PCEP Extensions for Segment RoutingCisco Systems, Inc.2000 Innovation DriveKanataOntarioK2K 3E8Canadamsiva@cisco.comCisco Systems, Inc.Pegasus ParcDe kleetlaan 6aDIEGEMBRABANT 1831BELGIUMcfilsfil@cisco.comIndividual444 San Antonio Rd, 10APalo AltoCA94306USAjefftant.ietf@gmail.comNokiaCopernicuslaan 50 Antwerp 2018CA95134BELGIUMwim.henderickx@alcatel-lucent.comMetaswitch Networks100 Church StreetEnfieldMiddlesexUKjon.hardwick@metaswitch.comPCESegment Routing (SR) enables any head-end node to select any path without relying on a hop-by-hop signaling technique (e.g., LDP or RSVP-TE). It depends only on "segments" that are advertised by Link-State Interior Gateway Protocols (IGPs). A Segment Routed Path can be derived from a variety of mechanisms, including an IGP Shortest Path Tree (SPT), explicit configuration, or a Path Computation Element (PCE). This document specifies extensions to the Path Computation Element Protocol (PCEP) that allow a stateful PCE to compute and initiate Traffic Engineering (TE) paths, as well as a PCC to request a path subject to certain constraint(s) and optimization criteria in SR networks.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .
SR technology leverages the source routing and tunneling paradigms. A source node can choose a path without relying on hop-by-hop signaling protocols such as LDP or RSVP-TE. Each path is specified as a set of "segments" advertised by link-state routing protocols (IS-IS or OSPF). provides an introduction to SR architecture. The corresponding IS-IS and OSPF extensions are specified in and , respectively. SR architecture defines a "segment" as a piece of information advertised by a link-state routing protocols, e.g. an IGP prefix or an IGP adjacency. Several types of segments are defined. A Node segment represents an ECMP-aware shortest-path computed by IGP to a specific node, and is always global within SR/IGP domain. An Adjacency Segment represents unidirectional adjacency. An Adjacency Segment is local to the node which advertises it. Both Node segments and Adjacency segments can be used for SR Traffic Engineering (SR-TE).The SR architecture can be applied to the MPLS forwarding plane without any change, in which case an SR path corresponds to an MPLS Label Switching Path (LSP). This document is relevant to MPLS forwarding plane only and assumes that a 32-bit Segment Identifier (SID) represents an absolute value of MPLS label entry. In this document, "Node-SID" and "Adjacency-SID" denote Node Segment Identifier and Adjacency Segment Identifier respectively.A Segment Routed path (SR path) can be derived from an IGP Shortest Path Tree (SPT). SR-TE paths may not follow IGP SPT. Such paths may be chosen by a suitable network planning tool and provisioned on the ingress node of the SR-TE path. describes Path Computation Element Protocol (PCEP) for communication between a Path Computation Client (PCC) and a Path Computation Element (PCE) or between one a pair of PCEs. A PCE or a PCC operating as a PCE (in hierarchical PCE environment) computes paths for MPLS Traffic Engineering LSPs (MPLS-TE LSPs) based on various constraints and optimization criteria. specifies extensions to PCEP that allow a stateful PCE to compute and recommend network paths in compliance with and defines objects and TLVs for MPLS-TE LSPs. Stateful PCEP extensions provide synchronization of LSP state between a PCC and a PCE or between a pair of PCEs, delegation of LSP control, reporting of LSP state from a PCC to a PCE, controlling the setup and path routing of an LSP from a PCE to a PCC. Stateful PCEP extensions are intended for an operational model in which LSPs are configured on the PCC, and control over them is delegated to the PCE.A mechanism to dynamically initiate LSPs on a PCC based on the requests from a stateful PCE or a controller using stateful PCE is specified in . Such mechanism is useful in Software Driven Networks (SDN) applications, such as on demand engineering, or bandwidth calendaring.It is possible to use a stateful PCE for computing one or more SR-TE paths taking into account various constraints and objective functions. Once a path is chosen, the stateful PCE can initiate an SR-TE path on a PCC using PCEP extensions specified in using the SR specific PCEP extensions specified in this document. Additionally, using procedures described in this document, a PCC can request an SR path from either stateful or a stateless PCE. This specification relies on the PATH-SETUP-TYPE TLV and procedures specified in .The following terminologies are used in this document:
