Internet Engineering Task Force (IETF) S. Sivabalan
Request for Comments: 8664 C. Filsfils
Updates: 8408 Cisco Systems, Inc.
Category: Standards Track J. Tantsura
ISSN: 2070-1721 Apstra, Inc.
W. Henderickx
Nokia
J. Hardwick
Metaswitch Networks
October
December 2019
Path Computation Element Communication Protocol (PCEP) Extensions for
Segment Routing
Abstract
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.
Status of This Memo
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 at
https://www.rfc-editor.org/info/rfc8664.
Copyright Notice
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
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publication of this document. Please review these documents
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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.
Table of Contents
1. Introduction
2. Terminology
2.1. Requirements Language
3. Overview of PCEP Operation in SR Networks
4. Object Formats
4.1. The OPEN Object
4.1.1. The Path Setup Type Capability TLV
4.1.2. The SR PCE Capability Sub-TLV
4.2. The RP/SRP Object
4.3. ERO
4.3.1. SR-ERO Subobject
4.3.2. NAI Associated with SID
4.4. RRO
4.5. METRIC Object
5. Procedures
5.1. Exchanging the SR PCE Capability
5.2. ERO Processing
5.2.1. SR-ERO Validation
5.2.2. Interpreting the SR-ERO
5.3. RRO Processing
6. Management Considerations
6.1. Controlling the Path Setup Type
6.2. Migrating a Network to Use PCEP Segment-Routed Paths
6.3. Verification of Network Operation
6.4. Relationship to Existing Management Models
7. Security Considerations
8. IANA Considerations
8.1. PCEP ERO and RRO Subobjects
8.2. New NAI Type Registry
8.3. New SR-ERO Flag Registry
8.4. PCEP-Error Object
8.5. PCEP TLV Type Indicators
8.6. PATH-SETUP-TYPE-CAPABILITY Sub-TLV Type Indicators
8.7. New Path Setup Type
8.8. New Metric Type
8.9. SR PCE Capability Flags
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Compatibility with Early Implementations
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
Segment Routing (SR) leverages the source-routing paradigm. Using
SR, a source node steers a packet through a path without relying on
hop-by-hop signaling protocols such as LDP or RSVP-TE. Each path is
specified as an ordered list of instructions called "segments". Each
segment is an instruction to route the packet to a specific place in
the network or to perform a function on the packet. A database of
segments can be distributed through the network using a routing
protocol (such as IS-IS or OSPF) or by any other means. Several
types of segments are defined. A node segment uniquely identifies a
specific node in the SR domain. Each router in the SR domain
associates a node segment with an ECMP-aware shortest path to the
node that it identifies. An adjacency segment represents a
unidirectional adjacency. An adjacency segment is local to the node
that advertises it. Both node segments and adjacency segments can be
used for SR.
[RFC8402] describes the SR architecture. The corresponding IS-IS and
OSPF extensions are specified in [RFC8667] and [RFC8665],
respectively.
The SR architecture can be implemented using either an MPLS
forwarding plane [RFC8660] or an IPv6 forwarding plane [IPv6-SRH].
The MPLS forwarding plane can be applied to SR without any change; in
which case, an SR path corresponds to an MPLS Label Switching Path
(LSP). This document is relevant to the MPLS forwarding plane only.
In this document, "Node-SID" and "Adjacency-SID" "Adj-SID" denote the Node Segment
Identifier and Adjacency Segment Identifier, respectively.
An SR path can be derived from an IGP Shortest Path Tree (SPT).
Segment Routing Traffic-Engineering (SR-TE) paths may not follow an
IGP SPT. Such paths may be chosen by a suitable network planning
tool and provisioned on the ingress node of the SR-TE path.
[RFC5440] describes the Path Computation Element Communication
Protocol (PCEP) for communication between a Path Computation Client
(PCC) and a Path Computation Element (PCE) or between a pair of PCEs.
A PCE computes paths for MPLS Traffic-Engineering (MPLS-TE) LSPs
based on various constraints and optimization criteria. [RFC8231]
specifies extensions to PCEP that allow a stateful PCE to compute and
recommend network paths in compliance with [RFC4657]. It also
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, and control of the set up 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 [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 Segment Routing capability and to specify that the path
setup type of an LSP is segment routing. Segment Routing. This specification also
updates [RFC8408] to clarify the use of sub-TLVs in the PATH-SETUP-
TYPE-CAPABILITY TLV. See Section 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].
