draft-ietf-opsec-urpf-improvements-00.txt		draft-ietf-opsec-urpf-improvements-01.txt
	OPSEC Working Group K. Sriram		OPSEC Working Group K. Sriram
	Internet-Draft D. Montgomery		Internet-Draft D. Montgomery
	Intended status: Best Current Practice USA NIST		Intended status: Best Current Practice USA NIST
	Expires: October 22, 2018 J. Haas		Expires: April 24, 2019 J. Haas
	Juniper Networks, Inc.		Juniper Networks, Inc.
	April 20, 2018		October 21, 2018
	Enhanced Feasible-Path Unicast Reverse Path Filtering		Enhanced Feasible-Path Unicast Reverse Path Filtering
	draft-ietf-opsec-urpf-improvements-00		draft-ietf-opsec-urpf-improvements-01
	Abstract		Abstract
	This document identifies a need for improvement of the unicast		This document identifies a need for improvement of the unicast
	Reverse Path Filtering techniques (uRPF) [BCP84] for source address		Reverse Path Filtering techniques (uRPF) [BCP84] for source address
	validation (SAV) [BCP38]. The strict uRPF is inflexible about		validation (SAV) [BCP38]. The strict uRPF is inflexible about
	directionality, the loose uRPF is oblivious to directionality, and		directionality, the loose uRPF is oblivious to directionality, and
	the current feasible-path uRPF attempts to strike a balance between		the current feasible-path uRPF attempts to strike a balance between
	the two [BCP84]. However, as shown in this draft, the existing		the two [BCP84]. However, as shown in this draft, the existing
	feasible-path uRPF still has short comings. This document describes		feasible-path uRPF still has short comings. This document describes
	skipping to change at page 1, line 43 ¶		skipping to change at page 1, line 43 ¶
	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 October 22, 2018.		This Internet-Draft will expire on April 24, 2019.
	Copyright Notice		Copyright Notice
	Copyright (c) 2018 IETF Trust and the persons identified as the		Copyright (c) 2018 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 24 ¶		skipping to change at page 2, line 24 ¶
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3		1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
	2. Review of Existing Source Address Validation Techniques . . . 3		2. Review of Existing Source Address Validation Techniques . . . 3
	2.1. SAV using Access Control List . . . . . . . . . . . . . . 4		2.1. SAV using Access Control List . . . . . . . . . . . . . . 4
	2.2. SAV using Strict Unicast Reverse Path Filtering . . . . . 4		2.2. SAV using Strict Unicast Reverse Path Filtering . . . . . 4
	2.3. SAV using Feasible-Path Unicast Reverse Path Filtering . 5		2.3. SAV using Feasible-Path Unicast Reverse Path Filtering . 5
	2.4. SAV using Loose Unicast Reverse Path Filtering . . . . . 6		2.4. SAV using Loose Unicast Reverse Path Filtering . . . . . 6
			2.5. SAV using VRF Table . . . . . . . . . . . . . . . . . . . 7
	3. SAV using Enhanced Feasible-Path uRPF . . . . . . . . . . . . 7		3. SAV using Enhanced Feasible-Path uRPF . . . . . . . . . . . . 7
	3.1. Description of the Method . . . . . . . . . . . . . . . . 7		3.1. Description of the Method . . . . . . . . . . . . . . . . 7
	3.1.1. Algorithm A: Enhanced Feasible-Path uRPF . . . . . . 8		3.1.1. Algorithm A: Enhanced Feasible-Path uRPF . . . . . . 8
	3.2. Operational Recommendations . . . . . . . . . . . . . . . 9		3.2. Operational Recommendations . . . . . . . . . . . . . . . 9
	3.3. A Challenging Scenario . . . . . . . . . . . . . . . . . 9		3.3. A Challenging Scenario . . . . . . . . . . . . . . . . . 9
	3.4. Algorithm B: Enhanced Feasible-Path uRPF with Additional		3.4. Algorithm B: Enhanced Feasible-Path uRPF with Additional
	Flexibility Across Customer Cone . . . . . . . . . . . . 10		Flexibility Across Customer Cone . . . . . . . . . . . . 10
	3.5. Implementation Considerations . . . . . . . . . . . . . . 11		3.5. Augmenting RPF Tables with ROA and IRR Data . . . . . . . 11
	3.5.1. Impact on FIB Memory Size Requirement . . . . . . . . 11		3.6. Implementation Considerations . . . . . . . . . . . . . . 11
	3.6. Summary of Recommendations . . . . . . . . . . . . . . . 12		3.6.1. Impact on FIB Memory Size Requirement . . . . . . . . 11
			3.6.2. Coping with BGP's Transient Behavior . . . . . . . . 12
			3.7. Summary of Recommendations . . . . . . . . . . . . . . . 13
	4. Security Considerations . . . . . . . . . . . . . . . . . . . 13		4. Security Considerations . . . . . . . . . . . . . . . . . . . 13
	5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13		5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
	6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13		6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
	7. Informative References . . . . . . . . . . . . . . . . . . . 13		7. Informative References . . . . . . . . . . . . . . . . . . . 14
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
	1. Introduction		1. Introduction
	This internet draft identifies a need for improvement of the unicast		This internet draft identifies a need for improvement of the unicast
	Reverse Path Filtering (uRPF) techniques [RFC2827] for source address		Reverse Path Filtering (uRPF) techniques [RFC2827] for source address
