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rfc:rfc4389

Network Working Group D. Thaler Request for Comments: 4389 M. Talwar Category: Experimental Microsoft

                                                              C. Patel
                                                     All Play, No Work
                                                            April 2006
               Neighbor Discovery Proxies (ND Proxy)

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 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.  Bridging some types of
 media requires network-layer support, however.  This document
 describes these cases and specifies the IP-layer support that enables
 bridging under these circumstances.

Thaler, et al. Experimental [Page 1] RFC 4389 ND Proxy April 2006

Table of Contents

 1. Introduction ....................................................3
    1.1. SCENARIO 1: Wireless Upstream ..............................3
    1.2. SCENARIO 2: PPP Upstream ...................................4
    1.3. Inapplicable Scenarios .....................................5
 2. Terminology .....................................................5
 3. Requirements ....................................................5
    3.1. Non-requirements ...........................................6
 4. Proxy Behavior ..................................................7
    4.1. Forwarding Packets .........................................7
         4.1.1. Sending Packet Too Big Messages .....................8
         4.1.2. Proxying Packets with Link-Layer Addresses ..........8
         4.1.3. IPv6 ND Proxying ....................................9
                4.1.3.1. ICMPv6 Neighbor Solicitations ..............9
                4.1.3.2. ICMPv6 Neighbor Advertisements .............9
                4.1.3.3. ICMPv6 Router Advertisements ...............9
                4.1.3.4. ICMPv6 Redirects ..........................10
    4.2. Originating Packets .......................................10
 5. Example ........................................................11
 6. Loop Prevention ................................................12
 7. Guidelines to Proxy Developers .................................12
 8. IANA Considerations ............................................13
 9. Security Considerations ........................................13
 10. Acknowledgements ..............................................14
 11. Normative References ..........................................14
 12. Informative References ........................................15
 Appendix A: Comparison with Naive RA Proxy ........................16

Thaler, et al. Experimental [Page 2] RFC 4389 ND Proxy April 2006

1. Introduction

 In the IPv4 Internet today, it is common for Network Address
 Translators (NATs) [NAT] to be used to easily connect one or more
 leaf links to an existing network without requiring any coordination
 with the network service provider.  Since NATs modify IP addresses in
 packets, they are problematic for many IP applications.  As a result,
 it is desirable to address the problem (for both IPv4 and IPv6)
 without the need for NATs, while still maintaining the property that
 no explicit cooperation from the router is needed.
 One common solution is IEEE 802 bridging, as specified in [BRIDGE].
 It is expected that whenever possible links will be bridged at the
 link layer using classic bridge technology [BRIDGE] as opposed to
 using the mechanisms herein.  However, classic bridging at the data-
 link layer has the following limitations (among others):
 o    It requires the ports to support promiscuous mode.
 o    It requires all ports to support the same type of link-layer
      addressing (in particular, IEEE 802 addressing).
 As a result, two common scenarios, described below, are not solved,
 and it is these two scenarios we specifically target in this
 document.  While the mechanism described herein may apply to other
 scenarios as well, we will concentrate our discussion on these two
 scenarios.

1.1. SCENARIO 1: Wireless Upstream

 The following figure illustrates a likely example:
          |         +-------+           +--------+
    local |Ethernet |       | Wireless  | Access |
          +---------+   A   +-)))   (((-+        +--> rest of network
    hosts |         |       |   link    | Point  |
          |         +-------+           +--------+
 In this scenario, the access point has assigned an IPv6 subnet prefix
 to the wireless link, and uses link-layer encryption so that wireless
 clients may not see each other's data.
 Classic bridging requires the bridge (node A in the above diagram) to
 be in promiscuous mode.  In this wireless scenario, A cannot put its
 wireless interface into promiscuous mode, since one wireless node
 cannot see traffic to/from other wireless nodes.