2. Terminology
The following terminology is used in this document:
ERO: Explicit Route Object
IGP: Interior Gateway Protocol
IS-IS: Intermediate System to Intermediate System
LSR: Label Switching Router
MSD: Base MPLS Imposition Maximum SID Depth, as defined in
[RFC8491]
NAI: Node or Adjacency Identifier
OSPF: Open Shortest Path First
PCC: Path Computation Client
PCE: Path Computation Element
PCEP: Path Computation Element Communication Protocol
RRO: Record Route Object
SID: Segment Identifier
SR: Segment Routing
SR-DB: Segment Routing Database: the collection of SRGBs, SRLBs, and
SIDs and the objects they map to, advertised by a link-state
IGP
SR-TE: Segment Routing Traffic Engineering
SRGB: Segment Routing Global Block
SRLB: Segment Routing Local Block
2.1. Requirements Language
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.
3. Overview of PCEP Operation in SR Networks
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:
* An ordered set of IP addresses representing network nodes/links.
* An ordered set of SIDs, with or without the corresponding IP
addresses.
* An ordered set of MPLS labels, with or without corresponding IP
addresses.
The PCC converts these into an MPLS label stack and next hop, as
described in Section 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-TE
signaling to use an SR-TE path by sending a PCUpd to the PCC that
delegated the LSP to it [RFC8231]. A PCC can update an undelegated
LSP that is initially established via RSVP-TE signaling to use an SR-
TE path as follows. First, it requests an SR-TE path from a PCE by
sending a Path Computation Request (PCReq) message. If it receives a
suitable path, it establishes the path in the data plane and then
tears down the original RSVP-TE path. If the PCE is stateful, then
the PCC sends PCRpt messages indicating that the new path 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 PCC MAY 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:
* Defines a new ERO subobject, a new RRO subobject, 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 for the ERO subobject.
* Defines a new path setup type to be used in the PATH-SETUP-TYPE
and PATH-SETUP-TYPE-CAPABILITY TLVs [RFC8408].
* Defines a new sub-TLV for the PATH-SETUP-TYPE-CAPABILITY TLV.
The extensions specified in this document complement the existing
PCEP specifications to support SR-TE paths. As such, the PCEP
messages (e.g., PCReq, PCRep, PCRpt, PCUpd, PCInitiate, etc.) are
formatted according to [RFC5440], [RFC8231], [RFC8281], and any other
applicable PCEP specifications.
4. Object Formats
4.1. The OPEN Object
4.1.1. The Path Setup Type Capability TLV
[RFC8408] defines the PATH-SETUP-TYPE-CAPABILITY TLV for use in the
OPEN object. The PATH-SETUP-TYPE-CAPABILITY TLV contains an optional
list of sub-TLVs, which are intended to convey parameters that are
associated with the path setup types supported by a PCEP speaker.
This specification updates [RFC8408] as follows. It creates a new
registry that defines the valid type indicators of the sub-TLVs of
the PATH-SETUP-TYPE-CAPABILITY TLV (see Section 8.6). A PCEP speaker
MUST NOT include a sub-TLV in the PATH-SETUP-TYPE-CAPABILITY TLV
unless it appears in this registry. If a PCEP speaker receives a
sub-TLV whose type indicator does not match one of those from the
registry or is not recognized by the speaker, then the speaker MUST
ignore the sub-TLV.
4.1.2. The SR PCE Capability Sub-TLV
This document defines a new Path Setup Type (PST) for SR, as follows:
PST = 1: Traffic-engineering path is set up using Segment
Routing.
A PCEP speaker SHOULD 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 it MUST 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: SR-PCE-CAPABILITY Sub-TLV Format
The code point codepoint for the TLV type is 26. The TLV length is 4 octets.
The 32-bit value is formatted as follows.
Reserved: MUST be set to zero by the sender and MUST be ignored by
the receiver.
Flags: 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.
N: A PCC sets this flag bit to 1 to indicate that it is
capable of resolving a Node or Adjacency Identifier (NAI)
to a SID.
X: A PCC sets this flag bit to 1 to indicate that it does not
impose any limit on the MSD.
Maximum SID Depth (MSD): specifies the maximum number of SIDs (MPLS
label stack depth in the context of this document) that a PCC is
capable of imposing on a packet. Section 5.1 explains the
relationship between this field and the X flag. X-Flag.
4.2. The RP/SRP Object
To set up an SR-TE LSP using SR, the Request Parameter (RP) or
Stateful PCE Request Parameter (SRP) object MUST include the PATH-
SETUP-TYPE TLV, specified in [RFC8408], with the PST set to 1 (and
path setup using SR-TE).
The LSP-IDENTIFIERS TLV MAY be present for the above PST type.
4.3. ERO
An SR-TE path consists of one or more SIDs 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 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 it MUST
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 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.
