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.¶

Draft [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 analysis the situations that the SDN controller needs to get the interconnected topology information between different AS domains, defines the new Stub Link NLRI and some new TLVs within the BGP-LS protocol to transfer the key information related to them. 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 NLRI types(Node NLRI, Link NLRI, IPv4 Topology Prefix NLRI, IPv6 Topology Prefix NLRI) to transfer the topology and prefix information. For inter-as link, the two ends of the link locates in different IGP domains, then it is not appropriate to transfer their information within the current defined NLRI types.¶

This draft defines one new NLRI type, called Stub Link NLRI, which is coded as the following format:¶

      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 "Direct" protocol.¶

The semantics of "Local Node Descriptors" and "Stub Link Descriptors" are same as that defined in [RFC9552] for "Node Descriptors" and "Link Descriptor".¶

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.¶

8. Topology Reconstruction.

When SDN controller gets such information from BGP-LS protocol, it should find the corresponding associated router information, build the link between these two border routers. After iterating the above procedures for all of the stub links, the SDN controller can then retrieve the connection topology between different domains automatically.¶

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            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|TBD(suggestion 7)| Stub Link NLRI |  Allocation from IANA      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 7:  Stub Link NLRI Codepoint

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

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