< draft-ietf-rift-applicability-02.txt		draft-ietf-rift-applicability-03.txt >
	RIFT WG Yuehua. Wei, Ed.		RIFT WG Yuehua. Wei, Ed.
	Internet-Draft Zheng. Zhang		Internet-Draft Zheng. Zhang
	Intended status: Informational ZTE Corporation		Intended status: Informational ZTE Corporation
	Expires: 12 April 2021 Dmitry. Afanasiev		Expires: 16 April 2021 Dmitry. Afanasiev
	Yandex		Yandex
	Tom. Verhaeg		Tom. Verhaeg
	Juniper Networks		Juniper Networks
	Jaroslaw. Kowalczyk		Jaroslaw. Kowalczyk
	Orange Polska		Orange Polska
	P. Thubert		P. Thubert
	Cisco Systems		Cisco Systems
	9 October 2020		13 October 2020
	RIFT Applicability		RIFT Applicability
	draft-ietf-rift-applicability-02		draft-ietf-rift-applicability-03
	Abstract		Abstract
	This document discusses the properties, applicability and operational		This document discusses the properties, applicability and operational
	considerations of RIFT in different network scenarios. It intends to		considerations of RIFT in different network scenarios. It intends to
	provide a rough guide how RIFT can be deployed to simplify routing		provide a rough guide how RIFT can be deployed to simplify routing
	operations in Clos topologies and their variations.		operations in Clos topologies and their variations.
	Status of This Memo		Status of This Memo
	skipping to change at page 1, line 41 ¶		skipping to change at page 1, line 41 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on 12 April 2021.		This Internet-Draft will expire on 16 April 2021.
	Copyright Notice		Copyright Notice
	Copyright (c) 2020 IETF Trust and the persons identified as the		Copyright (c) 2020 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 (https://trustee.ietf.org/		Provisions Relating to IETF Documents (https://trustee.ietf.org/
	license-info) in effect on the date of publication of this document.		license-info) in effect on the date of publication of this document.
	Please review these documents carefully, as they describe your rights		Please review these documents carefully, as they describe your rights
	skipping to change at page 4, line 13 ¶		skipping to change at page 4, line 13 ¶
	and RIFT protocol concepts.		and RIFT protocol concepts.
	3.1. Overview of RIFT		3.1. Overview of RIFT
	RIFT is a dynamic routing protocol for Clos and fat-tree network		RIFT is a dynamic routing protocol for Clos and fat-tree network
	topologies. It defines a link-state protocol when "pointing north"		topologies. It defines a link-state protocol when "pointing north"
	and path-vector protocol when "pointing south".		and path-vector protocol when "pointing south".
	It floods flat link-state information northbound only so that each		It floods flat link-state information northbound only so that each
	level obtains the full topology of levels south of it. That		level obtains the full topology of levels south of it. That
	information is never flooded East-West or back South again. So a top		information is never flooded east-west or back South again. So a top
	tier node has full set of prefixes from the SPF calculation.		tier node has full set of prefixes from the SPF calculation.
	In the southbound direction the protocol operates like a "fully		In the southbound direction the protocol operates like a "fully
	summarizing, unidirectional" path vector protocol or rather a		summarizing, unidirectional" path vector protocol or rather a
	distance vector with implicit split horizon whereas the information		distance vector with implicit split horizon whereas the information
	propagates one hop south and is 're-advertised' by nodes at next		propagates one hop south and is 're-advertised' by nodes at next
	lower level, normally just the default route.		lower level, normally just the default route.
