wdiff draft-ietf-pce-segment-routing-12.txt draft-ietf-pce-segment-routing-13.txt

PCE S. Sivabalan
Internet-Draft C. Filsfils
Intended status: Standards Track Cisco Systems, Inc.
Expires: December 31, 2018 April 15, 2019 J. Tantsura
Individual
W. Henderickx
Nokia
J. Hardwick
Metaswitch Networks
June 29,
October 12, 2018

PCEP Extensions for Segment Routing
draft-ietf-pce-segment-routing-12
draft-ietf-pce-segment-routing-13

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). A Segment Routed Path can
be derived from a variety of mechanisms, including an IGP Shortest
Path Tree (SPT), explicit configuration, or a Path Computation
Element (PCE). This document specifies extensions to the Path
Computation Element Communication Protocol (PCEP) that allow a
stateful PCE to compute and initiate Traffic Engineering (TE) paths,
as well as a PCC to request a path subject to certain constraints and
optimization criteria in SR networks.

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.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on December 31, 2018. April 15, 2019.

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.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 5
3. Overview of PCEP Operation in SR Networks . . . . . . . . . . 5
4. SR-Specific PCEP Message Extensions . . . . . . . . . . . . . 7
5. Object Formats . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. The OPEN Object . . . . . . . . . . . . . . . . . . . . . 7
5.1.1. The SR PCE Capability sub-TLV . . . . . . . . . . . . 7
5.2. The RP/SRP Object . . . . . . . . . . . . . . . . . . . . 8
5.3. ERO . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 9
5.3.1. SR-ERO Subobject . . . . . . . . . . . . . . . . . . 9
5.3.2. NAI Associated with SID . . . . . . . . . . . . . . . 11
5.4. RRO . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.5. METRIC Object . . . . . . . . . . . . . . . . . . . . . . 13
6. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 13 14
6.1. Exchanging the SR PCE Capability . . . . . . . . . . . . 13 14
6.2. ERO Processing . . . . . . . . . . . . . . . . . . . . . 15
6.2.1. SR-ERO Validation . . . . . . . . . . . . . . . . . . 15
6.2.2. Interpreting the SR-ERO . . . . . . . . . . . . . . . 17
6.3. RRO Processing . . . . . . . . . . . . . . . . . . . . . 20 19
7. Backward Compatibility . . . . . . . . . . . . . . . . . . . 20 19
8. Management Considerations . . . . . . . . . . . . . . . . . . 21 20
8.1. Controlling the Path Setup Type . . . . . . . . . . . . . 21 20
8.2. Migrating a Network to Use PCEP Segment Routed Paths . . 22 21
8.3. Verification of Network Operation . . . . . . . . . . . . 23 22
8.4. Relationship to Existing Management Models . . . . . . . 24 23
9. Security Considerations . . . . . . . . . . . . . . . . . . . 24 23
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
10.1. PCEP Objects . . . . . . . . ERO and RRO subobjects . . . . . . . . . . . . . . 24
10.2. New NAI Type Registry . . . . . . . . . . . . . . . . . 24
10.3. New SR-ERO Flag Registry . . . . . . . . . . . . . . . . 25 24
10.4. PCEP-Error Object . . . . . . . . . . . . . . . . . . . 25
10.5. PCEP TLV Type Indicators . . . . . . . . . . . . . . . . 27 26
10.6. New Path Setup Type . . . . . . . . . . . . . . . . . . 27 26
10.7. New Metric Type . . . . . . . . . . . . . . . . . . . . 27
10.8. SR PCE Capability Flags . . . . . . . . . . . . . . . . 27
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 28 27
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
13.1. Normative References . . . . . . . . . . . . . . . . . . 28
13.2. Informative References . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31 30

1. Introduction

Segment Routing (SR) technology leverages the source routing and
tunneling paradigms. A source node can choose a path without relying
on hop-by-hop signaling protocols such as LDP or RSVP-TE. Each path
is specified as a set of "segments" advertised by link-state routing
protocols (IS-IS or OSPF). [I-D.ietf-spring-segment-routing] [RFC8402] provides an introduction to the
SR architecture. The corresponding IS-IS and OSPF extensions are
specified in [I-D.ietf-isis-segment-routing-extensions] and
[I-D.ietf-ospf-segment-routing-extensions], respectively. The SR
architecture defines a "segment" as a piece of information advertised
by a link-state routing protocols, e.g., an IGP prefix or an IGP
adjacency. Several types of segments are defined. A Node segment
represents an ECMP-aware shortest-path computed by IGP to a specific
node, and is always identified uniquely within the SR/IGP domain. An
Adjacency Segment represents a unidirectional adjacency. An
Adjacency Segment is local to the node which advertises it. Both
Node segments and Adjacency segments can be used for SR Traffic
Engineering (SR-TE).

