Network Working Group J. Arkko
Internet-Draft Ericsson
Expires: December 18, 2003 June 19, 2003
SEcure Neighbor Discovery (SEND)
draft-arkko-send-ndopt-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
IPv6 nodes use the Neighbor Discovery (ND) protocol to discover other
nodes on the link, to determine each other's link-layer addresses, to
find routers and to maintain reachability information about the paths
to active neighbors. If not secured, ND protocol is vulnerable to
various attacks. This document specifies security mechanisms for ND.
Contrary to the original ND specifications, these mechanisms do not
make use of IPsec.
The purpose of this draft is to present an alternative to the current
approach in the Working Group.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Neighbor and Router Discovery Overview . . . . . . . . . . 6
4. Secure Neighbor Discovery Overview . . . . . . . . . . . . 9
5. Neighbor Discovery Options . . . . . . . . . . . . . . . . 10
5.1 CGA Option . . . . . . . . . . . . . . . . . . . . . 10
5.1.1 Processing Rules for Senders . . . . . . . . .12
5.1.2 Processing Rules for Receivers . . . . . . . .12
5.2 Signature Option . . . . . . . . . . . . . . . . . . 13
5.2.1 Processing Rules for Senders . . . . . . . . .14
5.2.2 Processing Rules for Receivers . . . . . . . .15
5.2.3 Configuration . . . . . . . . . . . . . . . .15
5.3 Timestamp Option . . . . . . . . . . . . . . . . . . 17
5.4 Nonce Option . . . . . . . . . . . . . . . . . . . . 18
5.5 Proxy Neighbor Discovery . . . . . . . . . . . . . . 19
6. Authorization Delegation Discovery . . . . . . . . . . . . 20
6.1 Delegation Chain Solicitation Message Format . . . . 20
6.2 Delegation Chain Advertisement Message Format . . . 22
6.3 Trusted Root Option . . . . . . . . . . . . . . . . 24
6.4 Certificate Option . . . . . . . . . . . . . . . . . 25
6.5 Router Authorization Certificate Format . . . . . . 26
6.5.1 Field Values . . . . . . . . . . . . . . . . .27
6.6 Processing Rules for Routers . . . . . . . . . . . . 28
6.7 Processing Rules for Hosts . . . . . . . . . . . . . 29
7. Securing Neighbor Discovery with SEND . . . . . . . . . . 32
7.1 Neighbor Solicitation Messages . . . . . . . . . . . 32
7.1.1 Sending Secure Neighbor Solicitations . . . .32
7.1.2 Receiving Secure Neighbor Solicitations . . .32
7.2 Neighbor Advertisement Messages . . . . . . . . . . 32
7.2.1 Sending Secure Neighbor Advertisements . . . .32
7.2.2 Receiving Secure Neighbor Advertisements . . .33
7.3 Other Requirements . . . . . . . . . . . . . . . . . 33
8. Securing Router Discovery with SEND . . . . . . . . . . . 34
8.1 Router Solicitation Messages . . . . . . . . . . . . 34
8.1.1 Sending Secure Router Solicitations . . . . .34
8.1.2 Receiving Secure Router Solicitations . . . .34
8.2 Router Advertisement Messages . . . . . . . . . . . 34
8.2.1 Sending Secure Router Advertisements . . . . .34
8.2.2 Receiving Secure Router Advertisements . . . .35
8.3 Redirect Messages . . . . . . . . . . . . . . . . . 35
8.3.1 Sending Redirects . . . . . . . . . . . . . .35
8.3.2 Receiving Redirects . . . . . . . . . . . . .35
8.4 Other Requirements . . . . . . . . . . . . . . . . . 36
9. Co-Existence of SEND and ND . . . . . . . . . . . . . . . 37
10. Performance Considerations . . . . . . . . . . . . . . . . 38
11. Security Considerations . . . . . . . . . . . . . . . . . 39
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11.1 Threats to the Local Link Not Covered by SEND . . . 39
11.2 How SEND Counters Threats to Neighbor Discovery . . 39
11.2.1 Neighbor Solicitation/Advertisement Spoofing .39
11.2.2 Neighbor Unreachability Detection Failure . .41
11.2.3 Duplicate Address Detection DoS Attack . . . .41
11.2.4 Router Solicitation and Advertisement Attacks 41
11.2.5 Replay Attacks . . . . . . . . . . . . . . . .41
11.2.6 Neighbor Discovery DoS Attack . . . . . . . .42
11.3 Attacks against SEND Itself . . . . . . . . . . . . 42
12. IANA Considerations . . . . . . . . . . . . . . . . . . . 44
13. Comparison to AH-Based Approach . . . . . . . . . . . . . 45
Normative References . . . . . . . . . . . . . . . . . . . 48
Informative References . . . . . . . . . . . . . . . . . . 50
Author's Address . . . . . . . . . . . . . . . . . . . . . 51
A. Contributors . . . . . . . . . . . . . . . . . . . . . . . 52
B. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 53
C. IPR Considerations . . . . . . . . . . . . . . . . . . . . 54
Intellectual Property and Copyright Statements . . . . . . 55
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1. Introduction
IPv6 defines the Neighbor Discovery (ND) protocol in RFC 2461 [6].
Nodes on the same link use the ND protocol to discover each other's
presence, to determine each other's link-layer addresses, to find
routers and to maintain reachability information about the paths to
active neighbors. The ND protocol is used both by hosts and routers.
Its functions include Router Discovery (RD), Address Auto-
configuration, Address Resolution, Neighbor Unreachability Detection
(NUD), Duplicate Address Detection (DAD), and Redirection.
RFC 2461 called for the use of IPsec for protecting the ND messages.
However, it turns out that in this particular application IPsec can
only be used with a manual configuration of security associations due
to chicken-and-egg problems in using IKE [23, 21] before ND is
operational. Furthermore, the number of such manually configured
security associations needed for protecting ND is impractically large
[24]. Finally, RFC 2461 did not specify detailed instructions for
using IPsec to secure ND.
Section 4 describes our overall approach to securing ND. This
approach involves the use of new ND options to carry public-key based
signatures. A zero-configuration mechanism is used for showing
address ownership, and routers are certified by a trusted root. The
formats, procedures, and cryptographic mechanisms for the
zero-configuration mechanism are described in a related specification
[27].
Section 6 describes the mechanism for distributing certificate chains
to establish authorization delegation chain to a common trusted root.
The new ND options are discussed in Section 5, and Section 8 show how
to apply these components to securing Neighbor and Router Discovery.
Finally, Section 9 discusses the co-existence of secure and
non-secure Neighbor Discovery on the same link, Section 10 discusses
performance considerations, and Section 11 discusses security
considerations for SEND.
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2. Terms
Authorization Certificate (AC)
The signer of an authorization certificate has authorized the
entity designated in the certificate for a specific task or
service.
Authorization Delegation Discovery (ADD)
This is a process through which SEND nodes can acquire a
certificate chain from a peer node to a trusted root.
Cryptographically Generated Addresses (CGAs)
A technique [27, 31] where the address of the node is
cryptographically generated from the public key of the node and
some other parameters using a one-way hash function.
Duplicate Address Detection (DAD)
This mechanism defined in RFC 2462 [7] assures that two IPv6 nodes
on the same link are not using the same addresses.
Internet Control Message Protocol version 6 (ICMPv6)
The IPv6 control signaling protocol. Neighbor Discovery is a part
of ICMPv6.
Neighbor Discovery (ND)
The IPv6 Neighbor Discovery protocol [6].
Neighbor Unreachability Detection (NUD)
This mechanism defined in RFC 2461 [6] is used for tracking the
reachability of neighbors.
Nonce
Nonces are random numbers generated by a node. In SEND, they are
used to ensure that a particular advertisement is linked to the
solicitation that triggered it.
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3. Neighbor and Router Discovery Overview
IPv6 Neighbor and Router Discovery have several functions. Many of
these functions are overloaded on a few central message types such as
the ICMPv6 Neighbor Discovery message. In this section we explain
some of these tasks and their effects in order to understand better
how the messages should be treated. Where this section and the
original Neighbor Discovery RFCs are in conflict, the original RFCs
take precedence.
