draft-ietf-lsr-ospf-prefix-originator-02.txt		draft-ietf-lsr-ospf-prefix-originator-03.txt
	LSR Working Group A. Wang		LSR Working Group A. Wang
	Internet-Draft China Telecom		Internet-Draft China Telecom
	Intended status: Standards Track A. Lindem		Intended status: Standards Track A. Lindem
	Expires: February 26, 2020 Cisco Systems		Expires: February 28, 2020 Cisco Systems
	J. Dong		J. Dong
	Huawei Technologies		Huawei Technologies
	K. Talaulikar
	P. Psenak		P. Psenak
			K. Talaulikar
	Cisco Systems		Cisco Systems
	August 25, 2019		August 27, 2019
	OSPF Extension for Prefix Originator		OSPF Extension for Prefix Originator
	draft-ietf-lsr-ospf-prefix-originator-02		draft-ietf-lsr-ospf-prefix-originator-03
	Abstract		Abstract
	This document describes Open Shortest Path First (OSPF) v2 and OSPFv3		This document describes Open Shortest Path First (OSPF) v2 and OSPFv3
	encodings to advertise the router-id of the originator of inter-area		encodings to advertise the router-id of the originator of inter-area
	prefixes for OSPFv2 and OSPFv3 Link-State Advertisement (LSA), which		prefixes for OSPFv2 and OSPFv3 Link-State Advertisement (LSA), which
	are needed in several use cases in multi-area OSPF use cases.		are needed in several use cases in multi-area OSPF use cases.
	Status of This Memo		Status of This Memo
	skipping to change at page 1, line 39 ¶		skipping to change at page 1, line 39 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on February 26, 2020.		This Internet-Draft will expire on February 28, 2020.
	Copyright Notice		Copyright Notice
	Copyright (c) 2019 IETF Trust and the persons identified as the		Copyright (c) 2019 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 20 ¶		skipping to change at page 2, line 20 ¶
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	2. Conventions used in this document . . . . . . . . . . . . . . 3		2. Conventions used in this document . . . . . . . . . . . . . . 3
	3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3		3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
	4. Conventions used in this document . . . . . . . . . . . . . . 4		4. Conventions used in this document . . . . . . . . . . . . . . 4
	5. Scenario Description . . . . . . . . . . . . . . . . . . . . 4		5. Scenario Description . . . . . . . . . . . . . . . . . . . . 4
	6. Prefix Source Router-ID sub-TLV . . . . . . . . . . . . . . . 5		6. Prefix Source Router-ID sub-TLV . . . . . . . . . . . . . . . 5
	7. Extended LSA Elements of Procedure . . . . . . . . . . . . . 6		7. Extended LSA Elements of Procedure . . . . . . . . . . . . . 6
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 6		8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
	9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6		9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
	10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 7		10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 7
	11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7		11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
	11.1. Normative References . . . . . . . . . . . . . . . . . . 7		11.1. Normative References . . . . . . . . . . . . . . . . . . 7
	11.2. Informative References . . . . . . . . . . . . . . . . . 8		11.2. Informative References . . . . . . . . . . . . . . . . . 8
	Appendix A. Inter-Area Topology Retrieval Process . . . . . . . 8		Appendix A. Inter-Area Topology Retrieval Process . . . . . . . 9
	Appendix B. Special Considerations on Inter-Area Topology		Appendix B. Special Considerations on Inter-Area Topology
	Retrieval . . . . . . . . . . . . . . . . . . . . . 9		Retrieval . . . . . . . . . . . . . . . . . . . . . 9
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
	1. Introduction		1. Introduction
	[I-D.ietf-ospf-mpls-elc] defines mechanisms to Entropy Readable Label		[I-D.ietf-ospf-mpls-elc] defines mechanisms to Entropy Readable Label
	Depth (ERLD) for ingress Label Switching Router (LSR) to discover		Depth (ERLD) for ingress Label Switching Router (LSR) to discover
	each LSR's capability of performing Entropy Label (EL) -based load-		each LSR's capability of performing Entropy Label (EL) -based load-
	balancing in Multi Protocol Label Switch (MPLS) networks. The		balancing in Multi Protocol Label Switch (MPLS) networks. The
	ingress LSR can use this information to push the appropriate label		ingress LSR can use this information to push the appropriate label
	stack for specific traffic, especially in segment routing		stack for specific traffic, especially in segment routing
	environments and other stacked LSPs scenarios.		environments and other stacked LSPs scenarios.
