| Internet-Draft | BGP Classful Transport Planes | June 2023 |
| Vairavakkalai & Venkataraman | Expires 22 December 2023 | [Page] |
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 a 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 administered 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 endpoints as well as Inter-AS option-A, option-B, option-C variations. Examples of these variations are given in Appendix A.¶
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.¶
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 22 December 2023.¶
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 extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
The mechanisms defined in this document enable brownfield networks deployed using existing technologies like RSVP-TE and greenfield networks that use technologies like SPRING to 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 endpoints. Multiple tunnels may exist between the same pair of endpoints.¶
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 heterogeneous; 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-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 including 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:¶
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 de facto 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.¶
This document realizes "Intent" as defined in Intent-based Networking: Concepts and Definitions [RFC9315] and prescribes procedures that use the transport class as a construct to express intent. The procedures defined in this document provide homogenous building blocks to achieve Intent-based Networking.¶
The document Intent-aware Routing using Color [Intent-Routing-Color] describes various use cases and applications of procedures described in this document.¶
LSP: Label Switched Path.¶
TE : Traffic Engineering.¶
TC: Transport Class.¶
SN : Service Node. A router that sends or receives BGP Service routes (e.g. SAFIs 1, 128) with self as nexthop.¶
eSN : Egress Service Node. A router that sends BGP Service routes (e.g. SAFIs 1, 128) with self as nexthop.¶
iSN : Ingress Service Node. A router that receives BGP Service routes (e.g. SAFIs 1, 128).¶
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.¶
BGP LU: BGP Labeled Unicast family (SAFI 4)¶
BGP CT: BGP Classful Transport family (SAFI 76)¶
ASN: Autonomous System Number.¶
RT : Route-Target extended community.¶
RD : Route-Distinguisher.¶
RTC : Route Target Constrain.¶
VRF: Virtual Router Forwarding Table.¶
CsC: Carrier serving Carrier VPN.¶
PNH : Protocol-Nexthop address carried in a BGP Update message.¶
MNH : BGP MultiNexthop attribute.¶
FEC : Forwarding Equivalence Class.¶
RSVP-TE : Resource Reservation Protocol - Traffic Engineering.¶
SR : Segment Routing.¶
SRTE : Segment Routing Traffic Engineering.¶
SID : SR Segment Identifier.¶
EP : Endpoint, a loopback address in the network.¶
SEP : Service Endpoint, 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.¶
Intent: A set of operational goals (that a network should meet) and outcomes (that a network is supposed to deliver) defined in a declarative manner without specifying how to achieve or implement them.¶
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).¶
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 (TRDB): 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 consisting of transport tunnels belonging to the same Transport Class. Tunnels of the 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.¶
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 including 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 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.¶
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:¶
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.¶
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 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.¶
This section defines the Nexthop Resolution Scheme construct that is used to specify how a service route or a BGP CT route can resolve its nexthop using its associated Mapping Community over a specific TRDB or an ordered set of TRDBs.¶
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 [RFC9012], or the transport-target:0:100 described in section 4 in this document.¶
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.¶
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].¶
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:
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 Distinguisher:
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 Nexthop 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 nexthop with an 8-octet RD set to zero).¶
When the length of the Nexthop Address field is 12 the nexthop address is of type VPN-IPv4 with 8-octet RD set to zero.¶
