wdiff rfc9832.original rfc9832.txt

Network Working Group

Internet Engineering Task Force (IETF) K. Vairavakkalai, Ed.
Internet-Draft
Request for Comments: 9832 N. Venkataraman, Ed.
Intended status:
Category: Experimental Juniper Networks, Inc.
Expires: 1 September 2025 28 February
ISSN: 2070-1721 August 2025

BGP Classful Transport Planes
draft-ietf-idr-bgp-ct-39

Abstract

This document specifies a mechanism referred to as "Intent Driven "Intent-Driven
Service Mapping". The mechanism uses BGP to express intent based intent-based
association of overlay routes with underlay routes having specific
Traffic Engineering (TE) characteristics satisfying a certain Service
Level Agreement (SLA). This is achieved by defining new constructs
to group underlay routes with sufficiently similar TE characteristics
into identifiable classes (called "Transport Classes"), Classes" or "TCs"), that
overlay routes use as an ordered set to resolve reachability
(Resolution Schemes) towards service endpoints. These constructs can
be used, for example, to realize the "IETF Network Slice" defined in
the TEAS Network Slices framework.

Additionally, this document specifies protocol procedures for BGP
that enable dissemination of service mapping information in a network
that may span multiple cooperating administrative domains. These
domains may be administered either by the same provider or by closely
coordinating providers. A new BGP address family that leverages RFC
4364 ("BGP/MPLS the
procedures described in "BGP/MPLS IP Virtual Private Networks (VPNs)") procedures (VPNs)"
(RFC 4364) and follows the NLRI encoding described in RFC 8277
("Using BGP to Bind MPLS Labels to Address Prefixes") NLRI encoding is defined to
enable each advertised underlay route to be identified by its class.
This new address family is called "BGP Classful Transport", a.k.a., BGP CT.

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. Transport" (or "BGP
CT").

Status of This Memo

This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 not an Internet Standards Track specification; it is
published for examination, experimental implementation, and BCP 79.

Internet-Drafts are working documents
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents valid approved by the IESG are candidates for a maximum any level of six months
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 updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on 1 September 2025.
https://www.rfc-editor.org/info/rfc9832.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info)
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Abbreviations
2.2. Definitions and Notations . . . . . . . . . . . . . . . . 8
2.3. Requirements Language
3. Architecture Overview . . . . . . . . . . . . . . . . . . . . 10
4. Transport Class . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Classifying TE tunnels . . . . . . . . . . . . . . . . . 13 Tunnels
4.2. Transport Route Database . . . . . . . . . . . . . . . . 15 (TRDB)
4.3. "Transport Class" Route Target Extended Community . . . . 15
5. Resolution Scheme . . . . . . . . . . . . . . . . . . . . . . 17
5.1. Mapping Community . . . . . . . . . . . . . . . . . . . . 18
6. BGP Classful Transport Family . . . . . . . . . . . . . . . . 19
6.1. NLRI Encoding . . . . . . . . . . . . . . . . . . . . . . 19
6.2. Next Hop Encoding . . . . . . . . . . . . . . . . . . . . 19
6.3. Carrying multiple Multiple Encapsulation Information . . . . . . . 20
6.4. Comparison with Other Families using RFC-8277 Using Encoding . 20 from RFC 8277
7. Protocol Procedures . . . . . . . . . . . . . . . . . . . . . 22
7.1. Preparing the network Network to deploy Deploy Classful Transport
planes . . . . . . . . . . . . . . . . . . . . . . . . . 22 Planes
7.2. Originating Classful Transport Routes . . . . . . . . . . 22
7.3. Processing Classful Transport Routes by Ingress Nodes . . 23
7.4. Readvertising Classful Transport Route by Border Nodes . 24
7.5. Border Nodes Receiving Classful Transport Routes on EBGP . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.6. Avoiding Path Hiding Through Route Reflectors . . . . . . 25
7.7. Avoiding Loops Between Route Reflectors in Forwarding
Path . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Paths
7.8. Ingress Nodes Receiving Service Routes with a Mapping
Community . . . . . . . . . . . . . . . . . . . . . . . 25
7.9. Best Effort Best-Effort Transport Class . . . . . . . . . . . . . . . 26
7.10. Interaction with BGP Attributes Specifying Next Hop Address
and Color . . . . . . . . . . . . . . . . . . . . . . . 27
7.11. Applicability to Flowspec Redirect to IP . . . . . . . . 27 Redirect-to-IP
7.12. Applicability to IPv6 . . . . . . . . . . . . . . . . . . 28
7.13. SRv6 Support . . . . . . . . . . . . . . . . . . . . . . 28
7.14. Error Handling Error-Handling Considerations . . . . . . . . . . . . . . 29
8. Illustration of BGP CT Procedures . . . . . . . . . . . . . . 29
8.1. Reference Topology . . . . . . . . . . . . . . . . . . . 29
8.2. Service Layer Service-Layer Route Exchange . . . . . . . . . . . . . . 31
8.3. Transport Layer Transport-Layer Route Propagation . . . . . . . . . . . . 32
8.4. Data Plane View . . . . . . . . . . . . . . . . . . . . . 35
8.4.1. Steady State . . . . . . . . . . . . . . . . . . . . 35
8.4.2. Local Repair of Primary Path . . . . . . . . . . . . 35
8.4.3. Absorbing Failure of the Primary Path: Fallback to Best
Effort
Best-Effort Tunnels . . . . . . . . . . . . . . . . . . . 36
9. Scaling Considerations . . . . . . . . . . . . . . . . . . . 36
9.1. Avoiding Unintended Spread of BGP CT Routes Across Domains . . . . . . . . . . . . . . . . . . . . . . . . . 36
9.2. Constrained Distribution of PNHs to SNs (On-Demand Next
Hop) . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.3. Limiting The the Visibility Scope of PE Loopback as PNHs . . 38
10. Operations and Manageability Considerations . . . . . . . . . 39
10.1. MPLS OAM . . . . . . . . . . . . . . . . . . . . . . . . 39
10.2. Usage of Route Distinguisher RD and Label Allocation Label-Allocation Modes . . . . . . . . . . . . . . . . . . . . . . . . . 40
10.3. Managing Transport Route Transport-Route Visibility . . . . . . . . . . 41
11. Deployment Considerations. . . . . . . . . . . . . . . . . . 44 Considerations
11.1. Coordination Between Domains Using Different Community
Namespaces . . . . . . . . . . . . . . . . . . . . . . . 44
11.2. Managing Intent at Service and Transport layers. . . . . 44 Layers
11.2.1. Service Layer Service-Layer Color Management . . . . . . . . . . . 44
11.2.2. Non-Agreeing Color Transport Domains . . . . . . . . 45
11.2.3. Heterogeneous Agreeing Color Transport Domains . . . 46
11.3. Migration Scenarios. . . . . . . . . . . . . . . . . . . 49 Scenarios
11.3.1. BGP CT Islands Connected via BGP LU Domain . . . . . 49
11.3.2. BGP CT - CT: Interoperability between Between MPLS and Other
Forwarding Technologies . . . . . . . . . . . . . . . 51
11.4. MTU Considerations . . . . . . . . . . . . . . . . . . . 54
11.5. Use of DSCP . . . . . . . . . . . . . . . . . . . . . . 54
12. Applicability to Network Slicing . . . . . . . . . . . . . . 55
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 55
13.1. New BGP SAFI . . . . . . . . . . . . . . . . . . . . . . 56
13.2. New Format for BGP Extended Community . . . . . . . . . 56
13.2.1. Existing Registries . . . . . . . . . . . . . . . . 57
13.2.2. New Registries . . . . . . . . . . . . . . . . . . . 57
13.3. MPLS OAM Code Points . . . . . . . . . . . . . . . . . . 58
14. Registries maintained by this document . . . . . . . . . . . 59
14.1. Transport Class ID . . . . . . . . . . . . . . . . . . . 59 Registry
15. Security Considerations . . . . . . . . . . . . . . . . . . . 60
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 61
16.1. Normative References . . . . . . . . . . . . . . . . . . 61
16.2. Informative References . . . . . . . . . . . . . . . . . 64
Appendix A. Extensibility considerations . . . . . . . . . . . . 66 Considerations
A.1. Signaling Intent over a PE-CE Attachment Circuit . . . . . 66
A.2. BGP CT Egress TE . . . . . . . . . . . . . . . . . . . . 66
Appendix B. Applicability to Intra-AS and different Different Inter-AS
deployments. . . . . . . . . . . . . . . . . . . . . . . 67
Deployments
B.1. Intra-AS usecase . . . . . . . . . . . . . . . . . . . . 67 Use Case
B.1.1. Topology . . . . . . . . . . . . . . . . . . . . . . 67
B.1.2. Transport Layer . . . . . . . . . . . . . . . . . . . 67
B.1.3. Service Layer route exchange . . . . . . . . . . . . 68 Service-Layer Route Exchange
B.2. Inter-AS option Option A usecase . . . . . . . . . . . . . . . . 69 Use Case
B.2.1. Topology . . . . . . . . . . . . . . . . . . . . . . 69
B.2.2. Transport Layer . . . . . . . . . . . . . . . . . . . 69
B.2.3. Service Layer route exchange . . . . . . . . . . . . 70 Route Exchange
B.3. Inter-AS option Option B usecase . . . . . . . . . . . . . . . . 71 Use Case
B.3.1. Topology . . . . . . . . . . . . . . . . . . . . . . 71
B.3.2. Transport Layer . . . . . . . . . . . . . . . . . . . 71
B.3.3. Service Layer route exchange . . . . . . . . . . . . 72 Service-Layer Route Exchange
Appendix C. Why reuse RFC RFCs 8277 and RFC 4364? . . . . . . . . . . 73
C.1. Update packing considerations . . . . . . . . . . . . . . 74 Packing Considerations
Appendix D. Scaling using Using BGP MPLS Namespaces . . . . . . . . . 75
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Co-Authors . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Other Contributors . . . . . . . . . . . . . . . . . . . . . . 76
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 77
Contributors
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 77

1. Introduction

Provider networks typically span across multiple domains where each
domain can either represent an Autonomous System (AS) or an Interior
Gateway Protocol (IGP) region within an AS. In these networks,
several services are provisioned between different pairs of service
endpoints (e.g., Provider Edge (PE) nodes), nodes) that can either be either in the
same domain or across different domains.

[RFC9315] defines "Intent" as, "A as:

| A set of operational goals (that a network should meet) and
| outcomes (that a network is supposed to deliver) defined in a
| declarative manner without specifying how to achieve or implement them.".
| them.

This document prescribes constructs and procedures to realize
"Intent",
"Intent" and enable provider networks to be able to forward service traffic
based on service specific intent, service-specific intent from end-to-end across service
endpoints.

The mechanisms described in this document achieve "Intent Driven "Intent-Driven
Service Mapping" between any pair of service endpoints by:

* Provisioning end-to-end "intent-aware" paths using BGP. For
example, low latency path, best effort a low-latency path or a best-effort path.

* Expressing a desired intent. For example, use low latency a low-latency path
with a fallback to the best effort best-effort path.

* Forwarding service traffic "only" using end-to-end "intent-aware"
paths honoring that desired intent.

The constructs and procedures defined in this document apply equally
to intra-AS as well as and inter-AS (a.k.a. multi-AS) deployments in the style
of Option A, Option B B, and Option C (Section 10, 10 of [RFC4364]) style deployments in
provider networks.

Such networks provision intra-domain transport tunnels between a pair
of endpoints, typically a service node or a border node that service
traffic traverses through. These tunnels are signaled using various
tunneling protocols depending on the forwarding architecture used in
the domain, which can be Multiprotocol Label Switching (MPLS),
Internet Protocol version 4 (IPv4), or Internet Protocol version 6
(IPv6).

The mechanisms defined in this document allow different tunneling
technologies to become Transport Class TC aware. These can be applied homogeneously
to intra-domain tunneling technologies used in existing brownfield
networks as well as new greenfield networks. For clarity, only some
tunneling technologies are detailed in this document. In some examples
examples, only MPLS Traffic Engineering (TE) examples are is described. Other
tunneling technologies have been described in detail in other
documents and (and only an overview has been included in this document. document).
For example, the details for Segment Routing over IPv6 (SRv6) are
provided in [BGP-CT-SRv6], [BGP-CT-SRv6] and an overview is provided in
Section 7.13.

Customers need to be able to express desired Intent to the network,
and the network needs to have constructs able to enact the customer's
intent. The network constructs defined in this document are used to
classify and group these intra-domain tunnels based on various
characteristics, like TE characteristics (e.g., low latency), low-latency), into
identifiable classes that can pass "intent-aware" traffic. These
constructs enable services to signal their intent to use one or more
identifiable classes, classes and mechanisms to selectively map traffic onto
"intent-aware" tunnels for these classes.

This document introduces a new BGP address family called "BGP
Classful Transport", that which extends/stitches intent-aware intra-domain
tunnels belonging to the same class across domain boundaries, boundaries to
establish end-to-end intent-aware paths between service endpoints.

[Intent-Routing-Color] describes various use cases and applications
of the procedures described in this document.

Appendix C provides an outline of the design philosophy behind this
specification. In particular, readers who are already familiar with
one or more BGP VPN technologies may want to review this appendix
before reading the main body of the specification.

2. Terminology

2.1. Abbreviations

ABR: Area Border Router (Readvertises (readvertises BGP CT or BGP LU routes with
next hop
NH self)

AFI: Address Family Identifier

AS: Autonomous System

ASBR: Autonomous System Border Router

ASN: Autonomous System Number

BGP VPN: VPNs built using RD, RD or RT; architecture described in RFC4364
[RFC4364]

BGP LU: BGP Labeled Unicast family (AFI/SAFIs 1/4, 2/4)

BGP CT: BGP Classful Transport family (AFI/SAFIs 1/76, 2/76)

BN: Border Node

CBF: Class Based Class-Based Forwarding

CCA: Community Carrying Attribute

CsC: Carriers' Carriers (serving the Carrier serving Carrier VPN VPN)

DSCP: Differentiated Services Code Point

EP: Endpoint of (of a tunnel, e.g. e.g., a loopback address in the network network)

EPE: Egress Peer Engineering

eSN: Egress Service Node

FEC: Forwarding Equivalence Class

FRR: Fast ReRoute (Pre-programmed next hop Reroute (Preprogrammed NH leg in forwarding)

iSN: Ingress Service Node

L-ISIS: Labeled ISIS (RFC (see RFC 8667)

LSP: Label Switched Path

MPLS: Multi Protocol Multiprotocol Label Switching

NH: Next Hop

NLRI: Network Layer Reachability Information

PE: Provider Edge

PIC: Prefix scale Independent Convergence

PNH: Protocol Next Hop address (address carried in a BGP Update message UPDATE message)

RD: Route Distinguisher

RD:EP : BGP CT Prefix consisting of

RD:EP: Route Distinguisher and Endpoint (in a BGP Prefix)

RSVP-TE: Resource Reservation Protocol - Traffic Engineering

RT: Route Target (as used in Route Target extended community community)

RTC: Route Target Constrain (RFC 4684) [RFC4684]

SAFI: Subsequent Address Family Identifier

SID: Segment Identifier

SLA: Service Level Agreement

SN: Service Node

SR: Segment Routing

SRTE: Segment Routing Traffic Engineering

TC: Transport Class

TC ID: Transport Class Identifier

TC-BE: Best Effort Transport Class - Best Effort

TE: Traffic Engineering

TEA: Tunnel Encapsulation Attribute, attribute Attribute (attribute type code 23 23)

TRDB: Transport Route Database

UHP: Ultimate Hop Pop Popping

VRF: Virtual Routing and Forwarding table

2.1. (used with a table)

2.2. Definitions and Notations

BGP Community Carrying Attribute (CCA) : CCA:
A BGP attribute that carries community. Examples of BGP CCA are: CCAs are
COMMUNITIES (attribute code 8), EXTENDED COMMUNITIES (attribute
code 16), IPv6 Address Specific Extended Community (attribute code
25), and LARGE_COMMUNITY (attribute code 32).

color:0:100 :

color:0:100:
This notation denotes a Color extended community Extended Community as defined in RFC 9012
[RFC9012] with the Flags "Flags" field set to 0 and the color "Color" field
set to 100.

End to End

End-to-End Tunnel:
A tunnel spanning several adjacent tunnel domains created by
"stitching" them together using MPLS labels or an equivalent
identifier based on the forwarding architecture.

Import processing: Receive side
Receive-side processing of an overlay route, including things like import policy
import-policy application, resolution scheme
selection resolution-scheme selection, and next hop NH
resolution.

Mapping Community:
Any BGP CCA (e.g., Community, Extended Community) on an overlay
route that maps to a Resolution Scheme. For example, color:0:100,
transport-target:0:100.

Provider Namespace:
Internal Infrastructure address space in Provider a provider network
managed by the Operator.

Resolution Scheme:
A construct comprising of an ordered set of TRDBs to resolve next hop reachability, NH
reachability for realizing a desired intent.

Service Family:
A BGP address family used for advertising routes for destinations
in "data traffic". For example, AFI/SAFIs 1/1 or 1/128.

Service Prefix:
A destinations destination in "data traffic". Routes to these prefixes are
carried in a Service family.

Transport Family:
A BGP address family used for advertising tunnels, which are are, in turn
turn, used by service routes for resolution. For example, AFI/SAFIs AFI/
SAFIs 1/4 or 1/76.

Transport Tunnel : Tunnel:
A tunnel over which a service may place traffic. Such a tunnel
can be provisioned or signaled using a variety of means. For
example, Generic Routing Encapsulation (GRE), UDP, LDP, RSVP-TE,
IGP FLEX-ALGO Flexible Algorithm (FLEX-ALGO), or SRTE.

Transport, Transport Layer:
A layer in the network that contains Transport Tunnels and
Transport Families.

Tunnel Route:
A Route to Tunnel Destination/Endpoint that is installed at the
headend (ingress) of the tunnel.

Tunnel Domain:
A domain of the network containing Service Nodes (SNs) SNs and Border Nodes (BNs) BNs under a single
administrative control that has tunnels between them.

Brownfield network:
An existing network that is already in service, deploying a chosen
set of technologies and hardware. Enhancements and upgrades to
such network deployments protect return on
investment, investment and should
consider continuity of service.

Greenfield network:
A new network deployment which that can make choice choices of new technology
or hardware as needed, needed with fewer constraints than brownfield
network.

Transport Class:
A construct to group transport tunnels offering similar SLA (Ref: Sec SLAs (see
Section 4.1).

Transport Class RT:
A Route Target Extended Community extended community used to identify a specific
Transport Class.

transport-target:0:100 :

transport-target:0:100:
This notation denotes a Transport Class RT Route Target extended
community as defined in this document with the "Transport Class
ID" field set to 100.

Transport Route Database:
At the SN and BN, a Transport Class has an associated Transport Route Database TRDB that
collects its Tunnel Routes. tunnel routes.

Transport Plane:
An end-to-end plane consisting of transport tunnels belonging to
the same Transport Class.