In IPv6, many of the tasks traditionally done at lower layers such as
ARP have been moved to the IP layer. As a consequence, unified
mechanisms can be applied across link layers, security mechanisms or
other extensions can be adopted more easily, and a clear separation
of the roles between the IP and link layer can be achieved.
The main functions of IPv6 Neighbor Discovery are as follows:
o Neighbor Unreachability Detection (NUD) is used for tracking the
reachability of neighbors, both hosts and routers. NUD is defined
in Section 7.3 of RFC 2461 [6]. NUD is security-sensitive,
because an attacker could falsely claim that reachability exists
when it in fact does not.
o Duplicate Address Detection (DAD) is used for preventing address
collisions [7]. A node that intends to assign a new address to
one of its interfaces runs first the DAD procedure to verify that
other nodes are not using the same address. Since the outlined
rules forbid the use of an address until it has been found unique,
no higher layer traffic is possible until this procedure has
completed. Thus, preventing attacks against DAD can help ensure
the availability of communications for the node in question.
o Address Resolution is similar to IPv4 ARP [20]. The Address
Resolution function resolves a node's IPv6 address to the
corresponding link-layer address for nodes on the link. Address
Resolution is defined in Section 7.2 of RFC 2461 [6] and it is
used for hosts and routers alike. Again, no higher level traffic
can proceed until the sender knows the hardware address of the
destination node or the next hop router. Note that like its
predecessor in ARP, IPv6 Neighbor Discovery does not check the
source link layer address 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 needs to be passed
from layer to layer.
o Address Autoconfiguration is used for automatically assigning
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addresses to a host [7]. This allows hosts to operate without
configuration related to IP connectivity. The Address
Autoconfiguration mechanism is stateless, where the hosts use
prefix information delivered to them during Router Discovery to
create addresses, and then test these addresses for uniqueness
using the DAD procedure. A stateful mechanism, DHCPv6 [25],
provides additional Autoconfiguration features. Router and Prefix
Discovery and Duplicate Address Detection have an effect to the
Address Autoconfiguration tasks.
o The Redirect function is used for automatically redirecting hosts
to an alternate router. Redirect is specified in Section 8 of RFC
2461 [6]. It is similar to the ICMPv4 Redirect message [19].
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 [6]. The main purpose of Router
Discovery is to find neighboring routers that are willing to
forward packets on behalf of hosts. 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 to the destination node directly.
Typically, address autoconfiguration and other tasks can not
proceed until suitable routers and prefixes have been found.
The Neighbor Discovery messages follow the ICMPv6 message format and
ICMPv6 types from 133 to 137. The IPv6 Next Header value for ICMPv6
is 58. The actual Neighbor Discovery message includes an ND message
header consisting of ICMPv6 header and ND message-specific data, and
zero or more ND options:
<------------ND Message----------------->
*-------------------------------------------------------------*
| IPv6 Header | ICMPv6 | ND message- | ND Message |
| Next Header = 58 | Header | specific | Options |
| (ICMPv6) | | data | |
*-------------------------------------------------------------*
<--ND Message header--->
The ND message options are formatted in the Type-Length-Value format.
All IPv6 ND protocol functions are realized using the following
messages:
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ICMPv6 Type Message
------------------------------------
133 Router Solicitation (RS)
134 Router Advertisement (RA)
135 Neighbor Solicitation (NS)
136 Neighbor Advertisement (NA)
137 Redirect
The functions of the ND protocol are realized using these messages as
follows:
o Router Discovery uses the RS and RA messages.
o Duplicate Address Detection uses the NS and NA messages.
o Address Autoconfiguration uses the NS, NA, RS, and RA messages.
o Address Resolution uses the NS and NA messages.
o Neighbor Unreachability Detection uses the NS and NA messages.
o Redirect uses the Redirect message.
The destination addresses used in these messages are as follows:
o Neighbor Solicitation: The destination address is either the
solicited-node multicast address, unicast address, or an anycast
address.
o Neighbor Advertisement: The destination address is either a
unicast address or the All Nodes multicast address [1].
o Router Solicitation: The destination address is typically the All
Routers multicast address [1].
o Router Advertisement: The destination address can be either a
unicast or the All Nodes multicast address [1]. Like the
solicitation message, the advertisement is also local to the link
only.
o Redirect: This message is always sent from the router's link-local
address to the source address of the packet that triggered the
Redirect. Hosts verify that the IP source address of the Redirect
is the same as the current first-hop router for the specified ICMP
Destination Address. Rules in [1] dictate that unspecified,
anycast, or multicast addresses may not be used as source
addresses. Therefore, the destination address will always be a
unicast address.
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4. Secure Neighbor Discovery Overview
New Neighbor Discovery options are used in to protect Neighbor and
Router Discovery messages. This specification introduces these
options, an authorization delegation discovery process, an address
ownership proof mechanism, and requirements for the use of these
components for Neighbor Discovery.
The components of the solution specified in this document are as
follows:
o Trusted roots are expected to certify the authority of routers. A
host and a router must have at least one common trusted root
before the host can adopt the router as its default router.
Optionally, an authorization certificate can specify the prefixes
for which the router is allowed to act as a router. Delegation
Chain Solicitation and Advertisement messages are used to discover
a certificate chain to the trusted root without requiring the
actual Router Discovery messages to carry lengthy certificate
chains.
o Cryptographically Generated Addresses are used to assure that the
sender of a Neighbor or Router Advertisement is the owner of an
the claimed address. A public-private key pair needs to be
generated by all nodes before they can claim an address.
o A new Neighbor Discovery option, the Signature option is used to
protect all messages relating to Neighbor and Router discovery.
Public key signatures are used. The trust to the public key is
established either with the authorization delegation process or
the address ownership proof mechanism, depending on configuration
and the type of the message protected.
o In order to prevent replay attacks, the new Neighbor Discovery
options Timestamp and Nonce are used. Given that Neighbor and
Router Discovery messages are in some cases sent to multicast
addresses, the Timestamp option offers replay protection without
previously established state or sequence numbers. In addition,
solicitation - advertisement pairs are protected through the Nonce
option.
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5. Neighbor Discovery Options
The following new ND mechanisms are required in SEND:
o The CGA option can be present in all Neighbor Discovery messages.
o The Signature option is required in all Neighbor Discovery
messages.
o The Timestamp option is required in all Neighbor Discovery
advertisements and Redirects.
o The Nonce option is required in all Neighbor Discovery
solicitations, and for all solicited advertisements.
o Proxy Neighbor Discovery is not supported in this specification
(it will be specified in a future document).
5.1 CGA Option
The CGA option allows the verification of the sender's CGA. The
format of the CGA 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Key Information .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The meaning of the fields is described below:
Type
TBD <To be assigned by IANA> for CGA.
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Length
The length of the option in units of 8 octets, i.e., 2.
Reserved
This is an 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 Information
This variable length field contains the public key of the sender.
It also may contain some other additional information which is
necessary when CGA is used.
The contents of the Key Information field are represented as ASN.1
DER-encoded data item of the following type:
SendKeyInformation ::= CGAParameters
CGAParameters ::= SEQUENCE {
publicKey SubjectPublicKeyInfo,
auxParameters CGAAuxParameters }
(The normative definition of the type CGAParameters is in in
[27]).
The verification of the CGA is based on the contents of the
CGAParameters structure.
This specification requires that if both the CGA option and the
Signature option are present, then the publicKey field in the
former option MUST be the public key referred to in the Key Hash
field in the latter option. Packets received with two different
keys MUST be silently discarded. Note that a future extension may
provide a mechanism which allows the owner of an address and the
signer to be different parties.
The length of the Key Information field is determined by the ASN.1
encoding.
Padding
This variable length field begins after the ASN.1 encoding of the
previous field has ends, and continues to the end of the option,
as specified by the Length field.
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5.1.1 Processing Rules for Senders
A node sending a message using the CGA option MUST construct the
message as follows:
The Key Information field in the Authentication Data field is set to
the SendKeyInformation structure according to the rules presented
above and in [27]. The used public key is taken from configuration.
An address MUST be constructed as specified in [27]. Depending on
the type of the message, this address appears in different places:
Redirect
The address MUST be the source address of the message.
