draft-ietf-ipv6-router-selection-01.txt		draft-ietf-ipv6-router-selection-02.txt
	IPng Working Group Richard Draves		IPng Working Group R. Draves
	Internet Draft Microsoft Research		Internet Draft Microsoft Research
	Document: draft-ietf-ipv6-router-selection-01.txt March 1, 2002		Document: draft-ietf-ipv6-router-selection-02.txt R. Hinden
			Nokia
			June 10, 2002
	Default Router Preferences and More-Specific Routes		Default Router Preferences, More-Specific Routes, and Load Sharing
	Status of this Memo		Status of this Memo
	This document is an Internet-Draft and is in full conformance with		This document is an Internet-Draft and is in full conformance with
	all provisions of Section 10 of RFC 2026 [1].		all provisions of Section 10 of RFC 2026 [1].
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF), its areas, and its working groups. Note that		Task Force (IETF), its areas, and its working groups. Note that
	other groups may also distribute working documents as Internet-		other groups may also distribute working documents as Internet-
	Drafts.		Drafts.
	skipping to change at line 31		skipping to change at line 33
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	The list of current Internet-Drafts can be accessed at		The list of current Internet-Drafts can be accessed at
	http://www.ietf.org/ietf/1id-abstracts.txt.		http://www.ietf.org/ietf/1id-abstracts.txt.
	The list of Internet-Draft Shadow Directories can be accessed at		The list of Internet-Draft Shadow Directories can be accessed at
	http://www.ietf.org/shadow.html.		http://www.ietf.org/shadow.html.
	Abstract		Abstract
	This document describes an optional extension to Neighbor Discovery		This document describes two changes to Neighbor Discovery. The first
	Router Advertisement messages for communicating default router		change is an optional extension to Router Advertisement messages for
	preferences and more-specific routes from routers to hosts. This		communicating default router preferences and more-specific routes
	improves the ability of hosts to pick an appropriate router,		from routers to hosts. This improves the ability of hosts to pick an
	especially when the host is multi-homed and the routers are on		appropriate router, especially when the host is multi-homed and the
	different links. The preference values and specific routes		routers are on different links. The preference values and specific
	advertised to hosts require administrative configuration; they are		routes advertised to hosts require administrative configuration;
	not automatically derived from routing tables.		they are not automatically derived from routing tables. The second
			change is a mandatory modification of the conceptual sending
			algorithm to support load-sharing among equivalent routers.
	1. Introduction		1. Introduction
	Neighbor Discovery [2] specifies a conceptual model for hosts that		Neighbor Discovery [2] specifies a conceptual model for hosts that
	includes a Default Router List and a Prefix List. Hosts send Router		includes a Default Router List and a Prefix List. Hosts send Router
	Solicitation messages and receive from routers Router Advertisement		Solicitation messages and receive Router Advertisement messages from
	messages. Hosts populate their Default Router List and Prefix List		routers. Hosts populate their Default Router List and Prefix List
	based on information in the Router Advertisement messages. A		based on information in the Router Advertisement messages. A
	conceptual sending algorithm uses the Prefix List to determine if a		conceptual sending algorithm uses the Prefix List to determine if a
	destination address is on-link and the Default Router List to select		destination address is on-link and the Default Router List to select
	a router for off-link destinations.		a router for off-link destinations.
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	In some network topologies where the host has multiple routers on		In some network topologies where the host has multiple routers on
	its Default Router List, the choice of router for an off-link		its Default Router List, the choice of router for an off-link
	destination is important. In some situations, one router may provide		destination is important. In some situations, one router may provide
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	much better performance than another for a destination. In other		much better performance than another for a destination. In other
	situations, choosing the wrong router may result in a failure to		situations, choosing the wrong router may result in a failure to
	communicate. (A later section gives specific examples of these		communicate. (A later section gives specific examples of these
	scenarios.)		scenarios.)
	This document describes an optional extension to Neighbor Discovery		This document describes an optional extension to Neighbor Discovery
	Router Advertisement messages for communicating default router		Router Advertisement messages for communicating default router
	preferences and more-specific routes from routers to hosts. This		preferences and more-specific routes from routers to hosts. This
	improves the ability of hosts to pick an appropriate router for an		improves the ability of hosts to pick an appropriate router for an
	off-link destination.		off-link destination.
	skipping to change at line 93		skipping to change at line 96
	6-over-4 [3] or configured tunnel [4] network connections.		6-over-4 [3] or configured tunnel [4] network connections.
	This document requires that the preference values and specific		This document requires that the preference values and specific
	routes advertised to hosts require explicit administrative		routes advertised to hosts require explicit administrative
	configuration. They are not automatically derived from routing		configuration. They are not automatically derived from routing
	tables. In particular, the preference values are not routing metrics		tables. In particular, the preference values are not routing metrics
	and it is not recommended that routers "dump out" their entire		and it is not recommended that routers "dump out" their entire
	routing tables to hosts.		routing tables to hosts.
