draft-ietf-6man-ipv6-subnet-model-03.txt		draft-ietf-6man-ipv6-subnet-model-04.txt
	Network Working Group H. Singh		Network Working Group H. Singh
	Internet-Draft W. Beebee		Internet-Draft W. Beebee
	Intended status: Standards Track Cisco Systems, Inc.		Intended status: Standards Track Cisco Systems, Inc.
	Expires: September 7, 2009 E. Nordmark		Expires: November 8, 2009 E. Nordmark
	Sun Microsystems		Sun Microsystems
	March 6, 2009		May 7, 2009
	IPv6 Subnet Model: the Relationship between Links and Subnet Prefixes		IPv6 Subnet Model: the Relationship between Links and Subnet Prefixes
	draft-ietf-6man-ipv6-subnet-model-03		draft-ietf-6man-ipv6-subnet-model-04
	Status of this Memo		Status of this Memo
	This Internet-Draft is submitted to IETF in full conformance with the		This Internet-Draft is submitted to IETF in full conformance with the
	provisions of BCP 78 and BCP 79. This document may contain material		provisions of BCP 78 and BCP 79. This document may contain material
	from IETF Documents or IETF Contributions published or made publicly		from IETF Documents or IETF Contributions published or made publicly
	available before November 10, 2008. The person(s) controlling the		available before November 10, 2008. The person(s) controlling the
	copyright in some of this material may not have granted the IETF		copyright in some of this material may not have granted the IETF
	Trust the right to allow modifications of such material outside the		Trust the right to allow modifications of such material outside the
	IETF Standards Process. Without obtaining an adequate license from		IETF Standards Process. Without obtaining an adequate license from
	skipping to change at page 1, line 44		skipping to change at page 1, line 44
	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."
	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.
	This Internet-Draft will expire on September 7, 2009.		This Internet-Draft will expire on November 8, 2009.
	Copyright Notice		Copyright Notice
	Copyright (c) 2009 IETF Trust and the persons identified as the		Copyright (c) 2009 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents in effect on the date of		Provisions Relating to IETF Documents in effect on the date of
	publication of this document (http://trustee.ietf.org/license-info).		publication of this document (http://trustee.ietf.org/license-info).
	Please review these documents carefully, as they describe your rights		Please review these documents carefully, as they describe your rights
	skipping to change at page 2, line 30		skipping to change at page 2, line 30
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. Host Behavior . . . . . . . . . . . . . . . . . . . . . . . . 4		2. Host Behavior . . . . . . . . . . . . . . . . . . . . . . . . 4
	3. Host Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 7		3. Host Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 7
	4. Observed Incorrect Implementation Behavior . . . . . . . . . . 9		4. Observed Incorrect Implementation Behavior . . . . . . . . . . 9
	5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 9		5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 9
	6. Security Considerations . . . . . . . . . . . . . . . . . . . 9		6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
	8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 9		8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 10
	9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10		9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
	10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10		10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
	10.1. Normative References . . . . . . . . . . . . . . . . . . 10		10.1. Normative References . . . . . . . . . . . . . . . . . . 10
	10.2. Informative References . . . . . . . . . . . . . . . . . 10		10.2. Informative References . . . . . . . . . . . . . . . . . 10
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
	1. Introduction		1. Introduction
	IPv4 implementations typically associate a netmask with an address		IPv4 implementations typically associate a netmask with an address
	when an IPv4 address is assigned to an interface. That netmask		when an IPv4 address is assigned to an interface. That netmask
	together with the IPv4 address designates an on-link prefix.		together with the IPv4 address designates an on-link prefix.
