draft-ietf-grow-bgp-gshut-00.txt		draft-ietf-grow-bgp-gshut-01.txt
	Network Working Group Pierre Francois		Network Working Group Pierre Francois
	Internet-Draft Universite catholique de Louvain		Internet-Draft Universite catholique de Louvain
	Intended status: Informational Bruno Decraene		Intended status: Informational Bruno Decraene
	Expires: December 17, 2009 France Telecom		Expires: April 29, 2010 France Telecom
	Cristel Pelsser		Cristel Pelsser
	NTT Corporation		Internet Initiative Japan
	Clarence Filsfils		Clarence Filsfils
	Cisco Systems		Cisco Systems
	June 15, 2009		October 26, 2009
	Graceful BGP session shutdown		Graceful BGP session shutdown
	draft-ietf-grow-bgp-gshut-00		draft-ietf-grow-bgp-gshut-01
	Status of this Memo		Status of this Memo
	This Internet-Draft is submitted to IETF in full conformance with the		This Internet-Draft is submitted to IETF in full conformance with the
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	copyright in some of this material may not have granted the IETF		copyright in some of this material may not have granted the IETF
	Trust the right to allow modifications of such material outside the		Trust the right to allow modifications of such material outside the
	IETF Standards Process. Without obtaining an adequate license from		IETF Standards Process. Without obtaining an adequate license from
	skipping to change at page 1, line 47		skipping to change at page 1, line 47
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
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	This Internet-Draft will expire on December 17, 2009.		This Internet-Draft will expire on April 29, 2010.
	Copyright Notice		Copyright Notice
	Copyright (c) 2009 IETF Trust and the persons identified as the		Copyright (c) 2009 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 in effect on the date of		Provisions Relating to IETF Documents in effect on the date of
	publication of this document (http://trustee.ietf.org/license-info).		publication of this document (http://trustee.ietf.org/license-info).
	Please review these documents carefully, as they describe your rights		Please review these documents carefully, as they describe your rights
	and restrictions with respect to this document.		and restrictions with respect to this document.
	skipping to change at page 3, line 16		skipping to change at page 3, line 16
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
	2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4		2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
	3. Packet loss upon manual eBGP session shutdown . . . . . . . . 5		3. Packet loss upon manual eBGP session shutdown . . . . . . . . 5
	4. Practices to avoid packet losses . . . . . . . . . . . . . . . 5		4. Practices to avoid packet losses . . . . . . . . . . . . . . . 5
	4.1. Improving availability of alternate paths . . . . . . . . 6		4.1. Improving availability of alternate paths . . . . . . . . 6
	4.2. Graceful shutdown procedures for eBGP sessions . . . . . . 6		4.2. Graceful shutdown procedures for eBGP sessions . . . . . . 6
	4.2.1. Outbound traffic . . . . . . . . . . . . . . . . . . . 6		4.2.1. Outbound traffic . . . . . . . . . . . . . . . . . . . 6
	4.2.2. Inbound traffic . . . . . . . . . . . . . . . . . . . 7		4.2.2. Inbound traffic . . . . . . . . . . . . . . . . . . . 7
	4.3. Graceful shutdown procedures for iBGP sessions . . . . . . 9		4.3. Graceful shutdown procedures for iBGP sessions . . . . . . 9
	5. Forwarding modes and forwarding loops . . . . . . . . . . . . 9		5. Forwarding modes and forwarding loops . . . . . . . . . . . . 10
	6. Dealing with Internet policies . . . . . . . . . . . . . . . . 10		6. Dealing with Internet policies . . . . . . . . . . . . . . . . 10
	7. Effect of the g-shut procedure on the convergence . . . . . . 10		7. Link Up cases . . . . . . . . . . . . . . . . . . . . . . . . 11
	7.1. Maintenance of an eBGP session . . . . . . . . . . . . . . 10		7.1. Unreachability local to the ASBR . . . . . . . . . . . . . 11
	7.1.1. Propagation on the other eBGP sessions of the		7.2. iBGP convergence . . . . . . . . . . . . . . . . . . . . . 11
	g-shut initiator . . . . . . . . . . . . . . . . . . . 10		8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 12
	7.1.2. Propagation on the other iBGP sessions of the		9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
	g-shut initiator . . . . . . . . . . . . . . . . . . . 11		10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
	7.1.3. Propagation of updates in an iBGP full-mesh . . . . . 11		11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
	7.1.4. Propagation of updates from iBGP to iBGP in a RR		Appendix A. Summary of operations . . . . . . . . . . . . . . . . 13
	hierarchy . . . . . . . . . . . . . . . . . . . . . . 11		A.1. Pre-configuration . . . . . . . . . . . . . . . . . . . . 13
	7.2. Maintenance of an iBGP session . . . . . . . . . . . . . . 12		A.2. Operations at maintenance time . . . . . . . . . . . . . . 14
	7.3. Applicability of the g-shut procedure . . . . . . . . . . 13		Appendix B. Alternative techniques with limited applicability . . 14
	7.4. Summary of operations . . . . . . . . . . . . . . . . . . 13		B.1. In-filter reconfiguration . . . . . . . . . . . . . . . . 14
	7.4.1. Pre-configuration . . . . . . . . . . . . . . . . . . 13		B.2. Multi Exit Discriminator tweaking . . . . . . . . . . . . 15
	7.4.2. Operations at maintenance time . . . . . . . . . . . . 13		B.3. IGP distance Poisoning . . . . . . . . . . . . . . . . . . 16
	8. Link Up cases . . . . . . . . . . . . . . . . . . . . . . . . 13		Appendix C. Effect of the g-shut procedure on the convergence . . 16
	8.1. Unreachability local to the ASBR . . . . . . . . . . . . . 13		C.1. Maintenance of an eBGP session . . . . . . . . . . . . . . 16
	8.2. iBGP convergence . . . . . . . . . . . . . . . . . . . . . 14		C.1.1. Propagation on the other eBGP sessions of the
	9. Alternative techniques with limited applicability . . . . . . 15		g-shut initiator . . . . . . . . . . . . . . . . . . . 16
	9.1. In-filter reconfiguration . . . . . . . . . . . . . . . . 15		C.1.2. Propagation on the other iBGP sessions of the
	9.2. Multi Exit Discriminator tweaking . . . . . . . . . . . . 16		g-shut initiator . . . . . . . . . . . . . . . . . . . 16
	9.3. IGP distance Poisoning . . . . . . . . . . . . . . . . . . 16		C.1.3. Propagation of updates in an iBGP full-mesh . . . . . 17
	10. IANA considerations . . . . . . . . . . . . . . . . . . . . . 16		C.1.4. Propagation of updates from iBGP to iBGP in a RR
	11. Security Considerations . . . . . . . . . . . . . . . . . . . 17		hierarchy . . . . . . . . . . . . . . . . . . . . . . 17
	12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17		C.2. Maintenance of an iBGP session . . . . . . . . . . . . . . 18
	13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
	1. Introduction		1. Introduction
	Routing changes in BGP can be caused by planned, manual, maintenance		Routing changes in BGP can be caused by planned, manual, maintenance
	operations. This document discusses operational procedures to be		operations. This document discusses operational procedures to be
	applied in order to reduce or eliminate losses of packets during the		applied in order to reduce or eliminate losses of packets during the
	maintenance. These losses come from the transient lack of		maintenance. These losses come from the transient lack of
	reachability during the BGP convergence following the shutdown of an		reachability during the BGP convergence following the shutdown of an
	eBGP peering session between two Autonomous System Border Routers		eBGP peering session between two Autonomous System Border Routers
	(ASBR).		(ASBR).
