Internet Engineering Task Force (IETF) S. Krishnan

Internet Engineering Task Force (IETF) S. Krishnan
Request for Comments: 6496 Ericsson
Category: Experimental J. Laganier
ISSN: 2070-1721 Juniper Networks
M. Bonola
Rome Tor Vergata University
A. Garcia-Martinez
UC3M
February 2012

Secure Proxy ND Support for SEcure Neighbor Discovery (SEND)

Abstract

SEcure Neighbor Discovery (SEND) specifies a method for securing
Neighbor Discovery (ND) signaling against specific threats. As
defined today, SEND assumes that the node sending an ND message is
the owner of the address from which the message is sent and/or
possesses a key that authorizes the node to act as a router, so that
it is in possession of the private key or keys used to generate the
digital signature on each message. This means that the Proxy ND
signaling performed by nodes that do not possess knowledge of the
address owner's private key and/or knowledge of a router's key cannot
be secured using SEND. This document extends the current SEND
specification in order to secure Proxy ND operation.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6496.

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Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction ....................................................2
2. Requirements Notation ...........................................3
3. Terminology .....................................................3
4. Secure Proxy ND Overview ........................................4
5. Secure Proxy ND Specification ...................................5
5.1. Proxy Signature Option .....................................6
5.2. Modified SEND Processing Rules .............................8
5.2.1. Processing Rules for Senders ........................8
5.2.2. Processing Rules for Receivers ......................9
5.3. Proxying Link-Local Addresses .............................11
6. Application Scenarios ..........................................11
6.1. Scenario 1: Mobile IPv6 ...................................11
6.2. Scenario 2: Proxy Mobile IPv6 .............................13
6.3. Scenario 3: RFC 4389 Neighbor Discovery Proxy .............16
7. Backward Compatibility with RFC 3971 Nodes and Non-SEND Nodes ..17
7.1. Backward Compatibility with RFC 3971 Nodes ................17
7.2. Backward Compatibility with Non-SEND Nodes ................18
8. Security Considerations ........................................20
9. IANA Considerations ............................................22
10. Acknowledgements ..............................................22
11. References ....................................................22
11.1. Normative References .....................................22
11.2. Informative References ...................................23

1. Introduction

SEcure Neighbor Discovery (SEND) [RFC3971] specifies a method for
securing Neighbor Discovery (ND) signaling [RFC4861] against specific
threats [RFC3756]. As defined today, SEND assumes that the node
sending an ND message is the owner of the address from which the
message is sent and/or possesses a key that authorizes the node to

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act as a router, so that it is in possession of the private key or
keys used to generate the digital signature on each message. This
means that the Proxy ND signaling performed by nodes that do not
possess knowledge of the address owner's private key and/or knowledge
of a router's key cannot be secured using SEND.

This document extends the current SEND specification with support for
Proxy ND. From this point on, we refer to such an extension as
"Secure Proxy ND Support for SEND".

2. Requirements Notation

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 [RFC2119].

3. Terminology

Secure ND Proxy

A node acting on behalf of another node and authorized to secure a
Neighbor Discovery Protocol (NDP) message without knowing the
private key related to the source address of the other node or the
key related to the router authorization.

Proxied IPv6 address

An IPv6 address that does not belong to the Secure ND Proxy and
for which the Secure ND Proxy is performing advertisements.

Non-SEND node

An IPv6 node that does not implement the SEND [RFC3971]
specification but uses the ND protocol defined in [RFC4861] and
[RFC4862], without additional security.

RFC 3971 node

An IPv6 node that does not implement the specification defined in
this document for Secure Proxy ND support but uses the SEND
specification as defined in [RFC3971].

Secure Proxy ND (SPND) node

An IPv6 node that receives and validates messages according to the
specification defined in this document for Secure Proxy ND
support.

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Translated NDP message

An NDP message issued by a Secure ND Proxy as a result of a
received NDP message originated by the owner of the address or
originated by another node acting on behalf of the owner of the
address.

Synthetic NDP message

An NDP message issued by a Secure ND Proxy that is not the result
of a received NDP message.

4. Secure Proxy ND Overview

The original SEND specification [RFC3971] has implicitly assumed that
only the node sending an ND message is the owner of the address from
which the message is sent. This assumption does not allow proxying
of ND messages, since the advertiser is required to generate a valid
RSA Signature option, which in turn requires possession of the
public-private key pair that was used to generate a Cryptographically
Generated Address (CGA), or that was associated to a router
certificate.

