Internet Engineering Task Force (IETF) Z. Li

Internet Engineering Task Force (IETF) Z. Li
Request for Comments: 9689 D. Dhody
Category: Informational Huawei Technologies
ISSN: 2070-1721 Q. Zhao
Etheric Networks
K. He
Tencent Holdings Ltd.
B. Khasanov
MTS Web Services (MWS)
December 2024

Use Cases for a PCE as a Central Controller (PCECC)

Abstract

The PCE is a core component of a Software-Defined Networking (SDN)
system. It can be used to compute optimal paths for network traffic
and update existing paths to reflect changes in the network or
traffic demands. The PCE was developed to derive Traffic Engineering
(TE) paths in MPLS networks, which are supplied to the headend of the
paths using the Path Computation Element Communication Protocol
(PCEP).

SDN has much broader applicability than signalled MPLS TE networks,
and the PCE may be used to determine paths in a range of use cases
including static Label-Switched Paths (LSPs), Segment Routing (SR),
Service Function Chaining (SFC), and most forms of a routed or
switched network. Therefore, it is reasonable to consider PCEP as a
control protocol for use in these environments to allow the PCE to be
fully enabled as a central controller.

A PCE as a Central Controller (PCECC) can simplify the processing of
a distributed control plane by blending it with elements of SDN
without necessarily completely replacing it. This document describes
general considerations for PCECC deployment and examines its
applicability and benefits, as well as its challenges and
limitations, through a number of use cases. PCEP extensions, which
are required for the PCECC use cases, are covered in separate
documents.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

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). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9689.

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the
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Table of Contents

1. Introduction
2. Terminology
3. Use Cases
3.1. PCECC for Label Management
3.2. PCECC and SR
3.2.1. PCECC SID Allocation for SR-MPLS
3.2.2. PCECC for SR-MPLS Best Effort (BE) Paths
3.2.3. PCECC for SR-MPLS TE Paths
3.2.4. PCECC for SRv6
3.3. PCECC for Static TE LSPs
3.4. PCECC for Load Balancing (LB)
3.5. PCECC and Inter-AS TE
3.6. PCECC for Multicast LSPs
3.6.1. PCECC for the Setup of P2MP/MP2MP LSPs
3.6.2. PCECC for the End-to-End Protection of P2MP/MP2MP LSPs
3.6.3. PCECC for the Local Protection of P2MP/MP2MP LSPs
3.7. PCECC for Traffic Classification
3.8. PCECC for SFC
3.9. PCECC for Native IP
3.10. PCECC for BIER
4. IANA Considerations
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Appendix A. Other Use Cases of the PCECC
A.1. PCECC for Network Migration
A.2. PCECC for L3VPN and PWE3
A.3. PCECC for Local Protection (RSVP-TE)
A.4. Using Reliable P2MP TE-Based Multicast Delivery for
Distributed Computations (MapReduce-Hadoop)
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

The PCE [RFC4655] was developed to offload the path computation
function from routers in an MPLS Traffic Engineering (TE) network.
It can compute optimal paths for traffic across a network and can
also update the paths to reflect changes in the network or traffic
demands. The role and function of the PCE have grown to cover
several other uses (such as GMPLS [RFC7025] or Multicast) and to
allow delegated stateful control [RFC8231] and PCE-initiated use of
network resources [RFC8281].

According to [RFC7399], Software-Defined Networking (SDN) refers to a
separation between the control elements and the forwarding components
so that software running in a centralized system, called a
"controller", can act to program the devices in the network to behave
in specific ways. A required element in an SDN architecture is a
component that plans how the network resources will be used and how
the devices will be programmed. It is possible to view this
component as performing specific computations to place traffic flows
within the network given knowledge of the availability of the network
resources, how other forwarding devices are programmed, and the way
that other flows are routed. This is the function and purpose of a
PCE, and the way that a PCE integrates into a wider network control
system (including an SDN system) is presented in [RFC7491].

[RFC8283] outlines the architecture for the PCE as a central
controller, extending the framework described in [RFC4655], and
demonstrates how PCEP can serve as a general southbound control
protocol between the PCE and Path Computation Client (PCC).
[RFC8283] further examines the motivations and applicability of PCEP
as a Southbound Interface (SBI) and introduces the implications for
the protocol.

[RFC9050] introduces the procedures and extensions for PCEP to
support the PCECC architecture [RFC8283].

This document describes the various use cases for the PCECC
architecture.

2. Terminology

The following terminology is used in this document.

AS: Autonomous System

ASBR: Autonomous System Border Router

BGP-LS: Border Gateway Protocol - Link State [RFC9552]

IGP: Interior Gateway Protocol (in this document, we assume IGP as
either Open Shortest Path First (OSPF) [RFC2328] [RFC5340] or
Intermediate System to Intermediate System (IS-IS) [RFC1195])

LSP: Label-Switched Path

PCC: Path Computation Client (as per [RFC4655], any client
application requesting a path computation to be performed by a PCE)

PCE: Path Computation Element (as per [RFC4655], an entity such as a
component, application, or network node that is capable of computing
a network path or route based on a network graph and applying
computational constraints)

PCECC: PCE as a Central Controller (an extension of a PCE to support
SDN functions as per [RFC8283])

PST: Path Setup Type [RFC8408]

RR: Route Reflector [RFC4456]

SID: Segment Identifier [RFC8402]

SR: Segment Routing [RFC8402]

SRGB: Segment Routing Global Block [RFC8402]

SRLB: Segment Routing Local Block [RFC8402]

TE: Traffic Engineering [RFC9522]

3. Use Cases

[RFC8283] describes various use cases for a PCECC such as:

* use of a PCECC to set up static TE LSPs (the PCEP extension for
this use case is in [RFC9050])

* use of a PCECC in SR [RFC8402]

* use of a PCECC to set up Multicast Point-to-Multipoint (P2MP) LSPs

* use of a PCECC to set up Service Function Chaining (SFC) [RFC7665]

* use of a PCECC in optical networks

Section 3.1 describes the general case of a PCECC being in charge of
managing MPLS label space, which is a prerequisite for further use
cases. Further, various use cases (SR, Multicast, etc.) are
described in the following sections to showcase scenarios that can
benefit from the use of a PCECC.

It is interesting to note that some of the use cases listed can also
be supported via BGP instead of PCEP. However, within the scope of
this document, the focus is on the use of PCEP.

3.1. PCECC for Label Management

As per [RFC8283], in some cases, the PCECC can take responsibility
for managing some part of the MPLS label space for each of the
routers that it controls, and it may take wider responsibility for
partitioning the label space for each router and allocating different
parts for different uses, communicating the ranges to the router
using PCEP.

[RFC9050] describes a mode where LSPs are provisioned as explicit
label instructions at each hop on the end-to-end path. Each router
along the path must be told what label forwarding instructions to
program and what resources to reserve. The controller uses PCEP to
communicate with each router along the path of the end-to-end LSP.
For this to work, the PCECC will take responsibility for managing
some part of the MPLS label space for each of the routers that it
controls. An extension to PCEP could be done to allow a PCC to
inform the PCE of such a label space to control (see [PCE-ID] for a
possible PCEP extension to support the advertisement of the MPLS
label space for the PCE to control).

[RFC8664] specifies extensions to PCEP that allow a stateful PCE to
compute, update, or initiate SR-TE paths. [PCECC-SR] describes the
mechanism for a PCECC to allocate and provision the node/prefix/
adjacency label (Segment Routing Identifier (SID)) via PCEP. To make
such an allocation, the PCE needs to be aware of the label space from
the Segment Routing Global Block (SRGB) or Segment Routing Local
Block (SRLB) [RFC8402] of the node that it controls. A mechanism for
a PCC to inform the PCE of such a label space to control is needed
within the PCEP. The full SRGB/SRLB of a node could be learned via
existing IGP or BGP-LS [RFC9552] mechanisms.

Further, there have been proposals for a global label range in MPLS
as well as the use of PCECC architecture to learn the label space of
each node to determine and provision the global label range.

+------------------------------+ +------------------------------+
| PCE DOMAIN 1 | | PCE DOMAIN 2 |
| +--------+ | | +--------+ |
| | | | | | | |
| | PCECC1 | ---------PCEP---------- | PCECC2 | |
| | | | | | | |
| | | | | | | |
| +--------+ | | +--------+ |
| ^ ^ | | ^ ^ |
| / \ PCEP | | PCEP / \ |
| V V | | V V |
| +--------+ +--------+ | | +--------+ +--------+ |
| | Node11 | | Node1n | | | | Node21 | | Node2n | |
| | | ...... | | | | | | ...... | | |
| | PCECC | | PCECC | | | | PCECC | |PCECC | |
| |Enabled | | Enabled| | | |Enabled | |Enabled | |
| +--------+ +--------+ | | +--------+ +--------+ |
| | | |
+------------------------------+ +------------------------------+

Figure 1: PCECC for MPLS Label Management

As shown in Figure 1:

* The PCC will advertise the PCECC capability to the PCECC
[RFC9050].

* The PCECC could also learn the label range set aside by the PCC
(via [PCE-ID]).

* Optionally, the PCECC could determine the shared MPLS global label
range for the network.

- In the case that the shared global label range needs to be
negotiated across multiple domains, the central controllers of
these domains will also need to negotiate a common global label
range across domains.