2.3. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Architecture Overview

This section describes the BGP CT architecture with a brief
illustration.
illustration:

INET [RR21]--------------<<---[RR11]
Service / / | IP1,color:0:100
[PE21] <<--------+ : [SN11] <<-----+ ^ IP2,color:0:200
\ ___ : \ ___ | IP3,100:200
\ _( .) : \ _( .) | ^^^^^^^^^^^
+-- ( _) --[BN21]===[BN11]--- ( _)-[PE11] Mapping
(.__) : (.__) Community
Inter-AS-Link
:
[.......AS2:SR-TE........]:[.......AS1:RSVP-TE......]
---------Traffic Direction----------->

Intents at SN11 and PE21:

Scheme1: color:0:100, (TRDB[TC-100], TRDB[TC-BE])
Scheme2: color:0:200, (TRDB[TC-200], TRDB[TC-BE])
Scheme3: 100:200, (TRDB[TC-100], TRDB[TC-200])
^^^^^^^ ^^^^ ^^^^^^
Resolution Schemes Transport Route DB Transport Class

Figure 1: BGP CT Overview with Example Topology

To achieve end-to-end "Intent Driven "Intent-Driven Service Mapping", this document
defines the following constructs and BGP extensions:

* The "Transport Class" (Section 4) construct (see Section 4) to group underlay
tunnels.

* The "Resolution Scheme" (Section 5) construct (see Section 5) for overlay
routes with Mapping Community Communities to resolve next hop NH reachability from
either one or an ordered set of Transport Classes.

* The "BGP Classful Transport" (Section (see Section 6) address family to
extend these constructs to adjacent domains.

Figure 1 depicts the intra-AS and inter-AS application of these
constructs. Interactions between SN1 and PE11 describe the Intra-AS
usage. Interactions between PE21 and PE11 describe the Inter-AS
usage.

The example topology is an Inter-AS option C network (Section 10, 10 of
[RFC4364])
network with two AS domains, domains; each domain contains tunnels serving
two Intents, e.g. e.g., 'low-latency' denoted by color 100 and 'high-bandwidth' 'high-
bandwidth' denoted by color 200. AS1 is a an RSVP-TE network, network; AS2 is a
an SRTE network. BGP CT and BGP LU are transport families used
between the two AS domains. IP1, IP2, and IP3 are service prefixes
(AFI/SAFI: 1/1) behind egress PE11.

PE21, SN11 SN11, and PE11 are the SNs in this network. SN11 is an ingress
PE with intra domain intra-domain reachability to PE11. PE21 is an ingress PE
with inter domain inter-domain reachability to PE11.

The tunneling mechanisms are made "Transport Class" aware. They
publish their underlay tunnels for a Transport Class into an
associated "Transport Route Database" (TRDB) (Section TRDB (see Section 4.2). In Figure 1, RSVP-TE publishes
its underlay tunnels into TRDBs created for Transport Class Classes 100 and
200 at BN11 and SN11 within AS1; Similarly, SR-TE publishes its
underlay tunnels into TRDBs created for Transport Class Classes 100 and 200
at PE21 within AS2.

Resolution Schemes are used to realize Intent. A Resolution Scheme
is identified by its "Mapping Community", Community" and contains an ordered list
of transport classes. Overlay routes carry an indication of the
desired Intent using a BGP community community, which assumes the role of
"Mapping Community".

Egress SN "PE11" advertises service routes with desired Mapping
Community e.g.
Community, e.g., color:0:100.

For the Intra-AS case, SN1 maps this intra-AS route on RSVP-TE
tunnels with TC ID 100 by using the Resolution Scheme for
color:0:100.

For the Inter-AS case, the underlay route in a TRDB is advertised in
BGP to extend an underlay tunnel to adjacent domains. A new BGP
transport family called "BGP Classful Transport", also known as BGP
CT (AFI/SAFIs 1/76, 2/76) 2/76), is defined for this purpose. BGP CT makes
it possible to advertise multiple tunnels to the same destination
address, thus avoiding the need for multiple loopbacks on the Egress
Service Node (eSN). eSN.

The BGP CT address family carries transport prefixes across tunnel
domain boundaries. Its design and operation are analogous to BGP LU
(AFI/SAFIs 1/4 or 2/4). It disseminates "Transport Class"
information for the transport prefixes across the participating
domains while avoiding the need of per-transport class loopback.
This is not possible with BGP LU without using per-color loopback.
This dissemination makes the end-to-end network a "Transport Class"
aware tunneled network.

In Figure 1, BGP CT routes are originated at BN11 in AS1 with next
hop NH
"self" towards BN21 in AS2 to extend available RSVP-TE tunnels for
Transport Class Classes 100 and 200 in AS1. BN21 propagates these routes
with next hop NH "self" to PE21, which resolves the BGP CT routes over SRTE
tunnels belonging to same transport class. Thus class, thus forming a BGP CT
tunnel for each TC ID at PE21.

PE21 maps the Inter-AS service routes received with color:0:100 from
AS1 on BGP CT tunnel with TC ID 100 by using the Resolution Scheme
for color:0:100. Note that this procedure is same as that followed
by SN1 in the Intra-AS case.

The following text illustrates how CT architecture provides tiered
fallback options at a per-route granularity. Figure 1, 1 shows the
Resolution Schemes in use, which make the following next hop NH resolution
happen at SN11 (Intra-AS) and PE21 (Inter-AS) for the service routes
of prefixes IP1, IP2, and IP3:

* Resolve IP1 next hop NH over available tunnels in TRDB for Transport Class
100 with fallback to TRDB for best effort.

* Resolve IP2 next hop NH over available tunnels in TRDB for Transport Class
200 with fallback to TRDB for best effort.

* Resolve IP3 next hop NH over available tunnels in TRDB for Transport Class
100 with fallback to TRDB for Transport Class 200.

In Figure 1, SN11 resolves IP1, IP2 IP2, and IP3 directly over RSVP-TE
tunnels in AS1. PE21 resolves IP1, IP2 IP2, and IP3 over extended BGP CT
tunnels that resolve over SR-TE tunnels in AS2.

This document describes procedures using MPLS forwarding
architecture. However, these procedures would work in a similar
manner for non-MPLS forwarding architectures as well. Section 7.13
describes the application of BGP CT over the SRv6 data plane.

4. Transport Class

Transport Class is a construct that groups transport tunnels offering
similar SLA SLAs within the administrative domain of a provider network
or closely coordinated provider networks.

A Transport Class is uniquely identified by a 32-bit "Transport Class
ID",
ID" that is assigned by the operator. The operator consistently
provisions a Transport Class on participating nodes (SNs and BNs) in
a domain with its unique Transport Class ID.

A Transport Class is also configured with RD and import/export RT
attributes. Creation of a Transport Class instantiates its
corresponding TRDB and Resolution Schemes on that node.

All nodes within a domain agree on a common Transport Class ID
namespace. However, two co-operating cooperating domains may not always agree on
the same namespace. Procedures to manage differences in Transport
Class ID namespaces between co-operating cooperating domains are specified in
Section 11.2.2.

Transport Class ID conveys the Color of tunnels in a Transport Class.
The terms 'Transport "Transport Class ID' ID" and 'Color' "Color" are used interchangeably
in this document.

4.1. Classifying TE tunnels Tunnels

TE tunnels can be classified into a Transport Class based on the TE
attributes they possess and the TE characteristics that the operator
defines for that Transport Class. Due to the fact that multiple TE
tunneling protocols exist, their TE attributes and characteristics
may not be equal but sufficiently similar. Some examples of such
classifications are as follows:

* Tunnels (RSVP-TE, IGP FLEX-ALGO, SR-TE) that support latency
sensitive routing.

* RSVP-TE Tunnels tunnels that only go over admin-group with Green links.

* Tunnels (RSVP-TE, SR-TE) that offer Fast Reroute. FRR.

* Tunnels (RSVP-TE, SR-TE) that share resources in the network based
on Shared Risk Link Groups defined by TE policy.

* Tunnels (RSVP-TE, SR-TE, BGP CT) that avoid certain nodes in the
network based on RSVP-TE ERO, Explicit Route Object (ERO), SR-TE policy
policy, or BGP policy.

An operator may configure a an SN/BN to classify a tunnel into an
appropriate Transport Class. How exactly these tunnels are made
Transport Class aware is implementation specific and outside the
scope of this document.

When a tunnel is made Transport Class aware, it causes the Tunnel
Route to be installed in the corresponding TRDB of that Transport
Class. These routes are used to resolve overlay routes, including
BGP CT. The BGP CT routes may be further readvertised to adjacent
domains to extend these tunnels. While readvertising BGP CT routes,
the "Transport Class" identifier is encoded as part of the Transport
Class RT, which is a new Route Target extended community defined in
Section 4.3.

An SN/BN receiving the transport routes via BGP with sufficient
signaling information to identify a Transport Class can associate
those tunnel routes to with the corresponding Transport Class. For
example, in BGP CT family routes, the Transport Class RT indicates
the Transport Class. For BGP LU family routes, import processing
based on Communities communities or Inter-AS source-peer may be used to place the
route in the desired Transport Class.

When the tunnel route is received via [SRTE] [RFC9830] with "Color:Endpoint"
as the NLRI that encodes the Transport Class as an integer 'Color' in
its Policy Color field, the 'Color' is mapped to a Transport Class
during the import processing. The SRTE tunnel route for this
'Endpoint' is installed in the corresponding TRDB. The SRTE tunnel
will be extended by a BGP CT advertisement with NLRI 'RD:Endpoint',
Transport Class RT RT, and a new label. The MPLS swap route thus
installed for the new label will pop the label and forward the
decapsulated traffic into the path determined by the SRTE route for
further encapsulation.

[PCEP-SRPOLICY] extends the Path Computation Element Communication
Protocol (PCEP) to signal attributes of an SR Policy which that include
Color. This Color is mapped to a Transport Class thus associating
the SR Policy with the desired Transport Class.

Similarly, [PCEP-RSVP-COLOR] extends PCEP to carry the Color
attribute for its use with RSVP-TE LSPs . LSPs. This Color is mapped to a
Transport Class thus associating the RSVP-TE LSP with the desired
Transport Class.

4.2. Transport Route Database

A Transport Route Database (TRDB)

A TRDB is a logical collection of transport routes pertaining to the
same Transport Class. In any node, every Transport Class has an
associated TRDB. Resolution Schemes resolve next hop NH reachability for EP
using the transport routes within the scope of the TRDBs.

Tunnel endpoint EP addresses (EP) in a TRDB belong to the "Provider
Namespace" provider namespace
representing the core transport region.

An implementation may realize the TRDB as a "Routing Table" referred
to in Section 9.1.2.1 of RFC4271 (https://www.rfc-editor.org/rfc/
rfc4271#section-9.1.2.1) [RFC4271], which is used only for resolving next hop
NH reachability in the control plane. An implementation may choose a
different datastructure to realize this logical construct while still
adhering to the procedures defined in this document. The tunnel
routes in a TRDB require no footprint in the forwarding plane unless
they are used to resolve a next hop. an NH.

SNs or BNs originate routes for the "Classful Transport" address
family from the TRDB. These routes have "RD:Endpoint" in the NLRI,
carry a Transport Class RT, and an MPLS label or equivalent
identifier in different forwarding architecture. "Classful
Transport" family routes received with Transport Class RT are
installed into their respective TRDB.

4.3. "Transport Class" Route Target Extended Community

This section defines a new type of Route Target, Target called a "Transport
Class" Route Target Extended Community; also extended community (also known as a Transport
Target. "Transport
Target"). The procedures for use of this extended community with BGP
CT routes (AFI/SAFI: 1/76 or 2/76) are described below.

The "Transport Class" Route Target Extended Community extended community is a transitive
extended community EXT-COMM [RFC4360] of extended type, which has the format
as shown in Figure 2.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type= 0xa | SubType= 0x02 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport Class ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: "Transport Class" Route Target Extended Community

Type: A 1-octet field that MUST be set to 0xa to indicate 'Transport
Class'.

SubType: A 1-octet field that MUST be set to 0x2 to indicate 'Route
Target'.

Reserved: A 2-octet reserved bits field.

This field MUST be set to zero on transmission.

This field SHOULD be ignored on reception, reception and MUST be left
unaltered.

Transport Class ID: This field is encoded in 4 octets.

This field contains the "Transport Class" identifier, which is an
unsigned 32-bit integer.

This document reserves the Transport class ID value 0 to represent "Best Effort
the "Best-Effort Transport Class ID".

Figure 2:

A "Transport Class" Route Target Extended Community

A Transport Class Route Target Extended extended community with TC ID 100 is
denoted as "transport-target:0:100".

The VPN route import/export mechanisms specified in BGP/MPLS IP VPNs
[RFC4364]
(see [RFC4364]) and the Constrained Route Distribution mechanisms
specified in Route Target Constrain [RFC4684] (see [RFC4684]) are applied using
the Route Target extended community. These mechanisms are applied to
BGP CT routes (AFI/SAFI: 1/76 or 2/76) using the "Transport Class
Route Target
Extended extended community".

A BGP speaker that implements procedures described in this document
and Route Target Constrain [RFC4684] MUST also apply the RTC procedures to the Transport
Class Route Target Extended extended communities carried on BGP CT routes
(AFI/SAFI: 1/76 or 2/76). An RTC route is generated for each Route
Target imported by locally provisioned Transport Classes.

Further, when processing RT membership NLRIs containing a Transport
Class Route Target Extended extended community received from external BGP
peers, it is necessary to consider multiple EBGP External BGP (EBGP) paths
for a given RTC prefix for building the outbound route filter, and filter: not
just the best path. An implementation MAY provide configuration to
control how many EBGP RTC paths are considered.

The Transport Class Route Target Extended extended community is carried on BGP
CT family routes and is used to associate them with appropriate TRDBs
at receiving BGP speakers. The Transport Target is carried unaltered
on the BGP CT route across BGP CT negotiated sessions except for
scenarios described in Section 11.2.2. Implementations should
provide policy mechanisms to perform match, strip, or rewrite
operations on a Transport Target just like any other BGP community.

Defining a new type code for the Transport Class Route Target
Extended
extended community avoids conflicting with any VPN Route Target
assignments already in use for service families.

This document also reserves the Non-Transitive version of the
Transport Class extended community (Section (see Section 13.2.1.1.2) for
future use. The "Non-Transitive Transport Class" Route Target Extended Community
extended community is not used. If received, it is considered
equivalent in functionality to the Transitive Transport Class Route
Target Extended Community, extended community, except for the difference in Transitive
bit flag.

5. Resolution Scheme

A Resolution Scheme is a construct that consists of a specific TRDB
or an ordered set of TRDBs. An overlay route is associated with a
resolution scheme during import processing, processing based on the Mapping
Community in the route.

Resolution Schemes enable a BGP speaker to resolve next hop NH reachability
for overlay routes over the appropriate underlay tunnels within the
scope of the TRDBs. Longest Prefix Match (LPM) of the
next hop NH is
performed within the identified TRDB.

An implementation may provide an option for the overlay route to
resolve over less preferred less-preferred Transport Classes, should the resolution
over a primary Transport Class fail.

To accomplish this, the "Resolution Scheme" is configured with the
primary Transport Class, Class and an ordered list of fallback Transport
Classes. Two Resolution Schemes are considered equivalent in Intent
if they consist of the same ordered set of TRDBs.

Operators must ensure that Resolution Schemes for a mapping community
are provisioned consistently on various nodes participating in a BGP
CT network, network based on desired Intent and transport classes available in
that domain.

5.1. Mapping Community

A "Mapping Community" is used to signal the desired Intent on an
overlay route. At an ingress node receiving the route, it maps the
overlay route to a "Resolution Scheme" used to resolve the route's
next hop.
NH.

A Mapping Community is a "role" and not a new type of community; any
BGP Community Carrying Attribute (e.g. (e.g., Community or Extended
Community) may play this role, besides role in addition to the other roles it may
already be playing. For example, the Transport Class Route Target Extended
Community
extended community plays a dual role, being a role: as Route Target as well as and a Mapping
Community.

Operator provisioning ensures that the ingress and egress SNs agree
on the BGP CCA and community namespace to use for the Mapping
Community.

A Mapping Community maps to exactly one Resolution Scheme at a
receiving BGP speaker. An implementation SHOULD allow associating the
association of multiple Mapping Communities to a Resolution Scheme.
This helps with renumbering and migration scenarios.

An example of a mapping community is "color:0:100", described in
[RFC9012], or the "transport-target:0:100" described in Section 4.3
in this document. 4.3.

The first community on the overlay route that matches a Mapping
Community of a locally configured Resolution Scheme is considered the
effective Mapping Community for the route. The Resolution Scheme
thus found is used when resolving the route's PNH. If a route
contains more than one Mapping Community, it indicates that the route
considers these distinct Mapping Communities as equivalent in Intent.

If more than one distinct Mapping Communities Community on an overlay route map
to distinct Resolution Schemes with dissimilar Intents at a receiving
node, it is considered a configuration error.

Since a route can carry multiple communities, but only a single
Resolution Scheme can be in effect for the route on any given router,
it is incumbent on the operator to ensure that communities attached
to a route will map to the desired Resolution Scheme at each point in
the network.

It should be noted that the Mapping Community role does not require
applying Route Target Constrain procedures specified in RFC 4684. [RFC4684].

6. BGP Classful Transport Family

The BGP Classful Transport (BGP CT) family uses the existing Address
Family Identifier (AFI) of IPv4 or IPv6 and a new SAFI 76 "Classful
Transport" that applies to both IPv4 and IPv6 AFIs.

The AFI/SAFI 1/76 MUST be negotiated as per the Multiprotocol
Extensions capability described in Section 8 of [RFC4760] to be able
to send and receive BGP CT routes for IPv4 endpoint prefixes.

The AFI/SAFI 2/76 MUST be negotiated as per the Multiprotocol
Extensions capability described in Section 8 of [RFC4760] to be able
to send and receive BGP CT routes for IPv6 endpoint prefixes.

6.1. NLRI Encoding

The "Classful Transport" SAFI NLRI has the same encoding as specified
in Section 2 of [RFC8277].

When the AFI/SAFI is 1/76, the Classful Transport NLRI Prefix
consists of an 8-byte RD followed by an IPv4 prefix. When AFI/SAFI
is 2/76, the Classful Transport NLRI Prefix consists of an 8-byte RD
followed by an IPv6 prefix.

The procedures described for AFI/SAFIs 1/4 or 1/128 in Section 2 of
[RFC8277] apply for AFI/SAFI 1/76 also. The procedures described for
AFI/SAFIs 2/4 or 2/128 in Section 2 of [RFC8277] apply for AFI/SAFI
2/76 also.

BGP CT routes MAY carry multiple labels in the NLRI, NLRI by negotiating
the Multiple Labels Capability as described in Section 2.1 of
[RFC8277]
[RFC8277].

Properties received on a Classful Transport route include the
Transport Class Route Target extended community, which is used to
associate the route with the correct TRDBs on SNs and BNs in the
network, and either an IPv4 or an IPv6 next hop. NH.

6.2. Next Hop Encoding

When the length of the Next hop Address field is 4, the next hop
address is of type an IPv4 address.

When the length of the Next hop Address field is 16 (or 32), the next
hop address is of type an IPv6 address (potentially followed by the link-
local IPv6 address of the next hop). This follows Section 3 in
[RFC2545] of
[RFC2545].

When the length of Next hop Address field is 24 (or 48), the next hop
address is of type a VPN-IPv6 with an 8-octet RD set to zero (potentially
followed by the link-local VPN-IPv6 address of the next hop with an
8-octet RD set to zero). This follows Section 3.2.1.1 in
[RFC4659] of [RFC4659].

When the length of the Next hop Address field is 12, the next hop
address is of type a VPN-IPv4 with 8-octet RD set to zero.

If the length of the Next hop Address field contains any other
values, it is considered an error and is handled via BGP session
reset as per Section 7.11 of [RFC7606].

6.3. Carrying multiple Multiple Encapsulation Information

To ease interoperability between nodes supporting different
forwarding technologies, a BGP CT route allows carrying multiple
encapsulation information.

