wdiff draft-ietf-lsr-ospf-prefix-originator-05.txt draft-ietf-lsr-ospf-prefix-originator-06.txt

LSR Working Group A. Wang
Internet-Draft China Telecom
Intended status: Standards Track A. Lindem
Expires: May 28, 2020 January 1, 2021 Cisco Systems
J. Dong
Huawei Technologies
P. Psenak
K. Talaulikar
Cisco Systems
November 25, 2019
June 30, 2020

OSPF Prefix Originator Extension
draft-ietf-lsr-ospf-prefix-originator-05 Extensions
draft-ietf-lsr-ospf-prefix-originator-06

Abstract

This document defines Open Shortest Path First (OSPF) encodings OSPF extensions to
advertise include information
associated with the router-id of node originating a prefix along with the originator of inter-area prefixes for
OSPFv2 and OSPFv3 Link-State Advertisements (LSAs). The source
originator is needed in several multi-area OSPF use cases. prefix
advertisement.

Status of This Memo

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provisions of BCP 78 and BCP 79.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document
1.1. Requirements Language . . . . . . . . . . . . . . 3
3. Terminology . . . . 3
2. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 3
4. Inter-Area
2.1. Prefix Source Advertisement Use Cases Router-ID Sub-TLV . . . . . . 4
5. External Prefix Source Advertisement Use Cases . . . . . . . 5
6. 4
2.2. Prefix Source Router-ID sub-TLV Originator Sub-TLV . . . . . . . . . . . . . . . 6
7. . 4
3. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 7
7.1. Inter-Area Prefixes . . . . . . . . . . . . . . . . . . . 7
7.2. External Prefixes . . . . . . . . . . . . . . . . . . . . 7
8. 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. 7
6. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 8
11. 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. 7
7.2. Informative References . . . . . . . . . . . . . . . . . 10 8
Appendix A. Inter-Area Topology Retrieval Process . . . . . . . 10 9
Appendix B. Special Considerations on Inter-Area Topology
Retrieval . . . . . . . . . . . . . . . . . . . . . 11 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 10

1. Introduction

[I-D.ietf-ospf-mpls-elc] defines mechanisms to advertise Entropy
Readable Label Depth (ERLD) for ingress Label Switching Routers (LSR)
to discover other LSR's capability of performing Entropy Label based
load-balancing

Prefix attributes are advertised in MPLS networks. The ingress LSR can use this
information to construct OSPFv2 [RFC2328] using the appropriate label stack for specific
traffic requirements, especially in segment routed networks
Extended Prefix Opaque Link State Advertisement (LSA) [RFC7684] and other
deployments requiring stacked LSPs.

However,
in inter-area scenarios, OSPFv3 [RFC5340] using the Area Border Router (ABR) does
not advertise various Extended Prefix LSA types
[RFC8362].

The identification of the originating OSPF router-id for inter-area prefixes.
An OSPF router for a prefix in one area doesn't know OSPF
varies by the origin area type of inter-area
prefixes the prefix and can't determine is currently not always
possible. For intra-area prefixes, the originating router that originated these
prefixes or is
identified by the ERLD capabilities advertising Router ID field of the destination. Therefore, it
is necessary to advertise area-scoped LSA
used for those prefix advertisements. However, for the originator of these inter-area
prefixes
to ensure advertised by the ingress LSR can construct Area Border Router (ABR), the appropriate label stack.

More generally, [RFC8476] defines a mechanism to advertise multiple
types advertising
Router ID field of supported Maximum SID Depths (MSD) at node and/or link
granularity. This their area-scoped LSAs is set to the ABR itself
and the information will be referred when about the head-end router starts to send traffic to destination prefixes. In inter-area
scenario, it originating the prefix
advertisement is also necessary lost in this process of prefix propagation across
areas. For Autonomous System (AS) external prefixes, the originating
router may be considered as the Autonomous System Border Router
(ASBR) and is identified by the advertising Router ID field of the
AS-scoped LSA used. However, the actual originating router for the sender to learn
prefix may be a remote router outside the
capabilities OSPF domain. Similarly,
when an ABR performs translation of Not-So-Stubby Area (NSSA)
[RFC3101] LSAs to AS-external LSAs, the receivers information associated with
the inter-area
prefixes.

There NSSA ASBR (or the router outside the OSPF domain) is another scenario where knowing not conveyed
across the OSPF domain.

