< draft-ietf-rtgwg-mrt-frr-architecture-00.txt		draft-ietf-rtgwg-mrt-frr-architecture-01.txt >
	Routing Area Working Group A. Atlas, Ed.		Routing Area Working Group A. Atlas, Ed.
	Internet-Draft R. Kebler		Internet-Draft R. Kebler
	Intended status: Standards Track M. Konstantynowicz		Intended status: Standards Track Juniper Networks
	Expires: July 29, 2012 Juniper Networks		Expires: September 13, 2012 G. Enyedi
	G. Enyedi
	A. Csaszar		A. Csaszar
	Ericsson		Ericsson
			M. Konstantynowicz
	R. White		R. White
	Cisco Systems		Cisco Systems
	M. Shand		M. Shand
	January 26, 2012		March 12, 2012
	An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees		An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
	draft-ietf-rtgwg-mrt-frr-architecture-00		draft-ietf-rtgwg-mrt-frr-architecture-01
	Abstract		Abstract
	As IP and LDP Fast-Reroute are increasingly deployed, the coverage		As IP and LDP Fast-Reroute are increasingly deployed, the coverage
	limitations of Loop-Free Alternates are seen as a problem that		limitations of Loop-Free Alternates are seen as a problem that
	requires a straightforward and consistent solution for IP and LDP,		requires a straightforward and consistent solution for IP and LDP,
	for unicast and multicast. This draft describes an architecture		for unicast and multicast. This draft describes an architecture
	based on redundant backup trees where a single failure can cut a		based on redundant backup trees where a single failure can cut a
	point-of-local-repair from the destination only on one of the pair of		point-of-local-repair from the destination only on one of the pair of
	redundant trees.		redundant trees.
	skipping to change at page 2, line 7 ¶		skipping to change at page 2, line 7 ¶
	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 http://datatracker.ietf.org/drafts/current/.		Drafts is at http://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 July 29, 2012.		This Internet-Draft will expire on September 13, 2012.
	Copyright Notice		Copyright Notice
	Copyright (c) 2012 IETF Trust and the persons identified as the		Copyright (c) 2012 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
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://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 3, line 12 ¶		skipping to change at page 3, line 12 ¶
	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. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
	1.1. Goals for Extending IP Fast-Reroute coverage beyond LFA . 4		1.1. Goals for Extending IP Fast-Reroute coverage beyond LFA . 4
	2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5		2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
	3. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 6		3. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 6
	4. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . . 8		4. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . . 8
	4.1. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . 9		5. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . . 9
	4.2. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . 10		5.1. LDP Unicast Forwarding - Avoid Tunneling . . . . . . . . . 10
	4.2.1. LDP Unicast Forwarding - Avoid Tunneling . . . . . . . 11		5.2. IP Unicast Traffic . . . . . . . . . . . . . . . . . . . . 10
	4.2.1.1. Protocol Extensions and Considerations: LDP . . . 12		6. Protocol Extensions and Considerations: OSPF and ISIS . . . . 12
	4.2.2. IP Unicast Traffic . . . . . . . . . . . . . . . . . . 12		7. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . . . 14
	4.2.2.1. Protocol Extensions and Considerations: OSPF		8. Inter-Area and ABR Forwarding Behavior . . . . . . . . . . . . 15
	and ISIS . . . . . . . . . . . . . . . . . . . . . 13		9. Issues with Area Abstraction . . . . . . . . . . . . . . . . . 18
	4.2.3. Inter-Area and ABR Forwarding Behavior . . . . . . . . 13		10. Partial Deployment and Islands of Compatible MRT FRR
	4.2.4. Issues with Area Abstraction . . . . . . . . . . . . . 15		routers . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
	4.2.5. Partial Deployment and Islands of Compatible MRT		11. Network Convergence and Preparing for the Next Failure . . . . 21
	FRR routers . . . . . . . . . . . . . . . . . . . . . 16		11.1. Micro-forwarding loop prevention and MRTs . . . . . . . . 21
	4.2.6. Network Convergence and Preparing for the Next		11.2. MRT Recalculation . . . . . . . . . . . . . . . . . . . . 22
	Failure . . . . . . . . . . . . . . . . . . . . . . . 17		12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
	4.2.6.1. Micro-forwarding loop prevention and MRTs . . . . 17		13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
	4.2.6.2. MRT Recalculation . . . . . . . . . . . . . . . . 17		14. Security Considerations . . . . . . . . . . . . . . . . . . . 23
	5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18		15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18		15.1. Normative References . . . . . . . . . . . . . . . . . . . 23
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 18		15.2. Informative References . . . . . . . . . . . . . . . . . . 23
	8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
	8.1. Normative References . . . . . . . . . . . . . . . . . . . 18
	8.2. Informative References . . . . . . . . . . . . . . . . . . 19
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
	1. Introduction		1. Introduction
	There is still work required to completely provide IP and LDP Fast-		There is still work required to completely provide IP and LDP Fast-
	Reroute[RFC5714] for unicast and multicast traffic. This draft		Reroute[RFC5714] for unicast and multicast traffic. This draft
	proposes an architecture to provide 100% coverage.		proposes an architecture to provide 100% coverage for unicast
			traffic. The associated multicast architecture is described in
			[I-D.atlas-rtgwg-mrt-mc-arch].
	Loop-free alternates (LFAs)[RFC5286] provide a useful mechanism for		Loop-free alternates (LFAs)[RFC5286] provide a useful mechanism for
	link and node protection but getting complete coverage is quite hard.		link and node protection but getting complete coverage is quite hard.
	[LFARevisited] defines sufficient conditions to determine if a		[LFARevisited] defines sufficient conditions to determine if a
	network provides link-protecting LFAs and also proves that augmenting		network provides link-protecting LFAs and also proves that augmenting
	a network to provide better coverage is NP-hard.		a network to provide better coverage is NP-hard.
	[I-D.ietf-rtgwg-lfa-applicability] discusses the applicability of LFA		[I-D.ietf-rtgwg-lfa-applicability] discusses the applicability of LFA
	to different topologies with a focus on common PoP architectures.		to different topologies with a focus on common PoP architectures.
	While Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is defined as		While Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is defined as
	an architecture, in practice, it has proved too complicated and		an architecture, in practice, it has proved too complicated and
	stateful to spark substantial interest in implementation or		stateful to spark substantial interest in implementation or
	deployment. Academic implementations [LightweightNotVia] exist and		deployment. Academic implementations [LightweightNotVia] exist and
	have found the address management complexity high (but no		have found the address management complexity high (but no
	standardization has been done to reduce this).		standardization has been done to reduce this).
	A different approach is needed and that is what is described here.		A different approach is needed and that is what is described here.
	It is based on the idea of using disjoint backup topologies as		It is based on the idea of using disjoint backup topologies as
	realized by Maximally Redundant Trees (described in		realized by Maximally Redundant Trees (described in
	[LightweightNotVia]); the general architecture could also apply to		[LightweightNotVia]); the general architecture can also apply to
	future improved redundant tree algorithms.		future improved redundant tree algorithms.