To be able to separate the roles of owner and advertiser, the
following extensions to the SEND protocol are defined:

o A Secure Proxy ND certificate, which is a certificate authorizing
an entity to act as an ND proxy. It is an X.509v3 certificate in
which the purpose for which the certificate is issued has been
specified explicitly, as described in a companion document
[RFC6494]. Briefly, Secure Proxy ND certificates include one or
more KeyPurposeId values that can be used for authorizing proxies
to sign Router Advertisement (RA) and Redirect messages, or to
sign Neighbor Advertisement (NA), Neighbor Solicitation (NS), or
Router Solicitation (RS) messages on behalf of other nodes. The
inclusion of this value allows the certificate owner to perform
proxying of SEND messages for a range of addresses indicated in
the same certificate. This certificate can be exchanged through
the Authorization Delegation Discovery process defined in
[RFC3971].

o A new Neighbor Discovery option called the Proxy Signature (PS)
option. This option contains the hash value of the public key of
the proxy, and the digital signature of the SEND message computed
with the private key of the proxy. The hash of the public key of
the proxy is computed over the public key contained in the Secure

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Proxy ND certificate. When an ND message contains a PS option, it
MUST NOT contain CGA or RSA Signature options. The PS option MUST
be appended to any NDP message (NA, NS, RS, RA, and Redirect) to
secure it.

o A modification of the SEND processing rules for all ND messages:
NA, NS, RS, RA, and Redirect. When any of these messages
containing a PS option is validated, it is considered secure.

These extensions are applied in the following way:

o A Secure ND Proxy that proxies ND messages on behalf of a node can
use the PS option to protect the proxied messages. This Secure ND
Proxy becomes part of the trusted infrastructure just like a SEND
router.

o The messages to be secured with the PS option are built according
to [RFC4861] if they are synthesized by the Secure ND Proxy, or
they result from the processing rules defined in [RFC4389] if they
are translated ND messages.

o In order to allow nodes to successfully validate secured proxied
messages, the nodes MUST be aware of the Secure Proxy ND
certificate (in the format described in [RFC6494]) and MUST apply
the modified processing rules specified in this document. We call
these nodes 'SPND nodes'. Note that the rules for generating ND
messages in SPND nodes do not change, so these nodes behave as
defined in [RFC3971] when they send ND messages.

o To allow SPND nodes to know the certification path required to
validate the public key of the proxy, devices responding to CPS
(Certification Path Solicitation) messages with CPA (Certification
Path Advertisement) messages as defined in Section 6 of the SEND
specification [RFC3971] are extended to support the certificate
format specified in [RFC6494], and are configured with the
appropriate certification path.

5. Secure Proxy ND Specification

A Secure ND Proxy performs all the operations described in the SEND
specification [RFC3971] with the addition of new processing rules to
ensure that the receiving node can identify an authorized proxy
generating a translated or synthetic SEND message for a proxied
address.

This is accomplished by signing the message with a private key of the
authorized Secure ND Proxy. The signature of the Secure ND Proxy is
included in a new option called the PS option. The signature is

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performed over all the Neighbor Discovery Protocol (NDP) options
present in the message, and the PS option is appended as the last
option in the message.

5.1. Proxy Signature Option

The Proxy Signature option allows signatures based on public keys to
be attached to NDP messages. The format of the PS option is
described in the following diagram:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Key Hash |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Digital Signature .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: PS Option Layout

Type

32

Length

The length of the option (including the Type, Length, Reserved,
Key Hash, Digital Signature, and Padding fields) in units of
8 octets.

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Reserved

A 16-bit field reserved for future use. The value MUST be
initialized to zero by the sender, and MUST be ignored by the
receiver.

Key Hash

A 128-bit field containing the most significant (leftmost)
128 bits of a SHA-1 [SHA1] hash of the public key used for
constructing the signature. Its purpose is to associate the
signature to a particular key known by the receiver. Such a key
MUST be the same one within the corresponding Secure Proxy ND
certificate.

Digital Signature

A variable-length field containing a PKCS#1 v1.5 signature,
constructed by using the sender's private key over the following
sequence of octets:

1. The 128-bit CGA Message Type tag [RFC3972] value for Secure
Proxy ND, 0x09F5 2BE5 3B62 4C76 CB96 4E7F CDC9 2804. (The tag
value has been generated randomly by the editor of this
specification.)

2. The 128-bit Source Address field from the IP header.

3. The 128-bit Destination Address field from the IP header.

4. The 8-bit Type, 8-bit Code, and 16-bit Checksum fields from
the ICMP header.

5. The NDP message header, starting from the octet after the ICMP
Checksum field and continuing up to, but not including, NDP
options.

6. All NDP options preceding the Proxy Signature option.

The signature value is computed with the RSASSA-PKCS1-v1_5
algorithm and SHA-1 hash, as defined in [RSA]. This field starts
after the Key Hash field. The length of the Digital Signature
field is determined by the ASN.1 BER coding of the PKCS#1 v1.5
signature.

