Internet Engineering Task Force (IETF) Q. Zhao
Request for Comments: 8306 D. Dhody, Ed.
Obsoletes: 6006 R. Palleti
Category: Standards Track Huawei Technologies
ISSN: 2070-1721 D. King
Old Dog Consulting
November 2017 Extensions to
the Path Computation Element Communication Protocol (PCEP)
for Point-to-Multipoint Traffic Engineering Label Switched Paths
Point-to-point Multiprotocol Label Switching (MPLS) and Generalized
MPLS (GMPLS) Traffic Engineering Label Switched Paths (TE LSPs) may
be established using signaling techniques, but their paths may first
need to be determined. The Path Computation Element (PCE) has been
identified as an appropriate technology for the determination of the
paths of point-to-multipoint (P2MP) TE LSPs.
This document describes extensions to the PCE Communication Protocol
(PCEP) to handle requests and responses for the computation of paths
for P2MP TE LSPs.
This document obsoletes RFC 6006.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Table of Contents
1. Introduction ....................................................41.1. Terminology ................................................51.2. Requirements Language ......................................52. PCC-PCE Communication Requirements ..............................53. Protocol Procedures and Extensions ..............................63.1. P2MP Capability Advertisement ..............................73.1.1. IGP Extensions for P2MP Capability Advertisement ....73.1.2. Open Message Extension ..............................73.2. Efficient Presentation of P2MP LSPs ........................83.3. P2MP Path Computation Request/Reply Message Extensions .....93.3.1. The Extension of the RP Object ......................93.3.2. The P2MP END-POINTS Object .........................113.4. Request Message Format ....................................133.5. Reply Message Format ......................................15
The Path Computation Element (PCE) as defined in [RFC4655] is an
entity that is capable of computing a network path or route based on
a network graph and applying computational constraints. A Path
Computation Client (PCC) may make requests to a PCE for paths to be
[RFC4875] describes how to set up point-to-multipoint (P2MP) Traffic
Engineering Label Switched Paths (TE LSPs) for use in Multiprotocol
Label Switching (MPLS) and Generalized MPLS (GMPLS) networks.
The PCE has been identified as a suitable application for the
computation of paths for P2MP TE LSPs [RFC5671].
The PCE Communication Protocol (PCEP) is designed as a communication
protocol between PCCs and PCEs for point-to-point (P2P) path
computations and is defined in [RFC5440]. However, that
specification does not provide a mechanism to request path
computation of P2MP TE LSPs.
A P2MP LSP is comprised of multiple source-to-leaf (S2L) sub-LSPs.
These S2L sub-LSPs are set up between ingress and egress Label
Switching Routers (LSRs) and are appropriately overlaid to construct
a P2MP TE LSP. During path computation, the P2MP TE LSP may be
determined as a set of S2L sub-LSPs that are computed separately and
combined to give the path of the P2MP LSP, or the entire P2MP TE LSP
may be determined as a P2MP tree in a single computation.
This document relies on the mechanisms of PCEP to request path
computation for P2MP TE LSPs. One Path Computation Request message
from a PCC may request the computation of the whole P2MP TE LSP, or
the request may be limited to a subset of the S2L sub-LSPs. In the
extreme case, the PCC may request the S2L sub-LSPs to be computed
individually; the PCC is responsible for deciding whether to signal
individual S2L sub-LSPs or combine the computation results to signal
the entire P2MP TE LSP. Hence, the PCC may use one Path Computation
Request message or may split the request across multiple path
This document obsoletes [RFC6006] and incorporates the following
o Erratum IDs 3819, 3830, 3836, 4867, and 4868 for [RFC6006]
o Erratum ID 4956 for [RFC5440]
All changes from [RFC6006] are listed in Appendix A.
Terminology used in this document:
TE LSP: Traffic Engineering Label Switched Path.
LSR: Label Switching Router.
OF: Objective Function. A set of one or more optimization criteria
used for the computation of a single path (e.g., path cost
minimization) or for the synchronized computation of a set of
paths (e.g., aggregate bandwidth consumption minimization).
