Circuit Style Segment Routing Policies
draft-ietf-spring-cs-sr-policy-02
| Document | Type | Active Internet-Draft (spring WG) | |
|---|---|---|---|
| Authors | Christian Schmutzer , Zafar Ali , Praveen Maheshwari , Reza Rokui , Andrew Stone | ||
| Last updated | 2024-04-18 | ||
| Replaces | draft-schmutzer-spring-cs-sr-policy | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-spring-cs-sr-policy-02
Network Working Group C. Schmutzer, Ed.
Internet-Draft Z. Ali, Ed.
Intended status: Informational Cisco Systems, Inc.
Expires: 20 October 2024 P. Maheshwari
Airtel India
R. Rokui
Ciena
A. Stone
Nokia
18 April 2024
Circuit Style Segment Routing Policies
draft-ietf-spring-cs-sr-policy-02
Abstract
This document describes how Segment Routing (SR) policies can be used
to satisfy the requirements for bandwidth, end-to-end recovery and
persistent paths within a segment routing network. SR policies
satisfying these requirements are called "circuit-style" SR policies
(CS-SR policies).
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
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on 20 October 2024.
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/
license-info) in effect on the date of publication of this document.
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Reference Model . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Managing Bandwidth . . . . . . . . . . . . . . . . . . . 5
4. CS-SR Policy Characteristics . . . . . . . . . . . . . . . . 6
5. CS-SR Policy Creation . . . . . . . . . . . . . . . . . . . . 7
5.1. Policy Creation when using PCEP . . . . . . . . . . . . . 7
5.2. Policy Creation when using BGP . . . . . . . . . . . . . 8
5.3. Maximum Segment Depth . . . . . . . . . . . . . . . . . . 9
6. Recovery Schemes . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Unprotected . . . . . . . . . . . . . . . . . . . . . . . 10
6.2. 1:1 Protection . . . . . . . . . . . . . . . . . . . . . 11
6.2.1. Reversion . . . . . . . . . . . . . . . . . . . . . . 12
6.3. Restoration . . . . . . . . . . . . . . . . . . . . . . . 13
6.3.1. 1+R Restoration . . . . . . . . . . . . . . . . . . . 13
6.3.2. 1:1+R Restoration . . . . . . . . . . . . . . . . . . 15
7. Operations, Administration, and Maintenance (OAM) . . . . . . 17
7.1. Connectivity Verification . . . . . . . . . . . . . . . . 17
7.2. Performance Measurement . . . . . . . . . . . . . . . . . 17
7.3. Candidate Path Validity Verification . . . . . . . . . . 18
8. External Commands . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Candidate Path Switchover . . . . . . . . . . . . . . . . 18
8.2. Candidate Path Re-computation . . . . . . . . . . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
12.1. Normative References . . . . . . . . . . . . . . . . . . 19
12.2. Informative References . . . . . . . . . . . . . . . . . 19
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
Segment routing does allow for a single network to carry both typical
IP (connection-less) services and connection-oriented transport
services commonly referred to as "private lines". IP services
typically require ECMP and TI-LFA, while transport services delivered
via pseudowires (defined by the PWE3 and PALS workgroups) do require:
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* Persistent end-to-end traffic engineered paths that provide
predictable and identical latency in both directions
* A requested amount of bandwidth per path to ensure no impact on
the Service Level Agreement (SLA) due to changing network load
from other services
* Fast end-to-end protection and restoration mechanisms
* Monitoring and maintenance of path integrity
* Data plane remaining up while control plane is down
Such a "transport centric" behavior is referred to as "circuit-style"
in this document.
This document describes how SR policies
[I-D.ietf-spring-segment-routing-policy] and the use of adjacency-
SIDs defined in the SR architecture [RFC8402] together with a
stateful Path Computation Element (PCE) [RFC8231] can be used to
satisfy those requirements. It includes how end-to-end recovery and
path integrity monitoring can be implemented.
SR policies that satisfy those requirements are called "circuit-
style" SR policies (CS-SR policies).
2. Terminology
* BSID : Binding Segment Identifier
* CS-SR : Circuit-Style Segment Routing
* ID : Identifier
* LSP : Label Switched Path
* LSPA : LSP attributes
* OAM : Operations, Administration and Maintenance
* OF : Objective Function
* PCE : Path Computation Element
* PCEP : Path Computation Element Communication Protocol
* PT : Protection Type
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* SID : Segment Identifier
* SLA : Service Level Agreement
* SR : Segment Routing
* STAMP : Simple Two-Way Active Measurement Protocol
* TI-LFA : Topology Independent Loop Free Alternate
* TLV : Type Length Value
3. Reference Model
The reference model for CS-SR policies is following the Segment
Routing Architecture [RFC8402] and SR Policy Architecture
[I-D.ietf-spring-segment-routing-policy] and is depicted in Figure 1.
