| Internet-Draft | SRMPLS FRR Extension | September 2023 |
| Chen, et al. | Expires 16 March 2024 | [Page] |
The current SR FRR such as TI-LFA provides fast re-route protection for the failure of a node on an SR-MPLS path by the neighbor upstream node as point of local repair (PLR) of the failed node. However, once the IGP converges, the SR FRR is no longer sufficient to forward traffic of the path around the failure, since the non-neighbor upstream node of the failed node will no longer have a route to the failed node. This document describes a simple mechanism to extend the fast re-route protection for the failure on an SR-MPLS path after the IGP converges. The mechanism protects the node SID, adjacency SID and binding SID of the failed node on the path.¶
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[I-D.ietf-rtgwg-segment-routing-ti-lfa] describes an SR FRR mechanism that provides fast re-route protection for the failure of a node on an SR-TE path by the neighbor upstream node as point of local repair (PLR) of the failed node. However, once the IGP converges, the SR FRR is no longer sufficient to forward traffic of the path around the failure, since the non-neighbor upstream node of the failed node will no longer have a route to the failed node and drop the traffic.¶
[I-D.ietf-spring-segment-protection-sr-te-paths] proposes a solution in which a hold-down timer is configured on every node in a network. After the IGP converges on a node failure, when a node is going to delete the route to the failed node, instead of programming a route delete, it programs a tunnel/path to the node consisting of the Node SID of the nearside neighbor of the failed node followed by the original path in the packet. The modified path will be in force until the hold-down timer expires.¶
This document describes a simple mechanism to extend the fast re-route protection for the failure on an SR-MPLS path after the IGP converges. The mechanism protects the node SID, adjacency SID and binding SID of the failed node on the path.¶
This section illustrates the extension to SR-MPLS FRR for the failure on SR-MPLS paths after the IGP converges through examples. It shows the procedure on every related node on each path without any failure, with a failure before and after the IGP converges on the failure.¶
Figure 1 shows an example topology with two SR-MPLS paths: Path 1 and Path 2 for explaining the extension. They go through the same nodes and links, but represented differently. The former does not have any binding SID (BSID). The latter has a BSID of node N (BSID-N).¶
[ P1 ]-----[ P3 ]-----[ N ]-----[ Q1 ]
/ | \ / | \ / | \ / | \
/ | \ / | \ / | \ / | \
[CE1]------[A] | X | X | X | [C]-----[CE2]
\ | / \ | / \ | / \ | /
\ | / \ | / \ | / \ | /
[ P2 ]-----[ P4 ]-----[ N1 ]-----[ Q2 ]
Path 1:A->P1->N->Q1->C, indicated at A by {SID-P1,SID-N,SID-Q1,SID-C}
Path 2:A->P1->N->Q1->C, indicated at A by {SID-P1,SID-N,BSID-N} and
BSID-N associated with SID list {SID-Q1,SID-C}