	+-----------+ +-----------+		+-----------+ +-----------+
	| ToF | | ToF | LEVEL 2		| ToF | | ToF | LEVEL 2
	+ +-----+--+--+ +-+--+------+		+ +-----+--+--+ +-+--+------+
	| | | | | | | | | ^		| | | | | | | | | ^
	+ | | | +-------------------------+ |		+ | | | +-------------------------+ |
	Distance | +-------------------+ | | | | |		Distance | +-------------------+ | | | | |
	Vector | | | | | | | | +		Vector | | | | | | | | +
	South | | | | +--------+ | | | Link+State		South | | | | +--------+ | | | Link-state
	+ | | | | | | | | Flooding		+ | | | | | | | | Flooding
	| | | +-------------+ | | | North		| | | +-------------+ | | | North
	v | | | | | | | | +		v | | | | | | | | +
	+-+--+-+ +------+ +-------+ +--+--+-+ |		+-+--+-+ +------+ +-------+ +--+--+-+ |
	|SPINE | |SPINE | | SPINE | | SPINE | | LEVEL 1		|SPINE | |SPINE | | SPINE | | SPINE | | LEVEL 1
	+ ++----++ ++---+-+ +--+--+-+ ++----+-+ |		+ ++----++ ++---+-+ +--+--+-+ ++----+-+ |
	+ | | | | | | | | | ^ N		+ | | | | | | | | | ^ N
	Distance | +-------+ | | +--------+ | | | E		Distance | +-------+ | | +--------+ | | | E
	Vector | | | | | | | | | +------>		Vector | | | | | | | | | +------>
	South | +-------+ | | | +-------+ | | | |		South | +-------+ | | | +-------+ | | | |
	skipping to change at page 4, line 51 ¶		skipping to change at page 4, line 51 ¶
	v ++--++ +-+-++ ++-+-+ +-+--++ +		v ++--++ +-+-++ ++-+-+ +-+--++ +
	|LEAF| |LEAF| |LEAF| |LEAF | LEVEL 0		|LEAF| |LEAF| |LEAF| |LEAF | LEVEL 0
	+----+ +----+ +----+ +-----+		+----+ +----+ +----+ +-----+
	Figure 1: Rift overview		Figure 1: Rift overview
	A middle tier node has only information necessary for its level,		A middle tier node has only information necessary for its level,
	which are all destinations south of the node based on SPF		which are all destinations south of the node based on SPF
	calculation, default route and potential disaggregated routes.		calculation, default route and potential disaggregated routes.
	RIFT combines the advantage of both Link-State and Distance Vector:		RIFT combines the advantage of both link-state and distance vector:
	* Fastest Possible Convergence		* Fastest Possible Convergence
	* Automatic Detection of Topology		* Automatic Detection of Topology
	* Minimal Routes/Info on TORs		* Minimal Routes/Info on TORs
	* High Degree of ECMP		* High Degree of ECMP
	* Fast De-commissioning of Nodes		* Fast De-commissioning of Nodes
	* Maximum Propagation Speed with Flexible Prefixes in an Update		* Maximum Propagation Speed with Flexible Prefixes in an Update
	And RIFT eliminates the disadvantages of Link-State or Distance		And RIFT eliminates the disadvantages of link-state or distance
	Vector:		vector:
	* Reduced and Balanced Flooding		* Reduced and Balanced Flooding
	* Automatic Neighbor Detection		* Automatic Neighbor Detection
	So there are two types of link state database which are "north		So there are two types of link-state database which are "north
	representation" N-TIEs and "south representation" S-TIEs. The N-TIEs		representation" N-TIEs and "south representation" S-TIEs. The N-TIEs
	contain a link state topology description of lower levels and S-TIEs		contain a link-state topology description of lower levels and S-TIEs
	carry simply default routes for the lower levels.		carry simply default routes for the lower levels.
	There are a bunch of more advantages unique to RIFT listed below		There are a bunch of more advantages unique to RIFT listed below
	which could be understood if you read the details of [RIFT].		which could be understood if you read the details of [RIFT].
	* True ZTP		* True ZTP
	* Minimal Blast Radius on Failures		* Minimal Blast Radius on Failures
	* Can Utilize All Paths Through Fabric Without Looping		* Can Utilize All Paths Through Fabric Without Looping
	skipping to change at page 6, line 15 ¶		skipping to change at page 6, line 15 ¶
	3.2. Applicable Topologies		3.2. Applicable Topologies
	Albeit RIFT is specified primarily for "proper" Clos or "fat-tree"		Albeit RIFT is specified primarily for "proper" Clos or "fat-tree"
	structures, it already supports PoD concepts which are strictly		structures, it already supports PoD concepts which are strictly
	speaking not found in original Clos concepts.		speaking not found in original Clos concepts.
