Internet DRAFT - draft-ietf-spring-segment-protection-sr-te-paths
draft-ietf-spring-segment-protection-sr-te-paths
Routing area S. Hegde
Internet-Draft C. Bowers
Intended status: Informational Juniper Networks Inc.
Expires: 12 August 2024 S. Litkowski
Cisco Systems
X. Xu
China Mobile Inc.
F. Xu
Tencent
9 February 2024
Segment Protection for SR-TE Paths
draft-ietf-spring-segment-protection-sr-te-paths-06
Abstract
Segment routing supports the creation of explicit paths using Adj-
Segment-ID (SID), Node-SIDs, and BSIDs. 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 SR-MPLS 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.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 12 August 2024.
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Copyright Notice
Copyright (c) 2024 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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Node Failures Along SR-TE Paths . . . . . . . . . . . . . . . 3
2.1. Segment protection for explicit paths with Node-SIDs . . 4
2.2. Segment Protection for Anycast-SIDs . . . . . . . . . . . 5
2.3. Segment protection for explicit paths with Adj-SIDs . . . 6
3. Detailed Solution using Context Tables . . . . . . . . . . . 7
3.1. Building Context Tables . . . . . . . . . . . . . . . . . 7
3.2. Segment protection for Node-SIDs . . . . . . . . . . . . 8
3.3. Segment protection for Adj-SIDs . . . . . . . . . . . . . 9
3.4. Segment protection for edge nodes . . . . . . . . . . . . 10
3.4.1. Detailed Example for Segment protection for edge
nodes . . . . . . . . . . . . . . . . . . . . . . . . 11
4. Determining node can be bypassed . . . . . . . . . . . . . . 12
5. Nearside Tunneling for Node-SID/Prefix-SIDs . . . . . . . . . 13
5.1. Interaction with micro-loop avoidance . . . . . . . . . . 14
6. Optimization Considerations . . . . . . . . . . . . . . . . . 14
6.1. Segment Protection Example with Common SRGB . . . . . . . 15
7. Alternate path protection mechanisms . . . . . . . . . . . . 17
8. Operational Considerations . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
12.1. Normative References . . . . . . . . . . . . . . . . . . 18
12.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
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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
[I-D.ietf-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 Adj-
SIDs and Node-SIDs. In order to distinguish the node failures of the
segment endpoints (mid points) in an SR-TE path from the usual node
protection mechanisms described in various LFA mechansims, this
document uses the term Segment Protection. Binding SIDs [RFC8402]
are used to provide network scalability and opacity. When a node
advertising Binding-SID goes down the traffic needs to be protected.
In order to protect binding-SID, the protecting node would need to
learn the binding SID of the protected node. Such signaling
mechanisms are out of scope of this draft
2. Node Failures Along SR-TE Paths
The topology shown in Figure 1. illustrates a example network
topology with Segment Routing enabled on each node.
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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
* Numbers on the links represent the symmetric link cost
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. Segment protection for explicit paths with Node-SIDs
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).
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 segment protection, R7 will
need to understand the meaning of label 3005 in order to determine
where the packet is headed after R8.
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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. Segment 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 |
+------------+
* Numbers on the links represent the symmetric link cost
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
packet sent by R1 with this label stack will follow the path from
R1->R5 along R1->R7->R8->R4->R5.
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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 segment-protecting SR-TE paths can be constructed
through the use of Anycast-SIDs, as opposed to the mechanism
described in this document.
2.3. Segment protection for explicit paths with Adj-SIDs
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 |
+------------+
* Numbers on the links represent the symmetric link cost
Figure 3: Explicit path using an Adj-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 Adj-SIDs,
as shown in Figure 3. The diagram shows the label stack needed to
instantiate this path, as well as several Adj-SIDs advertised by
nodes involved in this path. When a packet leaving R1 with this
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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.ietf-rtgwg-segment-routing-ti-lfa]section 6.2. 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
looking into all the nexthops for the Prefix-SID and choosing a
loop-free path as explained in Section 3.2
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- All the Adj-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. Segment 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 shown 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. Segment protection for Adj-SIDs
This section gives an example of how to constuct node-protecting
backup paths when the SR-TE path uses Adj-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 Adj-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 Adj-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 Adj- 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
Adj-SID. R3 recognizes that R8 has advertised this Adj-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. Segment protection for edge nodes
The segment 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 EPE-SIDs as described in [RFC9086] are used to choose egress
interface among a set of egress paths. EPE-SID can be a bottom-most
label in a SR-TE path. EPE-SIDs are not understood in the IGP
domain. In order to support the procedures described in this
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document, EPE-SIDs should always be added after Anycast-SID for the
nodes that advertised the EPE-SIDs. Same EPE-SID should be
configured on all these Anycast nodes so that in case of node
failure, the traffic is correctly forwarded by the other protector
nodes. If a Node-SID is used instead of an Anycast SID, above the
EPE-SID in the label stack, if procedures in this document are in
use, it may cause packets to be dropped.