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Padding

This variable-length field contains padding. The length of the
padding field is determined by the length of the Proxy Signature
option minus the length of the other fields.

5.2. Modified SEND Processing Rules

This specification modifies the sender and receiver processing rules
defined in the SEND specification [RFC3971].

5.2.1. Processing Rules for Senders

A Secure ND Proxy MUST NOT use a key to sign NDP message types that
do not correspond to the authorization granted to the considered key.
NA, NS, and RS messages MUST be signed with a key corresponding to a
Secure Proxy ND certificate with a KeyPurposeId value [RFC6494] of
id-kp-sendProxiedOwner, and the source addresses of the messages MUST
be encompassed in the prefix associated to the certificate. RA and
Redirect messages MUST be signed with a key corresponding to a Secure
Proxy ND certificate with a KeyPurposeId value of
id-kp-sendProxiedRouter. The prefix included in the RA message for
on-link determination and/or stateless address autoconfiguration, and
the Target Address of the Redirect message, MUST be encompassed in
the prefix associated to that certificate.

A secured NDP message sent by a Secure ND Proxy for a proxied address
MUST contain a PS option and MUST NOT contain either CGA or RSA
Signature options. Section 7 discusses in which cases an NDP message
has to be secured in a scenario including non-SEND nodes.

The input of this process is a message obtained in either of the
following ways:

a. If the Secure ND Proxy generates synthetic SEND messages for a
proxied address, the message MUST be constructed as described in
the Neighbor Discovery for IP version 6 specification [RFC4861].

b. If the Secure ND Proxy translates secured messages, first the
authenticity of the intercepted message MUST be verified. If the
intercepted message is a SEND message, it MUST be validated as
specified in Section 5 of the SEND specification [RFC3971]. If
the intercepted message contains a PS option, the authenticity of
the message MUST be verified as detailed in Section 5.2.2 of this
specification. After validation, the CGA, RSA, or PS options of
the original message MUST be removed. Then, the message to be
translated MUST be processed according to the ND Proxy
specification [RFC4389]. In this way, it is determined whether

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the message received should be proxied or not; the proxy
interface status is updated if needed, the outgoing interface is
determined, the link-layer header and the link-layer address
within the payload are modified if required, etc.

A Secure ND Proxy then modifies the input message as follows:

1. Timestamp and Nonce options MUST be included according to the
rules specified in SEND [RFC3971]. The value in the Timestamp
option MUST be generated by the proxy. If the proxy is
translating a message that includes a nonce, the Nonce value in
the proxied message MUST be the same as in the intercepted
message. If the proxy is synthesizing a solicitation message,
the Nonce value MUST be generated by the proxy. If the proxy is
synthesizing an advertisement message, the Nonce value MUST
correspond to the solicitation message to which the proxy is
responding.

2. The Proxy Signature option MUST be added as the last option in
the message.

3. The data MUST be signed as explained in Section 5.1.

5.2.2. Processing Rules for Receivers

Any SEND message without a Proxy Signature option MUST be treated as
specified in the SEND specification [RFC3971].

A SEND message including a Proxy Signature option MUST be processed
as specified below:

1. The receiver MUST ignore any RSA and CGA options, as well as any
options that might come after the first PS option. The options
are ignored for both signature verification and NDP processing
purposes.

2. The Key Hash field MUST indicate the use of a known public key.
A valid certification path (see [RFC6494] Section 9) between the
receiver's trust anchor and the sender's public key MUST be
known. The Secure Proxy ND X.509v3 certificate MUST contain an
extended key usage extension including the appropriate
KeyPurposeId value and prefix for the message to validate:

* For RA messages, a KeyPurposeId value of
id-kp-sendProxiedRouter MUST exist for the certificate, and
the prefix included in the RA message for on-link
determination and/or stateless address autoconfiguration MUST
be encompassed in the prefix associated to that certificate.

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* For Redirect messages, a KeyPurposeId value of
id-kp-sendProxiedRouter MUST exist for the certificate, and
the prefix included in the Target Address of the Redirect
message MUST be encompassed in the prefix associated to that
certificate.

* For NA, NS, and RS messages, a KeyPurposeId value of
id-kp-sendProxiedOwner MUST exist for the certificate, and the
source addresses of the messages MUST be encompassed in the
prefix associated to the certificate.

If any of these tests fail, the verification fails.

3. The Digital Signature field MUST have correct encoding;
otherwise, the verification of the message including the PS
option fails.

4. The Digital Signature verification MUST show that the signature
has been calculated as specified in Section 5.1; otherwise, the
verification of the message including the PS option fails.