This document also uses the terminology defined in [RFC4655],
[RFC4875], and [RFC5440].
1.2. 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.
2. PCC-PCE Communication Requirements
This section summarizes the PCC-PCE communication requirements as met
by the protocol extension specified in this document for P2MP MPLS-TE
LSPs. The numbering system in the list below corresponds to the
requirement numbers (e.g., R1, R2) used in [RFC5862].
1. The PCC MUST be able to specify that the request is a P2MP path
2. The PCC MUST be able to specify that objective functions are to
be applied to the P2MP path computation request.
3. The PCE MUST have the capability to reject a P2MP path
computation request and indicate non-support of P2MP path
4. The PCE MUST provide an indication of non-support of P2MP path
computation by back-level PCE implementations.
5. A P2MP path computation request MUST be able to list multiple
6. A P2MP path computation response MUST be able to carry the path
of a P2MP LSP.
7. By default, the path returned by the PCE SHOULD use the
8. It MUST be possible for a single P2MP path computation request or
response to be conveyed by a sequence of messages.
9. It MUST NOT be possible for a single P2MP path computation
request to specify a set of different constraints, traffic
parameters, or quality-of-service requirements for different
destinations of a P2MP LSP.
10. P2MP path modification and P2MP path diversity MUST be supported.
11. It MUST be possible to reoptimize existing P2MP TE LSPs.
12. It MUST be possible to add and remove P2MP destinations from
13. It MUST be possible to specify a list of applicable branch nodes
to use when computing the P2MP path.
14. It MUST be possible for a PCC to discover P2MP path computation
15. The PCC MUST be able to request diverse paths when requesting a
3. Protocol Procedures and Extensions
The following section describes the protocol extensions required to
satisfy the requirements specified in Section 2 ("PCC-PCE
Communication Requirements") of this document.
3.1. P2MP Capability Advertisement
3.1.1. IGP Extensions for P2MP Capability Advertisement
[RFC5088] defines a PCE Discovery (PCED) TLV carried in an OSPF
Router Information Link State Advertisement (LSA) as defined in
[RFC7770] to facilitate PCE discovery using OSPF. [RFC5088]
specifies that no new sub-TLVs may be added to the PCED TLV. This
document defines a flag in the OSPF PCE Capability Flags to indicate
the capability of P2MP computation.
Similarly, [RFC5089] defines the PCED sub-TLV for use in PCE
discovery using IS-IS. This document will use the same flag for the
OSPF PCE Capability Flags sub-TLV to allow IS-IS to indicate the
capability of P2MP computation.
The IANA assignment for a shared OSPF and IS-IS P2MP Capability Flag
is documented in Section 6.9 ("OSPF PCE Capability Flag") of this
PCEs wishing to advertise that they support P2MP path computation
would set the bit (10) accordingly. PCCs that do not understand this
bit will ignore it (per [RFC5088] and [RFC5089]). PCEs that do not
support P2MP will leave the bit clear (per the default behavior
defined in [RFC5088] and [RFC5089]).
PCEs that set the bit to indicate support of P2MP path computation
MUST follow the procedures in Section 3.3.2 ("The P2MP END-POINTS
Object") to further qualify the level of support.
3.1.2. Open Message Extension
Based on the Capabilities Exchange requirement described in
[RFC5862], if a PCE does not advertise its P2MP capability during
discovery, PCEP should be used to allow a PCC to discover, during the
Open Message Exchange, which PCEs are capable of supporting P2MP path
To satisfy this requirement, we extend the PCEP OPEN object by
defining an optional TLV to indicate the PCE's capability to perform
P2MP path computations.
IANA has allocated value 6 from the "PCEP TLV Type Indicators"
subregistry, as documented in Section 6.1 ("PCEP TLV Type
Indicators"). The description is "P2MP capable", and the length
value is 2 bytes. The value field is set to default value 0.