	Addresses that are covered by this prefix are viewed as on-link i.e.,		Addresses that are covered by this prefix are viewed as on-link i.e.,
	traffic to these addresses is not sent to a router. See section		traffic to these addresses is not sent to a router. See section
	3.3.1 in [RFC1122]. Prior to the deployment of Classless Inter-		3.3.1 in [RFC1122]. Prior to the deployment of Classless Inter-
	Domain Routing (CIDR), an address's netmask could be derived directly		Domain Routing (CIDR), an address's netmask could be derived directly
	from the address. In the absence of specifying a specific netmask		from the address. In the absence of specifying a specific netmask
	when assigning a address, some implementations would fall back to		when assigning an address, some implementations would fall back to
	deriving the netmask from the class of the address.		deriving the netmask from the class of the address.
	The behavior of IPv6 as specified in Neighbor Discovery [RFC4861] is		The behavior of IPv6 as specified in Neighbor Discovery [RFC4861] is
	quite different. The on-link determination is separate from the		quite different. The on-link determination is separate from the
	address assignment. A host can have IPv6 addresses without any		address assignment. A host can have IPv6 addresses without any
	related on-link prefixes or have on-link prefixes that are not		related on-link prefixes or have on-link prefixes that are not
	related to any IPv6 addresses that are assigned to the host. Any		related to any IPv6 addresses that are assigned to the host. Any
	assigned address on an interface should initially be considered as		assigned address on an interface should initially be considered as
	having no internal structure as shown in [RFC4291].		having no internal structure as shown in [RFC4291].
	skipping to change at page 3, line 39		skipping to change at page 3, line 39
	The reception of a Prefix Information Option (PIO) with the L-bit set		The reception of a Prefix Information Option (PIO) with the L-bit set
	[RFC4861] and a non-zero valid lifetime creates (or updates) an entry		[RFC4861] and a non-zero valid lifetime creates (or updates) an entry
	in the Prefix List. All the prefixes that are on the Prefix List,		in the Prefix List. All the prefixes that are on the Prefix List,
	i.e., have not yet timed out, are considered to be on-link.		i.e., have not yet timed out, are considered to be on-link.
	The on-link definition in the Terminology section of [RFC4861], as		The on-link definition in the Terminology section of [RFC4861], as
	modified by this document, defines the complete list of cases where		modified by this document, defines the complete list of cases where
	an address is considered on-link. Individual address entries can be		an address is considered on-link. Individual address entries can be
	expired by the Neighbor Unreachability Detection mechanism.		expired by the Neighbor Unreachability Detection mechanism.
	A host only performs address resolution for IPv6 addresses that are		IPv6 packets sent using the Conceptual Sending Algorithm as described
	on-link. Packets to any other address are sent to a default router.		in [RFC4861] only trigger address resolution for IPv6 addresses that
	If there is no default router, then the node should send an ICMPv6		are on-link. Note that transmission of ND messages is not governed
	Destination Unreachable indication as specified in [RFC4861] - more		by the Conceptual Sending Algorithm. Packets to any other address
	details are provided in the Host Behavior and Rules section. (Note		are sent to a default router. If there is no default router, then
	that [RFC4861] changed the behavior when the Default Router List is		the node should send an ICMPv6 Destination Unreachable indication as
	empty. The behavior in the old version of Neighbor Discovery		specified in [RFC4861] - more details are provided in the Host
	[RFC2461] was different when there were no default routers.)		Behavior and Rules section. (Note that [RFC4861] changed the
			behavior when the Default Router List is empty. The behavior in the
			old version of Neighbor Discovery [RFC2461] was different when there
			were no default routers.) Note that ND is scoped to a single link.
			All Neighbor Solicitaton responses are assumed to be sent our the
			same interface on which the corresponding query was received.
	Failure of host implementations to correctly implement the IPv6		Failure of host implementations to correctly implement the IPv6
	subnet model can result in lack of IPv6 connectivity. See the		subnet model can result in lack of IPv6 connectivity. See the
	Observed Incorrect Implementation Behavior section for details.		Observed Incorrect Implementation Behavior section for details.
			This document deprecates the last two bullets from the definition of
			on-link from [RFC4861] to address security concerns arising from
			particular ND implementations.
