Internet DRAFT - draft-ietf-idr-bgp-ct
draft-ietf-idr-bgp-ct
Network Working Group K. Vairavakkalai, Ed.
Internet-Draft N. Venkataraman, Ed.
Intended status: Experimental Juniper Networks, Inc.
Expires: 11 October 2023 9 April 2023
BGP Classful Transport Planes
draft-ietf-idr-bgp-ct-03
Abstract
This document specifies a mechanism, referred to as "Intent Driven
Service Mapping" to express association of overlay routes with
underlay routes satisfying a certain SLA using BGP. The document
describes a framework for classifying underlay routes into transport
classes and mapping service routes to specific transport class.
The "Transport class" construct maps to a desired SLA and can be used
to realize the "Topology Slice" in 5G Network slicing architecture.
This document specifies BGP protocol procedures that enable
dissemination of such service mapping information that may span
multiple cooperating administrative domains. These domains may be
administetered by the same provider or by closely co-ordinating
provider networks.
A new BGP transport layer address family (SAFI 76) is defined for
this purpose that uses RFC-4364 technology and follows RFC-8277 NLRI
encoding. This new address family is called "BGP Classful
Transport", aka BGP CT.
BGP CT makes it possible to advertise multiple tunnels to the same
destination address, thus avoiding need of multiple loopbacks on the
egress node.
It carries transport prefixes across tunnel domain boundaries (e.g.
in Inter-AS Option-C networks), which is parallel to BGP LU (SAFI 4)
. It disseminates "Transport class" information for the transport
prefixes across the participating domains, which is not possible with
BGP LU. This makes the end-to-end network a "Transport Class" aware
tunneled network.
Just like BGP LU (SAFI 4), BGP CT family (SAFI 76) is used in inter-
AS option-C networks. The Service Mapping procedures described in
this document apply in the same manner to Intra-AS service end points
as well as Inter-AS option-A, option-B, option-C variations.
Examples of these variations are given in Appendix A.
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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 RFC 2119 [RFC2119] RFC 8174 [RFC8174] when, and only when, they
appear in all capitals, as shown here.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 11 October 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Transport Class . . . . . . . . . . . . . . . . . . . . . . . 8
4. "Transport Class" Route Target Extended Community . . . . . . 9
5. Transport Route Database . . . . . . . . . . . . . . . . . . 11
6. Nexthop Resolution Scheme . . . . . . . . . . . . . . . . . . 11
7. BGP Classful Transport Family NLRI . . . . . . . . . . . . . 12
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7.1. Carrying multiple encapsulation information . . . . . . . 14
8. Usage of Route Distinguisher and Label Allocation Modes . . . 15
9. Comparison with other families using RFC-8277 encoding . . . 16
10. Protocol Procedures . . . . . . . . . . . . . . . . . . . . . 17
10.1. Preparing the network to deploy Classful Transport
planes . . . . . . . . . . . . . . . . . . . . . . . . 17
10.2. Origination of Classful Transport route . . . . . . . . 17
10.3. Ingress node receiving Classful Transport route . . . . 18
10.4. Border node readvertising Classful Transport route with
nexthop self . . . . . . . . . . . . . . . . . . . . . 18
10.5. Border node receiving Classful Transport route on
EBGP . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.6. Avoiding path-hiding through Route Reflectors . . . . . 19
10.7. Avoiding loop between Route Reflectors in forwarding
path . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.8. Ingress node receiving service route with Mapping
Community . . . . . . . . . . . . . . . . . . . . . . . 20
10.9. Coordinating between domains using different community
namespaces . . . . . . . . . . . . . . . . . . . . . . 20
10.10. Best effort transport class . . . . . . . . . . . . . . 20
11. Flowspec Redirect to IP . . . . . . . . . . . . . . . . . . . 21
12. BGP CT Egress TE . . . . . . . . . . . . . . . . . . . . . . 21
13. Interaction with BGP attributes specifying nexthop address and
color . . . . . . . . . . . . . . . . . . . . . . . . . . 22
14. Signaling Intent across PE-CE link . . . . . . . . . . . . . 22
14.1. Using DSCP in MultiNexthop attribute . . . . . . . . . . 23
14.2. Using MPLS Label in MultiNexthop attribute . . . . . . . 23
15. Scaling considerations . . . . . . . . . . . . . . . . . . . 24
15.1. Avoiding unintended spread of BGP CT routes across
domains . . . . . . . . . . . . . . . . . . . . . . . . 24
15.2. Constrained distribution of PNHs to SNs (On Demand
Nexthop) . . . . . . . . . . . . . . . . . . . . . . . . 24
15.3. Limiting scope of visibility of PE loopback as PNHs . . 26
16. OAM considerations . . . . . . . . . . . . . . . . . . . . . 26
17. Applicability to Network Slicing . . . . . . . . . . . . . . 27
18. SRv6 support . . . . . . . . . . . . . . . . . . . . . . . . 27
19. Illustration of BGP CT procedures in Inter AS option-C . . . 28
19.1. Topology . . . . . . . . . . . . . . . . . . . . . . . . 28
19.2. Service Layer route exchange . . . . . . . . . . . . . . 29
19.3. Transport Layer route propagation . . . . . . . . . . . 30
19.4. Data plane view . . . . . . . . . . . . . . . . . . . . 33
19.4.1. Steady state . . . . . . . . . . . . . . . . . . . . 33
19.4.2. Local repair of primary path . . . . . . . . . . . . 34
19.4.3. Absorbing failure of primary path. Fallback to
best-effort tunnels. . . . . . . . . . . . . . . . . 34
20. Deployment considerations. . . . . . . . . . . . . . . . . . 35
20.1. Managing Transport Route Visibility . . . . . . . . . . 35
20.2. Managing Intent at Service and Transport layers. . . . . 37
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20.2.1. Service layer Color Management . . . . . . . . . . . 37
20.2.2. Non-Agreeing Color Domains . . . . . . . . . . . . . 38
20.2.3. Heterogeneous Agreeing Color Domains . . . . . . . . 39
20.3. Migration scenarios. . . . . . . . . . . . . . . . . . . 41
20.3.1. BGP CT islands connected via BGP LU domain. . . . . 41
20.3.2. BGP CT - Interop between MPLS and other forwarding
technologies. . . . . . . . . . . . . . . . . . . . . 43
21. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
21.1. New BGP SAFI . . . . . . . . . . . . . . . . . . . . . . 46
21.2. New Format for BGP Extended Community . . . . . . . . . 46
21.2.1. Existing registries to be modified . . . . . . . . . 46
21.2.2. New registries to be created . . . . . . . . . . . . 47
21.3. MPLS OAM code points . . . . . . . . . . . . . . . . . . 48
21.4. Best Effort Transport Class ID . . . . . . . . . . . . . 49
22. Security Considerations . . . . . . . . . . . . . . . . . . . 49
23. Normative References . . . . . . . . . . . . . . . . . . . . 49
Appendix A. Applicability to Intra AS and different Inter AS
deployments. . . . . . . . . . . . . . . . . . . . . . . 53
A.1. Intra AS usecase . . . . . . . . . . . . . . . . . . . . 53
A.1.1. Topology . . . . . . . . . . . . . . . . . . . . . . 53
A.1.2. Transport Layer . . . . . . . . . . . . . . . . . . . 53
A.1.3. Service Layer route exchange . . . . . . . . . . . . 54
A.2. Inter AS option-A usecase . . . . . . . . . . . . . . . . 55
A.2.1. Topology . . . . . . . . . . . . . . . . . . . . . . 55
A.2.2. Transport Layer . . . . . . . . . . . . . . . . . . . 55
A.2.3. Service Layer route exchange . . . . . . . . . . . . 56
A.3. Inter AS option-B usecase . . . . . . . . . . . . . . . . 57
A.3.1. Topology . . . . . . . . . . . . . . . . . . . . . . 57
A.3.2. Transport Layer . . . . . . . . . . . . . . . . . . . 57
A.3.3. Service Layer route exchange . . . . . . . . . . . . 58
Appendix B. Why reuse RFC 8277 and RFC 4364? . . . . . . . . . . 59
B.1. Update packing considerations . . . . . . . . . . . . . . 60
Appendix C. Scaling using BGP MPLS Namespaces . . . . . . . . . 61
C.1. Illustration. . . . . . . . . . . . . . . . . . . . . . . 61
C.2. Topology . . . . . . . . . . . . . . . . . . . . . . . . 62
C.3. Context Protocol Nexthop Address (CPNH) . . . . . . . . . 63
C.4. Service Forwarding Helper, and changes to transport
layer. . . . . . . . . . . . . . . . . . . . . . . . . . 63
C.5. BGP MPLS Namespace Address family (AFI:16399,
SAFI:128) . . . . . . . . . . . . . . . . . . . . . . . . 64
C.6. Changes to Service Layer route exchange . . . . . . . . . 64
C.7. Analysis of forwarding behavior . . . . . . . . . . . . . 64
Appendix D. BGP CT deployment in SRv6 networks . . . . . . . . . 65
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Co-Authors . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Other Contributors . . . . . . . . . . . . . . . . . . . . . . 69
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 70
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 70
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1. Introduction
The mechanisms defined in this document enable brownfiled networks
deployed using existing technologies like RSVP-TE and greenfield
networks that use technologies like SPRING achieve 'Intent Driven
Service Mapping'.
To facilitate this, the tunnels in a network can be grouped by the
purpose they serve into a "Transport Class". These tunnels could be
created using any signaling protocol including but not limited to
LDP, RSVP-TE, BGP LU or SPRING. The tunnels may use MPLS, IPv4 or
IPv6 forwarding and carry one of the signaled payload types (e.g.
MPLS). Tunnels may exist between different pair of end points.
Multiple tunnels may exist between the same pair of end points.
A Transport Class consists of tunnels created by various protocols
that satisfy the properties of the class. For example, a "Gold"
transport class may consist of tunnels that traverse the shortest
path with fast re-route protection. A "Silver" transport class may
hold tunnels that traverse shortest paths without protection. A "To
NbrAS Foo" transport class may hold tunnels that exit to neighboring
AS Foo and so on.
The extensions specified in this document can be used to create a BGP
transport tunnel that potentially spans domains while preserving its
Transport Class. Examples of domain are Autonomous System (AS) or
IGP area. Within each domain, there is a second level underlay
tunnel used by BGP to cross the domain. The second level underlay
tunnels could be hetrogeneous; each domain may use a different type
of tunnel (e.g. MPLS, IP, GRE or SRv6) and use a different signaling
mechanism. A domain boundary is demarcated by a rewrite of BGP
nexthop to 'self' while readvertising BGP CT transport routes.
Examples of domain boundary are inter-AS links and inter-region ABRs.
The path uses MPLS label-switching when crossing domain boundaries
and uses the native intra-AS tunnel of the desired transport class
when traversing within a domain.
Overlay routes carry sufficient indication of the desired Transport
Classes in the form of a BGP community called the "Mapping
community". The "route resolution" procedure on the ingress node
selects an appropriate tunnel whose destination matches (LPM) the
nexthop of the overlay route belonging to the corresponding Transport
Class. If the overlay route is carried in BGP, the protocol nexthop
(or PNH) is carried as an attribute of the route.
The PNH of the overlay route is also referred to as "Service
Endpoint" (SEP). The SEP may exist in the same domain as the service
ingress node or lie in a different domain, which is adjacent or non-
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adjacent. In the former case, reachability to the SEP is provided by
an intra-domain tunneling protocol and in the latter case,
reachability to the SEP is via BGP transport families (e.g. SAFI 4
or 76).
In this architecture, the intra-domain transport protocols (e.g.
RSVP-TE, SRTE) are also "Transport Class aware". They publish
ingress routes in the Transport Route Database associated with the
Transport Class at the tunnel ingress node. These routes are used to
resolve BGP routes inluding BGP CT which may be further readvertised
to adjacent domains to extend this tunnel. How exactly the transport
protocols are made transport class aware is outside the scope of this
document.
This document describes mechanisms to:
Model a "Transport Class" as a "Transport Route Database" on a
router and to collect tunnel ingress routes of a certain class.
Enable service routes to resolve over an intended Transport Class
by virtue of carrying the appropriate "Mapping Community", which
results in using the corresponding Transport Route Database for
finding nexthop reachability.
Publish tunnel ingress routes in a Transport Route Database via
BGP without any path hiding using BGP VPN and Add-path procedures,
such that overlay routes in the receiving domains can also resolve
over tunnels of the associated Transport Class.
Provide a way for cooperating domains to reconcile any differences
in extended community namespaces and interoperate between
different transport signaling protocols in each domain.
In this document we focus mainly on MPLS as the intra-domain
transport tunnel forwarding technology, but the mechanisms described
here would work in similar manner for non-MPLS (e.g. IP, GRE, UDP or
SRv6) transport tunnel forwarding technologies too.
This document assumes MPLS forwarding as the defacto standard when
crossing domain boundaries. However mechanisms specified in this
document can also support different forwarding technologies (e.g.
SRv6). Section 17 (SRv6 support) in this document describes the
application of BGP CT over SRv6 data plane.
The document Intent-aware Routing using Color [Intent-Routing]
describes various use cases and applications of procedures described
in this document.
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2. Terminology
LSP: Label Switched Path.
TE : Traffic Engineering.
SN : Service Node. A router that sends or receives BGP Service
routes (e.g. SAFIs 1, 128) with self as nexthop.
BN : Border Node. A router that sends or receives BGP Transport
routes (e.g. SAFI 4, 76) with self as nexthop.
TN : Transport Node, P-router.
BGP VPN : VPNs built using RFC4364 mechanisms.
