RFC 3971
SEcure Neighbor Discovery (SEND)
Network Working Group J. Arkko, Ed. Request for Comments: 3971 Ericsson Category: Standards Track J. Kempf DoCoMo Communications Labs USA B. Zill Microsoft P. Nikander Ericsson March 2005 SEcure Neighbor Discovery (SEND) Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2005).Abstract
IPv6 nodes use the Neighbor Discovery Protocol (NDP) to discover other nodes on the link, to determine their link-layer addresses to find routers, and to maintain reachability information about the paths to active neighbors. If not secured, NDP is vulnerable to various attacks. This document specifies security mechanisms for NDP. Unlike those in the original NDP specifications, these mechanisms do not use IPsec.
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Specification of Requirements . . . . . . . . . . . . . 4 2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Neighbor and Router Discovery Overview. . . . . . . . . . . . 6 4. Secure Neighbor Discovery Overview. . . . . . . . . . . . . . 8 5. Neighbor Discovery Protocol Options . . . . . . . . . . . . . 9 5.1. CGA Option. . . . . . . . . . . . . . . . . . . . . . . 10 5.1.1. Processing Rules for Senders. . . . . . . . . . 11 5.1.2. Processing Rules for Receivers. . . . . . . . . 12 5.1.3. Configuration . . . . . . . . . . . . . . . . . 13 5.2. RSA Signature Option. . . . . . . . . . . . . . . . . . 14 5.2.1. Processing Rules for Senders. . . . . . . . . . 16 5.2.2. Processing Rules for Receivers. . . . . . . . . 16 5.2.3. Configuration . . . . . . . . . . . . . . . . . 17 5.2.4. Performance Considerations. . . . . . . . . . . 18 5.3. Timestamp and Nonce Options . . . . . . . . . . . . . . 19 5.3.1. Timestamp Option. . . . . . . . . . . . . . . . 19 5.3.2. Nonce Option. . . . . . . . . . . . . . . . . . 20 5.3.3. Processing Rules for Senders. . . . . . . . . . 21 5.3.4. Processing Rules for Receivers. . . . . . . . . 21 6. Authorization Delegation Discovery. . . . . . . . . . . . . . 24 6.1. Authorization Model . . . . . . . . . . . . . . . . . . 24 6.2. Deployment Model. . . . . . . . . . . . . . . . . . . . 25 6.3. Certificate Format. . . . . . . . . . . . . . . . . . . 26 6.3.1. Router Authorization Certificate Profile. . . . 26 6.3.2. Suitability of Standard Identity Certificates . 29 6.4. Certificate Transport . . . . . . . . . . . . . . . . . 29 6.4.1. Certification Path Solicitation Message Format. 30 6.4.2. Certification Path Advertisement Message Format 32 6.4.3. Trust Anchor Option . . . . . . . . . . . . . . 34 6.4.4. Certificate Option. . . . . . . . . . . . . . . 36 6.4.5. Processing Rules for Routers. . . . . . . . . . 37 6.4.6. Processing Rules for Hosts. . . . . . . . . . . 38 6.5. Configuration . . . . . . . . . . . . . . . . . . . . . 39 7. Addressing. . . . . . . . . . . . . . . . . . . . . . . . . . 40 7.1. CGAs. . . . . . . . . . . . . . . . . . . . . . . . . . 40 7.2. Redirect Addresses. . . . . . . . . . . . . . . . . . . 40 7.3. Advertised Subnet Prefixes. . . . . . . . . . . . . . . 40 7.4. Limitations . . . . . . . . . . . . . . . . . . . . . . 41 8. Transition Issues . . . . . . . . . . . . . . . . . . . . . . 42 9. Security Considerations . . . . . . . . . . . . . . . . . . . 44 9.1. Threats to the Local Link Not Covered by SEND . . . . . 44 9.2. How SEND Counters Threats to NDP. . . . . . . . . . . . 45 9.2.1. Neighbor Solicitation/Advertisement Spoofing. . 45 9.2.2. Neighbor Unreachability Detection Failure . . . 46 9.2.3. Duplicate Address Detection DoS Attack. . . . . 46
9.2.4. Router Solicitation and Advertisement Attacks . 46
9.2.5. Replay Attacks. . . . . . . . . . . . . . . . . 47
9.2.6. Neighbor Discovery DoS Attack . . . . . . . . . 48
9.3. Attacks against SEND Itself . . . . . . . . . . . . . . 48
10. Protocol Values . . . . . . . . . . . . . . . . . . . . . . . 49
10.1. Constants . . . . . . . . . . . . . . . . . . . . . . . 49
10.2. Variables . . . . . . . . . . . . . . . . . . . . . . . 49
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
