Internet DRAFT - draft-ietf-spring-node-protection-for-sr-te-paths
draft-ietf-spring-node-protection-for-sr-te-paths
Routing area S. Hegde
Internet-Draft C. Bowers
Intended status: Informational Juniper Networks Inc.
Expires: March 25, 2021 S. Litkowski
Cisco Systems
X. Xu
Alibaba Inc.
F. Xu
Tencent
September 21, 2020
Node Protection for SR-TE Paths
draft-ietf-spring-node-protection-for-sr-te-paths-00
Abstract
Segment routing supports the creation of explicit paths using
adjacency-sids, node-sids, and binding-sids. It is important to
provide fast reroute (FRR) mechanisms to respond to failures of links
and nodes in the Segment-Routed Traffic-Engineered(SR-TE) path. A
point of local repair (PLR) can provide FRR protection against the
failure of a link in an SR-TE path by examining only the first (top)
label in the SR label stack. In order to protect against the failure
of a node, a PLR may need to examine the second label in the stack as
well, in order to determine SR-TE path beyond the failed node. This
document specifies how a PLR can use the first and second label in
the label stack describing an SR-TE path to provide protection
against node failures.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 25, 2021.
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Copyright Notice
Copyright (c) 2020 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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Node Failures Along SR-TE Paths . . . . . . . . . . . . . . . 3
2.1. Node protection for node-sid explicit paths . . . . . . . 3
2.2. Node-Protection for Anycast-SIDs . . . . . . . . . . . . 4
2.3. Node-protection for adj-sid explicit paths . . . . . . . 5
3. Detailed Solution using Context Tables . . . . . . . . . . . 6
3.1. Building Context Tables . . . . . . . . . . . . . . . . . 6
3.2. Node protection for node SIDs . . . . . . . . . . . . . . 7
3.3. Node protection for adjacency SIDs . . . . . . . . . . . 8
3.4. Node protection for edge nodes . . . . . . . . . . . . . 9
4. Hold timers for Node-SID/Prefix-SIDs and Adjacency-SIDs . . . 10
4.1. Interaction with micro-loop avoidance . . . . . . . . . . 10
5. Optimization Considerations . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
It is possible for a routing device to completely go out of service
abruptly due to power failure, hardware failure or software crashes.
Node protection is an important property of the Fast Reroute
mechanism. It provides protection against a node failure by
rerouting traffic around the failed node. For example, the
mechanisms described in Loop Free Alternates ([RFC5286]), Remote Loop
Free Alternates ([RFC8102]), and
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[I-D.bashandy-rtgwg-segment-routing-ti-lfa] can be used to provide
node protection to ensure minimal traffic loss after a node failure.
Section 2 describes problems with SR-TE paths and the need for a
specialized mechanism to provide node protection for SR-TE paths.
Section 3 describes the solution applied to paths built using
adjacency-sids and node-sids.
2. Node Failures Along SR-TE Paths
The topology shown in Figure 1. illustrates a example network
topology with SPRING enabled on each node.
Node Node Node Node Node
sid:1 sid:2 sid:3 sid:4 sid:5
+----+ 10 +----+ 10 +----+ 10 +----+ 10 +----+
| R1 |--------| R2 |--------| R3 |--------| R4 |--------| R5 |
+----+ +----+ +----+ +----+ +----+
\ \ /
\ 10 \ 100 / 60
\ \ /
\ +----+ +----+
+--| R7 |------------------| R8 |
+----+ 30 +----+
/ Node Node Label stack:
/ sid:7 sid:8 +------------+
+----+ SRGB: | 1008 (top)|
| R6 | 3000-4000 +------------+
+----+ | 3005 |
Node +------------+
sid:6
Figure 1: Example topology. The segment index for each node is shown
in the diagram. All nodes have SRGB = [1000-2000], except for R8
which has SRGB = [3000-4000]. A label stack that represents the path
R1->R7->R8->R4->R5 is shown as well.
2.1. Node protection for node-sid explicit paths
Consider an explicit path in the topology in Figure 1 from R1->R5 via
R1->R7->R8->R4->R5. This path can be built using the shortest paths
from R1-to-R8 and R8-to-R5. The label stack to instantiate this path
contains two node-sids 1008 and 3005. The 1008 label will take the
packet from R1 to R8 via R7 and get popped. The next label in the
stack 3005 will take the packet from R8 to the destination R5 via R4.
If the node R8 goes down, it is not possible for R7 to perform FRR
without examining the second label in the incoming label stack
(3005).