5. The Nonce option MUST be processed as specified in [RFC3971]
Section 5.3.4, except for replacing 'RSA Signature option' with
'PS option'; if these tests fail, the verification of the message
including the PS option fails.

6. The Timestamp option MUST be processed as specified in [RFC3971]
Section 5.3.4, except for replacing 'RSA Signature option' with
'PS option'. If these tests fail, the verification of the
message including the PS option fails. The receiver SHOULD store
the peer-related timing information specified in [RFC3971]
Sections 5.3.4.1 and 5.3.4.2 (RDlast, TSlast) separately for each
different proxy (which could be identified by the different Key
Hash values of the proxied message) and separately from the
timing information associated to the IP address of a node for
which the message is proxied. In this way, a message received
for the first time from a proxy (i.e., for which there is no
information stored in the cache) for which the Timestamp option
is checked SHOULD be checked as a message received from a new
peer (as in [RFC3971] Section 5.3.4.2).

7. Messages with the Override bit [RFC4861] set MUST override an
existing cache entry regardless of whether it was created as a
result of an RSA Signature option or a PS option validation.
When the Override bit is not set, the advertisement MUST NOT
update a cached link-layer address created securely by means of
RSA Signature option or PS option validation.

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Messages for which the verification fails MUST be silently discarded
if the node has been configured to accept only secured ND messages.
The messages MAY be accepted if the host has been configured to
accept both secured and unsecured messages but MUST be treated as an
unsecured message.

5.3. Proxying Link-Local Addresses

SEND [RFC3971] relies on certificates to prove that routers are
authorized to announce a certain prefix. However, Neighbor Discovery
[RFC4861] states that routers do not announce the link-local prefix
(fe80::/64). Hence, it is not required for a SEND certificate to
hold an X.509 extension for IP addresses that authorizes the
fe80::/64 prefix. However, some Secure Proxy ND scenarios
([RFC4389], [RFC5213]) impose providing the proxying function for the
link-local address of a node. When Secure ND Proxy functionality for
a link-local address is required, either a list of link-local
addresses, or the fe80::/64 prefix MUST be explicitly authorized to
be proxied in the corresponding certificate.

6. Application Scenarios

In this section, we describe three different application scenarios
for which Secure Proxy ND support for SEND can be applied. Note that
the particular way in which Secure Proxy ND support is applied (which
ND messages are proxied, in which direction, how the interaction with
non-SEND hosts and RFC 3971 hosts is handled, etc.) largely depends
on the particular scenario considered. In the first two scenarios
presented below, ND messages are synthesized on behalf of off-link
nodes. In the third one, ND messages are translated from the
messages received in other interfaces of the proxy.

6.1. Scenario 1: Mobile IPv6

The description of the problems for deploying SEND in this scenario
is presented in [RFC5909].

The Mobile IPv6 (MIPv6) protocol [RFC6275] allows a Mobile Node (MN)
to move from one link to another while maintaining reachability at a
stable address, the so-called MN's Home Address (HoA). When an MN
attaches to a foreign network, all the packets sent to the MN's HoA
by a Correspondent Node (CN) on the home link or a router are
intercepted by the Home Agent (HA) on that home link, encapsulated,
and tunneled to the MN's registered Care-of Address (CoA).

To deploy Secure Proxy ND in this scenario, i.e., to secure the HA
operation, a Secure Proxy ND certificate with a KeyPurposeId value of
id-kp-sendProxiedOwner for the prefix of the home link is required.

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The Secure ND Proxy is configured with the private key associated to
this certificate. When a NS is intercepted by the HA on the home
link, the HA checks whether the Target Address within the NS matches
with any of the MN's Home Addresses in the binding cache, and if so,
it replies with a Neighbor Advertisement (NA) constructed as
described in [RFC4861], containing its own link-layer address (HA_LL)
as the Target Link-Layer Address Option (TLLAO). Then, a timestamp
(generated by the proxy) and nonce (if appropriate, according to
[RFC3971]) MUST be included. Finally, a PS option signing the
message MUST be included as the last option of the message.

Node (N) Home Agent (HA) Mobile Node (MN)
on Home Link on Home Link on Foreign Link
| | |
| SRC = N | |
| DST = solicited_node (MN) | |
| ICMPv6 NS | |
| TARGET = MN | |
| SLLAO = N_LL | |
| [CGA] | |
| RSA signature | |
|-------------------------->| |
| | |
| SRC = HA | |
| DST = N | |
| ICMPv6 NA | |
| TARGET = MN | |
| TLLAO = HA_LL | |
| PS signature | |
|<--------------------------| |
| | |
| traffic | |
| dest = MN HoA | |
|-------------------------->| |
| | |
| | tunneled traffic |
| | dest = MN CoA |
| |------------------------->|
| | |

Figure 2: Proxy ND Role of the Home Agent in MIPv6

A node receiving the NA containing the PS option (e.g., the CN in the
home link, or a router) MUST apply the rules defined in
Section 5.2.2. Note that in this case the Override bit of the NA
message is used to control which messages should prevail on each

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case: the message generated by the proxy when the MN moves from the
home network, or the MN if it comes back to the home link, as defined
in the MIPv6 specification [RFC6275].