Neighbor Solicitation
The address MUST be the Target Address for solicitations sent for
the purpose of Duplicate Address Detection, and source address of
the message otherwise.
Neighbor Advertisement
The address MUST be the source address of the message.
Router Solicitation
The address MUST be the source address of the message, unless it
is the unspecified address.
Router Advertisement
The address MUST be the source address of the message.
5.1.2 Processing Rules for Receivers
A message containing a Signature option MUST be checked as follows:
If the use of CGA has been configured, we require the receiving node
to verify the source address of the packet using the algorithm
described in Section 5 of [27]. The inputs for the algorithm are the
contents of the CGAParameters structure from the Key Information
field, the source address of the packet, and the minimum acceptable
Sec value from the security association. If the CGA verification is
successful, the recipient proceeds with the cryptographically more
time consuming check of the signature.
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Note that a receiver which does not support CGA or has not specified
its use in its security associations can still verify packets using
trusted roots, even if CGA had been used on a packet. The CGA
property of the address is simply left untested.
5.2 Signature Option
The Signature option allows public-key based signatures to be
attached to Neighbor Discovery messages. Both trusted root
authentication and CGAs can be used. The format of the Signature
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Key Hash |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Digital Signature .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The meaning of the fields is described below:
Type
TBD <To be assigned by IANA> for Signature.
Length
The length of the option in units of 8 octets, i.e., 2.
Reserved
This is an 16-bit field reserved for future use. The value MUST
be initialized to zero by the sender, and MUST be ignored by the
receiver..
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Key Hash
This 128 bit field contains a SHA1 hash of the public key used for
the constructing the signature. Its purpose is to associate the
signature to a particular key known by the receiver. Such a key
can be either stored in the certificate cache of the receiver, or
be received in the CGA option in the same message.
Digital Signature
This variable length field contains the signature made using the
sender's private key, over the the whole packet as defined by the
usual AH rules [3]. The signature is made using the RSA algorithm
and MUST be encoded as private key encryption in PKCS #1 format
[17].
This field starts after the Key Hash field. The length of the
Digital Signature field is determined by the PKCS #1 encoding.
Padding
This variable length field begins after the PKCS #1 encoding of
the previous field has ends, and continues to the end of the
option, as specified by the Length field.
5.2.1 Processing Rules for Senders
A node sending a message using the Signature option MUST construct
the message as follows:
o The message is constructed in its entirety.
o The Signature option is added as the last option in the message.
o For the purpose of constructing a signature, the following data
items are appended:
* The source address of the message.
* The destination address of the message.
* The contents of the message, starting from the ICMPv6 header,
up to and including the Key Information field in the Signature
option. The Signature and the Padding fields are not included.
o The message, in the form defined above, is signed using the
configured private key, and the resulting PCKS #1 signature is put
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to the Digital Signature field.
5.2.2 Processing Rules for Receivers
A message containing a Signature option MUST be checked as follows:
o The Signature option appears as the last option.
o The Key Information and Digital Signature fields have correct
encoding, and do not exceed the length of the Authentication Data
field.
o The Digital Signature verification shows that it has been
calculated as specified in the previous section.
o If the use of a trusted root has been configured, a valid
authorization delegation chain is known between the receiver's
trusted root and the sender's public key.
Note that the receiver may verify just the CGA property of a
packet, even if the sender has used a trusted root as well.
Messages that do not pass all the above tests MUST be silently
discarded.
5.2.3 Configuration
All nodes that support the reception of the Signature option MUST
record the following configuration information:
authorization method
This parameter determines the mechanisms through which the
authority of the sender is determined. It can have four values:
trusted root
The authority of the sender is verified as described in Section
6.5. The sender may have 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 [27]. The sender may have additional authority
through a trusted root, but this is neither required nor
verified.
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trusted root and CGA
Both the trusted root and the CGA verification is required.
trusted root or CGA
Either the trusted root or the CGA verification is required.
root
The public key of the trusted root, if authorization method is not
set CGA.
minbits
The minimum acceptable key length for peer public keys (and any
intermediaries between the trusted root and the peer). The
default SHOULD be 1024 bits. Implementations MAY also set an
upper limit in order to limit the amount of computation they need
to perform when verifying packets that use these security
associations.
minSec
The minimum acceptable Sec value, if CGA verification is required
(see Section 2 in [27]. This parameter is intended to facilitate
future extensions and experimental work. The minSec value SHOULD
always be set to zero.
All nodes that support the sending of the Signature option MUST
record the following configuration information:
keypair
A public-private key pair. If authorization delegation is in use,
there must exist a delegation chain from a trusted root to this
key pair.
CGA flag
A flag that indicates whether or not the CGA is used.
CGA parameters
Optionally any information required to construct CGAs, including
the used Sec value and nonce, and the CGA itself.
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5.3 Timestamp Option
The purpose of the Timestamp option is to ensure that unsolicited
advertisements and redirects have not been replayed. The format of
the Timestamp 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 +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where the fields are as follows:
Type
TBD <To be assigned by IANA> for Timestamp.
Length
The length of the option in units of 8 octets, i.e., 2.
Reserved
This is 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
This 64 bit unsigned integer field contains a timestamp. The
format is 64 bits, and the contents are the number of milliseconds
since January 1, 1970 00:00 UTC.
Senders SHOULD set the Timestamp field to the current time.
Receivers SHOULD be configured with an allowed Delta value. They
SHOULD maintain a cache of the last received timestamp value from
each specific source address within this time period. Receivers
SHOULD then check the Timestamp field as follows:
o A packet with a Timestamp field value beyond the current time plus
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or minus the allowed Delta value MUST be silently discarded.
Recommended default value for the allowed Delta is 3,600 seconds.
o A packet accepted according to the above rule MUST be checked
against the last received timestamp value from the given source
address. A packet that has already been seen from the same source
with the same or lower Timestamp field value MUST be silently
discarded.
o If packet passes both of the above tests, a new timestamp value
MUST be registered in the cache for the given source address.
o If the cache becomes full, the receiver SHOULD temporarily reduce
the Delta value for that source address so that all messages
within that value can still be stored.
5.4 Nonce Option
The purpose of the Nonce option is to ensure that an advertisement is
a fresh response to a solicitation sent earlier by this same node.
The format of the Nonce option is as 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 ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where the fields are as follows:
Type
TBD <To be assigned by IANA> for Nonce.
Length
The length of the option (including the Type, Length, and Nonce
fields) in units of 8 octets.
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Nonce
This field contains a random number selected by the sender of the
solicitation message. The length of the number MUST be at least 6
bytes.
5.5 Proxy Neighbor Discovery
The Target Address in Neighbor Advertisement is required to be equal
to the source address of the packet, except in the case of proxy
Neighbor Discovery. Proxy Neighbor Discovery is discussed in another
specification.
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6. Authorization Delegation Discovery
Several protocols, including IPv6 Neighbor Discovery, allow a node to
automatically configure itself based on information it learns shortly
after connecting to a new link. It is particularly easy for "rogue"
routers to be configured, and it is particularly difficult for a
network node to distinguish between valid and invalid sources of
information when the node needs this information before communicating
off-link.
Since the newly-connected node likely can not communicate off-link,
it can not be responsible for searching information to help validate
the router; however, given a chain of appropriately signed
certificates, it can check someone else's search results and conclude
that a particular message comes from an authorized source.
Similarly, the router, which is already connected to the network, can
if necessary communicate off-link and construct the certificate
chain.
The Secure Neighbor Discovery protocol introduces two new ICMPv6
messages that are used between hosts and routers to allow the client
to learn the certificate chain with the assistance of the router.
Where hosts have certificates from a trusted root, these messages MAY
also optionally be used between hosts to acquire the peer's
certificate chain.
The Delegation Chain Solicitation message is sent by hosts when they
wish to request the certificate chain between a router and the one of
the hosts' trusted roots. The Delegation Chain Advertisement message
is sent as an answer to this message, or periodically to the All
Nodes multicast address. These messages are separate from the rest
of the Neighbor Discovery in order to reduce the effect of the
potentially voluminous certificate chain information to other
messages.
