PCE Working Group C. Li
Internet-Draft M. Chen
Intended status: Standards Track Huawei Technologies
Expires: January 25, 2020 W. Cheng
China Mobile
J. Dong
Z. Li
Huawei Technologies
R. Gandhi
Cisco Systems, Inc.
Q. Xiong
ZTE Corporation
July 24, 2019
Path Computation Element Communication Protocol (PCEP) Extension for
Path Segment in Segment Routing (SR)
draft-li-pce-sr-path-segment-07
Abstract
The Path Computation Element (PCE) provides path computation
functions in support of traffic engineering in Multiprotocol Label
Switching (MPLS) and Generalized MPLS (GMPLS) networks.
The Source Packet Routing in Networking (SPRING) architecture
describes how Segment Routing (SR) can be used to steer packets
through an IPv6 or MPLS network using the source routing paradigm. 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).
Path identification is needed for several use cases such as
performance measurement in Segment Routing (SR) network. This
document specifies extensions to the Path Computation Element
Protocol (PCEP) to support requesting, replying, reporting and
updating the Path Segment ID (Path SID) between PCEP speakers.
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/.
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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 January 25, 2020.
Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
3. Overview of Path Segment Extensions in PCEP . . . . . . . . . 4
4. Objects and TLVs . . . . . . . . . . . . . . . . . . . . . . 5
4.1. The OPEN Object . . . . . . . . . . . . . . . . . . . . . 5
4.1.1. The SR PCE Capability sub-TLV . . . . . . . . . . . . 5
4.1.2. The SRv6 PCE Capability sub-TLV . . . . . . . . . . . 6
4.1.3. PCECC-CAPABILITY sub-TLV . . . . . . . . . . . . . . 6
4.2. LSP Object . . . . . . . . . . . . . . . . . . . . . . . 7
4.2.1. Path Segment TLV . . . . . . . . . . . . . . . . . . 7
4.3. FEC Object . . . . . . . . . . . . . . . . . . . . . . . 9
4.4. CCI Object . . . . . . . . . . . . . . . . . . . . . . . 10
5. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Stateful PCE Operation . . . . . . . . . . . . . . . . . 11
5.1.1. Ingress PCC-Initiated Path Segment Allocation . . . . 12
5.1.2. PCE Initiated Path Segment Allocation . . . . . . . . 14
5.2. PCECC Based Operation . . . . . . . . . . . . . . . . . . 15
5.2.1. PCE Controlled Label Spaces Advertisement . . . . . . 15
5.2.2. PCECC based Path Segment Allocation . . . . . . . . . 15
6. Dataplane Considerations . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7.1. SR PCE Capability Flags . . . . . . . . . . . . . . . . . 18
7.2. SRv6 PCE Capability Flags . . . . . . . . . . . . . . . . 18
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7.3. New LSP Flag Registry . . . . . . . . . . . . . . . . . . 18
7.4. New PCEP TLV . . . . . . . . . . . . . . . . . . . . . . 18
7.4.1. Path Segment TLV . . . . . . . . . . . . . . . . . . 19
7.5. New CCI Flag Registry . . . . . . . . . . . . . . . . . . 19
7.6. New FEC Type Registry . . . . . . . . . . . . . . . . . . 20
7.7. PCEP Error Type and Value . . . . . . . . . . . . . . . . 20
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
[RFC5440] describes the Path Computation Element (PCE) Communication
Protocol (PCEP). PCEP enables the communication between a Path
Computation Client (PCC) and a PCE, or between PCE and PCE, for the
purpose of computation of Multiprotocol Label Switching (MPLS) as
well as Generalzied MPLS (GMPLS) Traffic Engineering Label Switched
Path (TE LSP) characteristics.
[RFC8231] specifies a set of extensions to PCEP to enable stateful
control of TE LSPs within and across PCEP sessions in compliance with
[RFC4657]. It includes mechanisms to effect LSP State
Synchronization between PCCs and PCEs, delegation of control over
LSPs to PCEs, and PCE control of timing and sequence of path
computations within and across PCEP sessions. The model of operation
where LSPs are initiated from the PCE is described in [RFC8281].
[I-D.ietf-pce-pcep-extension-for-pce-controller] specify the
procedures and PCEP protocol extensions for using the PCE as the
central controller for static LSPs, where LSPs can be provisioned as
explicit label instructions at each hop on the end-to-end path.