The inclusion of this TLV in an OPEN object indicates that the sender
can perform P2MP path computations.
The capability TLV is meaningful only for a PCE, so it will typically
appear only in one of the two Open messages during PCE session
establishment. However, in the case of PCE cooperation (e.g.,
inter-domain), when a PCE behaving as a PCC initiates a PCE session
it SHOULD also indicate its path computation capabilities.
3.2. Efficient Presentation of P2MP LSPs
When specifying additional leaves or when optimizing existing P2MP TE
LSPs as specified in [RFC5862], it may be necessary to pass existing
P2MP LSP route information between the PCC and PCE in the request and
reply messages. In each of these scenarios, we need path objects for
efficiently passing the existing P2MP LSP between the PCE and PCC.
We specify the use of the Resource Reservation Protocol Traffic
Engineering (RSVP-TE) extensions Explicit Route Object (ERO) to
encode the explicit route of a TE LSP through the network. PCEP ERO
sub-object types correspond to RSVP-TE ERO sub-object types. The
format and content of the ERO are defined in [RFC3209] and [RFC3473].
The Secondary Explicit Route Object (SERO) is used to specify the
explicit route of an S2L sub-LSP. The path of each subsequent S2L
sub-LSP is encoded in a P2MP_SECONDARY_EXPLICIT_ROUTE object SERO.
The format of the SERO is the same as the format of an ERO as defined
in [RFC3209] and [RFC3473].
The Secondary Record Route Object (SRRO) is used to record the
explicit route of the S2L sub-LSP. The class of the P2MP SRRO is the
same as the class of the SRRO as defined in [RFC4873].
The SERO and SRRO are used to report the route of an existing TE LSP
for which a reoptimization is desired. The format and content of the
SERO and SRRO are defined in [RFC4875].
PCEP Object-Class and Object-Type values for the SERO and SRRO have
Object-Class Value 29
Object-Type 0: Reserved
Reference RFC 8306
Object-Class Value 30
Object-Type 0: Reserved
Reference RFC 8306
The IANA assignments are documented in Section 6.5 ("PCEP Objects").
Since the explicit path is available for immediate signaling by the
MPLS or GMPLS control plane, the meanings of all of the sub-objects
and fields in this object are identical to those defined for the ERO.
3.3. P2MP Path Computation Request/Reply Message Extensions
This document extends the existing P2P RP (Request Parameters) object
so that a PCC can signal a P2MP path computation request to the PCE
receiving the PCEP request. The END-POINTS object is also extended
to improve the efficiency of the message exchange between the PCC and
PCE in the case of P2MP path computation.
3.3.1. The Extension of the RP Object
The PCE path computation request and reply messages will need the
following additional parameters to indicate to the receiving PCE
(1) that the request and reply messages have been fragmented across
multiple messages, (2) that they have been requested for a P2MP path,
and (3) whether the route is represented in the compressed or
This document adds the following flags to the RP object:
The F-bit is added to the flag bits of the RP object to indicate to
the receiver that the request is part of a fragmented request or
is not a fragmented request.
o F (RP fragmentation bit - 1 bit):
0: This indicates that the RP is not fragmented or it is the last
piece of the fragmented RP.
1: This indicates that the RP is fragmented and this is not the
last piece of the fragmented RP. The receiver needs to wait
for additional fragments until it receives an RP with the same
RP-ID and with the F-bit set to 0.
The N-bit is added in the flag bits field of the RP object to signal
the receiver of the message that the request/reply is for P2MP or
is not for P2MP.
o N (P2MP bit - 1 bit):
0: This indicates that this is not a Path Computation Request
(PCReq) or Path Computation Reply (PCRep) message for P2MP.
1: This indicates that this is a PCReq or PCRep message for P2MP.
The E-bit is added in the flag bits field of the RP object to signal
the receiver of the message that the route is in the compressed
format or is not in the compressed format. By default, the path
returned by the PCE SHOULD use the compressed format.
o E (ERO-compression bit - 1 bit):
0: This indicates that the route is not in the compressed format.