Explicit Route Object Interior Gateway Protocol Intermediate System to Intermediate System Label Switching Router Maximum SID Depth Node or Adjacency Identifier Open Shortest Path First Path Computation Client Path Computation Element Path Computation Element Protocol Record Route Object Segment Identifier Segment Routing Segment Routed Traffic Engineering Traffic Engineering DatabaseIn SR networks, an ingress node of an SR path appends all outgoing packets with an SR header consisting of a list of SIDs (or MPLS labels in the context of this document). The header has all necessary information to guide the packets from the ingress node to the egress node of the path, and hence there is no need for any signaling protocol.In a PCEP session, LSP information is carried in the Explicit Route Object (ERO), which consists of a sequence of subobjects. Various types of ERO subobjects have been specified in , , and . In SR networks, an ingress node of an SR path appends all outgoing packets with an SR header consisting of a list of SIDs (or MPLS labels in the context of this document). SR-TE LSPs computed by a PCE can be represented in one of the following forms:
An ordered set of IP address(es) representing network nodes/links: In this case, the PCC needs to convert the IP address(es) into the corresponding MPLS labels by consulting its Traffic Engineering Database (TED).An ordered set of SID(s).An ordered set of both MPLS label(s) and IP address(es): In this case, the PCC needs to convert the IP address(es) into the corresponding SID(s) by consulting its TED.This document defines a new ERO subobject denoted by "SR-ERO subobject" capable of carrying a SID as well as the identity of the node/adjacency represented by the SID. SR-capable PCEP speakers should be able to generate and/or process such ERO subobject. An ERO containing SR-ERO subobjects can be included in the PCEP Path Computation Reply (PCRep) message defined in , the PCEP LSP Initiate Request message (PCInitiate) defined in , as well as in the PCEP LSP Update Request (PCUpd) and PCEP LSP State Report (PCRpt) messages defined in defined in .When a PCEP session between a PCC and a PCE is established, both PCEP speakers exchange their capabilites to indicate their ability to support SR-specific functionality. Furthermore, an LSP initially established via RSVP-TE signaling can be updated with SR-TE path. This capability is useful when a network is migrated from RSVP-TE to SR-TE technology. Similarly, an LSP initially created with SR-TE signaling can be updated using RSVP-TE if necessary.A PCC MAY include an RRO object containing the recorded LSP in PCReq and PCRpt messages as specified in and respectively. This document defines a new RRO subobject for SR networks. Methods used by a PCC to record SR-TE LSP are outside the scope of this document.In summary, this document:
Defines a new PCEP capability, new ERO subobject, new RRO subobject, a new TLV, and new PCEP error codes.Specifies how two PCEP speakers can establish a PCEP session that can carry information about SR-TE paths.Specifies processing rules of ERO subobject.Defines a new path setup type carried in the PATH-SETUP-TYPE TLV for SR-TE LSP.The extensions specified in this document complement the existing PCEP specifications to support SR-TE path. As such, the PCEP messages (e.g., Path Computation Request, Path Computation Reply, Path Computation Report, Path Computation Update, Path Computation Initiate, etc.,) MUST be formatted according to , , , and any other applicable PCEP specifications.As defined in , a PCEP message consists of a common header followed by a variable length body made up of mandatory and/or optional objects. This document does not require any changes in the format of PCReq and PCRep messages specified in , PCInitiate message specified in , and PCRpt and PCUpd messages specified in . However, PCEP messages pertaining to SR-TE LSP MUST include PATH-SETUP-TYPE TLV in the RP or SRP object to clearly identify that SR-TE LSP is intended. In other words, a PCEP speaker MUST not infer whether or not a PCEP message pertains to SR-TE LSP from any other object or TLV.This document defines a new optional TLV for use in the OPEN Object.The SR-PCE-CAPABILITY TLV is an optional TLV associated with the OPEN Object to exchange SR capability of PCEP speakers. The format of the SR-PCE-CAPABILITY TLV is shown in the
following figure:The code point for the TLV type is to be defined by IANA. The TLV length is 4 octets.The 32-bit value is formatted as follows. The "Maximum SID Depth" (1 octet) field (MSD) specifies the maximum number of SIDs (MPLS label stack depth in context of this document) that a PCC is capable of imposing on a packet. The "Reserved" (2 octets) field is unused, and MUST be set to zero on transmission and ignored on reception. The "Flags" field is 1 octect long, and this document defines the following flag:
L-flag: A PCC sets this flag to 1 to indicate that it does not impose any limit on MSD.By including the SR-PCE-CAPABILITY TLV in the OPEN message destined to a PCE, a PCC indicates that it is capable of supporting the head-end functions for SR-TE LSP. By including the TLV in the OPEN message destined to a PCC, a PCE indicates that it is capable of computing SR-TE paths.The number of SIDs that can be imposed on a packet depends on PCC's data plane's capability. An MSD value MUST be non-zero otherwise the receiver of the SR-PCE-CAPABILITY TLV MUST assume that the sender is not capable of imposing a MSD of any depth and hence is not SR-TE capable.Note that the MSD value exchanged via SR-PCE-CAPABILITY TLV indicates the SID/label imposition limit for the PCC node. However, if a PCE learns MSD value of a PCC node via different means, e.g routing protocols, as specified in: ; ; , then it ignores the MSD value in the SR-PCE-CAPABILITY TLV. Furthermore, whenever a PCE learns MSD for a link via different means, it MUST use that value for that link regardless of the MSD value exchanged via SR-PCE-CAPABILITY TLV.Once an SR-capable PCEP session is established with a non-zero MSD value, the corresponding PCE MUST NOT send SR-TE paths with number of SIDs exceeding that MSD value. If a PCC needs to modify the MSD value, the PCEP session MUST be closed and re-established with the new MSD value. If a PCEP session is established with a non-zero MSD value, and the PCC receives an SR-TE path containing more SIDs than specified in the MSD value, the PCC MUST send a PCErr message with Error-Type 10 (Reception of an invalid object) and Error-Value 3 (Unsupported number of Segment ERO). If a PCEP session is established with an MSD value of zero, then the PCC MAY specify an MSD for each path computation request that it sends to the PCE.The MSD value inside SR Capability TLV is meaningful only in the OPEN message sent from a PCC to a PCE. As such, a PCE does not need to set MSD value in outbound message to a PCC. Similarly, a PCC ignores any MSD value received from a PCE. If a PCE receives multiple SR-PCE-CAPABILITY TLVs in an OPEN message, it processes only the first TLV received.In order to setup an SR-TE LSP using SR, RP or SRP object MUST include
PATH-SETUP-TYPE TLV specified in . This document defines a new Path Setup Type (PST) for SR as follows:
PST = 1: Path is setup using Segment Routing Traffic Engineering technique.LSP-IDENTIFIERS TLV MAY be present for the above PST type.An SR-TE path consists of one or more SID(s) where each SID MAY be associated with the identifier that represents the node or adjacency corresponding to the SID. This identifier is referred to as the 'Node or Adjacency Identifier' (NAI). As described later, a NAI can be represented in various formats (e.g., IPv4 address, IPv6 address, etc). Furthermore, a NAI is used for troubleshooting purposes and, if necessary, to derive SID value as described below.The ERO object specified in is used to carry SR-TE path information. In order to carry SID and/or NAI, this document defines a new ERO subobject referred to as "SR-ERO subobject" whose format is specified in the following section. An ERO object carrying an SR-TE path consists of one or more ERO subobject(s), and MUST carry only SR-ERO subobject(s). Note that an SR-ERO subobject does not need to have both SID and NAI. However, at least one of them MUST be present.When building the MPLS label stack from ERO, a PCC MUST assume that SR-ERO subobjects are organized as a last-in-first-out stack. The first subobject relative to the beginning of ERO contains the information about the topmost label. The last subobject contains information about the bottommost label.An SR-ERO subobject consists of a 32-bit header followed by the SID and the NAI associated with the SID. The SID is a 32-bit number. The size of the NAI depends on its respective type, as described in the following sections.The fields in the SR-ERO Subobject are as follows:
indicates whether the subobject represents a loose-hop in the LSP . If this flag is unset, a PCC MUST not overwrite the SID value present in the SR-ERO subobject. Otherwise, a PCC MAY expand or replace one or more SID value(s) in the received SR-ERO based on its local policy.is the type of the SR-ERO subobject. This document defines the SR-ERO subobject type, and requests a new codepoint from IANA.contains the total length of the subobject in octets, including the L, Type and Length fields. Length MUST be at least 8, and MUST be a multiple of 4. As mentioned earlier, an SR-ERO subobject MUST have at least SID or NAI. The length should take into consideration SID or NAI only if they are not null. The flags described below used to indicate whether SID or NAI field is null.indicates the type of information associated with the SID contained in the object body. When ST value is 0, SID MUST NOT be null and NAI MUST be null. Other ST values are described later in this document.