An MPLS Label is carried using the encoding in [RFC8277]. A node
that does not support MPLS forwarding advertises the special label 3
(Implicit NULL) in the RFC 8277 MPLS Label field. field (see [RFC8277]). The
Implicit NULL label carried in BGP CT route indicates to a receiving
node that it should not impose any BGP CT label for this route.

The SID information for SR with respect to the MPLS Data Plane data plane is
carried as specified in the Prefix SID attribute defined as part of
Section 3 in of [RFC8669].

The SID information for SR with respect to SRv6 Data Plane data plane is carried
as specified in Section 7.13.

UDP tunneling information is carried using the Tunnel Encapsulation
Attribute as specified in [RFC9012].

6.4. Comparison with Other Families using RFC-8277 Using Encoding from RFC 8277

AFI/SAFI 1/128 (MPLS-labeled VPN address) is an RFC8277 encoded a family encoded using
[RFC8277] that carries service prefixes in the NLRI, where the
prefixes come from the customer namespaces and are contextualized
into separate user virtual service RIBs called VRFs as per [RFC4364].

AFI/SAFI 1/4 (BGP LU) is an RFC8277 encoded a family encoded using [RFC8277] that
carries transport prefixes in the NLRI, where the prefixes come from
the provider namespace.

AFI/SAFI 1/76 (Classful Transport SAFI) is an RFC8277 encoded a family encoded using
[RFC8277] that carries transport prefixes in the NLRI, where the
prefixes come from the provider namespace and are contextualized into
separate TRDB, following mechanisms similar to RFC 4364 [RFC4364] procedures.

It is worth noting that AFI/SAFI 1/128 has been used to carry
transport prefixes in "L3VPN Inter-AS Carrier's carrier" scenario as
defined in Section 10 of [RFC4364], where BGP LU/LDP prefixes in CsC
VRF are advertised in AFI/SAFI 1/128 towards the remote-end client
carrier.

In this document, SAFI 76 (BGP CT) is used instead of reusing SAFI
128 (L3VPN) for AFIs 1 or 2 to carry these transport routes because
it is operationally advantageous to segregate transport and service
prefixes into separate address families. For example, such an
approach allows operators to safely enable a "per-prefix" label label-
allocation scheme for Classful Transport prefixes, typically with a
number of routes in the hundreds of thousands or less, without
affecting SAFI 128 service prefixes prefixes, which may represent millions of
routes,
routes at the time of writing. The "per prefix" label allocation "per-prefix" label-allocation
scheme localizes routing churn during topology changes.

Service routes continue to be carried in their existing AFI/SAFIs
without any change. For example, L3VPN (AFI/SAFI: 1/128 and 2/128),
EVPN (AFI/SAFI: 25/70 ), VPLS (AFI/SAFI: Virtual Private LAN Service (VPLS) (AFI/
SAFI: 25/65), or Internet (AFI/SAFI: 1/1 or 2/1). These service
routes can resolve over BGP CT (AFI/SAFI: 1/76 or 2/76) transport
routes.

A new SAFI 76 for AFI 1 and AFI 2 also facilitates having a different
distribution path of the transport family routes in a network than
the service route distribution path. Service routes (Inet-VPN SAFI
128) are exchanged over an EBGP multihop session between ASes with
next hop
the NH unchanged; whereas Classful Transport routes (SAFI 76) are
advertised over EBGP single-hop sessions with "next hop a "NH self" rewrite
over inter-AS links.

The BGP CT SAFI 76 for AFI 1 and 2 is conceptually similar to BGP LU
SAFI 4, 4 in that it carries transport prefixes. The only difference is
that it also carries in a Route Target an indication of which
Transport Class the transport prefix belongs to, to and uses the RD to
disambiguate multiple instances of the same transport prefix in a BGP
Update.
UPDATE message.

7. Protocol Procedures

This section summarizes the procedures followed by various nodes
speaking Classful Transport family.

7.1. Preparing the network Network to deploy Deploy Classful Transport planes Planes

It is the responsibility of the operators to decide the Transport
Classes to enable and use in their network. They are also expected
to allocate a Transport Class Route Target to identify each Transport
Class.

Operators configure the Transport Classes on the SNs and BNs in the
network with Transport Class Route Targets and appropriate
Route-Distinguishers. Route
Distinguishers.

Implementations MAY provide automatic generation and assignment of
RD, RT values. They MAY also provide a way to manually override the
automatic mechanism in order to deal with any conflicts that may
arise with existing RD, RT values in different network domains
participating in the deployment.

7.2. Originating Classful Transport Routes

BGP CT routes are sent only to BGP peers that have negotiated the
Multiprotocol Extensions capability described in Section 8 of
[RFC4760] to be able to send and receive BGP CT routes.

At the ingress node of the tunnel's home domain, the tunneling
protocols install tunnel routes in the TRDB associated with the
Transport Class to which the tunnel belongs.

The egress node of the tunnel, i.e. i.e., the tunnel endpoint (EP),
originates the BGP CT route with RD:EP in the NLRI, a Transport Class RT
RT, and a PNH as the EP. This BGP CT route will be resolved over the
tunnel route in TRDB at the ingress node. When the tunnel is up, the
Classful Transport BGP route will become usable and get
re-advertised readvertised
by the ingress node to BGP peers in neighboring domains.

Alternatively, the ingress node of the tunnel, which is also an
ASBR/ABR ASBR/
ABR in a tunnel's home domain, may originate the BGP CT route for the
tunnel destination with NLRI RD:EP, RD:EP in the NLRI, attaching a Transport
Class Route Target that identifies the Transport Class. This BGP CT
route is advertised to EBGP peers and IBGP peers in neighboring
domains.

This originated route SHOULD NOT be advertised to the IBGP core that
contains the tunnel. This may be implemented by mechanisms such as
policy configuration. The impact of not prohibiting such
advertisements is outside the scope of this document.

Unique

A unique RD SHOULD be used by the originator of a Classful Transport
route to disambiguate the multiple BGP advertisements for a transport
endpoint. An administrator may use duplicate RDs based on local
choice, understanding the impact on path diversity and
troubleshooting, as described in Section 10.2.

7.3. Processing Classful Transport Routes by Ingress Nodes

Upon receipt of a BGP CT route with a PNH EP that is not directly
connected (e.g. (e.g., an IBGP-route), a Mapping Community (the Transport
Class RT) on the route is used to decide to which resolution scheme
this route is to be mapped.

The resolution scheme for a Transport Class RT with Transport Class
ID "C1" contains the TRDB of a Transport Class with same ID. The
administrator MAY customize the resolution scheme for Transport Class
ID "C1" to map to a different ordered list of TRDBs. If the
resolution scheme for TC ID "C1" is not found, the resolution scheme
containing the "Best Effort" "Best-Effort" transport class TRDB is used.

The routes in the TRDBs associated with a selected resolution scheme
are used to resolve the received PNH EP. The order of TRDBs in the
resolution scheme is followed when resolving the received PNH, such
that a route in a backup TRDB is used only when a matching route was
not found for EP in the primary TRDBs preceding it. This achieves
the fallback desired by the resolution scheme.

If the resolution process does not find a matching route for the EP
in any of the associated TRDBs, the received BGP CT route MUST be
considered unresolvable. (See RFC 4271, Section 9.1.2.1). 9.1.2.1 of [RFC4271].)

The received BGP CT route MUST be added to the TRDB corresponding to
the Transport Class "C1", ID "C1" if the transport class is provisioned
locally. This step applies only if the Transport Class RT is
received on a BGP CT family route. The RD in the BGP CT NLRI prefix
RD:EP is ignored when the BGP CT route for EP is added to the TRDB, TRDB so
that overlay routes can resolve over this BGP CT tunnel route by
performing a lookup for the EP. Please note that a TRDB is a logical
database of tunnel routes belonging to the same Transport Class ID, hence ID;
hence, it uses only uses the EP as the lookup key
without (without RD or TC ID. ID).

If no Mapping Community was is found on a BGP CT route, the best
effort best-effort
resolution scheme is used for resolving to resolve the route's next hop, and the
BGP CT route is not added to any TRDB.

7.4. Readvertising Classful Transport Route by Border Nodes

This section describes the MPLS label handling when readvertising a
BGP CT route with Next Hop set to Self. "NH self". When readvertising a BGP CT route with Next Hop set to Self,
"NH self", a BN allocates an MPLS label to advertise upstream in the
Classful Transport NLRI. The BN also installs an MPLS route for that
label that swaps the incoming label with the label received from the
downstream BGP speaker (or pops the incoming label if the label
received from the downstream BGP speaker was Implicit-NULL). The
MPLS route then pushes received traffic to the transport tunnel or
direct interface that the Classful Transport route's PNH resolved
over.

The label SHOULD be allocated with "per-prefix" label allocation label-allocation
semantics. The IP prefix in the TRDB context (Transport-Class,
IP-prefix) IP-
prefix) is used as the key to do per-prefix "per-prefix" label allocation. This
helps in avoiding BGP CT route churn throughout the CT network when
an instability (e.g., link failure) is experienced in a domain. The
failure is not propagated further than the BN closest to the failure.
If a different label allocation label-allocation mode is used, the impact on end to end-to-
end convergence should be considered.

The value of the advertised MPLS label is locally significant, significant and is
dynamic by default. A BN may provide an option to allocate a value
from a statically provisioned range. This can be achieved using a
locally configured export policy, policy or via mechanisms such as the ones
described in related to BGP Prefix-SID [RFC8669]. as described in BGP (see
[RFC8669]).

7.5. Border Nodes Receiving Classful Transport Routes on EBGP

If a route is received with a PNH that is known to be directly
connected (for example, an EBGP single-hop neighbor address), the
directly connected interface is checked for MPLS forwarding
capability. No other next hop resolution process is performed since
the inter-AS link can be used for any Transport Class.

If the inter-AS links need to honor Transport Class, then the BN MUST
follow the procedures of an Ingress node (Section 7.3) and perform
the next hop resolution process. In order to make the link Transport
Class aware, the route to the directly connected PNH is installed in
the TRDB belonging to the associated Transport Class.

7.6. Avoiding Path Hiding Through Route Reflectors

When multiple instances of a given RD:EP exist with different
forwarding characteristics, then BGP ADD-PATH [RFC7911] (see [RFC7911]) is helpful.

When multiple BNs exist such that they advertise a an "RD:EP" prefix to
Route Reflectors (RRs), the RRs may hide all but one of the BNs,
unless BGP ADD-PATH [RFC7911] (see [RFC7911]) is used for the Classful
Transport family. This is similar to L3VPN Option B scenarios.

Hence, BGP ADD-PATH [RFC7911] (see [RFC7911]) SHOULD be used for the Classful
Transport family, family to avoid path-hiding path hiding through RRs so that the RR
sends multiple CT routes for RD:EP to its clients. This improves the
convergence time when the path via one of the multiple BNs fails.

7.7. Avoiding Loops Between Route Reflectors in Forwarding Path Paths

A pair of redundant ABRs, each acting as an RR with the next hop self, set
to itself, may choose each other as the best path instead of the
upstream ASBR, causing a traffic forwarding traffic-forwarding loop.

This problem can happen for routes of any BGP address family,
including BGP CT and BGP LU.

Using one or more of the approaches described in [BGP-FWD-RR]
softens lowers
the possibility of such loops in a network with redundant ABRs.

7.8. Ingress Nodes Receiving Service Routes with a Mapping Community

Upon receipt of a BGP service route (for example, AFI/SAFI: 1/1, 2/1)
with a PNH as the EP that is not directly connected (for example, an
IBGP-route), a Mapping Community (for example, a Color Extended
Community) on the route is used to decide to which resolution scheme
this route is to be mapped.

The resolution scheme for a Color Extended Community with Color "C1"
contains a TRDB for a Transport Class with same ID, ID followed by the Best Effort
Best-Effort TRDB. The administrator MAY customize the resolution
scheme to map to a different ordered list of TRDBs. If the
resolution scheme for TC ID "C1" is not found, the resolution scheme
containing the "Best Effort" "Best-Effort" transport class TRDB is used.

If no Mapping Community was found on the overlay route, the "Best
Effort" resolution scheme is used for resolving the route's next hop.
This behavior is backward compatible to behavior of an implementation
that does not follow procedures described in this document.

The routes in the TRDBs associated with the selected resolution
scheme are used to resolve the received PNH EP. The order of TRDBs
in a resolution scheme is followed when resolving the received PNH,
such that a route in a backup TRDB is used only when a matching route
was not found for the EP in the primary TRDBs preceding it. This
achieves the fallback desired by the resolution scheme.

If the resolution process does not find a Tunnel Route for the EP in
any of the Transport Route Databases, the service route MUST be
considered unresolvable unresolvable. (See RFC 4271, Section 9.1.2.1). 9.1.2.1 of [RFC4271]).

Note: For an illustration of above procedures in a an MPLS network,
refer to Section 8.

7.9. Best Effort Best-Effort Transport Class

It is also possible to represent 'Best effort' a 'Best-effort' SLA also as a Transport
Class. Today, At the time of writing, BGP LU is used to extend the best best-
effort intra
domain intra-domain tunnels to other domains.

Alternatively, BGP CT may also be used to carry the best effort best-effort
tunnels. This document reserves the Transport Class ID value 0 to
represent "Best Effort the "Best-Effort Transport Class ID". However,
implementations SHOULD provide configuration to use a different value
for this purpose. Procedures to manage differences in Transport
Class ID namespaces between domains are provided in Section 11.2.2.

The "Best Effort "Best-Effort Transport Class ID" value is used in the "Transport
Class ID" field of the Transport Route Target Extended
Community extended community that
is attached to the BGP CT route that advertises a
best effort best-effort tunnel
endpoint. The Thus, the RT thus formed is called the
"Best Effort "Best-Effort Transport
Class Route Target".

When a BN or SN receives a BGP CT route with Best Effort Best-Effort Transport
Class Route Target as the mapping community, the Best effort Best-effort
resolution scheme is used for resolving the BGP next hop, and the
resultant route is installed in the best effort best-effort transport route
database. If no best effort best-effort tunnel was found to resolve the BGP next
hop, the BGP CT route MUST be considered unusable, unusable and not be
propagated further.

When a BGP speaker receives an overlay route without any explicit
Mapping Community, and absent local policy, the best effort best-effort
resolution scheme is used for resolving the BGP next hop on the
route. This behavior is backward compatible to behavior of an
implementation that does not follow procedures described in this
document.

Implementations MAY provide configuration to selectively install BGP
CT routes to the Forwarding Information Base (FIB), (FIB) to provide
reachability for control plane control-plane peering towards endpoints in other
domains.

7.10. Interaction with BGP Attributes Specifying Next Hop Address and
Color

The Tunnel Encapsulation Attribute, described in [RFC9012] [RFC9012], can be
used to request a specific type of tunnel encapsulation. This
attribute may apply to BGP service routes or transport routes, routes
including BGP Classful Transport family routes.

It should be noted that in such cases "Transport Class ID/Color" can
exist in multiple places on the same route, and a precedence order
needs to be established to determine which Transport Class the
route's next hop should resolve over. This document specifies the
following order of precedence, more specific precedence with more-specific scoping of Color
preferred to less specific less-specific scoping:

* Color SubTLV, sub-TLV in the Tunnel Encapsulation Attribute.

* Transport Target Extended community, extended community on a BGP CT route.

* Color Extended community, Community on a BGP service route.

Color specified in the Color subTLV sub-TLV in a TEA is a more specific more-specific
indication of "Transport Class ID/Color" than Mapping Community
(Transport Target) on a BGP CT transport route, which is which, in turn turn, is
more specific than a Service route scoped Service-route-scoped Mapping Community (Color
Extended community). Community).

Any BGP attributes or mechanisms defined in future that carry
Transport Class ID/Color on the route are expected to specify the
order of precedence relative to the above.

7.11. Applicability to Flowspec Redirect to IP Redirect-to-IP

Flowspec routes using Redirect to IP redirect-to-IP next hop is are described in
[FLOWSPEC-REDIR-IP]
[FLOWSPEC-REDIR-IP].

Such Flowspec BGP routes with Redirect to IP redirect-to-IP next hop MAY be attached
with a Mapping Community (e.g. (e.g., Color:0:100), which allows
redirecting the flow traffic over a tunnel to the IP next hop
satisfying the desired SLA (e.g. (e.g., Transport Class color 100).

The Flowspec BGP family acts as just another service that can make
use of the BGP CT architecture to achieve Flow based flow-based forwarding with
SLAs.

7.12. Applicability to IPv6

BGP CT procedures apply equally to IPv4 IPv4- and IPv6 enabled IPv6-enabled Intra-AS or
Inter-AS Option A, B, and C network. networks. This section describes the
applicability of BGP CT to IPv6 at various layers.

A network that is BGP CT enabled network supports IPv6 service families (for
example, AFI/SAFI 2/1 or 2/128) and IPv6 transport signaling
protocols like SRTEv6, LDPv6, or RSVP-TEv6.

Procedures in this document also apply to a network with Pure IPv6
core, that uses MPLS forwarding for intra-domain tunnels and inter-AS
links. The BGP CTv6 family (AFI/SAFI: 2/76) is used to carry global
IPv6 address tunnel endpoints in the NLRI. Service family routes
(for example, AFI/SAFI: 1/1, 2/1, 1/128, and 2/128) are also
advertised with those Global IPv6 addresses as next hop.

Procedures in this document also apply to a 6PE network with an IPv4
core, that which uses MPLS forwarding for intra-domain tunnels and Inter-AS Inter-
AS links. The BGP CTv6 family (AFI/SAFI: 2/76) is used to carry IPv4
Mapped IPv6 address tunnel endpoints in the NLRI. IPv6 Service
family routes (for example, AFI/SAFI: 2/1, 2/128) are also advertised
with those IPv4 Mapped IPv6 addresses as next hop.

The PE-CE attachment circuits may use IPv4 addresses only, IPv6
addresses only, or both IPv4 and IPv6 addresses.

7.13. SRv6 Support

The BGP CT family (AFI/SAFI 2/76) may be used to set up inter-domain
tunnels of a certain Transport Class, Class when using a Segment Routing
over IPv6 (SRv6) data plane on the inter-AS links or as an intra-AS
tunneling mechanism.

Details of SRv6 Endpoint behaviors used by BGP CT and the procedures
are specified and illustrated in a separate document [BGP-CT-SRv6], along with
illustration. (see
[BGP-CT-SRv6]). As noted in that document, a BGP CT route update for
SRv6 includes a BGP attribute containing SRv6 SID information (e.g.
Prefix SID (e.g.,
a BGP Prefix-SID [RFC9252]) with the Transposition scheme disabled.

7.14. Error Handling Error-Handling Considerations

If a BGP speaker receives both Transitive (Section 13.2.1.1.1) and Non-Transitive (Section 13.2.1.1.2) (see
Section 13.2.1.1.1 and Section 13.2.1.1.2, respectievely) versions of
a Transport Class extended community on a route, only the Transitive
one is used.

If a BGP speaker considers a received "Transport Class" extended
community (Transitive (the Transitive or Non-Transitive version), version) or any other
part of a BGP CT route invalid for some reason, but is able to
successfully parse the NLRI and attributes, Treat-as-withdraw the treat-as-withdraw
approach from [RFC7606] is used. The route is kept as Unusable, with
appropriate diagnostic information, to aid troubleshooting.

8. Illustration of BGP CT Procedures

This section illustrates BGP CT procedures in an Inter-AS Option C
MPLS network.