The SR architecture can be implemented using either an MPLS
forwarding plane [I-D.ietf-spring-segment-routing-mpls] or an IPv6
forwarding plane [I-D.ietf-6man-segment-routing-header]. 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" denote Node Segment
Identifier and Adjacency Segment Identifier respectively.

A Segment Routed path (SR path) can be derived from an IGP Shortest
Path Tree (SPT). SR-TE paths may not follow 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 LSPs (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] and defines
objects and TLVs for MPLS-TE LSPs. Stateful PCEP extensions provide
synchronization of LSP state between a PCC and a PCE or between a
pair of PCEs, delegation of LSP control, reporting of LSP state from
a PCC to a PCE, controlling the setup and path routing of an LSP from
a PCE to a PCC. Stateful PCEP extensions are intended for an
operational model in which LSPs are configured on the PCC, and
control over them is delegated to the PCE.

A mechanism to dynamically initiate LSPs on a PCC based on the
requests from a stateful PCE or a controller using stateful PCE is
specified in [RFC8281]. This mechanism is useful in Software Defined
Networking (SDN) applications, such as on-demand engineering, or
bandwidth calendaring.

It is possible to use a stateful PCE for computing one or more SR-TE
paths taking into account various constraints and objective
functions. Once a path is chosen, the stateful PCE can initiate an
SR-TE path on a PCC using PCEP extensions specified in [RFC8281]
using the SR specific PCEP extensions specified in this document.
Additionally, using procedures described in this document, a PCC can
request an SR path from either a stateful or a stateless PCE.

This specification relies on the procedures specified in
[I-D.ietf-pce-lsp-setup-type] [RFC8408] to
exchange the segment routing capability and to specify that the path
setup type of an LSP is segment routing.

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
[I-D.ietf-spring-segment-routing-policy].

2. Terminology

The following terminologies are used in this document:

ERO: Explicit Route Object

IGP: Interior Gateway Protocol

IS-IS: Intermediate System to Intermediate System

LSR: Label Switching Router

MSD: Maximum SID Depth

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 (as defined in
[I-D.ietf-spring-segment-routing-policy])

SR-TE: Segment Routed Routing Traffic Engineering

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. In
SR networks, an 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). SR-TE paths computed
by a PCE can be represented in an ERO in one of the following forms:

o An ordered set of IP addresses representing network nodes/links:
In this case, the PCC needs to convert the IP addresses into the
corresponding MPLS labels by consulting its Link State Database
(LSDB). nodes/links.

o An ordered set of SIDs, with or without the corresponding IP
addresses.

o An ordered set of MPLS labels and labels, with or without corresponding IP addresses: In this case, the
address.

The PCC needs to convert the IP addresses converts these into the corresponding SIDs
by consulting its LSDB. an MPLS label stack and next hop, as
described in Section 6.2.2.

This document defines a new ERO subobject denoted by "SR-ERO
subobject" capable of carrying a SID as well as the identity of the
node/adjacency represented by the SID. SR-capable PCEP speakers
should be able to generate and/or process such ERO subobject. An ERO
containing SR-ERO subobjects can be included in the PCEP Path
Computation Reply (PCRep) message defined in [RFC5440], the PCEP LSP
Initiate Request message (PCInitiate) defined in [RFC8281], as well
as in the PCEP LSP Update Request (PCUpd) and PCEP LSP State Report
(PCRpt) messages defined in [RFC8231].

When a PCEP session between a PCC and a PCE is established, both PCEP
speakers exchange their capabilites 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 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 can be updated 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 an 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:

o Defines a new ERO subobject, a new RRO subobject and new PCEP
error codes.

o Specifies how two PCEP speakers can establish a PCEP session that
can carry information about SR-TE paths.

o Specifies processing rules for the ERO subobject.

o Defines a new path setup type to be used in the PATH_SETUP_TYPE PATH-SETUP-TYPE
and PATH_SETUP_TYPE_CAPABILITY PATH-SETUP-TYPE-CAPABILITY TLVs
([I-D.ietf-pce-lsp-setup-type]). ([RFC8408]).

o Defines a new sub-TLV for the PATH_SETUP_TYPE_CAPABILITY 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., Path Computation Request, Path Computation Reply,
Path Computation Report, Path Computation Update, Path Computation
Initiate, etc.,) MUST be formatted according to [RFC5440], [RFC8231],
[RFC8281], and any other applicable PCEP specifications.

4. SR-Specific PCEP Message Extensions

As defined in [RFC5440], a PCEP message consists of a common header
followed by a variable length body made up of mandatory and/or
optional objects. This document does not require any changes in the
format of the PCReq and PCRep messages specified in [RFC5440],
PCInitiate message specified in [RFC8281], and PCRpt and PCUpd
messages specified in [RFC8231].