Segment routing (SR) [RFC8402] leverages the source routing and
tunneling paradigms and supports steering packets into an explicit
forwarding path at the ingress node.
An SR path needs to be identified in some use cases such as
performance measurement. For identifying an SR path,
[I-D.ietf-spring-mpls-path-segment] introduces a new segment that is
referred to as Path Segment.
[I-D.ietf-pce-segment-routing] specifies extensions to the Path
Computation Element Protocol (PCEP) [RFC5440] for SR networks, that
allow a stateful PCE to compute and initiate SR-TE paths, as well as
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a PCC to request, report or delegate SR paths.
[I-D.ietf-pce-segment-routing-ipv6] extend PCEP to support SR paths
for IPv6 data plane.
[I-D.zhao-pce-pcep-extension-pce-controller-sr] specifies the
procedures and PCEP protocol extensions when a PCE-based controller
is also responsible for configuring the forwarding actions on the
routers (SR SID distribution in this case), in addition to computing
the paths for packet flows in a segment routing network and telling
the edge routers what instructions to attach to packets as they enter
the network.
This document specifies a mechanism to carry the SR path
identification information in PCEP messages [RFC5440] [RFC8231]
[RFC8281]. The SR path identifier can be a Path Segment in SR-MPLS
[I-D.ietf-spring-mpls-path-segment], or a Path Segment in SRv6
[I-D.li-spring-srv6-path-segment] or other IDs that can identify an
SR path. This document also extends the PCECC-SR mechanism to inform
the Path Segment to the egress PCC.
2. Terminology
This memo makes use of the terms defined in [RFC4655],
[I-D.ietf-pce-segment-routing], and [RFC8402].
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 Path Segment Extensions in PCEP
This document specifies a mechanism of encoding (and allocating) Path
Segment in PCEP extensions. For supporting Path Segment in PCEP,
several TLVs and flags are defined. The formats of the objects and
TLVs are described in Section 4. The procedures of Path Segment
allocation are described in Section 5.
There are various modes of operations, such as -
o The Path Segment can be allocated by Egress PCC. The PCE should
request the Path Segment from Egress PCC.
o The PCE can allocate a Path Segment on its own accord and inform
the ingress/egress PCC, useful for PCE-initiated LSPs.
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o Ingress PCC can also request PCE to allocate the Path Segment, in
this case, the PCE would either allocate and inform the assigned
Path Segment to the ingress/egress PCC using PCEP messages, or
first request egress PCC for Path Segment and then inform it to
the ingress PCC.
The path information to the ingress PCC and PCE is exchanged via an
extension to [I-D.ietf-pce-segment-routing] and
[I-D.ietf-pce-segment-routing-ipv6]. The Path Segment information to
the egress PCC can be informed via an extension to the PCECC-SR
procedures [I-D.zhao-pce-pcep-extension-pce-controller-sr].
For the PCE to allocate a Path Segment, the PCE SHOULD be aware of
the MPLS label space from the PCCs. This is done via mechanism as
described in [I-D.li-pce-controlled-id-space]. Otherwise, the PCE
should request the egress PCC for Path Segment allocation.
4. Objects and TLVs
4.1. The OPEN Object
4.1.1. The SR PCE Capability sub-TLV
[I-D.ietf-pce-segment-routing] defined a new Path Setup Type (PST)
and SR-PCE-CAPABILITY sub-TLV for SR. PCEP speakers use this sub-TLV
to exchange information about their SR capability. The TLV defines a
Flags field that includes one bit (L-flag) to indicate Local
Significance [I-D.ietf-pce-segment-routing].
This document adds an additional flag for Path Segment allocation, as
follows -
o P (Path Segment Identification bit): A PCEP speaker sets this flag
to 1 to indicate that it has the capability to encode SR path
identification (Path Segment, as per
[I-D.ietf-spring-mpls-path-segment]).
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=TBD11 | Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Flags |P|N|X| MSD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: P-flag in SR-PCE-CAPABILITY TLV
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The figure is included for the ease of the reader and can be removed
at the time of publication.
4.1.2. The SRv6 PCE Capability sub-TLV
[I-D.ietf-pce-segment-routing-ipv6] defined a new Path Setup Type
(PST) and SRv6-PCE-CAPABILITY sub-TLV for SRv6. PCEP speakers use
this sub-TLV to exchange information about their SRv6 capability.
The TLV includes a Flags field and one bit (L-flag) was allocated in
[I-D.ietf-pce-segment-routing-ipv6].
