| Internet-Draft | SID Algorithm in PCEP | February 2022 |
| Tokar, et al. | Expires 26 August 2022 | [Page] |
The Algorithm associated with a prefix Segment-ID (SID) defines the path computation Algorithm used by Interior Gateway Protocols (IGPs). This information is available to controllers such as the Path Computation Element (PCE) via topology learning. This document proposes an approach for informing headend routers regarding the Algorithm associated with each prefix SID used in PCE-computed paths, as well as signalling a specific SID algorithm as a constraint to the PCE.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
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 working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 26 August 2022.¶
Copyright (c) 2022 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. 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 described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
A PCE can compute SR-TE paths using SIDs with different Algorithms depending on the use-case, constraints, etc. While this information is available on the PCE, there is no method of conveying this information to the headend router.¶
Similarly, the headend can also compute SR-TE paths using different Algorithms, and this information also needs to be conveyed to the PCE for collection or troubleshooting purposes. In addition, in the case of multiple (redundant) PCEs, when the headend receives a path from the primary PCE, it needs to be able to report the complete path information - including the Algorithm - to the backup PCE so that in HA scenarios, the backup PCE can verify the prefix SIDs appropriately.¶
An operator may also want to constrain the path computed by the PCE to a specific SID Algorithm, for example, in order to only use SID Algorithms for a low-latency path. A new TLV is introduced for this purpose.¶
Refer to [RFC8665] and [RFC8667] for details about the prefix SID Algorithm.¶
This document is extending:¶
A new TLV for signalling SID Algorithm constraint to the PCE is also introduced, to be carried inside the LSPA object, which is defined in [RFC5440].¶
The mechanisms described in this document are equally applicable to both SR-MPLS and SRv6.¶
The following terminologies are used in this document:¶
A new flag S is proposed in the SR PCE Capability Sub-TLV introduced in Section 4.1.2 of [RFC8664] in Path Computation Element Communication Protocol (PCEP) to indicate support for SID Algorithm field in the SR-ERO subobject.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=26 | Length=4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Flags |S|N|X| MSD | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
A new flag S is proposed in the SRv6 PCE Capability sub-TLV introduced in 4.1.1 of [I-D.ietf-pce-segment-routing-ipv6] in Path Computation Element Communication Protocol (PCEP) to indicate support for SID Algorithm field in the SRv6-ERO subobject.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=TBD1 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Flags |S|N|X| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MSD-Type | MSD-Value | MSD-Type | MSD-Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // ... // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MSD-Type | MSD-Value | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
The SR-ERO subobject encoding is extended with new flag "A" to indicate if the Algorithm field is included after other optional fields.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |L| Type=36 | Length | NT | Flags |A|V|F|S|C|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // NAI (variable, optional) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
The SRv6-ERO subobject encoding is extended with new flag "A" to indicate if the Algorithm field is included after other optional fields.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |L| Type=TBD3 | Length | NT | Flags |A|V|T|F|S| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Endpoint Behavior | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | SRv6 SID (optional) | | (128-bit) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // NAI (variable, optional) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID Structure (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
A new TLV for the LSPA Object with TLV type=TBD3 is introduced to carry the SID Algorithm constraint. This TLV SHOULD only be used when PST (Path Setup type) = SR or SRv6.¶
The format of the SID Algorithm TLV is as follows:¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=TBD3 | Length=4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Flags |L| Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The code point for the TLV type is TBD3. The TLV length is 4 octets.¶
The 32-bit value is formatted as follows.¶
This document defines the following flag bits. The other bits MUST be set to zero by the sender and MUST be ignored by the receiver.¶
PCEP speaker MAY set the A flag and include the Algorithm field in SR-ERO or SRv6-ERO subobject if the S flag was advertised by both PCEP speakers.¶
If PCEP peer receives SR-ERO subobject with the A flag set or with the SID Algorithm included, but the S flag was not advertised, then such PCEP message must be rejected with PCError as described in Section 7.2 of [RFC5440]¶
The Algorithm field MUST be included after optional SID, NAI or SID structure and length of SR-ERO or SRv6-ERO subobject MUST be increased with additional 4 bytes for Reserved and Algorithm field.¶
In order to signal a specific SID Algorithm constraint to the PCE, the headend MUST encode the SID ALGORITHM TLV inside the LSPA object.¶
When the PCE receives a SID Algorithm constraint, it MUST only take prefix SIDs with the specified Algorithm into account during path computation. However, if the L flag is set in the SID Algorithm TLV, the PCE MAY insert prefix SIDs with a different Algorithm in order to successfully compute a path.¶
If the PCE is unable to find a path with the given SID Algorithm constraint, it MUST bring the LSP down.¶
SID Algorithm does not replace the Objective Function defined in [RFC5541]. The SID Algorithm constraint acts as a filter, restricting which SIDs may be used as a result of the path computation function.¶
No additional security measure is required.¶
IANA maintains a sub-registry, named "SR Capability Flag Field", within the "Path Computation Element Protocol (PCEP) Numbers" registry to manage the Flags field of the SR-PCE-CAPABILITY TLV. IANA is requested to make the following assignment:¶
| Value | Description | Reference |
|---|---|---|
| TBD1 | SID Algorithm Capability | This document |
IANA was requested in [I-D.ietf-pce-segment-routing-ipv6] to create a sub-registry, named "SRv6 PCE Capability Flags", within the "Path Computation Element Protocol (PCEP) Numbers" registry to manage the Flags field of SRv6-PCE-CAPABILITY sub-TLV. IANA is requested to make the following assignment:¶
| Value | Description | Reference |
|---|---|---|
| TBD2 | SID Algorithm Capability | This document |
IANA maintains a sub-registry, named "SR-ERO Flag Field", within the "Path Computation Element Protocol (PCEP) Numbers" registry to manage the Flags field of the SR-ERO Subobject. IANA is requested to make the following assignment:¶
| Value | Description | Reference |
|---|---|---|
| TBD3 | SID Algorithm Flag | This document |
IANA was requested in [I-D.ietf-pce-segment-routing-ipv6], named "SRv6-ERO Flag Field", within the "Path Computation Element Protocol (PCEP) Numbers" registry to manage the Flags field of the SRv6-ERO subobject. IANA is requested to make the following assignment:¶
| Value | Description | Reference |
|---|---|---|
| TBD4 | SID Algorithm Flag | This document |
IANA is requested to allocate a new TLV type for the new LSPA TLV specified in this document.¶
| Value | Description | Reference |
|---|---|---|
| TBD5 | SID Algorithm | This document |
Mike Koldychev Cisco Systems Kanata, Ontario Canada Email: mkoldych@cisco.com¶