draft-ietf-idr-bgp-flowspec-oid-08.txt		draft-ietf-idr-bgp-flowspec-oid-09.txt
	Network Working Group J. Uttaro		Network Working Group J. Uttaro
	Internet-Draft AT&T		Internet-Draft AT&T
	Updates: 5575bis (if approved) J. Alcaide		Updates: 5575bis (if approved) J. Alcaide
	Intended status: Standards Track C. Filsfils		Intended status: Standards Track C. Filsfils
	Expires: November 10, 2019 D. Smith		Expires: January 9, 2020 D. Smith
	Cisco		Cisco
	P. Mohapatra		P. Mohapatra
	Sproute Networks		Sproute Networks
	May 9, 2019		July 8, 2019
	Revised Validation Procedure for BGP Flow Specifications		Revised Validation Procedure for BGP Flow Specifications
	draft-ietf-idr-bgp-flowspec-oid-08		draft-ietf-idr-bgp-flowspec-oid-09
	Abstract		Abstract
	This document describes a modification to the validation procedure		This document describes a modification to the validation procedure
	defined in RFC 5575bis for the dissemination of BGP flow		defined in RFC 5575bis for the dissemination of BGP Flow
	specifications. RFC 5575bis requires that the originator of the flow		Specifications. RFC 5575bis requires that the originator of the Flow
	specification matches the originator of the best-match unicast route		Specification matches the originator of the best-match unicast route
	for the destination prefix embedded in the flow specification. This		for the destination prefix embedded in the Flow Specification. This
	allows only BGP speakers within the data forwarding path (such as		allows only BGP speakers within the data forwarding path (such as
	autonomous system border routers) to originate BGP flow		autonomous system border routers) to originate BGP Flow
	specifications. Though it is possible to disseminate such flow		Specifications. Though it is possible to disseminate such Flow
	specifications directly from border routers, it may be operationally		Specifications directly from border routers, it may be operationally
	cumbersome in an autonomous system with a large number of border		cumbersome in an autonomous system with a large number of border
	routers having complex BGP policies. The modification proposed		routers having complex BGP policies. The modification proposed
	herein enables flow specifications to be originated from a		herein enables Flow Specifications to be originated from a
	centralized BGP route controller.		centralized BGP route controller.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	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 November 10, 2019.		This Internet-Draft will expire on January 9, 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.
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	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
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Requirements Language . . . . . . . . . . . . . . . . . . . . 2		1. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
	2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 2		2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4		3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
	4. Revised Validation Procedure . . . . . . . . . . . . . . . . 5		4. Revised Validation Procedure . . . . . . . . . . . . . . . . 5
	5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7		4.1. Revision of Route Feasibility . . . . . . . . . . . . . . 5
	6. Security Considerations . . . . . . . . . . . . . . . . . . . 7		4.2. Revision of AS_PATH Validation . . . . . . . . . . . . . 6
	7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7		5. Other RFC5575bis Considerations . . . . . . . . . . . . . . . 7
	8. Normative References . . . . . . . . . . . . . . . . . . . . 7		6. Topology Considerations . . . . . . . . . . . . . . . . . . . 8
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
			8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
			9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
			10. Normative References . . . . . . . . . . . . . . . . . . . . 10
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
	1. Requirements Language		1. Requirements Language
	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 RFC 2119 [RFC2119].		document are to be interpreted as described in RFC 2119 [RFC2119].
	2. Motivation		2. Introduction
			[RFC5575bis] defined a new BGP [RFC4271] capability that can be used
			to distribute traffic Flow Specifications amongst BGP speakers in
			support of traffic filtering. The primary intention of [RFC5575bis]
			is to enable downstream autonomous systems to signal traffic
			filtering policies to upstream autonomous systems. In this way,
			traffic is filtered closer to the source and the upstream autonomous
			system(s) avoid carrying the traffic to the downstream autonomous
			system only to be discarded. [RFC5575bis] also enables more granular
			traffic filtering based upon upper layer protocol information (e.g.,
			protocol port numbers) as opposed to coarse IP destination prefix-
			based filtering. Flow specification NLRIs received from a BGP peer
			are subject to validity checks before being considered feasible and
			subsequently installed within the respective Adj-RIB-In.