RT : Route-Target extended community.
RD : Route-Distinguisher.
VRF: Virtual Router Forwarding Table.
CsC: Carrier serving Carrier VPN.
PNH : Protocol-Nexthop address carried in a BGP Update message.
EP : End point, a loopback address in the network.
SEP : Service End point, the PNH of a Service route.
LPM : Longest Prefix Match.
SLA: Service Level Agreement.
EPE: Egress Peer Engineering.
UHP Label: Ultimate Hop Pop label.
PHP Label: Penultimate Hop Pop label.
Service Family : BGP address family used for advertising routes for
"data traffic" as opposed to tunnels (e.g. SAFI 1 or 128).
Transport Family : BGP address family used for advertising tunnels,
which are in turn used by service routes for resolution (e.g. SAFI 4
or 76).
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Transport Tunnel : A tunnel over which a service may place traffic
(e.g. GRE, UDP, LDP, RSVP-TE or SPRING).
Tunnel Ingress Route: Route to Tunnel Destination/Endpoint installed
at the headend (ingress) of the tunnel by the tunneling protocol.
Tunnel Domain : A domain of the network containing SNs and BNs under
a single administrative control that has tunnels between them. An
end-to-end tunnel spanning several adjacent tunnel domains can be
created by "stitching" them together using labels.
Transport Class : A group of transport tunnels offering the same SLA.
Transport Class RT : A Route-Target extended community used to
identify a specific Transport Class.
Transport Route Database : At the SN and BN, a Transport Class has an
associated Transport Route Database that collects its tunnel ingress
routes.
Transport Plane : An end to end plane comprising of transport tunnels
belonging to same Transport Class. Tunnels of same Transport Class
are stitched together by BGP CT route readvertisements with nexthop
self to enable Label-Swap forwarding across domain boundaries.
Mapping Community : BGP Community/Extended-community on a BGP route
that maps it to resolve over a Transport Class. E.g. color:0:100,
transport-target:0:100.
3. Transport Class
A Transport Class is defined as a set of transport tunnels that share
the same SLA. It is encoded as the Transport Class RT, which is a
new Route-Target extended community.
A Transport Class is configured at SN and BN with RD and Route Target
attributes. Creation of a Transport Class instantiates its
corresponding Transport Route Database.
The operator may configure an SN/BN to classify a tunnel into an
appropriate Transport Class, which causes the tunnel's ingress route
to be installed in the corresponding Transport Route Database. These
routes are used to resolve BGP routes inluding BGP CT which may be
further readvertised to adjacent domains to extend this tunnel.
Alternatively, a router receiving the transport routes in BGP with
appropriate signaling information can associate those ingress routes
to the appropriate Transport Class. E.g. for Classful Transport
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family (SAFI 76) routes, the Transport Class RT indicates the
Transport Class. For BGP LU family(SAFI 4) routes, import processing
based on Communities or inter-AS source-peer may be used to place the
route in the desired Transport Class.
When the ingress route is received via SRTE [SRTE] with
"Color:Endpoint" as the NLRI that encodes the Transport Class as an
integer 'Color', the 'Color' is mapped to a Transport Class during
import processing. SRTE ingress route for 'Endpoint' is installed in
the corresponding Transport Route Database. The SRTE tunnel will be
extended by a BGP CT advertisement with NLRI 'RD:Endpoint', Transport
Class RT and a new label. The MPLS swap route thus installed for the
new label will pop the label and deliver decapsulated traffic into
the path determined by SRTE route.
RFC8664 [RFC8664] extends PCEP to carry SRTE Color. This color
association learnt from PCEP is also mapped to a Transport Class thus
associating the PCEP signaled SRTE LSP with the desired Transport
Class.
Similarly, PCEP-RSVP-COLOR [PCEP-RSVP-COLOR] extends PCEP to carry
RSVP Color. This color association learnt from PCEP is also mapped
to a Transport Class thus associating the PCEP signaled RSVP-TE LSP
with the desired Transport Class.
4. "Transport Class" Route Target Extended Community
This document defines a new type of Route Target, called "Transport
Class" Route Target Extended Community.
"Transport Class" Route Target extended community is a transitive
extended community EXT-COMM [RFC4360] of extended-type, with a new
Format (Type high = 0xa) and SubType as 0x2 (Route Target).
This new Route Target Format has the following encoding:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type= 0xa | SubType= 0x02 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport Class ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig 1: "Transport Class" Route Target Extended Community
Type: 1 octet
Type field contains value 0xa.
SubType: 1 octet
Subtype field contain 0x2. This indicates 'Route Target'.
Transport Class ID: 4 octets
The least significant 32-bits of the value field contain the
"Transport Class" identifier, which is an unsigned non-zero
32-bit integer.
This document reserves the Transport class ID value 0 to
represent "Best Effort Transport Class ID"
The remaining 2 octets after SubType field are Reserved. They MUST
be set to zero on transmission, SHOULD be ignored on reception and
left unaltered.
The "Transport class" Route Target Extended community follows the
mechanisms for VPN route import/export as specified in BGP VPN
[RFC4364] and follows the Constrained Route Distribution mechanisms
as specified in Route Target Constraints [RFC4684]
A BGP speaker that implements RT Constraint Route Target Constraints
[RFC4684] MUST apply the RT Constraint procedures to the "Transport
class" Route Target Extended community as well.
The Transport Class Route Target Extended community is carried on
Classful Transport family routes and allows associating them with
appropriate Transport Route Databases at receiving BGP speakers.
Use of the Transport Class Route Target Extended community with a new
Type code avoids conflicts with any VPN Route Target assignments
already in use for service families.
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5. Transport Route Database
A Transport Route Database is a logical collection of transport
routes pertaining to the same Transport Class. Tunnel endpoint
addresses in this database belong to the "Provider Namespace".
Overlay routes that want to use a specific Transport Class confine
the scope of nexthop resolution to the set of routes contained in the
corresponding Transport Route Database.
The Transport Route Database can be realized as a "Routing Table"
referred in Section 9.1.2.1 of RFC4271 (https://www.rfc-
editor.org/rfc/rfc4271#section-9.1.2.1) which is a control plane only
database. However, an implementation may choose a different
methodology to realize this logical construct in such a way that it
supports the procedures defined in this document.
SN or BN originate routes for 'Classful Transport' address family
from the Transport Route Database. These routes have NLRI
"RD:Endpoint", Transport Class RT and an MPLS label. 'Classful
Transport' family routes received with Transport Class RT are
imported into its corresponding Transport Route Database.
6. Nexthop Resolution Scheme
An implementation may provide an option for the service route to
resolve over less preferred Transport Classes, should the resolution
over preferred or "primary" Transport Class fail.
To accomplish this, the set of service routes may be associated with
a user-configured "Resolution Scheme" that consists of the primary
Transport Class and an optional ordered list of fallback Transport
Classes.
A community known as "Mapping Community" is configured for a
"resolution scheme". Mapping community is a "role" and not a new
type of community; any BGP community or extended community may play
this role. A Mapping Community maps to exactly one Resolution
Scheme. A Resolution Scheme comprises of one primary transport class
and optionally, one or more fallback transport classes. The
Resolution scheme is used to realize the desired Intent.
An example of mapping community is color:0:100, described in RFC
9012, or the transport-target:0:100 described in section 4 in this
document.
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A BGP route is associated with a resolution scheme during import
processing. The first community on the 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.
So, the first community that maps to a Resolution Scheme is chosen as
the effective mapping community.
A transport route received in BGP Classful Transport family SHOULD
use a Resolution Scheme that contains the primary Transport Class
without any fallback to best effort tunnels. The primary Transport
Class is identified by the Transport Class RT carried on the route.
Thus, Transport Class RT serves as the Mapping Community for BGP CT
routes.
A service route received in a BGP service family MAY map to a
Resolution Scheme that contains the primary Transport Class
identified by the Mapping Community on the route and a fallback to
best effort Transport Class. The primary Transport Class is
identified by the Mapping Community carried on the route. For e.g.
the Extended Color community may serve as the Mapping Community for
service routes. Color:0:<n> MAY map to a Resolution Scheme that has
primary Transport Class <n> and a fallback to best-effort Transport
Class.
The Resolution Scheme mechanism not only works with SPRING transport
protocols to realize Intent based forwarding, but also with existing
tunneling technologies like RSVP TE, GRE, UDP, etc. Not assuming a
specific tunneling technology makes the BGP CT architecture backward
and forward compatible with existing and newer tunneling protocols,
respectively. It is compatible with SPRING, but there is no specific
dependency on SPRING. It is more generic and has broader
applicability.
7. BGP Classful Transport Family NLRI
The Classful Transport (CT) family will use the existing AFI of IPv4
or IPv6 and a new SAFI 76 "Classful Transport" that will apply to
both IPv4 and IPv6 AFIs. These AFI, SAFI pair of values MUST be
negotiated in Multiprotocol Extensions capability described in
[RFC4760] to be able to send and receive BGP CT routes.
The "Classful Transport" SAFI NLRI itself is encoded as specified in
https://tools.ietf.org/html/rfc8277#section-2 [RFC8277].
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When 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.
Procedures described for SAFI 4 or SAFI 128 in
https://tools.ietf.org/html/rfc8277#section-2 [RFC8277] apply for
SAFI 76 as well. BGP CT routes MAY carry multiple labels in the
NLRI, by negotiating the Multiple Labels Capability as described in
https://www.rfc-editor.org/rfc/rfc8277#section-2.1 [RFC8277]
For easy reference, the following figure illustrates a BGP Classful
Transport family NLRI when single Label is advertised (Multiple
Labels Capability is not negotiated):
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Label |Rsrv |S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Route Distinguisher (8 bytes) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4/IPv6 Prefix ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig 2: SAFI 76 "Classful Transport" NLRI
Length: 1 octet
The Length field consists of a single octet. It specifies the
length in bits of the remainder of the NLRI field.
Note that the length will always be the sum of 20 (number of bits
in Label field), plus 3 (number of bits in Rsrv field), plus 1
(number of bits in S field), plus the length in bits of the
Prefix (RD:IP prefix).
In an MP_REACH_NLRI attribute whose SAFI is 76, the Prefix
(RD + IP prefix) will be 96 bits or less if the AFI is 1
and will be 192 bits or less if the AFI is 2.
As specified in [RFC4760], the actual length of the NLRI field
will be the number of bits specified in the Length field, rounded
up to the nearest integral number of octets.
Label:
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The Label field is a 20-bit field containing an MPLS label value
(see [RFC3032]).
Rsrv:
This 3-bit field SHOULD be set to zero on transmission and MUST be
ignored on reception.
S:
When single label is advertised, this 1-bit field MUST be set to
one on transmission and MUST be ignored on reception.
Route Distinguihser:
8 byte RD as defined in [RFC4364 Sec 4.2].
IPv4/IPv6 Prefix:
IPv4 prefix, if AFI/SAFI 1/76.
IPv6 prefix, if AFI/SAFI 2/76.
Attributes on a Classful Transport route include the Transport Class
Route-Target extended community, which is used to associate the route
with the correct Transport Route Databases on SNs and BNs in the
network.
SAFI 76 routes can be sent with either IPv4 or IPv6 nexthop. The
type of nexthop is inferred from the length of the nexthop.
When the length of Next Hop Address field is 24 (or 48) the nexthop
address is of type VPN-IPv6 with 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).
When the length of the Next Hop Address field is 12 the nexthop
address is of type VPN-IPv4 with 8-octet RD set to zero.
7.1. Carrying multiple encapsulation information
To allow interoperating with nodes supporting different forwarding
technologies, a BGP CT route allows carrying multiple encapsulation
information.
MPLS Label is carried using RFC 8277 [RFC8277] encoding. A node that
does not support MPLS forwarding advertises the special label 3
(Implicit Null) in the RFC 8277 MPLS Label field.
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SRv6 SID is carried using Prefix SID attribute as specified in RFC
9252 [RFC9252], without Transposition Scheme. The Transposition
Length is set to 0 and Transposition Offset is set to 0 to indicate
nothing is transposed and that the entire SRv6 SID value is encoded
in the SID Information Sub-TLV.
UDP tunneling information is carried using TEA attribute as specified
in RFC 9012 [RFC9012].
8. Usage of Route Distinguisher and Label Allocation Modes
RD aids in troubleshooting a BGP CT network by uniquely identifying
the originator of a route across a multi domain network.
Use of RD also allows the option for signaling forwarding diversity
within the same Transport Class. The same Egress PE can advertise
multiple BGP CT routes for an EP belonging to the same Transport
Class.
E.g. multiple RDx:EP1 prefixes can be advertised for an EP1 to
different set of BGP peers in order to collect traffic statistics for
them. In absense of RD, duplicated Transport Class/Color values will
be needed in the transport network to achieve such use cases.
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 nexthop self. BGP CT provides flexible RD and Label allocation
modes to address operational requirements in a multi-domain network.
The allocation of RD is done at the point of origin of the BGP CT
route. This can either be an Egress SN or a BN. The default RD
allocation mode is to use unique RD per originiating 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
nexthop self by a SN or a BN. An implementation may use different
label allocation modes with BGP-CT. The recommended label allocation
mode is per-prefix as it provides better traffic convergence
properties than per-nexthop label allocation mode. Furthermore, BGP-
CT offers two flavors for per-prefix label 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.
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The impacts on control plane and forwarding behavior for the above
modes are detailed with an example in Managing Transport Route
Visibility (Section 20.1)
9. Comparison with other families using RFC-8277 encoding
SAFI 128 (Inet-VPN) is an RFC8277 encoded family that carries service
prefixes in the NLRI, where the prefixes come from the customer
namespaces and are contexualized into separate user virtual service
RIBs called VRFs using RFC4364 procedures.