The Authorization Delegation Discovery process does not exclude other
forms of discovering the certificate chains. For instance, during
fast movements mobile nodes may learn information - including the
certificate chains - of the next router from the previous router.
6.1 Delegation Chain Solicitation Message Format
Hosts send Delegation Chain Solicitations in order to prompt routers
to generate Delegation Chain Advertisements quickly.
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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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
Source Address
An IP address assigned to the sending interface, or the
unspecified address if no address is assigned to the sending
interface.
Destination Address
Typically the all-routers multicast address, the solicited-node
multicast address, or the address of the hosts' default router.
Hop Limit
255
ICMP Fields:
Type
TBD <To be assigned by IANA> for Delegation Chain Solicitation.
Code
0
Checksum
The ICMP checksum [8]..
Identifier
This 16 bit unsigned integer field acts as an identifier to
help match advertisements to solicitations. The Identifier
field MUST NOT be zero.
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Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Valid Options:
Trusted Root
One or more trusted roots that the client is willing to accept.
Future versions of this protocol may define new option types.
Receivers MUST silently ignore any options they do not recognize
and continue processing the message.
6.2 Delegation Chain Advertisement Message Format
Routers send out Delegation Chain Advertisement messages
periodically, or in response to a Delegation Chain Solicitation.
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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Component |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
Source Address
MUST be a unicast address assigned to the interface from which
this message is sent.
Destination Address
Either the solicited-node multicast address of the receiver or
the all-nodes multicast address.
Hop Limit
255
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ICMP Fields:
Type
TBD <To be assigned by IANA> for Delegation Chain
Advertisement.
Code
0
Checksum
The ICMP checksum [8]..
Identifier
This 16 bit unsigned integer field acts as an identifier to
help match advertisements to solicitations. The Identifier
field MUST be zero for unsolicited advertisements and MUST NOT
be zero for solicited advertisements.
Component
This is a 16 bit unsigned integer field used for informing the
receiver which certificate is being sent, and how many are
still left to be sent in the whole chain.
A single advertisement MUST be broken into separately sent
components if there is more than one Certificate option, in
order to avoid excessive fragmentation at the IP layer. Unlike
the fragmentation at the IP layer, individual components of an
advertisement may be stored and taken in use before all the
components have arrived; this makes them slightly more reliable
and less prone to Denial-of-Service attacks.
The first message in a N-component advertisement has the
Component field set to N-1, the second set to N-2, and so on.
Zero indicates that there are no more components coming in this
advertisement.
The components MUST be ordered so that the trusted root end of
the chain is the one sent first, each certificate sent after it
can be verified with previously sent certificates, and the
certificate of the sender comes last.
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Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Valid Options:
Certificate
One certificate is provided in Certificate option, to establish
a (part of) certificate chain to a trusted root.
Trusted Root
Zero or more Trusted Root options may be included to help
receivers decide which advertisements are useful for them. If
present, these options MUST appear in the first component of a
multi-component advertisement.
Future versions of this protocol may define new option types.
Receivers MUST silently ignore any options they do not recognize
and continue processing the message.
6.3 Trusted Root Option
The format of the Trusted Root option is as 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 | Name Type | Name Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where the fields are as follows:
Type
TBD <To be assigned by IANA> for Trusted Root.
Length
The length of the option (including the Type, Length, Name Type,
Name Length, and Name fields) in units of 8 octets.
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Name Type
The type of the name included in the Name field. This
specification defines only one legal value for this field:
1 FQDN
Name Length
The length of the Name field, in bytes. Octets beyond this length
but within the length specified by the Length field are padding
and MUST be set to zero by senders and ignored by receivers.
Name
When the Name Type field is set to 1, the Name field contains the
Fully Qualified Domain Name of the trusted root, for example
"trustroot.operator.com".
6.4 Certificate Option
The format of the certificate option is as 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 | Cert Type | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Certificate ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where the fields are as follows:
Type
TBD <To be assigned by IANA> for Certificate.
Length
The length of the option (including the Type, Length, Cert Type,
Pad Length, and Certificate fields) in units of 8 octets.
Cert Type
The type of the certificate included in the Name field. This
specification defines only one legal value for this field:
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1 X.509 Certificate
Pad Length
The amount of padding beyond the end of the Certificate field but
within the length specified by the Length field. Padding MUST be
set to zero by senders and ignored by receivers.
Certificate
When the Cert Type field is set to 1, the Certificate field
contains an X.509 certificate [16].
6.5 Router Authorization Certificate Format
The certificate chain of a router terminates in a router
authorization certificate that authorizes a specific IPv6 node as a
router. Because authorization chains are not common practice in the
Internet at the time this specification is being written, the chain
MUST consist of standard Public Key Certificates (PKC, in the sense
of [11]) for identity from the trusted root shared with the host to
the router. This allows the host to anchor trust for the router's
public key in the trusted root. The last item in the chain is an
Authorization Certificate (AC, in the sense of [12]) authorizing the
router's right to route. Stronger certification is necessary here
than for CGAs because the right to route must be granted by an
authorizing agency. Future versions of this specification may
include provision for full authorization certificate chains, should
they become common practice.
SEND nodes MUST support the RFC 3281 X.509 attribute certificate
format [12] as the default format for router authorization
certificates, and MAY support other formats. Router authorization
certificates MUST be signed by the network operator or other trusted
third party whose PKC is above the router's PKC in the delegation
chain. Routers MAY advertise multiple ACs if the trust delegation
obtains from different trust roots, and the authorized prefixes in
those certificates MAY be disjoint. A router SHOULD only advertise
one AC corresponding to one trust root and all interfaces and
prefixes covered by that trust root MUST be in the AC.
In the attribute certificate, the GeneralName type MUST be either a
dNSName or iPAddress for the router, unless otherwise specified by
RFC 3281. If the GeneralName attribute is a dNSName, it MUST be
resolvable to a global unicast address assigned to the router. If
the GeneralName attribute is an iPAddress, it MUST be a global
unicast address assigned to the router. For purposes of facilitating
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renumbering, a dNSName SHOULD be used. However, hosts MUST NOT use a
dNSName or iPAddress for validating the certificate. The router's
public key hash, stored in the
acinfo.holder.objectDigestInfo.objectDigest field of the certificate
provides the definitive validation. As explained in Section 8.2, the
addresses from the certificate can be matched against the global
addresses claimed in the Router Advertisement.
6.5.1 Field Values
acinfo.holder.entityName
This field MAY contain one or several entityNames, of type dNSName
or iPAddress, referring to global address(es) belonging to the
router.
acinfo.objectDigestInfo.digestedObjectType
This field MUST be present and of type (1), publicKey.
acinfo.holder.digestAlgorithm
This field MUST indicate id-sha1 as indicated in RFC 3279 [10].
acinfo.objectDigestInfo.objectDigest
This field MUST be a SHA-1 digest over either a PKCS#1 [17] (RSA)
or an RFC 3279 Section 2.3.2 representation [10] (DSA)
representation of the router's public key. If this digest does
not match the digest of the router's public key from its PKC, a
node MUST discard the certificate.
acinfo.issuer.v2form.issuerName
The field MUST contain the distinguished name from the PKC used to
sign the router AC.
acinfo.attrCertValidityPeriod
A node MUST NOT accept a certificate if the validity period has
ended or has not yet started.
acinfo.attributes
This field MUST contain a list of prefixes that the router is
authorized to route, or the unspecified prefix if the router
is allowed to route any prefix. The field has the following
type:
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name: AuthorizedSubnetPrefix
OID: {id-rcert}
Syntax: iPAddress
values: Multiple allowed
Multiple prefix values are allowed.
The details of the above syntax are specified in Section 2.2.3.8
of [14].
If the router is authorized only to route specific prefixes, the
ipAddress values consist of IPv6 addresses in standard RFC 3513
[13] prefix format. One iPAddress value appears for each prefix
routed by the router. If the router is authorized to route any
prefix, a single ipAddress value appears with the value of the
unspecified address.
6.6 Processing Rules for Routers
Routers SHOULD possess a key pair and certificate from at least one
certificate authority.