			The validation procedure defined within [RFC5575bis] requires that
			the originator of the Flow Specification NLRI matches the originator
			of the best-match unicast route for the destination prefix embedded
			in the Flow Specification. This allows only BGP speakers within the
			data forwarding path (such as autonomous system border routers) to
			originate BGP Flow Specification NLRIs. Though it is possible to
			disseminate such Flow Specification NLRIs directly from border
			routers, it may be operationally cumbersome in an autonomous system
			with a large number of border routers having complex BGP policies.
			This document describes a modification to the [RFC5575bis] validation
			procedure allowing Flow Specification NLRIs to be originated from a
			centralized BGP route controller within the local autonomous system
			that is not in the data forwarding path. While the proposed
			modification cannot be used for inter-domain coordination of traffic
			filtering, it greatly simplifies distribution of intra-domain traffic
			filtering policies within an autonomous system which has a large
			number of border routers having complex BGP policies. By relaxing
			the validation procedure for iBGP, the proposed modification allows
			Flow Specifications to be distributed in a standard and scalable
			manner throughout an autonomous system.
			3. Motivation
	Step (a) of the validation procedure in [RFC5575bis], section 6 is		Step (a) of the validation procedure in [RFC5575bis], section 6 is
	defined with the underlying assumption that the flow specification		defined with the underlying assumption that the Flow Specification
	NLRI traverses the same path, in the inter-domain and intra-domain		NLRI traverses the same path, in the inter-domain and intra-domain
	route distribution graph, as that of the longest-match unicast route		route distribution graph, as that of the longest-match unicast route
	for the destination prefix embedded in the flow specification.		for the destination prefix embedded in the Flow Specification.
	In the case of inter-domain traffic filtering, for example, the flow		In the case of inter-domain traffic filtering, the Flow Specification
	specification originator at the egress border routers of ASN1 (RTR-D		originator at the egress border routers of an AS (e.g. RTR-D and
	and RTR-E in figure 1) matches the EBGP neighbor that advertised the		RTR-E of ASN1 in figure 1) matches the eBGP neighbor that advertised
	longest match destination prefix (RTR-F and RTR-G respectively).		the longest match destination prefix (see RTR-F and RTR-G
	Similarly, at the ingress border routers of ASN1 (RTR-A and RTR-B in		respectively in figure 1). Similarly, at the ingress border routers
	figure 1), the flow specification originator matches the egress IBGP		of ASN (see RTR-A and RTR-B of ASN1 in figure 1), the Flow
	border routers that had advertised the unicast route for the best-		Specification originator matches the egress iBGP border routers that
	match destination prefix (RTR-D and RTR-E respectively). This is		had advertised the unicast route for the best-match destination
	true even when ingress border routers select paths from different		prefix (see RTR-D and RTR-E respectively in figure 1). This is true
	egress border routers as best path based upon IGP distance (as an		even when ingress border routers select paths from different egress
	example, RTR-A chooses RTR-D's path as best; RTR-B chooses RTR-E as		border routers as best path based upon IGP distance. For example, in
	the best path).		figure 1:
			RTR-A chooses RTR-D's path as best
			RTR-B chooses RTR-E as the best path
	/ - - - - - - - - - - - - - -		/ - - - - - - - - - - - - - -
	| ASN1 |		| ASN1 |
	+-------+ +-------+		+-------+ +-------+
	| | | | | |		| | | | | |
	| RTR-A | | RTR-B |		| RTR-A | | RTR-B |
	| | | | | |		| | | | | |
	+-------+ +-------+		+-------+ +-------+
	| \ / |		| \ / |
	IBGP \ / IBGP		iBGP \ / iBGP
	| \ / |		| \ / |
	+-------+		+-------+
	| | | |		| | | |
	| RTR-C |		| RTR-C |