While typically the originator of inter-area
prefixes is useful. For example, Border Gateway Protocol Link-State
(BGP-LS) [RFC7752] describes mechanisms using the BGP protocol to
advertise Link-State information. This information can enable a
Software Definition Network (SDN) controller to automatically
determine the underlay network topology.

However, if the underlay network in OSPF is partitioned into multiple areas
and running the identified
by its OSPF protocol, Router ID, it is does not easy necessarily represent a reachable
address for the SDN controller router. The IPv4/IPv6 Router Address as defined in
[RFC3630] and [RFC5329] for OSPFv2 and OSPFv3 respectively provide an
address to rebuild the multi-area topology since ABR reach that connects multiple
areas will normally hide the detailed topology router.

The primary use case for these non-backbone
areas. If only the internal routers within backbone area run the
BGP-LS protocol, they just learn and report the summary network
information from the non-backbone areas. If the SDN controller can
learn extensions proposed in this document is
to be able to identify the originator of the inter-area prefixes, prefix in the network.
In cases where multiple prefixes are advertised by a given router, it
is possible to
rebuild the inter-area topology.

[RFC7794] introduces the Intermediate System also useful to Intermediate System
(IS-IS) "IPv4/IPv6 Source Router IDs" Type-Length-Value (TLV) be able to
advertise the source of associate all these prefixes leaked from with a different IS-IS level.
This TLV can be used in
single router even when prefixes are advertised outside of the above scenarios. Such solution can also
be applied area
in networks that run which they originated. It also helps to determine when the OSPF protocol, but existing OSPF
LSAs TLVs must be extended same
prefix is being originated by multiple routers across areas.

This document proposes extensions to include the OSPF protocol for inclusion
of information associated with the router originating the
prefix.

This draft provides such solution prefix
along with the prefix advertisement. These extensions do not change
the core OSPF route computation functionality. They provide useful
information for topology analysis and traffic engineering, especially
on a controller when this information is advertised as an attribute
of the OSPFv2 [RFC2328] prefixes via mechanisms such as Border Gateway Protocol Link-
State (BGP-LS) [RFC7752].

Applications related to use of the prefix originating node
information for topology reconstruction process on a controller and OSPFv3
[RFC5340] protocols.

2. Conventions used
the associated limitations are described in this document Appendix A and
Appendix B.

1.1. Requirements Language

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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Terminology

The following terms are used in this document:

o ABR: Area Border Router
o ASBR: Autonomous System Border Router

o ERLD: Entropy Readable Label Depth

o EL: Entropy Label

o IS-IS: Intermediate System to Intermediate System

o LSA: Link-State Advertisement

o MSD: Maximum SID Depths

o NLRI: Network Layer Reachability Information

o OSPF: Open Shortest Path First

o SID: Segment IDentifier

o SDN: Software Definition Network

4. Inter-Area Prefix Source Advertisement Use Cases

Figure 1 illustrates a topology where OSPF is running in multiple
areas. R0-R4 are routers in the backbone area, S1-S4 are internal
routers in area 1, and T1-T4 are internal routers in area

2. R1 and
R3 are ABRs between area 0 and area 1. R2 and R4 are ABRs between
area 0 and area 2. N1 is the network between router S1 and S2 and N2
is the network between router T1 and T2. Ls1 is Protocol Extensions

This document defines the loopback address
of Node S1 Prefix Source Router-ID and Lt1 is the loopback address of Node T1.

+-----------------+
|IP SDN Controller|
+--------+--------+
|
| BGP-LS
|
+---------------------+------+--------+-----+--------------+
| +--+ +--+ ++-+ ++-+ +-++ + -+ +--+|
| |S1+--------+S2+---+R1+---|R0+----+R2+---+T1+--------+T2||
| +-++ N1 +-++ ++-+ +--+ +-++ ++++ N2 +-++|
| | | | | || | |
| | | | | || | |
| +-++ +-++ ++-+ +-++ ++++ +-++|
| |S4+--------+S3+---+R3+-----------+R4+---+T3+--------+T4||
| +--+ +--+ ++-+ +-++ ++-+ +--+|
| | | |
| | | |
| Area 1 | Area 0 | Area 2 |
+---------------------+---------------+--------------------+

Figure 1: OSPF Inter-Area Prefix
Originator Scenario

If S1 wants to send traffic to prefix Lt1 that is connected to T1 in
another area, it should know the ERLD and MSD values associated with
the node T1, and then construct the right label stack at the ingress
node Sub-TLVs for traffic destined to prefix Lt1.