	skipping to change at page 4, line 34		skipping to change at page 4, line 34
	during the convergence process.		during the convergence process.
	The goal of the document is to meet the requirements described in		The goal of the document is to meet the requirements described in
	[REQS] at best, without changing the BGP protocol or BGP		[REQS] at best, without changing the BGP protocol or BGP
	implementations.		implementations.
	Still, it explains why reserving a community value for the purpose of		Still, it explains why reserving a community value for the purpose of
	BGP session graceful shutdown would reduce the management overhead		BGP session graceful shutdown would reduce the management overhead
	bound with the solution. It would also allow vendors to provide an		bound with the solution. It would also allow vendors to provide an
	automatic graceful shutdown mechanism that does not require any		automatic graceful shutdown mechanism that does not require any
	configuration at maintenance time.		router reconfiguration at maintenance time.
			The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
			"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
			document are to be interpreted as described in RFC 2119 [RFC2119].
	2. Terminology		2. Terminology
	g-shut initiator : a router on which the session shutdown is		g-shut initiator : a router on which the session shutdown is
	performed for the maintenance.		performed for the maintenance.
	g-shut neighbor : a router that peers with the g-shut initiator via		g-shut neighbor : a router that peers with the g-shut initiator via
	(one of) the session(s) to be shut down.		(one of) the session(s) to be shut down.
	Note that for the link-up case, we will refer to these nodes as g-no-		Note that for the link-up case, we will refer to these nodes as g-no-
	skipping to change at page 5, line 11		skipping to change at page 5, line 15
	Neighbor AS : the Autonomous System of the g-shut neighbor.		Neighbor AS : the Autonomous System of the g-shut neighbor.
	Affected path / Nominal / pre-convergence path : a BGP path via the		Affected path / Nominal / pre-convergence path : a BGP path via the
	peering link(s) undergoing the maintenance. This path will no longer		peering link(s) undergoing the maintenance. This path will no longer
	exist after the shutdown.		exist after the shutdown.
	Affected prefix : a prefix initially reached via an affected path.		Affected prefix : a prefix initially reached via an affected path.
	Affected router : a router having an affected prefix.		Affected router : a router having an affected prefix.
	Backup / alternate / post-convergence path : a path toward an		Backup / alternate / post-convergence path : a path towards an
	affected prefix that will be selected as the best path by an affected		affected prefix that will be selected as the best path by an affected
	router, when the link is shut down and the BGP convergence is		router, when the link is shut down and the BGP convergence is
	completed.		completed.
	Transient alternate path : a path towards an affected prefix that may		Transient alternate path : a path towards an affected prefix that may
	be transiently selected as best by an affected router during the		be transiently selected as best by an affected router during the
	convergence process but that is not a post-convergence path.		convergence process but that is not a post-convergence path.
	Loss of Connectivity (LoC) : the state when a router has no path		Loss of Connectivity (LoC) : the state when a router has no path
	towards an affected prefix.		towards an affected prefix.
	skipping to change at page 6, line 12		skipping to change at page 6, line 12
	This section describes means for an ISP to reduce the transient loss		This section describes means for an ISP to reduce the transient loss
	of packets upon a manual shutdown of a BGP session.		of packets upon a manual shutdown of a BGP session.
	4.1. Improving availability of alternate paths		4.1. Improving availability of alternate paths
	All solutions that increase the availability of alternate BGP paths		All solutions that increase the availability of alternate BGP paths
	in routers performing packet lookups in BGP tables [BestExternal]		in routers performing packet lookups in BGP tables [BestExternal]
	[AddPath] help in reducing the LoC bound with manual shutdown of eBGP		[AddPath] help in reducing the LoC bound with manual shutdown of eBGP
	sessions.		sessions.
	One solution increasing diversity in such a way that, at any single		One of such solutions increasing diversity in such a way that, at any
	step of the convergence process following the eBGP session shutdown,		single step of the convergence process following the eBGP session
	a BGP router does not receive a message withdrawing the only path it		shutdown, a BGP router does not receive a message withdrawing the
	currently knows for a given NLRI, allows for a simplified g-shut		only path it currently knows for a given NLRI, allows for a
	procedure. This simplified procedure would only tackle potential LoC		simplified g-shut procedure. This simplified procedure would only
	for the inbound traffic.		tackle potential LoC for the inbound traffic.