	validation (SAV) [RFC3704]. The strict uRPF is inflexible about		validation (SAV) [RFC3704]. The strict uRPF is inflexible about
	directionality, the loose uRPF is oblivious to directionality, and		directionality, the loose uRPF is oblivious to directionality, and
	the current feasible-path uRPF attempts to strike a balance between		the current feasible-path uRPF attempts to strike a balance between
	the two [RFC3704]. However, as shown in this draft, the existing		the two [RFC3704]. However, as shown in this draft, the existing
	feasible-path uRPF still has short comings. Even with the feasible-		feasible-path uRPF still has short comings. Even with the feasible-
	skipping to change at page 3, line 15 ¶		skipping to change at page 3, line 17 ¶
	This document describes an enhanced feasible-path uRPF technique,		This document describes an enhanced feasible-path uRPF technique,
	which aims to be more flexible (in a meaningful way) about		which aims to be more flexible (in a meaningful way) about
	directionality than the feasible-path uRPF. It is based on the		directionality than the feasible-path uRPF. It is based on the
	principle that if BGP updates for multiple prefixes with the same		principle that if BGP updates for multiple prefixes with the same
	origin AS were received on different interfaces (at border routers),		origin AS were received on different interfaces (at border routers),
	then incoming data packets with source addresses in any of those		then incoming data packets with source addresses in any of those
	prefixes should be accepted on any of those interfaces (presented in		prefixes should be accepted on any of those interfaces (presented in
	Section 3). For some challenging ISP-customer scenarios (see		Section 3). For some challenging ISP-customer scenarios (see
	Section 3.3), this document also describes a more relaxed version of		Section 3.3), this document also describes a more relaxed version of
	the enhanced feasible-path uRPF technique (presented in Section 3.4).		the enhanced feasible-path uRPF technique (presented in Section 3.4).
	Implementation considerations are discussed in Section 3.5.		Implementation considerations are discussed in Section 3.6.
	Note: Definition of Reverse Path Filtering (RPF) list: The list of		Note: Definition of Reverse Path Filtering (RPF) list: The list of
	permissible source address prefixes for incoming data packets on a		permissible source address prefixes for incoming data packets on a
	given interface.		given interface.
	Note: Throughout this document, the routes in consideration are		Note: Throughout this document, the routes in consideration are
	assumed to have been vetted based on prefix filtering [RFC7454] and		assumed to have been vetted based on prefix filtering [RFC7454] and
	possibly (in the future) origin validation [RFC6811].		possibly (in the future) origin validation [RFC6811].
	The enhanced feasible-path uRPF methods described here are expected		The enhanced feasible-path uRPF methods described here are expected
	skipping to change at page 3, line 46 ¶		skipping to change at page 3, line 48 ¶
	document are to be interpreted as described in RFC 2119 [RFC2119].		document are to be interpreted as described in RFC 2119 [RFC2119].
	2. Review of Existing Source Address Validation Techniques		2. Review of Existing Source Address Validation Techniques
	There are various existing techniques for mitigation against DDoS		There are various existing techniques for mitigation against DDoS
	attacks with spoofed addresses [RFC2827] [RFC3704]. There are also		attacks with spoofed addresses [RFC2827] [RFC3704]. There are also
	some techniques used for mitigating reflection attacks [RRL]		some techniques used for mitigating reflection attacks [RRL]
	[TA14-017A], which are used to amplify the impact in DDoS attacks.		[TA14-017A], which are used to amplify the impact in DDoS attacks.
	Employing a combination of these preventive techniques (as		Employing a combination of these preventive techniques (as
	applicable) in enterprise and ISP border routers, broadband and		applicable) in enterprise and ISP border routers, broadband and
	wireless access network, data centers, and DNS servers provides		wireless access network, data centers, and DNS/NTP servers provides
	reasonably effective protection against DDoS attacks.		reasonably effective protection against DDoS attacks.
	Source address validation (SAV) is performed in network edge devices		Source address validation (SAV) is performed in network edge devices
	such as border routers, Cable Modem Termination Systems (CMTS),		such as border routers, Cable Modem Termination Systems (CMTS),
	Digital Subscriber Line Access Multiplexers (DSLAM), and Packet Data		Digital Subscriber Line Access Multiplexers (DSLAM), and Packet Data
	Network (PDN) gateways in mobile networks. Ingress Access Control		Network (PDN) gateways in mobile networks. Ingress Access Control
	List (ACL) and unicast Reverse Path Filtering (uRPF) are techniques		List (ACL) and unicast Reverse Path Filtering (uRPF) are techniques
	employed for implementing SAV [RFC2827] [RFC3704] [ISOC].		employed for implementing SAV [RFC2827][RFC3704][ISOC].