	1.1. Goals for Extending IP Fast-Reroute coverage beyond LFA		1.1. Goals for Extending IP Fast-Reroute coverage beyond LFA
	Any scheme proposed for extending IPFRR network topology coverage		Any scheme proposed for extending IPFRR network topology coverage
	beyond LFA, apart from attaining basic IPFRR properties, should also		beyond LFA, apart from attaining basic IPFRR properties, should also
	aim to achieve the following usability goals:		aim to achieve the following usability goals:
	o ensure maximum physically feasible link and node disjointness		o ensure maximum physically feasible link and node disjointness
	regardless of topology,		regardless of topology,
	skipping to change at page 6, line 47 ¶		skipping to change at page 6, line 47 ¶
	of the shortest-path-first algorithm in tree-building and including		of the shortest-path-first algorithm in tree-building and including
	all links in the network as possibilities for one path or another		all links in the network as possibilities for one path or another
	should improve this. Modeling is underway to investigate and compare		should improve this. Modeling is underway to investigate and compare
	the MRT alternates to the optimal		the MRT alternates to the optimal
	[I-D.enyedi-rtgwg-mrt-frr-algorithm]. Providing shortest detour		[I-D.enyedi-rtgwg-mrt-frr-algorithm]. Providing shortest detour
	paths would require failure-specific detour paths to the		paths would require failure-specific detour paths to the
	destinations, but the state-reduction advantage of MRT lies in the		destinations, but the state-reduction advantage of MRT lies in the
	detour being established per destination (root) instead of per		detour being established per destination (root) instead of per
	destination AND per failure.		destination AND per failure.
	The specific algorithm to compute MRTs as well as the logic behind		The specific algorithms to compute MRTs as well as the logic behind
	that algorithm and alternative computational approaches are given in		that algorithm and alternative computational approaches are given in
	detail in [I-D.enyedi-rtgwg-mrt-frr-algorithm]. Those interested are		detail in [I-D.enyedi-rtgwg-mrt-frr-algorithm]. Those interested are
	highly recommended to read that document. This document describes		highly recommended to read that document. This document describes
	how the MRTs can be used and not how to compute them.		how the MRTs can be used and not how to compute them.
	The most important thing to understand about MRTs is that for each		The most important thing to understand about MRTs is that for each
	pair of destination-routed MRTs, there is a path from every node X to		pair of destination-routed MRTs, there is a path from every node X to
	the destination D on the Blue MRT that is as disjoint as possible		the destination D on the Blue MRT that is as disjoint as possible
	from the path on the Red MRT. The two paths along the two MRTs to a		from the path on the Red MRT. The two paths along the two MRTs to a
	given destination-root of a 2-connected graph are node-disjoint,		given destination-root of a 2-connected graph are node-disjoint and
	while in any non-2-connected graph, only the cut-vertices and cut-		link-disjoint, while in any non-2-connected graph, only the cut-
	edges can be contained by both of the paths.		vertices and cut-edges can be contained by both of the paths.
	For example, in Figure 1, there is a network graph that is		For example, in Figure 1, there is a network graph that is
	2-connected in (a) and associated MRTs in (b) and (c). One can		2-connected in (a) and associated MRTs in (b) and (c). One can
	consider the paths from B to R; on the Blue MRT, the paths are		consider the paths from B to R; on the Blue MRT, the paths are
	B->F->D->E->R or B->F->C->E->R. On the Red MRT, the path is B->A->R.		B->F->D->E->R or B->C->D->E->R. On the Red MRT, the path is B->A->R.
	These are clearly link and node-disjoint. These MRTs are redundant		These are clearly link and node-disjoint. These MRTs are redundant
	trees because the paths are disjoint.		trees because the paths are disjoint.
	[E]---[D]---| [E]<--[D]<--| [E]-->[D]---|		[E]---[D]---| [E]<--[D]<--| [E]-->[D]---|
	| | | | ^ | | |		| | | | ^ | | |
	| | | V | | V V		| | | V | | V V
	[R] [F] [C] [R] [F] [C] [R] [F] [C]		[R] [F] [C] [R] [F] [C] [R] [F] [C]
	| | | ^ ^ ^ | |		| | | ^ ^ ^ | |
	| | | | | | V |		| | | | | | V |
	[A]---[B]---| [A]-->[B]---| [A]---[B]<--|		[A]---[B]---| [A]-->[B]---| [A]---[B]<--|
	(a) (b) (c)		(a) (b) (c)
	a 2-connected graph Blue MRT towards R Red MRT towards R		a 2-connected graph Blue MRT towards R Red MRT towards R
	Figure 1: A 2-connected Network		Figure 1: A 2-connected Network
	By contrast, in Figure 2, the network in (a) is not 2-conneted. If		By contrast, in Figure 2, the network in (a) is not 2-connected. If
	F, G or the link F<->G failed, then the network would be partitioned.		F, G or the link F<->G failed, then the network would be partitioned.
	It is clearly impossible to have two link-disjoint or node-disjoint		It is clearly impossible to have two link-disjoint or node-disjoint
	paths from G, I or J to R. The MRTs given in (b) and (c) offer paths		paths from G, I or J to R. The MRTs given in (b) and (c) offer paths
	that are as disjoint as possible. For instance, the paths from B to		that are as disjoint as possible. For instance, the paths from B to
	R are the same as in Figure 1 and the path from G to R on the Blue		R are the same as in Figure 1 and the path from G to R on the Blue
	MRT is G->F->D->E->R and on the Red MRT is G->F->B->A->R.		MRT is G->F->D->E->R and on the Red MRT is G->F->B->A->R.
	[E]---[D]---|		[E]---[D]---|
	| | | |----[I]		| | | |----[I]
	| | | | |		| | | | |
	skipping to change at page 8, line 37 ¶		skipping to change at page 8, line 37 ¶
	Figure 2: A non-2-connected network		Figure 2: A non-2-connected network
	4. Maximally Redundant Trees (MRT) and Fast-Reroute		4. Maximally Redundant Trees (MRT) and Fast-Reroute
	In normal IGP routing, each router has its shortest-path-tree to all		In normal IGP routing, each router has its shortest-path-tree to all
	destinations. From the perspective of a particular destination, D,		destinations. From the perspective of a particular destination, D,
	this looks like a reverse SPT (rSPT). To use maximally redundant		this looks like a reverse SPT (rSPT). To use maximally redundant
	trees, in addition, each destination D has two MRTs associated with		trees, in addition, each destination D has two MRTs associated with
	it; by convention these will be called the blue and red MRTs.		it; by convention these will be called the blue and red MRTs.
			Any IP/LDP fast-reroute technique beyond LFA requires an additional
			dataplane procedure, such as an additional forwarding mechanism. The
			well-known options are tunneling (e.g.
			[I-D.ietf-rtgwg-ipfrr-notvia-addresses]), per-interface forwarding
			(e.g. Loop-Free Failure Insensitive Routing in [EnyediThesis]), and
			multi-topology forwarding. MRT is realized by using multi-topology
			forwarding. There is a Blue MRT forwarding topology and a Red MRT
			forwarding topology.
