| RFC 8664 | PCEP Extensions for Segment Routing | December 2019 |
| Sivabalan, et al. | Standards Track | [Page] |
Segment 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). An SR path can be derived from a variety of mechanisms, including an IGP Shortest Path Tree (SPT), an explicit configuration, or a Path Computation Element (PCE). This document specifies extensions to the Path Computation Element Communication Protocol (PCEP) that allow a stateful PCE to compute and initiate Traffic-Engineering (TE) paths, as well as a Path Computation Client (PCC) to request a path subject to certain constraints and optimization criteria in SR networks.¶
This document updates RFC 8408.¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8664.¶
Copyright (c) 2019 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
Segment Routing (SR) leverages the source-routing paradigm. UsingSR, a source node steers a packet through a path without relying onhop-by-hop signaling protocols such as LDP or RSVP-TE. Each path isspecified as an ordered list of instructions called "segments". Eachsegment is an instruction to route the packet to a specific place inthe network or to perform a function on the packet. A database ofsegments can be distributed through the network using a routingprotocol (such as IS-IS or OSPF) or by any other means. Several typesof segments are defined. A node segment uniquely identifies a specificnode in the SR domain. Each router in the SR domain associates a nodesegment with an ECMP-aware shortest path to the node that itidentifies. An adjacency segment represents a unidirectionaladjacency. An adjacency segment is local to the node that advertisesit. Both node segments and adjacency segments can be used for SR.¶
[RFC8402] describes the SR architecture. Thecorresponding IS-IS and OSPF extensions are specified in[RFC8667] and[RFC8665], respectively.¶
The SR architecture can be implemented using either an MPLSforwarding plane[RFC8660] or an IPv6 forwarding plane[IPv6-SRH]. The MPLS forwarding plane can be appliedto SR without any change; in which case, an SR path corresponds to anMPLS Label Switching Path (LSP). This document is relevant to the MPLSforwarding plane only. In this document, "Node-SID" and"Adj-SID" denote the Node Segment Identifier and AdjacencySegment Identifier, respectively.¶
An SR path can be derived from an IGP Shortest Path Tree(SPT). Segment Routing Traffic-Engineering (SR-TE) paths may notfollow an IGP SPT. Such paths may be chosen by a suitable networkplanning tool and provisioned on the ingress node of the SR-TEpath.¶
[RFC5440] describes the Path Computation ElementCommunication Protocol (PCEP) for communication between a PathComputation Client (PCC) and a Path Computation Element (PCE) orbetween a pair of PCEs. A PCE computes paths for MPLSTraffic-Engineering (MPLS-TE) LSPs based on various constraints andoptimization criteria.[RFC8231] specifies extensionsto PCEP that allow a stateful PCE to compute and recommend networkpaths in compliance with[RFC4657]. It also defines objectsand TLVs for MPLS-TE LSPs. Stateful PCEP extensions providesynchronization of LSP state between a PCC and a PCE or between a pairof PCEs, delegation of LSP control, reporting of LSP state from a PCCto a PCE, and control of the setup and path routing of an LSP from aPCE to a PCC. Stateful PCEP extensions are intended for an operationalmodel in which LSPs are configured on the PCC, and control over themis 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[RFC8281]. This mechanism is useful in Software-Defined Networking (SDN) applications, such as on-demand engineering or bandwidth calendaring[RFC8413].¶
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 the PCEP extensions specified in[RFC8281] and 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 a stateful or a stateless PCE.¶
This specification relies on the procedures specified in[RFC8408] to exchange the Segment Routing capability and to specify that the path setup type of an LSP is Segment Routing. This specification also updates[RFC8408] to clarify the use of sub-TLVs in the PATH-SETUP-TYPE-CAPABILITY TLV. SeeSection 4.1.1 for details.¶
This specification provides a mechanism for a network controller (acting as a PCE) to instantiate candidate paths for an SR Policy onto a head-end node (acting as a PCC) using PCEP. For more information on the SR Policy Architecture, see[SR-POLICY].¶