Thaler, et al. Experimental [Page 3] RFC 4389 ND Proxy April 2006

 IPv4 Address Resolution Protocol (ARP) proxying has been used for
 some years to solve this problem without involving NAT or requiring
 any change to the access point or router.  In this document, we
 describe equivalent functionality for IPv6 to remove this incentive
 to deploy NATs in IPv6.
 We also note that Prefix Delegation [PD] could also be used to solve
 this scenario.  There are, however, two disadvantages to this.
 First, if an implementation already supports IPv4 ARP proxying (which
 is indeed the case in a number of implementations today), then IPv6
 Prefix Delegation would result in separate IPv6 subnets on either
 side of the device, while a single IPv4 subnet would span both
 segments.  This topological discrepancy can complicate applications
 and protocols that use the concept of a local subnet.  Second, the
 extent to which Prefix Delegation is supported for any particular
 subscriber class is up to the service provider.  Hence, there is no
 guarantee that Prefix Delegation will work without explicit
 configuration or additional charge.  Bridging, on the other hand,
 allows the device to work with zero configuration, regardless of the
 service provider's policies, just as a NAT does.  Hence bridging
 avoids the incentive to NAT IPv6 just to avoid paying for, or
 requiring configuration to get, another prefix.

1.2. SCENARIO 2: PPP Upstream

 The following figure illustrates another likely example:
          |         +-------+           +--------+
    local |Ethernet |       | PPP link  |        |
          +---------+   A   +-----------+ Router +--> rest of network
    hosts |         |       |           |        |
          |         +-------+           +--------+
 In this scenario, the router has assigned a /64 to the PPP link and
 advertises it in an IPv6 Router Advertisement.
 Classic bridging does not support non-802 media.  The PPP Bridging
 Control Protocol [BCP] defines a mechanism for supporting bridging
 over PPP, but it requires both ends to be configured to support it.
 Hence IPv4 connectivity is often solved by making the proxy (node A
 in the above diagram) be a NAT or an IPv4 ARP proxy.  This document
 specifies a solution for IPv6 that does not involve NAT or require
 any change to the router.

Thaler, et al. Experimental [Page 4] RFC 4389 ND Proxy April 2006

1.3. Inapplicable Scenarios

 This document is not applicable to scenarios with loops in the
 physical topology, or where routers exist on multiple segments.
 These cases are detected and proxying is disabled (see Section 6).
 In addition, this document is not appropriate for scenarios where
 classic bridging can be applied, or when configuration of the router
 can be done.

2. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in BCP 14, RFC 2119
 [KEYWORDS].
 The term "proxy interface" will be used to refer to an interface
 (which could itself be a bridge interface) over which network-layer
 proxying is done as defined herein.
 In this document, we make no distinction between a "link" (in the
 classic IPv6 sense) and a "subnet".  We use the term "segment" to
 apply to a bridged component of the link.
 Finally, while it is possible that functionality equivalent to that
 described herein may be achieved by nodes that do not fulfill all the
 requirements in [NODEREQ], in the remainder of this document we will
 describe behavior in terms of an IPv6 node as defined in that
 document.

3. Requirements

 Proxy behavior is designed with the following requirements in mind:
 o    Support connecting multiple segments with a single subnet
      prefix.
 o    Support media that cannot be bridged at the link layer.
 o    Do not require any changes to existing routers.  That is,
      routers on the subnet may be unaware that the subnet is being
      bridged.

Thaler, et al. Experimental [Page 5] RFC 4389 ND Proxy April 2006

 o    Provide full connectivity between all nodes in the subnet.
      For example, if there are existing nodes (such as any routers
      on the subnet) that have addresses in the subnet prefix,
      adding a proxy must allow bridged nodes to have full
      connectivity with existing nodes on the subnet.
 o    Prevent loops.
 o    Also work in the absence of any routers.
 o    Support nodes moving between segments.  For example, a node
      should be able to keep its address without seeing its address
      as a duplicate due to any cache maintained at the proxy.
 o    Allow dynamic addition of a proxy without adversely
      disrupting the network.
 o    The proxy behavior should not break any existing classic
      bridges in use on a network segment.