SAFI 4 (BGP LU) is an RFC8277 encoded family that carries transport
prefixes in the NLRI, where the prefixes come from the provider
namespace.
SAFI 76 (Classful Transport) is an RFC8277 encoded family that
carries transport prefixes in the NLRI, where the prefixes come from
the provider namespace and are contexualized into separate Transport
Route Databases using RFC4364 procedures.
It is worth noting that SAFI 128 has been used to carry transport
prefixes in "L3VPN Inter-AS Carrier's carrier" scenario, where BGP
LU/LDP prefixes in CsC VRF are advertised in SAFI 128 towards the
remote-end client carrier.
In this document a new AFI/SAFI is used instead of reusing SAFI 128
to carry these transport routes because it is operationally
advantageous to segregate transport and service prefixes into
separate address families. E.g. It allows to safely enable "per-
prefix" label allocation scheme for Classful Transport prefixes
without affecting SAFI 128 service prefixes which may have huge
scale. The "per prefix" label allocation scheme keeps the routing
churn local during topology changes.
A new family also facilitates having a different readvertisement path
of the transport family routes in a network than the service route
readvertisement path. Service routes (Inet-VPN) are exchanged over
an EBGP multihop session between Autonomous systems with nexthop
unchanged; whereas Classful Transport routes are readvertised over
EBGP single hop sessions with "nexthop self" rewrite over inter-AS
links.
The Classful Transport family is similar in vein to BGP LU, in that
it carries transport prefixes. The only difference is that it also
carries in Route Target, an indication of which Transport Class the
transport prefix belongs to and uses RD to disambiguate multiple
instances of the same transport prefix in a BGP Update.
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10. Protocol Procedures
This section summarizes the procedures followed by various nodes
speaking Classful Transport family.
10.1. Preparing the network to deploy Classful Transport planes
Operator decides on the Transport Classes that exist in the
network and allocates a Transport Class Route Target to identify
each Transport Class.
Operator configures Transport Classes on the SNs and BNs in the
network with Transport Class Route Targets and unique 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.
10.2. Origination of Classful Transport route
At the ingress node of the tunnel's home domain, the tunneling
protocols install tunnel ingress routes in the Transport Route
Database associated with the Transport Class the tunnel belongs
to.
The egress node of the tunnel i.e. the tunnel endpoint originates
the BGP Classful Transport route with NLRI containing
RD:TunnelEndpoint, Transport Class RT and PNH TunnelEndpoint,
which will resolve over the tunnel route in Transport Route
Database at the ingress node. When the tunnel is up, the Classful
Transport BGP route will become usable and get re-advertised.
Alternatively, the ingress node may advertise this tunnel
destination into BGP as a Classful Transport family route with
NLRI RD:TunnelEndpoint, 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 route SHOULD NOT be advertised to the IBGP core that contains
the tunnel.
Unique RD SHOULD be used by the originator of a Classful Transport
route to disambiguate the multiple BGP advertisements for a
transport end point.
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10.3. Ingress node receiving Classful Transport route
On receiving a BGP Classful Transport route with a PNH that is not
directly connected (e.g. an IBGP-route), a Mapping Community on
the route (the Transport Class RT) indicates which Resolution
Scheme this route maps to. The resolution scheme for a Transport
Class RT with Transport class ID "C1" contains Transport Route
Database for Transport Class with same ID. In cases where
Transport class "C1" tunnels are not available in a domain the
administrator MAY customize the Resolution scheme to map to a
different set of transport class available in that domain.
The routes in the associated Transport Route Databases are used to
resolve the received PNH. If the resolution process does not find
a matching route in any of the associated Transport Route
Databases, the BGP CT route MUST be considered unusable for
forwarding purpose and be withdrawn.
10.4. Border node readvertising Classful Transport route with nexthop
self
The BN allocates an MPLS label to advertise upstream in Classful
Transport NLRI. The BN also installs an MPLS route for that label
that swaps the incoming label with a label received from the
downstream BGP speaker or pops the incoming label. It 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
semantics. RD is stripped from the BGP CT NLRI prefix when a BGP
CT route is added to a Transport Route Database. The IP prefix in
the Transport Route Database context (Transport-Class, IP-prefix)
is used as the key to do per-prefix label allocation. This helps
in avoiding BGP CT route churn through out the CT network when a
failure happens in a domain. The failure is not propagated
further than the BN closest to the failure.
The value of advertised MPLS label is locally significant, and is
dynamic by default. The BN may provide option to allocate a value
from a statically carved out range. This can be achieved using
locally configured export policy, or via mechanisms described in
BGP Prefix-SID [RFC8669].
10.5. Border node receiving Classful Transport route on EBGP
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If the route is received with PNH that is known to be directly
connected (e.g. EBGP single-hop peering address), the directly
connected interface is checked for MPLS forwarding capability. No
other nexthop resolution process is performed as the inter-AS link
can be used for any Transport Class.
If the inter-AS links should honor Transport Class, then the BN
SHOULD follow procedures of an Ingress node described above and
perform nexthop resolution process. The interface routes SHOULD
be installed in the Transport Route Database belonging to the
associated Transport Class.
10.6. Avoiding path-hiding through Route Reflectors
When multiple BNs exist such that theu advertise a RD:EP prefix to
RRs, the RRs may hide all but one of the BNs, unless ADDPATH
[RFC7911] is used for the Classful Transport family. This is
similar to L3VPN option-B scenarios. Hence ADDPATH SHOULD be used
for Classful Transport family, to avoid path-hiding through RRs.
10.7. Avoiding loop between Route Reflectors in forwarding path
Pair of redundant ABRs, each acting as an RR with nexthop self may
chose each other as best path instead of the upstream ASBR,
causing a traffic forwarding loop.
Implementations SHOULD provide a way to alter the tie-breaking
rule specified in BGP RR [RFC4456] to tie-break on CLUSTER_LIST
step before ROUTER-ID step, when performing path selection for BGP
CT routes. RFC4456 considers pure RR which is not in forwarding
path. When RR is in forwarding path and reflects routes with
nexthop self as is the case for ABR BNs in a BGP transport
network, this rule may cause loops. This document suggests the
following modification to the BGP Decision Process Tie Breaking
rules (Sect. 9.1.2.2, [RFC4271]) when doing path selection for BGP
CT family routes:
The following rule SHOULD be inserted between Steps e) and f): a
BGP Speaker SHOULD prefer a route with the shorter CLUSTER_LIST
length. The CLUSTER_LIST length is zero if a route does not carry
the CLUSTER_LIST attribute.
Some deployment considerations can also help in avoiding this
problem:
- IGP metric should be assigned such that "ABR to redundant ABR"
cost is inferior than "ABR to upstream ASBR" cost.
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- Tunnels belonging to non best effort Transport Classes SHOULD
NOT be provisioned between ABRs. This will ensure that the
route received from an ABR with nexthop self will not be usable
at a redundant ABR.
This avoids possibility of such loops altogether.
10.8. Ingress node receiving service route with Mapping Community
Service routes received with Mapping Community resolve using
Transport Route Databases determined by the Resolution Scheme. If
the resolution process does not find a Tunnel Ingress Route in any
of the Transport Route Databases, the service route MUST be
considered unusable for forwarding purpose and be withdrawn.
10.9. Coordinating between domains using different community namespaces
Cooperating option-C domains may sometimes not agree on RT, RD,
Mapping community or Transport Route Target values because of
differences in community namespaces (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, using per ASBR import policies. This is no different
than any other BGP VPN family. Mechanisms used in inter-AS VPN
deployments may be used with the Classful Transport family also.
The Resolution Schemes SHOULD allow association with multiple
Mapping Communities. This helps with renumbering, network mergers
or transitions.
Deploying unique RDs is strongly RECOMMENDED because it helps in
troubleshooting by uniquely identifying originator of a route and
avoids path-hiding.
This document defines a new format of Route-Target extended-
community to carry Transport Class, this avoids collision with
regular Route Target namespace used by service routes.
10.10. Best effort transport class
It is possible to represent 'Best effort' SLA also as a Transport
class. Today BGP LU (SAFI 4) is used to extend best effort intra
domain tunnels to other domains.
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Alternatively, BGP CT (SAFI 76) may be used to carry best effort
tunnels also. This document reserves the Transport class ID value
0 to represent "Best Effort Transport Class ID". However,
implementations SHOULD provide configuration to use a different
value for this purpose.
The 'Best Effort Transport Class ID" value is used in the
'Transport Class ID' field of Transport Route Target extended
community that is attached to the BGP CT route that advertises a
best effort tunnel endpoint. The route target thus formed is
called the "Best Effort Transport class route target".
When a BN or SN receives a BGP CT route with Best Effort Transport
class route target as the mapping community, the Best effort
Resolution scheme is used for resolving the BGP nexthop, and the
resultant route is installed in best effort transport route
database. if no best effort tunnel was found to resolve the BGP
nexthop, the BGP CT route MUST be considered unusable, and not be
propagated further.
When a BGP speaker receives an overlay route without any explicit
mapping community, the Best effort Resolution scheme is used for
resolving the BGP nexthop 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 FIB, to provide reachability for control
plane peering towards end points in other domains.
11. Flowspec Redirect to IP
Flowspec routes using Redirect to IP nexthop is described in BGP
Flow-Spec Redirect to IP Action [FLOWSPEC-REDIR-IP]
Such Flowspec BGP routes with Redirect to IP nexthop can be attached
with a Mapping Community (e.g. Color:0:100), which allows
redirecting the flow traffic over a tunnel to the IP nexthop
satisfying the desired SLA (e.g. Transport Class color 100).
Flowspec BGP family acts as just another service that can make use of
BGP CT architecture to achieve Flow based forwarding with SLAs.
12. BGP CT Egress TE
Mechanisms described in BGP LU EPE [BGP-LU-EPE] also applies to BGP
CT family.
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The Peer/32 or Peer/128 EPE route MAY be originated in BGP CT family
with appropriate Mapping Community (e.g. transport-target:0:100),
thus allowing an EPE path to the peer that satisfies the desired SLA.
13. Interaction with BGP attributes specifying nexthop address and
color
The Tunnel Encapsulation Attribute described in RFC9012 [RFC9012] can
be used to request a specific type of tunnel encapsulation. Usage of
this attribute may apply to BGP service routes or transport routes,
including BGP Classful Transport family routes.
Mechanisms described in BGP MultiNexthop Attribute [MULTI-NH-ATTR]
allow a BGP route to carry multiple nexthop addresses. It also
allows specifying 'Transport Class ID' as a qualifier for each
Nexthop address.
It should be noted that in such cases "Transport Class/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 nexthop should resolve over. This document suggests the
following order of precedence, more preferred first:
Transport Class ID SubTLV, in MultiNexthop Attribute.
Color SubTLV, in Tunnel Encapsulation Attribute.
Transport Target Extended community, on BGP CT route.
Color Extended community, on BGP service route.
The above precedence order follows more specific scoping of Color to
less specific scoping.
Transport Class ID specified for Nexthop-Leg subTLV in a MultiNextHop
attribute is more specific indication of Color than Color subTLV in a
TEA, which inturn is more specific than Mapping Community (Transport
Target) on a BGP CT transport route, which is inturn more specific
than a Service route scoped Mapping Community (Color Extended
community).
14. Signaling Intent across PE-CE link
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.
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This section describes the mechanisms that enable such signaling.
These procedures use existing service families (SAFI 1) on the PE-CE
link, with a new BGP attribute. It does not require a forklift
upgrade of the PE-CE session with a new set of address families.
---Gold----->
[CE1]-----[PE1]---[P]----[PE2]-----[CE2]
---Bronze--->
11.0.0.0 22.0.0.0
---- Traffic direction ---->
Figure 1: Intent on PE-CE link.
14.1. Using DSCP in MultiNexthop attribute
One such indication can be in form of DSCP code point (RFC2474
[RFC2474]) in IP header.
Let PE1 be configured to map DSCP1 to Gold Transport class, and DSCP2
to Bronze Transport class. Based on the DSCP code point received on
the IP traffic from CE1, the PE1 forwards the IP packet over a Gold
or Bronze tunnel. Thus the forwarding is not based on just the
destination IP address, but also the DSCP code point. This is known
as Class Based Forwarding (CBF). Today CBF is configured at the PE1
devices and CE1 doesn't receive any indication in BGP signaling
regarding what DSCP code points are being offered by the provider
network.
With a BGP MultiNexthop Attribute [MULTI-NH-ATTR] attached to a SAFI
1 service route, it is possible to extend the PE-CE BGP signalling to
communicate such information to the CE1. In above example, the MNH
contains two Nexthop Legs, described by two Forwarding Instruction
TLVs. Each Nexthop Leg contains the PE1 nexthop address in Endpoint
Identifier TLV ([MULTI-NH-ATTR] Sec 5.5.1.), the Transport class ID
(Color) ([MULTI-NH-ATTR] Sec 5.5.2.2.) for Gold, Bronze respectively,
and associated DSCP code-point ([MULTI-NH-ATTR] Sec 5.5.3.4.) to the
CE1 device. This allows the CE to discover what transport classes
exist in the provider network, and which DSCP codepoint to encode so
that traffic is forwarded using the desired transport class.
14.2. Using MPLS Label in MultiNexthop attribute
If the CE-PE link is MPLS enabled, a distinch MPLS label can also be
used to identify the transport class. PE1 can allocate a MPLS Label
L1 for the tuple "VPN Label, PNH Address, Transport class ID" and
advertise to CE1. When MPLS packet with label L1 is received from
CE1, the label is Popped, VPN Label if any is pushed, and then push
tunnel encap information related to "PNH address, Transport class
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ID". PE1 may thus do forwarding based on MPLS label without
performaing any IP lookup. This allows for PE1 to be a low IP FIB
device and still support CBF. The number of MPLS Labels will be
proportional to the number of unique VPN labels received.