+----------------+
+-------------->| PCE/controller |<------------+
| +----------------+ |
PCEP/BGP PCEP/BGP
| |
v <<<<<<<<<<<<<< CS-SR Policy >>>>>>>>>>>>> v
+-------+ +-------+
| |=========================================>| |
| A | SR-policy from A to Z | Z |
| |<=========================================| |
+-------+ SR-policy from Z to A +-------+
Figure 1: Circuit-style SR Policy Reference Model
By nature of CS-SR policies, paths will be computed and maintained by
a centralized entity providing a consistent simple mechanism for
initializing the co-routed bidirectional end to end paths, performing
bandwidth allocation control, as well as monitoring facilities to
ensure SLA compliance for the live of the CS-SR Policy.
When using PCEP as the communication protocol on the endpoints, the
centralized entity is a stateful PCE defined in [RFC8231]. When
using a MPLS data plane [RFC8660], PCEP extensions defined in
[RFC8664] will be used. When using a SRv6 data plane [RFC8754], PCEP
extensions defined in [I-D.ietf-pce-segment-routing-ipv6] will be
used.
When using BGP as the communication protocol on the endpoints, the
centralized entity does perform the same role. BGP extensions
defined in [I-D.ietf-idr-segment-routing-te-policy] will be used.
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In order to satisfy the requirements of CS-SR policies, each link in
the topology MUST have:
* An adjacency-SID which is:
- Manually allocated or persistent : to ensure that its value
does not change after a node reload
- Non-protected : to avoid any local TI-LFA protection to happen
upon interface/link failures
* The bandwidth available for CS-SR policies specified
* A per-hop behavior ([RFC3246] or [RFC2597]) that ensures that the
specified bandwidth is available to CS-SR policies at all times
independent of any other traffic
When using a MPLS data plane [RFC8660] existing IGP extensions
defined in [RFC8667] and [RFC8665] and BGP-LS defined in [RFC9085]
can be used to distribute the topology information including those
persistent and unprotected adjacency-SIDs.
When using a SRv6 data plane [RFC8754] the IGP extensions defined in
[I-D.ietf-lsr-isis-srv6-extensions] and
[I-D.ietf-lsr-ospfv3-srv6-extensions] and BGP-LS extensions in
[I-D.ietf-idr-bgpls-srv6-ext] apply.
3.1. Managing Bandwidth
In a network, resources are represented by links of certain
bandwidth. In a circuit switched network such as SONET/SDH, OTN or
DWDM resources (timeslots or a wavelength) are allocated for a
provisioned connection at the time of reservation even if no
communication is present. In a packet switched network resources are
only allocated when communication is present, i.e. packets are to be
sent. This allows for the total reservations to exceed the link
bandwidth as well in general for link congestion.
To satisfy the bandwidth requirement for CS-SR policies it must be
ensured that packets carried by CS-SR policies can be at all times
sent up to the reserved bandwidth on each hop along the path. This
is done by:
* Firstly, CS-SR policy bandwidth reservations per link must be
limited to euqal or less than the physical link bandwidth.
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* Secondly, ensuring traffic for each CS-SR policy is limited to the
bandwidth reserved for that CS-SR policy by traffic policing or
shaping
* Thirdly, ensuring that during times of link congestion only non-
CS-SR policy traffic is being buffered or dropped.
For the later several approaches can be considered:
* Allocate a dedicated physical link of bandwidth P to CS-SR
policies and allow CS-SR reservations up to bandwidth C. Consider
bandwidth N allocated for network control, ensure that P - N >= C
* Allocate a dedicate logical link (i.e. 801.q VLAN on ethernet) to
CS-SR policies on a physical link of bandwidth P. Limit the total
utilization across all other logical links to bandwidth O by
traffic policing or shaping and ensure that P - N - O >= C
* Allocate a dedicated Diffserv codepoint and queue to CS-SR
policies and limit the total utilization across all other queues
to bandwidth O by traffic policing or shaping and ensure that P -N
- O >= C
* Allocate a dedicate Diffserv codepoint and strict priority queue
to CS-SR policies and limit the total utilization across all
priority queues of higher or equal priority to bandwidth O by
traffic policing or shaping and ensure that P - N - O >= C
* Allocate a dedicate Diffserv codepoint and a strict priority queue
with a priority higher than all other queues to CS-SR policies and
limit the utilization of that priority queue by traffic policing
to C <= P - N
In addition CS-SR policy telemetry collection can be used to raise
alarms when bandwidth utilization thresholds are passed or to request
the reserved bandwidth to be adjusted.
