--- 1/draft-ietf-6man-ipv6-subnet-model-03.txt 2009-05-07 18:12:06.000000000 +0200 +++ 2/draft-ietf-6man-ipv6-subnet-model-04.txt 2009-05-07 18:12:06.000000000 +0200 @@ -1,20 +1,20 @@ Network Working Group H. Singh Internet-Draft W. Beebee Intended status: Standards Track Cisco Systems, Inc. -Expires: September 7, 2009 E. Nordmark +Expires: November 8, 2009 E. Nordmark Sun Microsystems - March 6, 2009 + May 7, 2009 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 This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from @@ -33,21 +33,21 @@ and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at 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 (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights @@ -65,38 +65,38 @@ Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Host Behavior . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Host Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Observed Incorrect Implementation Behavior . . . . . . . . . . 9 5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 - 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 9 + 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 10 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 10.1. Normative References . . . . . . . . . . . . . . . . . . 10 10.2. Informative References . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 1. Introduction IPv4 implementations typically associate a netmask with an address when an IPv4 address is assigned to an interface. That netmask together with the IPv4 address designates an on-link prefix. 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 3.3.1 in [RFC1122]. Prior to the deployment of Classless Inter- Domain Routing (CIDR), an address's netmask could be derived directly 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. The behavior of IPv6 as specified in Neighbor Discovery [RFC4861] is quite different. The on-link determination is separate from the address assignment. A host can have IPv6 addresses without any related on-link prefixes or have on-link prefixes that are not related to any IPv6 addresses that are assigned to the host. Any assigned address on an interface should initially be considered as having no internal structure as shown in [RFC4291]. @@ -106,79 +106,92 @@ 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 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. The on-link definition in the Terminology section of [RFC4861], as modified by this document, defines the complete list of cases where an address is considered on-link. Individual address entries can be expired by the Neighbor Unreachability Detection mechanism. - A host only performs address resolution for IPv6 addresses that are - on-link. Packets to any other address are sent to a default router. - If there is no default router, then the node should send an ICMPv6 - Destination Unreachable indication as specified in [RFC4861] - more - details are provided in the Host 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.) + IPv6 packets sent using the Conceptual Sending Algorithm as described + in [RFC4861] only trigger address resolution for IPv6 addresses that + are on-link. Note that transmission of ND messages is not governed + by the Conceptual Sending Algorithm. Packets to any other address + are sent to a default router. If there is no default router, then + the node should send an ICMPv6 Destination Unreachable indication as + specified in [RFC4861] - more details are provided in the Host + 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 subnet model can result in lack of IPv6 connectivity. See the 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. 2. Host Behavior - 1. The original ND specification [RFC4861] was unclear in its usage - of the term on-link in a few places. In IPv6, an address is - considered to be on-link (with respect to a specific link), if - the address has been assigned to an interface attached to that - link. Any node attached to the link can send a datagram directly - to an on-link address without forwarding the datagram through a - router. In IPv6, there are two ways to indicate an address is - on-link. First, a host maintains a Prefix List that identifies - ranges of addresses that are to be considered on-link. Second, - Redirects can identify individual destinations that are on-link; - such Redirects update the Destination Cache. + 1. The original Neighbor Discovery (ND) specification [RFC4861] was + unclear in its usage of the term on-link in a few places. In + IPv6, an address is considered to be on-link (with respect to a + specific link), if the address has been assigned to an interface + attached to that link. Any node attached to the link can send a + datagram directly to an on-link address without forwarding the + datagram through a router. In IPv6, there are two ways to + indicate an address is on-link. First, a host maintains a Prefix + List that identifies ranges of addresses that are to be + considered on-link. Second, Redirects can identify individual + destinations that are on-link; such Redirects update the + Destination Cache. The Prefix List is populated via the following means: - * Receipt of a Valid RA that specifies a prefix with the L-bit - set. Such a prefix is considered on-link for a period - specified in the Valid Lifetime and is added to the Prefix - List. (The link-local prefix is effectively considered a - permanent entry on the Prefix List.) + * Receipt of a Valid Router Advertisement (RA) that specifies a + prefix with the L-bit set. Such a prefix is considered on- + link for a period specified in the Valid Lifetime and is added + to the Prefix List. (The link-local prefix is effectively + considered a permanent entry on the Prefix List.) * Indication of an on-link prefix (which may be a /128) via manual configuration, or some other yet-to-be specified configuration mechanism. A Redirect can also signal whether an address is on-link. If a host originates a packet, but the first-hop router routes the received packet back out onto the same link, the router also sends the host a Redirect. If the Target and Destination Address 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 host updates its Destination Cache (but not its Prefix List -- though the impact is similar). - 2. Note that Redirects cannot signal that an address is off-link. - In section 8.1 of [RFC4861], a Redirect message is silently - discarded if it does not have an IP source address that is the - same as the current first-hop router for the specified ICMP - Destination Address. An ICMP Destination Address on the same - link would have no current first-hop router. Any Redirect - message received could not have an IP source address that is the - same as the current (null) first-hop router, so the Redirect MUST - be dropped. + 2. Note that Redirect Messages do not contain sufficient information + to signal that an address is off-link. Rather, they indicate a + preferred next-hop that is a more appropriate choice to use than + the originator of the Redirect. That alternate next-hop may be + the destination itself (in which case packets would flow directly + to a neighbor), or a router closer to the destination than the + current next-hop router (which is the originator of the + Redirect). Note, however, that the Redirect message itself does + 