csi Working Group                                               G. Daley
Intended status: Informational                               J-M. Combes
Expires: November 28, 2008                               Orange Labs R&D
                                                            May 27, 2008

          Securing Neighbour Discovery Proxy Problem Statement

Status of this Memo

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   Neighbour Discovery Proxy is used to provide an address presence on a
   link from nodes which are no themselves present.  It allows a node to
   receive packets directed at its address by allowing another device to
   neighbour advertise on its behalf.

   Neighbour Discovery Proxy is used in Mobile IPv6 and related
   protocols to provide reachability from nodes on the home network when
   a Mobile Node is not at home, by allowing the Home Agent to act as
   proxy.  It is also used as a mechanism to allow a global prefix to
   span multiple links, where proxies act as relays for neighbour
   discovery messages.

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   Neighbour Discovery Proxy currently cannot be secured using SEND.
   Today, SEND assumes that a node advertising an address is the address
   owner and in possession of appropriate public and private keys for
   that node.  This document describes how existing practice for proxy
   Neighbour Discovery relates to Secured Neighbour Discovery.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Scenarios  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  IPv6 Mobile Nodes and Neighbour Discovery Proxy  . . . . .  3
     2.2.  IPv6 Fixed Nodes and Neighbor Discovery Proxy  . . . . . .  5
     2.3.  Bridge-like ND proxies . . . . . . . . . . . . . . . . . .  5
   3.  Proxy ND and Mobility  . . . . . . . . . . . . . . . . . . . .  7
   4.  Proxy Neighbour Discovery and SEND . . . . . . . . . . . . . . 10
     4.1.  CGA signatures and Proxy Neighbour Discovery . . . . . . . 11
     4.2.  Non-CGA signatures and Proxy Neighbour Discovery . . . . . 11
     4.3.  Securing proxy DAD . . . . . . . . . . . . . . . . . . . . 12
   5.  Potential Approaches to Securing Proxy ND  . . . . . . . . . . 13
     5.1.  Secured Proxy ND and Mobile IPv6 . . . . . . . . . . . . . 14
       5.1.1.  Mobile IPv6 and Router-based authorization . . . . . . 14
       5.1.2.  Mobile IPv6 and per-address authorization  . . . . . . 14
       5.1.3.  Cryptographic based solutions  . . . . . . . . . . . . 15
       5.1.4.  'Point-to-Point' link model based solution . . . . . . 15
     5.2.  Secured Proxy ND and Bridge-like proxies . . . . . . . . . 15
       5.2.1.  Authorization Delegation . . . . . . . . . . . . . . . 15
       5.2.2.  Unauthorized routers and proxies . . . . . . . . . . . 16
       5.2.3.  Multiple proxy spans . . . . . . . . . . . . . . . . . 16
       5.2.4.  Routing Infrastructure Delegation  . . . . . . . . . . 17
       5.2.5.  Local Delegation . . . . . . . . . . . . . . . . . . . 17
       5.2.6.  Host delegation of trust to proxies  . . . . . . . . . 18
     5.3.  Proxying unsecured addresses . . . . . . . . . . . . . . . 19
   6.  Two or more nodes defending a same address . . . . . . . . . . 19
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
     8.1.  Router Trust Assumption  . . . . . . . . . . . . . . . . . 20
     8.2.  Certificate Transport  . . . . . . . . . . . . . . . . . . 20
     8.3.  Timekeeping  . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     10.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Changes from the previous versions  . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
   Intellectual Property and Copyright Statements . . . . . . . . . . 25

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1.  Introduction

   Neighbour Discovery Proxy is defined in IPv6 Neighbour Discovery
   [RFC4861].  It is used in networks where a prefix has to span
   multiple links [RFC4389] but also in Mobile IPv6 [RFC3775] (and so in
   Mobile IPv6 based protocols like NEMO [RFC3963], FMIPv6 [RFC4068] or
   HMIPv6 [RFC4140]) and in IKEv2 [RFC4306].  It allows a device which
   is not physically present on a link to have another advertise its
   presence, and forward on packets to the off-link device.

   Neighbour Discovery Proxy relies upon another device, the proxy, to
   monitor for Neighbour Solicitations (NS), and answer with Neighbour
   Advertisements (NA).  These proxy Neighbour Advertisements direct
   data traffic through the proxy.  Proxied traffic is then forwarded on
   to the end destination.

2.  Scenarios

   This section describes the different scenarios where the interaction
   between SEND and ND-Proxy raises issues.

2.1.  IPv6 Mobile Nodes and Neighbour Discovery Proxy

   When moving in the Internet, the aim of IPv6 mobility is to allow a
   device continued packet delivery, whether present on its home network
   or not.  The following text is focused on Mobile IPv6 but the issue
   is the same with Mobile IPv6 based protocols (e.g.  NEMO, HMIPv6).

   For Mobile IPv6 Mobile Nodes (MN), it is necessary to keep existing
   sessions going even when one leaves the home network.  If a neighbour
   is actively delivering packets to a Mobile Node which is at home,
   this neighbour will have a valid neighbour cache entry pointed at the
   MN's link-layer address on the Home link.

