RFC 3063 (rfc3063) - Page 2 of 44

MPLS Loop Prevention Mechanism

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.



Ohba, et al.                  Experimental