- The PCECC will need to set the shared global label range to all
PCC nodes in the network.

As per [RFC9050], the PCECC could also rely on the PCC to make label
allocations initially and use PCEP to distribute it to where it is
needed.

3.2. PCECC and SR

SR [RFC8402] leverages the source routing paradigm. Using SR, a
source node steers a packet through a path without relying on hop-by-
hop signalling protocols such as LDP [RFC5036] or RSVP-TE [RFC3209].
Each path is specified as an ordered list of instructions called
"segments". Each segment is an instruction to route the packet to a
specific place in the network or to perform a specific service on the
packet. A database of segments can be distributed through the
network using an intra-domain routing protocol (such as IS-IS or
OSPF), an inter-domain protocol (such as BGP), or by any other means.
PCEP could also be one of other protocols.

[RFC8664] specifies the PCEP extension specific to SR for SR over
MPLS (SR-MPLS). The PCECC may further use the PCEP for distributing
SR Segment Identifiers (SIDs) to the SR nodes (PCC) with some
benefits. If the PCECC allocates and maintains the SIDs in the
network for the nodes and adjacencies, and further distributes them
to the SR nodes directly via the PCEP session, then it is more
advantageous over the configurations on each SR node and flooding
them via IGP, especially in an SDN environment.

When the PCECC is used for the distribution of the Node-SID and Adj-
SID for SR-MPLS, the Node-SID is allocated from the SRGB of the node
and the Adj-SID is allocated from the SRLB of the node as described
in [PCECC-SR].

[RFC8355] identifies various protection and resiliency use cases for
SR. Path protection lets the ingress node be in charge of the
failure recovery (used for SR-TE [RFC8664]). Also, protection can be
performed by the node adjacent to the failed component, commonly
referred to as "local protection techniques" or "fast-reroute (FRR)
techniques". In the case of the PCECC, the protection paths can be
precomputed and set up by the PCE.

Figure 2 illustrates the use case where the Node-SID and Adj-SID are
allocated by the PCECC for SR-MPLS.

192.0.2.1/32
+----------+
| R1(1001) |
+----------+
|
+----------+
| R2(1002) | 192.0.2.2/32
+----------+
* | * *
* | * *
*link1| * *
192.0.2.4/32 * | *link2 * 192.0.2.5/32
+-----------+ 9001| * +-----------+
| R4(1004) | | * | R5(1005) |
+-----------+ | * +-----------+
* | *9003 * +
* | * * +
* | * * +
+-----------+ +-----------+
192.0.2.3/32 | R3(1003) | |R6(1006) |192.0.2.6/32
+-----------+ +-----------+
|
+-----------+
| R8(1008) | 192.0.2.8/32
+-----------+

Figure 2: SR Topology

3.2.1. PCECC SID Allocation for SR-MPLS

Each node (PCC) is allocated a Node-SID by the PCECC. The PCECC
needs to update the label mapping of each node to all the other nodes
in the domain. After receiving the label mapping, each node (PCC)
uses the local routing information to determine the next hop and
download the label forwarding instructions accordingly. The
forwarding behavior and the end result are the same as IGP shortest-
path SR forwarding based on Node-SIDs. Thus, from anywhere in the
domain, it enforces the ECMP-aware shortest-path forwarding of the
packet towards the related node.

The PCECC can allocate an Adj-SID for each adjacency in the network.
The PCECC sends a PCInitiate message to update the label mapping of
each adjacency to the corresponding nodes in the domain. Each node
(PCC) downloads the label forwarding instructions accordingly. The
forwarding behavior and the end result are similar to IGP-based Adj-
SID allocation and usage in SR.

These mechanisms are described in [PCECC-SR].

3.2.2. PCECC for SR-MPLS Best Effort (BE) Paths

When using PCECC for SR-MPLS Best Effort (BE) Paths, the PCECC needs
to allocate the Node-SID (without calculating the explicit path for
the SR path). The ingress router of the forwarding path needs to
encapsulate the destination Node-SID on top of the packet. All the
intermediate nodes will forward the packet based on the destination
Node-SID. It is similar to the LDP LSP.

R1 may send a packet to R8 simply by pushing an SR label with segment
{1008} (Node-SID for R8). The path will be based on the routing /
next hop calculation on the routers.

3.2.3. PCECC for SR-MPLS TE Paths

SR-TE paths may not follow an IGP shortest path tree (SPT). Such
paths may be chosen by a PCECC and provisioned on the ingress node of
the SR-TE path. The SR header consists of a list of SIDs (or MPLS
labels). The header has all necessary information so that the
packets can be guided from the ingress node to the egress node of the
path. Hence, there is no need for any signalling protocol. For the
case where a strict traffic engineering path is needed, all the Adj-
SIDs are stacked; otherwise, a combination of Node-SIDs or Adj-SIDs
can be used for the SR-TE paths.

As shown in Figure 3, R1 may send a packet to R8 by pushing an SR
header with segment list {1002, 9001, 1008}, where 1002 and 1008 are
the Node-SIDs of R2 and R8, respectively. 9001 is the Adj-SID for
link1. The path should be: "R1-R2-link1-R3-R8".

To achieve this, the PCECC first allocates and distributes SIDs as
described in Section 3.2.1. [RFC8664] describes the mechanism for a
PCE to compute, update, or initiate SR-MPLS TE paths.

192.0.2.1/32
+----------+
| R1 (1001)|
+----------+
| |
90011 | |90012
link1 | |link2
+----------+
| R2 (1002)| 192.0.2.2/32
+----------+
link3 * | * * link4
90023 * | * * 90024
*link5| * *
192.0.2.4/32 *90025 | *link6 * 192.0.2.5/32
+-----------+ | *90026+-----------+
| R4 (1004) | | * | R5 (1005) |
+-----------+ | * +-----------+
* | * +
link10 * | * link7 +
* | * +
+-----------+ +-----------+
192.0.2.3/32 | R3 (1003) | |R6 (1006) |192.0.2.6/32
+-----------+ +-----------+
| |
|link8 |
| |----------|link9
+-----------+
| R8 (1008) | 192.0.2.8/32
+-----------+

Figure 3: PCECC TE LSP Setup Example

Refer to Figure 3 for an example of TE topology, where 100x are Node-
SIDs and 900xx are Adj-SIDs.

* The SID allocation and distribution are done by the PCECC with all
Node-SIDs (100x) and all Adj-SIDs (900xx).

* Based on path computation request/delegation or PCE initiation,
the PCECC receives a request with constraints and optimization
criteria from a PCC.

* The PCECC will calculate the optimal path according to the given
constraints (e.g., bandwidth (BW)).

* The PCECC will provision the SR-MPLS TE LSP path
("R1-link1-R2-link6-R3-R8") at the ingress node: {90011, 1002,
90026, 1003, 1008}

* For the end-to-end protection, the PCECC can provision the
secondary path ("R1-link2-R2-link4-R5-R8"): {90012, 1002, 90024,
1005, 1008}.

3.2.3.1. PCECC for SR Policy

[RFC8402] defines SR architecture, which uses an SR Policy to steer
packets from a node through an ordered list of segments. The SR
Policy could be configured on the headend or instantiated by an SR
controller. The SR architecture does not restrict how the controller
programs the network. In this case, the focus is on PCEP as the
protocol for SR Policy delivery from the PCE to PCC.

An SR Policy architecture is described in [RFC9256]. An SR Policy is
a framework that enables the instantiation of an ordered list of
segments on a node for implementing a source routing policy for the
steering of traffic for a specific purpose (e.g., for a specific
Service Level Agreement (SLA)) from that node.

An SR Policy is identified through the tuple <headend, color,
endpoint>.

Figure 3 is used as an example of PCECC application for SR Policy
instantiation for SR-MPLS, where the Node-SIDs are 100x and the Adj-
SIDs are 900xx.

Let's assume that R1 needs to have two disjoint SR Policies towards
R8 based on different BWs. This means the possible paths are:

* POL1: {Headend R1, color 100, Endpoint R8; Candidate Path1:
Segment List 1: {90011, 1002, 90023, 1004, 1003, 1008}}

* POL2: {Headend R1, color 200, Endpoint R8; Candidate Path1:
Segment List 1: {90012, 1002, 90024, 1005, 1006, 1008}}

Each SR Policy (including the candidate path and segment list) will
be signalled to a headend (R1) via PCEP [PCEP-POLICY] with the
addition of an ASSOCIATION object. A Binding SID (BSID) [RFC8402]
can be used for traffic steering of labelled traffic into an SR
Policy; a BSID can be provisioned from the PCECC also via PCEP
[RFC9604]. For non-labelled traffic steering into the SR Policy POL1
or POL2, a per-destination traffic steering will be used by means of
the BGP Color Extended Community [RFC9012].

The procedure is as follows:

* The PCECC allocates Node-SIDs and Adj-SIDs using the mechanism
described in Section 3.2.1 for all nodes and links.

* The PCECC calculates disjoint paths for POL1 and POL2 and create
segment lists for them: {90011, 1002, 90023, 1004, 1003,
1008};{90012, 1002, 90024, 1005, 1006, 1008}.

* The PCECC forms both SR Policies POL1 and POL2.