4.3.1. SR-ERO Subobject
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) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SR-ERO Subobject Format
The fields in the SR-ERO subobject are as follows:
The 'L' Flag: L-Flag: Indicates whether the subobject represents a loose hop
in the LSP [RFC3209]. If this flag is set to zero, 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 values in
the received SR-ERO based on its local policy.
Type: Set to 36.
Length: Contains the total length of the subobject in octets. The
Length MUST be at least 8 and MUST be a multiple of 4. An SR-ERO
subobject MUST contain at least one SID or NAI. The flags
described below indicate whether the SID or NAI fields are absent.
NAI Type (NT): 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 and MUST be ignored
by the receiver. This document describes the following NT values:
NT=0 The NAI is absent.
NT=1 The NAI is an IPv4 node ID.
NT=2 The NAI is an IPv6 node ID.
NT=3 The NAI is an IPv4 adjacency.
NT=4 The NAI is an IPv6 adjacency with global IPv6 addresses.
NT=5 The NAI is an unnumbered adjacency with IPv4 node IDs.
NT=6 The NAI is an IPv6 adjacency with link-local IPv6 addresses.
Flags: Used to carry additional information pertaining to the SID.
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.
M: If this bit is set to 1, the SID value represents an MPLS
label stack entry as specified in [RFC3032]. Otherwise, the
SID value is an administratively configured value that
represents an index into an MPLS label space (either SRGB or
SRLB) per [RFC8402].
C: If the M bit and the C bit are both set to 1, then the TC, S,
and TTL fields in the MPLS label stack entry are specified by
the PCE. However, a PCC MAY choose to override these values
according to its local policy and MPLS forwarding rules. If
the M bit is set to 1 but the C bit is set to zero, then the
TC, S, and TTL fields MUST be ignored by the PCC. The PCC
MUST set these fields according to its local policy and MPLS
forwarding rules. If the M bit is set to zero, then the C
bit MUST be set to zero.
S: When this bit is set to 1, the SID value in the subobject
body is absent. In this case, the PCC is responsible for
choosing the SID value, e.g., by looking it up in the SR-DB
using the NAI that, in this case, MUST be present in the
subobject. If the S bit is set to 1, then the M and C bits
MUST be set to zero.
F: When this bit is set to 1, the NAI value in the subobject
body is absent. The F bit MUST be set to 1 if NT=0;
otherwise, it MUST be set to zero. The S and F bits MUST NOT
both be set to 1.
SID: The Segment Identifier. Depending on the M bit, it contains
either:
* A 4-octet index defining the offset into an MPLS label space
per [RFC8402] or
* A 4-octet MPLS label stack entry, where the 20 most significant
bits encode the label value per [RFC3032].
NAI: The NAI associated with the SID. The NAI's format depends on
the value in the NT field and is described in the following
section.
At least one SID and NAI MUST be included in the SR-ERO subobject,
and both MAY be included.
4.3.2. NAI Associated with SID
This document defines the following NAIs:
IPv4 Node ID: Specified as an IPv4 address. In this case, the NT
value is 1, and the NAI field length is 4 octets.
IPv6 Node ID: Specified as an IPv6 address. In this case, the NT
value is 2, and the NAI field length is 16 octets.
IPv4 Adjacency: 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: NAI for IPv4 Adjacency
IPv6 Global Adjacency: 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) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: NAI for IPv6 Global Adjacency
Unnumbered Adjacency with IPv4 NodeIDs: Specified as a pair of (node
ID, interface ID) tuples. In this case, the NT value is 5, and
the NAI field length is 16 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 Node-ID Node ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Node-ID Node ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: NAI for Unnumbered Adjacency with IPv4 Node IDs
IPv6 Link-Local Adjacency: 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: NAI for IPv6 Link-Local Adjacency
4.4. RRO
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) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: SR-RRO Subobject Format
The format of the SR-RRO subobject is the same as that of the SR-ERO
subobject, but without the L flag. L-Flag.
A PCC MUST 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.
4.5. METRIC Object
A PCC MAY 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:
T = 11: Maximum SID Depth of the requested path.
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 PCE MUST NOT return a path whose SID depth exceeds
the given metric value. If the PCC did not set the X flag X-Flag in its SR-
PCE-CAPABILITY TLV, then it MUST set the B bit to 1. If the PCC set
the X flag X-Flag in its SR-PCE-CAPABILITY TLV, then it MAY set the B bit to
1 or zero.
If a PCEP session is established with a non-zero default MSD value,
then the PCC MUST 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 it MUST consider the
request invalid and send a PCEP Error (PCErr) with Error-Type = 10
("Reception of an invalid object") and Error-Value Error-value = 9 ("MSD exceeds
the default for the PCEP session").