	2.1. SAV using Access Control List		2.1. SAV using Access Control List
	Ingress/egress Access Control Lists (ACLs) are maintained which list		Ingress/egress Access Control Lists (ACLs) are maintained which list
	acceptable (or alternatively, unacceptable) prefixes for the source		acceptable (or alternatively, unacceptable) prefixes for the source
	addresses in the incoming Internet Protocol (IP) packets. Any packet		addresses in the incoming Internet Protocol (IP) packets. Any packet
	with a source address that does not match the filter is dropped. The		with a source address that does not match the filter is dropped. The
	ACLs for the ingress/egress filters need to be maintained to keep		ACLs for the ingress/egress filters need to be maintained to keep
	them up to date. Updating the ACLs is an operator driven manual		them up to date. Updating the ACLs is an operator driven manual
	process, and hence operationally difficult or infeasible.		process, and hence operationally difficult or infeasible.
	skipping to change at page 4, line 35 ¶		skipping to change at page 4, line 40 ¶
	2.2. SAV using Strict Unicast Reverse Path Filtering		2.2. SAV using Strict Unicast Reverse Path Filtering
	In the strict unicast Reverse Path Filtering (uRPF) method, an		In the strict unicast Reverse Path Filtering (uRPF) method, an
	ingress packet at border router is accepted only if the Forwarding		ingress packet at border router is accepted only if the Forwarding
	Information Base (FIB) contains a prefix that encompasses the source		Information Base (FIB) contains a prefix that encompasses the source
	address, and forwarding information for that prefix points back to		address, and forwarding information for that prefix points back to
	the interface over which the packet was received. In other words,		the interface over which the packet was received. In other words,
	the reverse path for routing to the source address (if it were used		the reverse path for routing to the source address (if it were used
	as a destination address) should use the same interface over which		as a destination address) should use the same interface over which
	the packet was received. It is well known that this method has		the packet was received. It is well known that this method has
	limitations when networks are multi-homed and there is asymmetric		limitations when networks are multi-homed, routes are not
	routing of packets. Asymmetric routing occurs (see Figure 1) when a		symmetrically announced to all transit providers, and there is
	customer AS announces one prefix (P1) to one transit provider (ISP-a)		asymmetric routing of data packets. Asymmetric routing occurs (see
	and a different prefix (P2) to another transit provider (ISP-b), but		Figure 1) when a customer AS announces one prefix (P1) to one transit
	routes data packets with source addresses in the second prefix (P2)		provider (ISP-a) and a different prefix (P2) to another transit
	to the first transit provider (ISP-a) or vice versa.		provider (ISP-b), but routes data packets with source addresses in
			the second prefix (P2) to the first transit provider (ISP-a) or vice
			versa.
	+------------+ ---- P1[AS2 AS1] ---> +------------+		+------------+ ---- P1[AS2 AS1] ---> +------------+
	| AS2(ISP-a) | <----P2[AS3 AS1] ---- | AS3(ISP-b)|		| AS2(ISP-a) | <----P2[AS3 AS1] ---- | AS3(ISP-b)|
	+------------+ +------------+		+------------+ +------------+
	/\ /\		/\ /\
	\ /		\ /
	\ /		\ /
	\ /		\ /
	P1[AS1]\ /P2[AS1]		P1[AS1]\ /P2[AS1]
	\ /		\ /
	skipping to change at page 5, line 25 ¶		skipping to change at page 5, line 25 ¶
	| AS1(customer) |		| AS1(customer) |
	+-----------------------+		+-----------------------+
	P1, P2 (prefixes originated)		P1, P2 (prefixes originated)
	Consider data packets received at AS2		Consider data packets received at AS2
	(1) from AS1 with source address in P2, or		(1) from AS1 with source address in P2, or
	(2) from AS3 that originated from AS1		(2) from AS3 that originated from AS1
	with source address in P1:		with source address in P1:
	* Strict uRPF fails		* Strict uRPF fails
	* Feasible-path uRPF fails		* Feasible-path uRPF fails
	* Loose uRPF works (but ineffective in IPv4)		* Loose uRPF works (but not desirable)
	* Enhanced Feasible-path uRPF works best		* Enhanced Feasible-path uRPF works best
	Figure 1: Scenario 1 for illustration of efficacy of uRPF schemes.		Figure 1: Scenario 1 for illustration of efficacy of uRPF schemes.