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.¶
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].¶
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 absence 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 a unique RD per originating node for an EP. This mode allows for the ingress to uniquely identify each originated path. Alternatively, the same RD may be provisioned for multiple originators of the same EP. This mode can be used when the ingress does not require full visibility of all nodes originating an EP.¶
A label is allocated for a BGP CT route when it is advertised with 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.¶
The impacts on control plane and forwarding behavior for the above modes are detailed with an example in Managing Transport Route Visibility (Section 20.3)¶
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 contextualized 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 contextualized 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.¶
This section summarizes the procedures followed by various nodes speaking Classful Transport family.¶
Some deployment considerations can also help in avoiding this problem:¶
This avoids the possibility of such loops altogether.¶
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 MAY 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.¶
Mechanisms described in BGP LU EPE [BGP-LU-EPE] also applies to BGP CT family.¶
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.¶
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:¶
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 in turn is more specific than Mapping Community (Transport Target) on a BGP CT transport route, which is in turn more specific than a Service route scoped Mapping Community (Color Extended community).¶
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.¶
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 ---->
One such indication can be in form of DSCP code point (RFC2474 [RFC2474]) in IP header.¶
In RFC2474, a Forwarding Class Selector maps to a PHB (Per-hop Behavior). The Transport class construct is a PHB at transport layer.¶
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, 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 device roles 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 the above example, the MNH contains two Nexthop Legs, described by two Forwarding Instruction TLVs. Each Nexthop Leg contains PE1's peering self address in Endpoint Identifier TLV [MNH-EP] , the color Gold or Bronze encoded in the Transport class ID TLV [MNH-TC] , and associated DSCP code point indicating Gold or Bronze transport class encoded in the Payload Encapsulation Info TLV [MNH-ENCAP-DSCP] . 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 in the provided network.¶
If the PE-CE link is MPLS enabled, a distinct MPLS label can also be used to express Intent in data packets from CE. Enabling MPLS forwarding on PE-CE links comes with some security implications. This section gives details on these aspects.¶
Consider the ingress PE1 receiving a VPN prefix RD:Pfx1 received with VPN label VL1, nexthop as PE2 and a mapping community containing TC1 as 'Transport class ID'. PE1 can allocate a MPLS Label PVL1 for the tuple "VPN Label, PNH Address, Transport class ID" and advertise to CE1.¶
Label PVL1 may identifies a service function at any node in the network, e.g. a Firewall device or egress node PE2. And, for the same service prefix, a distinct label may be advertised to different CEs, such that incoming traffic from different CEs to the same service prefix can be diverted to a distinct devices in the network for further processing. This provides Ingress Peer Engineering control to the network.¶
PE1 installs a MPLS FIB route for PVL1 with nexthop as "Swap VL1, Push TL1 towards PE2". TL1 is the BGP CT label received for the tuple 'PE2, TC1'. In forwarding, when MPLS packet with label PVL1 is received from CE1, PVL1 Swaps to label VL1 and pushes the BGP CT label TL1. PE1 advertises the label "PVL1" in the MULTI_NH_ATTR to CE1. PE1 forwards based on MPLS label without performing any IP lookup. This allows for PE1 to be a low IP FIB device and still support CBF by using MPLS Label inferred PHB. The number of MPLS Labels consumed at PE1 for this approach will be proportional to the number of Service functions and Intents that are exposed to CE1.¶