12. References. . . . . . . . . . . . . . . . . . . . . . . . . . 50
12.1. Normative References. . . . . . . . . . . . . . . . . . 50
12.2. Informative References. . . . . . . . . . . . . . . . . 51
Appendices. . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
A. Contributors and Acknowledgments. . . . . . . . . . . . 53
B. Cache Management. . . . . . . . . . . . . . . . . . . . 53
C. Message Size When Carrying Certificates . . . . . . . . 54
Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . . 55
Full Copyright Statements . . . . . . . . . . . . . . . . . . . . 56
1. Introduction
IPv6 defines the Neighbor Discovery Protocol (NDP) in RFCs 2461 [4]
and 2462 [5]. Nodes on the same link use NDP to discover each
other's presence and link-layer addresses, to find routers, and to
maintain reachability information about the paths to active
neighbors. NDP is used by both hosts and routers. Its functions
include Neighbor Discovery (ND), Router Discovery (RD), Address
Autoconfiguration, Address Resolution, Neighbor Unreachability
Detection (NUD), Duplicate Address Detection (DAD), and Redirection.
The original NDP specifications called for the use of IPsec to
protect NDP messages. However, the RFCs do not give detailed
instructions for using IPsec to do this. In this particular
application, IPsec can only be used with a manual configuration of
security associations, due to bootstrapping problems in using IKE
[19, 15]. Furthermore, the number of manually configured security
associations needed for protecting NDP can be very large [20], making
that approach impractical for most purposes.
The SEND protocol is designed to counter the threats to NDP. These
threats are described in detail in [22]. SEND is applicable in
environments where physical security on the link is not assured (such
as over wireless) and attacks on NDP are a concern.
This document is organized as follows. Sections 2 and 3 define some
terminology and present a brief review of NDP, respectively. Section
4 describes the overall approach to securing NDP. This approach
involves the use of new NDP options to carry public key - based
signatures. A zero-configuration mechanism is used for showing
address ownership on individual nodes; routers are certified by a trust anchor [7]. The formats, procedures, and cryptographic mechanisms for the zero-configuration mechanism are described in a related specification [11]. The required new NDP options are discussed in Section 5. Section 6 describes the mechanism for distributing certification paths to establish an authorization delegation chain to a trust anchor. Finally, Section 8 discusses the co-existence of secured and unsecured NDP on the same link, and Section 9 discusses security considerations for SEcure Neighbor Discovery (SEND). The use of identity certificates provisioned on end hosts for authorizing address use is out of the scope for this document, as is the security of NDP when the entity defending an address is not the same as the entity claiming that address (also known as "proxy ND"). These are extensions of SEND that may be treated in separate documents, should the need arise.1.1. Specification of Requirements
In this document, several words are used to signify the requirements of the specification. These words are often capitalized. The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", and "MAY" are to be interpreted as described in [2].2. Terms
Authorization Delegation Discovery (ADD) A process through which SEND nodes can acquire a certification path from a peer node to a trust anchor. Certificate Revocation List (CRL) In one method of certificate revocation, an authority periodically issues a signed data structure called the Certificate Revocation List. This is a time-stamped list identifying revoked certificates, signed by the issuer, and made freely available in a public repository. Certification Path Advertisement (CPA) The advertisement message used in the ADD process.
Certification Path Solicitation (CPS)
The solicitation message used in the ADD process.
