Network Working Group                                  Dave Thaler
Internet-Draft                                   Christian Huitema
Expires: December 2002                                   Microsoft
                                                      29 June 2002

                Multi-link Subnet Support in IPv6

Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

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Copyright Notice

Copyright (C) The Internet Society (2002).  All Rights Reserved.

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Bridging multiple links into a single entity has several
operational advantages.  A single subnet prefix is sufficient to
support multiple physical links.  There is no need to allocate
subnet numbers to the different networks, simplifying management.
This document introduces the concept of a "multilink subnet",
defined as a collection of independent links, connected by
routers, but sharing a common subnet prefix.  It then specifies
the behavior of multilink subnet routers so that no changes to
host behavior are needed.

1.  Introduction

Not all link-layer media can be easily bridged.  Classic IEEE 802
bridging technology fails when the media does not naturally
support IEEE 802 addressing.  Furthermore, the operation becomes
problematic when the different links don't support the same MTU
size.  Finally, bridging cannot be easily implemented when the
network interface cannot be easily placed in "promiscuous" mode.

We define a "multilink subnet" as a collection of independent
links, connected by routers, but sharing a common subnet prefix.
This might be used, for example, in a home network where a router
connects a wired and a wireless link together to form a single

A multilink subnet can be implemented in either of two ways.  In a
simple scenario, a router can act as an asymmetric Neighbor
Discovery proxy, connecting links in a loop-free topology.  In a
more complex scenario, an arbitrary topology exists, and routers
within the subnet communicate using some means of exchanging host
routes.  In this draft, we will only specify the behavior of a
router in the simple scenario, while the behavior of hosts in
either scenario is the same (indeed hosts need not even know which
scenario they are in, or even whether they are on a multilink
subnet).  Appendix A will discuss some of the issues affecting
routers in the complex scenario.

2.  Terminology

multilink subnet:
      a collection of independent links, connected by routers, but

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      sharing a common subnet prefix.

subnet scope:
      multicast SCOP value 3, as specified in [ADDRARCH], which
      covers a (potentially multilink) subnet.  This is the next
      larger multicast scope above link scope.

multilink-subnet router (MSR):
      a router which has interfaces attached to different links in
      a multilink subnet, and which implements the rules in this
      document.  Such interfaces are boundaries for the link
      scope, but not for the subnet scope.

3.  Design Goals

Multilink subnet functionality is designed with the following
goals in mind:

   o Existing IPv6 end hosts should continue to work when
     connected to a multilink subnet, without requiring any change
     to their behavior.  For example, the host behavior parts of
     Router Discovery, Neighbor Discovery [ND], and Multicast
     Listener Discovery [MLD], must be supported.

   o Leave link-local address behavior unchanged.  Link-local
     behavior continues to function only within a link, not across
     a multilink subnet.  That is, sending and receiving unicast,
     anycast, and multicast traffic within the link should be
     supported in the normal fashion.

   o Also support sending and receiving unicast and anycast
     traffic at the site and global scopes.

   o Also support sending and receiving multicast traffic at the
     subnet scope and above.

   o Prevent routing loops.

   o Support nodes moving between links within the subnet, with a
     reasonably fast convergence time (on the same order as
     Neighbor Unreachability Detection).

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4.  Overview

This section gives an overview of multilink subnets.  We describe
the behavior of hosts (which is normal IPv6 host behavior with no
changes), and the resulting requirements for routers.

4.1.  Router Discovery

Router Discovery continues to work on a per-link basis, as
specified in [ND].  When sending Router Advertisements (RAs) with
a Prefix Information Option, there are two possibilities for how a
multilink subnet router can influence the Neighbor Discovery
procedure used.

4.1.1.  Making hosts not use ND

If the MSR sets the A (autonomous address-configuration) flag on,
and the L (on-link) flag off, then hosts on the link will attempt
stateless address configuration [ADDRCONF] in the given prefix,
but will not treat the prefix as being on-link.  As a result,
neighbor discovery is effectively disabled and packets to new
destinations always go to the router first, which will then either
forward them if the destination is off-link, or redirect them if
the destination is on-link.