	Host behavior is clarified in the Host Behavior and Rules section.		Host behavior is clarified in the Host Behavior and Rules section.
	2. Host Behavior		2. Host Behavior
	1. The original ND specification [RFC4861] was unclear in its usage		1. The original Neighbor Discovery (ND) specification [RFC4861] was
	of the term on-link in a few places. In IPv6, an address is		unclear in its usage of the term on-link in a few places. In
	considered to be on-link (with respect to a specific link), if		IPv6, an address is considered to be on-link (with respect to a
	the address has been assigned to an interface attached to that		specific link), if the address has been assigned to an interface
	link. Any node attached to the link can send a datagram directly		attached to that link. Any node attached to the link can send a
	to an on-link address without forwarding the datagram through a		datagram directly to an on-link address without forwarding the
	router. In IPv6, there are two ways to indicate an address is		datagram through a router. In IPv6, there are two ways to
	on-link. First, a host maintains a Prefix List that identifies		indicate an address is on-link. First, a host maintains a Prefix
	ranges of addresses that are to be considered on-link. Second,		List that identifies ranges of addresses that are to be
	Redirects can identify individual destinations that are on-link;		considered on-link. Second, Redirects can identify individual
	such Redirects update the Destination Cache.		destinations that are on-link; such Redirects update the
			Destination Cache.
	The Prefix List is populated via the following means:		The Prefix List is populated via the following means:
	* Receipt of a Valid RA that specifies a prefix with the L-bit		* Receipt of a Valid Router Advertisement (RA) that specifies a
	set. Such a prefix is considered on-link for a period		prefix with the L-bit set. Such a prefix is considered on-
	specified in the Valid Lifetime and is added to the Prefix		link for a period specified in the Valid Lifetime and is added
	List. (The link-local prefix is effectively considered a		to the Prefix List. (The link-local prefix is effectively
	permanent entry on the Prefix List.)		considered a permanent entry on the Prefix List.)
	* Indication of an on-link prefix (which may be a /128) via		* Indication of an on-link prefix (which may be a /128) via
	manual configuration, or some other yet-to-be specified		manual configuration, or some other yet-to-be specified
	configuration mechanism.		configuration mechanism.
	A Redirect can also signal whether an address is on-link. If a		A Redirect can also signal whether an address is on-link. If a
	host originates a packet, but the first-hop router routes the		host originates a packet, but the first-hop router routes the
	received packet back out onto the same link, the router also		received packet back out onto the same link, the router also
	sends the host a Redirect. If the Target and Destination Address		sends the host a Redirect. If the Target and Destination Address
	of the Redirect are the same, the Target Address is to be treated		of the Redirect are the same, the Target Address is to be treated
	as on-link as specified in Section 8 of [RFC4861]. That is, the		as on-link as specified in Section 8 of [RFC4861]. That is, the
	host updates its Destination Cache (but not its Prefix List --		host updates its Destination Cache (but not its Prefix List --
	though the impact is similar).		though the impact is similar).
	2. Note that Redirects cannot signal that an address is off-link.		2. Note that Redirect Messages do not contain sufficient information
	In section 8.1 of [RFC4861], a Redirect message is silently		to signal that an address is off-link. Rather, they indicate a
	discarded if it does not have an IP source address that is the		preferred next-hop that is a more appropriate choice to use than
	same as the current first-hop router for the specified ICMP		the originator of the Redirect. That alternate next-hop may be
	Destination Address. An ICMP Destination Address on the same		the destination itself (in which case packets would flow directly
	link would have no current first-hop router. Any Redirect		to a neighbor), or a router closer to the destination than the
	message received could not have an IP source address that is the		current next-hop router (which is the originator of the
	same as the current (null) first-hop router, so the Redirect MUST		Redirect). Note, however, that the Redirect message itself does
	be dropped.		not contain sufficient information to distinguish these cases.