A BGP MultiNexthop Attribute [MULTI-NH-ATTR] is attached to a SAFI 1 service route to convey the MPLS Label information to CE1. In the above example, the MNH contains two Nexthop Legs, described by two Forwarding Instruction TLVs. Each Nexthop Leg contains PE1's peering self address in Endpoint Identifier TLV [MNH-EP] , the color Gold or Bronze encoded in the Transport class ID TLV [MNH-TC] , and associated MPLS Label "PVL1" or "PVL2" encoded in the Payload Encapsulation Info TLV [MNH-ENCAP-MPLS] . 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.¶
The MPLS enabled PE-CE link is considered connecting to an untrusted domain. Such interfaces can be secured against MPLS label spoofing by a walled garden approach using "MPLS context tables".¶
The PE1-CE1 interface can be confined to a specific MPLS context table "A" corresponding to the BGP peer. Such that only the routes for labels advertised to CE1 are installed in MPLS context table "A".¶
This ensures that if CE1 sends MPLS packet with a label that was not advertised to the CE1, the packet will be dropped.¶
Further, the routes for labels PVL1, PVL2 installed in MPLS context table "A" can match on 'Bottom of stack' bit being 'one', ensuring a MPLS packet is accepted from CE1 only if it has no more than one label in the label stack.¶
However, the PE itself may not be able to perform any checks based on inner payload in the MPLS packet since it performs label swap forwarding. Such inner payload based checks may be offloaded to a downstream node that forwards and processes inner payload, e.g. a IP FIB router. These security aspects should be considered when using MPLS enabled CE devices.¶
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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:¶
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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, the Network Slice Controller can use the Mapping Community on the service route to map traffic to the desired Transport Slice.¶
This section describes how BGP CT family (SAFI 76) 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 routes with SRv6 data plane.¶
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 .¶
[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
This example shows a provider network that consists 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.¶
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.¶
Following tunnels exist for Bronze transport class.¶
These tunnels are either provisioned or auto-discovered to belong to transport class 100 or 200.¶
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.¶
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.¶
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 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 an 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.¶
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 address 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 address 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 visibility 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.¶
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.¶
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 yields the service route that pushes the VPN service label V-L1, BGP CT label B-L5, and labels for PE25_to_ABR23_gold tunnel. Thus, PE25 encapsulates the IP packet in 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.¶
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 the 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.¶
This section describes how the data plane at ASBR22 reacts when the 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).¶
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.¶
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 demonstrate the various levels of failsafe mechanisms available to protect traffic in a BGP CT network.¶
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 guaranteed. 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 Heterogeneous 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.¶
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 independently 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 community value representing a global service layer intent is mapped to a local transport layer intent available in each domain.¶
In this manner, it is recommended to keep color namespace management at the service layer and the transport layer decoupled from each other. In the following sections the service layer agrees on a single global namespace.¶
Non-agreeing color domains require a mapping community rewrite on each domain boundary. 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 -------->
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 sufficiently similar 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 to a 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 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.¶
In a heterogeneous 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.¶