4. CS-SR Policy Characteristics
A CS-SR policy has the following characteristics:
* Requested bandwidth : bandwidth to be reserved for the CS-SR
policy
* Bidirectional co-routed : a CS-SR policy between A and Z is an
association of an SR-Policy from A to Z and an SR-Policy from Z to
A following the same path(s)
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* Deterministic and persistent paths : segment lists with strict
hops using unprotected adjacency-SIDs
* Not automatically recomputed or reoptimized : the SID list of a
candidate path must not change automatically to a SID list
representing a different path (for example upon topology change)
* Multiple candidate paths in case of protection/restoration:
- Following the SR policy architecture, the highest preference
valid path is carrying traffic
- Depending on the protection/restoration scheme (Section 6),
lower priority candidate paths
o may be pre-computed
o may be pre-programmed
o may have to be disjoint
* Connectivity verification and performance measurement is activated
on each candidate path (Section 7)
5. CS-SR Policy Creation
5.1. Policy Creation when using PCEP
Considering the scenario illustrated in Figure 1 a CS-SR policy
between A and Z is configured both on A (with Z as endpoint) and Z
(with A as endpoint).
Both nodes A and Z act as PCC and delegate path computation to the
PCE using PCEP with the extensions defined in [RFC8664] and the
procedure described in Section 5.7.1 of [RFC8231]. The PCRpt message
sent from the headends to the PCE contains the following parameters:
* BANDWIDTH object (Section 7.7 of [RFC5440]) : to indicate the
requested bandwidth
* LSPA object (section 7.11 of [RFC5440]) : to indicate that no
local protection requirements
- L flag set to 0 : no local protection
- E flag set to 1 : protection enforcement (section 5 of
[I-D.ietf-pce-local-protection-enforcement])
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* ASSOCIATION object ([RFC8697]) :
- Type : Double-sided Bidirectional with Reverse LSP Association
([I-D.ietf-pce-sr-bidir-path])
- Bidirectional Association Group TLV ([RFC9059]) :
o R flag is always set to 0 (forward path)
o C flag is always set to 1 (co-routed)
If the SR-policies are configured with more than one candidate path,
a PCEP request is sent per candidate path. Each PCEP request does
include the "SR Policy Association" object (type 6) as defined in
[I-D.ietf-pce-segment-routing-policy-cp] to make the PCE aware of the
candidate path belonging to the same policy.
The signaling extensions described in
[I-D.sidor-pce-circuit-style-pcep-extensions] are used to ensure that
* Path determinism is achieved by the PCE only using segment lists
representing a strict hop by hop path using unprotected adjacency-
SIDs.
* Path persistency across node reloads in the network is achieved by
the PCE only including manually configured adjacency-SIDs in its
path computation response.
* Persistency across network changes is achieved by the PCE not
performing periodic nor network event triggered re-optimization.
Bandwidth adjustment can be requested after initial creation by
signaling both requested and operational bandwidth in the BANDWIDTH
object but the PCE is not allowed to respond with a changed path.
As discussed in section 3.2 of [I-D.ietf-pce-multipath] it may be
necessary to use load-balancing across multiple paths to satisfy the
bandwidth requirement of a candidate path. In such a case the PCE
will notify the PCC to install multiple segment lists using the
signaling procedures described in section 5.3 of
[I-D.ietf-pce-multipath].
5.2. Policy Creation when using BGP
Again considering the scenario illustrated in Figure 1, there is no
CS-SR policy configuration required on A nor Z in order to create the
CS-SR policy between A and Z.
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The centralized controller is instructed (i.e. by an application via
a API call) to create the CS-SR policy, for which the controller does
perform path computation and is requesting A via BGP to instante a
SR-policy (with Z as endpoint) and requesting Z via BGP to
instantiate a SR-policy (with Z as endpoint).
To instantiate the SR-policies in A and Z the BGP extensions defined
in [I-D.ietf-idr-segment-routing-te-policy] are used.
No signaling extensions are required for the following:
* Path determinism is achieved by the controller only using segment
lists representing a strict hop by hop path using unprotected
adjacency-SIDs.
* Path persistency across node reloads in the network is achieved by
the controller only including manually configured adjacency-SIDs
in its path computation response.
* Persistency across network changes is achieved by the controller
not performing periodic nor network event triggered re-
optimization.
If there are more than one candidate paths per SR-policy required,
multiple NLRIs with different distinguisher values (see section 2.1
of [I-D.ietf-idr-segment-routing-te-policy]) have to be included in
the BGP UPDATE message.
To achieve load-balancing across multiple paths to satisfy the
bandwidth requirement of a candidate path, multiple Segment List Sub-
TLVs have to be included in the SR Policy Sub-TLV. See section 2.1
of [I-D.ietf-idr-segment-routing-te-policy]
The endpoints A and Z report the SR-policy states back to the
centralized controller via BGP-LS using the extension defined in
[I-D.ietf-idr-bgp-ls-sr-policy].
5.3. Maximum Segment Depth
A Segment Routed path defined by a segment list is constrained by
maximum segment depth (MSD), which is the maximum number of segments
a router can impose onto a packet. [RFC8491], [RFC8476], [RFC8814]
and [RFC8664] provide the necessary capabilities for a PCE to
determine the MSD capability of a router. The MSD constraint is
typically resolved by leveraging a label stack reduction technique,
such as using Node SIDs and/or BSIDs (SR architecture [RFC8402]) in a
segment list, which represents one or many hops in a given path.