Cryptographically Generated Address (CGA)
A technique [11] whereby an IPv6 address of a node is
cryptographically generated by using a one-way hash function from
the node's public key and some other parameters.
Distinguished Encoding Rules (DER)
An encoding scheme for data values, defined in [12].
Duplicate Address Detection (DAD)
A mechanism assuring that two IPv6 nodes on the same link are not
using the same address.
Fully Qualified Domain Name (FQDN)
A fully qualified domain name consists of a host and domain name,
including the top-level domain.
Internationalized Domain Name (IDN)
Internationalized Domain Names can be used to represent domain
names that contain characters outside the ASCII set. See RFC 3490
[9].
Neighbor Discovery (ND)
The Neighbor Discovery function of the Neighbor Discovery Protocol
(NDP). NDP contains functions besides ND.
Neighbor Discovery Protocol (NDP)
The IPv6 Neighbor Discovery Protocol [7, 8].
The Neighbor Discovery Protocol is a part of ICMPv6 [6].
Neighbor Unreachability Detection (NUD)
A mechanism used for tracking the reachability of neighbors.
Non-SEND node
An IPv6 node that does not implement this specification but uses
only the Neighbor Discovery protocol defined in RFCs 2461 and
2462, as updated, without security.
Nonce
An unpredictable random or pseudo-random number generated by a
node and used exactly once. In SEND, nonces are used to assure
that a particular advertisement is linked to the solicitation that
triggered it.
Router Authorization Certificate
An X.509v3 [7] public key certificate using the profile specified
in Section 6.3.1.
SEND node
An IPv6 node that implements this specification.
Router Discovery (RD)
Router Discovery allows the hosts to discover what routers exist
on the link, and what subnet prefixes are available. Router
Discovery is a part of the Neighbor Discovery Protocol.
Trust Anchor
Hosts are configured with a set of trust anchors to protect Router
Discovery. A trust anchor is an entity that the host trusts to
authorize routers to act as routers. A trust anchor configuration
consists of a public key and some associated parameters (see
Section 6.5 for a detailed explanation of these parameters).
3. Neighbor and Router Discovery Overview
The Neighbor Discovery Protocol has several functions. Many of these
are overloaded on a few central message types, such as the ICMPv6
Neighbor Advertisement message. In this section, we review some of
these tasks and their effects in order to better understand how the
messages should be treated. This section is not normative, and if
this section and the original Neighbor Discovery RFCs are in
conflict, the original RFCs, as updated, take precedence.
The main functions of NDP are as follows:
o The Router Discovery function allows IPv6 hosts to discover the
local routers on an attached link. Router Discovery is described
in Section 6 of RFC 2461 [4]. The main purpose of Router
Discovery is to find neighboring routers willing to forward
packets on behalf of hosts. Subnet prefix discovery involves
determining which destinations are directly on a link; this
information is necessary in order to know whether a packet should
be sent to a router or directly to the destination node.
o The Redirect function is used for automatically redirecting a host
to a better first-hop router, or to inform hosts that a
destination is in fact a neighbor (i.e., on-link). Redirect is
specified in Section 8 of RFC 2461 [4].
o Address Autoconfiguration is used for automatically assigning
addresses to a host [5]. This allows hosts to operate without
explicit configuration related to IP connectivity. The default
autoconfiguration mechanism is stateless. To create IP addresses,
hosts use any prefix information delivered to them during Router
Discovery and then test the newly formed addresses for uniqueness.
A stateful mechanism, DHCPv6 [18], provides additional
autoconfiguration features.
o Duplicate Address Detection (DAD) is used for preventing address
collisions [5]: for instance, during Address Autoconfiguration. A
node that intends to assign a new address to one of its interfaces
first runs the DAD procedure to verify that no other node is using
the same address. As the rules forbid the use of an address until
it has been found unique, no higher layer traffic is possible
until this procedure has been completed. Thus, preventing attacks
against DAD can help ensure the availability of communications for
the node in question.
o The Address Resolution function allows a node on the link to
resolve another node's IPv6 address to the corresponding link-
layer address. Address Resolution is defined in Section 7.2 of
RFC 2461 [4], and it is used for hosts and routers alike. Again,
no higher level traffic can proceed until the sender knows the
link layer address of the destination node or the next hop router.