	However, in inter-area scenarios, the Area Border Router (ABR) does		However, in inter-area scenarios, the Area Border Router (ABR) does
	not advertise the originating OSPF router-id for inter-area prefixes.		not advertise the originating OSPF router-id for inter-area prefixes.
	An OSPF router in one area doesn't know where the prefixes really		An OSPF router in one area doesn't know where the prefixes really
	came from and can't determine the router that originated inter-area		came from and can't determine the router that originated inter-area
	prefixes and then can't judge the ERLD capabilities of the		prefixes and then can't judge the ERLD capabilities of the
	destination. It is necessary to transfer the originator information		destination. It is necessary to transfer the originator information
	of these inter-area prefixes to ensure the ingress LSR constructs the		of these inter-area prefixes to ensure the ingress LSR constructs the
	right Label stack.		right label stack.
	More generally, draft [RFC8476] defines a mechanism to advertise		More generally, [RFC8476] defines a mechanism to advertise multiple
	multiple types of supported Maximum SID Depths (MSD) at node and/or		types of supported Maximum SID Depths (MSD) at node and/or link
	link granularity. This information will be referred when the head-		granularity. This information will be referred when the head-end
	end router starts to send traffic to destination prefixes. In inter-		router starts to send traffic to destination prefixes. In inter-area
	area scenario, it is also necessary for the sender to learn the		scenario, it is also necessary for the sender to learn the
	capabilities of the receivers associated with the inter-area		capabilities of the receivers associated with the inter-area
	prefixes.		prefixes.
	There is also another scenario where knowing the originator of inter-		There is also another scenario where knowing the originator of inter-
	area prefixes is useful. For example, Border Gateway Protocol Link-		area prefixes is useful. For example, Border Gateway Protocol Link-
	State (BGP-LS) [RFC7752] describes mechanisms using the BGP protocol		State (BGP-LS) [RFC7752] describes mechanisms using the BGP protocol
	to advertise Link-State information. This can enable an Soft		to advertise Link-State information. This can enable an Software
	Definition Network (SDN) controller to collect the underlay network		Definition Network (SDN) controller to collect the underlay network
	topology automatically.		topology automatically.
	But if the underlay network is divided into multiple areas and		But if the underlay network is divided into multiple areas and
	running the OSPF protocol, it is not easy for the SDN controller to		running the OSPF protocol, it is not easy for the SDN controller to
	rebuild the multi-area topology, because normally an ABR that		rebuild the multi-area topology, because normally an ABR that
	connects multiple areas will hide the detailed topology information		connects multiple areas will hide the detailed topology information
	for these non-backbone areas. If only the router in backbone area		for these non-backbone areas. If only the internal routers within
	runs the BGP-LS protocol, it just learn and report the summary		backbone area run the BGP-LS protocol, they just learn and report the
	network information from the non-backbone areas. If the SDN		summary network information from the non-backbone areas. If the SDN
	controller can learn the originator of the inter-area prefixes, it is		controller can learn the originator of the inter-area prefixes, it is
	possible for them to rebuild the inter-area topology automatically.		possible to rebuild the inter-area topology automatically.
	[RFC7794] introduces the Intermediate System to Intermediate System		[RFC7794] introduces the Intermediate System to Intermediate System
	(IS-IS) "IPv4/IPv6 Source Router IDs" Type-Length-Value (TLV) to		(IS-IS) "IPv4/IPv6 Source Router IDs" Type-Length-Value (TLV) to
	advertise the source of the prefixes redistributed from a different		advertise the source of the prefixes redistributed from a different
	IS-IS level. This TLV can be used in the above scenarios. Such		IS-IS level. This TLV can be used in the above scenarios. Such
	solution can also be applied in networks that run the OSPF protocol,		solution can also be applied in networks that run the OSPF protocol,
	but the related LSAs TLV must be extended.		but the related LSAs TLV must be extended.