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As described in Section 4, adjacency-SIDs without local protection
are to be used for CS-SR policies to ensure no ECMP, no rerouting due
to topological changes nor localized protection is being invoked on
the traffic, as the alternate path may not be providing the desired
SLA.
If a CS-SR Policy path requires SID List reduction, a Node SID cannot
be utilized as it is eligible for traffic rerouting following IGP re-
convergence. However, a BSID can be programmed to a transit node, if
the following requirements are met:
* The BSID is unprotected, hence only has one candidate path
* The BSID follows the rerouting and optimization characteristics
defined in Section 4 which implies the SID list of the candidate
path MUST only use unprotected adjacency-SIDs.
This ensures that any CS-SR policies in which the BSID provides
transit for do not get rerouted due to topological changes or
protected due to failures. A BSID may be pre-programmed in the
network or automatically injected in the network by a PCE.
6. Recovery Schemes
Various protection and restoration schemes can be implemented. The
terms "protection" and "restoration" are used with the same subtle
distinctions outlined in section 1 of [RFC4872], [RFC4427] and
[RFC3386] respectively.
* Protection : another candidate path is computed and fully
established in the data plane and ready to carry traffic
* Restoration : a candidate path may be computed and may be
partially established but is not ready to carry traffic
The term "failure" is used to represent both "hard failures" such
complete loss of connectivity detected by Section 7.1 or degradation,
a packet loss ratio, beyond a configured acceptable threshold.
6.1. Unprotected
In the most basic scenario no protection nor restoration is required.
The CS-SR policy has only one candidate path configured. This
candidate path is established, activated and is carrying traffic.
When using PCEP, a PCRpt message is sent from the PCC to the PCE with
the O field in the LSP object set to 2.
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When using BGP, a BGP-LS update with a SR Policy Candidate Path NLRI
is sent from the endpoint to the centralized controller having
* C flag set to 1 to indicate the candidate path was provisioned by
the controller
* A flag set to 1 to indicate the candidate path is active and
carrying traffic
In case of a failure along the path the CS-SR policy will go down and
traffic will not be recovered.
Typically two CS-SR policies are deployed either within the same
network with disjoint paths or in two completely separate networks
and the overlay service is responsible for traffic recovery.
6.2. 1:1 Protection
For fast recovery against failures the CS-SR policy has two candidate
paths. Both paths are established but only the candidate with higher
preference is activated and is carrying traffic.
When using PCEP, the PCRpt message for the candidate path with higher
preference will have the O field in the LSP object set to 2. For the
candidate path with the lower preference the O field in the LSP
object is set to 1.
Appropriate routing of the protect path diverse from the working path
can be requested from the PCE by using the "Disjointness Association"
object (type 2) defined in [RFC8800] in the PCRpt messages. The
disjoint requirements are communicated in the "DISJOINTNESS-
CONFIGURATION TLV"
* L bit set to 1 for link diversity
* N bit set to 1 for node diversity
* S bit set to 1 for SRLG diversity
* T bit set to enforce strict diversity
The P bit may be set for first candidate path to allow for finding
the best working path that does satisfy all constraints without
considering diversity to the protect path.
The "Objective Function (OF) TLV" as defined in section 5.3 of
[RFC8800] may also be added to minimize the common shared resources.
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When using BGP, the controller is already aware of the disjoint
requirements and does consider them while computing both paths. Two
NLRIs with different distinguisher values and different preference
values are included in the BGP UPDATE sent to the headend routers.
A BGP-LS update is sent to the controller with a SR Policy Candidate
Path NLRI for the candidate path with higher preference with
* C flag set to 1 to indicate that candidate path was provisioned by
the controller
* A flag set to 1 to indicate the candidate path is active and
carrying traffic
and another SR Policy Candidate Path NLRI for the candidate path with
lower preference with
* C flag set to 1 to indicate the candidate path was provisioned by
the controller
* B flag set to 1 to indicate the role of backup path
Upon a failure impacting the candidate path with higher preference
carrying traffic, the candidate path with lower preference is
activated immediately and traffic is now sent across it.
When using PCEP a PCRpt message for the higher preference candidate
path is sent to the PCE with the O field changed from 2 to 0 and a
PCRpt message for the lower preference candidate path with the O
field change from 1 to 2.
When using BGP a BGP-LS update is sent to the controller with a SR
Policy Candidate Path NLRI for the candidate path with higher
preference with the A flag cleared and for another BGP-LS update for
the candidate path with lower preference with the B flag cleared and
A flag set to 1.
Protection switching is bidirectional. As described in Section 7.1,
both headends will generate and receive their own loopback mode test
packets, hence even a unidirectional failure will always be detected
by both headends without protection switch coordination required.
6.2.1. Reversion
Two cases are to be considered when the failure(s) impacting a
candidate path with higher preference are cleared:
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* Revertive switching : re-activate the highere preference candidate
path and start sending traffic over it
* Non-revertive switching : do not activate the higher preference
candidate path and keep sending traffic via the lower preference
candidate path
When using PCEP, for revertive switching a PCRpt message for the
recovered higher preference candidate path is sent to the PCE with
the O field changed from 0 to 2 and send a PCRpt message for the
lower preference candidate path with the O field changed from 2 to 1.
