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Network Working Group                                          D. ThalerRequest for Comments: 4903                   Internet Architecture BoardCategory: Informational                                        June 2007Multi-Link Subnet IssuesStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The IETF Trust (2007).Abstract   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   approach.Table of Contents1. Introduction ....................................................22. Issues ..........................................................32.1. IP Model ...................................................32.2. TTL/Hop Limit Issues .......................................42.3. Link-scoped Multicast and Broadcast ........................62.4. Duplicate Address Detection Issues .........................73. Security Considerations .........................................84. Recommendations .................................................94.1. IP Link Model ..............................................94.2. IPv6 Address Assignment ...................................104.3. Duplicate Address Detection Optimizations .................125. Normative References ...........................................126. Informative References .........................................13Thaler                       Informational                      [Page 1]

RFC 4903                Multi-Link Subnet Issues               June 20071.  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         subnets").   [RFC925] was a proposal for option 2 that defined a specific type of   Address Resolution Protocol (ARP) proxy behavior, where the   forwarding plane had the properties of decrementing the Time To Live   (TTL) to prevent loops when forwarding, not forwarding packets   destined to 255.255.255.255, 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].   Instead, the IETF standardized option 3 with [RFC950], whereby hosts   were required to learn a subnet mask, and this became the IPv4 model.   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 versions 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 that preserves the notion that a subnet spans   only one link [RFC4389].Thaler                       Informational                      [Page 2]

RFC 4903                Multi-Link Subnet Issues               June 2007   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 multi-link 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 issues.   The key words "MUST", "RECOMMENDED", and "SHOULD" in this document   are to be interpreted as described in [RFC2119].2.  Issues2.1.  IP Model   The term "link" is generally used to refer to a topological area   bounded by routers that 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.   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 isThaler                       Informational                      [Page 3]

RFC 4903                Multi-Link Subnet Issues               June 2007   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 that 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 of 0 means node-local, 1 means link-local" (this of   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 that use the terms interchangeably due to the IP model   described earlier.Thaler                       Informational                      [Page 4]

RFC 4903                Multi-Link Subnet Issues               June 2007   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   the following:      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 multi-link 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 subnet.)   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 the following:      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 1.      o  Reverse tunneling for Mobile IPv4 [RFC3024] uses TTL 255         checked on receipt for Registration Requests sent to foreign         agents.Thaler                       Informational                      [Page 5]

RFC 4903                Multi-Link Subnet Issues               June 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   that 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], Section 4.2.2.11).   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:http://www.iana.org/assignments/multicast-addresseshttp://www.iana.org/assignments/ipv6-multicast-addresses   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: Distance Vector Multicast Routing Protocol         (DVMRP) [RFC1075], OSPF [RFC2328], RIP  [RFC2453][RFC2080],         Enhanced Interior Gateway Routing Protocol (EIGRP) [EIGRP],         etc.  These protocols exchange routes to subnet prefixes.      o  Address management protocols: Neighbor Discovery, DHCPv4         [RFC2131], Dynamic Host Configuration Protocol for IPv6         (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 (seeSection 2.4 below) within the subnet.         DHCP uses link-scoped multicast or broadcast to obtain an         address in the subnet.  Teredo states that the Teredo IPv4         Discovery Address is "an IPv4 multicast address used toThaler                       Informational                      [Page 6]

RFC 4903                Multi-Link Subnet Issues               June 2007         discover other Teredo clients on the same IPv4 subnet.  The         value of this address is 224.0.0.253", which is a link-scoped         multicast address.  It also says that "the client MUST silently         discard all local discovery bubbles [...] whose IPv4 source         address does not belong to the local IPv4 subnet".      o  Service discovery protocols: Simple Service Discovery Protocol         (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         prefix.   Note that protocols such as Bonjour and Teredo that drop packets that   don't come from an address within the subnet are not necessarily   broken by multi-link 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 multi-link 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 inSection 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."Thaler                       Informational                      [Page 7]

RFC 4903                Multi-Link Subnet Issues               June 2007   The last possibility, sometimes referred to as Duplicate Interface   Identifier Detection (DIID), has been a matter of much debate, and   the current work 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.   The existence of such implementations also causes problems with   multi-link 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 multi-link subnet, address conflicts   can occur.3.  Security Considerations   The notion of multi-link subnets can cause problems with any security   protocols that 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 that 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.Thaler                       Informational                      [Page 8]

RFC 4903                Multi-Link Subnet Issues               June 20074.  Recommendations4.1.  IP Link Model   There are two models that 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  Multi-access 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         (255.255.255.255) 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, etc.) in the middle         of the link.      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, etc.) in the middle of the         link.   A variant of the multi-access link model, which has fewer issues, but   still some, is the following:      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 that 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 inSection 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 beThaler                       Informational                      [Page 9]

RFC 4903                Multi-Link Subnet Issues               June 2007   analogous to supporting multicast routing across a series of local-   area links.  The issues discussed inSection 2.3 will arise, but may   be acceptable over a wide area until multicast routing is also   supported.   Note that the distinction of whether or not a link is a tunnel is   orthogonal to the choice of model; there exist tunnel links for all   link models mentioned above.   A multi-link subnet model should be avoided.  IETF working groups   using, or considering using, multi-link 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 multi-link subnet solution to   another model, the following issues should be considered:   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 inSection 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      Internet Group Management Protocol (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 inThaler                       Informational                     [Page 10]

RFC 4903                Multi-Link Subnet Issues               June 2007   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   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 that designers and maintainers of APIs that provide a   prefix length to applications 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 applicationsThaler                       Informational                     [Page 11]