	Using advertise-best-external [BestExternal] on ASBRs and RRs helps		Using advertise-best-external [BestExternal] on ASBRs and RRs helps
	in avoiding lack of alternate paths in route reflectors upon a		in avoiding lack of alternate paths in route reflectors upon a
	convergence. Hence it reduces the LoC duration for the outbound		convergence. Hence it reduces the LoC duration for the outbound
	traffic of the ISP upon an eBGP Session shutdown by reducing the iBGP		traffic of the ISP upon an eBGP Session shutdown by reducing the iBGP
	path hunting.		path hunting.
	Still it does not ensure that BGP routers will always have at least		Still it does not ensure that BGP routers will always have at least
	one path towards affected prefixes during the convergence following		one path towards affected prefixes during the convergence following
	the event. This property may be verified in future revisions of		the event. This property may be verified in future revisions of
	[BestExternal], notably of its Section 4, hence the current proposal		[BestExternal], notably of its Section 3, hence the current proposal
	will be updated accordingly.		will be updated accordingly.
	Increasing diversity with [AddPath] might lead to the respect of this		Increasing diversity with [AddPath] might lead to the respect of this
	property, depending on the path propagation decision process that		property, depending on the path propagation decision process that
	add-path compliant routers would use.		add-path compliant routers would use.
	Note that the LoC for the inbound traffic of the maintained router,		Note that the LoC for the inbound traffic of the maintained router,
	induced by a lack of alternate path propagation within the iBGP		induced by a lack of alternate path propagation within the iBGP
	topology of a neighboring AS is not under the control of the operator		topology of a neighboring AS is not under the control of the operator
	performing the maintenance, hence the procedure described in		performing the maintenance, hence the procedure described in
	skipping to change at page 7, line 29		skipping to change at page 7, line 29
	switch to another path, as the input to the BGP decision process of		switch to another path, as the input to the BGP decision process of
	that router does not change. As a consequence, the g-shut initiator		that router does not change. As a consequence, the g-shut initiator
	will not send a withdraw message over its iBGP sessions when it		will not send a withdraw message over its iBGP sessions when it
	receives an alternate path over an iBGP session. It will however		receives an alternate path over an iBGP session. It will however
	modify the local-pref of the affected paths so that upstream routers		modify the local-pref of the affected paths so that upstream routers
	will switch to alternate ones.		will switch to alternate ones.
	When the actual shutdown of the session is performed, the g-shut		When the actual shutdown of the session is performed, the g-shut
	initiator will itself switch to the alternate paths.		initiator will itself switch to the alternate paths.
			In cases some BGP speakers in the AS override the local-pref
			attribute of paths received over iBGP sessions, the procedure
			described above will not work. In such cases, the recommended
			procedure is to tag the paths sent over the iBGP sessions of the
			g-shut initiator with an AS specific community. This AS specific
			community should lead to the setting of a low local-pref value. To
			be effective, the configuration related to this community MUST
			supplant or be applied after the already configured local-pref
			overriding.
	4.2.2. Inbound traffic		4.2.2. Inbound traffic
	The solution described for the outbound traffic can be applied at the		The solution described for the outbound traffic can be applied at the
	neighbor AS. This can be done either "manually" or by using a		neighbor AS. This can be done either "manually" or by using a
	community value dedicated to this task.		community value dedicated to this task.
	4.2.2.1. Phone call		4.2.2.1. Phone call
	The operator performing the maintenance of the eBGP session can		The operator performing the maintenance of the eBGP session can
	contact the operator at the other side of the peering link, and let		contact the operator at the other side of the peering link, and let
	skipping to change at page 8, line 24		skipping to change at page 8, line 36
	4.2.2.2.2. Operational action upon maintenance		4.2.2.2.2. Operational action upon maintenance
	Upon the manual shutdown, the output filter associated with the		Upon the manual shutdown, the output filter associated with the
	maintained eBGP session will be modified on the g-shut initiator so		maintained eBGP session will be modified on the g-shut initiator so
	as to tag all the paths advertised over the session with the GSHUT		as to tag all the paths advertised over the session with the GSHUT
	community.		community.
	4.2.2.2.3. Transitivity of the community		4.2.2.2.3. Transitivity of the community
	If the GSHUT community is an extended community, it SHOULD be set non		If the GSHUT community is an extended community, it SHOULD be chosen
	transitive.		non-transitive. In that case, the clarification described in
			[Clarification4360] is required.
	If a normal community is used, this community SHOULD be removed from		If a regular community is used, this community SHOULD be removed from
	the path by the ASBR of the peer receiving it. If not, the GSHUT		the path when the path is propagated over eBGP sessions.
	community MAY be removed from the path by all the ASBRs of the
	neighboring AS, before propagating the path to other peers.
	Not propagating the community further in the Internet reduces the		Not propagating the community further in the Internet reduces the
	amount of BGP churn and avoids rerouting in distant ASes that would		amount of BGP churn and avoids rerouting in distant ASes that would
	also recognize this community value. In other words, it helps		also recognize this community value. In other words, from a routing
	concealing the convergence at the maintenance location.		stability perspective, it helps concealing the convergence at the
			maintenance location. From a security perspective, it prevents
			malignant ASes from using the community over paths propagated through
			intermediate ASes that do not support the feature, in order to
			perform inbound traffic engineering at the first AS recognizing the
			community.
			ASes which support the g-shut procedure SHOULD remove the community
			value(s) that they use for g-shut from the paths received from
			neighboring ASes that do not support the procedure or to whom the
			service is not provided.
	There are cases where an interdomain exploration is to be performed		There are cases where an interdomain exploration is to be performed
	to recover the reachability, e.g., in the case of a shutdown in		to recover the reachability, e.g., in the case of a shutdown in
	confederations where the alternate paths will be found in another AS		confederations where the alternate paths will be found in another AS
	of the confederation. In such scenarios, the community value SHOULD		of the confederation. In such scenarios, the community value SHOULD
	be allowed to transit through the confederation but MAY be removed		be allowed to transit through the confederation but SHOULD be removed
	from the paths advertised outside of the confederation.		from the paths advertised outside of the confederation.