3.1. Non-requirements

 The following items are not considered requirements, as they are not
 met by classic bridges:
 o    Show up as a hop in a traceroute.
 o    Use the shortest path between two nodes on different
      segments.
 o    Be able to use all available interfaces simultaneously.
      Instead, bridging technology relies on disabling redundant
      interfaces to prevent loops.
 o    Support connecting media on which Neighbor Discovery is not
      possible.  For example, some technologies such as [6TO4] use
      an algorithmic mapping from IPv6 address to the underlying
      link-layer (IPv4 in this case) address, and hence cannot
      support bridging arbitrary IP addresses.
 The following additional items are not considered requirements for
 this document:
 o    Support network-layer protocols other than IPv6.  We do not
      preclude such support, but it is not specified in this
      document.

Thaler, et al. Experimental [Page 6] RFC 4389 ND Proxy April 2006

 o    Support Redirects for off-subnet destinations that point to a
      router on a different segment from the redirected host.
      While this scenario may be desirable, no solution is
      currently known that does not have undesirable side effects
      outside the subnet.  As a result, this scenario is outside
      the scope of this document.

4. Proxy Behavior

 Network-layer support for proxying between multiple interfaces SHOULD
 be used only when classic bridging is not possible.
 When a proxy interface comes up, the node puts it in "all-multicast"
 mode so that it will receive all multicast packets.  It is common for
 interfaces not to support full promiscuous mode (e.g., on a wireless
 client), but all-multicast mode is generally still supported.
 As with all other interfaces, IPv6 maintains a neighbor cache for
 each proxy interface, which will be used as described below.

4.1. Forwarding Packets

 When a packet from any IPv6 source address other than the unspecified
 address is received on a proxy interface, the neighbor cache of that
 interface SHOULD be consulted to find an entry for the source IPv6
 address.  If no entry exists, one is created in the STALE state.
 When any IPv6 packet is received on a proxy interface, it must be
 parsed to see whether it is known to be of a type that negotiates
 link-layer addresses.  This document covers the following types:
 Neighbor Solicitations, Neighbor Advertisements, Router
 Advertisements, and Redirects.  These packets are ones that can carry
 link-layer addresses, and hence must be proxied (as described below)
 so that packets between nodes on different segments can be received
 by the proxy and have the correct link-layer address type on each
 segment.
 When any other IPv6 multicast packet is received on a proxy
 interface, in addition to any normal IPv6 behavior such as being
 delivered locally, it is forwarded unchanged (other than using a new
 link-layer header) out all other proxy interfaces on the same link.
 (As specified in [BRIDGE], the proxy may instead support multicast
 learning and filtering, but this is OPTIONAL.)  In particular, the
 IPv6 Hop Limit is not updated, and no ICMP errors (except as noted in
 Section 4.1.1 below) are sent as a result of attempting this
 forwarding.

Thaler, et al. Experimental [Page 7] RFC 4389 ND Proxy April 2006

 When any other IPv6 unicast packet is received on a proxy interface,
 if it is not locally destined then it is forwarded unchanged (other
 than using a new link-layer header) to the proxy interface for which
 the next hop address appears in the neighbor cache.  Again the IPv6
 Hop Limit is not updated, and no ICMP errors (except as noted in
 Section 4.1.1 below) are sent as a result of attempting this
 forwarding.  To choose a proxy interface to forward to, the neighbor
 cache is consulted, and the interface with the neighbor entry in the
 "best" state is used.  In order of least to most preferred, the
 states (per [ND]) are INCOMPLETE, STALE, DELAY, PROBE, REACHABLE.  A
 packet is never forwarded back out the same interface on which it
 arrived; such a packet is instead silently dropped.
 If no cache entry exists (as may happen if the proxy has previously
 evicted the cache entry or if the proxy is restarted), the proxy
 SHOULD queue the packet and initiate Neighbor Discovery as if the
 packet were being locally generated.  The proxy MAY instead silently
 drop the packet.  In this case, the entry will eventually be re-
 created when the sender re-attempts Neighbor Discovery.
 The link-layer header and the link-layer address within the payload
 for each forwarded packet will be modified as follows:
 1)   The source address will be the address of the outgoing
      interface.
 2)   The destination address will be the address in the neighbor
      entry corresponding to the destination IPv6 address.
 3)   The link-layer address within the payload is substituted with
      the address of the outgoing interface.