All Illustrations illustrations in this document make use of [RFC6890] IP address
ranges. ranges as
described in [RFC6890]. The range 192.0.2.0/24 is used to represent
transport endpoints like loopback addresses. The range
203.0.113.0/24 is used to represent service route prefixes advertised
in AFI/SAFIs: 1/1 or 1/128.

Though this section illustrates using the use of IPv4, as described in
Section 7.12 7.12, these procedures work equally for IPv6 as-well. as well.

8.1. Reference Topology

[RR26] [RR27] [RR16]
| | |
| | |
| +--[ABR23]--+ | +--[ASBR21]-[ASBR13]--+ | +--[PE11]--+
| | | | | \ / | | | |
[CE41]-[PE25]-[P28] [P29] \/ [P15] [CE31]
| | | /\ | | |
| | | / \ | | |
| | | / \ | | |
+--[ABR24]--+ +--[ASBR22]-[ASBR14]--+ +--[PE12]--+

: AS2 : AS2 : :
AS4 : region-1 : region-2 : AS1 : AS3
: : : :

203.0.113.41 ---------- Traffic Direction ------------> 203.0.113.31

Figure 3: Multi-Domain BGP CT Network

This example shows a provider MPLS network that consists of two ASes,
AS1 and AS2. They are serving AS2, that serve customers AS3, AS4 AS3 and AS4, respectively.
Traffic The
traffic direction being described is from CE41 to CE31. CE31 may
request a specific SLA (for example, mapped (mapped to Gold for this example), when
traversing these provider networks.

AS2 is further divided into two regions. There are three tunnel
domains in the provider's space:

* AS1 uses ISIS Flex-Algo [RFC9350] intra-
domain (see[RFC9350]) intra-domain tunnels.

* AS2 uses RSVP-TE intra-domain tunnels.

MPLS forwarding is used within these domains and on inter-domain
links.

The network exposes two Transport Classes: "Gold" with Transport
Class ID 100, 100 and "Bronze" with Transport Class ID 200. These
Transport Classes are provisioned at the PEs and the Border nodes (ABRs,
(ABRs and ASBRs) in the network.

The following tunnels exist for the Gold Transport Class. Class:

* PE25_to_ABR23_gold - RSVP-TE tunnel

* PE25_to_ABR24_gold - RSVP-TE tunnel

* ABR23_to_ASBR22_gold - RSVP-TE tunnel

* ASBR13_to_PE11_gold - SRTE tunnel

* ASBR14_to_PE11_gold - SRTE tunnel

The following tunnels exist for Bronze Transport Class. Class:

* PE25_to_ABR23_bronze - RSVP-TE tunnel

* ABR23_to_ASBR21_bronze - RSVP-TE tunnel

* ABR23_to_ASBR22_bronze - RSVP-TE tunnel

* ABR24_to_ASBR21_bronze - RSVP-TE tunnel

* ASBR13_to_PE12_bronze - ISIS FlexAlgo tunnel

* ASBR14_to_PE11_bronze - ISIS FlexAlgo tunnel

These tunnels are either provisioned or auto-discovered autodiscovered to belong to
Transport Classes Class IDs 100 or 200.

8.2. Service Layer Service-Layer Route Exchange

Service nodes PE11, PE11 and PE12 negotiate service families (AFI: 1 and
SAFIs 1, 128) on the BGP session with RR16. Service helpers RR16 and
RR26 exchange these service routes with the next hop unchanged over a
multihop EBGP session between the two AS. ASes. PE25 negotiates service
families (AFI: 1 and SAFIs 1, 128) with RR26.

The PEs see each other as the next hop in the BGP Update UPDATE message for
the service family routes. BGP ADD-PATH send and receive is are enabled
on both directions on the EBGP multihop session between RR16 and RR26
for AFI:1 and SAFIs 1, 128. BGP ADD-PATH send is negotiated in the
RR to PE direction in each AS. This is to avoid path hiding of the path-hiding
service routes at RR; the RR, i.e., AFI/SAFI 1/1 routes advertised by
both PE11 and
PE12. Or, PE12 or AFI/SAFI 1/128 routes originated by both PE11
and PE12 using the same RD.

Forwarding happens using service routes installed at service nodes
PE25, PE11, and PE12 only. Service routes received from CEs are not
present in any other nodes' FIB in the network.

As an example, CE31 advertises a route for prefix 203.0.113.31 with
the next hop as self itself to PE11, PE11 and PE12. CE31 can attach a Mapping
Community Color:0:100 on this route, route to indicate its request for a
Gold SLA. Or, PE11 can attach the same using locally configured
policies.

Consider,

Consider CE31 is getting VPN service from PE11. The RD1:203.0.113.31
route is readvertised in AFI/SAFI 1/128 by PE11 with the next hop self set
to itself (192.0.2.11) and label V-L1, V-L1 to RR16 with the Mapping
Community Color:0:100 attached. RR16 advertises this route with the
BGP ADD-PATH ID set to RR26 RR26, which readvertises to PE25 with the next
hop unchanged. Now, PE25 can resolve the PNH 192.0.2.11 using
transport routes received in BGP CT or BGP LU.

Using BGP ADD-PATH, service routes advertised by PE11 and PE12 for
AFI:1 SAFIs 1, 128 reach PE25 via RR16, RR26 with the next hop
unchanged, as PE11 or PE12.

The IP FIB at the PE25 VRF will have a route for 203.0.113.31 with a
next hop when resolved, resolved that points to a Gold tunnel in the ingress
domain.

8.3. Transport Layer Transport-Layer Route Propagation

Egress nodes PE11, PE11 and PE12 negotiate a BGP CT family with transport
ASBRs
ASBR13, ASBR13 and ASBR14. These egress nodes originate BGP CT routes
for tunnel endpoint addresses, addresses that are advertised as a next hop in
BGP service routes. In this example, both PEs participate in
transport classes Gold and Bronze. The protocol procedures are
explained using the Gold SLA transport plane and plane; the Bronze SLA
transport plane is used to highlight the path hiding path-hiding aspects.

For Gold tunnels, PE11 is provisioned with transport class 100, RD
value 192.0.2.11:100 192.0.2.11:100, and a transport-target:0:100 for Gold tunnels. And a transport-target:0:100. For Bronze
tunnels, PE11 is provisioned with Transport class
200 with 200, RD value
192.0.2.11:200, and transport route target 0:200
for Bronze tunnels. 0:200. Similarly, for
Gold tunnels, PE12 is provisioned with transport class 100, RD value 192.0.2.12:100
192.0.2.12:100, and a transport-target:0:100 for
Gold tunnels. And transport-target:0:100. For Bronze tunnels,
PE12 is provisioned with transport class 200, RD value 192.0.2.12:200 with
transport-target:0:200 for Bronze tunnels.
192.0.2.12:200, and transport-target:0:200. Note that that, in this
example, the BGP CT routes carry only the transport class route
target,
target and no IP address format route target.

The RD value originated by an egress node is not modified by any BGP
speakers when the route is readvertised to the ingress node. Thus,
the RD can be used to identify the originator (unique RD provisioned)
or set of originators (RD reused on multiple nodes).

Similarly, these transport classes are also configured on ASBRs, ABRs
ABRs, and PEs with same Transport Route Target and unique RDs.

ASBR13 and ASBR14 negotiate BGP CT family with transport ASBRs
ASBR21, ASBR21
and ASBR22 in neighboring AS. ASBR21, ASes. ASBR21 and ASBR22 negotiate BGP CT
family with RR27 in region 2, which reflects BGP CT routes to ABR23, ABR23
and ABR24. ABR23, ABR23 and ABR24 negotiate BGP CT family with Ingress node
PE25 in region 1. The BGP LU family is also negotiated on these
sessions alongside the BGP CT family. The BGP LU family carries "best effort"
"best-effort" transport class routes, routes; BGP CT carries Gold, Gold and Bronze
transport class routes.

PE11 is provisioned to originate a BGP CT route for endpoint PE11,
with a Gold SLA. This route is sent with NLRI RD prefix
192.0.2.11:100:192.0.2.11, Label B-L0, next hop 192.0.2.11 192.0.2.11, and a
route target
Route Target extended community transport-target:0:100. Label B-L0
can either be Implicit Null (Label 3) or an a UHP label.

This route is received by ASBR13 and it resolves over the tunnel
ASBR13_to_PE11_gold. The route is then readvertised by ASBR13 in BGP
CT family to ASBRs ASBR21, ASBR22 according to export policy. This
route is sent with same NLRI RD prefix 192.0.2.11:100:192.0.2.11,
Label B-L1, the next hop self, set to itself, and transport-target:0:100.
An MPLS swap route is installed at ASBR13 for B-L1 with a next hop
pointing to ASBR13_to_PE11_gold tunnel.

Similarly, ASBR14 also receives a BGP CT route for
192.0.2.11:100:192.0.2.11 from PE11 PE11, and it resolves over the tunnel
ASBR14_to_PE11_gold. The route is then readvertised by ASBR14 in the
BGP CT family to ASBRs ASBR21, ASBR21 and ASBR22 according to export policy.
This route is sent with the same NLRI RD prefix
192.0.2.11:100:192.0.2.11, Label B-L2, next hop self, set to itself, and
transport-target:0:100. An MPLS swap route is installed at ASBR14
for B-L1 with a next hop pointing to ASBR14_to_PE11_gold tunnel.

In the Bronze plane, the BGP CT route with a Bronze SLA to endpoint
PE11 is originated by PE11 with a an NLRI containing RD prefix
192.0.2.11:200:192.0.2.11,
192.0.2.11:200:192.0.2.11 and an appropriate label. The use of
distinct RDs for Gold and Bronze allows both Gold and Bronze
advertisements to traverse path selection path-selection pinchpoints without any
path hiding at RRs or ASBRs. And route target Route Target extended community
transport-target:0:200 lets the route resolve over Bronze tunnels in
the network, similar to the process being described for the Gold SLA
path.

Moving back to the Gold plane, ASBR21 receives the Gold SLA BGP CT
routes for NLRI RD prefix 192.0.2.11:100:192.0.2.11 over the single single-
hop EBGP sessions from ASBR13, ASBR14, ASBR13 and ASBR14 and can compute ECMP/FRR
towards them. ASBR21 readvertises the BGP CT route for
192.0.2.11:100:192.0.2.11 with a next hop self set to itself (loopback
address 192.0.2.21) to RR27, advertising a new label label: B-L3. An MPLS
swap route is installed for label B-L3 at ASBR21 to swap to received label
B-L1,
labels B-L1 and B-L2 and forward to ASBR13, ASBR13 and ASBR14 respectively, respectively;
this is an ECMP route. RR27 readvertises this BGP CT route to ABR23, ABR23
and ABR24 with the label and next hop unchanged.

Similarly, ASBR22 receives BGP CT route 192.0.2.11:100:192.0.2.11
over the single hop single-hop EBGP sessions from ASBR13, ASBR13 and ASBR14, and it
readvertises with the next hop self set to itself (loopback address
192.0.2.22) to RR27, advertising a new label label: B-L4. An MPLS swap
route is installed for label B-L4 at ASBR22 to swap to received label B-L1,
labels B-L1 and B-L2 and forward to ASBR13, ASBR14 ASBR13 and ASBR14, respectively.
RR27 also readvertises this BGP CT route also to ABR23, ABR23 and ABR24 with the
label and next hop unchanged.

BGP ADD-PATH is enabled for the BGP CT family on the sessions between
RR27 and ASBRs, the ASBRs and ABRs such that routes for
192.0.2.11:100:192.0.2.11 with the next hops ASBR21 and ASBR22 are
reflected to ABR23, ABR23 and ABR24 without any path hiding. Thus, ABR23 is
given visibility of both available next hops for the Gold SLA.

ABR23 receives the route with next hop 192.0.2.21, 192.0.2.21 and label B-L3 from
RR27. The route target "transport-target:0:100" on this route acts
as the Mapping Community, Community and instructs ABR23 to strictly resolve the
next hop using transport class 100 routes only. ABR23 is unable to
find a route for 192.0.2.21 with transport class 100. Thus, it
considers this route unusable and does not propagate it further.
This prunes ASBR21 from the Gold SLA tunneled path.

ABR23 also receives the route with next hop 192.0.2.22, 192.0.2.22 and label B-L4
from RR27. The route target "transport-target:0:100" on this route
acts as the Mapping Community, Community and instructs ABR23 to strictly resolve
the next hop using transport class 100 routes only. ABR23
successfully resolves the next hop to point to ABR23_to_ASBR22_gold
tunnel. ABR23 readvertises this BGP CT route with the next hop self set
to itself (loopback address 192.0.2.23) and a new label B-L5 to PE25. Swap
A swap route for B-L5 is installed by ABR23 to swap to label B-L4, B-L4 and
forward into ABR23_to_ASBR22_gold tunnel.

PE25 receives the BGP CT route for prefix 192.0.2.11:100:192.0.2.11
with label B-L5, next hop 192.0.2.23 192.0.2.23, and transport-target:0:100 from
RR26. And it It similarly resolves the next hop 192.0.2.23 over transport
class 100, pushing labels associated with PE25_to_ABR23_gold tunnel.

In this manner, the Gold transport LSP "ASBR13_to_PE11_gold" in the
egress domain is extended by BGP CT until the ingress node PE25 in
the ingress domain, to create an end-to-end Gold SLA path. MPLS swap
routes are installed at ASBR13, ASBR22 ASBR22, and ABR23, when propagating
the PE11 BGP CT Gold transport class route 192.0.2.11:100:192.0.2.11
with next hop self set to itself towards PE25.

The

Thus formed, the BGP CT LSP thus formed, originates in PE25, PE25 and terminates in
ASBR13 (assuming PE11 advertised Implicit Null), traversing over the
Gold underlay LSPs in each domain. ASBR13 uses UHP to stitch the BGP
CT LSP into the "ASBR13_to_PE11_gold" LSP to traverse the last
domain, thus satisfying Gold SLA end-to-end.

When PE25 receives service routes from RR26 with next hop 192.0.2.11
and mapping community Color:0:100, it resolves over this BGP CT route
192.0.2.11:100:192.0.2.11. Thus, pushing label B-L5, B-L5 and pushing as
the top label the labels associated with PE25_to_ABR23_gold tunnel.

8.4. Data Plane View

8.4.1. Steady State

This section describes how the data plane looks in steady state.

CE41 transmits an IP packet with the destination as 203.0.113.31. On
receiving this packet, PE25 performs a lookup in the IP FIB
associated with the CE41 interface. This lookup yields the service
route that pushes the VPN service label V-L1, BGP CT label B-L5, and
labels for PE25_to_ABR23_gold tunnel. Thus, PE25 encapsulates the IP
packet in an MPLS packet with label labels V-L1 (innermost), B-L5, and top
label as PE25_to_ABR23_gold tunnel. This MPLS packet is thus
transmitted to ABR23 using the Gold SLA.

ABR23 decapsulates the packet received on PE25_to_ABR23_gold tunnel
as required, required and finds the MPLS packet with label B-L5. It performs a
lookup for label B-L5 in the global MPLS FIB. This yields the route
that swaps label B-L5 with label B-L4, B-L4 and pushes the top label
provided by ABR23_to_ASBR22_gold tunnel. Thus, ABR23 transmits the
MPLS packet with label B-L4 to ASBR22, ASBR22 on a tunnel that satisfies the
Gold SLA.

ASBR22 similarly performs a lookup for label B-L4 in the global MPLS
FIB, finds the route that swaps label B-L4 with label B-L2, and
forwards it to ASBR13 over the directly connected MPLS-enabled
interface. This interface is a common resource not dedicated to any
specific transport class, in this example.

ASBR13 receives the MPLS packet with label B-L2, B-L2 and performs a lookup
in the MPLS FIB, finds the route that pops label B-L2, and pushes
labels associated with ASBR13_to_PE11_gold tunnel. This transmits
the MPLS packet with VPN label V-L1 to PE11 using a tunnel that
preserves the Gold SLA in AS 1.

PE11 receives the MPLS packet with V-L1, V-L1 and performs VPN forwarding.
Thus forwarding,
thus transmitting the original IP payload from CE41 to CE31. The
payload has traversed path satisfying the Gold SLA end-to-end.

8.4.2. Local Repair of Primary Path

This section describes how the data plane at ASBR22 reacts when the
link between ASBR22 and ASBR13 experiences a failure, failure and an alternate
path exists.

Assuming the ASBR22_to_ASBR13 link goes down, such that traffic with a Gold SLA
going to PE11 needs will need repair. ASBR22 has an alternate BGP CT route
for 192.0.2.11:100:192.0.2.11 from ASBR14. This has been
preprogrammed in forwarding by ASBR22 as an FRR backup next hop for
label B-L4. This allows the Gold SLA traffic to be locally repaired
at ASBR22 without the failure event propagated in the BGP CT network.
In this case, ingress node PE25 will not know there was a failure,
and traffic restoration will be independent of prefix scale (PIC).

8.4.3. Absorbing Failure of the Primary Path: Fallback to Best Effort Best-Effort
Tunnels

This section describes how the data plane reacts when a Gold path
experiences a failure, failure but no alternate path exists.

Assume tunnel ABR23_to_ASBR22_gold goes down, such that now no end-
to-end Gold path exists in the network. This makes the BGP CT route
for RD prefix 192.0.2.11:100:192.0.2.11 is unusable at ABR23. This
makes ABR23 send a BGP withdrawal for 192.0.2.11:100:192.0.2.11 to
PE25.

The withdrawal for 192.0.2.11:100:192.0.2.11 allows PE25 to react to
the loss of the Gold path to 192.0.2.11. Assuming PE25 is
provisioned to use best effort a best-effort transport class as the backup path,
this withdrawal of a BGP CT route allows PE25 to adjust the next hop
of the VPN Service-route to push the labels provided by the BGP LU
route. That repairs the traffic to go via the best effort best-effort path.
PE25 can also be provisioned to use the Bronze transport class as the
backup path. The repair will happen in similar manner in that case
as-well.
as well.

Traffic repair to absorb the failure happens at ingress node PE25, PE25 in
a service prefix scale independent manner. This is called PIC. manner (PIC). The repair time
will be proportional to time taken for withdrawing the BGP CT route.

These examples demonstrate the various levels of failsafe mechanisms
available to protect traffic in a BGP CT network.

9. Scaling Considerations

9.1. Avoiding Unintended Spread of BGP CT Routes Across Domains

[RFC8212] suggests BGP speakers require explicit configuration of
both BGP Import and Export Policies in order to receive or send
routes over EBGP sessions.

It is recommended to follow this for BGP CT routes. It will prohibit
unintended advertisement of transport routes throughout the BGP CT
transport domain, which may span across multiple AS domains. This
will conserve usage of resources for MPLS label labels and next hop
resources hops in the
network. An ASBR of a domain can be provisioned to allow routes with
only the Transport Route Targets that are required by SNs in the
domain.

9.2. Constrained Distribution of PNHs to SNs (On-Demand Next Hop)

This section describes how the number of Protocol Next hops Hops (PNHs)
advertised to a an SN or BN can be constrained using BGP Classful
Transport and Route Target Constrain (RTC) RTC (see [RFC4684].

An egress SN MAY advertise a BGP CT route for RD:eSN with two Route
Targets: transport-target:0:<TC> and a an RT carrying <eSN>:<TC>, where
TC is the Transport Class identifier, identifier and eSN is the IP address used
by the SN as BGP next hop in its service route advertisements.