In another scenario, If R0 has some method to learn the originator inclusion of
network N1 the Router ID and reports such a reachable
address information to IP SDN controller, then it
is possible for the controller to reconstruct the topology in router originating the
non-backbone areas. The topology reconstruction process and its
limitations are described in prefix as a prefix
attribute.

2.1. Prefix Source Router-ID Sub-TLV

For OSPFv2, the Appendix A and Appendix B.

5. External Prefix Source Advertisement Use Cases

Figure 2 illustrates a topology where OSPF is running in different
domain that Router-ID Sub-TLV is connected by an Autonomous System Border Router
(ASBR). A, B, and C are routers in the Domain 1; C, D, and E are
routers in Domain 2. Router C is optional Sub-
TLV of the ASBR between OSPFv2 Extended Prefix TLV [RFC7684]. For OSPFv3, the two domains.

When router E receives an external prefix, it will redistribute it as
Prefix Source Router-ID Sub-TLV is an AS-External LSA within domain 2. When C receives such LSA, optional Sub-TLV of the
originator information for such external prefix will be lost Intra-
Area-Prefix TLV, Inter-Area-Prefix TLV, and External-Prefix TLV
[RFC8362] when it
encodes the originating either an IPv4 [RFC5838] or an IPv6 prefix information with the current LSA format field. In
some situations, it will be helpful if C can advertise such
originator information.

+-------------------------------------------------------------------+
| | External
advertisement.

The Prefix Source Router-ID Sub-TLV has 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | +---+ +---+ +---+ +---+ +-|-+ |
| | A +-------+ B +----------| C +---------+ D +---------+ E |------|
| +---+ +---+ +---+ +---+ +---+ | Length | Domain 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain 2 OSPF Router ID |
+-------------------------------------------------------------------+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: OSPF External 1: Prefix Originator Scenario

From Source Router-ID Sub-TLV Format

Where:

o Type: 4 for OSPFv2 and 27 for OSPFv3

o Length: 4

o OSPF Router ID : the above scenarios, we can conclude OSPF Router ID of the OSPF router that it is useful to define
originated the prefix advertisement in the OSPF domain.

A prefix originator sub TLV .

6. advertisement MAY include more than one Prefix Source
Router-ID sub-TLV

[RFC7684] and [RFC8362] respectively define TLV-based LSAs for OSPFv2
and OSPFv3. These documents facilitate addition sub-TLV, one corresponding to each of new attributes
for prefixes and provide the basis for a sub-TLV to advertise Equal-Cost Multi-
Path (ECMP) nodes that originated the
"Prefix given prefix.

A received Prefix Source Router-ID Sub-TLV with OSPF Router ID". ID set to
0 MUST be considered invalid and ignored. Additionally, reception of
such Sub-TLV SHOULD be logged as an error (subject to rate-limiting).

2.2. Prefix Originator Sub-TLV

For OSPFv2, this sub-TLV the Prefix Originator Sub-TLV is a sub-TLV an optional Sub-TLV of
the OSPFv2 Extended Prefix TLV which SHOULD be included in the "OSPFv2
Extended Prefix Opaque LSA" [RFC7684] for inter-area prefixes. [RFC7684]. For OSPFv3, this sub-TLV the Prefix
Originator Sub-TLV is a sub-TLV an optional Sub-TLV of "Inter-Area-Prefix TLV", which
SHOULD be included in the "E-Inter-Area-Prefix-LSA" [RFC8362]. Intra-Area-Prefix
TLV, Inter-Area-Prefix TLV, and External-Prefix TLV [RFC8362] when
originating either an IPv4 [RFC5838] or an IPv6 prefix advertisement.

The "Prefix Source Router-ID" sub-TLV Prefix Originator Sub-TLV has 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Source Router-ID Router Address (4 or 16 octects) |
+---------------------------------------------------------------+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: 2: Prefix Source Router-ID sub-TLV Originator Sub-TLV Format

Where:

o Source Router-ID Sub-TLV Type: 4 (IANA TEMPORARY allocation)
[RFC7684] or 27 (IANA TEMPORARY allocation) [RFC8362] TBD1 for OSPFv2 and TBD2 for OSPFv3

o Length: 4 or 16

o Value: Router-ID of OSPFv2/OSPFv3 Router Address: A reachable IPv4 or IPv6 router address for the
router that is originated the source IPv4 or IPv6 prefix advertisement.
Such an address would be semantically equivalent to what may be
advertised in the OSPFv2 Router Address TLV [RFC3630] or in the
OSPFv3 Router IPv6 Address TLV [RFC5329].