	We use Router Advertisement messages, instead of some other protocol		We use Router Advertisement messages, instead of some other protocol
	like RIP [5], is that Router Advertisements are an existing		like RIP [5], because Router Advertisements are an existing
	standard, stable protocol for router-to-host communication.		standard, stable protocol for router-to-host communication.
	Piggybacking this information on existing message traffic from		Piggybacking this information on existing message traffic from
	routers to hosts reduces network overhead. Neighbor Discovery is to		routers to hosts reduces network overhead. Neighbor Discovery is to
	unicast routing as Multicast Listener Discovery is to multicast		unicast routing as Multicast Listener Discovery is to multicast
	routing. In both cases, a single simple protocol insulates the host		routing. In both cases, a single simple protocol insulates the host
	from the variety of router-to-router protocols. In addition, RIP is		from the variety of router-to-router protocols. In addition, RIP is
	unsuitable because it does not carry route lifetimes so it requires		unsuitable because it does not carry route lifetimes so it requires
	frequent message traffic with greater processing overheads.		frequent message traffic with greater processing overheads.
			This document also describes a mandatory change in host behavior.
			Neighbor DiscoveryÆs conceptual sending algorithm is modified to
			require hosts to select randomly among equivalent routers. This
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			distributes traffic to different destinations among the routers.
			Traffic to a single destination continues to use a single router,
			because of the Destination Cache.
	The mechanisms specified here are backwards-compatible, so that		The mechanisms specified here are backwards-compatible, so that
	hosts that do not implement them continue to function as well as		hosts that do not implement them continue to function as well as
	they did previously.		they did previously.
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	1.1. Conventions used in this document		1.1. Conventions used in this document
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
	this document are to be interpreted as described in RFC-2119 [6].		this document are to be interpreted as described in RFC-2119 [6].
	2. Message Formats		2. Message Formats
	2.1. Preference Values		2.1. Preference Values
	skipping to change at line 154		skipping to change at line 164
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Reachable Time |		| Reachable Time |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Retrans Timer |		| Retrans Timer |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Options ...		| Options ...
	+-+-+-+-+-+-+-+-+-+-+-+-		+-+-+-+-+-+-+-+-+-+-+-+-
	Fields:		Fields:
			Draves Expires January 2003 3
	Prf (Default Router Preference)		Prf (Default Router Preference)
	2-bit signed integer. Indicates whether or not to prefer		2-bit signed integer. Indicates whether or not to prefer
	this router over other default routers. If Router		this router over other default routers. If Router
	Lifetime is zero, it MUST be initialized to zero by the		Lifetime is zero, it MUST be initialized to zero by the
	sender and MUST be ignored by the receiver. If the		sender and MUST be ignored by the receiver. If the
	Reserved (10) value is received, the receiver should		Reserved (10) value is received, the receiver should
	treat the RA as having a zero Router Lifetime.		treat the RA as having a zero Router Lifetime.
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	Resvd (Reserved)		Resvd (Reserved)
	A 3-bit unused field. It MUST be initialized to zero by		A 3-bit unused field. It MUST be initialized to zero by
	the sender and MUST be ignored by the receiver.		the sender and MUST be ignored by the receiver.
	Possible Options:		Possible Options:
	Route Information		Route Information
	These options specify prefixes that are reachable via		These options specify prefixes that are reachable via
	the router.		the router.
	Discussion:		Discussion:
	Note that in addition to the preference value in the message header,		Note that in addition to the preference value in the message header,
	a Router Advertisement can also contain a Route Information Option		a Router Advertisement can also contain a Route Information Option
	for ::/0, with a preference value and lifetime. Encoding a		for ::/0, with a preference value and lifetime. Encoding a
	preference value in the Router Advertisement header has some		preference value in the Router Advertisement header has some
	advantages:		advantages:
	1. It allows for a distinction between "best default router" and		1. It allows for a distinction between "best default router" and
	"best router for default", as described below.		"best router for default", as described below in section 5.1.
	2. When the best default router is also the best router for		2. When the best default router is also the best router for
	default (which will be a common case), encoding the preference		default (which will be a common case), encoding the preference
	value in the message header is more efficient than having to send		value in the message header is more efficient than having to send
	a separate option.		a separate option.
	2.3. Route Information Option		2.3. Route Information Option
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	skipping to change at line 209		skipping to change at line 219
	+ +		+ +
	| |		| |
	+ Prefix +		+ Prefix +
	| |		| |
	+ +		+ +
	| |		| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Fields:		Fields:
			Draves Expires January 2003 4
	Type TBD		Type TBD
	Length 1, 2, or 3 depending on Prefix Length. If Prefix Length		Length 8-bit unsigned integer. The length of the option
	is greater than 64, then Length must be at least 3. If		(including the Type and Length fields) in units of
	Prefix Length is greater than 0, then Length must be at		8 octets.
	least 2. If Prefix Length is zero, then Length may be 1.
