BGP-LS Extension for Inter-AS Topology Retrieval

BGP-LS describes the use of BGP protocol for advertisement of the Link-State topology information. It enables applications such as an SDN controllers to collect the underlay network topology. covers the advertisement of topology information from within Interior Gateway Protocol (IGP) domain. If the network has more than one IGP domain, and these domains interconnect with each other via inter-AS links, there is no mechanism within the current BGP-LS to advertise the interconnect topology information. defines extensions for exporting BGP peering node topology information (including peers, interfaces, and peering ASes) in a way that is used to compute 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. Considering there will be several border routers on the network boundary, such solution restricts its deployment flexibility. This document defines the Inter-AS Link NLRI and some new TLVs for BGP-LS to cover scenarios where an SDN controller needs to get the interconnection topology information between different AS domains when sourced from IGPs.

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 terms are defined in this document: IDCs: Internet Data Centers MAN: Metro-Area-Network SDN: Software Defined 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 inter-AS links. In Figure 1, S2(in IGP domain A) and T1(in IGP domain B) are connected to the IP SDN Controller via BGP-LS, but they can only report the topology information among the IGP A and IGP B themselves, and can't report the inter-as topology information among them because there is no IGP protocol runs on the inter-AS links. The border routers, SB1/SB3 in IGP A and TB2/TB4 in IGP B know the inter-AS links among them, and can advertise such information via underlying OSPF or IS-IS , but there is no place in current BGP-LS protocol to transfer such information.

defines four NLRI types (Node, Link, IPv4 Topology Prefix, and IPv6 Topology Prefix) to transfer the topology and prefix information. For inter-AS link, as the two ends of the link belong in different IGP domains and the link does not run an IGP protocol, it is not appropriate to advertise their information within the existing NLRI types listed above. This document defines a new NLRI type 7, see ) within the BGP-LS NLRI, referred to as the Inter-AS Link NLRI. The Inter-AS Link NLRI is encoded in the format shown in Figure 2 as explained below:

This document specifies the advertisement of Inter-AS Links using the Inter-AS Link NLRI when originating the information from the underlying OSPF and IS-IS advertisements. This section describes the encoding of the Inter-AS Link NLRI while the more detailed procedures for sourcing of this information from the underlying IGP are described in . The "Protocol-ID" is set to the value indicating the source protocol of the Inter-AS Link information, as specified in Section 5.2. As the information is sourced from OSPF or IS-IS, the value MUST correspond to one of IGP values as specified in . The semantics of "Identifier" field are the same as defined in and will be set to a value that is identical to the "Identifier" value of the IGP domain associated with the ASBR of the inter-AS link. Therefore, the "Identifier" values for the two half-links (refer section 5.2.2 of ) of the inter-AS link could be different depending on the configuration of Identifiers for the two IGP domains. The "Local Node Descriptors" field is encoded using the TLV 256 defined in section 5.2.1.2 of to identify the ASBR associated with the specific half-link of the inter-AS link. The following TLVs MUST be included as the Local Node Descriptors: - Autonomous System (TLV 512) . - OSPF Area-ID (TLV 514) to be included only in the case of OSPF, when the Inter-AS TE LSA from which information is sourced is being flooded with an area-scope. It is not included when the LSA is flooded with AS-scope. - IGP Router ID (TLV 515) encoded for either OSPF or IS-IS, depending on the source protocol as specified in section 5.2.1.4 of . - One or both of IPv4 and IPv6 Router-ID of the ASBR using TLV 1028 and/or 1029 , depending on whether the ASBR is configured with one or both of the IPv4 and IPv6 TE Router-IDs. (Note: while introduced these TLVs for use in the BGP-LS attribute, this document also leverages the same TLVs for use in the NLRI.) Inter-AS Link Descriptors are encoded as TLVs that identify the specific half-link of the inter-AS link. Section 6 of this document introduces the TLVs that MUST be included as the Inter-AS Link Descriptors: - Remote AS Number (TLV 270), and - One or both of IPv4 and IPv6 Remote ASBR ID using TLV 271 and/or TLV 272, depending on whether the Remote ASBR is configured with one or both of the IPv4 and IPv6 TE Router-IDs. Additionally, the following TLVs MUST be included as Inter-AS Link Descriptors if they are being advertised in the underlying IGP advertisement of the inter-AS link as they help identify individual links when there are more than one inter-AS links between two ASBRs. - Link Local/Remote Identifiers (TLV 258) - IPv4 Interface Address (TLV 259) - IPv4 Neighbor Address (TLV 260) - IPv6 Interface Address (TLV 261) - IPv6 Neighbor Address (TLV 262) Use of any other TLVs as Local Node Descriptors or Inter-AS Link Descriptors may cause challenges in the correlation of the two Inter-AS Link NLRI half-links when the BGP-LS Producer implementations vary.

This document introduces three TLVs for inclusion as Inter-AS Link Descriptors within the Inter-AS Link NLRI for the advertisement of inter-AS link information via BGP-LS.

The encoding of these TLVs are aligned with the corresponding advertisements in and , which keeps the BGP-LS protocol agnostic to the underly protocol.

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 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), the left (high-order) 2 octets MUST be set to 0.

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 and is 4 octets in length. Its format is as follows:

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 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 Link NLRI.

Advertisement of Inter-AS Links along with their TE information is done is done in IGPs as follows: - In OSPFv2 via the Inter-AS-TE-v2 LSA - In OSPFv3 via the Inter-AS-TE-v3 LSA - In IS-IS via the Inter-AS Reachability Information TLV (TLV 141) The routers that connect to the SDN controller via the BGP-LS protocol within each domain will advertise the information from the above IGPs for the inter-AS link. To retrieval the Inter-AS topology, the Autonomous System TLV(TLV 512), Remote AS number, IPv6 Remote ASBR ID and/or IPv4 Remote ASBR ID MUST be presented within the inter-AS Link NLRI of each inter-AS link. When advertising these Inter-AS Links from the IGPs into BGP-LS as Inter-AS Links, the sourcing of information for the Inter-AS Link NLRI except for the Inter-AS Link Descriptors follows the same procedures as specified in . The information about the Remote AS Number and the IPv4/IPv6 Remote ASBR IDs specified in are derived from the Remote AS Number and IPv4/IPv6 Remote ASBR ID TLVs specified for OSPF and IS-IS in and respectively. The rest of the Inter-AS Link Descriptor TLVs of the Inter-AS Link NLRI are sourced from the base OSPF/ISIS TE TLVs that were originally introduced for normal IGP links and which are also encoded for the inter-AS TE links as specified in and ; their procedures are therefore same as in . The OSPF/ISIS Inter-AS Link advertisements also include various link properties (e.g., TE metric, Admin Groups, SRLGs, etc.) which are encoded using the same TLVs as for normal IGP links. These link properties are advertised using their corresponding BGP-LS TLVs as specified in and other BGP-LS extensions in the BGP-LS Attribute associated with the Inter-AS Link NLRI of that specific link.

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 Inter-AS Link characteristics does not cause issues within a "walled garden". However, the Inter-AS 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 requests IANA to update the allocated codepoints from under the "Border Gateway Protocol - Link State (BGP-LS) Parameters" registry group as follows:

IANA has allocated codepoint for the Inter-AS Link NLRI type from the "BGP-LS NLRI Types" registry in the “Border Gateway Protocol – Link State (BGP-LS) Parameter” Group:

IANA has allocated codepoints for the following TLVs from "BGP-LS NLRI and Attribute TLVs" registry in the “Border Gateway Protocol – Link State (BGP-LS) Parameter” Group:

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.