A prefix advertisement MAY include more than one Prefix Originator
sub-TLV, one corresponding to each of the Equal-Cost Multi-Path
(ECMP) nodes that originated the given prefix.

This sub-TLV provides

A received Prefix Originator Sub-TLV that has an invalid length (not
4 or 16) or a Reachable Address containing an invalid IPv4 or IPv6
address (dependent on address family of the same functionality associated prefix) MUST
be considered invalid and ignored. Additionally, reception of such
Sub-TLV SHOULD be logged as an error (subject to rate-limiting).

[RFC7794] provides similar functionality for the IS-IS "IPv4/IPv6
Source Router" TLV defined in [RFC7794].

7. Intermediate System
to Intermediate System (IS-IS) protocol.

3. Elements of Procedure

The following sections describe

This section describes the procedure to include the newly
defined "Source Router-ID Sub-TLV" in the related LSA for inter-area
prefixes and external prefixes respectively.

7.1. Inter-Area Prefixes

When an ABR, for example R2 in Figure 1, receives a Router-LSA advertisement in area 2, it SHOULD originate of the corresponding
"OSPFv2 Extended Prefix Opaque LSA" for OSPFv2 or "E-Inter-Area-
Prefix-LSA" for OSPFv3 that includes the
Source Router-ID sub-TLV for and Prefix Originator Sub-TLVs along with the network prefixes. For example, prefix
advertisement.

The OSPF Router ID of the Prefix Source Router-ID is set to identify the source router OSPF
Router ID of the node originating the prefix Lt1 and other inter-area prefixes in Figure 1.

When a router in another area, e.g., S1, receives such LSA, it the OSPF domain.

If the originating node is advertising an OSPFv2 Router Address TLV
[RFC3630] or an OSPFv3 Router IPv6 Address TLV [RFC5329], then
can ascertain that prefix Lt1
value is associated with node T1 and obtain set in the ERLD or MSD value from T1's Router-Information LSA [RFC7770] and
construct Router Address field of the right label stack at Prefix Originator
Sub-TLV. When the ingress orignating node S1 is not advertising such an
address, implementations MAY support mechanisms to determine a
reachable address belonging to the originating node to set in the
Router Address field. Such mechanisms are outside the scope of this
document.

Implementations MAY support the selection of specific prefixes for traffic
destined
which the originating node information needs to be included with
their prefix Lt1. advertisements.

When a router in another an ABR generates inter-area prefix advertisements into its non-
backbone areas corresponding to an inter-area prefix advertisement
from the backbone area, e.g., R0, receives such LSA, it learns the only way to determine the originating
node information is based on the Prefix Source Router-id Router-ID and includes it Prefix
Originator Sub-TLVs present in the inter-area prefix information
advertised to advertisement
originated into the backbone area by an SDN controller as described in
[I-D.ietf-idr-bgp-ls-segment-routing-ext]. ABR for another non-backbone
area. The SDN controller can
then use such information ABR performs its prefix calculation to build determine the inter-area topology according set
of nodes that contribute to the process described in the Appendix A. The topology retrieval
process may not suitable for some environments as stated in
Appendix B.

7.2. External Prefixes

When an ASBR, for example C in Figure 2, receives an AS-External LSA
for an external best prefix in domain 2, it SHOULD extract reachability. It MUST
use the prefix originator information only from this set of nodes.
The ABR MUST NOT include the "Advertising Router" field from Prefix Source Router-ID or the LSA header.
When Prefix
Originator Sub-TLVs when it is unable to determine the information of
the best originating node.

Implementations MAY provide control on ABRs to selectively disable
the propagation of the originating node information across area
boundaries.

Implementations MAY support the propagation of the originating node
information along with a redistributed prefix is advertised into the OSPF domain 1 as an AS-External LSA,
router C may
from another routing domain. The details of such mechanisms are
outside the scope of this document. Such implementations MAY also advertise
provide control on whether the Router Address in the Source Router-ID using a AS-scoped
OSPFv2 Extended Prefix Opaque LSA
Originator Sub-TLV is set as the ABSR node address or as a Sub-TLV in the OSPFv3 AS-
External LSA.