5. Object Formats

5.1. The OPEN Object

5.1.1. The SR PCE Capability sub-TLV

This document defines a new Path Setup Type (PST) for SR, as follows:

o PST = 1: Path is setup using Segment Routing Traffic Engineering.

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|L| MSD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: SR-PCE-CAPABILITY sub-TLV format

The code point for the TLV type is 26. The TLV length is 4 octets.

The 32-bit value is formatted as follows. The "Maximum SID Depth" (1
octet) field (MSD) specifies the maximum number of SIDs (MPLS label
stack depth in the context of this document) that a PCC is capable of
imposing on a packet. The "Reserved" (2 octets) field is unused, and

Reserved: MUST be set to zero on transmission by the sender and MUST be ignored on reception. The
"Flags" field is 1 octet long, and this by
the receiver.

Flags: This document defines the following flags:

o L flag: A PCC sets this flag bits. The other
bits MUST be set to 1 to indicate that it does not
impose any limit on zero by the MSD.

o N flag: 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.

* L: A PCC sets this 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 6.1 explains the
relationship between this field and the L bit.

5.2. The RP/SRP Object

To set up an SR-TE LSP using SR, the RP or SRP object MUST include
the PATH-SETUP-TYPE TLV, specified in [I-D.ietf-pce-lsp-setup-type], [RFC8408], with the PST set to
1 (path setup using SR-TE).

The LSP-IDENTIFIERS TLV MAY be present for the above PST type.

5.3. 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
'Node or Adjacency Identifier' (NAI). As described later, a NAI can
be represented in various formats (e.g., IPv4 address, IPv6 address,
etc). Furthermore, a NAI is used for troubleshooting purposes and,
if necessary, to derive SID value as described below.

The ERO specified in [RFC5440] is used to carry SR-TE path
information. In order to carry SID and/or NAI, this document defines
a new ERO subobject referred to as "SR-ERO subobject" whose format is
specified in the following section. An ERO carrying an SR-TE path
consists of one or more ERO subobjects, and MUST carry only SR-ERO
subobjects. Note that an SR-ERO subobject does not need to have both
SID and NAI. However, at least one of them MUST be present.

When building the MPLS label stack from ERO, a PCC MUST assume that
SR-ERO subobjects are organized as a last-in-first-out stack. The
first subobject relative to the beginning of ERO contains the
information about the topmost label. The last subobject contains
information about the bottommost label.

5.3.1. SR-ERO Subobject

An SR-ERO subobject consists of a 32-bit header followed by the SID
and/or the NAI associated with the SID. The SID is a 32-bit number.
The size of the NAI depends on its respective type, formatted as described shown in the following sections. At least one of the SID and the NAI MUST be
included in the SR-ERO subobject, and both MAY be included. 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 subobject format

The fields in the SR-ERO Subobject are as follows:

The 'L' Flag indicates 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 is set

Type: Set to 36.

Length contains

Length: Contains the total length of the subobject in octets,
including the L, Type and Length fields. The Length MUST be at
least 8, and MUST be a multiple of 4. As mentioned earlier, an An SR-ERO subobject MUST
contain at least one of a SID or an NAI. The length should
include the SID and NAI fields if and only if they are not absent.
The flags described below indicate whether the SID or NAI fields
are absent.

NAI Type (NT) indicates (NT): Indicates the type and format of the NAI associated
with the SID contained in
the object body. 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.

NT=5 The NAI is an unnumbered adjacency with IPv4 node IDs.

Flags is used

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 which acts as
represents an index into an MPLS label space. 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 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 up in its LSDB the SR-DB using
the NAI which, 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, and
otherwise MUST be set to zero. The S and F bits MUST NOT both
be set to 1.

SID is the

SID: The Segment Identifier.

NAI Depending on the M bit, it contains
either:

* A 4 octet index defining the offset into an MPLS label space
per [RFC8402].

* A 4 octet MPLS label, 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 of the SID and the NAI MUST be included in the SR-ERO
subobject, and both MAY be included.

5.3.2. NAI Associated with SID

This document defines the following NAIs:

'IPv4 Node ID' is specified as an IPv4 address. In this case, the
NT value is 1. 1 and the NAI field length is 4 octets.

'IPv6 Node ID' is specified as an IPv6 address. In this case, the
NT value is 2. 2 and the NAI field length is 16 octets.

'IPv4 Adjacency' is specified as a pair of IPv4 addresses. In this
case, the NT value is 3. 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 Adjacency' is specified as a pair of IPv6 addresses. In this
case, the NT value is 4. 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 bytes) octets) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Remote IPv6 address (16 bytes) octets) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: NAI for IPv6 adjacency

'Unnumbered Adjacency with IPv4 NodeIDs' is specified as a pair of
Node ID / Interface ID tuples. In this case, the NT value is 5. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Node-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: NAI for Unnumbered adjacency with IPv4 Node IDs

5.4. RRO

A PCC can record reports an SR-TE LSP and report the LSP to a PCE via by sending a PCRpt message, per
[RFC8231]. The RRO on this message represents the
RRO. 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 6: 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.