A router MUST silently discard any received Delegation Chain
Solicitation messages that do not satisfy all of the following
validity checks:
o The IP Hop Limit field has a value of 255, i.e., the packet could
not possibly have been forwarded by a router.
o If the message includes an IP Authentication Header, the message
authenticates correctly.
o ICMP Checksum is valid.
o ICMP Code is 0.
o ICMP length (derived from the IP length) is 8 or more octets.
o Identifier field is non-zero.
o All included options have a length that is greater than zero.
The contents of the Reserved field, and of any unrecognized options,
MUST be ignored. Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values. The
contents of any defined options that are not specified to be used
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with Router Solicitation messages MUST be ignored and the packet
processed in the normal manner. The only defined option that may
appear is the Trusted Root option. A solicitation that passes the
validity checks is called a "valid solicitation".
Routers MAY send unsolicited Delegation Chain Advertisements for
their trusted root. When such advertisements are sent, their timing
MUST follow the rules given for Router Advertisements in RFC 2461
[6]. The only defined option that may appear is the Certificate
option. At least one such option MUST be present. Router SHOULD
also include at least one Trusted Root option to indicate the trusted
root on which the Certificate is based.
In addition to sending periodic, unsolicited advertisements, a router
sends advertisements in response to valid solicitations received on
an advertising interface. A router MUST send the response to the
all-nodes multicast address, if the source address in the
solicitation was the unspecified address. If the source address was
a unicast address, the router MUST send the response to the
solicited-node multicast address corresponding to the source address.
In a solicited advertisement, the router SHOULD include suitable
Certificate options so that a delegation chain to the solicited root
can be established. The root is identified by the FQDN from the
Trusted Root option being equal to an FQDN in the AltSubjectName
field of the root's certificate. The router SHOULD include the
Trusted Root option(s) in the advertisement for which the delegation
chain was found.
If the router is unable to find a chain to the requested root, it
SHOULD send an advertisement without any certificates. In this case
the router SHOULD include the Trusted Root options which were
solicited.
Rate limitation of Delegation Chain Advertisements is performed as
specified for Router Advertisements in RFC 2461 [6].
6.7 Processing Rules for Hosts
Hosts SHOULD possess the certificate of at least one certificate
authority, and MAY possess their own key pair and certificate from
this authority.
A host MUST silently discard any received Delegation Chain
Advertisement messages that do not satisfy all of the following
validity checks:
o IP Source Address is a unicast address. Note that routers may use
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multiple addresses, so this address not sufficient for the unique
identification of routers.
o IP Destination Address is either the all-nodes multicast address
or the solicited-node multicast address corresponding to one of
the unicast addresses assigned to the host.
o The IP Hop Limit field has a value of 255, i.e., the packet could
not possibly have been forwarded by a router.
o If the message includes an IP Authentication Header, the message
authenticates correctly.
o ICMP Checksum is valid.
o ICMP Code is 0.
o ICMP length (derived from the IP length) is 16 or more octets.
o All included options have a length that is greater than zero.
The contents of the Reserved field, and of any unrecognized options,
MUST be ignored. Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values. The
contents of any defined options that are not specified to be used
with Delegation Chain Advertisement messages MUST be ignored and the
packet processed in the normal manner. The only defined option that
may appear is the Certificate option. An advertisement that passes
the validity checks is called a "valid advertisement".
Hosts SHOULD store certificate chains retrieved in Delegation Chain
Discovery messages if they start from a root trusted by the host.
The certificates chains SHOULD be verified before storing them.
Routers are required to send the certificates one by one, starting
from the trusted root end of the chain. Except for temporary
purposes to allow for message loss and reordering, hosts SHOULD NOT
store certificates received in a Delegation Chain Advertisement
unless they contain a certificate which can be immediately verified
either to the trusted root or to a certificate which has been
verified earlier.
Note that it may be useful to cache this information and implied
verification results for use over multiple attachments to the
network. In order to use an advertisement for the verification of a
specific Neighbor Discovery message, the host matches the key hash in
acinfo.Holder.objectDigestInfo to the public key carried in the IPsec
AH Authentication Data field.
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When an interface becomes enabled, a host may be unwilling to wait
for the next unsolicited Delegation Chain Advertisement. To obtain
such advertisements quickly, a host SHOULD transmit up to
MAX_RTR_SOLICITATIONS Delegation Chain Solicitation messages each
separated by at least RTR_SOLICITATION_INTERVAL seconds. Delegation
Chain Solicitations SHOULD be sent after any of the following events:
o The interface is initialized at system startup time.
o The interface is reinitialized after a temporary interface failure
or after being temporarily disabled by system management.
o The system changes from being a router to being a host, by having
its IP forwarding capability turned off by system management.
o The host attaches to a link for the first time.
o A movement has been indicated by lower layers or has been inferred
from changed information in a Router Advertisement.
o The host re-attaches to a link after being detached for some time.
o A Router Advertisement has been received with a public key that is
not stored in the hosts' cache of certificates, or there is no
authorization delegation chain to the host's trusted root.
Delegation Chain Solicitations MUST NOT be sent if the host has a
currently valid certificate chain for the router to a trusted root,
including the Attribute Certificate for the desired router (or host).
A host MUST send Delegation Chain Solicitations either to the
All-Routers multicast address, if it has not selected a default
router yet, or to the default router's IP address if it has already
been selected.
If two hosts communicate with the solicitations and advertisements,
the solicitations MUST be sent to the solicited-node multicast
address of the receiver. The advertisements MUST be sent as
specified above for routers.
Delegation Chain Solicitations SHOULD be rate limited and timed
similarly with Router Solicitations, as specified in RFC 2461 [6].
When processing a possible advertisement sent as a response to a
solicitation, the host MAY prefer to process first those
advertisements with the same Identifier field value as in the
solicitation. This makes Denial-of-Service attacks against the
mechanism harder (see Section 11.3).
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7. Securing Neighbor Discovery with SEND
This section describes how to use the mechanisms from Section 5,
Section 6, and the reference [27] in order to provide security for
Neighbor Discovery.
7.1 Neighbor Solicitation Messages
All Neighbor Solicitation messages are protected with SEND.
7.1.1 Sending Secure Neighbor Solicitations
Secure Neighbor Solicitation messages are sent as described in RFC
2461 and 2462, with the additional requirements listed in the
following.
All Neighbor Solicitation messages sent MUST contain the Nonce,
Timestamp and Signature options, and MAY contain the CGA option. The
Signature option MUST be configured with the sender's key pair,
setting the authorization method and additional information as is
desired.
7.1.2 Receiving Secure Neighbor Solicitations
Received Neighbor Solicitation messages are processed as described in
RFC 2461 and 2462, with the additional SEND-related requirements
listed in the following.
Neighbor Solicitation messages received without the Nonce, Timestamp,
or Signature option MUST be silently discarded. The Signature option
MUST be configured with the expected authorization method, the
minimum allowable key size, and optionally with the information
related to the trusted root and the acceptable minSec value.
7.2 Neighbor Advertisement Messages
All Neighbor Advertisement messages are protected with SEND.
7.2.1 Sending Secure Neighbor Advertisements
Secure Neighbor Advertisement messages are sent as described in RFC
2461 and 2462, with the additional requirements listed in the
following.
All Neighbor Advertisement messages sent MUST be sent with the
Timestamp and Signature options and MAY be sent with the CGA option.
The Signature option MUST be configured with the sender's key pair,
setting the authorization method and additional information as is
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desired.
Neighbor Advertisements sent in response to a Neighbor Solicitation
MUST contain a copy of the Nonce option included in the solicitation.
7.2.2 Receiving Secure Neighbor Advertisements
Received Neighbor Advertisement messages are processed as described
in RFC 2461 and 2462, with the additional SEND-related requirements
listed in the following.
Neighbor Advertisement messages received without the Timestamp and
Signature options MUST be silently discarded. The Signature option
MUST be configured with the expected authorization mechanism (CGA or
trusted root), the minimum allowable key size, and optionally with
the information related to the trusted root and the acceptable minSec
value.
Received Neighbor Advertisements sent to a unicast destination
address without a Nonce option MUST be silently discarded.