	MRTs are practical to maintain redundancy even after a single link or		MRTs are practical to maintain redundancy even after a single link or
	node failure. If a pair of MRTs is computed rooted at each		node failure. If a pair of MRTs is computed rooted at each
	destination, all the destinations remain reachable along one of the		destination, all the destinations remain reachable along one of the
	MRTs in the case of a single link or node failure.		MRTs in the case of a single link or node failure.
	When there is a link or node failure affecting the rSPT, each node		When there is a link or node failure affecting the rSPT, each node
	will still have at least one path via one of the MRTs to reach the		will still have at least one path via one of the MRTs to reach the
	destination D. For example, in Figure 2, C would normally forward		destination D. For example, in Figure 2, C would normally forward
	traffic to R across the C<->R link. If that C<->R link fails, then C		traffic to R across the C<->R link. If that C<->R link fails, then C
	could use either the Blue MRT path C->D->E->R or the Red MRT path		could use either the Blue MRT path C->D->E->R or the Red MRT path
	skipping to change at page 9, line 25 ¶		skipping to change at page 9, line 35 ¶
	Tunneling[RFC5715], then the whole IGP area needs to have alternates		Tunneling[RFC5715], then the whole IGP area needs to have alternates
	available so that the micro-loop prevention mechanism, which requires		available so that the micro-loop prevention mechanism, which requires
	slower network convergence, can take the necessary time without		slower network convergence, can take the necessary time without
	impacting traffic badly.		impacting traffic badly.
	As described in [RFC5286], when a worse failure than is anticipated		As described in [RFC5286], when a worse failure than is anticipated
	happens, using LFAs that are not downstream neighbors can cause		happens, using LFAs that are not downstream neighbors can cause
	micro-looping. An example is given of link-protecting alternates		micro-looping. An example is given of link-protecting alternates
	causing a loop on node failure. Even if a worse failure than		causing a loop on node failure. Even if a worse failure than
	anticipated happened, the use of MRT alternates will not cause		anticipated happened, the use of MRT alternates will not cause
	looping. Therefore, while node-protecting LFAs may be prefered,		looping. Therefore, while node-protecting LFAs may be prefered, an
	there are advantages to using MRT alternates when such a node-		advantage to using MRT alternates when such a node-protecting LFA is
	protecting LFA is not a downstream path.		not a downstream path is the certainty that no alternate-induced
			looping will occur.
	4.1. Multi-homed Prefixes
	One advantage of LFAs that is necessary to preserve is the ability to
	protect multi-homed prefixes against ABR failure. For instance, if a
	prefix from the backbone is available via both ABR A and ABR B, if A
	fails, then the traffic should be redirected to B. This can also be
	done for backups via MRT.
	This generalizes to any multi-homed prefix. A multi-homed prefix
	could be:
	o An out-of-area prefix announced by more than one ABR,
	o An AS-External route announced by 2 or more ASBRs,
	o A prefix with iBGP multipath to different ASBRs,
	o etc.
	For each prefix, the two lowest total cost ABRs are selected and a
	proxy-node is created connected to those two ABRs. If there exist
	multiple multi-homed prefixes that share the same two best
	connectivity, then a single proxy-node can be used to represent the
	set. An example of this is shown in Figure 3.
	2 2 2 2
	A----B----C A----B----C
	2 | | 2 2 | | 2
	| | | |
	[ABR1] [ABR2] [ABR1] [ABR2]
	| | | |
	p,10 p,15 10 |---[P]---| 15
	(a) Initial topology (b)with proxy-node
	A<---B<---C A--->B--->C
	| ^ ^ |
	V | | V
	[ABR1] [ABR2] [ABR1] [ABR2]
	| |
	|-->[P] [P]<--|
	(c) Blue MRT (d) Red MRT
	Figure 3: Prefixes Advertised by Multiple ABRs
	The proxy-nodes and associated links are added to the network
	topology after all real links have been assigned to a direction and
	before the actual MRTs are computed. Proxy-nodes cannot be transited
	when computing the MRTs. In addition to computing the pair of MRTs
	associated with each router destination D in the area, a pair of MRTs
	can be computed for each such proxy-node to fully protect against ABR
	failure.
	Each ABR or attaching router must remove the MRT marking[see
	Section 4.2] and then forward the traffic outside of the area (or
	island of MRT-fast-reroute-supporting routers).
	When directing traffic along an MRT towards a multi-homed prefix, if
	a topology-identifier label[see Section 4.2.1] is not used, then the
	proxy-node must be named and either additional LDP labels or IP
	addresses associated with it.
	4.2. Unicast Forwarding with MRT Fast-Reroute		5. Unicast Forwarding with MRT Fast-Reroute
	With LFA, there is no need to tunnel unicast traffic, whether IP or		With LFA, there is no need to tunnel unicast traffic, whether IP or
	LDP. The traffic is simply sent to an alternate. The behavior with		LDP. The traffic is simply sent to an alternate. As mentioned
	MRT Fast-Reroute is different depending upon whether IP or LDP		earlier in Section 4, MRT needs multi-topology forwarding.
	unicast traffic is considered.		Unfortunately, neither IP nor LDP provide extra bits for a packet to
			indicate its topology.
	Logically, one could use the same IP address or LDP FEC and then also
	use 2 bits to express the topology to use. The topology options are
	(00) IGP/SPT, (01) blue MRT, (10) red MRT. Unfortunately, there just
	aren't 2 spare bits available in the IPv4 or IPv6 header. This has
	different consequences for IP and LDP because LDP can just add a
	topology label on top or take 2 spare bits from the label space.
	Once the MRTs are computed, the two sets of MRTs are seen by the		Once the MRTs are computed, the two sets of MRTs are seen by the
	forwarding plane as essentially two additional topologies. The same		forwarding plane as essentially two additional topologies. The same
	considerations apply for forwarding along the MRTs as for handling		considerations apply for forwarding along the MRTs as for handling
	multiple topologies.		multiple topologies.
	4.2.1. LDP Unicast Forwarding - Avoid Tunneling		5.1. LDP Unicast Forwarding - Avoid Tunneling
	For LDP, it is very desirable to avoid tunneling because, for at		For LDP, it is very desirable to avoid tunneling because, for at
	least node protection, tunneling requires knowledge of remote LDP		least node protection, tunneling requires knowledge of remote LDP
	label mappings and thus requires targeted LDP sessions and the		label mappings and thus requires targeted LDP sessions and the
	associated management complexity. There are two different mechanisms		associated management complexity. There are two different mechanisms
	that can be used.		that can be used.
	1. Option A - Encode Topology in Labels: In addition to sending a		1. Option A - Encode MT-ID in Labels: In addition to sending a
	single label for a FEC, a router would provide two additional		single label for a FEC, a router would provide two additional
	labels with their associated MRT colors. This is simple, but		labels with the MT-IDs associated with the Blue MRT or Red MRT
	reduces the label space for other uses. It also increases the		forwarding topologies. This is very simple for hardware support.
	memory to store the labels and the communication required by LDP.		It does reduce the label space for other uses. It also increases
			the memory to store the labels and the communication required by
			LDP.