	Further, the specification explains and supports operations of multi-		Further, the specification explains and supports operations of multi-
	plane Clos variants where the protocol relies on set of rings to		plane Clos variants where the protocol relies on set of rings to
	allow the reconciliation of topology view of different planes as most		allow the reconciliation of topology view of different planes as most
	desirable solution making proper disaggregation viable in case of		desirable solution making proper disaggregation viable in case of
	failures. This observations hold not only in case of RIFT but in the		failures. These observations hold not only in case of RIFT but in
	generic case of dynamic routing on Clos variants with multiple planes		the generic case of dynamic routing on Clos variants with multiple
	and failures in bi-sectional bandwidth, especially on the leafs.		planes and failures in bi-sectional bandwidth, especially on the
			leafs.
	3.2.1. Horizontal Links		3.2.1. Horizontal Links
	RIFT is not limited to pure Clos divided into PoD and multi-planes		RIFT is not limited to pure Clos divided into PoD and multi-planes
	but supports horizontal links below the top of fabric level. Those		but supports horizontal links below the top of fabric level. Those
	links are used however only as routes of last resort northbound when		links are used however only as routes of last resort northbound when
	a spine loses all northbound links or cannot compute a default route		a spine loses all northbound links or cannot compute a default route
	through them.		through them.
	A possible configuration is a "ring" of horizontal links at a level.		A possible configuration is a "ring" of horizontal links at a level.
	skipping to change at page 7, line 8 ¶		skipping to change at page 7, line 8 ¶
	implementations can be extended to support vertical "shortcuts" as		implementations can be extended to support vertical "shortcuts" as
	proposed by e.g. [I-D.white-distoptflood]. The RIFT specification		proposed by e.g. [I-D.white-distoptflood]. The RIFT specification
	itself does not provide the exact details since the resulting		itself does not provide the exact details since the resulting
	solution suffers from either much larger blast radius with increased		solution suffers from either much larger blast radius with increased
	flooding volumes or in case of maximum aggregation routing bow-tie		flooding volumes or in case of maximum aggregation routing bow-tie
	problems.		problems.
	3.2.3. Generalizing to any Directed Acyclic Graph		3.2.3. Generalizing to any Directed Acyclic Graph
	RIFT is an anisotropic routing protocol, meaning that it has a sense		RIFT is an anisotropic routing protocol, meaning that it has a sense
	of direction (Northbound, Southbound, East-West) and that it operates		of direction (northbound, southbound, east-west) and that it operates
	differently depending on the direction.		differently depending on the direction.
	* Northbound, RIFT operates as a Link State IGP, whereby the control		* Northbound, RIFT operates as a link-state IGP, whereby the control
	packets are reflooded first all the way North and only interpreted		packets are reflooded first all the way North and only interpreted
	later. All the individual fine grained routes are advertised.		later. All the individual fine grained routes are advertised.
	* Southbound, RIFT operates as a Distance Vector IGP, whereby the		* Southbound, RIFT operates as a distance vector IGP, whereby the
	control packets are flooded only one hop, interpreted, and the		control packets are flooded only one hop, interpreted, and the
	consequence of that computation is what gets flooded on more hop		consequence of that computation is what gets flooded on more hop
	South. In the most common use-cases, a ToF node can reach most of		South. In the most common use-cases, a ToF node can reach most of
	the prefixes in the fabric. If that is the case, the ToF node		the prefixes in the fabric. If that is the case, the ToF node
	advertises the fabric default and disaggregates the prefixes that		advertises the fabric default and disaggregates the prefixes that
	it cannot reach. On the other hand, a ToF Node that can reach		it cannot reach. On the other hand, a ToF Node that can reach
	only a small subset of the prefixes in the fabric will preferably		only a small subset of the prefixes in the fabric will preferably
	advertise those prefixes and refrain from aggregating.		advertise those prefixes and refrain from aggregating.