   As seen in Figure 1, solicitors send a multicast solicitation to the
   solicited nodes address of the absent node (based on the unicast

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            Absent Mobile       Proxy         Solicitor

                               +-----+     SL2=s,DL2=sol(a),SLL=s
                               |     |<================
                               |     |
                               |     |================>
                               +-----+  NA:SL3=P,DL3=S,TA=A,

      SL3: Source      IPv6 Address         NS: Neighbour Solicitation
      DL3: Destination IPv6 Address         NA: Neighbour Advertisement
      SL2: Source Link-Layer Address        RS: Router Solicitation
      DL2: Destination Link-Layer Address   RA: Router Advertisement
      TA:  Target Address
      SLL/TLL:  Source/Target Link-Layer Address Option

                                 Figure 1

   The Proxy, which listens to this address responds with a Neighbour
   Advertisement which originates at its own IPv6 address and has the
   proxy's address as the Target Link-Layer Address, but contains the
   absent mobile in the Target Address field of the Neighbour
   Advertisement.  In this case, no solicitations are proxied, as the
   advertisements originate within the proxy itself.

   If Cryptographically Generated Addressing (CGA) [RFC3972] is
   available, the MN may be able to secure its neighbour cache bindings
   while at home using Secured Neighbour Discovery (SEND) [RFC3971].
   SEND assumes that the address owner is the advertiser and therefore
   has access to the keys required to sign advertisements about the
   address.  Movement away from the home link requires that a proxy
   undertake Neighbour Discovery.

   In Mobile IPv6, the role of the proxy is undertaken by the Home
   Agent.  While the Home agent has a security association with the MN,
   it as proxy will not have access to the public-private key pair used
   to generate the MN's cryptographic address.  This prevents Proxy
   Neighbour Discovery from using SEND as defined [RFC3971].

   Where a host moves from the home network to a visited network, the
   proxy needs to override existing valid neighbour cache entries which
   may have SEND protection.  This is needed in order to redirect
   traffic to use the proxy's link-layer address, allowing packets to
   flow onto the tunnel connecting the Home Agent/Proxy and the MN.
   With current specifications, any solicitation or advertisement sent
   by the proxy will not be able to update the MN's home address if the

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   existing NC entry is protected by SEND.  Such existing neighbour
   cache entries will time-out after Neighbour Unreachability Detection

   Where secured proxy services are not able to be provided, a proxy's
   advertisement may be overridden by a bogus proxy without it even
   knowing the attack has occurred.

2.2.  IPv6 Fixed Nodes and Neighbor Discovery Proxy

   This scenario is a sub-case from the previous one.  The IPv6 node
   will never be on the link where the ND messages are proxied.  This is
   case with IKEv2 [RFC4306] when a node needs an IP address in the
   network protected by a security gateway and this latest assigns it
   dynamically using Configuration Payload during IKEv2 exchanges.  The
   security gateway will have to proxy ND messages to be able to
   intercept messages, sent to the node, to tunnel them to this latest.

2.3.  Bridge-like ND proxies

   Where proxies exist between two segments, messages may be sent by the
   proxy on the far link, in order to gain or pass on neighbour
   information.  The proxy in this case forwards messages while
   modifying their source and destination MAC addresses, and rewrites
   their Link-Layer Address Options solicited and override flags.  This
   is defined in Bridge Like ND Proxy (ND Proxy) [RFC4389].

   This rewriting is incompatible with SEND signed messages for a number
   of reasons:

   o  Rewriting elements within the message will break the digital

   o  The source IP address of the packets is the packet's origin, not
      the proxy's address.  The proxy is unable to generate another
      signature for this address, as it doesn't have the CGA private key

   Proxy modification of SEND solicitations and advertisements require
   removal of (at least) CGA and Signature options, and may also need
   new options with proxy capabilities if non-CGA signatures are added
   to SEND.

   While bridge-like ND proxies aim to provide as little interference
   with ND mechanisms as possible, SEND has been designed to prevent
   modification or spoofing of advertisements by devices on the link.

   Of particular note is the fact that ND Proxy performs a different

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   kind of proxy neighbour discovery to Mobile IPv6 [RFC3775] [RFC4389].
   The Mobile IPv6 RFC specifies that the Home Agent as proxy sends
   Neighbour Advertisements from its own address with the Target Address
   set to the absent Mobile Node's address.  The Home Agent's own link-
   layer address is placed in the Target Link-Layer address option

   ND Proxy resends messages containing their original address, even
   after modification [RFC4389].  Figure 2 describes packet formats for
   proxy neighbour solicitation and advertisement as specified by the

            Advertisor          Proxy         Solicitor

     NS:SL3=S,DL3=Sol(A),TA=A,          NS:SL3=S,DL3=Sol(A),TA=A,
        SL2=p,DL2=sol(A),SLL=p +-----+      SL2=s,DL2=sol(a),SLL=s
            <==================|     |<================
                               |     |
            ==================>|     |================>
     NA:SL3=A,DL3=S,TA=A,      +-----+  NA:SL3=A,DL3=S,TA=A
        SL2=a,DL2=p,TLL=a                  SL2=p,DL2=s,TLL=p

                                 Figure 2

   In order to use the same security procedures for both ND Proxy and
   Mobile IPv6, changes may be needed to the proxying procedures in
   [RFC4389], as well as changes to SEND.