A BGP MultiNexthop Attribute [MULTI-NH-ATTR] is attached to a SAFI 1
service route to convey the MPLS Label information to CE1. In above
example, the MNH contains two Nexthop Legs, described by two
Forwarding Instruction TLVs. Each Nexthop Leg contains the PE1
nexthop address in Endpoint Identifier TLV ([MULTI-NH-ATTR] Sec
5.5.1.), the Transport class ID (Color) ([MULTI-NH-ATTR] Sec
5.5.2.2.) for Gold, Bronze respectively, and associated MPLS Label
([MULTI-NH-ATTR] Sec 5.5.3.1.) to the CE1 device. This allows the CE
to discover what transport classes exist in the provider network, and
which MPLS Label to encode so that traffic is forwarded using the
desired transport class.
15. Scaling considerations
15.1. Avoiding unintended spread of BGP CT routes across domains
RFC8212 [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 MPLS label and nexthop
resources 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.
15.2. Constrained distribution of PNHs to SNs (On Demand Nexthop)
This section describes how the number of Protocol Nexthops
advertised to a SN or BN can be constrained using BGP Classsful
Transport and Route Target Constraints (RTC) [RFC4684].
An egress SN MAY advertise BGP CT route for RD:eSN with two Route
Targets: transport-target:0:<TC> and a RT carrying <eSN>:<TC>.
Where TC is the Transport Class identifier, and eSN is the IP-
address used by SN as BGP nexthop in its service route
advertisements.
Note that such use of the 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
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transport route for an egress node eSN to only the set of ingress
nodes that need it. When only RT of transport-target:0:<TC> is
used, the pruning happens in granularity of Transport Class ID
(Color), and not BGP nexthop; BGP CT routes will not be advertised
into domains with PEs that dont import 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
BGP extended community attribute (BGP attribute code 16).
The RT carrying <eSN>:<TC> MAY be an IP-address specific regular
RT (BGP attribute code 16), IPv6-address specific RT (BGP
attribute code 25), or a Wide-communities based RT (BGP attribute
code 34) as described in Route Target Constrain Extension
[RTC-Ext]. This document recommends using Wide-communities based
RT for the same.
An ingress SN MAY import BGP CT routes with Route Target carrying
<eSN>:<TC>. The ingress SN MAY learn the eSN values either by
configuration, or it MAY discover them from the BGP nexthop field
in the BGP VPN service routes received from eSN. A BGP ingress SN
receiving a BGP service route with nexthop of eSN SHOULD generate
a RTC/Extended-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 path of route propogation of BGP CT routes is 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 Nexthop" of BGP CT routes,
which help with the scaling of MPLS forwarding state at SN and BN.
However, the amount of state carried in RTC family may become
proportional to 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 home region of
ingress-SN, or the BNs of a super core.
Such a BN in the core of the network SHOULD import BGP CT routes
with Transport-Target:0:<TC> and generate a RTC route for <Origin
ASN>:0:<TC>/96, while not propagating the more specific RTC
requests for specific PNHs. This will let the BN learn transport
routes to all eSN nodes. But confine their propagation to
ingress-SNs.
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15.3. Limiting scope of visibility of PE loopback as PNHs
It may be even more desirable to limit the number of PNHs that are
globaly visible in the network. This is possible using mechanism
described in MPLS Namespaces [MPLS-NAMESPACES]
Such that advertisement of PE loopback addresses as 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, swapping the SN scoped service label with a CPNH scoped
private namespace label.
This provides much greater advantage in terms of scaling and
convergence. Changes to implement this feature are required only
on the local region's BNs and RRs.
16. OAM considerations
Standard MPLS OAM procedures specified in [RFC8029] also apply to BGP
Classful Transport.
The 'Target FEC Stack' sub-TLV for IPv4 Classful Transport has a Sub-
Type of [TBD], 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) and a prefix length
encoded as follows:
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 2: Classful Transport IPv4 FEC
The 'Target FEC Stack' sub-TLV for IPv6 Classful Transport has a Sub-
Type of [TBD], 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 follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Distinguisher |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 prefix |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Classful Transport IPv6 FEC
17. Applicability to Network Slicing
In Network Slicing, the Transport Slice Controller (TSC) sets up the
Topology (e.g. RSVP-TE, SR-TE tunnels with desired characteristics)
and resources (e.g. polices/shapers) in a transport network to create
a Transport Slice. The Transport Class construct described in this
document represents the "Topology Slice" portion of this equation.
The TSC can use the Transport Class Identifier (Color value) to
provision a transport tunnel in a specific Topology Slice.
Further, Network Slice Controller can use the Mapping Community on
the service route to map traffic to the desired Transport Slice.
18. SRv6 support
This section describes how BGP CT may be used to set up inter domain
tunnels of a certain Transport Class, when using Segment Routing over
IPv6 (SRv6) data plane on the inter AS links or as an intra AS
tunneling mechanism.
[RFC8986] specifies the SRv6 Endpoint behaviors (End USD, End.BM,
End.B6.Encaps). [SRV6-INTER-DOMAIN] specifies the SRv6 Endpoint
behaviors (END.REPLACE, END.REPLACEB6 and END.DB6). These are
leveraged for BGP CT with SRv6 data plane.
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The BGP Classful Transport route update for SRv6 MUST include an
attribute containing SRv6 SID information. This may be either the
BGP Prefix-SID attribute as specified in [RFC9252]or the BGP
MultiNexthop attribute as specified in BGP MultiNexthop Attribute
[MULTI-NH-ATTR] section 5.5.3.3. If the Prefix-SID attribute is
used, it MUST NOT include SRv6 SID structure for Transposition
described in [RFC9252].
It should be noted that prefixes carried in BGP CT family are
transport layer end-points, e.g. PE loopback addresses. Thus the
SRv6 SID carried in a BGP CT route is also a transport layer
identifier. For an illustration of BGP CT deployment in SRv6
newtorks, please refer to Appendix D Appendix D .
19. Illustration of BGP CT procedures in Inter AS option-C
19.1. Topology
[RR26] [RR27] [RR16]
| | |
| | |
|+-[ABR23]--+|+--[ASBR21]---[ASBR13]-+|+--[PE11]--+
|| ||| ` / ||| |
[CE41]--[PE25]--[P28] [P29] `/ [P15] [CE31]
| | | /` | | |
| | | / ` | | |
| | | / ` | | |
+--[ABR24]--+ +--[ASBR22]---[ASBR14]-+ +--[PE12]--+
| | | |
+ + + +
CE | region-1 | region-2 | |CE
AS4 ...AS2... AS1 AS3
41.41.41.41 ------------ Traffic Direction ----------> 31.31.31.31
Figure 4: Multi-Domain BGP CT Network
This example shows a provider network that comprises of two
Autonomous systems, AS1, AS2. They are serving customers AS3, AS4
respectively. Traffic direction being described is CE41 to CE31.
CE31 may request a specific SLA (e.g. Gold for this traffic), when
traversing these provider networks.
AS2 is further divided into two regions. So, there are three tunnel
domains in provider space. AS1 uses ISIS Flex-Algo intra-domain
tunnels, whereas AS2 uses RSVP-TE intra-domain tunnels.
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The network has two Transport classes: Gold with transport class id
100, Bronze with transport class id 200. These transport classes are
provisioned at the PEs and the Border nodes (ABRs, ASBRs) in the
network.
Following tunnels exist for Gold transport 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 - ISIS FlexAlgo tunnel
ASBR14_to_PE11_gold - ISIS FlexAlgo tunnel
Following tunnels exist for Bronze transport 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 to belong to
transport class 100 or 200.
19.2. Service Layer route exchange
Service nodes PE11, PE12 negotiate service families (SAFIs 1, 128) on
the BGP session with RR16. Service helpers RR16 and RR26 exchange
these service routes with nexthop unchanged over a multihop EBGP
session between the two AS. PE25 negotiates service families (SAFIs
1, 128) with RR26.
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The PEs see each other as nexthop in the BGP Update for service
family (SAFIs 1, 128) routes. Addpath send, receive is enabled on
both directions on the EBGP multihop session between RR16 and RR26
for SAFIs 1, 128. Addpath send is negotiated in the RR to PE
direction in each AS. This is to avoid path hiding of service routes
at RR. E.g. SAFI 1 routes advertised by both PE11 and PE12. Or,
SAFI 128 routes originated by both PE11 and PE12 using same RD.
Forwarding happens using service routes installed at service nodes
PE25, PE11, 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 31.31.31.31 with
nexthop as self to PE11, PE12. CE31 can attach a Mapping Community
Color:0:100 on this route, to indicate its request for Gold SLA. Or,
PE11 can attach the same using locally configured policies.
Consider CE31 is getting VPN service from PE11. The RD1:31.31.31.31
route is readvertised in SAFI 128 by PE11 with nexthop self (1.1.1.1)
and label V-L1, to RR16 with the Mapping Community Color:0:100
attached. RR16 advertises this route with Addpath-ID to RR26 which
readvertises to PE25 with nexthop unchanged. Now PE25 can resolve
the PNH 1.1.1.1 using transport routes received in BGP CT or BGP LU.
Using Addpath, service routes advertised by PE11 and PE12 for SAFIs
1, 128 reach PE25 via RR16, RR26 with the nexthop unchanged, as PE11
or PE12.
The IP FIB at PE25 VRF will have a route for 31.31.31.31 with a
nexthop when resolved, that points to a Gold tunnel in ingress
domain.
19.3. Transport Layer route propagation
Egress nodes PE11, PE12 negotiate BGP CT family with transport ASBRs
ASBR13, ASBR14. These egress nodes originate BGP CT routes for
tunnel endpoint addresses, that are advertised as nexthop in BGP
service routes. In this example both PEs participate in transport
classes Gold and Bronze. The protocol procedures are explained using
Gold SLA plane and the Bronze SLA plane is used to highlight the path
hiding aspects.
PE11 is provisioned with transport class 100, RD value 1.1.1.1:10 and
a transport-target:0:100 for Gold tunnels. And a Transport class 200
with RD value 1.1.1.1:20, and transport route target 0:200 for Bronze
tunnels. Similarly, PE12 is provisioned with transport class 100, RD
value 1.1.1.2:10 and a transport-target:0:100 for Gold tunnels. And
transport class 200, RD value 1.1.1.2:20 with transport-target:0:200
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for Bronze tunnels. Note that in this example the BGP CT routes
carry only the transport class route 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
and PEs with same Transport Route Target and unique RDs.
ASBR13 and ASBR14 negotiate BGP CT family with transport ASBRs
ASBR21, ASBR22 in neighboring AS. They negotiate BGP CT family with
RR27 in region 2, which reflects BGP CT routes to ABR23, ABR24.
ABR23, ABR24 negotiate BGP CT family with Ingress node PE25 in region
1. BGP LU family is also negotiated on these sessions alongside BGP
CT family. BGP LU carries "best effort" transport class routes, BGP
CT carries gold, bronze transport class routes.
PE11 is provisioned to originate BGP CT route with Gold SLA to
endpoint PE11. This route is sent with NLRI RD prefix
1.1.1.1:10:1.1.1.1, Label B-L0, nexthop 1.1.1.1 and a route target
extended community transport-target:0:100. Label B-L0 can either be
Implicit Null (Label 3) or a Ultimate Hop Pop (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 1.1.1.1:10:1.1.1.1, Label
B-L1, nexthop self, and transport-target:0:100. MPLS swap route is
installed at ASBR13 for B-L1 with a nexthop pointing to
ASBR13_to_PE11_gold tunnel.
Similarly ASBR14 also receives BGP CT route for 1.1.1.1:10:1.1.1.1
from PE11 and it resolves over the tunnel ASBR14_to_PE11_gold. The
route is then readvertised by ASBR14 in BGP CT family to ASBRs
ASBR21, ASBR22 according to export policy. This route is sent with
same NLRI RD prefix 1.1.1.1:10:1.1.1.1, Label B-L2, nexthop self, and
transport-target:0:100. MPLS swap route is installed at ASBR14 for
B-L1 with a nexthop pointing to ASBR14_to_PE11_gold tunnel.
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In the Bronze plane, BGP CT route with Bronze SLA to endpoint PE11 is
originated by PE11 with a NLRI containing RD prefix
1.1.1.1:20:1.1.1.1, and appropriate label. The RD allows both Gold
and Bronze advertisements traverse path selection pinchpoints without
any path hiding at RRs or ASBRs. And 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 Gold SLA
path.
Moving back to the Gold plane, ASBR21 receives the Gold SLA BGP CT
routes for NLRI RD prefix 1.1.1.1:10:1.1.1.1 over the single hop EBGP
sessions from ASBR13, ASBR14, and can compute ECMP/FRR towards them.
ASBR21 readvertises BGP CT route for 1.1.1.1:10:1.1.1.1 with nexthop
self (loopback adderss 2.2.2.1) to RR27, advertising a new label
B-L3. MPLS swap route is installed for label B-L3 at ASBR21 to swap
to received label B-L1, B-L2 and forward to ASBR13, ASBR14
respectively. RR27 readvertises this BGP CT route to ABR23, ABR24
with label and nexthop unchanged.