This document adds an additional flag for Path Segment allocation, as
follows -
o P (Path Segment Identification bit): A PCEP speaker sets this flag
to 1 to indicate that it has the capability to encode SRv6 path
identification.(Path Segment, as per
[I-D.li-spring-srv6-path-segment]).
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=TBD1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Flags |P|L|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value | MSD-Type | MSD-Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ... //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: P-flag in SRv6-PCE-CAPABILITY TLV
The figure is included for the ease of the reader and can be removed
at the time of publication.
4.1.3. PCECC-CAPABILITY sub-TLV
Along with the SR sub-TLVs, the PCECC Capability as per
[I-D.zhao-pce-pcep-extension-pce-controller-sr] should be advertised
if the PCE allocates the Path Segment and acts as a Central
Controller that manages the Label space.
The PCECC Capability should also be advertised on the egress PCEP
session, along with the SR sub-TLVs. This is needed to ensure that
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the PCE can use the PCECC objects/mechanism to request/inform the
egress PCC of the Path Segment as described in this document.
4.2. LSP Object
The LSP Object is defined in Section 7.3 of [RFC8231]. This document
adds the following flags to the LSP Object:
o P (PCE Allocation bit): If the bit is set to 1, it indicates that
the PCC requests PCE to allocate resource for this LSP. With the
resource TLV, a PCE can undertsand what kind of resource should be
allocated, such as Path Segment and Binding Segment. A PCC would
set this bit to 1 and include a PATH-SEGMENT TLV in the LSP object
to request for allocation of Path Segment by the PCE in the PCReq
or PCRpt message. A PCE would also set this bit to 1 and include
a PATH-SEGMENT TLV to indicate that the Path Segment is allocated
by PCE and encoded in the PCRep, PCUpd or PCInitiate message.
Further, a PCE would set this bit to 0 and include a PATH-SEGMENT
TLV in the LSP object to indicate that the Path Segment should be
allocated by the PCC as described in Section 5.1.1.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PLSP-ID | Flag|P|C| O |A|R|S|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// TLVs //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: P-flag in LSP Object
The figure is included for the ease of the reader and can be removed
at the time of publication.
4.2.1. Path Segment TLV
The PATH-SEGMENT TLV is an optional TLV for use in the LSP Object for
Path Segment allocation. The type of this TLV is to be allocated by
IANA (TBA4). The format is shown below.
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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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ST | Flag |L| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ (Variable length) Path Segment ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: The PATH-SEGMENT TLV Format
The type (16-bit) of the TLV is TBA4 (to be allocated by IANA). The
length (16-bit) has a fixed value of 8 octets. The value contains
the following fields:
o ST (The Segment type - 8 bits): The ST field specifies the type of
the Path Segment field, which carries a Path Segment corresponding
to the SR path.
* 0: MPLS Path Segment, which is an MPLS label as defined in
[I-D.ietf-spring-mpls-path-segment]. The PST type MUST be set
to SR (MPLS).
* 1: SRv6 Path Segment, which is a 128 bit IPv6 address as
defined in [I-D.li-spring-srv6-path-segment]. The PST type
MUST be set to SRv6.
o Flags (8 bits): Two flags are currently defined:
* L-Bit (Local/Global - 1 bit): If set, then the Path Segment
carried by the PATH-SEGMENT TLV has local significance. If not
set, then the Path Segment carried by this TLV has global
significance (i.e. Path Segment is global within an SR
domain).
* The unassigned bits MUST be set to 0 and MUST be ignored at
receipt.
o Reserved (16 bits): MUST be set to 0 and MUST be ignored at
receipt.
o Path Segment: The Path Segment of an SR path. The Path Segment
type is indicated by the ST field. When the ST is 0, it is a MPLS
Path Segment [I-D.ietf-spring-mpls-path-segment] in the MPLS label
format. When the ST field is 1, it is a 128-bit SRv6 Path Segment
as defined in [I-D.li-spring-srv6-path-segment].
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In general, only one instance of PATH-SEGMENT TLV will be included in
LSP object. If more than one PATH-SEGMENT TLV is included, the first
one is processed and others MUST be ignored. Multiple Path Segment
allocation for use cases like alternate-making will be considered in
future version of this draft.
When the Path Segment allocation is enable, a PATH-SEGMENT TLV MUST
be included in the LSP object.