	2. Option B - Create Topology-Identification Labels: Use the label-		2. Option B - Create Topology-Identification Labels: Use the label-
	stacking ability of MPLS and specify only two additional labels -		stacking ability of MPLS and specify only two additional labels -
	one for each associated MRT color - by a new FEC type. When		one for each associated MRT color - by a new FEC type. When
	sending a packet onto an MTR, first swap the LDP label and then		sending a packet onto an MRT, first swap the LDP label and then
	push the topology-identification label for that MTR color. When		push the topology-identification label for that MRT color. When
	receiving a packet with a topology-identification label, pop it		receiving a packet with a topology-identification label, pop it
	and use it to guide the next-hop selection in combination with		and use it to guide the next-hop selection in combination with
	the next label in the stack; then swap the remaining label, if		the next label in the stack; then swap the remaining label, if
	appropriate, and push the topology-identification label for the		appropriate, and push the topology-identification label for the
	next-hop. This has minimal usage of additional labels, memory		next-hop. This has minimal usage of additional labels, memory
	and LDP communication. It does increase the size of packets and		and LDP communication. It does increase the size of packets and
	the complexity of the required label operations and look-ups.		the complexity of the required label operations and look-ups.
	This can use the same mechanisms as are needed for context-aware		This can use the same mechanisms as are needed for context-aware
	label spaces.		label spaces.
	Note that with LDP unicast forwarding, regardless of whether		Note that with LDP unicast forwarding, regardless of whether
	topology-identification label or encoding topology in label is used,		topology-identification label or encoding topology in label is used,
	no additional loopbacks per router are required as are required in		no additional loopbacks per router are required. This is because LDP
	the IP unicast forwarding case. This is because LDP labels are used		labels are used on a hop-by-hop basis to identify MRT-blue and MRT-
	on a hop-by-hop basis to identify MRT-blue and MRT-red forwarding		red forwading topologies.
	trees.
	For greatest hardware compatibility, routers should support Option B
	of encoding the topology in the labels.
	4.2.1.1. Protocol Extensions and Considerations: LDP
	This captures an initial understanding of what may need to be
	specified.
	1. Specify Topology in Label: When sending a Label Mapping, have the
	ability to send a Label TLV and multiple Topology-Label TLVs.
	The Topology-Label TLV would specify MRT and the associated MRT
	color.
	2. Topology-Identification Labels: Define a new FEC type that		For greatest hardware compatibility, routers implementing MRT LDP
	describes the topology for MRT and the associated MRT color.		fast-reroute MUST support Option A of encoding the MT-ID in the
			labels. The extensions to indicate an MT-ID for a FEC are described
			in Section 3.2.1 of [I-D.ietf-mpls-ldp-multi-topology]
	4.2.2. IP Unicast Traffic		5.2. IP Unicast Traffic
	For IP, there is no currently practical alternative except tunneling.		For IP, there is no currently practical alternative except tunneling.
	The tunnel egress could be the original destination in the area, the		The tunnel egress could be the original destination in the area, the
	next-next-hop, etc.. If the tunnel egress is the original		next-next-hop, etc.. If the tunnel egress is the original
	destination router, then the traffic remains on the redundant tree		destination router, then the traffic remains on the redundant tree
	with sub-optimal routing. If the tunnel egress is the next-next-hop,		with sub-optimal routing. If the tunnel egress is the next-next-hop,
	then protection of multi-homed prefixes and node-failure for ABRs is		then protection of multi-homed prefixes and node-failure for ABRs is
	not available. Selection of the tunnel egress is a router-local		not available. Selection of the tunnel egress is a router-local
	decision.		decision.
	skipping to change at page 13, line 15 ¶		skipping to change at page 11, line 40 ¶
	2. Tunnel IP packets in IP. Each router supporting this option		2. Tunnel IP packets in IP. Each router supporting this option
	would announce two additional loopback addresses and their		would announce two additional loopback addresses and their
	associated MRT color. Those addresses are used as destination		associated MRT color. Those addresses are used as destination
	addresses for MRT-blue and MRT-red IP tunnels respectively. They		addresses for MRT-blue and MRT-red IP tunnels respectively. They
	allow the transit nodes to identify the traffic as being		allow the transit nodes to identify the traffic as being
	forwarded along either MRT-blue or MRT-red tree topology to reach		forwarded along either MRT-blue or MRT-red tree topology to reach
	the tunnel destination. Announcements of these two additional		the tunnel destination. Announcements of these two additional
	loopback addresses per router with their MRT color requires IGP		loopback addresses per router with their MRT color requires IGP
	extensions.		extensions.
	For proxy-nodes associated with one or more multi-homed prefixes, the		For greatest hardware compatibility and ease in removing the MRT-
	problem is harder because there is no router associated with the		topology marking at area/level boundaries, routers that support MPLS
	proxy-node, so its loopbacks can't be known or used. In this case,		and implement IP MRT fast-reroute SHOULD support Option A - using an
	each router attached to the proxy-node could announce two common IP		LDP label that indicates the destination and MT-ID.
	addresses with their associated MRT colors. This would require
	configuration as well as the previously mentioned IGP extensions.
	Similarly, in the LDP case, two additional FEC bindings could be
	announced.
	4.2.2.1. Protocol Extensions and Considerations: OSPF and ISIS		For proxy-nodes associated with one or more multi-homed prefixes,
			there is no router associated with the proxy-node, so its loopbacks
			can't be known or used. Instead, the loopback addresses of the two
			routers that are attached to the proxy-node can be used. One of
			those routers will be on the Red MRT and the other on the Blue MRT.
			The MRT-red loopback of the first router would be used to reach the
			router on the Red MRT and similarly the MRT-blue loopback of the
			second router would be used. The routers connected to the proxy-node
			are the end of the area/level and can decapsulate the traffic and
			properly forward it into the next area.
			6. Protocol Extensions and Considerations: OSPF and ISIS
	This captures an initial understanding of what may need to be		This captures an initial understanding of what may need to be
	specified.		specified. In cases of partial deployment, it is necessary for a
			router to determine a consistent set of routers to include in the
			island of MRT support. To facilitate this, each router can announce
			both what its capabilities are and what it requires from other
			routers to add them to the MRT island. Generally, there will be a
			set of information advertised about the MRT support. This
			information has only area/level-wide scope.
	o Capabilities: Does a router support MRT? Does the router do MRT		MRT Island Creation ID: This identifies the process that the router
	tunneling with LDP or IP or GRE or...?		uses to form an MRT Island. By advertising an ID for the process,
			it is possible to have different processes in the future. It may
			be desirable to advertise a list ordered by preference to allow
			transitions.
	o Topology Association: A router needs to advertise a loopback and		MRT Algorithm ID: This identifies the particular MRT algorithm used
	associate it with an MRT whether blue or red. Additional		by the router. By having an Algorithm ID, it is possible to
	flexibility for future uses would be good.		change the algorithm used or use different ones in different
			networks. It may be desirable to advertise a list ordered by
			preference to allow transitions.
	o Proxy-nodes for Multi-homed Prefixes: We need a way to advertise		Red MRT MT-ID: This specifies the MT-ID to be associated with the
	common addresses with MRT for multi-homed prefixes' proxy-nodes.		Red MRT forwarding topology. It is needed for use in signaling.