4.1.1. Sending Packet Too Big Messages

 Whenever any IPv6 packet is to be forwarded out an interface whose
 MTU is smaller than the size of the packet, the ND proxy drops the
 packet and sends a Packet Too Big message back to the source, as
 described in [ICMPv6].

4.1.2. Proxying Packets with Link-Layer Addresses

 Once it is determined that the packet is either multicast or else is
 not locally destined (if unicast), the special types enumerated above
 (ARP, etc.) that carry link-layer addresses are handled by generating
 a proxy packet that contains the proxy's link-layer address on the
 outgoing interface instead.  Such link-layer addresses occur in the

Thaler, et al. Experimental [Page 8] RFC 4389 ND Proxy April 2006

 link-layer header itself, as well as in the payloads of some
 protocols.  As with all forwarded packets, the link-layer header is
 new.
 Section 4.1.3 enumerates the currently known cases where link-layer
 addresses must be changed in payloads.  For guidance on handling
 future protocols, Section 7, "Guidelines to Proxy Developers",
 describes the scenarios in which the link-layer address substitution
 in the payload should be performed.  Note that any change to the
 length of a proxied packet, such as when the link-layer address
 length changes, will require a corresponding change to the IPv6
 Payload Length field.

4.1.3. IPv6 ND Proxying

 When any IPv6 packet is received on a proxy interface, it must be
 parsed to see whether it is known to be one of the following types:
 Neighbor Solicitation, Neighbor Advertisement, Router Advertisement,
 or Redirect.

4.1.3.1. ICMPv6 Neighbor Solicitations

 If the received packet is an ICMPv6 Neighbor Solicitation (NS), the
 NS is processed locally as described in Section 7.2.3 of [ND] but no
 NA is generated immediately.  Instead the NS is proxied as described
 above and the NA will be proxied when it is received.  This ensures
 that the proxy does not interfere with hosts moving from one segment
 to another since it never responds to an NS based on its own cache.

4.1.3.2. ICMPv6 Neighbor Advertisements

 If the received packet is an ICMPv6 Neighbor Advertisement (NA), the
 neighbor cache on the receiving interface is first updated as if the
 NA were locally destined, and then the NA is proxied as described in
 4.1.2 above.

4.1.3.3. ICMPv6 Router Advertisements

 The following special processing is done for IPv6 Router
 Advertisements (RAs).
 A new "Proxy" bit is defined in the existing Router Advertisement
 flags field as follows:
 +-+-+-+-+-+-+-+-+
 |M|O|H|Prf|P|Rsv|
 +-+-+-+-+-+-+-+-+

Thaler, et al. Experimental [Page 9] RFC 4389 ND Proxy April 2006

 where "P" indicates the location of the Proxy bit, and "Rsv"
 indicates the remaining reserved bits.
 The proxy determines an "upstream" proxy interface, typically through
 a (zero-configuration) physical choice dictated by the scenario (see
 Scenarios 1 and 2 above), or through manual configuration.
 When an RA with the P bit clear arrives on the upstream interface,
 the P bit is set when the RA is proxied out all other ("downstream")
 proxy interfaces (see Section 6).
 If an RA with the P bit set has arrived on a given interface
 (including the upstream interface) within the last 60 minutes, that
 interface MUST NOT be used as a proxy interface; i.e., proxy
 functionality is disabled on that interface.
 Furthermore, if any RA (regardless of the value of the P bit) has
 arrived on a "downstream" proxy interface within the last 60 minutes,
 that interface MUST NOT be used as a proxy interface.
 The RA is processed locally as well as proxied as described in
 Section 4.1.2, unless such proxying is disabled as noted above.

