IDR Working Group A. Wang
Internet-Draft China Telecom
Intended status: Standards Track H. Chen
Expires: 16 August 2025 Individual
K. Talaulikar
Cisco Systems
S. Zhuang
Huawei Technologies
C. Lin
New H3C Technologies
12 February 2025
BGP-LS Extension for Inter-AS Topology Retrieval
draft-ietf-idr-bgpls-inter-as-topology-ext-17
Abstract
This document describes the process to distribute Border Gateway
Protocol- Link State (BGP-LS) key parameters for inter-domain links
between two Autonomous Systems. This document defines a new type
within the BGP-LS NLRI for a Stub Link and three new type-length-
values (TLVs) for BGP-LS Link descriptor. These additions to BGP-LS
let Software Definition Network (SDN) controllers retrieve the
network topology automatically under various inter-AS environments.
Such extension and process can enable the network operator to collect
the interconnect information between different domains and then
calculate the overall network topology automatically based on the
information provided by BGP-LS protocol.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 16 August 2025.
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Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Inter-AS Domain Scenarios . . . . . . . . . . . . . . . . . . 3
5. Stub Link NLRI . . . . . . . . . . . . . . . . . . . . . . . 4
6. IGP Information for Inter-AS Link . . . . . . . . . . . . . . 6
7. BGP-LS Extensions for Inter-AS Link . . . . . . . . . . . . . 6
7.1. Remote AS Number TLV . . . . . . . . . . . . . . . . . . 7
7.2. IPv4 Remote ASBR ID . . . . . . . . . . . . . . . . . . . 8
7.3. IPv6 Remote ASBR ID . . . . . . . . . . . . . . . . . . . 8
8. Topology Reconstruction. . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10.1. New BGP-LS NLRI type . . . . . . . . . . . . . . . . . . 10
10.2. New Stub Link Descriptors . . . . . . . . . . . . . . . 10
11. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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.
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[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.
2. Conventions used in this document
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 [RFC2119] .
3. Terminology
The following terms are defined in this document:
* IDCs: Internet Data Centers
* MAN: Metrio-Area-Network
* SDN: Software Definition Network
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.
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+-----------------+
+----+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, see__Section 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:
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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
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• 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.
6. IGP Information for Inter-AS Link
[RFC9346] and [RFC5392] define IS-IS and OSPF extensions respectively
to deal with the reasons for reporting inter-AS link. Three sub-TLVs
relating to Inter-Domain Links (Remote AS Numberã€IPv4 Remote ASBR IDã€
and IPv6 Remote ASBR ID) are defined in these documents. .
These IGP TLVs are flooded within the IGP domain automatically. This
document specifies that these MAY also be carried within the newly
defined Stub Link NLRI within the BGP-LS protocol, as the descriptors
for the inter-AS stub link. The "Local Node Descriptors" in the Stub
Link NLRI within the BGP-LS NLRI should describe the characteristics
of ASBRs that are connected across the inter-AS links.
If the SDN controller knows these information via one of the interior
router that runs BGP-LS protocol, the SDN controller can rebuild the
inter-AS topology correctly according to the procedure described in
Section 8
7. BGP-LS Extensions for Inter-AS Link
This section proposes to add three new TLVs to be supported in the
Stub Link NLRI in the BGP-LS NLRI. These new TLVs allow BGP-LS to
transfer inter-AS information gathered by the SDN controller.
The following Link Descriptor TLVs are added into the BGP-LS protocol
:
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+-----------+---------------------+--------------+----------------+
| TLV Code | Description |IS-IS/OSPF TLV| Reference |
| Point | | /Sub-TLV | (RFC/Section) |
+-----------+---------------------+--------------+----------------+
| TBD2 |Remote AS Number | 24/21 | [RFC9346]/3.3.1|
| | | | [RFC5392]/3.3.1|
| TBD3 |IPv4 Remote ASBR ID | 25/22 | [RFC9346]/3.3.2|
| | | | [RFC5392]/3.3.2|
| TBD4 |IPv6 Remote ASBR ID | 26/24 | [RFC9346]/3.3.3|
| | | | [RFC5392]/3.3.3|
+-----------+---------------------+--------------+----------------+
Figure 3: Stub Link Descriptor TLVs
Detail encoding of these TLVs are synchronized with the corresponding
parts in [RFC9346] and [RFC5392], which keeps the BGP-LS protocol
agnostic to the underly protocol.