* The PCECC sends both POL1 and POL2 to R1 via PCEP.

* The PCECC optionally allocates BSIDs for the SR Policies.

* The traffic from R1 to R8, which fits to color 100, will be
steered to POL1 and follows the path:
"R1-link1-R2-link3-R4-R3-R8". The traffic from R1 to R8, which
fits color 200, will be steered to POL2 and follows the path:
"R1-link2-R2-link4-R5-R6-R8". Due to the possibility of having
many segment lists in the same candidate path of each POL1/POL2,
the PCECC could provision more paths towards R8 and traffic will
be balanced either as ECMP or as weighted-ECMP (W-ECMP). This is
the advantage of SR Policy architecture.

Note that an SR Policy can be associated with multiple candidate
paths. A candidate path is selected when it is valid and it is
determined to be the best path of the SR Policy as described in
[RFC9256].

3.2.4. PCECC for SRv6

As per [RFC8402], with SR, a node steers a packet through an ordered
list of instructions, called segments. SR can be applied to the IPv6
architecture with the Segment Routing Header (SRH) [RFC8754]. A
segment is encoded as an IPv6 address. An ordered list of segments
is encoded as an ordered list of IPv6 addresses in the routing
header. The active segment is indicated by the destination address
of the packet. Upon completion of a segment, a pointer in the new
routing header is incremented and indicates the next segment.

As per [RFC8754], an SR over IPv6 (SRv6) Segment is a 128-bit value.
"SRv6 SID" or simply "SID" are often used as a shorter reference for
"SRv6 Segment". [RFC8986] defines the SRv6 SID as consisting of
LOC:FUNCT:ARG.

[RFC9603] extends [RFC8664] to support SR for the IPv6 data plane.
Further, a PCECC could be extended to support SRv6 SID allocation and
distribution. An example of how PCEP extensions could be extended
for SRv6 for a PCECC is described in [PCECC-SRv6].

2001:db8::1
+----------+
| R1 |
+----------+
|
+----------+
| R2 | 2001:db8::2
+----------+
* | * *
* | * *
*link1| * *
2001:db8::4 * | *link2 * 2001:db8::5
+-----------+ | * +-----------+
| R4 | | * | R5 |
+-----------+ | * +-----------+
* | * * +
* | * * +
* | * * +
+-----------+ +-----------+
2001:db8::3 | R3 | |R6 |2001:db8::6
+-----------+ +-----------+
|
+-----------+
| R8 | 2001:db8::8
+-----------+

Figure 4: PCECC for SRv6

In this case, as shown in Figure 4, the PCECC could assign the SRv6
SID (in the form of an IPv6 address) to be used for node and
adjacency. Later, the SRv6 path in the form of a list of SRv6 SIDs
could be used at the ingress. Some examples:

* The best path towards R8: SRv6 SID-List={2001:db8::8}

* The path towards R8 via R5: SRv6 SID-List={2001:db8::5,
2001:db8::8}

The rest of the procedures and mechanisms remain the same as SR-MPLS.

3.3. PCECC for Static TE LSPs

As described in Section 3.1.2 of [RFC8283], the PCECC architecture
supports the provisioning of static TE LSPs. To achieve this, the
existing PCEP can be used to communicate between the PCECC and nodes
along the path to provision explicit label instructions at each hop
on the end-to-end path. Each router along the path must be told what
label-forwarding instructions to program and what resources to
reserve. The PCECC keeps a view of the network and determines the
paths of the end-to-end LSPs, and the controller uses PCEP to
communicate with each router along the path of the end-to-end LSP.

192.0.2.1/32
+----------+
| R1 |
+----------+
| |
|link1 |
| |link2
+----------+
| R2 | 192.0.2.2/32
+----------+
link3 * | * * link4
* | * *
*link5| * *
192.0.2.4/32 * | *link6 * 192.0.2.5/32
+-----------+ | * +-----------+
| R4 | | * | R5 |
+-----------+ | * +-----------+
* | * * +
link10 * | * *link7 +
* | * * +
+-----------+ +-----------+
192.0.2.3/32 | R3 | |R6 |192.0.2.6/32
+-----------+ +-----------+
| |
|link8 |
| |link9
+-----------+
| R8 | 192.0.2.8/32
+-----------+

Figure 5: PCECC TE LSP Setup Example

Refer to Figure 5 for an example TE topology.

* Based on path computation request/delegation or PCE initiation,
the PCECC receives a request with constraints and optimization
criteria.

* The PCECC will calculate the optimal path according to the given
constraints (e.g., BW).

* The PCECC will provision each node along the path and assign
incoming and outgoing labels from R1 to R8 with the path as
"R1-link1-R2-link3-R4-link10-R3-link8-R8":

- R1: Outgoing label 1001 on link 1

- R2: Incoming label 1001 on link 1

- R2: Outgoing label 2003 on link 3

- R4: Incoming label 2003 on link 3

- R4: Outgoing label 4010 on link 10

- R3: Incoming label 4010 on link 10

- R3: Outgoing label 3008 on link 8

- R8: Incoming label 3008 on link 8

* This can also be represented as: {R1, link1, 1001}, {1001, R2,
link3, 2003}, {2003, R4, link10, 4010}, {4010, R3, link8, 3008},
{3008, R8}.

* For the end-to-end protection, the PCECC programs each node along
the path from R1 to R8 with the secondary path: {R1, link2, 1002},
{1002, R2, link4, 2004}, {2004, R5, link7, 5007}, {5007, R3,
link9, 3009}, {3009, R8}.

* It is also possible to have a bypass path for the local protection
set up by the PCECC. For example, use the primary path as above,
then to protect the node R4 locally, the PCECC can program the
bypass path like this: {R2, link5, 2005}, {2005, R3}. By doing
this, the node R4 is locally protected at R2.

3.4. PCECC for Load Balancing (LB)

Very often, many service providers use TE tunnels for solving issues
with non-deterministic paths in their networks. One example of such
applications is the usage of TEs in the mobile backhaul (MBH).
Consider the topology as shown in Figure 6 (where AGG 1...AGG N are
Aggregation routers, and Core 1...Core N are Core routers).

TE1 ----------->
+--------+ +------+ +-----+ +-------+ +------+ +---+
|Access |----|Access|----|AGG 1|----|AGG N-1|----|Core 1|--|SR1|
|SubNode1| |Node1 | +-----+ +-------+ +------+ +---+
+--------+ +------+ | | | ^ |
| Access | Access | AGG Ring 1| | |
| SubRing 1 | Ring 1 | | | | |
+--------+ +------+ +-----+ | | |
|Access | |Access| |AGG 2| | | |
|SubNode2| |Node2 | +-----+ | | |
+--------+ +------+ | | | | |
| | | | | | |
| | | +---TE2---|-+ |
+--------+ +------+ +-----+ +-------+ +------+ +---+
|Access | |Access|----|AGG 3|----| AGG N |----|Core N|--|SRn|
|SubNodeN|----|NodeN | +-----+ +-------+ +------+ +---+
+--------+ +------+

Figure 6: PCECC LB Use Case

This MBH architecture uses L2 access rings and sub-rings. L3 starts
at the aggregation layer. For the sake of simplicity, the figure
shows only one access sub-ring. The access ring and aggregation ring
are connected by Nx10GE interfaces. The aggregation domain runs its
own IGP. There are two egress routers (AGG N-1 and AGG N) that are
connected to the Core domain (Core 1...Core N) via L2 interfaces.
The Core also has connections to service routers; RSVP-TE or SR-TE is
used for MPLS transport inside the ring. There could be at least two
tunnels (one way) from each AGG router to egress AGG routers. There
are also many L2 access rings connected to AGG routers.

Service deployment is made by means of Layer 2 Virtual Private
Networks (L2VPNs), Virtual Private LAN Services (VPLSs), Layer 3
Virtual Private Networks (L3VPNs), or Ethernet VPNs (EVPNs). Those
services use MPLS TE (or SR-TE) as transport towards egress AGG
routers. TE tunnels could be used as transport towards service
routers in case of architecture based on seamless MPLS
[MPLS-SEAMLESS].

Load Balancing (LB) between TE tunnels involves distributing network
traffic across multiple TE tunnels to optimize the use of available
network resources, enhance performance, and ensure reliability. Some
common techniques include Equal-Cost Multipath (ECMP) and Unequal-
Cost Multipath (UCMP) based on the BW of the TE tunnels.

There is a need to solve the following tasks:

* Perform automatic LB amongst TE tunnels according to current
traffic loads.

* Manage TE BW by guaranteeing BW for specific services (such as
High-Speed Internet (HSI), IPTV, etc.) and enabling time-based BW
reservation (such as Bandwidth on Demand (BoD)).

* Simplify the development of TE tunnels by automation without any
manual intervention.

* Provide flexibility for service router placement (anywhere in the
network by the creation of transport LSPs to them).

In this section, the focus is on LB tasks. LB tasks could be solved
by means of the PCECC in the following ways:

* Applications, network services, or operators can ask the SDN
controller (PCECC) for LSP-based LB between AGG X and AGG N/AGG
N-1 (egress AGG routers that have connections to the core). Each
of these will have associated constraints (such as BW, inclusion
or exclusion of specific links or nodes, number of paths,
Objective Function (OF), need for disjoint LSP paths, etc.).