1: This indicates that the route is in the compressed format.
The IANA assignments are documented in Section 6.2 ("Request
Parameter Bit Flags") of this document.
3.3.2. The P2MP END-POINTS Object
The END-POINTS object is used in a PCReq message to specify the
source IP address and the destination IP address of the path for
which a path computation is requested. To represent the end points
for a P2MP path efficiently, we define two types of END-POINTS
objects for the P2MP path:
o Old leaves whose path can be modified/reoptimized.
o Old leaves whose path must be left unchanged.
With the P2MP END-POINTS object, the PCE Path Computation Request
message is expanded in a way that allows a single request message to
list multiple destinations.
In total, there are now four possible types of leaves in a
o New leaves to add (leaf type = 1)
o Old leaves to remove (leaf type = 2)
o Old leaves whose path can be modified/reoptimized (leaf type = 3)
o Old leaves whose path must be left unchanged (leaf type = 4)
A given END-POINTS object gathers the leaves of a given type. The
type of leaf in a given END-POINTS object is identified by the
END-POINTS object leaf type field.
Using the P2MP END-POINTS object, the END-POINTS portion of a request
message for the multiple destinations can be reduced by up to 50% for
a P2MP path where a single source address has a very large number of
Note that a P2MP path computation request can mix the different types
of leaves by including several END-POINTS objects per RP object as
shown in the PCReq Routing Backus-Naur Form (RBNF) [RFC5511] format
in Section 3.4 ("Request Message Format").
The END-POINTS object body has a variable length. These are
o multiples of 4 bytes for IPv4
o multiples of 16 bytes, plus 4 bytes, for IPv6
3.4. Request Message Format
As per [RFC5440], a Path Computation Request message (also referred
to as a PCReq message) is a PCEP message sent by a PCC to a PCE to
request a path computation. A PCReq message may carry more than one
path computation request.
As per [RFC5541], the OF object MAY be carried within a PCReq
message. If an objective function is to be applied to a set of
synchronized path computation requests, the OF object MUST be carried
just after the corresponding SVEC (Synchronization Vector) object and
MUST NOT be repeated for each elementary request.
The PCReq message is encoded as follows using RBNF as defined in
Below is the message format for the request message:
<PCReq Message> ::= <Common Header>
<svec-list> ::= <SVEC>
<request-list> ::= <request>[<request-list>]
<request> ::= <RP>
<RRO-List> ::= (<RRO>|<SRRO>)[<RRO-List>]
<metric-list> ::= <METRIC>[<metric-list>]
Figure 3: The Message Format for the Request Message
Note that we preserve compatibility with the definition of <request>
provided in [RFC5440]. At least one instance of <END-POINTS> MUST be
present in this message.
We have documented the IANA assignment of additional END-POINTS
Object-Type values in Section 6.5 ("PCEP Objects") of this document.
3.5. Reply Message Format
The PCEP Path Computation Reply message (also referred to as a
PCRep message) is a PCEP message sent by a PCE to a requesting PCC in
response to a previously received PCReq message. PCEP supports the
bundling of multiple replies to a set of path computation requests
within a single PCRep message.
The PCRep message is encoded as follows using RBNF as defined in
Below is the message format for the reply message:
<PCRep Message> ::= <Common Header>
<response-list> ::= <response>[<response-list>]
<response> ::= <RP>
<path> ::= (<ERO>|<SERO>) [<path>]
<attribute-list> ::= [<OF>]
Figure 4: The Message Format for the Reply Message
The optional END-POINTS object in the reply message is used to
specify which paths are removed, changed, not changed, or added for
the request. The path is only needed for the end points that are
added or changed.
If the E-bit (ERO-Compress bit) was set to 1 in the request, then the
path will be formed by an ERO followed by a list of SEROs.
Note that we preserve compatibility with the definition of <response>
provided in [RFC5440] and with the optional
<end-point-path-pair-list> and <path>.