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
3.4.1. Detailed Example for Segment protection for edge nodes
sid:1 sid:2 sid:3 sid:4 sid:5
1000-2000 1000-2000 1000-2000 1000-2000 1000-2000
R2:1024 R3:1034 R8:1044 R5:1064
R4:2014 =========================
+----+ 10 +----+ 10 +----+ 10 +----+ 10 +----+ Primary
| PE1|----| R2 |----| R3 |-------| R4 |-- | PE2| context 192.0.2.1
+----+ +----+ +----+ +----+ +----+\sid 10
\ \ / \+-----+
\ 10 \ 100 / 60 /| CE1 |
\ \ / / +-----+
\ +----+ +----+ R4:1054 +-----+sid 10
+--| R7 |---------| R8 | --------| PE3 |context 192.0.2.1
+----+ 30 +----+ +-----+ Protector
/ sid:7 sid:8 sid:9 mirror SID 100
/ 1000-2000 3000-4000 1000-2000
/ 10
+----+
| R6 |
+----+
sid:6
1000-2000
R4's Context Table for PE2 (context.PE2, partial)
+=============+=============================================+
| In label | Outgoing label action |
+=============+=============================================+
| 1010 | swap 1100(mirror sid), push 1010 fwd to R8 |
+-------------+---------------------------------------------+
* Numbers on the links represent the symmetric link cost
Figure 6: Node protection for edge nodes Adj-SIDs
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The segment protection mechanisms that are described in previous
sections depend on the assumption that the label below the top label
in the label stack are understood in the IGP domain. If the edge
node goes down, the label below the top label representing the edge
node could be BGP service label or labels representing other
applications. Service mirroring use case is described in [RFC8402]
section 5.1. The Customer edges are multi-homed to provider edges
and one of the PE's acts in primary role and the other in protector
role. The two PEs advertise a context ip address for each customer
site and attaches a Anycast-SID to the context. The protector PE
advertises a binding sid with M bit set (Mirror-SID)which implies
mirroring capability for the context. Protector PE builds the
context table for the BGP service labels advertised by the primary PE
for the same context. The BGP service resolves on a transport that
has stack of labels with context-sid at the bottom of the label
stack. Any penultimate node of PE2 builds a context table for PE2 as
explained in the section Section 3.1. This context table contains
the sid for the context-id and output action is to pop the top label
and replace with the Mirror-SID that the protector PE advertised for
the context 192.0.2.1. As shown in the example Section 3.4.1 the SID
10 attached to context-id 192.0.2.1 has been programmed in the
context.PE2 on the penultimate router R4. The action is to swap 1010
with Mirror-SID 1100 and push 1010 which is PE2's context SID. When
packet reaches PE2, it has top label of 1100 which is a Mirror-
SID(context label)on PE2 and directs the protector PE to lookup the
context table of Primary PE for the BGP service labels.
4. Determining node can be bypassed
In certain scenarios, the node in the label stack may represent an
important function such as firewall filter which must be performed.
Bypassing such a functionality may cause major security issues. When
segment protection mechanisms described in this document are applied,
it's possible that if the firewall goes down, traffic is re-routed
via the next label in the stack. There are multiple ways this
problem could be solved.
The procedures described in this document should be optional and
should be enabled when devices are configured to apply the procedures
and examine next label in the stack. The feature should be
controllable on a per neighbor granularity. When certain devices
offer a critical function, the neighbors of the devices may disable
the segment protection for this particular neighbor providing
critical functions.