In the remainder of this document, we will refer to this model as
the "off-link" model, since hosts initially treat all addresses in
the subnet as being off-link.

4.1.2.  Making hosts use ND

If the MSR sets both the A and the L flags, then hosts on the link
will perform stateless address configuration and neighbor
discovery as usual.  However, since Neighbor Solicitations (NSs)
from existing hosts are sent to a link-scoped solicited-node
multicast address, they will never reach nodes on other links
within the subnet.  Instead, MSRs must either know the location of
the destination a priori, or else be able to relay such NS's to
other links.  We will return to this discussion in Section 5.

In the remainder of this document, we will refer to this model as
the "on-link" model, since hosts treat all addresses in the subnet

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as being on-link.

4.1.3.  Effects on Duplicate Address Detection

In either approach above, existing nodes will still do Duplicate
Address Detection using the link-scoped solicited-node multicast

Two important issues arise that must be addressed:

1) If two nodes on different links in the subnet simultaneously
   attempt DAD for the same address, care must be taken to so that
   the collision is detected correctly.

2) If a node moves from one link to another link in the same
   subnet, and performs DAD in its new location, care must be
   taken so that MSRs can distinguish between such a move, and a
   legitimate duplicate, so that after the move, the node can
   retain its address.

Because of these issues, routers MUST NOT use cached information
to respond on behalf of off-link nodes.

Another problem arises from the statement in [ND] that: "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".

Collisions would result if the interface identifier were unique on
the link, but not across the entire multilink subnet.  To avoid
this, MSRs must get involved in duplicate address detection even
for link-local addresses, to ensure that all addresses are unique
across a multilink subnet.

4.2.  Neighbor Discovery

Neighbor Discovery is used differently, depending on whether the
on-link or off-link model is used, as described in the previous

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Off-link model
     If the subnet is treated as being off-link, all packets are
     sent to a default router.  It is then the default router's
     responsibility to figure out the next-hop of the packets.  If
     the next-hop is on-link, it sends a Redirect to the source.

On-link model
     If the subnet is treated as being on-link, nodes will send
     NS's to the solicited node multicast address.  (If a node has
     interfaces attached to multiple links in the subnet, NS's MAY
     be sent on each link.)  If the next-hop is off-link, a router
     will respond with a proxy Neighbor Advertisement (NA)
     containing its own link-layer address.

In either case, it is the router's responsibility to determine
whether a destination in the subnet is on-link.

5.  ND-Proxy Behavior

A simple ND-Proxy router will have one or more interfaces on which
it acts as a router, and optionally an interface on which it acts
as a "host", proxying for all nodes on its router interfaces.  We
will hereafter refer to an interface in this latter mode as a
"proxy-mode" interface.  (This configuration is consistent with
that of an IGMP/MLD Proxy [IGMPPROXY].)

An ND-Proxy MAY support automatic configuration as follows.
First, it starts out as a normal host, discovering routers (if
any) on each interface.  Then, it switches to router-mode on all
interfaces on which no routers were discovered.  If any interfaces
with routers were found, it chooses one and acts in proxy-mode on
that interface.  In Router Advertisements sent on its router-mode
interfaces, the ND-Proxy advertises itself as a default router,
and includes copies of the Prefix Information options it learned
on its proxy-mode interface, possibly after changing the parity of
the L flag, depending on whether it wants nodes to use the on-link
or the off-link model.

5.1.  Basic Unicast

In this section, we step through an example of basic unicast
communication, assuming that address configuration has already
completed, and the router's routing table and neighbor cache

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already have any required information.  Section 5.2 will then
explain how it obtains the required information when a cache miss

In the simple scenario depicted in Figure 1 below, two links, (1)
and (2) on a common subnet with global prefix G, are connected by
an ND-Proxy B.  Node A has link-layer address a on link 1, and has
acquired global IPv6 address Ga.  ND-Proxy B is in router-mode on
link 1, where it has link-layer address b1, and IPv6 address Gb1.
It is in either router-mode or proxy-mode (e.g., if C is a router)
on link 2, where it has link-layer address b2 and IPv6 address
Gb2.  Node C has link-layer address c on link 2, and IPv6 address
Gc.  Node D has link-layer address d on link 1, and IPv6 address

                              | C |
                  |Router-mode or proxy-mode
                | B |
     |                          |
   +-+-+                      +-+-+
   | A |                      | D |
   +---+                      +---+

                Figure 1: Simple ND-Proxy Scenario

Off-link model
     When A wants to start communication with Gc, it finds that
     the destination address matches no on-link prefix, and so
     sends the packet directly to its default router B.  B first
     applies its usual packet validation rules (including
     decrementing the Hop Count in the IPv6 header).  B knows that
     C is on-link to link 2, with link-layer address c, and so it
     forwards the packet to C.