RFC 4903                Multi-Link Subnet Issues               June 2007   believing they are on separate subnets, rather than on a multi-link   subnet.4.3.  Duplicate Address Detection Optimizations   One of the reasons sometimes cited for wanting a multi-link 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 multi-link 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   layer 2 intermediate device (bridge, switch, access point, etc.).   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.  Normative References   [RFC791]  Postel, J., "Internet Protocol", STD 5,RFC 791, September             1981.   [RFC950]  Mogul, J. and J. Postel, "Internet Standard Subnetting             Procedure", STD 5,RFC 950, August 1985.   [RFC1812] Baker, F., Ed., "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.Thaler                       Informational                     [Page 12]

RFC 4903                Multi-Link Subnet Issues               June 2007   [RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor             Discovery for IP Version 6 (IPv6)",RFC 2461, December             1998.   [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.   [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             2005.   [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 2006.6.  Informative References   [2462BIS] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless             Address Autoconfiguration", Work in Progress, May 2005.   [BRIDGE]  T. Jeffree, editor, "Media Access Control (MAC) Bridges",             ANSI/IEEE Std 802.1D, 2004,http://standards.ieee.org/getieee802/download/802.1D-2004.pdf.   [DEERING] Deering, S., "IP Multicast Extensions for 4.3BSD UNIX and             related systems (MULTICAST 1.2 Release)", June 1989.http://www.kohala.com/start/mcast.api.txtThaler                       Informational                     [Page 13]

RFC 4903                Multi-Link Subnet Issues               June 2007   [EIGRP]   Cisco, "Enhanced Interior Gateway Routing Protocol", Cisco             Document ID 16406, September 2005.http://www.cisco.com/warp/public/103/eigrp-toc.html   [LINUX]   Juan-Mariano de Goyeneche, "Multicast over TCP/IP HOWTO",             March 1998.http://www.linux.com/howtos/Multicast-HOWTO-2.shtml   [LLMNR]   Aboba, B., Thaler, D., and L. Esibov, "Link-local Multicast             Name Resolution (LLMNR)",RFC 4795, January 2007.   [MDNS]    Cheshire, S. and M. Krochmal, "Multicast DNS", June 2005.http://files.multicastdns.org/draft-cheshire-dnsext-multicastdns.txt   [MLSR]    Thaler, D. and C. Huitema, "Multi-link Subnet Support in             IPv6", Proceedings of IETF 54, June 2002.http://www.ietf.org/proceedings/02jul/I-D/draft-ietf-ipv6-multilink-subnets-00.txt   [RBRIDGE] Perlman, R., Gai, S., and D. Dutt, "Rbridges: Base Protocol             Specification", Work in Progress, March 2007.   [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, and End-to-End             Services Task Force, "Protocol Standard for a NetBIOS             Service on a TCP/UDP Transport: Concepts and Methods", STD             19,RFC 1001, March 1987.   [RFC1027] Carl-Mitchell, S. and J. Quarterman, "Using ARP to             Implement Transparent Subnet Gateways",RFC 1027, October             1987.   [RFC1075] Waitzman, D., Partridge, C., and S. Deering, "Distance             Vector Multicast Routing Protocol",RFC 1075, November             1988.   [RFC1884] Hinden, R., Ed., and S. Deering, Ed., "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.Thaler                       Informational                     [Page 14]

RFC 4903                Multi-Link Subnet Issues               June 2007   [RFC2328] Moy, J., "OSPF Version 2", STD 54,RFC 2328, April 1998.   [RFC2373] Hinden, R. and S. Deering, "IP Version 6 Addressing             Architecture",RFC 2373, July 1998.   [RFC2453] Malkin, G., "RIP Version 2", STD 56,RFC 2453, November             1998.   [RFC3024] Montenegro, G., 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., Ed., Bound, J., Volz, B., Lemon, T., Perkins,             C., and M. Carney, "Dynamic Host Configuration Protocol for             IPv6 (DHCPv6)",RFC 3315, July 2003.   [RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6             (IPv6) Addressing Architecture",RFC 3513, April 2003.   [RFC3753] Manner, J., Ed., and 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., Ed., and L. Costa, Ed., "Multicast Listener             Discovery Version 2 (MLDv2) for IPv6",RFC 3810, June 2004.   [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through             Network Address Translations (NATs)",RFC 4380, February             2006.   [RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery             Proxies (ND Proxy)",RFC 4389, April 2006.   [SCOPID]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., Onoe,             A., and B. Zill, "IPv6 Scoped Address Architecture",             Proceedings of IETF 54, July 2002.http://www.ietf.org/proceedings/02jul/I-D/draft-ietf-ipngwg-scoping-arch-04.txt   [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.aspThaler                       Informational                     [Page 15]

RFC 4903                Multi-Link Subnet Issues               June 2007   [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".http://www.microsoft.com/technet/itsolutions/network/deploy/depovg/tcpip2k.mspx   [UNP]     Stevens, W. Richard, "Unix Network Programming, Volume 1,             Second Edition", Prentice Hall, 1998.   [WSDISC]  Microsoft, "Web Services Dynamic Discovery (WS-Discovery)",             2005.http://specs.xmlsoap.org/ws/2005/04/discovery/ws-discovery.pdfIAB Members at the time of this writing   Bernard Aboba   Loa Andersson   Brian Carpenter   Leslie Daigle   Elwyn Davies   Kevin Fall   Olaf Kolkman   Kurtis Lindqvist   David Meyer   David Oran   Eric Rescorla   Dave Thaler   Lixia ZhangAuthor's Address   Dave Thaler   Microsoft   One Microsoft Way   Redmond, WA 98052   Phone: +1 425 703 8835   EMail: dthaler@microsoft.comThaler                       Informational                     [Page 16]

RFC 4903                Multi-Link Subnet Issues               June 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.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 inBCP 78 andBCP 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 athttp://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-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Thaler                       Informational                     [Page 17]