Note that the source link layer address on link layer frames is
not checked against the information learned through Address
Resolution. This allows for an easier addition of network
elements such as bridges and proxies and eases the stack
implementation requirements, as less information has to be passed
from layer to layer.
o Neighbor Unreachability Detection (NUD) is used for tracking the
reachability of neighboring nodes, both hosts and routers. NUD is
defined in Section 7.3 of RFC 2461 [4]. NUD is security
sensitive, because an attacker could claim that reachability
exists when in fact it does not.
The NDP messages follow the ICMPv6 message format. All NDP functions
are realized by using the Router Solicitation (RS), Router
Advertisement (RA), Neighbor Solicitation (NS), Neighbor
Advertisement (NA), and Redirect messages. An actual NDP message
includes an NDP message header, consisting of an ICMPv6 header and ND
message-specific data, and zero or more NDP options. The NDP message
options are formatted in the Type-Length-Value format.
<------------NDP Message---------------->
*-------------------------------------------------------------*
| IPv6 Header | ICMPv6 | ND Message- | ND Message |
| Next Header = 58 | Header | specific | Options |
| (ICMPv6) | | data | |
*-------------------------------------------------------------*
<--NDP Message header-->
4. Secure Neighbor Discovery Overview
To secure the various functions in NDP, a set of new Neighbor
Discovery options is introduced. They are used to protect NDP
messages. This specification introduces these options, an
authorization delegation discovery process, an address ownership
proof mechanism, and requirements for the use of these components in
NDP.
The components of the solution specified in this document are as
follows:
o Certification paths, anchored on trusted parties, are expected to
certify the authority of routers. A host must be configured with
a trust anchor to which the router has a certification path before
the host can adopt the router as its default router.
Certification Path Solicitation and Advertisement messages are
used to discover a certification path to the trust anchor without
requiring the actual Router Discovery messages to carry lengthy
certification paths. The receipt of a protected Router
Advertisement message for which no certification path is available
triggers the authorization delegation discovery process.
o Cryptographically Generated Addresses are used to make sure that
the sender of a Neighbor Discovery message is the "owner" of the
claimed address. A public-private key pair is generated by all
nodes before they can claim an address. A new NDP option, the CGA
option, is used to carry the public key and associated parameters.
This specification also allows a node to use non-CGAs with
certificates that authorize their use. However, the details of
such use are beyond the scope of this specification and are left
for future work.
o A new NDP option, the RSA Signature option, is used to protect all
messages relating to Neighbor and Router discovery.
Public key signatures protect the integrity of the messages and
authenticate the identity of their sender. The authority of a
public key is established either with the authorization delegation
process, by using certificates, or through the address ownership
proof mechanism, by using CGAs, or with both, depending on
configuration and the type of the message protected.
Note: RSA is mandated because having multiple signature algorithms
would break compatibility between implementations or increase
implementation complexity by forcing the implementation of
multiple algorithms and the mechanism to select among them. A
second signature algorithm is only necessary as a recovery
mechanism, in case a flaw is found in RSA. If this happens, a
stronger signature algorithm can be selected, and SEND can be
revised. The relationship between the new algorithm and the RSA-
based SEND described in this document would be similar to that
between the RSA-based SEND and Neighbor Discovery without SEND.
Information signed with the stronger algorithm has precedence over
that signed with RSA, in the same way that RSA-signed information
now takes precedence over unsigned information. Implementations
of the current and revised specs would still be compatible.
o In order to prevent replay attacks, two new Neighbor Discovery
options, Timestamp and Nonce, are introduced. Given that Neighbor
and Router Discovery messages are in some cases sent to multicast
addresses, the Timestamp option offers replay protection without
any previously established state or sequence numbers. When the
messages are used in solicitation-advertisement pairs, they are
protected with the Nonce option.