* The PCECC could calculate multiple (say N) LSPs according to given
constraints. The calculation is based on the results of the OF
[RFC5541], constraints, endpoints, same or different BW, different
links (in case of disjoint paths), and other constraints.

* Depending on the given LSP PST, the PCECC will download
instructions to the PCC. At this stage, it is assumed the PCECC
is aware of the label space it controls and SID allocation and
distribution is already done in the case of SR.

* The PCECC will send a PCInitiate message [RFC8281] towards the
ingress AGG X router (PCC) for each of N LSPs and receive a PCRpt
message [RFC8231] back from PCCs. If the PST is a PCECC-SR, the
PCECC will include a SID stack as per [RFC8664]. If the PST is
set to "PCECC" type, then the PCECC will assign labels along the
calculated path and set up the path by sending central controller
instructions in a PCEP message to each node along the path of the
LSP as per [RFC9050]. Then, the PCECC will send a PCUpd message
to the ingress AGG X router with information about the new LSP.
AGG X (PCC) will respond with a PCRpt with LSP status.

* AGG X as an ingress router now has N LSPs towards AGG N and AGG
N-1, which are available for installation to the router's
forwarding table and for LB traffic between them. Traffic
distribution between those LSPs depends on the particular
realization of the hash function on that router.

* Since the PCECC is aware of the Traffic Engineering Database (TED)
(TE state) and the LSP Database (LSP-DB), it can manage and
prevent possible over-subscriptions and limit the number of
available load-balance states. Via a PCECC mechanism, the control
can take quick actions into the network by directly provisioning
the central control instructions.

3.5. PCECC and Inter-AS TE

There are various signalling options for establishing Inter-AS TE
LSPs: contiguous TE LSPs [RFC5151], stitched TE LSPs [RFC5150], and
nested TE LSPs [RFC4206].

The requirements for PCE-based Inter-AS setup [RFC5376] describe the
approach and PCEP functionality that is needed for establishing
Inter-AS TE LSPs.

[RFC5376] also gives an Inter-AS and Intra-AS PCE Reference Model (as
shown in Figure 7) that is provided below in shortened form for the
sake of simplicity.

Inter-AS Inter-AS
PCC <-->PCE1<--------->PCE2
:: :: ::
:: :: ::
R1----ASBR1====ASBR3---R3---ASBR5
| AS1 | | PCC |
| | | AS2 |
R2----ASBR2====ASBR4---R4---ASBR6
:: ::
:: ::
Intra-AS Intra-AS
PCE3 PCE4

Figure 7: Shortened Form of the Inter-AS and Intra-AS PCE
Reference Model

The PCECC belonging to the different domains can cooperate to set up
Inter-AS TE LSPs. The stateful Hierarchical PCE (H-PCE) mechanism
[RFC8751] could also be used to establish a per-domain PCECC LSP
first. These could be stitched together to form an Inter-AS TE LSP
as described in [PCE-INTERDOMAIN].

For the sake of simplicity, here the focus is on a simplified Inter-
AS case when both AS1 and AS2 belong to the same service provider
administration. In that case, Inter-AS and Intra-AS PCEs could be
combined in one single PCE if such combined PCE performance is enough
to handle the load. The PCE will require interfaces (PCEP and BGP-
LS) to both domains. PCECC redundancy mechanisms are described in
[RFC8283]. Thus, routers (PCCs) in AS1 and AS2 can send PCEP
messages towards the same PCECC. In Figure 8, the PCECC maintains a
BGP-LS session with Route Reflectors (RRs) in each AS. This allows
the RRs to redistribute routes to other BGP routers (clients) without
requiring a full mesh. The RRs act as a BGP-LS Propagator, and the
PCECC acts as a BGP-LS Consumer [RFC9552].

+----BGP-LS------+ +------BGP-LS-----+
| | | |
+-PCEP-|----++-+-------PCECC-----PCEP--++-+-|-------+
+-:------|----::-:-+ +--::-:-|-------:---+
| : | :: : | | :: : | : |
| : RR1 :: : | | :: : RR2 : |
| v v: : | LSP1 | :: v v |
| R1---------ASBR1=======================ASBR3--------R3 |
| | v : | | :v | |
| +----------ASBR2=======================ASBR4---------+ |
| | Region 1 : | | : Region 1 | |
|----------------:-| |--:-------------|--|
| | v | LSP2 | v | |
| +----------ASBR5=======================ASBR6---------+ |
| Region 2 | | Region 2 |
+------------------+ <--------------> +-------------------+
MPLS Domain 1 Inter-AS MPLS Domain 2
<=======AS1=======> <========AS2=======>

Figure 8: Particular Case of Inter-AS PCE

In the case of the PCECC Inter-AS TE scenario (as shown in Figure 8),
where the service provider controls both domains (AS1 and AS2), each
of them has its own IGP and MPLS transport. There is a need to set
up Inter-AS LSPs for transporting different services on top of them
(such as Voice, L3VPN, etc.). Inter-AS links with different
capacities exist in several regions. The task is not only to
provision those Inter-AS LSPs with given constraints but also to
calculate the path and pre-setup the backup Inter-AS LSPs that will
be used if the primary LSP fails.

As per Figure 8, LSP1 from R1 to R3 goes via ASBR1 and ASBR3, and it
is the primary Inter-AS LSP. LSP2 from R1 to R3 that goes via ASBR5
and ASBR6 is the backup one. In addition, there could also be a
bypass LSP setup to protect against ASBR or Inter-AS link failures.

After the addition of PCECC functionality to the PCE (SDN
controller), the PCECC-based Inter-AS TE model should follow the
PCECC use case for TE LSP including the requirements described in
[RFC5376] with the following details:

* Since the PCECC needs to know the topology of both domains AS1 and
AS2, the PCECC can utilize the BGP-LS peering with BGP routers (or
RRs) in both domains.

* The PCECC needs to establish PCEP connectivity with all routers in
both domains (see also Section 4 of [RFC5376]).

* After the operator's application or service orchestrator creates a
request for tunnel creation of a specific service, the PCECC will
receive that request via the Northbound Interface (NBI) (note that
the NBI type is implementation-dependent; it could be NETCONF/
YANG, REST, etc.). Then, the PCECC will calculate the optimal
path based on the OF and given constraints (i.e., PST, BW, etc.).
These constraints include those from [RFC5376], such as priority,
AS sequence, preferred ASBR, disjoint paths, and protection type.
In this step, we will have two paths: "R1-ASBR1-ASBR3-R3,
R1-ASBR5-ASBR6-R3".

* The PCECC will use central control download instructions to the
PCC based on the PST. At this stage, it is assumed the PCECC is
aware of the label space it controls, and in the case of SR, the
SID allocation and distribution is already done.

* The PCECC will send a PCInitiate message [RFC8281] towards the
ingress router R1 (PCC) in AS1 and receive the PCRpt message
[RFC8231] back from it.

- If the PST is SR-MPLS, the PCECC will include the SID stack as
per [RFC8664]. Optionally, a BSID or BGP Peering-SID [RFC9087]
can also be included on the AS boundary. The backup SID stack
can be installed at ingress R1, but more importantly, each node
along the SR path could also do the local protection just based
on the top segment.

- If the PST is a PCECC, the PCECC will assign labels along the
calculated paths ("R1-ASBR1-ASBR3-R3", "R1-ASBR5-ASBR6-R3") and
sets up the path by sending central controller instructions in
a PCEP message to each node along the path of the LSPs as per
[RFC9050]. After these steps, the PCECC will send a PCUpd
message to the ingress R1 router with information about new
LSPs and R1 will respond by a PCRpt with LSP(s) status.

* After that step, R1 now has primary and backup TEs (LSP1 and LSP2)
towards R3. It is up to the router implementation for how to
switchover to backup LSP2 if LSP1 fails.

3.6. PCECC for Multicast LSPs

The multicast LSPs can be set up via the RSVP-TE P2MP or Multipoint
LDP (mLDP) protocols. The setup of these LSPs may require manual
configurations and complex signalling when the protection is
considered. By using the PCECC solution, the multicast LSP can be
computed and set up through a centralized controller that has the
full picture of the topology and BW usage for each link. It not only
reduces the complex configurations comparing the distributed RSVP-TE
P2MP or mLDP signalling, but also it can compute the disjoint primary
path and secondary P2MP path efficiently.

3.6.1. PCECC for the Setup of P2MP/MP2MP LSPs

It is assumed the PCECC is aware of the label space it controls for
all nodes and makes allocations accordingly.

+----------+
| R1 | Root Node of the multicast LSP
+----------+
|9000 (link0)
+----------+
Transit Node | R2 |
branch +----------+
* | * *
9001* | * *9002
link1 * | * *link2
+-----------+ | * +-----------+
| R4 | | * | R5 | Transit Nodes
+-----------+ | * +-----------+
* | * * +
9003* | * * +9004
link3 * | * * +link4
+-----------+ +-----------+
| R3 | | R6 | Leaf Node
+-----------+ +-----------+
9005| link5
+-----------+
| R8 | Leaf Node
+-----------+

Figure 9: Using a PCECC for the Setup of P2MP/MP2MP LSPs

The P2MP examples (based on Figure 9) are explained here, where R1 is
the root and the routers R8 and R6 are the leaves.