6.2. Scenario 2: Proxy Mobile IPv6

Proxy Mobile IPv6 [RFC5213] is a network-based mobility management
protocol that provides IP mobility management support for MNs without
requiring that MNs be involved in the mobility-related signaling.
The IP mobility management is totally hidden to the MN in a Proxy
Mobile IPv6 domain, and it is performed by two functional entities:
the Local Mobility Anchor (LMA) and the Mobile Access Gateway (MAG).

When the MN connects to a new access link, it sends a multicast
Router Solicitation (RS). The MAG on the new access link, upon
detecting the MN's attachment, signals the LMA requesting an update
of the binding state of the MN (by means of a Proxy Binding Update
(PBU)). Once the signaling is completed (it receives a Proxy Binding
Ack (PBA)), the MAG replies to the MN with a Router Advertisement
(RA) containing the home network prefix(es) that were assigned to
that mobility session, making the MN believe it is still on the same
link, so the IPv6 address reconfiguration procedure is not triggered
(Figure 3).

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MN new MAG LMA
| | |
MN Attached | |
| | |
| MN Attached Event from MN/Network |
| | |
| SRC = MN | |
| DST = all routers | |
| ICMPv6 RS | |
| [CGA] | |
| RSA signature | |
|--------------------->| |
| | |
| |--- PBU ------------->|
| | |
| | Accept PBU
| | |
| |<------------- PBA ---|
| | |
| Accept PBA |
| | |
| |==== Bi-Dir Tunnel ===|
| | |
| SRC = MAG4MN | |
| DST = MN | |
| ICMPv6 RA | |
| SLL = MAG_LL | |
| PS | |
|<---------------------| |
| | |
| | |
| | |

Figure 3: Mobile Node's Handover in PMIPv6

To avoid potential link-local address collisions between the MAG and
the MN after a handoff to a new link, the Proxy Mobile IPv6
specification [RFC5213] requires that the MAG's link-local address on
the link to which the MN is attached be generated by the LMA when the
MN first attaches to a PMIPv6 domain, and be provided to the new MN's
serving MAG after each handoff. Thus, from the MN's point of view,
the MAG's link-local address remains constant for the duration of
that MN's session.

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The approach described above and the current SEND specification are
incompatible, since sharing the same link-local address on different
MAGs would require all MAGs of a PMIPv6 domain to construct the CGA
and the RSA Signature option with the same public-private key pair,
which is not an acceptable security policy.

Using different public-private key pairs on different MAGs would mean
that different MAGs use different CGAs as link-local addresses.
Thus, the serving MAG's link-local address would change after each
handoff of the MN, which is in contradiction with the way MAG link-
local address assignment occurs in a PMIPv6 domain.

To provide SEND protection, each MAG MUST be configured to act as a
proxy by means of a certificate associated to the PMIPv6 domain,
authorizing each MAG to securely proxy NA and RS messages by means of
a KeyPurposeId value of id-kp-sendProxiedOwner. In addition, the
certificate MUST also authorize the MAG to advertise prefixes by
associating to the same certificate a KeyPurposeId value of
id-kp-sendProxiedRouter. Note that the inclusion of multiple
KeyPurposeId values is supported by [RFC6494].

When a MAG replies to an RS with an RA, the source address MUST be
equal to the MAG link-local address associated to the MN in this
PMIPv6 domain, with its own link-layer address as the source link-
layer address. Then, a timestamp (generated by the proxy) and nonce
(if appropriate, according to [RFC3971]) MUST be included. Finally,
a PS option signing the message MUST be included as the last option
of the message. This procedure is followed for any other ND message
that could be generated by the MAG to the MN.

A node receiving a message from the MAG containing the PS option MUST
apply the processing rules defined in Section 5.2.2. Note that
unsolicited messages sent by the MAG should be validated by the host
according to timestamp values specific to the MAG serving the link,
not to any other MAG to which the host has been connected before in
other links, according to processing step number 6 of Section 5.2.2.

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6.3. Scenario 3: RFC 4389 Neighbor Discovery Proxy

The problems for deploying SEND in this scenario are presented in
[RFC5909].