8. address
of the actual node outside the OSPF domain that owns the prefix.

When translating the NSSA prefix advertisements [RFC3101] to the AS
external prefix advertisements, the NSSA ABR, follows the same
procedures as an ABR generating inter-area prefix advertisements for
the propagation of the originating node information.

4. Security Considerations

Since this document extends the "OSPFv2 OSPFv2 Extended Prefix LSA" and
"OSPFv3 E-Inter-Area-Prefix LSA", LSA, the
security considerations for [RFC7684] and [RFC8362] are applicable.

Modification of the "Prefix Source Sub-TLV" could be used for a
Denial-of-Service attack and could inhibit the use cases described in
Section 4. If Similarly,
since this document extends the OSPF domain is vulnerable to such attacks, OSPF
authentication should be used as defined for OSPFv2 in [RFC5709] and
[RFC7474] and for OSPFv3 in [RFC7166].

Additionally, advertisement of the prefix source for inter-area
prefixes facilitates reconstruction of E-Intra-Area-Prefix-LSA, E-
Inter-Area-Prefix-LSA, E-AS-External LSA and E-NSSA-LSA, the OSPF topology for other
areas. Network operators may consider their topologies to be
sensitive confidential data. For OSPFv3, IPsec can be used to
provide confidentiality [RFC4552]. Since there is no standard
defined security
considerations for native OSPFv2 IPsec, some form of secure tunnel is
required to provide confidentiality.

9. [RFC8362] are applicable.

5. IANA Considerations

This specification defines document requests IANA for the Prefix Source Router-ID sub-TLV as
described in Section 6. This value should be added to allocation of the both
existing "OSPFv2 Extended Prefix TLV Sub-TLVs" and "OSPFv3 Extended-
LSA Sub-TLVs" registries.

The following sub-TLV is added to codepoint from
the "OSPFv2 Extended Prefix TLV Sub-TLVs" registry under the "Open
Shortest Path First v2 (OSPFv2) Parameters" registry. The allocation policy is IETF Review as defined
in [RFC7684]

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: 3: Code Point Points in "OSPFv2 OSPFv2 Extended Prefix TLV Sub-TLVs"

The following sub-TLV is added to Sub-TLVs

This document requests IANA for the allocation of the codepoint from
the "OSPFv3 Extended-LSA Extended Prefix TLV Sub-TLVs" registry under the "Open
Shortest Path First v3 (OSPFv3) Parameters" registry. The allocation policy is IETF Review as defined in
[RFC8362]

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: 4: Code Point Points in "OSPFv3 OSPFv3 Extended-LSA Sub-TLVs"

10. Sub-TLVs

6. Acknowledgement

Many thanks to Les Ginsberg for his suggestions on this draft. Also
thanks to Jeff Tantsura, Rob Shakir, Gunter Van De Velde, Goethals
Dirk, Smita Selot, Shaofu Peng, and John E Drake for their valuable
comments.

11.

7. References

11.1.

7.1. 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>.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.

[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3",

[RFC3101] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option",
RFC 4552, 3101, DOI 10.17487/RFC4552, June 2006,
<https://www.rfc-editor.org/info/rfc4552>. 10.17487/RFC3101, January 2003,
<https://www.rfc-editor.org/info/rfc3101>.

[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.

[RFC5709] Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
Authentication", RFC 5709, DOI 10.17487/RFC5709, October
2009, <https://www.rfc-editor.org/info/rfc5709>.

[RFC7166] Bhatia, M., Manral, V., and A. Lindem, "Supporting
Authentication Trailer for OSPFv3", RFC 7166,
DOI 10.17487/RFC7166, March 2014,
<https://www.rfc-editor.org/info/rfc7166>.

[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.

[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.

[RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
February 2016, <https://www.rfc-editor.org/info/rfc7770>.

[RFC7794] Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
March 2016, <https://www.rfc-editor.org/info/rfc7794>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.

[RFC8476] Tantsura, J., Chunduri, U., Aldrin, S.,

7.2. Informative References

[RFC3630] Katz, D., Kompella, K., and P. Psenak,
"Signaling Maximum SID Depth (MSD) Using OSPF", D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 8476, 3630,
DOI 10.17487/RFC8476, December 2018,
<https://www.rfc-editor.org/info/rfc8476>.