IGP protocol extensions are proposed in
[I-D.li-rtgwg-enhanced-ti-lfa] which define a "no bypass" flag for
the SIDs. The nodes that indicate critical functions may advertise
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SIDs with "NB" bit set. Segment protection procedures described in
this document should not be applied on these SIDs and in case of
failure either link protecting backup paths can be programmed or
packet can be dropped with no protection.
5. Nearside Tunneling for Node-SID/Prefix-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.
Nearside-Tunneling is a mechanism that tunnels the packets that are
affected by the failure to the node that is neighbor of the failure
and is closest to the node computing the failure event. Nearside
Tunneling is explained in the [RFC5715] section 6.2.
When a node in the network experiances another node being deleted,
instead of programming a route delete, it programs a 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 for a duration called hold-down time. This hold-
down time should correspond to the time taken for a controller/PCE/
headend to learn the failure, recompute the paths avoiding the
failure and program them on the headend. For the hold-down time
period, based on the route programmed on the node that experiances
route deletion, packet will be sent to the nearside neighbor of the
failed node, followed by lookup on the next label in the stack. If
the nearside node supports the procedures described in this document,
packet will be forworded bypassing the failed node as described in
previous sections.
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If the nearside node does not support the procedures described in
this draft then traffic will be dropped. Solving partial upgrade
scenarios are out-of-scope of this document. The solutions described
in this document ensure the behaviour of partially upgraded network
is not worse than the behaviour when the procedures described in this
document are not deployed on any node.
The protection mechanisms are expected to work well when there is
single network event. If there are simultaneous network events, the
protection mechansims do not guarantee that the traffic will not be
impacted. When a node is running hold-down timer and is holding
Node-SID and other routes in forwarding plane, if there is another
link-down/link-up event or metric change event is received, the hold-
down should be aborted and the global convergence procedures should
be excecuted.
5.1. 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-
failure shortest path and take the node-protecting backup path at the
PLR of the failed node.
6. 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 Adj-SIDs advertised by the
protected neighbor. Although the scale of IGP domain is limited,
this may result in considerable additional memory consumption on the
routers. 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 Segment Routing Global Block (SRGB)
and all Adj-SIDs in the network are advertised as global Adj-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.
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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.
6.1. Segment Protection Example with Common SRGB
Node Node Node Node Node
sid:1000 sid:1001 sid:1002 sid:1003 sid:1004
+----+2001 2100+----+2102 2201+----+2203 2302+----+2304 2403+----+
| R0 |---------| R1 |---------| R2 |-----------| R3 |------------| R4 |
+----+ 1 +----+ 1 +----+ 1 +----+ 1 +----+
\ 2005 \ 2206 / 2306 2407 |
\ \ / |
\ 1 \ 10 / 6 1 |
\ \ / |
\ 2602 \ / 2603 2704 |
\ 2500+----+ 2506 2605+----+2607 2706+----+
+----| R5 |-----------------| R6 |---------------------| R7 |
+----+ 3 +----+ 1 +----+
Node Node Node
sid:1005 sid:1006 sid:1007
* Numbers on the links represent the symmetric link cost
* All nodes have SRGB = [400000-405000] size 5000
R2's Routing Table (partial)
+=============+=============================================+
| In label | Outgoing label action |
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+=============+=============================================+
| 4001003 | Primary: pop, fwd to R3 |
| | Backup: pop, lookup ilm table or ip table |
| | based on BOS bit |
+-------------+---------------------------------------------+
| 4001007 | Primary: swap 401007, fwd to R6 |
| | Backup: Swap 401007, Push 401005(top),fwd R1|
+-------------+---------------------------------------------+
| 4002203 | Primary: pop, fwd to R3 |
| | Backup: pop, lookup ilm table or ip table |
| | based on BOS bit |
+-------------+---------------------------------------------+
Label Stack 1:
+-------------+
|4001003 (top)|
+-------------+
| 4001007 |
+-------------+ Label Stack 2:
+-------------+
|4001003 (top)|
+-------------+
| 4001007 |
+-------------+
Figure 7: Common SRGB
The diagram Figure 7 shows an example where optimized Segment
Protection mechanism is deployed. All the nodes have a common SRGB
of 400000 to 4005000. The Node-SIDs are in the range 1001, 1002 etc
and the global Adj-SIDs are in the range 2001, 2005 and so on. R2's
partial ILM table consisting of primary and backup nexthops is also
shown in the diagram. Node-SID of R3 which is represented by label
4001003 has a primary nexthop pointing to R3 and backup nexthop which
pops the label and looks up ILM table with next label in the packet.