Link 1 Link 2

Host A ND Proxy (P) Host B
| | |
| SRC = A | |
| DST = solicited_node (B) | |
| ICMPv6 NS | |
| TARGET = B | |
| SLLAO = A_LL | |
|------------------------->| |
| | SRC = A |
| | DST = solicited_node (B) |
| | ICMPv6 NS |
| | TARGET = B |
| | SLLAO = P_LL |
| |------------------------->|
| | |
| | SRC = B |
| | DST = A |
| | ICMPv6 NA |
| | TARGET = B |
| | TLLAO = B_LL |
| |<-------------------------|
| SRC = B | |
| DST = A | |
| ICMPv6 NA | |
| TARGET = B | |
| TLLAO = P_LL | |
|<-------------------------| |
| | |

Figure 4: RFC 4389 Neighbor Discovery Proxy Operation

The Neighbor Discovery (ND) Proxy specification [RFC4389] provides a
method by which multiple link-layer segments are bridged into a
single segment and specifies the IP-layer support that enables
bridging under these circumstances.

A Secure ND Proxy MUST parse any IPv6 packet it receives on a proxy
interface to check whether it contains one of the following NDP
messages: NS, NA, RS, RA, or Redirect. The Secure ND Proxy MUST
verify the authenticity of the received ND message, according to
[RFC3971], or according to Section 5.2.2 if it contains a PS option.

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Then, after removing the CGA, RSA, or PS options, the message to be
translated MUST be processed according to the ND Proxy specification
[RFC4389]. This includes performing loop prevention checks,
determining the outgoing interface for the proxied message, changing
the source link-layer address to the address of the outgoing
interface, changing source link-layer addresses contained in the
payload (that is, in a Source Link-Layer Address Option (SLLAO) or a
Target Link-Layer Address Option (TLLAO)), maintaining the
destination link-layer address as the address in the neighbor entry
corresponding to the destination IPv6 address, setting the P bit for
proxied RA messages, etc. Note that besides link-layer addresses and
the P bit of a RA, no other field of the received message is changed
when proxied by an [RFC4389] proxy.

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. If no neighbor
cache entry is present, the Secure ND Proxy SHOULD queue the packet
and initiate a Neighbor Discovery signaling as if the NS message were
locally generated.

Note that to be able to sign any NS, NA, RS, RA, or Redirect message,
the key used MUST correspond to a certificate with KeyPurposeId
values of id-kp-sendProxiedOwner and id-kp-sendProxiedRouter.

In order to deploy this scenario, nodes in proxied segments MUST know
the certificate-authorizing proxy operation. To do so, it could be
required that at least one device per proxied segment (maybe the
proxy itself) be configured to propagate the required certification
path to authorize proxy operation by means of a CPS/CPA exchange.

7. Backward Compatibility with RFC 3971 Nodes and Non-SEND Nodes

In this section, we discuss the interaction of Secure ND Proxies and
SPND nodes with RFC 3971 nodes and non-SEND nodes. As stated in
[RFC3971], network operators may want to run a mixture of nodes
accepting secured and unsecured NDP messages at the same time.
Secure ND Proxies and SPND nodes SHOULD support the use of secured
and unsecured NDP messages at the same time.

7.1. Backward Compatibility with RFC 3971 Nodes

RFC 3971 nodes, i.e., SEND nodes not compliant with the modifications
required in Section 5, cannot correctly interpret a PS option
received in a proxied ND message. These SEND nodes silently discard
the PS option, as specified in [RFC4861] for any unknown option. As

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a result, these messages will be treated as unsecured, as described
in Section 8 ("Transitions Issues") of the SEND specification
[RFC3971].

When RFC 3971 nodes and SPND nodes exchange ND messages (without
proxy intervention), in either direction, messages are generated
according to the SEND specification [RFC3971], so these nodes
interoperate seamlessly.

In the scenarios in which the proxy translates ND messages, the
messages to translate can either be originated in an RFC 3971 node or
in an SPND node, without interoperability issues (note that the
difference between RFC 3971 nodes and SPND nodes only affects the
ability to process received NDP messages containing a PS option, not
the way they generate messages secured by SEND).

A configuration option MAY exist in a Secure ND Proxy to specify the
RFC 3971 nodes to which it is connected, so that the proxied messages
sent to these nodes are not processed according to the Secure Proxy
ND specification, for performance reasons.

7.2. Backward Compatibility with Non-SEND Nodes

Non-SEND nodes receiving NDP packets silently discard PS options, as
specified in [RFC4861] for any unknown option. Therefore, these
nodes interpret messages proxied by a Secure ND Proxy as any other ND
message.

When non-SEND nodes and SPND nodes exchange ND messages (without
proxy intervention), in either direction, the rules specified in
Section 8 of [RFC3971] apply.