			But that does not matter, because the receiver of such a message
			does the same in either case, updating its Neighbor Cache as
			defined in Section 8.1 of [RFC4861].
	3. IPv6 also defines the term "neighbor" and "link" to refer to		3. IPv6 also defines the term "neighbor" and "link" to refer to
	nodes attached to the same link and that can send packets		nodes attached to the same link and that can send packets
	directly to each other. Received ND packets that pass the		directly to each other. Received ND packets that pass the
	required validation tests can only come from a neighbor attached		required validation tests can only come from a neighbor attached
	to the link on which the ND packet was received. Unfortunately,		to the link on which the ND packet was received. Unfortunately,
	[RFC4861] is imprecise in its definition of on-link and states		[RFC4861] is imprecise in its definition of on-link and states
	that a node considers an address to be on-link if:		that a node considers an address to be on-link if:
	- a Neighbor Advertisement message is received for the		- a Neighbor Advertisement message is received for the
	skipping to change at page 5, line 32		skipping to change at page 5, line 45
	Neither of these tests are acceptable definitions for an address		Neither of these tests are acceptable definitions for an address
	to be considered as on-link as defined above, and this document		to be considered as on-link as defined above, and this document
	deprecates and removes both of them from the formal definition of		deprecates and removes both of them from the formal definition of
	on-link. Neither of these tests should be used as justification		on-link. Neither of these tests should be used as justification
	for modifying the Prefix List or Destination Cache for an		for modifying the Prefix List or Destination Cache for an
	address.		address.
	The conceptual sending algorithm of [RFC4861] defines a Prefix		The conceptual sending algorithm of [RFC4861] defines a Prefix
	List and Neighbor Cache. The combination of Prefix List and		List and Neighbor Cache. The combination of Prefix List and
	Neighbor Cache form what many implementations consider to be the		Neighbor Cache form what many implementations consider to be the
	"IP routing table" for a host. Note that the Neighbor Cache is a		IP data forwarding table for a host. Note that the Neighbor
	separate data structure referenced by the Destination Cache, but		Cache is a separate data structure referenced by the Destination
	entries in the Neighbor Cache are not necessarily in the		Cache, but entries in the Neighbor Cache are not necessarily in
	Destination Cache. It is quite possible (and intentional) that		the Destination Cache. It is quite possible (and intentional)
	entries be added to the Neighbor Cache for addresses that would		that entries be added to the Neighbor Cache for addresses that
	not be considered on-link as-defined above. For example, upon		would not be considered on-link as-defined above. For example,
	receipt of a valid NS, Section 7.2.3 of [RFC4861] states:		upon receipt of a valid NS, Section 7.2.3 of [RFC4861] states:
	If an entry does not already exist, the node SHOULD create a		If an entry does not already exist, the node SHOULD create a
	new one and set its reachability state to STALE as specified		new one and set its reachability state to STALE as specified
	in Section 7.3.3. If an entry already exists, and the cached		in Section 7.3.3. If an entry already exists, and the cached
	link-layer address differs from the one in the received Source		link-layer address differs from the one in the received Source
	Link-Layer option, the cached address should be replaced by		Link-Layer option, the cached address should be replaced by
	the received address, and the entry's reachability state MUST		the received address, and the entry's reachability state MUST
	be set to STALE.		be set to STALE.
	The intention of the above feature is to add an address to the		The intention of the above feature is to add an address to the
	skipping to change at page 6, line 35		skipping to change at page 7, line 5
	examination, to be inconsistent with the updated definition of		examination, to be inconsistent with the updated definition of
	on-link because the text does not ensure that the source address		on-link because the text does not ensure that the source address
	is already deemed on-link through other methods:		is already deemed on-link through other methods:
	If the Source Address is not the unspecified address and, on-		If the Source Address is not the unspecified address and, on-
	link layers that have addresses, the solicitation includes a		link layers that have addresses, the solicitation includes a
	Source Link-Layer Address option, then the recipient SHOULD		Source Link-Layer Address option, then the recipient SHOULD
	create or update the Neighbor Cache entry for the IP Source		create or update the Neighbor Cache entry for the IP Source
	Address of the solicitation.		Address of the solicitation.