	This draft provides such solution for the OSPFv2 and OSPFv3		This draft provides such solution for the OSPFv2 and OSPFv3
	protocols.		protocols.
	skipping to change at page 4, line 10 ¶		skipping to change at page 4, line 10 ¶
	o ERLD: Entropy Readable Label Depth		o ERLD: Entropy Readable Label Depth
	o EL: Entropy Label		o EL: Entropy Label
	o IS-IS: Intermediate System to Intermediate System		o IS-IS: Intermediate System to Intermediate System
	o LSA: Link-State Advertisement		o LSA: Link-State Advertisement
	o MSD: Maximum SID Depths		o MSD: Maximum SID Depths
			o NLRI: Network Layer Reachability Information
	o OSPF: Open Shortest Path First		o OSPF: Open Shortest Path First
	o SID: Segment IDentifier		o SID: Segment IDentifier
			o SDN: Software Definition Network
	4. Conventions used in this document		4. Conventions used in this document
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in [RFC2119] .		document are to be interpreted as described in [RFC2119] .
	5. Scenario Description		5. Scenario Description
	Figure 1 illustrates the topology scenario when OSPF is running in		Figure 1 illustrates the topology scenario when OSPF is running in
	multi-area. R0-R4 are routers in backbone area, S1-S4,T1-T4 are		multi-area. R0-R4 are routers in backbone area, S1-S4,T1-T4 are
	skipping to change at page 5, line 14 ¶		skipping to change at page 5, line 36 ¶
	If S1 wants to send traffic to prefix Lt1 that is connected T1 in		If S1 wants to send traffic to prefix Lt1 that is connected T1 in
	another area, it should know the ERLD, and MSD values that are		another area, it should know the ERLD, and MSD values that are
	associated with the node T1, and then construct the right label stack		associated with the node T1, and then construct the right label stack
	at the ingress node for the target traffic.		at the ingress node for the target traffic.
	In another scenario, If R0 has some method to learn the originator of		In another scenario, If R0 has some method to learn the originator of
	network N1 and reports such information to IP SDN controller, then it		network N1 and reports such information to IP SDN controller, then it
	is possible for the controller to retrieval the topology in non-		is possible for the controller to retrieval the topology in non-
	backbone area. The topology retrieval process and its usage		backbone area. The topology retrieval process and its usage
	limitation are described in the Appendix A and Appendix B.		limitation are described in the Appendix A and Appendix B .
	From the above scenarios, we can conclude it is useful to introduce		From the above scenarios, we can conclude it is useful to introduce
	and define the prefix originator sub TLV within OSPF.		and define the prefix originator sub TLV within OSPF.
	6. Prefix Source Router-ID sub-TLV		6. Prefix Source Router-ID sub-TLV
	[RFC7684] and [RFC8362] define the TLV extensions for OSPFv2 and		[RFC7684] and [RFC8362] define the TLV extensions for OSPFv2 and
	OSPFv3 respectively. These documents facilitate addition of new		OSPFv3 respectively. These documents facilitate addition of new
	attributes for prefixes. Based on these formats, we can define new		attributes for prefixes. Based on these formats, we can define new
	sub-TLV to advertise the "Prefix Source Router ID", as that defined		sub-TLV to advertise the "Prefix Source Router ID", as that defined
	skipping to change at page 5, line 45 ¶		skipping to change at page 6, line 21 ¶
	+---------------------------------------------------------------+		+---------------------------------------------------------------+
	Figure 2: Prefix Source Router-ID sub-TLV Format		Figure 2: Prefix Source Router-ID sub-TLV Format
	o Source Router-ID Sub-TLV Type: TBD1[RFC7684] or TBD2 [RFC8362]		o Source Router-ID Sub-TLV Type: TBD1[RFC7684] or TBD2 [RFC8362]
	o Length: 4		o Length: 4
	o Value: Router-ID of OSPFv2/OSPFv3 source router		o Value: Router-ID of OSPFv2/OSPFv3 source router
	For OSPFv2, this sub-TLV is a sub-TLV of OSPFv2 Extended Prefix TLV,		For OSPFv2, this sub-TLV is a sub-TLV of OSPFv2 Extended Prefix TLV,
	which is included in the "OSPFv2 Extended Prefix Opaque LSA"		which SHOULD be included in the "OSPFv2 Extended Prefix Opaque LSA" .