A Secure ND Proxy SHOULD support the use of secured and unsecured NDP
messages at the same time, although it MAY have a configuration that
causes proxying to not be performed for unsecured NDP messages. A
Secure ND Proxy MAY also have a configuration option whereby it
disables secure ND proxying completely. This configuration SHOULD be
switched off by default; that is, security is provided by default.
In the following paragraphs, we discuss the recommended behavior of
the Secure ND Proxy regarding the protection level to provide to
proxied messages in a mixed scenario involving SPND/RFC 3971 nodes
and non-SEND nodes. In particular, two different situations occur,
depending on whether the proxied nodes are RFC 3971 or SPND nodes, or
non-SEND nodes.

As a rule of thumb, if the proxied nodes can return to the link in
which the proxy operates, the Secure ND Proxy MUST only generate PS
options on behalf of nodes with SEND capabilities (i.e., those nodes

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that could use SEND to defend their messages if present on the same
link as the proxy -- in other words, either RFC 3971 nodes or SPND
nodes). This is relevant to allow nodes to prefer secured
information over an unsecured one, and to properly execute the
Duplicate Address Detection (DAD) procedure, as specified in
[RFC3971]. Therefore, in this case, the Secure ND Proxy MUST
synthesize/translate messages containing the PS option for SPND and
RFC 3971 hosts, and MUST NOT synthesize/translate messages containing
the PS option for non-SEND nodes. Note that ND advertisements in
response to solicitations generated by a Secure ND Proxy must either
be secured or not secured, according to the previous considerations
(i.e., according to the nature of the proxied node), and not
according to the secure or unsecure nature of the solicitation
message.

In order to apply this rule, the Secure ND Proxy needs to know the
security capabilities of the proxied node. The way this information
is acquired depends on the application scenario, and it is discussed
next:

o For scenarios in which ND messages are translated for nodes that
can arrive to the link in which the proxy operates, the rule can
be easily applied: only for messages validated in the Secure ND
Proxy according to the SEND specification [RFC3971], or according
to Section 5.2.2 of this specification for messages containing a
PS option (which means that another proxy previously checked that
the original message was secured), the message MUST be proxied
securely by the inclusion of a PS option. Unsecured ND messages
could be proxied if unsecured operation is enabled in the proxy,
but the message generated by the Secure ND Proxy for the received
message MUST NOT include a PS option.

o For scenarios in which ND messages are synthesized on behalf of
remote nodes, different considerations should be made according to
the particular application scenario.

* For MIPv6, if the MN can return to the home link, it is
required that the proxy know whether the node could use SEND to
defend its address or not. A HA including the PS option for
proxying a non-SEND MN would make ND messages sent by the proxy
be more preferred than an ND message of the non-SEND MN when
the MN returns to the home link (even if the proxied messages
have the Override bit set to 1). Not using the PS option for
an RFC 3971 or SPND MN would make the address in the home link
more vulnerable when the MN is away than when it is in the home
link, defeating the purpose of the Secure Proxy ND mechanism.
Therefore, in this case, the HA MUST know the SEND capabilities

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of the MN, MUST use the PS option if the MN is an SPND or
RFC 3971 host, and MUST NOT use the PS option for non-SEND
hosts.

* For the Proxy Mobile IPv6 scenario, a node moving from a link
in which the PS option has been used to protect a link-layer
address to a link in which ND messages are not protected by
SEND would prevent the MN from acquiring the new information
until the cached information expires. However, in this case,
it is reasonable to consider that all MAGs provide the same
security for protecting ND messages, and that either all MAGs
or no MAGs will behave as a Secure ND Proxy, so configuration
is expected to be easier.

A configuration option MAY exist in a Secure ND Proxy to specify the
non-SEND nodes to which it is connected, so that the proxied messages
sent to these nodes are not processed according to the Secure Proxy
ND specification, for performance reasons.

8. Security Considerations

The mechanism described in this document introduces a new PS option
allowing a Secure ND Proxy to synthesize or translate a SEND message
for a proxied address, to redirect traffic for given target
addresses, or to advertise prefix information by means of RA
messages. An SPND node only accepts such a message if it includes a
valid PS option generated by a properly authorized Secure ND Proxy
(with a certificate containing a KeyPurposeId with value
id-kp-sendProxiedOwner for protecting NA, NS, and RS messages, or
containing a KeyPurposeId value of id-kp-sendProxiedRouter for
protecting RA and Redirect messages). Such a message has protection
against the threats presented in Section 9 of [RFC3971] equivalent to
a message signed with an RSA Signature option.

The security of proxied ND messages not including a PS option is the
same as an unsecured ND message. The security of a proxied ND
message received by a non-SEND host or RFC 3971 host is the same as
an unsecured ND message.