7.1. Remote AS Number TLV
A new TLV, 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 TBD2 (see Section 10 ) and is 4
octets in length. The format is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote AS Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Remote AS Number TLV Format
The Remote AS number field has 4 octets. When only 2 octets are used
for the AS number, 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.
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7.2. IPv4 Remote ASBR ID
A new TLV, which is 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 could be any stable and routable IPv4 address of the remote
ASBR. Use of the TE Router ID as specified in the Traffic
Engineering router ID TLV [RFC9346] is RECOMMENDED.
The IPv4 remote ASBR ID TLV is TLV type TBD3 (see Section 10) and is
4 octets in length. The format of the IPv4 remote ASBR ID TLV is as
follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: IPv4 Remote ASBR ID TLV Format
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 (not even an IPv4 TE Router ID), the IPv6 remote ASBR ID TLV
MUST be included instead. An IPv4 remote ASBR ID TLV and IPv6 remote
ASBR ID TLV MAY both be present in an inter-AS stub link NLRI.
7.3. IPv6 Remote ASBR ID
A new TLV, which is 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 could be any stable and routable IPv6 address of the remote
ASBR. Use of the TE Router ID as specified in the IPv6 Traffic
Engineering router ID TLV [RFC9346] is RECOMMENDED.
The IPv6 remote ASBR ID TLV is TLV type TBD4 (see Section 10) and is
16 octets in length. The format of the IPv6 remote ASBR ID TLV is as
follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: IPv6 Remote ASBR ID TLV Format
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. An
IPv4 remote ASBR ID TLV and IPv6 remote ASBR ID TLV MAY both be
present in an inter-AS stub link NLRI.
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.
9. Security Considerations
BGP-LS security is described in [RFC9552]. This addition to BGP-LS
focuses on the case when one network operated by a single entity has
several IGP domains that are composited by its backbone network and
several MANs (Metro-Area- Networks) and Internet Data Centers (IDCs).
The configuration of these networks operated by the single
administrative entity creates a “walled gardenâ€. Within this single
Administrative Domain, the network operator needs to monitor and
engineer traffic flows that traverse such a network that spans
multiple Autonomous Systems. The network operators can obtain this
information on inter-as topology via the process described in this
draft. Using the passive-interface features or configuring the
Traffic Engineering (TE) parameters on the interconnect links will
not provide the real-time Information for this single Administrative
Domain.
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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.
10.1. New BGP-LS NLRI type
This document defines a new value in the registry "BGP-LS NLRI
Types":
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code Point | Description | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|TBD1(suggestion 7)| Stub Link NLRI | Allocation from IANA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Stub Link NLRI Codepoint
10.2. New Stub Link Descriptors
This document defines three new values in the registry "BGP-LS NLRI
and Attribute TLVs":
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
11. Acknowledgement
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.
12. References
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12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5392] Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5392, DOI 10.17487/RFC5392,
January 2009, <https://www.rfc-editor.org/info/rfc5392>.
[RFC9346] Chen, M., Ginsberg, L., Previdi, S., and D. Xiaodong, "IS-
IS Extensions in Support of Inter-Autonomous System (AS)
MPLS and GMPLS Traffic Engineering", RFC 9346,
DOI 10.17487/RFC9346, February 2023,
<https://www.rfc-editor.org/info/rfc9346>.
[RFC9552] Talaulikar, K., Ed., "Distribution of Link-State and
Traffic Engineering Information Using BGP", RFC 9552,
DOI 10.17487/RFC9552, December 2023,
<https://www.rfc-editor.org/info/rfc9552>.
12.2. Informative References
[RFC9086] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
Ray, S., and J. Dong, "Border Gateway Protocol - Link
State (BGP-LS) Extensions for Segment Routing BGP Egress
Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August
2021, <https://www.rfc-editor.org/info/rfc9086>.
Authors' Addresses
Aijun Wang
China Telecom
Beiqijia Town, Changping District
Beijing
Beijing, 102209
China
Email: wangaj3@chinatelecom.cn
Huaimo Chen
Individual
Boston, MA
United States of America
Email: hchen.ietf@gmail.com
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Ketan Talaulikar
Cisco Systems
India
Email: ketant.ietf@gmail.com
Shunwan Zhuang
Huawei Technologies
Huawei Building, No.156 Beiqing Rd.
Beijing
100095
China
Email: zhuangshunwan@huawei.com
Changwang Lin
New H3C Technologies
Huawei Building, No.156 Beiqing Rd.
Beijing
China
Email: linchangwang.04414@h3c.com
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