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 -------->
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.¶
This problem can be solved by two approaches, described below.¶
In this approach, duplicate tunnels that satisfy Gold SLA are configured in domains AS1 and AS3, but they are given fine grained colors 101, 102.¶
These tunnels will be installed in TRDBs corresponding to transport classes of color 101, 102.¶
Service routes received with mapping community (eg: transport-target or color community) can resolve over these tunnels in the TRDB with matching color by using resolution schemes.¶
This approach consumes more resources in the transport and forwarding layer, because of the duplicate tunnels.¶
In this approach, resolution schemes in domains AS1, AS3 are customized to map the received mapping community (eg: transport-target or color community) over available Gold SLA tunnels. This conserves resource usage with no additional state in transport and forwarding plane.¶
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.¶
Similarly, at ASBR21, the BGP CT routes received with transport-target:0:101 will be added to color 101 TRDB and strictly resolve over tunnel routes in the same TRDB. This route is re-advertised to ASBR11 with transport-target:0:101.¶
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 in turn 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.¶
This section explains how end-to-end SLA can be achieved while transiting a domain that does not support BGP CT (SAFI 76). BGP LU (SAFI 4) is used in such domains to connect the BGP CT islands.¶
+----------EBGP Multihop CT-------------+
| |
AS3 | AS2 | AS1
[PE31-----ASBR31]--------[ASBR22---ASBR21]-------[ASBR11---PE11]
<-EBGP LU-> <-EBGP LU->
<---IBGP CT---> <---IBGP LU---> <---IBGP CT--->
---------Packet Forwarding Direction--------->
In the above topology, there are three AS domains. AS1 and AS3 supports BGP CT. AS2 is a domain that does not support BGP CT.¶
Nodes in AS1, AS2 and AS3 negotiate BGP LU family on IBGP sessions within the domain. Nodes in AS1 and AS3 negotiate BGP CT family on IBGP sessions within the domain. ASBR11 and ASBR21 as well as ASBR22 and ASBR31 negotiate BGP LU family on the EBGP session over directly connected interdomain links. ASBR11 and ASBR31 have reachability to each other’s loopbacks through BGP LU. ASBR11 and ASBR31 negotiate BGP CT family over a multihop EBGP session formed using BGP LU reachability.¶
The following tunnels exist for Gold transport class¶
Following tunnels exist for Bronze transport class¶
These tunnels are provisioned to belong to transport class gold and bronze, and are advertised between ASBR31 and ASBR11 with Nexthop self.¶
In AS2, that does not support BGP CT, a separate loopback may be used on ASBR22 and ASBR21 to represent gold and bronze SLAs viz ASBR22_lpbk_gold, ASBR22_lpbk_bronze, ASBR21_lpbk_gold and ASBR21_lpbk_bronze.¶
Further, the following tunnels exist in AS2 to satisfy the different SLAs, using per SLA loopback endpoints:¶
RD:PE11 BGP CT route is originated from PE11 towards ASBR11 with transport-target gold. ASBR11 readvertises this route with nexthop set to ASBR11_lpbk_gold on the EBGP multihop session towards ASBR31. ASBR11 originates BGP LU route for endpoint ASBR11_lpbk_gold on EBGP session to ASBR21 with a 'gold SLA' community, and BGP LU route for ASBR11_lpbk_bronze with a 'bronze SLA' community. The SLA community is used by ASBR31 to publish the BGP LU routes in the corresponding BGP CT TRDBs.¶
ASBR21 readvertises the BGP LU route for endpoint ASBR11_lpbk_gold to ASBR22 with nexthop set by local policy config to the unique loopback ASBR21_lpbk_gold by matching the 'gold SLA' community received as part BGP LU advertisement from ASBR11. ASBR22 receives this route and resolves the nexthop over the ASBR22_to_ASBR21_lpbk_gold RSVP-TE tunnel. On successful resolution, ASBR22 readvertises this BGP LU route to ASBR31 with nexthop self and a new label.¶
ASBR31 adds the ASBR11_lpbk_gold route received via EBGP LU from ASBR22 to gold TRDB based on the received 'gold SLA' community. ASBR31 uses this gold TRDB route to resolve the nexthop ASBR11_lpbk_gold received on BGP CT route with transport-target gold, for the prefix RD:PE11 received over the EBGP multihop CT session, thus preserving the end-to-end SLA. Now ASBR31 readvertises the BGP CT route for RD:PE11 with nexthop as self thus stitching with the BGP LU LSP in AS2. 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 because SLA specific tunnels and loopbacks dont exist in AS2, traffic can be carried over available SLAs (eg: best-effort SLA) by rewriting the nexthop to ASBR21 loopback assigned to the available SLA. This eases migration in case of heterogeneous color domains as-well.¶