Similarly, ASBR22 receives BGP CT route 1.1.1.1:10:1.1.1.1 over the
single hop EBGP sessions from ASBR13, ASBR14, and readvertises with
nexthop self (loopback adderss 2.2.2.2) to RR27, advertising a new
label B-L4. MPLS swap route is installed for label B-L4 at ASBR22 to
swap to received label B-L1, B-L2 and forward to ASBR13, ASBR14
respectively. RR27 readvertises this BGP CT route also to ABR23,
ABR24 with label and nexthop unchanged.
Addpath is enabled for BGP CT family on the sessions between RR27 and
ASBRs, ABRs such that routes for 1.1.1.1:10:1.1.1.1 with the nexthops
ASBR21 and ASBR22 are reflected to ABR23, ABR24 without any path
hiding. Thus giving ABR23 visibiity of both available nexthops for
Gold SLA.
ABR23 receives the route with nexthop 2.2.2.1, label B-L3 from RR27.
The route target "transport-target:0:100" on this route acts as
Mapping Community, and instructs ABR23 to strictly resolve the
nexthop using transport class 100 routes only. ABR23 is unable to
find a route for 2.2.2.1 with transport class 100. Thus it considers
this route unusable and does not propagate it further. This prunes
ASBR21 from Gold SLA tunneled path.
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ABR23 also receives the route with nexthop 2.2.2.2, label B-L4 from
RR27. The route target "transport-target:0:100" on this route acts
as Mapping Community, and instructs ABR23 to strictly resolve the
nexthop using transport class 100 routes only. ABR23 successfully
resolves the nexthop to point to ABR23_to_ASBR22_gold tunnel. ABR23
readvertises this BGP CT route with nexthop self (loopback address
2.2.2.3) and a new label B-L5 to PE25. Swap route for B-L5 is
installed by ABR23 to swap to label B-L4, and forward into
ABR23_to_ASBR22_gold tunnel.
PE25 receives the BGP CT route for prefix 1.1.1.1:10:1.1.1.1 with
label B-L5, nexthop 2.2.2.3 and transport-target:0:100 from RR26.
And it similarly resolves the nexthop 2.2.2.3 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
egress-domain is extended by BGP CT until the ingress-node PE25 in
ingress domain, to create an end-to-end Gold SLA path. MPLS swap
routes are installed at ASBR13, ASBR22 and ABR23, when propagating
the PE11 BGP CT Gold transport class route 1.1.1.1:10:1.1.1.1 with
nexthop self towards PE25.
The BGP CT LSP thus formed, originates in 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 nexthop 1.1.1.1 and
mapping community Color:0:100, it resolves over this BGP CT route
1.1.1.1:10:1.1.1.1. Thus pushing label B-L5, and pushing as top
label the labels associated with PE25_to_ABR23_gold tunnel.
19.4. Data plane view
19.4.1. Steady state
This section describes how the data plane looks like in steady state.
CE41 transmits an IP packet with destination as 31.31.31.31. On
receiving this packet PE25 performs a lookup in the IP FIB associated
with the CE41 interface. This lookup yeids 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
MPLS packet with label V-L1(innermost), B-L5, and top label as
PE25_to_ABR23_gold tunnel. This MPLS packet is thus transmitted to
ABR23 using Gold SLA.
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ABR23 decapsulates the packet received on PE25_to_ABR23_gold tunnel
as required, and finds the MPLS packet with label B-L5. It performs
lookup for label B-L5 in the global MPLS FIB. This yields the route
that swaps label B-L5 with label B-L4, and pushes top label provided
by ABR23_to_ASBR22_gold tunnel. Thus ABR23 transmits the MPLS packet
with label B-L4 to ASBR22, on a tunnel that satisfies Gold SLA.
ASBR22 similarly performs a lookup for label B-L4 in global MPLS FIB,
finds the route that swaps label B-L4 with label B-L2, and forwards
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, and performs a
lookup in 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 Gold SLA in AS 1.
PE11 receives the MPLS packet with V-L1, and performs VPN forwarding.
Thus transmitting the original IP payload from CE41 to CE31. The
payload has traversed path satisfying Gold SLA end-to-end.
19.4.2. Local repair of primary path
This section describes how the data plane at ASBR22 reacts when link
between ASBR22 and ASBR13 experiences a failure, and an alternate
path exists.
Assuming ASBR22_to_ASBR13 link goes down, such that traffic with Gold
SLA going to PE11 needs repair. ASBR22 has an alternate BGP CT route
for 1.1.1.1:10:1.1.1.1 from ASBR14. This has been preprogrammed in
forwarding by ASBR22 as FRR backup nexthop 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).
19.4.3. Absorbing failure of primary path. Fallback to best-effort
tunnels.
This section describes how the data plane reacts when gold path
experiences a failure, but no alternate path exists.
Assuming tunnel ABR23_to_ASBR22_gold goes down, such that now end-to-
end Gold path does not exist in the network. This makes the BGP CT
route for RD prefix 1.1.1.1:10:1.1.1.1 unusable at ABR23. This makes
ABR23 send a BGP withdrawal for 1.1.1.1:10:1.1.1.1 to PE25.
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Withdrawal for 1.1.1.1:10:1.1.1.1 allows PE25 to react to the loss of
gold path to 1.1.1.1. Assuming PE25 is provisioned to use best-
effort transport class as the backup path, this withdrawal of BGP CT
route allows PE25 to adjust the nexthop of the VPN Service-route to
push the labels provided by the BGP LU route. That repairs the
traffic to go via best effort path. PE25 can also be provisioned to
use Bronze transport class as the backup path. The repair will
happen in similar manner in that case as-well.
Traffic repair to absorb the failure happens at ingress node PE25, in
a service prefix scale independent manner. This is called PIC
(Prefix scale Independent Convergence). The repair time will be
proportional to time taken for withdrawing the BGP CT route.
The above examples demostrate the various levels of failsafe
mechanisms available to protect traffic in a BGP CT network.
20. Deployment considerations.
20.1. Managing Transport Route Visibility
This section details the usage of BGP-CT RD and label allocation
modes to calibrate the level of path visibility and the amount of
route churn in a multi-domain network.
Consider a multi-domain BGP-CT network as illustrated in the figure
below.
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|-----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)
|-----AS3-----| |-------AS2------|
Figure 5: Multi-Domain Network
The following table details the BGP-CT route and path visibility at
PE31-- for each TC.
+--------+------+-------+-------+---------+---------+
|EP-type |Origin|RD-Mode|PP-Mode|CT Routes|CT Labels|
+--------+------+-------+-------+---------+---------+
|Unicast |SN |Unique |TC,EP | 16 | 8 |
|Unicast |SN |Unique |RD,EP | 16 | 16 |
|Unicast |BN |Unique |TC,EP | 16 | 8 |
|Unicast |BN |Unique |RD,EP | 16 | 16 |
|--------|------|-------|-------|---------|---------|
|Anycast |SN |Unique |TC,EP | 16 | 2 |
|Anycast |SN |Unique |RD,EP | 16 | 16 |
|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,IP | 2 | 2 |
+--------+------+-------+-------+---------+---------+
Figure 6: Route and Path Visibility at Ingress Node
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In the above example, both route churn and TE granularity are
directly proportional to the number of CT labels received.
Above table demonstrates that BGP CT allows an operator to control
how much path visibility and forwarding diversity is desired in the
network, for Unicast and Anycast endpoints.
20.2. Managing Intent at Service and Transport layers.
Illustration of BGP CT Procedures (Section 19) shows multiple domains
that agree on a color name space (Agreeing Color Domains) and contain
tunnels with equivalent set of colors (Homogenous Color Domains).
However in the real world, this may not always be gauranteed. Two
domains may independently manage their color namespaces, these are
known as Non-Agreeing Color Domains. Two domains may have tunnels
with unequal set of colors, these are known as Heterogenous Color
Domains.
This section describes how BGP CT is deployed in such scenarios to
preserve end to end Intent. Example described in this section use
Inter AS option C domains. But similar mechanisms will work for
Inter AS option A and Inter AS option B scenarios as-well.
20.2.1. Service layer Color Management
At the service layer, it is recommended that a global color namespace
be maintained across multiple co-operating 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
independantly maintains its own local transport color namespace.
As explained in Nexthop Resolution Scheme (Section 6) , mapping
community carried on service route maps to a resolution scheme. The
mapping community values for the service route can be abstract and
does not require to match the transport color namespace. This
abstract mapping commnunity value representing a global service layer
intent is mapped to an local transport layer intent available in each
domain.
In this manner, it is recommended to keep color namespace management
at service layer and the transport layer decoupled from each other.
In the following sections the service layer agrees on a single global
namespace.
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20.2.2. Non-Agreeing Color Domains
Non-agreeing color domains require a mapping community rewrite on
each domain boundry. This rewrite helps to map one domain's
namespace to another.
The below 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)
----------- Packet Forwarding Direction -------->
Figure 7: Transport Layer with Non-agreeing Color Domains
In the above topology we have three Autonomous Systems. All the
nodes in the topology supports BGP CT.
In AS1 Gold SLA is represented by color 100 and Bronze by 200.
In AS2 Gold SLA is represented by color 300 and Bronze by 400.
In AS3 Gold SLA is represented by color 500 and Bronze by 600.
Though the color values are different, they map to tunnels with same
TE characteristics in each domain.
The service route carries an abstract mapping community that maps to
the required SLA. For example, Service routes that need to resolve
over gold transport tunnels, carries a mapping community
color:0:100500. In AS3 it maps to a resolution scheme containing
TRDB with color 500 whereas in AS2 it maps a to TRDB with color 300
and in AS1 it maps to a TRDB with color 100. Co-ordination is needed
to provision the resolution schemes in each domain as explained
above.
At the AS boundary the transport-class route-target is rewritten for
the BGP CT routes. In the above topology, At ASBR31 the transport-
target:0:500 for gold tunnels is rewritten to transport-target:0:300
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and then advertised to ASBR22. Similarly the transport-target:0:300
for gold tunnels are re-written 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 above topology works as
explained in Section 19.
Transport-target re-write requires co-ordination of color values
between domains in the transport layer. This method avoids the need
to re-write service route mapping community, keeping the service
layer homogenous and simple to manage. Co-ordinating transport-class
route-target between adjacent domains is easier than co-ordinating
service layer colors deployed in various non-adjacent domains.
20.2.3. Heterogeneous Agreeing Color Domains
In a heterogenous domains scenario, it might not be possible to map a
service layer intent to the matching transport color as the color
might not be locally available in a domain.
In this model, resolution schemes are customized to map the received
mapping community (eg: transport-target or color community) to
locally available TRDBs that are acceptable to realize the desired
intent.
The below example illustrates how traffic is stitched, when a transit
AS contains more shades for an SLA paths compared to Ingress and
Egress domains. This example shows how service routes can traverse
through finer shades when available and take coarse shades otherwise.
<---------- Service Routes SAFI-128 ------------------------
Gold1(101)
Gold2(102)
Gold(100) Gold(100)
[PE11]------[ASBR11]----[ASBR21--------[ASBR22]----[ASBR31--------[PE31]
AS1-Metro-Ingress AS2-Core AS3-Metro-Egress
----------- Packet Forwarding Direction -------->
Figure 8: Tranport Layer with Heterogenous Color Domains
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In the above topology we have three Autonomous Systems. All the
nodes in the topology support BGP CT.
In AS1 Gold SLA is represented by color 100.
In AS2 Gold has finer shades: Gold1 by color 101 and Gold2 by color
102.
In AS3 Gold SLA is represented by color 100.
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 and
it maps to color 101 TRDB.
To facilitate this mapping every SN/BN in all AS provision required
transport classes viz. 100, 101 and 102. SN and BN in AS1 and AS3
are provisioned with customized resolution schemes that resolve
routes with transport-target:0:101 or transport-target:0:102 strictly
over color 100 TRDB.
PE31 is provisioned to originate BGP CT route with color 101 for
endpoint PE31. This route is sent with NLRI RD prefix RD1:PE31 and
route target extended community transport-target:0:101.
At ASBR31, the route target "transport-target:0:101" on this BGP CT
route instructs to add the route to color 101 TRDB. ASBR31 is
provisioned with 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 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 to ASBR21 with
transport-target:0:101.
Similarily 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 to
ASBR11 with transport-target:0:101.
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At ASBR11, the route target "transport-target:0:101" on this BGP CT
route instructs 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 from color 101 TRDB to PE11 with route-
target:0:101.
At PE11, the route target "transport-target:0:101" on this BGP CT
route instructs 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 it directly resolves over the BGP CT route in color 101
TRDB, which inturn resolves over tunnel routes in color 100 TRDB.
In this manner, PE11 can put traffic on tunnels with color 101, color
102 in the core domain, and color 100 in the metro domains.
20.3. Migration scenarios.
20.3.1. BGP CT islands connected via BGP LU domain.
This section explains how 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-MHOP-CT-----------+
| |
AS3 | AS2 | AS1
[PE31-----ASBR31]--------[ASBR22---ASBR21]-------[ASBR11---PE11]
<-EBGP-LU-> <-EBGP-LU->
<---IBGP-CT---> <---IBGP-LU---> <---IBGP-CT--->
---------Packet Forwarding Direction--------->
Figure 9: BGP CT in AS1 and AS3 connected by BGP LU in AS2
In the above topology there are three ASs. AS1 and AS3 supports BGP-
CT. AS2 is a domain that does not support BGP CT.
Nodes in AS1,AS2 and AS3 negotiate IBGP-LU within the domain. Nodes
in AS1 and AS3 negotiate IBGP CT within the domain. ASBR11 and
ASBR21 as well as ASBR22 and ASBR31 negotiate EBGP-LU over directly
connected interdomain links. ASBR11 and ASBR31 have reachability to
each other’s loopbacks through BGP-LU. ASBR11 and ASBR31 negotiate
BGP-CT over a multihop EBGP session formed using BGP-LU reachability.