	2.3. SAV using Feasible-Path Unicast Reverse Path Filtering		2.3. SAV using Feasible-Path Unicast Reverse Path Filtering
	The feasible-path uRPF helps partially overcome the problem		The feasible-path uRPF helps partially overcome the problem
	identified with the strict uRPF in the multi-homing case. The		identified with the strict uRPF in the multi-homing case. The
	feasible-path uRPF is similar to the strict uRPF, but in addition to		feasible-path uRPF is similar to the strict uRPF, but in addition to
	inserting the best-path prefix, additional prefixes from alternative		inserting the best-path prefix, additional prefixes from alternative
	announced routes are also included in the RPF table. This method		announced routes are also included in the RPF table. This method
	relies on announcements for the same prefixes (albeit some may be		relies on announcements for the same prefixes (albeit some may be
	prepended to effect lower preference) propagating to all routers		prepended to effect lower preference) propagating to all routers
	performing feasible-path uRPF checks. Therefore, in the multi-homing		performing feasible-path uRPF checks. Therefore, in the multi-homing
	scenario, if the customer AS announces routes for both prefixes (P1,		scenario (see Figure 2), if the customer AS announces routes for both
	P2) to both transit providers (with suitable prepends if needed for		prefixes (P1, P2) to both transit providers (with suitable prepends
	traffic engineering), then the feasible-path uRPF method works (see		if needed for traffic engineering), then the feasible-path uRPF
	Figure 2). It should be mentioned that the feasible-path uRPF works		method works. It should be mentioned that the feasible-path uRPF
	in this scenario only if customer routes are preferred at AS2 and AS3		works in this scenario only if customer routes are preferred at AS2
	over a shorter non-customer route.		and AS3 over a shorter non-customer route.
	+------------+ routes for P1, P2 +-----------+		+------------+ routes for P1, P2 +-----------+
	| AS2(ISP-a) |<-------------------->| AS3(ISP-b)|		| AS2(ISP-a) |<-------------------->| AS3(ISP-b)|
	+------------+ (p2p) +-----------+		+------------+ (p2p) +-----------+
	/\ /\		/\ /\
	\ /		\ /
	P1[AS1]\ /P2[AS1]		P1[AS1]\ /P2[AS1]
	\ /		\ /
	P2[AS1 AS1 AS1]\ /P1[AS1 AS1 AS1]		P2[AS1 AS1 AS1]\ /P1[AS1 AS1 AS1]
	\ /		\ /
	skipping to change at page 6, line 25 ¶		skipping to change at page 6, line 25 ¶
	| AS1(customer) |		| AS1(customer) |
	+-----------------------+		+-----------------------+
	P1, P2 (prefixes originated)		P1, P2 (prefixes originated)
	Consider data packets received at AS2 via AS3		Consider data packets received at AS2 via AS3
	that originated from AS1 and have source address in P1:		that originated from AS1 and have source address in P1:
	* Feasible-path uRPF works (if customer route to P1		* Feasible-path uRPF works (if customer route to P1
	is preferred at AS3 over shorter path)		is preferred at AS3 over shorter path)
	* Feasible-path uRPF fails (if shorter path to P1		* Feasible-path uRPF fails (if shorter path to P1
	is preferred at AS3 over customer route)		is preferred at AS3 over customer route)
	* Loose uRPF works (but ineffective in IPv4)		* Loose uRPF works (but not desirable)
	* Enhanced Feasible-path uRPF works best		* Enhanced Feasible-path uRPF works best
	Figure 2: Scenario 2 for illustration of efficacy of uRPF schemes.		Figure 2: Scenario 2 for illustration of efficacy of uRPF schemes.
	However, the feasible-path uRPF method has limitations as well. One		However, the feasible-path uRPF method has limitations as well. One
	form of limitation naturally occurs when the recommendation of		form of limitation naturally occurs when the recommendation of
	propagating the same prefixes to all routers is not followed.		propagating the same prefixes to all routers is not followed.
	Another form of limitation can be described as follows. In Scenario		Another form of limitation can be described as follows. In Scenario
	2 (described above, illustrated in Figure 2), it is possible that the		2 (described above, illustrated in Figure 2), it is possible that the
	second transit provider (ISP-b or AS3) does not propagate the		second transit provider (ISP-b or AS3) does not propagate the
	skipping to change at page 7, line 7 ¶		skipping to change at page 7, line 7 ¶
	In the loose unicast Reverse Path Filtering (uRPF) method, an ingress		In the loose unicast Reverse Path Filtering (uRPF) method, an ingress
	packet at the border router is accepted only if the FIB has one or		packet at the border router is accepted only if the FIB has one or
	more prefixes that encompass the source address. That is, a packet		more prefixes that encompass the source address. That is, a packet
	is dropped if no route exists in the FIB for the source address.		is dropped if no route exists in the FIB for the source address.
	Loose uRPF sacrifices directionality. This method is not effective		Loose uRPF sacrifices directionality. This method is not effective
	for prevention of address spoofing since there is little unrouted		for prevention of address spoofing since there is little unrouted
	address space in IPv4. It only drops packets if the spoofed address		address space in IPv4. It only drops packets if the spoofed address
	is unreachable in the current FIB (e.g. RFC 1918, unallocated,		is unreachable in the current FIB (e.g. RFC 1918, unallocated,
	allocated but currently not routed).		allocated but currently not routed).
			2.5. SAV using VRF Table
			The Virtual Routing and Forwarding (VRF) technology allows a router
			to maintain multiple routing table instances, separate from the
			global Routing Information Base (RIB) [Cisco3]. External BGP (eBGP)
			peering sessions send specific routes to be stored in a dedicated VRF
			table. The uRPF process queries the VRF table (instead of the FIB)
			for source address validation. VRF table can be dedicated per eBGP
			peer and used for uRPF for only that peer, resulting in strict mode
			operation. On the other hand, if loose mode uRPF is desired with
			VRF, then the VRF table can be global (contains VRF routes received
			on all eBGP sessions at the router).