The following terminology is used in this document:¶
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.¶
In an SR network, the ingress node of an SR path prepends an SR header to all outgoing packets. The SR header consists of a list of SIDs (or MPLS labels in the context of this document). The header has all necessary information so that, in combination with the information distributed by the IGP, the packets can be guided from the ingress node to the egress node of the path; hence, there is no need for any signaling protocol.¶
In PCEP messages, LSP route information is carried in the Explicit Route Object (ERO), which consists of a sequence of subobjects. SR-TE paths computed by a PCE can be represented in an ERO in one of the following forms:¶
The PCC converts these into an MPLS label stack and next hop, as described inSection 5.2.2.¶
This document defines a new ERO subobject denoted by "SR-ERO subobject" that is 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 an ERO subobject. An ERO containing SR-ERO subobjects can be included in the PCEP Path Computation Reply (PCRep) message defined in[RFC5440], the Path Computation LSP Initiate Request (PCInitiate) message defined in[RFC8281], and the Path Computation Update Request (PCUpd) and Path Computation State Report (PCRpt) messages for LSPs defined in[RFC8231].¶
When a PCEP session between a PCC and a PCE is established, both PCEP speakers exchange their capabilities to indicate their ability to support SR-specific functionality.¶
A PCE can update an LSP that is initially established via RSVP-TEsignaling to use an SR-TE path by sending a PCUpd to the PCC thatdelegated the LSP to it[RFC8231]. A PCC can update anundelegated LSP that is initially established via RSVP-TE signaling touse an SR-TE path as follows. First, it requests an SR-TE path from aPCE by sending a Path Computation Request (PCReq) message. If itreceives a suitable path, it establishes the path in the data planeand then tears down the original RSVP-TE path. If the PCE isstateful, then the PCC sends PCRpt messages indicating that the newpath is set up and the old path is torn down, per[RFC8231].¶
Similarly, a PCE or PCC can update an LSP initially created with an SR-TE path to use RSVP-TE signaling, if necessary. This capability is useful for rolling back a change when a network is migrated from RSVP-TE to SR-TE technology.¶
A PCCMAY include a Record Route Object (RRO) containing the recorded LSP in PCReq and PCRpt messages as specified in[RFC5440] and[RFC8231], respectively. This document defines a new RRO subobject for SR networks. The methods used by a PCC to record the SR-TE LSP are outside the scope of this document.¶
In summary, this document:¶
The extensions specified in this document complement the existingPCEP specifications to support SR-TE paths. As such, the PCEP messages(e.g., PCReq, PCRep, PCRpt, PCUpd, PCInitiate, etc.) are formattedaccording to[RFC5440],[RFC8231],[RFC8281], and any other applicable PCEP specifications.¶
[RFC8408] defines the PATH-SETUP-TYPE-CAPABILITY TLVfor use in the OPEN object. The PATH-SETUP-TYPE-CAPABILITY TLVcontains an optional list of sub-TLVs, which are intended to conveyparameters that are associated with the path setup types supported bya PCEP speaker.¶
This specification updates[RFC8408] as follows. Itcreates a new registry that defines the valid type indicators of thesub-TLVs of the PATH-SETUP-TYPE-CAPABILITY TLV (seeSection 8.6). A PCEP speakerMUST NOTinclude a sub-TLV in the PATH-SETUP-TYPE-CAPABILITY TLV unless itappears in this registry. If a PCEP speaker receives a sub-TLV whosetype indicator does not match one of those from the registry or is notrecognized by the speaker, then the speakerMUST ignore thesub-TLV.¶
This document defines a new Path Setup Type (PST) for SR, as follows:¶
A PCEP speakerSHOULD indicate its support of the function described in this document by sending a PATH-SETUP-TYPE-CAPABILITY TLV in the OPEN object with this new PST included in the PST list.¶
This document also defines the SR-PCE-CAPABILITY sub-TLV. PCEP speakers use this sub-TLV to exchange information about their SR capability. If a PCEP speaker includes PST=1 in the PST list of the PATH-SETUP-TYPE-CAPABILITY TLV, then itMUST also include the SR-PCE-CAPABILITY sub-TLV inside the PATH-SETUP-TYPE-CAPABILITY TLV.¶