For Example consider a path from R0 to R7 with a label stack
consisting of 4001003 and 4001007. When the node R3 fails, R2 which
is the PLR, will pop the label 4001003 and lookup for next label in
the same table. Next label in this example is 4001007. Based on the
primary nexthop for 4001007, traffic is forwarded to R6. Another
example label stack consists of global Adj-SID of 4002203 (Adj-SID
from R2->R3). As shown in the partial ILM table on R2, 4002203 also
has a backup nexthop which pops the label and looks-up next label in
the packet.On R3's failure, traffic will get forwarded via R6.
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7. Alternate path protection mechanisms
The current document describes protection mechanisms when nodes that
are mid-points in an SR-TE path fail. The solution described here
focuses on triggering protection locally on the Point of local
repair. There are other path protection mechanisms which provide
end-to-end path protection. In end-to-end path protection mechanism,
path liveness is monitored using liveness detection protocols such as
S-BFD[RFC7880]. A backup path is pre-programmed on the head end of
the SR-TE path. When the S-BFD running on a particular SR-TE path
detects path failure, the head end of SR-TE path switches the traffic
from primary path to backup path. The granularity of failure
detection timers configured on the headend depend on the scale of SR-
TE tunnels on the device and also capabilty of the device to support
fast switchover.
8. Operational Considerations
The procedures described in this document should be configurable and
applied only when enabled explicitly. In order to satisfy scenarios
described in Section 4, the feature should be controllable on the per
neighbor basis. The optimisation procedures described in Section 6,
should be applied only when the entire network has a common SRGB and
all nodes advertise global Adj-SIDs. This optimization should be
applied based on explicit configuration.
9. 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 considerations for SR-MPLS with label stacking is
described in detail in [RFC8402] are applicable for this document as
well. This document introduces the context table lookup for the
labels in the label stack. As described in [RFC8402] MPLS packet
filtering at the boundaries ensures the operations on the MPLS labels
inside the domain is safe includingcontext table lookup operation.
The security procedures applicable to IGP protocols are also
applicable to segment routing extensions as described in [RFC8667]
and [RFC8665] and ensure required protection for the segment
protection procedures described in this document.
10. IANA Considerations
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11. Acknowledgments
The authors would like to thank Peter Psenak, Bruno Decraene,
Alexander Vainshtein and Huzibo, Dhruv Dhody Ketan Talaulikar for
their review and suggestions. Thanks to Bharath R for suggesting
Node-SID hold down mechanisms.
12. References
12.1. 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>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
12.2. Informative References
[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", Work in Progress, Internet-Draft, draft-
bashandy-rtgwg-segment-routing-uloop-16, 17 December 2023,
<https://datatracker.ietf.org/doc/html/draft-bashandy-
rtgwg-segment-routing-uloop-16>.
[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute using Segment Routing", Work in Progress,
Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
13, 16 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
segment-routing-ti-lfa-13>.
[I-D.li-rtgwg-enhanced-ti-lfa]
Li, C., Hu, Z., Zhu, Y., and S. Hegde, "Enhanced Topology
Independent Loop-free Alternate Fast Re-route", Work in
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Progress, Internet-Draft, draft-li-rtgwg-enhanced-ti-lfa-
09, 19 October 2023,
<https://datatracker.ietf.org/doc/html/draft-li-rtgwg-
enhanced-ti-lfa-09>.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, DOI 10.17487/RFC5715, January
2010, <https://www.rfc-editor.org/info/rfc5715>.
[RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
Pallagatti, "Seamless Bidirectional Forwarding Detection
(S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
<https://www.rfc-editor.org/info/rfc7880>.
[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>.
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.
[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>.
[RFC9086] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
Ray, S., and J. Dong, "Border Gateway Protocol - Link
State (BGP-LS) Extensions for Segment Routing BGP Egress
Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August
2021, <https://www.rfc-editor.org/info/rfc9086>.
Authors' Addresses
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Shraddha Hegde
Juniper Networks Inc.
Exora Business Park
Bangalore 560103
KA
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
China Mobile Inc.
Beijing
China
Email: xuxiaohu_ietf@hotmail.com
Feng Xu
Tencent
China
Email: oliverxu@tencent.com
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