	Similarly, the following text from section 6.2.5 of [RFC4861]		Similarly, the following text from section 6.2.5 of [RFC4861] may
	may also seem inconsistent: If there is no existing Neighbor		also seem inconsistent:
	Cache entry for the solicitation's sender, the router creates
	one, installs the link- layer address and sets its		If there is no existing Neighbor Cache entry for the
	reachability state to STALE as specified in Section 7.3.3.		solicitation's sender, the router creates one, installs the
			link-layer address and sets its reachability state to STALE as
			specified in Section 7.3.3.
	However, the text in the aforementioned sections of [RFC4861],		However, the text in the aforementioned sections of [RFC4861],
	upon closer inspection, is actually consistent with the		upon closer inspection, is actually consistent with the
	deprecation of the last two bullets of the on-link definition		deprecation of the last two bullets of the on-link definition
	because there are two different ways in which on-link		because there are two different ways in which on-link
	determination can affect the state of ND: through updating the		determination can affect the state of ND: through updating the
	Prefix List or the Neighbor Cache. Through deprecating the last		Prefix List or the Neighbor Cache. Through deprecating the last
	two bullets of the on-link definition, the Prefix List is		two bullets of the on-link definition, the Prefix List is
	explicitly not to be changed when a node receives an NS, NA, or		explicitly not to be changed when a node receives an NS, NA, or
	RS. The Neighbor Cache can still be updated through receipt of		RS. The Neighbor Cache can still be updated through receipt of
	skipping to change at page 7, line 20		skipping to change at page 7, line 38
	the scope of ND processing from one link stays local to that		the scope of ND processing from one link stays local to that
	link. That is, when responding to a NS, the NA would be sent out		link. That is, when responding to a NS, the NA would be sent out
	on the same link on which it was received. Likewise, a host		on the same link on which it was received. Likewise, a host
	would not respond to a received NS for an an address assigned to		would not respond to a received NS for an an address assigned to
	an interface on a different link. Although implementions may		an interface on a different link. Although implementions may
	choose to implement Neighbor Discovery using a single data		choose to implement Neighbor Discovery using a single data
	structure that merges the Neighbor Caches of all interfaces, an		structure that merges the Neighbor Caches of all interfaces, an
	implementation's behavior must be consistent with the above		implementation's behavior must be consistent with the above
	model.		model.
	6. Note that the receipt of a link-local IPv6 multicast packet which
	is not an ND packet indicates direct reachability on a link, but
	is not specifically treated by [RFC4861].
	7. Note that the receipt of a packet with the Hop Limit field
	unchanged (the Hop Limit could be specified in a packet-type
	specific document) which is not an ND packet indicates direct
	reachability on a link, but is not specifically treated by
	[RFC4861].
	3. Host Rules		3. Host Rules
	A correctly implemented IPv6 host MUST adhere to the following rules:		A correctly implemented IPv6 host MUST adhere to the following rules:
	1. The assignment of an IPv6 address, whether through IPv6 stateless		1. The assignment of an IPv6 address, whether through IPv6 stateless
	address autoconfiguration [RFC4862], DHCPv6 [RFC3315], or manual		address autoconfiguration [RFC4862], DHCPv6 [RFC3315], or manual
	configuration MUST NOT implicitly cause a prefix derived from		configuration MUST NOT implicitly cause a prefix derived from
	that address to be treated as on-link and added to the Prefix		that address to be treated as on-link and added to the Prefix
	List. A host considers a prefix to be on-link only through		List. A host considers a prefix to be on-link only through
	explicit means, such as those specified in the on-link definition		explicit means, such as those specified in the on-link definition
	skipping to change at page 8, line 38		skipping to change at page 8, line 40
	Unreachable indication (for example, a locally delivered		Unreachable indication (for example, a locally delivered
	error message) as specified in the Terminology section of		error message) as specified in the Terminology section of
	[RFC4861].		[RFC4861].