	3. SAV using Enhanced Feasible-Path uRPF		3. SAV using Enhanced Feasible-Path uRPF
	3.1. Description of the Method		3.1. Description of the Method
	Enhanced feasible-path uRPF adds greater operational robustness and		Enhanced feasible-path uRPF adds greater operational robustness and
	efficacy to existing uRPF methods discussed in Section 2. The		efficacy to existing uRPF methods discussed in Section 2. The
	technique is based on the principle that if BGP updates for multiple		technique is based on the principle that if BGP updates for multiple
	prefixes with the same origin AS were received on different		prefixes with the same origin AS were received on different
	interfaces (at border routers), then incoming data packets with		interfaces (at border routers), then incoming data packets with
	source addresses in any of those prefixes should be accepted on any		source addresses in any of those prefixes should be accepted on any
	of those interfaces. It can be best explained with an example as		of those interfaces. It can be best explained with an example as
	follows:		follows:
	Let us say, a border router of ISP-A has in its Adj-RIB-Ins		Let us say, a border router of ISP-A has in its Adj-RIB-Ins [RFC4271]
	[RFC4271]. the set of prefixes {Q1, Q2, Q3} each of which has AS-x		the set of prefixes {Q1, Q2, Q3} each of which has AS-x as its origin
	as its origin and AS-x is in ISP-A's customer cone. Further, the		and AS-x is in ISP-A's customer cone. Further, the border router
	border router received a route for prefix Q1 over a customer facing		received a route for prefix Q1 over a customer facing interface,
	interface, while it learned routes for prefixes Q2 and Q3 from a		while it learned routes for prefixes Q2 and Q3 from a lateral peer
	lateral peer and an upstream transit provider, respectively. In this		and an upstream transit provider, respectively. In this example
	example scenario, the enhanced feasible-path uRPF method requires Q1,		scenario, the enhanced feasible-path uRPF method requires Q1, Q2, and
	Q2, and Q3 be included in the RPF list for the customer interface in		Q3 be included in the RPF list for the customer interface in
	consideration. Loose uRPF (see Section 2.4) is recommended to be		consideration. Loose uRPF (see Section 2.4) is recommended to be
	applied to the peer and provider interfaces in consideration.		applied to the peer and provider interfaces in consideration.
	Thus, enhanced feasible-path uRPF defines feasible paths for customer		Thus, enhanced feasible-path uRPF defines feasible paths for customer
	interfaces in a more generalized but precise way (as compared to		interfaces in a more generalized but precise way (as compared to
	feasible-path uRPF).		feasible-path uRPF).
	Looking back at Scenarios 1 and 2 (Figure 1 and Figure 2), the		Looking back at Scenarios 1 and 2 (Figure 1 and Figure 2), the
	enhanced feasible-path uRPF provides comparable or better performance		enhanced feasible-path uRPF provides comparable or better performance
	than the other uRPF methods. Scenario 3 (Figure 3) further		than the other uRPF methods. Scenario 3 (Figure 3) further
	skipping to change at page 9, line 29 ¶		skipping to change at page 9, line 41 ¶
	o A non-stub AS SHOULD also announce at least one of the prefixes it		o A non-stub AS SHOULD also announce at least one of the prefixes it
	originates to each of its transit provider ASes.		originates to each of its transit provider ASes.
	o Additionally, from the routes it has learned from customers, a		o Additionally, from the routes it has learned from customers, a
	non-stub AS SHOULD announce at least one route per origin AS to		non-stub AS SHOULD announce at least one route per origin AS to
	each of its transit provider ASes.		each of its transit provider ASes.
	(Note: It is worth noting that in the above recommendations if "at		(Note: It is worth noting that in the above recommendations if "at
	least one" is replaced with "all", then even traditional feasible-		least one" is replaced with "all", then even traditional feasible-
	path uRPF will work as effectively.)		path uRPF would work effectively. But the latter recommendation
			("all") does not seem practical.)
	3.3. A Challenging Scenario		3.3. A Challenging Scenario
	It should be observed that in the absence of ASes adhering the above		It should be observed that in the absence of ASes adhering the above
	recommendations, the following example scenario may be constructed		recommendations, the following example scenario may be constructed
	which poses a challenge for the enhanced feasible-path uRPF (as well		which poses a challenge for the enhanced feasible-path uRPF (as well
	as for traditional feasible-path uRPF). In the scenario illustrated		as for traditional feasible-path uRPF). In the scenario illustrated
	in Figure 4, since routes for neither P1 nor P2 are propagated on the		in Figure 4, since routes for neither P1 nor P2 are propagated on the
	AS2-AS4 interface, the enhanced feasible-path uRPF at AS4 will reject		AS2-AS4 interface, the enhanced feasible-path uRPF at AS4 will reject
	data packets received on that interface with source addresses in P1		data packets received on that interface with source addresses in P1
	or P2.		or P2. (Please see slide #10 in [sriram-urpf] for an additional
			scenario.)