5. Procedures
5.1. Exchanging the SR PCE Capability
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 speaker MUST send a PCErr message
with Error-Type = 10 (Reception ("Reception of an invalid object) object") and Error-Value Error-
value = 12 (Missing ("Missing PCE-SR-CAPABILITY sub-TLV) sub-TLV") and MUST then close
the PCEP session. If a PCEP speaker receives a PATH-SETUP-TYPE-CAPABILITY PATH-SETUP-TYPE-
CAPABILITY TLV with a SR-PCE-CAPABILITY sub-TLV, but the PST list
does not contain PST=1, then the PCEP speaker MUST ignore the SR-PCE-CAPABILITY sub-
TLV. SR-PCE-
CAPABILITY sub-TLV.
If a PCC sets the N flag N-Flag to 1, then the PCE MAY send an SR-ERO
subobject containing an NAI and no SID (see Section 5.2). Otherwise,
the PCE MUST 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 X-Flag to 1, then the
MSD is not used and MUST be set to zero. If a PCE receives an SR-
PCE-CAPABILITY sub-TLV with the X flag X-Flag set to 1, then it MUST ignore
the MSD field and assume that the sender can impose a SID stack of
any depth. If a PCC sets the X flag 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 PCC MUST set the MSD to a number greater than zero.
If a PCE receives an SR-PCE-CAPABILITY sub-TLV with the X flag X-Flag and
MSD both set to zero, then it MUST send a PCErr message with Error-
Type = 10 (Reception ("Reception of an invalid object) object") and Error-Value Error-value = 21 (Maximum
("Maximum SID depth must be non-zero) non-zero") and MUST 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 PCE SHOULD interpret this to mean
that all network interfaces on the PCC have the given MSD. However,
the PCE MAY 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 it MUST 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 it MUST 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 PCE MUST NOT send SR-TE paths with a number
of SIDs exceeding that MSD value. If a PCC needs to modify the MSD
value, it MUST 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 PCC MUST send a PCErr message with
Error-Type = 10 (Reception ("Reception of an invalid object) object") and Error-Value Error-value =
3
(Unsupported ("Unsupported number of Segment ERO subobjects). SR-ERO subobjects"). 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, by
including a "maximum SID depth" METRIC object on the request, as
defined in Section 4.5.
The N flag, X flag, 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 PCE MUST set the N flag N-Flag to zero, X flag X-Flag to 1, and MSD
value to zero in an outbound message to a PCC. Similarly, a PCC MUST
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.
5.2. ERO Processing
5.2.1. SR-ERO Validation
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 PCC MUST validate that the Length field, S
bit, F bit, and NT field are consistent, as follows.
* If NT=0, the F bit MUST be 1, the S bit MUST be zero, and the
Length MUST be 8.
* If NT=1, the F bit MUST be zero. If the S bit is 1, the Length
MUST be 8; otherwise, the Length MUST be 12.
* If NT=2, the F bit MUST be zero. If the S bit is 1, the Length
MUST be 20; otherwise, the Length MUST be 24.
* If NT=3, the F bit MUST be zero. If the S bit is 1, the Length
MUST be 12; otherwise, the Length MUST be 16.
* If NT=4, the F bit MUST be zero. If the S bit is 1, the Length
MUST be 36; otherwise, the Length MUST be 40.
* If NT=5, the F bit MUST be zero. If the S bit is 1, the Length
MUST be 20; otherwise, the Length MUST be 24.
* If NT=6, the F bit MUST be zero. If the S bit is 1, the Length
MUST be 44; otherwise, the Length MUST be 48.
If a PCC finds that the NT field, Length field, S bit, and F bit are
not consistent, it MUST consider the entire ERO invalid and MUST send
a PCErr message with Error-Type = 10 ("Reception of an invalid
object") and Error-Value Error-value = 11 ("Malformed object").