Note that such use of the IP address specific IP-address-specific route target <eSN>:<TC>
is optional in a BGP CT network. It is required only if there is a
requirement to prune the propagation of the transport route for an
egress node eSN to only the set of ingress nodes that need it. When
only the RT of transport-target:0:<TC> is used, the pruning happens
in granularity of Transport Class ID (Color), and not BGP next hop; a BGP
CT route will only be advertised into a domain with at least one PE
that imports its transport class.

The transport-target:0:<TC> is the new type of route target
(Transport Class RT) defined in this document. It is carried in the
BGP extended community attribute (BGP attribute code 16).

The RT carrying <eSN>:<TC> MAY be an IP-address specific IP-address-specific regular RT
(BGP attribute code 16), or IPv6-address specific RT (BGP attribute
code 25). It should be noted that the Local Administrator field of
these RTs can only carry two octets of
information, and thus information; thus, the <TC>
field in this approach is limited to a 2 octets 2-octet value. Future
protocol extensions extension work is needed to define a BGP CCA that can
accomodate an IPv4/IPv6 address along with a 4 octet 4-octet Local
Administrator field.

An ingress SN MAY import BGP CT routes with a Route Target carrying
<eSN>:<TC>. The ingress SN may learn the eSN values either by
configuration, configuration
or it may discover them from the BGP next hop field in the BGP VPN
service routes received from the eSN. A BGP ingress SN receiving a
BGP service route with a next hop of eSN generates a an RTC route for
Route Target prefix <Origin ASN>:<eSN>/[80|176] in order to learn BGP
CT transport routes to reach eSN. This allows constrained
distribution of the transport routes to the PNHs actually required by
iSN.

When RTC is in use use, as described here, BGP CT routes will be
constrained to follow the same path of propagation as the RTC routes.
Therefore, a BN would learn the RTC routes advertised by ingress SNs
and propagate further. This will allow constraining distribution of
BGP CT routes for a PNH to only the necessary BNs in the network,
closer to the egress SN.

When the path of route propagation of BGP CT routes is the same as
the RTC routes, a BN would learn the RTC routes advertised by ingress
SNs and propagate further. This will allow constraining distribution
of BGP CT routes for a PNH to only the necessary BNs in the network,
closer to the egress SN.

This mechanism provides "On Demand "On-Demand Next hop" Hop" of BGP CT routes, which help
helps with the scaling of MPLS forwarding state at the SN and BN.

However, the amount of state carried in RTC family may become
proportional to the number of PNHs in the network. To strike a
balance, the RTC route advertisements for <Origin ASN>:<eSN>/[80|176]
MAY be confined to the BNs in the home region of an ingress SN, or
the BNs of a super core.

Such a BN in the core of the network imports BGP CT routes with
Transport-Target:0:<TC> and generates an RTC route for <Origin
ASN>:0:<TC>/96, while not propagating the more specific RTC requests
for specific PNHs. This lets the BN learn transport routes to all
eSN nodes but confine confines their propagation to ingress SNs.

9.3. Limiting The the Visibility Scope of PE Loopback as PNHs

It may be even more desirable to limit the number of PNHs that are
globally visible in the network. This is possible using the
mechanism described in Appendix D, such that advertisement of PE
loopback addresses as next-hop next hops in BGP service routes is confined to
the region they belong to. An anycast IP-address IP address called a "Context
Protocol Nexthop Address" (CPNH) Nexthop" (or "CPNH") address abstracts the SNs in a region
from other regions in the network.

Such that advertisement of PE loopback addresses as next-hop next hop in BGP
service routes is confined to the region they belong to. An anycast
IP-address called "Context Protocol Nexthop Address" (CPNH) abstracts
the SNs in a region from other regions in the network.

This provides much greater advantage in terms of scaling, convergence
and security. Changes to implement this feature are required only on
the local region's BNs and RRs, so legacy PE devices can also benefit
from this approach.

10. Operations and Manageability Considerations

10.1. MPLS OAM

MPLS OAM Operations, Administration, and Maintenance (OAM) procedures
specified in [RFC8029] also apply to BGP Classful Transport.

The 'Target Target FEC Stack' Stack sub-TLV for IPv4 Classful Transport has a Sub-
Type of 31744, 31744 and a length of 13. The Value field consists of the RD
advertised with the Classful Transport prefix, the IPv4 prefix (with
trailing 0 bits to make 32 bits in all) all), and a prefix length encoded
as shown in Figure 4.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Distinguisher |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: Classful Transport IPv4 FEC

The 'Target Target FEC Stack' Stack sub-TLV for IPv6 Classful Transport has a Sub-
Type of 31745, 31745 and a length of 25. The Value field consists of the RD
advertised with the Classful Transport prefix, the IPv6 prefix (with
trailing 0 bits to make 128 bits in all) and a prefix length encoded
as shown in Figure 5.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Distinguisher |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 prefix |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: Classful Transport IPv6 FEC

These prefix layouts are inherited from Sections 3.2.5, 3.2.5 and 3.2.6 in
[RFC8029] of
[RFC8029].

10.2. Usage of Route Distinguisher RD and Label Allocation Label-Allocation Modes

RDs aid in troubleshooting provider networks that deploy BGP CT, by
uniquely identifying the originator of a route across an
administrative domain that may either span multiple domains within a
provider network or span closely coordinated provider networks.

The use of RDs also provides an option for signaling forwarding
diversity within the same Transport Class. A An SN can advertise an EP
with the same Transport Class in multiple BGP CT routes with unique
RDs.

For example, unique "RDx:EP1" prefixes can be advertised by an SN for
an EP1 to different upstream BNs with unique forwarding specific forwarding-specific
encapsulation (e.g., Label), a Label) in order to collect traffic statistics
at the SN for each BN. In the absence of an RD, duplicated Transport Class/
Class / Color values will be needed in the transport network to
achieve such use cases.

The allocation of RDs is done at the point of origin of the BGP CT
route. This can either be either an Egress SN or a BN. The default RD
allocation mode is to use a unique RD per originating node for an EP.
This mode allows for the ingress to uniquely identify each originated
path. Alternatively, the same RD may be provisioned for multiple
originators of the same EP. This mode can be used when the ingress
does not require full visibility of all nodes originating an EP.

A label is allocated for a BGP CT route when it is advertised with
the next hop self set to itself by a an SN or a BN. An implementation may
use different
label allocation label-allocation modes with BGP CT. The Per-prefix is the
recommended label allocation label-allocation mode is per-prefix as it provides better traffic
convergence properties than per-next hop label allocation a per-NH label-allocation mode.
Furthermore, BGP CT offers two flavors for per-prefix label allocation.
allocation:

* The first flavor assigns a label for each unique "RD, EP".

* The second flavor assigns a label for each unique "Transport
Class, EP" while ignoring the RD.

In a BGP CT network, the number of routes at an Ingress PE is a
function of unique EPs multiplied by BNs in the ingress domain that
do
have the next hop self. set to themselves. BGP CT provides flexible RD and Label allocation
label-allocation modes to address operational requirements in a
multi-domain network. The impacts on the control plane and
forwarding behavior for these modes are detailed with an example in Managing Transport Route
Visibility (Section 10.3)
Section 10.3.

10.3. Managing Transport Route Transport-Route Visibility

This section details the usage of BGP CT RD and label allocation label-allocation
modes to calibrate the level of path visibility and the amount of
route and label scale in a multi-domain network.

Consider a multi-domain BGP CT network as illustrated in the
following Figure 6:

...................... .............................
: AS3 : : AS1 :
: : : :
: +----------ASBR11 +--PE11 (EP1) :
: | : : \ / :
: +----ASBR31 : : [P]----PE12 (EP2) :
: | | : : / | \ :
: | +----------ASBR12 | +--PE13 (EP3) :
: | : : | :
: | : : +-----PE14 (EP4) :
: PE31--[P] : : :
: | : : :
: | : : :
: | +----------ASBR21 +--PE21 (EP5) :
: | | : : \ / :
: +----ASBR32 : : [P]----PE22 (EP6) :
: | : : / | \ :
: +----------ASBR22 | +--PE22 (EP7) :
: : : | :
: : : +-----PE24 (EP8) :
...................... .............................
----------- Traffic Direction -------->

Figure 6: Managing Transport Route Transport-Route Visibility in Multi Domain Network Multi-Domain
Networks

The following table provides a comparison of the BGP CT route and
label scale, scale for varying endpoint path endpoint-path visibility at ingress node PE31
for each TC. It analyzes scenarios where Unicast or Anycast EPs
(EP-type) (EP-
type) may be originated by different node roles (Origin), using
different RD allocation modes (RD-Mode), (RD-Modes), and different Per-Prefix
Label allocation
label-allocation modes (PP-Mode). (PP-Modes).

+--------+------+-------+-------+---------+---------+
|EP-type |Origin|RD-Mode|PP-Mode|CT Routes|CT Labels|
+--------+------+-------+-------+---------+---------+
|Unicast |SN |Unique |TC,EP | 8 | 8 |
|Unicast |SN |Unique |RD,EP | 8 | 8 |
|Unicast |BN |Unique |TC,EP | 16 | 8 |
|Unicast |BN |Unique |RD,EP | 16 | 16 |
|--------|------|-------|-------|---------|---------|
|Anycast |SN |Unique |TC,EP | 8 | 2 |
|Anycast |SN |Unique |RD,EP | 8 | 8 |
|Anycast |SN |Same |TC,EP | 2 | 2 |
|Anycast |SN |Same |RD,EP | 2 | 2 |
|Anycast |BN |Unique |TC,EP | 4 | 2 |
|Anycast |BN |Unique |RD,EP | 4 | 4 |
|Anycast |BN |Same |TC,EP | 2 | 2 |
|Anycast |BN |Same |RD,EP | 2 | 2 |
+--------+------+-------+-------+---------+---------+

Figure 7: Route and Path Visibility at Ingress Node

In the table shown in Figure 7, route scale at ingress node PE31 is proportional to path
diversity in the ingress domain (number of ASBRs) and point of
origination of the BGP CT route. TE granularity at ingress node PE31
is proportional to the number of unique CT labels received, which
depends on PP-mode the PP-Mode and the path diversity in the ingress domain.

Deploying unique RDs is strongly RECOMMENDED because it helps in
troubleshooting by uniquely identifying the originator of a route and
avoids path-hiding. path hiding.

In typical deployments deployments, originating BGP CT routes at the egress node
(SN) is recommended. In this model, using either an "RD, EP" or "TC,
EP" Per-Prefix label allocation label-allocation mode repairs traffic locally at the
nearest BN for any failures in the network, network because the label value
does not change.

Originating at BNs with unique RDs induces more routes than when
originating at egress SNs. In this model, use of the "TC, EP" Per-Prefix
label allocation Per-
Prefix label-allocation mode repairs traffic locally at the nearest
BN for any failures in the network, network because the label value does not
change.

The previous table in

Figure 7 demonstrates that BGP CT allows an operator to control how
much path visibility and forwarding diversity is desired in the network,
network for both Unicast and Anycast endpoints.

11. Deployment Considerations. Considerations

11.1. Coordination Between Domains Using Different Community Namespaces

Cooperating Inter-AS Option C domains may sometimes not agree on RT,
RD, Mapping community community, or Transport Route Target values because of
differences in community namespaces (e.g. (e.g., during network mergers or
renumbering for expansion). Such deployments may deploy mechanisms
to map and rewrite the Route Target values on domain boundaries, boundaries using per ASBR
per-ASBR import policies. This is no different than any other BGP
VPN family. Mechanisms used in inter-AS VPN deployments may be
leveraged with the Classful Transport family also.

A resolution scheme allows association with multiple Mapping
Communities. This minimizes service disruption during renumbering,
network merger merger, or transition scenarios.

The Transport Class Route Target Extended Community extended community is useful to
avoid collision with regular Route Target namespace used by service
routes.

11.2. Managing Intent at Service and Transport layers.

Illustration of BGP CT Procedures (Section 8) Layers

Section 8 shows multiple domains that agree on a color name space namespace
(Agreeing Color Domains) and contain tunnels with an equivalent set
of colors (Homogenous Color Domains).

However, in the real world, this may not always be guaranteed. Two
domains may independently manage their color namespaces; these are
known as Non-Agreeing Color Domains. Two domains may have tunnels
with unequal sets of colors; these are known as Heterogeneous Color
Domains.

This section describes how BGP CT is deployed in such scenarios to
preserve end-to-end Intent. Examples described in this section use
Inter-AS Option C domains. Similar mechanisms will work for Inter-AS
Option A and Inter-AS Option B scenarios as well.

11.2.1. Service Layer Service-Layer Color Management

At the service layer, it is recommended that a global color namespace
be maintained across multiple co-operating cooperating domains. BGP CT allows
indirection using resolution schemes to be able to maintain a global
namespace in the service layer. This is possible even if each domain
independently maintains its own local transport color namespace.

As explained in Next Hop Resolution Scheme (Section 5) , Section 5, a mapping community carried on a service
route maps to a resolution scheme. The mapping community values for
the service route can be abstract and are not required to match the
transport color namespace. This abstract mapping community value
representing a global service layer service-layer intent is mapped to a local transport layer
transport-layer intent available in each domain.

In this manner, it is recommended to keep color namespace management
at the service layer and the transport layer decoupled from each
other. In the following sections sections, the service layer agrees on a
single global namespace.

11.2.2. Non-Agreeing Color Transport Domains

Non-agreeing color domains

Non-Agreeing Color Domains require a mapping community rewrite on
each domain boundary. This rewrite helps to map one domain's color
namespace to another domain's color namespace.

The following example illustrates how traffic is stitched and SLA is
preserved when domains don't use the same namespace at the transport
layer. Each domain specifies the same SLA using different color
values.

..................... ....................... ......................
: Gold(100) : : Gold(300) : : Gold(500) :
: : : : : :
: [PE11]----[ASBR11]---[ASBR21------[ASBR22]---[ASBR31-------[PE31]:
: : : : : :
: AS1 : : AS2 : : AS3 :
: : : : : :
: Bronze(200) : : Bronze(400) : : Bronze(600) :
..................... ....................... ......................

----------- Traffic Direction -------->

Figure 8: Transport Layer with Non-agreeing Non-Agreeing Color Domains

In the topology shown in Figure 8, we have three Autonomous Systems.
All the nodes in the topology support BGP CT.

* In AS1 AS1, the Gold SLA is represented by color 100 and Bronze by
200.

* In AS2 AS2, the Gold SLA is represented by color 300 and Bronze by
400.

* In AS3 AS3, the Gold SLA is represented by color 500 and Bronze by
600.

Though the color values are different, they map to tunnels with
sufficiently similar TE characteristics in each domain.

The service route carries an abstract mapping community that maps to
the required SLA. For example, Service service routes that need to resolve
over Gold transport tunnels, tunnels carry a mapping community color:0:100500.
In AS3 AS3, it maps to a resolution scheme containing a TRDB with color 500 whereas
500; in AS2 AS2, it maps to a TRDB with color 300 300; and in AS1 AS1, it maps to
a TRDB with color 100. Coordination is needed to provision the
resolution schemes in each domain domain, as explained previously.

At the AS boundary, the transport-class route-target is rewritten for
the BGP CT routes. In the previous topology, at ASBR31, the
transport-target:0:500 for Gold tunnels is rewritten to transport-
target:0:300 and then advertised to ASBR22. Similarly, the
transport-target:0:300 for Gold tunnels are re-written rewritten to transport-
target:0:100 at ASBR21 before advertising to ASBR11. At PE11, the
transport route received with transport-target:0:100 will be added to
the color 100 TRDB. The service route received with mapping
community color:0:100500 at PE1 maps to the Gold TRDB and resolves
over this transport route.

Inter-domain traffic forwarding in the previous topology works as
explained in Section 8.

Transport-target re-write rewrite requires co-ordination coordination of color values
between domains in the transport layer. This method avoids the need
to re-write rewrite service route mapping communities, keeping the service
layer homogenous and simple to manage. Coordinating Transport Class
RT between two adjacent color domains at a time is easier than
coordinating service layer service-layer colors deployed in a global mesh of non-
adjacent color domains. This basically allows localizing the problem
to a pair of adjacent color domains and solving it.

11.2.3. Heterogeneous Agreeing Color Transport Domains

In a heterogeneous domains heterogeneous-domain scenario, it might not be possible to map a service layer
service-layer intent to the matching transport color, as the color
might not be locally available in a domain.

The following example illustrates how traffic is stitched, stitched when a
transit AS contains more shades for an SLA path compared to Ingress
and Egress domains. This example shows how service routes can
traverse through finer shades when available and take coarse shades
otherwise.

..................... ....................... ......................
: : : Gold1(101) : : :
: Gold(100) : : Gold2(102) : : Gold(100) :
: : : : : :
: [PE11]----[ASBR11]---[ASBR21------[ASBR22]---[ASBR31-------[PE31]:
: : : : : :
: Metro Ingress : : Core : : Metro Egress :
: : : : : :
: AS1 : : AS2 : : AS3 :
..................... ....................... ......................

----------- Traffic Direction -------->

Figure 9: Transport Layer with Heterogenous Color Domains

In the preceding topology shown in Figure 9, we have three Autonomous Systems. All the nodes in the
topology support BGP CT.

* In AS1 AS1, the Gold SLA is represented by color 100.

* In AS2 AS2, Gold has finer shades: Gold1 by color 101 and Gold2 by
color 102.

* In AS3 AS3, the Gold SLA is represented by color 100.

This problem can be solved by the two following approaches: approaches described in
Sections 11.2.3.1 and 11.2.3.2.

11.2.3.1. Duplicate Tunnels Approach

In this approach, duplicate tunnels that satisfy the Gold SLA are
configured in domains AS1 and AS3, but they are given fine grained fine-grained
colors 101 and 102.

These tunnels will be installed in TRDBs corresponding to transport
classes of color colors 101 and 102.

Overlay routes received with a mapping community (e.g.: (e.g., transport-
target or color community) can resolve over these tunnels in the TRDB
with matching color colors by using resolution schemes.

This approach consumes more resources in the transport and forwarding
layer,
layer because of the duplicate tunnels.

11.2.3.2. Customized Resolution Schemes Approach

In this approach, resolution schemes in domains AS1 and AS3 are
customized to map the received mapping community (e.g., transport-
target or color community) over available Gold SLA tunnels. This
conserves resource usage with no additional state in the transport or
forwarding planes.

Service routes advertised by PE31 that need to resolve over Gold1
transport tunnels carry a mapping community color:0:101. In AS3 and
AS1, where Gold1 is not available, it is mapped to color 100 TRDB
using a customized resolution scheme. In AS2, Gold1 is available available,
and it maps to color 101 TRDB.

Similarly, service routes advertised by PE31 that need to resolve
over Gold2 transport tunnels carry a mapping community color:0:102.
In AS3 and AS1, where Gold2 is not available, it is mapped to color
100 TRDB using a customized resolution scheme. In AS2, Gold2 is
available
available, and it maps to color 102 TRDB.

To facilitate this, SN/BN SNs/BNs in all three AS ASes provision the transport
classes 100, 101 101, and 102. SN SNs and BN BNs in AS1 and AS3 are
provisioned with customized resolution schemes that resolve routes
with transport-target:0:101 or transport-target:0:102 using color 100
TRDB.

PE31 is provisioned to originate BGP CT route routes with color 101 for
endpoint PE31. This route is sent with an NLRI RD prefix RD1:PE31
and
route target Route Target extended community transport-target:0:101.

Similarly, PE31 is provisioned to originate BGP CT route routes with color
102 for endpoint PE31. This route is sent with an NLRI RD prefix
RD2:PE31 and route target Route Target extended community transport-target:0:102.