	In the general case, what gets advertised South is in more		In the general case, what gets advertised South is in more
	details:		details:
	1. A fabric default that aggregates all the prefixes that are		1. A fabric default that aggregates all the prefixes that are
	reachable within the fabric, and that could be a default route		reachable within the fabric, and that could be a default route
	or a prefix that is dedicated to this particular fabric.		or a prefix that is dedicated to this particular fabric.
	2. The loopback addresses of the Northbound nodes, e.g., for		2. The loopback addresses of the northbound nodes, e.g., for
	inband management.		inband management.
	3. The disaggregated prefixes for the dynamic exceptions to the		3. The disaggregated prefixes for the dynamic exceptions to the
	fabric Default, advertised to route around the black hole that		fabric Default, advertised to route around the black hole that
	may form		may form
	* East-West routing can optionally be used, with specific		* east-west routing can optionally be used, with specific
	restrictions. It is useful in particular when a sibling has		restrictions. It is useful in particular when a sibling has
	access to the fabric default but this node does not.		access to the fabric default but this node does not.
	A Directed Acyclic Graph (DAG) provides a sense of North (the		A Directed Acyclic Graph (DAG) provides a sense of North (the
	direction of the DAG) and of South (the reverse), which can be used		direction of the DAG) and of South (the reverse), which can be used
	to apply RIFT. For the purpose of RIFT, an edge in the DAG that has		to apply RIFT. For the purpose of RIFT, an edge in the DAG that has
	only incoming vertices is a ToF node.		only incoming vertices is a ToF node.
	There are a number of caveats though:		There are a number of caveats though:
	* The DAG structure must exist before RIFT starts, so there is a		* The DAG structure must exist before RIFT starts, so there is a
	need for a companion protocol to establish the logical DAG		need for a companion protocol to establish the logical DAG
	structure.		structure.
	* A generic DAG does not have a sense of East and West. The		* A generic DAG does not have a sense of east and west. The
	operation specified for East-West links and the Southbound		operation specified for east-west links and the southbound
	reflection between nodes are not applicable.		reflection between nodes are not applicable.
	* In order to aggregate and disaggregate routes, RIFT requires that		* In order to aggregate and disaggregate routes, RIFT requires that
	all the ToF nodes share the full knowledge of the prefixes in the		all the ToF nodes share the full knowledge of the prefixes in the
	fabric. This can be achieved with a ring as suggested by the RIFT		fabric. This can be achieved with a ring as suggested by the RIFT
	main specification, by some preconfiguration, or using a		main specification, by some preconfiguration, or using a
	synchronization with a common repository where all the active		synchronization with a common repository where all the active
	prefixes are registered.		prefixes are registered.
	3.3. Use Cases		3.3. Use Cases
	skipping to change at page 18, line 7 ¶		skipping to change at page 18, line 7 ¶
	| +--------+		| +--------+
	| |		| |
	+-+---+-+		+-+---+-+
	|Spine11|		|Spine11|
	+-------+		+-------+
	Figure 7: Fallen spine		Figure 7: Fallen spine
	4.6. Positive vs. Negative Disaggregation		4.6. Positive vs. Negative Disaggregation
	Disaggregation is the procedure whereby [RIFT] advertises more a		Disaggregation is the procedure whereby [RIFT] advertises a more
	specific route Southwards as an exception to the aggregated fabric-		specific route Southwards as an exception to the aggregated fabric-
	default North. Disaggregation is useful when a prefix within the		default North. Disaggregation is useful when a prefix within the
	aggregation is reachable via some of the parents but not the others		aggregation is reachable via some of the parents but not the others
	at the same level of the fabric. It is mandatory when the level is		at the same level of the fabric. It is mandatory when the level is
	the ToF since a ToF node that cannot reach a prefix becomes a black		the ToF since a ToF node that cannot reach a prefix becomes a black
	hole for that prefix. The hard problem is to know which prefixes are		hole for that prefix. The hard problem is to know which prefixes are
	reachable by whom.		reachable by whom.