	Currently, those proxy-nodes aren't named or considered.		All routers in the MRT Island MUST agree on a value.
			Blue MRT MT-ID: This specifies the MT-ID to be associated with the
			Blue MRT forwarding topology. It is needed for use in signaling.
			All routers in the MRT Island MUST agree on a value.
			GADAG Root Election Priority: This specifies the priority of the
			router for being used as the GADAG root of its island. A GADAG
			root is elected from the set of routers with the highest priority;
			ties are broken based upon highest Router ID. The sensitivity of
			the MRT Algorithms to GADAG root selection is still being
			evaluated. This provides the network operator with a knob to
			force particular GADAG root selection.
			Forwarding Mechanism for IP: This specifies which forwarding
			mechanisms the router supports for IP traffic. An MRT island must
			support a common set of forwarding mechanisms, which may be less
			than the full set advertised. Multiple forwarding mechanisms may
			be specified, such as IP-in-IPv4, IP-in-IPv6 or IP-in-LDP-
			Destination-Topology Label. None is also an option.
			Forwarding Mechanism for LDP: This specifies which forwarding
			mechanisms the router supports for LDP traffic. An MRT island
			must support a common set of forwarding mechanisms, which may be
			less than the full set advertised. The expected mechanisms are
			"Encode MT-ID in Labels" or None.
			Red MRT Loopback Address: This provides the router's loopback
			address to reach the router via the Red MRT forwarding topology.
			It can, of course, be specified for both IPv4 and IPv6.
			Blue MRT Loopback Address: This provides the router's loopback
			address to reach the router via the Blue MRT forwarding topology.
			It can, of course, be specified for both IPv4 and IPv6.
			MRT Capabilities Available: This is the set of capabilities that
			the router is configured to support.
			MRT Capabilities Required: This is the set of capabilities that
			other routers must have available to be added into the MRT island.
			MRT Capability: Computes MRTs: The router can compute MRTs.
			MRT Capability: IP Fast-Reroute: The router can use the computed
			MRTs for IP fast-reroute.
			MRT Capability: LDP Fast-Reroute: The router can use the computed
			MRTs for LDP fast-reroute.
			MRT Capability: PIM Fast-Reroute: The router can use the computed
			MRTs for PIM fast-reroute.
			MRT Capability: mLDP Fast-Reroute: The router can use the computed
			MRTs for mLDP fast-reroute.
			MRT Capability: PIM Global Protection: The router can use the
			computed MRTs for PIM Global Protection 1+1.
			MRT Capability: mLDP Global Protection: The router can use the
			computed MRTs for mLDP Global Protection 1+1.
			The assumption is that a router will form 1 MRT island, compute MRTs
			within that island, and then use those MRTs for the different
			purposes. Including a router that, for instance, doesn't support
			mLDP Global Protection would mean that the whole MRT island could not
			support it. In a fully deployed case, of course, the whole area/
			level would support MRT and the complexities of MRT island formation
			would be minimal.
			If a router wanted to form multiple MRT islands for different
			application purposes, that could be done by specifying different Red
			MRT MT-ID and Blue MRT MT-IDs.
	As with LFA, it is expected that OSPF Virtual Links will not be		As with LFA, it is expected that OSPF Virtual Links will not be
	supported.		supported.
	4.2.3. Inter-Area and ABR Forwarding Behavior		7. Multi-homed Prefixes
			One advantage of LFAs that is necessary to preserve is the ability to
			protect multi-homed prefixes against ABR failure. For instance, if a
			prefix from the backbone is available via both ABR A and ABR B, if A
			fails, then the traffic should be redirected to B. This can also be
			done for backups via MRT.
			This generalizes to any multi-homed prefix. A multi-homed prefix
			could be:
			o An out-of-area prefix announced by more than one ABR,
			o An AS-External route announced by 2 or more ASBRs,
			o A prefix with iBGP multipath to different ASBRs,
			o etc.
			For each prefix, the two lowest total cost ABRs are selected and a
			proxy-node is created connected to those two ABRs. If there exist
			multiple multi-homed prefixes that share the same two best
			connectivity, then a single proxy-node can be used to represent the
			set. An example of this is shown in Figure 3.
			2 2 2 2
			A----B----C A----B----C
			2 | | 2 2 | | 2
			| | | |
			[ABR1] [ABR2] [ABR1] [ABR2]
			| | | |
			p,10 p,15 10 |---[P]---| 15
			(a) Initial topology (b)with proxy-node
			A<---B<---C A--->B--->C
			| ^ ^ |
			V | | V
			[ABR1] [ABR2] [ABR1] [ABR2]
			| |
			|-->[P] [P]<--|
			(c) Blue MRT (d) Red MRT
			Figure 3: Prefixes Advertised by Multiple ABRs
			The proxy-nodes and associated links are added to the network
			topology after all real links have been assigned to a direction and
			before the actual MRTs are computed. Proxy-nodes cannot be transited
			when computing the MRTs. In addition to computing the pair of MRTs
			associated with each router destination D in the area, a pair of MRTs
			can be computed for each such proxy-node to fully protect against ABR
			failure.
			Each ABR or attaching router must remove the MRT marking[see
			Section 5] and then forward the traffic outside of the area (or
			island of MRT-fast-reroute-supporting routers).
			If ASBR protection is desired, this has additonal complexities if the
			ASBRs are in different areas. Similarly, protecting labeled BGP
			traffic in the event of an ASBR failure has additional complexities
			due to the per-ASBR label spaces involved.
			8. Inter-Area and ABR Forwarding Behavior
	In regular forwarding, packets destined outside the area arrive at		In regular forwarding, packets destined outside the area arrive at
	the ABR and the ABR forwards them into the other area because the		the ABR and the ABR forwards them into the other area because the
	next-hops from the area with the best route (according to tie-		next-hops from the area with the best route (according to tie-
	breaking rules) are used by the ABR. The question is then what to do		breaking rules) are used by the ABR. The question is then what to do
	with packets marked with an MRT that are received by the ABR.		with packets marked with an MRT that are received by the ABR.
	The only option that doesn't require forwarding based upon incoming		For unicast fast-reroute, the need to stay on an MRT forwarding
	interface is to forward an MRT marked packet in the area with the		topology terminates at the ABR/LBR whose best route is via a
	best route along its associated MRT. If the packet came from that		different area/level. It is highly desirable to go back to the
	area, this correctly avoids the failure. If the packet came from a		default forwarding topology when leaving an area/level. There are
	different area, at least this gets the packet to the destination even		three basic reasons for this. First, the default topology uses
	though it is along an MRT rather than the shortest-path.		shortest paths; the packet will thus take the shortest possible route
			to the destination. Second, this allows failures that might appear
			in multiple areas (e.g. ABR/LBR failures) to be separately
			identified and repaired around. Third, the packet can be fast-
			rerouted again, if necessary, due to a failure in a different area.
			An ABR/LBR that receives a packet marked with an MRT towards a
			destination in another area should forward the MRT marked packet in
			the area with the best route along its associated MRT. If the packet
			came from that area, this correctly avoids the failure.
