| Internet-Draft | cs-srte | September 2021 |
| Schmutzer, et al. | Expires 3 April 2022 | [Page] |
This document describes how Segment Routing (SR) policies can be used to satisfy the requirements for strict bandwidth guarantees, 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).¶
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There are several applications that require strict bandwidth guarantees, end-to-end recovery and persistent paths through the network. 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 adjacency-SIDs defined in the SR architecture [RFC8402] together with a stateful Path Computation Element (PCE) [RFC8231] can be used to statisfy those requirements. It includes how end-to-end recovery and path integrity monitoring can be implemented.¶
SR policies that satisy those requirements are called "circuit-style" SR policies (CS-SR policies).¶
The requirements of circuit-style SR policies (CS-SR policies) are as follows:¶
CS-SR policies are following the segment routing architecture [RFC8402] and SR policy architecture [I-D.ietf-spring-segment-routing-policy].¶
By nature of CS-SR policies, paths wil be computed and maintained by 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.¶
A simplified architecture is shown in Figure 1.¶
+--------------+
+-------------->| PCE |<--------------+
| +--------------+ |
| |
| |
v <<<<<<<<<<<<<< CS-SR Policy >>>>>>>>>>>>> v
+-------+ +-------+
| |=========================================>| |
| A | SR-policy from A to Z | Z |
| |<=========================================| |
+-------+ SR-policy from Z to A +-------+
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¶
Existing IGP extensions defined in [RFC8667] and [RFC8665] can be used to distribute the topology information including those persistent and unprotected Adj-SIDs when using a MPLS dataplane [RFC8660]. When using a SRv6 dataplane [RFC8754] the IGP extensions defined in [I-D.ietf-lsr-isis-srv6-extensions] and [I-D.ietf-lsr-ospfv3-srv6-extensions] apply.¶
A CS-SR policy had the following characteristics:¶
Multiple candiate paths in case of protection/restoration:¶
A CS-SR policy between A and Z is configured both on A (with Z as endpoint) and Z (with A as endpoint) as shown in Figure 1.¶
Both nodes A and Z act as PCC and delegate path computation to the PCE using the extensions defined in [RFC8664]. Considering a CS-SR policy that has no protection/restoration requirement, the PCRpt message sent from the headends to the PCE contains the following parameters:¶
LSPA object (section 7.11 of [RFC5440]) : to indicate the local protection requirements¶
If the SR-policies are configured with more than one candiate 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 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 dataplane operations will make those packets get switched along the forward path to the tailend and along the reverse path back to the headend.¶
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]¶
The same STAMP session used for liveliness monitoring 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.¶
The same STAMP session can also be used to estimate round-trip loss as described in section 5 of [I-D.ietf-spring-stamp-srpm].¶
Various protection and restoration schemes can be implemented. The terms "protection" and "restoration" are used with same subtle distinctions outlined in section 1 of [RFC4872], [RFC4427] and [RFC3386] respectively.¶
When protection and/or restoration is required the SR polices are configured with two or more candidate paths. The PCRpt messages include the "Path Protection Association" object (type 1) defined in [RFC8745].¶
In the most basic scenario no protection nor restoration is required. The SR-policy only has one candidate path configured.¶
In case of a failure the CS-SR policy will go down and traffic will not be recovered.¶
For fast recovery against failures a second candidate with a lower preference is configured. Its path is pre-computed and pre-programmed so that upon detection of a failure traffic can be immediately directed to this alternate path by the headend.¶
For the highest preference candidate path the parameters of the "Path Protection Association TLV" defined in [RFC8745] are set as follows:¶
For the lower preference candidate path the parameters are as follows:¶
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"¶
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.¶
As described in Section 8.1, both headends will generate and receive their own loopback mode test packets, hence even a unidirectional failure will always be detected by both headends and therefore no protection switch coordination is required.¶
TO BE ADDED¶
This document has no IANA actions.¶
The author's want to thank Samuel Sidor, Mike Koldychev, Rakesh Gandhi for providing their review comments.¶
Contributors' Addresses¶
Brent Foster Cisco Systems, Inc. Email: brfoster@cisco.com Bertrand Duvivier Cisco System, Inc. Email: bduvivie@cisco.com Stephane Litkowski Cisco Systems, Inc. Email: slitkows@cisco.com¶