Carrying SR-Algorithm Information in PCE-based Networks.

3.1.1. SR PCE Capability Sub-TLV

A new flag S is proposed in the SR PCE Capability Sub-TLV introduced in Section 4.1.2 of [RFC8664] to indicate support for SR-Algorithm. If S flag is set, PCEP peer indicates support for Algorithm field in SR-ERO Subject and SR-Algorithm constraint only for Traffic-engineering paths with Segment Routing Path Setup Type. It is not indicating support for these extensions for other Path Setup Types.¶

 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      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.1.2. SRv6 PCE Capability sub-TLV

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] to indicate support for SR-Algorithm. If S flag is set, PCEP peer indicates support for Algorithm field in SRv6-ERO Subobject and SR-Algorithm constraint only for Traffic-engineering paths with SRv6 Path Setup Type. It is not indicating support for these extensions for other Path Setup Types.¶

 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=27            |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Reserved           |             Flags       |S|N|X|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   MSD-Type    | MSD-Value     |   MSD-Type    |   MSD-Value   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//                             ...                             //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   MSD-Type    | MSD-Value     |            Padding            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.5.1. Path Min Delay Metric value

[RFC7471] and [RFC8570] define "Min/Max Unidirectional Link Delay Sub-TLV" to advertise the link minimum and maximum delay in microseconds in a 24-bit field.¶

[RFC5440] defines the METRIC object with a 32-bit metric value encoded in IEEE floating point format.¶

The encoding for the Path Min Delay metric value is quantified in units of microseconds and encoded in IEEE floating point format.¶

The conversion from 24-bit integer to 32-bit IEEE floating point could introduce some loss of precision.¶

3.5.2. Path Min Delay Metric

The minimum Link Delay metric is defined in [RFC7471] and [RFC8570] as "Min Unidirectional Link Delay". The Path Min Link Delay metric represents measured minimum link delay value over a configurable interval.¶

The Path Min Delay metric type of the METRIC object in PCEP represents the sum of the Min Link Delay metric of all links along a P2P path.¶

• A Min Link Delay metric of link L is denoted D(L).¶

• A path P of a P2P LSP is a list of K links {Lpi,(i=1...K)}.¶

• A Path Min Delay metric for the P2P path P = Sum {D(Lpi), (i=1...K)}.¶

3.5.3. P2MP Path Min Delay Metric

The P2MP Path Min Delay metric type of the METRIC object in PCEP encodes the Path Min Delay metric for the destination that observes the worst delay metric among all destinations of the P2MP tree.¶

• A P2MP tree T comprises a set of M destinations {Dest_j, (j=1...M)}.¶

• The P2P Path Min Delay metric of the path to destination Dest_j is denoted by PMDM(Dest_j).¶

• The P2MP Path Min Delay metric for the P2MP tree T = Maximum{PMDM(Dest_j), (j=1...M)}.¶

3.5.4. Path Bandwidth Metric value

The section 4 of [I-D.ietf-lsr-flex-algo-bw-con] defines new metric type "Bandwidth Metric", which MAY be advertised in their link metric advertisements.¶

When performing Flexible Algorithm path computation as described in section 4.2.1, procedures described in section 4.1 and 5 from [I-D.ietf-lsr-flex-algo-bw-con] MUST be followed with automatic metric calculation attempted.¶

When performing path computation for other algorithms and Generic Metric sub-TLV with Bandwidth metric type is not advertised for the link then PCE implementation MAY have local policy to specify attributes similar to section 4.1.3 and 4.1.4 in [I-D.ietf-lsr-flex-algo-bw-con] and compute metric value automatically or the link MAY be treated as if the metric value is not available for other metric types (e.g. use default value instead). If Bandwidth metric value is advertised for the link, then PCE MUST use value advertised and compute path metric as described in Section 3.5.5 and 3.5.6.¶

The Path Bandwidth metric value is encoded in IEEE floating point format.¶

The conversion from 24-bit integer to 32-bit IEEE floating point could introduce some loss of precision.¶

3.5.5. Path Bandwidth Metric

The Path Bandwidth metric type of the METRIC object in PCEP represents the sum of the Bandwidth Metric of all links along a P2P path. Note: the link Bandwidth Metric utilized in the formula may be the original metric advertised on the link, which may have a value inversely proportional to the link capacity.¶

