Cisco Systems, Inc.

Canada rgandhi@cisco.com

PCE Working Group Networks may experience transmission bit errors due to various factors, such as poor fiber quality. The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Client (PCC) requests. This document describes the extensions to PCEP to include link Bit Error Rate (BER) metric as a constraint for end-to-end path computation. The PCEP extension utilize link BER metric advertisements defined for IS-IS and OSPF in draft-gandhi-lsr-ber.

Networks may experience transmission bit errors due to various factors, such as poor fiber quality. The bit error can be a single bit error or a burst of bit errors at a time. Bit errors include layer-2 bit errors (e.g., causing Cyclic Redundancy Check (CRC) errors) or layer-3 and layer-4 bit errors (e.g., causing checksum failures). It is important that link BER metric is considered during the path selection process to provide service level assurance. The Traffic Engineering Database (TED) is populated with network performance information like link latency, delay variation, packet loss, as well as parameters related to bandwidth (residual bandwidth, available bandwidth, and utilized bandwidth) via TE Metric Extensions in OSPF or IS-IS or via a management system. describes how a Path Computation Element (PCE) can use that information for path selection for explicitly routed Label Switched Paths (LSPs). A Path Computation Client (PCC) can request a PCE to provide a path meeting end-to-end network performance criteria. defines extensions to PCEP to compute service-aware Label Switched Paths (LSPs). This document extends the Path Computation Element Communication Protocol (PCEP) to include link Bit Error Rate (BER) metric as a constraint for end-to-end path computation. The PCEP extensions utilize link BER metric advertisements defined for IS-IS and OSPF in .

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 when, and only when, they appear in all capitals, as shown here.

BER: Bit Error Rate EMA: Exponential Moving Average IGP: Interior Gateway Protocol IS-IS: Intermediate System to Intermediate System OF: Objective Function OSPF: Open Shortest Path First PCC: Path Computation Client PCE: Path Computation Element PER: Rate of Packets with Bit Errors TE: Traffic Engineering TED: Traffic Engineering Database

This document further extends the service-aware path computation extensions for LSPs defined in to include link Bit Error Rate (BER) metric as a constraint for end-to-end path computation. Note that the PCEP extensions defined in this document are equally applicable to Segment Routing path computation defined in . The procedure for multipath Segment Routing will be added in a future revision. The PCEP extensions for including link PER metric will be added in a future revision.

This section defines PCEP extensions (see ) to support link-BER-based path computation.

The METRIC object is defined in Section 7.8 of . It comprises metric-value and metric-type (T field), and a flags field containing a number of bit flags (B bit and C bit). This document specifies additional types for the METRIC object to enable encoding of BER metric types. The following metric types (T field) for the METRIC object ae defined. o Metric Type T=TBA1: Path BER Metric o Metric Type T=TBA2: P2MP Path BER Metric The following terminology is used and expanded along the way. o A network comprises a set of N links {Li, (i=1...N)}. o A path P of a point-to-point (P2P) LSP is a list of K links {Lpi,(i=1...K)}.

The Path BER metric (as a percentage) as a metric-value in the METRIC object in PCEP encodes a function of the unidirectional BER metric of all links along a P2P path. This follows the composition function described in Section 5.1.5 of . o The percentage link BER of link L is denoted PR(L). o The fractional link BER of link L is denoted FR(L) = PR(L) / 100. o The percentage path BER of a path P is denoted PR(P), where PR(P) = (1 - ((1-FR(Lp1)) * (1-FR(Lp2)) * .. * (1-FR(LpK)))) * 100. A PCC MAY use the Path BER metric type in a PCReq message to request a path that meets the end-to-end BER metric requirement. In this case, the B bit MUST be set to suggest a bound (a maximum) for the Path BER metric that MUST NOT be exceeded for the PCC to consider the computed path as acceptable. The Path BER metric MUST be less than or equal to the value specified in the metric-value field. A PCC can also use the Path BER metric type to ask the PCE to optimize the path BER metric during path computation. In this case, the C bit MUST be set and the B bit MUST be cleared. A PCE MAY use the Path BER metric type in a PCRep message along with a NO-PATH object in the case where the PCE cannot compute a path meeting this constraint. defines the METRIC object with a 32-bit metric value encoded in IEEE floating point format (see (IEEE.754)). The encoding for the Path BER metric value is quantified in units of percentage and encoded in IEEE floating point format. For use in the PCEP METRIC object, the 24-bit unsigned integer BER value as defined is converted to a 32-bit IEEE floating point value. This conversion follows the procedure specified in [IEEE.754.2019].

defines a mechanism to specify an objective function (OF) that is used by a PCE when it computes a path. For Path BER Metric, the following new OF is defined when using the Path BER metric (TBA1). Objective Function Code: TBA3 Name: Minimum BER Path (MBERP) Description: Find a path P such that PR(P) is minimized. If the objective function defined in this document is unknown/ unsupported by a PCE, then the procedure as defined in Section 3.1.1 of is followed.

The P2MP Path BER metric in a metric-value of the METRIC object in PCEP encodes the Path BER Metric for the destination that observes the worst path BER metric among all destinations of the P2MP tree. Specifically, extending on the above-mentioned terminology: o A P2MP tree T comprises a set of M destinations {Dest_j, (j=1...M)}. o The P2P Path BER metric of the path to destination Dest_j is denoted by PRM(Dest_j). o The P2MP Path BER metric for the P2MP tree T = Maximum {PRM(Dest_j), (j=1...M)}.

For P2MP path computation using the P2MP Path BER metric (TBA2), the objective-function procedures defined in apply. In particular, this document reuses the P2MP objective functions defined in and does not define a new P2MP-specific objective function code for BER.

Flexible Algorithm (Flex-Algo) in Path Computation Element Communication Protocol (PCEP) enables Path Computation Elements (PCEs) to compute constrained paths using user-defined IGP algorithm metrics. defines PCEP extensions for carrying SR-Algorithm for TE. A new Flexible Algorithm Definition (FAD) type is defined for PCEP based on link BER metric for path computation using the IGP Flex-Algo definition specified in . Additional details will be added in a future revision of this document.

The new metric type and objective functions defined in this document can also be used with the stateful PCE extensions defined in for PCE-initiated LSPs. The format of PCEP messages described in uses (intended-attribute-list) and (attribute-list), respectively, (where the (intended-attribute-list) is the attribute-list defined in Section 6.5 of and extended in this document) for link BER parameters. A stateful PCE can also determine which LSPs should be reoptimized based on network events or triggers from external monitoring systems. For example, when a particular link deteriorates and its BER metric increases, this can trigger the stateful PCE to automatically determine which LSPs are impacted and should be reoptimized.

The security considerations specified in , , , and apply to the procedure and extensions defined in this document.

IANA maintains the "Path Computation Element Protocol (PCEP) Numbers" registry. Within this registry, IANA maintains a subregistry for "METRIC Object T Field". New metric types are defined in this document for the METRIC object (specified in ). IANA is requested to allocate the values for these METRIC types.

Metric Types

Value	Description	Reference
TBA1	Path BER Metric	This document
TBA2	P2MP Path BER Metric	This document

IANA also maintains a subregistry for "PCEP Objective Function (OF) Codes". This document defines a new objective function code (specified in ). IANA is requested to allocate the value for this OF code. Objective Function Code

Value	Description	Reference
TBA3	Minimum BER Path (MBERP)	This document

TBA.