A PCC MUST assume that order the SR-RRO subobjects are organized such that the first subobject
relative to the beginning of the RRO contains identifies the
information about first segment
visited by the topmost label, SR-TE LSP, and the last subobject contains
information about identifies the bottommost label final
segment of the SR-TE LSP. LSP, that is, its endpoint.

5.5. METRIC Object

If a PCEP session is established with an MSD value of zero, then the

A PCC MAY specify the MSD for 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 purpose, as follows:

o T = 11: Maximum SID Depth of the requested path.

The

If the PCC sets includes a METRIC object of this type on a path
computation request, then the metric-value to PCE MUST minimize the MSD for this SID depth of the
computed path. The PCC MUST
set If the B (bound) bit is set to to 1 in the METRIC
object, which specifies that then the PCE MUST NOT return a path whose SID depth for exceeds
the computed path given metric-value. If the PCC did not set the L bit in its SR-
PCE-CAPABILITY TLV, then it MUST NOT exceed set the metric-value. B bit to 1. If the PCC set
the L bit 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. 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 with a
non-zero MSD value that
is greater than the session's default MSD, then it MUST consider the
request invalid and send a PCErr with Error-Type = 10 ("Reception of
an invalid object") and Error-Value 9 ("Default MSD is specified ("MSD exceeds the default for
the PCEP session").

6. Procedures

6.1. 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 of an invalid object) and Error-Value
TBD1 (to be assigned by IANA) (Missing PCE-SR-CAPABILITY sub-TLV) and
MUST 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 speaker MUST ignore
the SR-PCE-CAPABILITY sub-TLV.

If a PCC sets the N flag to 1, then the PCE MAY send NAI to the PCC
within the an SR-ERO
subobject containing NAI and no SID (see Section 6.2). Otherwise,
the PCE MUST NOT send an SR-ERO subobject containing NAI to the PCC. and no SID.

The number of SIDs that can be imposed on a packet depends on the
PCC's data plane's capability. If a PCC sets the L 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 L flag set to 1 then it MUST
ignore the MSD field and MUST assume that the sender can impose a SID
stack of any depth. If a PCC sets the L flag to zero, then it sets
the MSD field to the maximum number of SIDs that it can impose on a
packet. If a PCE receives an SR-PCE-CAPABILITY sub-TLV with the L
flag and MSD both set to zero then it MUST assume that the PCC is not
capable of imposing a SID stack of any depth and hence is not SR-TE
capable, unless it learns a non-zero MSD for the PCC through some
other means.

Note that the MSD value exchanged via the SR-PCE-CAPABILITY sub-TLV
indicates the SID/label imposition limit for the PCC node. However,
if a PCE learns the MSD value of a PCC node via different means, e.g
routing protocols, as specified in:
[I-D.ietf-isis-segment-routing-msd];

[I-D.ietf-ospf-segment-routing-msd];
[I-D.ietf-idr-bgp-ls-segment-routing-msd], then it ignores the MSD
value in the SR-PCE-CAPABILITY sub-TLV. Furthermore, whenever a PCE
learns the MSD for a link via different means, it MUST use that value
for that link regardless of the MSD value exchanged in the SR-PCE-
CAPABILITY sub-TLV.

Once an SR-capable PCEP session is established with a non-zero MSD
value, the corresponding PCE MUST NOT send SR-TE paths with 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 of an invalid object) and Error-Value 3
(Unsupported number of Segment 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 5.5.

The N flag, L 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 to zero, the L 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.

6.2. ERO Processing

6.2.1. 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,
the S bit, the F bit and the NT field are consistent, as follows.

o If NT=0, the F bit MUST be 1, the S bit MUST be zero and the
Length MUST be 8.

o 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.

o 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.

o 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.

o 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.

o 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 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 = 11 ("Malformed object").

If a PCC does not recognise 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 = TBD2 ("Unsupported NAI Type in Segment ERO 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 = 6
("Both SID and NAI are absent in 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 = 4 ("Unsupported
parameter").

If a PCC receives an SR-ERO subobject in which the S bit is set to 1
and either or both of the M or C bits 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 = 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 (that is, it represents 1, then the subobject contains an MPLS
label. The PCC MAY choose not to accept a label
value), its value (20 most significant bits) provided by the PCE,
based on it local policy. The PCC MUST be larger than 15,
unless NOT accept MPLS label value 3
(Implicit NULL), but it is a MAY accept other special purpose label, such as an Entropy Label
Indicator (ELI). MPLS label
values. If a the PCC receives decides not to accept an invalid MPLS label value, it
MUST send a PCErr message with Error-Type = 10 ("Reception of an
invalid object") and Error Value = 2 ("Bad label value").

