[Search] [pdf|bibtex] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 rfc4903                              Informational
Internet Draft                                              D. Thaler
January 23, 2006                          Internet Architecture Board
Expires July 2007

                        Multilink Subnet Issues

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six
   months 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

   The list of Internet-Draft Shadow Directories can be accessed at

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   There have been several proposals around the notion that a subnet
   may span multiple links connected by routers.  This memo documents
   the issues and potential problems that have been raised with such an

Thaler                    Expires July 2007                         1
Draft                  Multilink Subnet Issues           January 2007

Table of Contents

   1.   Introduction.................................................2
   2.   Issues.......................................................3
   2.1.   IP Model...................................................3
   2.2.   TTL/Hop Limit Issues.......................................4
   2.3.   Link-scoped multicast and broadcast........................6
   2.4.   Duplicate Address Detection Issues.........................7
   3.   Security Considerations......................................8
   4.   Recommendations..............................................8
   4.1.   IP Link Model..............................................8
   4.2.   IPv6 Address Assignment...................................10
   4.3.   Duplicate Address Detection Optimizations.................11
   5.   IANA Considerations.........................................12
   6.   Normative References........................................12
   7.   Informative References......................................13
   IAB Members at the time of this writing..........................15
   Author's Address.................................................16
   Full Copyright Statement.........................................17
   Intellectual Property............................................17

1. Introduction

   The original IPv4 address definition [RFC791] consisted of a Network
   field, identifying a network number, and a Local Address field,
   identifying a host within that network.  As organizations grew to
   want many links within their network, their choices were (from
   [RFC950]) to:

     1. Acquire a distinct Internet network number for each cable;
     subnets are not used at all.

     2. Use a single network number for the entire organization, but
     assign host numbers without regard to which LAN a host is on
     ("transparent subnets").

     3. Use a single network number, and partition the host address
     space by assigning subnet numbers to the LANs ("explicit

   [RFC925] was a proposal for option 2 which defined a specific type
   of ARP proxy behavior, where the forwarding plane had the properties
   of decrementing the TTL to prevent loops when forwarding, not
   forwarding packets destined to, and supporting
   subnet broadcast by requiring that the ARP-based bridge maintain a
   list of recent broadcast packets.  This approach was never
   standardized, although [RFC1027] later documented an implementation
   of a subset of [RFC925].

IAB                     Expires February 2007                       2
Draft                  Multilink Subnet Issues           January 2007

   Instead, the IETF standardized option 3 with [RFC950], whereby hosts
   were required to learn a subnet mask, and this became the IPv4

   Over the recent past there have been several newer protocols
   proposing to extend the notion of a subnet to be able to span
   multiple links, similar to [RFC925].

   Early drafts of the IPv6 scoped address architecture [SCOPID]
   proposed a subnet scope above the link scope, to allow for multi-
   link subnets.  This notion was rejected by the WG due to the issues
   discussed in this memo, and as a result the final version [RFC4007]
   has no such notion.

   There was also a proposal to define multi-link subnets [MLSR] for
   IPv6.  However this notion was abandoned by the IPv6 WG due to the
   issues discussed in this memo, and that proposal was replaced by a
   different mechanism which preserves the notion that a subnet spans
   only one link [RFC4389].

   However, other WGs continued to allow for this concept even though
   it had been rejected in the IPv6 WG.  Mobile IPv6 [RFC3775] allows
   tunnels to mobile nodes to use the same subnet as a home link, with
   the Home Agent doing layer 3 forwarding between them.

   The notion also arises in Mobile Ad-hoc NETworks (MANETs) with
   proposals that an entire MANET is a subnet, with routers doing
   layer 3 forwarding within it.

   The use of multilink subnets has also been considered by other
   working groups, including NetLMM, 16ng, and Autoconf, and by other
   external organizations such as WiMax.

   In this memo we document the issues raised in the IPv6 WG which
   motivated the abandonment of the multi-link subnet concept, so that
   designers of other protocols can (and should) be aware of the

   The key words "MUST", "RECOMMENDED", and "SHOULD" in this document
   are to be interpreted as described in [RFC2119].

2. Issues

2.1. IP Model

   The term "link" is generally used to refer to a topological area
   bounded by routers which decrement the IPv4 TTL or IPv6 Hop Limit
   when forwarding the packet.  A link-local address prefix is defined
   in both IPv4 [RFC3927] and IPv6 [RFC4291].