If the label space is maintained by PCC itself, and the Path Segment
is allocated by Egress PCC, then the PCE should request the Path
Segment from Egress PCC as described in Section 5.1.1. In this case,
the PCE should send a PCUpdate or PCInitiate message to the egress
PCC to request the Path Segment. The P-flag in LSP should be unset
in this case.
If a PCEP node does not recognize the PATH-SEGMENT TLV, it would
behave in accordance with [RFC5440] and ignore the TLV. If a PCEP
node recognizes the TLV but does not support the TLV, it MUST send
PCErr with Error-Type = 2 (Capability not supported).
4.3. FEC Object
The FEC Object [I-D.zhao-pce-pcep-extension-pce-controller-sr] is
used to specify the FEC information and MAY be carried within
PCInitiate or PCRpt message for the PCECC-SR operations. The PCE
MUST inform the Path Identification information to the Egress PCC.
To do this, this document extends the procedures of
[I-D.zhao-pce-pcep-extension-pce-controller-sr] by defining a new FEC
object type for Path.
FEC Object-Type is TBA6 'Path'.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// TLV(s) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: The path FEC object Format
One or more following TLV(s) are allowed in the 'path' FEC object -
o SYMBOLIC-PATH-NAME TLV: As defined in [RFC8231], it is a human-
readable string that identifies an LSP in the network.
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o LSP-IDENTIFIERS TLVs: As defined in [RFC8231], it is optional for
SR, but could be used to encode the source, destination and other
identification information for the path.
o SPEAKER-ENTITY-ID TLV: As defined in [RFC8232], a unique
identifier for the PCEP speaker, it is used to identify the
Ingress PCC.
Either SYMBOLIC-PATH-NAME TLV or LSP-IDENTIFIERS TLV MUST be
included. SPEAKER-ENTITY-ID TLV is optional. Only one instance of
each TLV is processed, if more than one TLV of each type is included,
the first one is processed and others MUST be ignored.
4.4. CCI Object
The Central Control Instructions (CCI) Object is used by the PCE to
specify the forwarding instructions is defined in
[I-D.ietf-pce-pcep-extension-for-pce-controller]. Further
[I-D.zhao-pce-pcep-extension-pce-controller-sr] defined a CCI object
type for SR.
The Path Segment information is encoded directly in the CCI SR
object. The Path Segment TLV as described in the Section 4.2.1, MUST
also be included in the CCI SR object as the TLV (as it includes
additional information regarding the Path Segment identifier).
This document adds the following flags to the CCI Object:
o C (PCC Allocation bit): If the bit is set to 1, it indicates that
the allocation needs to be done by the PCC for this central
controller instruction. A PCE set this bit to request the PCC to
make an allocation from its SR label space. A PCC would set this
bit to indicate that it has allocated the CC-ID and report it to
the PCE.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CC-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | Algorithm | Flags |C|N|E|V|L|O|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// SID/Label/Index (variable) //
+---------------------------------------------------------------+
| |
// Optional TLV //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: The CCI object for SR
(Editor's Note - An update is planned for
[I-D.zhao-pce-pcep-extension-pce-controller-sr] in the next revision
detailing this procedure, and the above text might move there.)
5. Operations
The Path Segment allocation and encoding is as per the stateful PCE
operations for segment routing. The procedures are as per the
corresponding extensions defined in [I-D.ietf-pce-segment-routing]
and [I-D.ietf-pce-segment-routing-ipv6] (which are further based on
[RFC8231] and [RFC8281]). The additional operations for Path Segment
are defined in this section.
To notify (or request) the Path Segment to the Egress PCC, the
procedures are as per the PCECC-SR
[I-D.zhao-pce-pcep-extension-pce-controller-sr] (which is based on
[I-D.ietf-pce-pcep-extension-for-pce-controller]). The additional
operations are defined in this section.
5.1. Stateful PCE Operation
As defined in [I-D.ietf-spring-mpls-path-segment], a Path Segment can
be allocated by the egress PCC. In this case, the label space is
maintained on the PCC itself.
This section describes the mechanism of Path Segment allocation by
using PCInitiate and PCUpd message in Stateful PCE model.
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5.1.1. Ingress PCC-Initiated Path Segment Allocation
The ingress PCC could request the Path Segment to be allocated by the
PCE via PCRpt message. The delegate flag (D-flag) MUST also be set
for this LSP. Also, the P-flag in the LSP object MUST be set.