SR-MPLS Path 1: A->P1->N->Q1->C is indicated at node A by node SID of P1 (SID-P1), node SID of N (SID-N), node SID of Q1 (SID-Q1) and node SID of C (SID-C). SR-MPLS Path 2 is indicated at node A by {SID-P1,SID-N,BSID-N}, where BSID-N (binding SID of N) is associated with SID list {SID-Q1,SID-C}.¶
Figure 2 shows the result of executing procedure on each related node of SR-MPLS path 1 without any failure.¶
1.{SID-P1,SID-N,SID-Q1,SID-C}Pkt
| 2.{SID-N,SID-Q1,SID-C}Pkt
| | 3.{SID-N,SID-Q1,SID-C}Pkt
| | | 4.{SID-Q1,SID-C}Pkt
| | | |
| v v v
| [ P1 ]-----[ P3 ]-----[ N ]-----[ Q1 ]
v / | \ / | \ / | \ / | \5.{SID-C}Pkt
Pkt / | \ / | \ / | \ / | \
[CE1]------[A] | X | X | X | [C]-----[CE2]
\ | / \ | / \ | / \ | / 6.Pkt
\ | / \ | / \ | / \ | /
[ P2 ]-----[ P4 ]-----[ N1 ]-----[ Q2 ]
Path 1:A->P1->N->Q1->C, indicated at A by {SID-P1,SID-N,SID-Q1,SID-C}
The results from nodes A, P1, P3, N, Q1 and C are as follows.¶
Figure 3 shows the result of executing procedure on each related node of SR-MPLS path 1 with no BSID when node N failed and before the IGP converges on the failure.¶
1.{SID-P1,SID-N,SID-Q1,SID-C}Pkt
|
| 2.{SID-N,SID-Q1,SID-C}Pkt
| |
| v
| [ P1 ]-----[ P3 ] [ N ] [ Q1 ]
v / | \ / | \3.{SID-Q1, / | \5.{SID-C}Pkt
Pkt / | \ / | \ SID-C}Pkt / | \
[CE1]------[A] | X | X | X | [C]-----[CE2]
\ | / \ | / \ | / \ | / 6.Pkt
\ | / \ | / \ | /4.{SID-Q1,SID-C}Pkt
[ P2 ]-----[ P4 ]-----[ N1 ]-----[ Q2 ]
Path 1:A->P1->N->Q1->C, indicated at A by {SID-P1,SID-N,SID-Q1,SID-C}
The results from nodes A, P1, Q1 and C are the same as those described in Section 2.1.1. The results from neighbor upstream node P3 of N and node N1 are as follows.¶
Figure 4 shows the result of executing procedure on each related node on SR-MPLS path 1 with no BSID when node N failed and after the IGP converges on the failure.¶
1.{SID-P1,SID-N,SID-Q1,SID-C}Pkt
|
| [ P1 ]-----[ P3 ] [ N ] [ Q1 ]
v / | \2.{SID-Q1,SID-C}Pkt / | \5.{SID-C}Pkt
Pkt / | \ / | \ / | \
[CE1]------[A] | X | \ / | [C]-----[CE2]
\ | / \ | \ / | / 6.Pkt
\ | / \ | \ /4.{SID-Q1,SID-C}Pkt
[ P2 ]-----[ P4 ]-----[ N1 ]-----[ Q2 ]
^
|
3.{SID-Q1,SID-C}Pkt
Path 1:A->P1->N->Q1->C, indicated at A by {SID-P1,SID-N,SID-Q1,SID-C}
The results from nodes A, N1, Q1 and C are the same as those described above in Section 2.1.2. The results from nodes P1 and P4 are as follows.¶
Figure 5 shows the result of executing procedure on each related node of SR-MPLS path 2 with BSID-N without any failure.¶
1.{SID-P1,SID-N,BSID-N}Pkt
| 2.{SID-N,BSID-N}Pkt
| | 3.{SID-N,BSID-N}Pkt
| | | 4.{SID-Q1,SID-C}Pkt
| | | |
| v v v
| [ P1 ]-----[ P3 ]-----[ N ]-----[ Q1 ]
v / | \ / | \ / | \ / | \5.{SID-C}Pkt
Pkt / | \ / | \ / | \ / | \
[CE1]------[A] | X | X | X | [C]-----[CE2]
\ | / \ | / \ | / \ | / 6.Pkt
\ | / \ | / \ | / \ | /
[ P2 ]-----[ P4 ]-----[ N1 ]-----[ Q2 ]
Path 2:A->P1->N->Q1->C, indicated at A by {SID-P1,SID-N,BSID-N} and
BSID-N associated with SID list {SID-Q1,SID-C}
The results from nodes A, P1, P3, and N are as follows.¶
The results from nodes Q1 and C are the same as those described in Section 2.1.1.¶
Figure 6 shows the result of executing procedure on each related node of SR-MPLS path 2 with BSID-N when node N failed and before the IGP converges on the failure.¶
1.{SID-P1,SID-N,BSID-N}Pkt
|
| 2.{SID-N,BSID-N}Pkt