Following

The following description explains the remaining procedures with
color 101 as an example.

At ASBR31, the route target "transport-target:0:101" on this BGP CT
route instructs gives instruction to add the route to color 101 TRDB. ASBR31
is provisioned with a customized resolution scheme that resolves the
routes carrying mapping community transport-target:0:101 to resolve
using color 100 TRDB. This route is then re-advertised readvertised from color 101
TRDB to ASBR22 with route-target:0:101.

At ASBR22, the BGP CT routes received with transport-target:0:101
will be added to color 101 TRDB and strictly resolve over tunnel
routes in the same TRDB. This route is re-advertised readvertised to ASBR21 with
transport-target:0:101.

Similarly, at ASBR21, the BGP CT routes received with transport-
target:0:101 will be added to color 101 TRDB and strictly resolve
over tunnel routes in the same TRDB. This route is re-advertised readvertised to
ASBR11 with transport-target:0:101.

At ASBR11, the route target "transport-target:0:101" on this BGP CT
route instructs gives instruction to add the route to color 101 TRDB. ASBR11
is provisioned with a customized resolution scheme that resolves the
routes carrying transport-target:0:101 to use color 100 TRDB. This
route is then re-advertised readvertised from color 101 TRDB to PE11 with
transport-target:0:101.

At PE11, the route target "transport-target:0:101" on this BGP CT
route instructs gives instruction to add the route to color 101 TRDB. PE11 is
provisioned with a customized resolution scheme that resolves the
routes carrying transport-target:0:101 to use color 100 TRDB.

When PE11 receives the service route with the mapping community
color:0:101
color:0:101, it directly resolves over the BGP CT route in color 101
TRDB, which which, in turn turn, resolves over tunnel routes in color 100 TRDB.

Similar processing is done for color 102 routes also at ASBR31,
ASBR22, ASBR21, ASBR11 ASBR11, and PE11.

In doing so, PE11 can forward traffic via tunnels with color 101,
color 102 in the core domain, domain and color 100 in the metro domains.

11.3. Migration Scenarios. Scenarios

11.3.1. BGP CT Islands Connected via BGP LU Domain

This section explains how an end-to-end SLA can be achieved while
transiting a domain that does not support BGP CT. BGP LU is used in
such domains to connect the BGP CT islands.

+----------EBGP Multihop CT-------------+
| |
AS3 | AS2 | AS1
[PE31-----ASBR31]--------[ASBR22---ASBR21]-------[ASBR11---PE11]

<--EBGP LU--> <--EBGP LU-->
<--IBGP CT--> <--IBGP LU--> <--IBGP CT-->

---------Traffic Direction--------->

Figure 10: BGP CT in AS1 and AS3 connected Connected by BGP LU in AS2

In the preceding topology shown in Figure 10, there are three AS
domains.
domains: AS1 and AS3 support BGP CT, while AS2 does not support BGP
CT.

Nodes in AS1, AS2, and AS3 negotiate BGP LU family on IBGP sessions
within the domain. Nodes in AS1 and AS3 negotiate BGP CT family on
IBGP sessions within the domain. ASBR11 and ASBR21 as well as ASBR22
and ASBR31 negotiate BGP LU family on the EBGP session over directly
connected inter-domain links. ASBR11 and ASBR31 have reachability to
each other’s other's loopbacks through BGP LU. ASBR11 and ASBR31 negotiate
BGP CT family over a multihop EBGP session formed using BGP LU
reachability.

The following tunnels exist for the Gold Transport Class

* PE11_to_ASBR11_gold - RSVP-TE tunnel

* ASBR11_to_PE11_gold - RSVP-TE tunnel

* PE31_to_ASBR31_gold - SRTE tunnel

* ASBR31_to_PE31_gold - SRTE tunnel

The following tunnels exist for the Bronze Transport Class

* PE11_to_ASBR11_bronze - RSVP-TE tunnel

* ASBR11_to_PE11_bronze - RSVP-TE tunnel

* PE31_to_ASBR31_bronze - SRTE tunnel

* ASBR31_to_PE31_bronze - SRTE tunnel

These tunnels are provisioned to belong to Transport Classes Gold and
Bronze, and they are advertised between ASBR31 and ASBR11 with Next the
next hop
self. set to themselves.

In AS2, that which does not support BGP CT, a separate loopback may be
used on ASBR22 and ASBR21 to represent Gold and Bronze SLAs, viz. namely
ASBR22_lpbk_gold, ASBR22_lpbk_bronze, ASBR21_lpbk_gold ASBR21_lpbk_gold, and
ASBR21_lpbk_bronze.

Furthermore, the following tunnels exist in AS2 to satisfy the
different SLAs, SLAs using per SLA loopback per-SLA-loopback endpoints:

* ASBR21_to_ASBR22_lpbk_gold - RSVP-TE tunnel

* ASBR22_to_ASBR21_lpbk_gold - RSVP-TE tunnel

* ASBR21_to_ASBR22_lpbk_bronze - RSVP-TE tunnel

* ASBR22_to_ASBR21_lpbk_bronze - RSVP-TE tunnel

The RD:PE11 BGP CT route is originated from PE11 towards ASBR11 with
transport-target 'gold.' ASBR11 readvertises this route with the
next hop set to ASBR11_lpbk_gold on the EBGP multihop session towards
ASBR31. ASBR11 originates a BGP LU route for endpoint
ASBR11_lpbk_gold on an EBGP session to ASBR21 with a 'gold SLA' community,
community and a BGP LU route for ASBR11_lpbk_bronze with a 'bronze
SLA' community. The SLA community is used by ASBR31 to publish the
BGP LU routes in the corresponding BGP CT TRDBs.

ASBR21 readvertises the BGP LU route for endpoint ASBR11_lpbk_gold to
ASBR22 with the next hop set by local policy config to the unique
loopback ASBR21_lpbk_gold by matching the 'gold SLA' community
received as part of BGP LU advertisement from ASBR11. ASBR22
receives this route and resolves the next hop over the
ASBR22_to_ASBR21_lpbk_gold RSVP-TE tunnel. On successful resolution,
ASBR22 readvertises this BGP LU route to ASBR31 with the next hop self set
to itself and a new label.

ASBR31 adds the ASBR11_lpbk_gold route received via EBGP LU from
ASBR22 to a 'gold' TRDB based on the received 'gold SLA' community.
ASBR31 uses this 'gold' TRDB route to resolve the next hop
ASBR11_lpbk_gold received on the BGP CT route with transport-target
'gold,' for the prefix RD:PE11 received over the EBGP multihop CT
session, thus preserving the end-to-end SLA. Now ASBR31 readvertises
the BGP CT route for RD:PE11 with the next hop as self set to itself, thus
stitching with the BGP LU LSP in AS2. Intra-domain traffic
forwarding in AS1 and AS3 follows the procedures as explained in Illustration of CT
Procedures (Section 8)
Section 8.

In cases where an SLA cannot be preserved in AS2 because SLA specific SLA-specific
tunnels and loopbacks don't exist in AS2, traffic can be carried over
available SLAs (e.g.: best effort (e.g., best-effort SLA) by rewriting the next hop to
an ASBR21 loopback assigned to the available SLA. This eases
migration in case of a heterogeneous color domains as-well. domain as well.

11.3.2. BGP CT - CT: Interoperability between Between MPLS and Other Forwarding
Technologies

This section describes how nodes supporting dissimilar encapsulation
technologies can interoperate with each other when using the BGP CT family.

11.3.2.1. Interop Interoperation Between MPLS and SRv6 Nodes. Nodes

BGP speakers may carry MPLS label labels and SRv6 SID SIDs in BGP CT SAFI 76
for
AFIs AFI 1 or 2 routes using protocol encoding as described in Carrying
Multiple Encapsulation information (Section 6.3)
Section 6.3.

MPLS Labels are carried using RFC 8277 encoding, the encoding described in [RFC8277],
and SRv6 SID is SIDs are carried using the Prefix SID attribute as specified
in Section 7.13.

RR1---+
\ +-------R2 [MPLS + SRv6]
\ |
R1--------P-------R3 [MPLS only]
[MPLS + SRv6] |
+-------R4 [SRv6 only]

<---- Bidirectional Traffic ----->

Figure 11: BGP CT Interop between Interoperation Between MPLS and SRv6 nodes Nodes

This example shows a provider network with a mix of devices with that have
different forwarding capabilities. R1 and R2 support forwarding both
MPLS and SRv6 packets. R3 supports forwarding MPLS packets only. R4
supports forwarding SRv6 packets only. All these nodes have a BGP
session with Route Reflector RR1 RR1, which reflects routes between these
nodes with the next hop unchanged. The BGP CT family is negotiated
on these sessions.

R1 and R2 send and receive both MPLS label labels and SRv6 SID SIDs in the BGP
CT control plane routes. This allows them to be ingress and egress
for both MPLS and SRv6 data planes. The MPLS label is carried using RFC 8277
encoding,
the encoding described in [RFC8277], and an SRv6 SID is carried using
the Prefix SID attribute as specified in Section 7.13, 7.13 without the
Transposition Scheme. In this way, either MPLS or SRv6 forwarding
can be used between R1 and R2.

R1 and R3 send and receive an MPLS label in the BGP CT control plane
routes using RFC 8277 encoding. the encoding described in [RFC8277]. This allows them
to be ingress and egress for MPLS data plane. R1 will carry an SRv6
SID in Prefix-SID the Prefix SID attribute, which will not be used by R3. In
order to interoperate with MPLS only MPLS-only device R3, R1 MUST NOT use the
SRv6 Transposition scheme described in [RFC9252]. The encoding
suggested in Section 7.13 is used by R1. MPLS forwarding will be
used between R1 and R3.

R1 and R4 send and receive SRv6 SID SIDs in the BGP CT control plane
routes using the BGP Prefix-SID Prefix SID attribute, without a Transposition
Scheme. This allows them to be ingress and egress for the SRv6 data
plane. R4 will carry the special MPLS Label label with a value of 3
(Implicit-NULL) in RFC
8277 encoding, the encoding described in [RFC8277], which tells
R1 not to push any MPLS label for this BGP CT route towards R4. The
MPLS Label label advertised by R1 in RFC 8277 an NLRI as described in [RFC8277] will
not be used by R4. SRv6 forwarding will be used between R1 and R4.

Note that, in this example that example, R3 and R4 cannot communicate directly
with each other, other because they don't support a common forwarding
technology. The BGP CT routes received at R3, R3 and R4 from each other
will remain unusable, unusable due to incompatible forwarding technology.

11.3.2.2. Interop Between Nodes Supporting MPLS and UDP Tunneling

This section describes how nodes supporting MPLS forwarding can
interoperate with other nodes supporting UDP (or IP) tunneling, tunneling when
using BGP CT family.

MPLS Labels are carried using RFC 8277 encoding, the encoding described in [RFC8277],
and UDP (or IP) tunneling information is carried using the TEA
attribute or the Encapsulation Extended Community as specified in
[RFC9012].

RR1---+
\ +-------R2 [MPLS + UDP]
\ |
R1--------P-------R3 [MPLS only]
[MPLS + UDP] |
+-------R4 [UDP only]

<---- Bidirectional Traffic ----->

Figure 12: BGP CT Interop between Between MPLS and UDP tunneling nodes Tunneling Nodes

In this example, R1 and R2 support forwarding both MPLS and UDP
tunneled packets. R3 supports forwarding MPLS packets only. R4
supports forwarding UDP tunneled packets only. All these nodes have
BGP session with Route Reflector RR1 RR1, which reflects routes between
these nodes with the next hop unchanged. The BGP CT family is
negotiated on these sessions.

R1 and R2 send and receive both MPLS label labels and UDP tunneling info in
the BGP CT control plane routes. This allows them to be ingress and
egress for both MPLS and UDP tunneling data planes. The MPLS label
is carried using RFC 8277 encoding. the encoding described in [RFC8277]. As specified
in [RFC9012], UDP tunneling information is carried using TEA attribute the Tunnel
Encasulation Attribute (code 23) or the "barebones" Tunnel TLV
carried in Encapsulation Extended Community. Either MPLS or UDP tunneled
tunnel forwarding can be used between R1 and R2.

R1 and R3 send and receive MPLS label labels in the BGP CT control plane
routes using RFC 8277 encoding. the encoding described in [RFC8277]. This allows them
to be ingress and egress for MPLS data plane. R1 will carry UDP
tunneling info in TEA
attribute, the TEA, which will not be used by R3. MPLS
forwarding will be used between R1 and R3.

R1 and R4 send and receive UDP tunneling info in the BGP CT control
plane routes using the BGP TEA attribute. TEA. This allows them to be ingress and
egress for UDP tunneled data plane. R4 will carry special MPLS Label
with value 3 (Implicit-NULL) in RFC 8277 encoding, the encoding described in [RFC8277],
which tells R1 not to push any MPLS label for this BGP CT route
towards R4. The MPLS Label advertised by R1 will not be used by R4.
UDP tunneled forwarding will be used between R1 and R4.

11.4. MTU Considerations

Operators should coordinate the MTU of the intra-domain tunnels used
to prevent Path MTU discovery problems that could appear in
deployments. The encapsulation overhead due to the MPLS label stack
or equivalent tunnel header in different forwarding architecture
should also be considered when determining the Path MTU of the end-
to-end BGP CT tunnel.

The document

[INTAREA-TUNNELS] discusses these considerations in more detail.

11.5. Use of DSCP

BGP CT specifies procedures for Intent Driven Intent-Driven Service Mapping in a
service provider network, network and defines the 'Transport Class' construct
to represent an Intent.

It may be desirable to allow a CE device to indicate in the data
packet it sends what treatment it desires (the Intent) when the
packet is forwarded within the provider network.

Such an indication can be in the form of a DSCP code point [RFC2474] (see [RFC2474]) in
the IP header.

In RFC2474, [RFC2474], a Forwarding Class Selector maps to a PHB (Per-hop
Behavior). The Transport Class construct is a PHB at the transport
layer.

----Gold----->
[CE1]-----[PE1]---[P]----[PE2]-----[CE2]
---Bronze---->
203.0.113.11 203.0.113.22
-----Traffic direction---->

Figure 13: Example Topology with DSCP on PE-CE Links

Let PE1 be configured to map DSCP1 to the Gold Transport class, class and
DSCP2 to the Bronze Transport class. Based on the DSCP code point received on
the IP traffic from the CE device, PE1 forwards the IP packet over a
Gold or Bronze TC tunnel. Thus, the forwarding is not based on just
the destination IP address, address but also the DSCP code point. DSCP. This is known as Class Based
Class-Based Forwarding (CBF).

CBF is configured at the PE1 device, mapping the DSCP values to
respective Transport Classes. This mapping (DSCP peering agreement)
is communicated to CE device devices by out of band out-of-band mechanisms. This allows
the administrator of CE1 to discover what transport classes exist in
the provider network, network and which DSCP codepoint to encode so that traffic is
forwarded using the desired Transport Class in the provided network.
When the IP packet exits the provider network to CE2, PE2 resets the
DSCP code point based on the DSCP peering agreement with CE2.

12. Applicability to Network Slicing

In Network Slicing, the IETF Network Slice Controller (IETF NSC) (NSC) is
responsible for customizing and setting up the underlying transport
(e.g.
(e.g., RSVP-TE, SRTE tunnels with desired characteristics) and
resources (e.g., polices/shapers) policies/shapers) in a transport network to create
an IETF Network Slice.

The Transport Class construct described in this document can be used
to realize the "IETF Network Slice" described in Section 4 of
[RFC9543]
[RFC9543].

The NSC can use the Transport Class Identifier (Color value) to
provision a transport tunnel in a specific IETF Network Slice.

Furthermore, the NSC can use the Mapping Community on the service
route to map traffic to the desired IETF Network Slice.

13. IANA Considerations

This document makes the following requests of IANA.

13.1. New BGP SAFI

IANA has assigned a BGP SAFI code 76 for "Classful Transport". Value
76. IANA is requested to update the reference to this document. "Classful Transport SAFI".

Registry Group: Subsequent Address Family Identifiers (SAFI)
Parameters

Registry Name: SAFI Values

+=======+=========================+===========+
| Value | Description
-------------+-------------------------- | Reference |
+=======+=========================+===========+
| 76 | Classful Transport SAFI | RFC 9832 |
+-------+-------------------------+-----------+

Table 1

This will be used to create new AFI,SAFI AFI/SAFI pairs for IPv4, IPv4 and IPv6
Classful Transport families. viz: families, namely:

* "IPv4, Classful Transport". Transport" AFI/SAFI = "1/76" for carrying IPv4
Classful Transport prefixes.

* "IPv6, Classful Transport". Transport" AFI/SAFI = "2/76" for carrying IPv6
Classful Transport prefixes.

13.2. New Format for BGP Extended Community

IANA has assigned a Format type (Type high = 0xa) of Extended
Community EXT-COMM [RFC4360] for the Transport Class from the following
registries:
registries in the "Border Gateway Protocol (BGP) Extended
Communities" registry group:

* the "BGP Transitive Extended Community Types" registry, registry and

* the "BGP Non-Transitive Extended Community Types" registry.

The same low-order six bits have been assigned for both allocations.

IANA is requested to update the reference to this document.

This document uses this new Format with subtype 0x2 (route target),
as a transitive extended community. The Route Target thus formed is
called "Transport Class" route target Route Target extended community.

The Non-Transitive Transport Class Extended extended community with subtype
0x2 (route target) is called the "Non-Transitive Transport Class
route target
Route Target extended community".

Taking a reference of [RFC7153] , [RFC7153], the following assignments in the following
subsections have been
made: made.

13.2.1. Existing Registries

13.2.1.1. Registries for the "Type" Field

13.2.1.1.1. Transitive Types

This registry contains values of the high-order octet (the "Type"
field) of a Transitive Extended Community.

Registry Group: Border Gateway Protocol (BGP) Extended Communities

Registry Name: BGP Transitive Extended Community Types

+============+=================+
| Type Value | Name
--------------+--------------- |
+============+=================+
| 0x0a | Transport Class |
+------------+-----------------+

Table 2

(Sub-Types are defined in the "Transitive Transport Class Extended
Community Sub-Types"
registry) registry.)

13.2.1.1.2. Non-Transitive Types

This registry contains values of the high-order octet (the "Type"
field) of a Non-transitive Non-Transitive Extended Community.

Registry Group: Border Gateway Protocol (BGP) Extended Communities

Registry Name: BGP Non-Transitive Extended Community Types

+============+================================+
| Type Value | Name
--------------+-------------------------------- |
+============+================================+
| 0x4a | Non-Transitive Transport Class |
+------------+--------------------------------+

Table 3

(Sub-Types are defined in the "Non-Transitive Transport Class
Extended Community Sub-Types"
registry) registry.)

13.2.2. New Registries

13.2.2.1. Transitive Transport Class Extended Community Sub-Types
Registry

IANA is requested to add has added the following subregistry under the “Border "Border Gateway
Protocol (BGP) Extended Communities”: Communities" registry group:

Registry Group: Border Gateway Protocol (BGP) Extended Communities

Registry Name: Transitive Transport Class Extended Community Sub-Types Sub-
Types

Note: This registry contains values of the second octet (the
"Sub-Type" "Sub-
Type" field) of an extended community when the value of the first
octet (the "Type" field) is 0x0a.