	[RFC7684].
	For OSPFv3, this sub-TLV is a sub-TLV of "Inter-Area-Prefix TLV",		For OSPFv3, this sub-TLV is a sub-TLV of "Inter-Area-Prefix TLV",
	which is included in the "E-Inter-Area-Prefix-LSA".		which SHOULD be included in the "E-Inter-Area-Prefix-LSA".
	7. Extended LSA Elements of Procedure		7. Extended LSA Elements of Procedure
	When an ABR, for example R2 in Fig.1, receives the Router-LSA		When an ABR, for example R2 in Figure 1, receives the Router-LSA
	announcement in area 2, it should originate the corresponding "OSPFv2		announcement in area 2, it should originate the corresponding "OSPFv2
	Extended Prefix Opaque LSA" for OSPFv2 or "E-Inter-Area-Prefix-LSA"		Extended Prefix Opaque LSA" for OSPFv2 or "E-Inter-Area-Prefix-LSA"
	for OSPFv3 that includes the Source Router-ID sub-TLV for the network		for OSPFv3 that includes the Source Router-ID sub-TLV for the network
	prefixes, e.g., for prefix Lt1, N2. etc., which identifies the source		prefixes, e.g., for prefix Lt1, N2. etc., which identifies the source
	router that advertised the prefix.		router that advertised the prefix.
	When S1 in another area receives such LSA, it then can learn that		When S1 in another area receives such LSA, it then can learn that
	prefix Lt1 is associated with node T1, check the ERLD, or MSD value		prefix Lt1 is associated with node T1, check the ERLD, or MSD value
	according to its necessity, and construct the right label stack at		according to its necessity, and construct the right label stack at
	the ingress node S1 for the traffic destined to Lt1.		the ingress node S1 for the traffic destined to Lt1.
	skipping to change at page 6, line 30 ¶		skipping to change at page 7, line 4 ¶
	includes it in the prefix information advertised to an SDN controller		includes it in the prefix information advertised to an SDN controller
	as described in[I-D.ietf-idr-bgp-ls-segment-routing-ext]. The SDN		as described in[I-D.ietf-idr-bgp-ls-segment-routing-ext]. The SDN
	controller can then use such information to build the inter-area		controller can then use such information to build the inter-area
	topology according to the process described in the Appendix A. The		topology according to the process described in the Appendix A. The
	topology retrieval process may not suitable for some environments as		topology retrieval process may not suitable for some environments as
	stated in Appendix B.		stated in Appendix B.
	8. Security Considerations		8. Security Considerations
	Security concerns for OSPF are addressed in [RFC5709]		Security concerns for OSPF are addressed in [RFC5709]
	Advertisement of the additional information defined in this document		Advertisement of the additional information defined in this document
	introduces no new security concerns		introduces no new security concerns
	9. IANA Considerations		9. IANA Considerations
	This specification defines one Prefix Source Router-ID sub-TLV as		This specification defines one Prefix Source Router-ID sub-TLV as
	described in Section 6. This value should be added to the existing		described in Section 6. This value should be added to the existing
	OSPFv2 Extended Prefix TLV Sub-TLVs registry and OSPFv3 Extended-LSA		"OSPFv2 Extended Prefix TLV Sub-TLVs" registry and "OSPFv3 Extended-
	Sub-TLVs registry respectively.		LSA Sub-TLVs registry" respectively.