7.3 Other Requirements
Upon receiving a message for which the receiver has no certificate
chain to a trusted root, the receiver MAY use Authorization
Delegation Discovery to learn the certificate chain of the peer.
Hosts that use stateless address autoconfiguration MUST generate a
new CGA as specified in Section 4 of [27] for each new
autoconfiguration run.
It is outside the scope of this specification to describe the use of
trusted root authorization between hosts with dynamically changing
addresses. Such dynamically changing addresses may be the result of
stateful or stateless address autoconfiguration or through the use of
RFC 3041 [9]. If the CGA method is not used, hosts would be required
to exchange certificate chains that terminate in a certificate
authorizing a host to use an IP address having a particular interface
identifier. This specification does not specify the format of such
certificates, since there are currently a few cases where such
certificates are required by the link layer and it is up to the link
layer to provide certification for the interface identifier. This
may be the subject of a future specification. It is also outside the
scope of this specification to describe how stateful address
autoconfiguration works with the CGA method.
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8. Securing Router Discovery with SEND
This section describes how to use the mechanisms from Section 5,
Section 6, and the reference [27] in order to provide security for
Router Discovery.
8.1 Router Solicitation Messages
All Router Solicitation messages are protected with SEND.
8.1.1 Sending Secure Router Solicitations
Secure Router Solicitation messages are sent as described in RFC
2461, with the additional requirements listed in the following.
Router Solicitation messages sent with an unspecified source address
MUST have the Nonce and Timestamp options. Other Router
Solicitations MUST have the Nonce, Timestamp, and Signature options.
The Signature option MUST be configured with the sender's key pair,
setting the authorization method and additional information as is
desired.
8.1.2 Receiving Secure Router Solicitations
Received Router Solicitation messages are processed as described in
RFC 2461, with the additional SEND-related requirements listed in the
following.
Router Solicitation message sent with an unspecified source address
and without the Nonce and Timestamp options MUST be silently
discarded. Router Solicitation messages received with another type
of source address but without the Nonce, Timestamp, and Signature
options MUST be silently discarded. The Signature option MUST be
configured with the expected authorization mechanism (CGA or trusted
root), the minimum allowable key size, and optionally with the
information related to the trusted root and the acceptable minSec
value.
8.2 Router Advertisement Messages
All Router Advertisement messages are protected with SEND.
8.2.1 Sending Secure Router Advertisements
Secure Router Advertisement messages are sent as described in RFC
2461, with the additional requirements listed in the following.
All Router Advertisement messages sent MUST contain a Timestamp and
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Signature options. The Signature option SHOULD be configured to
protect the advertisement with the trusted root authorization method
and MAY be configured to additionally protect it with the CGA
authorization method.
Router Advertisements sent in response to a Router Solicitation MUST
contain a copy of the Nonce option included in the solicitation.
8.2.2 Receiving Secure Router Advertisements
Received Router Advertisement messages are processed as described in
RFC 2461, with the additional SEND-related requirements listed in the
following.
Router Advertisement messages received without the Timestamp and
Signature options MUST be silently discarded. The Signature option
SHOULD be configured to require the trusted-root authorization method
and they MAY additionally be configured to require CGA
authentication.
Received Router Advertisements sent to a unicast destination address
without a Nonce option MUST be silently discarded.
8.3 Redirect Messages
All Redirect messages are protected with SEND.
8.3.1 Sending Redirects
Secure Redirect messages are sent as described in RFC 2461, with the
additional requirements listed in the following.
All Redirect messages sent MUST contain the Timestamp and Signature
options. The security associations used for this MUST be configured
with the sender's key pair, setting the authorization method and
additional information as is desired.
8.3.2 Receiving Redirects
Received Redirect messages are processed as described in RFC 2461,
with the additional SEND-related requirements listed in the
following.
Redirect messages received without the Timestamp and Signature
options MUST be silently discarded. The Signature option MUST be
configured with the expected authorization mechanism (CGA or trusted
root), the minimum allowable key size, and optionally with the
information related to the trusted root and the acceptable minSec
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value.
If only CGA-based security associations are used, hosts MUST follow
the rules defined below when receiving Redirect messages:
1. The Redirect message MUST be protected as discussed above.
2. The receiver MUST verify that the Redirect message comes from an
IP address to which the host may have earlier sent the packet
that the Redirect message now partially returns. That is, the
source address of the Redirect message must be the default router
for traffic sent to the destination of the returned packet. If
this is not the case, the message MUST be silently discarded.
This step prevents a bogus router from sending a Redirect message
when the host is not using the bogus router as a default router.
8.4 Other Requirements
The certificate for a router MAY specify the global IP address(es) of
the router. If so, only these addresses can appear in advertisements
where the Router Address (R) bit [15] is set. All hosts MUST have
the certificate of a trusted root.
Hosts SHOULD use Authorization Delegation Discovery to learn the
certificate chain of their default router or peer host, as explained
in Section 6. The receipt of a protected Router Advertisement
message for which no router Authorization Certificate and certificate
chain is available triggers Authorization Delegation Discovery.
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9. Co-Existence of SEND and ND
During the transition to secure links or as a policy consideration,
network operators may want to run a particular link with a mixture of
secure and insecure nodes. However, all routers are required to
support SEND. The following behaviour is mandated:
o Router Solicitations SHOULD be accepted without the Nonce,
Timestamp, CGA, and Signature options. The router SHOULD respond
according to the rules outlined in Section 8.2 except that a Nonce
option is not sent.
o Neighbor Solicitations SHOULD be accepted without the Nonce,
Timestamp, CGA, and Signature options. The receiver SHOULD
respond according to the rules outlined in Section 7.2 except that
a Nonce option is not sent.
o Neighbor Advertisements SHOULD be accepted without the Timestamp,
CGA and Signature options. The receiver SHOULD act according to
the RFC 2461 [6] and RFC 2462 [7] rules, but take precedence for
information sent using SEND over plain ND.
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10. Performance Considerations
The computations related to AH_RSA_Sig transform are computationally
relatively expensive operations.
In the application for which AH_RSA_Sig has been designed, however,
hosts typically have the need to perform only a few operations as
they enter a link, and a few operations as they find a new on-link
peer with which to communicate.
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 operations on a
router is on the order of a few dozen operations per second, some of
which can be precomputed as discussed below. A large number of
router solicitations may cause higher demand for performing
asymmetric operations, although RFC 2461 limits the rate at which
responses to solicitations can be sent.
Signatures related to the use of the AH_RSA_Sig transform MAY be
precomputed for Multicast Neighbor and Router Advertisements.
Typically, solicited advertisements are sent to the unicast address
from which the solicitation was sent. Given that the IPv6 header is
covered by the AH integrity protection, it is typically not possible
to precompute solicited advertisements.
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11. Security Considerations
11.1 Threats to the Local Link Not Covered by SEND
SEND does not compensate for an insecure link layer. In particular,
there is no cryptographic binding in SEND between the link layer
frame address and the IPv6 address. On an insecure link layer that
allows nodes to spoof the link layer address of other nodes, an
attacker could disrupt IP service by sending out a Neighbor
Advertisement having the source address on the link layer frame of a
victim, a valid CGA with valid AH signature corresponding to itself,
and a Target Link-layer Address extension corresponding to the
victim. The attacker could then proceed to cause a traffic stream to
bombard the victim in a DoS attack. To protect against such attacks,
link layer security MUST be used. An example of such for 802 type
networks is port-based access control [35].
Prior to participating in Neighbor Discovery and Duplicate Address
Detection, nodes must subscribe to the All Nodes Multicast Group and
Solicited Node Multicast Group for the address that they are claiming
RFC 2461 [6]. Subscribing to a multicast group requires that the
nodes use MLD [22]. MLD contains no provision for security. An
attacker could send an MLD Done message to unsubscribe a victim from
the Solicited Node Multicast address. However, the victim should be
able to detect such an attack because the router sends a
Multicast-Address-Specific Query to determine whether any listeners
are still on the address, at which point the victim can respond to
avoid being dropped from the group. This technique will work if the
router on the link has not been compromised. Other attacks using MLD
are possible, but they primarily lead to extraneous (but not
overwhelming) traffic.