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Note that in the absence of a failure, R7 does not need to understand
the meaning of the second label (3005) in order to perform normal
forwarding. However, in order to support node protection, R7 will
need to understand the meaning of label 3005 in order to determine
where the packet is headed after R8.
The mechanisms used to detect whether a node failed or a link failed,
is outside the scope of this document. The possible options for node
failure detection capabilities of a device and resultant forwarding
state is described in section 5.2 in [RFC8679] are applicable to this
draft as well.
2.2. Node-Protection for Anycast-SIDs
A prefix segment advertised as a node SID may only be advertised by
one node in the network. Instead, an anycast prefix segment may be
advertised by more than one node. In some situations, one can use
anycast SIDs to construct SR-TE paths that are protected against node
failure, without the need for the mechanism described in this
document.
+----+ 10 +----+ 10 +----+ 10 +----+ 10 +----+
| R1 |--------| R2 |--------| R3 |--------| R4 |--------| R5 |
+----+ +----+ +----+ +----+ +----+
\ \ / |
\ 10 \100 60/ |
\ \ / |
\ +----+ 30 +----+ |
+--| R7 |------------------| R8 | |
+----+ +----+ |
/ \ Anycast +
/ \ sid:100 /
+----+ \ /
| R6 | \ 40 +----+ /60
+----+ +---------------| R9 |+ Label stack:
+----+ +------------+
Anycast | 1100 (top)|
sid:100 +------------+
| 1005 |
+------------+
Figure 2: Topology illustrating use of anycast-sids to protect
against node failures. All nodes have SRGB = [1000-2000].
An example of this is shown in Figure 2. In this example, R8 and R9
advertise an anycast SID of 100. The label stack in this example =
[1100, 1005];. The top label (1100) corresponds to the anycast SID
advertised by both R8 and R9. In the absence of a failure, the
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packet sent by R1 with this label stack will follow the path from
R1->R5 along R1->R7->R8->R4->R5.
If R7 is performing a per-prefix LFA calculation [RFC5286], then R7
will install a backup next-hop to R9 for this anycast SID, protecting
against the failure of the primary next-hop to R8. This backup path
does not pass through R8, so it is would not be affected by a
complete failure of node R8. As illustrated by this example, for
some topologies node-protecting SR-TE paths can be constructed
through the use of anycast SIDs, as opposed to the mechanism
described in this document.
2.3. Node-protection for adj-sid explicit paths
Adj-sid:
R3-R8:9044
Node Node Node Node Node
sid:1 sid:2 sid:3 sid:4 sid:5
+----+ 10 +----+ 10 +----+ 10 +----+ 10 +----+
| R1 |--------| R2 |--------| R3 |--------| R4 |--------| R5 |
+----+ +----+ +----+ +----+ +----+
\ \ / |
\ 10 \ 100 / 60 | 10
\ \ / |
\ +----+ +----+ +----+
+--| R7 |------------------| R8 |---------------| R9 |
+----+ 30 +----+ 10 +----+
/ Node Node Node
/ sid:7 sid:8 sid:9
+----+ SRGB:
| R6 | 3000-4000 Label stack:
+----+ +------------+
Node Adj-sids: | 1003 (top)|
sid:6 R8-R4:9054 +------------+
| 9044 |
+------------+
| 9054 |
+------------+
| 1005 |
+------------+
Figure 3: Explicit path using an adjacency sid. All nodes have SRGB
= [1000-2000], except for R8 which has SRGB = [3000-4000].
Consider an explicit path from R1->R5 via R1->R2->R3->R8->R4->R5.
This path can be built using a combination of node sids and adjacency
sids, as shown in Figure 3. The diagram shows the label stack needed
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to instantiate this path, as well as several adjacency sids
advertised by nodes involved in this path. When a packet leaving R1
with this label stack reaches R3, the top label is 9044, which will
take the packet to R8. The next-next-hop in the path is R4. To
provide protection for the failure of node R8, R3 would need to send
the the packet to R4 without going through R8. However, the only way
R3 can learn that the packet needs to go to the R4 is to examine the
next label in the stack, label 9054. Since R3 knows that R8 has
advertised label 9054 as the adjacency segment for the link from R8
to R4, R3 knows that a backup path can merge back into the original
explicit path at R4.
3. Detailed Solution using Context Tables
This section provides a detailed description of how to construct
node-protecting backup paths for SR-TE paths using context tables.