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	Prefix Length		Prefix Length
	8-bit unsigned integer. The number of leading bits in		8-bit unsigned integer. The number of leading bits in
	the Prefix that are valid. The value ranges from 0 to		the Prefix that are valid. The value ranges from 0 to
	128.		128.
	Prf (Route Preference)		Prf (Route Preference)
	2-bit signed integer. Indicates whether or not to prefer		2-bit signed integer. Indicates whether or not to prefer
	this router for the prefix over others. If the Reserved		this router for the prefix over others. If the Reserved
	(10) value is received, the Route Information Option		(10) value is received, the Route Information Option
	MUST be ignored.		MUST be ignored.
	skipping to change at line 238		skipping to change at line 247
	Resvd (Reserved)		Resvd (Reserved)
	Two 3-bit unused fields. They MUST be initialized to		Two 3-bit unused fields. They MUST be initialized to
	zero by the sender and MUST be ignored by the receiver.		zero by the sender and MUST be ignored by the receiver.
	Route Lifetime		Route Lifetime
	32-bit unsigned integer. The length of time in seconds		32-bit unsigned integer. The length of time in seconds
	(relative to the time the packet is sent) that the		(relative to the time the packet is sent) that the
	prefix is valid for route determination. A value of all		prefix is valid for route determination. A value of all
	one bits (0xffffffff) represents infinity.		one bits (0xffffffff) represents infinity.
	Prefix An IP address or a prefix of an IP address. The Prefix		Prefix Variable-length field containing an IP address or a
	Length field contains the number of valid leading bits		prefix of an IP address. The Prefix Length field
	in the prefix. The bits in the prefix after the prefix		contains the number of valid leading bits in the prefix.
	length are reserved and MUST be initialized to zero by		The bits in the prefix after the prefix length (if any)
	the sender and ignored by the receiver.		are reserved and MUST be initialized to zero by the
			sender and ignored by the receiver.
	The Prefix field is 0, 8, or 16 octets depending on		The Length field is 1, 2, or 3 depending on Prefix Length. If Prefix
	Length.		Length is greater than 64, then Length must be 3. If Prefix Length
			is greater than 0, then Length must be 2 or 3. If Prefix Length is
			zero, then Length must be 1, 2, or 3.
			The Prefix field is 0, 8, or 16 octets depending on Length.
	Routers SHOULD NOT include in a Router Advertisement two Route		Routers SHOULD NOT include in a Router Advertisement two Route
	Information Options with the same Prefix and Prefix Length. If a		Information Options with the same Prefix and Prefix Length. If a
	host processes a Router Advertisement carrying multiple Router		host processes a Router Advertisement carrying multiple Router
	Information Options with the same Prefix and Prefix Length, it MUST		Information Options with the same Prefix and Prefix Length, it MUST
	process one of the options (unspecified which one) and it MUST		process one of the options (unspecified which one) and it MUST
	effectively ignore the rest. It MUST NOT retain some information		effectively ignore the rest. It MUST NOT retain some information
	(like preference) from one option and other information (like		(like preference) from one option and other information (like
	lifetime) from another option.		lifetime) from another option.
	Discussion:		Discussion:
			Draves Expires January 2003 5
	There are several reasons for using a new Route Information Option,		There are several reasons for using a new Route Information Option,
	instead of using flag bits to overload the existing Prefix		instead of using flag bits to overload the existing Prefix
	Information Option:		Information Option:
	1. Prefixes will typically only show up in one or the other kind		1. Prefixes will typically only show up in one or the other kind
	of option, not both, so a new option does not introduce		of option, not both, so a new option does not introduce
	duplication.		duplication.
	2. The Route Information Option is typically 16 octets while the		2. The Route Information Option is typically 16 octets while the
	Prefix Information Option is 32 octets.		Prefix Information Option is 32 octets.
	Draves Expires September 2002 5
	3. Using a new option may improve backwards-compatibility with		3. Using a new option may improve backwards-compatibility with
	some host implementations.		some host implementations.
	3. Conceptual Model of a Host		3. Conceptual Model of a Host
	There are three possible conceptual models for host implementation		There are three possible conceptual models for host implementation
	of default router preferences and more-specific routes,		of default router preferences and more-specific routes,
	corresponding to different levels of support. We refer to these as		corresponding to different levels of support. We refer to these as
	host A, host B, and host C. Note that these are really classes of		host A, host B, and host C. Note that these are really classes of
	hosts, not individual hosts.		hosts, not individual hosts.
	skipping to change at line 300		skipping to change at line 314
	Route Information Options.		Route Information Options.