5. Neighbor Discovery Protocol Options
The options described in this section MUST be supported.
5.1. CGA Option
The CGA option allows the verification of the sender's CGA. The format of the CGA option is described as follows: 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 | Pad Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . CGA Parameters . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Padding . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 11 Length The length of the option (including the Type, Length, Pad Length, Reserved, CGA Parameters, and Padding fields) in units of 8 octets. Pad Length The number of padding octets beyond the end of the CGA Parameters field but within the length specified by the Length field. Padding octets MUST be set to zero by senders and ignored by receivers. Reserved An 8-bit field reserved for future use. The value MUST be initialized to zero by the sender and MUST be ignored by the receiver.
CGA Parameters
A variable-length field containing the CGA Parameters data
structure described in Section 4 of [11].
This specification requires that if both the CGA option and the
RSA Signature option are present, then the public key found from
the CGA Parameters field in the CGA option MUST be that referred
by the Key Hash field in the RSA Signature option. Packets
received with two different keys MUST be silently discarded. Note
that a future extension may provide a mechanism allowing the owner
of an address and the signer to be different parties.
Padding
A variable-length field making the option length a multiple of 8,
containing as many octets as specified in the Pad Length field.
5.1.1. Processing Rules for Senders
If the node has been configured to use SEND, the CGA option MUST be
present in all Neighbor Solicitation and Advertisement messages and
MUST be present in Router Solicitation messages unless they are sent
with the unspecified source address. The CGA option MAY be present
in other messages.
A node sending a message using the CGA option MUST construct the
message as follows:
The CGA Parameter field in the CGA option is filled according to
the rules presented above and in [11]. The public key in the
field is taken from the configuration used to generate the CGA,
typically from a data structure associated with the source
address. The address MUST be constructed as specified in Section
4 of [11]. Depending on the type of the message, this address
appears in different places, as follows:
Redirect
The address MUST be the source address of the message.
Neighbor Solicitation
The address MUST be the Target Address for solicitations sent for
Duplicate Address Detection; otherwise it MUST be the source
address of the message.
Neighbor Advertisement
The address MUST be the source address of the message.
Router Solicitation
The address MUST be the source address of the message. Note that
the CGA option is not used when the source address is the
unspecified address.
Router Advertisement
The address MUST be the source address of the message.
5.1.2. Processing Rules for Receivers
Neighbor Solicitation and Advertisement messages without the CGA
option MUST be treated as unsecured (i.e., processed in the same way
as NDP messages sent by a non-SEND node). The processing of
unsecured messages is specified in Section 8. Note that SEND nodes
that do not attempt to interoperate with non-SEND nodes MAY simply
discard the unsecured messages.
Router Solicitation messages without the CGA option MUST also be
treated as unsecured, unless the source address of the message is the
unspecified address.
Redirect, Neighbor Solicitation, Neighbor Advertisement, Router
Solicitation, and Router Advertisement messages containing a CGA
option MUST be checked as follows:
If the interface has been configured to use CGA, the receiving
node MUST verify the source address of the packet by using the
algorithm described in Section 5 of [11]. The inputs to the
algorithm are the claimed address, as defined in the previous
section, and the CGA Parameters field.
If the CGA verification is successful, the recipient proceeds with
a more time-consuming cryptographic check of the signature. Note
that even if the CGA verification succeeds, no claims about the
validity of the use can be made until the signature has been
checked.
A receiver that does not support CGA or has not specified its use for
a given interface can still verify packets by using trust anchors,
even if a CGA is used on a packet. In such a case, the CGA property
of the address is simply left unverified.
5.1.3. Configuration
All nodes that support the verification of the CGA option MUST record the following configuration information: minbits The minimum acceptable key length for public keys used in the generation of CGAs. The default SHOULD be 1024 bits. Implementations MAY also set an upper limit for the amount of computation needed when verifying packets that use these security associations. The upper limit SHOULD be at least 2048 bits. Any implementation should follow prudent cryptographic practice in determining the appropriate key lengths. All nodes that support the sending of the CGA option MUST record the following configuration information: CGA parameters Any information required to construct CGAs, as described in [11].