If a PCC does not recognize or support the value in the NT field, it
MUST consider the entire ERO invalid and MUST send a PCErr message
with Error-Type = 10 ("Reception of an invalid object") and Error-
Value
value = 13 ("Unsupported NAI Type in Segment ERO 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), it MUST
consider the entire ERO invalid and send a PCErr message with Error-
Type = 10 ("Reception of an invalid object") and Error-Value 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, it MUST
consider the entire ERO invalid and send a PCErr message with Error-
Type = 4 ("Not supported object") and Error-Value 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, it MUST
consider the entire ERO invalid and send a PCErr message with Error-
Type = 10 ("Reception of an invalid object") and Error-Value 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 PCC MAY choose not to accept a label provided by the PCE,
based on its local policy. The PCC MUST NOT accept MPLS label value
3 (Implicit NULL), but it MAY accept other special-purpose MPLS label
values. If the PCC decides not to accept an MPLS label value, it
MUST send a PCErr message with Error-Type = 10 ("Reception of an
invalid object") and Error Value 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, it MAY overwrite those fields with values chosen according to
its own policy. If the PCC does not overwrite them, it MUST send a
PCErr message with Error-Type = 10 ("Reception of an invalid object")
and Error-Value 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 PCC MUST consider the entire ERO invalid and MUST
send a PCErr message with Error-Type = 10 ("Reception of an invalid
object") and Error-Value 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 Adjacency-SID, Adj-SID, then the L flag L-Flag MUST NOT be
set. If the L flag L-Flag is set for an Adjacency-SID, Adj-SID, then the PCC MUST send a
PCErr message with Error-Type = 10 ("Reception of an invalid object")
and Error-Value 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 it MUST send a
PCErr message with Error-Type = 10 ("Reception of an invalid object")
and Error-Value 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 it MUST send a PCErr message with
Error-Type = 10 ("Reception of an invalid object") and Error-Value 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 PCC MUST NOT update the LSP.
5.2.2. Interpreting the SR-ERO
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.
* If the subobjects contain SID index values, then the PCC converts
them into the corresponding MPLS labels by following the procedure
defined in [RFC8660].
* If the subobjects contain NAIs only, the PCC first converts each
NAI into a SID index value and then proceeds as above. To convert
an NAI to a SID index, the PCC looks for a fully specified prefix
or adjacency matching the fields in the NAI. If the PCC finds a
matching prefix/adjacency, and the matching prefix/adjacency has a
SID associated with it, then the PCC uses that SID. If the PCC
cannot find a matching prefix/adjacency, or if the matching
prefix/adjacency has no SID associated with it, the PCC behaves as
specified in Section 5.2.2.1.
* If the subobjects contain MPLS labels, then the PCC looks up the
offset of the first subobject's label in its SRGB or SRLB. This
gives the first SID. The PCC pushes the labels in any remaining
subobjects onto the packet (with the final subobject specifying
the bottom-of-stack label).
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.
5.2.2.1. Handling Errors During SR-ERO Conversion
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 the PCC cannot find a SID index in the SR-DB, it MUST send a
PCErr message with Error-Type = 10 ("Reception of an invalid
object") and Error-Value Error-value = 14 ("Unknown SID").
* If the PCC cannot find an NAI in the SR-DB, it MUST send a PCErr
message with Error-Type = 10 ("Reception of an invalid object")
and Error-Value Error-value = 15 ("NAI cannot be resolved to a SID").
* If the PCC needs to convert a SID into an MPLS label value but
cannot find the corresponding router's SRGB in the SR-DB, it MUST
send a PCErr message with Error-Type = 10 ("Reception of an
invalid object") and Error-Value Error-value = 16 ("Could not find SRGB").
* If the PCC finds that a router's SRGB is not large enough for a
SID index value, it MUST send a PCErr message with Error-Type = 10
("Reception of an invalid object") and Error-Value Error-value = 17 ("SID
index exceeds SRGB size").
* If the PCC needs to convert a SID into an MPLS label value but
cannot find the corresponding router's SRLB in the SR-DB, it MUST
send a PCErr message with Error-Type = 10 ("Reception of an
invalid object") and Error-Value Error-value = 18 ("Could not find SRLB").
* If the PCC finds that a router's SRLB is not large enough for a
SID index value, it MUST send a PCErr message with Error-Type = 10
("Reception of an invalid object") and Error-Value Error-value = 19 ("SID
index exceeds SRLB size").
* If the number of labels in the computed label stack exceeds the
maximum number of SIDs that the PCC can impose on the packet, it
MUST send a PCErr message with Error-Type = 10 ("Reception of an
invalid object") and Error-Value Error-value = 3 ("Unsupported number of
Segment SR-
ERO subobjects").
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 PCC MUST NOT
update the LSP.
5.3. RRO Processing
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, it MUST consider the entire RRO invalid and send a
PCErr message with Error-Type = 10 ("Reception of an invalid object")
and Error-Value 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 it MUST send a
PCErr message with Error-Type = 10 ("Reception of an invalid object")
and Error-Value 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 it MUST send a PCErr message with
Error-Type = 10 ("Reception of an invalid object") and Error-Value Error-value =
20 ("Inconsistent SIDs in SR-ERO / SR-RRO subobjects").
6. Management Considerations
This document adds a new path setup type to PCEP to allow LSPs to be
set up using segment routing Segment Routing techniques. This path setup type may be
used with PCEP alongside other path setup types, such as RSVP-TE, or
it may be used exclusively.