   An additional (and undocumented) requirement for bridge-like proxying
   is the operation of router discovery.  Router Discovery packets may
   similarly modify neighbour cache state, and require protection from

   In Figure 3, the router discovery messages propagate without
   modification to the router address, but elements within the message
   change.  This is consistent with the description of Neighbour
   Discovery above.

            Advertisor          Proxy         Solicitor

     RS:SL3=S,DL3=AllR,                 RS:SL3=S,DL3=AllR,
        SL2=p,DL2=sol(A),SLL=p +-----+     SL2=s,DL2=allr,SLL=s
            <==================|     |<================
                               |     |
            ==================>|     |================>
     RA:SL3=A,DL3=S,           +-----+  RA:SL3=A,DL3=S,
        SL2=a,DL2=p,SLL=a                 SL2=p,DL2=s,SLL=p

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                                 Figure 3

   Once again, these messages may not be signed with a CGA signature by
   the re-advertisor, because it does not own the source address.

   Additionally, multicast Authorization Delegation Discovery ICMPv6
   messages need to be exchanged for bridge-like ND proxies to prove
   their authority to forward.  Unless the proxy receives explicit
   authority to act as a router, or the router knows of its presence, no
   authorization may be made.  This explicit authorization requirement
   may be at odds with zero configuration goal of ND proxying [RFC4389].

   An alternative (alluded to in an appendix of ND Proxy) suggests that
   the proxy send Router Advertisements (RA) from its own address.  As
   described by ND Proxy, this is insufficient for providing proxied
   Neighbour Advertisement service, but may be matched with neighbour
   solicitation and advertisement services using the proxy's source
   address in the same way as Mobile IPv6 [RFC4389] [RFC3775].  This
   means that all router and neighbour advertisements would come from
   the proxied address, but may contain a target address which allows
   proxied neighbour presence to be established with peers on other
   segments.  Router Discovery in this case has the identity of the
   original (non-proxy) router completely obscured in router discovery

   The resultant proxy messages would have no identifying information
   indicating their origin, which means that proxying between multiple
   links would require state to be stored on outstanding solicitations
   (effectively a ND only NAT).  This level of state storage may be

   Mobile IPv6 does not experience this issue when supplying its own
   address, since ND messages are never forwarded on to the absent node
   (the Home Agent having sufficient information to respond itself).

   Authorization from a router may still be required for Router
   Advertisement, and will be discussed in Section 5.2.

3.  Proxy ND and Mobility

   Whenever a mobile device moves off a link and requires another device
   to forward packets from that address to the MN's new location, proxy
   Neighbour Discovery is required.

   In the Mobile IPv6 case, where the Mobile Node moves away from home,
   a Home Agent needs to be able to override existing neighbour cache
   entries in order to redirect packet flow over a tunnel to the Mobile

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   Node's Care-of-Address (CoA) [RFC3775].

   In Fast Handovers for Mobile IPv6, local neighbours or routers with
   existing valid neighbour cache states need to be told the PAR's link-
   layer address when the MN is departing for a new link, or after
   arrival on the new link when tunnel forwarding begins [RFC4068].
   This allows the MN to maintain reachability to the hosts on that link
   until it is able to send Mobile IPv6 Binding signalling subsequent to
   address configuration on the new network.

   As shown in Figure 4, after the mobile node departs, the Home Agent
   or Proxy sends an overriding Neighbour advertisement, in order to
   update existing neighbour cache entries.

            Absent Mobile       Proxy         Solicitor

               Binding Update  |     |
              ---------------->|     |
               or Fast BU      |     |================>
                               +-----+  NA:SL3=P,DL3=AllN,TA=A,

                                 Figure 4

   Where the proxy forwards between segments of a network, nodes may
   move between segments [RFC4389].  For this scenario, the proxy is
   responsible for updating neighbour cache entries as incorrect state
   is left in them after the move.

   Devices which were on the same segment as the moving node,
   subsequently have incorrect neighbour cache state, as they now need
   to traverse the proxy to get to the other node.  Devices which were
   previously being proxied may now be on the same segment as the mobile
   node, and may go direct.

   As illustrated in Figure 5, the nodes may have incorrect neighbour
   cache state, even if the proxy knows of the departure to another

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            Mobile Node         Proxy          Mobile Node - M
            (Departed)            P            (New Location)

             + - - +                            +-----+ NC:
             '     '     NC:          NC:       |     | N -> n
             + - - +     N -> n+-----+M -> m    +-----+
                |              |     |             |
             ------------------|     |--------------------
                  |            |     |
               +-----+NC:      +-----+
               |     |M -> m

              Neighbour - N

                                 Figure 5

   While neighbour cache state times out, and causes devices to probe
   for the location of a peer, long delays may occur before timeouts of
   neighbour cache state [RFC4861].  In cases where these delays are too
   long, the proxy may have to override the neighbour cache entries of
   hosts which were previously on the same segment as the moving node.