	+----------+		+----------+
	| AS4(ISP4)|		| AS4(ISP4)|
	+----------+		+----------+
	/\ /\		/\ /\
	/ \ P1[AS3 AS1]		/ \ P1[AS3 AS1]
	P1 and P2 not / \ P2[AS3 AS1]		P1 and P2 not / \ P2[AS3 AS1]
	propagated / \ (C2P)		propagated / \ (C2P)
	(C2P) / \		(C2P) / \
	+----------+ +----------+		+----------+ +----------+
	skipping to change at page 11, line 17 ¶		skipping to change at page 11, line 20 ¶
	5. Then, Set Z = Union(P,Q) represents the RPF list for every		5. Then, Set Z = Union(P,Q) represents the RPF list for every
	customer interface in Set I.		customer interface in Set I.
	6. Apply loose uRPF method for SAV on all peer and provider		6. Apply loose uRPF method for SAV on all peer and provider
	interfaces.		interfaces.
	When Algorithm B (which is more flexible than Algorithm A) is		When Algorithm B (which is more flexible than Algorithm A) is
	employed, the type of limitation identified in Figure 4 (Section 3.3)		employed, the type of limitation identified in Figure 4 (Section 3.3)
	goes away.		goes away.
	3.5. Implementation Considerations		3.5. Augmenting RPF Tables with ROA and IRR Data
			It is worth emphasizing that an indirect part of the proposal in the
			draft is that RPF filters may be augmented from secondary sources.
			Hence, the construction of RPF lists using a method proposed in this
			document (Algorithm A or B) can be augmented with data from Route
			Origin Authorization (ROA) [RFC6482] as well as Internet Routing
			Registry (IRR) data. Prefixes from registered ROAs or IRR route
			objects that include ASes in an ISP's customer cone SHOULD be used to
			augment the appropriate RPF tables. This will help make the RPF
			tables more robust about source addresses that may be legitimately
			used by customers of the ISP.
			3.6. Implementation Considerations
			3.6.1. Impact on FIB Memory Size Requirement
	The existing RPF checks in edge routers take advantage of existing		The existing RPF checks in edge routers take advantage of existing
	line card implementations to perform the RPF functions. For		line card implementations to perform the RPF functions. For
	implementation of the enhanced feasible-path uRPF, the general		implementation of the enhanced feasible-path uRPF, the general
	necessary feature would be to extend the line cards to take arbitrary		necessary feature would be to extend the line cards to take arbitrary
	RPF lists that are not necessarily the same as the existing FIB		RPF lists that are not necessarily the same as the existing FIB
	contents. In the algorithms (Section 3.1.1 and Section 3.4)		contents. In the algorithms (Section 3.1.1 and Section 3.4)
	described here, the RPF lists are constructed by applying a set of		described here, the RPF lists are constructed by applying a set of
	rules to all received BGP routes (not just those selected as best		rules to all received BGP routes (not just those selected as best
	path and installed in FIB).		path and installed in FIB). The concept of uRPF querying an RPF
			table (instead of the FIB) is similar to uRPF querying VRF table (see
	3.5.1. Impact on FIB Memory Size Requirement		(Section 2.5).
	The techniques described here require that there should be FIB memory		The techniques described in this document require that there should
	(i.e., TCAM) available to store the RPF lists in line cards. For an		be additional memory (i.e., TCAM) available to store the RPF lists in
	ISP's AS, the RPF list size for each line card will roughly and		line cards. For an ISP's AS, the RPF list size for each line card
	conservatively equal the total number of prefixes in its customer		will roughly and conservatively equal the total number of prefixes in
	cone (assuming the algorithm in Section 3.4 is used). (Note: Most		its customer cone (assuming the algorithm in Section 3.4 is used).
	ISP customer cone scenarios would not require the algorithm in		(Note: Most ISP customer cone scenarios would not require the
	Section 3.4, but instead be served best by the algorithm in		algorithm in Section 3.4, but instead be served best by the algorithm
	Section 3.1.1, which requires much less FIB memory.) The following		in Section 3.1.1, which requires much less FIB memory.) The
	table shows the measured customer cone sizes for various types of		following table shows the measured customer cone sizes for various
	ISPs [sriram-ripe63]:		types of ISPs [sriram-ripe63]:
	+---------------------------------+---------------------------------+		+---------------------------------+---------------------------------+
	| Type of ISP | Measured Customer Cone Size in |		| Type of ISP | Measured Customer Cone Size in |
	| | # Prefixes (in turn this is an |		| | # Prefixes (in turn this is an |
	| | estimate for RPF list size on |		| | estimate for RPF list size on |
	| | line card) |		| | line card) |
	+---------------------------------+---------------------------------+		+---------------------------------+---------------------------------+
	| Very Large Global ISP | 32392 |		| Very Large Global ISP | 32392 |
	| ------------------------------- | ------------------------------- |		| ------------------------------- | ------------------------------- |
	| Very Large Global ISP | 29528 |		| Very Large Global ISP | 29528 |
	skipping to change at page 12, line 31 ¶		skipping to change at page 12, line 37 ¶
	Table 1: Customer cone sizes (# prefixes) for various types of ISPs.		Table 1: Customer cone sizes (# prefixes) for various types of ISPs.