6.1. Controlling the Path Setup Type
The following factors control which path setup type is used for a
given LSP.
* The available path setup types are constrained to those that are
supported by, or enabled on, the PCEP speakers. The PATH-SETUP-
TYPE-CAPABILITY TLV indicates which path setup types a PCEP
speaker supports. To use segment routing Segment Routing as a path setup type, it
is a prerequisite that the PCC and PCE both include PST=1 in the
list of supported path setup types in this TLV and also include
the SR-PCE-CAPABILITY sub-TLV.
* When a PCE initiates an LSP, it proposes which path setup type to
use by including it in the PATH-SETUP-TYPE TLV in the SRP object
of the PCInitiate message. The PCE chooses the path setup type
based on the capabilities of the network nodes on the path and on
its local policy. The PCC MAY choose to accept the proposed path
setup type or to reject the PCInitiate request, based on its local
policy.
* When a PCC requests a path for an LSP, it can nominate a preferred
path setup type by including it in the PATH-SETUP-TYPE TLV in the
RP object of the PCReq message. The PCE MAY choose to reply with
a path of the requested type, reply with a path of a different
type, or reject the request, based on the capabilities of the
network nodes on the path and on its local policy.
The operator can influence the path setup type as follows.
* Implementations MUST allow the operator to enable and disable the
segment routing
Segment Routing path setup type on a PCEP-speaking device.
Implementations MAY also allow the operator to enable and disable
the RSVP-TE path setup type.
* PCE implementations MUST allow the operator to specify that an LSP
should be instantiated using segment routing Segment Routing or RSVP-TE as the
proposed path setup type.
* PCE implementations MAY allow the operator to configure a
preference for the PCE to propose paths using segment routing Segment Routing or
RSVP-TE in the absence of a specified path setup type.
* PCC implementations MUST allow the operator to specify that a path
requested for an LSP nominates segment routing Segment Routing or RSVP-TE as the
path setup type.
* PCC implementations MAY allow the operator to configure a
preference for the PCC to nominate segment routing Segment Routing or RSVP-TE as
the path setup type if none is specified for an LSP.
* PCC implementations SHOULD allow the operator to configure a PCC
to refuse to set up an LSP using an undesired path setup type.
6.2. Migrating a Network to Use PCEP Segment-Routed Paths
This section discusses the steps that the operator takes when
migrating a network to enable PCEP to set up paths using segment
routing Segment
Routing as the path setup type.
* The operator enables the segment routing Segment Routing PST on the PCE servers.
* The operator enables the segment routing Segment Routing PST on the PCCs.
* The operator resets each PCEP session. The PCEP sessions come
back up with segment routing Segment Routing enabled.
* If the operator detects a problem, they can roll the network back
to its initial state by disabling the segment routing Segment Routing PST on the
PCEP speakers and resetting the PCEP sessions.
Note that the data plane is unaffected if a PCEP session is reset.
Any LSPs that were set up before the session reset will remain in
place and will still be present after the session comes back up.
An implementation SHOULD allow the operator to manually trigger a
PCEP session to be reset.
An implementation MAY automatically reset a PCEP session when an
operator reconfigures the PCEP speaker's capabilities. However, note
that if the capabilities at both ends of the PCEP session are not
reconfigured simultaneously, then the session could be reset twice,
which could lead to unnecessary network traffic. Therefore, such
implementations SHOULD allow the operator to override this behavior
and wait instead for a manual reset.
Once segment routing Segment Routing is enabled on a PCEP session, it can be used as
the path setup type for future LSPs.
User traffic is not automatically migrated from existing LSPs onto
segment-routed LSPs just by enabling the segment routing Segment Routing PST in PCEP.
The migration of user traffic from existing LSPs onto segment routing Segment Routing
LSPs is beyond the scope of this document.
6.3. Verification of Network Operation
The operator needs the following information to verify that PCEP is
operating correctly with respect to the segment routing Segment Routing path setup
type.
* An implementation SHOULD allow the operator to view whether the
PCEP speaker sent the segment routing Segment Routing PST capability to its peer.
If the PCEP speaker is a PCC, then the implementation SHOULD also
allow the operator to view the values of the L L-Flag and N flags N-Flag
that were sent and the value of the MSD field that was sent.
* An implementation SHOULD allow the operator to view whether the
peer sent the segment routing Segment Routing PST capability. If the peer is a
PCC, then the implementation SHOULD also allow the operator to
view the values of the L L-Flag and N flags N-Flag and MSD fields that the
peer sent.
* An implementation SHOULD allow the operator to view whether the
segment routing
Segment Routing PST is enabled on the PCEP session.