	Host C uses a Routing Table instead of a Default Router List. (The		Host C uses a Routing Table instead of a Default Router List. (The
	Routing Table may also subsume the Prefix List, but that is beyond		Routing Table may also subsume the Prefix List, but that is beyond
	the scope of this document.) Entries in the Routing Table have a		the scope of this document.) Entries in the Routing Table have a
	prefix, prefix length, preference value, lifetime, and next-hop		prefix, prefix length, preference value, lifetime, and next-hop
	router. Host C uses both the Default Router Preference value in the		router. Host C uses both the Default Router Preference value in the
	Router Advertisement header and Route Information Options.		Router Advertisement header and Route Information Options.
	When host C receives a Router Advertisement, it modifies its Routing		When host C receives a Router Advertisement, it modifies its Routing
	Table as follows. If a route's lifetime is zero, the route is		Table as follows. If the received route's lifetime is zero, the
	removed from the Routing Table if present. If a route's lifetime is		route is removed from the Routing Table if present. If a route's
	non-zero, the route is added to the Routing Table if not present and		lifetime is non-zero, the route is added to the Routing Table if not
	the route's lifetime and preference is updated if the route is		present and the route's lifetime and preference is updated if the
	already present. A route is located in the Routing Table based on		route is already present. A route is located in the Routing Table
	prefix, prefix length, and next-hop router. When processing a Router		based on prefix, prefix length, and next-hop router. When processing
	Advertisment, host C first updates a ::/0 route based on the Router		a Router Advertisement, host C first updates a ::/0 route based on
	Lifetime and Default Router Preference in the Router Advertisement		the Router Lifetime and Default Router Preference in the Router
	message header. Then as host C processes Route Information Options		Advertisement message header. Then as host C processes Route
	in the Router Advertisement message body, it updates its routing		Information Options in the Router Advertisement message body, it
	table for each such option. The Router Preference and Lifetime		updates its routing table for each such option. The Router
	values in a ::/0 Route Information Option override the preference		Preference and Lifetime values in a ::/0 Route Information Option
	and lifetime values in the Router Advertisement header.
			Draves Expires January 2003 6
			override the preference and lifetime values in the Router
			Advertisement header.
	For example, suppose a host receives a Router Advertisement from		For example, suppose a host receives a Router Advertisement from
	router X with a Router Lifetime of 100 seconds and Default Router		router X with a Router Lifetime of 100 seconds and Default Router
	Preference of Medium. The body of the Router Advertisement contains		Preference of Medium. The body of the Router Advertisement contains
	a Route Information Option for ::/0 with a Route Lifetime of 200		a Route Information Option for ::/0 with a Route Lifetime of 200
	seconds and a Route Preference of Low. After processing the Router		seconds and a Route Preference of Low. After processing the Router
	Advertisement, host A will have an entry for router X in its Default		Advertisement, host A will have an entry for router X in its Default
	Router List with lifetime 100 seconds. If host B receives the same		Router List with lifetime 100 seconds. If host B receives the same
	Router Advertisement, it will have an entry in its Default Router		Router Advertisement, it will have an entry in its Default Router
	List for router X with Medium preference and lifetime 100 seconds.		List for router X with Medium preference and lifetime 100 seconds.
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	Host C will have an entry in its Routing Table for ::/0 -> router X,		Host C will have an entry in its Routing Table for ::/0 -> router X,
	with Low preference and lifetime 200 seconds.		with Low preference and lifetime 200 seconds. Host C MAY have a
			transient state, during processing of the Router Advertisement, in
			which it has an entry in its Routing Table for ::/0 -> router X with
			Medium preference and lifetime 100 seconds.
	3.2. Conceptual Sending Algorithm for Hosts		3.2. Conceptual Sending Algorithm for Hosts
	Host A uses the conceptual sending algorithm described in Neighbor		Host A uses the conceptual sending algorithm described in Neighbor
	Discovery [2].		Discovery [2], modified slightly to support load sharing as
			described in section 3.5.
	When host B does next-hop determination and consults its Default		When host B does next-hop determination and consults its Default
	Router List, it first prefers reachable routers over non-reachable		Router List, it primarily prefers reachable routers over non-
	routers and second uses the router preference values. If all default		reachable routers and secondarily uses the router preference values.
	routers are not reachable, then it SHOULD round-robin among them all
	regardless of preference value.
	When host C does next-hop determination and consults its Routing		When host C does next-hop determination and consults its Routing
	Table for an off-link destination, it first prefers reachable		Table for an off-link destination, it first prefers reachable
	routers over non-reachable routers, second uses longest-matching-		routers over non-reachable routers, second uses longest-matching-
	prefix, and third uses route preference values.		prefix, and third uses route preference values.
	If there are no reachable routers with routes matching the		If there are no routes matching the destination (i.e., no default
	destination, then host C SHOULD round-robin among all routers with		routes and no more-specific routes), then if host C has a single
	routes matching the destination regardless of preference value or		interface then it SHOULD assume the destination is on-link to that
	prefix length.		interface. If host C has multiple interfaces then it SHOULD discard
			the packet and report a Destination Unreachable / No Route To
	If there are no routes matching the destination, then if host C has		Destination error to the upper layer.
	a single interface then it SHOULD assume the destination is on-link.