			How does an ABR/LBR ensure that MRT-marked packets do not arrive at
			the ABR/LBR? There are two different mechanisms depending upon the
			forwarding mechanism being used.
			If the LDP label encodes the MT-ID as well as the destination, then
			the ABR/LBR is responsible for advertising a particular label to each
			neighbor. Additionally, an LDP label is associated with an MT-ID due
			to the MT FEC that was used and not due to any intrisic particular
			value for the label. Assume that an ABR/LBR has allocated three
			labels for a particular destination; those labels are L_primary,
			L_blue, and L_red. When the ABR/LBR advertises label bindings to
			routers in the area with the best route to the destination, the ABR/
			LBR provides L_primary for the default topology, L_blue for the Blue
			MRT MT-ID and L_red for the Red MRT MT-ID, exactly as expected.
			However, when the ABR/LBR advertises label bindings to routers in
			other areas, the ABR/LBR advertises L_primary for the default
			topology, for the Blue MRT MT-ID, and for the Red MRT MT-ID. The
			ABR/LBR installs next-hops from the best area for L_primary based on
			the default topology, for L_blue based on the Blue MRT forwarding
			topology, and for L_red based on the Red MRT forwarding topology.
			Therefore, packets from the non-best area will arrive at the ABR/LBR
			with a label L_primary and will be forwarded into the best area along
			the default topology. By controlling what labels are advertised, the
			ABR/LBR can thus enforce that packets exiting the area do so on the
			shortest-path default topology.
			If IP-in-IP forwarding is used, then the ABR/LBR behavior is
			dependent upon the outermost IP address. If the outermost IP address
			is an MRT loopback address of the ABR/LBR, then the packet is
			decapsulated and forwarded based upon the inner IP address, which
			should go on the default SPT topology. If the outermost IP address
			is not an MRT loopback address of the ABR/LBR, then the packet is
			simply forwarded along the associated forwarding topology. A PLR
			sending traffic to a destination outside its local area/level will
			pick the MRT and use the associated MRT loopback address of the ABR/
			LBR immediately before the proxy-node on that MRT.
			Thus, regardless of which of these two forwarding mechanisms are
			used, there is no need for additional computation or per-area
			forwarding state.
	+----[C]---- --[D]--[E] --[D]--[E]		+----[C]---- --[D]--[E] --[D]--[E]
	| \ / \ / \		| \ / \ / \
	p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+		p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+
	| / \ / | \ / |		| / \ / | \ / |
	+----[B]---- --[F]--[G] | --[F]--[G] |		+----[B]---- --[F]--[G] | --[F]--[G] |
	| |		| |
	| other |		| other |
	+----------[p]-------+		+----------[p]-------+
	area		area
	skipping to change at page 14, line 44 ¶		skipping to change at page 18, line 5 ¶
	/ | | \		/ | | \
	[F]->[G] V V -<[F]<-[G]		[F]->[G] V V -<[F]<-[G]
	| |		| |
	| |		| |
	[p]<------+ +--------->[p]		[p]<------+ +--------->[p]
	(d) Blue MRT in Area 0 (e) Red MRT in Area 0		(d) Blue MRT in Area 0 (e) Red MRT in Area 0
	Figure 4: ABR Forwarding Behavior and MRTs		Figure 4: ABR Forwarding Behavior and MRTs
	To avoid using an out-of-area MRT, special action can be taken by the		The other potential forwarding mechanisms require additional
	penultimate router along the in-local-area MRT immediately before the		computation by the penultimate router along the in-local-area MRT
	ABR is reached. The penultimate router can determine that the ABR		immediately before the ABR/LBR is reached. The penultimate router
	will forward the packet out of area and, in that case, the		can determine that the ABR/LBR will forward the packet out of area/
	penultimate router can remove the MRT marking but still forward the		level and, in that case, the penultimate router can remove the MRT
	packet along the MRT next-hop to reach the ABR. For instance, in		marking but still forward the packet along the MRT next-hop to reach
	Figure 4, if node H fails, node E has to put traffic towards prefix p		the ABR. For instance, in Figure 4, if node H fails, node E has to
	onto the red MRT. But since node D knows that ABR1 will use a best		put traffic towards prefix p onto the red MRT. But since node D
	from another area, it is safe for D to remove the MRT marking and		knows that ABR1 will use a best from another area, it is safe for D
	just send the packet to ABR1 still on the red MRT but unmarked. ABR1		to remove the MRT marking and just send the packet to ABR1 still on
	will use the shortest path in Area 10.		the red MRT but unmarked. ABR1 will use the shortest path in Area
			10.
	In all cases for ISIS and most cases for OSPF, the penultimate router		In all cases for ISIS and most cases for OSPF, the penultimate router
	can determine what decision the adjacent ABR will make. The one case		can determine what decision the adjacent ABR will make. The one case
	where it can't be determined is when two ASBRs are in different non-		where it can't be determined is when two ASBRs are in different non-
	backbone areas attached to the same ABR, then the ASBR's Area ID may		backbone areas attached to the same ABR, then the ASBR's Area ID may
	be needed for tie-breaking (prefer the route with the largest OPSF		be needed for tie-breaking (prefer the route with the largest OPSF
	area ID) and the Area ID isn't announced as part of the ASBR link-		area ID) and the Area ID isn't announced as part of the ASBR link-
	state advertisement (LSA). In this one case, suboptimal forwarding		state advertisement (LSA). In this one case, suboptimal forwarding
	along the MRT in the other area would happen. If this is a realistic		along the MRT in the other area would happen. If this is a realistic
	deployment scenario, OSPF extensions could be considered.		deployment scenario, OSPF extensions could be considered.
	4.2.4. Issues with Area Abstraction		9. Issues with Area Abstraction
	MRT fast-reroute provides complete coverage in a area that is		MRT fast-reroute provides complete coverage in a area that is
	2-connected. Where a failure would partition the network, of course,		2-connected. Where a failure would partition the network, of course,
	no alternate can protect against that failure. Similarly, there are		no alternate can protect against that failure. Similarly, there are
	ways of connecting multi-homed prefixes that make it impractical to		ways of connecting multi-homed prefixes that make it impractical to
	protect them without excessive complexity.		protect them without excessive complexity.
	50		50
	|----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0:		|----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0:
	| | ABR 1, ABR 2, C, D		| | ABR 1, ABR 2, C, D
	skipping to change at page 16, line 26 ¶		skipping to change at page 19, line 30 ¶
	p ---[ASBR X]-X-[A]---[B]---[ABR 1]---[D]---[ASBR Y]--- p		p ---[ASBR X]-X-[A]---[B]---[ABR 1]---[D]---[ASBR Y]--- p
	BGP prefers ASBR X for prefix p		BGP prefers ASBR X for prefix p
	Figure 6: Failure of path towards ASBR preferred by BGP		Figure 6: Failure of path towards ASBR preferred by BGP
	The fine details of how to solve multi-area external prefix cases, or		The fine details of how to solve multi-area external prefix cases, or
	identifying certain cases as too unlikely and too complex to protect		identifying certain cases as too unlikely and too complex to protect
	is for further consideration.		is for further consideration.
