rfc9855v2.txt   rfc9855.txt 
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By utilizing SR, TI-LFA eliminates the need to establish Targeted By utilizing SR, TI-LFA eliminates the need to establish Targeted
Label Distribution Protocol sessions with remote nodes for leveraging Label Distribution Protocol sessions with remote nodes for leveraging
the benefits of Remote Loop-Free Alternates (RLFAs) [RFC7490] the benefits of Remote Loop-Free Alternates (RLFAs) [RFC7490]
[RFC7916] or Directed Loop-Free Alternates (DLFAs) [RFC5714]. All [RFC7916] or Directed Loop-Free Alternates (DLFAs) [RFC5714]. All
the Segment Identifiers (SIDs) required are present within the Link the Segment Identifiers (SIDs) required are present within the Link
State Database (LSDB) of the IGP. Consequently, there is no longer a State Database (LSDB) of the IGP. Consequently, there is no longer a
necessity to prefer LFAs over RLFAs or DLFAs, nor is there a need to necessity to prefer LFAs over RLFAs or DLFAs, nor is there a need to
minimize the number of RLFA or DLFA repair nodes. minimize the number of RLFA or DLFA repair nodes.
Utilizing SR makes the requirement unnecessary to establish an Utilizing SR also eliminates the need to establish an additional
additional state within the network for enforcing explicit Fast state within the network for enforcing explicit Fast Reroute (FRR)
Reroute (FRR) paths. This spares the nodes from maintaining a paths. This spares the nodes from maintaining a supplementary state
supplementary state and frees the operator from the necessity to and frees the operator from the necessity to implement additional
implement additional protocols or protocol sessions solely to augment protocols or protocol sessions solely to augment protection coverage.
protection coverage.
TI-LFA also brings the benefit of the ability to provide a backup TI-LFA also brings the benefit of the ability to provide a backup
path that follows the expected post-convergence path considering a path that follows the expected post-convergence path considering a
particular failure, which reduces the need of locally configured particular failure, which reduces the need of locally configured
policies that influence the backup path selection [RFC7916]. The policies that influence the backup path selection [RFC7916]. The
easiest way to express the expected post-convergence path in a loop- easiest way to express the expected post-convergence path in a loop-
free manner is to encode it as a list of adjacency segments. free manner is to encode it as a list of adjacency segments.
However, this may create a long segment list that some hardware may However, this may create a long segment list that some hardware may
not be able to program. One of the challenges of TI-LFA is to encode not be able to program. One of the challenges of TI-LFA is to encode
the expected post-convergence path by combining adjacency segments the expected post-convergence path by combining adjacency segments
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* Section 2 defines the main notations used in the document. They * Section 2 defines the main notations used in the document. They
are in line with [RFC5714]. are in line with [RFC5714].
* Section 3 defines the main principles of TI-LFA backup path * Section 3 defines the main principles of TI-LFA backup path
computation. computation.
* Section 4 suggests to compute the P-Space and Q-Space properties * Section 4 suggests to compute the P-Space and Q-Space properties
defined in Section 2 for the specific case of nodes lying over the defined in Section 2 for the specific case of nodes lying over the
post-convergence paths towards the protected destinations. post-convergence paths towards the protected destinations.
* Using the properties defined in Section 4, Section 5 describes how * Section 5 describes how to compute protection lists that encode a
to compute protection lists that encode a loop-free post- loop-free post-convergence path towards the destination using the
convergence path towards the destination. properties defined in Section 4
* Section 6 defines the segment operations to be applied by the PLR * Section 6 defines the segment operations to be applied by the PLR
to ensure consistency with the forwarding state of the repair to ensure consistency with the forwarding state of the repair
node. node.
* Section 7 discusses aspects that are specific to the dataplane. * Section 7 discusses aspects that are specific to the dataplane.
* Section 8 discusses the relationship between TI-LFA and the SR * Section 8 discusses the relationship between TI-LFA and the SR
algorithm. algorithm.
* Certain considerations are needed when adjacency segments are used * Section 9 provides an overview of the certain considerations that
in a repair list. Section 9 provides an overview of these are needed when adjacency segments are used in a repair list.
considerations.
* Section 10 discusses security considerations. * Section 10 discusses security considerations.
* Appendix A highlights advantages of using the expected post- * Appendix A highlights advantages of using the expected post-
convergence path during FRR. convergence path during FRR.
* By implementing the algorithms detailed in this document within * Appendix B summarizes the measurements from implementing the
actual service provider and large enterprise network environments, algorithms detailed in this document within actual service
real-life measurements are presented regarding the number of SIDs provider and large enterprise network environments. Real-life
utilized by repair paths. These measurements are summarized in measurements are presented regarding the number of SIDs utilized
Appendix B. by repair paths.
2. Terminology 2. Terminology
2.1. Abbreviations and Notations 2.1. Abbreviations and Notations
DLFA: Directed Loop-Free Alternate DLFA: Directed Loop-Free Alternate
FRR: Fast Reroute FRR: Fast Reroute
IGP: Interior Gateway Protocol IGP: Interior Gateway Protocol
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