	In the general case, [RIFT] solves that problem by interconnecting		In the general case, [RIFT] solves that problem by interconnecting
	the ToF nodes so they can exchange the full list of prefixes that		the ToF nodes so they can exchange the full list of prefixes that
	exist in the fabric and figure when a ToF node lacks reachability and		exist in the fabric and figure when a ToF node lacks reachability and
	to existing prefix. This requires additional ports at the ToF,		to existing prefix. This requires additional ports at the ToF,
	typically 2 ports per ToF node to form a ToF-spanning ring. [RIFT]		typically 2 ports per ToF node to form a ToF-spanning ring. [RIFT]
	also defines the Southbound Reflection procedure that enables a		also defines the southbound reflection procedure that enables a
	parent to explore the direct connectivity of its peers, meaning their		parent to explore the direct connectivity of its peers, meaning their
	own parents and children; based on the advertisements received from		own parents and children; based on the advertisements received from
	the shared parents and children, it may enable the parent to infer		the shared parents and children, it may enable the parent to infer
	the prefixes its peers can reach.		the prefixes its peers can reach.
	When a parent lacks reachability to a prefix, it may disaggregate the		When a parent lacks reachability to a prefix, it may disaggregate the
	prefix negatively, i.e., advertise that this parent can be used to		prefix negatively, i.e., advertise that this parent can be used to
	reach any prefix in the aggregation except that one. The Negative		reach any prefix in the aggregation except that one. The Negative
	Disaggregation signaling is simple and functions transitively from		Disaggregation signaling is simple and functions transitively from
	ToF to ToP and then from Top to Leaf. But it is hard for a parent to		ToF to ToP and then from Top to Leaf. But it is hard for a parent to
	skipping to change at page 19, line 44 ¶		skipping to change at page 19, line 44 ¶
	on sending a portion of the traffic to the black hole in the		on sending a portion of the traffic to the black hole in the
	meantime. In the case of Negative Disaggregation, the last ToF		meantime. In the case of Negative Disaggregation, the last ToF
	node(s) that injects the route may also incur an incast issue; this		node(s) that injects the route may also incur an incast issue; this
	problem would occur if a prefix that becomes totally unreachable is		problem would occur if a prefix that becomes totally unreachable is
	disaggregated, but doing so is mostly useless and is not recommended.		disaggregated, but doing so is mostly useless and is not recommended.
	4.7. Mobile Edge and Anycast		4.7. Mobile Edge and Anycast
	When a physical or a virtual node changes its point of attachement in		When a physical or a virtual node changes its point of attachement in
	the fabric from a previous-leaf to a next-leaf, new routes must be		the fabric from a previous-leaf to a next-leaf, new routes must be
	installed that supercede the old ones. Since the flooding flows		installed that supersede the old ones. Since the flooding flows
	Northwards, the nodes (if any) between the previous-leaf and the		Northwards, the nodes (if any) between the previous-leaf and the
	common parent are not immediately aware that the path via previous-		common parent are not immediately aware that the path via previous-
	leaf is obsolete, and a stale route may exist for a while. The		leaf is obsolete, and a stale route may exist for a while. The
	common parent needs to select the freshest route advertisement in		common parent needs to select the freshest route advertisement in
	order to install the correct route via the next-leaf. This requires		order to install the correct route via the next-leaf. This requires
	that the fabric determines the sequence of the movements of the		that the fabric determines the sequence of the movements of the
	mobile node.		mobile node.