11.2. Informative References

[I-D.ietf-idr-bgp-ls-segment-routing-ext]
Previdi, S., Talaulikar, 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.

[RFC5329] Ishiguro, K., Filsfils, C., Gredler, H., Manral, V., Davey, A., and M. Chen, "BGP Link-State extensions for Segment
Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16
(work in progress), June 2019.

[I-D.ietf-ospf-mpls-elc]
Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, A. Lindem, Ed.,
"Traffic Engineering Extensions to OSPF Version 3",
RFC 5329, DOI 10.17487/RFC5329, September 2008,
<https://www.rfc-editor.org/info/rfc5329>.

[RFC5838] Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and M. Bocci, "Signaling Entropy Label Capability and
Entropy Readable Label-stack Depth Using OSPF", draft-
ietf-ospf-mpls-elc-12 (work in progress), October 2019.
R. Aggarwal, "Support of Address Families in OSPFv3",
RFC 5838, DOI 10.17487/RFC5838, April 2010,
<https://www.rfc-editor.org/info/rfc5838>.

[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.

Appendix A. Inter-Area Topology Retrieval Process

When an IP SDN Controller receives BGP-LS [RFC7752] information, it
should compare the prefix Network Layer Reachability Information
(NLRI) that is included in the BGP-LS NLRI. When it encounters the
same prefix but with different source router ID, it should extract
the corresponding area-ID, rebuild the link between these two source
routers in the non-backbone area. Below is one example that based on
the Figure 1: 5 which illustrates a topology where OSPF is running in
multiple areas.

Figure 5: OSPF Inter-Area Prefix Originator Scenario

R0-R4 are routers in the backbone area, S1-S4 are internal routers in
area 1, and T1-T4 are internal routers in area 2. R1 and R3 are ABRs
between area 0 and area 1. R2 and R4 are ABRs between area 0 and
area 2. N1 is the network between router S1 and S2 and N2 is the
network between router T1 and T2. Ls1 is the loopback address of
Node S1 and Lt1 is the loopback address of Node T1.

Assuming we want to rebuild the connection between router S1 and
router S2 located in area 1:

a. Normally, router S1 will advertise prefix N1 within its router-
LSA.

b. When this router-LSA reaches the ABR router R1, it will convert
it into summary-LSA, add the Prefix Source Router-ID sub-TLV,
which is router id of S1 Sub-TLV and the
Prefix Originator Sub-TLV, as described in this example. Section 3.

c. R1 then floods this extension summary-LSA to R0, which is using
the BGP-LS protocol with IP SDN Controller. The controller then
knows the prefix for N1 is from S1.

d. Router S2 will perform a similar process, and the controller will
also learn that prefix N1 is also from S2.

e. Then it can reconstruct the link between S1 and S2, using the
prefix N1. The topology within Area 1 can then be reconstructed
accordingly.

Iterating the above process continuously, the IP SDN controller can
retrieve a detailed topology that spans multiple areas.

Appendix B. Special Considerations on Inter-Area Topology Retrieval

The above topology retrieval process can be applied in the case where
each point-to-point or multi-access link connecting routers is
assigned a unique prefix. However, there are some situations where
this heuristic cannot be applied. Specifically, the cases where the
link is unnumbered or the prefix corresponding to the link is an
anycast prefix.

The Appendix A heuristic to rebuild the topology can normally be used
if all links are numbered. For anycast prefixes, if it corresponds
to the loopback interface and has a host prefix length, i.e., 32 for
IPv4 prefixes and 128 for IPv6 prefixes, Appendix A can also applied
since these anycast prefixes are not required to reconstruct the
topology.

Authors' Addresses
Aijun Wang
China Telecom
Beiqijia Town, Changping District
Beijing 102209
China

Email: wangaj3@chinatelecom.cn

Acee Lindem
Cisco Systems
301 Midenhall Way
Cary, NC 27513
USA

Email: acee@cisco.com

Jie Dong
Huawei Technologies
Beijing
China

Email: jie.dong@huawei.com

Peter Psenak
Cisco Systems
Pribinova Street 10
Bratislava, Eurovea Centre, Central 3 81109
Slovakia

Email: ppsenak@cisco.com

Ketan Talaulikar
Cisco Systems
S.No. 154/6, Phase I, Hinjawadi
Pune 411 057
India

Email: ketant@cisco.com