* Based on the P2MP path computation request/delegation or PCE
initiation, the PCECC receives the request with constraints and
optimization criteria.

* The PCECC will calculate the optimal P2MP path according to given
constraints (i.e., BW).

* The PCECC will provision each node along the path and assign
incoming and outgoing labels from R1 to {R6, R8} with the path as
"R1-link0-R2-link2-R5-link4-R6" and
"R1-link0-R2-link1-R4-link3-R3-link5-R8":

- R1: Outgoing label 9000 on link0

- R2: Incoming label 9000 on link0

- R2: Outgoing label 9001 on link1 (*)

- R2: Outgoing label 9002 on link2 (*)

- R5: Incoming label 9002 on link2

- R5: Outgoing label 9004 on link4

- R6: Incoming label 9004 on link4

- R4: Incoming label 9001 on link1

- R4: Outgoing label 9003 on link3

- R3: Incoming label 9003 on link3

- R3: Outgoing label 9005 on link5

- R8: Incoming label 9005 on link5

* This can also be represented as: {R1, 6000}, {6000, R2, {9001,
9002}}, {9001, R4, 9003}, {9002, R5, 9004} {9003, R3, 9005},
{9004, R6}, {9005, R8}. The main difference (*) is in the branch
node instruction at R2, where two copies of a packet are sent
towards R4 and R5 with 9001 and 9002 labels, respectively.

The packet forwarding involves the following:

Step 1. R1 sends a packet to R2 simply by pushing the label of 9000
to the packet.

Step 2. When R2 receives the packet with label 9000, it will forward
it to R4 by swapping label 9000 to 9001. At the same time,
it will replicate the packet and swap the label 9000 to 9002
and forward it to R5.

Step 3. When R4 receives the packet with label 9001, it will forward
it to R3 by swapping 9001 to 9003. When R5 receives the
packet with the label 9002, it will forward it to R6 by
swapping 9002 to 9004.

Step 4. When R3 receives the packet with label 9003, it will forward
it to R8 by swapping it to 9005. When R5 receives the
packet with label 9002, it will be swapped to 9004 and sent
to R6.

Step 5. When R8 receives the packet with label 9005, it will pop the
label. When R6 receives the packet with label 9004, it will
pop the label.

3.6.2. PCECC for the End-to-End Protection of P2MP/MP2MP LSPs

This section describes the end-to-end managed path protection service
as well as the local protection with the operation management in the
PCECC network for the P2MP/MP2MP LSP.

An end-to-end protection principle can be applied for computing
backup P2MP or MP2MP LSPs. During the computation of the primary
multicast trees, the PCECC could also take the computation of a
secondary tree into consideration. A PCECC could compute the primary
and backup P2MP (or MP2MP) LSPs together or sequentially.

+----+ +----+
Root Node of LSP | R1 |--| R11|
+----+ +----+
/ +
10/ +20
/ +
+----------+ +-----------+
Transit Node | R2 | | R3 |
+----------+ +-----------+
| \ + +
| \ + +
10| 10\ +20 20+
| \ + +
| \ +
| + \ +
+-----------+ +-----------+ Leaf Nodes
| R4 | | R5 | (Downstream LSR)
+-----------+ +-----------+

Figure 10: PCECC for the End-to-End Protection of P2MP/MP2MP LSPs

In Figure 10, when the PCECC sets up the primary multicast tree from
the root node R1 to the leaves, which is R1->R2->{R4, R5}, it can set
up the backup tree at the same time, which is R1->R11->R3->{R4, R5}.
Both of them (the primary forwarding tree and secondary forwarding
tree) will be downloaded to each router along the primary path and
the secondary path. The traffic will be forwarded through the
R1->R2->{R4, R5} path normally, but when a node in the primary tree
fails (say R2), the root node R1 will switch the flow to the backup
tree, which is R1->R11->R3->{R4, R5}. By using the PCECC, path
computation, label downloading, and finally forwarding can be done
without the complex signalling used in the P2MP RSVP-TE or mLDP.

3.6.3. PCECC for the Local Protection of P2MP/MP2MP LSPs

In this section, we describe the local protection service in the
PCECC network for the P2MP/MP2MP LSP.

While the PCECC sets up the primary multicast tree, it can also build
the backup LSP between the Point of Local Repair (PLR), protected
node, and Merge Points (MPs) (the downstream nodes of the protected
node). In the cases where the amount of downstream nodes is huge,
this mechanism can avoid unnecessary packet duplication on the PLR
and protect the network from traffic congestion risks.

+------------+
| R1 | Root Node
+------------+
.
.
.
+------------+ Point of Local Repair /
| R10 | Switchover Point
+------------+ (Upstream LSR)
/ +
10/ +20
/ +
+----------+ +-----------+
Protected Node | R20 | | R30 |
+----------+ +-----------+
| \ + +
| \ + +
10| 10\ +20 20+
| \ + +
| \ +
| + \ +
+-----------+ +-----------+ Merge Point
| R40 | | R50 | (Downstream LSR)
+-----------+ +-----------+
. .
. .

Figure 11: PCECC for the Local Protection of P2MP/MP2MP LSPs

In Figure 11, when the PCECC sets up the primary multicast path
around the PLR node R10 to protect node R20, which is R10->R20->{R40,
R50}, it can set up the backup path R10->R30->{R40, R50} at the same
time. Both the primary forwarding path and the secondary bypass
forwarding path will be downloaded to each router along the primary
path and the secondary bypass path. The traffic will be forwarded
through the R10->R20->{R40, R50} path normally, and when there is a
node failure for node R20, the PLR node R10 will switch the flow to
the backup path, which is R10->R30->{R40, R50}. By using the PCECC,
path computation, label downloading, and finally forwarding can be
done without the complex signalling used in the P2MP RSVP-TE or mLDP.

3.7. PCECC for Traffic Classification

As described in [RFC8283], traffic classification is an important
part of traffic engineering. It is the process of looking into a
packet to determine how it should be treated while it is forwarded
through the network. It applies in many scenarios, including the
following:

* MPLS traffic engineering (where it determines what traffic is
forwarded into which LSPs),

* SR (where it is used to select which set of forwarding
instructions (SIDs) to add to a packet), and

* SFC (where it indicates how a packet should be forwarded across
which service function path).

In conjunction with traffic engineering, traffic classification is an
important enabler for LB. Traffic classification is closely linked
to the computational elements of planning for the network functions
because it determines how traffic is balanced and distributed through
the network. Therefore, selecting what traffic classification
mechanism should be performed by a router is an important part of the
work done by a PCECC.

The description of traffic flows by the combination of multiple Flow
Specification components and their dissemination as traffic Flow
Specifications is described for BGP in [RFC8955]. When a PCECC is
used to initiate tunnels (such as TE LSPs or SR paths) using PCEP, it
is important that the headend of the tunnels understands what traffic
to place on each tunnel. [RFC9168] specifies a set of extensions to
PCEP to support the dissemination of Flow Specification components
where the instructions are passed from the PCECC to the routers using
PCEP.

Along with traffic classification, there are a few more questions
about the tunnels set up by the PCECC that need to be considered:

* how to use it,

* whether it is a virtual link,

* whether to advertise it in the IGP as a virtual link, and

* what bits of this information to signal to the tail end.

These are out of the scope of this document.

3.8. PCECC for SFC

Service Function Chaining (SFC) is described in [RFC7665]. It is the
process of directing traffic in a network such that it passes through
specific hardware devices or virtual machines (known as service
function nodes) that can perform particular desired functions on the
traffic. The set of functions to be performed and the order in which
they are to be performed is known as a service function chain. The
chain is enhanced with the locations at which the service functions
are to be performed to derive a Service Function Path (SFP). Each
packet is marked as belonging to a specific SFP, and that marking
lets each successive service function node know which functions to
perform and to which service function node to send the packet next.
To operate an SFC network, the service function nodes must be
configured to understand the packet markings, and the edge nodes must
be told how to mark packets entering the network. Additionally, it
may be necessary to establish tunnels between service function nodes
to carry the traffic. Planning an SFC network requires LB between
service function nodes and traffic engineering across the network
that connects them. As per [RFC8283], these are operations that can
be performed by a PCECC, and that controller can use PCEP to program
the network and install the service function chains and any required
tunnels.

A possible mechanism could add support for SFC-based central control
instructions. The PCECC will be able to instruct each Service
Function Forwarder (SFF) along the SFP.

* Service Path Identifier (SPI): Uniquely identifies an SFP.

* Service Index (SI): Provides location within the SFP.

* Provide SFC Proxy handling instruction.

The PCECC can play the role of setting the traffic classification
rules (as per Section 3.7) at the classifier to impose the Network
Service Header (NSH) [RFC8300]. It can also download the forwarding
instructions to each SFF along the way so that they could process the
NSH and forward accordingly. This includes instructions for the
service classifier that handles the context header, metadata, etc.
This metadata/context is shared amongst SFs and classifiers, between
SFs, and between external systems (such as a PCECC) and SFs. As
described in [RFC7665], the SFC encapsulation enables the sharing of
metadata/context information along the SFP.

It is also possible to support SFC with SR in conjunction with or
without an NSH such as described in [RFC9491] and [SR-SERVICE].
PCECC techniques can also be used for service-function-related
segments and SR service policies.