The format of the SR-PCE-CAPABILITY sub-TLV is shown in the following figure:¶
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 |N|X| MSD |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The codepoint for the TLV type is 26. The TLV length is 4 octets.¶
The 32-bit value is formatted as follows.¶
This document defines the following flag bits. The other bitsMUST be set to zero by the sender andMUST be ignored by the receiver.¶
To set up an SR-TE LSP using SR, the Request Parameter (RP) or Stateful PCE Request Parameter (SRP) objectMUST include the PATH-SETUP-TYPE TLV, specified in[RFC8408], with the PST set to 1 (and path setup using SR-TE).¶
The LSP-IDENTIFIERS TLVMAY be present for the above PST type.¶
An SR-TE path consists of one or more SIDs where each SIDMAY be associated with the identifier that represents the node or adjacency corresponding to the SID. This identifier is referred to as the NAI. As described later, an NAI can be represented in various formats (e.g., IPv4 address, IPv6 address, etc). Furthermore, an NAI is used for troubleshooting purposes and, if necessary, to derive a SID value as described below.¶
The ERO specified in[RFC5440] is used to carry SR-TE path information. In order to carry a SID and/or NAI, this document defines a new ERO subobject referred to as the "SR-ERO subobject", whose format is specified in the following section. An ERO carrying an SR-TE path consists of one or more ERO subobjects, and itMUST carry only SR-ERO subobjects. Note that an SR-ERO subobject does not need to have both the SID and NAI. However, at least one of themMUST be present.¶
When building the MPLS label stack from ERO, a PCCMUST 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 is formatted as shown in the following diagram.¶
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 |F|S|C|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // NAI (variable, optional) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields in the SR-ERO subobject are as follows:¶
Indicates the type and format of the NAI contained in the object body, if any is present. If the F bit is set to zero (see below), then the NT field has no meaning andMUST be ignored by the receiver. This document describes the following NT values:¶
Used to carry additional information pertaining to the SID. This document defines the following flag bits. The other bitsMUST be set to zero by the sender andMUST be ignored by the receiver.¶
The Segment Identifier. Depending on the M bit, it contains either:¶
At least one SID and NAIMUST be included in the SR-ERO subobject, and bothMAY be included.¶
This document defines the following NAIs:¶
Specified as a pair of IPv4 addresses. In this case, the NT value is 3, and the NAI field length is 8 octets. The format of the NAI is shown in the following figure:¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Local IPv4 address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Remote IPv4 address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Specified as a pair of global IPv6 addresses. It is used to describe an IPv6 adjacency for a link that uses global IPv6 addresses. Each global IPv6 address is configured on a specific router interface, so together they identify an adjacency between a pair of routers. In this case, the NT value is 4, and the NAI field length is 32 octets. The format of the NAI is shown in the following figure:¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Local IPv6 address (16 octets) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Remote IPv6 address (16 octets) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Specified as apair of (node ID, interface ID) tuples. In this case, the NT value is5, and the NAI field length is 16 octets. The format of the NAI isshown in the following figure:¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Local Node ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Local Interface ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Remote Node ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Remote Interface ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Specified as a pair of (global IPv6 address, interface ID) tuples. It is used to describe an IPv6 adjacency for a link that uses only link-local IPv6 addresses. Each global