On receiving a stateful path computation request with Path Segment
allocation request from an ingress PCC, a stateful PCE requests the
egress PCC to allocate a Path Segment.
The PATH-SEGMENT TLV MUST be included in an LSP object in the
PCInitiate message sent from the PCE to the egress to request Path
Segment allocation by the egress PCC. The P flag in LSP object MUST
be set to 0. This PCInitiate message to egress PCC would be the
similar to the one sent to ingress PCC as per
[I-D.ietf-pce-segment-routing], but the egress PCC would only
allocate the Path Segment and would not trigger the initiation/update
operation.
If the value of Path Segment is 0x0 it indicates that the PCE is
requesting a Path Segment for this LSP. If the Path Segment is set
to a value 'n' and the P flag is unset in the LSP object, it
indicates that the PCE requests a specific value 'n' of Path Segment.
If the Path Segment is allocated successfully, the egress PCC reports
the Path Segment via PCRpt message with PATH-SEGMENT TLV in LSP
object. Else, it MUST send a PCErr message with Error-Type = TBA7
("Path SID failure") and Error Value = 1 ("Invalid SID"). If the
value of Path Segment is valid, but the PCC is unable to allocate the
Path Segment, it MUST send a PCErr message with Error-Type = TBA7
("Path label/SID failure") and Error Value = 2 ("Unable to allocate
the specified label/SID").
Once the PCE receives the PCRpt message, it can obtain the Path
Segment information from the egress PCC and then update the path with
Path Segment by sending PCUpd message.
If the Path Segment is updated successfully, the ingress PCC will
acknowledge with a PCRpt message to the PCE. In case of error, as
described in [I-D.ietf-pce-segment-routing], an PCErr message with
Error-Type = TBA7 ("Path SID failure") and Error Value = 1 ("Invalid
SID") will be sent back to the PCE. The PCE MUST roll back the Path
Segment vlaue to the previous value by sending a PCUpd message to
synchronize with the egress PCC.
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Ingress Egress
+-+-+ +-+-+ +-+-+
|PCC| |PCE| |PCC|
+-+-+ +-+-+ +-+-+
1) LSP State | ---- PCRpt ----> | |
Delegate | Delegate=1 | |
| P=1 |2) PCE update |
| | the LSP-DB and |
| | request Path SID |
| | |
| | --- PCInitiate ---> | Egress
| | PATH-SEGMENT | allocates
| | TLV in LSP | a Path-SID
| | | from its
| | <----- PCRpt ------ | space
| | Path SID |
| | |
|<---- PCUpd ---- |3)Paths update with |
| PATH-SEGMENT TLV | Path SID |
| | |
4) LSP State | ----- PCRpt ---> | |
Report | | |
| | |
Figure 7: Ingress PCC-Initiated Path Segment Allocation
If the ingress PCC wishes to withdraw or modify a previously reported
Path Segment value, it MUST send a PCRpt message without any PATH-
SEGMENT TLV or with the PATH-SEGMENT TLV containing the new Path
Segment respectively. In this case, the PCE should synchronize with
egresss PCC via PCUpd message.
The Path Segment MUST be withdrawn when the corresponding LSP is
removed. When the LSP is deleted, the PCE MUST request the egress
PCC to withdraw the LSP and associated Path Segment via PCInitiate
message with the R flag is set in the SRP object.
If an egress PCC receives a valid Path Segment value from a PCE which
is different than the current Path Segment, it MUST try to allocate
the new value. If the new Path Segment is successfully allocated,
the egress PCC MUST report the new value to the PCE. Otherwise, it
MUST send a PCErr message with Error-Type = TBA7 ("Path label/SID
failure") and Error Value = 2 ("Unable to allocate the specified
label/SID").
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5.1.2. PCE Initiated Path Segment Allocation
A stateful PCE also can initiate or update an LSP with Path Segment
actively via requesting the egress PCC to allocate a Path Segment.
If a PCE wishes to modify a previously requested Path Segment value
or allocate a Path Segment for an PCE-Initiated LSP, it MUST request
the egress PCC to allocate a new value by sending a PCUpd message to
the egress PCC with PATH-SEGMENT TLV containing the new Path Segment
value. Also, the P flag in LSP object is unset. Absence of the
PATH-SEGMENT TLV in PCUpd message means that the PCE wishes to
withdraw the Path Segment.
The mechanism of requesting Path Segment is as per Section 5.1.1.