	4.2.5. Partial Deployment and Islands of Compatible MRT FRR routers		10. Partial Deployment and Islands of Compatible MRT FRR routers
	A natural concern with new functionality is how to have it be useful		A natural concern with new functionality is how to have it be useful
	when it is not deployed across an entire IGP area. In the case of		when it is not deployed across an entire IGP area. In the case of
	MRT FRR, where it provides alternates when appropriate LFAs aren't		MRT FRR, where it provides alternates when appropriate LFAs aren't
	available, there are also deployment scenarios where it may make		available, there are also deployment scenarios where it may make
	sense to only enable some routers in an area with MRT FRR. A simple		sense to only enable some routers in an area with MRT FRR. A simple
	example of such a scenario would be a ring of 6 or more routers that		example of such a scenario would be a ring of 6 or more routers that
	is connected via two routers to the rest of the area.		is connected via two routers to the rest of the area.
	First, a computing router S must determine its local island of		First, a computing router S must determine its local island of
	skipping to change at page 17, line 7 ¶		skipping to change at page 20, line 13 ¶
	like a multi-homed prefix with caveats to avoid looping. For LDP		like a multi-homed prefix with caveats to avoid looping. For LDP
	labels including both destination and topology, the routers at the		labels including both destination and topology, the routers at the
	borders of the local island need to originate labels for the original		borders of the local island need to originate labels for the original
	FEC and the associated MRT-specific labels. Packets sent to an LDP		FEC and the associated MRT-specific labels. Packets sent to an LDP
	label marked as blue or red MRT to a destination outside the local		label marked as blue or red MRT to a destination outside the local
	island will have the last router in the local island swap the label		island will have the last router in the local island swap the label
	to one for the destination and forward the packet along the outgoing		to one for the destination and forward the packet along the outgoing
	interface on the MRT towards a router outside the local island that		interface on the MRT towards a router outside the local island that
	was represented by the proxy-node.		was represented by the proxy-node.
	For IP in IP encapsulations, remote destinations may not be		For IP in IP encapsulations, remote destinations' loopback addresses
	advertising additional IP loopback addresses for the MRTs. In that		for the MRTs cannot be used, even if they were available. Instead,
	case, a router attached to a proxy-node, which represents		the MRT loopback address of the router attached to a proxy-node,
	destinations outside the local island, must advertise IP addresses		which represents destinations outside the local island, can be used.
	associated with that proxy-node. Packets sent to an address		Packets sent to the router's MRT loopback address will have their
	associated with a proxy-node will have their outer IP header removed		outer IP header removed and will need to be explicitly forwarded
	by the router attached to the proxy-node and be forwarded by the		along the outgoing interface on the MRT towards a router outside the
	router along the outgoing interface on the MRT towards a router		local island that was represented by the proxy-node. This behavior
	outside the local island that was represented by the proxy-node.		requires essentially remembering the MT-ID indicated by the outer IP
			address. An alternate option would be to advertise different
			loopback addresses to be associated with the proxy-node; the outer IP
			address would still be removed but it would indicate the outgoing
			interface to use and no lookup would be necessary on the internal IP
			address while maintaining MT-ID context.
	4.2.6. Network Convergence and Preparing for the Next Failure		A key question is which routers outside the MRT island can packets be
			forwarded to so that they are not forwarded back into the MRT island.
			An example of the necessary network graph transformations are given
			in Figure 7. There are two parts to the computation. First, the MRT
			island is collapsed into a single node; this assumes that the cost of
			transiting the MRT island is nothing and is pessimistic but allows
			for simpler computation. Then, for each destination (other than the
			MRT island), the routers adjacent to the MRT island are checked to
			see if they are loop-free with respect to the MRT island and the
			destination. The two loop-free neighbors of the MRT island that are
			closest to the destination are selected. Then, a graph of just the
			MRT island is augmented with proxy-nodes that are attached via the
			outgoing interfaces to the selected loop-free neighbors. Finally,
			the MRTs rooted at each proxy-node are computed on that augmented MRT
			island graph. Essentially, the MRT island must have a loop-free
			neighbor to be able to have an alternate.
			[G]---[E]---(B)---(C)---(D)
			| \ | | |
			| \ | | |
			| \ | | |
			[H]---[F]---(A)---(S)----|
			(1) Network Graph with Partial Deployment
			[E],[F],[G],[H] : No support for MRT-FRR
			(A),(B),(C),(D),(S): MRT Island - supports MRT-FRR
			[G]---[E]----| |---(B)---(C)---(D)
			| \ | | | | |
			| \ | ( MRT Island ) [ proxy ] | |
			| \ | | | | |
			[H]---[F]----| |---(A)---(S)----|
			(2) Graph for determining (3) Graph for MRT computation
			loop-free neighbors
			Figure 7: Computing alternates to destinations outside the MRT Island
			Naturally, there are more complicated options to improve coverage,
			such as connecting multiple MRT islands across tunnels, but it is not
			clear that the additional complexity is necessary.
			11. Network Convergence and Preparing for the Next Failure
	After a failure, MRT detours ensure that packets reach their intended		After a failure, MRT detours ensure that packets reach their intended
	destination while the IGP has not reconverged onto the new topology.		destination while the IGP has not reconverged onto the new topology.
	As link-state updates reach the routers, the IGP process calculates		As link-state updates reach the routers, the IGP process calculates
	the new shortest paths. Two things need attention: micro-loop		the new shortest paths. Two things need attention: micro-loop
	prevention and MRT re-calculation.		prevention and MRT re-calculation.
	4.2.6.1. Micro-forwarding loop prevention and MRTs		11.1. Micro-forwarding loop prevention and MRTs
	As is well known[RFC5715], micro-loops can occur during IGP		As is well known[RFC5715], micro-loops can occur during IGP
	convergence; such loops can be local to the failure or remote from		convergence; such loops can be local to the failure or remote from
	the failure. Managing micro-loops is an orthogonal issue to having		the failure. Managing micro-loops is an orthogonal issue to having
	alternates for local repair, such as MRT fast-reroute provides.		alternates for local repair, such as MRT fast-reroute provides.
	There are two possible micro-loop prevention mechanism discussed in		There are two possible micro-loop prevention mechanism discussed in
	[RFC5715]. The first is Ordered FIB [I-D.ietf-rtgwg-ordered-fib].		[RFC5715]. The first is Ordered FIB [I-D.ietf-rtgwg-ordered-fib].
	The second is Farside Tunneling which requires tunnels or an		The second is Farside Tunneling which requires tunnels or an
	alternate topology to reach routers on the farside of the failure.		alternate topology to reach routers on the farside of the failure.
	Since MRTs provide an alternate topology through which traffic can be		Since MRTs provide an alternate topology through which traffic can be
	sent and which can be manipulated separately from the SPT, it is		sent and which can be manipulated separately from the SPT, it is
	possible that MRTs could be used to support Farside Tunneling.		possible that MRTs could be used to support Farside Tunneling.
	Details of how to do so are outside of this document.		Details of how to do so are outside of this document.