IPv6 address is configured on a specific router, so together they identify a pair of adjacent routers. The interface IDs identify the link that the adjacency is formed over. In this case, the NT value is 6, and the NAI field length is 40 octets. The format of the NAI is shown in the following figure:¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Local IPv6 address (16 octets) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Local Interface ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Remote IPv6 address (16 octets) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Remote Interface ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A PCC reports an SR-TE LSP to a PCE by sending a PCRpt message, per[RFC8231]. The RRO on this message represents the SID list that was applied by the PCC, that is, the actual path taken by the LSP. The procedures of[RFC8231] with respect to the RRO apply equally to this specification without change.¶
An RRO contains one or more subobjects called "SR-RRO subobjects", whose format is shown below:¶
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=36 | Length | NT | Flags |F|S|C|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // NAI (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the SR-RRO subobject is the same as that of the SR-ERO subobject, but without the L-Flag.¶
A PCCMUST order the SR-RRO subobjects such that the first subobject relative to the beginning of the RRO identifies the first segment visited by the SR-TE LSP, and the last subobject identifies the final segment of the SR-TE LSP, that is, its endpoint.¶
A PCCMAY request that PCE optimizes an individual path computation request to minimize the SID depth of the computed path by using the METRIC object defined in[RFC5440]. This document defines a new type for the METRIC object to be used for this purpose, as follows:¶
If the PCC includes a METRIC object of this type on a path computation request, then the PCE minimizes the SID depth of the computed path. If the B (bound) bit is set to 1 in the METRIC object, then the PCEMUST NOT return a path whose SID depth exceeds the given metric value. If the PCC did not set the X-Flag in its SR-PCE-CAPABILITY TLV, then itMUST set the B bit to 1. If the PCC set the X-Flag in its SR-PCE-CAPABILITY TLV, then itMAY set the B bit to 1 or zero.¶
If a PCEP session is established with a non-zero default MSD value, then the PCCMUST NOT send an MSD METRIC object with an MSD greater than the session's default MSD. If the PCE receives a path computation request with an MSD METRIC object on such a session that is greater than the session's default MSD, then itMUST consider the request invalid and send a PCEP Error (PCErr) with Error-Type = 10 ("Reception of an invalid object") and Error-value = 9 ("MSD exceeds the default for the PCEP session").¶
A PCC indicates that it is capable of supporting the head-end functions for SR-TE LSP by including the SR-PCE-CAPABILITY sub-TLV in the Open message that it sends to a PCE. A PCE indicates that it is capable of computing SR-TE paths by including the SR-PCE-CAPABILITY sub-TLV in the Open message that it sends to a PCC.¶
If a PCEP speaker receives a PATH-SETUP-TYPE-CAPABILITY TLV with a PST list containing PST=1, and supports that path setup type, then it checks for the presence of the SR-PCE-CAPABILITY sub-TLV. If that sub-TLV is absent, then the PCEP speakerMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 12 ("Missing PCE-SR-CAPABILITY sub-TLV") andMUST then close the PCEP session. If a PCEP speaker receives a PATH-SETUP-TYPE-CAPABILITY TLV with a SR-PCE-CAPABILITY sub-TLV, but the PST list does not contain PST=1, then the PCEP speakerMUST ignore the SR-PCE-CAPABILITY sub-TLV.¶
If a PCC sets the N-Flag to 1, then the PCEMAY send an SR-ERO subobject containing an NAI and no SID (seeSection 5.2). Otherwise, the PCEMUST NOT send an SR-ERO subobject containing an NAI and no SID.¶
The number of SIDs that can be imposed on a packet depends on the PCC's data-plane capability. If a PCC sets the X-Flag to 1, then the MSD is not used andMUST be set to zero. If a PCE receives an SR-PCE-CAPABILITY sub-TLV with the X-Flag set to 1, then itMUST ignore the MSD field and assume that the sender can impose a SID stack of any depth. If a PCC sets the X-Flag to zero, then it sets the MSD field to the maximum number of SIDs that it can impose on a packet. In this case, the PCCMUST set the MSD to a number greater than zero. If a PCE receives an SR-PCE-CAPABILITY sub-TLV with the X-Flag and MSD both set to zero, then itMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 21 ("Maximum SID depth must be non-zero") andMUST then close the PCEP session.¶