For non-revertive switching only a PCRpt message for the recovered
higher preference candidate path with the O field set to 1 is sent.
When using BGP and revertive switching a BGP-LS update is sent to the
controller with a SR Policy Candidate Path NLRI for the recovered
higher preference candidate path with the A flag set to 1 and another
BGP-LS update for the lower preference candidate path with the A flag
cleared and B flag set to 1. For non-revertive switching only a BGP-
LS update for the higher preference candidate path with the B flag
set to 1 is sent.
6.3. Restoration
6.3.1. 1+R Restoration
Compared to 1:1 protection described in Section 6.2, this restoration
scheme avoids pre-allocating protection bandwidth in steady state,
while still being able to recover traffic flow in case of a network
failure in a deterministic way (maintain required bandwidth
commitment)
When using PCEP, the CS-SR policy is configured with two candidate
paths. The candidate path with higher preference is established,
activated (O field in LSP object is set to 2) and is carrying
traffic.
The second candidate path with lower preference is only established
and activated (PCRpt message to the PCE with O field in LSP object is
set to 2) upon a failure impacting the first candidate path in order
to send traffic over an alternate path through the network around the
failure with potentially relaxed constraints but still satisfying the
bandwidth commitment.
The second candidate path is generally only requested from the PCE
and activated after a failure, but may also be requested and pre-
established during CS-SR policy creation with the downside of
bandwidth being set aside ahead of time.
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As soon as failure(s) that brought the first candidate path down are
cleared, the second candidate path is getting deactivated (PCRpt
message to the PCE with O field in LSP object is set to 1) or torn
down. The first candidate path is activated (PCRpt message to the
PCE with O field in LSP object is set to 2) and traffic sent across
it.
When using BGP, the controller does compute one path and does include
one NLRI in the BGP UPDATE message sent to the headend routers to
instantiate the CS-SR policy with one candidate path active and
carrying traffic.
A BGP-LS update with a SR Policy Candidate Path NLRI is sent to the
controller with
* C flag set to 1 to indicate the candidate path was provisioned by
the controller
* A flag set to 1 to indicate the candidate path is active and
carrying traffic
Upon the controller detecting the failure of the CS-SR policy's
candidate path, another path is computed and added as second
candidate path to the CS-SR policy by sending a BGP UPDATE message to
the headend routers with a NLRI distinguisher value being different
and preference being lower compared to the first candidate path.
A BGP-LS update with a SR Policy Candidate Path NLRI for the
candidate path with higher preference is sent to the controller with
* A flag is cleared to indicate the candidate path is no longer
active and not carrying traffic anymore
and another SR Policy Candidate Path NLRI for the candidate path with
lower preference with
* C flag set to 1 to indicate the candidate path was provisioned by
the controller
* A flag set to 1 to indicate the candidate path is active and
carrying traffic
The second candidate path is generally only instantiated by the
controller and activated after a failure, but may also be
instantiated and pre-established during CS-SR policy creation with
the downside of bandwidth being set aside ahead of time. If so, a
BGP-LS update with a SR Policy Candidate Path NLRI is sent to the
controller with
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* C flag set to 1 to indicate the candidate path was provisioned by
the controller
* B flag set to 1 to indicate the role of backup path
Once the controller has detected the failure(s) that brought the
first candidate path down are cleared, a BGP-LS update with a SR
Policy Candidate Path NLRI for the first candidate path is sent to
the controller with
* A flag set to 1 to indicate the candidate path became active and
is carrying traffic again
The second candidate path is getting removed by a BGP UPDATE message
withdrawing the NLRI of the second candidate path.
Restoration and reversion behavior is bidirectional. As described in
Section 7.1, both headends use connectivity verification in loopback
mode and therefore even in case of unidirectional failures both
headends will detect the failure or clearance of the failure and
switch traffic away from the failed or to the recovered candidate
path.
6.3.2. 1:1+R Restoration
For further resiliency in case of multiple concurrent failures that
could affect both candidate paths of 1:1 protection described in
Section 6.2, a third candidate path with a preference lower than the
other two candidate paths is added to the CS-SR policy to enable
restoration.
When using PCEP, the third candidate path will generally only be
established, activated (PCRpt message to the PCE with O field in LSP
object is set to 2) and carry traffic after failure(s) have impacted
both the candidate path with highest and second highest preference.
The third candidate path may also be requested and pre-computed
already whenever either the first or second candidate path went down
due to a failure with the downside of bandwidth being set aside ahead
of time.
As soon as failure(s) that brought either the first or second
candidate path down are cleared the third candidate path is getting
deactivated (PCRpt message to the PCE with O field in LSP object is
set to 1), the candidate path that recovered is activated (PCRpt
message to the PCE with O field in LSP object is set to 2) and
traffic sent across it.