If both M and C bits of an SR-ERO subobject are set to 1, and if a
PCC finds erroneous setting in one or more of TC, S, and TTL fields,
it MAY overwrite those fields with values chosen according to its own
policy. If the PCC does not overwite overwrite them, it MUST 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 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 = 11 ("Malformed object").

If the first SR-ERO represents an MPLS label value then the NAI field
MUST NOT be absent (that is, the F bit MUST be zero). The PCC needs
the NAI field to determine the first hop router in the segment routed
path. If the NAI is not present then the PCC MUST send a PCErr
message with Error-Type = 10 ("Reception of an invalid object") and
Error Value = TBD9 ("Cannot derive a next hop from SR-ERO").

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 (that is, it represents an index
value), zero, then the SID MUST be a node-SID, an adjacency-SID or a
binding-SID. If the SID is not one of these types, the PCC MUST send subobject contains a PCErr message with Error-Type = 10 ("Reception of an invalid
object") and Error Value = TBD10 ("Bad SID type in SR-ERO").
index value. If the SID is an Adjacency-SID then the L flag MUST NOT
be set. If the L flag is set for an Adjacency-SID then the PCC MUST
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 it MUST 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, a SID representing 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 = TBD11 TBD9 ("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.

6.2.2. Interpreting the SR-ERO

The PCC creates SR-ERO contains a segment routed path by converting the sequence of subobjects. According to
[I-D.ietf-spring-segment-routing-policy], each SR-ERO subobjects into 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 SR-ERO
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 following subsections explain how the PCC converts the SR-ERO subobject sequence to an MPLS label stack and uses a next hop.

6.2.2.1. SR-ERO subobjects contain MPLS Labels

If the SR-ERO subobjects contain SIDs with MPLS label values, then
proceed as follows:

(a) Initialize next_hop to null. Initialize label_stack to an empty
label stack.

(b) Get the first SR-ERO subobject from the ERO. Append its label
value to label_stack, setting the TC, S and TTL fields according
to the C bit and/or local policy. Set current_router and
next_hop different procedure to the router identified by the NAI. If the NAI is
absent from the first SR-ERO, then do this is an error, and conversion, depending
on the ERO
should have failed information that the validation checks of Section 6.2.1.

(c) Loop through PCE has provided in the remaining SR-ERO subobjects. For each SR-ERO
subobject, append it to label_stack, setting the TC, S and TTL
fields according to the C bit and/or local policy.

6.2.2.2. SR-ERO subobjects contain Index SIDs

o If the SR-ERO subobjects contain SIDs with SID index values, then proceed
as follows:

(a) Initialize current_router to the local router. Initialize
next_hop to null. Initialize label_stack to an empty label
stack.

(b) Get the first SR-ERO subobject from the ERO and look the SID
index up in the LSDB.

* If the SID is a node-SID, set current_router to the node
identified by the node-SID, compute the shortest path to that
node and set next_hop to the next hop from the shortest path.
If next_hop is PCC converts
them into the router identified corresponding MPLS labels by following the node-SID, and
that router advertised its node-SID with the P flag clear
(indicating that PHP is allowed), then do not add a label to
label_stack. Otherwise, look up the next_hop router's SRGB procedure
defined in the LSDB. Get the label that is at offset node-SID
relative to the SRGB base label and append it to label_stack.

* If the SID is an adjacency-SID, set next_hop to the
corresponding routing adjacency. Do not add a label the
label_stack. Set current_router to the adjacent router.

* [I-D.ietf-spring-segment-routing-mpls].

o If the SID is a binding-SID, subobjects contain NAI only, then append the binding SID's
associated label stack to label_stack. Set next_hop to the PCC first hop router in the binding SID tunnel. Set
current_router to the router that is the endpoint of the
binding-SID tunnel.

* Any other type of SID is an error, and the SR-ERO should have
failed the validation checks of Section 6.2.1.

(c) Loop through the remaining SR-ERO subobjects. For converts
each SR-ERO
subobject, look the NAI into a SID index value by looking it up in the LSDB.

* If the SID is a node-SID, then look up the current_router's
SRGB in the LSDB. Get the label that is at offset node-SID
relative to the SRGB base label its local
database, and append it to label_stack.

* then proceeds as above.

o If the SID is an adjacency-SID, subobjects contain MPLS labels, then look up the
current_router's SRLB in the LSDB. Get PCC looks up the label that is at
offset adjacency-SID relative to of the SRLB base first subobject's label and
append it to label_stack.

* If in its SRGB or SRLB. This
gives the SID is a binding-SID, then look up first SID. The PCC pushes the
current_router's SRGB labels in any remaining
subobjects onto the LSDB. Get packet (with the label that is at
offset binding-SID relative to final subobject specifying
the SRGB base label and append
it to label_stack.

* Any other type of SID is an error, bottom-of-stack label) and the SR-ERO should have
failed the validation checks of Section 6.2.1.