Note that the MSD value exchanged via the SR-PCE-CAPABILITY sub-TLV indicates the SID/label imposition limit for the PCC node. It is anticipated that, in many deployments, the PCCs will have network interfaces that are homogeneous with respect to MSD (that is, each interface has the same MSD). In such cases, having a per-node MSD on the PCEP session is sufficient; the PCESHOULD interpret this to mean that all network interfaces on the PCC have the given MSD. However, the PCEMAY also learn a per-node MSD and a per-interface MSD from the routing protocols, as specified in[RFC8491],[RFC8476], and[MSD-BGP]. If the PCE learns the per-node MSD of a PCC from a routing protocol, then itMUST ignore the per-node MSD value in the SR-PCE-CAPABILITY sub-TLV and use the per-node MSD learned from the routing protocol instead. If the PCE learns the MSD of a network interface on a PCC from a routing protocol, then itMUST use the per-interface MSD instead of the MSD value in the SR-PCE-CAPABILITY sub-TLV when it computes a path that uses that interface.¶
Once an SR-capable PCEP session is established with a non-zero MSD value, the corresponding PCEMUST NOT send SR-TE paths with a number of SIDs exceeding that MSD value. If a PCC needs to modify the MSD value, itMUST close the PCEP session and re-establish it 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 PCCMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 3 ("Unsupported number of SR-ERO subobjects"). If a PCEP session is established with an MSD value of zero, then the PCCMAY specify an MSD for each path computation request that it sends to the PCE, by including a "maximum SID depth" METRIC object on the request, as defined inSection 4.5.¶
The N-Flag, X-Flag, and MSD value inside the SR-PCE-CAPABILITY sub-TLV are meaningful only in the Open message sent from a PCC to a PCE. As such, a PCEMUST set the N-Flag to zero, X-Flag to 1, and MSD value to zero in an outbound message to a PCC. Similarly, a PCCMUST ignore any MSD value received from a PCE. If a PCE receives multiple SR-PCE-CAPABILITY sub-TLVs in an Open message, it processes only the first sub-TLV received.¶
If a PCC does not support the SR PCE Capability and thus cannot recognize the SR-ERO or SR-RRO subobjects, it will respond according to the rules for a malformed object per[RFC5440].¶
On receiving an SR-ERO, a PCCMUST validate that the Length field, S bit, F bit, and NT field are consistent, as follows.¶
If a PCC finds that the NT field, Length field, S bit, and F bit are not consistent, itMUST consider the entire ERO invalid andMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 11 ("Malformed object").¶
If a PCC does not recognize or support the value in the NT field, itMUST consider the entire ERO invalid andMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 13 ("Unsupported NAI Type in the SR-ERO/SR-RRO subobject").¶
If a PCC receives an SR-ERO subobject in which the S and F bits are both set to 1 (that is, both the SID and NAI are absent), itMUST consider the entire ERO invalid and send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 6 ("Both SID and NAI are absent in the SR-ERO subobject").¶
If a PCC receives an SR-ERO subobject in which the S bit is set to 1 and the F bit is set to zero (that is, the SID is absent and the NAI is present), but the PCC does not support NAI resolution, itMUST consider the entire ERO invalid and send a PCErr message with Error-Type = 4 ("Not supported object") and Error-value = 4 ("Unsupported parameter").¶
If a PCC receives an SR-ERO subobject in which the S bit is set to 1 and either (or both) the M bit or the C bit is set to 1, itMUST consider the entire ERO invalid and send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 11 ("Malformed object").¶