   The term "subnet" is generally used to refer to a topological area
   that uses the same address prefix, where that prefix is not further
   subdivided except into individual addresses.

IAB                     Expires February 2007                       3
Draft                  Multilink Subnet Issues           January 2007

   In December 1995, the original IP Version 6 Addressing Architecture
   [RFC1884] was published, stating: "IPv6 continues the IPv4 model
   that a subnet is associated with one link.  Multiple subnets may be
   assigned to the same link."

   Thus it explicitly acknowledges that the current IPv4 model has been
   that a subnet is associated with one link, and that IPv6 does not
   change this model.  Furthermore, a subnet is sometimes considered to
   be only a subset of a link, when multiple subnets are assigned to
   the same link.

   The IPv6 addressing architecture has since been updated three times,
   first in July 1998 [RFC2373], then April 2003 [RFC3513], and finally
   in February 2006 [RFC4291].  All updates include the language:
   "Currently IPv6 continues the IPv4 model that a subnet prefix is
   associated with one link.  Multiple subnet prefixes may be assigned
   to the same link."

   Clearly the notion of a multi-link subnet would be a change to the
   existing IP model.

   Similarly, the Mobility Related Terminology [RFC3753] defines a
   Foreign subnet prefix as "A bit string that consists of some number
   of initial bits of an IP address which identifies a node's foreign
   link within the Internet topology" with a similar definition for a
   Home subnet prefix.  These both state that the subnet prefix
   identifies a (singular) link.

2.2. TTL/Hop Limit Issues

   Since a link is bounded by routers that decrement the IPv4 TTL or
   IPv6 Hop Limit, there may be issues with applications and protocols
   that make any assumption about the relationship between TTL/Hop
   Limit and subnet prefix.

   There are two main cases which may arise.  Some applications and
   protocols may send packets with a TTL/Hop Limit of 1.  Other
   applications and protocols may send packets with a TTL/Hop Limit of
   255, and verify that the value is 255 on receipt.  Both are ways of
   limiting communication to within a single link, although the effects
   of these two approaches are quite different.  Setting TTL/Hop Limit
   to 1 ensures that packets that are sent do not leave the link, but
   it does not prevent an off-link attacker from sending a packet that
   can reach the link.  Checking that TTL/Hop Limit is 255 on receipt
   prevents a receiver from accepting packets from an off-link sender,
   but it doesn't prevent a sent packet from being forwarded off-link.

   As for assumptions about the relationship between TTL/Hop Limit and
   subnet, let's look at some example references familiar to many
   protocol and application developers.

   Stevens' "Unix Network Programming, 2nd ed." [UNP] states on page
   490 "a TTL if 0 means node-local, 1 means link-local" (this of

IAB                     Expires February 2007                       4
Draft                  Multilink Subnet Issues           January 2007

   course being true by the definition of link).  Then page 498 states,
   regarding IP_MULTICAST_TTL and IPV6_MULTICAST_HOPS, "If this is not
   specified, both default to 1, which restricts the datagram to the
   local subnet."  Here, Unix programmers learn that TTL=1 packets are
   restricted to a subnet (as opposed to a link).  This is typical of
   many documents which use the terms interchangeably due to the IP
   Model described earlier.

   Similarly, "TCP/IP Illustrated, Volume 1" [TCPILL] states on page
   182: "By default, multicast datagrams are sent with a TTL of 1. This
   restricts the datagram to the same subnet."

   Steve Deering's original multicast README file [DEERING] contained
   the statement "multicast datagrams with initial TTL 1 are restricted
   to the same subnet", and similar statements now appear in many
   vendors' documentation, including documentation for Windows (e.g.,
   [TCPIP2K]) and Linux (e.g., [LINUX] says a TTL of 1 is "Restricted
   to the same subnet. Won't be forwarded by a router.")

   The above are only some examples.  There is no shortage of places
   where application developers are being taught that a subnet is
   confined to a single link, and so we must expect that arbitrary
   applications may embed such assumptions.