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The following tunnels exist for 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
Following tunnels exist for 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 class gold and
bronze, and are advertised between ASBR31 and ASBR11 with Nexthop
self.
Further in AS2 the following tunnels exist to satisfy the different
SLAs, using per SLA endpoint:
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
RD:PE11 BGP CT route is originated from PE11 towards ASBR11 with
transport-target gold. ASBR11 readvertises this route with Nexthop
as SLA endpoint ASBR11_lpbk_gold on the EBGP CT MHOP session towards
ASBR31. ASBR11 originates ASBR11_lpbk_gold in EBGP LU towards ABR21
with gold SLA community. Similarily for ASBR11 originates per SLA
loopback routes for each SLA it supports attaching the community for
that SLA. This SLA community is used by ASBR31 to leak the LU routes
into their respective CT TRDBs.
ASBR21 readvertises the ASBR11_lpbk_gold BGP-LU route to ASBR22 with
the nexthop as a unique loopback (ASBR21_lpbk_gold) representing gold
SLA. ASBR22 on receiving these endpoints resolve them over the
appropriate SLA transport tunnels by virtue of per SLA tunnel
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endpoint provisioned in AS2. ASBR22, on succesful resolution,
readvertises this BGP-LU routes to ASBR31 with nexthop self and a new
label.
ASBR31 imports ASBR11_lpbk_gold route received via EBGP-LU from
ASBR22 to gold TRDB based on the received SLA community. ASBR31 uses
this gold TRDB route to resolve the NH ASBR11_lpbk_gold of RD:PE11
route received over the MHOP EBGP-MHOP-CT session with transport-
target gold, thus preserving the end-to-end SLA. Now ASBR31
readvertises RD:PE11 route with nexthop as self thus stitching with
the BGP-LU LSP. Intradomain traffic forwarding in AS1 and AS3
follows the procedures as explained in Illustration of CT Procedures
(Section 19)
In cases where an SLA cannot be preserved in AS2, it can be carried
over available SLAs (ex: best-effort SLA) by rewriting the nexthop to
ASBR21 loopback assigned to that endpoint. This eases migration in
case of heterogenous color domains.
20.3.2. BGP CT - Interop between MPLS and other forwarding
technologies.
This section describes how nodes supporting dissimilar encapsulation
technologies can interoperate with each other when using BGP CT
family.
20.3.2.1. Interop between MPLS and SRv6 nodes.
BGP speakers may carry MPLS label and SRv6 SID in BGP CT SAFI 76
routes using protocol encoding as described in Carrying Multiple
Encapsulation information (Section 7.1)
MPLS Labels are carried using RFC 8277 encoding, and SRv6 SID is
carried using Prefix SID attribute as specified in RFC 9252
RR1--+
\ +-------R2 [MPLS + SRv6]
\ |
R1--------P-------R3 [MPLS only]
[MPLS + SRv6] |
+-------R4 [SRv6 only]
<---- Bidirectional Traffic ---->
Figure 10: BGP CT Interop between MPLS and SRv6 nodes
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This example shows a provider network with a mix of devices with
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 BGP
session with Route Reflector RR1 which reflects routes between these
nodes with nexthop unchanged. BGP CT family is negotiated on these
sessions.
R1 and R2 send and receive both MPLS label and SRv6 SID in the BGP CT
control plane routes. This allows them to be ingress and egress for
both MPLS and SRv6 data planes. MPLS Label is carried using RFC 8277
encoding, and SRv6 SID is carried using Prefix SID attribute as
specified in RFC 9252, without Transposition Scheme. The
Transposition Length is set to 0 and Transposition Offset is set to 0
to indicate nothing is transposed and that the entire SRv6 SID value
is encoded in the SID Information Sub-TLV. In this way, either MPLS
or SRv6 forwarding can be used between R1 and R2.
R1 and R3 send and receive MPLS label in the BGP CT control plane
routes using RFC 8277 encoding. This allows them to be ingress and
egress for MPLS data plane. R1 will carry SRv6 SID in Prefix-SID
attribute, which will not be used by R3. In order to interoperate
with MPLS only device R3, R1 MUST NOT use SRv6 Transposition scheme
described in RFC 9252 that overloads the RFC 8277 MPLS-Label field
with SRv6 Transposition information. MPLS forwarding will be used
between R1 and R3.
R1 and R4 send and receive SRv6 SID in the BGP CT control plane
routes using BGP Prefix-SID attribute, without Transposition Scheme.
This allows them to be ingress and egress for SRv6 data plane. R4
will carry the special MPLS Label with value 3 (Implicit-NULL) in RFC
8277 encoding, which tells R1 not to push any MPLS label towards R4.
The MPLS Label advertised by R1 in RFC 8277 NLRI will not be used by
R4. SRv6 forwarding will be used between R1 and R4.
Note in this example that R3 and R4 cannot communicate directly with
each other, because they dont support a common forwarding technology.
The BGP CT routes received at R3, R4 from each other will remain
unusable, due to incompatible forwarding technology.
20.3.2.2. 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, when
using BGP CT family.
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MPLS Labels are carried using RFC 8277 encoding, and UDP (or IP)
tunneling information is carried using TEA attribute as specified in
RFC 9012
RR1--+
\ +-------R2 [MPLS + UDP]
\ |
R1--------P-------R3 [MPLS only]
[MPLS + UDP] |
+-------R4 [UDP only]
<---- Bidirectional Traffic ---->
Figure 11: BGP CT Interop between MPLS and UDP 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 which reflects routes between
these nodes with nexthop unchanged. BGP CT family is negotiated on
these sessions.
R1 and R2 send and receive both MPLS label 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. MPLS Label is
carried using RFC 8277 encoding. UDP tunneling information is
carried using TEA attribute as specified in RFC 9012. Either MPLS or
UDP tunneled forwarding can be used between R1 and R2.
R1 and R3 send and receive MPLS label in the BGP CT control plane
routes using RFC 8277 encoding. This allows them to be ingress and
egress for MPLS data plane. R1 will carry UDP tunneling info in TEA
attribute, 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 BGP TEA attribute. 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, which tells
R1 not to push any MPLS label towards R4. The MPLS Label advertised
by R1 will not be used by R4. UDP tunneled forwarding will be used
between R1 and R4.
Note in this example that R3 and R4 cannot communicate directly with
each other, because they dont support a common forwarding technology.
The BGP CT routes received at R3, R4 from each other will remain
unusable, due to incompatible forwarding technology.
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21. IANA Considerations
This document makes following requests of IANA.
21.1. New BGP SAFI
Please assign a new BGP SAFI code for "Classful Transport". Value
76.
Registry Group: Subsequent Address Family Identifiers (SAFI) Parameters
Registry Name: SAFI Values
Value Description
-------------+--------------------------
76 Classful-Transport SAFI
This will be used to create new AFI,SAFI pairs for IPv4, IPv6
Classful Transport families. viz:
* "Inet, Classful Transport". AFI/SAFI = "1/76" for carrying IPv4
Classful Transport prefixes.
* "Inet6, Classful Transport". AFI/SAFI = "2/76" for carrying IPv6
Classful Transport prefixes.
21.2. New Format for BGP Extended Community
Please assign a new Format (Type high = 0xa) of extended community
EXT-COMM [RFC4360] called "Transport Class" from the following
registries:
the "BGP Transitive Extended Community Types" registry, and
the "BGP Non-Transitive Extended Community Types" registry.
Please assign the same low-order six bits for both allocations.
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
extended community.
Taking reference of RFC7153 [RFC7153] , following requests are made:
21.2.1. Existing registries to be modified
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21.2.1.1. Registries for the "Type" Field
21.2.1.1.1. 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
(Sub-Types are defined in the
"Transitive Transport Class Extended Community Sub-Types"
registry)
21.2.1.1.2. Non-Transitive Types
This registry contains values of the high-order octet (the "Type"
field) of a Non-transitive Extended Community.
Registry Group: Border Gateway Protocol (BGP) Extended Communities
Registry Name: BGP Non-Transitive Extended Community Types
Type Value Name
--------------+------------------
0x4a Transport Class
(Sub-Types are defined in the
"Non-Transitive Transport Class Extended Community Sub-Types"
registry)
21.2.2. New registries to be created
21.2.2.1. Transitive Transport Class Extended Community Sub-Types
Registry
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Registry Group: Border Gateway Protocol (BGP) Extended Communities
Registry Name: Transitive Transport Class Extended Community Sub-Types
Note:
This registry contains values of the second octet (the
"Sub-Type" field) of an extended community when the value of the
first octet (the "Type" field) is 0x0a.
Range Registration Procedures
-----------------+----------------------------
0x00-0xBF First Come First Served
0xC0-0xFF IETF Review
Sub-Type Value Name
-----------------+--------------
0x02 Route Target
21.2.2.2. Non-Transitive Transport Class Extended Community Sub-Types
Registry
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" field) of an extended community when the value of the
first octet (the "Type" field) is 0x4a.
Range Registration Procedures
-----------------+----------------------------
0x00-0xBF First Come First Served
0xC0-0xFF IETF Review
Sub-Type Value Name
-----------------+--------------
0x02 Route Target
21.3. MPLS OAM code points
The following two code points are sought for Target FEC Stack sub-
TLVs:
* IPv4 BGP Classful Transport
* IPv6 BGP Classful Transport
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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
21.4. Best Effort Transport Class ID
This document reserves the Transport class ID value 0 to represent
"Best Effort Transport Class ID". This is used in the 'Transport
Class ID' field of Transport Route Target extended community that
represents best effort transport class. Please create a new registry
for this.
Registry Group: BGP CT Parameters
Registry Name: Transport Class ID
Value Name
-----------------+--------------------------------
0 Best Effort Transport Class ID
22. Security Considerations
Mechanisms described in this document carry Transport routes in a new
BGP address family. That minimizes possibility of these routes
leaking outside the expected domain or mixing with service routes.
When redistributing between SAFI 4 and SAFI 76 Classful Transport
routes, 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 4 routes in a separate transport
RIB, distinct from service RIB that contain SAFI 1 service routes.
23. Normative References
[BGP-CT-UPDATE-PACKING-TEST]
Vairavakkalai, Ed., "BGP CT Update packing Test Results",
23 November 2022, <https://raw.githubusercontent.com/ietf-
wg-idr/draft-ietf-idr-bgp-ct/main/update-packing-test-
results.txt>.
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[BGP-LU-EPE]
Gredler, Ed., "Egress Peer Engineering using BGP-LU", 6
July 2021, <https://datatracker.ietf.org/doc/html/draft-
gredler-idr-bgplu-epe-14>.
[FLOWSPEC-REDIR-IP]
Simpson, Ed., "BGP Flow-Spec Redirect to IP Action", 2
February 2015, <https://datatracker.ietf.org/doc/html/
draft-ietf-idr-flowspec-redirect-ip-02>.
[Intent-Routing]
Hegde, Ed., "Intent-aware Routing using Color", 14 July
2022, <https://datatracker.ietf.org/doc/html/draft-hr-
spring-intentaware-routing-using-color-00#section-6.3.2>.
[MPLS-NAMESPACES]
Vairavakkalai, Ed., "BGP signalled MPLS-namespaces", 11
June 2021, <https://datatracker.ietf.org/doc/html/draft-
kaliraj-bess-bgp-sig-private-mpls-labels-05#section-6.1>.
[MULTI-NH-ATTR]
Vairavakkalai, Ed., "BGP MultiNexthop Attribute", 28
December 2021, <https://datatracker.ietf.org/doc/html/
draft-kaliraj-idr-multinexthop-attribute-04#section-
5.5.2.2>.
[PCEP-RSVP-COLOR]
Rajagopalan, Ed., "Path Computation Element Protocol(PCEP)
Extension for RSVP Color", 15 January 2021,
<https://datatracker.ietf.org/doc/html/draft-rajagopalan-
pcep-rsvp-color-00>.
[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>.
[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>.
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[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>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.
[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>.
[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>.
[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>.
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[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>.
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/info/rfc8664>.
[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>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
[RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012,
DOI 10.17487/RFC9012, April 2021,
<https://www.rfc-editor.org/info/rfc9012>.
[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>.
[RTC-Ext] Zhang, Z., Ed., "Route Target Constrain Extension", 12
July 2020, <https://tools.ietf.org/html/draft-zzhang-idr-
bgp-rt-constrains-extension-00#section-2>.
[SRTE] Previdi, S., Ed., "Advertising Segment Routing Policies in
BGP", 18 November 2019, <https://tools.ietf.org/html/
draft-ietf-idr-segment-routing-te-policy-08>.
[SRV6-INTER-DOMAIN]
K A, Ed., "SRv6 inter-domain mapping SIDs", 10 January
2021, <https://datatracker.ietf.org/doc/html/draft-salih-
spring-srv6-inter-domain-sids-00>.
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Appendix A. Applicability to Intra AS and different Inter AS
deployments.
As described in BGP VPN [RFC4364] Section 10, 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 (SAFI 4) or BGP CT (SAFI 76).
Illustration of CT Procedures (Section 19) illustrates how constructs
of BGP CT work in an Inter AS option-C deployment. The BGP CT
constructs: SAFI 76, Transport Class and Resolution Scheme are used
in an option-C deploymnent.
In Intra AS and Inter AS option-A, option-B scenarios, SAFI 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, B deployment scenarios.
A.1. Intra AS usecase
A.1.1. Topology
[RR11]
|
+
[CE21]---[PE11]-------[P1]------[PE12]------[CE31]
| |
+ +
| |
AS2 ...AS1... AS3
10.21.21.21 ---- Traffic Direction ----> 10.31.31.31
Figure 12: BGP CT Intra-AS.