If a PCC receives an SR-ERO subobject in which the S bit is set to zero and the M bit is set to 1, then the subobject contains an MPLS label. The PCCMAY choose not to accept a label provided by the PCE, based on its local policy. The PCCMUST NOT accept MPLS label value 3 (Implicit NULL), but itMAY accept other special-purpose MPLS label values. If the PCC decides not to accept an MPLS label value, itMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 2 ("Bad label value").¶
If both the M and C bits of an SR-ERO subobject are set to 1, and if a PCC finds an erroneous setting in one or more of the TC, S, and TTL fields, itMAY overwrite those fields with values chosen according to its own policy. If the PCC does not overwrite them, itMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 4 ("Bad label format").¶
If the M bit of an SR-ERO subobject is set to zero but the C bit is set to 1, then the PCCMUST consider the entire ERO invalid andMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 11 ("Malformed object").¶
If a PCC receives an SR-ERO subobject in which the S bit is set to zero and the M bit is set to zero, then the subobject contains a SID index value. If the SID is an Adj-SID, then the L-FlagMUST NOT be set. If the L-Flag is set for an Adj-SID, then the PCCMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 11 ("Malformed object").¶
If a PCC detects that the subobjects of an ERO are a mixture of SR-ERO subobjects and subobjects of other types, then itMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 5 ("ERO mixes SR-ERO subobjects with other subobject types").¶
The SR-ERO subobjects can be classified according to whether they contain a SID representing an MPLS label value or an index value, or no SID. If a PCC detects that the SR-ERO subobjects are a mixture of more than one of these types, then itMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 20 ("Inconsistent SIDs in SR-ERO/SR-RRO subobjects").¶
If an ERO specifies a new SR-TE path for an existing LSP and the PCC determines that the ERO contains SR-ERO subobjects that are not valid, then the PCCMUST NOT update the LSP.¶
The SR-ERO contains a sequence of subobjects. Each SR-ERO subobject in the sequence identifies a segment that the traffic will be directed to, in the order given. That is, the first subobject identifies the first segment the traffic will be directed to, the second subobject represents the second segment, and so on.¶
The PCC interprets the SR-ERO by converting it to an MPLS label stack plus a next hop. The PCC sends packets along the segment-routed path by prepending the MPLS label stack onto the packets and sending the resulting, modified packet to the next hop.¶
The PCC uses a different procedure to do this conversion, depending on the information that the PCE has provided in the subobjects.¶
For all cases above, after the PCC has imposed the label stack on the packet, it sends the packet to the segment identified by the first SID.¶
There are several errors that can occur during the process of converting an SR-ERO sequence to an MPLS label stack and a next hop. The PCC deals with them as follows.¶
If an ERO specifies a new SR-TE path for an existing LSP and the PCC encounters an error while processing the ERO, then the PCCMUST NOT update the LSP.¶
The syntax-checking rules that apply to the SR-RRO subobject are identical to those of the SR-ERO subobject, except as noted below.¶
If a PCEP speaker receives an SR-RRO subobject in which both SID and NAI are absent, itMUST consider the entire RRO invalid and send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 7 ("Both SID and NAI are absent in the SR-RRO subobject").¶
If a PCE detects that the subobjects of an RRO are a mixture of SR-RRO subobjects and subobjects of other types, then itMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 10 ("RRO mixes SR-RRO subobjects with other subobject types").¶
The SR-RRO subobjects can be classified according to whether they contain a SID representing an MPLS label value or an index value, or no SID. If a PCE detects that the SR-RRO subobjects are a mixture of more than one of these types, then itMUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 20 ("Inconsistent SIDs in SR-ERO / SR-RRO subobjects").¶
This document adds a new path setup type to PCEP to allow LSPsto be set up using Segment Routing techniques. This path setuptype may be used with PCEP alongside other path setup types,such as RSVP-TE, or it may be used exclusively.¶
The following factors control which path setup type is used fora given LSP.¶