   Those devices now resident on the same segment as the mobile node
   will have the proxy's link-layer address in its neighbour cache.  In
   ND Proxy, it is indicated that packets are never forwarded back to
   the same segment upon which they arrived (potentially to prevent
   forwarding loops) [RFC4389].

   Similarly, if the mobile node is unaware of its movement, it too may
   have incorrect neighbour cache entries for devices which it is now on
   the same segment as.  This is shown below in Figure 6.

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            Mobile Node         Proxy          Mobile Node - M
            (Departed)            P            (New Location)

             + - - +                            +-----+ NC:
             '     '        NC:       NC:       |     | N -> p2
             + - - +           +-----+M -> m    +-----+
                |              |     |N -> n       |
             ------------------|     |--------------------
                               |     |           |
                             P2+-----+P1      +-----+ NC:
                                              |     | M -> p1

                                              Neighbour - N

                                 Figure 6

   For the remaining duration of their incorrect neighbour cache entry
   (up to around 35 seconds), all packets will be dropped.  Therefore,
   these devices may need to be updated with the present node's link-
   layer address.

   Procedures regarding updating caches rely upon Section 7.2.6 of IPv6
   Neighbour Discovery [RFC4861], which allows proxies to neighbour
   advertise to all-nodes with the override flag set when becoming a
   proxy or addresses change.

   For either environment, updates are required to neighbour cache
   entries which may be for SEND nodes.  These advertisements must
   therefore have enough authority to override neighbour cache entries
   even though they are secured.

4.  Proxy Neighbour Discovery and SEND

   There are currently no existing secured Neighbour Discovery
   procedures for proxied addresses, and all Neighbour Advertisements
   from SEND nodes are required to have equal source and target
   addresses, and be signed by the transmitter (section 7.4 of

   Signatures over SEND messages are required to be applied on the CGA
   source address of the message, and there is no way of indicating that
   a message is proxied.

   Even if the message is able to be transmitted from the original
   owner, differences in link-layer addressing and options require

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   modification by a proxy.  If a message is signed with a CGA-based
   signature, the proxy is unable to regenerate a signature over the
   changed message as it lacks the keying material.

   Therefore, a router wishing to provide proxy Neighbour Advertisement
   service can not use existing SEND procedures on those messages.

   A host may wish to establish a session with a device which is not on-
   link but is proxied.  As a SEND host, it prefers to create neighbour
   cache entries using secured procedures.  Since SEND signatures cannot
   be applied to an existing proxy Neighbour Advertisement, it must
   accept non-SEND advertisements in order to receive proxy Neighbour

   Neighbour Cache spoofing of another node therefore becomes trivial,
   as any address may be proxy advertised to the SEND node, and
   overridden only if the node is there to protect itself.  When a node
   is present to defend itself, it may also be difficult for the
   solicitor determine the difference between a proxy-spoofing attack,
   and a situation where a proxied device returns to a link and
   overrides other proxy advertisers [RFC4861].

4.1.  CGA signatures and Proxy Neighbour Discovery

   SEND defines one public-key and signature format for use with
   Cryptographically Generated Addresses (CGAs) [RFC3971].  CGAs are
   intended to tie address ownership to a particular Public/Private key

   In SEND as defined today, Neighbour Discovery Messages (including the
   IP Addresses from the IPv6 header) are signed with the same key used
   to generate the CGA.  This means that message recipients have proof
   that the signer of the message owned the address.

   Where a proxy replaces the message source with its own CGA, the
   existing CGA option and RSA signature option need to be replaced with
   the proxy's.  Such a message will validate using SEND, except that
   the Target Address field will not match the IPv6 Source Address in
   Neighbour Advertisements [RFC3971].

   Additional authorization information may be needed to prove that the
   proxy is indeed allowed to advertise for the target address, as is
   described in Section 5.

4.2.  Non-CGA signatures and Proxy Neighbour Discovery

   Where a proxy retains the original source address in a proxied
   message, existing SEND-CGA checks will fail, since fields within the

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   message will be changed.  In order to achieve secured proxy neighbour
   discovery in this case, new signature authentication mechanisms may
   be needed for SEND.

   SEND provides interfaces for extension of SEND to non-CGA based
   authorization.  Messages are available for Authorization Delegation
   Discovery, which is able to carry arbitrary PKIX/X.509 certificates

   There is no specification though of keying information option formats
   analogous to the SEND CGA Option [RFC3971].  The existing option
   allows a host to verify message integrity by specifying a key and
   algorithm for digital signature, without providing authorization for
   functions other than CGA ownership.