5.2. RSA Signature Option
The RSA Signature option allows public key-based signatures to be attached to NDP messages. The format of the RSA Signature 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 . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 12 Length The length of the option (including the Type, Length, Reserved, Key Hash, Digital Signature, and Padding fields) in units of 8 octets. 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 [14] hash of the public key used for constructing
the signature. The SHA-1 hash is taken over the presentation used
in the Public Key field of the CGA Parameters data structure
carried in the CGA option. Its purpose is to associate the
signature to a particular key known by the receiver. Such a key
can either be stored in the certificate cache of the receiver or
be received in the CGA option in the same message.
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 [11] value for SEND, 0x086F
CA5E 10B2 00C9 9C8C E001 6427 7C08. (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 RSA Signature option.
The signature value is computed with the RSASSA-PKCS1-v1_5
algorithm and SHA-1 hash, as defined in [13].
This field starts after the Key Hash field. The length of the
Digital Signature field is determined by the length of the RSA
Signature option minus the length of the other fields (including
the variable length Pad field).
Padding
This variable-length field contains padding, as many bytes long as
remain after the end of the signature.
5.2.1. Processing Rules for Senders
If the node has been configured to use SEND, Neighbor Solicitation, Neighbor Advertisement, Router Advertisement, and Redirect messages MUST contain the RSA Signature option. Router Solicitation messages not sent with the unspecified source address MUST contain the RSA Signature option. A node sending a message with the RSA Signature option MUST construct the message as follows: o The message is constructed in its entirety, without the RSA Signature option. o The RSA Signature option is added as the last option in the message. o The data to be signed is constructed as explained in Section 5.2, under the description of the Digital Signature field. o The message, in the form defined above, is signed by using the configured private key, and the resulting PKCS#1 v1.5 signature is put in the Digital Signature field.5.2.2. Processing Rules for Receivers
Neighbor Solicitation, Neighbor Advertisement, Router Advertisement, and Redirect messages without the RSA Signature option MUST be treated as unsecured (i.e., processed in the same way as NDP messages sent by a non-SEND node). See Section 8. Router Solicitation messages without the RSA Signature option MUST also be treated as unsecured, unless the source address of the message is the unspecified address. Redirect, Neighbor Solicitation, Neighbor Advertisement, Router Solicitation, and Router Advertisement messages containing an RSA Signature option MUST be checked as follows: o The receiver MUST ignore any options that come after the first RSA Signature option. (The options are ignored for both signature verification and NDP processing purposes.) o The Key Hash field MUST indicate the use of a known public key, either one learned from a preceding CGA option in the same message, or one known by other means.
o The Digital Signature field MUST have correct encoding and MUST
not exceed the length of the RSA Signature option minus the
Padding.
o The Digital Signature verification MUST show that the signature
has been calculated as specified in the previous section.
o If the use of a trust anchor has been configured, a valid
certification path (see Section 6.3) between the receiver's trust
anchor and the sender's public key MUST be known.
Note that the receiver may verify just the CGA property of a
packet, even if, in addition to CGA, the sender has used a trust
anchor.
Messages that do not pass all the above tests MUST be silently
discarded if the host 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. The receiver MAY also otherwise
silently discard packets (e.g., as a response to an apparent CPU
exhausting DoS attack).
5.2.3. Configuration
All nodes that support the reception of the RSA Signature options
MUST allow the following information to be configured for each
separate NDP message type:
authorization method
This parameter determines the method through which the authority
of the sender is determined. It can have four values:
trust anchor
The authority of the sender is verified as described in
Section 6.3. The sender may claim additional authorization
through the use of CGAs, but this is neither required nor
verified.
CGA
The CGA property of the sender's address is verified as
described in [11]. The sender may claim additional
authority through a trust anchor, but this is neither
required nor verified.
trust anchor and CGA
Both the trust anchor and the CGA verification is required.
trust anchor or CGA
Either the trust anchor or the CGA verification is required.
anchor
The allowed trust anchor(s), if the authorization method is not
set to CGA.