   Some examples of protocols today that are known to embed some
   assumption about the relationship between TTL and subnet prefix are:

     o Neighbor Discovery (ND) [RFC2461] uses messages with Hop Limit
       255 checked on receipt, to resolve the link-layer address of any
       IP address in the subnet.

     o Older clients of Apple's Bonjour [MDNS] use messages with TTL
       255 checked on receipt, and only respond to queries from
       addresses in the same subnet.  (Note that multilink subnets do
       not necessarily break this, as this behavior is to constrain
       communication to within a subnet, where a subnet is only a
       subset of a link; however it will not work across a multi-link

   Some other examples of protocols today that are known to use a TTL 1
   or 255, but do not appear to explicitly have any assumption about the
   relationship to subnet prefixes (other than the well-known link-local
   prefix) include:

     o Link-Local Multicast Name Resolution [LLMNR] uses a TTL/Hop
       Limit of 1 for TCP.

     o Multicast Listener Discovery (MLD) [RFC3810] uses a Hop Limit of

     o Reverse tunneling for Mobile IPv4 [RFC3024] uses TTL 255 checked
       on receipt for Registration Requests sent to foreign agents.

IAB                     Expires February 2007                       5
Draft                  Multilink Subnet Issues           January 2007

     o [RFC3927] discusses the use of TTL=1 and TTL=255 within the IPv4
       link-local address prefix.

   It is unknown whether any implementations of such protocols exist
   that add such assumptions about the relationship to subnet prefixes
   for other reasons.

2.3. Link-scoped multicast and broadcast

   Because multicast routing is not ubiquitous, the notion of a subnet
   which spans multiple links tends to result in cases where multicast
   does not work across the subnet.  Per [RFC2644], the default
   behavior is that routers do not forward directed broadcast packets
   either, nor do they forward limited broadcasts (see [RFC1812]

   There are many protocols and applications today that use link-scoped
   multicast.  The list of such applications and protocols that have
   been assigned their own link-scoped multicast group address (and may
   also have assumptions about the TTL/Hop Limit as noted above) can be
   found at:



   In addition, an arbitrarily large number of other applications may
   be using the all-1's broadcast address, or the all-hosts link-scoped
   multicast address, rather than their own group address.

   The well-known examples of protocols using link-scoped multicast or
   broadcast generally fall into one of the following groups:

     o Routing protocols: DVMRP [DVMRP], OSPF [RFC2328], RIP
       [RFC2453][RFC2080], EIGRP [EIGRP], etc.  These protocols
       exchange routes to subnet prefixes.

     o Address management protocols: Neighbor Discovery, DHCPv4
       [RFC2131], DHCPv6 [RFC3315], Teredo [RFC4380], etc.  By their
       nature this group tends to embed assumptions about the
       relationship between a link and a subnet prefix.  For example,
       ND uses link-scoped multicast to resolve the link-layer address
       of an IP address in the same subnet prefix, and to do duplicate
       address detection (see section 2.4 below) within the subnet.
       DHCP uses link-scoped multicast or broadcast to obtain an
       address in the subnet.  Teredo states: "An IPv4 multicast
       address used to discover other Teredo clients on the same IPv4
       subnet.  The value of this address is", which is a
       link-scoped multicast address.  It also says "the client MUST
       silently discard all local discovery bubbles [...] whose IPv4
       source address does not belong to the local IPv4 subnet".

IAB                     Expires February 2007                       6
Draft                  Multilink Subnet Issues           January 2007

     o Service discovery protocols: SSDP [SSDP], Bonjour, WS-Discovery
       [WSDISC], etc.  These often do not define any explicit
       assumption about the relationship to subnet prefix.

     o Name resolution protocols: NetBios [RFC1001], Bonjour, LLMNR,
       etc.  Most often these do not define any explicit assumption
       about the relationship to subnet prefix, but Bonjour only
       responds to queries from addresses within the same subnet

   Note that protocols such as Bonjour and Teredo which drop packets
   which don't come from an address within the subnet are not
   necessarily broken by multilink subnets, as this behavior is meant to
   constrain the behavior to within a subnet, when a link is larger than
   a single subnet.

   However, regardless of whether any assumption about the relationship
   to subnet prefixes exists, all protocols mentioned above or on the
   IANA assignments lists will not work across a multilink subnet
   without protocol-specific proxying functionality in routers, and
   adding proxying for an arbitrary number of protocols and applications
   does not scale.  Furthermore, it may hinder the development and use
   of future protocols using link-scoped multicast.

2.4. Duplicate Address Detection Issues

   Duplicate Address Detection (DAD) uses link-scoped multicast in IPv6
   and link-scoped broadcast in IPv4 and so has the issues mentioned in
   Section 2.3 above.