This section describes how nodes supporting dissimilar encapsulation technologies can interoperate with each other when using BGP CT family.¶
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 [RFC9252].¶
RR1--+
\ +-------R2 [MPLS + SRv6]
\ |
R1--------P-------R3 [MPLS only]
[MPLS + SRv6] |
+-------R4 [SRv6 only]
<---- Bidirectional Traffic ---->
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 [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. 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 [RFC9252]. 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. 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 don't support a common forwarding technology. The BGP CT routes received at R3, R4 from each other will remain unusable, due to incompatible forwarding technology.¶
This section describes how nodes supporting MPLS forwarding can interoperate with other nodes supporting UDP (or IP) tunneling, when using BGP CT family.¶
MPLS Labels are carried using RFC 8277 encoding, and UDP (or IP) tunneling information is carried using TEA attribute or the Encapsulation Extended Community as specified in RFC 9012 [RFC9012].¶
RR1--+
\ +-------R2 [MPLS + UDP]
\ |
R1--------P-------R3 [MPLS only]
[MPLS + UDP] |
+-------R4 [UDP only]
<---- Bidirectional Traffic ---->
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. As specified in RFC 9012 [RFC9012], UDP tunneling information is carried using TEA attribute (code 23) or the "barebones" Tunnel TLV carried in Encapsulation Extended Community. 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 don't support a common forwarding technology. The BGP CT routes received at R3, R4 from each other will remain unusable, due to incompatible forwarding technology.¶
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.¶
|-----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------|
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 |
+--------+------+-------+-------+---------+---------+
In the above example, both route churn and TE granularity are directly proportional to the number of CT labels received.¶
The 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.¶
This document makes the following requests of IANA.¶
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:¶
Please assign a new Format (Type high = 0xa) of extended community EXT-COMM [RFC4360] called "Transport Class" from the following registries:¶
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:¶
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)¶
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)
¶
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
¶
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
¶
The following two code points are sought for Target FEC Stack sub-TLVs:¶
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
¶
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¶
Mechanisms described in this document carry Transport routes in a new BGP address family. That minimizes the 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.¶
In scenarios where MPLS is enabled on link to a device in an untrusted domain, e.g. a PE-CE link or ASBR-ASBR inter-AS link, security can be provided against MPLS label spoofing by using MPLS context tables as described in MPLS enabled CE (Section 14.2). Such that only MPLS traffic with labels advertised to the BGP speaker are allowed to forward. However, the PE may not be able to perform any checks based on inner payload in the MPLS packet since it performs label swap forwarding. Such inner payload based checks may be offloaded to a downstream node that forwards and processes inner payload, e.g. a IP FIB router. These security aspects should be considered when using MPLS enabled CE devices¶
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 deployment.¶
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.¶
[RR11]
|
+
[CE21]---[PE11]-------[P1]------[PE12]------[CE31]
| |
+ +
| |
AS2 ...AS1... AS3
10.21.21.21 ---- Traffic Direction ----> 10.31.31.31
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.¶
AS1 uses RSVP-TE intra-domain tunnels between PE11 and PE12. And LDP tunnels for best effort traffic.¶
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.¶
Following tunnels exist for Bronze transport class.¶
These tunnels are provisioned to belong to transport class 100 or 200.¶
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.¶
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.¶
[RR11] [RR21]
| |
+ +
[CE31]---[PE11]----[P1]----[ASBR11]---[ASBR21]---[P2]---[PE21]----[CE41]
| | |
+ + +
| | |
AS3 ..AS1.. ..AS2.. AS4
10.31.31.31 ---- Traffic Direction ----> 10.41.41.41