   The digital signature in SEND is transported in the RSA Signature
   Option.  As currently specified, the signature operation is performed
   over a CGA Message type, and infers support for CGA verification.
   Clarification or changing of this issue for non-CGA operations may be

   Within SEND, more advanced functions such as routing may be
   authorized by certificate path verification using Authorization
   Delegation Discovery.

   With non-CGA signatures and authentication, certificate contents for
   authorization may need to be determined, as outlined in Section 5.

   While SEND provides for extensions to new non-CGA methods, existing
   SEND hosts may silently discard messages with unverifiable RSA
   signature options (Section 5.2.2 of [RFC3971]), if configured only to
   accept SEND messages.  In cases where unsecured neighbour cache
   entries are still accepted, messages from new algorithms will be
   treated as unsecured.

4.3.  Securing proxy DAD

   Initiation of Proxy Neighbour Discovery also requires Duplicate
   Address Detection (DAD) checks of the address [RFC4862].  These DAD
   checks need to be performed by sending Neighbour Solicitations, from
   the unspecified source address, with the target being the proxied

   In existing SEND procedures, the address which is used for CGA tests
   on DAD NS is the target address.  A Proxy which originates this
   message while the proxied address owner is absent is unable to
   generate a CGA-based signature for this address and must undertake
   DAD with an unsecured NS.  It may be possible that the proxy can

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   ensure that responding NA's are secured though.

   Where bridge-like ND proxy operations are being performed, DAD NS's
   may be copied from the original source, without modification
   (considering they have an unspecified source address and contain no
   link-layer address options) [RFC4389]

   If non-CGA based signatures are available, then the signature over
   the DAD NS doesn't need to have a CGA relationship to the Target
   Address, but authorization for address configuration needs to be
   shown using certificates.  Where SEND-only nodes do not understand
   the signature format.

5.  Potential Approaches to Securing Proxy ND

   SEND nodes already have the concept of delegated authority through
   requiring external authorization of routers to perform their routing
   and advertisement roles.  The authorization of these routers takes
   the form of delegation certificates.

   Proxy Neighbour Discovery requires a delegation of authority on
   behalf of the absent address owner, to the proxier.  Without this
   authority, other devices on the link have no reason to trust an

   For bridge-like proxies, it is assumed that there is no preexisting
   trust between the host owning the address and the proxy.  Therefore,
   authority may necessarily be dynamic or based on topological roles
   within the network [RFC4389].

   Existing trust relationships lend themselves to providing authority
   for proxying in two alternative ways.

   First, the SEND router authorization mechanisms described above
   provide delegation from the organization responsible for routing in
   an address domain, to the certified routers.  It may be argued that
   routers so certified may be trusted to provide service for nodes
   which form part of a link's address range, but are themselves absent.
   Devices which are proxies could either be granted the right to proxy
   by the network's router, or be implicitly allowed to proxy by virtue
   of being an authorized router.

   Second, where the proxied address is itself a CGA, the holder of the
   public and private keys is seen to be authoritative about the
   address' use.  If this address owner was able to sign the proxier's
   address and public key information, it would be possible to identify
   that the proxy is known and trusted by the CGA address owner for

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   proxy service.  This method requires that the proxied address know or
   learn the proxy's address and public key, and that the certificate
   signed by the proxied node's is passed to the proxy, either while
   they share the same link, or at a later stage.

   In both methods, the original address owner's advertisements need to
   override the proxy if it suddenly returns, and therefore timing and
   replay protection from such messages need to be carefully considered.

5.1.  Secured Proxy ND and Mobile IPv6

   Mobile IPv6 has a security association between the Mobile Node and
   Home Agent.  The Mobile Node sends a Binding Update to the Home
   Agent, to indicate that it is not at home.  This implies that the
   Mobile Node wishes the Home Agent to begin proxy Neighbour Discovery
   operations for its home address(es).

5.1.1.  Mobile IPv6 and Router-based authorization

   A secured Proxy Neighbour Advertisements proposal based on existing
   router trust would require no explicit authorization signalling
   between HA and MN to allow proxying.  Hosts on the home link will
   believe proxied advertisements solely because they come from a
   trusted router.

   Where the home agent operates as a router without explicit trust to
   route from the advertising routing infrastructure (such as in a home,
   with a router managed by an ISP), more explicit proxying
   authorization may be required, as described in Section 5.2.

5.1.2.  Mobile IPv6 and per-address authorization

   Where proxy Neighbour Discovery is delegated by the MN to the home
   agent, the MN needs to learn the public key for the Home Agent, so
   that it can generate a certificate authorizing the public-private
   key-pair to be used in proxying.  It may conceivably either do this
   using Certificate Path Solicitations over a home tunnel, over the
   Internet, or Router Discovery while still at home [RFC3971]

   When sending its Binding Update to the HA, the MN would need to
   provide a certificate containing the subject(proxy-HA)'s public key
   and address, the issuer(MN)'s CGA and public key, and timestamps
   indicating when the authority began and when it ends.  This
   certificate would need to be passed near to binding time, possibly in
   a Certificate Path Advertisement [RFC3971].  Messaging or such an
   exchange mechanism would have to be developed.