	On the one hand, a classical sequence counter provides a total order		On the one hand, a classical sequence counter provides a total order
	for a while but it will eventually wrap. On the other hand, a		for a while but it will eventually wrap. On the other hand, a
	skipping to change at page 20, line 31 ¶		skipping to change at page 20, line 31 ¶
	routes. Otherwise, the sequence counter from the mobile node, if		routes. Otherwise, the sequence counter from the mobile node, if
	available, is used. One caveat is that the sequence counter must not		available, is used. One caveat is that the sequence counter must not
	wrap within the precision of the timing protocol. Another is that		wrap within the precision of the timing protocol. Another is that
	the mobile node may not even provide a sequence counter, in which		the mobile node may not even provide a sequence counter, in which
	case the mobility itself must be slower than the precision of the		case the mobility itself must be slower than the precision of the
	timing.		timing.
	Mobility must not be confused with Anycast. In both cases, a same		Mobility must not be confused with Anycast. In both cases, a same
	address is injected in RIFT at different leaves. In the case of		address is injected in RIFT at different leaves. In the case of
	mobility, only the freshest route must be conserved, since mobile		mobility, only the freshest route must be conserved, since mobile
	node changed its point of attachement for a leaf to the next. In the		node changed its point of attachment for a leaf to the next. In the
	case of anycast, the node may be either multihomed (attached to		case of anycast, the node may be either multihomed (attached to
	multiple leaves in parallel) or reachable beyond the fabric via		multiple leaves in parallel) or reachable beyond the fabric via
	multiple routes that are redistributed to different leaves; either		multiple routes that are redistributed to different leaves; either
	way, in the case of anycast, the multiple routes are equally valid		way, in the case of anycast, the multiple routes are equally valid
	and should be conserved. Without further information from the		and should be conserved. Without further information from the
	redistributed routing protocol, it is impossible to sort out a		redistributed routing protocol, it is impossible to sort out a
	movement from a redistribution that happens asynchronously on		movement from a redistribution that happens asynchronously on
	different leaves. [RIFT] expects that anycast addresses are		different leaves. [RIFT] expects that anycast addresses are
	advertised within the timing precision, which is typically the case		advertised within the timing precision, which is typically the case
	with a low-precision timing and a multihomed node. Beyond that time		with a low-precision timing and a multihomed node. Beyond that time
	skipping to change at page 23, line 42 ¶		skipping to change at page 23, line 42 ¶
	| | | | | | | |		| | | | | | | |
	+---+ +---+ ...............+---+ +---+		+---+ +---+ ...............+---+ +---+
	SV(1) SV(2) SV(n+1) SV(n)		SV(1) SV(2) SV(n+1) SV(n)
	Figure 10: Dual-homing servers		Figure 10: Dual-homing servers
	In the single plane, the worst condition is disaggregation of every		In the single plane, the worst condition is disaggregation of every
	other servers at the same level. Suppose the links from ToR1 to all		other servers at the same level. Suppose the links from ToR1 to all
	the leaves become not available. All the servers' routes are		the leaves become not available. All the servers' routes are
	disaggregated and the FIB of the servers will be expanded with n-1		disaggregated and the FIB of the servers will be expanded with n-1
	more spicific routes.		more specific routes.
	Sometimes, pleople may prefer to disaggregate from ToR to servers		Sometimes, people may prefer to disaggregate from ToR to servers from
	from start on, i.e. the servers have couple tens of routes in FIB		start on, i.e. the servers have couple tens of routes in FIB from
	from start on beside default routes to avoid breakages at rack level.		start on beside default routes to avoid breakages at rack level.
	Full disaggregation of the fabric could be achieved by configuration		Full disaggregation of the fabric could be achieved by configuration
	supported by RIFT.		supported by RIFT.
	4.11. Fabric With A Controller		4.11. Fabric With A Controller
	There are many different ways to deploy the controller. One		There are many different ways to deploy the controller. One
	possibility is attaching a controller to the RIFT domain from ToF and		possibility is attaching a controller to the RIFT domain from ToF and
	another possibility is attaching a controller from the leaf.		another possibility is attaching a controller from the leaf.
	+------------+		+------------+
End of changes. 23 change blocks.
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