	Th following new sub-TLV is added to the registry of "OSPFv2 Extended		The following new sub-TLV is added to the registry of "OSPFv2
	Prefix TLV Sub-TLVs". The allocation policy is IETF Review that		Extended Prefix TLV Sub-TLVs". The allocation policy is IETF Review
	defined in [RFC7684]		that defined in [RFC7684]
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++
	| Code Point | Description | Status |		| Code Point | Description | Status |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++
	| TBD | Prefix Source Sub-TLV | Allocation from IANA |		| TBD | Prefix Source Sub-TLV | Allocation from IANA |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++
	Figure 3: Prefix Source sub-TLV CodePoint from OSPFv2 Extended Prefix TLV Sub-TLVs		Figure 3: CodePoint in "OSPFv2 Extended Prefix TLV Sub-TLVs"
	The following sub-TLV is added to the registry of "OSPFv3 Extended-
	LSA Sub-TLVs". The allocation is IETF Review that defined in		The following new sub-TLV is added to the registry of "OSPFv3
	[RFC8362]		Extended-LSA Sub-TLVs". The allocation policy is IETF Review that
			defined in [RFC8362]
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++
	| Code Point | Description | Status |		| Code Point | Description | Status |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++
	| TBD | Prefix Source Sub-TLV | Allocation from IANA |		| TBD | Prefix Source Sub-TLV | Allocation from IANA |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++++++++
	Figure 4: Prefix Source sub-TLV CodePoint from OSPFv3 Extended-LSA Sub-TLVs		Figure 4: CodePoint in "OSPFv3 Extended-LSA Sub-TLVs"
	10. Acknowledgement		10. Acknowledgement
	Many thanks to Les Ginsberg for his valuable suggestions on this		Many thanks to Les Ginsberg for his valuable suggestions on this
	draft. And also thanks Jeff Tantsura,Rob Shakir, Van De Velde		draft. And also thanks Jeff Tantsura,Rob Shakir, Van De Velde
	Gunter, Goethals Dirk, Shaofu Peng, John E Drake for their valuable		Gunter, Goethals Dirk, Shaofu Peng, John E Drake for their valuable
	comments on this draft.		comments on this draft.
	11. References		11. References
	skipping to change at page 8, line 32 ¶		skipping to change at page 9, line 8 ¶
	[I-D.ietf-ospf-mpls-elc]		[I-D.ietf-ospf-mpls-elc]
	Xu, X., Kini, S., Psenak, P., Filsfils, C., and S.		Xu, X., Kini, S., Psenak, P., Filsfils, C., and S.
	Litkowski, "Signaling Entropy Label Capability and Entropy		Litkowski, "Signaling Entropy Label Capability and Entropy
	Readable Label-stack Depth Using OSPF", draft-ietf-ospf-		Readable Label-stack Depth Using OSPF", draft-ietf-ospf-
	mpls-elc-08 (work in progress), May 2019.		mpls-elc-08 (work in progress), May 2019.
	Appendix A. Inter-Area Topology Retrieval Process		Appendix A. Inter-Area Topology Retrieval Process
	When an IP SDN Controller receives this information, it should		When an IP SDN Controller receives this information, it should
	compare the prefix NLRI that included in the BGP-LS packet. When it		compare the prefix Network Layer Reachability Information (NLRI) that
	encounters the same prefix but with different source router ID, it		included in the BGP-LS packet. When it encounters the same prefix
	should extract the corresponding area-ID, rebuild the link between		but with different source router ID, it should extract the
	these two different source routers in non-backbone area. Belows is		corresponding area-ID, rebuild the link between these two different
	one example that based on the Fig.1:		source routers in non-backbone area. Belows is one example that
			based on the Figure 1:
	Assuming we want to rebuild the connection between router S1 and		Assuming we want to rebuild the connection between router S1 and
	router S2 that locates in area 1:		router S2 that locates in area 1:
	a. Normally, router S1 will advertise prefix N1 within its router-		a. Normally, router S1 will advertise prefix N1 within its router-
	LSA.		LSA.
	b. When this router-LSA reaches the ABR router R1, it will convert		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,		it into summary-LSA, add the Prefix Source Router-ID sub-TLV,
	which is router id of S1 in this example.		which is router id of S1 in this example.