Once the PCE receives the PCRpt message, it can obtain the Path
Segment information from the egress PCC and then update or initiate
an LSP with Path Segment.
If the SR-Path is setup, the ingress PCC will acknowledge with a
PCRpt message to the PCE. In case of error, as described in
[I-D.ietf-pce-segment-routing], an PCErr message will be sent back to
the PCE. The PCE MUST request the egress PCC to withdraw the LSP and
associated Path Segment via PCInitiate message with the R flag is set
in the SRP object.
If the Path Segment is updated successfully, the ingress PCC will
acknowledge with a PCRpt message to the PCE. In case of error, an
PCErr message with Error-Type = TBA7 ("Path SID failure") and Error
Value = 1 ("Invalid SID") will be sent back to the PCE. The PCE MUST
roll back the Path Segment vlaue to the previous value by sending a
PCUpd message to synchronize with the egress PCC.
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Ingress Egress
+-+-+ +-+-+ +-+-+
|PCC| |PCE| |PCC|
+-+-+ +-+-+ +-+-+
1) LSP State | ---- PCRpt ----> | |
Delegate if| Delegate=1 | |
the LSP exists| |2)PCE actively update|
| | the LSP-DB and |
| | request Path SID |
| | |
| | --- PCInitiate ---> | Egress
| | PATH-SEGMENT | allocates
| | TLV in LSP | a Path-SID
| | | from its
| | <----- PCRpt ------ | space
| | Path SID |
| | |
|<-- PCUpd/PCInit -- |3)Paths update with |
| PATH-SEGMENT TLV | Path SID |
| | |
4) LSP State | ----- PCRpt ---> | |
Report | | |
| | |
Figure 8: Stateful PCE-Initiated Path Segment Allocation
5.2. PCECC Based Operation
5.2.1. PCE Controlled Label Spaces Advertisement
For allocating the Path Segments to SR paths by the PCEs, the PCE
controlled label space MUST be known at PCEs via configurations or
any other mechanisms. The PCE controlled label spaces MAY be
advertised as described in [I-D.li-pce-controlled-id-space].
5.2.2. PCECC based Path Segment Allocation
5.2.2.1. PCECC-Initiated
The PCE could allocate the Path Segment on its own for a PCE-
Initiated (or delegated LSP). The allocated Path Segment needs to be
informed to the Ingress and Egress PCC. The PCE would use the
PCInitiate message [RFC8281] or PCUpd message [RFC8231] towards the
Ingress PCC and MUST include the PATH-SEGMENT TLV in the LSP object.
The PCE would further inform the egress PCC about the Path Segment
allocated by the PCE using the PCInitiate message as described in
[I-D.zhao-pce-pcep-extension-pce-controller-sr].
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Ingress Egress
+-+-+ +-+-+ +-+-+
|PCC| |PCE| |PCC|
+-+-+ +-+-+ +-+-+
| | |
| <--PCInitiate--- |1)Initiate LSP with |
| PATH-SEGMENT TLV | Path SID |
| | |
2)LSP delegation |---PCRpt, D=1---> | (Confirm) |
| | |
|3) PCE informs the | --- PCInitiate ---> |
| Path SID to Egress| FEC=Path |
| | |
| | <-------- PCRpt --- |
| | |
Figure 9: PCE allocated Path Segment on its own
5.2.2.2. Ingress PCC-Initiated PCECC
The ingress PCC could request the Path Segment to be allocated by the
PCE via PCRpt message as per [RFC8231]. The delegate flag (D-flag)
MUST also be set for this LSP. Also, the P-flag in the LSP object
MUST be set.
A PATH-SEGMENT TLV MUST be included in the LSP object. If the value
of Path Segment is 0x0, it indicates that the Ingress PCC is
requesting a Path Segment for this LSP. If the Path Segment is set
to a value 'n', it indicates that the ingress PCC requests a specific
value 'n' of Path Segment.
If the Path Segment is allocated successfully, the PCE would further
respond to Ingress PCC with PCUpd message as per [RFC8231] and MUST
include the PATH-SEGMENT TLV in a LSP object. Else, it MUST send a
PCErr message with Error-Type = TBA7 ("Path SID failure") and Error
Value = 1 ("Invalid SID"). If the value of Path Segment is valid,
but the PCC is unable to allocate the Path Segment, it MUST send a
PCErr message with Error-Type = TBA7 ("Path label/SID failure") and
Error Value = 2 ("Unable to allocate the specified label/SID").