4.1.3.4. ICMPv6 Redirects

 If the received packet is an ICMPv6 Redirect message, then the
 proxied packet should be modified as follows.  If the proxy has a
 valid (i.e., not INCOMPLETE) neighbor entry for the target address on
 the same interface as the redirected host, then the Target Link-Layer
 Address (TLLA) option in the proxied Redirect simply contains the
 link-layer address of the target as found in the proxy's neighbor
 entry, since the redirected host may reach the target address
 directly.  Otherwise, if the proxy has a valid neighbor entry for the
 target address on some other interface, then the TLLA option in the
 proxied packet contains the link-layer address of the proxy on the
 sending interface, since the redirected host must reach the target
 address through the proxy.  Otherwise, the proxy has no valid
 neighbor entry for the target address, and the proxied packet
 contains no TLLA option, which will cause the redirected host to
 perform Neighbor Discovery for the target address.

4.2. Originating Packets

 Locally originated packets that are sent on a proxy interface also
 follow the same rules as packets received on a proxy interface.  If
 no neighbor entry exists when a unicast packet is to be locally
 originated, an interface can be chosen in any implementation-specific
 fashion.  Once the neighbor is resolved, the actual interface will be

Thaler, et al. Experimental [Page 10] RFC 4389 ND Proxy April 2006

 discovered and the packet will be sent on that interface.  When a
 multicast packet is to be locally originated, an interface can be
 chosen in any implementation-specific fashion, and the packet will
 then be forwarded out other proxy interfaces on the same link as
 described in Section 4.1 above.

5. Example

 Consider the following topology, where A and B are nodes on separate
 segments which are connected by a proxy P:
      A---|---P---|---B
       a    p1 p2    b
 A and B have link-layer addresses a and b, respectively.  P has
 link-layer addresses p1 and p2 on the two segments.  We now walk
 through the actions that happen when A attempts to send an initial
 IPv6 packet to B.
 A first does a route lookup on the destination address B.  This
 matches the on-link subnet prefix, and a destination cache entry is
 created as well as a neighbor cache entry in the INCOMPLETE state.
 Before the packet can be sent, A needs to resolve B's link-layer
 address and sends a Neighbor Solicitation (NS) to the solicited-node
 multicast address for B.  The Source Link-Layer Address (SLLA) option
 in the solicitation contains A's link-layer address.
 P receives the solicitation (since it is receiving all link-layer
 multicast packets) and processes it as it would any multicast packet
 by forwarding it out to other segments on the link.  However, before
 actually sending the packet, it determines if the packet being sent
 is one that requires proxying.  Since it is an NS, it creates a
 neighbor entry for A on interface 1 and records its link-layer
 address.  It also creates a neighbor entry for B (on an arbitrary
 proxy interface) in the INCOMPLETE state.  Since the packet is
 multicast, P then needs to proxy the NS out all other proxy
 interfaces on the subnet.  Before sending the packet out interface 2,
 it replaces the link-layer address in the SLLA option with its own
 link-layer address, p2.
 B receives this NS, processing it as usual.  Hence it creates a
 neighbor entry for A mapping it to the link-layer address p2.  It
 responds with a Neighbor Advertisement (NA) sent to A containing B's
 link-layer address b.  The NA is sent using A's neighbor entry, i.e.,
 to the link-layer address p2.

Thaler, et al. Experimental [Page 11] RFC 4389 ND Proxy April 2006

 The NA is received by P, which then processes it as it would any
 unicast packet; i.e., it forwards this out interface 1, based on the
 neighbor cache.  However, before actually sending the packet out, it
 inspects it to determine if the packet being sent is one that
 requires proxying.  Since it is an NA, it updates its neighbor entry
 for B to be REACHABLE and records the link-layer address b.  P then
 replaces the link-layer address in the TLLA option with its own
 link-layer address on the outgoing interface, p1.  The packet is then
 sent out interface 1.
 A receives this NA, processing it as usual.  Hence it creates a
 neighbor entry for B on interface 2 in the REACHABLE state and
 records the link-layer address p1.

6. Loop Prevention

 An implementation MUST ensure that loops are prevented by using the P
 bit in RAs as follows.  The proxy determines an "upstream" proxy
 interface, typically through a (zero-configuration) physical choice
 dictated by the scenario (see Scenarios 1 and 2 above), or through
 manual configuration.  As described in Section 4.1.3.3, only the
 upstream interface is allowed to receive RAs, and never from other
 proxies.  Proxy functionality is disabled on an interface otherwise.
 Finally, a proxy MUST wait until it has sent two P bit RAs on a given
 "downstream" interface before it enables forwarding on that
 interface.