	For some super large global ISPs that are at the core of the		For some super large global ISPs that are at the core of the
	Internet, the customer cone size (# prefixes) can be as high as a few		Internet, the customer cone size (# prefixes) can be as high as a few
	hundred thousand [CAIDA]. But uRPF is most effective when deployed		hundred thousand [CAIDA]. But uRPF is most effective when deployed
	at ASes at the edges of the Internet where the customer cone sizes		at ASes at the edges of the Internet where the customer cone sizes
	are smaller as shown in Table 1.		are smaller as shown in Table 1.
	A very large global ISP's router line card is likely to have a FIB		A very large global ISP's router line card is likely to have a FIB
	size large enough to accommodate 2 to 6 million routes [cisco1].		size large enough to accommodate 2 to 6 million routes [Cisco1].
	Similarly, the line cards in routers corresponding to a large global		Similarly, the line cards in routers corresponding to a large global
	ISP, a mid-size global ISP, and a regional ISP are likely to have FIB		ISP, a mid-size global ISP, and a regional ISP are likely to have FIB
	sizes large enough to accommodate about 1 million, 0.5 million, and		sizes large enough to accommodate about 1 million, 0.5 million, and
	100K routes, respectively [cisco2]. Comparing these FIB size numbers		100K routes, respectively [Cisco2]. Comparing these FIB size numbers
	with the corresponding RPF list size numbers in Table 1, it can be		with the corresponding RPF list size numbers in Table 1, it can be
	surmised that the conservatively estimated RPF list size is only a		surmised that the conservatively estimated RPF list size is only a
	small fraction of the anticipated FIB memory size under relevant ISP		small fraction of the anticipated FIB memory size under relevant ISP
	scenarios.		scenarios.
	3.6. Summary of Recommendations		3.6.2. Coping with BGP's Transient Behavior
			BGP routing announcements can exhibit transient behavior. Routes may
			be withdrawn temporarily and then re-announced due to transient
			conditions such as BGP session reset or link failure-recovery. To
			cope with this, hysteresis should be introduced in the maintenance of
			the RPF lists. Changes to the RPF lists SHOULD be delayed by a pre-
			determined amount (TBD) when responding to route withdrawals. This
			should help suppress the effects due to the transients in BGP.
			3.7. Summary of Recommendations
	Depending on the scenario, an ISP or enterprise AS operator should		Depending on the scenario, an ISP or enterprise AS operator should
	follow one of the following recommendations concerning uRPF/SAV:		follow one of the following recommendations concerning uRPF/SAV:
	1. For directly connected networks, i.e., subnets directly connected		1. For directly connected networks, i.e., subnets directly connected
	to the AS and not multi-homed, the AS in consideration SHOULD		to the AS and not multi-homed, the AS in consideration SHOULD
	perform ACL-based SAV.		perform ACL-based SAV.
	2. For a directly connected single-homed stub AS (customer), the AS		2. For a directly connected single-homed stub AS (customer), the AS
	in consideration SHOULD perform SAV based on the strict uRPF		in consideration SHOULD perform SAV based on the strict uRPF
	skipping to change at page 13, line 34 ¶		skipping to change at page 14, line 8 ¶
	Section 3.4).		Section 3.4).
	5. IANA Considerations		5. IANA Considerations
	This document does not request new capabilities or attributes. It		This document does not request new capabilities or attributes. It
	does not create any new IANA registries.		does not create any new IANA registries.
	6. Acknowledgements		6. Acknowledgements
	The authors would like to thank Job Snijders, Marco Marzetti, Marco		The authors would like to thank Job Snijders, Marco Marzetti, Marco
	d'Itri, Nick Hilliard, Gert Doering, Igor Gashinsky, Barry Greene,		d'Itri, Nick Hilliard, Gert Doering, Igor Gashinsky, Igor Lubashev,
	and Joel Jaeggli for comments and suggestions.		Barry Greene, Amir Herzberg, Ruediger Volk, Jared Mauch, Oliver
			Borchert, Mehmet Adalier, and Joel Jaeggli for comments and
			suggestions.
	7. Informative References		7. Informative References
	[CAIDA] "Information for AS 174 (COGENT-174)", CAIDA Spoofer		[CAIDA] "Information for AS 174 (COGENT-174)", CAIDA Spoofer
	Project , <https://spoofer.caida.org/as.php?asn=174>.		Project , <https://spoofer.caida.org/as.php?asn=174>.
	[cisco1] "Internet Routing Table Growth Causes ROUTING-FIB-		[Cisco1] "Internet Routing Table Growth Causes ROUTING-FIB-
	4-RSRC_LOW Message on Trident-Based Line Cards", Cisco		4-RSRC_LOW Message on Trident-Based Line Cards", Cisco
	Trouble-shooting Tech-notes , January 2014,		Trouble-shooting Tech-notes , January 2014,
	<https://www.cisco.com/c/en/us/support/docs/routers/asr-		<https://www.cisco.com/c/en/us/support/docs/routers/asr-
	9000-series-aggregation-services-routers/116999-problem-		9000-series-aggregation-services-routers/116999-problem-
	line-card-00.html>.		line-card-00.html>.