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5.1.3.  Cryptographic based solutions

   Specific cryptographic algorithms may help to allow trust between
   entities of a same group.

   This is the case, for example, with ring signature algorithms, a type
   of signature generated using the private key of any entity from the
   same group but to check the signature, the public keys of all group
   members are required.  Applied to SEND, the addresses are
   cryptographically generated using multiple public keys and the
   Neighbor Discovery messages are signed with an RSA ring signature.

5.1.4.  'Point-to-Point' link model based solution

   Another approach is to use the 'Point-to-Point' link model.

   In this model, one prefix is provided per MN and only a MN and the HA
   are on a same link.  The consequence is the HA no more needs to act
   as ND Proxy.

   One way to design such a solution is to use virtual interfaces, on
   the MN and the HA, and a virtual link between them.  Addresses
   generated on the virtual interfaces will only be advertised on the
   virtual link.  For Mobile IPv6, this results to use a virtual Home
   Network where the MN will never come back.

5.2.  Secured Proxy ND and Bridge-like proxies

   In link-extension environments, the role of a proxy is more
   explicitly separated from that of a router.  In SEND, routers may
   expect to be authorized by the routing infrastructure to advertise,
   and provide this authority to hosts in order to allow them to change
   forwarding state.

   Proxies are not part of the traditional infrastructure of the
   Internet, and hosts or routers may not have an explicit reason to
   trust them, except that they can forward packets to regions where
   otherwise they could not reach.

5.2.1.  Authorization Delegation

   If a proxy can convince a device that it should be trusted to perform
   proxying function, it may require that device to vouch for its
   operation in dealing with other devices.  It may do this by receiving
   a certificate, signed by the originating device that the proxy is
   believed capable of proxying under certain circumstances.

   This allows nodes receiving proxied neighbour discovery packets to

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   quickly check if the proxy is authorized for the operation.  There
   are several bases for such trust, and requirements in proxied
   environments, which are discussed below.

5.2.2.  Unauthorized routers and proxies

   Routers advertising on networks without routers may have to operate
   with no explicit authorization, and SEND hosts will configure these
   if there's no other option [RFC3971].  While proxies may similarly
   attempt to advertise without authority, this provides no security for
   the routing infrastructure.  Any device can set up to be a SEND
   proxy/router so long as it signs its own ND messages from its CGA.

   This may not help in the case that a proxy attempts to update
   neighbour cache entries for SEND node which moves between links,
   since the SEND node's authority to advertise its own CGA address
   would not be superceded by a proxy with no credentials.

5.2.3.  Multiple proxy spans

   Proxies may have multiple levels of nesting, which allow the network
   to connect between non-adjacent segments.

   In this case, authority delegated at one point will have to be
   redelegated (possibly in a diluted form) to proxies further away from
   the origin of the trust.

       Trust        ProxyA             ProxyB      Distant
       Origin - T                                   Node - D

        +-----+                                    +-----+
        |     |                                    |     |
        +-----+     +-----+            +-----+     +-----+
           |        |     |            |     |        |
        ------------|     |------------|     |----------
                    |     |            |     |
                    +-----+            +-----+
          ==========>     ==============>    ==========>
          Deleg(A,T)    Deleg(B,Deleg(A,T))   Advertise(D, Deleg(B,

                                 Figure 7

   As shown in Figure 7, the Proxy A needs to redelegate authority to
   proxy for T to B, this allows it to proxy advertisements back to D,
   which target T.

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5.2.4.  Routing Infrastructure Delegation

   Where it is possible for the proxy to pre-establish trust with the
   routing infrastructure, or at least to the local router, it may be
   possible to authorize proxying as a function of routing within the
   subnet.  The router or CA may then be able to certify proxying for
   only a subset of the prefixes which is itself certified for.

   If a router or CA provides certification for a particular prefix, it
   may be able to indicate that only proxying is supported, so that
   neighbour cache entries of routers connected to internet
   infrastructure are never overridden by the proxy, if the router is
   present on a segment.

   Hosts understanding such certificates may allow authorized proxies
   and routers to override host SEND/CGA when assuming proxy roles, if
   the host is absent.

   Proxy certificate signing could be done either dynamically (requiring
   exchanges of identity and authorization information), or statically
   when the network is set up.

5.2.5.  Local Delegation

   Where no trust tie exists between the authority which provides the
   routing infrastructure and the provider of bridging and proxying
   services, it may still be possible for SEND hosts to trust the
   bridging provider to authorize proxying operations.

   SEND itself requires that routers be able to show authorization, but
   doesn't require routers to have a single trusted root.

   A local bridging/proxying authority trust delegation may be possible.
   It would be possible for this authority to pass out local use
   certificates, allowing proxying on a specific subnet or subnets, with
   a separate authorization chain to that for the routers with Internet

   This would require little modification to SEND, other than addition
   of router based proxy authority (as in Section 5.2.4), and proxies
   would in effect be treated as routers by SEND hosts [RFC3971].
   Distribution of keying and trust material for the initial bootstrap
   of proxies would not be provided though (and may be static).