	| | RC | |		| | RC | |
	+-------+		+-------+
	| / \ |		| / \ |
	/ \		/ \
	| IBGP / \ IBGP |		| iBGP / \ iBGP |
	+-------+ +-------+		+-------+ +-------+
	| | RTR-D | | RTR-E | |		| | RTR-D | | RTR-E | |
	| | | |		| | | |
	| | | | | |		| | | | | |
	+-------+ +-------+		+-------+ +-------+
	| | | |		| | | |
	- - -|- - - - - - - - -|- - -/		- - -|- - - - - - - - -|- - -/
	| EBGP EBGP |		| eBGP eBGP |
	- - -|- - - - - - - - -|- - -/		- - -|- - - - - - - - -|- - -/
	| | | |		| | | |
	+-------+ +-------+		+-------+ +-------+
	| | | | | |		| | | | | |
	| RTR-F | | RTR-G |		| RTR-F | | RTR-G |
	| | | | | |		| | | | | |
	+-------+ +-------+		+-------+ +-------+
	| ASN2 |		| ASN2 |
	/ - - - - - - - - - - - - - -		/ - - - - - - - - - - - - - -
	Figure 1		Figure 1
	It is highly desirable that each ASN is able to protect itself		It is highly desirable that the mechanisms exist to protect each ASN
	independently from network security attacks using the BGP flow		independently from network security attacks using the BGP Flow
	specification NLRI for intra-domain purposes only. Network operators		Specification NLRI for intra-domain purposes only. Network operators
	often deploy a dedicated Security Operations Center (SOC) within		often deploy a dedicated Security Operations Center (SOC) within
	their ASN to monitor and detect such security attacks. To mitigate		their ASN to monitor and detect such security attacks. To mitigate
	attacks in a scalable intra-domain manner, operators require the		attacks within a domain (AS or group of ASes), operators require the
	ability to originate intra-domain flow specification NLRIs from a		ability to originate intra-domain Flow Specification NLRIs from a
	central BGP route controller that is not within the data forwarding		central BGP route controller that is not within the data forwarding
	plane. In this way, operators can direct border routers within their		plane. In this way, operators can direct border routers within their
	ASN with specific attack mitigation actions (drop the traffic,		ASN with specific attack mitigation actions (drop the traffic,
	forward to a clean-pipe center, etc.). To originate a flow		forward to a clean-pipe center, etc.).
	specification NLRI, a central BGP route controller must set itself as
	the originator in the flowspec NLRI. This is necessary given the
	route controller is originating the flow specification not reflecting
	it, and to avoid the complexity of having to determine the egress
	border router whose path was chosen as the best in each of the
	ingress border routers. It thus becomes necessary to modify step (a)
	of the [RFC5575bis] validation procedure such that an IBGP peer that
	is not within the data forwarding plane may originate flow
	specification NLRIs.
	3. Introduction
	[RFC5575bis] defined a new BGP capability that can be used to		To originate a Flow Specification NLRI, a central BGP route
	distribute traffic flow specifications amongst BGP speakers in		controller must set itself as the originator in the Flow
	support of traffic filtering. The primary intention of [RFC5575bis]		Specification NLRI. This is necessary given the route controller is
	is to enable downstream autonomous systems to signal traffic		originating the Flow Specification rather than reflecting it, and to
	filtering policies to upstream autonomous systems. In this way,		avoid the complexity of having to determine the egress border router
	traffic is filtered closer to the source and the upstream autonomous		whose path was chosen as the best in each of the ingress border
	system(s) avoid carrying the traffic to the downstream autonomous		routers. Thus, it is necessary to modify step (a) of the
	system only to be discarded. [RFC5575bis] also enables more granular		[RFC5575bis] validation procedure such that an iBGP peer that is not
	traffic filtering based upon upper layer protocol information (e.g.,		within the data forwarding plane may originate Flow Specification
	protocol port numbers) as opposed to coarse IP destination prefix-		NLRIs.