	4.2.6.2. MRT Recalculation		11.2. MRT Recalculation
	When a failure event happens, traffic is put by the PLRs onto the MRT		When a failure event happens, traffic is put by the PLRs onto the MRT
	topologies. After that, each router recomputes its shortest path		topologies. After that, each router recomputes its shortest path
	tree (SPT) and moves traffic over to that. Only after all the PLRs		tree (SPT) and moves traffic over to that. Only after all the PLRs
	have switched to using their SPTs and traffic has drained from the		have switched to using their SPTs and traffic has drained from the
	MRT topologies should each router install the recomputed MRTs into		MRT topologies should each router install the recomputed MRTs into
	the FIBs.		the FIBs.
	At each router, therefore, the sequence is as follows:		At each router, therefore, the sequence is as follows:
	skipping to change at page 18, line 22 ¶		skipping to change at page 22, line 38 ¶
	5. Wait configured period for all routers to be using their SPTs and		5. Wait configured period for all routers to be using their SPTs and
	traffic to drain from the MRTs.		traffic to drain from the MRTs.
	6. Install new MRTs.		6. Install new MRTs.
	While the recomputed MRTs are not installed in the FIB, protection		While the recomputed MRTs are not installed in the FIB, protection
	coverage is lowered. Therefore, it is important to recalculate the		coverage is lowered. Therefore, it is important to recalculate the
	MRTs and install them as quickly as possible.		MRTs and install them as quickly as possible.
	It is for further study whether MRT re-calculation is possible in an		The installation of the MRTs can be staged such that the affected or
	incremental fashion, such that the sections of the MRT in use after a		broken MRTs are updated first and then the unbroken.
	failure are not changed.
	5. Acknowledgements		12. Acknowledgements
	The authors would like to thank Hannes Gredler, Jeff Tantsura, Ted		The authors would like to thank Hannes Gredler, Jeff Tantsura, Ted
	Qian, Kishore Tiruveedhula, Santosh Esale, Nitin Bahadur, Harish		Qian, Kishore Tiruveedhula, Santosh Esale, Nitin Bahadur, Harish
	Sitaraman and Raveendra Torvi for their suggestions and review.		Sitaraman and Raveendra Torvi for their suggestions and review.
	6. IANA Considerations		13. IANA Considerations
	This doument includes no request to IANA.		This doument includes no request to IANA.
	7. Security Considerations		14. Security Considerations
	This architecture is not currently believed to introduce new security		This architecture is not currently believed to introduce new security
	concerns.		concerns.
	8. References		15. References
	8.1. Normative References		15.1. Normative References
	[I-D.enyedi-rtgwg-mrt-frr-algorithm]		[I-D.enyedi-rtgwg-mrt-frr-algorithm]
	Atlas, A., Envedi, G., and A. Csaszar, "Algorithms for		Enyedi, G., Atlas, A. and A. Csaszar, "Algorithms for
	computing Maximally Redundant Trees for IP/LDP Fast-		computing Maximally Redundant Trees for IP/LDP Fast-
	Reroute", draft-enyedi-rtgwg-mrt-frr-algorithm-00 (work in		Reroute", draft-enyedi-rtgwg-mrt-frr-algorithm-01 (work
	progress), October 2011.		in progress), March 2012.
	[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast		[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
	Reroute: Loop-Free Alternates", RFC 5286, September 2008.		Reroute: Loop-Free Alternates", RFC 5286, September 2008.
	[RFC5384] Boers, A., Wijnands, I., and E. Rosen, "The Protocol
	Independent Multicast (PIM) Join Attribute Format",
	RFC 5384, November 2008.
	[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",		[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
	RFC 5714, January 2010.		RFC 5714, January 2010.
	8.2. Informative References		15.2. Informative References
			[EnyediThesis]
			Enyedi, G., "Novel Algorithms for IP Fast Reroute",
			Department of Telecommunications and Media Informatics,
			Budapest University of Technology and Economics Ph.D.
			Thesis, February 2011,
			<http://timon.tmit.bme.hu/theses/thesis_book.pdf>.
			[I-D.atlas-rtgwg-mrt-mc-arch]
			Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G.
			Envedi, "An Architecture for Multicast Protection Using
			Maximally Redundant Trees",
			draft-atlas-rtgwg-mrt-mc-arch-00 (work in progress),
			March 2012.
			[I-D.ietf-mpls-ldp-multi-topology]
			Zhao, Q., Fang, L., Zhou, C., Li, L., and N. So, "LDP
			Extensions for Multi Topology Routing",
			draft-ietf-mpls-ldp-multi-topology-03 (work in progress),
			March 2012.
	[I-D.ietf-rtgwg-ipfrr-notvia-addresses]		[I-D.ietf-rtgwg-ipfrr-notvia-addresses]
	Bryant, S., Previdi, S., and M. Shand, "IP Fast Reroute		Bryant, S., Previdi, S., and M. Shand, "IP Fast Reroute
	Using Not-via Addresses",		Using Not-via Addresses",
	draft-ietf-rtgwg-ipfrr-notvia-addresses-08 (work in		draft-ietf-rtgwg-ipfrr-notvia-addresses-08 (work in
	progress), December 2011.		progress), December 2011.
	[I-D.ietf-rtgwg-lfa-applicability]		[I-D.ietf-rtgwg-lfa-applicability]
	Filsfils, C. and P. Francois, "LFA applicability in SP		Filsfils, C. and P. Francois, "LFA applicability in SP
	networks", draft-ietf-rtgwg-lfa-applicability-06 (work in		networks", draft-ietf-rtgwg-lfa-applicability-06 (work in
	skipping to change at page 20, line 22 ¶		skipping to change at page 25, line 4 ¶
	Email: akatlas@juniper.net		Email: akatlas@juniper.net
	Robert Kebler		Robert Kebler
	Juniper Networks		Juniper Networks
	10 Technology Park Drive		10 Technology Park Drive
	Westford, MA 01886		Westford, MA 01886
	USA		USA
	Email: rkebler@juniper.net		Email: rkebler@juniper.net
	Maciek Konstantynowicz
	Juniper Networks
	Email: maciek@juniper.net
	Gabor Sandor Enyedi		Gabor Sandor Enyedi
	Ericsson		Ericsson
	Konyves Kalman krt 11.		Konyves Kalman krt 11.
	Budapest 1097		Budapest 1097
	Hungary		Hungary
	Email: Gabor.Sandor.Enyedi@ericsson.com		Email: Gabor.Sandor.Enyedi@ericsson.com
	Andras Csaszar		Andras Csaszar
	Ericsson		Ericsson
	Konyves Kalman krt 11		Konyves Kalman krt 11
	Budapest 1097		Budapest 1097
	Hungary		Hungary
	Email: Andras.Csaszar@ericsson.com		Email: Andras.Csaszar@ericsson.com
			Maciek Konstantynowicz
			Cisco Systems
			Email: maciek@bgp.nu
	Russ White		Russ White
	Cisco Systems		Cisco Systems
	Email: russwh@cisco.com		Email: russwh@cisco.com
	Mike Shand		Mike Shand
	Email: mike@mshand.org.uk		Email: mike@mshand.org.uk
End of changes. 50 change blocks.
	212 lines changed or deleted		408 lines changed or added
This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/