BGP SR Policy Extensions for Network Resource Partition

The concept of Segment Routing (SR) policy is defined in . An SR Policy is a set of candidate paths, each consisting of one or more segment lists. The headend of an SR Policy may learn multiple candidate paths for an SR Policy. The header of a packet steered in an SR Policy is augmented with an ordered list of segments associated with that SR Policy. The BGP extensions to distribute SR Policy candidate paths are defined in . discusses the general framework, components, and interfaces for requesting and operating network slices using IETF technologies. It also introduces the concept of Network Resource Partition (NRP), which is a subset of the resources and associated policies in the underlay network. The network slices defined in can be realized by mapping one or more connectivity constructs to an NRP. describes the framework and the candidate component technologies for providing NRP-based enhanced VPN services based on Traffic Engineering and VPN technologies. Enhanced VPN can be used for the realization of network slice services defined in . As described in , one data plane approach to enable NRP and network slice is to carry a dedicated NRP selector ID in the data packet to identify the NRP the packet belongs to, so that the packet can be processed and forwarded using the subset of network resources allocated to the NRP. In networks where there are multiple NRPs, an SR Policy may be associated with a particular NRP. describes the association of SR Policy candidate path with NRP under the SR Policy architecture. This document defines the extensions to BGP to specify the control plane NRP ID that is associated with an SR Policy candidate path. The control plane NRP ID is linked to the NRP identifier used in data plane (denoted as NRP Selector ID).

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 following terminology is used in this document: Network Resource Partition (NRP): refer to the definition in NRP Identifier (NRP ID): refer to the definition in NRP Selector ID: refer to the definition in

In order to specify the NRP the candidate path of SR Policy is associated with, a new sub-TLV called "NRP ID" sub-TLV is defined in the BGP Tunnel Encapsulation Attribute . The NRP ID sub-TLV can be carried in the BGP Tunnel Encapsulation Attribute with the tunnel type set to SR Policy. The use of the NRP ID sub-TLV in other tunnel types is outside the scope of this document. The NRP ID sub-TLV has the following format:

where: Type: 123 (assigned by IANA) Length: The Length field MUST be set to 6 octets Flags: 1-octet flag field. None is defined in this document. The flags MUST be set to zero on transmission and MUST be ignored on receipt. Reserved: 1 octet of reserved bits. It MUST be set to zero on transmission and MUST be ignored on receipt. NRP ID: A 32-bit identifier in the control plane which uniquely identifies an NRP in an NRP domain . Value 0 is reserved, which MUST NOT be used by the sender. On receipt of the NRP ID TLV, if the NRP ID is 0, the NRP ID TLV MUST be ignored. The encoding structure of BGP SR Policy (the Subsequent Address Family Identifier (SAFI) is 73) Network Layer Reachability Information (NLRI) and attributes with the NRP ID sub-TLV is expressed as below:

Attributes: Tunnel Encaps Attribute (23) Tunnel Type: SR Policy (15) Binding SID SRv6 Binding SID Preference Priority Policy Name Policy Candidate Path Name Explicit NULL Label Policy (ENLP) NRP ID Segment List Weight Segment Segment ... ... Figure 2. SR Policy Encoding with NRP ID sub-TLV]]> The NRP ID sub-TLV is OPTIONAL and MUST NOT appear more than once for one SR Policy candidate path.

When a candidate path of SR Policy is instantiated within an NRP, and a domain-wide data plane NRP Selector ID is used for identifying the resources of the NRP, the originating node of SR Policy MUST include the NRP ID sub-TLV in the BGP Tunnel Encapsulation Attribute of the BGP SR Policy. The setting of other fields and attributes in BGP SR Policy follows the mechanism as defined in . On reception of an SR Policy NLRI with NRP ID sub-TLV in the BGP Tunnel Encapsulation Attribute , a BGP speaker determines if it is valid and usable according to the rules defined in Section 4.2 of . The selected best routes of the SR Policy SAFI is passed on to the SR Policy Module (SRPM) for further processing as described in Section 4.2.2 of .

The error handling of the BGP Update messages for BGP SR Policy SAFI with the NRP extensions defined in this document follows the procedures in section 5 of . The NRP ID sub-TLV is considered malformed if the length field of the sub-TLV is not 6 octets. If the NRP ID sub-TLV appears more than once, or its format is considered malformed, the NRP ID information associated with the BGP SR Policy NLRI is considered malformed and the "treat-as-withdraw" strategy of MUST be applied.

The mechanism specified in this document adds additional information to the SR Policy candidate paths advertisement in BGP. As the number of NRP increases, the number of SR Policies and candidate paths may also increase, and the amount of information exchanged between the network controller and the headend nodes for provisioning SR Policy would increase proportional to the number of SR Policies and candidate paths. However, there is no change to the SR Policy advertisment procedure, and no change to BGP best-path selection. As SR Policy candidate paths advertised using BGP are only installed by the corresponding headend nodes, the increase of SR Policy and candidate path information does not impact the state maintained on the transit nodes. Although the updates to SR Policy architecture in allow different candidate paths in one SR Policy to be associated with different NRPs, in normal network scenarios it is considered that no matter which candidate path is active, the association between an SR Policy and NRP is consistent. In such case all candidate paths of one SR Policy SHOULD be associated with the same NRP. The cases where different candidate paths of an SR Policy are associated with different NRPs is valid but NOT RECOMMENDED.

The security considerations of BGP data integrity and authentication in apply to this document. The security considerations of SR Policy architecture and using BGP for SR Policy information advertisement in apply to this document. The security considerations of SR Policy NRP extensions apply to this document. Incorrect association of NRP ID with SR Policy can affect traffic isolation and resource guarantee. And the adverisement of the associated NRP ID of SR Policy constitutes a risk to confidentiality of mission-critical or commercially sensitive information. It is the responsibility of the network operator to ensure that only trusted nodes (that include both routers and controller applications) within the SR domain are configured to send and receive such information, and to ensure the correctness of NRP association.

IANA has assigned the sub-TLV type as defined in Section 2 from "BGP Tunnel Encapsulation Attribute sub-TLVs" registry in the "Border Gateway Protocol (BGP) Tunnel Encapsulation" registry group.

The authors would like to thank Guoqi Xu, Lei Bao, Haibo Wang, Shunwan Zhuang and Susan Hares for their review and discussion of this document.