The end result of this description is externally visible forwarding
behavior that can be specified as a packet arriving at a PLR with a
particular incoming label stack and leaving the PLR on a particular
outgoing interface with a particular outgoing label stack. There may
be other methods of arriving at the same externally visible
forwarding behavior as described in draft
[I-D.bashandy-rtgwg-segment-routing-ti-lfa]. It is not the intent of
this document to exclude other methods, as long as the externally
visible forwarding behavior is the same as produced by this method.
3.1. Building Context Tables
[RFC5331] introduced the concept of Context Specific Label Spaces and
there are various applications making use of this concept.A context
label table on a router represents the Label Forwarding Information
Base (LFIB) from the point of view of a particular neighbor . Context
tables are built by constructing incoming label mappings advertised
by the neighbor and the actions corresponding to those labels. The
labels advertised by each node are local to the node and may not be
unique across the segment routing domain. The context tables are
separate tables built on a per-neighbor basis on every node to ensure
they represent LFIBs of a particular neighbor.
When a PLR needs to protect an SR-TE path against the failure of a
neighbor N, it creates a context table associated with N. This
context table is populated with the following segment routing
forwarding entries:
- All the Prefix-SIDs of the network. The programmed incoming
label map uses the SRGB of N to compute the input label value.
The NHLFE (Next Hop Label Forwarding Entry) is then constructed by
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looking into all the nexthops for the prefix-SID and choosing a
loop-free path as explained in Section 3.2
- All the Adjacency SIDs advertised by N. The NHLFE is
constructed as explained in Section 3.3
The following section illustrates how the context table is
constructed to allow the PLR to provide node-protecting paths for the
next-next hops in the topology shown in Figure 1 and Figure 3.
3.2. Node protection for node SIDs
Figure 4 shows the routing table entries on R7 corresponding to the
node SIDs to reach R1 and R8 for the topology in Figure 1. In the
absence of a failure, a packet with a label stack whose top label is
1008 will have its top label popped by R7 (assuming PHP behavior),
and R7 will forward the packet to R8. When the interface to R8 is
down, the backup next-hop entry is used. R7 will pop the top label
of 1008, and use the context table that R7 computed for R8 to
evaluate the next label on the stack.
R7's Routing Table (partial)
Transits routes for Node SIDs for R1 and R8
+=============+=============================================+
| In label | Outgoing label action |
+=============+=============================================+
| 1001 | Primary: pop, fwd to R1 |
| | Backup: pop, lookup context.r1 |
+-------------+---------------------------------------------+
| 1008 | Primary: pop, fwd to R8 |
| | Backup: pop, lookup context.r8 |
+-------------+---------------------------------------------+
R7's Context Table for R8 (context.r8, partial)
+=============+=============================================+
| In label | Outgoing label action |
+=============+=============================================+
| 3004 | swap 1004, fwd to R1 |
+-------------+---------------------------------------------+
| 3005 | swap 1005, fwd to R1 |
+-------------+---------------------------------------------+
| 3008 | drop |
+-------------+---------------------------------------------+
Figure 4: Building node-protecting backup paths for SR-TE paths
involving node SIDs
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R7 builds context table for R8 using the following process. R7
computes the mapping of incoming label to node-sid that R8 expects to
see based on the SRGB advertised by R8. In the example in Figure 1,
R7 can determine that R8 interprets in incoming label of 3005 as
mapping to the the node SID for R5.
R7 then computes a loop-free backup path to reach R5 which is node-
protecting with respect to the failure of R8. In this example, the
backup path computed by R7 to reach R5 without passing through R8 can
be achieved forwarding the packet to R1 with a top label of 1005,
corresponding to the node SID for R5 in the context of R1's SRGB.
The loop-free path computation may be based on a mechanism such as
LFA, R-LFA, TI-LFA, or constraint based SPF avoiding failure. To
populate the context table for R8, R7 maps the out label actions
corresponding to the backup path to R5 to the incoming label 3005.
This results in the entry for label 3005 showin in context.r8 in
Figure 4.
Therefore, when a packet arrives at R7 with label stack = [1008,
3005], and the link from R7 to R8 has recently failed, R7 will use
backup next-hop entry for label 1008 in its main routing table.
Based on this entry, R7 will pop label 1008, and use context.r8 to
lookup the new top label = 3005. R7 will swap label 3005 for 1005
and forward the packet to R1. This will get the packet to R5 on a
node protecting backup path.
Note that R7 activates the node-protecting backup path when it
detects that the link to R8 has failed. R7 does not know that node
R8 has actually failed. However, the node-protecting backup path is
computed assuming that the failure of the link to R8 implies that R8
has failed.