   In addition, [RFC2462] contains the statement:

     "Thus, for a set of addresses formed from the same interface
     identifier, it is sufficient to check that the link-local address
     generated from the identifier is unique on the link. In such
     cases, the link-local address MUST be tested for uniqueness, and
     if no duplicate address is detected, an implementation MAY choose
     to skip Duplicate Address Detection for additional addresses
     derived from the same interface identifier."

   The last possibility, sometimes referred to as Duplicate Interface
   Identifier Detection (DIID), has been a matter of much debate, and
   the current draft in progress [2462BIS] states:

     Each individual unicast address SHOULD be tested for uniqueness.
     Note that there are implementations deployed that only perform
     Duplicate Address Detection for the link-local address and skip
     the test for the global address using the same interface
     identifier as that of the link-local address.  Whereas this
     document does not invalidate such implementations, this kind of
     "optimization" is NOT RECOMMENDED, and new implementations MUST
     NOT do that optimization.

IAB                     Expires February 2007                       7
Draft                  Multilink Subnet Issues           January 2007

   The existence of such implementations also causes problems with
   multilink subnets.  Specifically, a link-local address is only valid
   within a link, and hence is only tested for uniqueness within a
   single link.  If the same interface identifier is then assumed to be
   unique across all links within a multilink subnet, address conflicts
   can occur.

3. Security Considerations

   The notion of multilink subnets can cause problems with any security
   protocols which either rely on the assumption that a subnet only
   spans a single link, or can leave gaps in the security solution
   where protocols are only defined for use on a single link.

   Secure Neighbor Discovery (SEND) [RFC3971], in particular, is
   currently only defined within a single link.  If a subnet were to
   span multiple links, SEND would not work as currently specified,
   since it secures Neighbor Discovery messages which include link-
   layer addresses, and if forwarded to other links, the link-layer
   address of the sender will be different.  This same problem also
   exists in cases where a subnet does not span multiple links but
   where Neighbor Discovery is proxied within a link.  Section 9 of
   [RFC4389] discusses some possible future directions in this regard.

   Furthermore, as noted above some applications and protocols (ND,
   Bonjour, Mobile IPv4, etc.) mitigate against off-link spoofing
   attempts by requiring a TTL or Hop Limit of 255 on receipt.  If this
   restriction were removed, or if alternative protocols were used,
   then off-link spoofing attempts would become easier, and some
   alternative way to mitigate such attacks would be needed.

4. Recommendations

4.1. IP Link Model

   There are two models which do not have the issues pointed out in the
   rest of the document.

   The IAB recommends that protocol designers use one of the following
   two models:

    o Multiaccess link model: In this model, there can be multiple
      nodes on the same link, including zero or more routers.  Data
      packets sent to the IPv4 link-local broadcast address
      ( or to a link-local multicast address can be
      received by all other interested nodes on the link.  Two nodes on
      the link are able to communicate without any IPv4 TTL or IPv6 Hop
      Limit decrement.  There can be any number of layer 2 devices
      (bridges, switches, access points, whatever) in the middle of the

IAB                     Expires February 2007                       8
Draft                  Multilink Subnet Issues           January 2007

    o Point-to-point link model: In this model, there are exactly two
      nodes on the same link.  Data packets sent to the IPv4 link-local
      broadcast address or to a link-local multicast address can be
      received by the other node on the link.  The two nodes are able
      to communicate without any IPv4 TTL or IPv6 Hop Limit decrement.
      There can be any number of layer 2 devices (bridges, switches,
      access points, whatever) in the middle of the link.

   A variant of the multi-access link model, which has fewer issues,
   but still some, is:

    o Non-broadcast multi-access (NBMA) model: Same as the multi-access
      link model, except that no broadcast or multicast packets can be
      sent, even between two nodes on the same link.  As a result, no
      protocols or applications which make use of broadcast or
      multicast will work.

   Links that appear as NBMA links at layer 3 are problematic.
   Instead, if a link is an NBMA link at layer 2, then protocol
   designers should define some mechanism such that it appears as
   either the multi-access link model or point-to-point link model at
   layer 3.

   One use of an NBMA link is when the link itself is intended as a
   wide-area link (e.g., a tunnel such as 6to4 [RFC3056]) where none of
   the groups of functionality in section 2.3 are required across the
   wide area.  Admittedly, the definition of wide-area is somewhat
   subjective.  Support for multicast on a wide-area link would be
   analogous to supporting multicast routing across a series of local-
   area links.  The issues discussed in section 2.3 will arise, but may
   be acceptable over a wide area until multicast routing is also

   Note that the distinction of whether a link is a tunnel or not is
   orthogonal to the choice of model; there exist tunnel links for all
   link models mentioned above.