is used to carry any additional information pertaining to SID. Currently, the following flag bits are defined:M: When this bit is set, the SID value represents an MPLS label stack entry as specified in where only the label value is specified by the PCE. Other fields (TC, S, and TTL) fields MUST be considered invalid, and PCC MUST set these fields according to its local policy and MPLS forwarding rules.C: When this bit as well as the M bit are set, then the SID value represents an MPLS label stack entry as specified in , where all the entry's fields (Label, TC, S, and TTL) are specified by the PCE. However, a PCC MAY choose to
override TC, S, and TTL values according its local policy and MPLS forwarding rules.S: When this bit is set, the SID value in the subobject body is null. In this case, the PCC is responsible for choosing the SID value, e.g., by looking up its TED using the NAI which, in this case, MUST be present in the subobject.F: When this bit is set, the NAI value in the subobject body is null.is the Segment Identifier.
contains the NAI associated with the
SID. Depending on the value of ST, the NAI can have different format as described in the following section.This document defines the following NAIs:
is specified as an IPv4 address. In this case, ST value is 1, and the Length is 8 or 12 depending on either SID or NAI or both are included in the subobject.is specified as an IPv6 address. In this case, ST and Length are 2, and Length is 8, 20, or 24 depending on either SID or NAI or both are included in the subobject.is specified as a pair of IPv4 addresses. In this case, ST value is 3. The Length is 8, 12, or 16 depending on either SID or NAI or both are included in the subobject, and the format of the NAI is shown in the following figure:
is specified as a pair of IPv6 addresses. In this case, ST valie is 4. The Length is 8, 36 or 40 depending on whether SID or NAI or both included in the subobject,and the format of the NAI is shown in the following figure:
is specified as a pair of Node ID / Interface ID tuples. In this case, ST value is 5. The Length is 8, 20, or 24 depending on whether SID or NAI or both included in the subobject, and the format of the NAI is shown
in the following figure:
A PCEP speaker that does not recognize the SR-ERO subobject in PCRep, PCInitiate, PCUpd or PCRpt messages MUST reject the entire PCEP message and MUST send a PCErr message with Error-Type=3 ("Unknown Object") and Error-Value=2 ("Unrecognized object Type") or Error-Type=4 ("Not supported object") and Error-Value=2 ("Not supported object Type"), defined in .When the SID represents an MPLS label (i.e. the M bit is set), its value (20 most significant bits) MUST be larger than 15, unless it is special purpose label, such as an Entropy Label Indicator (ELI). If a PCEP speaker receives an invalid value, it MUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error Value = TBD ("Bad label value"). If both M and C bits of an SR-ERO subobject are set, and if a PCEP speaker finds erroneous setting in one or more of TC, S, and
TTL fields, it MUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-Value = TBD ("Bad label format").If a PCC receives a stack of SR-ERO subobjects, and the number of stack exceeds the maximum number of SIDs that the PCC can impose on the packet, it MAY send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-Value = TBD ("Unsupported number of Segment ERO subobjects").When a PCEP speaker detects that all subobjects of ERO are not identical, and if it does not handle such ERO, it MUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-Value = TBD ("Non-identical ERO subobjects").If a PCEP speaker receives an SR-ERO subobject in which both SID and NAI are absent, it MUST consider the entire ERO object invalid and send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-Value = TBD ("Both SID and NAI are absent in ERO subobject").When a PCEP speaker receives an SR-ERO subobject in which ST is 0, SID MUST be present and NAI MUST NOT be present(i.e., S-flag MUST be 0, F-flag MUST be 1, and the Length MUST be 8). Otherwise, it MUST consider the entire ERO object invalid and send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-Value = TBD ("Malformed object"). The PCEP speaker MAY include the malformed SR-ERO object in the PCErr message as well.A PCC can record SR-TE LSP and report the LSP to a PCE via RRO. An RRO object contains one or more subobjects called "SR-RRO subobjects" whose format is shown below:The format of SR-RRO subobject is the same as that of SR-ERO subobject without L flag.A PCC MUST assume that SR-RRO subobjects are organized such that the first subobject relative to the