All nodes that support sending RSA Signature options MUST record the
following configuration information:
keypair
A public-private key pair. If authorization delegation is in
use, a certification path from a trust anchor to this key pair
must exist.
CGA flag
A flag that indicates whether CGA is used or not. This flag
may be per interface or per node. (Note that in future
extensions of the SEND protocol, this flag may also be per
subnet prefix.)
5.2.4. Performance Considerations
The construction and verification of the RSA Signature option is
computationally expensive. In the NDP context, however, hosts
typically only have to perform a few signature operations as they
enter a link, a few operations as they find a new on-link peer with
which to communicate, or Neighbor Unreachability Detection with
existing neighbors.
Routers are required to perform a larger number of operations,
particularly when the frequency of router advertisements is high due
to mobility requirements. Still, the number of required signature
operations is on the order of a few dozen per second, some of which
can be precomputed as explained below. A large number of router
solicitations may cause a higher demand for performing asymmetric
operations, although the base NDP protocol limits the rate at which
multicast responses to solicitations can be sent.
Signatures can be precomputed for unsolicited (multicast) Neighbor and Router Advertisements if the timing of the future advertisements is known. Typically, solicited neighbor advertisements are sent to the unicast address from which the solicitation was sent. Given that the IPv6 header is covered by the signature, it is not possible to precompute solicited advertisements.5.3. Timestamp and Nonce Options
5.3.1. Timestamp Option
The purpose of the Timestamp option is to make sure that unsolicited advertisements and redirects have not been replayed. The format of this option is described in the following: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Timestamp + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 13 Length The length of the option (including the Type, Length, Reserved, and Timestamp fields) in units of 8 octets; i.e., 2. Reserved A 48-bit field reserved for future use. The value MUST be initialized to zero by the sender and MUST be ignored by the receiver.
Timestamp
A 64-bit unsigned integer field containing a timestamp. The value
indicates the number of seconds since January 1, 1970, 00:00 UTC,
by using a fixed point format. In this format, the integer number
of seconds is contained in the first 48 bits of the field, and the
remaining 16 bits indicate the number of 1/64K fractions of a
second.
Implementation note: This format is compatible with the usual
representation of time under UNIX, although the number of bits
available for the integer and fraction parts may vary.
5.3.2. Nonce Option
The purpose of the Nonce option is to make sure that an advertisement
is a fresh response to a solicitation sent earlier by the node. The
format of this option is described in the following:
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 | Nonce ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
14
Length
The length of the option (including the Type, Length, and Nonce
fields) in units of 8 octets.
Nonce
A field containing a random number selected by the sender of the
solicitation message. The length of the random number MUST be at
least 6 bytes. The length of the random number MUST be selected
so that the length of the nonce option is a multiple of 8 octets.
5.3.3. Processing Rules for Senders
If the node has been configured to use SEND, all solicitation messages MUST include a Nonce. When sending a solicitation, the sender MUST store the nonce internally so that it can recognize any replies containing that particular nonce. If the node has been configured to use SEND, all advertisements sent in reply to a solicitation MUST include a Nonce, copied from the received solicitation. Note that routers may decide to send a multicast advertisement to all nodes instead of a response to a specific host. In such a case, the router MAY still include the nonce value for the host that triggered the multicast advertisement. (Omitting the nonce value may cause the host to ignore the router's advertisement, unless the clocks in these nodes are sufficiently synchronized so that timestamps function properly.) If the node has been configured to use SEND, all solicitation, advertisement, and redirect messages MUST include a Timestamp. Senders SHOULD set the Timestamp field to the current time, according to their real time clocks.5.3.4. Processing Rules for Receivers
The processing of the Nonce and Timestamp options depends on whether a packet is a solicited advertisement. A system may implement the distinction in various ways. Section 5.3.4.1 defines the processing rules for solicited advertisements. Section 5.3.4.2 defines the processing rules for all other messages. In addition, the following rules apply in all cases: o Messages received without at least one of the Timestamp and Nonce options MUST be treated as unsecured (i.e., processed in the same way as NDP messages sent by a non-SEND node). o Messages received with the RSA Signature option but without the Timestamp option MUST be silently discarded. o Solicitation messages received with the RSA Signature option but without the Nonce option MUST be silently discarded. o Advertisements sent to a unicast destination address with the RSA Signature option but without a Nonce option SHOULD be processed as unsolicited advertisements.