6.2.2.3. SR-ERO subobjects contain NAI only

If the SR-ERO subobjects do not contain SIDs (that is, contain only
NAI), then look each NAI up in directs the LSDB packet to find the corresponding SID
index. Then proceed as described above for SID index values.

6.2.2.4.
segment identified by the first SID.

6.2.2.1. 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.

o If the PCC cannot find a SID index in the LSDB, SR-DB, it MUST send a
PCErr message with Error-Type = 10 ("Reception of an invalid
object") and Error-Value = TBD3 ("Unknown SID").

o If the PCC cannot find an NAI in the LSDB, SR-DB, it MUST send a PCErr
message with Error-Type = 10 ("Reception of an invalid object")
and Error-Value = TBD4 ("NAI cannot be resolved to a SID").

o If the PCC needs to convert a SID into an MPLS label value but
cannot find an the corresponding router's SRGB in the LSDB, SR-DB, it MUST
send a PCErr message with Error-Type = 10 ("Reception of an
invalid object") and Error-Value = TBD5 ("Could not find SRGB").

o 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 = TBD6 ("SID
index exceeds SRGB size").

o If the PCC needs to convert a SID into an MPLS label value but
cannot find an the corresponding router's SRLB in the LSDB, SR-DB, it MUST
send a PCErr message with Error-Type = 10 ("Reception of an
invalid object") and Error-Value = TBD7 ("Could not find SRLB").

o 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 = TBD8 ("SID
index exceeds SRLB size").

o If the number of labels in label_stack 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 = 3 ("Unsupported number of
Segment 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.

6.3. 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 = 7 ("Both SID and NAI are absent in SR-RRO
subobject").

If a PCE detects that all the subobjects of the an RRO are not identical, a mixture of SR-
RRO subobjects and if subobjects of other types, then it does not support such MUST 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 a SID representing
an RRO, 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 = 10 ("Non-identical RRO TBD9 ("Inconsistent SIDs in SR-ERO
/ SR-RRO subobjects").

7. Backward Compatibility

A PCEP speaker that does not support the SR PCEP capability cannot
recognize the SR-ERO or SR-RRO subobjects. As such, it responds
according to the rules for a malformed object, per [RFC5440].

Some implementations, which are compliant with an earlier version of
this specification, do not send the PATH-SETUP-TYPE-CAPABILITY TLV in
their OPEN objects. Instead, to indicate that they support SR, these
implementations include the SR-CAPABILITY-TLV as a top-level TLV in
the OPEN object. Unfortunately, some of these implementations made
it into the field before this document was published in its final
form. Therefore, if a PCEP speaker receives an OPEN object in which
the SR-CAPABILITY-TLV appears as a top-level TLV, then it MUST
interpret this 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) and 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 MUST ignore the top-level SR-CAPABILITY-TLV and process
only the PATH-SETUP-TYPE-CAPABILITY TLV.

8. Management Considerations

This document adds a new path setup type to PCEP to allow LSPs to be
set up using 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.

8.1. Controlling the Path Setup Type

The following factors control which path setup type is used for a
given LSP.

o 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 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.

o 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.

o 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 PCInitiate PCReq message. The PCE MAY choose to reply with
a path of the requested type, or to reply with a path of a
different type, or to 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.

o Implementations MUST allow the operator to enable and disable the
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.

o PCE implementations MUST allow the operator to specify that an LSP
should be instantiated using segment routing or RSVP-TE as the
proposed path setup type.

o PCE implementations MAY allow the operator to configure a
preference for the PCE to propose paths using segment routing or
RSVP-TE in the absence of a specified path setup type.

o PCC implementations MUST allow the operator to specify that a path
requested for an LSP nominates segment routing or RSVP-TE as the
path setup type.

o PCC implementations MAY allow the operator to configure a
preference for the PCC to nominate segment routing or RSVP-TE as
the path setup type if none is specified for an LSP.

o PCC implementations SHOULD allow the operator to configure a PCC
to refuse to set up an LSP using an undesired path setup type.

8.2. 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 as the path setup type.

o The operator enables the segment routing PST on the PCE servers.

o The operator enables the segment routing PST on the PCCs.

o The operator resets each PCEP session. The PCEP sessions come
back up with segment routing enabled.

o If the operator detects a problem, they can roll the network back
to its initial state by disabling the 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 behaviour
and wait instead for a manual reset.

Once 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 be migrated from existing LSPs onto
segment routed LSPs just by enabling the segment routing PST in PCEP.
The migration of user traffic from existing LSPs onto segment routing
LSPs is beyond the scope of this document.