   Within small domains, key management and distribution may be a
   tractable problem, so long as these operations are are simple enough
   to perform.

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   Since these domains may be small, it may be necessary to provide
   certificate chains for trust anchors which weren't requested in
   Certificate Path Solicitations, if the proxy doesn't have a trust
   chain to any requested trust anchor.

   This is akin to 'suggesting' an appropriate trusted root.  It may
   allow for user action in allowing trust extension when visiting
   domains without ties to a global keying infrastructure.  In this
   case, the trust chain would have to start with a self-signed
   certificate from the original CA.

5.2.6.  Host delegation of trust to proxies

   Unlike Mobile IPv6, for bridge-like proxied networks, there is no
   existing security association upon which to transport proxying
   authorization credentials.

   Proxies need then to convince neighbours to delegate proxy authority
   to them, in order to proxy-advertise to nodes on different segments.
   It will be difficult without additional information to distinguish
   between legitimate proxies, and devices which have no need or right
   to proxy (and may wish two network segments to appear to be

   When proxy advertising, proxies must not only identify that proxying
   needs to occur, but provide proof that they are allowed to do so, so
   that SEND Neighbour Cache entries may be updated.  Unless the
   authorization to update such entries is tied to address ownership
   proofs from the proxied host or the verifiable routing
   infrastructure, spoofing may occur.

   When a host received a proxied neighbour advertisement, it would be
   necessary to check authorization in the same way that authorization
   delegation discovery is performed in SEND.

   Otherwise, certificate transport will be required to exchange
   authorization between proxied nodes and proxies.

   Proxies would have to be able to delegate this authorization to
   downstream proxies, as described in Section 5.2.3.

   Movement between segments could be controlled with increasing
   certificate sequence numbers and timestamps.  The timestamp of the
   root authority (in this case, the CGA address owner) would be most
   significant.  Where ties exist, the shortest chain would supercede,
   as this would indicate a proxy closer to the proxied node.

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5.3.  Proxying unsecured addresses

   Where the original neighbour discovery message is unsecured, there is
   an argument for not providing secured proxy service for that node.

   In both the Mobile IPv6 and extended networks cases, the node may
   arrive back at the network and require other hosts to map their
   existing neighbour cache entry to the node's link-layer address.  The
   re-arriving node's overriding of link-layer address mappings will
   occur without SEND in this case.

   It is notable that without SEND protection any node may spoof the
   arrival, and effectively steal service across an extended network.
   This is the same as in the non-proxy case, and is not made
   significantly worse by the proxy's presence (although the identity of
   the attacker may be masked if source addresses are being replaced).

   If signatures over the proxied messages were to be used, re-arrival
   and override of the neighbour cache entries would have to be allowed,
   so the signatures would indicate that at least the proxy wasn't
   spoofing (even if the original sender was).

   For non-SEND/CGA routers, though, it may be possible for secured
   proxies to send signed router advertisement messages, in order to
   ensure that routers aren't spoofed, and subsequently switched to
   being on different parts of the extended network.

   This has problems in that the origin is again unsecured, and any node
   on the network could spoof router advertisement for an unsecured
   address.  These spoofed messages may become almost indistinguishable
   (except for the non-CGA origin address) from unspoofed messages from
   SEND routers.

   Given these complexities, the simplest method is to allow unsecured
   devices to be spoofed from any port on the network, as is the case

6.  Two or more nodes defending a same address

   The previous part of this document focused on the case where two
   nodes defend a same address (i.e. the node and the proxy).  But,
   there are scenarios where two or more nodes are defending a same
   address.  This is at least the case for:

   o  Nodes having the same address, as the MAG's ingress link-local
      address in PMIPv6 [I-D.ietf-netlmm-mn-ar-if].

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   o  Nodes having a common anycast address [RFC4291].

   The problem statement, described previously in this document, applies
   for these cases and the issues are the same from a signalling point
   of view.

   In the first case, [I-D.ietf-netlmm-mn-ar-if] assumes that the
   security material used by SEND (i.e. public-private key pair) is
   shared between all the MAGs.  For the second case, there is no
   solution today.  But, in a same way, it should be possible to assume
   that the nodes having a common anycast address could also share the
   security material.

   It is important to notice that when many nodes defending a same
   address are not in the same administrative domain (e.g.  MAGs in
   different administrative domains but in a same PMIPv6 domain
   [I-D.ietf-netlmm-proxymip6]), sharing the security material used by
   SEND may raise a security issue.

7.  IANA Considerations

   No new options or messages are defined in this document.

8.  Security Considerations

8.1.  Router Trust Assumption

   Router based authorization for Secured Proxy ND may occur without the
   knowledge or consent of a device.  It is susceptible to the 'Good
   Router Goes Bad' attack described in [RFC3756].