	If host C has multiple interfaces then it SHOULD discard the packet
	and report a Destination Unreachable / No Route To Destination error
	to the upper layer.
	For example: suppose host C has four entries in its Routing Table:
	::/0 -> router W with Medium preference
	2001::/16 -> router X with Medium preference
	3ffe::/16 -> router Y with High preference
	3ffe::/16 -> router Z with Low preference
	and host C is sending to 3ffe::1, an off-link destination. If all
	routers are reachable, then router Y will be chosen. If router Y is
	not reachable, then router Z will be chosen. If routers Y and Z are
	not reachable, then router W will be chosen. If routers W, Y, and Z
	are all not reachable, then host C should round-robin among the
	three routers. Router X will never be chosen because its prefix does
	not match the destination.
	3.3. Destination Cache Management		3.3. Destination Cache Management
	When a host processes a Router Advertisement and updates its		When host C processes a Router Advertisement and updates its
	conceptual routing table, it SHOULD invalidate or remove Destination		conceptual Routing Table, it SHOULD invalidate or remove Destination
	Cache Entries and redo next-hop determination for destinations		Cache Entries and redo next-hop determination for destinations
	affected by the routing table changes. The host MAY implement this		affected by the Routing Table changes. The host MAY implement this
	requirement by flushing its entire Destination Cache.		requirement by flushing its entire Destination Cache.
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	3.4. Router Reachability Probing		3.4. Router Reachability Probing
	When a host avoids using a non-reachable router X and instead uses a		When a host avoids using a non-reachable router X and instead uses
	reachable router Y, and the host would have used router X if router		another router Y, and the host would have used router X if router X
	X were reachable, then the host SHOULD probe router XÆs reachability		were reachable, then the host SHOULD probe router X's reachability
	by sending a Neighbor Solicitation. These probes MUST be rate-
	limited to no more than one per minute per router.		Draves Expires January 2003 7
			by sending a Neighbor Solicitation. A host MUST NOT probe a router's
			reachability in the absence of useful traffic that the host would
			have sent to the router if it were reachable. In any case, these
			probes MUST be rate-limited to no more than one per minute per
			router.
	This requirement allows the host to discover when router X becomes		This requirement allows the host to discover when router X becomes
	reachable and to start using router X at that point. Otherwise, the		reachable and to start using router X at that point. Otherwise, the
	host might not notice router XÆs reachability and continue to use		host might not notice router XÆs reachability and continue to use
	the less-desirable router Y.		the less-desirable router Y.
	3.5. Host Load Sharing		3.5. Host Load Sharing
	Sometimes a host has a choice of multiple "equivalent" routers for a		Sometimes a host has a choice of multiple "equivalent" routers for a
	destination. We say that two routers are equivalent for a		destination. We say that two routers are equivalent for a
	destination if they have the same reachability, the same matching		destination if they have the same reachability, the same matching
	prefix length (if the host supports a Routing Table), and the same		prefix length (if the host supports a Routing Table), and the same
	preference values (if the host supports preference values).		preference values (if the host supports preference values).
	When a host chooses from multiple equivalent routers, it SHOULD		When a host chooses from multiple equivalent routers, it MUST choose
	choose randomly.		randomly.
	This has the effect of distributing load for new destinations among		This has the effect of distributing load for new destinations among
	the equivalent routers. Note that traffic to a single destination		the equivalent routers. Note that traffic to a single destination
	will use a single router as long as the Destination Cache Entry for		will use a single router as long as the Destination Cache Entry for
	the destination is not deleted. Random selection, instead of round-		the destination is not deleted. Random selection, instead of round-
	robin, is used to avoid synchronization issues.		robin, is used to avoid synchronization issues.
			3.6. Example
			For example: suppose host C has five entries in its Routing Table:
			::/0 -> router W with Medium preference
			2001::/16 -> router X with Medium preference
			3ffe::/16 -> router Y1 with High preference
			3ffe::/16 -> router Y2 with High preference
			3ffe::/16 -> router Z with Low preference
			and host C is sending to 3ffe::1, an off-link destination. If all
			routers are reachable, then the host will choose randomly between
			routers Y1 and Y2. If routers Y1 and Y2 are not reachable, then
			router Z will be chosen and the reachability of routers Y1 and Y2
			will be probed. If routers Y1, Y2, and Z are not reachable, then
			router W will be chosen and the reachability of routers Y1, Y2, and
			Z will be probed. If routers W, Y1, Y2, and Z are all not reachable,
			then host C should round-robin among Y1 and Y2 while probing the
			reachability of W and Z. Router X will never be chosen because its
			prefix does not match the destination.
