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Versions: 00                                                            
Internet Draft                                                 D. Thaler
February 15, 2006                                              Microsoft
Expires August 2006

           Issues With Protocols Proposing Multilink Subnets

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2006).  All Rights Reserved.


   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

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.

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     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

   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.

   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

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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 [RFC3513].

   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 twice, first
   in July 1998 [RFC2373] and again in April 2003 [RFC3513].  Both
   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.

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

     o Apple's Bonjour [MDNS] uses messages with TTL 255 checked on
       receipt, and only responds 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 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:

     o [LLMNR] uses a TTL/Hop Limit of 1.

     o MLDv2 [RFC3810] uses a Hop Limit of 1.

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

     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 broadcast packets either.

   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, OSPF, RIP, EIGRP, etc.  These
       protocols exchange routes to subnet prefixes.

     o Addressing protocols: ND, DHCPv4, DHCPv6, Teredo, etc.  By their
       nature this group tends to embed assumptions about the
       relationship between a link and a subnet prefix.  For example,
       ND [RFC2461] 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 [RFC4380] 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".

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

     o Name resolution protocols: NetBios [RFC1001], Bonjour [MDNS],
       LLMNR, etc.  Most often these do not define any explicit
       assumption about the relationship to subnet prefix, but [MDNS]
       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 list 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, [RFC2461] 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 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.

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   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 [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.  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 against such attacks would be needed.

4. IANA Considerations

   This document has no actions for IANA.

5. 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.

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

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

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

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   [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.

6. Informative References

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

   [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-45.txt,
             October 2005.

   [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-

   [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.

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

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

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

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   [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.

   [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.

Authors' Addresses

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

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