     When A wants to communicate with Dc, it again finds that the

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     destination address matches no on-link prefix, and so sends
     the packet directly to its default router B.  B knows that D
     is on-link to the same link as A, and so responds with a

On-link model
     When A wants to start communication with Gc, it finds that
     the destination address matches an on-link prefix, and so
     sends an NS to the solicited-node multicast address Sc
     constructed from Gc.  The NS message is received by the ND-
     Proxy B, which listens on all multicast groups.  B knows that
     C is on-link to link 2, and responds to A with an NA
     containing its own link-layer address b1 as the Target Link-
     Layer Address.

     After this, A can send packets to the address Gc.  The
     packets will be sent to the link address b1; they will be
     received by B, which will apply its usual validation rules
     (including decrementing the Hop Count in the IPv6 header),
     and forward them to the address c on link 2.

     When A wants to communicate with Gd, it again finds that the
     destination address matches an on-link prefix, and so sends
     an NS to its solicited-node multicast address.  D receives
     the NS and responds.  B also receives the NS, but knows that
     D is on the same link as A, and so does not respond.

Note that we did not assume that the links had to use IEEE 802
addresses, or in fact any form of consistent link-layer
addressing.  B can also handle MTU discovery procedures, returning
an ICMP messages if either A or C sends a packet that is too long.

5.2.  Router Configuration

The previous section assumed that the router's routing table and
neighbor cache already had any required information.  We now
describe how this can be done.

Like any other router, an MSR can acquire routes (including the
subnet prefix) by using manual configuration or a routing
protocol.  An MSR with all interfaces in the same subnet MAY
acquire its information solely based on RAs received from another

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router (which is not an MSR), in the same way a host would.  It
can then advertise the same prefix/route information on other
links in the subnet, using either the on-link or off-link model.

To receive Neighbor Solicitations for nodes for which it is
proxying, the MSR must be able to receive NS's sent to any
multicast group on any of its links within the subnet (including
its proxy-mode interface).

When a Neighbor Solicitation is received, and a corresponding
entry is not found in its neighbor cache, the MSR will attempt to
resolve by sending a Neighbor Solicitation on each attached link
in the subnet, except that in the on-link model one is not sent
back on the link from which an NS was just received.  After
sending an NS, the router SHOULD suppress sending of any other
NS's for the same target address for a short interval (which must
be less than ND's RetransTimer), and while it is resolving a next-
hop, remember each node sending an NS for the same target address.
If a Neighbor Advertisement is received, a proxy NA is sent to
each nodes from which the MSR received an NS.

A Neighbor Advertisement would be sent in response to an NS only
by (a) the actual node with the target address, or (b) an MSR
which has received an NA in response to a relayed NS it sent as a
result of receiving the first NS.  Specifically, a Neighbor
Advertisement MUST NOT be sent just because the MSR has a neighbor
cache entry for the target.  When an MSR receives an NA, it sends
an NA to all nodes from which it received NSs above.

As specified in [ND], proxy Neighbor Advertisements sent by MSR's
on behalf of remote targets always have the Override bit clear.

5.3.  Multicast

Most current multicast routing protocols are based on a "Reverse-
Path Forwarding" check.  That is, they drop a packet if the packet
does not arrive on the link towards a given address (e.g., the
source address, or a Rendezvous Point address associated with the
group address).  Thus, sourcing multicast will work as long as a
router can tell which link is towards any address within the
subnet.  Note that in particular, simply using the subnet route is
not sufficient in a multilink subnet.  If an MSR's longest-match
RPF lookup matches the subnet route for the multilink subnet, it
means the source is in the subnet, and the neighbor cache is

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consulted (as for unicast) to find the link towards the source.

