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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5942

Network Working Group                                           H. Singh

Internet-Draft                                                 W. Beebee

Updates: 4861 (if approved)                          Cisco Systems, Inc.

Intended status: Standards Track                             E. Nordmark

Expires: September 10, 2010                             Sun Microsystems

                                                           March 9, 2010



 IPv6 Subnet Model: the Relationship between Links and Subnet Prefixes

                  draft-ietf-6man-ipv6-subnet-model-09


Abstract


   IPv6 specifies a model of a subnet that is different than the IPv4

   subnet model.  The subtlety of the differences has resulted in

   incorrect implementations that do not interoperate.  This document

   spells out the most important difference; that an IPv6 address isn't

   automatically associated with an IPv6 on-link prefix.  This document

   also updates (partially due to security concerns caused by incorrect

   implementations) a part of the definition of on-link from [RFC4861].


Status of this Memo


   This Internet-Draft is submitted to IETF in full conformance with the

   provisions of BCP 78 and BCP 79.


   Internet-Drafts are working documents of the Internet Engineering

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   This Internet-Draft will expire on September 10, 2010.


Copyright Notice


   Copyright (c) 2010 IETF Trust and the persons identified as the

   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal

   Provisions Relating to IETF Documents

   (http://trustee.ietf.org/license-info) in effect on the date of

   publication of this document.  Please review these documents

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   This document may contain material from IETF Documents or IETF

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Table of Contents


   1.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  3

   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3

   3.  Host Behavior  . . . . . . . . . . . . . . . . . . . . . . . .  4

   4.  Host Rules . . . . . . . . . . . . . . . . . . . . . . . . . .  8

   5.  Observed Incorrect Implementation Behavior . . . . . . . . . .  9

   6.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 10

   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10

   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10

   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 10

   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10

   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11

     11.1.  Normative References  . . . . . . . . . . . . . . . . . . 11

     11.2.  Informative References  . . . . . . . . . . . . . . . . . 11

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12




































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1.  Requirements Language


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

   document are to be interpreted as described in RFC 2119 [RFC2119].



2.  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.  Nodes

   consider addresses covered by an on-link prefix to be directly

   attached to the same link as the sending node, i.e., they send

   traffic for such addresses directly rather than to a router.  See

   section 3.3.1 in [RFC1122].  Prior to the development of subnetting

   [RFC0950] and Classless Inter-Domain Routing (CIDR) [RFC1519], an

   address's netmask could be derived directly from the address simply

   by determining whether it was a Class A, B or C address.  Today,

   assigning an address to an interface also requires specifying a

   netmask to use.  In the absence of specifying a specific netmask 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 has 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].


   In IPv6, by default, a host treats only the link-local prefix as on-

   link.


   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 prefixes on a host's Prefix List, i.e., have

   not yet timed out, are considered to be on-link by that host.


   The on-link definition in the Terminology section of [RFC4861], as

   modified by this document, defines the complete list of cases where a

   host considers an address to be on-link.  Individual address entries

   can be expired by the Neighbor Unreachability Detection mechanism.


   IPv6 packets sent using the Conceptual Sending Algorithm as described

   in [RFC4861] only trigger address resolution for IPv6 addresses that

   the sender considers to be on-link.  Packets to any other address are




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   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.  In the old version

   of Neighbor Discovery [RFC2461], if the Default router List is empty,

   rather than sending the ICMPv6 Destination Unreachable indication,

   the [RFC2461] node assumed that the destination was on-link.")  Note

   that ND is scoped to a single link.  All Neighbor Solicitation

   responses are assumed to be sent out the same interface on which the

   corresponding query was received without using the Conceptual Sending

   Algorithm.


   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.