   A multilink subnet model should be avoided.  IETF working groups
   using, or considering using, multilink subnets today should
   investigate moving to one of the other models.  For example, the
   Mobile IPv6 WG should investigate having the Home Agent not
   decrement the Hop Limit, and forward multicast traffic.

   When considering changing an existing multilink subnet solution to
   another model, the following issues should be considered:

IAB                     Expires February 2007                       9
Draft                  Multilink Subnet Issues           January 2007

   Loop prevention: If physical loops cannot exist within the subnet,
        then removing the TTL/Hop Limit decrement is not an issue.
        Otherwise, protocol designers can (for example) retain the
        decrement but use a separate prefix per link, or use some form
        of bridging protocol instead (e.g., [BRIDGE] or [RBRIDGE]).

   Limiting broadcast (including all-hosts multicast): If there is no
        efficiency requirement to prevent broadcast from going to other
        on-link hosts, then flooding it within the subnet is not an
        issue.  Otherwise, protocol designers can (for example) use a
        separate prefix per link, or flood broadcast other than ARP
        within the subnet (ARP is covered below in section 4.3).

   Limiting the scope of other multicast (including IPv6 Neighbor
        Discovery): If there is no efficiency requirement to prevent
        multicast from going to other on-link hosts, then flooding
        multicast within the subnet is not an issue.  Otherwise,
        protocol designers can (for example) use a separate prefix per
        link, or use IGMP/MLD snooping [RFC4541] instead.

4.2. IPv6 Address Assignment

   In IPv6, the Prefix Information Option in a Router Advertisement
   (RA) is defined for use by a router to advertise an on-link prefix.
   That is, it indicates that a prefix is assigned to the link over
   which the RA is sent/received.  That is, the router and the node
   both have an on-link route in their routing table (or on-link Prefix
   List, in the conceptual model of a host in [RFC2461]), and any
   addresses used in the prefix are assigned to an interface (on any
   node) attached to that.

   In contrast, DHCPv6 Prefix Delegation (DHCP-PD) [RFC3633] is defined
   for use by a client to request a prefix for use on a different
   link.  Section 12.1 of RFC 3633 states:

     Upon the receipt of a valid Reply message, for each IA_PD the
     requesting router assigns a subnet from each of the delegated
     prefixes to each of the links to which the associated interfaces
     are attached, with the following exception: the requesting router
     MUST NOT assign any delegated prefixes or subnets from the
     delegated prefix(es) to the link through which it received the
     DHCP message from the delegating router.

   Hence, the upstream router has a route in its routing table that is
   not on-link, but points to the client; the prefix is assigned to a
   link other than the one over which DHCP-PD was done; and any

IAB                     Expires February 2007                      10
Draft                  Multilink Subnet Issues           January 2007

   addresses used in the prefix are assigned to an interface (on any
   node) attached to that other link.

   The IAB believes that the distinction between these two cases
   (assigning a prefix to the same link vs. another link) is important,
   and that the IETF protocols noted above are appropriate for the two
   scenarios noted.  The IAB recommends that other protocol designers
   remain consistent with the IETF-defined scopes of these protocols
   (e.g., not using DHCP-PD to assign a prefix to the same link, or
   using RAs to assign a prefix to another link).

   In addition, the Prefix Information Option contains an L (on-link)
   flag.  Normally this flag is set, indicating that this prefix can be
   used for on-link determination.  When not set the advertisement
   makes no statement about on-link or off-link properties of the
   prefix.  For instance, the prefix might be used for address
   configuration with some of the addresses belonging to the prefix
   being on-link and others being off-link.  Care must be taken when
   the L flag is not set.  Specifically, some platforms allow
   applications to retrieve the prefix length associated with each
   address of the node.  If an implementation were to return the prefix
   length used for address configuration, then applications may
   incorrectly assume that TTL=1 is sufficient for communication, and
   that link-scoped multicast will reach other addresses in the prefix.
   As a result, the IAB recommends to designers and maintainers of APIs
   that provide a prefix length to applications, that they address this
   issue.  For example, they might indicate that no prefix length
   exists when the prefix is not on-link.  If the API is not capable of
   reporting that one does not exist, then they might choose to report
   a value of 128 when the prefix is not on-link.  This would result in
   such applications believing they are on separate subnets, rather
   than on a multilink subnet.