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.¶
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.¶
Following tunnels exist for Bronze transport class.¶
These tunnels are provisioned to belong to transport class 100 or 200.¶
Service nodes PE11, ASBR11 negotiate service family (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 the route reaches PE11 and resolves over PE11_to_ASBR11_gold RSVP TE tunnel.¶
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.¶
[RR13] [RR23]
| |
+ +
[CE31]---[PE11]----[P1]----[ASBR12]---[ASBR21]---[P2]---[PE22]----[CE41]
| | |
+ + +
| | |
AS3 ..AS1.. ..AS2.. AS4
10.31.31.31 ---- Traffic Direction ----> 10.41.41.41
This example shows two provider network Autonomous systems AS1, AS2. They serve L3VPN customers AS3, AS4 respectively. The ASBRs ASBR12 and ASBR21 don't 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.¶
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.¶
Following tunnels exist for Bronze transport class.¶
These tunnels are provisioned to belong to transport class 100 or 200.¶
Service nodes PE11, ASBR12 negotiate service family (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.¶
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.¶
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.¶
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 deployments. Overloading RFC 8277 NLRI MPLS Label field with information related to non MPLS data plane leads to backward compatibility issues.¶
BGP CT carries transport class as an attribute. This means routes that don't 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-Color] 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 and 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].¶
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.¶
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-Color] 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.¶
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 this region as 1 BGP nexthop, and not as 1000 BGP nexthops.¶
[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 ----
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 throughout 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.¶
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.¶
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.¶
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 a 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.¶
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.¶
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.¶
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.¶
This section describes BGP CT deployment in SRv6 multi-domain network using Inter-AS Option C architecture.¶
This approach uses stacking of service SRv6 SID over transport SRv6 SID. Transport layer SIDs of types End, End.B6.Encaps defined in [RFC8986], and type END.REPLACE* defined in [SRV6-INTER-DOMAIN] are carried in SAFI 76. Service SID is carried in a service family like SAFI 1 or SAFI 128.¶
In this approach, the number of Service SIDs required at the egress SN is equal to service functions (e.g. Prefix, VRF or Nexthop) and the number of Transport SIDs are equal to the number of transport classes.¶
AS1 AS2
---gold---> ----gold-->
CE1---[PE1---P---ASBR1]-----[ASBR2---P---PE2]---CE2
--bronze--> --bronze-->
-------Forwarding Direction----->
In the above topology, there are two AS domains, AS1 and AS2. These are pure IPv6 domains, with no MPLS enabled. Inter-AS links between AS1 and AS2 are also enabled with IPv6 forwarding.¶
Intra-AS nodes in AS1 and AS2 speak IBGP CT (AFI: 2, SAFI: 76) and ISIS-SRv6 between them. The Inter-AS nodes ASBR1, ASBR2 speak EBGP CT (AFI: 2, SAFI:76) between them. Transport Classes Gold (100) and Bronze (200) are provisioned in all PEs and ASBRs. All BGP CT advertisements in this example carry a MPLS label value of 3 (Implicit Null) in the NLRI encoding.¶
Reachability between PE1 and PE2 is formed using BGP CT family. Service families like IPv4 unicast (AFI: 1, SAFI: 1) and L3VPN (AFI: 2, SAFI: 128) is negotiated on multihop EBGP session between PE1 and PE2. These service routes carry service SID to identify service functions at the advertising PE, and mapping community to identify the desired Intent.¶
The following SRv6 locators are provisioned:¶
The following transport layer SRv6 End SIDs are provisioned or dynamically allocated on demand:¶
Architecturally, the forwarding semantic of End.REPLACE SID operation is similar to Label SWAP operation in MPLS data plane. When a route received with End SID (e.g. PE2-SRv6-gold or PE2-SRv6-bronze transport SIDs) is readvertised with nexthop self, a IPv6 forwarding entry is emitted with a forwarding semantic of End.B6.Encaps operation, which means: Update IPv6 DA with Next Segment in SRH, and Encapsulate SRv6 SID corresponding to the correct transport class. This can be seen in IPv6 FIB of ASBR2 during "BGP CT processing at ASBR2" in the following illustration:¶