	4. Router Configuration		4. Router Configuration
	Routers should not advertise preferences or routes by default. In		Routers should not advertise preferences or routes by default. In
	particular, they should not "dump out" their entire routing table to		particular, they should not "dump out" their entire routing table to
	hosts. Routers MAY have a configuration mode where a filter is		hosts. Routers MAY have a configuration mode where a filter is
			Draves Expires January 2003 8
	applied to their routing table to obtain the routes that are		applied to their routing table to obtain the routes that are
	advertised to hosts.		advertised to hosts.
	The preference values (both Default Router Preferences and Route		The preference values (both Default Router Preferences and Route
	Preferences) should not be routing metrics or automatically derived		Preferences) should not be routing metrics or automatically derived
	from metrics: the preference values should be configured. The High		from metrics: the preference values should be configured. The High
	and Low (non-default) preference values should only be used when		and Low (non-default) preference values should only be used when
	someone with knowledge of both routers and the network topology		someone with knowledge of both routers and the network topology
	configures them explicitly. For example, it could be a common		configures them explicitly. For example, it could be a common
	network administrator, or it could be a customer request to		network administrator, or it could be a customer request to
	different administrators managing the routers.		different administrators managing the routers.
	As one exception to this general rule, the administrator of a router		As one exception to this general rule, the administrator of a router
	that does not have a connection to the internet, or is connected		that does not have a connection to the internet, or is connected
	through a firewall that blocks general traffic, may configure the		through a firewall that blocks general traffic, may configure the
	router to advertise a Low Default Router Preference.		router to advertise a Low Default Router Preference.
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	An administrator of a router may configure the router to advertise		An administrator of a router may configure the router to advertise
	specific routes for directly connected subnets and any shorter		specific routes for directly connected subnets and any shorter
	prefixes (eg, site, NLA, or TLA prefixes) for networks to which the		prefixes (eg, site, NLA, or TLA prefixes) for networks to which the
	router belongs.		router belongs.
	For example, if a home user sets up a tunnel into a firewalled		For example, if a home user sets up a tunnel into a firewalled
	corporate network, the access router on the corporate network end of		corporate network, the access router on the corporate network end of
	the tunnel can advertise itself as a default router, but with a Low		the tunnel can advertise itself as a default router, but with a Low
	preference. Furthermore the corporate router can advertise a		preference. Furthermore the corporate router can advertise a
	specific route for the corporate site prefix. The net result is that		specific route for the corporate site prefix. The net result is that
	destinations in the corporate network will be reached via the		destinations in the corporate network will be reached via the
	tunnel, and general internet destinations will be reached via the		tunnel, and general internet destinations will be reached via the
	home ISP. Without these mechanisms, the home machine might choose to		home ISP. Without these mechanisms, the home machine might choose to
	send internet traffic into the corporate network or corporate		send internet traffic into the corporate network or corporate
	traffic into the internet, leading to communication failure because		traffic into the internet, leading to communication failure because
	of the firewall.		of the firewall.
	Routers SHOULD NOT send more than 17 Route Information Options in		Routers SHOULD NOT send more than 17 Route Information Options in
	Router Advertisements per link.		Router Advertisements per link. This arbitrary bound is meant to
			reinforce that relatively few and carefully selected routes should
			be advertised to hosts.
	5. Examples		5. Examples
	5.1. Best Default Router vs Best Route for Default		5.1. Best Default Router vs Best Route for Default
	The best default router is not quite the same thing as the best		The best default router is not quite the same thing as the best
	router for default. The best default router is the router that will		router for default. The best default router is the router that will
	generate the fewest number of redirects for the host's traffic. The		generate the fewest number of redirects for the host's traffic. The
	best router for default is the router with the best route toward the		best router for default is the router with the best route toward the
	wider internet.		wider internet.
	For example, suppose a situation where you have a link with two		For example, suppose a situation where you have a link with two
	routers X and Y. Router X is the best for 2002::/16. (It's your 6to4		routers X and Y. Router X is the best for 2002::/16. (It's your 6to4
	site gateway.) Router Y is the best for ::/0. (It connects to the		site gateway.) Router Y is the best for ::/0. (It connects to the
	native IPv6 internet.) Router X forwards native IPv6 traffic to		native IPv6 internet.) Router X forwards native IPv6 traffic to
			Draves Expires January 2003 9
	router Y; router Y forwards 6to4 traffic to router X. But most		router Y; router Y forwards 6to4 traffic to router X. But most
	traffic from this site is sent to 2002:/16 destinations. In this		traffic from this site is sent to 2002:/16 destinations. In this
	scenario, router X is the best default router and router Y is the		scenario, router X is the best default router and router Y is the
	best router for default.		best router for default.
	To make host A work well, both routers should advertise themselves		To make host A work well, both routers should advertise themselves
	as default routers. In particular, if router Y goes down host A		as default routers. In particular, if router Y goes down host A
	should send traffic to router X to maintain 6to4 connectivity, so		should send traffic to router X to maintain 6to4 connectivity, so
	router X as well as router Y needs to be a default router.		router X as well as router Y needs to be a default router.