To allow hosts to receive multicast data, an ND-Proxy acts as an
MLD-Proxy using the rules described in [IGMPPROXY].

5.4.  Disabling ND-Proxying

The above rules assume only one ND-proxy is acting as a router on
each link.  Consequently, an ND-Proxy MUST stop acting as a router
on a given interface if it receives an RA from another device on
that interface.

6.  Connecting ND-Proxies

ND-Proxies can be connected, either in parallel (connecting
different leaf links) or in series (subject to the constraints
below), using the same rules specified above.  The following
figure illustrates this.

                           to Internet
                              | R |
                                |                       |
   --+------------+-------------+----------+---(1)--    |  +---+
     |            |                        |            +--+ F |
   +-+-+        +-+-+                    +-+-+          |  +---+
   | C |        | A |                    | B +----------+
   +---+        +-+-+                    +-+-+          |
                  |                        |            |  +---+
      --(2)-------+-------+----   ---+-----+---(3)--    +--+ G |
                          |          |                  |  +---+
                        +-+-+      +-+-+                |
                        | D |      | E |               (4)
                        +-+-+      +---+                |
                        | H |

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               Figure 2: Multiple ND-Proxy Scenario

Figure 2 shows a complex tree topology with MSRs A, B, and D
connecting five links into a single subnet with hosts C, D, E, F,
and G.  R is a normal router that provides connectivity to an
internet, and sends RAs on link 1.

MSR's A and B have both discovered router R on link 1, and are
acting in proxy-mode on that link, while acting in router-mode on
their other links.

MSR D is acting in proxy-mode on link 2, and in router-mode on
link 5.  In this configuration, all hosts can communicate with
each other and with the Internet.  However, the automatic
configuration mechanism described in section X above does not
always work for D.  If D comes up before A, then it would act in
router-mode on link 2, preventing A from doing so.  As a result,
chaining ND-Proxies should only be done where some means of
preventing this exists.  For example, D could either:

   o be manually configured to be in proxy-mode on link 2, or

   o could have one of its ports permanently labelled as being its
     proxy-mode port, so the correct orientation is obvious either
     when connecting cables (if homogenous media types are used on
     different interfaces) or when first obtaining the device (if
     its purpose is to connect heterogeneous media types).

7.  Security Considerations

In general, the security considerations issues and recommendations
are the same as those described in Section 11 of [ND].  In
addition, the following points are worth noting.

In the off-link model, the source address of the relevant ICMP
messages is the address of the actual source.  As a result,
messages can be authenticated, if some means of securing Router
Discovery is used.

In the on-link model, Neighbor Discovery messages are proxied, and
hence Neighbor Discovery is harder to secure.  As a result, when
secure Neighbor Discovery is required, the off-link model should
be used.

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8.  Appendix A: Complex Scenario Issues

In the network depicted in Figure 2, we have now three links, and
also three multilink-subnet routers (MSRs), B, E, and F.

   +---+                      +---+
   | A |                      | D |
   +-+-+                      +-+-+
     |                          |
                  |                        |
                +-+-+                    +-+-+
                | B |                    | E |
                +-+-+                    +-+-+
                  |                        |
                              | F |
                   | C |

              Figure 3: Arbitrary Topology Scenario

The network is sufficiently complex to expose several problems:

   o If A sends an NS packet, that packet is received by both B
     and E.  Depending on the inter-router communication
     mechanism, this could lead to duplicate transmissions on link
     2, and possibly to random behaviors, or to loops.

   o If A sends a multicast packet, and that packet is relayed by
     both B and E, it would lead to duplicate traffic, or even
     potential loops.  It may not be relayed at all, if neither B
     nor E realize there is a group member hidden behind F.

There are multiple possible approaches to solving the above
problems which might meet our design goals.  We discuss two
examples below.