8.2.  Certificate Transport

   The certification delegation relies upon transfer of the new
   credentials to the proxying HA in order to undertake ND proxy on its
   behalf.  Since the Binding cannot come into effect until DAD has
   taken place, the delegation of the proxying authority necessarily
   predates the return of the Binding Ack, as described in [RFC3775].
   In the above described case, the home tunnel which comes into
   creation as part of the binding process may be required for
   Certificate Path Solicitation or Advertisement transport [RFC3971].
   This constitutes a potential chicken-and-egg problem.  Either
   modifications to initial home binding semantics or certificate
   transport are required.  This may be trivial if signed, non-
   repudiable certificates are sent in the clear between the MN's CoA
   and the HA without being tunneled.

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8.3.  Timekeeping

   All of the presented methods rely on accurate timekeeping on the
   receiver nodes of Neighbour Discovery Timestamp Options and

   For router-authorized proxy ND, a neighbour may not know that a
   particular ND message is replayed from the time when the proxied host
   was still on-link, since the message's timestamp falls within the
   valid timing window.  Where the router advertises its secured proxy
   NA, a subsequent replay of the old message will override the NC entry
   created by the proxy.

   Creating the neighbour cache entry in this way, without reference to
   accurate subsequent timing, may only be done once.  Otherwise the
   receiver will notice that the timestamp of the advertisement is old
   or doesn't match.

   One way of creating a sequence of replayable messages which have
   timestamps likely to be accepted is to pretend to do an unsecured DAD
   on the address each second while the MN is at home.  The attacker
   saves each DAD defence in a sequence.  The granularity of SEND
   timestamp matching is around 1 second, so the attacker has a set of
   SEND NA's to advertise, starting at a particular timestamp, and valid
   for as many seconds as the original NA gathering occurred.

   This sequence may then be played against any host which doesn't have
   a timestamp history for that MN, by tracking the number of seconds
   elapsed since the initial transmission of the replayed NA to that
   victim, and replaying the appropriate cached NA.

   Where certificate based authorization of ND proxy is in use, the
   origination/starting timestamp of the delegated authority may be used
   to override a replayed (non-proxy) SEND NA, while also ensuring that
   the Proxy NA's timestamp (provided by the proxy) is fresh.  A
   returning MN would advertise a more recent timestamp than the
   delegated authority and thus override it.  This method is therefore
   not subject to the above attack, since the proxy advertisement's
   certificate will have a timestamp greater than any replayed messages,
   preventing it from being overridden.

9.  Acknowledgments

   James Kempf and Dave Thaler particularly contributed to work on this
   document.  Contributions to discussion on this topic helped to
   develop this document.  Thanks go to Jari Arkko, Vijay Devarapalli,
   and Mohan Parthasarathy.

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   Jean-Michel Combes is partly funded by MobiSEND, a research project
   supported by the French 'National Research Agency' (ANR).

10.  References

10.1.  Normative References

              Laganier, J., Narayanan, S., and P. McCann, "Interface
              between a Proxy MIPv6 Mobility Access Gateway and a Mobile
              Node", draft-ietf-netlmm-mn-ar-if-03 (work in progress),
              February 2008.

              Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
              and B. Patil, "Proxy Mobile IPv6",
              draft-ietf-netlmm-proxymip6-16 (work in progress),
              May 2008.

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

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

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

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

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

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

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

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

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10.2.  Informative References

   [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
              Discovery (ND) Trust Models and Threats", RFC 3756,
              May 2004.

   [RFC3963]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
              Thubert, "Network Mobility (NEMO) Basic Support Protocol",
              RFC 3963, January 2005.

   [RFC4068]  Koodli, R., "Fast Handovers for Mobile IPv6", RFC 4068,
              July 2005.

   [RFC4140]  Soliman, H., Castelluccia, C., El Malki, K., and L.
              Bellier, "Hierarchical Mobile IPv6 Mobility Management
              (HMIPv6)", RFC 4140, August 2005.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

Appendix A.  Changes from the previous versions

   To be removed prior to publication as an RFC.

   Previous version: draft-daley-send-spnd-prob-02

   o  Integration of the "Two or more nodes defending a same address"
      section in the core document.

   o  Addition of the "Cryptographic based solutions" section.

   o  Addition of the "'Point-to-Point' link model based solution"

   o  Update of the references.

   Previous version: draft-daley-send-spnd-prob-01

   o  Reorganisation of the draft structure.

   o  Addition of the "Fixed Nodes and Neighbor Discovery Proxy"

   o  Update of the references.

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   o  Addition of the "Two or more nodes defending a same address"

   o  Addition of the "Changes from the previous version" Appendix.

Authors' Addresses

   Greg Daley
   55 Pakington St
   Kew, Victoria  3101

   Phone: +61 405 494849
   Email: hoskuld@hotmail.com

   Jean-Michel Combes
   Orange Labs R&D
   38 rue du General Leclerc
   92794 Issy-les-Moulineaux Cedex 9

   Email: jeanmichel.combes@gmail.com

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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

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

   This document and the information contained herein are provided on an

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