	[cisco2] "Cisco Nexus 7000 Series NX-OS Unicast Routing		[Cisco2] "Cisco Nexus 7000 Series NX-OS Unicast Routing
	Configuration Guide, Release 5.x (Chapter: Managing the		Configuration Guide, Release 5.x (Chapter 15: Managing the
	Unicast RIB and FIB)", Cisco Configuration Guides , June		Unicast RIB and FIB)", Cisco Configuration Guides , March
	2018, <https://www.cisco.com/c/en/us/td/docs/switches/data		2018, <https://www.cisco.com/c/en/us/td/docs/switches/data
	center/sw/5_x/nx-		center/sw/5_x/nx-
	os/unicast/configuration/guide/l3_cli_nxos/		os/unicast/configuration/guide/l3_cli_nxos/
	l3_manage-routes.html#22859>.		l3_NewChange.html>.
			[Cisco3] "Unicast reverse path forwarding enhancements for the
			Internet service provider", Cisco white paper , 2005,
			<https://www.cisco.com/c/dam/en_us/about/security/
			intelligence/urpf.pdf>.
	[ISOC] Vixie (Ed.), P., "Addressing the challenge of IP		[ISOC] Vixie (Ed.), P., "Addressing the challenge of IP
	spoofing", ISOC report , September 2015,		spoofing", ISOC report , September 2015,
	<https://www.us-cert.gov/ncas/alerts/TA14-017A>.		<https://www.us-cert.gov/ncas/alerts/TA14-017A>.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	skipping to change at page 14, line 36 ¶		skipping to change at page 15, line 14 ¶
	[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed		[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
	Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March		Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
	2004, <https://www.rfc-editor.org/info/rfc3704>.		2004, <https://www.rfc-editor.org/info/rfc3704>.
	[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A		[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
	Border Gateway Protocol 4 (BGP-4)", RFC 4271,		Border Gateway Protocol 4 (BGP-4)", RFC 4271,
	DOI 10.17487/RFC4271, January 2006,		DOI 10.17487/RFC4271, January 2006,
	<https://www.rfc-editor.org/info/rfc4271>.		<https://www.rfc-editor.org/info/rfc4271>.
			[RFC6482] Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
			Origin Authorizations (ROAs)", RFC 6482,
			DOI 10.17487/RFC6482, February 2012,
			<https://www.rfc-editor.org/info/rfc6482>.
	[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.		[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
	Austein, "BGP Prefix Origin Validation", RFC 6811,		Austein, "BGP Prefix Origin Validation", RFC 6811,
	DOI 10.17487/RFC6811, January 2013,		DOI 10.17487/RFC6811, January 2013,
	<https://www.rfc-editor.org/info/rfc6811>.		<https://www.rfc-editor.org/info/rfc6811>.
	[RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations		[RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
	and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,		and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
	February 2015, <https://www.rfc-editor.org/info/rfc7454>.		February 2015, <https://www.rfc-editor.org/info/rfc7454>.
	[RRL] "Response Rate Limiting in the Domain Name System",		[RRL] "Response Rate Limiting in the Domain Name System",
	Redbarn blog , <http://www.redbarn.org/dns/ratelimits>.		Redbarn blog , <http://www.redbarn.org/dns/ratelimits>.
	[sriram-ripe63]		[sriram-ripe63]
	Sriram, K. and R. Bush, "Estimating CPU Cost of BGPSEC on		Sriram, K. and R. Bush, "Estimating CPU Cost of BGPSEC on
	a Router", Presented at RIPE-63; also at IETF-83 SIDR WG		a Router", Presented at RIPE-63; also, at IETF-83 SIDR WG
	Meeting, March 2012,		Meeting, March 2012,
	<http://www.ietf.org/proceedings/83/slides/		<http://www.ietf.org/proceedings/83/slides/
	slides-83-sidr-7.pdf>.		slides-83-sidr-7.pdf>.
			[sriram-urpf]
			Sriram, K., "Enhanced Feasible-Path Unicast Reverse Path
			Filtering", Presented at the IETF-101 OPSEC WG Meeting ,
			March 2018,
			<https://datatracker.ietf.org/meeting/101/materials/
			slides-101-opsec-draft-sriram-opsec-urpf-improvements-00>.
	[TA14-017A]		[TA14-017A]
	"UDP-Based Amplification Attacks", US-CERT alert		"UDP-Based Amplification Attacks", US-CERT alert
	TA14-017A , January 2014,		TA14-017A , January 2014,
	<https://www.us-cert.gov/ncas/alerts/TA14-017A>.		<https://www.us-cert.gov/ncas/alerts/TA14-017A>.
	Authors' Addresses		Authors' Addresses
	Kotikalapudi Sriram		Kotikalapudi Sriram
	USA National Institute of Standards and Technology		USA National Institute of Standards and Technology
	100 Bureau Drive		100 Bureau Drive
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