The following service layer SRv6 End.DT4 SIDs are provisioned:¶
The locators for above provisioned SRv6 SIDs will be advertised via ISIS between Intra-AS nodes and the established SRv6 tunnel to the node's loopback will be installed into the corresponding TRDB based on color.¶
The SRv6 tunnel ingress routes are published in the Gold and Bronze TRDBs at ASBR2 as shown below:¶
Gold TRDB routes at ASBR2
[ISIS SRv6] PE2-LPBK
NH: Encap "Gold-SRv6-Tunnel-to-PE2" tunnel
[ISIS SRv6] PE2-SRv6-gold
NH: Encap "Gold-SRv6-Tunnel-to-PE2" tunnel
Bronze TRDB routes at ASBR2
[ISIS SRv6] PE2-LPBK
NH: Encap "Bronze-SRv6-Tunnel-to-PE2" tunnel
[ISIS SRv6] PE2-SRv6-bronze:
NH: Encap "Bronze-SRv6-Tunnel-to-PE2" tunnel
ASBR2: IPv6 FIB for SRv6
[ISIS SRv6] PE2-SRv6-gold,
NH: Encap "Gold-SRv6-Tunnel-to-PE2"
[ISIS SRv6] PE2-SRv6-bronze,
NH: Encap "Bronze-SRv6-Tunnel-to-PE2"
The illustrations that follow, show how the BGP CT route for gold transport plane is originated, import processing done and propagated through this network. Similar processing is followed for the bronze transport plane route as well.¶
Firstly, PE2 originates BGP CT route for its transport layer endpoints like Loopback address with SRv6 SID information to ASBR2 as shown below.¶
IBGP CT routes from PE2 to ASBR2
RD1:PE2-LPBK,
transport-target:0:100,
Prefix-SID: PE2-SRv6-gold
NH: PE2-LPBK
RD2:PE2-LPBK,
transport-target:0:200,
Prefix-SID: PE2-SRv6-bronze
NH: PE2-LPBK
PE2: IPv6 FIB for SRv6
[BGP CT] PE2-SRv6-S1-DT4
NH: Decap, Perform service S1
When ASBR2 receives the IBGP CT advertisement for gold route from PE2, it performs import processing and nexthop resolution for the endpoint PE2-LPBK in the gold TRDB based on its transport-target:0:100. This would resolve over the ISIS-SRv6 route in gold TRDB and pick "Gold-SRv6-Tunnel-to-PE2" tunnel.¶
On successful resolution, a IPv6 transit route for ASBR2-SRv6-PE2-gold-replace/128 is installed in the global IPv6 FIB with "Gold-SRv6-Tunnel-to-PE2" tunnel as nexthop, enabling SRv6 forwarding for gold SLA. The BGP CT routes for RD1:PE2-LPBK is further advertised towards ASBR1 via EBGP CT as shown below. During this readvertisement, the nexthop is set to self, and SID is rewritten to ASBR2-SRv6-gold-Replace.¶
EBGP CT routes from ASBR2 to ASBR1
RD1:PE2-LPBK,
transport-target:0:100,
Prefix-SID: ASBR2-SRv6-PE2-gold-Replace,
NH: ASBR2_InterAS_Link
RD2:PE2-LPBK,
transport-target:0:200,
Prefix-SID: ASBR2-SRv6-PE2-bronze-Replace,
NH: ASBR2_InterAS_Link
ASBR2: IPv6 FIB for SRv6
[BGP CT] ASBR2-SRv6-PE2-gold-Replace
NH: UpdateIPv6DA(SRH.NextSegment), Encap "Gold-SRv6-Tunnel-to-PE2"
[BGP CT] ASBR2-SRv6-PE2-bronze-Replace
NH: UpdateIPv6DA(SRH.NextSegment), Encap "Bronze-SRv6-Tunnel-to-PE2"
When ASBR1 receives this EBGP CT advertisement from ASBR2, an IPv6 route for ASBR1-SRv6-gold-Replace/128 is installed with a nexthop of ASBR1_InterAS_Link in the global IPv6 FIB, enabling SRv6 forwarding for gold SLA. The BGP CT route for RD1:PE2-LPBK is further advertised to PE1 via IBGP CT, with nexthop set to self, and SID rewritten to ASBR1-SRv6-gold-Replace.¶
IBGP CT routes from ASBR1 to PE1
RD1:PE2-LPBK,
transport-target:0:100,
Prefix-SID: ASBR1-SRv6-PE2-gold-Replace,
NH: ASBR1-LPBK
RD2:PE2-LPBK,
transport-target:0:200,
Prefix-SID: ASBR1-SRv6-PE2-bronze-Replace,
NH: ASBR1-LPBK
ASBR1: IPv6 FIB for SRv6
[BGP CT] ASBR1-SRv6-PE2-gold-Replace,
NH: ASBR2_InterAS_Link
SID op: ReplaceSID(ASBR2-SRv6-PE2-gold-Replace)
[BGP CT] ASBR1-SRv6-PE2-bronze-Replace,
NH: ASBR2_InterAS_Link
SID op: ReplaceSID(ASBR2-SRv6-PE2-bronze-Replace)
When PE1 receives this IBGP CT advertisement from ASBR1, it resolves the nexthop ASBR1-LPBK in the gold TRDB based on its transport-target:0:100. This would resolve over the ISIS-SRv6 route in gold TRDB and pick "Gold-SRv6-Tunnel-to-ASBR1".¶
This forms the end-to-end Gold SLA path from PE1 to PE2. The gold BGP CT route for PE2-LPBK is installed in gold TRDB, and can be used for resolving service route nexthops. The Transport layer SIDs are replaced at each border node, which reduces the number of SID decaps required at the egress PE.¶
Gold TRDB routes at PE1
[BGP CT] PE2-LPBK,
NH: ASBR1-SRv6-gold
SID op: EncapSID(ASBR1-SRv6-PE2-gold-Replace)
Bronze TRDB routes at PE1
[BGP CT] PE2-LPBK,
NH: ASBR1-SRv6-bronze
SID op: EncapSID(ASBR1-SRv6-PE2-bronze-Replace)
PE1: IPv6 FIB for SRv6
[BGP CT] PE2-LPBK,
NH: ASBR1-SRv6-gold
SID op: EncapSID(ASBR1-SRv6-PE2-gold-Replace)
[BGP CT] PE2-LPBK,
NH: ASBR1-SRv6-bronze
SID op: EncapSID(ASBR1-SRv6-PE2-bronze-Replace)