	based filtering. Flow specification NLRIs received from a BGP peer
	are subject to validity checks before being considered feasible and
	subsequently installed within the respective Adj-RIB-In. The
	validation procedure defined within [RFC5575bis] requires that the
	originator of the flow specification NLRI matches the originator of
	the best-match unicast route for the destination prefix embedded in
	the flow specification. This allows only BGP speakers [RFC4271]
	within the data forwarding path (such as autonomous system border
	routers) to originate BGP flow specification NLRIs. Though it is
	possible to disseminate such flow specification NLRIs directly from
	border routers, it may be operationally cumbersome in an autonomous
	system with a large number of border routers having complex BGP
	policies. This document describes a modification to the [RFC5575bis]
	validation procedure allowing flow specification NLRIs to be
	originated from a centralized BGP route controller within the local
	autonomous system that is not in the data forwarding path. While the
	proposed modification cannot be used for inter-domain coordination of
	traffic filtering, it greatly simplifies distribution of intra-domain
	traffic filtering policies in an autonomous system with a large
	number of border routers having complex BGP policies. By relaxing
	the validation procedure for IBGP, the proposed modification allows
	flow specifications to be distributed in a standard and scalable
	manner throughout an autonomous system.
	4. Revised Validation Procedure		4. Revised Validation Procedure
			4.1. Revision of Route Feasibility
	Step (a) of the validation procedure specified in [RFC5575bis],		Step (a) of the validation procedure specified in [RFC5575bis],
	section 6 is redefined as follows:		section 6 is redefined as follows:
	a. One of the following conditions MUST hold true.		a. One of the following conditions MUST hold true.
	* The originator of the flow specification matches the		1. The originator of the Flow Specification matches the
	originator of the best-match unicast route for the destination		originator of the best-match unicast route for the
	prefix embedded in the flow specification.		destination prefix embedded in the Flow Specification.
	* The AS_PATH attribute of the flow specification does not		2. The AS_PATH attribute of the Flow Specification does not
	contain AS_SET and/or AS_SEQUENCE segments.		contain AS_SET and/or AS_SEQUENCE segments.
	An AS_PATH without AS_SET and/or AS_SEQUENCE segments indicates that		1. This condition SHOULD be enabled by default. This
	the flow specification was originated inside the local AS [RFC4271]		default behavior should validate an empty AS_PATH.
	or inside the local confederation (in the case that the local AS
	belongs to a confederation of ASes) [RFC5065]. With this
	modification to the [RFC5575bis] validation procedure, it is now
	possible for an IBGP peer that is not within the data forwarding path
	to originate flow specification NLRIs. This applies whether the AS
	belongs or not to a confederation of ASes. Checking the (newly
	introduced) second condition above MAY be disabled by configuration
	on a BGP speaker. However, it SHOULD be enabled by default.
	Disabling the condition may be a good practice when the administrator
	knows with certainty that there are not flow specification NLRI
	originated inside the local AS (or local confederation). The default
	behavior is thus to validate an empty AS_PATH. In this context, an
	empty AS_PATH means that it does not have AS_SET and/or AS_SEQUENCE
	segments. Optionally, an implementation MAY also validate a specific
	non-empty AS_PATH. For instance, it could validate a flowspec NLRI
	whose AS_PATH contains only an AS_SEQUENCE of ASes known (via
	configuration) to belong to the same administrative domain.
	Further, [RFC5575bis] states that "BGP (flow specification)		2. This condition MAY be disabled by configuration on a BGP
	implementations MUST also enforce that AS_PATH attribute of a route		speaker.
	received via the External Border Gateway Protocol (EBGP) contains the
	neighboring AS in the left-most position of the AS_PATH attribute".
	This rule is not valid for all topologies. For example, it prevents
	the exchange of BGP flow specification NLRIs at Internet exchanges
	with BGP route servers. Therefore, this document also redefines the
	[RFC5575bis] AS_PATH validation procedure referenced above as
	follows:
	BGP flow specification implementations MUST enforce that the last AS		3. As an exception to this rule, a given AS_PATH with AS_SET
	added within the AS_PATH attribute of a EBGP learned flow		and/or AS_SEQUENCE segments MAY be validated by policy.
	specification NLRI MUST match the last AS added within the AS_PATH
	attribute of the best-match unicast route for the destination prefix
	embedded in the flow specification. This proposed modification
	enables the exchange of BGP flow specification NLRIs at Internet
	exchanges with BGP route servers while at the same time, for security
	reasons, prevents an EBGP peer from advertising an inter-domain flow
	specification for a destination prefix that it does not provide
	reachability information for. Note, comparing only the last ASes
	added is sufficient for EBGP learned flow specification NLRIs.