	On-link information concerning particular addresses and prefixes		On-link information concerning particular addresses and prefixes
	can make those specific addresses and prefixes on-link, but does		can make those specific addresses and prefixes on-link, but does
	not change the default behavior mentioned above for addresses and		not change the default behavior mentioned above for addresses and
	prefixes not specified. [RFC4943] provides justification for		prefixes not specified. [RFC4943] provides justification for
	these rules.		these rules.
	Using cached on-link determination information without first		4. Hosts MUST verify that on-link information is still valid after
	verifying that the information is still valid after IPv6 interface		IPv6 interface re-initialization before using cached on-link
	re-initialization may lead to lack of IPv6 network connectivity. For		determination information. Failure to do so may lead to lack of
	example, a host receives an RA from a router with on-link prefix A.		IPv6 network connectivity. For example, a host receives an RA
	The host powers down. During the power off, the router sends out		from a router with on-link prefix A. The host powers down.
	prefix A with on-link bit set and a zero lifetime to indicate a		During the power off, the router sends out prefix A with on-link
	renumbering. The host misses the renumbering. The host powers on		bit set and a zero lifetime to indicate a renumbering. The host
	and comes online. Then, the router sends an RA with no PIO. The		misses the renumbering. The host powers on and comes online.
	host uses cached on-link prefix A and issues NS's instead of sending		Then, the router sends an RA with no PIO. The host uses cached
	traffic to a default router. The "Observed Incorrect Implementation		on-link prefix A and issues NS's instead of sending traffic to a
	Behavior" section below describes how this can result in lack of IPv6		default router. The "Observed Incorrect Implementation Behavior"
			section below describes how this can result in lack of IPv6
	connectivity.		connectivity.
	4. Observed Incorrect Implementation Behavior		4. Observed Incorrect Implementation Behavior
	One incorrect implementation behavior illustrates the severe		One incorrect implementation behavior illustrates the severe
	consequences when the IPv6 subnet model is not understood by the		consequences when the IPv6 subnet model is not understood by the
	implementers of several popular host operating systems. In an access		implementers of several popular host operating systems. In an access
	concentrator network ([RFC4388]), a host receives a Router		concentrator network ([RFC4388]), a host receives a Router
	Advertisement Message with no on-link prefix advertised. The host		Advertisement Message with no on-link prefix advertised. The host
	incorrectly assumes an invented prefix is on-link and performs		incorrectly assumes an invented prefix is on-link and performs
	skipping to change at page 9, line 33		skipping to change at page 9, line 39
	address does not imply the existence of corresponding on-link		address does not imply the existence of corresponding on-link
	prefixes. One should also look at API considerations for prefix		prefixes. One should also look at API considerations for prefix
	length as described in last paragraph of section 4.2 of [RFC4903].		length as described in last paragraph of section 4.2 of [RFC4903].
	This document also updates the definition of on-link from [RFC4861]		This document also updates the definition of on-link from [RFC4861]
	by retracting the last two bullets.		by retracting the last two bullets.
	6. Security Considerations		6. Security Considerations
	This document addresses a security concern present in [RFC4861]. As		This document addresses a security concern present in [RFC4861]. As
	a result, the last bullet of the on-link definition in [RFC4861] has		a result, the last bullet of the on-link definition in [RFC4861] has
	been retracted.		been retracted. US-CERT Vulnerability Note VU#472363 lists the
			implementations affected.
	7. IANA Considerations		7. IANA Considerations
	None.		None.
	8. Contributors		8. Contributors
	Thomas Narten contributed significant text and provided substantial		Thomas Narten contributed significant text and provided substantial
	guidance to the production of this document.		guidance to the production of this document.
End of changes. 16 change blocks.
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