	When the local-pref value of a path is conserved upon its propagation		When the local-pref value of a path is conserved upon its propagation
	from one AS of the confederation to the other, there is no need to		from one AS of the confederation to the other, there is no need to
	have the GSHUT community be propagated throughout that confederation.		have the GSHUT community be propagated throughout that confederation.
	4.2.2.2.4. Easing the configuration for G-SHUT		4.2.2.2.4. Easing the configuration for G-SHUT
	From a configuration burden viewpoint, it would be much easier to		From a configuration burden viewpoint, it is much easier to use a
	reserve a value for the GSHUT community.		single dedicated value for the GSHUT community.
	First, on the g-shut initiator, an operator would have a single		First, on the g-shut initiator, an operator would have a single
	configuration rule to be applied at the maintenance time, which would		configuration rule to be applied at the maintenance time, which would
	not depend on the identity of its peer. This would make the		not depend on the identity of its peer. This would make the
	maintenance operations less error prone.		maintenance operations less error prone.
	Second, on the g-shut neighbor, a simple filter related to g-shut can		Second, on the g-shut neighbor, a simple filter related to g-shut can
	be applied to all iBGP sessions. Additionnaly, this filter doesn't		be applied to all iBGP sessions. Additionnaly, this filter does not
	need to be updated each time neighboring ASes are added or removed.		need to be updated each time neighboring ASes are added or removed.
			The FCFS community value 0xFFFF0000 has been reserved for this
			purpose [BGPWKC].
	4.3. Graceful shutdown procedures for iBGP sessions		4.3. Graceful shutdown procedures for iBGP sessions
	If the iBGP topology is viable after the maintenance of the session,		If the iBGP topology is viable after the maintenance of the session,
	i.e, if all BGP speakers of the AS have an iBGP signaling path for		i.e, if all BGP speakers of the AS have an iBGP signaling path for
	all prefixes advertised on this g-shut iBGP session, then the		all prefixes advertised on this g-shut iBGP session, then the
	shutdown of an iBGP session does not lead to transient		shutdown of an iBGP session does not lead to transient
	unreachability.		unreachability.
	However, in the case of a shutdown of a router, a reconfiguration of		However, in the case of a shutdown of a router, a reconfiguration of
	the out-filters of the g-shut initiator should be performed to set a		the out-filters of the g-shut initiator MAY be performed to set a low
	low local-pref value for the paths originated by the g-shut initiator		local-pref value for the paths originated by the g-shut initiator
	(e.g, BGP aggregates redistributed from other protocols, including		(e.g, BGP aggregates redistributed from other protocols, including
	static routes).		static routes).
	This behavior is equivalent to the recommended behavior for paths		This behavior is equivalent to the recommended behavior for paths
	"redistributed" from eBGP sessions to iBGP sessions in the case of		"redistributed" from eBGP sessions to iBGP sessions in the case of
	the shutdown of an ASBR.		the shutdown of an ASBR.
	5. Forwarding modes and forwarding loops		5. Forwarding modes and forwarding loops
	If the AS applying the solution does not rely on encapsulation to		If the AS applying the solution does not rely on encapsulation to
	forward packets from the Ingress Border Router to the Egress Border		forward packets from the Ingress Border Router to the Egress Border
	Router, then transient forwarding loops and consequent packet losses		Router, then transient forwarding loops and consequent packet losses
	can occur during the convergence process, even if the procedure		can occur during the convergence process, even if the procedure
	described above is applied. Hence if zero LoC is required,		described above is applied. Hence if zero LoC is required,
	encapsulation is required between ASBRs of the AS.		encapsulation is required between ASBRs of the AS.
	Using the out-filter reconfiguration avoids the forwarding loops		Using the out-filter reconfiguration avoids the forwarding loops
	between the g-shut initiator and its directly connected upstream		between the g-shut initiator and its directly connected upstream
	neighbors. Indeed, when this reconfiguration is applied, the g-shut		neighboring routers. Indeed, when this reconfiguration is applied,
	initiator keeps using its own external path and lets the upstream		the g-shut initiator keeps using its own external path and lets the
	routers converge to the alternate ones. During this phase, no		upstream routers converge to the alternate ones. During this phase,
	forwarding loops can occur between the g-shut initiator and its		no forwarding loops can occur between the g-shut initiator and its
	upstream neighbors as the g-shut initiator keeps using the affected		upstream neighbors as the g-shut initiator keeps using the affected
	paths via its eBGP peering links. When all the upstream routers have		paths via its eBGP peering links. When all the upstream routers have
	switched to alternate paths, the transition performed by the g-shut		switched to alternate paths, the transition performed by the g-shut
	initiator when the session is actually shut down, will be loopfree.		initiator when the session is actually shut down, will be loopfree.
	Transient forwarding loops between other routers will not be avoided		Transient forwarding loops between other routers will not be avoided
	with this procedure.		with this procedure.
	6. Dealing with Internet policies		6. Dealing with Internet policies
	A side gain of the maintenance solution is that it can also reduce		A side gain of the maintenance solution is that it can also reduce
	skipping to change at page 10, line 31		skipping to change at page 11, line 9
	propagation of withdraw messages over eBGP sessions with shared-cost		propagation of withdraw messages over eBGP sessions with shared-cost
	peers and providers.		peers and providers.
	Proceeding like this reduces both BGP churn and traffic shifting as		Proceeding like this reduces both BGP churn and traffic shifting as
	routers will less likely switch to transient paths.		routers will less likely switch to transient paths.
	In the above example, it also prevents transient unreachabilities in		In the above example, it also prevents transient unreachabilities in
	the neighboring AS that are due to the sending of "abrupt" withdraw		the neighboring AS that are due to the sending of "abrupt" withdraw
	messages to shared-cost peers and providers.		messages to shared-cost peers and providers.