	Requiring a full AS_PATH match would limit origination of inter-
	domain flow specifications to the origin (or first) AS of the best-
	match unicast route for the destination prefix embedded in the flow
	specification only. As such, a full AS_PATH validity check may
	prevent transit ASes from originating inter-domain flow
	specifications, which is not desirable.
	This document also clarifies proper handling when the BGP flow		Explanation:
	specification does not embed a destination prefix component. The
	default behavior SHOULD be not to perform any validation procedure.
	Further, support for two-octet AS number space is out of the scope of
	this document.
	In this context, AS_PATH attribute is defined as the reconstructed AS		In this context, an empty AS_PATH means that it does not have
	Path information (by combining AS_PATH and AS4_PATH attributes, if		AS_SET and/or AS_SEQUENCE segments, and local domain means the
	the BGP speaker is a NEW speaker and receives the route from an OLD		local AS [RFC4271] or the local confederation of ASes (in the case
	speaker), according to section 4.2.3 of [RFC6793].		that the local AS belongs to a confederation of ASes [RFC5065]).
	[RFC5575bis] references "the best-match unicast route for the		Thus, receiving a Flow Specification with an empty AS_PATH
	destination prefix embedded in the flow specification". For clarity,		indicates that the Flow Specification was originated inside the
	this route is defined hereby as the best path of the unicast network		local domain.
	that covers destination prefix embedded in the flow specification
	with the longer prefix-length. In other words, we consider only the
	best-match network and we do not consider unicast non-best paths
	(even if it is received from the same peer than the flowspec route).
	Note that, per [RFC5575bis], originator may refer to the BGP		With the above modification to the [RFC5575bis] validation
	ORIGINATOR_ID attribute or the transport address of the peer from		procedure, a BGP peer within the local domain that is not within
	which we received the update. If the later, a network must be		the data forwarding path can originate a Flow Specification.
	designed so it has a congruent topology. Otherwise, using two
	peering sessions between the same pair of BGP speakers, one for
	unicast and one for flowspec, will cause the flowspec validation
	procedure to fail. Consider, for example, the case where a BGP route
	reflector receives the NLRIs from a route reflector client, thus not
	receiving the ORIGINATOR_ID attribute. If the speaker belongs to a
	confederation [RFC5065] and we are receiving a flowspec route from
	different peers than its best match unicast route, the flowspec
	validation procedure will fail as well. Consider also a
	misconfiguration where flowspec address-family is not configured for
	a particular peering between different member-AS (but it is
	configured for unicast). Even if we receive the flowspec route via a
	redundant peer, we may receive the unicast route and the flowspec
	from different peers, and thus flowspec validation will fail. Thus,
	with the (newly introduced) second condition above applied,
	incongruent topologies are supported.
	Note that if the flowspec NLRI is learned from another AS (and thus		Disabling the new condition above (a.2.2) may be a good practice
	the AS_PATH is not empty), the original validation procedures defined		when the operator knows with certainty that there is not a Flow
	in [RFC5575bis] still apply and incongruent topologies may cause		Specification originated inside the local domain.
	validation rules to fail.
	5. IANA Considerations		Also, policy may be useful to validate a specific set of non-empty
			AS_PATHs (a.2.3). For example, it could validate a Flow
			Specification whose AS_PATH contains only an AS_SEQUENCE with ASes
			that are all known to belong to the same administrative domain.
			4.2. Revision of AS_PATH Validation
			[RFC5575bis] states:
			o BGP implementations MUST also enforce that the AS_PATH attribute
			of a route received via the External Border Gateway Protocol
			(eBGP) contains the neighboring AS in the left-most position of
			the AS_PATH attribute.