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When using BGP, the third candidate path will generally only be
instantiated by the controller and activated after failure(s) have
impacted both the candidate path with highest and second highest
preference, but may also be instantiated and pre-established during
CS-SR policy creation with the downside of bandwidth being set aside
ahead of time.
Assuming the case where both candidate paths are down, a BGP-LS
update is sent with SR Policy Candidate Path NLRIs for the first and
second candidate path with
* A flag cleared
and a SR Policy Candidate Path NLRI for the third candidate path with
* C flag set to 1 to indicate the candidate path was provisioned by
the controller
* A flag set to 1 to indicate the candidate path is active and
carrying traffic
Assuming the case where only one candidate path is down, a BGP-LS
update is sent with a SR Policy Candidate Path NLRI for the failed
candidate path with
* A flag cleared
a SR Policy Candidate Path NLRI for the second candidate path with
* A flag set to 1 to indicate it is active and carrying traffic
network
and another SR Policy Candidate Path NLRI for the newly installed
third candidate path with
* C flag set to 1 to indicate the candidate path was provisioned by
the controller
* B flag set to 1 to indicate the role of backup path
Once the controller has detected the failure(s) that brought either
the first or the second candidate path down are cleared, a BGP-LS
update with a SR Policy Candidate Path NLRI for the recovered
candidate path is sent to the controller with
* A flag set to 1 to indicate the candidate path became active and
is carrying traffic again
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The third candidate path is getting removed by a BGP UPDATE message
withdrawing the NLRI of the third candidate path.
Again restoration and reversion behavior is bidirectional. As
described in Section 7.1, both headends use connectivity verification
in loopback mode and therefore even in case of unidirectional
failures both headends will detect the failure or clearance of the
failure and switch traffic away from the failed or to the recovered
candidate path.
7. Operations, Administration, and Maintenance (OAM)
7.1. Connectivity Verification
The proper operation of each segment list is validated by both
headends using STAMP in loopback measurement mode as described in
section 4.2.3 of [I-D.ietf-spring-stamp-srpm].
As the STAMP test packets are including both the segment list of the
forward and reverse path, standard segment routing data plane
operations will make those packets get switched along the forward
path to the tailend and along the reverse path back to the headend.
When using PCEP, the headend forms the bidirectional SR Policy
association using the procedure described in
[I-D.ietf-pce-sr-bidir-path] and receives the information about the
reverse segment list from the PCE as described in section 4.5 of
[I-D.ietf-pce-multipath]
When using BGP, the controller does inform the headend routers about
the reverse segment list using the Reverse Segment List Sub-TLV
defined in section 4.1 of [I-D.ietf-idr-sr-policy-path-segment].
7.2. Performance Measurement
The same STAMP session is used to estimate round-trip loss as
described in section 5 of [I-D.ietf-spring-stamp-srpm].
The same STAMP session used for connectivity verification can be used
to measure delay. As loopback mode is used only round-trip delay is
measured and one-way has to be derived by dividing the round-trip
delay by two.
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7.3. Candidate Path Validity Verification
A stateful PCE/controller is in sync with the network topology and
the CS-SR Policies provisioned on the headend routers. As described
in Section 4 a path must not be automatically recomputed after or
optimized for topology changes. However there may be a requirement
for the stateful PCE/controller to tear down a path if the path no
longer satisfies the original requirements, detected by stateful PCE/
controller, such as insufficient bandwidth, diversity constraint no
longer met or latency constraint exceeded.
The headend may measure the actual bandwidth utilization of a CS-SR
policy to take local action and/or report it as requested bandwidth
via PCEP or BGP-LS to the stateful PCE/controller. Typical actions
are raising alarms or adjusting the reserved bandwidth.
For a CS-SR policy configured with multiple candidate paths, a
headend may switch to another candidate path if the stateful PCE/
controller decided to tear down the active candidate path.
8. External Commands
8.1. Candidate Path Switchover
It is very common to allow operators to trigger a switch between
candidate paths even if no failure is present. I.e. to proactively
drain a resource for maintenance purposes. Operator triggered
switching between candidate paths is unidirectional and has to be
requested on both headends.
8.2. Candidate Path Re-computation
While no automatic re-optimization or pre-computation of CS-SR policy
candidate paths is allowed as specified in Section 4, network
operators trying to optimize network utilization may explicitly
request a candidate path to be re-computed at a certain point in
time.
9. Security Considerations
TO BE ADDED
10. IANA Considerations
This document has no IANA actions.
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11. Acknowledgements
The author's want to thank Samuel Sidor, Mike Koldychev, Rakesh
Gandhi and Tarek Saad for providing their review comments and all
contributors for their inputs and support.
12. References
12.1. Normative References
[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/rfc/rfc2119>.
12.2. Informative References
[I-D.ietf-idr-bgp-ls-sr-policy]
Previdi, S., Talaulikar, K., Dong, J., Gredler, H., and J.