When a message including a PS option is received by an SPND node, any
CGA or RSA options also included in the message are removed and the
remaining message further processed. Although properly formed
proxied messages MUST NOT include PS and CGA/RSA options at the same
time, discarding them if they appear does not affect security. If
the PS option is validated, then the information included in the
message has been validly generated by a proxy, and should be honored
(remember that anti-replay protection is provided by means of Nonce
and Timestamp options). If the PS option is not validated, then it

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is treated as an unsecured message. In any case, there is no gain
for an attacker from appending false or old CGA/RSA information to a
message secured by a Secure ND Proxy.

A compromised Secure ND Proxy provisioned with an authorization
certificate with a KeyPurposeId value of id-kp-sendProxiedRouter is
able, like a compromised router, to siphon off traffic from the host,
or mount a man-in-the-middle attack, for hosts communicating to off-
link hosts. A compromised Secure ND Proxy provisioned with an
authorization certificate with a KeyPurposeId value of
id-kp-sendProxiedOwner can siphon off traffic or mount a man-in-the-
middle attack for communication between on-link hosts, even if the
hosts use SEND. Note that different application scenarios may
require one type of authorization, the other, or both. To minimize
security risks, authorization capabilities MUST NOT exceed the ones
strictly required by the application scenario to be deployed.

The messages for which a Secure ND Proxy performs its function and
the link for which this function is performed MUST be configured
appropriately for each proxy and scenario. This configuration is
especially relevant if Secure Proxy ND is used for translating ND
messages from one link to another.

Section 7 discusses the security considerations resulting from the
decision to append or omit the PS option, depending on the SEND-
awareness of the proxied nodes.

Protection against replay attacks from unsolicited messages such as
NA, RA, and Redirects is provided by means of the Timestamp option.
When Secure ND Proxy is used, each host, and each proxy acting on
behalf of that host, are considered to be different peers in terms of
timestamp verification. Since the information provided by the host
and a proxy, including different link-layer addresses, may be
different, a replay attack could affect the operation of a third
node: replaying messages issued by a host that is no longer in the
link can prevent the use of a proxy, and replaying messages of a
proxy when the host is back in the link can prevent communication
with the host. This kind of attack can be performed until the
timestamp of the peer (either the host or a proxy) is no longer valid
for the receiver. The window of vulnerability is in general larger
for the first message received from a new peer than for subsequent
messages received from the same peer (see [RFC3971]). A more
detailed analysis of the possible attacks related to the Timestamp
option is described in Section 6.3 of [RFC5909].

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9. IANA Considerations

IANA has allocated the following a new IPv6 Neighbor Discovery Option
type for the PS option, as 32. The value has been allocated from the
namespace specified in the IANA "IPv6 Neighbor Discovery Option
Formats" registry located at
http://www.iana.org/assignments/icmpv6-parameters.

IANA has also allocated the following new 128-bit value under the
"Cryptographically Generated Addresses (CGA) Message Type Name Space"
registry [RFC3972]:

0x09F5 2BE5 3B62 4C76 CB96 4E7F CDC9 2804.

10. Acknowledgements

The text has benefited from feedback provided by Jari Arkko, Jean-
Michel Combes, Roque Gagliano, Tony Cheneau, Marcelo Bagnulo, Alexey
Melnikov, Sandra Murphy, and Sean Turner.

The work of Alberto Garcia-Martinez was supported in part by the T2C2
project (TIN2008-06739-C04-01, granted by the Spanish Science and
Innovation Ministry).

11. References

11.1. Normative References

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

[RFC3971] Arkko, J., Ed., 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.

[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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[RFC5213] Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
RFC 5213, August 2008.

[RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275, July 2011.

[RFC6494] Gagliano, R., Krishnan, S., and A. Kukec, "Certificate
Profile and Certificate Management for SEcure Neighbor
Discovery (SEND)", RFC 6494, February 2012.

[RSA] RSA Laboratories, "PKCS #1 v2.1: RSA Cryptography
Standard", June 2002.

[SHA1] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-1 , April 1995.

11.2. Informative References

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

[RFC5909] Combes, J-M., Krishnan, S., and G. Daley, "Securing
Neighbor Discovery Proxy: Problem Statement", RFC 5909,
July 2010.

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Authors' Addresses

Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada

Phone: +1 514 345 7900 x42871
EMail: [email protected]

Julien Laganier
Juniper Networks
1094 North Mathilda Avenue
Sunnyvale, CA 94089
USA

Phone: +1 408 936 0385
EMail: [email protected]

Marco Bonola
Rome Tor Vergata University
Via del Politecnico, 1
Rome I-00133
Italy

Phone:
EMail: [email protected]

Alberto Garcia-Martinez
U. Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
Spain

Phone: +34 91 6248782
EMail: [email protected]
URI: http://www.it.uc3m.es/

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