BGP-LS Extension for Inter-AS Topology Retrieval

BGP-LS specifies the methodology for using the BGP protocol to transfer Link-State information. This method enables SDN controllers to automatically collect underlay network topology, but it typically retrieves only information within a single Interior Gateway Protocol (IGP) domain. If an operator manages multiple IGP domains that interconnect with one another, no mechanism exists within the current BGP-LS protocol to transfer inter-domain topology information.. defines extensions for exporting BGP peering node topology information (including peers, interfaces, and peering ASes) in a manner exploitable for computing efficient BGP Peering Engineering policies and strategies. This information can also be used to compute interconnection topology among different IGP domains, but it requires every border router to run the BGP-LS protocol and report such information to SDN controllers. Given the large number of border routers at the network boundary, this solution limits deployment flexibility. This document analyzes scenarios in which SDN controllers require inter-domain topology information between different Autonomous Systems (ASes). After describing these scenarios, this document defines a new Stub Link type within the BGP-LS NLRI to describe inter-AS links and new TLVs for this new BGP-LS type. The SDN controller can then automatically deduce the multi-domain topology using information from the BGP-LS protocol.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 .

The following terms are defined in this document: IDCs: Internet Data Centers MAN: Metrio-Area-Network SDN: Software Definition Network

Figure 1 illustrates the multi-domain scenarios discussed in this document. Typically, the SDN Controller can retrieve the topology of IGP A and IGP B individually via the BGP-LS protocol, but it cannot obtain topology connection information between these two IGP domains, as IGP protocols are generally not run on the connected links.

defines four types within the BGP Link-State NLRI (Node NLRI, Link NLRI, IPv4 Topology Prefix NLRI, and IPv6 Topology Prefix NLRI) for transferring topology and prefix information. For inter-AS links, the two ends of a link reside in different IGP domains; thus, it is not appropriate to transfer their information using the currently defined NLRI types. This document defines a new NLRI type 7, see ) within the BGP Link-State (BGP-LS) NLRI, referred to as the Stub Link NLRI. The Stub Link NLRI is encoded in the format shown in Figure 2 and is explained below:

The "Protocol-ID" SHOULD be set to the value indicating the source protocol of the stub link information, as specified in Section 5.2. Local Node Descriptors: define the ASBRs attached to the inter-AS stub links, and use the "Local Node Descriptor" specified in , Section 5.2.1.4. The following Node Descriptor sub-TLVs from are valid for inclusion in the local Node descriptor: AS system, OSPF Area-ID, IGP Router-ID. Stub Link Descriptors: define the Stub Links that have only one end located in an IGP domain, using the "Link Descriptor definition" specified in ,Section 5.2.2 with the exceptions noted below. The Stub Link Descriptors support the inclusion of the following sub-TLVs: • Link/Local Identifier (TLV 258, ) • IPv4 Interface Address (TLV 259, ) • IPv4 Neighbor Address (TLV 260, ) • IPv6 Interface Address (TLV 261, ) • IPv6 Neighbor Address (TLV 262, ) • Remote-AS Number (TLV 270, [This document], section ) • IPv4 Remote ASBR ID (TLV 271, [This document], section ) • IPv6 Remote ASBR ID (TLV 272, [This document], section ) This newly defined NLRI can be used to describe links that have only one end located within an IGP domain, as described in the following sections. To match the stub link connecting the two bordering Autonomous Systems (ASes), the Remote AS Number TLV, the IPv4 Remote ASBR ID TLV (if the neighboring ASBR has an IPv4 address), and/or the IPv6 Remote ASBR ID TLV (if the neighboring ASBR has an IPv6 address) MUST be included. Correspondingly, the IPv4 Router-ID of Local Node TLV (1028) and/or the IPv6 Router-ID of Local Node TLV (1029) MUST be included as Link Attribute TLVs of the Stub Link NLRI. The Node and Link Descriptor sub-TLVs, as well as Node and Link attributes defined in MAY be included in the NLRI if necessary. The interface and neighbor address sub-TLVs SHOULD be included in the Local Node Descriptors to differentiate parallel links between two ASBRs.

and define IS-IS and OSPF extensions, respectively, to address the requirements for reporting inter-AS links. Three sub-TLVs related to Inter-Domain Links (Remote AS Number, IPv4 Remote ASBR ID, and IPv6 Remote ASBR ID) are defined in these documents. These IGP TLVs are automatically flooded within an IGP domain. This document specifies that these MAY also be carried within the newly defined Stub Link NLRI in the BGP-LS protocol, as descriptors for inter-AS stub links.