3.3. Node protection for adjacency SIDs
This section gives an example of how to constuct node-protecting
backup paths when the SR-TE path uses adjacency SIDs. Figure 5 shows
some of the routing table entries for R3 corresponding to the sample
network shown in Figure 3. When the top label of the label stack is
an adjacency SID, the PLR needs to recognize that in order to provide
a node-protecting backup path, it needs to pop the top label and
examine the next label in the context of the next-hop router
identified by the top label adjacency SID. In this example, when R3
is constructing its routing table, it recognizes that label 9044
corresponds to a next-hop of R8, so it installs a backup entry,
corresponding to the failure of the link to R8, when pops label 9044,
and then examines the new top label in the context of R8.
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R3's Routing Table (partial)
Transit route for Adj SID
+=============+=============================================+
| In label | Outgoing label action |
+=============+=============================================+
| 9044 | Primary: pop, fwd to R8 |
| | Backup: pop, lookup context.r8 |
+-------------+---------------------------------------------+
R3's Context Table for R8 (context.r8, partial)
+=============+=============================================+
| In label | Outgoing label action |
+=============+=============================================+
| 3005 | swap 1005, fwd to R4 |
+-------------+---------------------------------------------+
| 9054 | pop, fwd to R4 |
+-------------+---------------------------------------------+
Figure 5: Building node-protecting backup paths for SR-TE paths
involving adjacency SIDs
R3 constructs its context table for R8 by determining which labels R8
expects to receive to accomplish different forwarding actions. The
entry for incoming label 3005 in context.r8 in Figure 5 corresponds
to a node SID. This entry is computed using the methods described in
Section 3.2
The entry for incoming label 9054 in context.r8 corresponds to an
adjacency SID. R3 recognizes that R8 has advertised this adjacency
SID for the link from R8 to R4 in Figure 3. So R3 determines the
outgoing label action needed to reach R4 without passing through R8.
This can be accomplished by popping the label 9054, and forwarding
the packet directly on the link from R3 to R4.
3.4. Node protection for edge nodes
The node protection mechanism described in the previous sections
depends on the assumption that the label immediately below the top
label in the label stack is understood in the IGP domain. When the
provider edge routers exchange service labels via BGP or some other
non-IGP mechanism the bottom label is not understood in the IGP
domain.
The egress node protection mechanisms described in the draft
[RFC8679] is applicable to this usecase and no additional changes
will be required for SR based networks
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4. Hold timers for Node-SID/Prefix-SIDs and Adjacency-SIDs
SR-TE paths may be computed by a controller or by the head-end
router. When there is a node failure in the network, the controller
or head-end router has to learn about the failure, recompute the
label stacks of any affected SR-TE paths, and get the new label
stacks programmed into the forwarding plane of the head-end router.
This process may be slow compared to the speed with which routers in
the network react to the event. After learning about a node failure,
the non-PLR routers in the network will no longer be able to compute
a path to reach the failed node. If no special precautions are
taken, these non-PLR routers will remove the forwarding entries
corresponding the Node-SID and Prefix-SIDs advertised by the failed
node. If the head-end router is still sending traffic with that
Node-sid/prefix-sid in the stack, traffic can be blackholed at a non-
PLR router. In this case, the node-protection FRR mechanisms do not
bring full benefit.
In order to solve the above problem, hold timers are recommended.
The hold-timer corresponds to the maximum time that a combination of
controller and head-end router or a head-end router alone takes to
compute and install label stacks corresponding to a new SR-TE paths
in the event of a node failure. The hold times should be applied to
forwarding entries for Node-SIDs and Prefix-SIDs that are advertised
by single node in the network. If the Node-SID or Prefix-SID becomes
unreachable, the event and resulting forwarding changes should not
communicated to the forwarding planes on all configured routers
(including PLRs for the failed node) until the hold-timer expires.
The traffic will continue to follow the previous path and get FRR
protection on the PLR.
A route corresponding to a global adjacency SID advertised by a node
that becomes unreachable should also be left in the forwarding table
for the duration of the hold-timer.
The node-protecting backup forwarding entry on the PLR corresponding
to the local adjacency-SID from the PLR to the failed node should
also be left in the forwarding table for the duration of the hold-
timer.