The operator can influence the path setup type as follows.¶
This section discusses the steps that the operator takes when migrating anetwork to enable PCEP to set up paths using Segment Routing as the pathsetup type.¶
Note that the data plane is unaffected if a PCEP session is reset. AnyLSPs that were set up before the session reset will remain in place andwill still be present after the session comes back up.¶
An implementationSHOULD allow the operator to manually trigger a PCEPsession to be reset.¶
An implementationMAY automatically reset a PCEP session whenan operator reconfigures the PCEP speaker's capabilities. However, note thatif the capabilities at both ends of the PCEP session are not reconfiguredsimultaneously, then the session could be reset twice, which could lead tounnecessary network traffic. Therefore, such implementationsSHOULD allowthe operator to override this behavior and wait instead for a manual reset.¶
Once Segment Routing is enabled on a PCEP session, it can be used as thepath setup type for future LSPs.¶
User traffic is not automatically migrated from existing LSPs ontosegment-routed LSPs just by enabling the Segment Routing PST in PCEP. Themigration of user traffic from existing LSPs onto Segment Routing LSPs isbeyond the scope of this document.¶
The operator needs the following information to verify that PCEP isoperating correctly with respect to the Segment Routing path setup type.¶
The PCEP YANG module is defined in[PCE-PCEP-YANG]. In the future, this YANG module should be extended or augmented to provide the following additional information relating to Segment Routing:¶
The PCEP MIB[RFC7420] could also be updated to include thisinformation.¶
The security considerations described in[RFC5440],[RFC8231],[RFC8281], and[RFC8408] areapplicable to this specification. No additional security measures are required.¶
Note that this specification enables a network controller to instantiate a path in the network without the use of a hop-by-hop signaling protocol (such as RSVP-TE). This creates an additional vulnerability if the security mechanisms of[RFC5440],[RFC8231], and[RFC8281] are not used. If there is no integrity protection on the session, then an attacker could create a path that is not subjected to the further verification checks that would be performed by the signaling protocol.¶
Note that this specification adds the MSD field to the Open message (seeSection 4.1.2), which discloses how many MPLS labels the sender can push onto packets that it forwards into the network. If the security mechanisms of[RFC8231] and[RFC8281] are not used with strong encryption, then an attacker could use this new field to gain intelligence about the capabilities of the edge devices in the network.¶
This document defines a new subobject type for the PCEP ERO and a new subobject type for the PCEP RRO. The codepoints for subobject types of these objects are maintained in the "Resource Reservation Protocol (RSVP) Parameters" registry, under the EXPLICIT_ROUTE and ROUTE_RECORD objects, respectively.¶
| Object | Subobject | Subobject Type |
|---|---|---|
| EXPLICIT_ROUTE | SR-ERO (PCEP specific) | 36 |
| ROUTE_RECORD | SR-RRO (PCEP specific) | 36 |
IANA has created a new sub-registry within the "Path ComputationElement Protocol (PCEP) Numbers" registry called "PCEP SR-ERO NAITypes". The allocation policy for this new registry is by IETFReview[RFC8126]. The new registry contains thefollowing values:¶
| Value | Description | Reference |
|---|---|---|
| 0 | NAI is absent. | This document |
| 1 | NAI is an IPv4 node ID. | This document |
| 2 | NAI is an IPv6 node ID. | This document |
| 3 | NAI is an IPv4 adjacency. | This document |
| 4 | NAI is an IPv6 adjacency with global IPv6 addresses. | This document |
| 5 | NAI is an unnumbered adjacency with IPv4 node IDs. | This document |
| 6 | NAI is an IPv6 adjacency with link-local IPv6 addresses. | This document |
| 7-15 | Unassigned |
IANA has created a new sub-registry, named "SR-ERO Flag Field", within the "Path Computation Element Protocol (PCEP) Numbers" registry to manage the Flag field of the SR-ERO subobject. New values are to be assigned by Standards Action[RFC8126]. Each bit should be tracked with the following qualities:¶
The following values are defined in this document:¶
| Bit | Description | Reference |
|---|---|---|
| 0-7 | Unassigned | |