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8.1.  Method 1: Flooding Neighbor Solicitations

Neighbor Soliticitations and Advertisements could be proxied out
all interfaces and hence flooded across the subnet.  To prevent
loops, some mechanism such as a new "Local Distance" option in
NA's would be needed.  The use of Neighbor Discovery would then be
equivalent to a simple means of exchanging host routes between
MSRs, and could be used regardless of which routing protocol is

To route actual packets, an MSR's route lookup would determine
that the longest matching route is on-link to multiple links.  The
router would consult its (conceptual) neighbor cache, and use the
next-hop with the lowest Local Distance.  The same procedure would
apply to multicast packets as well, when the router would look up
the RPF address.

8.2.  Method 2: Proactively populate host routes

MSR's could inject host routes into a routing protocol used within
(at least) the subnet upon detecting a new node on a directly-
connected link (e.g., when DAD completes on an Ethernet, or when
IPv6CP completes on a PPP link).

Once host routes exist, either the off-link or the on-link model
could be used.  In addition, multicast works with no changes,
since host routes would be used for RPF checks.

Another advantage is that since all resolution is done by MSR's "a
priori", no additional delay is incurred when A wants to
communicate with A.  If the on-link model is used, no neighbor
discovery delay exists at all.  Packets are immediately forwarded
along the correct path.  This approach avoids bursty-source

Since host routes are cached state, they cannot, however, be used
for duplicate address detection, due to the issues described in
Section 4.1.3.  That is, the presence of a host route does not
imply a duplicate, since the node may have just moved.  The lack
of a host route does not imply uniqueness, since another node may
be simultaneously choosing the same address.  As a result, DAD
requires additional mechanisms, such as flooding neighbor
discovery messages as in Method 1, or provided by a specialized
routing protocol.

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Work in progress in the Mobile Ad-hoc Networks (manet) WG may
provide solutions to the above problems in the future.

9.  Acknowledgements

Steve Deering, Brian Zill, Hesham Soliman, and Karim El-Malki
participated in discussions that led to this draft.  The term
"multilink subnet" was coined by Steve Deering.

10.  Authors' Addresses

     Dave Thaler
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA  98052-6399
     Phone: +1 425 703 8835

     Christian Huitema
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA  98052-6399

11.  References

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

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

     Fenner, B., He, H., Haberman, B., and H. Sandick, "IGMP-based
     Multicast Forwarding (IGMP Proxying)", draft-ietf-magma-igmp-
     proxy-00.txt, November, 2001.

     Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
     Discovery (MLD) for IPv6", RFC 2710, October 1999.

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[ND] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
     for IP Version 6 (IPv6)", RFC 2461, December 1998.

12.  Full Copyright Statement

Copyright (C) The Internet Society (2002).  All Rights Reserved.

This document and translations of it may be copied and furnished
to others, and derivative works that comment on or otherwise
explain it or assist in its implmentation may be prepared, copied,
published and distributed, in whole or in part, without
restriction of any kind, provided that the above copyright notice
and this paragraph are included on all such copies and derivative
works.  However, this document itself may not be modified in any
way, such as by removing the copyright notice or references to the
Internet Society or other Internet organizations, except as needed
for the purpose of developing Internet standards in which case the
procedures for copyrights defined in the Internet Standards
process must be followed, or as required to translate it into
languages other than English.

The limited permissions granted above are perpetual and will not
be revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on

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Table of Contents

1: Introduction .............................................    2
2: Terminology ..............................................    2
3: Design Goals .............................................    3
4: Overview .................................................    4
4.1: Router Discovery .......................................    4
4.1.1: Making hosts not use ND ..............................    4
4.1.2: Making hosts use ND ..................................    4
4.1.3: Effects on Duplicate Address Detection ...............    5
4.2: Neighbor Discovery .....................................    5
5: ND-Proxy Behavior ........................................    6
5.1: Basic Unicast ..........................................    6
5.2: Router Configuration ...................................    8
5.3: Multicast ..............................................    9
5.4: Disabling ND-Proxying ..................................   10
6: Connecting ND-Proxies ....................................   10
7: Security Considerations ..................................   11
8: Appendix A: Complex Scenario Issues ......................   12
8.1: Method 1: Flooding Neighbor Solicitations ..............   13
8.2: Method 2: Proactively populate host routes .............   13
9: Acknowledgements .........................................   14
10: Authors' Addresses ......................................   14
11: References ..............................................   14
12: Full Copyright Statement ................................   15

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