Tantsura, "Advertisement of Segment Routing Policies using
BGP Link-State", Work in Progress, Internet-Draft, draft-
ietf-idr-bgp-ls-sr-policy-04, 20 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
ls-sr-policy-04>.
[I-D.ietf-idr-bgpls-srv6-ext]
Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
Bernier, D., and B. Decraene, "Border Gateway Protocol -
Link State (BGP-LS) Extensions for Segment Routing over
IPv6 (SRv6)", Work in Progress, Internet-Draft, draft-
ietf-idr-bgpls-srv6-ext-14, 17 February 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
bgpls-srv6-ext-14>.
[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
D. Jain, "Advertising Segment Routing Policies in BGP",
Work in Progress, Internet-Draft, draft-ietf-idr-segment-
routing-te-policy-26, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
segment-routing-te-policy-26>.
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[I-D.ietf-idr-sr-policy-path-segment]
Li, C., Li, Z., Yin, Y., Cheng, W., and K. Talaulikar, "SR
Policy Extensions for Path Segment and Bidirectional
Path", Work in Progress, Internet-Draft, draft-ietf-idr-
sr-policy-path-segment-09, 19 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
policy-path-segment-09>.
[I-D.ietf-lsr-isis-srv6-extensions]
Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
Z. Hu, "IS-IS Extensions to Support Segment Routing over
the IPv6 Data Plane", Work in Progress, Internet-Draft,
draft-ietf-lsr-isis-srv6-extensions-19, 14 November 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-lsr-
isis-srv6-extensions-19>.
[I-D.ietf-lsr-ospfv3-srv6-extensions]
Li, Z., Hu, Z., Talaulikar, K., and P. Psenak, "OSPFv3
Extensions for Segment Routing over IPv6 (SRv6)", Work in
Progress, Internet-Draft, draft-ietf-lsr-ospfv3-srv6-
extensions-15, 21 June 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-lsr-
ospfv3-srv6-extensions-15>.
[I-D.ietf-pce-local-protection-enforcement]
Stone, A., Aissaoui, M., Sidor, S., and S. Sivabalan,
"Local Protection Enforcement in the Path Computation
Element Communication Protocol (PCEP)", Work in Progress,
Internet-Draft, draft-ietf-pce-local-protection-
enforcement-11, 23 June 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
local-protection-enforcement-11>.
[I-D.ietf-pce-multipath]
Koldychev, M., Sivabalan, S., Saad, T., Beeram, V. P.,
Bidgoli, H., Yadav, B., Peng, S., and G. S. Mishra, "PCEP
Extensions for Signaling Multipath Information", Work in
Progress, Internet-Draft, draft-ietf-pce-multipath-11, 8
April 2024, <https://datatracker.ietf.org/doc/html/draft-
ietf-pce-multipath-11>.
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[I-D.ietf-pce-segment-routing-ipv6]
Li, C., Kaladharan, P., Sivabalan, S., Koldychev, M., and
Y. Zhu, "Path Computation Element Communication Protocol
(PCEP) Extensions for IPv6 Segment Routing", Work in
Progress, Internet-Draft, draft-ietf-pce-segment-routing-
ipv6-25, 4 April 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
segment-routing-ipv6-25>.
[I-D.ietf-pce-segment-routing-policy-cp]
Koldychev, M., Sivabalan, S., Barth, C., Peng, S., and H.
Bidgoli, "Path Computation Element Communication Protocol
(PCEP) Extensions for Segment Routing (SR) Policy
Candidate Paths", Work in Progress, Internet-Draft, draft-
ietf-pce-segment-routing-policy-cp-15, 17 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
segment-routing-policy-cp-15>.
[I-D.ietf-pce-sr-bidir-path]
Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
"Path Computation Element Communication Protocol (PCEP)
Extensions for Associated Bidirectional Segment Routing
(SR) Paths", Work in Progress, Internet-Draft, draft-ietf-
pce-sr-bidir-path-13, 13 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-sr-
bidir-path-13>.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", Work in
Progress, Internet-Draft, draft-ietf-spring-segment-
routing-policy-22, 22 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
segment-routing-policy-22>.
[I-D.ietf-spring-stamp-srpm]
Gandhi, R., Filsfils, C., Voyer, D., Chen, M., and R. F.
Foote, "Performance Measurement Using Simple Two-Way
Active Measurement Protocol (STAMP) for Segment Routing
Networks", Work in Progress, Internet-Draft, draft-ietf-
spring-stamp-srpm-14, 4 April 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
stamp-srpm-14>.
[I-D.sidor-pce-circuit-style-pcep-extensions]
Sidor, S., Maheshwari, P., Stone, A., Jalil, L., and S.
Peng, "PCEP extensions for Circuit Style Policies", Work
in Progress, Internet-Draft, draft-sidor-pce-circuit-
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style-pcep-extensions-06, 15 December 2023,
<https://datatracker.ietf.org/doc/html/draft-sidor-pce-
circuit-style-pcep-extensions-06>.