7. Guidelines to Proxy Developers

 Proxy developers will have to accommodate protocols or protocol
 options (for example, new ICMP messages) that are developed in the
 future, or protocols that are not mentioned in this document (for
 example, proprietary protocols).  This section prescribes guidelines
 that can be used by proxy developers to accommodate protocols that
 are not mentioned herein.
 1)   If a link-layer address carried in the payload of the
      protocol can be used in the link-layer header of future
      messages, then the proxy should substitute it with its own
      address.  For example, the link-layer address in NA messages is
      used in the link-layer header for future messages, and,
      hence, the proxy substitutes it with its own address.
      For multicast packets, the link-layer address substituted
      within the payload will be different for each outgoing
      interface.

Thaler, et al. Experimental [Page 12] RFC 4389 ND Proxy April 2006

 2)   If the link-layer address in the payload of the protocol will
      never be used in any link-layer header, then the proxy should
      not substitute it with its own address.  No special actions
      are required for supporting these protocols.  For example,
      [DHCPv6] is in this category.

8. IANA Considerations

 This document defines a new bit in the RA flags (the P bit).  There
 is currently no registration procedure for such bits, so IANA should
 not take any action.

9. Security Considerations

 Unsecured Neighbor Discovery has a number of security issues, which
 are discussed in detail in [PSREQ].  RFC 3971 [SEND] defines security
 mechanisms that can protect Neighbor Discovery.
 Proxies are susceptible to the same kind of security issues that
 plague hosts using unsecured Neighbor Discovery.  These issues
 include hijacking traffic and denial-of-service within the subnet.
 Malicious nodes within the subnet can take advantage of this
 property, and hijack traffic.  In addition, a Neighbor Discovery
 proxy is essentially a legitimate man-in-the-middle, which implies
 that there is a need to distinguish proxies from unwanted man-in-
 the-middle attackers.
 This document does not introduce any new mechanisms for the
 protection of proxy Neighbor Discovery.  That is, it does not provide
 a mechanism from authorizing certain devices to act as proxies, and
 it does not provide extensions to SEND to make it possible to use
 both SEND and proxies at the same time.  We note that RFC 2461 [ND]
 already defines the ability to proxy Neighbor Advertisements, and
 extensions to SEND are already needed to cover that case, independent
 of this document.
 Note also that the use of proxy Neighbor Discovery may render it
 impossible to use SEND both on the leaf subnet and on the external
 subnet.  This is because the modifications performed by the proxy
 will invalidate the RSA Signature Option in a secured Neighbor
 Discovery message, and cause SEND-capable nodes to either discard the
 messages or treat them as unsecured.  The latter is the desired
 operation when SEND is used together with this specification, and it
 ensures that SEND nodes within this environment can selectively
 downgrade themselves to unsecure Neighbor Discovery when proxies are
 present.

Thaler, et al. Experimental [Page 13] RFC 4389 ND Proxy April 2006

 In the following, we outline some potential paths to follow when
 defining a secure proxy mechanism.
 It is reasonable for nodes on the leaf subnet to have a secure
 relationship with the proxy and to accept ND packets either from the
 owner of a specific address (normal SEND) or from a trusted proxy
 that it can verify (see below).
 For nodes on the external subnet, there is a trade-off between
 security (where all nodes have a secure relationship with the proxy)
 and privacy (where no nodes are aware that the proxy is a proxy).  In
 the case of a point-to-point external link (Scenario 2), however,
 SEND may not be a requirement on that link.
 Verifying that ND packets come from a trusted proxy requires an
 extension to the SEND protocol and is left for future work [SPND],
 but is similar to the problem of securing Router Advertisements that
 is supported today.  For example, a rogue node can send a Router
 Advertisement to cause a proxy to disable its proxy behavior, and
 hence cause denial-of-service to other nodes; this threat is covered
 in Section 4.2.1 of [PSREQ].
 Alternative designs might involve schemes where the right for
 representing a particular host is delegated to the proxy, or where
 multiple nodes can make statements on behalf of one address
 [RINGSIG].