• A Bandwidth Metric of link L is denoted B(L).¶

• A path P of a P2P LSP is a list of K links {Lpi,(i=1...K)}.¶

• A Path Bandwidth metric for the P2P path P = Sum {B(Lpi), (i=1...K)}.¶

3.5.6. P2MP Path Bandwidth Metric

The Bandwidth metric type of the METRIC object in PCEP encodes the Path Bandwidth metric for the destination that observes the worst bandwidth metric among all destinations of the P2MP tree.¶

• A P2MP tree T comprises a set of M destinations {Dest_j, (j=1...M)}.¶

• The P2P Bandwidth metric of the path to destination Dest_j is denoted by BM(Dest_j).¶

• The P2MP Path Bandwidth metric for the P2MP tree T = Maximum{BM(Dest_j), (j=1...M)}.¶

3.5.7. User Defined Metric

The section 2 of [I-D.ietf-lsr-flex-algo-bw-con] defined new metric type range for "User defined metric", which MAY be advertised in their link metric advertisements. These are user defined and can be assigned by an operator for local use.¶

The encoding for the User Defined metric values is encoded in IEEE floating point format.¶

The conversion from 24-bit integer to 32-bit IEEE floating point could introduce some loss of precision.¶

Proposed metric type range was chosen to allow mapping with values assigned in "IGP Metric-Type Registry". For example, the User Defined metric type 130 of the METRIC object in PCEP can represent the sum of the User Defined Metric 130 of all links along a P2P or P2MP path.¶

User Defined Metric are equally applicable to P2P and P2MP paths.¶

4.2.1. Flexible Algorithm Path computation

This section is applicable only to Flexible Algorithms range of SR-Algorithm values.¶

The PCE MUST follow IGP Flexible Algorithm path computation logic as described in [RFC9350]. That includes using same ordered rules to select FAD if multiple FADs are available, considering node participation of specified SR-Algorithm in the path computation, using ASLA specific link attributes and other rules for Flexible Algorithm path computation described in that document.¶

The PCE MUST optimize computed path based on metric type specified in the FAD, metric type included in PCEP messages from PCC MUST be ignored. The PCE SHOULD use metric type from FAD in messages sent to the PCC. If corresponding metric type is not defined in PCEP, PCE SHOULD skip encoding of metric object for optimization metric.¶

There are corresponding metric types in PCEP for IGP and TE metric from FAD introduced in [RFC9350], but there were no corresponding metric types defined for "Min Unidirectional Link Delay" from [RFC9350] and "Bandwidth Metric", "User Defined Metric" from [I-D.ietf-lsr-flex-algo-bw-con]. Section 3.5 of this document is introducing them. Note that the defined "Path Bandwidth Metric" is accumulative and is different from the Bandwidth Object defined in [RFC5440]¶

The PCE MUST use constraints specified in the FAD and also constraints directly included in PCEP messages from PCC. The PCE implementation MAY decide to ignore specific constraints received from PCC based on existing processing rules for PCEP Objects and TLVs, e.g. P flag described in Section 7.2 of [RFC5440] and processing rules described in [I-D.ietf-pce-stateful-pce-optional]. If the PCE does not support specified combination of constraints, it MAY respond with PCEP message with PCInitiate or PCUpdate message with empty ERO or PCRep with NO-PATH object. PCC MUST NOT include constraints from FAD in PCEP message sent to PCE as it can result in undesired behavior in various cases. PCE SHOULD NOT include constraints from FAD in PCEP messages sent to PCC.¶

4.2.2. Path computation with SID filtering

The SR-Algorithm constraint acts as a filter, restricting which SIDs may be used as a result of the path computation function. Path computation is done based on optimization metric type and constraints specified in PCEP message received from PCC.¶

If the specified SR-Algorithm is Flexible Algorithm, the PCE MUST ensure that IGP path of Flexible Algorithm SIDs is congruent with computed path.¶

4.2.3. New Metric types

All the rules of processing the METRIC object as explained in [RFC5440] and [RFC8233] are applicable to new metric types defined in this document.¶