3.9. PCECC for Native IP

[RFC8735] describes the scenarios and simulation results for the
"Centralized Control Dynamic Routing (CCDR)" solution, which
integrates the advantage of using distributed protocols (IGP/BGP) and
the power of a centralized control technology (PCE/SDN), providing
traffic engineering for native IP networks. [RFC8821] defines the
framework for CCDR traffic engineering within a native IP network,
using multiple BGP sessions and a PCE as the centralized controller.
It requires the PCECC to send the instructions to the PCCs to build
multiple BGP sessions, distribute different prefixes on the
established BGP sessions, and assign the different paths to the BGP
next hops. The PCEP is used to transfer the key parameters between
the PCE and the underlying network devices (PCC) using the PCECC
technique. The central control instructions from the PCECC to PCC
will identify which prefix should be advertised on which BGP session.
There are PCEP extensions defined in [PCEP-NATIVE] for it.

+------+
+----------+ PCECC+-------+
| +------+ |
| |
PCEP | BGP Session 1(lo11/lo21)| PCEP
+-------------------------+
| |
| BGP Session 2(lo12/lo22)|
+-------------------------+
PF12 | | PF22
PF11 | | PF21
+---+ +-----+-----+ +-----+-----+ +---+
|SW1+---------+(lo11/lo12)+-------------+(lo21/lo22)+-----------+SW2|
+---+ | R1 +-------------+ R2 | +---+
+-----------+ +-----------+

Figure 12: PCECC for Native IP

In the case as shown in Figure 12, the PCECC will instruct both R1
and R2 how to form BGP sessions with each other via PCEP and which IP
prefixes need to be advertised via which BGP session.

3.10. PCECC for BIER

Bit Index Explicit Replication (BIER) [RFC8279] defines an
architecture where all intended multicast receivers are encoded as a
BitMask in the multicast packet header within different
encapsulations. A router that receives such a packet will forward
that packet based on the bit position in the packet header towards
the receiver(s) following a precomputed tree for each of the bits in
the packet. Each receiver is represented by a unique bit in the
BitMask.

BIER-TE [RFC9262] shares architecture and packet formats with BIER.
BIER-TE forwards and replicates packets based on a BitString in the
packet header, but every BitPosition of the BitString of a BIER-TE
packet indicates one or more adjacencies. BIER-TE paths can be
derived from a PCE and used at the ingress (a possible mechanism is
described in [PCEP-BIER]).

The PCECC mechanism could be used for the allocation of bits for the
BIER router for BIER as well as for the adjacencies for BIER-TE.
PCECC-based controllers can use PCEP to instruct the BIER-capable
routers on the meaning of the bits as well as other fields needed for
BIER encapsulation. The PCECC could be used to program the BIER
router with various parameters used in the BIER encapsulation (such
as BIER sub-domain-id, BFR-id, etc.) for both node and adjacency.

A possible way to use the PCECC and PCEP extension is described in
[PCECC-BIER].

4. IANA Considerations

This document has no IANA actions.

5. Security Considerations

[RFC8283] describes how the security considerations for a PCECC are a
little different from those for any other PCE system. PCECC
operations rely heavily on the use and security of PCEP, so due
consideration should be given to the security features discussed in
[RFC5440] and the additional mechanisms described in [RFC8253]. It
further lists the vulnerability of a central controller architecture,
such as a central point of failure, denial of service, and a focus on
interception and modification of messages sent to individual Network
Elements (NEs).

As per [RFC9050], the use of Transport Layer Security (TLS) in PCEP
is recommended, as it provides support for peer authentication,
message encryption, and integrity. It further provides mechanisms
for associating peer identities with different levels of access and/
or authoritativeness via an attribute in X.509 certificates or a
local policy with a specific accept-list of X.509 certificates. This
can be used to check the authority for the PCECC operations.

It is expected that each new document that is produced for a specific
use case will also include considerations of the security impacts of
the use of a PCECC on the network type and services being managed.

6. References

6.1. Normative References

[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>.

[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015,
<https://www.rfc-editor.org/info/rfc7665>.

[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>.

[RFC8253] Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
"PCEPS: Usage of TLS to Provide a Secure Transport for the
Path Computation Element Communication Protocol (PCEP)",
RFC 8253, DOI 10.17487/RFC8253, October 2017,
<https://www.rfc-editor.org/info/rfc8253>.

[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>.

[RFC8283] Farrel, A., Ed., Zhao, Q., Ed., Li, Z., and C. Zhou, "An
Architecture for Use of PCE and the PCE Communication
Protocol (PCEP) in a Network with Central Control",
RFC 8283, DOI 10.17487/RFC8283, December 2017,
<https://www.rfc-editor.org/info/rfc8283>.

[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>.

6.2. Informative References

[MAP-REDUCE]
Lee, K., Choi, T., Ganguly, A., Wolinsky, D., Boykin, P.,
and R. Figueiredo, "Parallel Processing Framework on a P2P
System Using Map and Reduce Primitives",
DOI 10.1109/IPDPS.2011.315, May 2011,
<https://leeky.me/publications/mapreduce_p2p.pdf>.

[MPLS-DC] Afanasiev, D. and D. Ginsburg, "MPLS in DC and inter-DC
networks: the unified forwarding mechanism for network
programmability at scale", March 2014,
<https://www.slideshare.net/DmitryAfanasiev1/yandex-
nag201320131031>.

[MPLS-SEAMLESS]
Leymann, N., Ed., Decraene, B., Filsfils, C.,
Konstantynowicz, M., Ed., and D. Steinberg, "Seamless MPLS
Architecture", Work in Progress, Internet-Draft, draft-
ietf-mpls-seamless-mpls-07, 28 June 2014,
<https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
seamless-mpls-07>.

[PCE-ID] Li, C., Shi, H., Ed., Wang, A., Cheng, W., and C. Zhou,
"Path Computation Element Communication Protocol (PCEP)
extension to advertise the PCE Controlled Identifier
Space", Work in Progress, Internet-Draft, draft-ietf-pce-
controlled-id-space-01, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
controlled-id-space-01>.

[PCE-INTERDOMAIN]
Dugeon, O., Meuric, J., Lee, Y., and D. Ceccarelli, "PCEP
Extension for Stateful Inter-Domain Tunnels", Work in
Progress, Internet-Draft, draft-ietf-pce-stateful-
interdomain-05, 5 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
stateful-interdomain-05>.

[PCE-PROTECTION]
Barth, C. and R. Torvi, "PCEP Extensions for RSVP-TE
Local-Protection with PCE-Stateful", Work in Progress,
Internet-Draft, draft-cbrt-pce-stateful-local-protection-
01, 29 June 2018, <https://datatracker.ietf.org/doc/html/
draft-cbrt-pce-stateful-local-protection-01>.

[PCECC-BIER]
Chen, R., Zhu, C., Xu, B., Chen, H., and A. Wang, "PCEP
Procedures and Protocol Extensions for Using PCE as a
Central Controller (PCECC) of BIER", Work in Progress,
Internet-Draft, draft-chen-pce-pcep-extension-pce-
controller-bier-06, 8 July 2024,
<https://datatracker.ietf.org/doc/html/draft-chen-pce-
pcep-extension-pce-controller-bier-06>.

[PCECC-SR] Li, Z., Peng, S., Negi, M. S., Zhao, Q., and C. Zhou, "PCE
Communication Protocol (PCEP) Extensions for Using PCE as
a Central Controller (PCECC) for Segment Routing (SR) MPLS
Segment Identifier (SID) Allocation and Distribution.",
Work in Progress, Internet-Draft, draft-ietf-pce-pcep-
extension-pce-controller-sr-09, 4 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
pcep-extension-pce-controller-sr-09>.

[PCECC-SRv6]
Li, Z., Peng, S., Geng, X., and M. S. Negi, "PCE
Communication Protocol (PCEP) Extensions for Using the PCE
as a Central Controller (PCECC) for Segment Routing over
IPv6 (SRv6) Segment Identifier (SID) Allocation and
Distribution.", Work in Progress, Internet-Draft, draft-
ietf-pce-pcep-extension-pce-controller-srv6-03, 18 August
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
pce-pcep-extension-pce-controller-srv6-03>.

[PCEP-BIER]
Chen, R., Zhang, Z., Chen, H., Dhanaraj, S., Qin, F., and
A. Wang, "PCEP Extensions for Tree Engineering for Bit
Index Explicit Replication (BIER-TE)", Work in Progress,
Internet-Draft, draft-ietf-pce-bier-te-01, 10 October
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
pce-bier-te-01>.

[PCEP-NATIVE]
Wang, A., Khasanov, B., Fang, S., Tan, R., and C. Zhu,
"Path Computation Element Communication Protocol (PCEP)
Extensions for Native IP Networks", Work in Progress,
Internet-Draft, draft-ietf-pce-pcep-extension-native-ip-
40, 10 September 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
pcep-extension-native-ip-40>.

[PCEP-POLICY]
Koldychev, M., Sivabalan, S., Barth, C., Peng, S., and H.
Bidgoli, "Path Computation Element Communication Protocol
(PCEP) Extensions for Segment Routing (SR) Policy
Candidate Paths", Work in Progress, Internet-Draft, draft-
ietf-pce-segment-routing-policy-cp-18, 14 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
segment-routing-policy-cp-18>.