8.3. Verification of Network Operation

The operator needs the following information to verify that PCEP is
operating correctly with respect to the segment routing path setup
type.

o An implementation SHOULD allow the operator to view whether the
PCEP speaker sent the 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 value values of the L flag and N flags that was
were sent, and the value of the MSD field that was sent.

o An implementation SHOULD allow the operator to view whether the
peer sent a the 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 flag and N flags and MSD fields that the peer sent
sent.

o An implementation SHOULD allow the operator to view whether the
segment routing PST is enabled on the PCEP session.

o If one PCEP speaker advertises the segment routing PST capability,
but the other does not, then the implementation SHOULD create a
log to inform the operator of the capability mismatch.

o 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.

o 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.)

o If a PCEP speaker rejects a segment routed path, then it SHOULD
create a log to inform the operator, giving the reson reason for the
decision (local policy, MSD exceeded etc.)

8.4. Relationship to Existing Management Models

The PCEP YANG module [I-D.ietf-pce-pcep-yang] should include:

o advertised PST capabilities and MSD per PCEP session.

o the PST configured for, and used by, each LSP.

The PCEP MIB [RFC7420] could also be updated to include this
information.

9. Security Considerations

The security considerations described in [RFC5440], [RFC8281] and
[I-D.ietf-pce-lsp-setup-type]
[RFC8408] are applicable to this specification. No additional
security measure is 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] and [RFC8281]
are not used, because an attacker could create a path which 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 5.1.1) which discloses how many MPLS labels the sender
can push onto packets that it forwards into the network. If the
security mechanisms of [RFC5440] and [RFC8281] are not used then an
attacker could use this new field to gain intelligence about the
capabilities of the edge devices in the network.

10. IANA Considerations

10.1. PCEP Objects ERO and RRO subobjects

This document defines a new subobject type for the PCEP explicit
route object (ERO), and a new subobject type for the PCEP record
route object (RRO). The code points for subobject types of these
objects is maintained in the RSVP parameters registry, under the
EXPLICIT_ROUTE and ROUTE_RECORD objects. IANA is requested to
confirm the early allocation of the following code points in the RSVP
Parameters registry for each of the new subobject types defined in
this document.

Object Subobject Subobject Type
--------------------- -------------------------- ------------------
EXPLICIT_ROUTE SR-ERO (PCEP-specific) 36
ROUTE_RECORD SR-RRO (PCEP-specific) 36

10.2. New NAI Type Registry

IANA is requested to create 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
should be by IETF Review. The new registry should contain 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. This document
5 NAI is an unnumbered This document
adjacency with IPv4 node IDs.

10.3. New SR-ERO Flag Registry

IANA is requested to create 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:

o Bit number (counting from bit 0 as the most significant bit)

o Capability description
o 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 This document
and TTL in addition
to an MPLS label (C)
11 SID specifies an MPLS This document
label (M)

10.4. PCEP-Error Object

IANA is requested to confirm the early allocation of the code-points
in the PCEP-ERROR Object Error Types and Values registry for the
following new error-values:

Error-Type Meaning
---------- -------
10 Reception of an invalid object.

Error-value = 2: Bad label value
Error-value = 3: Unsupported number
of SR-ERO
subobjects
Error-value = 4: Bad label format
Error-value = 5: ERO mixes SR-ERO
subobjects with
other subobject
types
Error-value = 6: Both SID and NAI
are absent in SR-
ERO subobject
Error-value = 7: Both SID and NAI
are absent in SR-
RRO subobject
Error-value = 9: Default MSD is
specified exceeds the
default for the
PCEP session
Error-value = 10: RRO mixes SR-RRO
subobjects with
other subobject
types

Error-value = TBD1: Missing PCE-SR-
CAPABILITY sub-TLV
Error-value = TBD2: Unsupported NAI
Type in SR-ERO
subobject
Error-value = TBD3: Unknown SID
Error-value = TBD4: NAI cannot be
resolved to a SID
Error-value = TBD5: Could not find SRGB
Error-value = TBD6: SID index exceeds
SRGB size
Error-value = TBD7: Could not find SRLB
Error-value = TBD8: SID index exceeds
SRLB size
Error-value = TBD9: Cannot derive a
next hop from SR-
ERO
Error-value = TBD10: Bad SID type in SR-
ERO
Error-value = TBD11: Inconsistent SIDs
in SR-ERO / SR-RRO
subobjects

Note to IANA: this draft originally had an early allocation for
Error-value=11 (Malformed object) in the above list. However, we
have since moved the definition of that code point to draft-ietf-pce-
lsp-setup-type and we included an instruction in that draft for you
to update the reference in the indicated registry. Please ensure
that this has happened when you process the present draft. RFC8408.

Note to IANA: some Error-values in the above list were defined after
the early allocation took place, and so do not currently have a code
point assigned. Please assign code points from the indicated
registry and replace each instance of "TBD1", "TBD2" etc. in this
document with the respective code points.

Note to IANA: some of the Error-value descriptive strings above have
changed since the early allocation. Please refresh the registry.