* If one PCEP speaker advertises the segment routing Segment Routing PST capability,
but the other does not, then the implementation SHOULD create a
log to inform the operator of the capability mismatch.
* An implementation SHOULD allow the operator to view the PST that
was proposed, or requested, for an LSP and the PST that was
actually used.
* If a PCEP speaker decides to use a different PST to the one that
was proposed, or requested, for an LSP, then the implementation
SHOULD create a log to inform the operator that the expected PST
has not been used. The log SHOULD give the reason for this choice
(local policy, equipment capability, etc.).
* If a PCEP speaker rejects a segment routing Segment Routing path, then it SHOULD
create a log to inform the operator, giving the reason for the
decision (local policy, MSD exceeded, etc.).
6.4. Relationship to Existing Management Models
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: Segment Routing:
* The advertised PST capabilities and MSD per PCEP session.
* The PST configured for, and used by, each LSP.
The PCEP MIB [RFC7420] could also be updated to include this
information.
7. Security Considerations
The security considerations described in [RFC5440], [RFC8231],
[RFC8281], and [RFC8408] are applicable 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
(see Section 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.
8. IANA Considerations
8.1. PCEP ERO and RRO Subobjects
This document defines a new subobject type for the PCEP ERO and a new
subobject type for the PCEP RRO. The code points 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 |
+----------------+------------------------+----------------+
Table 1
8.2. New NAI Type Registry
IANA has created a new sub-registry within the "Path Computation
Element Protocol (PCEP) Numbers" registry called "PCEP SR-ERO NAI
Types". The allocation policy for this new registry is by IETF
Review [RFC8126]. The new registry contains the following 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 | This document |
| | global IPv6 addresses. | |
+-------+-------------------------------+---------------+
| 5 | NAI is an unnumbered | This document |
| | adjacency with IPv4 node IDs. | |
+-------+-------------------------------+---------------+
| 6 | NAI is an IPv6 adjacency with | This document |
| | link-local IPv6 addresses. | |
+-------+-------------------------------+---------------+
| 7-15 | Unassigned | |
+-------+-------------------------------+---------------+
Table 2
8.3. New SR-ERO Flag Registry
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:
* Bit number (counting from bit 0 as the most significant bit)
* Capability description
* Defining RFC
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 | This document |
| | addition to an MPLS label (C) | |
+-----+---------------------------------+---------------+
| 11 | SID specifies an MPLS label (M) | This document |
+-----+---------------------------------+---------------+
Table 3
8.4. PCEP-Error Object
IANA has allocated the following code points codepoints in the "PCEP-ERROR Object
Error Types and Values" registry for the following new error-
values: Error-values:
+------------+-----------------+---------------------------------+
| Error-Type | Meaning | Error-Value 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-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 |
+------------+-----------------+---------------------------------+
Table 4
8.5. PCEP TLV Type Indicators
IANA has allocated the following code point 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. See Appendix A for
details.
+-------+--------------------------------+---------------+
| Value | Meaning | Reference |
+=======+================================+===============+
| 26 | SR-PCE-CAPABILITY (deprecated) | This document |
+-------+--------------------------------+---------------+
Table 5
8.6. PATH-SETUP-TYPE-CAPABILITY Sub-TLV Type Indicators
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 |
+-------+-------------------+---------------+
Table 6
8.7. New Path Setup Type
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 code point codepoint within this registry,
as follows:
+-------+-------------------------------+-----------+
| Value | Description | Reference |
+=======+===============================+===========+
| 1 | Traffic-engineering path is | This |
| | set up using Segment Routing. | document |
+-------+-------------------------------+-----------+
Table 7
8.8. New Metric Type
IANA has allocated the following code point codepoint in the PCEP "METRIC Object
T Field" registry:
+-------+-------------------------+---------------+
| Value | Description | Reference |
+=======+=========================+===============+
| 11 | Segment-ID (SID) Depth. | This document |
+-------+-------------------------+---------------+
Table 8
8.9. SR PCE Capability Flags
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:
* Bit number (counting from bit 0 as the most significant bit)
* Capability description
* Defining RFC
The following values are defined in this document:
+-----+------------------------------+-----------+
| Bit | Description | Reference |
+=====+==============================+===========+
| 0-5 | Unassigned | |
+-----+------------------------------+-----------+
| 6 | Node or Adjacency Identifier | This |
| | (NAI) is supported (N) | document |
+-----+------------------------------+-----------+
| 7 | Unlimited Maximum SID Depth | This |
| | (X) | document |
+-----+------------------------------+-----------+
Table 9
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<https://www.rfc-editor.org/info/rfc3032>.