[ISIS SRv6] ASBR1-SRv6-gold,
NH: Encap "Gold-SRv6-Tunnel-to-ASBR1"
[ISIS SRv6] ASBR1-SRv6-bronze,
NH: Encap "Bronze-SRv6-Tunnel-to-ASBR1"
Furthermore, any service routes received with nexthop as PE2-LPBK and Mapping Community as Color:0:100 indicating Gold SLA will use the Resolution Scheme associated with its Mapping Community to resolve over the PE2-LPBK CT route installed in the gold TRDB, and push the SRv6-gold SID stack to reach PE2.¶
Similarly, any service routes received with nexthop as PE2-LPBK and Mapping Community as Color:0:200 indicating Bronze SLA will use the Resolution Scheme associated with its Mapping Community to resolve over the PE2-LPBK CT route installed in the bronze TRDB, and push the SRv6-bronze SID stack to reach PE2. This is shown below.¶
BGP Service routes advertisement from PE2 to PE1:
SVC_PFX1,
color:0:100,
Prefix-SID: PE2-SRv6-S1-DT4,
NH: PE2-LPBK
SVC_PFX2,
color:0:200,
Prefix-SID: PE2-SRv6-S1-DT4,
NH: PE2-LPBK
PE1: Service routes FIB
[BGP INET] SVC_PFX1, color:0:100
NH: EncapSID "PE2-SRv6-S1-DT4, ASBR1-SRv6-gold-Repace, Gold-SRv6-Tunnel-to-ASBR1(outer)"
[BGP INET] SVC_PFX2, color:0:200
NH: EncapSID "PE2-SRv6-S1-DT4, ASBR1-SRv6-bronze-Replace, Bronze-SRv6-Tunnel-to-ASBR1(outer)"
The operational, scaling and convergence aspects of this approach are similar to the aspects of applying BGP CT procedures to the MPLS data plane.¶
CPR is defined in the document: Colorful Prefix Routing for SRv6 based services [Colorful-Prefix-Routing-SRv6], and uses IPv6 Unicast (AFI/SAFI = 2/1) as a transport family. CPR mechanism does not use BGP CT (SAFI 76) address family.¶
CPR uses color encoded SRv6 service SIDs to determine the intent-aware transport paths for the service, without a separate transport SRv6 SID. It routes using "Colorful Prefix" locators in the transport layer, which are carried in the IPv6 Unicast BGP family.¶
A Nexthop Resolution Scheme similar to that of BGP CT Section 6 is used on IPv6 unicast family to resolve “Colorful Prefix” locator routes that carry a mapping community to intent-aware paths in each domain.¶
By virtue of the CPR SID allocation scheme, the service SIDs inherit the Intent of the corresponding Colorful Prefix route just by performing longest prefix match in forwarding plane.¶
The CPR approach can be used to support intent driven routing while minimizing SRv6 encapsulation overhead, at the cost of careful SID numbering and planning. The state in the transport network is a function of total number of Colorful Prefixes.¶
In the CPR approach, typically one service SID is allocated for each service function (e.g. VRF) which is associated with a specific intent. In some special scenarios, for example, when different service routes in the same VRF are with different intents, a unique service SID would need to be allocated for each intent associated with the VRF.¶
However, the CPR mechanism preserves BGP PIC (Prefix scale Independent Convergence) for the egress SN failure scenario where only Colorful Prefix routes need to be withdrawn.¶
CPR achieves strict Intent based forwarding for the service routes. Fallback to best effort transport class is achieved by numbering all SRv6 Colorful Prefix locators at the egress SN to fall in the same subnet as the SRv6 locator that uses best effort transport class. Customized intent fallback between different color transport classes may be achieved by allocating a CPR prefix for each such intent fallback policy, and advertising that CPR prefix with an appropriate mapping community, that maps to a customized resolution scheme. Alternatively, the intent fallback policy may be provisioned on the ingress nodes directly.¶
Further, IPv6 unicast family is widely deployed to carry Internet Service routes. Repurposing IPv6 unicast family to carry Transport routes also may impact the operational complexity and security aspects in the network.¶
The authors thank Jeff Haas, John Scudder, Susan Hares, Dongjie (Jimmy), Moses Nagarajah, Jeffrey (Zhaohui) Zhang, Joel Harpern, Jingrong Xie, Navaneetha Krishnan, Ravi M R, Chandrasekar Ramachandran, Shradha Hegde, Sunil Malali, William J Britto, R Shilpa, Ashish Kumar (FE), Sunil Kumar Rawat, Abhishek Chakraborty, Richard Roberts, Krzysztof Szarkowicz, John E Drake, Srihari Sangli, Jim Uttaro, Luay Jalil, Keyur Patel, Ketan Talaulikar, Dhananjaya Rao, Swadesh Agarwal, Robert Raszuk, Ahmed Darwish, Aravind Srinivas Srinivasa Prabhakar, Moshiko Nayman, Chris Trip, Vijay Kestur, Santosh Kolenchery for all the valuable discussions, constructive criticisms, and review comments.¶
The decision to not reuse SAFI 128 and create a new address-family to carry these transport-routes was based on suggestion made by Richard Roberts and Krzysztof Szarkowicz.¶