beginning of RRO contains the information about the topmost label, and the last subobject contains information about the bottommost label of the SR-TE LSP.Processing rules of SR-RRO subobject are identical to those of SR-ERO subobject.If a PCEP speaker receives an SR-RRO subobject in which both SID and NAI are absent, it MUST consider the entire RRO object invalid and send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-Value = TBD ("Both SID and NAI are absent in RRO subobject").If a PCE detects that all subobjects of RRO are not identical, and if it does not handle such RRO, it MUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-Value = TBD ("Non-identical RRO subobjects").If a PCEP session is established with an MSD value of zero, then the PCC MAY specify the MSD for an individual path computation request using the METRIC object defined in . This document defines a new type for the METRIC object to be used for this purpose as follows:
T = TBD (suggested value 11): Maximum SID Depth of the requested path.The PCC sets the metric-value to the MSD for this path. The PCC MUST set the B (bound) bit to 1 in the METRIC object, which specifies that the SID depth for the computed path MUST NOT exceed the metric-value.If a PCEP session is established with a non-zero MSD value, then the PCC MUST NOT send an MSD METRIC object. If the PCE receives a path computation request with an MSD METRIC object on a session with a non-zero MSD value then it MUST consider the request invalid and send a PCErr with Error-Type = 10 ("Reception of an invalid object") and Error-Value TBD ("Default MSD is specified for the PCEP session").A PCEP speaker that does not support the SR PCEP capability cannot recognize the SR-ERO or SR-RRO subobjects. As such, it MUST send a PCEP error with Error-Type = 4 (Not supported object) and Error-Value = 2 (Not supported object Type) as per .PCEP implementation:Can enable SR PCEP capability either by default or via explicit configuration.May generate PCEP error due to unsupported number of SR-ERO or SR-RRO subobjects either by default or via explicit configuration.A PCEP MIB module is defined in needs be extended to cover additional functionality provided by and . Such extension will cover the new functionality specified in this document.The security considerations described in and are applicable to this specification. No additional security measure is required.This document defines a new sub-object type for the PCEP
explicit route object (ERO), and a new sub-object type for the PCEP record route object (RRO). The code points for sub-object types of these objects is maintained in the RSVP parameters registry, under the EXPLICIT_ROUTE and ROUTE_RECORD objects. IANA is requested to allocate code points in the RSVP Parameters registry for each of the new sub-object types defined in this document, as follows:ObjectSub-ObjectSub-Object Type-----------------------------------------------------------------EXPLICIT_ROUTESR-ERO (PCEP-specific)TBD (recommended 36)ROUTE_RECORDSR-RRO (PCEP-specific)TBD (recommended 36)IANA is requested to allocate code-points in the PCEP-ERROR Object Error Types and Values registry for the following new error-values:
MeaningReception of an invalid object.
Bad label valueUnsupported number of Segment ERO subobjectsBad label formatNon-identical ERO subobjectsBoth SID and NAI are absent in ERO subobjectBoth SID and NAI are absent in RRO subobjectDefault MSD is specified for the PCEP sessionNon-identical RRO subobjectsMalformed objectIANA is requested to allocate a new code point in the PCEP TLV Type Indicators registry, as follows:ValueMeaning Reference -------------------------------------------------------------------TBD (recommended 26)SR-PCE-CAPABILITYThis document defines the PATH-SETUP-TYPE TLV and requests that IANA creates a registry to manage the value of the PATH_SETUP_TYPE TLV's PST field. IANA is requested to allocate a new code point in the PCEP PATH_SETUP_TYPE TLV PST field registry, as follows:ValueDescription Reference -------------------------------------------------------------------1Traffic engineering path is setup using Segment Routing technique.This documentIANA is requested to allocate a new code point in the PCEP METRIC object T field registry, as follows:ValueDescription Reference -------------------------------------------------------------------TBD (recommended 11)Segment-ID (SID) Depth.This documentThe following people contributed to this document:
- Lakshmi Sharma- Jan Medved- Edward Crabbe- Robert Raszuk- Victor LopezWe like to thank Ina Minei, George Swallow, Marek Zavodsky, Dhruv Dhody, Ing-Wher Chen and Tomas Janciga for the valuable comments.