	7. Effect of the g-shut procedure on the convergence		7. Link Up cases
	This section describes the effect of applying the solution.
	7.1. Maintenance of an eBGP session
	This section describes the effect of applying the solution for the
	shutdown of an eBGP session.
	7.1.1. Propagation on the other eBGP sessions of the g-shut initiator
	Nothing is propagated on the other eBGP sessions when the out-filters
	reconfiguration step is applied. The reconfiguration is indeed only
	defined for its iBGP sessions.
	The reconfiguration of the iBGP out-filters will trigger the
	reception of alternate paths at the g-shut initiator. As the eBGP
	in-filters have not been modified at that step, the old paths are
	still preferred by the g-shut initiator.
	7.1.2. Propagation on the other iBGP sessions of the g-shut initiator
	During the out-filter reconfiguration, path updates are propagated
	with a reduced local-pref value for the affected paths. As a
	consequence, Route Reflectors and distant ASBRs select and propagate
	alternate paths through the iBGP topology as they no longer select
	the old paths as best.
	When the shut-down is performed, for each affected prefix, the g-shut
	initiator propagates on its iBGP sessions:
	. The alternate path, if the best path was received over an eBGP
	sessions.
	. A withdraw, if the best path was received over an iBGP sessions.
	7.1.3. Propagation of updates in an iBGP full-mesh
	No transient LoC can occur if a reconfiguration of the iBGP out-
	filters on the g-shut initiator is performed.
	7.1.4. Propagation of updates from iBGP to iBGP in a RR hierarchy
	Upon the reception of the update of a primary path with a lower
	local-pref value from a client, a Route Reflector RR1 will either
	propagate the update, or select an alternate path, depending on the
	fact that the updated primary path is still the best one w.r.t. the
	state of the Adj-Rib-In of RR1.
	If the updated primary path is still the best, then the RR will
	propagate an update for this path to the iBGP neighbors to which it
	previously advertised the path. Hence it cannot cause transient lack
	of path in the Adj-Rib-In of its iBGP neighbors.
	If an alternate path is picked, and this path was also originated by
	a client of RR1, an update will also be propagated to the same
	neighbors as the one to which the primary path was initially
	propagated. Hence it cannot cause transient lack of path in the Adj-
	Rib-In of its iBGP neighbors.
	If an alternate path is picked, and this path was received from a
	member of its Route-Reflector iBGP full-mesh, then a withdraw message
	is sent. As the alternate path has been sent over each session of
	the iBGP full-mesh, the propagation of a withdraw for the primary
	path of RR1 is done to routers that are expected to know the
	alternate path picked by RR1.
	The following example describes a situation where some corner case
	timings could lead to transient unreachability from some members of
	the iBGP full-mesh.
	1. A Route Reflector RR1 only knew about the primary path upon
	the shutdown.
	2. A member of its RR full-mesh, RR2, propagates an update of
	the old path with a lower local-pref.
	3. Another member of its RR full-mesh, RR3 processes the
	update, selects an alternate path, and propagates an update in
	the mesh.
	4. RR2 receives the alternate path, selects it as best, and
	hence withdraws the updated old path on the iBGP sessions of the
	mesh.
	5. If for any reason, RR1 receives and processes the withdraw
	generated in step 4 before processing the update generated in
	step 3, RR1 transiently suffers from unreachability for the
	affected prefix.
	In such corner cases, the solution improves the iBGP convergence
	behavior/LoC but does not ensure 0 packet loss, as we cannot define a
	simple solution relying only on a reconfiguration of the filters of
	the g-shut initiator. Improving the availability of alternate paths
	in Route Reflectors, using [BestExternal], or [AddPath], seems to be
	the most pragmatic solution to these corner cases.
	The use of [BestExternal] in the iBGP full-mesh between RRs can solve
	these corner cases by ensuring that within an AS, the advertisement
	of a new path is not translated into the withdraw of a former path.
	Indeed, "best-external" ensures that an ASBR does not withdraw a
	previously advertised (eBGP) path when it receives an additional,
	preferred path over an iBGP session. Also, "best-intra-cluster"
	ensures that a RR does not withdraw a previously advertised (iBGP)
	path to its non clients (e.g. other RRs in a mesh of RR) when it
	receives a new, preferred path over an iBGP session.
	7.2. Maintenance of an iBGP session
	If the shutdown does not temper with the viability of the iBGP
	topology, the described procedure is sufficient to avoid LoC.
	7.3. Applicability of the g-shut procedure
	The applicability of the procedure described in this draft to the
	cases presented in [REQS] can be shown by combining the effects
	described in this section. A complete case by case analysis will be
	provided in the next versions of the draft.
	7.4. Summary of operations
	This section summarizes the configurations and actions to be
	performed to support the g-shut procedure for eBGP peering links.
	7.4.1. Pre-configuration
	On each ASBR supporting the g-shut procedure, set-up an out-filter
	applied on all iBGP sessions of the ASBR, that :
	. sets the local-pref of the paths tagged with the g-shut community
	to a low value
	. removes the g-shut community from the path.
	7.4.2. Operations at maintenance time
	On the g-shut initiator :
	. Apply an in-filter on the maintained BGP session to tag the paths
	received over the session with the g-shut community.
	. Apply an out-filter on the maintained BGP session to tag the paths
	propagated over the session with the g-shut community.
	. Wait for convergence to happen.
	. Perform a BGP session shutdown.
	8. Link Up cases
	We identify two potential causes for transient packet losses upon an		We identify two potential causes for transient packet losses upon an
	eBGP link up event. The first one is local to the g-shut initiator,		eBGP link up event. The first one is local to the g-no-shut
	the second one is due to the BGP convergence following the injection		initiator, the second one is due to the BGP convergence following the
	of new best paths within the iBGP topology.		injection of new best paths within the iBGP topology.
