP Loop prevention/detection ......................... 15 5.2 Using old path while looping on new path .............. 15 5.3 How to deal with ordered downstream allocation ........ 15 5.4 How to realize load splitting ......................... 15 6 Why this works ........................................ 16 6.1 Why a thread with unknown hop count is extended ....... 16 6.2 Why a rewound thread cannot contain a loop ............ 17 6.2.1 Case1: LSP with known link hop counts ................. 17 6.2.1 Case2: LSP with unknown link hop counts ............... 17 6.3 Why L3 loop is detected ............................... 17 6.4 Why L3 loop is not mis-detected ....................... 17 6.5 How a stalled thread automatically recovers from loop . 18 6.6 Why different colored threads do not chase each other . 18 7 Loop prevention examples .............................. 19 7.1 First example ......................................... 19 7.2 Second example ........................................ 23 8 Thread control block .................................. 24 8.1 Finite state machine .................................. 25 9 Comparison with path-vector/diffusion method .......... 28 10 Security Considerations ............................... 29 11 Intellectual Property Considerations .................. 29 12 Acknowledgments ....................................... 29 13 Authors' Addresses .................................... 30 14 References ............................................ 30 Appendix A Further discussion of the algorithm ............. 31 Full Copyright Statement ..................................... 44 1. Introduction This paper presents a simple mechanism, based on "threads", which can be used to prevent MPLS from setting up label switched paths (LSPs) which have loops. When an LSR finds that it has a new next hop for a particular FEC (Forwarding Equivalence Class) [1], it creates a thread and extends it downstream. Each such thread is assigned a unique "color", such that no two threads in the network can have the same color.
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