4.1. Interaction with micro-loop avoidance
During network convergence, the micro-loop avoidance mechansims as
described in [I-D.bashandy-rtgwg-segment-routing-uloop] may be
applied.For the failed node, all the nodes in the network should
consistently detect the failure and maintain the pre-failure shortest
path in the forwarding plane so that the traffic can follow pre-
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failure shortest path and take the node-protecting backup path at the
PLR of the failed node.
5. Optimization Considerations
The solution described in this document requires that a PLR build a
context table for each neighbor for which node-protection is desired.
The context table for each protected neighbor needs to contain route
entries for all of the prefix-SIDs in the network, as well as the
route entries corresponding to the adjacency SIDs advertised by the
protected neighbor. This may result in considerable additional
memory consumption. It is possible to take advantage of an
optimization that allows the PLR to avoid creating context-tables
when all of the nodes in the network advertise the same SRGB and all
adjacency SIDs in the network are advertised as global adjacency
SIDs. In this case, all labels in the stack representing an SR-TE
path are globally unique. Protection for node failure cases in such
a deployment can be achieved by doing a lookup of the first label and
potentially a second lookup of the second label using a common route
table with primary and backup entries for all prefix-SIDs as well as
for all of the global adj-SIDs.
The primary route entries for global adj-SIDs not advertised by the
PLR will be the shortest path to the node advertising the global adj-
SID. The backup route entries for these global adj-SIDs will
generally correspond to the node-protecting backup path to the node
advertising the global adj-SID. However, for a global adj-SID
advertised by the direct neighbor of the PLR the node-protecting
backup route entry will correspond to the backup path to the node on
the far end of the adj-SID.
With the common route table constructed in this manner, when the PLR
receives a packet whose first label is a global adj-SID advertised by
the failed neighbor of the PLR, the lookup of the first label will
produce the correct backup path directly. When the PLR receives a
packet whose first label is the node-SID of the failed neighbor, or
an adj-SID advertised by the PLR corresponding to the failed
neighbor, the route entry will instruct the PLR to lookup the second
label using the common route table. Finally, when the PLR receives a
packet whose first label is a global adj-SID or a node-SID advertised
by a node which is neither the PLR nor the failed neighbor, then the
usual link-protecting backup path will be produced based on a lookup
of the first label only.
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6. Security Considerations
The procedures described in this document will in most common cases
be deployed inside a single ownership IGP domain. No new security
risks are exposed due to the procedures described in this document.
The security procedures applicable to IGP protocols will provide the
desired protection.
7. IANA Considerations
8. Acknowledgments
The authors would like to thank Peter Psenak, Bruno Decraene,
Alexander Vainshtein and Huzibo for their review and suggestions.
9. References
9.1. Normative References
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space",
RFC 5331, DOI 10.17487/RFC5331, August 2008,
<https://www.rfc-editor.org/info/rfc5331>.
9.2. Informative References
[I-D.bashandy-rtgwg-segment-routing-ti-lfa]
Bashandy, A., Filsfils, C., Decraene, B., Litkowski, S.,
Francois, P., daniel.voyer@bell.ca, d., Clad, F., and P.
Camarillo, "Topology Independent Fast Reroute using
Segment Routing", draft-bashandy-rtgwg-segment-routing-ti-
lfa-05 (work in progress), October 2018.
[I-D.bashandy-rtgwg-segment-routing-uloop]
Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B.,
Francois, P., and P. Psenak, "Loop avoidance using Segment
Routing", draft-bashandy-rtgwg-segment-routing-uloop-09
(work in progress), June 2020.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC8102] Sarkar, P., Ed., Hegde, S., Bowers, C., Gredler, H., and
S. Litkowski, "Remote-LFA Node Protection and
Manageability", RFC 8102, DOI 10.17487/RFC8102, March
2017, <https://www.rfc-editor.org/info/rfc8102>.
[RFC8679] Shen, Y., Jeganathan, M., Decraene, B., Gredler, H.,
Michel, C., and H. Chen, "MPLS Egress Protection
Framework", RFC 8679, DOI 10.17487/RFC8679, December 2019,
<https://www.rfc-editor.org/info/rfc8679>.
Authors' Addresses
Shraddha Hegde
Juniper Networks Inc.
Exora Business Park
Bangalore, KA 560103
India
Email: shraddha@juniper.net
Chris Bowers
Juniper Networks Inc.
Email: cbowers@juniper.net
Stephane Litkowski
Cisco Systems
Email: slitkows.ietf@gmail.com
Xiaohu Xu
Alibaba Inc.
Beijing
China
Email: xiaohu.xxh@alibaba-inc.com
Feng Xu
Tencent
China
Email: oliverxu@tencent.com
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