3.  Host Behavior


   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 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.  However, in order for a node to know that a destination

       is on-link, it must obtain configuration information to that

       effect.  In IPv6, there are two main ways of maintaining

       information about on-link destinations.  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:








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       *  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.  It should be noted that ND does not have a way to indicate a

       destination is "off-link".  Rather, a destination is assumed to

       be off-link, unless there is explicit information indicating that

       it is on-link.  Such information may later expire or be changed,

       in which case a destination may revert back to being considered

       off-link, but that is different than there being an explicit

       mechanism for signaling that a destination is off-link.  Redirect

       Messages do not contain sufficient information to signal that an

       address is off-link.  Instead, Redirect Messages indicate a

       preferred next-hop that is a more appropriate choice to use than

       the originator of the Redirect.




   3.  IPv6 also defines the term "neighbor" 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

          (target) address, or





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          - any Neighbor Discovery message is received from the address.


       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, Destination Cache, and Default Router List.  The

       combination of Prefix List, Destination Cache, and Default Router

       List form what many implementations consider to be the 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

       Neighbor Cache, even though it might not be considered on-link

       per the Prefix List.  The benefit of such a step is to have the

       receiver populate the Neighbor Cache with an address it will

       almost certainly be sending packets to shortly, thus avoiding the

       need for an additional round of ND to perform address resolution.

       But because there is no validation of the address being added to

       the Neighbor Cache, an intruder could spoof the address and cause

       a receiver to add an address for a remote site to its Neighbor

       Cache.  This vulnerability is a specific instance of the broad

       set of attacks that are possible by an on-link neighbor

       [RFC3756].  This causes no problems in practice, so long as the

       entry only exists in the Neighbor Cache and the address is not

       considered to be on-link by the IP forwarding code (i.e., the

       address is not added to the Prefix List and is not marked as on-




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       link in the Destination Cache).




   4.  After the update to the on-link definition in [RFC4861], certain

       text from section 7.2.3 of [RFC4861] may appear, upon a cursory

       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.


       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 Destination 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

       an NS, NA, or RS.




   5.  [RFC4861] is written from the perspective of a host with a single

       interface on which Neighbor Discovery is run.  All ND traffic

       (whether sent or received) traverses the single interface.  On

       hosts with multiple interfaces, care must be taken to ensure 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

       on the same link on which it was received.  Likewise, a host

       would not respond to a received NS for an address assigned to an

       interface on a different link.  Although implementations may

       choose to implement Neighbor Discovery using a single data

       structure that merges the Neighbor Caches of all interfaces, an




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       implementation's behavior must be consistent with the above

       model.



4.  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

       in the Terminology section of [RFC4861], as modified by this

       document, or via manual configuration.  Note that the requirement

       for manually configured addresses is not explicitly mentioned in

       [RFC4861].




   2.  In the absence of other sources of on-link information, including

       Redirects, if the RA advertises a prefix with the on-link(L) bit

       set and later the Valid Lifetime expires, the host MUST then

       consider addresses of the prefix to be off-link, as specified by

       the PIO paragraph of section 6.3.4 of [RFC4861].




   3.  In the absence of other sources of on-link information, including

       Redirects, if the RA advertises a prefix with the on-link(L) bit

       set and later the Valid Lifetime expires, the host MUST then

       update its Prefix List with respect to the entry.  In most cases,

       this will result in the addresses covered by the prefix

       defaulting back to being considered off-link, as specified by the

       PIO paragraph of section 6.3.4 of [RFC4861].  However, there are

       cases where an address could be covered by multiple entries in

       the Prefix List, where expiration of one prefix would result in

       destinations then being covered by a different entry.




   4.  Implementations compliant with [RFC4861] MUST adhere to the

       following rules.  If the Default Router List is empty and there

       is no other source of on-link information about any address or

       prefix:






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       1.  The host MUST NOT assume that all destinations are on-link.


       2.  The host MUST NOT perform address resolution for non-link-

           local addresses.


       3.  Since the host cannot assume the destination is on-link, and

           off-link traffic cannot be sent to a default router (since

           the Default Router List is empty), address resolution cannot

           be performed.  This case is specified in the last paragraph

           of section 4 of [RFC4943]: when there is no route to

           destination, the host should send an ICMPv6 Destination

           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.