11.2 How SEND Counters Threats to Neighbor Discovery
The SEND protocol is designed to counter the threats to IPv6 Neighbor
Discovery outlined in [29]. The following subsections contain a
regression of the SEND protocol against the threats, to illustrate
what aspects of the protocol counter each threat.
11.2.1 Neighbor Solicitation/Advertisement Spoofing
This threat is defined in Section 4.1.1 of [29]. The threat is that
a spoofed Neighbor Solicitation or Neighbor Advertisement causes a
false entry in a node's Neighbor Cache. There are two cases:
1. Entries made as a side effect of a Neighbor Solicitation or
Router Solicitation. There are two cases:
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1. A router receiving a Router Solicitation with a firm IPv6
source address and a Target Link-Layer Address extension
inserts an entry for the IPv6 address into its Neighbor
Cache.
2. A node doing Duplicate Address Detection (DAD) that receives
a Neighbor Solicitation for the same address regards the
situation as a collision and ceases to solicit for the
address.
2. Entries made as a result of a Neighbor Advertisement sent as a
response to a Neighbor Solicitation for purposes of on-link
address resolution.
11.2.1.1 Solicitations with Effect
SEND counters the threat of solicitations with effect in the
following ways:
1. As discussed in Section 5, SEND nodes preferably send Router
Solicitations with a firm IPv6 address and AH header, which the
router can verify, so the Neighbor Cache binding is correct. If
a SEND node must send a Router Solicitation with the unspecified
address, the router will not update its Neighbor Cache, as per
RFC 2461.
2. When SEND nodes are performing DAD, they use the tentative
address as the source address on the Neighbor Solicitation
packet, and include an IPv6 AH header. This allows the receiving
SEND node to verify the solicitation.
See Section 11.2.5, below, for discussion about replay protection and
timestamps.
11.2.1.2 Address Resolution
SEND counters attacks on address resolution by requiring that the
responding node include an AH header with a signature on the packet,
and that the node's interface identifier either be a CGA or that the
node be able to produce a certificate authorizing that node to use
the interface identifier.
The Neighbor Solicitation and Advertisement pairs implement a
challenge-response protocol, as explained in Section 7 and discussed
in Section 11.2.5 below.
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11.2.2 Neighbor Unreachability Detection Failure
This attack is described in Section 4.1.2 of [29]. SEND counters
this attack by requiring a node responding to Neighbor Solicitations
sent as NUD probes to include an AH header and proof of authorization
to use the interface identifier in the address being probed. If
these prerequisites are not met, the node performing NUD discards the
responses.
11.2.3 Duplicate Address Detection DoS Attack
This attack is described in Section 4.1.3 of [29]. SEND counters
this attack by requiring the Neighbor Advertisements sent as
responses to DAD to include an AH header and proof of authorization
to use the interface identifier in the address being tested. If
these prerequisites are not met, the node performing DAD discards the
responses.
When a SEND node is used on a link that also connects to non-SEND
nodes, the SEND node defends its addresses by sending unprotected
Neighbor Solicitations with an unspecified address, as explained in
Section 9. However, the SEND node ignores any unprotected Neighbor
Solicitations or Advertisements that may be send by the non-SEND
nodes. This protects the SEND node from DAD DoS attacks by non-SEND
nodes or attackers simulating to non-SEND nodes, at the cost of a
potential address collision between a SEND node and non-SEND node.
The probability and effects of such an address collision are
discussed in [27].
11.2.4 Router Solicitation and Advertisement Attacks
These attacks are described in Sections 4.2.1, 4.2.4, 4.2.5, 4.2.6,
and 4.2.7 of [29]. SEND counters these attacks by requiring Router
Advertisements to contain an AH header, and that the signature in the
header be calculated using the public key of a host that can prove
its authorization to route the subnet prefixes contained in any
Prefix Information Options. The router proves it authorization by
showing an attribute certificate containing the specific prefix or
the indication that the router is allowed to route any prefix. A
Router Advertisement without these protections is dropped as part of
the IPsec processing.
SEND does not protect against brute force attacks on the router, such
as DoS attacks, or compromise of the router, as described in Sections
4.4.2 and 4.4.3 of [29].
11.2.5 Replay Attacks
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This attack is described in Section 4.3.1 of [29]. SEND protects
against attacks in Router Solicitation/Router Advertisement and
Neighbor Solicitation/Neighbor Advertisement transactions by
including a Nonce option in the solicitation and requiring the
advertisement to include a matching option. Together with the
signatures this forms a challenge-response protocol. SEND protects
against attacks from unsolicited messages such as Neighbor
Advertisements, Router Advertisements, and Redirects by including a
timestamp into the AH header. A window of vulnerability for replay
attacks exists until the timestamp expires.
When timestamps are used, SEND nodes are protected against replay
attacks as long as they cache the state created by the message
containing the timestamp. The cached state allows the node to
protect itself against replayed messages. However, once the node
flushes the state for whatever reason, an attacker can re-create the
state by replaying an old message while the timestamp is still valid.
Since most SEND nodes are likely to use fairly coarse grained
timestamps, as explained in Section 5.3, this may affect some nodes.
11.2.6 Neighbor Discovery DoS Attack
This attack is described in Section 4.3.2 of [29]. In this attack,
the attacker bombards the router with packets for fictitious
addresses on the link, causing the router to busy itself with
performing Neighbor Solicitations for addresses that do not exist.
SEND does not address this threat because it can be addressed by
techniques such as rate limiting Neighbor Solicitations, restricting
the amount of state reserved for unresolved solicitations, and clever
cache management. These are all techniques involved in implementing
Neighbor Discovery on the router.
11.3 Attacks against SEND Itself
The CGAs have a 59-bit hash value. The security of the CGA mechanism
has been discussed in [27].
Some Denial-of-Service attacks against ND and SEND itself remain.
For instance, an attacker may try to produce a very high number of
packets that a victim host or router has to verify using asymmetric
methods. While safeguards are required to prevent an excessive use
of resources, this can still render SEND non-operational.
Security associations based on the use of asymmetric cryptography can
be vulnerable to Denial-of-Service attacks, particularly when the
attacker can guess the SPIs and destination addresses used in the
security associations. In SEND this is easy, as both the SPIs and
the addresses (such as all nodes multicast address) are standardized.
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Due to the use of multicast, one packet sent by the attacker will be
processed by multiple receivers.
When CGA protection is used, SEND deals with these attacks using the
verification process described in Section 5.2.2. In this process a
simple hash verification of the CGA property of the address is
performed first before performing the more expensive signature
verification.
When trusted roots and certificates are used for address validation
in SEND, the defenses are not quite as effective. Implementations
SHOULD track the resources devoted to the processing of packets
received with the AH_RSA_Sig transform, and start selectively
dropping packets if too many resources are spent. Implementations
MAY also drop first packets that are not protected with CGA.
The Authorization Delegation Discovery process may also be vulnerable
to Denial-of-Service attacks. An attack may target a router by
request a large number of delegation chains to be discovered for
different roots. Routers SHOULD defend against such attacks by
caching discovered information (including negative responses) and by
limiting the number of different discovery processes they engage in.
Attackers may also target hosts by sending a large number of
unnecessary certificate chains, forcing hosts to spend useless memory
and verification resources for them. Hosts defend against such
attacks by limiting the amount of resources devoted to the
certificate chains and their verification. Hosts SHOULD also
prioritize advertisements sent as a response to their requests above
multicast advertisements.
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12. IANA Considerations
This document defines two new ICMP message types, used in
Authorization Delegation Discovery. These messages must be assigned
ICMPv6 type numbers from the informational message range:
o The Delegation Chain Solicitation message, described in Section
6.1.
o The Delegation Chain Advertisement message, described in Section
6.2.
This document defines two new Neighbor Discovery [6] options, which
must be assigned Option Type values within the option numbering space
for Neighbor Discovery messages:
o The Trusted Root option, described in Section 6.3.
o The Certificate option, described in Section 6.4.
o The CGA option, described in Section 5.1.
o The Signature option, described in Section 5.2.
o The Timestamp option, described in Section 5.3.
o The Nonce option, described in Section 5.4.
This document defines a new name space for the Name Type field in the
Trusted Root option. Future values of this field can be allocated
using standards action [5].