Table 4

+----------------+--------------+
| Sub-Type Value | Name
-----------------+-------------- |
+----------------+--------------+
| 0x02 | Route Target |
+----------------+--------------+

Table 5

13.2.2.2. Non-Transitive Transport Class Extended Community Sub-Types
Registry

IANA is requested to add has added the following subregistry under the “Border "Border Gateway
Protocol (BGP) Extended Communities”: Communities" registry group:

Registry Group: Border Gateway Protocol (BGP) Extended Communities

Registry Name: Non-Transitive Transport Class Extended Community
Sub-Types

Note: This registry contains values of the second octet (the
"Sub-Type" "Sub-
Type" field) of an extended community when the value of the first
octet (the "Type" field) is 0x4a.

Table 6

+================+==============+
| Sub-Type Value | Name
-----------------+-------------- |
+================+==============+
| 0x02 | Route Target |
+----------------+--------------+

Table 7

13.3. MPLS OAM Code Points

The following two code points have been assigned for Target FEC Stack
sub-TLVs:

* IPv4 BGP Classful Transport

* IPv6 BGP Classful Transport

Registry Group: Multiprotocol Label Switching (MPLS) Label Switched
Paths (LSPs) Ping Parameters

Registry Name: Sub-TLVs for TLV Types 1, 16, and 21

+==========+=============================+
| Sub-Type | Name
-----------------+------------------------------ |
+==========+=============================+
| 31744 | IPv4 BGP Classful Transport |
+----------+-----------------------------+
| 31745 | IPv6 BGP Classful Transport

IANA is requested to update |
+----------+-----------------------------+

Table 8

14. Transport Class ID Registry

This RFC documents the reference to "Transport Class ID" registry and its assigned
values. The value ranges in this document.

14. Registries maintained registry are either assigned by
this document

14.1. or reserved for Private Use. Because the registry is
complete, it is being published in this RFC rather than as an IANA-
maintained registry. However, note that IANA-related terminology
[BCP26] is used here.

Registry Name: Transport Class ID

This document reserves

The registration procedures are as follows:

+==============+========================+
| Value | Registration Procedure |
+==============+========================+
| 0 | IETF Review |
+--------------+------------------------+
| 1-4294967295 | Private Use |
+--------------+------------------------+

Table 9

As shown in the table below, the Transport class Class ID value 0 is
Reserved to represent
"Best Effort the "Best-Effort Transport Class ID". This is
used in the 'Transport Class ID' field of a Transport Route Target
extended community that represents best effort the best-effort transport class.

Since all value ranges in this registry are already assigned or
Private use, this registry will be maintained by this document. IANA
does not need to maintain this registry.

Registry Group: BGP Classful Transport (BGP CT)

Registry Name: Transport Class ID

+==============+================================+
| Value | Name
-----------------+-------------------------------- |
+==============+================================+
| 0 Best Effort | Best-Effort Transport Class ID |
+--------------+--------------------------------+
| 1-4294967295 | Private Use

Reference: This document.

Registration Procedure(s)

Value Registration Procedure
-----------------+--------------------------------
0 IETF Review
1-4294967295 Private Use |
+--------------+--------------------------------+

Table 10

As noted in Sec Sections 4 and Sec 7.10, 'Transport Class ID' is
interchangeable with 'Color'. For purposes of backward compatibility
with usage of a 'Color' field in a Color extended community Extended Community as
specified in [RFC9012] and [RFC9256], the range 1-4294967295 uses
'Private Use' as the Registration Procedure.

15. Security Considerations

This document uses [RFC4760] the mechanisms from [RFC4760] to define new BGP
address families (AFI/SAFI : 1/76 and 2/76) that carry transport transport-
layer endpoints. These address families are explicitly configured
and negotiated between BGP speakers, which confines the propagation
scope of this reachability information. These routes stay in the
part of network where the new address family is negotiated, negotiated and don't
leak out into the Internet.

Furthermore, procedures defined in Section 9.1 mitigate the risk of
unintended propagation of BGP CT routes across Inter-AS boundaries
even when a BGP CT family is negotiated. BGP import and export
policies are used to control the BGP CT reachability information
exchanged across AS boundaries. This mitigates the risk of
advertising internal loopback addresses outside the administrative
control of the provider network.

This document does not change the underlying security issues inherent
in the existing BGP protocol, such as those described in [RFC4271]
and [RFC4272].

Additionally, BGP sessions SHOULD be protected using the TCP
Authentication Option [RFC5925] and the Generalized TTL Security
Mechanism [RFC5082].

Using a separate BGP family and new RT (Transport Class RT) minimizes
the possibility of these routes mixing with service routes.

If redistributing between SAFI 76 and SAFI 4 routes for AFIs 1 or 2,
there is a possibility of SAFI 4 routes mixing with SAFI 1 service
routes. To avoid such scenarios, it is RECOMMENDED that
implementations support keeping SAFI 76 and SAFI 4 transport routes
in separate transport RIBs, distinct from service RIB that contain
SAFI 1 service routes.

BGP CT routes distribute label binding using [RFC8277] for the MPLS
dataplane and hence
data plane; hence, its security considerations apply.

BGP CT routes distribute SRv6 SIDs for SRv6 dataplanes and hence data planes; hence, the
security considerations of Section 9.3 of [RFC9252] apply. Moreover,
the SRv6 SID transposition scheme is disabled in BGP CT, as described
in Section 7.13, to mitigate the risk of misinterpreting transposed
SRv6 SID information as an MPLS label.

As [RFC4272] discusses, BGP is vulnerable to traffic-diversion
attacks. This SAFI routes route adds a new means by which an attacker could
cause the traffic to be diverted from its normal path. Potential
consequences include "hijacking" of traffic (insertion of an
undesired node in the path, which allows for inspection or
modification of traffic, or avoidance of security controls) or denial
of service (directing traffic to a node that doesn't desire to
receive it).

In order to mitigate the risk of the diversion of traffic from its
intended destination, BGPsec solutions ([RFC8205] and Origin
Validation [RFC8210][RFC6811]) may be extended in future to work for
non-Internet SAFIs (SAFIs other than 1).

The restriction of the applicability of the BGP CT SAFI 76 to its
intended well-defined scope and utilizing [RFC8212] limits the
likelihood of traffic diversions.

16. References

16.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.

[RFC2545] Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
Extensions for IPv6 Inter-Domain Routing", RFC 2545,
DOI 10.17487/RFC2545, March 1999,
<https://www.rfc-editor.org/info/rfc2545>.

[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.

[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.

[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <https://www.rfc-editor.org/info/rfc4360>.

[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>.

[RFC4659] De Clercq, J., Ooms, D., Carugi, M., and F. Le Faucheur,
"BGP-MPLS IP Virtual Private Network (VPN) Extension for
IPv6 VPN", RFC 4659, DOI 10.17487/RFC4659, September 2006,
<https://www.rfc-editor.org/info/rfc4659>.

[RFC4684] Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
R., Patel, K., and J. Guichard, "Constrained Route
Distribution for Border Gateway Protocol/MultiProtocol
Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684,
November 2006, <https://www.rfc-editor.org/info/rfc4684>.

[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.

[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
<https://www.rfc-editor.org/info/rfc5082>.

[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.

[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.

[RFC7153] Rosen, E. and Y. Rekhter, "IANA Registries for BGP
Extended Communities", RFC 7153, DOI 10.17487/RFC7153,
March 2014, <https://www.rfc-editor.org/info/rfc7153>.

[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.

[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.

[RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
Switched (MPLS) Data-Plane Failures", RFC 8029,
DOI 10.17487/RFC8029, March 2017,
<https://www.rfc-editor.org/info/rfc8029>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8212] Mauch, J., Snijders, J., and G. Hankins, "Default External
BGP (EBGP) Route Propagation Behavior without Policies",
RFC 8212, DOI 10.17487/RFC8212, July 2017,
<https://www.rfc-editor.org/info/rfc8212>.

[RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address
Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
<https://www.rfc-editor.org/info/rfc8277>.

[RFC8669] Previdi, S., Filsfils, C., Lindem, A., Ed., Sreekantiah,
A., and H. Gredler, "Segment Routing Prefix Segment
Identifier Extensions for BGP", RFC 8669,
DOI 10.17487/RFC8669, December 2019,
<https://www.rfc-editor.org/info/rfc8669>.

[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>.

[RFC9252] Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene,
B., Zhuang, S., and J. Rabadan, "BGP Overlay Services
Based on Segment Routing over IPv6 (SRv6)", RFC 9252,
DOI 10.17487/RFC9252, July 2022,
<https://www.rfc-editor.org/info/rfc9252>.

[SRTE]

[RFC9830] Previdi, S., Filsfils, C., Talaulikar, Ed. K., Ed., Mattes,
P., and S. Previdi, D. Jain, "Advertising Segment Routing Policies in
BGP", 7 November 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
policy-safi-10>. RFC 9830, DOI 10.17487/RFC9830, August 2025,
<https://www.rfc-editor.org/info/rfc9830>.

16.2. Informative References

[BCP26] Best Current Practice 26,
<https://www.rfc-editor.org/info/bcp26>.
At the time of writing, this BCP comprises the following:

Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[BGP-CT-SRv6]
Vairavakkalai, K., Ed. and N. Venkataraman, Ed., "BGP CT -
Adaptation to SRv6 dataplane", 25 April Work in Progress, Internet-
Draft, draft-ietf-idr-bgp-ct-srv6-06, 9 November 2024,
<https://tools.ietf.org/html/draft-ietf-idr-bgp-ct-
srv6-05>.
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
ct-srv6-06>.

[BGP-CT-UPDATE-PACKING-TEST]
Vairavakkalai, Ed., "BGP CT Update packing Test Results",
"update-packing-test-results.txt", 1a75d4d, 25 June 2023, <https://raw.githubusercontent.com/ietf-wg-
idr/draft-ietf-idr-bgp-
ct/1a75d4d10d4df0f1fd7dcc041c2c868704b092c7/update-
packing-test-results.txt>.
<https://github.com/ietf-wg-idr/draft-ietf-idr-bgp-
ct/blob/main/update-packing-test-results.txt>.

[BGP-FWD-RR]
Vairavakkalai, K., Ed. and N. Venkataraman, Ed., "BGP
Route Reflector with Next Hop Self", Work in Forwarding Path", Progress,
Internet-Draft, draft-ietf-idr-bgp-fwd-rr-03, 17 March September
2024,
<https://tools.ietf.org/html/draft-ietf-idr-bgp-fwd-rr-
02>. <https://datatracker.ietf.org/doc/html/draft-ietf-
idr-bgp-fwd-rr-03>.

[BGP-LU-EPE]
Gredler, H., Ed., Vairavakkalai, K., Ed., R, C.,
Rajagopalan, B., Aries, E., and L. Fang, "Egress Peer
Engineering using BGP-LU", 16
June 2023, <https://datatracker.ietf.org/doc/html/draft-
gredler-idr-bgplu-epe-15>. Work in Progress, Internet-
Draft, draft-gredler-idr-bgplu-epe-16, 14 October 2024,
<https://datatracker.ietf.org/doc/html/draft-gredler-idr-
bgplu-epe-16>.

[FLOWSPEC-REDIR-IP]
Uttaro, J., Haas, J., akarch@cisco.com, Ray, S.,
Mohapatra, P., Henderickx, W., Simpson, Ed., A., and M. Texier,
"BGP Flow-Spec Redirect to IP Redirect-to-IP Action", 8 Work in Progress,
September 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-idr-flowspec-redirect-ip-03>.

[INTAREA-TUNNELS]
Touch, Ed. J. D. and M. Townsley, Ed., "IP Tunnels in the Internet
Architecture", 26 March 2023,
<https://datatracker.ietf.org/doc/draft-ietf-intarea-
tunnels/13/>. Work in Progress, Internet-Draft, draft-
ietf-intarea-tunnels-15, 9 May 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-intarea-
tunnels-15>.

[Intent-Routing-Color]
Hegde, Ed., "Intent-aware S., Rao, D., Uttaro, J., Bogdanov, A., and L.
Jalil, "Problem statement for Inter-domain Intent-aware
Routing using Color", 23 October
2023, <https://datatracker.ietf.org/doc/html/draft-hr-
spring-intentaware-routing-using-color-03>. Work in Progress, Internet-Draft,
draft-hr-spring-intentaware-routing-using-color-04, 31
January 2025, <https://datatracker.ietf.org/doc/html/
draft-hr-spring-intentaware-routing-using-color-04>.

[MNH] Vairavakkalai, K., Ed., Jeganathan, J. M., Nanduri, M.,
and A. R. Lingala, "BGP MultiNexthop Attribute", 17 Work in
Progress, Internet-Draft, draft-ietf-idr-multinexthop-
attribute-04, 25 March
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
idr-multinexthop-attribute-00>. 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
multinexthop-attribute-04>.

[MPLS-NS] Vairavakkalai, K., Ed., Jeganathan, J. M., Ramadenu, P.,
and I. Means, "BGP signalled Signaled MPLS namespaces", Namespaces", Work in
Progress, Internet-Draft, draft-kaliraj-bess-bgp-sig-
private-mpls-labels-09, 9 November 2024, <https://datatracker.ietf.org/doc/html/
draft-kaliraj-bess-bgp-sig-private-mpls-labels-09>.
<https://datatracker.ietf.org/doc/html/draft-kaliraj-bess-
bgp-sig-private-mpls-labels-09>.

[PCEP-RSVP-COLOR]
Rajagopalan, Ed. and Pavan. B., Beeram, Ed., V. P., Peng, S., Koldychev, M.,
and G. S. Mishra, "Path Computation Element Protocol(PCEP) Protocol
(PCEP) Extension for RSVP Color", 17 Work in Progress, Internet-
Draft, draft-ietf-pce-pcep-color-12, 26 February 2025, <https://datatracker.ietf.org/doc/html/
draft-ietf-pce-pcep-color-11>.
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
pcep-color-12>.

[PCEP-SRPOLICY]
Koldychev, Ed., M., Sivabalan, Ed., and S., Sidor, S., Barth, Ed., "PCEP C., Peng,
S., and H. Bidgoli, "Path Computation Element
Communication Protocol (PCEP) Extensions for SR Segment
Routing (SR) Policy Candidate Paths", 9 February
2024, <https://www.ietf.org/archive/id/draft-ietf-pce-
segment-routing-policy-cp-14.html>. Work in Progress,
Internet-Draft, draft-ietf-pce-segment-routing-policy-cp-
27, 4 April 2025, <https://datatracker.ietf.org/doc/html/
draft-ietf-pce-segment-routing-policy-cp-27>.

[RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
"Special-Purpose IP Address Registries", BCP 153,
RFC 6890, DOI 10.17487/RFC6890, April 2013,
<https://www.rfc-editor.org/info/rfc6890>.

[RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8205, DOI 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.

[RFC8210] Bush, R. and R. Austein, "The Resource Public Key
Infrastructure (RPKI) to Router Protocol, Version 1",
RFC 8210, DOI 10.17487/RFC8210, September 2017,
<https://www.rfc-editor.org/info/rfc8210>.

[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>.

[RFC9315] Clemm, A., Ciavaglia, L., Granville, L. Z., and J.
Tantsura, "Intent-Based Networking - Concepts and
Definitions", RFC 9315, DOI 10.17487/RFC9315, October
2022, <https://www.rfc-editor.org/info/rfc9315>.

[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
DOI 10.17487/RFC9350, February 2023,
<https://www.rfc-editor.org/info/rfc9350>.

[RFC9543] Farrel, A., Ed., Drake, J., Ed., Rokui, R., Homma, S.,
Makhijani, K., Contreras, L., and J. Tantsura, "A
Framework for Network Slices in Networks Built from IETF
Technologies", RFC 9543, DOI 10.17487/RFC9543, March 2024,
<https://www.rfc-editor.org/info/rfc9543>.

Appendix A. Extensibility considerations Considerations

A.1. Signaling Intent over a PE-CE Attachment Circuit

It may be desirable to allow a CE device to indicate in the data
packet it sends what treatment it desires (the Intent) when the
packet is forwarded within the provider network.

Section

Appendix A.10 in BGP MultiNexthop Attribute of [MNH] describes some mechanisms that enable such
signaling.

A.2. BGP CT Egress TE

Mechanisms described in [BGP-LU-EPE] also applies apply to the BGP CT family.

The Peer/32 or Peer/128 EPE route MAY be originated in the BGP CT
family with the appropriate Mapping Community (e.g. transport-target:0:100), (e.g., transport-
target:0:100), thus allowing an EPE path to the peer that satisfies
the desired SLA.

Appendix B. Applicability to Intra-AS and different Different Inter-AS
deployments.
Deployments

As described in BGP VPN [RFC4364] Section 10, 10 of [RFC4364], in an Option C network,
service routes (VPN-IPv4) are neither maintained nor distributed by
the ASBRs. Transport routes are maintained in the ASBRs and
propagated in BGP LU or BGP CT.

Illustration of CT Procedures (Section 8)

Section 8 illustrates how constructs of BGP CT work in an inter-AS
Option C deployment. The BGP CT constructs: AFI/SAFI 1/76, Transport Class
Class, and Resolution Scheme are used in an inter-AS Option C
deployment.

In Intra-AS and Inter-AS option A, A and option B scenarios, AFI/SAFI
1/76 may not be used, but the Transport Class and Resolution Scheme
mechanisms are used to provide service mapping.

This section illustrates how BGP CT constructs work in Intra-AS and
Inter-AS Option A, A and B deployment scenarios.

B.1. Intra-AS usecase Use Case

B.1.1. Topology

[RR11]
|
+
[CE21]---[PE11]-------[P1]------[PE12]------[CE31]

: :
AS2 : ...AS1... : AS3
: :

203.0.113.21 ---- Traffic Direction ----> 203.0.113.31

Figure 14: BGP CT Intra-AS

This example in

Figure 14 shows a provider network Autonomous system System, AS1. It serves
customers AS2, AS2 and AS3. Traffic direction being described is CE21 to
CE31. CE31 may request a specific SLA (e.g. (e.g., Gold for this traffic), traffic)
when traversing this provider network.

B.1.2. Transport Layer

AS1 uses RSVP-TE intra-domain tunnels between PE11 and PE12. And it
uses LDP tunnels for best effort best-effort traffic.

The network has two Transport classes: Gold with Transport Class ID
100,
100 and Bronze with Transport Class ID 200. These transport classes
are provisioned at the PEs. This creates the Resolution Schemes for
these transport classes at these PEs.

Following

The following tunnels exist for the Gold transport class. class:

* PE11_to_PE12_gold - RSVP-TE tunnel

* PE12_to_PE11_gold - RSVP-TE tunnel

Following

The following tunnels exist for Bronze transport class. class:

* PE11_to_PE12_bronze - RSVP-TE tunnel

These tunnels are provisioned to belong to transport class 100 or
200.

B.1.3. Service Layer route exchange Service-Layer Route Exchange

Service nodes PE11, PE11 and PE12 negotiate service families (AFI/SAFI
1/128) on the BGP session with RR11. Service helper RR11 reflects
service routes between the two PEs with the next hop unchanged.
There are no tunnels for transport-class 100 or 200 from RR11 to the
PEs.

Forwarding happens using service routes at service nodes PE11, PE11 and
PE12. Routes received from CEs are not present in any other nodes'
FIB in the provider network.

CE31 advertises a route route, for example example, prefix 203.0.113.31 with the
next hop
self set to itself to PE12. CE31 can attach a Mapping Community
Color:0:100 on this route, route to indicate its request for a Gold SLA.
Or, PE12 can attach the same using locally configured policies.