   5.  Hosts MUST verify that on-link information is still valid after

       IPv6 interface re-initialization.  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.



5.  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.  This invented

   prefix typically is a /64 that was written by the developer of the

   API as a "default" prefix length when a length isn't specified.  This

   may cause the API to seem to work in the case of a network interface




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   initiating SLAAC, however it can cause connectivity problems in NBMA

   networks.  Having incorrectly assumed an invented prefix, the host

   performs address resolution when the host should send all non-link-

   local traffic to a default router.  Neither the router nor any other

   host will respond to the address resolution, preventing this host

   from sending IPv6 traffic.



6.  Conclusion


   This document clarifies and summarizes the relationship between links

   and subnet prefixes described in [RFC4861].  Configuration of an IPv6

   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.



7.  Security Considerations


   This document addresses a security concern present in [RFC4861].  As

   a result, the last two bullets of the on-link definition in [RFC4861]

   have been retracted.  US-CERT Vulnerability Note VU#472363 lists the

   implementations affected.



8.  IANA Considerations


   None.



9.  Contributors


   Thomas Narten contributed significant text and provided substantial

   guidance to the production of this document.



10.  Acknowledgements


   Thanks (in alphabetical order) to Adeel Ahmed, Jari Arkko, Ralph

   Droms, Alun Evans, Dave Forster, Prashanth Krishnamurthy, Suresh

   Krishnan, Josh Littlefield, Bert Manfredi, David Miles, Madhu Sudan,

   Jinmei Tatuya, Dave Thaler, Bernie Volz, and Vlad Yasevich for their

   consistent input, ideas and review during the production of this

   document.  The security problem related to an NS message that

   provides one reason for invalidating a part of the on-link definition

   was found by David Miles.  Jinmei Tatuya found the security problem




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   to also exist with an RS message.



11.  References


11.1.  Normative References


   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate

              Requirement Levels", BCP 14, RFC 2119, March 1997.


   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,

              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,

              September 2007.


11.2.  Informative References


   [RFC0950]  Mogul, J. and J. Postel, "Internet Standard Subnetting

              Procedure", STD 5, RFC 950, August 1985.


   [RFC1122]  Braden, R., "Requirements for Internet Hosts -

              Communication Layers", STD 3, RFC 1122, October 1989.


   [RFC1519]  Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless

              Inter-Domain Routing (CIDR): an Address Assignment and

              Aggregation Strategy", RFC 1519, September 1993.


   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor

              Discovery for IP Version 6 (IPv6)", RFC 2461,

              December 1998.


   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,

              and M. Carney, "Dynamic Host Configuration Protocol for

              IPv6 (DHCPv6)", RFC 3315, July 2003.


   [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor

              Discovery (ND) Trust Models and Threats", RFC 3756,

              May 2004.


   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing

              Architecture", RFC 4291, February 2006.


   [RFC4388]  Woundy, R. and K. Kinnear, "Dynamic Host Configuration

              Protocol (DHCP) Leasequery", RFC 4388, February 2006.


   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless

              Address Autoconfiguration", RFC 4862, September 2007.


   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903,




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              June 2007.


   [RFC4943]  Roy, S., Durand, A., and J. Paugh, "IPv6 Neighbor

              Discovery On-Link Assumption Considered Harmful",

              RFC 4943, September 2007.



Authors' Addresses


   Hemant Singh

   Cisco Systems, Inc.

   1414 Massachusetts Ave.

   Boxborough, MA  01719

   USA


   Phone: +1 978 936 1622

   Email: shemant@cisco.com

   URI:   http://www.cisco.com/



   Wes Beebee

   Cisco Systems, Inc.

   1414 Massachusetts Ave.

   Boxborough, MA  01719

   USA


   Phone: +1 978 936 2030

   Email: wbeebee@cisco.com

   URI:   http://www.cisco.com/



   Erik Nordmark

   Sun Microsystems

   17 Network Circle

   Menlo Park, CA 94025

   USA


   Phone: +1 650 786 2921

   Email: erik.nordmark@sun.com













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