			This rule prevents the exchange of BGP Flow Specification NLRIs at
			Internet exchanges with BGP route servers. Therefore, this document
			also redefines the [RFC5575bis] AS_PATH validation procedure
			referenced above as follows:
			o BGP Flow Specification implementations MUST enforce that the AS in
			the left-most position of the AS_PATH attribute of a Flow
			Specification route received via the External Border Gateway
			Protocol (eBGP) matches the AS in the left-most position of the
			AS_PATH attribute of the best-match unicast route for the
			destination prefix embedded in the Flow Specification NLRI.
			Explanation:
			For clarity, the AS in the left-most position of the AS_PATH means
			the AS that was last added to the AS_SEQUENCE.
			This proposed modification enables the exchange of BGP Flow
			Specification NLRIs at Internet exchanges with BGP route servers
			while at the same time, for security reasons, prevents an eBGP
			peer from advertising an inter-domain Flow Specification for a
			destination prefix that it does not provide reachability
			information for.
			Comparing only the last ASes added is sufficient for eBGP learned
			Flow Specification NLRIs. Requiring a full AS_PATH match would
			limit origination of inter-domain Flow Specifications to the
			origin AS of the best-match unicast route for the destination
			prefix embedded in the Flow Specification only. As such, a full
			AS_PATH validity check may prevent transit ASes from originating
			inter-domain Flow Specifications, which is not desirable.
			Redefinition of this AS_PATH validation rule for a Flow
			Specification does not mean that the original rule in [RFC5575bis]
			cannot be enforced as well. Its enforcement remains optional per
			[RFC4271]. That is, we can enforce the first AS in the AS_PATH to
			be the same as the neighbor AS for any address-family route
			(including a Flow Specification).
			Using the new rule to validate a Flow Specification received from
			an Internal Border Gateway Protocol (iBGP) peer is out of the
			scope of this document. Note that in most scenarios such
			validation would be redundant.
			Using the new rule to validate a Flow Specification route received
			from an External Border Gateway Protocol (eBGP) peer belonging to
			the same local domain (in the case that the local AS belongs to a
			confederation of ASes) is out of the scope of this document. Note
			that although it's possible, its utility is dubious.
			5. Other RFC5575bis Considerations
			This section clarifies some of the terminology and rules referenced
			in [RFC5575bis]. Namely:
			o [RFC5575bis] references "the best-match unicast route for the
			destination prefix embedded in the Flow Specification". For
			clarity, this route is defined hereby as the best path of the
			unicast network with the longer prefix-length that covers the
			destination prefix embedded in the Flow Specification.
			Explanation:
			We consider only the best-match network and we do not consider
			unicast non-best paths (even if it is received from the same peer
			as the Flow Specification).
			o If a BGP Flow Specification does not embed a destination prefix
			component, then the default behavior SHOULD be not to perform the
			validation procedures.
			Explanation:
			The reason for this is because the best-match unicast route is not
			defined in this case.
			o In the context of this document and [RFC5575bis], AS_PATH
			attribute is defined as the reconstructed AS path information (by
			combining AS_PATH and AS4_PATH attributes, if the BGP speaker is a
			NEW speaker and receives the route from an OLD speaker), according
			to section 4.2.3 of [RFC6793].
			o Support for two-octet AS only implementations is out of the scope
			of this document (i.e. it's assumed that the BGP speaker supports
			[RFC6793]).
			6. Topology Considerations
			[RFC5575bis] indicates that the originator may refer to the
			originator path attribute (ORIGINATOR_ID) or (if the attribute is not
			present) the transport address of the peer from which we received the
			update. If the latter applies, a network should be designed so it
			has a congruent topology.
			With the additional second condition (a.2) in the validation
			procedure, non-congruent topologies are supported within the local
			domain if the Flow Specification is originated within the local
			domain.
			Explanation:
			Consider the following scenarios without the second condition
			(a.2) being added to the validation procedure:
			1. Consider a topology with two BGP speakers with two peering
			sessions between them, one for unicast and one for Flow
			Specification. This is a non-congruent topology. Let's
			assume that the ORIGINATOR_ID attribute was not received (e.g.
			a route reflector receiving routes from its clients). In this
			case, the Flow Specification validation procedure will fail
			because of the first condition (a.1).