| |
| v
| [ P1 ]-----[ P3 ] [ N ] [ Q1 ]
v / | \ / | \3.{SID-Q1, / | \5.{SID-C}Pkt
Pkt / | \ / | \ SID-C}Pkt / | \
[CE1]------[A] | X | X | X | [C]-----[CE2]
\ | / \ | / \ | / \ | / 6.Pkt
\ | / \ | / \ | /4.{SID-Q1,SID-C}Pkt
[ P2 ]-----[ P4 ]-----[ N1 ]-----[ Q2 ]
Path 2:A->P1->N->Q1->C, indicated at A by {SID-P1,SID-N,BSID-N} and
BSID-N associated with SID list {SID-Q1,SID-C}
The results from nodes A, P1, Q1 and C are the same as those described in Section 2.2.1. The results from neighbor upstream node P3 of N and node N1 are as follows.¶
Figure 7 shows the result of executing procedure on each related node on SR-MPLS path 2 with BSID-N when node N failed and after the IGP converges on the failure.¶
1.{SID-P1,SID-N,BSID-N}Pkt
|
| [ P1 ]-----[ P3 ] [ N ] [ Q1 ]
v / | \2.{SID-Q1,SID-C}Pkt / | \5.{SID-C}Pkt
Pkt / | \ / | \ / | \
[CE1]------[A] | X | \ / | [C]-----[CE2]
\ | / \ | \ / | / 6.Pkt
\ | / \ | \ /4.{SID-Q1,SID-C}Pkt
[ P2 ]-----[ P4 ]-----[ N1 ]-----[ Q2 ]
^
|
3.{SID-Q1,SID-C}Pkt
Path 2:A->P1->N->Q1->C, indicated at A by {SID-P1,SID-N,BSID-N} and
BSID-N associated with SID list {SID-Q1,SID-C}
The results from nodes A, Q1 and C are the same as those described above in Section 2.2.2. The results from nodes P4 and N1 are the same as those described above in Section 2.1.3. The result from node P1 is as follows.¶
This section presents the procedures on a neighbor upstream node and a non-neighbor upstream node of node N on an SR MPLS path.¶
For an SR-MPLS path with the node SID of node N (SID-N), suppose that node X is an non-neighbor upstream node of node N along the path, wherein SID-N follows the node SID of node X (SID-X) or the adjacency SID to node X in the packet to be transported by the path. For example, SR-MPLS Path 1 and Path 2 in Figure 1 are from A to P1 to N to Q1 to C. Node P1 is the non-neighbor upstream node of N. At ingress node A of the path, node A adds the SIDs in the SID list into the packet to be transported by the path.¶
Without any failure or from the failure of node N to IGP convergence on the failure, the non-neighbor upstream node (such as P1) of node N pops its SID from the packet if any and sends the packet to the next hop node toward node N along the IGP shortest path to N.¶
After node N failed and from the IGP convergence on the failure to global reroute, the non-neighbor upstream node (such as P1) of node N pops its SID (such as SID-P1) from the packet if any, pops SID-N from the packet and does one of the following:¶
Suppose that node Y is the neighbor upstream node of node N on an SR-MPLS path. Without any failure, node Y sends the packet received from the path to node N according to the top SID in the packet. For example, SR-MPLS Path 1 and Path 2 in Figure 1 are from A to P1 to N to Q1 to C. Node P3 is the neighbor upstream node of N suppose that the shortest path from P1 to N is from P1 to P3 to N. When node Y detects the failure of node N, node Y pops SID-N from the packet if SID-N is the top SID of the packet, and does one of the following:¶
No requirements for IANA.¶
The authors would like to thank people for their comments to this work.¶