	skipping to change at page 9, line 12 ¶		skipping to change at page 9, line 36 ¶
	BGP-LS protocol with IP SDN Controller. The controller then		BGP-LS protocol with IP SDN Controller. The controller then
	knows the prefixes of N1 is from S1.		knows the prefixes of N1 is from S1.
	d. Router S2 will do the similar process, and the controller will		d. Router S2 will do the similar process, and the controller will
	also learn that prefixes N1 is also from S2.		also learn that prefixes N1 is also from S2.
	e. Then it can reconstruct the link between S1 and S2, using the		e. Then it can reconstruct the link between S1 and S2, using the
	prefix N1. The topology within Area 1 can then be reconstructed		prefix N1. The topology within Area 1 can then be reconstructed
	accordingly.		accordingly.
	Iterating the above process continuously, an IP SDN controller can		Iterating the above process continuously, the IP SDN controller can
	retrieve a detailed topology that spans multiple areas.		retrieve a detailed topology that spans multiple areas.
	Appendix B. Special Considerations on Inter-Area Topology Retrieval		Appendix B. Special Considerations on Inter-Area Topology Retrieval
	The above topology retrieval process can be applied in the case where		The above topology retrieval process can be applied in the case where
	each link between routers is assigned a unique prefix. However,		each link between routers is assigned a unique prefix. However,
	there are some situations where this heuristic cannot be applied.		there are some situations where this heuristic cannot be applied.
	Specifically, the cases where the link is unnumbered or the prefix		Specifically, the cases where the link is unnumbered or the prefix
	corresponding to the link is an anycast prefix and is not unique.		corresponding to the link is an anycast prefix.
	The Appendix A heuristic to rebuild the topology can normally be used		The Appendix A heuristic to rebuild the topology can normally be used
	if all links are numbered and the anycast prefixes correspond to		if all links are numbered. For anycast prefixes, if it corresponds
	loopbacks and have a host prefix length, i.e., 32 for IPv4 prefixes		to the loopback interface and has a host prefix length, i.e., 32 for
	and 128 for IPv6 prefixes.		IPv4 prefixes and 128 for IPv6 prefixes, Appendix A can also apply.
	Authors' Addresses		Authors' Addresses
	Aijun Wang		Aijun Wang
	China Telecom		China Telecom
	Beiqijia Town, Changping District		Beiqijia Town, Changping District
	Beijing 102209		Beijing 102209
	China		China
	Email: wangaj3@chinatelecom.cn		Email: wangaj3@chinatelecom.cn
	skipping to change at page 10, line 4 ¶		skipping to change at page 10, line 22 ¶
	Email: wangaj3@chinatelecom.cn		Email: wangaj3@chinatelecom.cn
	Acee Lindem		Acee Lindem
	Cisco Systems		Cisco Systems
	301 Midenhall Way		301 Midenhall Way
	Cary, NC 27513		Cary, NC 27513
	USA		USA
	Email: acee@cisco.com		Email: acee@cisco.com
	Jie Dong		Jie Dong
	Huawei Technologies		Huawei Technologies
	Beijing		Beijing
	China		China
	Email: jie.dong@huawei.com		Email: jie.dong@huawei.com
	Ketan Talaulikar
	Cisco Systems
	S.No. 154/6, Phase I, Hinjawadi
	Pune 411 057
	India
	Email: ketant@cisco.com
	Peter Psenak		Peter Psenak
	Cisco Systems		Cisco Systems
	Pribinova Street 10		Pribinova Street 10
	Bratislava, Eurovea Centre, Central 3 81109		Bratislava, Eurovea Centre, Central 3 81109
	Slovakia		Slovakia
	Email: ppsenak@cisco.com		Email: ppsenak@cisco.com
			Ketan Talaulikar
			Cisco Systems
			S.No. 154/6, Phase I, Hinjawadi
			Pune 411 057
			India
			Email: ketant@cisco.com
End of changes. 32 change blocks.
	53 lines changed or deleted		50 lines changed or added
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