			2. Consider a topology with a BGP speaker within a confederation
			of ASes, inside local AS X. ORIGINATOR_ID attribute is not
			advertised within the local domain. Let's assume the Flow
			Specification route is received from peer A and the best-match
			unicast route is received from peer B. Both peers belong in
			local AS Y. Both AS X and AS Y belong to the same local
			domain. The Flow Specification validation procedure will also
			fail because of the first condition (a.1).
			In the examples above, if Flow Specifications are originated in
			the same local domain, AS_PATH will not contain AS_SET and/or
			AS_SEQUENCE segments. When the second condition (a.2) in the
			validation procedure is used, the validation procedure will pass.
			Thus, non-congruent topologies are supported if the Flow
			Specification is originated in the same local domain.
			Even when the second condition (a.2) is used in the validation
			procedure, a Flow Specification originated in a different local
			domain needs a congruent topology. AS_SEQUENCE is not empty and
			the first condition (a.1) in the validation procedure needs to be
			evaluated. Because transport addresses for Flow Specification and
			unicast routes are different, the validation procedure will fail.
			This is true both across domains and within domains. Consider
			both cases:
			* Consider the first example. If the Flow Specification route is
			originated in another AS, the validation procedure will fail
			because the topology is non-congruent within the domain.
			* Consider the second example and modify it so AS X and AS Y
			belong to different local domains (no confederation of ASes
			exists). The validation procedure will fail because the
			topology is non-congruent across domains.
			7. IANA Considerations
	This memo includes no request to IANA.		This memo includes no request to IANA.
	6. Security Considerations		8. Security Considerations
	No new security issues are introduced by relaxing the validation		No new security issues are introduced by relaxing the validation
	procedure for IBGP learned flow specifications. With this proposal,		procedure for iBGP learned Flow Specifications. With this proposal,
	the security characteristics of BGP flow specifications remain		the security characteristics of BGP Flow Specifications remain
	equivalent to the existing security properties of BGP unicast		equivalent to the existing security properties of BGP unicast
	routing. Traffic flow specifications learned from IBGP peers are		routing. Traffic Flow Specifications learned from iBGP peers are
	trusted, hence, it is not required to validate that the originator of		trusted, hence, it is not required to validate that the originator of
	an intra-domain traffic flow specification matches the originator of		an intra-domain traffic Flow Specification matches the originator of
	the best-match unicast route for the flow destination prefix.		the best-match unicast route for the flow destination prefix.
	Conversely, this proposal continues to enforce the validation		Conversely, this proposal continues to enforce the validation
	procedure for EBGP learned traffic flow specifications. In this way,		procedure for eBGP learned traffic Flow Specifications. In this way,
	the security properties of [RFC5575bis] are maintained such that an		the security properties of [RFC5575bis] are maintained such that an
	EBGP peer cannot cause a denial-of-service attack by advertising an		eBGP peer cannot cause a denial-of-service attack by advertising an
	inter-domain flow specification for a destination prefix that it does		inter-domain Flow Specification for a destination prefix that it does
	not provide reachability information for.		not provide reachability information for.
	7. Acknowledgements		9. Acknowledgements
	The authors would like to thank Han Nguyen for his direction on this		The authors would like to thank Han Nguyen for his direction on this
	work as well as Waqas Alam, Keyur Patel, Robert Raszuk, Eric Rosen		work as well as Waqas Alam, Keyur Patel, Robert Raszuk, Eric Rosen
	and Shyam Sethuram for their review comments.		and Shyam Sethuram for their review comments.
	8. Normative References		10. Normative References
	[I-D.ietf-idr-rfc5575bis]		[I-D.ietf-idr-rfc5575bis]
	Hares, S., Loibl, C., Raszuk, R., McPherson, D., and M.		Hares, S., Loibl, C., Raszuk, R., McPherson, D., and M.
	Bacher, "Dissemination of Flow Specification Rules",		Bacher, "Dissemination of Flow Specification Rules",
	draft-ietf-idr-rfc5575bis-14 (work in progress), April		draft-ietf-idr-rfc5575bis-14 (work in progress), April
	2019.		2019.
	[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,
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