	8.1. Unreachability local to the ASBR		7.1. Unreachability local to the ASBR
	An ASBR that selects as best a path received over a newly brought up		An ASBR that selects as best a path received over a newly brought up
	eBGP session may transiently drop traffic. This can typically happen		eBGP session may transiently drop traffic. This can typically happen
	when the nexthop attribute differs from the IP address of the eBGP		when the nexthop attribute differs from the IP address of the eBGP
	peer, and the receiving ASBR has not yet resolved the MAC address		peer, and the receiving ASBR has not yet resolved the MAC address
	associated with the IP address of that "third party" nexthop.		associated with the IP address of that "third party" nexthop.
	A BGP speaker implementation could avoid such losses by ensuring that		A BGP speaker implementation could avoid such losses by ensuring that
	"third party" nexthops are resolved before installing paths using		"third party" nexthops are resolved before installing paths using
	these in the RIB.		these in the RIB.
	If the link up event corresponds to an eBGP session that is being		If the link up event corresponds to an eBGP session that is being
	manually brought up, over an already up multi-access link, then the		manually brought up, over an already up multi-access link, then the
	operator can ping third party nexthops that are expected to be used		operator can ping third party nexthops that are expected to be used
	before actually bringing the session up, or ping directed broadcast		before actually bringing the session up, or ping directed broadcast
	the subnet IP address of the link. By proceeding like this, the MAC		the subnet IP address of the link. By proceeding like this, the MAC
	addresses associated with these third party nexthops will be resolved		addresses associated with these third party nexthops will be resolved
	by the g-no-shut initiator.		by the g-no-shut initiator.
	8.2. iBGP convergence		7.2. iBGP convergence
	Similar corner cases as described in Section 7.1.4 for the link down		Similar corner cases as described in Appendix C.1.4 for the link down
	case, can occur during an eBGP link up event.		case, can occur during an eBGP link up event.
	A typical example for such transient unreachability for a given		A typical example for such transient unreachability for a given
	prefix is the following :		prefix is the following :
	1. A Route Reflector, RR1, is initially advertising the current		1. A Route Reflector, RR1, is initially advertising the current
	best path to the members of its iBGP RR full-mesh. It		best path to the members of its iBGP RR full-mesh. It
	propagated that path within its RR full-mesh. Another route		propagated that path within its RR full-mesh. Another route
	reflector of the full-mesh, RR2, knows only that path towards		reflector of the full-mesh, RR2, knows only that path towards
	the prefix.		the prefix.
	skipping to change at page 15, line 9		skipping to change at page 12, line 25
	advertisement of a new route is not translated into the withdraw of a		advertisement of a new route is not translated into the withdraw of a
	former route.		former route.
	Indeed, "best-external" ensures that an ASBR does not withdraw a		Indeed, "best-external" ensures that an ASBR does not withdraw a
	previously advertised (eBGP) path when it receives an additional,		previously advertised (eBGP) path when it receives an additional,
	preferred path over an iBGP session. Also, "best-intra-cluster"		preferred path over an iBGP session. Also, "best-intra-cluster"
	ensures that a RR does not withdraw a previously advertised (iBGP)		ensures that a RR does not withdraw a previously advertised (iBGP)
	path to its non clients (e.g. other RRs in a mesh of RR) when it		path to its non clients (e.g. other RRs in a mesh of RR) when it
	receives a new, preferred path over an iBGP session.		receives a new, preferred path over an iBGP session.
	9. Alternative techniques with limited applicability		8. IANA considerations
			Applying the g-shut procedure is rendered much easier with a reserved
			g-shut community value. The community value 0xFFFF0000 has been
			reserved from the FCFS community pool for this purpose.
			9. Security Considerations
			By providing the g-shut service to a neighboring AS, an ISP provides
			means to this neighbor to lower the local-pref value assigned to the
			paths received from this neighbor.
			The neighbor could abuse the technique and do inbound traffic
			engineering by declaring some prefixes as undergoing a maintenance so
			as to switch traffic to another peering link.
			If this behavior is not tolerated by the ISP, it SHOULD monitor the
			use of the g-shut community by this neighbor.
			ASes which support the g-shut procedure SHOULD remove the community
			value(s) that they use for g-shut from the paths received from
			neighboring ASes that do not support the procedure or to whom the
			service is not provided. Doing so prevents malignant ASes from using
			the community through intermediate ASes that do not support the
			feature, in order to perform inbound traffic engineering.
			10. Acknowledgments
			The authors wish to thank Olivier Bonaventure and Pradosh Mohapatra
			for their useful comments on this work.
			11. References
			[AddPath] D. Walton, A. Retana, and E. Chen, "Advertisement of
			Multiple Paths in BGP", draft-walton-bgp-add-paths-06.txt
			(work in progress).
			[BestExternal]
			Marques, P., Fernando, R., Chen, E., and P. Mohapatra,
			"Advertisement of the best-external route to IBGP",
			draft-ietf-idr-best-external-00.txt, May 2009.
			[REQS] Decraene, B., Francois, P., Pelsser, C., Ahmad, Z., and T.
			Takeda, "Requirements for the graceful shutdown of BGP
			sessions",
			draft-ietf-grow-bgp-graceful-shutdown-requirements-
			01.txt, October 2009.
			[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
			Communities Attribute", RFC 4360, February 2006.
			[Clarification4360]
			Decraene, B., Vanbever, L., and P. Francois, "RFC 4360
			Clarification Request",
			draft-decraene-idr-rfc4360-clarification-00,
			October 2009.
			[BGPWKC] "http://www.iana.org/assignments/
			bgp-well-known-communities".
			[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
			Requirement Levels", BCP 14, RFC 2119, March 1997.
			Appendix A. Summary of operations
			This section summarizes the configurations and actions to be
			performed to support the g-shut procedure for eBGP peering links.
			A.1. Pre-configuration
			On each ASBR supporting the g-shut procedure, set-up an out-filter
			applied on all iBGP sessions of the ASBR, that :
			. sets the local-pref of the paths tagged with the g-shut community
			to a low value
			. removes the g-shut community from the path.