10. Acknowledgements

 The authors wish to thank Jari Arkko for contributing portions of the
 Security Considerations text.

11. Normative References

 [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.
 [ICMPv6]    Conta, A. and S. Deering, "Internet Control Message
             Protocol (ICMPv6) for the Internet Protocol Version 6
             (IPv6) Specification", RFC 2463, December 1998.
 [KEYWORDS]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [ND]        Narten, T., Nordmark, E., and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", RFC 2461, December
             1998.

Thaler, et al. Experimental [Page 14] RFC 4389 ND Proxy April 2006

 [NODEREQ]   Loughney, J., Ed., "IPv6 Node Requirements", RFC 4294,
             April 2006.

12. Informative References

 [6TO4]      Carpenter, B. and K. Moore, "Connection of IPv6 Domains
             via IPv4 Clouds", RFC 3056, February 2001.
 [BCP]       Higashiyama, M., Baker, F., and T. Liao, "Point-to-Point
             Protocol (PPP) Bridging Control Protocol (BCP)", RFC
             3518, April 2003.
 [DHCPv6]    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.
 [NAT]       Srisuresh, P. and K. Egevang, "Traditional IP Network
             Address Translator (Traditional NAT)", RFC 3022, January
             2001.
 [PD]        Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
             Host Configuration Protocol (DHCP) version 6", RFC 3633,
             December 2003.
 [PSREQ]     Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
             Discovery (ND) Trust Models and Threats", RFC 3756, May
             2004.
 [RINGSIG]   Kempf, J. and C. Gentry, "Secure IPv6 Address Proxying
             using Multi-Key Cryptographically Generated Addresses
             (MCGAs)", Work in Progress, August 2005.
 [SEND]      Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
             "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.
 [SPND]      Daley, G., "Securing Proxy Neighbour Discovery Problem
             Statement", Work in Progress, February 2005.

Thaler, et al. Experimental [Page 15] RFC 4389 ND Proxy April 2006

Appendix A: Comparison with Naive RA Proxy

 It has been suggested that a simple Router Advertisement (RA) proxy
 would be sufficient, where the subnet prefix in an RA is "stolen" by
 the proxy and applied to a downstream link instead of an upstream
 link.  Other ND messages are not proxied.
 There are many problems with this approach.  First, it requires
 cooperation from all nodes on the upstream link.  No node (including
 the router sending the RA) can have an address in the subnet or it
 will not have connectivity with nodes on the downstream link.  This
 is because when a node on a downstream link tries to do Neighbor
 Discovery, and the proxy does not send the NS on the upstream link,
 it will never discover the neighbor on the upstream link.  Similarly,
 if messages are not proxied during Duplicate Address Detection (DAD),
 conflicts can occur.
 Second, if the proxy assumes that no nodes on the upstream link have
 addresses in the prefix, such a proxy could not be safely deployed
 without cooperation from the network administrator since it
 introduces a requirement that the router itself not have an address
 in the prefix.  This rules out use in situations where bridges and
 Network Address Translators (NATs) are used today, which is the
 problem this document is directly addressing.  Instead, where a
 prefix is desired for use on one or more downstream links in
 cooperation with the network administrator, Prefix Delegation [PD]
 should be used instead.

Thaler, et al. Experimental [Page 16] RFC 4389 ND Proxy April 2006

Authors' Addresses

 Dave Thaler
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052-6399
 Phone: +1 425 703 8835
 EMail: dthaler@microsoft.com
 Mohit Talwar
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052-6399
 Phone: +1 425 705 3131
 EMail: mohitt@microsoft.com
 Chirayu Patel
 All Play, No Work
 Bangalore, Karnataka 560038
 Phone: +91-98452-88078
 EMail: chirayu@chirayu.org

Thaler, et al. Experimental [Page 17] RFC 4389 ND Proxy April 2006

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Thaler, et al. Experimental [Page 18]

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