[RFC1925] Callon, R., "The Twelve Networking Truths", RFC 1925,
DOI 10.17487/RFC1925, April 1996,
<https://www.rfc-editor.org/rfc/rfc1925>.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<https://www.rfc-editor.org/rfc/rfc2597>.
[RFC3246] Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le
Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
<https://www.rfc-editor.org/rfc/rfc3246>.
[RFC3386] Lai, W., Ed. and D. McDysan, Ed., "Network Hierarchy and
Multilayer Survivability", RFC 3386, DOI 10.17487/RFC3386,
November 2002, <https://www.rfc-editor.org/rfc/rfc3386>.
[RFC4427] Mannie, E., Ed. and D. Papadimitriou, Ed., "Recovery
(Protection and Restoration) Terminology for Generalized
Multi-Protocol Label Switching (GMPLS)", RFC 4427,
DOI 10.17487/RFC4427, March 2006,
<https://www.rfc-editor.org/rfc/rfc4427>.
[RFC4872] Lang, J.P., Ed., Rekhter, Y., Ed., and D. Papadimitriou,
Ed., "RSVP-TE Extensions in Support of End-to-End
Generalized Multi-Protocol Label Switching (GMPLS)
Recovery", RFC 4872, DOI 10.17487/RFC4872, May 2007,
<https://www.rfc-editor.org/rfc/rfc4872>.
[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/rfc/rfc5440>.
[RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231,
DOI 10.17487/RFC8231, September 2017,
<https://www.rfc-editor.org/rfc/rfc8231>.
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[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/rfc/rfc8402>.
[RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
"Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
DOI 10.17487/RFC8476, December 2018,
<https://www.rfc-editor.org/rfc/rfc8476>.
[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
DOI 10.17487/RFC8491, November 2018,
<https://www.rfc-editor.org/rfc/rfc8491>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/rfc/rfc8660>.
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/rfc/rfc8664>.
[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/rfc/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/rfc/rfc8667>.
[RFC8697] Minei, I., Crabbe, E., Sivabalan, S., Ananthakrishnan, H.,
Dhody, D., and Y. Tanaka, "Path Computation Element
Communication Protocol (PCEP) Extensions for Establishing
Relationships between Sets of Label Switched Paths
(LSPs)", RFC 8697, DOI 10.17487/RFC8697, January 2020,
<https://www.rfc-editor.org/rfc/rfc8697>.
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[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/rfc/rfc8754>.
[RFC8800] Litkowski, S., Sivabalan, S., Barth, C., and M. Negi,
"Path Computation Element Communication Protocol (PCEP)
Extension for Label Switched Path (LSP) Diversity
Constraint Signaling", RFC 8800, DOI 10.17487/RFC8800,
July 2020, <https://www.rfc-editor.org/rfc/rfc8800>.
[RFC8814] Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G.,
and N. Triantafillis, "Signaling Maximum SID Depth (MSD)
Using the Border Gateway Protocol - Link State", RFC 8814,
DOI 10.17487/RFC8814, August 2020,
<https://www.rfc-editor.org/rfc/rfc8814>.
[RFC9059] Gandhi, R., Ed., Barth, C., and B. Wen, "Path Computation
Element Communication Protocol (PCEP) Extensions for
Associated Bidirectional Label Switched Paths (LSPs)",
RFC 9059, DOI 10.17487/RFC9059, June 2021,
<https://www.rfc-editor.org/rfc/rfc9059>.
[RFC9085] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
H., and M. Chen, "Border Gateway Protocol - Link State
(BGP-LS) Extensions for Segment Routing", RFC 9085,
DOI 10.17487/RFC9085, August 2021,
<https://www.rfc-editor.org/rfc/rfc9085>.
Contributors
Daniel Voyer
Bell Canada
Email: daniel.voyer@bell.ca
Luay Jalil
Verizon
Email: luay.jalil@verizon.com
Shuping Peng
Huawei Technologies
Email: pengshuping@huawei.com
Clarence Filsfils
Cisco Systems, Inc.
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Email: cfilsfil@cisco.com
Francois Clad
Cisco Systems, Inc.
Email: fclad@cisco.com
Tarek Saad
Cisco Systems, Inc.
Email: tsaad.net@gmail.com
Brent Foster
Cisco Systems, Inc.
Email: brfoster@cisco.com
Bertrand Duvivier
Cisco Systems, Inc.
Email: bduvivie@cisco.com
Stephane Litkowski
Cisco Systems, Inc.
Email: slitkows@cisco.com
Jie Dong
Huawei Technologies
Email: jie.dong@huawei.com
Authors' Addresses
Christian Schmutzer (editor)
Cisco Systems, Inc.
Email: cschmutz@cisco.com
Zafar Ali (editor)
Cisco Systems, Inc.
Email: zali@cisco.com
Praveen Maheshwari
Airtel India
Email: Praveen.Maheshwari@airtel.com
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Reza Rokui
Ciena
Email: rrokui@ciena.com
Andrew Stone
Nokia
Email: andrew.stone@nokia.com
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