			Optionally, add an AS specific g-shut community on the path to
			indicate that this path is to be shutdown. If some ingress ASBRs
			reset the local preference attribute, this AS specific g-shut
			community will be used to override other local preference changes.
			A.2. Operations at maintenance time
			On the g-shut initiator :
			. Apply an in-filter on the maintained eBGP session to tag the paths
			received over the session with the g-shut community.
			. Apply an out-filter on the maintained eBGP session to tag the
			paths propagated over the session with the g-shut community.
			. Wait for convergence to happen.
			. Perform a BGP session shutdown.
			Appendix B. Alternative techniques with limited applicability
	A few alternative techniques have been considered to provide g-shut		A few alternative techniques have been considered to provide g-shut
	capabilities but have been rejected due to their limited		capabilities but have been rejected due to their limited
	applicability. This section describe them for possible reference.		applicability. This section describe them for possible reference.
	9.1. In-filter reconfiguration		B.1. In-filter reconfiguration
	An In-filter reconfiguration on the eBGP session undergoing the		An In-filter reconfiguration on the eBGP session undergoing the
	maintenance could be performed instead of out-filter reconfigurations		maintenance could be performed instead of out-filter reconfigurations
	on the iBGP sessions of the g-shut initiator.		on the iBGP sessions of the g-shut initiator.
	Upon the application of the maintenance procedure, if the g-shut		Upon the application of the maintenance procedure, if the g-shut
	initiator has an alternate path in its Adj-Rib-In, it will switch to		initiator has an alternate path in its Adj-Rib-In, it will switch to
	it directly.		it directly.
	If this new path was advertised by an eBGP neighbor of the g-shut		If this new path was advertised by an eBGP neighbor of the g-shut
	skipping to change at page 16, line 21		skipping to change at page 15, line 40
	path first, the withdraw from ASBR2 does not trigger unreachability		path first, the withdraw from ASBR2 does not trigger unreachability
	in other nodes, as the old path is still available. Indeed, even		in other nodes, as the old path is still available. Indeed, even
	though it receives alternate paths, the g-shut initiator keeps using		though it receives alternate paths, the g-shut initiator keeps using
	its old path as best as the in-filter of the maintained eBGP session		its old path as best as the in-filter of the maintained eBGP session
	has not been modified yet.		has not been modified yet.
	Applying the out-filter reconfiguration also prevents packet loops		Applying the out-filter reconfiguration also prevents packet loops
	between the g-shut initiator and its direct neighbors when		between the g-shut initiator and its direct neighbors when
	encapsulation is not used between the ASBRs of the AS.		encapsulation is not used between the ASBRs of the AS.
	9.2. Multi Exit Discriminator tweaking		B.2. Multi Exit Discriminator tweaking
	The MED attribute of the paths to be avoided can be increased so as		The MED attribute of the paths to be avoided can be increased so as
	to force the routers in the neighboring AS to select other paths.		to force the routers in the neighboring AS to select other paths.
	The solution only works if the alternate paths are as good as the		The solution only works if the alternate paths are as good as the
	initial ones with respect to the Local-Pref value and the AS Path		initial ones with respect to the Local-Pref value and the AS Path
	Length value. In the other cases, increasing the MED value will not		Length value. In the other cases, increasing the MED value will not
	have an impact on the decision process of the routers in the		have an impact on the decision process of the routers in the
	neighboring AS.		neighboring AS.
	9.3. IGP distance Poisoning		B.3. IGP distance Poisoning
	The distance to the BGP nexthop corresponding to the maintained		The distance to the BGP nexthop corresponding to the maintained
	session can be increased in the IGP so that the old paths will be		session can be increased in the IGP so that the old paths will be
	less preferred during the application of the IGP distance tie-break		less preferred during the application of the IGP distance tie-break
	rule. However, this solution only works for the paths whose		rule. However, this solution only works for the paths whose
	alternates are as good as the old paths with respect to their Local-		alternates are as good as the old paths with respect to their Local-
	Pref value, their AS Path length, and their MED value.		Pref value, their AS Path length, and their MED value.
	Also, this poisoning cannot be applied when nexthop self is used as		Also, this poisoning cannot be applied when nexthop self is used as
	there is no nexthop specific to the maintained session to poison in		there is no nexthop specific to the maintained session to poison in
	the IGP.		the IGP.
	10. IANA considerations		Appendix C. Effect of the g-shut procedure on the convergence
	Applying the g-shut procedure is rendered much easier with a reserved		This section describes the effect of applying the solution.
	g-shut community value. Hence this draft suggests to reserve a
	community value, e.g., 0xFFFF0000, for this purpose.
	11. Security Considerations		C.1. Maintenance of an eBGP session
	By providing the g-shut service to a neighboring AS, an ISP provides		This section describes the effect of applying the solution for the
	means to this neighbor to lower the local-pref value assigned to the		shutdown of an eBGP session.
	paths received from this neighbor.
	The neighbor could abuse the technique and do inbound traffic		C.1.1. Propagation on the other eBGP sessions of the g-shut initiator
	engineering by declaring some prefixes as undergoing a maintenance so
	as to switch traffic to another peering link.
	If this behavior is not tolerated by the ISP, it SHOULD monitor the		Nothing is propagated on the other eBGP sessions when the out-filters
	use of the g-shut community by this neighbor.		reconfiguration step is applied. The reconfiguration is indeed only
			defined for its iBGP sessions.
	12. Acknowledgments		The reconfiguration of the iBGP out-filters will trigger the
			reception of alternate paths at the g-shut initiator. As the eBGP
			in-filters have not been modified at that step, the old paths are
			still preferred by the g-shut initiator.
	The authors wish to thank Olivier Bonaventure and Pradosh Mohapatra		C.1.2. Propagation on the other iBGP sessions of the g-shut initiator
	for their useful comments on this work.