This section proposes adding three new TLVs to be supported within the Stub Link NLRI of the BGP-LS NLRI. These new TLVs enable BGP-LS to transfer inter-AS information collected by the SDN controller. The following Link Descriptor TLVs are added to the BGP-LS protocol:

The detailed encoding of these TLVs is synchronized with the corresponding sections in and , which maintains BGP-LS protocol agnosticism to the underlying protocol.

A new TLV, referred to as the Remote AS Number TLV, is defined for inclusion in the Link Descriptor when advertising inter-AS links. The Remote AS Number TLV specifies the AS number of the neighboring AS to which the advertised link connects. The Remote AS Number TLV is TLV Type 270 (see ) and is 4 octets in length. Its format is as follows:

The Remote AS Number field has 4 octets. When only 2 octets are used for the AS number (for example, when such information is advertised from OSPF, as in current deployments), the left (high-order) 2 octets MUST be set to 0. The Remote AS Number TLV MUST be included when a router advertises an inter-AS link.

A new TLV, referred to as the IPv4 Remote ASBR ID TLV, is defined for inclusion in the Link Descriptor when advertising inter-AS links. The IPv4 Remote ASBR ID TLV specifies the IPv4 identifier of the remote ASBR to which the advertised inter-AS link connects. This can be any stable, routable IPv4 address of the remote ASBR. The use of the TE Router ID, as specified in the Traffic Engineering Router ID TLV is RECOMMENDED. The IPv4 Remote ASBR ID TLV is TLV Type 271 (see ) and is 4 octets in length. Its format is as follows:

The IPv4 Remote ASBR ID TLV MUST be included if the neighboring ASBR has an IPv4 address. If the neighboring ASBR does not have an IPv4 address (including no IPv4 TE Router ID), the IPv6 Remote ASBR ID TLV MUST be included instead. Both an IPv4 Remote ASBR ID TLV and an IPv6 Remote ASBR ID TLV MAY be present in an inter-AS Stub Link NLRI.

A new TLV, referred to as the IPv6 Remote ASBR ID TLV, is defined for inclusion in the Link Descriptor when advertising inter-AS links. The IPv6 Remote ASBR ID TLV specifies the IPv6 identifier of the remote ASBR to which the advertised inter-AS link connects. This can be any stable, routable IPv6 address of the remote ASBR. The use of the TE Router ID, as specified in the IPv6 Traffic Engineering Router ID TLV is RECOMMENDED. The IPv6 Remote ASBR ID TLV is TLV Type 272 (see ) and is 16 octets in length. Its format is as follows:

The IPv6 Remote ASBR ID TLV MUST be included if the neighboring ASBR has an IPv6 address. If the neighboring ASBR does not have an IPv6 address, the IPv4 Remote ASBR ID TLV MUST be included instead. Both an IPv4 Remote ASBR ID TLV and an IPv6 Remote ASBR ID TLV MAY be present in an inter-AS Stub Link NLRI.

BGP-LS security is specified in . This extension to BGP-LS focuses on scenarios where a single entity-operated network includes multiple IGP domains composed of its backbone network, several Metro-Area Networks (MANs), and Internet Data Centers (IDCs). The configuration of these networks, operated by a single administrative entity, creates a "walled garden". Within this single administrative domain, the network operator needs to monitor and engineer traffic flows traversing a network that spans multiple Autonomous Systems (ASes). The network operator can obtain this inter-AS topology information via the procedure described in this document. A single administrative domain consisting of two ASes that passes information about Stub Link characteristics does not cause issues within a "walled garden". However, the Stub Link NLRI and its characteristics (Link/Local Identifier, IPv4 Interface Address, IPv4 Neighbor Address, IPv6 Interface Address, IPv6 Neighbor Address, Multi-Topology Identifier, Remote-AS Number, IPv4 Remote ASBR ID, and IPv6 Remote ASBR ID) constitute critical network information. As such, operators SHOULD handle this critical information in a manner that restricts it to the walled garden.

This document defines: A new BGP NLRI Type: Stub Link NLRI. The codepoint is from the "BGP-LS NLRI Types" Three new Link Descriptors TLV: Remote AS Number TLV, IPv4 Remote ASBR ID, IPv6 Remote ASBR ID. The codepoint are from "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" registry.

This document defines a new value in the registry "BGP-LS NLRI Types":

This document defines three new values in the registry "BGP-LS NLRI and Attribute TLVs":

The author would like to thank Susan Hares, Acee Lindem, Jie Dong, Shaowen Ma, Jeff Tantsura and Dhruv Dhody for their valuable comments and suggestions.