BGP extensions for SRv6/MPLS Transport Interworking

The deployment of SRv6 into existing transport network require SRv6 to interwork with MPLS. describes SRv6 and MPLS interworking architecture in multi domain network where each domain run SRv6 or MPLS data plane independently. To accomplish it, section 7.1.2 of details BGP inter-domain routing procedure in which domain border router set next hop to self when propagating transport routes (for example, locator or loopback prefix of a PE) across domains. Setting next hop to self results in allocation of label or SRv6 SID depending on dataplane type of the domain where route is propagated. Signaling of label is already specified in . This document specifies method to signal SRv6 SID behaviors End.DTM, End.DTM46 and End.DPM specified in needed for SRv6 and MPLS interworking with BGP Labeled Unicast (BGP-LU) routes (SAFI value 4 routes as specified in ).

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 when, and only when, they appear in all capitals, as shown here.

The Prefix-SID attribute is an optional, transitive path attribute defined in to announce information about BGP Segment identifiers as list of TLVs. defined TLV type 1 to encode label index and TLV type 3 to encode Originator SRGB to be attached to prefixes of BGP-LU SAFI(). extended Prefix-SID attribute to carry SRv6 Segment Identifier that is encoded in SRv6 Service TLV and attached to BGP service routes.

This document defines a new TLV called "SRv6 Transport TLV" of the BGP Prefix-SID Attribute to announce SRv6 SID with prefixes of BGP-LU SAFI. SRv6 SID is encoded in "SRv6 Transport TLV" exactly like SRv6 Service SID TLV . This document uses the TLV to signal End.DTM, End.DTM46 and End.DPM. The usage of this TLV for other SRv6 SID behaviors and SAFI's is out of scope of this document and may be extended in future documents. Extension in this document lead to signaling of label in MPLS label field and SRv6 SID in SRv6 Transport TLV for BGP-LU route. Below section describes usage of both the information based on behavior of the SRv6 SID.

SRv6 SID is used to encapsulate the MPLS packet whose top label is the label value in MPLS field of the NLRI of the BGP-LU route. Draft defines End.DTM and End.DTM46 pseudo-code for this purpose. Domain border router that set next hop to self, allocates label bound to each prefix of BGP-LU route. Label is encoded in MPLS label field as specified in . SRv6 SID is encoded in "SRv6 Transport TLV" without transposition i.e. transposition length MUST be set to 0 in SRv6 SID Structure Sub-Sub-TLV. Behavior field MUST be set with SRv6 Endpoint Behavior codepoint value of End.DTM or End.DTM46. This behavior signals "AND" semantics i.e. push label signaled in NLRI and perform H.Encaps.M with DA as SRv6 SID signaled in the TLV. Same SRv6 SID can be attached to multiple BGP-LU routes by domain border router that sets next hop to self. Section 7.1.2.2.1 of describe control and dataplane state using SRv6 Transport TLV.

Domain border node that set next hop to self, allocates SRv6 SID and label for each prefix of BGP-LU SAFI. Draft defines End.DPM pseudo-code for this purpose. Label is encoded in MPLS label field as specified in . SRv6 SID is encoded in SRv6 Transport TLV exactly like SRv6 Service SID TLV . Behavior field MUST be set with SRv6 Endpoint Behavior codepoint value of End.DPM. Receiving node perform H.Encaps, where destination of IPv6 header is set to SRv6 SID for traffic destined to prefix in NLRI. Please refer to section 7.1.2.2.2 of for overall procedures when SRv6 SID is bound to prefix in NLRI. Signaling of label and SRv6 SID assist in migration by allowing receiving node to select relevant encapsulation. Similarly, it allows to advertise just label to legacy node. For example, in node 4 being SRv6 capable may select relevant encapsulation and node 44 being legacy continue MPLS encapsulation.

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A BGP speaker that receive Prefix-SID attribute with SRv6 Transport TLV with transport route observe following rules when advertising the route to other peers: If the nexthop is unchanged, the TLVs, including any unrecognized Types of Sub-TLV and Sub-Sub-TLV, SHOULD be propagated further. In addition, all Reserved fields in the TLV or Sub-TLV or Sub-Sub-TLV MUST be propagated unchanged. If the nexthop is modified, the TLV and associated sub-TLVs/Sub-Sub-TLVs SHOULD be updated based on local policy. For example, if upstream is MPLS domain, then TLVs carrying SRv6 SID should be removed and local MPLS label bound to address in NLRI is propagated further.