	To make host B work well, router X should in addition advertise		To make host B work well, router X should in addition advertise
	itself with a High default router preference. This will cause host B		itself with a High default router preference. This will cause host B
	to prefer router X, minimizing the number of redirects.		to prefer router X, minimizing the number of redirects.
	To make host C work well, router X should in addition advertise the		To make host C work well, router X should in addition advertise the
	::/0 route with Low preference and the 2002::/16 route with Medium		::/0 route with Low preference and the 2002::/16 route with Medium
	preference. Host C will end up with three routes in its routing		preference. Host C will end up with three routes in its routing
	table: ::/0 -> router X (Low), ::/0 -> router Y (Medium), 2002::/16		table: ::/0 -> router X (Low), ::/0 -> router Y (Medium), 2002::/16
	-> router X (Medium). It will send 6to4 traffic to router X and		-> router X (Medium). It will send 6to4 traffic to router X and
	other traffic to router Y. Host C will not cause any redirects.		other traffic to router Y. Host C will not cause any redirects.
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	Note that when host C processes the Router Advertisement from router		Note that when host C processes the Router Advertisement from router
	X, the Low preference for ::/0 overrides the High default router		X, the Low preference for ::/0 overrides the High default router
	preference. If the ::/0 specific route were not present, then host C		preference. If the ::/0 specific route were not present, then host C
	would apply the High default router preference to its ::/0 route to		would apply the High default router preference to its ::/0 route to
	router X.		router X.
	5.2. Multi-Homed Host and Isolated Network		5.2. Multi-Homed Host and Isolated Network
	Here's another scenario: a multi-homed host is connected to the		Here's another scenario: a multi-homed host is connected to the
	6bone/internet via router X on one link and to an isolated network		6bone/internet via router X on one link and to an isolated network
	skipping to change at line 519		skipping to change at line 545
	advertise a Route Information Option for the isolated network		advertise a Route Information Option for the isolated network
	prefix. It might not advertise itself as a default router at all		prefix. It might not advertise itself as a default router at all
	(zero Router Lifetime), or it might advertise itself as a default		(zero Router Lifetime), or it might advertise itself as a default
	router with Low preference. Router Y should advertise itself as a		router with Low preference. Router Y should advertise itself as a
	default router with Medium preference.		default router with Medium preference.
	6. Security Considerations		6. Security Considerations
	A malicious node could send Router Advertisement messages,		A malicious node could send Router Advertisement messages,
	specifying High Default Router Preference or carrying specific		specifying High Default Router Preference or carrying specific
			Draves Expires January 2003 10
	routes, with the effect of pulling traffic away from legitimate		routes, with the effect of pulling traffic away from legitimate
	routers. However, a malicious node could easily achieve this same		routers. However, a malicious node could easily achieve this same
	effect in other ways. For example, it could fabricate Router		effect in other ways. For example, it could fabricate Router
	Advertisement messages with zero Router Lifetime from the other		Advertisement messages with zero Router Lifetime from the other
	routers, causing hosts to stop using the other routes. Hence, this		routers, causing hosts to stop using the other routes. Hence, this
	document has no appreciable impact on Internet infrastructure		document has no new appreciable impact on Internet infrastructure
	security.		security.
	References		References
	1 S. Bradner, "The Internet Standards Process -- Revision 3", BCP		1 S. Bradner, "The Internet Standards Process -- Revision 3", BCP
	9, RFC 2026, October 1996.		9, RFC 2026, October 1996.
	2 T. Narten, E. Nordmark, W. Simpson. "Neighbor Discovery for IP		2 T. Narten, E. Nordmark, W. Simpson. "Neighbor Discovery for IP
	Version 6 (IPv6)", RFC 2461, December 1998.		Version 6 (IPv6)", RFC 2461, December 1998.
	3 B. Carpenter, K. Moore. "Connection of IPv6 Domains via IPv4		3 B. Carpenter, K. Moore. "Connection of IPv6 Domains via IPv4
	Clouds", RFC 3056, February 2001.		Clouds", RFC 3056, February 2001.
	Draves Expires September 2002 10
	4 R. Gilligan, E. Nordmark. "Transition Mechanisms for IPv6 Hosts		4 R. Gilligan, E. Nordmark. "Transition Mechanisms for IPv6 Hosts
	and Routers", RFC 1933, April 1996.		and Routers", RFC 1933, April 1996.
	5 G. Malkin, R. Minnear. "RIPng for IPv6", RFC 2080 , January 1997.		5 G. Malkin, R. Minnear. "RIPng for IPv6", RFC 2080 , January 1997.
	6 S. Bradner, "Key words for use in RFCs to Indicate Requirement		6 S. Bradner, "Key words for use in RFCs to Indicate Requirement
	Levels", BCP 14, RFC 2119, March 1997.		Levels", BCP 14, RFC 2119, March 1997.