	13. References		During the out-filter reconfiguration, path updates are propagated
			with a reduced local-pref value for the affected paths. As a
			consequence, Route Reflectors and distant ASBRs select and propagate
			alternate paths through the iBGP topology as they no longer select
			the old paths as best.
	[AddPath] D. Walton, A. Retana, and E. Chen, "Advertisement of		When the shut-down is performed, for each affected prefix, the g-shut
	Multiple Paths in BGP", draft-walton-bgp-add-paths-06.txt		initiator propagates on its iBGP sessions:
	(work in progress).
	[BestExternal]		. The alternate path, if the best path was received over an eBGP
	Marques, P., Fernando, R., Chen, E., and P. Mohapatra,		sessions.
	"Advertisement of the best-external route to IBGP",
	draft-marques-idr-best-external-00.txt, July 2008.
	[REQS] Decraene, B., Francois, P., Pelsser, C., and Z. Ahmad,		. A withdraw, if the best path was received over an iBGP sessions.
	"Requirements for the graceful shutdown of BGP sessions",
	draft-ietf-grow-bgp-graceful-shutdown-requirements-00.txt		C.1.3. Propagation of updates in an iBGP full-mesh
	, May 2009.
			No transient LoC can occur if a reconfiguration of the iBGP out-
			filters on the g-shut initiator is performed.
			C.1.4. Propagation of updates from iBGP to iBGP in a RR hierarchy
			Upon the reception of the update of a primary path with a lower
			local-pref value from a client, a Route Reflector RR1 will either
			propagate the update, or select an alternate path, depending on the
			fact that the updated primary path is still the best one w.r.t. the
			state of the Adj-Rib-In of RR1.
			If the updated primary path is still the best, then the RR will
			propagate an update for this path to the iBGP neighbors to which it
			previously advertised the path. Hence it cannot cause transient lack
			of path in the Adj-Rib-In of its iBGP neighbors.
			If an alternate path is picked, and this path was also originated by
			a client of RR1, an update will also be propagated to the same
			neighbors as the one to which the primary path was initially
			propagated. Hence it cannot cause transient lack of path in the Adj-
			Rib-In of its iBGP neighbors.
			If an alternate path is picked, and this path was received from a
			member of its Route-Reflector iBGP full-mesh, then a withdraw message
			is sent. As the alternate path has been sent over each session of
			the iBGP full-mesh, the propagation of a withdraw for the primary
			path of RR1 is done to routers that are expected to know the
			alternate path picked by RR1.
			The following example describes a situation where some corner case
			timings could lead to transient unreachability from some members of
			the iBGP full-mesh.
			1. A Route Reflector RR1 only knew about the primary path upon
			the shutdown.
			2. A member of its RR full-mesh, RR2, propagates an update of
			the old path with a lower local-pref.
			3. Another member of its RR full-mesh, RR3 processes the
			update, selects an alternate path, and propagates an update in
			the mesh.
			4. RR2 receives the alternate path, selects it as best, and
			hence withdraws the updated old path on the iBGP sessions of the
			mesh.
			5. If for any reason, RR1 receives and processes the withdraw
			generated in step 4 before processing the update generated in
			step 3, RR1 transiently suffers from unreachability for the
			affected prefix.
			In such corner cases, the solution improves the iBGP convergence
			behavior/LoC but does not ensure 0 packet loss, as we cannot define a
			simple solution relying only on a reconfiguration of the filters of
			the g-shut initiator. Improving the availability of alternate paths
			in Route Reflectors, using [BestExternal], or [AddPath], seems to be
			the most pragmatic solution to these corner cases.
			The use of [BestExternal] in the iBGP full-mesh between RRs can solve
			these corner cases by ensuring that within an AS, the advertisement
			of a new path is not translated into the withdraw of a former path.
			Indeed, "best-external" ensures that an ASBR does not withdraw a
			previously advertised (eBGP) path when it receives an additional,
			preferred path over an iBGP session. Also, "best-intra-cluster"
			ensures that a RR does not withdraw a previously advertised (iBGP)
			path to its non clients (e.g. other RRs in a mesh of RR) when it
			receives a new, preferred path over an iBGP session.
			C.2. Maintenance of an iBGP session
			If the shutdown does not temper with the viability of the iBGP
			topology, the described procedure is sufficient to avoid LoC.
	Authors' Addresses		Authors' Addresses
	Pierre Francois		Pierre Francois
	Universite catholique de Louvain		Universite catholique de Louvain
	Place Ste Barbe, 2		Place Ste Barbe, 2
	Louvain-la-Neuve 1348		Louvain-la-Neuve 1348
	BE		BE
	Email: pierre.francois@uclouvain.be		Email: pierre.francois@uclouvain.be
	URI: http://inl.info.ucl.ac.be/pfr		URI: http://inl.info.ucl.ac.be/pfr
	Bruno Decraene		Bruno Decraene
	France Telecom		France Telecom
	38-40 rue du General Leclerc		38-40 rue du General Leclerc
	92794 Issi Moulineaux cedex 9		92794 Issi Moulineaux cedex 9
	FR		FR
	Email: bruno.decraene@orange-ftgroup.com		Email: bruno.decraene@orange-ftgroup.com
	Cristel Pelsser		Cristel Pelsser
	NTT Corporation		Internet Initiative Japan
	9-11, Midori-Cho 3 Chrome		Jinbocho Mitsui Bldg.
	Musashino-Shi, Tokyo 180-8585		1-105 Kanda Jinbo-cho
			Tokyo 101-0051
	JP		JP
	Email: pelsser.cristel@lab.ntt.co.jp		Email: pelsser.cristel@iij.ad.jp
	Clarence Filsfils		Clarence Filsfils
	Cisco Systems		Cisco Systems
	De kleetlaan 6a		De kleetlaan 6a
	Diegem 1831		Diegem 1831
	BE		BE
	Email: cfilsfil@cisco.com		Email: cfilsfil@cisco.com
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