The active PCE would allocate the Path Segment as per the PATH-
SEGMENT flags and in case PATH-SEGMENT is not included, the PCE MUST
act based on the local policy.
The PCE would further inform the egress PCC about the Path Segment
allocated by the PCE using the PCInitiate message as described in
[I-D.zhao-pce-pcep-extension-pce-controller-sr].
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Ingress Egress
+-+-+ +-+-+ +-+-+
|PCC| |PCE| |PCC|
+-+-+ +-+-+ +-+-+
1) LSP State | ---- PCRpt ----> | |
Delegate | Delegate=1 | |
| P=1 |2) PCE update |
| | the LSP-DB and |
| | allocate Path SID |
|<---- PCUpd ---- |3)Paths update with |
| PATH-SEGMENT TLV | Path SID |
| | |
4) LSP State Report | ----- PCRpt ---> | |
| | |
|5) PCE informs the | --- PCInitiate ---> |
| Path SID to Egress| FEC=Path |
| | |
| | <-------- PCRpt --- |
| | |
Figure 10: Ingress PCC request Path Segment to PCE
6. Dataplane Considerations
As described in [I-D.ietf-spring-mpls-path-segment], in an SR-MPLS
network, when a packet is transmitted along an SR path, the labels in
the MPLS label stack will be swapped or popped. So that no label or
only the last label may be left in the MPLS label stack when the
packet reaches the egress node. Thus, the egress node cannot
determine from which SR path the packet comes. For this reason, it
introduces the Path Segment.
Apart from allocation and encoding of the Path Segment (described in
this document) for the LSP, it would also be included in the SID/
Label stack of the LSP (usually for processing by the egress). To
support this, the Path Segment MAY also be a part of SR-ERO as
prepared by the PCE as per [I-D.ietf-pce-segment-routing]. The PCC
MAY also include the Path Segment while preparing the label stack
based on the local policy and use-case.
It is important that the PCE learns the Maximum SID Depth (MSD) that
can be imposed at each node/link of a given SR path to ensure that
the SID stack depth does not exceed the number of SIDs the node is
capable of imposing. As a new type of segment, Path Segment will be
inserted in the SID list just like other SIDs. Thus, the PCE needs
to consider the affect of Path Segment when computing a LSP with Path
Segment allocation.
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7. IANA Considerations
7.1. SR PCE Capability Flags
SR PCE Capability TLV is defined in [I-D.ietf-pce-segment-routing],
and the registry to manage the Flag field of the SR PCE Capability
TLV is requested in [I-D.ietf-pce-segment-routing]. IANA is
requested to make the following allocation in the aforementioned
registry.
Bit Description Reference
TBA1 Path Segment Allocation is supported(P) This document
7.2. SRv6 PCE Capability Flags
SRv6 PCE Capability TLV is defined in defined in
[I-D.ietf-pce-segment-routing-ipv6], and the registry to manage the
Flag field of the SRv6 PCE Capability Flags is requested in
[I-D.ietf-pce-segment-routing-ipv6]. IANA is requested to make the
following allocation in the aforementioned registry.
Bit Description Reference
TBA2 Path Segment Allocation is supported(P) This document
7.3. New LSP Flag Registry
[RFC8231] defines the LSP object; per that RFC, IANA created a
registry to manage the value of the LSP object's Flag field. IANA
has allocated a new bit in the "LSP Object Flag Field" subregistry,
as follows:
Bit Description Reference
TBA3 Request for Path Segment Allocation(P) This document
7.4. New PCEP TLV
IANA is requested to add the assignment of a new allocation in the
existing "PCEP TLV Type Indicators" subregistry as follows:
Value Description Reference
TBA4 PATH-SEGMENT TLV This document
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7.4.1. Path Segment TLV
This document requests that a new subregistry named "PATH-SEGMENT TLV
Segment Type (ST) Field" to be created to manage the value of the ST
field in the PATH-SEGMENT TLV.
Value Description Reference
0 MPLS Path Segment(MPLS label) This document
1 SRv6 Path Segment(IPv6 address) This document
Further, this document also requests that a new subregistry named
"PATH-SEGMENT TLV Flag Field" to be created to manage the Flag field
in the PATH-SEGMENT TLV. New values are 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
Bit Description Reference
7 Local Signification(L) This document
7.5. New CCI Flag Registry
CCI object is defined in defined in
[I-D.ietf-pce-pcep-extension-for-pce-controller], further
[I-D.zhao-pce-pcep-extension-pce-controller-sr] defined a CCI object
type for SR. and the subregistry to manage the Flag field of the CCI
object for SR is requested in
[I-D.zhao-pce-pcep-extension-pce-controller-sr]. IANA is requested
to make the following allocation in the aforementioned subregistry.