This example shows a provider network Autonomous system AS1. It
serves customers AS2, AS3. Traffic direction being described is CE21
to CE31. CE31 may request a specific SLA (e.g. Gold for this
traffic), when traversing this provider network.
A.1.2. Transport Layer
AS1 uses RSVP-TE intra-domain tunnels between PE11 and PE12. And LDP
tunnels for best effort traffic.
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The network has two Transport classes: Gold with transport class id
100, 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 tunnels exist for Gold transport class.
PE11_to_PE12_gold - RSVP-TE tunnel
PE12_to_PE11_gold - RSVP-TE tunnel
Following tunnels exist for Bronze transport class.
PE11_to_PE12_bronze - RSVP-TE tunnel
PE11_to_PE12_bronze - RSVP-TE tunnel
These tunnels are provisioned to belong to transport class 100 or
200.
A.1.3. Service Layer route exchange
Service nodes PE11, PE12 negotiate service families (SAFI 128) on the
BGP session with RR11. Service helper RR11 reflects service routes
between the two PEs with nexthop 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, PE12.
Routes received from CEs are not present in any other nodes' FIB in
the provider network.
CE31 advertises a route for example prefix 10.31.31.31 with nexthop
self to PE12. CE31 can attach a Mapping Community Color:0:100 on
this route, to indicate its request for Gold SLA. Or, PE11 can
attach the same using locally configured policies.
Consider CE31 is getting VPN service from PE12. The RD:10.31.31.31
route is readvertised in SAFI 128 by PE12 with nexthop self
(10.12.12.12) and label V-L1, to RR11 with the Mapping Community
Color:0:100 attached. This SAFI 128 route reaches PE11 via RR11 with
the nexthop unchanged as PE12 and label V-L1. Now PE11 can resolve
the PNH 10.12.12.12 using PE11_to_PE12_gold RSVP TE LSP.
The IP FIB at PE11 VRF will have a route for 10.31.31.31 with a
nexthop when resolved using Resolution Scheme belonging to the
mapping community Color:0:100, points to a PE11_to_PE12_gold tunnel.
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BGP CT SAFI 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.
A.2. Inter AS option-A usecase
A.2.1. Topology
[RR11] [RR21]
| |
+ +
[CE31]---[PE11]----[P1]----[ASBR11]---[ASBR21]---[P2]---[PE21]----[CE41]
| | |
+ + +
| | |
AS3 ..AS1.. ..AS2.. AS4
10.31.31.31 ---- Traffic Direction ----> 10.41.41.41
Figure 13: BGP CT Inter-AS option A.
This example 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. Gold for this traffic), when traversing these
provider core networks.
A.2.2. Transport Layer
AS1 uses RSVP-TE intra-domain tunnels between PE11 and ASBR11. And
LDP tunnels for best effort traffic. AS2 uses SRTE intra-domain
tunnels between ASBR21 and PE21, and L-ISIS for 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
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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.
A.2.3. Service Layer route exchange
Service nodes PE11, ASBR11 negotiate service familiy (SAFI 128) on
the BGP session with RR11. Service helper RR11 reflects service
routes between the PE11 and ASBR11 with nexthop unchanged.
Similarly, in AS2 PE21, ASBR21 negotiate service family (SAFI 128) on
the BGP session with RR21, which reflects service routes between the
PE21 and ASBR21 with nexthop unchanged .
CE41 advertises a route for example prefix 10.41.41.41 with nexthop
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:10.41.41.41
route is readvertised in SAFI 128 by PE21 with nexthop self
(10.21.21.21) and label V-L1, to RR21 with the Mapping Community
Color:0:100 attached. This SAFI 128 route reaches ASBR21 via RR21
with the nexthop unchanged as PE21 and label V-L1. Now ASBR21 can
resolve the PNH 10.21.21.21 using ASBR21_to_PE21_gold SRTE LSP.
The IP FIB at ASBR21 VRF will have a route for 10.41.41.41 with a
nexthop resolved using Resolution Scheme associated with mapping
community Color:0:100, pointing to ASBR21_to_PE21_gold tunnel.
This route is readvertised by ASBR21 on BGP session inside VRF with
nexthop self. EBGP session peering on interface address. ASBR21
acts like a CE to ASBR11, and the above mentioned process repeats in
AS1, until route reaches PE11 and resolves over PE11_to_ASBR11_gold
RSVP TE tunnel.
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Traffic traverses as IP packet on the following legs: CE31-PE11,
ASBR11-ASBR21, PE21-CE41. And uses MPLS forwarding inside AS1, AS2
core.
BGP CT SAFI 76 is not used in this Inter AS option-A deployment. But
the Transport class and Resolution Scheme constructs are used to
preserve end-to-end SLA.
A.3. Inter AS option-B usecase
A.3.1. Topology
[RR13] [RR23]
| |
+ +
[CE31]---[PE11]----[P1]----[ASBR12]---[ASBR21]---[P2]---[PE22]----[CE41]
| | |
+ + +
| | |
AS3 ..AS1.. ..AS2.. AS4
10.31.31.31 ---- Traffic Direction ----> 10.41.41.41
Figure 14: BGP CT Inter-AS option B.
This example shows two provider network Autonomous systems AS1, AS2.
They serve L3VPN customers AS3, AS4 respectively. The ASBRs ASBR12
and ASBR21 dont have any IP VRFs . The inter AS link is MPLS
forwarding enabled.
Traffic direction being described is CE31 to CE41. CE41 may request
a specific SLA (e.g. Gold for this traffic), when traversing these
provider core networks.
A.3.2. Transport Layer
AS1 uses RSVP-TE intra-domain tunnels between PE11 and ASBR21. And
LDP tunnels for best effort traffic. AS2 uses SRTE intra-domain
tunnels between ASBR21 and PE22, and L-ISIS for 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_ASBR12_gold - RSVP-TE tunnel
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ASBR12_to_PE11_gold - RSVP-TE tunnel
PE22_to_ASBR21_gold - SRTE tunnel
ASBR21_to_PE22_gold - SRTE tunnel
Following tunnels exist for Bronze transport 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.
A.3.3. Service Layer route exchange
Service nodes PE11, ASBR12 negotiate service familiy (SAFI 128) on
the BGP session with RR13. Service helper RR13 reflects service
routes between the PE11 and ASBR12 with nexthop unchanged.
Similarly, in AS2 PE22, ASBR21 negotiate service family (SAFI 128) on
the BGP session with RR23, which reflects service routes between the
PE22 and ASBR21 with nexthop unchanged .
ASBR21 and ASBR12 negotiate SAFI 128 between them, and readvertise
L3VPN routes with nexthop self, allocating new labels. EBGP session
peering on interface address.
CE41 advertises a route for example prefix 10.41.41.41 with nexthop
self to PE22 VRF. CE41 can attach a Mapping Community Color:0:100 on
this route, to indicate its request for Gold SLA. Or, PE22 can
attach the same using locally configured policies.
Consider CE41 is getting VPN service from PE22. The RD:10.41.41.41
route is readvertised in SAFI 128 by PE22 with nexthop self
(10.22.22.22) and label V-L1, to RR23 with the Mapping Community
Color:0:100 attached. This SAFI 128 route reaches ASBR21 via RR23
with the nexthop unchanged as PE22 and label V-L1. Now ASBR21 can
resolve the PNH 10.22.22.22 using ASBR21_to_PE22_gold SRTE LSP.
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Next, ASBR21 readvertises the RD:10.41.41.41 route with nexthop self
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:10.41.41.41 route via RR13 to PE11
with nexthop self 10.12.12.12. PE1 resolves the nexthop 10.12.12.12
over PE11_to_ASBR11_gold RSVP TE tunnel.
Traffic traverses as IP packet on the following legs: CE31-PE11,
PE21-CE41. And uses MPLS forwarding on ASBR11-ASBR21 link, and
inside AS1, AS2 core.
BGP CT SAFI 76 is not used in this Inter AS option-B deployment. But
the Transport class and Resolution Scheme constructs are used to
preserve end-to-end SLA.
Appendix B. Why reuse RFC 8277 and RFC 4364?
RFC 4364 is one of the key design patterns produced by networking
industry. It introduced virtualization and allowed sharing of
resources in service provider space with multiple tenant networks,
providing isolated and secure Layer3 VPN services. This design
pattern has been reused since to provide other service layer
virtualizations like Layer2 virtualization (VPLS, L2VPN, EVPN), ISO
virtualization, ATM virtualization, Flowspec VPN.
It is to be noted that these services have different NLRI encoding.
L3VPN Service family that binds MPLS label to an IP prefix use RFC
8277 encoding, and others define different NLRI encodings.
BGP CT reuses RFC 4364 procedures to slice a transport network into
multiple transport planes that different service routes can bind to,
using Color.
BGP CT reuses RFC 8277 because it precisely fits the purpose. viz. In
a MPLS network, BGP CT needs to bind MPLS label for transport
endpoints which are IPv4 or IPv6 endpoints, and disambiguate between
multiple instances of those endpoints in multiple transport planes.
Hence use of RD:IP_Prefix and carrying a Label for it as specified in
RFC 8277 works well for this purpose.
Another advantage of using the precise encoding as defined in RFC
4364 and RFC 8277 is that it allows to interoperate with BGP speakers
that support SAFI 128. This can be useful during transition, until
all BGP speakers in the network support BGP CT.
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In future, if RFC 8277 evolves into a typed NLRI, that does not carry
Label in the NLRI, BGP CT will be compatible with that as-well. In
essence, BGP CT encoding is compatible with existing deployed
technologies (RFC 4364, RFC 8277) and will adapt to any changes RFC
8277 mechanisms undergo in future.
This is a more pragmatic approach which leverages the benefits of
time tested design patterns proposed in RFC 4364 and RFC 8277.
Moreover, this approach greatly reduces operational training costs
and protocol compatibility considerations as it complements and works
well with existing protocol machineries. This problem does not need
reinventing the wheel with brand new NLRI and procedures.
This is a more pragmatic approach, rather than abandoning time tested
design pattern like RFC 4364 and RFC 8277, just to invent something
completely new that is not backward compatible with existing
deployements. Overloading RFC 8277 NLRI MPLS Label field with
information related to non MPLS dataplane leads to backward
compatibility issues.
B.1. Update packing considerations
BGP CT carries transport class as an attribute. This means routes
that dont share the same transport class cannot be packed into same
Update message. Update packing in BGP CT will be similar to RFC 8277
family routes carrying attributes like communities or extended
communities. Service families like SAFI 128 have considerably more
scale than transport families like SAFI 4 or SAFI 76, which carry
only loopbacks. Update packing mechanisms that scale for SAFI 128
routes will scale similarly for SAFI 76 routes also.
The document Intent-aware Routing using Color [Intent-Routing]
section 6.3.2.1 suggests scaling numbers for 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, IPv6 endpoints with MPLS
label, and IPv6 endpoints with SRv6 SID were tested.
Tests were conducted with 1.9 million BGP CT route scale (387K
endpoints in 5 transport classes). Initial convergence time for all
cases was less than 2 minutes, This experiment proves that carrying
transport class information as an attribute keeps BGP convergence
within acceptable range. Details of the experiment and test results
are available in BGP CT Update packing Test Results
[BGP-CT-UPDATE-PACKING-TEST].
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Further, even in today's BGP LU deployments each egress node
originates BGP LU route for it's loopback, with some attributes like
community identifying the originating node or region, and AIGP
attribute. These attributes may be unique per egress node, thus do
not help with update packing in transport layer family routes.
Appendix C. Scaling using BGP MPLS Namespaces
This section describes how scaling is achieved in an Inter domain
MPLS network, where a domain is an AS or IGP area. Domain boundary
is demarcated by a BN performing BGP nexthop self action on the
transport route.
It considers the scenario suggested in the document Intent-aware
Routing using Color [Intent-Routing] section 6.3.2.1. 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 resources.
This section explains how mechanism described in MPLS Namespaces
[MPLS-NAMESPACES] 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
state is kept confined closer to network edge, and any churn is
confined within the region containing the point of failure, which
improves convergence.
In order to achieve these scaling benefits, new functionality is
required only at a Region's Border Nodes and the Regional RRs. All
other nodes can remain legacy nodes, and still get the scaling and
convergence benefits of this mechanism. This is mainly advantageous
to ingress and egress PE devices which may be low end devices not
capable of pushing deep label stacks or supporting large number of
ECMP nexthops. They can enjoy the scaling benefits without needing
software upgrades.
C.1. Illustration.
Let us consider the decomposition of this example network with 300K
nodes to be such that there are 300 domains containing 1000 nodes
each. The mechanism described here will reduce the forwarding
resource usage in all Border Nodes to become a function of number of
domains (300) instead of number of nodes (300K). Thus drastically
reducing MPLS transit routes from 1.5M to 1500. The Border Nodes and
Regional RRs in a Region do the job of abstracting the 1000 PE
loopbacks from the rest of the network. The rest of the network sees
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this region as 1 BGP nexthop, and not as 1000 BGP nexthops.
C.2. Topology
[RR11] [RR31]
| |
| |
[PE11]\ | /[BN11]--+ +--[BN31]\ | /[PE31]
\ | / \ / \ | /
[CE41]--[PE12]--[P11] [BN21] [P31]--[PE32]--[CE31]
.. / \ / \ / \ ..
.. / \[BN12]--+ +--[BN32]/ \ ..