	Acknowledgments		Acknowledgments
	The author would like to acknowledge the contributions of Balash		The authors would like to acknowledge the contributions of Balash
	Akbari, Steve Deering, Robert Elz, Tony Hain, Bob Hinden, Christian		Akbari, Steve Deering, Robert Elz, Tony Hain, Christian Huitema,
	Huitema, JINMEI Tatuya, Erik Nordmark, Dave Thaler, and Brian Zill.		JINMEI Tatuya, Erik Nordmark, Pekka Savola, Dave Thaler, and Brian
	The packet diagrams are derived from Neighbor Discovery [2]. The		Zill. The packet diagrams are derived from Neighbor Discovery [2].
	description of host load sharing is derived from Bob Hinden's draft		The description of host load sharing is derived from Bob Hinden's
	on the subject.		draft on the subject.
	Author's Addresses		Author's Addresses
	Richard Draves		Richard Draves
	Microsoft Research		Microsoft Research
	One Microsoft Way		One Microsoft Way
	Redmond, WA 98052		Redmond, WA 98052
	Phone: 1-425-706-2268		Phone: +1 425 706 2268
	Email: richdr@microsoft.com		Email: richdr@microsoft.com
			Robert M. Hinden
			Nokia
			313 Fairchild Drive
			Mountain View, CA 94043
			Phone: +1 650 625 2004
			Email: hinden@iprg.nokia.com
			Draves Expires January 2003 11
	Revision History		Revision History
			Changes from draft-ietf-ipv6-router-selection-01
			Added Bob Hinden as co-author.
			Various clarifications and textual improvements.
			Slightly simplified the specification of round-robining in next-hop
			determination, relying on router-reachability probing in some cases.
			Clarified that router reachability probing only happens when the
			host is sending packets that would have gone to that router if it
			were reachable.
			Changed load sharing to a mandatory requirement and added supporting
			text to the title, abstract, and introduction.
	Changes from draft-ietf-ipngwg-router-selection-00		Changes from draft-ietf-ipngwg-router-selection-00
	Specified reachability probing of otherwise more-preferred but		Specified reachability probing of otherwise more-preferred but
	currently unreachable routers.		currently unreachable routers.
	Changed the requirement of Destination Cache invalidation, from MAY		Changed the requirement of Destination Cache invalidation, from MAY
	to SHOULD, but allowing flushing of the entire Destination Cache.		to SHOULD, but allowing flushing of the entire Destination Cache.
	Added a section specifying load sharing among equivalent routers.		Added a section specifying load sharing among equivalent routers.
	skipping to change at line 590		skipping to change at line 642
	seen.		seen.
	Specified that routers SHOULD NOT send more than 17 Route		Specified that routers SHOULD NOT send more than 17 Route
	Information Options.		Information Options.
	Added discussion of Destination Cache invalidation, allowing but not		Added discussion of Destination Cache invalidation, allowing but not
	requiring it.		requiring it.
	Removed references to the fourth conceptual host model, host D.		Removed references to the fourth conceptual host model, host D.
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	Changes from draft-draves-ipngwg-router-selection-00		Changes from draft-draves-ipngwg-router-selection-00
	Made the option variable length. Must ignore prefix bits past prefix		Made the option variable length. Must ignore prefix bits past prefix
	length.		length.
	Added more allowable router configuration scenarios, weakening the		Added more allowable router configuration scenarios, weakening the
	requirement that one administrator must coordinate the configuration		requirement that one administrator must coordinate the configuration
	of all relevant routers.		of all relevant routers.
	Draves Expires September 2002 12		Draves Expires January 2003 12
	Full Copyright Statement		Full Copyright Statement
	Copyright (C) The Internet Society (2000). All Rights Reserved.		Copyright (C) The Internet Society (2000). All Rights Reserved.
	This document and translations of it may be copied and furnished to		This document and translations of it may be copied and furnished to
	others, and derivative works that comment on or otherwise explain it		others, and derivative works that comment on or otherwise explain it
	or assist in its implementation may be prepared, copied, published		or assist in its implementation may be prepared, copied, published
	and distributed, in whole or in part, without restriction of any		and distributed, in whole or in part, without restriction of any
	kind, provided that the above copyright notice and this paragraph		kind, provided that the above copyright notice and this paragraph
	are included on all such copies and derivative works. However, this		are included on all such copies and derivative works. However, this
	skipping to change at line 630		skipping to change at line 680
	The limited permissions granted above are perpetual and will not be		The limited permissions granted above are perpetual and will not be
	revoked by the Internet Society or its successors or assigns.		revoked by the Internet Society or its successors or assigns.
	This document and the information contained herein is provided on an		This document and the information contained herein is provided on an
	"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING		"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
	TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING		TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
	BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION		BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
	HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF		HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
	MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.		MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
	Draves Expires September 2002 13		Draves Expires January 2003 13
End of changes.
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