o An implementation MAY use some mechanism such as a timestamp cache
to strengthen resistance to replay attacks. When there is a very
large number of nodes on the same link, or when a cache filling
attack is in progress, it is possible that the cache holding the
most recent timestamp per sender will become full. In this case,
the node MUST remove some entries from the cache or refuse some
new requested entries. The specific policy as to which entries
are preferred over others is left as an implementation decision.
However, typical policies may prefer existing entries to new ones,
CGAs with a large Sec value to smaller Sec values, and so on. The
issue is briefly discussed in Appendix B.
o The receiver MUST be prepared to receive the Timestamp and Nonce
options in any order, as per RFC 2461 [4], Section 9.
5.3.4.1. Processing Solicited Advertisements
The receiver MUST verify that it has recently sent a matching
solicitation, and that the received advertisement contains a copy of
the Nonce sent in the solicitation.
If the message contains a Nonce option but the Nonce value is not
recognized, the message MUST be silently discarded.
Otherwise, if the message does not contain a Nonce option, it MAY be
considered an unsolicited advertisement and processed according to
Section 5.3.4.2.
If the message is accepted, the receiver SHOULD store the receive
time of the message and the timestamp time in the message, as
specified in Section 5.3.4.2.
5.3.4.2. Processing All Other Messages
Receivers SHOULD be configured with an allowed timestamp Delta value,
a "fuzz factor" for comparisons, and an allowed clock drift
parameter. The recommended default value for the allowed Delta is
TIMESTAMP_DELTA; for fuzz factor TIMESTAMP_FUZZ; and for clock drift,
TIMESTAMP_DRIFT (see Section 10.2).
To facilitate timestamp checking, each node SHOULD store the
following information for each peer:
o The receive time of the last received and accepted SEND message.
This is called RDlast.
o The time stamp in the last received and accepted SEND message.
This is called TSlast.
An accepted SEND message is any successfully verified Neighbor
Solicitation, Neighbor Advertisement, Router Solicitation, Router
Advertisement, or Redirect message from the given peer. The RSA
Signature option MUST be used in such a message before it can update
the above variables.
Receivers SHOULD then check the Timestamp field as follows:
o When a message is received from a new peer (i.e., one that is not
stored in the cache), the received timestamp, TSnew, is checked,
and the packet is accepted if the timestamp is recent enough to
the reception time of the packet, RDnew:
-Delta < (RDnew - TSnew) < +Delta
The RDnew and TSnew values SHOULD be stored in the cache as RDlast
and TSlast.
o If the timestamp is NOT within the boundaries but the message is a
Neighbor Solicitation message that the receiver should answer, the
receiver SHOULD respond to the message. However, even if it does
respond to the message, it MUST NOT create a Neighbor Cache entry.
This allows nodes that have large differences in their clocks to
continue communicating with each other by exchanging NS/NA pairs.
o When a message is received from a known peer (i.e., one that
already has an entry in the cache), the timestamp is checked
against the previously received SEND message:
TSnew + fuzz > TSlast + (RDnew - RDlast) x (1 - drift) - fuzz
If this inequality does not hold, the receiver SHOULD silently
discard the message. If, on the other hand, the inequality holds,
the receiver SHOULD process the message.
Moreover, if the above inequality holds and TSnew > TSlast, the
receiver SHOULD update RDlast and TSlast. Otherwise, the receiver
MUST NOT update RDlast or TSlast.
As unsolicited messages may be used in a Denial-of-Service attack to
make the receiver verify computationally expensive signatures, all
nodes SHOULD apply a mechanism to prevent excessive use of resources
for processing such messages.