[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, <https://www.rfc-editor.org/info/rfc1195>.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.

[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>.

[RFC3985] Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985,
DOI 10.17487/RFC3985, March 2005,
<https://www.rfc-editor.org/info/rfc3985>.

[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206,
DOI 10.17487/RFC4206, October 2005,
<https://www.rfc-editor.org/info/rfc4206>.

[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.

[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.

[RFC4655] Farrel, A., Vasseur, J.-P., 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>.

[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <https://www.rfc-editor.org/info/rfc5036>.

[RFC5150] Ayyangar, A., Kompella, K., Vasseur, JP., and A. Farrel,
"Label Switched Path Stitching with Generalized
Multiprotocol Label Switching Traffic Engineering (GMPLS
TE)", RFC 5150, DOI 10.17487/RFC5150, February 2008,
<https://www.rfc-editor.org/info/rfc5150>.

[RFC5151] Farrel, A., Ed., Ayyangar, A., and JP. Vasseur, "Inter-
Domain MPLS and GMPLS Traffic Engineering -- Resource
Reservation Protocol-Traffic Engineering (RSVP-TE)
Extensions", RFC 5151, DOI 10.17487/RFC5151, February
2008, <https://www.rfc-editor.org/info/rfc5151>.

[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.

[RFC5376] Bitar, N., Zhang, R., and K. Kumaki, "Inter-AS
Requirements for the Path Computation Element
Communication Protocol (PCECP)", RFC 5376,
DOI 10.17487/RFC5376, November 2008,
<https://www.rfc-editor.org/info/rfc5376>.

[RFC5541] Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
Objective Functions in the Path Computation Element
Communication Protocol (PCEP)", RFC 5541,
DOI 10.17487/RFC5541, June 2009,
<https://www.rfc-editor.org/info/rfc5541>.

[RFC7025] Otani, T., Ogaki, K., Caviglia, D., Zhang, F., and C.
Margaria, "Requirements for GMPLS Applications of PCE",
RFC 7025, DOI 10.17487/RFC7025, September 2013,
<https://www.rfc-editor.org/info/rfc7025>.

[RFC7399] Farrel, A. and D. King, "Unanswered Questions in the Path
Computation Element Architecture", RFC 7399,
DOI 10.17487/RFC7399, October 2014,
<https://www.rfc-editor.org/info/rfc7399>.

[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>.

[RFC7491] King, D. and A. Farrel, "A PCE-Based Architecture for
Application-Based Network Operations", RFC 7491,
DOI 10.17487/RFC7491, March 2015,
<https://www.rfc-editor.org/info/rfc7491>.

[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.

[RFC8300] Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
"Network Service Header (NSH)", RFC 8300,
DOI 10.17487/RFC8300, January 2018,
<https://www.rfc-editor.org/info/rfc8300>.

[RFC8355] Filsfils, C., Ed., Previdi, S., Ed., Decraene, B., and R.
Shakir, "Resiliency Use Cases in Source Packet Routing in
Networking (SPRING) Networks", RFC 8355,
DOI 10.17487/RFC8355, March 2018,
<https://www.rfc-editor.org/info/rfc8355>.

[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>.

[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/info/rfc8664>.

[RFC8735] Wang, A., Huang, X., Kou, C., Li, Z., and P. Mi,
"Scenarios and Simulation Results of PCE in a Native IP
Network", RFC 8735, DOI 10.17487/RFC8735, February 2020,
<https://www.rfc-editor.org/info/rfc8735>.

[RFC8751] Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., and D. King,
"Hierarchical Stateful Path Computation Element (PCE)",
RFC 8751, DOI 10.17487/RFC8751, March 2020,
<https://www.rfc-editor.org/info/rfc8751>.

[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.

[RFC8821] Wang, A., Khasanov, B., Zhao, Q., and H. Chen, "PCE-Based
Traffic Engineering (TE) in Native IP Networks", RFC 8821,
DOI 10.17487/RFC8821, April 2021,
<https://www.rfc-editor.org/info/rfc8821>.

[RFC8955] Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
Bacher, "Dissemination of Flow Specification Rules",
RFC 8955, DOI 10.17487/RFC8955, December 2020,
<https://www.rfc-editor.org/info/rfc8955>.

[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.

[RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012,
DOI 10.17487/RFC9012, April 2021,
<https://www.rfc-editor.org/info/rfc9012>.

[RFC9050] Li, Z., Peng, S., Negi, M., Zhao, Q., and C. Zhou, "Path
Computation Element Communication Protocol (PCEP)
Procedures and Extensions for Using the PCE as a Central
Controller (PCECC) of LSPs", RFC 9050,
DOI 10.17487/RFC9050, July 2021,
<https://www.rfc-editor.org/info/rfc9050>.

[RFC9087] Filsfils, C., Ed., Previdi, S., Dawra, G., Ed., Aries, E.,
and D. Afanasiev, "Segment Routing Centralized BGP Egress
Peer Engineering", RFC 9087, DOI 10.17487/RFC9087, August
2021, <https://www.rfc-editor.org/info/rfc9087>.

[RFC9168] Dhody, D., Farrel, A., and Z. Li, "Path Computation
Element Communication Protocol (PCEP) Extension for Flow
Specification", RFC 9168, DOI 10.17487/RFC9168, January
2022, <https://www.rfc-editor.org/info/rfc9168>.

[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.

[RFC9262] Eckert, T., Ed., Menth, M., and G. Cauchie, "Tree
Engineering for Bit Index Explicit Replication (BIER-TE)",
RFC 9262, DOI 10.17487/RFC9262, October 2022,
<https://www.rfc-editor.org/info/rfc9262>.

[RFC9491] Guichard, J., Ed. and J. Tantsura, Ed., "Integration of
the Network Service Header (NSH) and Segment Routing for
Service Function Chaining (SFC)", RFC 9491,
DOI 10.17487/RFC9491, November 2023,
<https://www.rfc-editor.org/info/rfc9491>.

[RFC9522] Farrel, A., Ed., "Overview and Principles of Internet
Traffic Engineering", RFC 9522, DOI 10.17487/RFC9522,
January 2024, <https://www.rfc-editor.org/info/rfc9522>.

[RFC9552] Talaulikar, K., Ed., "Distribution of Link-State and
Traffic Engineering Information Using BGP", RFC 9552,
DOI 10.17487/RFC9552, December 2023,
<https://www.rfc-editor.org/info/rfc9552>.

[RFC9603] Li, C., Ed., Kaladharan, P., Sivabalan, S., Koldychev, M.,
and Y. Zhu, "Path Computation Element Communication
Protocol (PCEP) Extensions for IPv6 Segment Routing",
RFC 9603, DOI 10.17487/RFC9603, July 2024,
<https://www.rfc-editor.org/info/rfc9603>.

[RFC9604] Sivabalan, S., Filsfils, C., Tantsura, J., Previdi, S.,
and C. Li, Ed., "Carrying Binding Label/SID in PCE-Based
Networks", RFC 9604, DOI 10.17487/RFC9604, August 2024,
<https://www.rfc-editor.org/info/rfc9604>.

[SR-SERVICE]
Clad, F., Ed., Xu, X., Ed., Filsfils, C., Bernier, D., Li,
C., Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W.,
and S. Salsano, "Service Programming with Segment
Routing", Work in Progress, Internet-Draft, draft-ietf-
spring-sr-service-programming-10, 23 August 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
sr-service-programming-10>.

Appendix A. Other Use Cases of the PCECC

This section lists some more use cases of the PCECC that were
proposed by operators and discussed within the working group but are
not in active development at the time of publication. They are
listed here for future consideration.

A.1. PCECC for Network Migration

One of the main advantages of the PCECC solution is its backward
compatibility. The PCE server can function as a proxy node of the
MPLS network for all the new nodes that no longer support the
signalling protocols.

As illustrated in the following example, the current network could
migrate to a total PCECC-controlled network gradually by replacing
the legacy nodes. During the migration, the legacy nodes still need
to use the existing MPLS signalling protocols such as LDP and RSVP-
TE, and the new nodes will set up their portion of the forwarding
path through the PCECC directly. With the PCECC function as the
proxy of these new nodes, MPLS signalling can populate through the
network for both old and new nodes.

The example described in this section is based on network
configurations illustrated in Figure 13:

+------------------------------------------------------------------+
| PCE DOMAIN |
| +-----------------------------------------------------+ |
| | PCECC | |
| +-----------------------------------------------------+ |
| ^ ^ ^ ^ |
| | PCEP | | PCEP | |
| V V V V |
| +--------+ +--------+ +--------+ +--------+ +--------+ |
| | Node1 | | Node2 | | Node3 | | Node4 | | Node5 | |
| | |...| |...| |...| |...| | |
| | Legacy |if1| Legacy |if2|Legacy |if3| PCECC |if4| PCECC | |
| | Node | | Node | |Enabled | |Enabled | | Enabled| |
| +--------+ +--------+ +--------+ +--------+ +--------+ |
| |
+------------------------------------------------------------------+

Figure 13: PCECC-Initiated LSP Setup in the Network Migration

In this example, there are five nodes for the TE LSP from the headend
(Node1) to the tail end (Node5), where Node4 and Node5 are centrally
controlled and other nodes are legacy nodes.