Bit Description Reference
TBA5 PCC is requested to This document
allocate resource(C)
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7.6. New FEC Type Registry
A new PCEP object called FEC is defined in
[I-D.zhao-pce-pcep-extension-pce-controller-sr]. IANA is requested
to allocate a new Object-Type for FEC object in the "PCEP Objects"
subregistry.
Value Description Reference
TBA6 SR path This document
7.7. PCEP Error Type and Value
IANA is requested to allocate code-points in the "PCEP-ERROR Object
Error Types and Values" subregistry for the following new error-types
and error-values:
Error-Type Meaning Reference
TBA7 Path SID failure: This document
Error-value = 1
Invalid SID
Error-value = 2
Unable to allocate
Path SID
8. Security Considerations
TBA
9. Acknowledgments
10. Contributors
The following people have substantially contributed to this document:
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Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka 560066
India
Email: dhruv.ietf@gmail.com
Zafar Ali
Cisco Systems, Inc.
Email: zali@cisco.com
11. References
11.1. 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>.
[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>.
[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>.
[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>.
[RFC8232] Crabbe, E., Minei, I., Medved, J., Varga, R., Zhang, X.,
and D. Dhody, "Optimizations of Label Switched Path State
Synchronization Procedures for a Stateful PCE", RFC 8232,
DOI 10.17487/RFC8232, September 2017,
<https://www.rfc-editor.org/info/rfc8232>.
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[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>.
[I-D.ietf-pce-segment-routing]
Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "PCEP Extensions for Segment Routing",
draft-ietf-pce-segment-routing-16 (work in progress),
March 2019.
[I-D.ietf-pce-segment-routing-ipv6]
Negi, M., Li, C., Sivabalan, S., Kaladharan, P., and Y.
Zhu, "PCEP Extensions for Segment Routing leveraging the
IPv6 data plane", draft-ietf-pce-segment-routing-ipv6-02
(work in progress), April 2019.
[I-D.zhao-pce-pcep-extension-pce-controller-sr]
Zhao, Q., Li, Z., Negi, M., and C. Zhou, "PCEP Procedures
and Protocol Extensions for Using PCE as a Central
Controller (PCECC) of SR-LSPs", draft-zhao-pce-pcep-
extension-pce-controller-sr-05 (work in progress), July
2019.
[I-D.ietf-pce-pcep-extension-for-pce-controller]
Zhao, Q., Li, Z., Negi, M., and C. Zhou, "PCEP Procedures
and Protocol Extensions for Using PCE as a Central
Controller (PCECC) of LSPs", draft-ietf-pce-pcep-
extension-for-pce-controller-02 (work in progress), July
2019.
[I-D.li-spring-srv6-path-segment]
Li, C., Cheng, W., Chen, M., Dhody, D., Li, Z., Dong, J.,
and R. Gandhi, "Path Segment for SRv6 (Segment Routing in
IPv6)", draft-li-spring-srv6-path-segment-02 (work in
progress), July 2019.
11.2. Informative References
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.
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[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>.
[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>.
[I-D.li-pce-controlled-id-space]
Li, C., Chen, M., Dong, J., Li, Z., Wang, A., Cheng, W.,
and C. Zhou, "PCE Controlled ID Space", draft-li-pce-
controlled-id-space-03 (work in progress), June 2019.
[I-D.ietf-spring-mpls-path-segment]
Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler,
"Path Segment in MPLS Based Segment Routing Network",
draft-ietf-spring-mpls-path-segment-00 (work in progress),
March 2019.
Authors' Addresses
Cheng Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: chengli13@huawei.com
Mach(Guoyi) Chen
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: Mach.chen@huawei.com
Weiqiang Cheng
China Mobile
China
Email: chengweiqiang@chinamobile.com
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Jie Dong
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: jie.dong@huawei.com
Zhenbin Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: lizhenbin@huawei.com
Rakesh Gandhi
Cisco Systems, Inc.
Canada
Email: rgandhi@cisco.com
Quan Xiong
ZTE Corporation
China
Email: xiong.quan@zte.com.cn
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