| Internet-Draft | SID Algorithm in PCEP | February 2021 |
| Tokar, et al. | Expires 23 August 2021 | [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.¶
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A PCE can compute SR-TE paths using prefix 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 introduces two new NAI types for the SR-ERO subobject, which is defined in [RFC8664]. 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:¶
The SR-ERO subobject encoding is extended with additional NAI types.¶
The following new NAI types (NT) are defined:¶
This document defines the following NAIs:¶
is specified as an IPv4 address and Algorithm identifier. In this case, the NT value is TBD1 and the NAI field length is 8 octets.¶
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Node IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
is specified as an IPv6 address and Algorithm identifier. In this case, the NT value is TBD2 and the NAI field length is 20 octets.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Node IPv6 address (16 octets) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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.¶
IPv4 prefix SIDs used by SR-TE paths with an associated Algorithm SHOULD be encoded with 'IPv4 Node ID with Algorithm' NAI.¶
IPv6 prefix SIDs used by SR-TE paths with an associated Algorithm SHOULD be encoded with 'IPv6 Node ID with Algorithm' NAI.¶
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 is requested to allocate new SR-ERO NAI types for the new NAI types specified in this document.¶
| Value | Description | Reference |
|---|---|---|
| TBD1 | IPv4 Node ID with Algorithm | This document |
| TBD2 | IPv6 Node ID with Algorithm | This document |
IANA is requested to allocate a new TLV type for the new LSPA TLV specified in this document.¶
| Value | Description | Reference |
|---|---|---|
| TBD3 | SID Algorithm | This document |
Mike Koldychev Cisco Systems Kanata, Ontario Canada EMail: mkoldych@cisco.com¶