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 nodes attached to the same link and that can send packets directly to each other. Received ND packets that pass the required validation tests can only come from a neighbor attached to the link on which the ND packet was received. Unfortunately, [RFC4861] is imprecise in its definition of on-link and states that a node considers an address to be on-link if: - a Neighbor Advertisement message is received for the @@ -189,27 +202,27 @@ Neither of these tests are acceptable definitions for an address to be considered as on-link as defined above, and this document deprecates and removes both of them from the formal definition of on-link. Neither of these tests should be used as justification for modifying the Prefix List or Destination Cache for an address. The conceptual sending algorithm of [RFC4861] defines a Prefix List and Neighbor Cache. The combination of Prefix List and Neighbor Cache form what many implementations consider to be the - "IP routing table" for a host. Note that the Neighbor Cache is a - separate data structure referenced by the Destination Cache, but - entries in the Neighbor Cache are not necessarily in the - Destination Cache. It is quite possible (and intentional) that - entries be added to the Neighbor Cache for addresses that would - not be considered on-link as-defined above. For example, upon - receipt of a valid NS, Section 7.2.3 of [RFC4861] states: + IP data forwarding table for a host. Note that the Neighbor + Cache is a separate data structure referenced by the Destination + Cache, but entries in the Neighbor Cache are not necessarily in + the Destination Cache. It is quite possible (and intentional) + that entries be added to the Neighbor Cache for addresses that + would not be considered on-link as-defined above. For example, + 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 new one and set its reachability state to STALE as specified 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 option, the cached address should be replaced by the received address, and the entry's reachability state MUST be set to STALE. The intention of the above feature is to add an address to the @@ -234,25 +247,27 @@ examination, to be inconsistent with the updated definition of on-link because the text does not ensure that the source address is already deemed on-link through other methods: If the Source Address is not the unspecified address and, on- link layers that have addresses, the solicitation includes a Source Link-Layer Address option, then the recipient SHOULD create or update the Neighbor Cache entry for the IP Source Address of the solicitation. - Similarly, the following text from section 6.2.5 of [RFC4861] - may also seem inconsistent: If there is no existing Neighbor - Cache entry for the 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. + Similarly, the following text from section 6.2.5 of [RFC4861] may + also seem inconsistent: + + If there is no existing Neighbor Cache entry for the + 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], upon closer inspection, is actually consistent with the deprecation of the last two bullets of the on-link definition because there are two different ways in which on-link determination can affect the state of ND: through updating the Prefix List or the Neighbor Cache. Through deprecating the last two bullets of the on-link definition, the Prefix List is explicitly not to be changed when a node receives an NS, NA, or RS. The Neighbor Cache can still be updated through receipt of @@ -265,30 +280,20 @@ 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 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 an interface on a different link. Although implementions may choose to implement Neighbor Discovery using a single data structure that merges the Neighbor Caches of all interfaces, an implementation's behavior must be consistent with the above 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 A correctly implemented IPv6 host MUST adhere to the following rules: 1. The assignment of an IPv6 address, whether through IPv6 stateless address autoconfiguration [RFC4862], DHCPv6 [RFC3315], or manual configuration MUST NOT implicitly cause a prefix derived from 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 explicit means, such as those specified in the on-link definition @@ -323,31 +328,32 @@ Unreachable indication (for example, a locally delivered error message) as specified in the Terminology section of [RFC4861]. On-link information concerning particular addresses and prefixes can make those specific addresses and prefixes on-link, but does not change the default behavior mentioned above for addresses and prefixes not specified. [RFC4943] provides justification for these rules. - Using cached on-link determination information without first - verifying that the information is still valid after IPv6 interface - re-initialization may lead to lack of IPv6 network connectivity. For - example, a host receives an RA from a router with on-link prefix A. - The host powers down. During the power off, the router sends out - prefix A with on-link bit set and a zero lifetime to indicate a - renumbering. The host misses the renumbering. The host powers on - and comes online. Then, the router sends an RA with no PIO. The - host uses cached on-link prefix A and issues NS's instead of sending - traffic to a default router. The "Observed Incorrect Implementation - Behavior" section below describes how this can result in lack of IPv6 + 4. Hosts MUST verify that on-link information is still valid after + IPv6 interface re-initialization before using cached on-link + determination information. Failure to do so may lead to lack of + IPv6 network connectivity. For example, a host receives an RA + from a router with on-link prefix A. The host powers down. + During the power off, the router sends out prefix A with on-link + bit set and a zero lifetime to indicate a renumbering. The host + misses the renumbering. The host powers on and comes online. + Then, the router sends an RA with no PIO. The host uses cached + on-link prefix A and issues NS's instead of sending traffic to a + default router. The "Observed Incorrect Implementation Behavior" + section below describes how this can result in lack of IPv6 connectivity. 4. Observed Incorrect Implementation Behavior One incorrect implementation behavior illustrates the severe consequences when the IPv6 subnet model is not understood by the implementers of several popular host operating systems. In an access concentrator network ([RFC4388]), a host receives a Router Advertisement Message with no on-link prefix advertised. The host incorrectly assumes an invented prefix is on-link and performs @@ -363,21 +369,22 @@ address does not imply the existence of corresponding on-link prefixes. One should also look at API considerations for prefix length as described in last paragraph of section 4.2 of [RFC4903]. This document also updates the definition of on-link from [RFC4861] by retracting the last two bullets. 6. Security Considerations This document addresses a security concern present in [RFC4861]. As 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 None. 8. Contributors Thomas Narten contributed significant text and provided substantial guidance to the production of this document.