| 8 | NAI is absent (F) | This document |
| 9 | SID is absent (S) | This document |
| 10 | SID specifies TC, S, and TTL in addition to an MPLS label (C) | This document |
| 11 | SID specifies an MPLS label (M) | This document |
IANA has allocated the following codepoints in the "PCEP-ERROR Object Error Types and Values" registry for the following new Error-values:¶
| Error-Type | Meaning | Error-value |
|---|---|---|
| 10 | Reception of an invalid object | |
| 2: Bad label value | ||
| 3: Unsupported number of SR-ERO subobjects | ||
| 4: Bad label format | ||
| 5: ERO mixes SR-ERO subobjects with other subobject types | ||
| 6: Both SID and NAI are absent in the SR-ERO subobject | ||
| 7: Both SID and NAI are absent in the SR-RRO subobject | ||
| 9: MSD exceeds the default for the PCEP session | ||
| 10: RRO mixes SR-RRO subobjects with other subobject types | ||
| 12: Missing PCE-SR-CAPABILITY sub-TLV | ||
| 13: Unsupported NAI Type in the SR-ERO/SR-RRO subobject | ||
| 14: Unknown SID | ||
| 15: NAI cannot be resolved to a SID | ||
| 16: Could not find SRGB | ||
| 17: SID index exceeds SRGB size | ||
| 18: Could not find SRLB | ||
| 19: SID index exceeds SRLB size | ||
| 20: Inconsistent SIDs in SR-ERO / SR-RRO subobjects | ||
| 21: MSD must be non-zero |
IANA has allocated the following codepoint in the "PCEP TLV Type Indicators" registry. Note that this TLV type indicator is deprecated but retained in the registry to ensure compatibility with early implementations of this specification. SeeAppendix A for details.¶
| Value | Meaning | Reference |
|---|---|---|
| 26 | SR-PCE-CAPABILITY (deprecated) | This document |
IANA has created a new sub-registry, named "PATH-SETUP-TYPE-CAPABILITY Sub-TLV Type Indicators", within the "Path Computation Element Protocol (PCEP) Numbers" registry to manage the type indicator space for sub-TLVs of the PATH-SETUP-TYPE-CAPABILITY TLV. New values are to be assigned by Standards Action[RFC8126]. The valid range of values in the registry is 0-65535. IANA has initialized the registry with the following values. All other values in the registry should be marked as "Unassigned".¶
| Value | Meaning | Reference |
|---|---|---|
| 0 | Reserved | This document |
| 26 | SR-PCE-CAPABILITY | This document |
A sub-registry within the "Path Computation Element Protocol (PCEP) Numbers" registry called "PCEP Path Setup Types" was created in[RFC8408]. IANA has allocated a new codepoint within this registry, as follows:¶
| Value | Description | Reference |
|---|---|---|
| 1 | Traffic-engineering path is set up using Segment Routing. | This document |
IANA has allocated the following codepoint in the PCEP "METRIC Object T Field" registry:¶
| Value | Description | Reference |
|---|---|---|
| 11 | Segment-ID (SID) Depth. | This document |
IANA has created a new sub-registry, named "SR Capability Flag Field", within the "Path Computation Element Protocol (PCEP) Numbers" registry to manage the Flag field of the SR-PCE-CAPABILITY TLV. New values are to be assigned by Standards Action[RFC8126]. Each bit should be tracked with the following qualities:¶
The following values are defined in this document:¶
| Bit | Description | Reference |
|---|---|---|
| 0-5 | Unassigned | |
| 6 | Node or Adjacency Identifier (NAI) is supported (N) | This document |
| 7 | Unlimited Maximum SID Depth (X) | This document |
An early implementation of this specification will send the SR-CAPABILITY-TLV as a top-level TLV in the OPEN object instead of sending the PATH-SETUP-TYPE-CAPABILITY TLV in the OPEN object. Implementations that wish to interoperate with such early implementations should also send the SR-CAPABILITY-TLV as a top-level TLV in their OPEN object and should interpret receiving this top-level TLV as though the sender had sent a PATH-SETUP-TYPE-CAPABILITY TLV with a PST list of (0, 1) (that is, both RSVP-TE and SR-TE PSTs are supported) with the SR-CAPABILITY-TLV as a sub-TLV. If a PCEP speaker receives an OPEN object in which both the SR-CAPABILITY-TLV and PATH-SETUP-TYPE-CAPABILITY TLV appear as top-level TLVs, then it should ignore the top-level SR-CAPABILITY-TLV and process only the PATH-SETUP-TYPE-CAPABILITY TLV.¶
We thank Ina Minei, George Swallow, Marek Zavodsky, Dhruv Dhody, Ing-Wher Chen, and Tomas Janciga for the valuable comments.¶
The following people contributed to this document:¶