BGP-LS Extension for Inter-AS Topology Retrieval

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Table of Contents

1. Introduction

BGP-LS [RFC9552] describes the methodology that using BGP protocol to transfer the Link-State information. Such method can enable SDN controller to collect the underlay network topology automatically, but normally it can only get the information within one Interior Gateway Protocol (IGP) domain. If the operator has more than one IGP domain, and these domains interconnect with each other, there is no mechanic within current BGP- LS to transfer the interconnect topology information.¶

[RFC9086] defines some extensions for exporting BGP peering node topology information (including its peers, interfaces and peering ASs) in a way that is exploitable in order to compute efficient BGP Peering Engineering policies and strategies. Such information can also be used to calculate the interconnection topology among different IGP domains, but it requires every border router to run BGP-LS protocol and report the information to SDN controller. Considering there will be plenty of border routers on the network boundary, such solution restricts its deployment flexibility.¶

This draft analyzes the situations during which the SDN controller needs to get the interconnected topology information between different AS domains. After describing these situations, this draft defines a new Stub Link type within the BGP-LS NLRI[RFC9552] to describe the Iner-AS link and some new TLVs for that new BGP-LS type. After that, the SDN controller can then deduce the multi-domain topology automatically based on the information from BGP-LS protocol.¶

4. Inter-AS Domain Scenarios

Figure 1 illustrates the multi-domain scenarios that this draft discusses. Normally, SDN Controller can get the topology of IGP A and IGP B individually via the BGP-LS protocol, but it can't get the topology connection information between these two IGP domains because there is generally no IGP protocol run on the connected links.¶

                      +-----------------+
                 +----+IP SDN Controller+----+
                 |    +-----------------+    |
                 |                           |
                 |BGP-LS                     |BGP-LS
                 |                           |
 +---------------+-----+               +-----+--------------+
 | +--+        +-++   ++-+           +-++   +|-+        +--+|
 | |S1+--------+S2+---+B1+-----------+B2+---+T1+--------+T2||
 | +-++   N1   +-++   ++-+           +-++   ++++   N2   +-++|
 |   |           |     |               |     ||           | |
 |   |           |     |               |     ||           | |
 | +-++        +-++   ++-+           +-++   ++++        +-++|
 | |S4+--------+S3+---+B3+-----------+B4+---+T3+--------+T4||
 | +--+        +--+   ++-+           +-++   ++-+        +--+|
 |                     |               |                    |
 |                     |               |                    |
 |       IGP A         |               |      IGP B         |
 +---------------------+               +--------------------+

             Figure 1: Inter-AS Domain Scenarios

5. Stub Link NLRI

[RFC9552] defines four types within the BGP Link-State NLRI (Node NLRI, Link NLRI, IPv4 Topology Prefix NLRI, and IPv6 Topology Prefix NLRI) to transfer the topology and prefix information. For inter-as link, the two ends of the link exist in different IGP domains, so it is not appropriate to transfer their information within the current defined NLRI types.¶

This draft defines one new NLRI type(TBD1, seeSection 10) within the BGP Link-State (BGP-LS) NLRI, called the Stub link NLRI. The Stub link NLRI is encoded in the format shown in Figure 2 and explained below:¶

      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
     +-+-+-+-+-+-+-+-+
     |  Protocol-ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Identifier                          |
     |                            (64 bits)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //              Local Node Descriptors (variable)              //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Stub Link Descriptors (variable)              //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 2: Stub Link NLRI Format

The "Protocol-ID" should be set to the value that indicates the source protocol of the stub link information, as indicated in [RFC9552] in section 5.2.¶

Local Node descriptors: define the ASBRs that are attached to the Inter-AS stub link, and use the [RFC9552] “Local Node Descriptor” in section 5.2.1.4. The following Node Descriptor sub-TLVs from [RFC9552] are valid for inclusion in the local Node descriptor: AS system, OSPF Area-ID, IGP Router-ID.¶

Stub Link Descriptors: define the Stub link which has only one end Located in the IGP domain using the [RFC9552] “Link Descriptor definition” in section 5.2.2 with the exceptions noted below.¶

The Stub link Descriptor supports the inclusion of the following subTLVs:¶

• Link/Local Identifier (TLV 258, [RFC9552]),¶

• IPv4 Interface Address (TLV 259, [RFC9552]),¶

• IPv4 Neighbor Address (TLV 260,[RFC9552]),¶

• IPv6 Interface Address (TLV 261, [RFC9552]),¶

• IPv6 Neighbor Address (TLV 262, [RFC9552]),¶

• Multi-topology identifier (TLV 263, [RFC9552]),¶

• Remote-AS Number (TLV TBD1, [This document], section Section 7.1),¶

• IPv4 Remote ASBR ID [TLV TBD2, [This document], section Section 7.2), and¶

• IPv6 Remote ASBR ID [TLV TBD2, [This document], section Section 7.3).¶

This newly defined NLRI can be used to describe the link that has only one end located within the IGP domain, as described in the following sections. The Node and Link Descriptor sub-TLVs and Node and Link attributes that are defined in [RFC9552] can be included in the NLRI if necessary. The interface and neighbor address sub- TLVs SHOULD be included in the Local Node Descriptors to differentiate the parallel links between two ASBRs.¶

8. Topology Reconstruction.

When SDN controller gets such information from BGP-LS protocol, it should find information from the associated router. Based on this information it can create a logical topology that contains the link between these two border routers. Iterating the above procedures for all of the stub links, the SDN controller can automatically retrieve the Inter-AS connection topology.¶

10. IANA Considerations

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.¶

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Code Point    |   Description  |          Status           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|TBD1(suggestion 7)| Stub Link NLRI |  Allocation from IANA     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 7:  Stub Link NLRI Codepoint

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Code Point      |   Description        |        Status       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|TBD2(suggestion 270)|  Remote AS Number    | Allocation from IANA|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|TBD3(suggestion 271)|  IPv4 Remote ASBR ID | Allocation from IANA|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|TBD4(suggestion 272)|  IPv6 Remote ASBR ID | Allocation from IANA|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 8:  BGP-LS Link Descriptors TLV

Authors' Addresses