Another new name space is allocated for the Cert Type field in the
Certificate option. Future values of this field can be allocated
using standards action [5].
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13. Comparison to AH-Based Approach
This approach has the following benefits compared to the current
Working Group document approach:
o The full implementation of the security mechanism, including
Nonces and CGAs, exists within one module. There is no need to
analyze the security of the mechanism across ND, IPsec, and
possibly the CGA layers.
o The CGA part of the solution can easily be separated into its own
optional specification, if IPR concerns can not be resolved. This
is possible because the CGA handling is done in its own option.
(The authorization method configuration flag is the only thing
common to the CGA and Signature options.)
o No extensions or modifications of IPsec processing are required:
SPD entries are not required to distinguish ICMP types, AH does
not need to support public keys or CGAs, and destination address
acgnostic security associations are not needed.
o It is not necessary to allocate a new multicast address to
represent the solicited-node multicast address for SEND nodes.
o It is not necessary to change the Neighbor Discovery behavior with
regards to the use of the unspecified address. Since all
information is available within the Neighbor Discovery messages,
unspecified source addresses can be used, still being able to
correlate the CGA property with the Target Address in a Neighbor
Solicitation during Duplicate Address Detection.
o The transition mechanisms for links with both SEND and non-SEND
nodes are significantly simpler. In particular, non-SEND nodes
will be able to receive DAD probes and other messages sent by the
SEND nodes.
o Only a single set of Neighbor Discovery messages from the router
needs to be transmitted on a link. This helps avoid extra
overhead for mobility beacons and other frequently occurring
messaging.
o Given that the asymmetric computations required in SEND are
computationally expensive, it is necessary to control the number
of these operations in order to avoid Denial-of-Service attacks.
This control is easier to arrange with "application layer"
information. For instance, a router need not verify more Router
Solicitations with an unspecified source address than it can
respond to according to the RFC 2461 rules.
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o There is no need for an API to communicate certificate chains
requests and certificate chains between the IPsec and Neighbor
Discovery modules.
Also, a good implementation of SEND would not require the user to
configure it (beyond perhaps enabling it). In order to achieve
this with IPsec, a set of policy entries needs to be automatically
created upon system start. This may require an additional API.
o There is no need for the CGA parameters to be stored both in the
IPsec and Neighbor Discovery modules, where they are needed for
the construction of AH headers and addresses, respectively.
o It is not necessary to change existing BITS or BITW IPsec
implementations to support SEND and AH_RSA_Sig. There are two
problems associated with such changes:
* A SEND implementation in such environment can not proceed until
this modification has been completed.
* Typical hardware that processes IPsec packets may not be easily
changed to process asymmetric transforms. (Of course such
packets can be passed to the main CPU at the node, assuming
this can easily be done in the given implementation.)
o In addition, many IPsec implementations are highly optimized
because they are on the fast path for packet processing. For
example, the Linux implementation runs in the kernel interrupt
thread. Some of the SEND modifications might require IPsec
processing to wait on a semaphore while, for example, a
certificate chain is fetched, an operation that takes place out of
band in regular IPsec processing because it is done using IKE.
While it is possible that the implemenation can be arranged so
that general IPsec processing isn't impacted, the resulting code
could increase in complexity.
The use of IPsec to protect ND is possible, but the limits and
capabilities of IPsec have to be stretched. Small changes in the ND
protocol (or our understanding of the issues) may cause a situation
which is no longer easily handled when the "application" and the
security exist at different layers. Although IPsec as defined in RFC
2402 just defines a header format, RFC 2401 and the ensuing years of
implementation have evolved a complex interconnected set of
components for IPsec which will require some modification to
accommodate SEND.
On the other hand, IPsec is the current solution for securing ND in
the original ND RFCs. Even if the current IPsec can be used only in
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very limited networks to secure ND, it could be argued that it is
logical to continue its use. Also, the existence of an asymmetric
transform in IPsec would be potentially useful in other contexts as
well.
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Normative References
[1] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[2] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[3] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[4] Piper, D., "The Internet IP Security Domain of Interpretation
for ISAKMP", RFC 2407, November 1998.
[5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[6] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[7] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[8] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998.
[9] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[10] Bassham, L., Polk, W. and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile", RFC
3279, April 2002.
[11] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002.
[12] Farrell, S. and R. Housley, "An Internet Attribute Certificate
Profile for Authorization", RFC 3281, April 2002.
[13] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
Addressing Architecture", RFC 3513, April 2003.
[14] Lynn, C., "X.509 Extensions for IP Addresses and AS
Identifiers", Internet-Draft (expired)
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draft-ietf-pkix-x509-ipaddr-as-extn-00, February 2002.
[15] Perkins, C., Johnson, D. and J. Arkko, "Mobility Support in
IPv6", draft-ietf-mobileip-ipv6-22 (work in progress), May
2003.
[16] International Organization for Standardization, "The Directory
- Authentication Framework", ISO Standard X.509, 2000.
[17] RSA Laboratories, "RSA Encryption Standard, Version 1.5", PKCS
1, November 1993.
[18] National Institute of Standards and Technology, "Secure Hash
Standard", FIPS PUB 180-1, April 1995, <http://
www.itl.nist.gov/fipspubs/fip180-1.htm>.
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Informative References
[19] Postel, J., "Internet Control Message Protocol", STD 5, RFC
792, September 1981.
[20] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37, RFC
826, November 1982.
[21] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[22] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
[23] Arkko, J., "Effects of ICMPv6 on IKE and IPsec Policies",
draft-arkko-icmpv6-ike-effects-01 (work in progress), June
2002.
[24] Arkko, J., "Manual SA Configuration for IPv6 Link Local
Messages", draft-arkko-manual-icmpv6-sas-01 (work in progress),
June 2002.
[25] Droms, R., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress),
November 2002.
[26] Kent, S., "IP Encapsulating Security Payload (ESP)",
draft-ietf-ipsec-esp-v3-04 (work in progress), March 2003.
[27] Aura, T., "Cryptographically Generated Addresses (CGA)",
draft-ietf-send-cga-00.txt (work in progress), May 2003.
[28] Arkko, J., Kempf, J., Sommerfeld, B. and B. Zill, "SEcure
Neighbor Discovery (SEND) Protocol",
draft-ietf-send-ipsec-00.txt (work in progress), February 2003.
[29] Nikander, P., "IPv6 Neighbor Discovery trust models and
threats", draft-ietf-send-psreq-00 (work in progress), October
2002.
[30] Montenegro, G. and C. Castelluccia, "SUCV Identifiers and
Addresses", draft-montenegro-sucv-03 (work in progress), July
2002.
[31] O'Shea, G. and M. Roe, "Child-proof Authentication for MIPv6",
Computer Communications Review, April 2001.
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[32] Nikander, P., "Denial-of-Service, Address Ownership, and Early
Authentication in the IPv6 World", Proceedings of the Cambridge
Security Protocols Workshop, April 2001.
[33] Arkko, J., Aura, T., Kempf, J., Mantyla, V., Nikander, P. and
M. Roe, "Securing IPv6 Neighbor Discovery", Wireless Security
Workshop, September 2002.
[34] Montenegro, G. and C. Castelluccia, "Statistically Unique and
Cryptographically Verifiable (SUCV) Identifiers and Addresses",
NDSS, February 2002.
[35] Institute of Electrical and Electronics Engineers, "Local and
Metropolitan Area Networks: Port-Based Network Access Control",
IEEE Standard 802.1X, September 2001.
Author's Address
Jari Arkko
Ericsson
Jorvas 02420
Finland
EMail: jari.arkko@ericsson.com
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Appendix A. Contributors
Most of the substantive material in this document has been derived
from the current official Working Group item [28]. The authors of
that document have deserve full credit for this document as well.
All errors are mine, however.
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Appendix B. Acknowledgements
The author would like to thank James Kempf, Pekka Nikander, Tuomas
Aura, Ran Atkinson for interesting discussions in this problem space.
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Appendix C. IPR Considerations
The optional CGA part of SEND uses public keys and hashes to prove
address ownership. Several IPR claims have been made about such
methods.
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