4.3. Duplicate Address Detection Optimizations

   One of the reasons sometimes cited for wanting a multilink subnet
   model (rather than a multi-access link model), is to minimize the
   ARP/ND traffic between end-nodes.  This is primarily a concern in
   IPv4 where ARP results in a broadcast that would be seen by all
   nodes, not just the node with the IPv4 address being resolved.  Even
   if this is a significant concern, the use of a multilink subnet
   model is not necessary.  The point-to-point link model is one way to
   avoid this issue entirely.

   In the multi-access link model, IPv6 ND traffic can be reduced by
   using well-known multicast learning techniques (e.g., [RFC4541] at a

IAB                     Expires February 2007                      11
Draft                  Multilink Subnet Issues           January 2007

   layer 2 intermediate device (bridge, switch, access point,

   Some have suggested that a layer 2 device could maintain an ARP or
   ND cache and service requests from that cache.  However, such a
   cache prevents any type of fast mobility between layer 2 ports, and
   breaks Secure Neighbor Discovery [RFC3971].  As a result, the IAB
   recommends to protocol designers that this approach be avoided,
   instead using an alternative such as layer 2 learning.  For IPv4
   (where no Secure ARP exists) the IAB recommends that protocol
   designers avoid having a device respond from its cache in cases
   where a node can legitimately move between layer 2 segments of the
   link without any layer 2 indications at the layer 2 intermediate
   device.  Also, since currently there is no guarantee that any device
   other than the end host knows all addresses of the end host,
   protocol designers should avoid any dependency on such an
   assumption.  For example, when no cache entry for a given request is
   found, protocol designers may specify that a node broadcast the
   request to all nodes.

5. IANA Considerations

   This document has no actions for IANA.

6. Normative References

   [RFC791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September

   [RFC950]  Mogul, J. and J. Postel, "Internet Standard Subnetting
             Procedure", STD 5, RFC 950, August 1985.

   [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC
             1812, June 1995.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", RFC 2461, December

   [RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
             Autoconfiguration", RFC 2462, December 1998.

   [RFC2644] Senie, D., "Changing the Default for Directed Broadcasts
             in Routers", BCP 34, RFC 2644, August 1999.

IAB                     Expires February 2007                      12
Draft                  Multilink Subnet Issues           January 2007

   [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
             Host Configuration Protocol (DHCP) version 6", RFC 3633,
             December 2003.

   [RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
             Configuration of IPv4 Link-Local Addresses", RFC 3927, May

   [RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
             "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4007] Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
             B. Zill. "IPv6 Scoped Address Architecture", RFC 4007,
             March 2005.

   [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
             Architecture", RFC 4291, February 2006.

   [RFC4541] Christensen, M., Kimball, K., and F. Solensky,
             "Considerations for Internet Group Management Protocol
             (IGMP) and Multicast Listener Discovery (MLD) Snooping
             Switches", RFC 4541, May 2006.

7. Informative References

   [2462BIS] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
             Address Autoconfiguration", draft-ietf-ipv6-rfc2462bis-
             08.txt, May 2005.

   [BRIDGE]  T. Jeffree, editor, "Media Access Control (MAC) Bridges",
             ANSI/IEEE Std 802.1D, 2004, http://standards.ieee.org/

   [DEERING] Deering, S., "IP Multicast Extensions for 4.3BSD UNIX and
             related systems (MULTICAST 1.2 Release)", June 1989.

   [DVMRP]   Waitzman, D., Partridge, C., and S. Deering, "Distance
             Vector Multicast Routing Protocol", RFC 1075, November

   [EIGRP]   Cisco, "Enhanced Interior Gateway Routing Protocol", Cisco
             Document ID 16406, September 2005.

   [LINUX]   Juan-Mariano de Goyeneche, "Multicast over TCP/IP HOWTO",
             March 1998.  http://www.linux.com/howtos/Multicast-HOWTO-

   [LLMNR]   Aboba, B., Thaler, D. and L. Esibov, "Linklocal Multicast
             Name Resolution (LLMNR)", draft-ietf-dnsext-mdns-47.txt,
             August 2006.