Consider,

Consider CE31 is getting VPN service from PE12. The RD:203.0.113.31
route is readvertised in AFI/SAFI 1/128 by PE12 with the next hop self set
to itself (192.0.2.12) and label V-L1, V-L1 to RR11 with the Mapping
Community Color:0:100 attached. This AFI/SAFI 1/128 route reaches
PE11 via RR11 with the next hop unchanged as PE12 and label V-L1.
Now PE11 can resolve the PNH 192.0.2.12 using the PE11_to_PE12_gold
RSVP TE LSP.

The IP FIB at PE11 VRF will have a route for 203.0.113.31 with a next
hop when resolved using the Resolution Scheme belonging to the
mapping community Color:0:100, points to a PE11_to_PE12_gold tunnel.

BGP CT AFI/SAFI 1/76 is not used in this Intra-AS deployment. But
the Transport class and Resolution Scheme constructs are used to
preserve end-to-end SLA.

B.2. Inter-AS option Option A usecase Use Case

B.2.1. Topology

[RR11] [RR21]
| |
+ +
[CE31]---[PE11]----[P1]----[ASBR11]---[ASBR21]---[P2]---[PE21]----[CE41]

: : :
AS3 : ..AS1.. : ..AS2.. : AS4
: : :

203.0.113.31 -------Traffic Direction------> 203.0.113.41

Figure 15: BGP CT Inter-AS option Option A

This example in Figure 15 shows two provider network Autonomous
systems AS1, AS2. They serve L3VPN customers AS3, AS4 respectively.
The ASBRs ASBR11 and ASBR21 have IP VRFs connected directly. The
inter-AS link is IP enabled with no MPLS forwarding.

Traffic direction being described is CE31 to CE41. CE41 may request
a specific SLA (e.g. (e.g., Gold for this traffic), when traversing these
provider core networks.

B.2.2. Transport Layer

AS1 uses RSVP-TE intra-domain tunnels between PE11 and ASBR11. And
LDP tunnels for best effort best-effort traffic. AS2 uses SRTE intra-domain
tunnels between ASBR21 and PE21, and L-ISIS for best effort best-effort tunnels.

The networks have two Transport classes: Gold with Transport Class ID
100, Bronze with Transport Class ID 200. These transport classes are
provisioned at the PEs and ASBRs. This creates the Resolution
Schemes for these transport classes at these PEs and ASBRs.

Following tunnels exist for Gold transport class.

* PE11_to_ASBR11_gold - RSVP-TE tunnel

* ASBR11_to_PE11_gold - RSVP-TE tunnel

* PE21_to_ASBR21_gold - SRTE tunnel

* ASBR21_to_PE21_gold - SRTE tunnel

Following tunnels exist for Bronze transport class.

* PE11_to_ASBR11_bronze - RSVP-TE tunnel

* ASBR11_to_PE11_bronze - RSVP-TE tunnel

* PE21_to_ASBR21_bronze - SRTE tunnel

* ASBR21_to_PE21_bronze - SRTE tunnel

These tunnels are provisioned to belong to transport class 100 or
200.

B.2.3. Service Layer route exchange Route Exchange

Service nodes PE11, ASBR11 negotiate service family (AFI/SAFI 1/128)
on the BGP session with RR11. Service helper RR11 reflects service
routes between the PE11 and ASBR11 with next hop unchanged.

Similarly, in AS2 PE21, ASBR21 negotiate service family (AFI/SAFI
1/128) on the BGP session with RR21, which reflects service routes
between the PE21 and ASBR21 with next hop unchanged.

CE41 advertises a route for example prefix 203.0.113.41 with next hop
self to PE21 VRF. CE41 can attach a Mapping Community Color:0:100 on
this route, to indicate its request for Gold SLA. Or, PE21 can
attach the same using locally configured policies.

Consider, CE41 is getting VPN service from PE21. The RD:203.0.113.41
route is readvertised in AFI/SAFI 1/128 by PE21 with next hop self
(203.0.113.21) and label V-L1 to RR21 with the Mapping Community
Color:0:100 attached. This AFI/SAFI 1/128 route reaches ASBR21 via
RR21 with the next hop unchanged as PE21 and label V-L1. Now ASBR21
can resolve the PNH 203.0.113.21 using ASBR21_to_PE21_gold SRTE LSP.

The IP FIB at ASBR21 VRF will have a route for 203.0.113.41 with a
next hop resolved using Resolution Scheme associated with mapping
community Color:0:100, pointing to ASBR21_to_PE21_gold tunnel.

This route is readvertised with the next hop self set to itself by ASBR21
to ASBR11 on a BGP session in the VRF. The single-hop EBGP session
endpoints are interface addresses. ASBR21 and ASBR11 act like a CE
to each other. The previously mentioned process repeats in AS1, AS1 until
the route reaches PE11 and resolves over the PE11_to_ASBR11_gold RSVP
TE tunnel.

Traffic traverses as an unlabeled IP packet on the following legs:
CE31-PE11, ASBR11-ASBR21, PE21-CE41. And it uses MPLS forwarding
inside
AS1, the AS1 and AS2 core.

BGP CT AFI/SAFI 1/76 is not used in this Inter-AS Option B
deployment. But the Transport class and Resolution Scheme constructs
are used to preserve an end-to-end SLA.

B.3. Inter-AS option Option B usecase Use Case

B.3.1. Topology

[RR13] [RR23]
| |
+ +
[CE31]---[PE11]----[P1]----[ASBR12]---[ASBR21]---[P2]---[PE22]----[CE41]

: : :
AS3 : ..AS1.. : ..AS2.. : AS4
: : :

203.0.113.31 ---- Traffic Direction ----> 203.0.113.41

Figure 16: BGP CT Inter-AS option Option B

This example in

Figure 16 shows two provider network Autonomous
systems Systems: AS1 and AS2.
They serve L3VPN customers AS3 and AS4 AS4, respectively. The ASBRs
ASBR12 and ASBR21 don't have any IP VRFs. The inter-AS link is MPLS MPLS-
forwarding enabled.

Traffic direction being described is CE31 to CE41. CE41 may request
a specific SLA (e.g. (e.g., Gold for this traffic), traffic) when traversing these
provider core networks.

B.3.2. Transport Layer

AS1 uses RSVP-TE intra-domain tunnels between PE11 and ASBR21. And ASBR21 and LDP
tunnels for best effort best-effort traffic. AS2 uses SRTE intra-domain tunnels
between ASBR21 and PE22, and PE22 along with L-ISIS for best effort best-effort tunnels.

The networks have two Transport classes: Gold with Transport Class ID
100,
100 and Bronze with Transport Class ID 200. These transport classes
are provisioned at the PEs and ASBRs. This creates the Resolution
Schemes for these transport classes at these PEs and ASBRs.

Following

The following tunnels exist for Gold transport class. class:

* PE11_to_ASBR12_gold - RSVP-TE tunnel

* ASBR12_to_PE11_gold - RSVP-TE tunnel

* PE22_to_ASBR21_gold - SRTE tunnel

* ASBR21_to_PE22_gold - SRTE tunnel

Following

The following tunnels exist for Bronze transport class. class:

* PE11_to_ASBR12_bronze - RSVP-TE tunnel

* ASBR12_to_PE11_bronze - RSVP-TE tunnel

* PE22_to_ASBR21_bronze - SRTE tunnel

* ASBR21_to_PE22_bronze - SRTE tunnel

These tunnels are provisioned to belong to transport class 100 or
200.

B.3.3. Service Layer route exchange Service-Layer Route Exchange

Service nodes PE11, PE11 and ASBR12 negotiate service family (AFI/SAFI
1/128) on the BGP session with RR13. Service helper RR13 reflects
service routes between the PE11 and ASBR12 with the next hop
unchanged.

Similarly, in AS2 PE22, ASBR21 negotiate negotiates service family (AFI/SAFI
1/128) on the BGP session with RR23, which reflects service routes
between the PE22 and ASBR21 with the next hop unchanged.

ASBR21 and ASBR12 negotiate AFI/SAFI 1/128 between them, them and
readvertise L3VPN routes with the next hop self, set to themselves,
allocating new labels. The single-hop EBGP session endpoints are
interface addresses.

CE41 advertises a route route, for example example, prefix 203.0.113.41 with the
next hop
self set to itself to PE22 VRF. CE41 can attach a Mapping
Community Color:0:100 on this route, route to indicate its request for the
Gold SLA. Or, PE22 can attach the same using locally configured
policies.

Consider,

Consider CE41 is getting VPN service from PE22. The RD:203.0.113.41
route is readvertised in AFI/SAFI 1/128 by PE22 with the next hop self set
to itself (192.0.2.22) and label V-L1 to RR23 with the Mapping
Community Color:0:100 attached. This AFI/SAFI 1/128 route reaches
ASBR21 via RR23 with the next hop unchanged as PE22 and label V-L1.
Now ASBR21 can resolve the PNH 192.0.2.22 using ASBR21_to_PE22_gold
SRTE LSP.

Next, ASBR21 readvertises the RD:203.0.113.41 route with the next hop
self
set to itself to ASBR12 with a newly allocated MPLS label V-L2.
Forwarding for this label is installed to Swap V-L1, and Push labels
for ASBR21_to_PE22_gold tunnel.

ASBR12 further readvertises the RD:203.0.113.41 route via RR13 to
PE11 with the next hop self set to itself, 192.0.2.12. PE11 resolves the
next hop 192.0.2.12 over PE11_to_ASBR12_gold RSVP TE tunnel.

Traffic traverses as the IP packet on the following legs: CE31-PE11
and PE21-CE41. And it uses MPLS forwarding on the ASBR11-ASBR21 link, link
and inside the AS1-AS2 core.

BGP CT AFI/SAFI 1/76 is not used in this Inter-AS Option B
deployment. But the Transport class and Resolution Scheme constructs
are used to preserve an end-to-end SLA.

Appendix C. Why reuse RFC RFCs 8277 and RFC 4364?

RFC 4364

[RFC4364] is one of the key design patterns produced by the
networking industry. It introduced virtualization and allowed
sharing of resources in the service provider space with multiple
tenant networks, providing isolated and secure Layer3 Layer 3 VPN services.
This design pattern has been reused since to provide other service service-
layer virtualizations like Layer2 Layer 2 virtualization (VPLS, L2VPN,
EVPN), ISO virtualization, ATM virtualization, and Flowspec VPN.

It is to be noted that these services have different NLRI encoding. encodings.
The L3VPN Service family that binds the MPLS label to an IP prefix use RFC
8277 encoding,
uses the encoding described in [RFC8277] and others define different
NLRI encodings.

BGP CT reuses RFC 4364 the procedures described in [RFC4364] to slice a
transport network into multiple transport planes that different
service routes can bind to, to using color.

BGP CT reuses RFC 8277 [RFC8277] because it precisely fits the purpose. viz. In
a That
is, in an MPLS network, BGP CT needs to bind the MPLS label for
transport
endpoints endpoints, which are IPv4 or IPv6 endpoints, and
disambiguate between multiple instances of those endpoints in
multiple transport planes. Hence, use of the RD:IP_Prefix and
carrying a Label for it as specified in RFC 8277 [RFC8277] works well for this
purpose.

Another advantage of using the precise encoding as defined in RFC
4364
[RFC4364] and RFC 8277 [RFC8277] is that it allows to interoperate interoperation with BGP
speakers that support SAFI 128 for AFIs 1 or 2. This can be useful
during
transition, transition until all BGP speakers in the network support BGP
CT.

In the future, if RFC 8277 [RFC8277] evolves into a typed NLRI, NLRI that does not
carry Label in the NLRI, BGP CT will be compatible with that as-well. as well.
In essence, BGP CT encoding is compatible with existing deployed
technologies (RFC 4364, RFC 8277) ([RFC4364] and [RFC8277]) and will adapt to any changes RFC
8277
mechanisms from [RFC8277] undergo in future.

This approach leverages the benefits of time tested time-tested design patterns
proposed in RFC 4364 [RFC4364] and RFC 8277. [RFC8277]. Moreover, this approach greatly
reduces operational training costs and protocol compatibility
considerations,
considerations as it complements and works well with existing
protocol machineries. This machineries: this problem does not need reinventing the
wheel with a brand new NLRI and
procedures.

BGP CT design also avoids overloading RFC 8277 the NLRI MPLS Label field from
[RFC8277] with information related to non MPLS the non-MPLS data plane, plane because
it leads to
backward compatibility backward-compatibility issues.

C.1. Update packing considerations Packing Considerations

BGP CT carries transport class as an attribute. This means routes
that don't share the same transport class cannot be packed into the
same
Update BGP UPDATE message. Update packing in BGP CT will be similar to RFC 8277
family routes from [RFC8277] carrying attributes like communities or
extended communities. Service families like AFI/SAFI 1/128 have
considerably more scale than transport families like AFI/SAFI 1/4 or
AFI/SAFI 1/76, which carry only loopbacks. Update packing mechanisms
that scale for AFI/SAFI 1/128 routes will scale similarly for AFI/SAFI AFI/
SAFI 1/76 routes also. routes.

Section 6.3.2.1 of [Intent-Routing-Color] suggests scaling numbers
for a transport network where BGP CT can be deployed. Experiments
were conducted with this scale to find the convergence time with BGP
CT for those scaling numbers. Scenarios involving BGP CT carrying
IPv4 and IPv6 endpoints with an MPLS label were tested. Tests with
BGP CT IPv6 endpoints and SRv6 SID are planned.

Tests were conducted with a 1.9 million BGP CT route scale (387K
endpoints in 5 transport classes). Initial convergence time for all
cases was less than 2 minutes, which compares favorably with user
expectation for such a scale. This experiment proves that carrying
transport class
transport-class information as an attribute keeps BGP convergence
within an acceptable range. Details of the experiment and test
results are available in BGP CT Update packing Test Results [BGP-CT-UPDATE-PACKING-TEST].

Furthermore, even in today's BGP LU deployments deployments, each egress node
originates a BGP LU route for it's its loopback, with some attributes like
community identifying the originating node or region, region and an AIGP
attribute. These attributes may be unique per egress node, thus node; thus,
they do not help with update packing in transport family routes.

Appendix D. Scaling using Using BGP MPLS Namespaces

This document considers the scaling scenario suggested in
Section 6.3.2.1 of [Intent-Routing-Color] where 300K nodes exist in
the network with 5 transport classes.

This may result in 1.5M transport layer routes and MPLS transit
routes in all Border Nodes in the network, which may overwhelm the
nodes' MPLS forwarding MPLS-forwarding resources.

Section 6.2 of [MPLS-NS] describes how MPLS Namespaces mechanism is
used to scale such a network. This approach reduces the number of
PNHs that are globally visible in the network, thus reducing
forwarding resource usage network wide. Service route Service-route state is kept
confined closer to network edge, and any churn is confined within the
region containing the point of failure, which improves convergence
also.

Acknowledgements

The authors thank Jeff Haas, John Scudder, Susan Hares, Dongjie
(Jimmy), Moses Nagarajah, Jeffrey (Zhaohui) Zhang, Joel Halpern,
Jingrong Xie, Mohamed Boucadair, Greg Skinner, Simon Leinen,
Navaneetha Krishnan, Ravi M R, Chandrasekar Ramachandran, Shradha
Hegde, Colby Barth, Vishnu Pavan Beeram, Sunil Malali, William J
Britto, R Shilpa, Ashish Kumar (FE), Sunil Kumar Rawat, Abhishek
Chakraborty, Richard Roberts, Krzysztof Szarkowicz, John E Drake,
Srihari Sangli, Jim Uttaro, Luay Jalil, Keyur Patel, Ketan
Talaulikar, Dhananjaya Rao, Swadesh Agarwal, Robert Raszuk, Ahmed
Darwish, Aravind Srinivas Srinivasa Prabhakar, Moshiko Nayman, Chris
Tripp, Gyan Mishra, Vijay Kestur, and Santosh Kolenchery for all the
valuable discussions, constructive criticisms, and review comments.

The decision to not reuse SAFI 128 and create a new address family to
carry these transport routes was based on suggestion made by Richard
Roberts and Krzysztof Szarkowicz.

Thanks to John Scudder for showing us with example how the Figures
can be enhanced using SVG format.

Contributors

Co-Authors

The following people contributed substantially to the content of this
document and should be considered coauthors:

Reshma Das
Juniper Networks, Inc.
1133 Innovation Way, Way
Sunnyvale, CA 94089
United States of America
Email: dreshma@juniper.net

Israel Means
AT&T
2212 Avenida Mara, Mara
Chula Vista, California 91914
United States of America
Email: israel.means@att.com

Csaba Mate
KIFU, Hungarian NREN
Budapest
35 Vaci street, Street
1134
Hungary
Email: ietf@nop.hu

Deepak J Gowda
Extreme Networks
55 Commerce Valley Drive West, Suite 300, 300
Thornhill, Toronto, Toronto Ontario L3T 7V9
Canada
Email: dgowda@extremenetworks.com

Other Contributors

We also acknowledge the contribution of the following individuals:

Balaji Rajagopalan
Juniper Networks, Inc.
Electra, Exora Business Park~Marathahalli - Sarjapur Outer Ring Road, Road
Bangalore 560103
KA
India
Email: balajir@juniper.net

Rajesh M
Juniper Networks, Inc.
Electra, Exora Business Park~Marathahalli - Sarjapur Outer Ring Road, Road
Bangalore 560103
KA
India
Email: mrajesh@juniper.net

Chaitanya Yadlapalli
AT&T
200 S Laurel Ave,
Middletown,, Ave
Middletown, NJ 07748
United States of America
Email: cy098d@att.com

Mazen Khaddam
Cox Communications Inc.
Atlanta, GA
United States of America
Email: mazen.khaddam@cox.com

Rafal Jan Szarecki
Google.
Google
1160 N Mathilda Ave, Bldg 5,
Sunnyvale,, 5
Sunnyvale, CA 94089
United States of America
Email: szarecki@google.com

Xiaohu Xu
China Mobile
Beijing
China
Email: xuxiaohu@cmss.chinamobile.com

Acknowledgements

The authors thank Jeff Haas, John Scudder, Susan Hares, Dongjie
(Jimmy), Moses Nagarajah, Jeffrey (Zhaohui) Zhang, Joel Halpern,
Jingrong Xie, Mohamed Boucadair, Greg Skinner, Simon Leinen,
Navaneetha Krishnan, Ravi M R, Chandrasekar Ramachandran, Shradha
Hegde, Colby Barth, Vishnu Pavan Beeram, Sunil Malali, William J
Britto, R Shilpa, Ashish Kumar (FE), Sunil Kumar Rawat, Abhishek
Chakraborty, Richard Roberts, Krzysztof Szarkowicz, John E Drake,
Srihari Sangli, Jim Uttaro, Luay Jalil, Keyur Patel, Ketan
Talaulikar, Dhananjaya Rao, Swadesh Agarwal, Robert Raszuk, Ahmed
Darwish, Aravind Srinivas Srinivasa Prabhakar, Moshiko Nayman, Chris
Tripp, Gyan Mishra, Vijay Kestur, Santosh Kolenchery for all the
valuable discussions, constructive criticisms, and review comments.

The decision to not reuse SAFI 128 and create a new address-family to
carry these transport-routes was based on suggestion made by Richard
Roberts and Krzysztof Szarkowicz.

Thanks to John Scudder for showing us with example how the Figures
can be enhanced using SVG format.

Authors' Addresses

Kaliraj Vairavakkalai (editor)
Juniper Networks, Inc.
1133 Innovation Way, Way
Sunnyvale, CA 94089
United States of America
Email: kaliraj@juniper.net

Natrajan Venkataraman (editor)
Juniper Networks, Inc.
1133 Innovation Way, Way
Sunnyvale, CA 94089
United States of America
Email: natv@juniper.net