[PE11000] [PE31000]
| | | |
AS4 | ..Domain1.. | ..Domain2.. | ..Domain3.. | AS3
| | (backbone) | |
<---- Traffic Direction ----
Figure 15: BGP MPLS Namespaces.
This topology shows a cross section of the network with focus on two
domains Domain1 and Domain3 connected via a backbone domain Domain2.
Rest of the domains are not shown for brevity. The border nodes have
forwarding state pertaining to all domains in the network. The
control plane and forwarding plane state in node BN21 can be examined
to determine the MPLS scaling characteristics of the network.
L3VPN Service routes are present only at ingress and egress PEs.
L3VPN family (SAFI 128) is negotiated between PE11..PE11000 and
regional route reflector RR11. RR11 has multihop EBGP peering with
RR31 and negotiates SAFI 128. RR31 further peers with all PEs
PE31..PE31000 in Domain3.
At the Transport layer - in Domain1, PE11..PE11000 negotiate BGP
families (SAFI 4, SAFI 76) with BN11, BN12. In Domain2, BN11 and
BN12 similarly negotiate the transport families with BN21, which in
turn peers with BN31 and BN32. In Domain3, BN31 and BN32 peer with
PEs PE31..PE31000. Each of these BNs change BGP nexthop to self,
when re advertising the SAFI 4, SAFI 76 transport routes.
When all nodes loopback addresses are visible through out the
network, it will result in 1.5M transport layer routes and MPLS
transit routes in BN21.
Following sections describe the control plane and forwarding plane
mechanics to reduce this to 1500 routes, when MPLS Namespaces is
deployed in this network.
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Traffic direction being described is CE41 to CE31. Reverse direction
would work in similar way.
Traffic direction being described is CE41 to CE31. Reverse direction
would work in similar way.
C.3. Context Protocol Nexthop Address (CPNH)
A MPLS Namespace is identified by a Context PNH address. In MPLS
forwarding, labels are locally significant to the node advertising
it. E.g. labels in default/global MPLS Namespace are scoped by the
node's loopback address. The labels belonging to a MPLS Namespace
are locally significant in scope of the Context PNH address.
A UHP label called as "Context Label" is advertised for the CPNH in a
transport protocol, which points to the MPLS Namespace forwarding
context. When Context label is received as outer label in a MPLS
packet, it is Popped, and lookup is performed for the MPLS label that
appears in the MPLS Namespace identified by the CPNH.
In this example, CPNH is an anycast IP address that represents set of
PEs in a domain. E.g. CPNH1 represent all PEs in Domain1. And
CPNH3 represents all PEs in Domain3.
C.4. Service Forwarding Helper, and changes to transport layer.
The border nodes BN11, BN12 maintain the forwarding context for MPLS
Namespace identified by CPNH1. They advertise CPNH1 in transport
layer routes like SAFI-4 or SAFI-76 with a UHP Context Label CL1.
Any transport layer protocol may be used to advertise the UHP Context
Label for the CPNH.
In this way, BN11 and BN12 serve as Service Forwarding Helpers for
CPNH1 MPLS Namespace. They attract traffic that remote devices send
towards the BGP nexthop CPNH1, and forward the MPLS packets received
with the MPLS labels belonging to the MPLS Namespace identified by
CPNH1.
The individual loopback addresses of the PEs need not be advertised
outside the local region. E.g. PE11..PE11000 are not advertised
beyond BN11, BN12. Only CPNH1 and RR11 addresses are advertised out.
RR1 is used for the control plane peering. and CPNH1 is used as an
forwarding anchor point.
Similarly, Domain3 advertises only RR31 and CPNH3 to Domain2. This
significantly reduces the transport route scale and MPLS forwarding
resource usage at the border nodes throughout the network.
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C.5. BGP MPLS Namespace Address family (AFI:16399, SAFI:128)
In Domain1, the regional route reflector RR11 negotiates MPLS
Namespace Signaling address family with the border nodes BN11, BN12.
RR11 is an external label allocator for the MPLS Namespace identified
by CPNH1. RR1 advertises in the MPLS Namespace address family, the
labels it allocated in scope of CPNH1. These routes are advertised
with a route target that identifies CPNH1. BN11 and BN12 use this
route target to import the label route into the forwarding context
associated with CPNH1.
Similarly, in Domain3, RR31 negotiates MPLS Namespace Signaling
address family with the border nodes BN31, BN32.
C.6. Changes to Service Layer route exchange
When RR11 re-advertises to RR31 a VPN route RD:Pfx1 received with
label VL1 from egress PE11 in Domain1, it sets BGP nexthop to CPNH1,
and advertises a new label PL1. This label PL1 is allocated within
the scope of CPNH1 namespace.
The label PL1 is advertised to BN1, BN2 in MPLS Namespace address
family with a route target identifying CPNH1, and BGP nexthop PE11
and label VL1 that were received from the egress PE. BN1 and BN2
resolve the path to that BGP nexthop PE11 and use as nexthop for the
PL1 route installed in CPNH1 forwarding context.
The remote PEs in Domain3 consume the BGP updates from Domain1
following regular procedures for SAFI 128. When resolving the BGP
nexthop CPNH1, they will push the context label that lands the
traffic into the correct forwarding context in one of the border
nodes.
C.7. Analysis of forwarding behavior
The forwarding behavior thus achieved is similar to Inter AS option-
b, without carrying any service routes at the border nodes. Further,
the MPLS namespace labels are installed in all the border nodes,
which allows for quicker traffic convergence in case of border node
failure. The number of border nodes can be increased in a scale out
manner, which gives a cookie cutter template to scale a network
region.
In conclusion, this mechanism provides both scaling and convergence
benefits for the MPLS network, and allows to support huge scale
networks.
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Appendix D. BGP CT deployment in SRv6 networks
This section describes BGP CT deployment in SRv6 multi-domain network
using Inter-AS Option C architecture.
AS1 AS2
CE1---[PE1-----ASBR1]-----[ASBR2-----PE2]---CE2
-------Forwarding Direction----->
Figure 16: BGP CT in SRv6 Only Dataplane
In the above topology there are two ASs, AS1 and AS2
Intra-AS Nodes in AS1 and AS2 speak IBGP-CT and ISIS-SRv6 between
them. The Inter-AS Nodes speak EBGP-CT between them. Transport
Classes Gold (100) and Bronze (200) are provisioned in SNs and BNs.
All BGP CT advertisements in this example will carry a MPLS label
value of 3 (Implicit Null).
The following SRv6 locators are provisioned
PE1-SRv6 - SRv6 Locator for PE1
PE2-SRv6 - SRv6 Locator for PE2
ASBR1-SRv6 - SRv6 Locator for ASBR1
ASBR2-SRv6 - SRv6 Locator for ASBR2
The following SRv6 End SIDs are provisioned
PE2-SRv6-gold - PE2 End SID for gold transport class
PE2-SRv6-bronze - PE2 End SID for bronze transport class
ASBR1-SRv6-gold - ASBR1 End SID for gold transport class
ASBR1-SRv6-bronze - ASBR1 End SID for bronze transport class
The above provisioned SRv6 End SIDs will be advertised via ISIS
between Intra-AS nodes and the established SRv6 tunnels will be
installed into the correct TRDB based on color. The illustrations
that follow, show how the import processing and BGP CT advertisements
are made. Firstly, PE2 advertises its SRv6 End Points to ASBR2 via
IBGP-CT as shown below.
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IBGP-CT: PE2--->ASBR2
RD1:PE2-SRv6-gold,
transport-target:0:100,
Prefix-SID: PE2-SRv6-gold,
NH: PE2-LPBK
RD2:PE2-SRv6-bronze,
transport-target:0:200,
Prefix-SID: PE2-SRv6-bronze,
NH: PE2-LPBK
Figure 17: BGP CT advertisements from PE2 to ASBR2
When ASBR2 receives the IBGP CT advertisement from PE2, it performs
import processing and nexthop resolution for the SRv6 End Point
PE2-SRv6-gold in the gold TRDB based on its transport target. On
successful resolution, a route for PE2-SRv6-gold is installed in the
global IPv6 FIB for enabling SRv6 forwarding. The BGP-CT route for
RD1:PE2-SRv6-gold is further advertised towards ASBR1 via EBGP CT as
shown below.
ASBR2: IPv6 FIB for SRv6
[BGP-CT]PE2-SRv6-gold,
NH: Gold SRv6-Tunnel-to-PE2
[BGP-CT]PE2-SRv6-bronze,
NH: Bronze SRv6-Tunnel-to-PE2
EBGP-CT: ASBR2--->ASBR1
RD1:PE2-SRv6-gold,
transport-target:0:100,
Prefix-SID: PE2-SRv6-gold,
NH: ASBR2_InterAS_Link
RD2:PE2-SRv6-bronze,
transport-target:0:200,
Prefix-SID: PE2-SRv6-bronze,
NH: ASBR2_InterAS_Link
Figure 18: BGP CT processing at ASBR2
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When ASBR1 receives the EBGP CT advertisement from ASBR2, a route for
PE2-SRv6-gold is installed with a nexthop of ASBR1_InterAS_Link in
the global IPv6 FIB for enabling SRv6 forwarding. The BGP-CT route
for RD1:PE2-SRv6-gold is further advertised with SID ASBR1-SRv6-gold
towards PE1 via IBGP CT as shown below.
ASBR1: IPv6 FIB for SRv6
[BGP-CT]PE2-SRv6-gold,
NH: ASBR2_InterAS_Link
[BGP-CT]PE2-SRv6-bronze,
NH: ASBR2_InterAS_Link
IBGP-CT: ASBR1--->PE1
RD1:PE2-SRv6-gold,
transport-target:0:100,
Prefix-SID: ASBR1-SRv6-gold,
NH: ASBR1-LPBK
RD2:PE2-SRv6-bronze,
transport-target:0:200,
Prefix-SID: ASBR1-SRv6-bronze,
NH: ASBR1-LPBK
Figure 19: BGP CT processing at ASBR1
When PE1 receives the IBGP CT advertisement from ASBR1, it resolves
the SID ASBR1-SRv6-gold in its corresponding TRDB over the SRv6
underlay tunnel installed by SRv6-ISIS. Furthermore, any service
routes requiring intent based resolution will use the Resolution
Scheme associated with its Mapping Community to resolve over the
PE2-SRv6-gold CT route installed in the gold TRDB.
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PE1: IPv6 FIB for SRv6
[BGP-INET] SVC_PFX1, color:0:100
NH: PE2-SRv6-gold, Gold SRv6-Tunnel-to-ASBR1
[BGP-INET] SVC_PFX2, color:0:200
NH: PE2-SRv6-bronze, Bronze SRv6-Tunnel-to-ASBR1
[BGP-CT]PE2-SRv6-gold,
NH: Gold SRv6-Tunnel-to-ASBR1
[BGP-CT]PE2-SRv6-bronze,
NH: Bronze SRv6-Tunnel-to-ASBR1
Figure 20: BGP CT processing at PE1
Similar processing is followed for the bronze transport plane as
well.
Contributors
Co-Authors
Israel Means
AT&T
2212 Avenida Mara,
Chula Vista, California 91914
United States of America
Email: israel.means@att.com
Csaba Mate
KIFU, Hungarian NREN
Budapest
35 Vaci street,
1134
Hungary
Email: ietf@nop.hu
Deepak J Gowda
Extreme Networks
55 Commerce Valley Drive West, Suite 300,
Thornhill, Toronto, Ontario L3T 7V9
Canada
Email: dgowda@extremenetworks.com
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Other Contributors
Balaji Rajagopalan
Juniper Networks, Inc.
Electra, Exora Business Park~Marathahalli - Sarjapur Outer Ring Road,
Bangalore 560103
KA
India
Email: balajir@juniper.net
Reshma Das
Juniper Networks, Inc.
1133 Innovation Way,
Sunnyvale, CA 94089
United States of America
Email: dreshma@juniper.net
Rajesh M
Juniper Networks, Inc.
Electra, Exora Business Park~Marathahalli - Sarjapur Outer Ring Road,
Bangalore 560103
KA
India
Email: mrajesh@juniper.net
Chaitanya Yadlapalli
AT&T
200 S Laurel Ave,
Middletown,, NJ 07748
United States of America
Email: cy098d@att.com
Gyan Mishra
Verizon Inc.
13101 Columbia Pike
Silver Spring, MD 20904
United States of America
Email: gyan.s.mishra@verizon.com
Mazen Khaddam
Cox Communications Inc.
Atlanta, GA
United States of America
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Email: mazen.khaddam@cox.com
Rafal Jan Szarecki
Google.
1160 N Mathilda Ave, Bldg 5,
Sunnyvale,, CA 94089
United States of America
Email: szarecki@google.com
Xiaohu Xu
Capitalonline.
Beijing
China
Email: xiaohu.xu@capitalonline.net
Acknowledgements
The authors thank Jeff Haas, John Scudder, Susan Hares, Moses
Nagarajah, Navaneetha Krishnan, Ravi M R, Chandrasekar Ramachandran,
Shradha Hegde, Richard Roberts, Krzysztof Szarkowicz, John E Drake,
Srihari Sangli, Vijay Kestur, Santosh Kolenchery, Robert Raszuk,
Ahmed Darwish, Aravind Srinivas Srinivasa Prabhakar, Moshiko Nayman,
Chris Trip for the valuable discussions 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.
Authors' Addresses
Kaliraj Vairavakkalai (editor)
Juniper Networks, Inc.
1133 Innovation Way,
Sunnyvale, CA 94089
United States of America
Email: kaliraj@juniper.net
Natrajan Venkataraman (editor)
Juniper Networks, Inc.
1133 Innovation Way,
Sunnyvale, CA 94089
United States of America
Email: natv@juniper.net
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