[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231,
DOI 10.17487/RFC8231, September 2017,
<https://www.rfc-editor.org/info/rfc8231>.
[RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for PCE-Initiated LSP Setup in a Stateful PCE
Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
<https://www.rfc-editor.org/info/rfc8281>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8408] Sivabalan, S., Tantsura, J., Minei, I., Varga, R., and J.
Hardwick, "Conveying Path Setup Type in PCE Communication
Protocol (PCEP) Messages", RFC 8408, DOI 10.17487/RFC8408,
July 2018, <https://www.rfc-editor.org/info/rfc8408>.
[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
DOI 10.17487/RFC8491, November 2018,
<https://www.rfc-editor.org/info/rfc8491>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, October December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
9.2. Informative References
[IPv6-SRH] Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
Matsushima, S., and d. daniel.voyer@bell.ca, D. Voyer, "IPv6 Segment Routing Header
(SRH)", Work in Progress, Internet-Draft,
draft-ietf-6man-segment-routing-header-25, 10 draft-ietf-6man-
segment-routing-header-26, 22 October 2019, <https://tools.ietf.org/html/draft-ietf-6man-
segment-routing-header-25>.
<https://tools.ietf.org/html/draft-ietf-6man-segment-
routing-header-26>.
[MSD-BGP] Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G.,
and N. Triantafillis, "Signaling MSD (Maximum SID Depth)
using Border Gateway Protocol Link-State", Work in
Progress, Internet-Draft, draft-ietf-idr-bgp-ls-segment-
routing-msd-09, 15 October 2019,
<https://tools.ietf.org/html/draft-ietf-idr-bgp-ls-
segment-routing-msd-09>.
[PCE-PCEP-YANG]
Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
YANG Data Model for Path Computation Element
Communications Protocol (PCEP)", Work in Progress,
Internet-Draft, draft-ietf-pce-pcep-yang-12, 1 July draft-ietf-pce-pcep-yang-13, 31 October
2019,
<https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-12>.
<https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-13>.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[RFC4657] Ash, J., Ed. and J.L. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, DOI 10.17487/RFC4657, September
2006, <https://www.rfc-editor.org/info/rfc4657>.
[RFC7420] Koushik, A., Stephan, E., Zhao, Q., King, D., and J.
Hardwick, "Path Computation Element Communication Protocol
(PCEP) Management Information Base (MIB) Module",
RFC 7420, DOI 10.17487/RFC7420, December 2014,
<https://www.rfc-editor.org/info/rfc7420>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8413] Zhuang, Y., Wu, Q., Chen, H., and A. Farrel, "Framework
for Scheduled Use of Resources", RFC 8413,
DOI 10.17487/RFC8413, July 2018,
<https://www.rfc-editor.org/info/rfc8413>.
[RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
"Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
DOI 10.17487/RFC8476, December 2018,
<https://www.rfc-editor.org/info/rfc8476>.
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, October December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, October December 2019,
<https://www.rfc-editor.org/info/rfc8667>.
[SR-POLICY]
Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d.,
bogdanov@google.com, b., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", Work in
Progress, Internet-Draft,
draft-ietf-spring-segment-routing-policy-03, 12 May draft-ietf-spring-segment-
routing-policy-05, 17 November 2019,
<https://tools.ietf.org/html/draft-ietf-spring-segment-
routing-policy-03>.
routing-policy-05>.
Appendix A. Compatibility with Early Implementations
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.
Acknowledgements
We thank Ina Minei, George Swallow, Marek Zavodsky, Dhruv Dhody, Ing-
Wher Chen, and Tomas Janciga for the valuable comments.
Contributors
The following people contributed to this document:
* Lakshmi Sharma
* Jan Medved
* Edward Crabbe
* Robert Raszuk
* Victor Lopez
Authors' Addresses
Siva Sivabalan
Cisco Systems, Inc.
2000 Innovation Drive
Kanata Ontario K2K 3E8
Canada
Email: msiva@cisco.com
Clarence Filsfils
Cisco Systems, Inc.
Pegasus Parc
Brabant 1831 De kleetlaan 6a
Belgium
Email: cfilsfil@cisco.com
Jeff Tantsura
Apstra, Inc.
333 Middlefield Rd #200
Menlo Park, CA 94025
United States of America
Email: jefftant.ietf@gmail.com
Wim Henderickx
Nokia
Copernicuslaan 50
95134 Antwerp 2018
Belgium
Email: wim.henderickx@alcatel-lucent.com wim.henderickx@nokia.com
Jon Hardwick
Metaswitch Networks
100 Church Street
Enfield
United Kingdom
Email: jonathan.hardwick@metaswitch.com