IAB                     Expires February 2007                      13
Draft                  Multilink Subnet Issues           January 2007

   [MDNS]    Cheshire, S. and M. Krochmal, "Multicast DNS", Internet
             Draft, June 2005.  http://files.multicastdns.org/draft-

   [MLSR]    Thaler, D. and C. Huitema, "Multi-link Subnet Support in
             IPv6", draft-ietf-ipv6-multilink-subnets-00.txt (expired),
             June 2002.  http://www.ietf.org/proceedings/02jul/I-

   [RBRIDGE] Perlman, R., and J. Touch, "Rbridges: Base Protocol
             Specification", draft-ietf-trill-rbridge-protocol-00.txt,
             May 2006.

   [RFC925]  Postel, J., "Multi-LAN address resolution", RFC 925,
             October 1984.

   [RFC1001] NetBIOS Working Group in the Defense Advanced Research
             Projects Agency, Internet Activities Board, End-to-End
             Services Task Force, "Protocol standard for a NetBIOS
             service on a TCP/UDP transport: Concepts and methods", RFC
             1001, March 1987.

   [RFC1027] Carl-Mitchell, S. and J. Quarterman, "Using ARP to
             implement transparent subnet gateways", RFC 1027, October

   [RFC1884] Hinden, R. and S. Deering, "IP Version 6 Addressing
             Architecture", RFC 1884, December 1995.

   [RFC2080] Malkin, G. and R. Minnear, "RIPng for IPv6", RFC 2080,
             January 1997.

   [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
             2131, March 1997.

   [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC2373] R. Hinden, S. Deering, "IP Version 6 Addressing
             Architecture", RFC 2373, July 1998.

   [RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453, November

   [RFC3024] G. Montenegro, Ed., "Reverse Tunneling for Mobile IP,
             revised", RFC 3024, January 2001.

   [RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
             via IPv4 Clouds", RFC 3056, February 2001.

   [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.

IAB                     Expires February 2007                      14
Draft                  Multilink Subnet Issues           January 2007

   [RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
             (IPv6) Addressing Architecture", RFC 3513, April 2003.

   [RFC3753] J. Manner, Ed., M. Kojo, Ed., "Mobility Related
             Terminology", RFC 3753, June 2004.

   [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
             in IPv6", RFC 3775, June 2004.

   [RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
             Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC4380] C. Huitema, "Teredo: Tunneling IPv6 over UDP through
             Network Address Translations (NATs)", RFC 4380, February

   [RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
             Proxies (ND Proxy)", RFC 4389, February 2006.

   [SCOPID]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., Onoe,
             A., and B. Zill, "IPv6 Scoped Address Architecture",
             Internet-Draft (Obsolete), March 2005.

   [SSDP]    Goland, Yaron Y., Cai, T., Leach, P., Gu, Y., and S.
             Albright, "Simple Service Discovery Protocol (SSDP)",
             1999.  http://www.upnp.org/resources/specifications.asp

   [TCPILL]  Stevens, W. Richard, "TCP/IP Illustrated, Volume 1",
             Addison-Wesley, 1994.

   [TCPIP2K] MacDonald, D. and W. Barkley, "Microsoft Windows 2000
             TCP/IP Implementation Details".

   [UNP]     Stevens, W. Richard, "Unix Network Programming, Volume 1,
             Second Edition", Prentice Hall, 1998.

   [WSDISC]  Microsoft, "Web Services Dynamic Discovery (WS-
             Discovery)", 2005.

IAB Members at the time of this writing

   Bernard Aboba
   Loa Andersson
   Brian Carpenter
   Leslie Daigle
   Elwyn Davies

IAB                     Expires February 2007                      15
Draft                  Multilink Subnet Issues           January 2007

   Kevin Fall
   Olaf Kolkman
   Kurtis Lindqvist
   David Meyer
   David Oran
   Eric Rescorla
   Dave Thaler
   Lixia Zhang

Author's Address

   Dave Thaler
   One Microsoft Way
   Redmond, WA 98052
   Phone: +1 425 703 8835
   Email: dthaler@microsoft.com

IAB                     Expires February 2007                      16
Draft                  Multilink Subnet Issues           January 2007

Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed
   to pertain to the implementation or use of the technology described
   in this document or the extent to which any license under such
   rights might or might not be available; nor does it represent that
   it has made any independent effort to identify any such rights.
   Information on the procedures with respect to rights in RFC
   documents can be found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository
   at http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-

IAB                     Expires February 2007                      17