10.5. PCEP TLV Type Indicators

IANA is requested to confirm the early allocation of the following
code point in the PCEP TLV Type Indicators registry.

Value Meaning Reference
------------------------- ---------------------------- --------------
26 SR-PCE-CAPABILITY This document

10.6. New Path Setup Type

[I-D.ietf-pce-lsp-setup-type]

[RFC8408] requests that IANA creates a sub-
registry sub-registry within the "Path
Computation Element Protocol (PCEP) Numbers" registry called "PCEP
Path Setup Types". IANA is requested to allocate a new code point
within this registry, as follows:

Value Description Reference
------------------------- ---------------------------- --------------
1 Traffic engineering path is This document
setup using Segment Routing.

10.7. New Metric Type

IANA is requested to confirm the early allocation of the following
code point in the PCEP METRIC object T field registry:

Value Description Reference
------------------------- ---------------------------- --------------
11 Segment-ID (SID) Depth. This document

10.8. SR PCE Capability Flags

IANA is requested to create 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:

o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC

The following values are defined in this document:

Bit Description Reference

0-5 Unassigned
6 Node or Adjacency This document
Identifier (NAI) is
supported (N)
7 Unlimited Maximum SID This document
Depth (L)

11. Contributors

The following people contributed to this document:

- Lakshmi Sharma
- Jan Medved
- Edward Crabbe
- Robert Raszuk
- Victor Lopez

12. Acknowledgements

We thank Ina Minei, George Swallow, Marek Zavodsky, Dhruv Dhody, Ing-
Wher Chen and Tomas Janciga for the valuable comments.

13. References

13.1. Normative References

[I-D.ietf-idr-bgp-ls-segment-routing-msd]
Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
"Signaling Maximum MSD (Maximum SID Depth Depth) using Border Gateway
Protocol Link-State", draft-ietf-idr-bgp-ls-segment-routing-msd-01 draft-ietf-idr-bgp-ls-segment-
routing-msd-02 (work in progress), October 2017. August 2018.

[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
Gredler, H., Litkowski, S., Decraene, B., and J. Tantsura,
"IS-IS Extensions for Segment Routing", draft-ietf-isis-
segment-routing-extensions-18
segment-routing-extensions-19 (work in progress), June July
2018.

[I-D.ietf-isis-segment-routing-msd]
Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling MSD (Maximum SID Depth) using IS-IS", draft-
ietf-isis-segment-routing-msd-12
ietf-isis-segment-routing-msd-16 (work in progress), May
September 2018.

[I-D.ietf-ospf-segment-routing-extensions]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", draft-ietf-ospf-segment-
routing-extensions-25 (work in progress), April 2018.

[I-D.ietf-ospf-segment-routing-msd]
Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
"Signaling MSD (Maximum SID Depth) using OSPF", draft-
ietf-ospf-segment-routing-msd-14
ietf-ospf-segment-routing-msd-20 (work in progress), May
2018.

[I-D.ietf-pce-lsp-setup-type]
Sivabalan, S., Tantsura, J., Minei, I., Varga, R., and J.
Hardwick, "Conveying path setup type in PCEP messages",
draft-ietf-pce-lsp-setup-type-10 (work in progress), May
August 2018.

[I-D.ietf-pce-pcep-yang]
Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
YANG Data Model for Path Computation Element
Communications Protocol (PCEP)", draft-ietf-pce-pcep-
yang-08 (work in progress), June 2018.

[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.

[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>.

[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>.

[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>.

13.2. Informative References

[I-D.ietf-6man-segment-routing-header]
Previdi, S.,
Filsfils, C., Previdi, S., Leddy, J., Matsushima, S., and
d. daniel.voyer@bell.ca, "IPv6 Segment Routing Header
(SRH)", draft-ietf-6man-segment-routing-header-13 draft-ietf-6man-segment-routing-header-14 (work in
progress), May June 2018.

[I-D.ietf-spring-segment-routing-mpls]
Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing with MPLS
data plane", draft-ietf-spring-segment-routing-mpls-14
(work in progress), June 2018.

[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d.,
bogdanov@google.com, b., and P. Mattes, "Segment Routing
Policy Architecture", draft-ietf-spring-segment-routing-
policy-01 (work in progress), June 2018.

[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. 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>.

[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>.

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
De kleetlaan 6a, DIEGEM BRABANT 1831
BELGIUM

Email: cfilsfil@cisco.com

Jeff Tantsura
Individual
444 San Antonio Rd, 10A
Palo Alto, CA 94306
USA

Email: jefftant.ietf@gmail.com

Wim Henderickx
Nokia
Copernicuslaan 50
Antwerp 2018, CA 95134
BELGIUM

Email: wim.henderickx@alcatel-lucent.com

Jon Hardwick
Metaswitch Networks
100 Church Street
Enfield, Middlesex
UK

Email: jonathan.hardwick@metaswitch.com