* Node1 sends a path request message for the setup of the LSP with
the destination as Node5.

* The PCECC sends a reply message to Node1 for LSP setup with the
path: (Node1, if1), (Node2, if2), (Node3, if3), (Node4, if4),
Node5.

* Node1, Node2, and Node3 will set up the LSP to Node5 using the
local labels as usual. With the help of the PCECC, Node3 could
proxy the signalling.

* Then, the PCECC will program the out-segment of Node3, the in-
segment/out-segment of Node4, and the in-segment for Node5.

A.2. PCECC for L3VPN and PWE3

As described in [RFC8283], various network services may be offered
over a network. These include protection services (including Virtual
Private Network (VPN) services such as L3VPN [RFC4364] or EVPNs
[RFC7432]) or pseudowires [RFC3985]. Delivering services over a
network in an optimal way requires coordination in the way where
network resources are allocated to support the services. A PCECC can
consider the whole network and all components of a service at once
when planning how to deliver the service. It can then use PCEP to
manage the network resources and to install the necessary
associations between those resources.

In the case of L3VPN, VPN labels could also be assigned and
distributed through PCEP among the Provider Edge (PE) router instead
of using the BGP protocols.

The example described in this section is based on network
configurations illustrated in Figure 14:

+-------------------------------------------+
| PCE DOMAIN |
| +-----------------------------------+ |
| | PCECC | |
| +-----------------------------------+ |
| ^ ^ ^ |
| PWE3/L3VPN|PCEP PCEP|LSP PWE3/L3VPN|PCEP |
| V V V |
+--------+ | +--------+ +--------+ +--------+ | +--------+
| CE | | | PE1 | | Node x | | PE2 | | | CE |
| |...... | |...| |...| |.....| |
| Legacy | |if1 | PCECC |if2|PCECC |if3| PCECC |if4 | Legacy |
| Node | | | Enabled| |Enabled | |Enabled | | | Node |
+--------+ | +--------+ +--------+ +--------+ | +--------+
| |
+-------------------------------------------+

Figure 14: PCECC for L3VPN and PWE3

In the case of PWE3, instead of using the LDP signalling protocols,
the label and port pairs assigned to each pseudowire can be assigned
through the PCECC among the PE routers and the corresponding
forwarding entries will be distributed into each PE router through
the extended PCEP and PCECC mechanism.

A.3. PCECC for Local Protection (RSVP-TE)

[PCE-PROTECTION] claims that there is a need for the PCE to maintain
and associate the local protection paths for the RSVP-TE LSP. Local
protection requires the setup of a bypass at the PLR. This bypass
can be PCC-initiated and delegated or PCE-initiated. In either case,
the PLR needs to maintain a PCEP session with the PCE. The bypass
LSPs need to be mapped to the primary LSP. This could be done
locally at the PLR based on a local policy, but there is a need for a
PCE to do the mapping as well to exert greater control.

This mapping can be done via PCECC procedures where the PCE could
instruct the PLR to the mapping and identify the primary LSP for
which bypass should be used.

A.4. Using Reliable P2MP TE-Based Multicast Delivery for Distributed
Computations (MapReduce-Hadoop)

The MapReduce model of distributed computations in computing clusters
is widely deployed. In Hadoop (https://hadoop.apache.org/) 1.0
architecture, MapReduce operations occur on big data in the Hadoop
Distributed File System (HDFS), where NameNode knows about resources
of the cluster and where actual data (chunks) for a particular task
are located (which DataNode). Each chunk of data (64 MB or more)
should have three saved copies in different DataNodes based on their
proximity.

The proximity level currently has a semi-manual allocation and is
based on Rack IDs (the assumption is that closer data is better
because of access speed / smaller latency).

The JobTracker node is responsible for computation tasks and
scheduling across DataNodes and also has Rack awareness. Currently,
transport protocols between NameNode/JobTracker and DataNodes are
based on IP unicast. It has simplicity as an advantage but has
numerous drawbacks related to its flat approach.

There is a need to go beyond one data center (DC) for Hadoop cluster
creation and move towards distributed clusters. In that case, one
needs to handle performance and latency issues. Latency depends on
the speed of light in the fiber links and on the latency introduced
by intermediate devices in between. The latter is closely correlated
with network device architecture and performance. The current
performance of routers based on Network Processing Unit (NPU) should
be enough for creating distributed Hadoop clusters with predicted
latency. The performance of software-based routers (mainly Virtual
Network Functions (VNFs)) with additional hardware features such as
the Data Plane Development Kit (DPDK) is promising but requires
additional research and testing.

The main question is how to create a simple but effective
architecture for a distributed Hadoop cluster.

There is research [MAP-REDUCE] that shows how usage of the multicast
tree could improve the speed of resource or cluster members'
discovery inside the cluster as well as increased redundancy in
communications between cluster nodes.

The conventional IP-based multicast may not be appropriate because it
requires an additional control plane (IGMP, PIM) and a lot of
signalling, which is not suitable for high-performance computations
that are very sensitive to latency.

P2MP TE tunnels are more suitable as a potential solution for the
creation of multicast-based communications between NameNode as the
root and DataNodes as leaves inside the cluster. These P2MP tunnels
could be dynamically created and turned down (with no manual
intervention). Here, the PCECC comes into play with the main
objective of creating an optimal topology for each particular request
for MapReduce computation and creating P2MP tunnels with needed
parameters such as BW and delay.

This solution will require the use of MPLS label-based forwarding
inside the cluster. The usage of label-based forwarding inside DC
was proposed by Yandex [MPLS-DC]. Technically, it is already
possible because MPLS on switches is already supported by some
vendors, and MPLS also exists on Linux and Open vSwitch (OVS).

A possible framework for this task is shown in Figure 15:

+--------+
| APP |
+--------+
| NBI (REST API,...)
|
PCEP +----------+ REST API
+---------+ +---| PCECC |----------+
| Client |---|---| | |
+---------+ | +----------+ |
| | | | | |
+-----|---+ |PCEP| |
+--------+ | | | | |
| | | | | |
| REST API | | | | |
| | | | | |
+-------------+ | | | | +----------+
| Job Tracker | | | | | | NameNode |
| | | | | | | |
+-------------+ | | | | +----------+
+------------------+ | +-----------+
| | | |
|---+-----P2MP TE--+-----|-----------| |
+-----------+ +-----------+ +-----------+
| DataNode1 | | DataNode2 | | DataNodeN |
|TaskTracker| |TaskTracker| .... |TaskTracker|
+-----------+ +-----------+ +-----------+

Figure 15: Using Reliable P2MP TE-Based Multicast Delivery for
Distributed Computations (MapReduce-Hadoop)

Communication between the JobTracker, NameNode, and PCECC can be done
via REST API directly or via a cluster manager such as Mesos.

* Phase 1: Distributed cluster resource discovery occurs during this
phase. JobTracker and NameNode should identify and find available
DataNodes according to computing requests from the application
(APP). NameNode should query the PCECC about available DataNodes,
and NameNode may provide additional constraints to the PCECC such
as topological proximity and redundancy level.

The PCECC should analyze the topology of the distributed cluster
and perform a constraint-based path calculation from the client
towards the most suitable NameNodes. The PCECC should reply to
NameNode with the list of the most suitable DataNodes and their
resource capabilities. The topology discovery mechanism for the
PCECC will be added later to that framework.

* Phase 2: The PCECC should create P2MP LSPs from the client towards
those DataNodes by means of PCEP messages following the previously
calculated path.

* Phase 3: NameNode should send this information to the client, and
the PCECC should inform the client about the optimal P2MP path
towards DataNodes via a PCEP message.

* Phase 4: The client sends data blocks to those DataNodes for
writing via the created P2MP tunnel.

When this task is finished, the P2MP tunnel could be turned down.

Acknowledgments

Thanks to Adrian Farrel, Aijun Wang, Robert Tao, Changjiang Yan,
Tieying Huang, Sergio Belotti, Dieter Beller, Andrey Elperin, and
Evgeniy Brodskiy for their useful comments and suggestions.

Thanks to Mach Chen and Carlos Pignataro for the RTGDIR review.
Thanks to Derrell Piper for the SECDIR review. Thanks to Sue Hares
for GENART review.

Thanks to Vishnu Pavan Beeram for being the document shepherd and Jim
Guichard for being the responsible AD.

Thanks to Roman Danyliw for the IESG review comments.

Contributors

Luyuan Fang
United States of America
Email: [email protected]

Chao Zhou
HPE
Email: [email protected]

Boris Zhang
Amazon
Email: [email protected]

Artsiom Rachytski
AWS
Germany
Email: [email protected]

Anton Hulida
AWS
Australia
Email: [email protected]

Authors' Addresses

Zhenbin (Robin) Li
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Beijing
100095
China
Email: [email protected]

Dhruv Dhody
Huawei Technologies
India
Email: [email protected]

Quintin Zhao
Etheric Networks
1009 S Claremont St.
San Mateo, CA 94402
United States of America
Email: [email protected]

King He
Tencent Holdings Ltd.
Shenzhen
China
Email: [email protected]

Boris Khasanov
MTS Web Services (MWS)
Andropova Ave. 18, building 9
Moscow
Russian Federation
Email: [email protected]